     ______   ___    ___
    /\  _  \ /\_ \  /\_ \
    \ \ \L\ \\//\ \ \//\ \      __     __   _ __   ___ 
     \ \  __ \ \ \ \  \ \ \   /'__`\ /'_ `\/\`'__\/ __`\
      \ \ \/\ \ \_\ \_ \_\ \_/\  __//\ \L\ \ \ \//\ \L\ \
       \ \_\ \_\/\____\/\____\ \____\ \____ \ \_\\ \____/
        \/_/\/_/\/____/\/____/\/____/\/___L\ \/_/ \/___/
                                       /\____/
                                       \_/__/     Version 4.4.3


                A game programming library.

             By Shawn Hargreaves, Mar 03, 2019.

                See the AUTHORS file for a
               complete list of contributors.



#include <std_disclaimer.h>

   "I do not accept responsibility for any effects, adverse or otherwise, 
    that this code may have on you, your computer, your sanity, your dog, 
    and anything else that you can think of. Use it at your own risk."



=======================================
============ Using Allegro ============
=======================================

See readme.txt for a general introduction, copyright details, and 
information about how to install Allegro and link your program with it.

int install_allegro(int system_id, int *errno_ptr, int (*atexit_ptr)());
   Initialises the Allegro library. You must call either this or 
   allegro_init() before doing anything other than using the Unicode 
   routines. If you want to use a text mode other than UTF-8, you can set
   it with set_uformat() before you call this. The other functions that can
   be called before this one will be marked explicitly in the documentation,
   like set_config_file().

   The available system ID codes will vary from one platform to another, but
   you will almost always want to pass SYSTEM_AUTODETECT. Alternatively,
   SYSTEM_NONE installs a stripped down version of Allegro that won't even
   try to touch your hardware or do anything platform specific: this can be
   useful for situations where you only want to manipulate memory bitmaps,
   such as the text mode datafile tools or the Windows GDI interfacing
   functions.

   The `errno_ptr' and `atexit_ptr' parameters should point to the errno
   variable and atexit function from your libc: these are required because
   when Allegro is linked as a DLL, it doesn't have direct access to your
   local libc data. `atexit_ptr' may be NULL, in which case it is your
   responsibility to call allegro_exit() manually. Example:

      install_allegro(SYSTEM_AUTODETECT, &errno, atexit);

   This function returns zero on success and non-zero on failure (e.g. no
   system driver could be used). Note: in previous versions of Allegro this
   function would abort on error.

int allegro_init();
   Macro which initialises the Allegro library. This is the same thing as
   calling install_allegro(SYSTEM_AUTODETECT, &errno, atexit).

void allegro_exit();
   Closes down the Allegro system. This includes returning the system to
   text mode and removing whatever mouse, keyboard, and timer routines have
   been installed. You don't normally need to bother making an explicit call
   to this function, because allegro_init() installs it as an atexit()
   routine so it will be called automatically when your program exits.

   Note that after you call this function, other functions like
   destroy_bitmap() will most likely crash. This is a problem for C++ global
   destructors, which usually get called after atexit(), so don't put Allegro
   calls in them. You can write the destructor code in another method which
   you can manually call before your program exits, avoiding this problem.

Macro END_OF_MAIN()
   In order to maintain cross-platform compatibility, you have to put this
   macro at the very end of your main function. This macro uses some `magic'
   to mangle your main procedure on platforms that need it like Windows,
   some flavours of UNIX or MacOS X. On the other platforms this macro 
   compiles to nothing, so you don't have to #ifdef around it. Example:

      int main(void)
      {
         allegro_init();
         /* more stuff goes here */
         ...
         return 0;
      }
      END_OF_MAIN()

extern char allegro_id[];
   Text string containing a date and version number for the library, in case 
   you want to display these somewhere.

extern char allegro_error[ALLEGRO_ERROR_SIZE];
   Text string used by set_gfx_mode(), install_sound() and other functions to
   report error messages. If they fail and you want to tell the user why, this
   is the place to look for a description of the problem. Example:

      void abort_on_error(const char *message)
      {
         if (screen != NULL)
            set_gfx_mode(GFX_TEXT, 0, 0, 0, 0);

         allegro_message("%s.\nLast Allegro error `%s'\n",
                         message, allegro_error);
         exit(-1);
      }
      ...
         if (some_allegro_function() == ERROR_CODE)
            abort_on_error("Error calling some function!");
#define ALLEGRO_VERSION
   Defined to the major version of Allegro. From a version number like
   4.1.16, this would be defined to the integer 4.
   
#define ALLEGRO_SUB_VERSION
   Defined to the middle version of Allegro. From a version number like
   4.1.16, this would be defined to the integer 1.
   
#define ALLEGRO_WIP_VERSION
   Defined to the minor version of Allegro. From a version number like
   4.1.16, this would be defined to the integer 16.
   
#define ALLEGRO_VERSION_STR
   Defined to a text string containing all version numbers and maybe some
   additional text. This could be `4.2.1 (SVN)' for an Allegro version
   obtained straight from the SVN repository.
   
#define ALLEGRO_DATE_STR
   Defined to a text string containing the year this version of Allegro was
   released, like `2004'.
   
#define ALLEGRO_DATE
   Defined to an integer containing the release date of Allegro in the packed
   format `yyyymmdd'. Example:

      const int year = ALLEGRO_DATE / 10000;
      const int month = (ALLEGRO_DATE / 100) % 100;
      const int day = ALLEGRO_DATE % 100;

      allegro_message("Year %d, month %d, day %d\n",
         year, month, day);

Macro AL_ID(a,b,c,d)
   This macro can be used to create a packed 32 bit integer from 8 bit
   characters, on both 32 and 64 bit machines. These can be used for various 
   things, like custom datafile objects or system IDs. Example:

      #define OSTYPE_LINUX       AL_ID('T','U','X',' ')

Macro MAKE_VERSION(a, b, c)
   This macro can be used to check if some Allegro version is (binary)
   compatible with the current version. It is safe to use > and < to
   check if one version is more recent than another.
   The third number is ignored if the second number is even, so
   MAKE_VERSION(4, 2, 0) is equivalent to MAKE_VERSION(4, 2, 1).
   This is because of our version numbering policy since 4.0.0:
   the second number is even for stable releases, which must be
   ABI-compatible with earlier versions of the same series.
   This macro is mainly useful for addon packages and libraries. See the 
   `ABI compatibility information' section of the manual for more detailed 
   information. Example:

      /* Check if the current version is compatible with Allegro 4.2.0 */
      #if (MAKE_VERSION(4, 2, 0) <= MAKE_VERSION(ALLEGRO_VERSION, \
                         ALLEGRO_SUB_VERSION, ALLEGRO_WIP_VERSION))
         /* Allegro 4.2.0 compatibility */
      #else
         /* Work-around */
      #endif
   
extern int os_type;
   Set by allegro_init() to one of the values:

      OSTYPE_UNKNOWN    - unknown, or regular MSDOS
      OSTYPE_WIN3       - Windows 3.1 or earlier
      OSTYPE_WIN95      - Windows 95
      OSTYPE_WIN98      - Windows 98
      OSTYPE_WINME      - Windows ME
      OSTYPE_WINNT      - Windows NT
      OSTYPE_WIN2000    - Windows 2000
      OSTYPE_WINXP      - Windows XP
      OSTYPE_WIN2003    - Windows 2003
      OSTYPE_WINVISTA   - Windows Vista
      OSTYPE_OS2        - OS/2
      OSTYPE_WARP       - OS/2 Warp 3
      OSTYPE_DOSEMU     - Linux DOSEMU
      OSTYPE_OPENDOS    - Caldera OpenDOS
      OSTYPE_LINUX      - Linux
      OSTYPE_SUNOS      - SunOS/Solaris
      OSTYPE_FREEBSD    - FreeBSD
      OSTYPE_NETBSD     - NetBSD
      OSTYPE_IRIX       - IRIX
      OSTYPE_DARWIN     - Darwin
      OSTYPE_QNX        - QNX
      OSTYPE_UNIX       - Unknown Unix variant
      OSTYPE_BEOS       - BeOS
      OSTYPE_MACOS      - MacOS
      OSTYPE_MACOSX     - MacOS X

extern int os_version;
extern int os_revision;
   The major and minor version of the Operating System currently running.
   Set by allegro_init(). If Allegro for some reason was not able to
   retrieve the version of the Operating System, os_version and
   os_revision will be set to -1. For example: Under Win98 SE (v4.10.2222)
   os_version will be set to 4 and os_revision to 10.

extern int os_multitasking;
   Set by allegro_init() to either TRUE or FALSE depending on whether your
   Operating System is multitasking or not.

void allegro_message(const char *text_format, ...);
   Outputs a message, using a printf() format string. Usually you want to
   use this to report messages to the user in an OS independent way when some
   Allegro subsystem cannot be initialised. But you must not use this function
   if you are in a graphic mode, only before calling set_gfx_mode(), or after
   a set_gfx_mode(GFX_TEXT). Also, this function depends on a system driver
   being installed, which means that it won't display the message at all on
   some platforms if Allegro has not been initialised correctly.

   On platforms featuring a windowing system, it will bring up a blocking GUI
   message box. If there is no windowing system, it will try to print the
   string to a text console, attempting to work around codepage differences
   by reducing any accented characters to 7-bit ASCII approximations.
   Example:

      if (allegro_init() != 0)
         exit(1);

      if (init_my_data() != 0) {
         allegro_message("Sorry, missing game data!\n");
         exit(2);
      }

void set_window_title(const char *name);
   On platforms that are capable of it, this routine alters the window title 
   for your Allegro program. Note that Allegro cannot set the window title 
   when running in a DOS box under Windows. Example:

      set_window_title("Allegro rules!");

int set_close_button_callback(void (*proc)(void));
   On platforms that have a close button, this routine installs a callback 
   function to handle the close event. In other words, when the user clicks 
   the close button on your program's window or any equivalent device, the
   function you specify here will be called.

   This function should not generally attempt to exit the program or save 
   any data itself. The function could be called at any time, and there is 
   usually a risk of conflict with the main thread of the program. Instead, 
   you should set a flag during this function, and test it on a regular 
   basis in the main loop of the program.

   Pass NULL as the `proc' argument to this function to disable the close
   button functionality, which is the default state.

   Note that Allegro cannot intercept the close button of a DOS box in 
   Windows.
   
   Also note that the supplied callback is also called under MacOS X when
   the user hits Command-Q or selects "Quit" from the application menu.
   Example:

      volatile int close_button_pressed = FALSE;

      void close_button_handler(void)
      {
         close_button_pressed = TRUE;
      }
      END_OF_FUNCTION(close_button_handler)
      ...
      
      allegro_init();
      LOCK_FUNCTION(close_button_handler);
      set_close_button_callback(close_button_handler);
      ...
         
      while (!close_button_pressed)
         do_stuff();

   Returns zero on success and non-zero on failure (e.g. the feature is not
   supported by the platform).

int desktop_color_depth();
   Finds out the currently selected desktop color depth. You can use this
   information to make your program use the same color depth as the desktop,
   which will likely make it run faster because the graphic driver won't be
   doing unnecessary color conversions behind your back.

   Under some OSes, switching to a full screen graphics mode may automatically
   change the desktop color depth. You have, therefore, to call this function
   before setting any graphics mode in order to retrieve the real desktop
   color depth. Example:

      allegro_init();
      ...
      if ((depth = desktop_color_depth()) != 0) {
         set_color_depth(depth);
      }

   Returns the color depth or zero on platforms where this information is not
   available or does not apply.

int get_desktop_resolution(int *width, int *height);
   Finds out the currently selected desktop resolution. You can use this
   information to avoid creating windows bigger than the current resolution.
   This is especially important for some windowed drivers which are unable
   to create windows bigger than the desktop. Each parameter is a pointer
   to an integer where one dimension of the screen will be stored.

   Under some OSes, switching to a full screen graphics mode may automatically
   change the desktop resolution. You have, therefore, to call this function
   before setting any graphics mode in order to retrieve the real desktop
   resolution. Example:

      int width, height;
      
      allegro_init();
      ...
      if (get_desktop_resolution(&width, &height) == 0) {
         /* Got the resolution correctly */
      }

   Returns zero on success, or a negative number if this information is not
   available or does not apply, in which case the values stored in the
   variables you provided for `width' and `height' are undefined.

void check_cpu();
   Detects the CPU type, setting the following global variables. You don't 
   normally need to call this, because allegro_init() will do it for you.

extern char cpu_vendor[];
   On Intel PCs, contains the CPU vendor name if known. On Mac OSX systems 
   this contains the PPC subtype name. On other platforms, this may be an empty
   string. You can read this variable after you have called check_cpu()
   (which is automatically called by allegro_init()).

extern int cpu_family;
   Contains the Intel type, where applicable. Allegro defines the following
   CPU family types:

      CPU_FAMILY_UNKNOWN  - The type of processor is unknown
      CPU_FAMILY_I386     - The processor is an Intel-compatible 386
      CPU_FAMILY_I486     - The processor is an Intel-compatible 486
      CPU_FAMILY_I586     - The processor is a Pentium or equivalent
      CPU_FAMILY_I686     - The processor is a Pentium Pro, II, III
                            or equivalent
      CPU_FAMILY_ITANIUM  - The processor is an Itanium processor
      CPU_FAMILY_POWERPC  - The processor is a PowerPC processor
      CPU_FAMILY_EXTENDED - The processor type needs to be read
                            from the cpu_model

   You can read this variable after you have called check_cpu()
   (which is automatically called by allegro_init()).

extern int cpu_model;
   Contains the CPU submodel, where applicable. Allegro defines at least the 
   following CPU family types (see include/allegro/system.h for a more 
   complete list):

      CPU_FAMILY_I586:
         CPU_MODEL_PENTIUM, CPU_MODEL_K5, CPU_MODEL_K6

      CPU_FAMILY_I686:
         CPU_MODEL_PENTIUMPRO, CPU_MODEL_PENTIUMII,
         CPU_MODEL_PENTIUMIIIKATMAI, CPU_MODEL_PENTIUMIIICOPPERMINE,
         CPU_MODEL_ATHLON, CPU_MODEL_DURON

      CPU_FAMILY_EXTENDED:
         CPU_MODEL_PENTIUMIV, CPU_MODEL_XEON,
         CPU_MODEL_ATHLON64, CPU_MODEL_OPTERON

      CPU_FAMILY_POWERPC:
         CPU_MODEL_POWERPC_x, for x=601-604, 620, 750, 7400, 7450

   You can read this variable after you have called check_cpu() (which is 
   automatically called by allegro_init()). Make sure you check the cpu_family
   and cpu_vendor so you know which models make sense to check.

extern int cpu_capabilities;
   Contains CPU flags indicating what features are available on the current 
   CPU. The flags can be any combination of these:

      CPU_ID       - Indicates that the "cpuid" instruction is
                     available. If this is set, then all Allegro CPU
                     variables are 100% reliable, otherwise there
                     may be some mistakes.
      CPU_FPU      - An FPU is available.
      CPU_IA64     - Running on Intel 64 bit CPU
      CPU_AMD64    - Running on AMD 64 bit CPU
      CPU_MMX      - Intel MMX  instruction set is available.
      CPU_MMXPLUS  - Intel MMX+ instruction set is available.
      CPU_SSE      - Intel SSE  instruction set is available.
      CPU_SSE2     - Intel SSE2 instruction set is available.
      CPU_SSE3     - Intel SSE3 instruction set is available.
      CPU_3DNOW    - AMD 3DNow! instruction set is available.
      CPU_ENH3DNOW - AMD Enhanced 3DNow! instruction set is
                     available.
      CPU_CMOV     - Pentium Pro "cmov" instruction is available.

   You can check for multiple features by OR-ing the flags together.
   For example, to check if the CPU has an FPU and MMX instructions
   available, you'd do:

      if ((cpu_capabilities & (CPU_FPU | CPU_MMX)) ==
          (CPU_FPU | CPU_MMX)) {
         printf("CPU has both an FPU and MMX instructions!\n");
      }

   You can read this variable after you have called check_cpu() (which is
   automatically called by allegro_init()).



=================================================================
============ Structures and types defined by Allegro ============
=================================================================

There are several structures and types defined by Allegro which are used
in many functions (like the BITMAP structure). This section of the manual
describes their useful content from a user point of view when they don't fit
very well any of the existing manual sections, and redirects you to the
appropriate section when it's already described there. Note that unless stated
otherwise, the contents shown here are just for read only purposes, there
might be other internal flags, but you shouldn't depend on them being
available in past/future versions of Allegro.

typedef long fixed
   This is a fixed point integer which can replace float with similar results
   and is faster than float on low end machines. Read chapter "Fixed point
   math routines" for the full explanation.
   
typedef struct BITMAP

   int w, h;               - size of the bitmap in pixels
   int clip;               - non-zero if clipping is turned on
   int cl, cr, ct, cb;     - clip rectangle left, right, top,
                             and bottom
   unsigned char *line[];  - pointers to the start of each line

   There is some other stuff in the structure as well, but it is liable to
   change and you shouldn't use anything except the above. The `w' and `h'
   fields can be used to obtain the size of an existing bitmap:

      bmp = load_bitmap("file.bmp", pal);
      allegro_message("Bitmap size: (%dx%d)\n", bmp->w, bmp->h);

   The clipping rectangle is inclusive on the left and top (0 allows drawing
   to position 0) but exclusive on the right and bottom (10 allows drawing
   to position 9, but not to 10). Note this is not the same format as that of
   the clipping API, which takes inclusive coordinates for all four corners.
   All the values of this structure should be regarded as read-only, with the
   exception of the line field, whose access is described in depth in the
   "Direct access to video memory" section of the manual. If you want to
   modify the clipping region, please refrain from changing this structure.
   Use set_clip_rect() instead.

typedef struct RLE_SPRITE

   int w, h;           - width and height in pixels
   int color_depth;    - color depth of the image

   RLE sprites store the image in a simple run-length encoded format, where
   repeated zero pixels are replaced by a single length count, and strings of
   non-zero pixels are preceded by a counter giving the length of the solid
   run. Read chapter "RLE sprites" for a description of the restrictions and
   how to obtain/use this structure.
   
typedef struct COMPILED_SPRITE

   short planar;        - set if it's a planar (mode-X) sprite
   short color_depth;   - color depth of the image
   short w, h;          - size of the sprite

   Compiled sprites are stored as actual machine code instructions that draw
   a specific image onto a bitmap, using mov instructions with immediate data
   values. Read chapter "Compiled sprites" for a description of the
   restrictions and how to obtain/use this structure.

typedef struct JOYSTICK_INFO

   int flags;                       - status flags for this
                                      joystick
   int num_sticks;                  - how many stick inputs?
   int num_buttons;                 - how many buttons?
   JOYSTICK_STICK_INFO stick[n];    - stick state information
   JOYSTICK_BUTTON_INFO button[n];  - button state information

   Read chapter "Joystick routines" for a description on how to obtain/use
   this structure.

typedef struct JOYSTICK_BUTTON_INFO

   int b;                           - boolean on/off flag
   char *name;                      - description of this
                                      button

   Read chapter "Joystick routines" for a description on how to obtain/use
   this structure.

typedef struct JOYSTICK_STICK_INFO

   int flags;                       - status flags for this
                                      input
   int num_axis;                    - how many axes do we
                                      have? (note the misspelling)
   JOYSTICK_AXIS_INFO axis[n];      - axis state information
   char *name;                      - description of this
                                      input

   Read chapter "Joystick routines" for a description on how to obtain/use
   this structure.

typedef struct JOYSTICK_AXIS_INFO

   int pos;                         - analogue axis position
   int d1, d2;                      - digital axis position
   char *name;                      - description of this axis

   Read chapter "Joystick routines" for a description on how to obtain/use
   this structure.

typedef struct GFX_MODE_LIST

   int num_modes;
   GFX_MODE *mode;

   Structure returned by get_gfx_mode_list, which contains an array of
   GFX_MODE structures.

typedef struct GFX_MODE

   int width, height, bpp;

   Structure contained in GFX_MODE_LIST.

#define PAL_SIZE
   Preprocessor constant equal to 256.

typedef PALETTE RGB[PAL_SIZE]
   Allegro palettes are arrays of PAL_SIZE RGB entries.

typedef struct RGB

   unsigned char r, g, b;

   Palette entry. It contains an additional field for the purpose of padding
   but you should not usually care about it. Read chapter "Palette routines"
   for a description on how to obtain/use this structure.

typedef struct V3D

   fixed x, y, z;       - position
   fixed u, v;          - texture map coordinates
   int c;               - color

   A vertex structure used by polygon3d and other polygon rendering
   functions. Read the description of polygon3d() for a description on how
   to obtain/use this structure.

typedef struct V3D_f

   float x, y, z;       - position
   float u, v;          - texture map coordinates
   int c;               - color

   Like V3D but using float values instead of fixed ones. Read the
   description of polygon3d_f() for a description on how to obtain/use this
   structure.

typedef struct COLOR_MAP

   unsigned char data[PAL_SIZE][PAL_SIZE];

   Read chapter "Transparency and patterned drawing", section "256-color
   transparency" for a description on how to obtain/use this structure.

typedef struct RGB_MAP

   unsigned char data[32][32][32];

   Read chapter "Converting between color formats" for a description on how
   to obtain/use this structure.

struct al_ffblk

   int attrib;       - actual attributes of the file found
   time_t time;      - modification time of file
   char name[512];   - name of file

   Read the description of al_findfirst for a description on how to
   obtain/use this structure.

typedef struct DATAFILE

   void *dat;     - pointer to the actual data
   int type;      - type of the data
   long size;     - size of the data in bytes
   void *prop;    - list of object properties

   Read chapter "Datafile routines", section "Using datafiles" for a
   description on how to obtain/use this structure.

typedef struct MATRIX

   fixed v[3][3];           - 3x3 scaling and rotation component
   fixed t[3];              - x/y/z translation component

   Fixed point matrix structure. Read chapter "3D math routines" for a
   description on how to obtain/use this structure.

typedef struct MATRIX_f

   float v[3][3];           - 3x3 scaling and rotation component
   float t[3];              - x/y/z translation component

   Floating point matrix structure. Read chapter "3D math routines" for a
   description on how to obtain/use this structure.

typedef struct QUAT

   float w, x, y, z;

   Read chapter "Quaternion math routines" for a description on how to
   obtain/use this structure.

typedef struct DIALOG

   int (*proc)(int, DIALOG *, int); - dialog procedure
                                      (message handler)
   int x, y, w, h;       - position and size of the object
   int fg, bg;           - foreground and background colors
   int key;              - ASCII keyboard shortcut
   int flags;            - flags about the status of the object
   int d1, d2;           - whatever you want to use them for
   void *dp, *dp2, *dp3; - pointers to more object-specific data

   This is the structure which contains a GUI object. Read chapter "GUI
   routines" for a description on how to obtain/use this structure.

typedef struct MENU

   char *text;          - the text to display for the menu item
   int (*proc)(void);   - called when the menu item is clicked
   struct MENU *child;  - nested child menu
   int flags;           - disabled or checked state
   void *dp;            - pointer to any data you need

   Structure used to hold an entry of a menu. Read chapter "GUI routines",
   section "GUI menus" for a description on how to obtain/use this structure.

typedef struct DIALOG_PLAYER
   A structure which holds GUI data used internally by Allegro. Read the
   documentation of init_dialog() for a description on how to obtain/use this
   structure.

typedef struct MENU_PLAYER
   A structure which holds GUI data used internally by Allegro. Read the
   documentation of init_menu() for a description on how to obtain/use this
   structure.
   
typedef struct FONT
   A structure holding an Allegro font, usually created beforehand with the
   grabber tool or Allegro's default font. Read chapter "Fonts" for a
   description on how to load/destroy fonts, and chapter "Text output" for a
   description on how to show text.

typedef struct BITMAP ZBUFFER
   Structure used by Allegro's 3d zbuffered rendering functions. You are not
   supposed to mix ZBUFFER with BITMAP even though it is currently possible
   to do so. This is just an internal representation, and it may change in
   the future.

typedef struct SAMPLE

   int bits;                   - 8 or 16
   int stereo;                 - sample type flag
   int freq;                   - sample frequency
   int priority;               - 0-255
   unsigned long len;          - length (in samples)
   unsigned long loop_start;   - loop start position
   unsigned long loop_end;     - loop finish position
   void *data;                 - raw sample data

   A sample structure, which holds sound data, used by the digital sample
   routines. You can consider all of these fields as read only except
   priority, loop_start and loop_end, which you can change them for example
   after loading a sample from disk.

   The priority is a value from 0 to 255 (by default set to 128) and controls
   how hardware voices on the sound card are allocated if you attempt to play
   more than the driver can handle. This may be used to ensure that the less
   important sounds are cut off while the important ones are preserved.

   The variables loop_start and loop_end specify the loop position in sample
   units, and are set by default to the start and end of the sample.

   If you are creating your own samples on the fly, you might also want to
   modify the raw data of the sample pointed by the data field. The sample
   data are always in unsigned format. This means that if you are loading a
   PCM encoded sound file with signed 16-bit samples, you would have to XOR
   every two bytes (i.e. every sample value) with 0x8000 to change the
   signedness.

typedef struct MIDI
   A structure holding MIDI data. Read chapter "Music routines (MIDI)" for a
   description on how to obtain/use this structure.

typedef struct AUDIOSTREAM

   int voice;  - the hardware voice used for the sample

   A structure holding an audiostream, which is a convenience wrapper around
   a SAMPLE structure to double buffer sounds too big to fit into memory, or
   do clever things like generating the sound wave real time.
   
   While you shouldn't modify directly the value of the voice, you can use
   all of the voice functions in chapter "Digital sample routines" to modify
   the properties of the sound, like the frequency.

typedef struct PACKFILE
   A packfile structure, similar to the libc FILE structure. Read chapter
   "File and compression routines" for a description on how to obtain/use
   this structure. Note that prior to version 4.1.18, some internal fields
   were accidentally documented - but PACKFILE should be treated as an
   opaque structure, just like the libc FILE type.

typedef struct PACKFILE_VTABLE

   int pf_fclose(void *userdata);
   int pf_getc(void *userdata);
   int pf_ungetc(int c, void *userdata);
   long pf_fread(void *p, long n, void *userdata);
   int pf_putc(int c, void *userdata);
   long pf_fwrite(const void *p, long n, void *userdata);
   int pf_fseek(void *userdata, int offset);
   int pf_feof(void *userdata);
   int pf_ferror(void *userdata);

   This is the vtable which must be provided for custom packfiles, which then
   can read from and write to wherever you like (eg. files in memory). You
   should provide all the entries of the vtable, even if they are empty stubs
   doing nothing, to avoid Allegro (or you) calling a NULL method at some
   point.

typedef struct LZSS_PACK_DATA
   Opaque structure for handling LZSS compression. Read chapter "File and
   compression routines for a description on how to obtain/use this
   structure.

typedef struct LZSS_UNPACK_DATA
   Opaque structure for handling LZSS decompression. Read chapter "File and
   compression routines for a description on how to obtain/use this
   structure.



==========================================
============ Unicode routines ============
==========================================

Allegro can manipulate and display text using any character values from 0 
right up to 2^32-1 (although the current implementation of the grabber can 
only create fonts using characters up to 2^16-1). You can choose between a 
number of different text encoding formats, which controls how strings are 
stored and how Allegro interprets strings that you pass to it. This setting 
affects all aspects of the system: whenever you see a function that returns 
a char * type, or that takes a char * as an argument, that text will be in 
whatever format you have told Allegro to use.

By default, Allegro uses UTF-8 encoded text (U_UTF8). This is a 
variable-width format, where characters can occupy anywhere from one to four
bytes. The nice thing about it is that characters ranging from 0-127 are 
encoded directly as themselves, so UTF-8 is upwardly compatible with 7-bit 
ASCII ("Hello, World!" means the same thing regardless of whether you 
interpret it as ASCII or UTF-8 data). Any character values above 128, such 
as accented vowels, the UK currency symbol, and Arabic or Chinese 
characters, will be encoded as a sequence of two or more bytes, each in the 
range 128-255. This means you will never get what looks like a 7-bit ASCII 
character as part of the encoding of a different character value, which 
makes it very easy to manipulate UTF-8 strings.

There are a few editing programs that understand UTF-8 format text files. 
Alternatively, you can write your strings in plain ASCII or 16-bit Unicode 
formats, and then use the Allegro textconv program to convert them into 
UTF-8.

If you prefer to use some other text format, you can set Allegro to work 
with normal 8-bit ASCII (U_ASCII), or 16-bit Unicode (U_UNICODE) instead, or 
you can provide some handler functions to make it support whatever other 
text encoding you like (for example it would be easy to add support for 32 
bit UCS-4 characters, or the Chinese GB-code format).

There is some limited support for alternative 8-bit codepages, via the 
U_ASCII_CP mode. This is very slow, so you shouldn't use it for serious 
work, but it can be handy as an easy way to convert text between different 
codepages. By default the U_ASCII_CP mode is set up to reduce text to a 
clean 7-bit ASCII format, trying to replace any accented vowels with their 
simpler equivalents (this is used by the allegro_message() function when it 
needs to print an error report onto a text mode DOS screen). If you want to 
work with other codepages, you can do this by passing a character mapping 
table to the set_ucodepage() function.

Note that you can use the Unicode routines before you call install_allegro() 
or allegro_init(). If you want to work in a text mode other than UTF-8, it 
is best to set it with set_uformat() just before you call these.

void set_uformat(int type);
   Sets the current text encoding format. This will affect all parts of 
   Allegro, wherever you see a function that returns a char *, or takes a 
   char * as a parameter. `type' should be one of these values:

      U_ASCII     - fixed size, 8-bit ASCII characters
      U_ASCII_CP  - alternative 8-bit codepage (see set_ucodepage())
      U_UNICODE   - fixed size, 16-bit Unicode characters
      U_UTF8      - variable size, UTF-8 format Unicode characters

   Although you can change the text format on the fly, this is not a good 
   idea. Many strings, for example the names of your hardware drivers and 
   any language translations, are loaded when you call allegro_init(), so if 
   you change the encoding format after this, they will be in the wrong 
   format, and things will not work properly. Generally you should only call 
   set_uformat() once, before allegro_init(), and then leave it on the same 
   setting for the duration of your program.

int get_uformat(void);
   Finds out what text encoding format is currently selected. This function
   is probably useful only if you are writing an Allegro addon dealing with
   text strings and you use a different codepath for each possible format.
   Example:

      switch(get_uformat()) {
         case U_ASCII:
            do_something();
            break;
         case U_UTF8:
            do_something_else();
            break;
         ...
      }

   Returns the currently selected text encoding format. See the documentation
   of set_uformat() for a list of encoding formats.

void register_uformat(int type,
                      int (*u_getc)(const char *s),
                      int (*u_getx)(char **s),
                      int (*u_setc)(char *s, int c),
                      int (*u_width)(const char *s),
                      int (*u_cwidth)(int c),
                      int (*u_isok)(int c));
   Installs a set of custom handler functions for a new text encoding 
   format. The `type' is the ID code for your new format, which should be a
   4-character string as produced by the AL_ID() macro, and which can later 
   be passed to functions like set_uformat() and uconvert(). The function 
   parameters are handlers that implement the character access for your new 
   type: see below for details of these.

void set_ucodepage(const unsigned short *table,
                   const unsigned short *extras);
   When you select the U_ASCII_CP encoding mode, a set of tables are used to 
   convert between 8-bit characters and their Unicode equivalents. You can 
   use this function to specify a custom set of mapping tables, which allows 
   you to support different 8-bit codepages.

   The `table' parameter points to an array of 256 shorts, which contain the
   Unicode value for each character in your codepage. The `extras' parameter,
   if not NULL, points to a list of mapping pairs, which will be used when
   reducing Unicode data to your codepage. Each pair consists of a Unicode
   value, followed by the way it should be represented in your codepage.
   The list is terminated by a zero Unicode value. This allows you to create
   a many->one mapping, where many different Unicode characters can be
   represented by a single codepage value (eg. for reducing accented vowels
   to 7-bit ASCII).

   Allegro will use the `table' parameter when it needs to convert an ASCII
   string to an Unicode string. But when Allegro converts an Unicode string
   to ASCII, it will use both parameters. First, it will loop through the
   `table' parameter looking for an index position pointing at the Unicode
   value it is trying to convert (ie. the `table' parameter is also used for
   reverse matching). If that fails, the `extras' list is used. If that fails
   too, Allegro will put the character `^', giving up the conversion.

   Note that Allegro comes with a default `table' and `extras' parameters
   set internally. The default `table' will convert 8-bit characters to `^'.
   The default `extras' list reduces Latin-1 and Extended-A characters to 7
   bits in a sensible way (eg. an accented vowel will be reduced to the same
   vowel without the accent).

int need_uconvert(const char *s, int type, int newtype);
   Given a pointer to a string (`s'), a description of the type of the string
   (`type'), and the type that you would like this string to be converted into
   (`newtype'), this function tells you whether any conversion is required. No
   conversion will be needed if `type' and `newtype' are the same, or if one
   type is ASCII, the other is UTF-8, and the string contains only character
   values less than 128. As a convenience shortcut, you can pass the value
   U_CURRENT as either of the type parameters, to represent whatever text
   encoding format is currently selected. Example:

      if (need_uconvert(text, U_UTF8, U_CURRENT)) {
         /* conversion is required */
      }

   Returns non-zero if any conversion is required or zero otherwise.

int uconvert_size(const char *s, int type, int newtype);
   Finds out how many bytes are required to store the specified string `s'
   after a conversion from `type' to `newtype', including the mandatory zero
   terminator of the string. You can use U_CURRENT for either `type' or
   `newtype' as a shortcut to represent whatever text encoding format is
   currently selected. Example:

      length = uconvert_size(old_string, U_CURRENT, U_UNICODE);
      new_string = malloc(length);
      ustrcpy(new_string, old_string);

   Returns the number of bytes required to store the string after conversion.

void do_uconvert(const char *s, int type,
                 char *buf, int newtype, int size);
   Converts the specified string `s' from `type' to `newtype', storing at most
   `size' bytes into the output `buf'. The type parameters can use the value
   U_CURRENT as a shortcut to represent the currently selected encoding 
   format. Example:

      char temp_string[256];
      do_uconvert(input_string, U_CURRENT, temp_string, U_ASCII, 256);

   Note that, even for empty strings, your destination string must have at
   least enough bytes to store the terminating null character of the string,
   and your parameter `size' must reflect this. Otherwise, the debug version
   of Allegro will abort at an assertion, and the release version of Allegro
   will overrun the destination buffer.
   
char *uconvert(const char *s, int type,
               char *buf, int newtype, int size);
   Higher level function running on top of do_uconvert(). This function 
   converts the specified string `s' from `type' to `newtype', storing at most
   `size' bytes into the output `buf' (including the terminating null
   character), but it checks before doing the conversion, and doesn't bother
   if the string formats are already the same (either both types are equal, or
   one is ASCII, the other is UTF-8, and the string contains only 7-bit ASCII
   characters).

   As a convenience, if `buf' is NULL it will convert the string into an
   internal static buffer and the `size' parameter will be ignored. You should
   be wary of using this feature, though, because that buffer will be
   overwritten the next time this routine is called, so don't expect the data
   to persist across any other library calls. The static buffer may hold less
   than 1024 characters, so you won't be able to convert large chunks of text.
   Example:

      char *p = uconvert(input_string, U_CURRENT, buffer, U_ASCII, 256);

   Returns a pointer to `buf' (or the static buffer if you used NULL) if a
   conversion was performed. Otherwise returns a copy of `s'. In any cases,
   you should use the return value rather than assuming that the string will
   always be moved to `buf'.

char *uconvert_ascii(const char *s, char buf[]);
   Helper macro for converting strings from ASCII into the current encoding 
   format. Expands to uconvert(s, U_ASCII, buf, U_CURRENT, sizeof(buf)).

char *uconvert_toascii(const char *s, char buf[]);
   Helper macro for converting strings from the current encoding format into 
   ASCII. Expands to uconvert(s, U_CURRENT, buf, U_ASCII, sizeof(buf)).

extern char empty_string[];
   You can't just rely on "" to be a valid empty string in any encoding 
   format. This global buffer contains a number of consecutive zeros, so it 
   will be a valid empty string no matter whether the program is running in 
   ASCII, Unicode, or UTF-8 mode.

int ugetc(const char *s);
   Low level helper function for reading Unicode text data. Example:

      int first_unicode_letter = ugetc(text_string);

   Returns the character pointed to by `s' in the current encoding format.

int ugetx(char **s);
int ugetxc(const char **s);
   Low level helper function for reading Unicode text data. ugetxc is provided
   for working with pointer-to-pointer-to-const char data. Example:

      char *p = string;
      int first_letter, second_letter, third_letter;
      first_letter = ugetx(&p);
      second_letter = ugetx(&p);
      third_letter = ugetx(&p);

   Returns the character pointed to by `s' in the current encoding format, and
   advances the pointer to the next character after the one just returned.

int usetc(char *s, int c);
   Low level helper function for writing Unicode text data. Writes the
   character `c' to the address pointed to by `s'.

   Returns the number of bytes written, which is equal to the width of the
   character in the current encoding format.

int uwidth(const char *s);
   Low level helper function for testing Unicode text data.

   Returns the number of bytes occupied by the first character of the
   specified string, in the current encoding format.

int ucwidth(int c);
   Low level helper function for testing Unicode text data.

   Returns the number of bytes that would be occupied by the specified
   character value, when encoded in the current format.

int uisok(int c);
   Low level helper function for testing Unicode text data. Finds out if the
   character value `c' can be encoded correctly in the current format, which
   can be useful if you are converting from Unicode to ASCII or another
   encoding format where the range of valid characters is limited.

   Returns non-zero if the value can be correctly encoded, zero otherwise.

int uoffset(const char *s, int index);
   Finds out the offset (in bytes from the start of the string) of the
   character at the specified `index' in the string `s'. A zero `index'
   parameter will return the first character of the string. If `index' is
   negative, it counts backward from the end of the string, so an `index' of
   `-1' will return an offset to the last character. Example:

      int from_third_letter = uoffset(text_string, 2);

   Returns the offset in bytes to the specified character.

int ugetat(const char *s, int index);
   Finds out the character value at the specified `index' in the string. A
   zero `index' parameter will return the first character of the string. If
   `index' is negative, it counts backward from the end of the string, so an
   `index' of `-1' will return the last character of the string. Example:

      int third_letter = ugetat(text_string, 2);

   Returns the character value at the specified index in the string.

int usetat(char *s, int index, int c);
   Replaces the character at the specified index in the string with value `c',
   handling any adjustments for variable width data (ie. if `c' encodes to a
   different width than the previous value at that location). If `index' is
   negative, it counts backward from the end of the string. Example:

      usetat(text_string, 2, letter_a);

   Returns the number of bytes by which the trailing part of the string was
   moved. This is of interest only with text encoding formats where
   characters have a variable length, like UTF-8.

int uinsert(char *s, int index, int c);
   Inserts the character `c' at the specified `index' in the string, sliding
   the rest of the data along to make room. If `index' is negative, it counts
   backward from the end of the string. Example:

      uinsert(text_string, 0, prefix_letter);

   Returns the number of bytes by which the trailing part of the string was
   moved.

int uremove(char *s, int index);
   Removes the character at the specified `index' within the string, sliding
   the rest of the data back to fill the gap. If `index' is negative, it
   counts backward from the end of the string. Example:

      int length_in_bytes = ustrsizez(text_string);
      ...
      length_in_bytes -= uremove(text_string, -1);

   Returns the number of bytes by which the trailing part of the string was
   moved.

int ustrsize(const char *s);
   Returns the size of the specified string in bytes, not including the 
   trailing null character.

int ustrsizez(const char *s);
   Returns the size of the specified string in bytes, including the trailing 
   null character.

int uwidth_max(int type);
   Low level helper function for working with Unicode text data. Returns the 
   largest number of bytes that one character can occupy in the given 
   encoding format. Pass U_CURRENT to represent the current format. Example:

      char *temp_buffer = malloc(256 * uwidth_max(U_UTF8));

int utolower(int c);
   This function returns `c', converting it to lower case if it is upper case.

int utoupper(int c);
   This function returns `c', converting it to upper case if it is lower case.

int uisspace(int c);
   Returns nonzero if `c' is whitespace, that is, carriage return, newline,
   form feed, tab, vertical tab, or space. Example:

      for (counter = 0; counter < ustrlen(text_string); counter++) {
         if (uisspace(ugetat(text_string, counter)))
            usetat(text_string, counter, '_');
      }

int uisdigit(int c);
   Returns nonzero if `c' is a digit.

      for (counter = 0; counter < ustrlen(text_string); counter++) {
         if (uisdigit(ugetat(text_string, counter)))
            usetat(text_string, counter, '*');
      }

char *ustrdup(const char *src)
   This functions copies the null-terminated string `src' into a newly
   allocated area of memory, effectively duplicating it. Example:

      void manipulate_string(const char *input_string)
      {
         char *temp_buffer = ustrdup(input_string);
         /* Now we can modify temp_buffer */
         ...

   Returns the newly allocated string. This memory must be freed by the
   caller. Returns NULL if it cannot allocate space for the duplicated string.

char *_ustrdup(const char *src, void* (*malloc_func)(size_t))
   Does the same as ustrdup(), but allows the user to specify a custom memory
   allocator function.

char *ustrcpy(char *dest, const char *src);
   This function copies `src' (including the terminating null character into
   `dest'. You should try to avoid this function because it is very easy to
   overflow the destination buffer. Use ustrzcpy instead.

   Returns the value of dest.

char *ustrzcpy(char *dest, int size, const char *src);
   This function copies `src' (including the terminating null character) into
   `dest', whose length in bytes is specified by `size' and which is
   guaranteed to be null-terminated even if `src' is bigger than `size'.

   Note that, even for empty strings, your destination string must have at
   least enough bytes to store the terminating null character of the string,
   and your parameter `size' must reflect this. Otherwise, the debug version
   of Allegro will abort at an assertion, and the release version of Allegro
   will overrun the destination buffer.

   Returns the value of `dest'.

char *ustrcat(char *dest, const char *src);
   This function concatenates `src' to the end of `dest`'. You should try to
   avoid this function because it is very easy to overflow the destination
   buffer, use ustrzcat instead.

   Returns the value of `dest'.

char *ustrzcat(char *dest, int size, const char *src);
   This function concatenates `src' to the end of `dest', whose length in
   bytes is specified by `size' and which is guaranteed to be null-terminated
   even when `src' is bigger than `size'.

   Note that, even for empty strings, your destination string must have at
   least enough bytes to store the terminating null character of the string,
   and your parameter `size' must reflect this. Otherwise, the debug version
   of Allegro will abort at an assertion, and the release version of Allegro
   will overrun the destination buffer.

   Returns the value of `dest'.

int ustrlen(const char *s);
   This function returns the number of characters in `s'. Note that this
   doesn't have to equal the string's size in bytes.

int ustrcmp(const char *s1, const char *s2);
   This function compares `s1' and `s2'.

   Returns zero if the strings are equal, a positive number if `s1' comes
   after `s2' in the ASCII collating sequence, else a negative number.

char *ustrncpy(char *dest, const char *src, int n);
   This function is like ustrcpy() except that no more than `n' characters
   from `src' are copied into `dest'. If `src' is shorter than `n' characters,
   null characters are appended to `dest' as padding until `n' characters have
   been written.

   Note that if `src' is longer than `n' characters, `dest' will not be
   null-terminated.

   The return value is the value of `dest'.

char *ustrzncpy(char *dest, int size, const char *src, int n);
   This function is like ustrzcpy() except that no more than `n' characters
   from `src' are copied into `dest' whose length in bytes is specified by
   `size' and which is guaranteed to be null-terminated even if `src' is
   bigger than `size'. If `src' is shorter than `n' characters,
   null characters are appended to `dest' as padding until `n' characters have
   been written. In any case, `dest' is guaranteed to be null-terminated.

   Note that, even for empty strings, your destination string must have at
   least enough bytes to store the terminating null character of the string,
   and your parameter `size' must reflect this. Otherwise, the debug version
   of Allegro will abort at an assertion, and the release version of Allegro
   will overrun the destination buffer.

   The return value is the value of `dest'.

char *ustrncat(char *dest, const char *src, int n);
   This function is like ustrcat() except that no more than `n' characters
   from `src' are appended to the end of `dest'. If the terminating null
   character in `src' is reached before `n' characters have been written, the
   null character is copied, but no other characters are written. If `n'
   characters are written before a terminating null is encountered, the
   function appends its own null character to `dest', so that `n+1' characters
   are written. You should try to avoid this function because it is very
   easy to overflow the destination buffer. Use ustrzncat instead.

   The return value is the value of `dest'.

char *ustrzncat(char *dest, int size, const char *src, int n);
   This function is like ustrzcat() except that no more than `n' characters
   from `src' are appended to the end of `dest'. If the terminating null
   character in `src' is reached before `n' characters have been written, the
   null character is copied, but no other characters are written. Note that
   `dest' is guaranteed to be null-terminated.

   The return value is the value of `dest'.

int ustrncmp(const char *s1, const char *s2, int n);
   This function compares up to `n' characters of `s1' and `s2'. Example:

      if (ustrncmp(prefix, long_string, ustrlen(prefix)) == 0) {
         /* long_string starts with prefix */
      }

   Returns zero if the substrings are equal, a positive number if `s1' comes
   after `s2' in the ASCII collating sequence, else a negative number.

int ustricmp(const char *s1, const char *s2);
   This function compares `s1' and `s2', ignoring case. Example:

      if (ustricmp(string, user_input) == 0) {
         /* string and user_input are equal (ignoring case) */
      }

   Returns zero if the strings are equal, a positive number if `s1' comes
   after `s2' in the ASCII collating sequence, else a negative number.

int ustrnicmp(const char *s1, const char *s2, int n);
   This function compares up to `n' characters of `s1' and `s2', ignoring
   case. Example:

      if (ustrnicmp(prefix, long_string, ustrlen(prefix)) == 0) {
         /* long_string starts with prefix (ignoring case) */
      }

   Returns zero if the strings are equal, a positive number if `s1' comes
   after `s2' in the ASCII collating sequence, else a negative number.

char *ustrlwr(char *s);
   This function replaces all upper case letters in `s' with lower case
   letters. Example:

      char buffer[] = "UPPER CASE STRING";
      allegro_message(ustrlwr(buffer));

   The return value is the value of `s'.

char *ustrupr(char *s);
   This function replaces all lower case letters in `s' with upper case
   letters. Example:

      char buffer[] = "lower case string";
      allegro_message(ustrupr(buffer));

   The return value is the value of `s'.

char *ustrchr(const char *s, int c);
   Finds the first occurrence of the character `c' in the string `s'. Example:

      char *p = ustrchr("one,two,three,four", ',');

   Returns a pointer to the first occurrence of `c' in `s', or NULL if no
   match was found. Note that if `c' is NULL, this will return a pointer to
   the end of the string.

char *ustrrchr(const char *s, int c);
   Finds the last occurrence of the character `c' in the string `s'. Example:

      char *p = ustrrchr("one,two,three,four", ',');

   Returns a pointer for the last occurrence of `c' in `s', or NULL if no
   match was found.

char *ustrstr(const char *s1, const char *s2);
   This function finds the first occurrence of string `s2' in string `s1'.
   Example:

      char *p = ustrstr("hello world", "world");

   Returns a pointer within `s1', or NULL if `s2' wasn't found.

char *ustrpbrk(const char *s, const char *set);
   This function finds the first character in `s' that matches any character
   in `set'. Example:

      char *p = ustrpbrk("one,two-three.four", "-. ");

   Returns a pointer to the first match, or NULL if none are found.

char *ustrtok(char *s, const char *set);
   This function retrieves tokens from `s' which are delimited by characters
   from `set'. To initiate the search, pass the string to be searched as `s'.
   For the remaining tokens, pass NULL instead. Warning: Since ustrtok alters
   the string it is parsing, you should always copy the string to a temporary
   buffer before parsing it. Also, this function is not re-entrant (ie. you
   cannot parse two strings at the same time). Example:

      char *word;
      char string[]="some-words with dashes";
      char *temp = ustrdup(string);
      word = ustrtok(temp, " -");
      while (word) {
         allegro_message("Found `%s'\n", word);
         word = ustrtok(NULL, " -");
      }
      free(temp);

   Returns a pointer to the token, or NULL if no more are found.

char *ustrtok_r(char *s, const char *set, char **last);
   Reentrant version of ustrtok. The `last' parameter is used to keep track
   of where the parsing is up to and must be a pointer to a char * variable
   allocated by the user that remains the same while parsing the same
   string. Example:

      char *word, *last;
      char string[]="some-words with dashes";
      char *temp = ustrdup(string);
      word = ustrtok_r(string, " -", &last);
      while (word) {
         allegro_message("Found `%s'\n", word);
         word = ustrtok_r(NULL, " -", &last);
      }
      free(temp);

   Returns a pointer to the token, or NULL if no more are found. You can free
   the memory pointed to by `last' once NULL is returned.

double uatof(const char *s);
   Convert as much of the string as possible to an equivalent double
   precision real number. This function is almost like `ustrtod(s, NULL)'.

   Returns the equivalent value, or zero if the string does not represent a
   number.

long ustrtol(const char *s, char **endp, int base);
   This function converts the initial part of `s' to a signed integer, setting
   `*endp' to point to the first unused character, if `endp' is not a NULL
   pointer. The `base' argument indicates what base the digits (or letters)
   should be treated as. If `base' is zero, the base is determined by looking
   for `0x', `0X', or `0' as the first part of the string, and sets the base
   used to 16, 16, or 8 if it finds one. The default base is 10 if none of
   those prefixes are found. Example:

      char *endp, *string = "456.203 askdfg";
      int number = ustrtol(string, &endp, 10);

   Returns the string converted as a value of type `long int'. If nothing was
   converted, returns zero with `*endp' pointing to the beginning of `s'.

double ustrtod(const char *s, char **endp);
   This function converts as many characters of `s' that look like a floating
   point number into one, and sets `*endp' to point to the first unused
   character, if `endp' is not a NULL pointer. Example:

      char *endp, *string = "456.203 askdfg";
      double number = ustrtod(string, &endp);

   Returns the string converted as a value of type `double'. If nothing was
   converted, returns zero with *endp pointing to the beginning of s.

const char *ustrerror(int err);
   This function returns a string that describes the error code `err', which
   normally comes from the variable `errno'. Example:

      PACKFILE *input_file = pack_fopen("badname", "r");
      if (input_file == NULL)
         allegro_message("%s\nSorry!\n", ustrerror(errno));

   Returns a pointer to a static string that should not be modified or
   freed. If you make subsequent calls to ustrerror(), the string will be
   overwritten.

int usprintf(char *buf, const char *format, ...);
   This function writes formatted data into the output buffer. A NULL
   character is written to mark the end of the string. You should try to avoid
   this function because it is very easy to overflow the destination buffer.
   Use uszprintf instead.

   Returns the number of characters written, not including the terminating
   null character.

int uszprintf(char *buf, int size, const char *format, ...);
   This function writes formatted data into the output buffer, whose length
   in bytes is specified by `size' and which is guaranteed to be NULL
   terminated. Example:

      char buffer[10];
      int player_score;
      ...
      uszprintf(buffer, sizeof(buffer), "Your score is: %d", player_score);

   Returns the number of characters that would have been written without
   eventual truncation (like with usprintf), not including the terminating
   null character.
   
int uvsprintf(char *buf, const char *format, va_list args);
   This is like usprintf(), but you pass the variable argument list directly,
   instead of the arguments themselves. You can use this function to implement
   printf like functions, also called variadic functions. You should try to
   avoid this function because it is very easy to overflow the destination
   buffer. Use uvszprintf instead.

   Returns the number of characters written, not including the terminating
   null character.

int uvszprintf(char *buf, int size, const char *format, va_list args);
   This is like uszprintf(), but you pass the variable argument list
   directly, instead of the arguments themselves. Example:

      #include <stdarg.h>

      void log_message(const char *format, ...)
      {
         char buffer[100];
         va_list parameters;
         
         va_start(parameters, format);
         uvszprintf(buffer, sizeof(buffer), format, parameters);
         va_end(parameters);
         
         append_buffer_to_logfile(log_name, buffer);
         send_buffer_to_other_networked_players(multicast_ip, buffer);
         and_also_print_it_on_the_screen(cool_font, buffer);
      }
      
      void some_other_function(void)
      {
         log_message("Hello %s, are you %d years old?\n", "Dave", 25);
      }

   Returns the number of characters that would have been written without
   eventual truncation (like with uvsprintf), not including the terminating
   null character.



================================================
============ Configuration routines ============
================================================

Various parts of Allegro, such as the sound routines and the 
load_joystick_data() function, require some configuration information. This 
data is stored in text files as a collection of `variable=value' lines,
along with comments that begin with a `#' character and continue to the end
of the line. The configuration file may optionally be divided into sections, 
which begin with a `[sectionname]' line. Each section has a unique
namespace, to prevent variable name conflicts, but any variables that aren't 
in a section are considered to belong to all the sections simultaneously.

Note that variable and section names cannot contain spaces.

By default the configuration data is read from a file called `allegro.cfg',
which can be located either in the same directory as the program executable, 
or the directory pointed to by the ALLEGRO environment variable. Under Unix, 
it also checks for `~/allegro.cfg', `~/.allegrorc', `/etc/allegro.cfg', and
`/etc/allegrorc', in that order; under BeOS only the last two are also
checked. MacOS X also checks in the Contents/Resources directory of the
application bundle, if any, before doing the checks above.

If you don't like this approach, you can specify any filename you like, or
use a block of binary configuration data provided by your program (which
could for example be loaded from a datafile). You can also extend the paths
searched for allegro resources with set_allegro_resource_path().

You can store whatever custom information you like in the config file, along 
with the standard variables that are used by Allegro (see below). Allegro
comes with a setup directory where you can find configuration programs. The
standalone setup program is likely to be of interest to final users. It
allows any user to create an `allegro.cfg' file without the need to touch a
text editor and enter values by hand. It also provides a few basic tests like
sound playing for sound card testing. You are welcome to include the setup
program with your game, either as is or with modified graphics to fit better
your game.

void set_config_file(const char *filename);
   Sets the configuration file to be used by all subsequent config
   functions. (Allegro will not search for this file in other locations
   as it does with allegro.cfg at the time of initialization.)
   
   All pointers returned by previous calls to get_config_string() and
   other related functions are invalidated when you call this function!
   You can call this function before install_allegro() to change the
   configuration file, but after set_uformat() if you want to use a text 
   encoding format other than the default.

void set_config_data(const char *data, int length);
   Specifies a block of data to be used by all subsequent config functions, 
   which you have already loaded from disk (eg. as part of some more 
   complicated format of your own, or in a grabber datafile). This routine 
   makes a copy of the information, so you can safely free the data after 
   calling it.

void override_config_file(const char *filename);
   Specifies a file containing config overrides. These settings will be used 
   in addition to the parameters in the main config file, and where a 
   variable is present in both files this version will take priority. This 
   can be used by application programmers to override some of the config 
   settings from their code, while still leaving the main config file free 
   for the end user to customise. For example, you could specify a 
   particular sample frequency and IBK instrument file, but the user could 
   still use an `allegro.cfg' file to specify the port settings and irq
   numbers.

   The override config file will not only take precedence when reading, but
   will also be used for storing values. When you are done with using the
   override config file, you can call override_config_file with a NULL
   parameter, so config data will be directly read from the current config
   file again.

   Note: The override file is completely independent from the current
   configuration. You can e.g. call set_config_file, and the override file
   will still be active. Also the flush_config_file function will only affect
   the current config file (which can be changed with set_config_file), never
   the overriding one specified with this function. The modified override
   config is written back to disk whenever you call override_config_file.

   Example:

      override_config_file("my.cfg");
      /* This will read from my.cfg, and if it doesn't find a
       * setting, will read from the current config file instead.
       */
      language = get_config_string("system", "language", NULL);
      /* This will always write to my.cfg, no matter if the
       * settings is already present or not.
       */
      set_config_string("system", "language", "RU");
      /* This forces the changed setting to be written back to
       * disk. Else it is written back at the next call to
       * override_config_file, or when Allegro shuts down.
       */
      override_config_file(NULL);

   Note that this function and override_config_data() are mutually exclusive,
   i.e. calling one will cancel the effects of the other.

void override_config_data(const char *data, int length);
   Version of override_config_file() which uses a block of data that has 
   already been read into memory. The length of the block has to be specified
   in bytes. Example:

      /* Force German as system language, Spanish keyboard map. */
      const char *override_data = "[system]\n"
         "language=DE\n"
         "keyboard=ES";
      override_config_data(override_data, ustrsize(override_data));

   Note that this function and override_config_file() are mutually exclusive,
   i.e. calling one will cancel the effects of the other.

void push_config_state();
   Pushes the current configuration state (filename, variable values, etc). 
   onto an internal stack, allowing you to select some other config source 
   and later restore the current settings by calling pop_config_state(). 
   This function is mostly intended for internal use by other library 
   functions, for example when you specify a config filename to the 
   save_joystick_data() function, it pushes the config state before 
   switching to the file you specified.

void pop_config_state();
   Pops a configuration state previously stored by push_config_state(), 
   replacing the current config source with it.

void flush_config_file();
   Writes the current config file to disk if the contents have changed 
   since it was loaded or since the latest call to the function.

void reload_config_texts(const char *new_language);
   Reloads the translated strings returned by get_config_text(). This is
   useful to switch to another language in your program at runtime. If you
   want to modify the `[system]' language configuration variable yourself, or
   you have switched configuration files, you will want to pass NULL to
   just reload whatever language is currently selected. Or you can pass a
   string containing the two letter code of the language you desire to
   switch to, and the function will modify the language variable. After you
   call this function, the previously returned pointers of get_config_text()
   will be invalid. Example:

      ...
      /* The user selects French from a language choice menu. */
      reload_config_texts("FR");

void hook_config_section(const char *section,
       int (*intgetter)(const char *name, int def),
       const char *(*stringgetter)(const char *name, const char *def),
       void (*stringsetter)(const char *name, const char *value));
   Takes control of the specified config file section, so that your hook 
   functions will be used to manipulate it instead of the normal disk file 
   access. If both the getter and setter functions are NULL, a currently 
   present hook will be unhooked. Hooked functions have the highest 
   priority. If a section is hooked, the hook will always be called, so you 
   can also hook a '#' section: even override_config_file() cannot override 
   a hooked section. Example:

      int decode_encrypted_int(const char *name, int def)
      {
         ...
      }
      
      const char *decode_encrypted_string(const char *name, const char *def)
      {
         ...
      }
      
      void encode_plaintext_string(const char *name, const char *value)
      {
         ...
      }
      
      int main(int argc, char *argv[])
      {
         ...
         /* Make it harder for users to tinker with the high scores. */
         hook_config_section("high_scores", decode_encrypted_int,
            decode_encrypted_string, encode_plaintext_string);
         ...
      } END_OF_MAIN()

int config_is_hooked(const char *section);
   Returns TRUE if the specified config section has been hooked. Example:

      if (config_is_hooked("high_scores")) {
         hook_config_section("high_scores, NULL, NULL, NULL);
      }

const char *get_config_string(const char *section,
                              const char *name, const char *def);
   Retrieves a string variable from the current config file. The section name
   may be set to NULL to read variables from the root of the file, or used to
   control which set of parameters (eg. sound or joystick) you are interested
   in reading. Example:

      const char *lang = get_config_string("system", "language", "EN");


   Returns a pointer to the constant string found in the configuration file.
   If the named variable cannot be found, or its entry in the config file is
   empty, the value of `def' is returned.

int get_config_int(const char *section, const char *name, int def);
   Reads an integer variable from the current config file. See the comments 
   about get_config_string().

int get_config_hex(const char *section, const char *name, int def);
   Reads an integer variable from the current config file, in hexadecimal 
   format. See the comments about get_config_string().

float get_config_float(const char *section, const char *name, float def);
   Reads a floating point variable from the current config file. See the 
   comments about get_config_string().

int get_config_id(const char *section, const char *name, int def);
   Reads a 4-letter driver ID variable from the current config file. See the 
   comments about get_config_string().

char **get_config_argv(const char *section, const char *name, int *argc);
   Reads a token list (words separated by spaces) from the current config
   file. The token list is stored in a temporary buffer that will be clobbered
   by the next call to get_config_argv(), so the data should not be expected
   to persist.

   Returns an argv style argument list and sets `argc' to the number of
   retrieved tokens. If the variable is not present, returns NULL and sets 
   argc to zero.

const char *get_config_text(const char *msg);
   This function is primarily intended for use by internal library code, but 
   it may perhaps be helpful to application programmers as well. It uses the 
   `language.dat' or `XXtext.cfg' files (where XX is a language code) to look 
   up a translated version of the parameter in the currently selected 
   language.

   This is basically the same thing as calling get_config_string() with 
   `[language]' as the section, `msg' as the variable name, and `msg' as the 
   default value, but it contains some special code to handle Unicode format 
   conversions. The `msg' parameter is always given in ASCII format, but the 
   returned string will be converted into the current text encoding, with 
   memory being allocated as required, so you can assume that this pointer 
   will persist without having to manually allocate storage space for each 
   string.

   Note that if you are planning on distributing your game on the Unix
   platform there is a special issue with how to deal with the `language.dat'
   file. Read section "Files shared by Allegro" of the chapter "Unix
   specifics" to learn more about this.

   Returns a suitable translation if one can be found or a copy of the 
   parameter if nothing else is available. 

void set_config_string(const char *section, const char *name,
                       const char *val);
   Writes a string variable to the current config file, replacing any 
   existing value it may have, or removes the variable if `val' is NULL. The
   section name may be set to NULL to write the variable to the root of the 
   file, or used to control which section the variable is inserted into. The 
   altered file will be cached in memory, and not actually written to disk 
   until you call allegro_exit(). Note that you can only write to files in 
   this way, so the function will have no effect if the current config 
   source was specified with set_config_data() rather than set_config_file().

   As a special case, variable or section names that begin with a '#' 
   character are treated specially and will not be read from or written to 
   the disk. Addon packages can use this to store version info or other 
   status information into the config module, from where it can be read with 
   the get_config_string() function.

void set_config_int(const char *section, const char *name, int val);
   Writes an integer variable to the current config file. See the comments 
   about set_config_string().

void set_config_hex(const char *section, const char *name, int val);
   Writes an integer variable to the current config file, in hexadecimal 
   format. See the comments about set_config_string().

void set_config_float(const char *section, const char *name, float val);
   Writes a floating point variable to the current config file. See the 
   comments about set_config_string().

void set_config_id(const char *section, const char *name, int val);
   Writes a 4-letter driver ID variable to the current config file. See the
   comments about set_config_string().

int list_config_entries(const char *section, const char ***names);
   This function can be used to get a list of all entries in the given config
   section. The names parameter is a pointer to an array of strings. If it
   points to a NULL pointer, the list will be allocated, else it will be
   re-allocated. You should free the list again with free_config_entries if you
   don't need it anymore, or you can pass it again to list_config_entries and
   the memory will be re-used. See the following example for how you can use it,
   it will print out the complete contents of the current configuration:

   int i, n;
   char const **sections = NULL;
   char const **entries = NULL;

   /* List all entries not in any section. */
   n = list_config_entries(NULL, &entries);
   for (i = 0; i 
      printf(" %s=\"%s\"\n", entries[i], get_config_string(
             NULL, entries[i], "-"));

   /* List all sections (and entries in them). */
   n = list_config_sections(&sections);
   /* loop through all section names */
   for (i = 0; i 
   {
      int j, m;
      printf("%s\n", sections[i]);
      m = list_config_entries(sections[i], &entries);
      /* loop through all entries in the section */
      for (j = 0; j 
      {
          printf(" %s=\"%s\"\n", entries[j], get_config_string(
             sections[i], entries[j], "-"));
      }
   }
   /* It is enough to free the arrays once at the end. */
   free_config_entries(&sections);
   free_config_entries(&entries);

   Returns the number of valid strings in the names array.

int list_config_sections(const char ***names);
   The names parameter is a pointer to an array of strings. If it points to a
   NULL pointer, the list will be allocated, else it will be re-allocated. After
   the function returns, it will contain the names of all sections in the
   current configuration. Use free_config_entries to free the allocated memory
   again. See list_config_entries for more information and an example how to use
   it.

   Returns the number of valid strings in the names array.

int free_config_entries(const char ***names);

   Once you are done with the string arrays filled in by list_config_entries and
   list_config_sections, you can free them again with this function. The passed
   array pointer will be set to NULL, and you directly can pass the same pointer
   again to list_config_entries or list_config_sections later - but you also
   could pass them again without freeing first, since the memory is re-allocated
   when the pointer is not NULL.

   See list_config_entries for an example of how to use it.

Standard config variables
-------------------------

Allegro uses these standard variables from the configuration file:

[system]
   Section containing general purpose variables:

system = x
   Specifies which system driver to use. This is currently only useful on 
   Linux, for choosing between the X-Windows ("XWIN") or console ("LNXC") 
   modes.

keyboard = x
   Specifies which keyboard layout to use. The parameter is the name of a 
   keyboard mapping file produced by the keyconf utility, and can either be 
   a fully qualified file path or a base name like `us' or `uk'. If the 
   latter, Allegro will look first for a separate config file with that name 
   (eg. `uk.cfg') and then for an object with that name in the `keyboard.dat'
   file (eg. `UK_CFG'). The config file or `keyboard.dat' file can be stored 
   in the same directory as the program, or in the location pointed to by 
   the ALLEGRO environment variable. Look in the `keyboard.dat' file to see 
   what mappings are currently available.

language = x
   Specifies which language file to use for error messages and other bits of 
   system text. The parameter is the name of a translation file, and can 
   either be a fully qualified file path or a base name like `en' or `es'. If
   the latter, Allegro will look first for a separate config file with a 
   name in the form `entext.cfg', and then for an object with that name in 
   the `language.dat' file (eg. `ENTEXT_CFG'). The config file or 
   `language.dat' file can be stored in the same directory as the program, or 
   in the location pointed to by the ALLEGRO environment variable.

   Look in the `language.dat' file to see which mappings are currently
   available. If there is none for your language, you can create it using the
   English one as model, and even send it to the Allegro development team to
   include it in future releases.

disable_screensaver = x
   Specifies whether to disable the screensaver: 0 to never disable it, 1 to
   disable it in fullscreen mode only and 2 to always disable it. Default is 1.

menu_opening_delay = x
   Sets how long the menus take to auto-open. The time is given in
   milliseconds (default is `300'). Specifying `-1' will disable the 
   auto-opening feature.

XInitThreads = x
   If this is set to 0, the X11 port will not call XInitThreads. This can have
   slight performance advantages and was required on some broken X11 servers,
   but it makes Allegro incompatible with other X11 libraries like Mesa.

[graphics]
   Section containing graphics configuration information, using the
   variables:

gfx_card = x
   Specifies which graphics driver to use when the program requests 
   GFX_AUTODETECT. Multiple possible drivers can be suggested with extra 
   lines in the form `gfx_card1 = x', `gfx_card2 = x', etc, or you can 
   specify different drivers for each mode and color depth with variables in 
   the form `gfx_card_24bpp = x', `gfx_card_640x480x16 = x', etc.

gfx_cardw = x
   Specifies which graphics driver to use when the program requests 
   GFX_AUTODETECT_WINDOWED. This variable functions exactly like
   gfx_card in all other respects. If it is not set, Allegro will look
   for the gfx_card variable.

disable_vsync = x
   Specifies whether to disable synchronization with the vertical blank when
   page-flipping (yes or no). Disabling synchronization may increase the
   frame rate on slow systems, at the expense of introducing flicker on fast
   systems.

vbeaf_driver = x
   DOS and Linux only: specifies where to look for the VBE/AF driver 
   (vbeaf.drv). If this variable is not set, Allegro will look in the same 
   directory as the program, and then fall back on the standard locations 
   (`c:\' for DOS, `/usr/local/lib', `/usr/lib', `/lib', and `/' for Linux, or 
   the directory specified with the VBEAF_PATH environment variable).

framebuffer = x
   Linux only: specifies what device file to use for the fbcon driver. If 
   this variable is not set, Allegro checks the FRAMEBUFFER environment 
   variable, and then defaults to `/dev/fb0'.

force_centering = x
   Unix/X11 only: specifies whether to force window centering in fullscreen
   mode when the XWFS driver is used (yes or no). Enabling this setting may
   cause some artifacts to appear on KDE desktops.

disable_direct_updating = x
   Windows only: specifies whether to disable direct updating when the 
   GFX_DIRECTX_WIN driver is used in color conversion mode (yes or no).
   Direct updating can cause artifacts to be left on the desktop when the 
   window is moved or minimized; disabling it results in a significant
   performance loss.

[mouse]
   Section containing mouse configuration information, using the variables:

mouse = x
   Mouse driver type. Available DOS drivers are:

      MICK - mickey mode driver (normally the best)
      I33  - int 0x33 callback driver
      POLL - timer polling (for use under NT)

   Linux console mouse drivers are:

      MS   - Microsoft serial mouse
      IMS  - Microsoft serial mouse with Intellimouse extension
      LPS2 - PS2 mouse
      LIPS - PS2 mouse with Intellimouse extension
      GPMD - GPM repeater data (Mouse Systems protocol)
      EV   - Event interfaces (EVDEV)

num_buttons = x
   Sets the number of mouse buttons viewed by Allegro. You don't normally
   need to set this variable because Allegro will autodetect it. You can only
   use it to restrict the set of actual mouse buttons to zero or positive
   values, negative values will be ignored.

emulate_three = x
   Sets whether to emulate a third mouse button by detecting chords of the 
   left and right buttons (yes or no). Defaults to no.

mouse_device = x
   Linux only: specifies the name of the mouse device file (eg. 
   `/dev/mouse').

ev_absolute = x
   Linux only: specifies the mode for the default EV input:
      0 - relative mode: pointer position changes if the input moves,
      1 - absolute mode: pointer position is the input position.
   If unspecified, the mode is relative.
   If the device supports several tools (such as a graphic tablet), the
   default input is the mouse. If the device has only one tool (e.g. a
   normal mouse) the default input is this tool. All additional tools
   work in absolute mode.

ev_min_x = x
ev_max_x = x
ev_min_y = x
ev_max_y = x
ev_min_z = x
ev_max_z = x
   Linux only: for absolute EV inputs, minimum and maximum value. By default
   this information is autodetected.  If you want to use only part of a
   tablet, you need to set the entries for X and Y axis by hand.

ev_abs_to_rel_x = x
ev_abs_to_rel_y = x
ev_abs_to_rel_z = x
   Linux only: scaling factor for tablet mouse speed.  Defaults to 1.
   This is used only when the input sends absolute events (tablet, joystick,
   etc.) and the cursor should behave like a mouse.
   If you are using a mouse on a tablet, you need to set these entries
   for X and Y axis (try numbers between 1 and 40).

mouse_accel_factor = x
   Windows only: specifies the mouse acceleration factor. Defaults to 1.
   Set it to 0 in order to disable mouse acceleration. 2 accelerates twice
   as much as 1.

[sound]
   Section containing sound configuration information, using the variables:

digi_card = x
   Sets the driver to use for playing digital samples.

midi_card = x
   Sets the driver to use for MIDI music.

digi_input_card = x
   Sets the driver to use for digital sample input.

midi_input_card = x
   Sets the driver to use for MIDI data input.

digi_voices = x
   Specifies the minimum number of voices to reserve for use by the digital 
   sound driver. How many are possible depends on the driver.

midi_voices = x
   Specifies the minimum number of voices to reserve for use by the MIDI 
   sound driver. How many are possible depends on the driver.

digi_volume = x
   Sets the volume for digital sample playback, from 0 to 255.

midi_volume = x
   Sets the volume for midi music playback, from 0 to 255.

quality = x
   Controls the sound quality vs. performance tradeoff for the sample mixing 
   code. This can be set to any of the values:
      0 - fast mixing of 8-bit data into 16-bit buffers
      1 - true 16-bit mixing (requires a 16-bit stereo sound card)
      2 - interpolated 16-bit mixing

flip_pan = x
   Toggling this between 0 and 1 reverses the left/right panning of samples, 
   which might be needed because some SB cards get the stereo image the wrong
   way round.

sound_freq = x
   DOS, Unix and BeOS: sets the sample frequency. With the SB driver, 
   possible rates are 11906 (any), 16129 (any), 22727 (SB 2.0 and above), 
   and 45454 (only on SB 2.0 or SB16, not the stereo SB Pro driver). On the 
   ESS Audiodrive, possible rates are 11363, 17046, 22729, or 44194. On the 
   Ensoniq Soundscape, possible rates are 11025, 16000, 22050, or 48000. On 
   the Windows Sound System, possible rates are 11025, 22050, 44100, or 
   48000. Don't worry if you set some other number by mistake: Allegro will 
   automatically round it to the closest supported frequency.

sound_bits = x
   Unix and BeOS: sets the preferred number of bits (8 or 16).

sound_stereo = x
   Unix and BeOS: selects mono or stereo output (0 or 1).

sound_port = x
   DOS only: sets the sound card port address (this is usually 220).

sound_dma = x
   DOS only: sets the sound card DMA channel (this is usually 1).

sound_irq = x
   DOS only: sets the sound card IRQ number (this is usually 7).

fm_port = x
   DOS only: sets the port address of the OPL synth (this is usually 388).

mpu_port = x
   DOS only: sets the port address of the MPU-401 MIDI interface (this is 
   usually 330).

mpu_irq = x
   DOS only: sets the IRQ for the MPU-401 (this is usually the same as 
   sound_irq).

ibk_file = x
   DOS only: specifies the name of a .IBK file which will be used to replace 
   the standard Adlib patch set.

ibk_drum_file = x
   DOS only: specifies the name of a .IBK file which will be used to replace 
   the standard set of Adlib percussion patches.

oss_driver = x
   Unix only: sets the OSS device driver name. Usually `/dev/dsp' or 
   `/dev/audio', but could be a particular device (e.g. `/dev/dsp2').

oss_numfrags = x
oss_fragsize = x
   Unix only: sets number of OSS driver fragments (buffers) and size of each 
   buffer in samples. Buffers are filled with data in the interrupts where 
   interval between subsequent interrupts is not less than 10 ms. If 
   hardware can play all information from buffers faster than 10 ms, then 
   there will be clicks, when hardware have played all data and library has 
   not prepared new data yet. On the other hand, if it takes too long for 
   device driver to play data from all buffers, then there will be delays 
   between action which triggers sound and sound itself.

oss_midi_driver = x
   Unix only: sets the OSS MIDI device name. Usually `/dev/sequencer'.

oss_mixer_driver = x
   Unix only: sets the OSS mixer device name. Usually `/dev/mixer'.

esd_server = x
   Unix only: where to find the ESD (Enlightened Sound Daemon) server.

alsa_card = x
alsa_pcmdevice = x
   Unix only: card number and PCM device for the ALSA 0.5 sound driver.

alsa_device = x
   Unix only: device name for the ALSA 0.9 sound driver. The format is
   <driver>[:<card>,<device>], for example: `hw:0,1'.

alsa_mixer_device = x
   Unix only: mixer device name for the ALSA 0.9 sound driver. The
   default is "default".

alsa_mixer_elem = x
   Unix only: mixer element name for the ALSA 0.9 driver. The default
   is PCM.

alsa_numfrags = x
   Unix only: number of ALSA driver fragments (buffers).

alsa_fragsize = x
   Unix only: size of each ALSA fragment, in samples.

alsa_rawmidi_card = x
   Unix only: card number and device for the ALSA 0.5 midi driver.

alsa_rawmidi_device = x
   Unix only: device for the ALSA 0.5 midi driver or device name for
   the ALSA 0.9 midi driver (see alsa_device for the format).

jack_client_name = x
   Sets the name with which Allegro should identify itself to the Jack
   audio server.

jack_buffer_size = x
   Forces a buffer size for the transfer buffer from Allegro's mixer
   to Jack.

be_midi_quality = x
   BeOS only: system MIDI synthesizer instruments quality. 0 uses low
   quality 8-bit 11 kHz samples, 1 uses 16-bit 22 kHz samples.

be_midi_freq = x
   BeOS only: MIDI sample mixing frequency in Hz. Can be 11025, 22050 or
   44100.

be_midi_interpolation = x
   BeOS only: specifies the MIDI samples interpolation method. 0 doesn't
   interpolate, it's fast but has the worst quality; 1 does a fast
   interpolation with better performances, but it's a bit slower than the
   previous method; 2 does a linear interpolation between samples, it is the
   slowest method but gives the best performances.

be_midi_reverb = x
   BeOS only: reverberation intensity, from 0 to 5. 0 disables it, 5 is the
   strongest one.

ca_midi_quality = x
   MacOS X only: CoreAudio MIDI synthesizer rendering quality, from 0 to 127.
   Higher qualities sound better but increase the CPU work load.

ca_midi_reverb = x
   MacOS X only: CoreAudio MIDI synthesizer reverberation intensity, from 0
   to 5. 0 equals to a small room (low reverb), 5 to a plate (high reverb).

patches = x
   Specifies where to find the sample set for the DIGMID driver. This can 
   either be a Gravis style directory containing a collection of .pat files 
   and a `default.cfg' index, or an Allegro datafile produced by the pat2dat 
   utility. If this variable is not set, Allegro will look either for a 
   `default.cfg' or `patches.dat' file in the same directory as the program, 
   the directory pointed to by the ALLEGRO environment variable, and the 
   standard GUS directory pointed to by the ULTRASND environment variable.

[midimap]
   If you are using the SB MIDI output or MPU-401 drivers with an external 
   synthesiser that is not General MIDI compatible, you can use the midimap 
   section of the config file to specify a patch mapping table for 
   converting GM patch numbers into whatever bank and program change 
   messages will select the appropriate sound on your synth. This is a real 
   piece of self-indulgence. I have a Yamaha TG500, which has some great 
   sounds but no GM patch set, and I just had to make it work somehow...

   This section consists of a set of lines in the form:

p<n> = bank0 bank1 prog pitch
   With this statement, n is the GM program change number (1-128), bank0 and 
   bank1 are the two bank change messages to send to your synth (on 
   controllers #0 and #32), prog is the program change message to send to 
   your synth, and pitch is the number of semitones to shift everything that 
   is played with that sound. Setting the bank change numbers to -1 will 
   prevent them from being sent.

   For example, the line:

      p36 = 0 34 9 12

   specifies that whenever GM program 36 (which happens to be a fretless 
   bass) is selected, Allegro should send a bank change message #0 with a 
   parameter of 0, a bank change message #32 with a parameter of 34, a 
   program change with a parameter of 9, and then should shift everything up 
   by an octave.

[joystick]
   Section containing joystick configuration information, using the
   variables:

joytype = x
   Specifies which joystick driver to use when the program requests 
   JOY_TYPE_AUTODETECT.

joystick_device = x
   BeOS and Linux only: specifies the name of the joystick device to be used
   (as reported in the system joystick preferences under BeOS). The first
   device found is used by default. If you want to specify the device for
   each joystick, use variables of the form joystick_device_n, where n is
   the joystick number.

throttle_axis = x
   Linux only: sets the axis number the throttle is located at. This
   variable will be used for every detected joystick. If you want to specify
   the axis number for each joystick individually, use variables of the form
   throttle_axis_n, where n is the joystick number.



========================================
============ Mouse routines ============
========================================

Allegro provides functions for reading the mouse state and displaying a mouse
cursor on-screen. You can read the absolute position of the mouse and the 
state of the mouse buttons from global variables. Additionally, you can read
the mouse position difference as mouse mickeys, which is the number of pixels
the cursor moved since the last time this information was read.

Allegro offers three ways to display the mouse cursor:

Standard Allegro cursor
   Allegro is responsible for drawing the mouse cursor from a timer. Use 
   set_mouse_sprite() and show_mouse() to define your own cursor and display 
   it on the screen.
   You need to call scare_mouse()/unscare_mouse() to hide the mouse cursor
   whenever you draw to the screen.

Custom operating system cursor (hardware cursor)
   Allegro will let the operating system draw the mouse cursor. Use
   set_mouse_sprite() and show_mouse() (or show_os_cursor) to define your
   own cursor and display it on the screen. Not all graphics drivers are
   capable of this and some may only be able to display cursors up to a
   certain size. Allegro will fall back on its own cursor drawing if it
   cannot let the OS handle this. On some platforms, the hardware cursor
   is incompatible with get_mouse_mickeys() and it is therefor disabled by
   default. In such cases you need to call enable_hardware_cursor() to
   enable it explicitly.

Default operating system cursor
   Allegro will not draw its own cursor, but use the operating system default
   cursor. You can use the select_mouse_cursor() function to select the cursor
   shape to display. As with custom operating system cursors, you need to call 
   enable_hardware_cursor() before you can use this. Or you can use the
   low level show_os_cursor() function.

Not all drivers will support all functionality. See the platform specific
information for more details.

int install_mouse();
   Installs the Allegro mouse handler. You must do this before using any 
   other mouse functions. 

   Returns -1 on failure, zero if the mouse handler is already installed (in
   which case this function does nothing) and the number of buttons on the
   mouse if the mouse handler has successfully been installed (ie. this is
   the first time a handler is installed or you have removed the previous
   one).

   Note that the number of mouse buttons returned by this function is more
   an indication than a physical reality. With most devices there is no way
   of telling how many buttons there are, and any user can override the
   number of mouse buttons returned by this function with a custom
   configuration file and the variable num_buttons. Even if this value is
   overridden by the user, the global mouse variables will still report
   whatever the hardware is sending.

void remove_mouse();
   Removes the mouse handler. You don't normally need to bother calling 
   this, because allegro_exit() will do it for you.

int poll_mouse();
   Wherever possible, Allegro will read the mouse input asynchronously (ie. 
   from inside an interrupt handler), but on some platforms that may not be 
   possible, in which case you must call this routine at regular intervals 
   to update the mouse state variables. To help you test your mouse polling 
   code even if you are programming on a platform that doesn't require it, 
   after the first time that you call this function Allegro will switch into 
   polling mode, so from that point onwards you will have to call this 
   routine in order to get any mouse input at all, regardless of whether the 
   current driver actually needs to be polled or not. 

   Returns zero on success, or a negative number on failure (ie. no mouse 
   driver installed).

int mouse_needs_poll();
   Returns TRUE if the current mouse driver is operating in polling mode.

void enable_hardware_cursor(void);
   After calling this function, Allegro will let the operating system draw the
   mouse cursor instead of doing it itself. This is not possible with all
   graphics drivers though: you'll need to check the gfx_capabilities flags
   after calling show_mouse() to see if this works. On some platforms, enabling
   the hardware cursor causes get_mouse_mickeys() to return only a limited
   range of values, so you should not call this function if you need mouse 
   mickeys.

void disable_hardware_cursor(void);
   After calling this function, Allegro will be responsible for drawing the
   mouse cursor rather than the operating system. On some platforms calling
   enable_hardware_cursor() makes the return values of get_mouse_mickeys()
   unreliable. After calling this function, get_mouse_mickeys() returns 
   reliable results again.

void select_mouse_cursor(int cursor);
   This function allows you to use the operating system's native mouse
   cursors rather than some custom cursor. You will need to enable this
   functionality by calling enable_hardware_cursor() beforehand. If the
   operating system does not support this functionality, or if it has
   not been enabled, then Allegro will substitute its own cursor images.
   You can change these substitute images using set_mouse_cursor_bitmap().

   Note that the effects of this function are not apparent until show_mouse()
   is called.

   To know whether the operating system's native cursor is being used,
   or if Allegro has made a substitution, you can check the GFX_SYSTEM_CURSOR
   flag in gfx_capabilities after calling show_mouse().

   The cursor argument selects the type of cursor to be displayed:
   
   MOUSE_CURSOR_NONE
   Selects an invisible mouse cursor. In that sense, it is similar to calling
   show_mouse(NULL);
   
   MOUSE_CURSOR_ALLEGRO
   Selects the custom Allegro cursor, i.e. the one that you set with
   set_mouse_sprite().
   
   MOUSE_CURSOR_ARROW
   The operating system default arrow cursor.
   
   MOUSE_CURSOR_BUSY
   The operating system default `busy' cursor (hourglass).
   
   MOUSE_CURSOR_QUESTION
   The operating system default `question' cursor (arrow with question mark).
   
   MOUSE_CURSOR_EDIT
   The operating system default `edit' cursor (vertical bar).
   
   Example:

   /* initialize mouse sub-system */
   install_mouse();
   enable_hardware_cursor();
   
   /* Set busy pointer */
   select_mouse_cursor(MOUSE_CURSOR_BUSY);
   show_mouse(screen);
   
   /* Initialize stuff */
   ...
   
   /* Set normal arrow pointer */
   select_mouse_cursor(MOUSE_CURSOR_ARROW);


void set_mouse_cursor_bitmap(int cursor, BITMAP *bmp);
   This function changes the cursor image Allegro uses if
   select_mouse_cursor() is called but no native operating system cursor
   can be used, e.g. because you did not call enable_hardware_cursor().

   The cursor argument can be one of:
   MOUSE_CURSOR_ALLEGRO
   MOUSE_CURSOR_ARROW
   MOUSE_CURSOR_BUSY
   MOUSE_CURSOR_QUESTION
   MOUSE_CURSOR_EDIT

   but not MOUSE_CURSOR_NONE.

   The bmp argument can either point to a valid bitmap or it can be NULL.
   Passing a bitmap makes Allegro use that image in place of its own
   default substitution (should the operating system's native cursor
   be unavailable). The bitmap must remain available for the duration
   in which it could be used. Passing NULL lets Allegro revert to its
   default substitutions.

   The effect of this function will not be apparent until show_mouse() is
   called.

extern volatile int mouse_x;
extern volatile int mouse_y;
extern volatile int mouse_z;
extern volatile int mouse_w;
extern volatile int mouse_b;
extern volatile int mouse_pos;
   Global variables containing the current mouse position and button state. 
   Wherever possible these values will be updated asynchronously, but if 
   mouse_needs_poll() returns TRUE, you must manually call poll_mouse() to 
   update them with the current input state. The `mouse_x' and `mouse_y' 
   positions are integers ranging from zero to the bottom right corner of 
   the screen. The `mouse_z' and `mouse_w' variables hold the current vertical
   and horizontal wheel position, when using an input driver that supports
   wheel mice. The `mouse_b' variable is a bitfield indicating the state of
   each button: bit 0 is the left button,  bit 1 the right, and bit 2 the
   middle button. Additional non standard mouse buttons might be available
   as higher bits in this variable. Usage example:

      if (mouse_b & 1)
         printf("Left button is pressed\n");

      if (!(mouse_b & 2))
         printf("Right button is not pressed\n");

   The `mouse_pos' variable has the current X coordinate in the upper 16 bits
   and  the Y in the lower 16 bits. This may be useful in tight polling loops 
   where a mouse interrupt could occur between your reading of the two
   separate variables, since you can copy this value into a local variable
   with a single instruction and then split it up at your leisure. Example:

   int pos, x, y;
   
   pos = mouse_pos;
   x = pos >> 16;
   y = pos & 0x0000ffff;



extern BITMAP *mouse_sprite;
extern int mouse_x_focus;
extern int mouse_y_focus;
   Global variables containing the current mouse sprite and the focus
   point.  These are read-only, and only to be modified using the
   set_mouse_sprite() and set_mouse_sprite_focus() functions.

void show_mouse(BITMAP *bmp);
   Tells Allegro to display a mouse pointer on the screen. This will only 
   work if the timer module has been installed. The mouse pointer will be 
   drawn onto the specified bitmap, which should normally be `screen' (see 
   later for information about bitmaps). To hide the mouse pointer, call 
   show_mouse(NULL). 
   
   Warning: if you draw anything onto the screen while the pointer is 
   visible, a mouse movement interrupt could occur in the middle of your 
   drawing operation. If this happens the mouse buffering and graphics drawing
   code will get confused and will leave 'mouse droppings' all over the 
   screen. To prevent this, you must make sure you turn off the mouse 
   pointer whenever you draw onto the screen. This is not needed if you are
   using a hardware cursor.

   Note: you must not be showing a mouse pointer on a bitmap at the time that
   the bitmap is destroyed with destroy_bitmap(), e.g. call show_mouse(NULL);
   before destroying the bitmap. This does not apply to `screen' since you
   never destroy `screen' with destroy_bitmap().

void scare_mouse();
   Helper for hiding the mouse pointer prior to a drawing operation. This 
   will temporarily get rid of the pointer, but only if that is really 
   required (ie. the mouse is visible, and is displayed on the physical 
   screen rather than some other memory surface, and it is not a hardware 
   or OS cursor). The previous mouse state is stored for subsequent calls to 
   unscare_mouse().

void scare_mouse_area(int x, int y, int w, int h);
   Like scare_mouse(), but will only hide the cursor if it is inside the 
   specified rectangle. Otherwise the cursor will simply be frozen in place 
   until you call unscare_mouse(), so it cannot interfere with your drawing.

void unscare_mouse();
   Undoes the effect of a previous call to scare_mouse() or 
   scare_mouse_area(), restoring the original pointer state.

int show_os_cursor(int cursor);
   In case you do not need Allegro's mouse cursor API, which automatically
   emulates a cursor in software if no other cursor is available, you can
   use this low level function to try to display or hide the system cursor
   directly. The cursor parameter takes the same values as
   select_mouse_cursor. This function is very similar to calling
   enable_hardware_cursor, select_mouse_cursor and show_mouse, but will
   not try to do anything if no system cursor is available.

   The most common use for this function is to just call it once at the
   beginning of the program to tell it to display the system cursor inside
   the Allegro window. The return value can be used to see if this
   succeeded or not. On some systems (e.g. DirectX fullscreen) this is not
   supported and the function will always fail, and in other cases only
   some of the cursors will work, or in the case of MOUSE_CURSOR_ALLEGRO,
   only certain bitmap sizes may be supported.

   You never should use show_os_cursor together with the function
   show_mouse and other functions affecting it (select_mouse_cursor,
   enable_hardware_cursor, disable_hardware_cursor, scare_mouse,
   unscare_mouse). They implement the standard high level mouse API, and
   don't work together with this low level function.

   Returns 0 if a system cursor is being displayed after the function
   returns, or -1 otherwise.

extern volatile int freeze_mouse_flag;
   If this flag is set, the mouse pointer won't be redrawn when the mouse 
   moves. This can avoid the need to hide the pointer every time you draw to 
   the screen, as long as you make sure your drawing doesn't overlap with 
   the current pointer position.

void position_mouse(int x, int y);
   Moves the mouse to the specified screen position. It is safe to call even 
   when a mouse pointer is being displayed.

void position_mouse_z(int z);
   Sets the mouse wheel position variable to the specified value.

void position_mouse_w(int w);
   Sets the horizontal mouse wheel position to the specified value.

void set_mouse_range(int x1, int y1, int x2, int y2);
   Sets the area of the screen within which the mouse can move. Pass the top 
   left corner and the bottom right corner (inclusive). If you don't call 
   this function the range defaults to (0, 0, SCREEN_W-1, SCREEN_H-1).

void set_mouse_speed(int xspeed, int yspeed);
   Sets the mouse speed. Larger values of xspeed and yspeed represent slower 
   mouse movement: the default for both is 2.

void set_mouse_sprite(BITMAP *sprite);
   You don't like Allegro's mouse pointer? No problem. Use this function to
   supply an alternative of your own. If you change the pointer and then want
   to get Allegro's lovely arrow back again, call set_mouse_sprite(NULL).

   As a bonus, set_mouse_sprite(NULL) uses the current palette in choosing
   colors for the arrow. So if your arrow mouse sprite looks ugly after
   changing the palette, call set_mouse_sprite(NULL).

void set_mouse_sprite_focus(int x, int y);
   The mouse focus is the bit of the pointer that represents the actual 
   mouse position, ie. the (mouse_x, mouse_y) position. By default this is 
   the top left corner of the arrow, but if you are using a different mouse 
   pointer you might need to alter it.

void get_mouse_mickeys(int *mickeyx, int *mickeyy);
   Measures how far the mouse has moved since the last call to this 
   function. The values of mickeyx and mickeyy will become negative if the 
   mouse is moved left or up, respectively. The mouse will continue to 
   generate movement mickeys even when it reaches the edge of the screen, so 
   this form of input can be useful for games that require an infinite range 
   of mouse movement.

   Note that the infinite movement may not work in windowed mode, since under 
   some platforms the mouse would leave the window, and may not work at all 
   if the hardware cursor is in use.

extern void (*mouse_callback)(int flags);
   Called by the interrupt handler whenever the mouse moves or one of the 
   buttons changes state. This function must be in locked memory, and must 
   execute _very_ quickly! It is passed the event flags that triggered the 
   call, which is a bitmask containing any of the values MOUSE_FLAG_MOVE, 
   MOUSE_FLAG_LEFT_DOWN, MOUSE_FLAG_LEFT_UP, MOUSE_FLAG_RIGHT_DOWN, 
   MOUSE_FLAG_RIGHT_UP, MOUSE_FLAG_MIDDLE_DOWN, MOUSE_FLAG_MIDDLE_UP, and 
   MOUSE_FLAG_MOVE_Z. Note that even if the mouse has more than three buttons,
   only the first three can be trapped using a callback.

int mouse_on_screen();
   This function can be useful to prevent having two mouse pointers on the 
   screen at the same time when running your program in windowed mode and 
   drawing the mouse pointer yourself. Other possible uses include the 
   ability to pause your game when the mouse goes off of the window, or 
   only scrolling the view when the pointer is near the edge of the window,
   but not while off of the window.
   
   Example :

   if (mouse_on_screen()) {draw_sprite(buffer , mouse_sprite , mouse_x , mouse_y);}



   Returns 0 if the mouse pointer is off of the screen, or non-zero otherwise.



========================================
============ Timer routines ============
========================================

Allegro can set up several virtual timer functions, all going at different
speeds.

Under DOS it will constantly reprogram the clock to make sure they are all
called at the correct times. Because they alter the low level timer chip
settings, these routines should not be used together with other DOS timer
functions like the DJGPP uclock() routine. Moreover, the FPU state is not
preserved across Allegro interrupts so you ought not to use floating point
or MMX code inside timer interrupt handlers.

Under other platforms, they are usually implemented using threads, which run
parallel to the main thread. Therefore timer callbacks on such platforms
will not block the main thread when called, so you may need to use
appropriate synchronisation devices (eg. mutexes, semaphores, etc.) when
accessing data that is shared by a callback and the main thread. (Currently
Allegro does not provide such devices.)

int install_timer();
   Installs the Allegro timer interrupt handler. You must do this before 
   installing any user timer routines, and also before displaying a mouse 
   pointer, playing FLI animations or MIDI music, and using any of the GUI 
   routines.

   Returns zero on success, or a negative number on failure (but you may
   decide not to check the return value as this function is very unlikely to
   fail).

void remove_timer();
   Removes the Allegro timer handler (and, under DOS, passes control of the
   clock back to the operating system). You don't normally need to bother
   calling this, because allegro_exit() will do it for you.

int install_int(void (*proc)(), int speed);
   Installs a user timer handler, with the speed given as the number of 
   milliseconds between ticks. This is the same thing as 
   install_int_ex(proc, MSEC_TO_TIMER(speed)). If you call this routine 
   without having first installed the timer module, install_timer() will be 
   called automatically. Calling again this routine with the same timer
   handler as parameter allows you to adjust its speed.

   Returns zero on success, or a negative number if there is no room to add
   a new user timer.

int install_int_ex(void (*proc)(), int speed);
   Adds a function to the list of user timer handlers or, if it is already 
   installed, retroactively adjusts its speed (i.e makes as though the speed
   change occurred precisely at the last tick). The speed is given in hardware
   clock ticks, of which there are 1193181 a second. You can convert from 
   other time formats to hardware clock ticks with the macros:

      SECS_TO_TIMER(secs)  - give the number of seconds between
                             each tick
      MSEC_TO_TIMER(msec)  - give the number of milliseconds
                             between ticks
      BPS_TO_TIMER(bps)    - give the number of ticks each second
      BPM_TO_TIMER(bpm)    - give the number of ticks per minute

   There can only be sixteen timers in use at a time, and some other parts of
   Allegro (the GUI code, the mouse pointer display routines, rest(), the FLI
   player, and the MIDI player) need to install handlers of their own, so you
   should avoid using too many at the same time. If you call this routine
   without having first installed the timer module, install_timer() will be
   called automatically.

   Your function will be called by the Allegro interrupt handler and not 
   directly by the processor, so it can be a normal C function and does not 
   need a special wrapper. You should be aware, however, that it will be 
   called in an interrupt context, which imposes a lot of restrictions on 
   what you can do in it. It should not use large amounts of stack, it must 
   not make any calls to the operating system, use C library functions, or 
   contain any floating point code, and it must execute very quickly. Don't 
   try to do lots of complicated code in a timer handler: as a general rule 
   you should just set some flags and respond to these later in your main 
   control loop.

   In a DOS protected mode environment like DJGPP, memory is virtualised and 
   can be swapped to disk. Due to the non-reentrancy of DOS, if a disk swap 
   occurs inside an interrupt handler the system will die a painful death, 
   so you need to make sure you lock all the memory (both code and data) 
   that is touched inside timer routines. Allegro will lock everything it 
   uses, but you are responsible for locking your handler functions. The 
   macros LOCK_VARIABLE (variable), END_OF_FUNCTION (function_name),
   END_OF_STATIC_FUNCTION (function_name), and LOCK_FUNCTION (function_name)
   can be used to simplify this task. For example, if you want an interrupt 
   handler that increments a counter variable, you should write:

      volatile int counter;

      void my_timer_handler()
      {
         counter++;
      }

      END_OF_FUNCTION(my_timer_handler)

   and in your initialisation code you should lock the memory:

      LOCK_VARIABLE(counter);
      LOCK_FUNCTION(my_timer_handler);

   Obviously this can get awkward if you use complicated data structures and 
   call other functions from within your handler, so you should try to keep 
   your interrupt routines as simple as possible.

   Returns zero on success, or a negative number if there is no room to add
   a new user timer.

Macro LOCK_VARIABLE(variable_name);
   Due to interrupts, you are required to lock all the memory used by your
   timer routines. See the description of install_int_ex() for a more
   detailed explanation and usage example.
   
Macro LOCK_FUNCTION(function_name);
   Due to interrupts, you are required to lock all the memory used by your
   timer routines. See the description of install_int_ex() for a more
   detailed explanation and usage example.
   
Macro END_OF_FUNCTION(function_name);
   Due to interrupts, you are required to lock all the code used by your
   timer routines. See the description of install_int_ex() for a more
   detailed explanation and usage example.
   
void remove_int(void (*proc)());
   Removes a function from the list of user interrupt routines. At program 
   termination, allegro_exit() does this automatically.

int install_param_int(void (*proc)(void *), void *param, int speed);
   Like install_int(), but the callback routine will be passed a copy of the 
   specified void pointer parameter. To disable the handler, use 
   remove_param_int() instead of remove_int().

int install_param_int_ex(void (*proc)(void *), void *param, int speed);
   Like install_int_ex(), but the callback routine will be passed a copy of 
   the specified void pointer parameter. To disable the handler, use 
   remove_param_int() instead of remove_int().

void remove_param_int(void (*proc)(void *), void *param);
   Like remove_int(), but for use with timer callbacks that have parameter 
   values. If there is more than one copy of the same callback active at a 
   time, it identifies which one to remove by checking the parameter value 
   (so you can't have more than one copy of a handler using an identical 
   parameter).

extern volatile int retrace_count;
   If the retrace simulator is installed, this count is incremented on each
   vertical retrace; otherwise, if the refresh rate is known, the count is
   incremented at the same rate (ignoring retraces); otherwise, it is
   incremented 70 times a second. This provides a way of controlling
   the speed of your program without installing user timer functions.

void rest(unsigned int time);
   This function waits for the specified number of milliseconds.

   Passing 0 as parameter will not wait, but just yield. This can be useful
   in order to "play nice" with other processes. Other values will cause
   CPU time to be dropped on most platforms. This will look better to
   users, and also does things like saving battery power and making fans
   less noisy.

   Note that calling this inside your active game loop is a bad idea, as
   you never know when the OS will give you the CPU back, so you could end
   up missing the vertical retrace and skipping frames. On the other hand,
   on multitasking operating systems it is good form to give up the CPU for
   a while if you will not be using it.

void rest_callback(long time, void (*callback)())
   Like rest(), but for non-zero values continually calls the specified
   function while it is waiting for the required time to elapse. If the
   provided `callback' parameter is NULL, this function does exactly the
   same thing as calling rest().



===========================================
============ Keyboard routines ============
===========================================

The Allegro keyboard handler provides both buffered input and a set of flags 
storing the current state of each key. Note that it is not possible to 
correctly detect every combination of keys, due to the design of the PC 
keyboard. Up to two or three keys at a time will work fine, but if you press 
more than that the extras are likely to be ignored (exactly which 
combinations are possible seems to vary from one keyboard to another).

On DOS, Allegro requires the user to specify the language of the keyboard
mapping because it is impossible to obtain this information from the OS,
otherwise the default US keyboard mapping will be used. Allegro comes with a
prepackaged `keyboard.dat' file which you can put along with your binary. If
this file is present, Allegro will be able to extract the keyboard mapping
information stored there. However, the end user still needs to select which
keyboard mapping to use. This can be accomplished through the keyboard variable
of the system section in a standard `allegro.cfg' configuration file. Read
chapter "Configuration routines" for more information about this.

int install_keyboard();
   Installs the Allegro keyboard interrupt handler. You must call this 
   before using any of the keyboard input routines. Once you have set up the 
   Allegro handler, you can no longer use operating system calls or C 
   library functions to access the keyboard.

   Note that on some platforms the keyboard won't work unless you have set a
   graphics mode, even if this function returns a success value before
   calling set_gfx_mode. This can happen in environments with graphic
   windowed modes, since Allegro usually reads the keyboard through the
   graphical window (which appears after the set_gfx_mode call). Example:

      allegro_init();
      install_timer();
      install_keyboard();
      /* We are not 100% sure we can read the keyboard yet! */
      if (set_gfx_mode(GFX_AUTODETECT, 640, 480, 0, 0) != 0)
         abort_on_error("Couldn't set graphic mode!");

      /* Now we are guaranteed to be able to read the keyboard. */
      readkey();

   Returns zero on success, or a negative number on failure (but you may
   decide not to check the return value as this function is very unlikely to
   fail).

void remove_keyboard();
   Removes the keyboard handler, returning control to the operating system. 
   You don't normally need to bother calling this, because allegro_exit() 
   will do it for you. However, you might want to call this during runtime if
   you want to change the keyboard mapping on those platforms were keyboard
   mappings are needed. You would first modify the configuration variable
   holding the keyboard mapping and then reinstall the keyboard handler.
   Example:

      remove_keyboard();
      /* Switch to Spanish keyboard mapping. */
      set_config_string("system", "keyboard", "es");
      install_keyboard();

void install_keyboard_hooks(int (*keypressed)(), int (*readkey)());
   You should only use this function if you *aren't* using the rest of the 
   keyboard handler. It should be called in the place of install_keyboard(), 
   and lets you provide callback routines to detect and read keypresses, 
   which will be used by the main keypressed() and readkey() functions. This 
   can be useful if you want to use Allegro's GUI code with a custom 
   keyboard handler, as it provides a way for the GUI to get keyboard input 
   from your own code, bypassing the normal Allegro input system.

int poll_keyboard();
   Wherever possible, Allegro will read the keyboard input asynchronously 
   (ie. from inside an interrupt handler), but on some platforms that may 
   not be possible, in which case you must call this routine at regular 
   intervals to update the keyboard state variables.

   To help you test your keyboard polling code even if you are programming
   on a platform that doesn't require it, after the first time that you
   call this function Allegro will switch into polling mode, so from that
   point onwards you will have to call this routine in order to get any
   keyboard input at all, regardless of whether the current driver actually
   needs to be polled or not.

   The keypressed(), readkey(), and ureadkey() functions call poll_keyboard()
   automatically, so you only need to use this function when accessing the
   key[] array and key_shifts variable.

   Returns zero on success, or a negative number on failure (ie. no keyboard
   driver installed).

int keyboard_needs_poll();
   Returns TRUE if the current keyboard driver is operating in polling mode.

extern volatile char key[KEY_MAX];
   Array of flags indicating the state of each key, ordered by scancode. 
   Wherever possible these values will be updated asynchronously, but if 
   keyboard_needs_poll() returns TRUE, you must manually call 
   poll_keyboard() to update them with the current input state. The 
   scancodes are defined in allegro/keyboard.h as a series of KEY_*
   constants (and are also listed below). For example, you could write:

      if (key[KEY_SPACE])
         printf("Space is pressed\n");

   Note that the array is supposed to represent which keys are physically
   held down and which keys are not, so it is semantically read-only.

   These are the keyboard scancodes:

      KEY_A ... KEY_Z,
      KEY_0 ... KEY_9,
      KEY_0_PAD ... KEY_9_PAD,
      KEY_F1 ... KEY_F12,

      KEY_ESC, KEY_TILDE, KEY_MINUS, KEY_EQUALS,
      KEY_BACKSPACE, KEY_TAB, KEY_OPENBRACE, KEY_CLOSEBRACE,
      KEY_ENTER, KEY_COLON, KEY_QUOTE, KEY_BACKSLASH,
      KEY_BACKSLASH2, KEY_COMMA, KEY_STOP, KEY_SLASH,
      KEY_SPACE,

      KEY_INSERT, KEY_DEL, KEY_HOME, KEY_END, KEY_PGUP,
      KEY_PGDN, KEY_LEFT, KEY_RIGHT, KEY_UP, KEY_DOWN,

      KEY_SLASH_PAD, KEY_ASTERISK, KEY_MINUS_PAD,
      KEY_PLUS_PAD, KEY_DEL_PAD, KEY_ENTER_PAD,

      KEY_PRTSCR, KEY_PAUSE,

      KEY_ABNT_C1, KEY_YEN, KEY_KANA, KEY_CONVERT, KEY_NOCONVERT,
      KEY_AT, KEY_CIRCUMFLEX, KEY_COLON2, KEY_KANJI,

      KEY_LSHIFT, KEY_RSHIFT,
      KEY_LCONTROL, KEY_RCONTROL,
      KEY_ALT, KEY_ALTGR,
      KEY_LWIN, KEY_RWIN, KEY_MENU,
      KEY_SCRLOCK, KEY_NUMLOCK, KEY_CAPSLOCK

      KEY_EQUALS_PAD, KEY_BACKQUOTE, KEY_SEMICOLON, KEY_COMMAND

   Finally, you may notice an `odd' behaviour of the KEY_PAUSE key. This key
   only generates an interrupt when it is pressed, not when it is released.
   For this reason, Allegro pretends the pause key is a `state' key, which
   is the only way to make it usable.

extern volatile int key_shifts;
   Bitmask containing the current state of shift/ctrl/alt, the special 
   Windows keys, and the accent escape characters. Wherever possible this 
   value will be updated asynchronously, but if keyboard_needs_poll() 
   returns TRUE, you must manually call poll_keyboard() to update it with 
   the current input state. This can contain any of the flags:

      KB_SHIFT_FLAG
      KB_CTRL_FLAG
      KB_ALT_FLAG
      KB_LWIN_FLAG
      KB_RWIN_FLAG
      KB_MENU_FLAG
      KB_COMMAND_FLAG
      KB_SCROLOCK_FLAG
      KB_NUMLOCK_FLAG
      KB_CAPSLOCK_FLAG
      KB_INALTSEQ_FLAG
      KB_ACCENT1_FLAG
      KB_ACCENT2_FLAG
      KB_ACCENT3_FLAG
      KB_ACCENT4_FLAG

   Example:

      if (key[KEY_W]) {
         if (key_shifts & KB_SHIFT_FLAG) {
            /* User is pressing shift + W. */
         } else {
            /* Hmmm... lower case W then. */
         }
      }

int keypressed();
   Returns TRUE if there are keypresses waiting in the input buffer. You can
   use this to see if the next call to readkey() is going to block or to 
   simply wait for the user to press a key while you still update the screen
   possibly drawing some animation. Example:

      while (!keypressed()) {
         /* Show cool animated logo. */
      }
      /* So he skipped our title screen. */

int readkey();
   Returns the next character from the keyboard buffer, in ASCII format. If 
   the buffer is empty, it waits until a key is pressed. You can see if there
   are queued keypresses with keypressed().

   The low byte of the return value contains the ASCII code of the key, and
   the high byte the scancode. The scancode remains the same whatever the
   state of the shift, ctrl and alt keys, while the ASCII code is affected by
   shift and ctrl in the normal way (shift changes case, ctrl+letter gives
   the position of that letter in the alphabet, eg. ctrl+A = 1, ctrl+B = 2,
   etc). Pressing alt+key returns only the scancode, with a zero ASCII code
   in the low byte. For example:

      int val;
      ...
      val = readkey();
      if ((val & 0xff) == 'd')     /* by ASCII code */
         allegro_message("You pressed 'd'\n");

      if ((val >> 8) == KEY_SPACE) /* by scancode */
         allegro_message("You pressed Space\n");

      if ((val & 0xff) == 3)       /* ctrl+letter */
         allegro_message("You pressed Control+C\n");

      if (val == (KEY_X << 8))     /* alt+letter */
         allegro_message("You pressed Alt+X\n");

   This function cannot return character values greater than 255. If you 
   need to read Unicode input, use ureadkey() instead.

int ureadkey(int *scancode);
   Returns the next character from the keyboard buffer, in Unicode format. 
   If the buffer is empty, it waits until a key is pressed. You can see if
   there are queued keypresses with keypressed(). The return value contains
   the Unicode value of the key, and if not NULL, the pointer argument will
   be set to the scancode. Unlike readkey(), this function is able to return
   character values greater than 255. Example:

      int val, scancode;
      ...
      val = ureadkey(&scancode);
      if (val == 0x00F1)
         allegro_message("You pressed n with tilde\n");

      if (val == 0x00DF)
         allegro_message("You pressed sharp s\n");

   You should be able to find Unicode character maps at
   http://www.unicode.org/. Remember that on DOS you must specify a custom
   keyboard map (like those found in `keyboard.dat') usually with the help of
   a configuration file specifying the language mapping (keyboard variable in
   system section of `allegro.cfg'), or you will get the default US keyboard
   mapping.

int scancode_to_ascii(int scancode);
   Converts the given scancode to an ASCII character for that key (mangling
   Unicode values), returning the unshifted uncapslocked result of pressing
   the key, or zero if the key isn't a character-generating key or the lookup
   can't be done. The lookup cannot be done for keys like the F1-F12 keys or the
   cursor keys, and some drivers will only return approximate values. Generally,
   if you want to display the name of a key to the user, you should use the
   scancode_to_name function.

   Example:

      int ascii;
      ...
      ascii = scancode_to_ascii(scancode);
      allegro_message("You pressed '%c'\n", ascii);

const char *scancode_to_name(int scancode);
   This function returns a string pointer containing the name of they key with
   the given scancode. This is useful if you e.g. let the user choose a key for
   some action, and want to display something more meaningful than just the
   scancode. Example:

      char const *keyname = scancode_to_name(scancode);
      allegro_message("You pressed the %s key.", keyname);

void simulate_keypress(int key);
   Stuffs a key into the keyboard buffer, just as if the user had pressed 
   it. The parameter is in the same format returned by readkey(). Example:

      simulate_keypress(KEY_SPACE << 8);
      if (readkey() == (KEY_SPACE << 8))
         allegro_message("You simulated Alt+Space\n");

void simulate_ukeypress(int key, int scancode);
   Stuffs a key into the keyboard buffer, just as if the user had pressed 
   it. The parameter is in the same format returned by ureadkey(). Example:

      /* We ignore the scancode simulation. */
      simulate_ukeypress(0x00DF, 0);
      if (ureadkey(&scancode) == 0x00DF)
         allegro_message("You simulated sharp s\n");

extern int (*keyboard_callback)(int key);
   If set, this function is called by the keyboard handler in response to 
   every keypress. It is passed a copy of the value that is about to be 
   added into the input buffer, and can either return this value unchanged, 
   return zero to cause the key to be ignored, or return a modified value to 
   change what readkey() will later return. This routine executes in an 
   interrupt context, so it must be in locked memory. Example:

      int enigma_scrambler(int key)
      {
         /* Add one to both the scancode and ascii values. */
         return (((key >> 8) + 1) 
      }
      END_OF_FUNCTION(enigma_scrambler)
      
      ...
      
         install_timer();
         LOCK_FUNCTION(enigma_scrambler);
         install_keyboard();
         keyboard_callback = enigma_scrambler;

   Note that this callback will be ignored if you also set the unicode
   keyboard callback.

extern int (*keyboard_ucallback)(int key, int *scancode);
   Unicode-aware version of keyboard_callback(). If set, this function is 
   called by the keyboard handler in response to every keypress. It is 
   passed the character value and scancode that are about to be added into 
   the input buffer, can modify the scancode value, and returns a new or 
   modified key code. If it both sets the scancode to zero and returns zero, 
   the keypress will be ignored. This routine executes in an interrupt 
   context, so it must be in locked memory. Example:

      int silence_g_key(int key, int *scancode)
      {
         if (key == 'g') {
            *scancode = 0;
            return 0;
         }
         return key;
      } END_OF_FUNCTION(silence_g_key)

      ...
      
         install_timer();
         LOCK_FUNCTION(silence_g_key);
         install_keyboard();
         keyboard_ucallback = silence_g_key;

   Note that this keyboard callback has priority over the non unicode
   callback. If you set both, only the unicode one will work.

extern void (*keyboard_lowlevel_callback)(int scancode);
   If set, this function is called by the keyboard handler in response to 
   every keyboard event, both presses (including keyboard repeat rate) and
   releases. It will be passed a raw keyboard scancode byte (scancodes are
   7 bits long), with the top bit (8th bit) clear if the key has been
   pressed or set if it was released. This routine executes in an interrupt
   context, so it must be in locked memory. Example:

      volatile int key_down, key_up;
      
      void keypress_watcher(int scancode)
      {
         if (scancode & 0x80) {
            key_up = 1;
         } else {
            key_down = 1;
         }
      } END_OF_FUNCTION(keypress_watcher)

      ...

         install_timer();
         LOCK_FUNCTION(silence_g_key);
         LOCK_VARIABLE(key_down);
         LOCK_VARIABLE(key_up);
         install_keyboard();
         keyboard_lowlevel_callback = keypress_watcher;
         /* Disable keyboard repeat to get typewriter effect. */
         set_keyboard_rate(0, 0);

      ...

         while (game_loop) {
            if (key_down) {
               key_down = 0;
               /* Play sample of typewriter key press. */
            }
            if (key_up) {
               key_up = 0;
               /* Play sample of typewriter key release. */
            }
         }
         
void set_leds(int leds);
   Overrides the state of the keyboard LED indicators. The parameter is a 
   bitmask containing any of the values KB_SCROLOCK_FLAG, KB_NUMLOCK_FLAG, 
   and KB_CAPSLOCK_FLAG, or -1 to restore the default behavior. Example:

      /* Cycle led indicators. */
      set_leds(KB_SCROLOCK_FLAG);
      rest(1000);
      set_leds(KB_CAPSLOCK_FLAG);
      rest(1000);
      set_leds(KB_NUMLOCK_FLAG);
      rest(1000);
      set_leds(-1);

   Note that the led behaviour cannot be guaranteed on some platforms, some
   leds might not react, or none at all. Therefore you shouldn't rely only on
   them to communicate information to the user, just in case it doesn't get
   through.

void set_keyboard_rate(int delay, int repeat);
   Sets the keyboard repeat rate. Times are given in milliseconds. Passing 
   zero times will disable the key repeat.

void clear_keybuf();
   Empties the keyboard buffer. Usually you want to use this in your program
   before reading keys to avoid previously buffered keys to be returned by
   calls to readkey() or ureadkey().

extern int three_finger_flag;
   The DJGPP keyboard handler provides an 'emergency exit' sequence which
   you can use to kill off your program. If you are running under DOS this 
   is the three finger salute, ctrl+alt+del. Most multitasking OS's will 
   trap this combination before it reaches the Allegro handler, in which 
   case you can use the alternative ctrl+alt+end. If you want to disable 
   this behaviour in release versions of your program, set this flag to 
   FALSE.

extern int key_led_flag;
   By default, the capslock, numlock, and scroll-lock keys toggle the 
   keyboard LED indicators when they are pressed. If you are using these 
   keys for input in your game (eg. capslock to fire) this may not be 
   desirable, so you can clear this flag to prevent the LED's being updated.



===========================================
============ Joystick routines ============
===========================================

Unlike keyboard or mouse input, which are usually read through hardware
interrupts by Allegro, joystick input functions have to be polled because
there are no hardware interrupts for them on most platforms. This doesn't
mean that you have to poll the joysticks on each line of code you want to
read their values, but you should make sure to poll them at least once per
frame in your game loop. Otherwise you face the possibility of reading stale
incorrect data.

int install_joystick(int type);
   Installs Allegro's joystick handler, and calibrates the centre position
   values. The type parameter should usually be JOY_TYPE_AUTODETECT, or see
   the platform specific documentation for a list of the available drivers.
   You must call this routine before using any other joystick functions, and
   you should make sure that all joysticks are in the middle position at the
   time. Example:

      textout_centre_ex(screen, font,
                        "Center the joystick and press a key",
                        SCREEN_W/2, SCREEN_H/2, red_color, -1);
      readkey();
      if (install_joystick(JOY_TYPE_AUTODETECT) != 0)
         abort_on_error("Error initialising joystick!");

   Returns zero on success. As soon as you have installed the joystick 
   module, you will be able to read the button state and digital (on/off 
   toggle) direction information, which may be enough for some games. If you 
   want to get full analogue input, though, you need to use the 
   calibrate_joystick() functions to measure the exact range of the inputs: 
   see below.

void remove_joystick();
   Removes the joystick handler. You don't normally need to bother calling 
   this, because allegro_exit() will do it for you.

int poll_joystick();
   The joystick handler is not interrupt driven, so you need to call this
   function every now and again to update the global position values. Example:

   do {
      /* Get joystick input */
      poll_joystick();
      
      /* Process input for the first joystick */
      if (joy[0].button[0].b)
         first_button_pressed();

      if (joy[0].button[1].b)
         second_button_pressed();
      ...
   } while(!done);

   Returns zero on success or a negative number on failure (usually because
   no joystick driver was installed).

extern int num_joysticks;
   Global variable containing the number of active joystick devices. The 
   current drivers support a maximum of eight controllers.

extern JOYSTICK_INFO joy[n];
   Global array of joystick state information, which is updated by the 
   poll_joystick() function. Only the first num_joysticks elements will 
   contain meaningful information. The JOYSTICK_INFO structure is defined as:

      typedef struct JOYSTICK_INFO
      {
         int flags;                       - status flags for this
                                            joystick
         int num_sticks;                  - how many stick inputs?
         int num_buttons;                 - how many buttons?
         JOYSTICK_STICK_INFO stick[n];    - stick state information
         JOYSTICK_BUTTON_INFO button[n];  - button state information
      } JOYSTICK_INFO;

   The button status is stored in the structure:

      typedef struct JOYSTICK_BUTTON_INFO
      {
         int b;                           - boolean on/off flag
         char *name;                      - description of this
                                            button
      } JOYSTICK_BUTTON_INFO;

   You may wish to display the button names as part of an input 
   configuration screen to let the user choose what game function will be 
   performed by each button, but in simpler situations you can safely assume 
   that the first two elements in the button array will always be the main 
   trigger controls.

   Each joystick will provide one or more stick inputs, of varying types. 
   These can be digital controls which snap to specific positions (eg. a 
   gamepad controller, the coolie hat on a Flightstick Pro or Wingman 
   Extreme, or a normal joystick which hasn't yet been calibrated), or they 
   can be full analogue inputs with a smooth range of motion. Sticks may 
   also have different numbers of axes, for example a normal directional 
   control has two, but the Flightstick Pro throttle is only a single axis, 
   and it is possible that the system could be extended in the future to 
   support full 3d controllers. A stick input is described by the structure:

      typedef struct JOYSTICK_STICK_INFO
      {
         int flags;                       - status flags for this
                                            input
         int num_axis;                    - how many axes do we
                                            have? (note the misspelling)
         JOYSTICK_AXIS_INFO axis[n];      - axis state information
         char *name;                      - description of this
                                            input
      } JOYSTICK_STICK_INFO;

   A single joystick may provide several different stick inputs, but you can 
   safely assume that the first element in the stick array will always be 
   the main directional controller.

   Information about each of the stick axis is stored in the substructure:

      typedef struct JOYSTICK_AXIS_INFO
      {
         int pos;                         - analogue axis position
         int d1, d2;                      - digital axis position
         char *name;                      - description of this axis
      } JOYSTICK_AXIS_INFO;

   This provides both analogue input in the pos field (ranging from -128 to 
   128 or from 0 to 255, depending on the type of the control), and digital 
   values in the d1 and d2 fields. For example, when describing the X-axis 
   position, the pos field will hold the horizontal position of the 
   joystick, d1 will be set if it is moved left, and d2 will be set if it is 
   moved right. Allegro will fill in all these values regardless of whether 
   it is using a digital or analogue joystick, emulating the pos field for 
   digital inputs by snapping it to the min, middle, and maximum positions, 
   and emulating the d1 and d2 values for an analogue stick by comparing the 
   current position with the centre point.

   The joystick flags field may contain any combination of the bit flags:

   JOYFLAG_DIGITAL
      This control is currently providing digital input.

   JOYFLAG_ANALOGUE
      This control is currently providing analogue input.

   JOYFLAG_CALIB_DIGITAL
      This control will be capable of providing digital input once it has 
      been calibrated, but is not doing this at the moment.

   JOYFLAG_CALIB_ANALOGUE
      This control will be capable of providing analogue input once it has 
      been calibrated, but is not doing this at the moment.

   JOYFLAG_CALIBRATE
      Indicates that this control needs to be calibrated. Many devices 
      require multiple calibration steps, so you should call the 
      calibrate_joystick() function from a loop until this flag is cleared.

   JOYFLAG_SIGNED
      Indicates that the analogue axis position is in signed format, ranging 
      from -128 to 128. This is the case for all 2d directional controls.

   JOYFLAG_UNSIGNED
      Indicates that the analogue axis position is in unsigned format, 
      ranging from 0 to 255. This is the case for all 1d throttle controls.

   Note for people who spell funny: in case you don't like having to type 
   "analogue", there are some #define aliases in allegro/joystick.h that
   will allow you to write "analog" instead.

const char *calibrate_joystick_name(int n);
   Pass the number of the joystick you want to calibrate as the parameter.

   Returns a text description for the next type of calibration that will be
   done on the specified joystick, or NULL if no more calibration is 
   required.

int calibrate_joystick(int n);
   Most joysticks need to be calibrated before they can provide full 
   analogue input. This function performs the next operation in the 
   calibration series for the specified stick, assuming that the joystick 
   has been positioned in the manner described by a previous call to 
   calibrate_joystick_name(), returning zero on success. For example, a 
   simple routine to fully calibrate all the joysticks might look like:

      int i;

      for (i=0; i<;num_joysticks; i++) {
         while (joy[i].flags & JOYFLAG_CALIBRATE) {
            char *msg = calibrate_joystick_name(i);
            textprintf_ex(..., "%s, and press a key\n", msg);
            readkey();
            if (calibrate_joystick(i) != 0) {
               textprintf_ex(..., "oops!\n");
               readkey();
               exit(1);
            }
         }
      }

   Returns zero on success, non-zero if the calibration could not be
   performed successfully.
   
int save_joystick_data(const char *filename);
   After all the headache of calibrating the joystick, you may not want to 
   make your poor users repeat the process every time they run your program. 
   Call this function to save the joystick calibration data into the 
   specified configuration file, from which it can later be read by 
   load_joystick_data(). Pass a NULL filename to write the data to the 
   currently selected configuration file.

   Returns zero on success, non-zero if the data could not be saved.

int load_joystick_data(const char *filename);
   Restores calibration data previously stored by save_joystick_data() or 
   the setup utility. This sets up all aspects of the joystick code: you 
   don't even need to call install_joystick() if you are using this 
   function. Pass a NULL filename to read the data from the currently 
   selected configuration file.

   Returns zero on success: if it fails the joystick state is undefined and
   you must reinitialise it from scratch.

int initialise_joystick();
   Deprecated. Use install_joystick() instead.



========================================
============ Graphics modes ============
========================================

Graphics modes are the common denominator for most Allegro programs. While it
is possible to write platform specific programs using Allegro which don't set
a graphic mode through the routines provided in this chapter, these are not
very common.

The first thing to note is that due to the wide range of supported platforms,
a graphic mode is the only way to safely communicate with the user. When
Allegro was a DOS only library (versions 3.x and previous), it was frequent
for programmers to use functions from the C standard library to communicate
with the user, like calling printf() before setting a graphic mode or maybe
scanf() to read the user's input. However, what would happen for such a game
running under Windows where there is no default console output or it may be
hidden from the user? Even if the game compiled successfully, it would be
unplayable, especially if there was vital information for the user in those
text only messages.

Allegro provides the allegro_message() function to deal with this problem,
but this is not a very user friendly method of communicating with the user
and its main purpose is displaying small error like messages when no graphic
mode is available. Therefore, the first thing your Allegro program should do
is set a graphic mode, and from there on, use Allegro's text output routines
to display messages to the user, just like `allegro/examples/exhello.c' does.

Setting a graphic mode involves deciding how to allocate the memory of the
video card for your program. On some platforms this means creating a virtual
screen bigger than the physical resolution to do hardware scrolling or page
flipping. Virtual screens can cause a lot of confusion, but they are really
quite simple. Warning: patronising explanation coming up, so you may wish to
skip the rest of this paragraph. Think of video memory as a rectangular piece
of paper which is being viewed through a small hole (your monitor) in a bit of
cardboard. Since the paper is bigger than the hole you can only see part of it
at any one time, but by sliding the cardboard around you can alter which
portion of the image is visible. You could just leave the hole in one position
and ignore the parts of video memory that aren't visible, but you can get all
sorts of useful effects by sliding the screen window around, or by drawing
images in a hidden part of video memory and then flipping across to display
them.

For example, you could select a 640x480 mode in which the monitor acts as a
window onto a 1024x1024 virtual screen, and then move the visible screen
around in this larger area (hardware scrolling). Initially, with the visible
screen positioned at the top left corner of video memory, this setup would
look like:

      (0,0)------------(640,0)----(1024,0)
        |                  |           |
        |  visible screen  |           |
        |                  |           |
      (0,480)----------(640,480)       |
        |                              |
        |   the rest of video memory   |
        |                              |
      (0,1024)--------------------(1024,1024)

With a virtual screen bigger than the visible screen you can perform smooth
CPU inexpensive scrolling: you draw your graphics once, and then only tell
the video card to show a different portion of the screen. However, virtual
screens are not supported on all platforms, and on some they might be
emulated through software, losing any performance. On top of that, many video
cards only allow horizontal scrolling in steps of 32 bytes. This is not a
problem if your game runs in 24 or 32 bit, but it tends to mean jerky
scrolling for other color depths.

The other reason you could use virtual screens for is page flipping. This
means showing one portion of the virtual screen while your program draws to
the hidden one. When you finish, you show the part you have been drawing to
and repeat the process with the area now hidden. The result is a perfectly
smooth screen update without flickering or other graphical artifacts.

Scrolling manually to one part of the video memory is one non portable way to
accomplish this. The portable way is to use functions like
create_system_bitmap(), create_video_bitmap(), show_video_bitmap(), etc. These
functions divide the memory of the video card in areas and switch between
them, a feature supported on all platforms and video cards (given that they
have enough memory for the screen resolutions you asked for).

The last thing you need to know about setting a graphic mode are drivers.
Each platform has a number of graphic drivers which support a different range
of hardware or behave in different ways. To avoid cluttering your own code
with #ifdefs and dealing with drivers added after you release your program,
Allegro provides several so called magic drivers. These magic drivers don't
really exists, they wrap around a specific kind of functionality.

The magic drivers you can use are:

GFX_AUTODETECT:
   Allegro will try to set the specified resolution with the current color
   depth in fullscreen mode. Failing that, it will try to repeat the same
   operation in windowed mode. If the call to set_gfx_mode() succeeds, you
   are guaranteed to have set the specified resolution in the current color
   depth, but you don't know if the program is running fullscreen or
   windowed.

GFX_AUTODETECT_FULLSCREEN:
   Allegro will try to set the specified resolution with the current color
   depth in fullscreen mode. If that is not possible, set_gfx_mode() will
   fail.

GFX_AUTODETECT_WINDOWED:
   Allegro will try to set the specified resolution with the current color
   depth in a windowed mode. If that is not possible, set_gfx_mode() will
   fail. When it comes to windowed modes, the `specified resolution' actually
   means the graphic area your program can draw on, without including window
   decorations (if any). Note that in windowed modes running with a color
   depth other than the desktop may result in non optimal performance due to
   internal color conversions in the graphic driver. Use
   desktop_color_depth() to your advantage in these situations.

GFX_SAFE:
   Using this driver Allegro guarantees that a graphic mode will always be
   set correctly. It will try to select the resolution that you request, and
   if that fails, it will fall back upon whatever mode is known to be
   reliable on the current platform (this is 320x200 VGA mode under DOS, a
   640x480 resolution under Windows, the actual framebuffer's resolution
   under Linux if it's supported, etc). If it absolutely cannot set any
   graphics mode at all, it will return negative as usual, meaning that
   there's no possible video output on the machine, and that you should abort
   your program immediately, possibly after notifying this to the user with
   allegro_message.

   This fake driver is useful for situations where you just want to get into
   some kind of workable display mode, and can't be bothered with trying
   multiple different resolutions and doing all the error checking yourself.
   Note however, that after a successful call to set_gfx_mode with this
   driver, you cannot make any assumptions about the width, height or color
   depth of the screen: your code will have to deal with this little detail.

   GFX_TEXT:
   Closes any previously opened graphics mode, making you unable to use the
   global variable `screen', and in those environments that have text modes,
   sets one previously used or the closest match to that (usually 80x25).
   With this driver the size parameters of set_gfx_mode don't mean anything,
   so you can leave them all to zero or any other number you prefer.


void set_color_depth(int depth);
   Sets the pixel format to be used by subsequent calls to set_gfx_mode() 
   and create_bitmap(). Valid depths are 8 (the default), 15, 16, 24, and 32 
   bits. Example:

      set_color_depth(32);
      if (set_gfx_mode(GFX_AUTODETECT, 640, 480, 0, 0) != 0) {
         abort_on_error("Couldn't set a 32 bit color resolution");
      }

   Note that the screen color depth won't change until the next successful
   call to set_gfx_mode().

int get_color_depth(void);
   Returns the current pixel format. This can be very useful to know in order
   to write generic functions which select a different code path internally
   depending on the color depth being used.

   Note that the function returns whatever value you may have set previously
   with set_color_depth(), which can be different from the current color
   depth of the screen global variable. If you really need to know the color
   depth of the screen, use bitmap_color_depth().

void request_refresh_rate(int rate);
   Requests that the next call to set_gfx_mode() try to use the specified 
   refresh rate, if possible. Not all drivers are able to control this at 
   all, and even when they can, not all rates will be possible on all 
   hardware, so the actual settings may differ from what you requested. 
   After you call set_gfx_mode(), you can use get_refresh_rate() to find out 
   what was actually selected. At the moment only the DOS VESA 3.0, X DGA 2.0
   and some Windows DirectX drivers support this function. The speed is
   specified in Hz, eg. 60, 70. To return to the normal default selection,
   pass a rate value of zero. Example:

      request_refresh_rate(60);
      if (set_gfx_mode(GFX_AUTODETECT, 640, 480, 0, 0) != 0)
         abort_on_error("Couldn't set graphic mode!");
      if (get_refresh_rate() != 60)
         abort_on_error("Couldn't set refresh rate to 60Hz!");

int get_refresh_rate(void);
   Returns the current refresh rate, if known (not all drivers are able to 
   report this information). Returns zero if the actual rate is unknown.

GFX_MODE_LIST *get_gfx_mode_list(int card);
   Attempts to create a list of all the supported video modes for a certain
   graphics driver, made up from the GFX_MODE_LIST structure, which has the
   following definition:

      typedef struct GFX_MODE_LIST
      {
         int num_modes;
         GFX_MODE *mode;
      } GFX_MODE_LIST;

   The mode entry  points to the actual list of video modes.

      typedef struct GFX_MODE
      {
         int width, height, bpp;
      } GFX_MODE;

   This list of video modes is terminated with an { 0, 0, 0 } entry.

   Note that the card parameter must refer to a _real_ driver. This function
   fails if you pass GFX_SAFE, GFX_AUTODETECT, or any other "magic" driver.

   Returns a pointer to a list structure of the type GFX_MODE_LIST or NULL
   if the request could not be satisfied.

void destroy_gfx_mode_list(GFX_MODE_LIST *mode_list);
   Removes the mode list created by get_gfx_mode_list() from memory. Use this
   once you are done with the generated mode list to avoid memory leaks in
   your program.

int set_gfx_mode(int card, int w, int h, int v_w, int v_h);
   Switches into graphics mode. The card parameter should usually be one of
   the Allegro magic drivers (read introduction of chapter "Graphics modes")
   or see the platform specific documentation for a list of the available
   drivers. The w and h parameters specify what screen resolution you want.
   The color depth of the graphic mode has to be specified before calling
   this function with set_color_depth().

   The v_w and v_h parameters specify the minimum virtual screen size, in 
   case you need a large virtual screen for hardware scrolling or page 
   flipping. You should set them to zero if you don't care about the virtual 
   screen size.
   
   When you call set_gfx_mode(), the v_w and v_h parameters represent the 
   minimum size of virtual screen that is acceptable for your program. The 
   range of possible sizes is usually very restricted, and Allegro may 
   end up creating a virtual screen much larger than the one you request.
   Allowed sizes are driver dependent and some drivers do not allow virtual 
   screens that are larger than the visible screen at all: don't assume
   that whatever you pass will always work.
   
   In mode-X the virtual width can be any multiple of eight greater than or 
   equal to the physical screen width, and the virtual height will be set 
   accordingly (the VGA has 256k of vram, so the virtual height will be 
   256*1024/virtual_width).

   Currently, using a big virtual screen for page flipping is considered bad
   practice.  There are platforms which don't support virtual screens bigger
   than the physical screen but can create different video pages to flip back
   and forth. This means that, if you want page flipping and aren't going to
   use hardware scrolling, you should call set_gfx_mode() with (0,0) as the
   virtual screen size and later create the different video pages with
   create_video_bitmap(). Otherwise your program will be limited to the
   platforms supporting hardware scrolling.

   After you select a graphics mode, the physical and virtual screen sizes 
   can be checked with the macros SCREEN_W, SCREEN_H, VIRTUAL_W, and 
   VIRTUAL_H.

   Returns zero on success. On failure returns a negative number and stores a
   description of the problem in allegro_error.

int set_display_switch_mode(int mode);
   Sets how the program should handle being switched into the background, 
   if the user tabs away from it. Not all of the possible modes will be
   supported by every graphics driver on every platform. The available modes
   are:

   SWITCH_NONE
      Disables switching. This is the default in single-tasking systems like 
      DOS. It may be supported on other platforms, but you should use it 
      with caution, because your users won't be impressed if they want to 
      switch away from your program, but you don't let them!

   SWITCH_PAUSE
      Pauses the program whenever it is in the background. Execution will be 
      resumed as soon as the user switches back to it. This is the default 
      in most fullscreen multitasking environments, for example the Linux 
      console, but not under Windows.

   SWITCH_AMNESIA
      Like SWITCH_PAUSE, but this mode doesn't bother to remember the 
      contents of video memory, so the screen, and any video bitmaps that 
      you have created, will be erased after the user switches away and then 
      back to your program. This is not a terribly useful mode to have, but 
      it is the default for the fullscreen drivers under Windows because 
      DirectDraw is too dumb to implement anything better.

   SWITCH_BACKGROUND
      The program will carry on running in the background, with the screen 
      bitmap temporarily being pointed at a memory buffer for the fullscreen 
      drivers. You must take special care when using this mode, because bad 
      things will happen if the screen bitmap gets changed around when your 
      program isn't expecting it (see below).

   SWITCH_BACKAMNESIA
      Like SWITCH_BACKGROUND, but this mode doesn't bother to remember the 
      contents of video memory (see SWITCH_AMNESIA). It is again the only 
      mode supported by the fullscreen drivers under Windows that lets the 
      program keep running in the background.

   Note that you should be very careful when you are using graphics routines 
   in the switching context: you must always call acquire_screen() before the
   start of any drawing code onto the screen and not release it until you are
   completely finished, because the automatic locking mechanism may not be
   good enough to work when the program runs in the background or has just
   been raised in the foreground.

   Returns zero on success, invalidating at the same time all callbacks
   previously registered with set_display_switch_callback(). Returns -1 if
   the requested mode is not currently possible.

int set_display_switch_callback(int dir, void (*cb)());
   Installs a notification callback for the switching mode that was 
   previously selected by calling set_display_switch_mode(). The direction 
   parameter can either be SWITCH_IN or SWITCH_OUT, depending whether you 
   want to be notified about switches away from your program or back to your 
   program. You can sometimes install callbacks for both directions at the 
   same time, but not every platform supports this. You can install several
   switch callbacks, but no more than eight on any platform.

   Returns zero on success, decreasing the number of empty callback slots by
   one. Returns -1 if the request is impossible for the current platform or
   you have reached the maximum number of allowed callbacks.

void remove_display_switch_callback(void (*cb)());
   Removes a notification callback that was previously installed by calling 
   set_display_switch_callback(). All the callbacks will automatically be 
   removed when you call set_display_switch_mode(). You can safely call this
   function even if the callback you want to remove is not installed.

int get_display_switch_mode();
   Returns the current display switching mode, in the same format passed to 
   set_display_switch_mode().

int is_windowed_mode(void);
   This function can be used to detect whether or not set_gfx_mode() selected
   a windowed mode. Example:

      if (set_gfx_mode(GFX_AUTODETECT, 640, 480, 0, 0) != 0)
         abort_on_error("Couldn't set graphic mode!");
      if (is_windowed_mode()) {
         /* Windowed mode stuff. */
      } else {
         /* Fullscreen mode stuff. */
      }

   Returns true if the current graphics mode is a windowed mode, or zero if
   it is a fullscreen mode. You should not call this function if you are not
   in graphics mode.

int get_gfx_mode_type(int graphics_card);
   This function lets you determine the types of operating modes that a
   specific graphics card driver operates in. It will tell you whether it is
   a windowed, fullscreen, definitely windowed or fullscreen, and/or a magic
   driver.
   
   The value returned is a bitfield consisting of these fields:
   GFX_TYPE_UNKNOWN
   GFX_TYPE_WINDOWED
   GFX_TYPE_FULLSCREEN
   GFX_TYPE_DEFINITE
   GFX_TYPE_MAGIC
   
   The return value will only be equivalent to GFX_TYPE_UNKNOWN when it is
   a driver unrecognized on that platform, or it is a bogus value. Test for
   the other types by using a bitwise AND. If the driver is windowed or
   fullscreen, it will also have the definite flag set. 
   For example,

   int gfx_type = get_gfx_mode_type(GFX_AUTODETECT_WINDOWED);

   gfx_type would have the GFX_TYPE_WINDOWED, GFX_TYPE_DEFINITE, and
   GFX_TYPE_MAGIC flags set.
   
   Allegro needs to be initialized first.
   
   Example:

   /* Accept the use of only windowed drivers in our selection dialog */
   int accept_windowed(int card , int w , int h , int color_depth)
   {
      if (get_gfx_mode_type(card) & GFX_TYPE_WINDOWED)
         return 0;
      return 1;
   }

   /* In main: */
   gfx_mode_select_filter(&card, &w, &h, &color_depth, accept_windowed);


   Returns a bitfield describing the graphics mode type.

int get_gfx_mode();
   This function will let you determine which graphics driver is currently
   set by allegro. If no graphics driver is set, it will return GFX_NONE.
   

   Returns the id of the current graphics driver if there is one, or GFX_NONE
   if none is set.

extern int gfx_capabilities;
   Bitfield describing the capabilities of the current graphics driver and 
   video hardware. This may contain combination any of the flags:

   GFX_CAN_SCROLL:
      Indicates that the scroll_screen() function may be used with this 
      driver.

   GFX_CAN_TRIPLE_BUFFER:
      Indicates that the request_scroll() and poll_scroll() functions may be 
      used with this driver. If this flag is not set, it is possible that 
      the enable_triple_buffer() function may be able to activate it.

   GFX_HW_CURSOR:
      Indicates that a hardware mouse cursor is in use. When this flag is 
      set, it is safe to draw onto the screen without hiding the mouse 
      pointer first. Note that not every cursor graphic can be implemented 
      in hardware: in particular VBE/AF only supports 2-color images up to 
      32x32 in size, where the second color is an exact inverse of the 
      first. This means that Allegro may need to switch between hardware and 
      software cursors at any point during the execution of your program, so 
      you should not assume that this flag will remain constant for long 
      periods of time. It only tells you whether a hardware cursor is in use 
      at the current time, and may change whenever you hide/redisplay the 
      pointer.
      
   GFX_SYSTEM_CURSOR
      Indicates that the mouse cursor is the default system cursor, not 
      Allegro's custom cursor.

   GFX_HW_HLINE:
      Indicates that the normal opaque version of the hline() function is 
      implemented using a hardware accelerator. This will improve the 
      performance not only of hline() itself, but also of many other 
      functions that use it as a workhorse, for example circlefill(), 
      triangle(), and floodfill().

   GFX_HW_HLINE_XOR:
      Indicates that the XOR version of the hline() function, and any other 
      functions that use it as a workhorse, are implemented using a hardware 
      accelerator.

   GFX_HW_HLINE_SOLID_PATTERN:
      Indicates that the solid and masked pattern modes of the hline() 
      function, and any other functions that use it as a workhorse, are 
      implemented using a hardware accelerator (see note below).

   GFX_HW_HLINE_COPY_PATTERN:
      Indicates that the copy pattern mode of the hline() function, and any 
      other functions that use it as a workhorse, are implemented using a 
      hardware accelerator (see note below).

   GFX_HW_FILL:
      Indicates that the opaque version of the rectfill() function, the 
      clear_bitmap() routine, and clear_to_color(), are implemented using a
      hardware accelerator.

   GFX_HW_FILL_XOR:
      Indicates that the XOR version of the rectfill() function is 
      implemented using a hardware accelerator.

   GFX_HW_FILL_SOLID_PATTERN:
      Indicates that the solid and masked pattern modes of the rectfill() 
      function are implemented using a hardware accelerator (see note below).

   GFX_HW_FILL_COPY_PATTERN:
      Indicates that the copy pattern mode of the rectfill() function is 
      implemented using a hardware accelerator (see note below).

   GFX_HW_LINE:
      Indicates that the opaque mode line() and vline() functions are 
      implemented using a hardware accelerator.

   GFX_HW_LINE_XOR:
      Indicates that the XOR version of the line() and vline() functions are 
      implemented using a hardware accelerator.

   GFX_HW_TRIANGLE:
      Indicates that the opaque mode triangle() function is implemented 
      using a hardware accelerator.

   GFX_HW_TRIANGLE_XOR:
      Indicates that the XOR version of the triangle() function is 
      implemented using a hardware accelerator.

   GFX_HW_GLYPH:
      Indicates that monochrome character expansion (for text drawing) is 
      implemented using a hardware accelerator.

   GFX_HW_VRAM_BLIT:
      Indicates that blitting from one part of the screen to another is 
      implemented using a hardware accelerator. If this flag is set, 
      blitting within the video memory will almost certainly be the fastest 
      possible way to display an image, so it may be worth storing some of 
      your more frequently used graphics in an offscreen portion of the 
      video memory.

   GFX_HW_VRAM_BLIT_MASKED:
      Indicates that the masked_blit() routine is capable of a hardware 
      accelerated copy from one part of video memory to another, and that 
      draw_sprite() will use a hardware copy when given a sub-bitmap of the 
      screen or a video memory bitmap as the source image. If this flag is 
      set, copying within the video memory will almost certainly be the 
      fastest possible way to display an image, so it may be worth storing 
      some of your more frequently used sprites in an offscreen portion of 
      the video memory.

      Warning: if this flag is not set, masked_blit() and draw_sprite() will 
      not work correctly when used with a video memory source image! You 
      must only try to use these functions to copy within the video memory 
      if they are supported in hardware.

   GFX_HW_MEM_BLIT:
      Indicates that blitting from a memory bitmap onto the screen is being 
      accelerated in hardware.

   GFX_HW_MEM_BLIT_MASKED:
      Indicates that the masked_blit() and draw_sprite() functions are being 
      accelerated in hardware when the source image is a memory bitmap and 
      the destination is the physical screen.

   GFX_HW_SYS_TO_VRAM_BLIT:
      Indicates that blitting from a system bitmap onto the screen is being 
      accelerated in hardware. Note that some acceleration may be present 
      even if this flag is not set, because system bitmaps can benefit from 
      normal memory to screen blitting as well. This flag will only be set 
      if system bitmaps have further acceleration above and beyond what is 
      provided by GFX_HW_MEM_BLIT.

   GFX_HW_SYS_TO_VRAM_BLIT_MASKED:
      Indicates that the masked_blit() and draw_sprite() functions are being 
      accelerated in hardware when the source image is a system bitmap and 
      the destination is the physical screen. Note that some acceleration 
      may be present even if this flag is not set, because system bitmaps 
      can benefit from normal memory to screen blitting as well. This flag 
      will only be set if system bitmaps have further acceleration above and 
      beyond what is provided by GFX_HW_MEM_BLIT_MASKED.

   GFX_HW_VRAM_STRETCH_BLIT:
      Indicates that stretched blitting of video bitmaps onto the screen is
      implemented using hardware acceleration. 

   GFX_HW_SYS_STRETCH_BLIT:
      Indicates that stretched blitting of system bitmaps onto the screen is
      implemented using hardware acceleration. 

   GFX_HW_VRAM_STRETCH_BLIT_MASKED:
      Indicates that masked stretched blitting (including stretch_sprite) of 
      video bitmaps onto the screen is implemented using hardware acceleration.
      NOTE: some display drivers may show artifacts when this function is used.
      If the image does not look correct try updating your video drivers.

   GFX_HW_SYS_STRETCH_BLIT_MASKED:
      Indicates that masked stretched blitting (including stretch_sprite) of 
      system bitmaps onto the screen is implemented using hardware acceleration.
      NOTE: some display drivers may show artefact's when this function is used.
      If the image does not look correct try updating your video drivers.

   Note: even if the capabilities information says that patterned drawing is 
   supported by the hardware, it will not be possible for every size of 
   pattern. VBE/AF only supports patterns up to 8x8 in size, so Allegro will 
   fall back on the original non-accelerated drawing routines whenever you 
   use a pattern larger than this.

   Note2: these hardware acceleration features will only take effect when 
   you are drawing directly onto the screen bitmap, a video memory bitmap, 
   or a sub-bitmap thereof. Accelerated hardware is most useful in a page 
   flipping or triple buffering setup, and is unlikely to make any 
   difference to the classic "draw onto a memory bitmap, then blit to the 
   screen" system.

int enable_triple_buffer();
   If the GFX_CAN_TRIPLE_BUFFER bit of the gfx_capabilities field is not 
   set, you can attempt to enable it by calling this function. In particular 
   if you are running in mode-X in a clean DOS environment, this routine 
   will enable the timer retrace simulator, which will activate the triple 
   buffering functions.

   Returns zero if triple buffering is enabled, -1 otherwise.

int scroll_screen(int x, int y);
   Attempts to scroll the hardware screen to display a different part of the 
   virtual screen (initially it will be positioned at 0, 0, which is the top 
   left corner). You can use this to move the screen display around in a
   large virtual screen space, or to page flip back and forth between two
   non-overlapping areas of the virtual screen. Note that to draw outside the
   original position in the screen bitmap you will have to alter the clipping
   rectangle with set_clip_rect().

   Mode-X scrolling is reliable and will work on any card, other drivers may
   not work or not work reliably. See the platform-specific section of the docs
   for more information.

   Allegro will handle any necessary vertical retrace synchronisation when 
   scrolling the screen, so you don't need to call vsync() before it. This 
   means that scroll_screen() has the same time delay effects as vsync().

   Returns zero on success. Returns non-zero if the graphics driver can't
   handle hardware scrolling or the virtual screen is not large enough.

int request_scroll(int x, int y);
   This function is used for triple buffering. It requests a hardware scroll 
   to the specified position, but returns immediately rather than waiting 
   for a retrace. The scroll will then take place during the next vertical 
   retrace, but you can carry on running other code in the meantime and use 
   the poll_scroll() routine to detect when the flip has actually taken 
   place.

   Triple buffering is only possible with certain drivers: you can look at the 
   GFX_CAN_TRIPLE_BUFFER bit in the gfx_capabilities flag to see if it will 
   work with the current driver.

   This function returns zero on success, non-zero otherwise.

int poll_scroll();
   This function is used for triple buffering. It checks the status of a 
   hardware scroll previously initiated by the request_scroll() routine.

   Returns non-zero if it is still waiting to take place, and zero if the
   requested scroll has already happened.

int show_video_bitmap(BITMAP *bitmap);
   Attempts to page flip the hardware screen to display the specified video 
   bitmap object, which must be the same size as the physical screen, and 
   should have been obtained by calling the create_video_bitmap() function. 
   
   Allegro will handle any necessary vertical retrace synchronisation when 
   page flipping, so you don't need to call vsync() before it. This means
   that show_video_bitmap() has the same time delay effects as vsync() by
   default. This can be adjusted with the "disable_vsync" config key in the
   [graphics] section of allegro.cfg. Example:

      int current_page;
      BITMAP *video_page[2];
      ...
      /* Create pages for page flipping */
      video_page[0] = create_video_bitmap(SCREEN_W, SCREEN_H);
      video_page[1] = create_video_bitmap(SCREEN_W, SCREEN_H);
      current_page = 0;
      ...
      /* draw the screen and flip pages */
      draw_screen(video_page[current_page]);
      show_video_bitmap(video_page[current_page]);
      current_page = (current_page+1)%2;
      ...

   Returns zero on success and non-zero on failure.

int request_video_bitmap(BITMAP *bitmap);
   This function is used for triple buffering. It requests a page flip to 
   display the specified video bitmap object, but returns immediately rather 
   than waiting for a retrace. The flip will then take place during the next 
   vertical retrace, but you can carry on running other code in the meantime 
   and use the poll_scroll() routine to detect when the flip has actually 
   taken place. Triple buffering is only possible on certain hardware: see 
   the comments about request_scroll(). Example:

      int current_page;
      BITMAP *video_page[3];
      ...
      /* Create pages for page flipping */
      video_page[0] = create_video_bitmap(SCREEN_W, SCREEN_H);
      video_page[1] = create_video_bitmap(SCREEN_W, SCREEN_H);
      video_page[2] = create_video_bitmap(SCREEN_W, SCREEN_H);
      current_page = 0;
      ...
      /* draw the screen and flip pages */
      draw_screen(video_page[current_page]);
      do {
      } while (poll_scroll());
      request_video_bitmap(video_page[current_page]);
      current_page = (current_page+1)%3;
      ...

   Returns zero on success and non-zero on failure.

void vsync();
   Waits for a vertical retrace to begin. The retrace happens when the
   electron beam in your monitor has reached the bottom of the screen and is
   moving back to the top ready for another scan. During this short period
   the graphics card isn't sending any data to the monitor, so you can do
   things to it that aren't possible at other times, such as altering the
   palette without causing flickering (snow). Allegro will automatically
   wait for a retrace before altering the palette or doing any hardware
   scrolling, though, so you don't normally need to bother with this
   function.



========================================
============ Bitmap objects ============
========================================

Once you have selected a graphics mode, you can draw things onto the display 
via the `screen' bitmap. All the Allegro graphics routines draw onto BITMAP
structures, which are areas of memory containing rectangular images, stored 
as packed byte arrays (in 8-bit modes one byte per pixel, in 15- and 16-bit
modes two bytes per pixel, in 24-bit modes 3 bytes per pixel and in 32-bit
modes 4 bytes per pixel). You can create and manipulate bitmaps in system
RAM, or you can write to the special `screen' bitmap which represents the
video memory in your graphics card.

Read chapter "Direct access to video memory" for information on how to get
direct access to the image memory in a bitmap.

Allegro supports several different types of bitmaps:

   The `screen' bitmap, which represents the hardware video memory. Ultimately
   you have to draw onto this in order for your image to be visible. It is
   destroyed by any subsequent calls to set_gfx_mode(), so you should never
   attempt to destroy it yourself.

   Memory bitmaps, which are located in system RAM and can be used to store
   graphics or as temporary drawing spaces for double buffered systems. These
   can be obtained by calling create_bitmap(), load_pcx(), or by loading a
   grabber datafile.

   Sub-bitmaps. These share image memory with a parent bitmap (which can be
   the screen, a video or system bitmap, a memory bitmap, or another
   sub-bitmap), so drawing onto them will also change their parent. They can
   be of any size and located anywhere within the parent bitmap, and can have
   their own clipping rectangles, so they are a useful way of dividing a
   bitmap into several smaller units, eg. splitting a large virtual screen
   into multiple sections (see examples/exscroll.c).

   Warning: Make sure not to destroy a bitmap before all of its sub-bitmaps,
   otherwise bad things will happen when you try to access one of these
   sub-bitmaps.

   Video memory bitmaps. These are created by the create_video_bitmap()
   function, and are usually implemented as sub-bitmaps of the screen object.
   They must be destroyed by destroy_bitmap() before any subsequent calls to
   set_gfx_mode().

   System bitmaps. These are created by the create_system_bitmap() function,
   and are a sort of halfway house between memory and video bitmaps. They
   live in system memory, so you aren't limited by the amount of video ram in
   your card, but they are stored in a platform-specific format that may
   enable better hardware acceleration than is possible with a normal memory
   bitmap (see the GFX_HW_SYS_TO_VRAM_BLIT and GFX_HW_SYS_TO_VRAM_BLIT_MASKED
   flags in gfx_capabilities). System bitmaps must be accessed in the same way
   as video bitmaps, using the bank switch functions and bmp_write*() macros.
   Not every platform implements this type of bitmap: if they aren't
   available, create_system_bitmap() will function identically to
   create_bitmap(). They must be destroyed by destroy_bitmap() before any
   subsequent calls to set_gfx_mode().


extern BITMAP *screen;
   Global pointer to a bitmap, sized VIRTUAL_W x VIRTUAL_H. This is created 
   by set_gfx_mode(), and represents the hardware video memory. Only a part 
   of this bitmap will actually be visible, sized SCREEN_W x SCREEN_H. 
   Normally this is the top left corner of the larger virtual screen, so you 
   can ignore the extra invisible virtual size of the bitmap if you aren't 
   interested in hardware scrolling or page flipping. To move the visible 
   window to other parts of the screen bitmap, call scroll_screen(). 
   Initially the clipping rectangle will be limited to the physical screen 
   size, so if you want to draw onto a larger virtual screen space outside 
   this rectangle, you will need to adjust the clipping.

   For example, to draw a pixel onto the screen you would write:

      putpixel(screen, x, y, color);

   Or to implement a double-buffered system:

      /* Make a bitmap in RAM. */
      BITMAP *bmp = create_bitmap(320, 200);
      /* Clean the memory bitmap. */
      clear_bitmap(bmp);
      /* Draw onto the memory bitmap. */
      putpixel(bmp, x, y, color);
      /* Copy it to the screen. */
      blit(bmp, screen, 0, 0, 0, 0, 320, 200);

   Warning: be very careful when using this pointer at the same time as any
   bitmaps created by the create_video_bitmap() function (see the description
   of this function for more detailed information). And never try to destroy
   it with destroy_bitmap().

#define SCREEN_W;
#define SCREEN_H;
   Global defines that return the width and height of the screen, or zero if
   the screen has not been initialised yet. Example:

      char buf[100];
      ...
      uszprintf(buf, sizeof(buf),
                "The screen size is %d x %d pixels",
                SCREEN_W, SCREEN_H);

#define VIRTUAL_W;
#define VIRTUAL_H;
   Global defines that return the width and height of the virtual screen, or
   zero if the screen has not been initialised yet. Example:

      char buf[100];
      ...
      uszprintf(buf, sizeof(buf),
                "The virtual screen size is %d x %d pixels",
                SCREEN_W, SCREEN_H);

BITMAP *create_bitmap(int width, int height);
   Creates a memory bitmap sized width by height. The bitmap will have
   clipping turned on, and the clipping rectangle set to the full size of the
   bitmap. The image memory will not be cleared, so it will probably contain
   garbage: you should clear the bitmap before using it. This routine always
   uses the global pixel format, as specified by calling set_color_depth().
   The minimum height of the BITMAP must be 1 and width can't be negative.
   Example:

      /* Create a 10 pixel tall bitmap, as wide as the screen. */
      BITMAP *bmp = create_bitmap(SCREEN_W, 10);
      if (!bmp)
         abort_on_error("Couldn't create bitmap!");
      /* Use the bitmap. */
      ...
      /* Destroy it when we don't need it any more. */
      destroy_bitmap(bmp);

   Returns a pointer to the created bitmap, or NULL if the bitmap could not
   be created. Remember to free this bitmap later to avoid memory leaks.

BITMAP *create_bitmap_ex(int color_depth, int width, int height);
   Creates a bitmap in a specific color depth (8, 15, 16, 24 or 32 bits per 
   pixel). Example:

      /* Create screen sized bitmap in 32 bits per pixel. */
      BITMAP *bmp = create_bitmap_ex(32, SCREEN_W, SCREEN_H);
      if (!bmp)
         abort_on_error("Couldn't create bitmap!");
      /* Use the bitmap. */
      ...
      /* Destroy it when we don't need it any more. */
      destroy_bitmap(bmp);

   Returns a pointer to the created bitmap, or NULL if the bitmap could not
   be created. Remember to free this bitmap later to avoid memory leaks.
   

BITMAP *create_sub_bitmap(BITMAP *parent, int x, y, width, height);
   Creates a sub-bitmap, ie. a bitmap sharing drawing memory with a 
   pre-existing bitmap, but possibly with a different size and clipping 
   settings. When creating a sub-bitmap of the mode-X screen, the x position 
   must be a multiple of four. The sub-bitmap width and height can extend 
   beyond the right and bottom edges of the parent (they will be clipped), 
   but the origin point must lie within the parent region.

   Returns a pointer to the created sub bitmap, or NULL if the sub bitmap
   could not be created. Remember to free the sub bitmap before freeing
   the parent bitmap to avoid memory leaks and potential crashes accessing
   memory which has been freed.

BITMAP *create_video_bitmap(int width, int height);
   Allocates a video memory bitmap of the specified size. This can be used
   to allocate offscreen video memory for storing source graphics ready for
   a hardware accelerated blitting operation, or to create multiple video
   memory pages which can then be displayed by calling show_video_bitmap().
   Read the introduction of this chapter for a comparison with other types
   of bitmaps and other specific details.

   Warning: video memory bitmaps are usually allocated from the same space
   as the screen bitmap, so they may overlap with it; it is therefore not
   a good idea to use the global screen at the same time as any surfaces
   returned by this function.

   Returns a pointer to the bitmap on success, or NULL if you have run out
   of video ram. Remember to destroy this bitmap before any subsequent
   call to set_gfx_mode().
   
BITMAP *create_system_bitmap(int width, int height);
   Allocates a system memory bitmap of the specified size. Read the
   introduction of this chapter for a comparison with other types of bitmaps
   and other specific details.

   Returns a pointer to the bitmap on success, NULL otherwise. Remember to
   destroy this bitmap before any subsequent call to set_gfx_mode().
   
void destroy_bitmap(BITMAP *bitmap);
   Destroys a memory bitmap, sub-bitmap, video memory bitmap, or system 
   bitmap when you are finished with it. If you pass a NULL pointer this
   function won't do anything. See above for the restrictions as to when you
   are allowed to destroy the various types of bitmaps.

   The bitmap must not have a mouse cursor shown on it at the time it is
   destroyed.

void lock_bitmap(BITMAP *bitmap);
   Under DOS, locks all the memory used by a bitmap. You don't normally need
   to call this function unless you are doing very weird things in your
   program.

int bitmap_color_depth(BITMAP *bmp);
   Returns the color depth of the specified bitmap (8, 15, 16, 24, or 32).
   Example:

      switch (bitmap_color_depth(screen)) {
         case 8:
            /* Access screen using optimized 8-bit code. */
            break;
         default:
            /* Use generic slow functions. */
            break;
      }

int bitmap_mask_color(BITMAP *bmp);
   Returns the mask color for the specified bitmap (the value which is 
   skipped when drawing sprites). For 256-color bitmaps this is zero, and 
   for truecolor bitmaps it is bright pink (maximum red and blue, zero
   green). A frequent use of this function is to clear a bitmap with the mask
   color so you can later use this bitmap with masked_blit() or
   draw_sprite() after drawing other stuff on it. Example:

      /* Replace mask color with another color. */
      for (y = 0; y h; y++)
         for (x = 0; x w; x++)
            if (getpixel(bmp, x, y) == bitmap_mask_color(bmp))
               putpixel(bmp, x, y, another_color);

int is_same_bitmap(BITMAP *bmp1, BITMAP *bmp2);
   Returns TRUE if the two bitmaps describe the same drawing surface, ie. 
   the pointers are equal, one is a sub-bitmap of the other, or they are 
   both sub-bitmaps of a common parent.

int is_planar_bitmap(BITMAP *bmp);
   Returns TRUE if bmp is a planar (mode-X or Xtended mode) screen bitmap.

int is_linear_bitmap(BITMAP *bmp);
   Returns TRUE if bmp is a linear bitmap, i.e. a bitmap that can be accessed
   linearly within each scanline (for example a memory bitmap, the DOS VGA
   or SVGA screen, Windows bitmaps, etc). Linear bitmaps can be used with the
   _putpixel(), _getpixel(), bmp_write_line(), and bmp_read_line() functions.

   Historically there were only linear and planar bitmaps for Allegro, so
   is_linear_bitmap() is actually an alias for !is_planar_bitmap().

int is_memory_bitmap(BITMAP *bmp);
   Returns TRUE if bmp is a memory bitmap, ie. it was created by calling 
   create_bitmap() or loaded from a grabber datafile or image file. Memory 
   bitmaps can be accessed directly via the line pointers in the bitmap 
   structure, eg. bmp->line[y][x] = color.

int is_screen_bitmap(BITMAP *bmp);
   Returns TRUE if bmp is the screen bitmap, or a sub-bitmap of the screen.

int is_video_bitmap(BITMAP *bmp);
   Returns TRUE if bmp is the screen bitmap, a video memory bitmap, or a 
   sub-bitmap of either.

int is_system_bitmap(BITMAP *bmp);
   Returns TRUE if bmp is a system bitmap object, or a sub-bitmap of one.

int is_sub_bitmap(BITMAP *bmp);
   Returns TRUE if bmp is a sub-bitmap.

void acquire_bitmap(BITMAP *bmp);
   Acquires the specified video bitmap prior to drawing onto it. You never need
   to call the function explicitly as it is low level, and will only give you a
   speed up if you know what you are doing. Using it wrongly may cause slowdown,
   or even lock up your program.

   Note: You do never need to use acquire_bitmap on a memory bitmap, i.e. a
   normal bitmap created with create_bitmap. It will simply do nothing in that
   case.

   It still can be useful, because e.g. under the current DirectDraw driver of
   Allegro, most drawing functions need to lock a video bitmap before drawing to
   it. But doing this is very slow, so you will get much better performance if
   you acquire the screen just once at the start of your main redraw function,
   then call multiple drawing operations which need the bitmap locked, and only
   release it when done.

   Multiple acquire calls may be nested, but you must make sure to match up the
   acquire_bitmap and release_bitmap calls. Be warned that DirectX and X11
   programs activate a mutex lock whenever a surface is locked, which prevents
   them from getting any input messages, so you must be sure to release all your
   bitmaps before using any timer, keyboard, or other non-graphics routines!

   Note that if you are using hardware accelerated VRAM->VRAM functions, you
   should not call acquire_bitmap(). Such functions need an unlocked target
   bitmap under DirectX, so there is now just the opposite case from before - if
   the bitmap is already locked with acquire_bitmap, the drawing
   operation has to unlock it.

   Note: For backwards compatibility, the unlocking behavior of such functions
   is permanent. That is, if you call acquire_bitmap first, then call e.g. an
   accelerated blit, the DirectX bitmap will be unlocked internally (it won't
   affect the nesting counter of acquire/release calls).

   There is no clear cross-platform way in this Allegro version to know which
   drawing operations need a locked/unlocked state. For example a normal
   rectfill most probably is accelerated under DirectX, and therefore needs the
   screen unlocked, but an XOR rectfill, or one with blending activated, most
   probably is not, and therefore locks the screen. And while the DirectX driver
   will do automatic unlocking, there is no such thing under X11, where the
   function is used to synchronize X11 calls from different threads. Your best
   bet is to never use acquire_bitmap - changes are you are doing something in
   the wrong way if you think you need it.

   Warning: This function can be very dangerous to use, since the whole program
   may get locked while the bitmap is locked. So the lock should only be held
   for a short time, and you should not call anything but drawing operations
   onto the locked video bitmap while a lock is in place. Especially don't call
   things like show_mouse (or scare_mouse which calls that) or readkey, since
   it will most likely deadlock your entire program.

void release_bitmap(BITMAP *bmp);
   Releases a bitmap that was previously locked by calling acquire_bitmap(). 
   If the bitmap was locked multiple times, you must release it the same 
   number of times before it will truly be unlocked.

void acquire_screen();
   Shortcut version of acquire_bitmap(screen);

void release_screen();
   Shortcut version of release_bitmap(screen);

void set_clip_rect(BITMAP *bitmap, int x1, int y1, int x2, int y2);
   Each bitmap has an associated clipping rectangle, which is the area of 
   the image that it is OK to draw onto. Nothing will be drawn to positions
   outside this space. This function sets the clipping rectangle for the
   specified bitmap. Pass the coordinates of the top-left and bottom-right
   corners of the clipping rectangle in this order; these are both inclusive,
   i.e. set_clip_rect(bitmap, 16, 16, 32, 32) will allow drawing to (16, 16)
   and (32, 32), but not to (15, 15) and (33, 33).

   Drawing operations will be performed (at least partially) on the bitmap as
   long as the first coordinates of its clipping rectangle are not greater
   than the second coordinates and its intersection with the actual image
   is non-empty. If either condition is not fulfilled, drawing will be turned
   off for the bitmap, e.g.

      set_clip_rect(bmp, 0, 0, -1, -1); /* disable drawing on bmp */

   Note that passing "out-of-bitmap" coordinates is allowed, but they are
   likely to be altered (and so the coordinates returned by get_clip_rect()
   will be different). However, such modifications are guaranteed to preserve
   the external effect of the clipping rectangle, that is not to modify the
   actual area of the image that it is OK to draw onto.

void get_clip_rect(BITMAP *bitmap, int *x1, int *y1, int *x2, int *y2);
   Returns the clipping rectangle for the specified bitmap.

void add_clip_rect(BITMAP *bitmap, int x1, int y1, int x2, int y2);
   Sets the clipping rectangle of the specified bitmap as the intersection of
   its current clipping rectangle and the rectangle described by the four
   coordinates.

void set_clip_state(BITMAP *bitmap, int state)
   Turns on (if state is non-zero) or off (if state is zero) clipping for the
   specified bitmap. Turning clipping off may slightly speed up some drawing
   operations (usually a negligible  difference, although every little helps)
   but will result in your program dying a horrible death if you try to draw
   beyond the edges of the bitmap.

int get_clip_state(BITMAP *bitmap)
   Returns non-zero if clipping is turned on for the specified bitmap and
   zero otherwise.

int is_inside_bitmap(BITMAP *bmp, int x, int y, int clip);
   Returns non-zero if point (x, y) lies inside the bitmap. If `clip' is
   non-zero, the function compares the coordinates with the clipping
   rectangle, that is it returns non-zero if the point lies inside the
   clipping rectangle or if clipping is disabled for the bitmap. If `clip'
   is zero, the function compares the coordinates with the actual dimensions
   of the bitmap.



=============================================
============ Loading image files ============
=============================================

Warning: when using truecolor images, you should always set the graphics
mode before loading any bitmap data! Otherwise the pixel format (RGB or BGR)
will not be known, so the file may be converted wrongly.

BITMAP *load_bitmap(const char *filename, RGB *pal);
   Loads a bitmap from a file. The palette data will be stored in the second
   parameter, which should be an array of 256 RGB structures. At present this
   function supports BMP, LBM, PCX, and TGA files, determining the type from
   the file extension.

   If the file contains a truecolor image, you must set the video mode or
   call set_color_conversion() before loading it. In this case, if the
   destination color depth is 8-bit, the palette will be generated by calling
   generate_optimized_palette() on the bitmap; otherwise, the returned
   palette will be generated by calling generate_332_palette().

   The pal argument may be NULL. In this case, the palette data are simply
   not returned. Additionally, if the file is a truecolor image and the
   destination color depth is 8-bit, the color conversion process will use
   the current palette instead of generating an optimized one.

   Example:

      BITMAP *bmp;
      PALETTE palette;
      ...
      bmp = load_bitmap("image.pcx", palette);
      if (!bmp)
         abort_on_error("Couldn't load image.pcx!");
      ...
      destroy_bitmap(bmp);

   Returns a pointer to the bitmap or NULL on error. Remember that you are
   responsible for destroying the bitmap when you are finished with it to
   avoid memory leaks.

BITMAP *load_bmp(const char *filename, RGB *pal);
   Loads an 8-bit, 16-bit, 24-bit or 32-bit Windows or OS/2 BMP file.

   Returns a pointer to the bitmap or NULL on error. Remember that you are
   responsible for destroying the bitmap when you are finished with it to
   avoid memory leaks.

BITMAP *load_bmp_pf(PACKFILE *f, RGB *pal);
   A version of load_bmp() which reads from a packfile. Example:

      PACKFILE *packfile;
      BITMAP *bmp;

      packfile = pack_fopen("mybitmap.bmp", F_READ);
      if (!packfile)
         abort_on_error("Couldn't open mybitmap.bmp");
         
      bmp = load_bmp_pf(packfile, pal);
      if (!bmp)
         abort_on_error("Error loading mybitmap.bmp");

   Returns a pointer to the bitmap or NULL on error. Remember that you are
   responsible for destroying the bitmap when you are finished with it to
   avoid memory leaks.

BITMAP *load_lbm(const char *filename, RGB *pal);
   Loads a 256-color IFF ILBM/PBM file.

   Returns a pointer to the bitmap or NULL on error. Remember that you are
   responsible for destroying the bitmap when you are finished with it to
   avoid memory leaks.

BITMAP *load_pcx(const char *filename, RGB *pal);
   Loads a 256-color or 24-bit truecolor PCX file.

   Returns a pointer to the bitmap or NULL on error. Remember that you are
   responsible for destroying the bitmap when you are finished with it to
   avoid memory leaks.

BITMAP *load_pcx_pf(PACKFILE *f, RGB *pal);
   A version of load_pcx() which reads from a packfile. Example:

      PACKFILE *packfile;
      BITMAP *bmp;

      packfile = pack_fopen("mybitmap.pcx", F_READ);
      if (!packfile)
         abort_on_error("Couldn't open mybitmap.pcx");
         
      bmp = load_bmp_pf(packfile, pal);
      if (!bmp)
         abort_on_error("Error loading mybitmap.pcx");

   Returns a pointer to the bitmap or NULL on error. Remember that you are
   responsible for destroying the bitmap when you are finished with it to
   avoid memory leaks.

BITMAP *load_tga(const char *filename, RGB *pal);
   Loads a 256-color, 15-bit hicolor, 24-bit truecolor, or 32-bit 
   truecolor+alpha TGA file.

   Returns a pointer to the bitmap or NULL on error. Remember that you are
   responsible for destroying the bitmap when you are finished with it to
   avoid memory leaks.

BITMAP *load_tga_pf(PACKFILE *f, RGB *pal);
   A version of load_tga() which reads from a packfile. Example:

      PACKFILE *packfile;
      BITMAP *bmp;

      packfile = pack_fopen("mybitmap.tga", F_READ);
      if (!packfile)
         abort_on_error("Couldn't open mybitmap.tga");
         
      bmp = load_bmp_pf(packfile, pal);
      if (!bmp)
         abort_on_error("Error loading mybitmap.tga");

   Returns a pointer to the bitmap or NULL on error. Remember that you are
   responsible for destroying the bitmap when you are finished with it to
   avoid memory leaks.

int save_bitmap(const char *filename, BITMAP *bmp, const RGB *pal);
   Writes a bitmap into a file, using the specified palette, which should be 
   an array of 256 RGB structures. The output format is determined from the
   filename extension: at present this function supports BMP, PCX and TGA
   formats.

   Two things to watch out for: on some video cards it may be faster to copy
   the screen to a memory bitmap and save the latter, and if you use this to
   dump the screen into a file you may end up with an image much larger than
   you were expecting, because Allegro often creates virtual screens larger
   than the visible screen. You can get around this by using a sub-bitmap to
   specify which part of the screen to save, eg:

      BITMAP *bmp;
      PALETTE pal;
      ...
      get_palette(pal);
      bmp = create_sub_bitmap(screen, 0, 0, SCREEN_W, SCREEN_H);
      save_bitmap("dump.pcx", bmp, pal);
      destroy_bitmap(bmp);

   Returns non-zero on error.
      
int save_bmp(const char *filename, BITMAP *bmp, const RGB *pal);
   Writes a bitmap into a 256-color or 24-bit truecolor BMP file.

   Returns non-zero on error.

int save_bmp_pf(PACKFILE *f, BITMAP *bmp, RGB *pal);
   A version of save_bmp which writes to a packfile.

int save_pcx(const char *filename, BITMAP *bmp, const RGB *pal);
   Writes a bitmap into a 256-color or 24-bit truecolor PCX file.

   Returns non-zero on error.

int save_pcx_pf(PACKFILE *f, BITMAP *bmp, RGB *pal);
   A version of save_pcx which writes to a packfile.

int save_tga(const char *filename, BITMAP *bmp, const RGB *pal);
   Writes a bitmap into a 256-color, 15-bit hicolor, 24-bit truecolor, or
   32-bit truecolor+alpha TGA file.

   Returns non-zero on error.

int save_tga_pf(PACKFILE *f, BITMAP *bmp, RGB *pal);
   A version of save_tga which writes to a packfile.

void register_bitmap_file_type(const char *ext,
          BITMAP *(*load)(const char *filename, RGB *pal),
          int (*save)(const char *filename, BITMAP *bmp, const RGB *pal));
   Informs the load_bitmap() and save_bitmap() functions of a new file type, 
   providing routines to read and write images in this format (either 
   function may be NULL). The functions you supply must follow the same
   prototype as load_bitmap() and save_bitmap(). Example:

      BITMAP *load_dump(const char *filename, RGB *pal)
      {
         ...
      }
      
      int save_dump(const char *filename, BITMAP *bmp, const RGB *pal)
      {
         ...
      }

         register_bitmap_file_type("dump", load_dump, save_dump);

void set_color_conversion(int mode);
   Specifies how to convert images between the various color depths when 
   reading graphics from external bitmap files or datafiles. The mode is a 
   bitmask specifying which types of conversion are allowed. If the 
   appropriate bit is set, data will be converted into the current pixel 
   format (selected by calling the set_color_depth() function), otherwise it 
   will be left in the same format as the disk file, leaving you to convert 
   it manually before the graphic can be displayed. The default mode is 
   total conversion, so that all images will be loaded in the appropriate
   format for the current video mode. Valid bit flags are:

      COLORCONV_NONE                // disable all format
                                    // conversions
      COLORCONV_8_TO_15             // expand 8-bit to 15-bit
      COLORCONV_8_TO_16             // expand 8-bit to 16-bit
      COLORCONV_8_TO_24             // expand 8-bit to 24-bit
      COLORCONV_8_TO_32             // expand 8-bit to 32-bit
      COLORCONV_15_TO_8             // reduce 15-bit to 8-bit
      COLORCONV_15_TO_16            // expand 15-bit to 16-bit
      COLORCONV_15_TO_24            // expand 15-bit to 24-bit
      COLORCONV_15_TO_32            // expand 15-bit to 32-bit
      COLORCONV_16_TO_8             // reduce 16-bit to 8-bit
      COLORCONV_16_TO_15            // reduce 16-bit to 15-bit
      COLORCONV_16_TO_24            // expand 16-bit to 24-bit
      COLORCONV_16_TO_32            // expand 16-bit to 32-bit
      COLORCONV_24_TO_8             // reduce 24-bit to 8-bit
      COLORCONV_24_TO_15            // reduce 24-bit to 15-bit
      COLORCONV_24_TO_16            // reduce 24-bit to 16-bit
      COLORCONV_24_TO_32            // expand 24-bit to 32-bit
      COLORCONV_32_TO_8             // reduce 32-bit RGB to 8-bit
      COLORCONV_32_TO_15            // reduce 32-bit RGB to 15-bit
      COLORCONV_32_TO_16            // reduce 32-bit RGB to 16-bit
      COLORCONV_32_TO_24            // reduce 32-bit RGB to 24-bit
      COLORCONV_32A_TO_8            // reduce 32-bit RGBA to 8-bit
      COLORCONV_32A_TO_15           // reduce 32-bit RGBA to 15-bit
      COLORCONV_32A_TO_16           // reduce 32-bit RGBA to 16-bit
      COLORCONV_32A_TO_24           // reduce 32-bit RGBA to 24-bit
      COLORCONV_DITHER_PAL          // dither when reducing to 8-bit
      COLORCONV_DITHER_HI           // dither when reducing to
                                    // hicolor
      COLORCONV_KEEP_TRANS          // keep original transparency

   For convenience, the following macros can be used to select common 
   combinations of these flags:

      COLORCONV_EXPAND_256          // expand 256-color to hi/truecolor
      COLORCONV_REDUCE_TO_256       // reduce hi/truecolor to 256-color
      COLORCONV_EXPAND_15_TO_16     // expand 15-bit hicolor to 16-bit
      COLORCONV_REDUCE_16_TO_15     // reduce 16-bit hicolor to 15-bit
      COLORCONV_EXPAND_HI_TO_TRUE   // expand 15/16-bit to 24/32-bit
      COLORCONV_REDUCE_TRUE_TO_HI   // reduce 24/32-bit to 15/16-bit
      COLORCONV_24_EQUALS_32        // convert between 24- and 32-bit
      COLORCONV_TOTAL               // everything to current format
      COLORCONV_PARTIAL             // convert 15 <-> 16-bit and
                                    // 24 <-> 32-bit
      COLORCONV_MOST                // all but hi/truecolor <-> 256
      COLORCONV_DITHER              // dither during all color reductions
      COLORCONV_KEEP_ALPHA          // convert everything to current format
                                    // unless it would lose alpha information

   If you enable the COLORCONV_DITHER flag, dithering will be performed 
   whenever truecolor graphics are converted into a hicolor or paletted 
   format, including by the blit() function, and any automatic conversions 
   that take place while reading graphics from disk. This can produce much 
   better looking results, but is obviously slower than a direct conversion.

   If you intend using converted bitmaps with functions like masked_blit() 
   or draw_sprite(), you should specify the COLORCONV_KEEP_TRANS flag. It 
   will ensure that the masked areas in the bitmap before and after the 
   conversion stay exactly the same, by mapping transparent colors to each 
   other and adjusting colors which would be converted to the transparent 
   color otherwise. It affects every blit() operation between distinct pixel 
   formats and every automatic conversion.

int get_color_conversion();
   Returns the current color conversion mode.



==========================================
============ Palette routines ============
==========================================

All the Allegro drawing functions use integer parameters to represent 
colors. In truecolor resolutions these numbers encode the color directly as 
a collection of red, green, and blue bits, but in a regular 256-color mode 
the values are treated as indexes into the current palette, which is a table 
listing the red, green and blue intensities for each of the 256 possible 
colors.

Palette entries are stored in an RGB structure, which contains red, green 
and blue intensities in the VGA hardware format, ranging from 0-63, and is 
defined as:

   typedef struct RGB
   {
      unsigned char r, g, b;
   } RGB;

It contains an additional field for the purpose of padding but you should
not usually care about it. For example:

   RGB black = { 0,  0,  0  };
   RGB white = { 63, 63, 63 };
   RGB green = { 0,  63, 0  };
   RGB grey  = { 32, 32, 32 };

The type PALETTE is defined to be an array of PAL_SIZE RGB structures, where
PAL_SIZE is a preprocessor constant equal to 256.

You may notice that a lot of the code in Allegro spells 'palette' as 
'pallete'. This is because the headers from my old Mark Williams compiler on 
the Atari spelt it with two l's, so that is what I'm used to. Allegro will 
happily accept either spelling, due to some #defines in allegro/alcompat.h
(which can be turned off by defining the ALLEGRO_NO_COMPATIBILITY symbol
before including Allegro headers).

void set_color(int index, const RGB *p);
   Sets the specified palette entry to the specified RGB triplet. Unlike the 
   other palette functions this doesn't do any retrace synchronisation, so 
   you should call vsync() before it to prevent snow problems. Example:

      RGB rgb;
      ...
      vsync();
      set_color(192, &rgb);

void _set_color(int index, const RGB *p);
   This is an inline version of set_color(), intended for use in the
   vertical retrace simulator callback function (retrace_proc, which is now
   deprecated).

   If you really must use _set_color from retrace_proc, note that it should
   only be used under DOS, in VGA mode 13h and mode-X. Some SVGA chipsets
   aren't VGA compatible (set_color() and set_palette() will use VESA calls on
   these cards, but _set_color() doesn't know about that).

void set_palette(const PALETTE p);
   Sets the entire palette of 256 colors. You should provide an array of 256 
   RGB structures. Unlike set_color(), there is no need to call vsync() 
   before this function. Example:

      BITMAP *bmp;
      PALETTE palette;
      ...
      bmp = load_bitmap(filename, palette);
      if (!bmp)
         abort_on_error("Couldn't load bitmap!");
      set_palette(palette);

void set_palette_range(const PALETTE p, int from, int to, int vsync);
   Sets the palette entries between from and to (inclusive: pass 0 and 255 
   to set the entire palette). If vsync is set it waits for the vertical 
   retrace, otherwise it sets the colors immediately. Example:

      PALETTE palette;
      ...
      /* Modify the first 16 entries. */
      change_first_16_colors(palette);
      /* Now update them waiting for vsync. */
      set_palette_range(palette, 0, 15, 1);

void get_color(int index, RGB *p);
   Retrieves the specified palette entry. Example:

      RGB color;
      ...
      get_color(11, &color);

void get_palette(PALETTE p);
   Retrieves the entire palette of 256 colors. You should provide an array 
   of 256 RGB structures to store it in. Example:

      PALETTE pal;
      ...
      get_palette(pal);

void get_palette_range(PALETTE p, int from, int to);
   Retrieves the palette entries between from and to (inclusive: pass 0 and 
   255 to get the entire palette).

void fade_interpolate(const PALETTE source, const PALETTE dest,
                      PALETTE output, int pos, int from, int to);
   Calculates a temporary palette part way between source and dest, 
   returning it in the output parameter. The position between the two 
   extremes is specified by the pos value: 0 returns an exact copy of 
   source, 64 returns dest, 32 returns a palette half way between the two, 
   etc. This routine only affects colors between from and to (inclusive: 
   pass 0 and 255 to interpolate the entire palette).

void fade_from_range(const PALETTE source, const PALETTE dest,
                     int speed, int from, int to);
   Gradually fades a part of the palette from the source palette to the dest 
   palette. The speed is from 1 (the slowest) up to 64 (instantaneous). This 
   routine only affects colors between from and to (inclusive: pass 0 and 
   255 to fade the entire palette).

   Note that this function will block your game while the fade is in effect,
   and it won't work right visually if you are not in an 8 bit color depth
   resolution.

void fade_in_range(const PALETTE p, int speed, int from, int to);
   Gradually fades a part of the palette from a black screen to the 
   specified palette. The speed is from 1 (the slowest) up to 64 
   (instantaneous). This routine only affects colors between from and to 
   (inclusive: pass 0 and 255 to fade the entire palette).

   Note that this function will block your game while the fade is in effect,
   and it won't work right visually if you are not in an 8 bit color depth
   resolution.

void fade_out_range(int speed, int from, int to);
   Gradually fades a part of the palette from the current palette to a black 
   screen. The speed is from 1 (the slowest) up to 64 (instantaneous). This 
   routine only affects colors between from and to (inclusive: pass 0 and 
   255 to fade the entire palette).

   Note that this function will block your game while the fade is in effect,
   and it won't work right visually if you are not in an 8 bit color depth
   resolution.

void fade_from(const PALETTE source, const PALETTE dest, int speed);
   Fades gradually from the source palette to the dest palette. The speed is 
   from 1 (the slowest) up to 64 (instantaneous).

   Note that this function will block your game while the fade is in effect,
   and it won't work right visually if you are not in an 8 bit color depth
   resolution.

void fade_in(const PALETTE p, int speed);
   Fades gradually from a black screen to the specified palette. The speed 
   is from 1 (the slowest) up to 64 (instantaneous).

   Note that this function will block your game while the fade is in effect,
   and it won't work right visually if you are not in an 8 bit color depth
   resolution.

void fade_out(int speed);
   Fades gradually from the current palette to a black screen. The speed is 
   from 1 (the slowest) up to 64 (instantaneous).

   Note that this function will block your game while the fade is in effect,
   and it won't work right visually if you are not in an 8 bit color depth
   resolution.

void select_palette(const PALETTE p);
   Ugly hack for use in various dodgy situations where you need to convert 
   between paletted and truecolor image formats. Sets the internal palette 
   table in the same way as the set_palette() function, so the conversion 
   will use the specified palette, but without affecting the display 
   hardware in any way. The previous palette settings are stored in an 
   internal buffer, and can be restored by calling unselect_palette(). If
   you call select_palette() again, however, the internal buffer will be
   overwritten.

void unselect_palette();
   Restores the palette tables that were in use before the last call to 
   select_palette().

void generate_332_palette(PALETTE pal);
   Constructs a fake truecolor palette, using three bits for red and green 
   and two for the blue. The load_bitmap() function fills the palette
   parameter with this if the file does not contain a palette itself (ie. you
   are reading a truecolor bitmap).

int generate_optimized_palette(BITMAP *bmp, PALETTE pal,
                               const char rsvd[PAL_SIZE]);
   Generates a 256-color palette suitable for making a reduced color version 
   of the specified truecolor image. The rsvd parameter points to a table 
   indicating which colors it is allowed to modify: zero for free colors
   which may be set to whatever the optimiser likes, negative values for
   reserved colors which cannot be used, and positive values for fixed
   palette entries that must not be changed, but can be used in the
   optimisation.

   Returns the number of different colors recognised in the provided bitmap,
   zero if the bitmap is not a truecolor image or there wasn't enough memory
   to perform the operation, and negative if there was any internal error in
   the color reduction code.

extern PALETTE default_palette;
   The default IBM BIOS palette. This will be automatically selected 
   whenever you set a new graphics mode. The palette contains 16 basic
   colors plus many gradients between them. If you want to see the values,
   you can write a small Allegro program which saves a screenshot with this
   palette, or open the grabber tool provided with Allegro and create a new
   palette object, which will use this palette by default.

extern PALETTE black_palette;
   A palette containing solid black colors, used by the fade routines.

extern PALETTE desktop_palette;
   The palette used by the Atari ST low resolution desktop. I'm not quite 
   sure why this is still here, except that the grabber and test programs 
   use it. It is probably the only Atari legacy code left in Allegro, and it 
   would be a shame to remove it :-)

   The contents of this palette are 16 colors repeated 16 times. Color entry
   zero is equal to color entry 16, which is equal to color entry 24, etc.

       Index      Color       RGB values
         0     White          63  63  63
         1     Red            63   0   0
         2     Green           0  63   0
         3     Yellow         63  63   0
         4     Blue            0   0  63
         5     Pink           63   0  63
         6     Cyan            0  63  63
         7     Grey           16  16  16
         8     Light grey     31  31  31
         9     Light red      63  31  31
        10     Light green    31  63  31
        11     Light yellow   63  63  31
        12     Light blue     31  31  63
        13     Light pink     63  31  63
        14     Light cyan     31  63  63
        15     Black           0   0   0



=================================================
============ Truecolor pixel formats ============
=================================================

In a truecolor video mode the red, green, and blue components for each pixel 
are packed directly into the color value, rather than using a palette lookup 
table. In a 15-bit mode there are 5 bits for each color, in 16-bit modes 
there are 5 bits each of red and blue and six bits of green, and both 24 and 
32-bit modes use 8 bits for each color (the 32-bit pixels simply have an 
extra padding byte to align the data nicely). The layout of these components 
can vary depending on your hardware, but will generally either be RGB or 
BGR. Since the layout is not known until you select the video mode you will 
be using, you must call set_gfx_mode() before using any of the following 
routines!

int makecol8(int r, int g, int b);
int makecol15(int r, int g, int b);
int makecol16(int r, int g, int b);
int makecol24(int r, int g, int b);
int makecol32(int r, int g, int b);
   These functions convert colors from a hardware independent form (red, 
   green, and blue values ranging 0-255) into various display dependent 
   pixel formats. Converting to 15, 16, 24, or 32-bit formats only takes a 
   few shifts, so it is fairly efficient. Converting to an 8-bit color 
   involves searching the palette to find the closest match, which is quite 
   slow unless you have set up an RGB mapping table (see below). Example:

      /* 16 bit color version of green. */
      int green_color = makecol16(0, 255, 0);

   Returns the requested RGB triplet in the specified color depth.

int makeacol32(int r, int g, int b, int a);
   Converts an RGBA color into a 32-bit display pixel format, which includes 
   an alpha (transparency) value. There are no versions of this routine for 
   other color depths, because only the 32-bit format has enough room to 
   store a proper alpha channel. You should only use RGBA format colors as 
   the input to draw_trans_sprite() or draw_trans_rle_sprite() after calling 
   set_alpha_blender(), rather than drawing them directly to the screen.

int makecol(int r, int g, int b);
   Converts colors from a hardware independent format (red, green, and blue 
   values ranging 0-255) to the pixel format required by the current video 
   mode, calling the preceding 8, 15, 16, 24, or 32-bit makecol functions as 
   appropriate. Example:

      /* Regardless of color depth, this will look green. */
      int green_color = makecol(0, 255, 0);

   Returns the requested RGB triplet in the current color depth.

int makecol_depth(int color_depth, int r, int g, int b);
   Converts colors from a hardware independent format (red, green, and blue 
   values ranging 0-255) to the pixel format required by the specified color 
   depth. Example:

      /* Compose the green color for 15 bit color depth. */
      int green_15bit = makecol_depth(15, 0, 255, 0);

   Returns the requested RGB triplet in the specified color depth.

int makeacol(int r, int g, int b, int a);
int makeacol_depth(int color_depth, int r, int g, int b, int a);
   Convert RGBA colors into display dependent pixel formats. In anything 
   less than a 32-bit mode, these are the same as calling makecol() or 
   makecol_depth(), but by using these routines it is possible to create 
   32-bit color values that contain a true 8 bit alpha channel along with 
   the red, green, and blue components. You should only use RGBA format 
   colors as the input to draw_trans_sprite() or draw_trans_rle_sprite() 
   after calling set_alpha_blender(), rather than drawing them directly to 
   the screen.

   Returns the requested RGBA quadruplet.

int makecol15_dither(int r, int g, int b, int x, int y);
int makecol16_dither(int r, int g, int b, int x, int y);
   Given both a color value and a pixel coordinate, calculate a dithered 15 
   or 16-bit RGB value. This can produce better results when reducing images 
   from truecolor to hicolor. In addition to calling these functions 
   directly, hicolor dithering can be automatically enabled when loading 
   graphics by calling the set_color_conversion() function, for example 
   set_color_conversion(COLORCONV_REDUCE_TRUE_TO_HI | COLORCONV_DITHER).
   
   Example:

      int pixel1, pixel2;

      /* The following two color values MAY be different. */
      pixel1 = makecol16_dither(255, 192, 64, 0, 0);
      pixel2 = makecol16_dither(255, 192, 64, 1, 0);

   Returns the RGB value dithered for the specified coordinate.

int getr8(int c);
int getg8(int c);
int getb8(int c);
int getr15(int c);
int getg15(int c);
int getb15(int c);
int getr16(int c);
int getg16(int c);
int getb16(int c);
int getr24(int c);
int getg24(int c);
int getb24(int c);
int getr32(int c);
int getg32(int c);
int getb32(int c);
   Given a color in a display dependent format, these functions extract one 
   of the red, green, or blue components (ranging 0-255). Example:

      int r, g, b, color_value;

      color_value = _getpixel15(screen, 100, 100);
      r = getr15(color_value);
      g = getg15(color_value);
      b = getb15(color_value);

int geta32(int c);
   Given a color in a 32-bit pixel format, this function extracts the alpha 
   component (ranging 0-255).

int getr(int c);
int getg(int c);
int getb(int c);
int geta(int c);
   Given a color in the format being used by the current video mode, these 
   functions extract one of the red, green, blue, or alpha components 
   (ranging 0-255), calling the preceding 8, 15, 16, 24, or 32-bit get 
   functions as appropriate. The alpha part is only meaningful for 32-bit 
   pixels. Example:

      int r, g, b, color_value;

      color_value = getpixel(screen, 100, 100);
      r = getr(color_value);
      g = getg(color_value);
      b = getb(color_value);

int getr_depth(int color_depth, int c);
int getg_depth(int color_depth, int c);
int getb_depth(int color_depth, int c);
int geta_depth(int color_depth, int c);
   Given a color in the format being used by the specified color depth, 
   these functions extract one of the red, green, blue, or alpha components 
   (ranging 0-255). The alpha part is only meaningful for 32-bit pixels.
   Example:

      int r, g, b, color_value, bpp;

      bpp = bitmap_color_depth(bitmap);
      color_value = getpixel(bitmap, 100, 100);
      r = getr_depth(bpp, color_value);
      g = getg_depth(bpp, color_value);
      b = getb_depth(bpp, color_value);
      
extern int palette_color[256];
   Table mapping palette index colors (0-255) into whatever pixel format is 
   being used by the current display mode. In a 256-color mode this just 
   maps onto the array index. In truecolor modes it looks up the specified 
   entry in the current palette, and converts that RGB value into the 
   appropriate packed pixel format. Example:

      set_color_depth(32);
      ...
      set_palette(desktop_palette);
      /* Put a pixel with the color 2 (green) of the palette */
      putpixel(screen, 100, 100, palette_color[2]);

#define MASK_COLOR_8 0
#define MASK_COLOR_15 (5.5.5 pink)
#define MASK_COLOR_16 (5.6.5 pink)
#define MASK_COLOR_24 (8.8.8 pink)
#define MASK_COLOR_32 (8.8.8 pink)
   Constants representing the colors used to mask transparent sprite pixels 
   for each color depth. In 256-color resolutions this is zero, and in 
   truecolor modes it is bright pink (maximum red and blue, zero green).



============================================
============ Drawing primitives ============
============================================

Except for _putpixel(), all these routines are affected by the current 
drawing mode and the clipping rectangle of the destination bitmap. Unless
specified otherwise, all coordinates for drawing operations are inclusive,
and they, as well as lengths, are specified in pixel units.

void clear_bitmap(BITMAP *bitmap);
   Clears the bitmap to color 0.

void clear_to_color(BITMAP *bitmap, int color);
   Clears the bitmap to the specified color. Example:

      /* Clear the screen to red. */
      clear_to_color(bmp, makecol(255, 0, 0));

void putpixel(BITMAP *bmp, int x, int y, int color);
   Writes a pixel to the specified position in the bitmap, using the current 
   drawing mode and the bitmap's clipping rectangle. Example: 

      putpixel(screen, 10, 30, some_color);

void _putpixel(BITMAP *bmp, int x, int y, int color);
void _putpixel15(BITMAP *bmp, int x, int y, int color);
void _putpixel16(BITMAP *bmp, int x, int y, int color);
void _putpixel24(BITMAP *bmp, int x, int y, int color);
void _putpixel32(BITMAP *bmp, int x, int y, int color);
   Like the regular putpixel(), but much faster because they are implemented 
   as an inline assembler functions for specific color depths. These won't 
   work in mode-X graphics modes, don't perform any clipping (they will 
   crash if you try to draw outside the bitmap!), and ignore the drawing 
   mode.

int getpixel(BITMAP *bmp, int x, int y);
   Reads a pixel from point (x, y) in the bitmap.

   Returns -1 if the point lies outside the bitmap (ignoring the clipping
   rectangle), otherwise the value of the pixel in the color format of the
   bitmap.

   Warning: -1 is also a valid value for pixels contained in 32-bit bitmaps
   with alpha channel (when R,G,B,A are all equal to 255) so you can't use
   the test against -1 as a predicate for such bitmaps. In this cases, the
   only reliable predicate is is_inside_bitmap().

   To extract the individual color components, use the getr() / getg() /
   getb() / geta() family of functions.

int _getpixel(BITMAP *bmp, int x, int y);
int _getpixel15(BITMAP *bmp, int x, int y);
int _getpixel16(BITMAP *bmp, int x, int y);
int _getpixel24(BITMAP *bmp, int x, int y);
int _getpixel32(BITMAP *bmp, int x, int y);
   Faster inline versions of getpixel() for specific color depths. These 
   won't work in mode-X, and don't do any clipping, so you must make sure 
   the point lies inside the bitmap.

   Returns the value of the pixel in the color format you specified.

void vline(BITMAP *bmp, int x, int y1, int y2, int color);
   Draws a vertical line onto the bitmap, from point (x, y1) to (x, y2).

   Note: vline() is implemented as an alias to another function.
   See ALLEGRO_NO_VHLINE_ALIAS in the `Differences between platforms'
   section for details.

void hline(BITMAP *bmp, int x1, int y, int x2, int color);
   Draws a horizontal line onto the bitmap, from point (x1, y) to (x2, y).

   Note: hline() is implemented as an alias to another function.
   See ALLEGRO_NO_VHLINE_ALIAS in the `Differences between platforms'
   section for details.

void do_line(BITMAP *bmp, int x1, y1, x2, y2, int d, 
             void (*proc)(BITMAP *bmp, int x, int y, int d));
   Calculates all the points along a line from point (x1, y1) to (x2, y2), 
   calling the supplied function for each one. This will be passed a copy of 
   the bmp parameter, the x and y position, and a copy of the d parameter, 
   so it is suitable for use with putpixel(). Example:

      void draw_dust_particle(BITMAP *bmp, int x, int y, int d)
      {
         ...
      }
      
         do_line(screen, 0, 0, SCREEN_W-1, SCREEN_H-2,
                 dust_strength, draw_dust_particle);

void line(BITMAP *bmp, int x1, int y1, int x2, int y2, int color);
   Draws a line onto the bitmap, from point (x1, y1) to (x2, y2).

void fastline(BITMAP *bmp, int x1, int y1, int x2, int y2, int color);
   Faster version of the previous function. Note that pixel correctness is
   not guaranteed for this function.

void triangle(BITMAP *bmp, int x1, y1, x2, y2, x3, y3, int color);
   Draws a filled triangle between the three points.

void polygon(BITMAP *bmp, int vertices, const int *points, int color);
   Draws a filled polygon with an arbitrary number of corners. Pass the 
   number of vertices and an array containing a series of x, y points (a 
   total of vertices*2 values). Example:

      int points[12] = { 50, 50,   100, 100,  100, 150,
                         50, 200,  0,   150,  0,   100 };
      ...
      clear_to_color(screen, makecol(255, 255, 255));
      polygon(screen, 6, points, makecol(0, 0, 0));

void rect(BITMAP *bmp, int x1, int y1, int x2, int y2, int color);
   Draws an outline rectangle with the two points as its opposite corners.

void rectfill(BITMAP *bmp, int x1, int y1, int x2, int y2, int color);
   Draws a solid, filled rectangle with the two points as its opposite 
   corners.

void do_circle(BITMAP *bmp, int x, int y, int radius, int d,
               void (*proc)(BITMAP *bmp, int x, int y, int d));
   Calculates all the points in a circle around point (x, y) with radius r, 
   calling the supplied function for each one. This will be passed a copy of 
   the bmp parameter, the x and y position, and a copy of the d parameter, 
   so it is suitable for use with putpixel(). Example:

      void draw_explosion_ring(BITMAP *bmp, int x, int y, int d)
      {
         ...
      }
      
         do_circle(screen, SCREEN_W/2, SCREEN_H/2,
                   SCREEN_H/16, flame_color,
                   draw_explosion_ring);

void circle(BITMAP *bmp, int x, int y, int radius, int color);
   Draws a circle with the specified centre and radius.

void circlefill(BITMAP *bmp, int x, int y, int radius, int color);
   Draws a filled circle with the specified centre and radius.

void do_ellipse(BITMAP *bmp, int x, int y, int rx, ry, int d,
                void (*proc)(BITMAP *bmp, int x, int y, int d));
   Calculates all the points in an ellipse around point (x, y) with radius 
   rx and ry, calling the supplied function for each one. This will be 
   passed a copy of the bmp parameter, the x and y position, and a copy of 
   the d parameter, so it is suitable for use with putpixel(). Example:

      void draw_explosion_ring(BITMAP *bmp, int x, int y, int d)
      {
         ...
      }
      
         do_ellipse(screen, SCREEN_W/2, SCREEN_H/2,
                   SCREEN_H/16, SCREEN_H/32, flame_color,
                   draw_explosion_ring);

void ellipse(BITMAP *bmp, int x, int y, int rx, int ry, int color);
   Draws an ellipse with the specified centre and radius.

void ellipsefill(BITMAP *bmp, int x, int y, int rx, int ry, int color);
   Draws a filled ellipse with the specified centre and radius.

void do_arc(BITMAP *bmp, int x, int y, fixed a1, fixed a2, int r, int d,
            void (*proc)(BITMAP *bmp, int x, int y, int d));
   Calculates all the points in a circular arc around point (x, y) with 
   radius r, calling the supplied function for each one. This will be passed 
   a copy of the bmp parameter, the x and y position, and a copy of the d
   parameter, so it is suitable for use with putpixel(). The arc will be 
   plotted in an anticlockwise direction starting from the angle a1 and 
   ending when it reaches a2. These values are specified in 16.16 fixed 
   point format, with 256 equal to a full circle, 64 a right angle, etc. 
   Zero is to the right of the centre point, and larger values rotate 
   anticlockwise from there. Example:

      void draw_explosion_ring(BITMAP *bmp, int x, int y, int d)
      {
         ...
      }
         do_arc(screen, SCREEN_W/2, SCREEN_H/2,
             itofix(-21), itofix(43), 50, flame_color,
             draw_explosion_ring);

void arc(BITMAP *bmp, int x, y, fixed ang1, ang2, int r, int color);
   Draws a circular arc with centre x, y and radius r, in an anticlockwise 
   direction starting from the angle a1 and ending when it reaches a2. These 
   values are specified in 16.16 fixed point format, with 256 equal to a 
   full circle, 64 a right angle, etc. Zero is to the right of the centre 
   point, and larger values rotate anticlockwise from there. Example:

      /* Draw a black arc from 4 to 1 o'clock. */
      arc(screen, SCREEN_W/2, SCREEN_H/2,
          itofix(-21), itofix(43), 50, makecol(0, 0, 0));

void calc_spline(const int points[8], int npts, int *x, int *y);
   Calculates a series of npts values along a Bezier spline, storing them in 
   the output x and y arrays. The Bezier curve is specified by the four x/y 
   control points in the points array: points[0] and points[1] contain the 
   coordinates of the first control point, points[2] and points[3] are the 
   second point, etc. Control points 0 and 3 are the ends of the spline, and 
   points 1 and 2 are guides. The curve probably won't pass through points 1 
   and 2, but they affect the shape of the curve between points 0 and 3 (the 
   lines p0-p1 and p2-p3 are tangents to the spline). The easiest way to 
   think of it is that the curve starts at p0, heading in the direction of 
   p1, but curves round so that it arrives at p3 from the direction of p2. 
   In addition to their role as graphics primitives, spline curves can be 
   useful for constructing smooth paths around a series of control points, 
   as in exspline.c.

void spline(BITMAP *bmp, const int points[8], int color);
   Draws a Bezier spline using the four control points specified in the 
   points array. Read the description of calc_spline() for information on
   how to build the points array.

void floodfill(BITMAP *bmp, int x, int y, int color);
   Floodfills an enclosed area, starting at point (x, y), with the specified 
   color.



==============================================
============ Blitting and sprites ============
==============================================

As far as Allegro is concerned, a bitmap and a sprite are the same thing,
but to many people the two words imply slightly different things.  The
function draw_sprite() is called so rather than draw_bitmap() partly because
it indicates that it uses a masked drawing mode (if it existed, you could
expect draw_bitmap() to be a simple block copy), and partly for historical
reasons. In Allegro 1.0 there were actually different structures for sprites
and bitmaps, each with their own set of abilities. Allegro 2.0 merged these
into a single more flexible structure, but retained some names like
draw_sprite().

In wider (non-Allegro) terms, the two words can mean quite different things.
Generally you can say that sprites are a subset of bitmaps, but even that
isn't true in 100% of cases.

BITMAP: a widely accepted term that will be understood by anyone even
remotely connected with computer graphics. It simply means an image built up
from a grid of pixels, ie. just about any picture that you are likely to come
across on a computer (vector graphics formats are the exception, but those
must be rendered into a bitmap format before they can be displayed by most
hardware). A more accurate term but slightly rarer term with the same meaning
is "pixmap" (pixel-map).

SPRITE: a particular usage of bitmapped images, restricted to video games
(other types of programmer probably won't be familiar with this term).
Originally on machines like the C64, sprites were a hardware feature that
allowed a number of small bitmap images to be loaded into special registers,
and they could then be superimposed over the main graphics display and moved
around just by modifying the position register. They were used for the moving
objects (player and enemy characters), and enabled the C64 to do much more
impressive things than would have been possible if all the drawing had to be
done directly by the puny CPU.

Later on, a lot of old C64 programmers upgraded to machines like the Atari
ST, which didn't have any special sprite hardware, but they carried on
referring to their main moving objects as sprites (the routine to draw such a
thing would obviously be called draw_sprite()).  A sprite is really just a
bitmap graphic which is drawn onto the screen, but when you call it a sprite
rather than a bitmap, this suggests it is a gameplay element that can move
freely around the world rather than being a static part of the environment,
and that it will be drawn in a masked overlay mode rather than as a solid
rectangle (there is also a strong implication that a sprite will be animated
by cycling through a number of frames, but that isn't always the case).

In recent years some people have started using "sprite" to refer to any
character graphics, even if they are not in fact drawn as 2d bitmaps, eg.
"this game uses 3d polygonal player sprites". This is a confusing misuse of
the word (Doom uses sprites, Quake does not), but it does happen.

The origin of the term "blit" is also rather interesting. This was originally
BitBlt, an abbreviation of BITmap BLock Transfer, which was a function
designed (possibly) by the people at Xerox who did so much of the pioneering
work on graphics display systems, and subsequently copied by virtually
everybody doing computer graphics (the Microsoft Windows GDI still provides a
BitBlt function with identical functionality to the original). This routine
was a workhorse for all sorts of drawing operations, basically copying bitmap
graphics from one place to another, but including a number of different ROP
modes (Raster OPerations) for doing things like XOR, inverting pixels, etc.
A whole family of related words grew up around the BitBlt function, but "blt"
is impossible to speak (try saying "bltter" or "bltting" :-) so people added
the vowel to make it easier to pronounce.

Therefore, the act of calling the BitBlt function came to be known as "doing a
blit". The obvious next step was to rename the function itself to blit(),
which generally took place at the same time as people decided to simplify the
original, removing the different ROP modes on the grounds that they aren't
needed for games coding and don't work well with anything higher than
monochrome images in any case. This leaves us with a function called blit(),
which is an abbreviation for "block transfer". A strong case could be made
for calling this blot() instead, but somehow that just doesn't sound the
same!

Anyway, all the routines in this chapter are affected by the
clipping rectangle of the destination bitmap.

void blit(BITMAP *source, BITMAP *dest, int source_x, int source_y,
          int dest_x, int dest_y, int width, int height);
   Copies a rectangular area of the source bitmap to the destination bitmap. 
   The source_x and source_y parameters are the top left corner of the area 
   to copy from the source bitmap, and dest_x and dest_y are the 
   corresponding position in the destination bitmap. This routine respects 
   the destination clipping rectangle, and it will also clip if you try to 
   blit from areas outside the source bitmap. Example:

      BITMAP *bmp;
      ...
      /* Blit src on the screen. */
      blit(bmp, screen, 0, 0, 0, 0, bmp->w, bmp->h);
      
      /* Now copy a chunk to a corner, slightly outside. /*
      blit(screen, screen, 100, 100, -10, -10, 25, 30);

   You can blit between any parts of any two bitmaps, even if the two memory 
   areas overlap (ie. source and dest are the same, or one is sub-bitmap of 
   the other). You should be aware, however, that a lot of SVGA cards don't 
   provide separate read and write banks, which means that blitting from one 
   part of the screen to another requires the use of a temporary bitmap in 
   memory, and is therefore extremely slow. As a general rule you should 
   avoid blitting from the screen onto itself in SVGA modes.

   In mode-X, on the other hand, blitting from one part of the screen to 
   another can be significantly faster than blitting from memory onto the 
   screen, as long as the source and destination are correctly aligned with 
   each other. Copying between overlapping screen rectangles is slow, but if 
   the areas don't overlap, and if they have the same plane alignment (ie. 
   (source_x%4) == (dest_x%4)), the VGA latch registers can be used for a 
   very fast data transfer. To take advantage of this, in mode-X it is often 
   worth storing tile graphics in a hidden area of video memory (using a 
   large virtual screen), and blitting them from there onto the visible part 
   of the screen.

   If the GFX_HW_VRAM_BLIT bit in the gfx_capabilities flag is set, the 
   current driver supports hardware accelerated blits from one part of the 
   screen onto another. This is extremely fast, so when this flag is set it 
   may be worth storing some of your more frequently used graphics in an 
   offscreen portion of the video memory.

   Unlike most of the graphics routines, blit() allows the source and
   destination bitmaps to be of different color depths, so it can be
   used to convert images from one pixel format to another. In this
   case, the behavior is affected by the COLORCONV_KEEP_TRANS and
   COLORCONV_DITHER* flags of the current color conversion mode: see
   set_color_conversion() for more information.

void stretch_blit(BITMAP *source, BITMAP *dest,
                  int source_x, source_y, source_width, source_height,
                  int dest_x, dest_y, dest_width, dest_height);
   Like blit(), except it can scale images (so the source and destination 
   rectangles don't need to be the same size) and requires the source and 
   destination bitmaps to be of the same color depth. This routine doesn't 
   do as much safety checking as the regular blit(): in particular you must 
   take care not to copy from areas outside the source bitmap, and you 
   cannot blit between overlapping regions, ie. you must use different 
   bitmaps for the source and the destination. Moreover, the source must 
   be a memory bitmap. Example:

      BITMAP *bmp;
      ...
      /* Stretch bmp to fill the screen. */
      stretch_blit(bmp, screen, 0, 0, bmp->w, bmp->h,
                   0, 0, SCREEN_W, SCREEN_H);

void masked_blit(BITMAP *source, BITMAP *dest, int source_x, int source_y,
                 int dest_x, int dest_y, int width, int height);
   Like blit(), but skips transparent pixels, which are marked by a zero in 
   256-color modes or bright pink for truecolor data (maximum red and blue, 
   zero green), and requires the source and destination bitmaps to be of 
   the same color depth. The source and destination regions must not overlap.
   Example:

      BITMAP *hud_overlay;
      ...
      /* Paint hud overlay on the screen. */
      masked_blit(hud_overlay, screen, 0, 0, 0, 0,
                  hud_overlay->w, hud_overlay->h);

   If the GFX_HW_VRAM_BLIT_MASKED bit in the gfx_capabilities flag is set, 
   the current driver supports hardware accelerated masked blits from one 
   part of the screen onto another. This is extremely fast, so when this 
   flag is set it may be worth storing some of your more frequently used 
   sprites in an offscreen portion of the video memory.

   Warning: if the hardware acceleration flag is not set, masked_blit() will 
   not work correctly when used with a source image in system or video 
   memory so the latter must be a memory bitmap.

void masked_stretch_blit(BITMAP *source, BITMAP *dest,
                         int source_x, source_y, source_w, source_h,
                         int dest_x, dest_y, dest_w, dest_h);
   Like masked_blit(), except it can scale images (so the source and 
   destination rectangles don't need to be the same size). This routine 
   doesn't do as much safety checking as the regular masked_blit(): in 
   particular you must take care not to copy from areas outside the source 
   bitmap. Moreover, the source must be a memory bitmap. Example:

      BITMAP *hud_overlay;
      ...
      /* Stretch hud overlay over the screen. */
      masked_stretch_blit(hud_overlay, screen, 0, 0,
                          hud_overlay->w, hud_overlay->h,
                          0, 0, SCREEN_W, SCREEN_H);

void draw_sprite_ex(BITMAP *bmp, BITMAP *sprite, int x, int y,
                    int mode, int flip);
   Draws the sprite image onto the destination bitmap using the specified
   mode argument, optionally flipping the sprite in the orientation specified
   by flip argument.
   The mode argument defines how is sprite going to be drawn on the
   destination bitmap:

   DRAW_SPRITE_NORMAL     - draws a masked sprite, like draw_sprite()
   DRAW_SPRITE_LIT        - draws a tinted sprite, like draw_lit_sprite()
   DRAW_SPRITE_TRANS      - draws a blended sprite, like draw_trans_sprite()


   The flip argument defines the flipping orientation:

   DRAW_SPRITE_NO_FLIP = 0    - do not perform flipping
   DRAW_SPRITE_H_FLIP         - flip horizontally
   DRAW_SPRITE_V_FLIP         - flip vertically
   DRAW_SPRITE_VH_FLIP        - flip both vertically and horizontally


void draw_sprite(BITMAP *bmp, BITMAP *sprite, int x, int y);
   Draws a copy of the sprite bitmap onto the destination bitmap at the 
   specified position. This is almost the same as blit(sprite, bmp, 0, 0, x, 
   y, sprite->w, sprite->h), but it uses a masked drawing mode where 
   transparent pixels are skipped, so the background image will show through 
   the masked parts of the sprite. Transparent pixels are marked by a zero 
   in 256-color modes or bright pink for truecolor data (maximum red and 
   blue, zero green). Example:

      BITMAP *spaceship;
      ...
      draw_sprite(screen, spaceship, x, y);

   If the GFX_HW_VRAM_BLIT_MASKED bit in the gfx_capabilities flag is set, 
   the current driver supports hardware accelerated sprite drawing when the 
   source image is a video memory bitmap or a sub-bitmap of the screen. This 
   is extremely fast, so when this flag is set it may be worth storing some 
   of your more frequently used sprites in an offscreen portion of the video 
   memory.

   Warning: if the hardware acceleration flag is not set, draw_sprite() will 
   not work correctly when used with a sprite image in system or video 
   memory so the latter must be a memory bitmap.

   Although generally not supporting graphics of mixed color depths, as a 
   special case this function can be used to draw 256-color source images 
   onto truecolor destination bitmaps, so you can use palette effects on 
   specific sprites within a truecolor program.

void stretch_sprite(BITMAP *bmp, BITMAP *sprite, int x, int y, int w, int h);
   Like draw_sprite(), except it can stretch the sprite image to the 
   specified width and height and requires the sprite image and destination 
   bitmap to be of the same color depth. Moreover, the sprite image must 
   be a memory bitmap. Example:

      /* Create tunnel like effect. */
      for (step = 1; step 
         int width = SCREEN_W / step;
         int height = SCREEN_H / step;
         stretch_sprite(screen, image, SCREEN_W / 2 - width / 2,
                        SCREEN_H / 2 - height / 2, width, height);
      }

void draw_sprite_v_flip(BITMAP *bmp, BITMAP *sprite, int x, int y);
void draw_sprite_h_flip(BITMAP *bmp, BITMAP *sprite, int x, int y);
void draw_sprite_vh_flip(BITMAP *bmp, BITMAP *sprite, int x, int y);
   These are like draw_sprite(), but they additionally flip the image
   vertically, horizontally, or both, respectively. Flipping vertically
   means that the y-axis is reversed, while flipping horizontally means
   that the x-axis is reversed, between the source and the destination.
   This produces exact mirror images, which is not the same as rotating the
   sprite (and it is a lot faster than the rotation routine). The sprite
   must be a memory bitmap. Example:

      if (key[KEY_RIGHT])
         draw_sprite(screen, hero_right, pos_x, pos_y);
      else if (key[KEY_LEFT])
         draw_sprite_h_flip(screen, hero_right, pos_x, pos_y);
      else
         draw_sprite(screen, hero_idle, pos_x, pos_y);

void draw_trans_sprite(BITMAP *bmp, BITMAP *sprite, int x, int y);
   Uses the global color_map table or truecolor blender functions to overlay
   the sprite on top of the existing image. This must only be used after you 
   have set up the color mapping table (for 256-color modes) or blender 
   functions (for truecolor modes). Because it involves reading as well as 
   writing the bitmap memory, translucent drawing is very slow if you draw 
   directly to video RAM, so wherever possible you should use a memory 
   bitmap instead. Example:

      /* Some one time initialisation code. */
      COLOR_MAP global_trans_table;
      create_trans_table(&global_trans_table, my_palette,
                         128, 128, 128, NULL);
      ...
      if (get_color_depth() == 8)
         color_map = &global_trans_table;
      else
         set_trans_blender(128, 128, 128, 128);

      draw_trans_sprite(buffer, ghost_sprite, x, y);

   The bitmap and sprite must normally be in the same color depth, but as a
   special case you can draw 32 bit RGBA format sprites onto any hicolor or
   truecolor bitmap, as long as you call set_alpha_blender() first, and you
   can draw 8-bit alpha images onto a 32-bit RGBA destination, as long as you
   call set_write_alpha_blender() first. As draw_sprite() this function skips
   transparent pixels, except if the source sprite is an 8-bit image; if this
   is the case, you should pay attention to properly set up your color map
   table for index 0.

void draw_lit_sprite(BITMAP *bmp, BITMAP *sprite, int x, int y, int color);
   In 256-color modes, uses the global color_map table to tint the sprite
   image to the specified color or to light it to the level specified by
   'color', depending on the function which was used to build the table
   (create_trans_table or create_light_table), and draws the resulting image
   to the destination bitmap. In truecolor modes, uses the blender functions
   to light the sprite image using the alpha level specified by 'color' (the
   alpha level which was passed to the blender functions is ignored) and
   draws the resulting image to the destination bitmap. The 'color' parameter
   must be in the range [0-255] whatever its actual meaning is. This must
   only be used after you have set up the color mapping table (for 256-color
   modes) or blender functions (for truecolor modes). Example:

      /* Some one time initialisation code. */
      COLOR_MAP global_light_table;
      create_light_table(&global_trans_table, my_palette,
                         10, 10, 60, NULL);
      ...
      if (get_color_depth() == 8)
         color_map = &global_light_table;
      else
         set_trans_blender(40, 40, 255, 255);

      /* Lit the cape with a blueish light. */
      draw_lit_sprite(buffer, colored_cape, x, y, 64);

void draw_gouraud_sprite(BITMAP *bmp, BITMAP *sprite, int x, int y,
                         int c1, int c2, int c3, int c4);
   More sophisticated version of draw_lit_sprite(): the 'color' parameter is
   not constant across the sprite image anymore but interpolated between the
   four specified corner colors. The corner values passed to this function
   indicate the strength of the color applied on them, ranging from 0 (no
   strength) to 255 (full strength). Example:

      /* Some one time initialisation code. */
      COLOR_MAP global_light_table;
      create_light_table(&global_trans_table, my_palette,
                         0, 0, 0, NULL);
      ...
      if (get_color_depth() == 8)
         color_map = &global_light_table;
      else
         set_trans_blender(0, 0, 0, 128);

      /* Enemies are in shadow unless lit by torch. */
      draw_gouraud_sprite(buffer, menacing_spy, x, y,
                          light_strength_on_corner_1,
                          light_strength_on_corner_2,
                          light_strength_on_corner_3,
                          light_strength_on_corner_4);

void draw_character_ex(BITMAP *bmp, BITMAP *sprite, int x, int y,
                       color, bg);
   Draws a copy of the sprite bitmap onto the destination bitmap at the 
   specified position, drawing transparent pixels in the background color
   (or skipping them if the background color is -1) and setting all other
   pixels to the specified color. Transparent pixels are marked by a zero
   in 256-color modes or bright pink for truecolor data (maximum red and
   blue, zero green). The sprite must be an 8-bit image, even if the
   destination is a truecolor bitmap. Example:

      BITMAP *logo;
      ...
      /* Draw the logo silhouette in red. */
      draw_character_ex(screen, logo, SCREEN_W / 2, SCREEN_H / 2,
                        makecol(255, 0, 0), -1);

void rotate_sprite(BITMAP *bmp, BITMAP *sprite, int x, int y, fixed angle);
   Draws the sprite image onto the bitmap. It is placed with its top left
   corner at the specified position, then rotated by the specified angle
   around its centre. The angle is a fixed point 16.16 number in the same
   format used by the fixed point trig routines, with 256 equal to a full
   circle, 64 a right angle, etc. All rotation functions can draw between any
   two bitmaps, even screen bitmaps or bitmaps of different color depth.

   Positive increments of the angle will make the sprite rotate clockwise
   on the screen, as demonstrated by the Allegro example.

void rotate_sprite_v_flip(BITMAP *bmp, BITMAP *sprite, int x, int y, fixed angle);
   Like rotate_sprite, but flips the image vertically before rotating it. To
   flip horizontally, use this routine but add itofix(128) to the angle. To
   flip in both directions, use rotate_sprite() and add itofix(128) to its
   angle.

void rotate_scaled_sprite(BITMAP *bmp, BITMAP *sprite, int x, int y,
                          fixed angle, fixed scale);
   Like rotate_sprite(), but stretches or shrinks the image at the same time 
   as rotating it.

void rotate_scaled_sprite_v_flip(BITMAP *bmp, BITMAP *sprite, int x, int y,
                                 fixed angle, fixed scale);
   Draws the sprite, similar to rotate_scaled_sprite() except that it flips
   the sprite vertically first.

void pivot_sprite(BITMAP *bmp, BITMAP *sprite, int x, int y,
                  int cx, int cy, fixed angle);
   Like rotate_sprite(), but aligns the point in the sprite given by (cx, cy)
   to (x, y) in the bitmap, then rotates around this point.

void pivot_sprite_v_flip(BITMAP *bmp, BITMAP *sprite, int x, int y,
                         int cx, int cy, fixed angle);
   Like rotate_sprite_v_flip(), but aligns the point in the sprite given by
   (cx, cy) to (x, y) in the bitmap, then rotates around this point.

void pivot_scaled_sprite(BITMAP *bmp, BITMAP *sprite, int x, int y,
                         int cx, int cy, fixed angle, fixed scale);
   Like rotate_scaled_sprite(), but aligns the point in the sprite given by
   (cx, cy) to (x, y) in the bitmap, then rotates and scales around this
   point.

void pivot_scaled_sprite_v_flip(BITMAP *bmp, BITMAP *sprite, int x, int y,
                                int cx, int cy, fixed angle, fixed scale);
   Like rotate_scaled_sprite_v_flip(), but aligns the point in the sprite
   given by (cx, cy) to (x, y) in the bitmap, then rotates and scales around
   this point.

void rotate_sprite_trans(BITMAP *bmp, BITMAP *sprite,
                          int x, int y, fixed angle);
   Draws the sprite image onto the bitmap. It is placed with its top left
   corner at the specified position, then rotated by the specified angle
   around its centre. The angle is a fixed point 16.16 number in the same
   format used by the fixed point trig routines, with 256 equal to a full
   circle, 64 a right angle, etc. All rotation functions can draw between any
   two bitmaps, even screen bitmaps or bitmaps of different color depth.

   Positive increments of the angle will make the sprite rotate clockwise
   on the screen, as demonstrated by the Allegro example.

void rotate_sprite_v_flip_trans(BITMAP *bmp, BITMAP *sprite, int x, int y, fixed angle);
   Like rotate_sprite_trans, but flips the image vertically before rotating it.
   To flip horizontally, use this routine but add itofix(128) to the angle. To
   flip in both directions, use rotate_sprite() and add itofix(128) to its
   angle.

void rotate_scaled_sprite_trans(BITMAP *bmp, BITMAP *sprite, int x, int y,
                          fixed angle, fixed scale);
   Like rotate_sprite_trans(), but stretches or shrinks the image at the same
   time as rotating it.

void rotate_scaled_sprite_v_flip_trans(BITMAP *bmp, BITMAP *sprite,
                                 int x, int y,
                                 fixed angle, fixed scale);
   Draws the sprite, similar to rotate_scaled_sprite_trans() except that it
   flips the sprite vertically first.

void pivot_sprite_trans(BITMAP *bmp, BITMAP *sprite, int x, int y,
                  int cx, int cy, fixed angle);
   Like rotate_sprite_trans(), but aligns the point in the sprite given by
   (cx, cy) to (x, y) in the bitmap, then rotates around this point.

void pivot_sprite_v_flip_trans(BITMAP *bmp, BITMAP *sprite, int x, int y,
                         int cx, int cy, fixed angle);
   Like rotate_sprite_v_flip_trans(), but aligns the point in the sprite given
   by (cx, cy) to (x, y) in the bitmap, then rotates around this point.

void pivot_scaled_sprite_trans(BITMAP *bmp, BITMAP *sprite, int x, int y,
                         int cx, int cy, fixed angle, fixed scale);
   Like rotate_scaled_sprite_trans(), but aligns the point in the sprite given
   by (cx, cy) to (x, y) in the bitmap, then rotates and scales around this
   point.

void pivot_scaled_sprite_v_flip_trans(BITMAP *bmp, BITMAP *sprite,
                                int x, int y,
                                int cx, int cy, fixed angle, fixed scale);
   Like rotate_scaled_sprite_v_flip_trans(), but aligns the point in the sprite
   given by (cx, cy) to (x, y) in the bitmap, then rotates and scales around
   this point.

void rotate_sprite_lit(BITMAP *bmp, BITMAP *sprite,
                          int x, int y, fixed angle);
   Draws the sprite image onto the bitmap. It is placed with its top left
   corner at the specified position, then rotated by the specified angle
   around its centre. The angle is a fixed point 16.16 number in the same
   format used by the fixed point trig routines, with 256 equal to a full
   circle, 64 a right angle, etc. All rotation functions can draw between any
   two bitmaps, even screen bitmaps or bitmaps of different color depth.

   Positive increments of the angle will make the sprite rotate clockwise
   on the screen, as demonstrated by the Allegro example.

void rotate_sprite_v_flip_lit(BITMAP *bmp, BITMAP *sprite, int x, int y, fixed angle);
   Like rotate_sprite_lit, but flips the image vertically before rotating it.
   To flip horizontally, use this routine but add itofix(128) to the angle. To
   flip in both directions, use rotate_sprite() and add itofix(128) to its
   angle.

void rotate_scaled_sprite_lit(BITMAP *bmp, BITMAP *sprite, int x, int y,
                          fixed angle, fixed scale);
   Like rotate_sprite_lit(), but stretches or shrinks the image at the same
   time as rotating it.

void rotate_scaled_sprite_v_flip_lit(BITMAP *bmp, BITMAP *sprite,
                                 int x, int y,
                                 fixed angle, fixed scale);
   Draws the sprite, similar to rotate_scaled_sprite_lit() except that it
   flips the sprite vertically first.

void pivot_sprite_lit(BITMAP *bmp, BITMAP *sprite, int x, int y,
                  int cx, int cy, fixed angle);
   Like rotate_sprite_lit(), but aligns the point in the sprite given by
   (cx, cy) to (x, y) in the bitmap, then rotates around this point.

void pivot_sprite_v_flip_lit(BITMAP *bmp, BITMAP *sprite, int x, int y,
                         int cx, int cy, fixed angle);
   Like rotate_sprite_v_flip_lit(), but aligns the point in the sprite given
   by (cx, cy) to (x, y) in the bitmap, then rotates around this point.

void pivot_scaled_sprite_lit(BITMAP *bmp, BITMAP *sprite, int x, int y,
                         int cx, int cy, fixed angle, fixed scale);
   Like rotate_scaled_sprite_lit(), but aligns the point in the sprite given
   by (cx, cy) to (x, y) in the bitmap, then rotates and scales around this
   point.

void pivot_scaled_sprite_v_flip_lit(BITMAP *bmp, BITMAP *sprite,
                                int x, int y,
                                int cx, int cy, fixed angle, fixed scale);
   Like rotate_scaled_sprite_v_flip_lit(), but aligns the point in the sprite
   given by (cx, cy) to (x, y) in the bitmap, then rotates and scales around
   this point.



=====================================
============ RLE sprites ============
=====================================

Because bitmaps can be used in so many different ways, the bitmap structure 
is quite complicated, and it contains a lot of data. In many situations, 
though, you will find yourself storing images that are only ever copied to 
the screen, rather than being drawn onto or used as filling patterns, etc. 
If this is the case you may be better off storing your images in RLE_SPRITE 
(read chapter "Structures and types defined by Allegro" for an internal
description of the RLE_SPRITE structure) or COMPILED_SPRITE (see next
chapter) structures rather than regular bitmaps.

RLE sprites store the image in a simple run-length encoded format, where 
repeated zero pixels are replaced by a single length count, and strings of 
non-zero pixels are preceded by a counter giving the length of the solid 
run. RLE sprites are usually much smaller than normal bitmaps, both because
of the run length compression, and because they avoid most of the overhead 
of the bitmap structure. They are often also faster than normal bitmaps, 
because rather than having to compare every single pixel with zero to 
determine whether it should be drawn, it is possible to skip over a whole 
run of zeros with a single add, or to copy a long run of non-zero pixels 
with fast string instructions.

Every silver lining has a cloud, though, and in the case of RLE sprites it 
is a lack of flexibility. You can't draw onto them, and you can't flip them, 
rotate them, or stretch them. In fact the only thing you can do with them is 
to blast them onto a bitmap with the draw_rle_sprite() function, which is 
equivalent to using draw_sprite() with a regular bitmap. You can convert 
bitmaps into RLE sprites at runtime, or you can create RLE sprite structures 
in grabber datafiles by making a new object of type 'RLE sprite'.

RLE_SPRITE *get_rle_sprite(BITMAP *bitmap);
   Creates an RLE sprite based on the specified bitmap (which must be a 
   memory bitmap). Remember to free this RLE sprite later to avoid memory
   leaks. Example:

      RLE_SPRITE *rle;
      BITMAP *bmp;
      ...
      /* Create RLE sprite from an existent bitmap. */
      rle = get_rle_sprite(bmp);
      if (!rle)
         abort_on_error("Couldn't create RLE sprite!");
         
      /* We don't need the bitmap any more.*/
      destroy_bitmap(bmp);
      
      /* Use the RLE sprite. */
      ...
      /* Destroy it when we don't need it any more. */
      destroy_rle_sprite(rle);

   Returns a pointer to the created RLE sprite, or NULL if the RLE sprite
   could not be created. Remember to free this RLE sprite later to avoid
   memory leaks.

void destroy_rle_sprite(RLE_SPRITE *sprite);
   Destroys an RLE sprite structure previously returned by get_rle_sprite().
   If you pass a NULL pointer this function won't do anything. Use this once
   you are done with an RLE sprite to avoid memory leaks in your program.

void draw_rle_sprite(BITMAP *bmp, const RLE_SPRITE *sprite,
                     int x, int y);
   Draws an RLE sprite onto a bitmap at the specified position. Example:

      RLE_SPRITE *rle_sprite;
      ...
      draw_rle_sprite(screen, rle_sprite, 100, 100);

void draw_trans_rle_sprite(BITMAP *bmp, const RLE_SPRITE *sprite,
                           int x, int y);
   Translucent version of draw_rle_sprite(). See the description of 
   draw_trans_sprite(). This must only be used after you have set up the 
   color mapping table (for 256-color modes) or blender functions (for 
   truecolor modes). The bitmap and sprite must normally be in the same 
   color depth, but as a special case you can draw 32-bit RGBA format 
   sprites onto any hicolor or truecolor bitmap, as long as you call 
   set_alpha_blender() first. Example:

      /* Some one time initialisation code. */
      COLOR_MAP global_trans_table;
      create_trans_table(&global_trans_table, my_palette,
                         128, 128, 128, NULL);
      ...
      if (get_color_depth() == 8)
         color_map = &global_trans_table;
      else
         set_trans_blender(128, 128, 128, 128);

      draw_trans_rle_sprite(buffer, rle_ghost_sprite, x, y);

void draw_lit_rle_sprite(BITMAP *bmp, const RLE_SPRITE *sprite,
                         int x, y, color);
   Tinted version of draw_rle_sprite(). See the description of
   draw_lit_sprite(). This must only be used after you have set up the color 
   mapping table (for 256-color modes) or blender functions (for truecolor 
   modes). Example:

      /* Some one time initialisation code. */
      COLOR_MAP global_light_table;
      create_light_table(&global_trans_table, my_palette,
                         10, 10, 60, NULL);
      ...
      if (get_color_depth() == 8)
         color_map = &global_light_table;
      else
         set_trans_blender(40, 40, 255, 255);

      /* Lit the cape with a blueish light. */
      draw_lit_rle_sprite(buffer, rle_colored_cape, x, y, 64);



==========================================
============ Compiled sprites ============
==========================================

Compiled sprites are stored as actual machine code instructions that draw a 
specific image onto a bitmap, using mov instructions with immediate data 
values. This is the fastest way to draw a masked image: on slow machines,
up to and including a 486, drawing compiled sprites can be about to five
times as fast as using draw_sprite() with a regular bitmap. On newer
machines the difference is usually negligible.

Compiled sprites are big, so if memory is tight you should use RLE sprites
instead, and what you can do with them is even more restricted than with RLE
sprites, because they don't support clipping. If you try to draw one off the
edge of a bitmap, you will corrupt memory and probably crash the system. You
can convert bitmaps into compiled sprites at runtime, or you can create
compiled sprite structures in grabber datafiles by making a new object of
type 'Compiled sprite' or 'Compiled x-sprite'.

COMPILED_SPRITE *get_compiled_sprite(BITMAP *bitmap, int planar);
   Creates a compiled sprite based on the specified bitmap (which must be a 
   memory bitmap). Compiled sprites are device-dependent, so you have to 
   specify whether to compile it into a linear or planar format. Pass FALSE
   as the second parameter if you are going to be drawing it onto memory 
   bitmaps or mode 13h and SVGA screen bitmaps, and pass TRUE if you are 
   going to draw it onto mode-X or Xtended mode screen bitmaps. Example:

      COMPILED_SPRITE *cspr;
      BITMAP *bmp;
      ...
      /* Create compiled sprite from an existent bitmap. */
      cspr = get_compiled_sprite(bmp, 0);
      if (!cspr)
         abort_on_error("Couldn't create compiled sprite!");
      
      /* We don't need the bitmap any more.*/
      destroy_bitmap(bmp);
      
      /* Use the compiled sprite. */
      ...
      /* Destroy it when we don't need it any more. */
      destroy_compiled_sprite(cspr);

   Returns a pointer to the created compiled sprite, or NULL if the compiled
   sprite could not be created. Remember to free this compiled sprite later
   to avoid memory leaks.

void destroy_compiled_sprite(COMPILED_SPRITE *sprite);
   Destroys a compiled sprite structure previously returned by 
   get_compiled_sprite(). If you pass a NULL pointer this function won't do
   anything. Use this once you are done with a compiled sprite to avoid
   memory leaks in your program.

void draw_compiled_sprite(BITMAP *bmp, const COMPILED_SPRITE *sprite,
                          int x, int y);
   Draws a compiled sprite onto a bitmap at the specified position. The 
   sprite must have been compiled for the correct type of bitmap (linear or 
   planar). This function does not support clipping.

   Hint: if not being able to clip compiled sprites is a problem, a neat 
   trick is to set up a work surface (memory bitmap, mode-X virtual screen, 
   or whatever) a bit bigger than you really need, and use the middle of it 
   as your screen. That way you can draw slightly off the edge without any 
   trouble... 



===============================
============ Fonts ============
===============================

Allegro provides routines for loading fonts directly from GRX format .fnt
files, 8x8 or 8x16 BIOS format .fnt files, from bitmap images, from datafiles
or you can import a multiple-range Unicode font by writing a .txt script that
specifies a number of different source files for each range of characters.

By default, Allegro can only use bitmapped (non-scalable) fonts. If you want
to use TrueType fonts, you will need to use an add-on library which allows
you to load them on the fly (like AllegTTF or Glyph Keeper, listed among
others at http://www.allegro.cc/) and render them directly, or generate a
bitmapped version of a TrueType font with tools like TTF2PCX
(http://www.talula.demon.co.uk/ttf2pcx/index.html).

void register_font_file_type(const char *ext,
          FONT *(*load)(const char *filename, RGB *pal, void *param));
   Informs the load_font() functions of a new file type, providing a routine 
   to read fonts in this format. The function you supply must follow the 
   following prototype:

      FONT *load_my_font(const char *filename, RGB *pal, void *param)
      {
         ...
      }

   The pal parameter can optionally be used to return a palette for the FONT.
   The parameter param can be anything you like: you can use this to pass
   information to your loading routine, such as for instance the font height,
   the character range to load or the index number of a font in a datafile.
   If you choose to write your own font loading code, your function should be
   prepared to deal with a value of NULL for either of these parameters.

FONT *load_font(const char *filename, RGB *pal, void *param);
   Loads a font from a file. At present, this supports loading fonts from
   a GRX format .fnt file, a 8x8 or 8x16 BIOS format .fnt file, a datafile or
   any bitmap format that can be loaded by load_bitmap().
   
   If the font contains palette information, then the palette is returned in
   the second parameter, which should be an array of 256 RGB structures 
   (a PALETTE). The pal argument may be NULL. In this case, the palette data,
   if present, is simply not returned.
   
   The third parameter can be used to pass specific information to a custom
   loader routine. Normally, you can just leave this as NULL. Note that
   another way of loading fonts is embedding them into a datafile and using
   the datafile related functions.

   Example:

      FONT *myfont;
      PALETTE palette;
      ...
      myfont = load_font("my_font.pcx", palette, NULL);
      if (!myfont)
         abort_on_error("Couldn't load font!");
      ...
      textout_centre_ex(screen, myfont, "This is my own pretty font!",
                        SCREEN_W / 2, SCREEN_H / 2, white, black);
      ...
      destroy_font(myfont);

   Returns a pointer to the font or NULL on error. Remember that you are
   responsible for destroying the font when you are finished with it to
   avoid memory leaks.

void destroy_font(FONT *f);
   Frees the memory being used by a font structure. Don't use this on the
   default global Allegro font or any text routines using it could crash. You
   should use this only on fonts you have loaded manually after you are done
   with them, to prevent memory leaks in your program.

void make_trans_font(FONT *f);
   This function converts a font to use transparency for drawing. That is, each
   glyph in the font will be drawn with draw_trans_sprite, so you can use the
   same blenders as with draw_trans_sprite to draw the font. One common use of
   this is to load a bitmap font with an alpha channel, and therefore get
   anti-aliased text output by using Allegro's alpha blender. Here's an example
   how to do that:

   
   FONT *f = load_font("alphafont.tga", NULL, NULL);
   make_trans_font(f);
   set_alpha_blender();
   textprintf_centre_ex(screen, f, 320, 240, -1, -1, "Anti-aliased Font!");

int is_trans_font(FONT *f)
   This function checks if the given font is a color font using
   draw_trans_sprite to render glyphs.

   Returns TRUE if the font uses transparency, FALSE if it does not.

int is_color_font(FONT *f)
   This function checks if the given font is a color font, as opposed to a
   monochrome font.

   Returns TRUE if the font is a color font, FALSE if it is not.

int is_mono_font(FONT *f)
   This function checks if the given font is a mono font, as opposed to a
   color font.

   Returns TRUE if the font is a monochrome font, FALSE if it is not.

int font_has_alpha(FONT *f)
   This function goes through all pixels of all glyphs in the font and looks for
   alpha values.

   Returns TRUE if any alpha pixels are found, else FALSE.

int *is_compatible_font(FONT *f1, FONT *f2)
   This function compares the two fonts, which you can use to find out if
   Allegro is capable of merging them.

   Returns TRUE if the two fonts are of the same general type (both are color
   fonts or both are monochrome fonts, for instance).

int get_font_ranges(FONT *f)
   Use this function to find out the number of character ranges in a font. You
   should query each of these ranges with get_font_range_begin() and
   get_font_range_end() to find out what characters are available in the
   font. Example:

      FONT *f;
      int range;
      int n;
      ...
      
      range = get_font_ranges(f);
      printf("The font has %d character ranges:\n", range);
      for (n = 0; n < range; n++)
         printf("Range %d from 0x%03x - 0x%03x\n",
                get_font_range_begin(f, n),
                get_font_range_end(f, n));

   Returns the number of continuous character ranges in a font, or -1 if that
   information is not available.

int get_font_range_begin(FONT *f, int range)
   This function allows you to find out the start of a specific character
   range for a font. You can pass -1 for the `range' parameter if you want to
   know the start of the whole font range, or a number from 0 to (but not
   including) get_font_ranges(f) to get the start of a specific character
   range in the font. Example:

      printf("The font has a character range of %d - %d\n",
             get_font_range_begin(font, -1),
             get_font_range_end(font, -1));

   Returns the first character in the font range, or -1 if that information
   is not available.

int get_font_range_end(FONT *f, int range)
   This function allows you to find out the index to the last character of a
   character range for a font. You can pass -1 for the range parameter if you
   want to know the start of the whole font range, or a number from 0 to (but
   not including) get_font_ranges(f) to get the start of a specific character
   range in the font. You should check the start and end of all font ranges to
   see if a specific character is actually available in the font. Not all
   characters in the range returned by get_font_range_begin(f, -1) and
   get_font_range_end(f, -1) need to be available! Example:

      printf("The font has a character range of %d - %d\n",
             get_font_range_begin(font, -1),
             get_font_range_end(font, -1));

   Returns the last character in the font range, or -1 if that information is
   not available.

FONT *extract_font_range(FONT *f, int begin, int end)
   This function extracts a character range from a font and returns a new font
   that contains only the range of characters selected by this function. You
   can pass -1 for either the lower or upper bound if you want to select all
   characters from the start or to the end of the font.
   Example:

      FONT *myfont;
      FONT *capitals;
      FONT *fontcopy;
      ...
      /* Create a font of only capital letters */
      capitals = extract_font_range(myfont, 'A', 'Z');

      /* Create a copy of the font */
      fontcopy = extract_font_range(myfont, -1, -1);
      ...
      destroy_font(capitals);
      destroy_font(fontcopy);

   Returns a pointer to the new font or NULL on error. Remember that you are
   responsible for destroying the font when you are finished with it to
   avoid memory leaks.

int transpose_font(FONT *f, int drange)
   This function transposes all characters in a font, effectively remapping the
   font. Example:

      FONT *myfont;
      FONT *capitals;
      ...
      /* Create a font of only capital letters */
      capitals = extract_font_range(myfont, 'A', 'Z');

      /* Now transpose the characters in the font so that they will be used */
      /*  for the lower case letters a-z */
      transpose_font(capitals, 'a'-'A');
      textout_ex(screen, capitals, "allcaps",
                 100, 100, makecol(255,255,255), 0);

   Returns 0 on success, -1 on failure.

FONT *merge_fonts(FONT *f1, FONT *f2)
   This function merges the character ranges from two fonts and returns a new
   font containing all characters in the old fonts. In general, you cannot
   merge fonts of different types (eg, TrueType fonts and bitmapped fonts),
   but as a special case, this function can promote a monochrome bitmapped
   font to a color font and merge those. Example:

      FONT *myfont;
      FONT *myfancy_font;
      FONT *lower_range;
      FONT *upper_range;
      FONT *capitals;
      FONT *combined_font;
      FONT *tempfont;
      ...
      /* Create a font that contains the capitals from  */
      /* the fancy font but other characters from myfont */
      lower_range = extract_font_range(myfont, -1, 'A'-1);
      upper_range = extract_font_range(myfont, 'Z'+1, -1);
      capitals = extract_font_range(myfancy_font, 'A', 'Z');

      tempfont = merge_fonts(lower_range, capitals);
      combined_font = merge_fonts(tempfont, upper_range);

      /* Clean up temporary fonts */
      destroy_font(lower_range);
      destroy_font(upper_range);
      destroy_font(capitals);
      destroy_font(tempfont);

   Returns a pointer to the new font or NULL on error. Remember that you are
   responsible for destroying the font when you are finished with it to
   avoid memory leaks.

FONT *load_dat_font(const char *filename, RGB *pal, void *param)
   Loads a FONT from an Allegro datafile. You can set param parameter to
   point to an array that holds two strings that identify the font and the 
   palette in the datafile by name.
   The first string in this list is the name of the font. You can pass NULL
   here to just load the first font found in the datafile. The second string
   can be used to specify the name of the palette associated with the font.
   This is only returned if the pal parameter is not NULL. If you pass NULL
   for the name of the palette, the last palette found before the font was
   found is returned.
   You can also pass NULL for param, which is treated as if you had passed
   NULL for both strings separately. In this case, the function will simply
   load the first font it finds from the datafile and the palette that
   precedes it.

   For example, suppose you have a datafile named `fonts.dat' with the
   following contents:

      FONT  FONT_1_DATA
      FONT  FONT_2_DATA
      FONT  FONT_3_DATA
      PAL   FONT_1_PALETTE
      PAL   FONT_2_PALETTE

   Then the following code will load FONT_1_DATA as a FONT and return
   FONT_1_PALETTE as the palette:

      FONT *f;
      PALETTE pal;
      char *names[] = { "FONT_1_DATA", "FONT_1_PALETTE" }
      
      f = load_dat_font("fonts.dat", pal, names);

   If instead you want to load the second font, FONT_2, from the datafile,
   you would use:

      FONT *f;
      PALETTE pal;
      char *names[] = { "FONT_2_DATA", "FONT_2_PALETTE" }
      
      f = load_dat_font("fonts.dat", pal, names);

   If you want to load the third font, but not bother with a palette, use:

      FONT *f;
      char *names[] = { "FONT_3_DATA", NULL }
      
      f = load_dat_font("fonts.dat", NULL, names);

   Returns a pointer to the font or NULL on error. Remember that you are
   responsible for destroying the font when you are finished with it to
   avoid memory leaks.

FONT *load_bios_font(const char *filename, RGB *pal, void *param)
   Loads a 8x8 or 8x16 BIOS format font. You shouldn't normally call this 
   routine directly.

   Returns a pointer to the font or NULL on error. Remember that you are
   responsible for destroying the font when you are finished with it to
   avoid memory leaks.

FONT *load_grx_font(const char *filename, RGB *pal, void *param)
   Loads a GRX format font. You shouldn't normally call this routine
   directly.

   Returns a pointer to the font or NULL on error. Remember that you are
   responsible for destroying the font when you are finished with it to
   avoid memory leaks.

FONT *load_grx_or_bios_font(const char *filename, RGB *pal, void *param)
   Loads either a BIOS or GRX format font. You shouldn't normally call this 
   routine directly.

   Returns a pointer to the font or NULL on error. Remember that you are
   responsible for destroying the font when you are finished with it to
   avoid memory leaks.

FONT *load_bitmap_font(const char *filename, RGB *pal, void *param)
   Tries to grab a font from a bitmap. The bitmap can be in any format that
   load_bitmap understands.

   The size of each character is determined by the layout of the image, which
   should be a rectangular grid containing all the ASCII characters from
   space (32) up to the tilde (126). The way the characters are separated
   depends on the color depth of the image file:

   paletted (8 bit) image file
      Use color 0 for the transparent portions of the characters and fill the 
      spaces between each letter with color 255.  

   High (15/16 bit) and true (24/32 bit) color image file
      Use bright pink (maximum red and blue, zero green) for the transparent
      portions of the characters and fill the spaces between each letter with
      bright yellow (maximum red and green, zero blue).

   
   Note that in each horizontal row the bounding boxes around the characters 
   should align and have the same height.
   
   Probably the easiest way to get to grips with how this works is to load up 
   the `demo.dat' file and export the TITLE_FONT into a PCX file. Have a look
   at the resulting picture in your paint program: that is the format a font
   should be in.

   Take care with high and true color fonts: Allegro will convert these to the
   current color depth when you load the font. If you try to use a font on
   a bitmap with a different color depth Allegro will do color conversions on
   the fly, which will be rather slow. For optimal performance you should
   set the color depth to the color depth you want to use before loading any
   fonts.

   Returns a pointer to the font or NULL on error. Remember that you are
   responsible for destroying the font when you are finished with it to
   avoid memory leaks.

FONT *grab_font_from_bitmap(BITMAP *bmp)
   This function is the work-horse of load_bitmap_font, and can be used to
   grab a font from a bitmap in memory. You can use this if you want to
   generate or modify a font at runtime. The bitmap should follow the layout
   described for load_bitmap_font.

   Returns a pointer to the font or NULL on error. Remember that you are
   responsible for destroying the font when you are finished with it to
   avoid memory leaks.

FONT *load_txt_font(const char *filename, RGB *pal, void *param)
   This function can be used to load scripted fonts. The script file
   contains a number of lines in the format "filename start end", which
   specify the source file for that range of characters, the Unicode value of
   the first character in the range, and the end character in the range
   (optional, if left out, the entire input file will be grabbed). If the
   filename is replaced by a hyphen, more characters will be grabbed from the
   previous input file.
   For example, the script:
   
      ascii.fnt 0x20 0x7F
      - 0xA0 0xFF
      dingbats.fnt 0x1000
   
   would import the first 96 characters from ascii.fnt as the range
   0x20-0x7F, the next 96 characters from ascii.fnt as the range 0xA0-0xFF,
   and the entire contents of dingbats.fnt starting at Unicode position
   0x1000.

   Returns a pointer to the font or NULL on error. Remember that you are
   responsible for destroying the font when you are finished with it to
   avoid memory leaks.



=====================================
============ Text output ============
=====================================

Allegro provides text output routines that work with both monochrome and 
color fonts, which can contain any number of Unicode character ranges. The 
grabber program can create fonts from sets of characters drawn in a bitmap 
file (see grabber.txt for more information), and can also import GRX or BIOS 
format font files. The font structure contains a number of hooks that can be 
used to extend it with your own custom drawing code: see the definition in 
allegro/text.h for details.

extern FONT *font;
   A simple 8x8 fixed size font (the mode 13h BIOS default). If you want to 
   alter the font used by the GUI routines, change this to point to one of 
   your own fonts. This font contains the standard ASCII (U+20 to U+7F), 
   Latin-1 (U+A1 to U+FF), and Latin Extended-A (U+0100 to U+017F) character 
   ranges.

extern int allegro_404_char;
   When Allegro cannot find a glyph it needs in a font, it will instead
   output the character given in allegro_404_char. By default, this is set to
   the caret symbol, `^', but you can change this global to use any other
   character instead. Example:

      /* Show unknown glyphs with an asterisk. */
      allegro_404_char = '*';

int text_length(const FONT *f, const char *str);
   Returns the length (in pixels) of a string in the specified font. Example:

      int width = text_length(font, "I love spam");
      ...
      bmp = create_bitmap(width, height);

int text_height(const FONT *f)
   Returns the height (in pixels) of the specified font. Example:

      int height = text_height(font);
      ...
      bmp = create_bitmap(width, height);

void textout_ex(BITMAP *bmp, const FONT *f, const char *s,
                int x, int y, int color, int bg);
   Writes the string `s' onto the bitmap at position x, y, using the
   specified font, foreground color and background color. If the background
   color is -1, then the text is written transparently. If the foreground
   color is -1 and a color font is in use, it will be drawn using the colors
   from the original font bitmap (the one you imported into the grabber
   program), which allows multicolored text output. For high and true color
   fonts, the foreground color is ignored and always treated as -1. Example:

      /* Show the program's version in blue letters. */
      textout_ex(screen, font, "v4.2.0-beta2", 10, 10,
                 makecol(0, 0, 255), -1);

void textout_centre_ex(BITMAP *bmp, const FONT *f, const char *s,
                       int x, y, int color, int bg);
   Like textout_ex(), but interprets the x coordinate as the centre rather
   than the left edge of the string. Example:

      /* Important texts go in the middle. */
      width = text_length("GAME OVER");
      textout_centre_ex(screen, font, "GAME OVER",
                        SCREEN_W / 2, SCREEN_H / 2,
                        makecol(255, 0, 0), makecol(0, 0, 0));

void textout_right_ex(BITMAP *bmp, const FONT *f, const char *s,
                      int x, int y, int color, int bg);
   Like textout_ex(), but interprets the x coordinate as the right rather
   than the left edge of the string. Example:

      textout_right_ex(screen, font, "Look at this color!",
                       SCREEN_W - 10, 10, my_yellow, -1);

void textout_justify_ex(BITMAP *bmp, const FONT *f, const char *s,
                        int x1, int x2, int y, int diff, int color,
                        int bg);
   Draws justified text within the region x1-x2. If the amount of spare
   space is greater than the diff value, it will give up and draw regular
   left justified text instead. Example:

      char *lines[] = {"Draws justified text",
                       "within the specified",
                       "x2-x1 area. But not",
                       "T H I S !", NULL};
      /* Show the justification marker. */
      vline(screen, 200, 0, SCREEN_H-1, makecol(0, 0, 0));
      /* Draw all the lines until we reach a NULL entry. */
      for (num = 0, y = 0; lines[num]; num++, y += text_height(font))
         textout_justify_ex(screen, font, lines[num], 0, 200,
                            y, 80, makecol(0, 0, 0),
                            makecol(255, 255, 255));

void textprintf_ex(BITMAP *bmp, const FONT *f, int x, int y,
                   int color, int bg, const char *fmt, ...);
   Formatted text output, using a printf() style format string. Due to an
   internal limitation, this function can't be used for extremely long texts.
   If you happen to reach this limit, you can work around it by using
   uszprintf() and textout_ex(), which don't have any. Example:

      int player_score;
      ...
      textprintf_ex(screen, font, 10, 10, makecol(255, 100, 200),
                    -1, "Score: %d", player_score);

void textprintf_centre_ex(BITMAP *bmp, const FONT *f, int x, int y,
                          int color, int bg, const char *fmt, ...);
   Like textprintf_ex(), but interprets the x coordinate as the centre rather
   than the left edge of the string. This function shares the text length
   limitation of textprintf_ex(). Example:

      textprintf_centre_ex(screen, font, SCREEN_W / 2, 120,
                           makecol(0, 100, 243), -1,
                           "Your best score so far was %d!",
                           total_max_points);

void textprintf_right_ex(BITMAP *bmp, const FONT *f, int x, y, color, bg,
                         const char *fmt, ...);
   Like textprintf_ex(), but interprets the x coordinate as the right rather
   than the left edge of the string. This function shares the text length
   limitation of textprintf_ex(). Example:

      textprintf_right_ex(screen, font, SCREEN_W - 10, 10,
                          makecol(200, 200, 20), -1,
                          "%d bullets left", player_ammo);

void textprintf_justify_ex(BITMAP *bmp, const FONT *f, int x1, x2, y,
                           diff, color, bg, const char *fmt, ...);
   Like textout_justify_ex(), but using a printf() style format string. This
   function shares the text length limitation of textprintf_ex(). Example:

      char *lines[] = {"Line %02d: Draws justified text",
                       "Line %02d: within the specified",
                       "Line %02d: x2-x1 area. But not",
                       "Line %02d: T H I S !", NULL};
      /* Show the justification marker. */
      vline(screen, 300, 0, SCREEN_H-1, makecol(0, 0, 0));
      /* Draw all the lines until we reach a NULL entry. */
      for (num = 0, y = 0; lines[num]; num++, y += text_height(font))
         textprintf_justify_ex(screen, font, 0, 300, y, 180,
                               makecol(0, 0, 0), makecol(255, 255, 255),
                               lines[num], num);



===========================================
============ Polygon rendering ============
===========================================

All the 3d functions that accept a `type' parameter are asking for a polygon
rendering mode, which can be any of the following POLYTYPE_* values. If the
CPU_MMX flag of the cpu_capabilities global variable is set, the GRGB and
truecolor *LIT routines will be optimised using MMX instructions. If the
CPU_3DNOW flag is set, the truecolor PTEX*LIT routines will take advantage of
the 3DNow! CPU extensions.

Using MMX for *LIT routines has a side effect: normally (without MMX), these
routines use the blender functions used also for other lighting functions,
set with set_trans_blender() or set_blender_mode(). The MMX versions only use
the RGB value passed to set_trans_blender() and do the linear interpolation
themselves. Therefore a new set of blender functions passed to
set_blender_mode() is ignored.

#define POLYTYPE_FLAT
   A simple flat shaded polygon, taking the color from the `c' value of the
   first vertex. This polygon type is affected by the drawing_mode() function,
   so it can be used to render XOR or translucent polygons.

#define POLYTYPE_GCOL
   A single-color gouraud shaded polygon. The colors for each vertex are taken
   from the `c' value, and interpolated across the polygon. This is very fast,
   but will only work in 256-color modes if your palette contains a smooth
   gradient between the colors. In truecolor modes it interprets the color as
   a packed, display-format value as produced by the makecol() function.

#define POLYTYPE_GRGB
   A gouraud shaded polygon which interpolates RGB triplets rather than a
   single color. In 256-color modes this uses the global rgb_map table to
   convert the result to an 8-bit paletted color, so it must only be used
   after you have set up the RGB mapping table! The colors for each vertex are
   taken from the `c' value, which is interpreted as a 24-bit RGB triplet
   (0xFF0000 is red, 0x00FF00 is green, and 0x0000FF is blue).

#define POLYTYPE_ATEX
   An affine texture mapped polygon. This stretches the texture across the
   polygon with a simple 2d linear interpolation, which is fast but not
   mathematically correct. It can look OK if the polygon is fairly small or
   flat-on to the camera, but because it doesn't deal with perspective
   foreshortening, it can produce strange warping artifacts. To see what this
   means, run Allegro's test program and see what happens to the polygon3d()
   test when you zoom in very close to the cube.

#define POLYTYPE_PTEX
   A perspective-correct texture mapped polygon. This uses the `z' value from
   the vertex structure as well as the u/v coordinates, so textures are
   displayed correctly regardless of the angle they are viewed from. Because
   it involves division calculations in the inner texture mapping loop, this
   mode is a lot slower than POLYTYPE_ATEX, and it uses floating point so it
   will be very slow on anything less than a Pentium (even with an FPU, a 486
   can't overlap floating point division with other integer operations like
   the Pentium can).

#define POLYTYPE_ATEX_MASK
#define POLYTYPE_PTEX_MASK
   Like POLYTYPE_ATEX and POLYTYPE_PTEX, but zero texture map pixels are
   skipped, allowing parts of the texture map to be transparent.

#define POLYTYPE_ATEX_LIT
#define POLYTYPE_PTEX_LIT
   Like POLYTYPE_ATEX and POLYTYPE_PTEX, but the global color_map table (for
   256-color modes) or blender function (for non-MMX truecolor modes) is used
   to blend the texture with a light level taken from the `c' value in the
   vertex structure. This must only be used after you have set up the color
   mapping table or blender functions!

#define POLYTYPE_ATEX_MASK_LIT
#define POLYTYPE_PTEX_MASK_LIT
   Like POLYTYPE_ATEX_LIT and POLYTYPE_PTEX_LIT, but zero texture map pixels
   are skipped, allowing parts of the texture map to be transparent.

#define POLYTYPE_ATEX_TRANS
#define POLYTYPE_PTEX_TRANS
   Render translucent textures. All the general rules for drawing
   translucent things apply. However, these modes have a major
   limitation: they only work with memory bitmaps or linear frame
   buffers (not with banked frame buffers). Don't even try, they do not
   check and your program will die horribly (or at least draw wrong
   things).

#define POLYTYPE_ATEX_MASK_TRANS
#define POLYTYPE_PTEX_MASK_TRANS
   Like POLYTYPE_ATEX_TRANS and POLYTYPE_PTEX_TRANS, but zero texture map 
   pixels are skipped.

void polygon3d(BITMAP *bmp, int type, BITMAP *texture, int vc, V3D *vtx[]);
void polygon3d_f(BITMAP *bmp, int type, BITMAP *texture, int vc, V3D_f *vtx[]);
   Draw 3d polygons onto the specified bitmap, using the specified rendering 
   mode. Unlike the regular polygon() function, these routines don't support 
   concave or self-intersecting shapes, and they can't draw onto mode-X 
   screen bitmaps (if you want to write 3d code in mode-X, draw onto a 
   memory bitmap and then blit to the screen). The width and height of the 
   texture bitmap must be powers of two, but can be different, eg. a 64x16 
   texture is fine, but a 17x3 one is not. The vertex count parameter (vc) 
   should be followed by an array containing the appropriate number of 
   pointers to vertex structures: polygon3d() uses the fixed point V3D 
   structure, while polygon3d_f() uses the floating point V3D_f structure. 
   These are defined as:

      typedef struct V3D
      {
         fixed x, y, z;       - position
         fixed u, v;          - texture map coordinates
         int c;               - color
      } V3D;
   
      typedef struct V3D_f
      {
         float x, y, z;       - position
         float u, v;          - texture map coordinates
         int c;               - color
      } V3D_f;

   How the vertex data is used depends on the rendering mode:

   The `x' and `y' values specify the position of the vertex in 2d screen
   coordinates.

   The `z' value is only required when doing perspective correct texture
   mapping, and specifies the depth of the point in 3d world coordinates.

   The `u' and `v' coordinates are only required when doing texture mapping,
   and specify a point on the texture plane to be mapped on to this vertex.
   The texture plane is an infinite plane with the texture bitmap tiled
   across it. Each vertex in the polygon has a corresponding vertex on the
   texture plane, and the image of the resulting polygon in the texture plane
   will be mapped on to the polygon on the screen.

   We refer to pixels in the texture plane as texels. Each texel is a block, 
   not just a point, and whole numbers for u and v refer to the top-left 
   corner of a texel. This has a few implications. If you want to draw a 
   rectangular polygon and map a texture sized 32x32 on to it, you would use 
   the texture coordinates (0,0), (0,32), (32,32) and (32,0), assuming the 
   vertices are specified in anticlockwise order. The texture will then be 
   mapped perfectly on to the polygon. However, note that when we set u=32, 
   the last column of texels seen on the screen is the one at u=31, and the 
   same goes for v. This is because the coordinates refer to the top-left 
   corner of the texels. In effect, texture coordinates at the right and 
   bottom on the texture plane are exclusive.

   There is another interesting point here. If you have two polygons side 
   by side sharing two vertices (like the two parts of folded piece of 
   cardboard), and you want to map a texture across them seamlessly, the 
   values of u and v on the vertices at the join will be the same for both 
   polygons. For example, if they are both rectangular, one polygon may use 
   (0,0), (0,32), (32,32) and (32,0), and the other may use (32,0), (32,32), 
   (64,32), (64,0). This would create a seamless join.

   Of course you can specify fractional numbers for u and v to indicate a 
   point part-way across a texel. In addition, since the texture plane is 
   infinite, you can specify larger values than the size of the texture. 
   This can be used to tile the texture several times across the polygon.

   The `c' value specifies the vertex color, and is interpreted differently
   by various rendering modes. Read the beginning of chapter "Polygon
   rendering" for a list of rendering types you can use with this function.

void triangle3d(BITMAP *bmp, int type, BITMAP *tex, V3D *v1, *v2, *v3);
void triangle3d_f(BITMAP *bmp, int type, BITMAP *tex, V3D_f *v1, *v2, *v3);
   Draw 3d triangles, using either fixed or floating point vertex structures.
   Unlike quad3d[_f](), triangle3d[_f]() functions are not wrappers of
   polygon3d[_f](). The triangle3d[_f]() functions use their own routines
   taking into account the constantness of the gradients. Therefore
   triangle3d[_f](bmp, type, tex, v1, v2, v3) is faster than
   polygon3d[_f](bmp, type, tex, 3, v[]).

   Read the beginning of chapter "Polygon rendering" for a list of rendering
   types you can use with this function.

void quad3d(BITMAP *bmp, int type, BITMAP *tex, V3D *v1, *v2, *v3, *v4);
void quad3d_f(BITMAP *bmp, int type, BITMAP *tex, V3D_f *v1, *v2, *v3, *v4);
   Draw 3d quads, using either fixed or floating point vertex structures. 
   These are equivalent to calling polygon3d(bmp, type, tex, 4, v[]) or
   polygon3d_f(bmp, type, tex, 4, v[]).

   Read the beginning of chapter "Polygon rendering" for a list of rendering
   types you can use with this function.
   
int clip3d_f(int type, float min_z, float max_z, int vc,
             const V3D_f *vtx[], V3D_f *vout[], V3D_f *vtmp[], int out[]);
   Clips the polygon given in `vtx'. The number of vertices is `vc', the
   result goes in `vout', and `vtmp' and `out' are needed for internal
   purposes. The pointers in `vtx', `vout' and `vtmp' must point to valid
   V3D_f structures.
   
   As additional vertices may appear in the process of clipping, so the 
   size of `vout', `vtmp' and `out' should be at least vc * (1.5 ^ n), where
   `n' is the number of clipping planes (5 or 6), and `^' denotes "to the
   power of".
   
   The frustum (viewing volume) is defined by -z<x<z, -z<y<z,
   0<min_z<z<max_z. If  max_z<=min_z, the z<max_z clipping is
   not done. As you can see, clipping is done in the camera space, with
   perspective in mind, so this routine should be called after you apply
   the camera matrix, but before the perspective projection. The routine
   will correctly interpolate u, v, and c in the vertex structure. However,
   no provision is made for high/truecolor GCOL.

   Returns the number of vertices after clipping is done.

int clip3d(int type, fixed min_z, fixed max_z, int vc,
           const V3D *vtx[], V3D *vout[], V3D *vtmp[], int out[]);
   Fixed point version of clip3d_f(). This function should be used with 
   caution, due to the limited precision of fixed point arithmetic and high 
   chance of rounding errors: the floating point code is better for most 
   situations.

   Returns the number of vertices after clipping is done.

Zbuffered rendering
-------------------

A Z-buffer stores the depth of each pixel that is drawn on a viewport.
When a 3D object is rendered, the depth of each of its pixels is compared
against the value stored into the Z-buffer: if the pixel is closer it is
drawn, otherwise it is skipped.

No polygon sorting is needed. However, backface culling should be done
because it prevents many invisible polygons being compared against the
Z-buffer. Z-buffered rendering is the only algorithm supported by Allegro
that directly solves penetrating shapes (see example exzbuf.c, for instance).
The price to pay is more complex (and slower) routines.

Z-buffered polygons are designed as an extension of the normal POLYTYPE_*
rendering styles. Just OR the POLYTYPE with the value POLYTYPE_ZBUF, and
the normal polygon3d(), polygon3d_f(), quad3d(), etc. functions will
render z-buffered polygons.

Example:

   polygon3d(bmp, POLYTYPE_ATEX | POLYTYPE_ZBUF, tex, vc, vtx);

Of course, the z coordinates have to be valid regardless of rendering style.

A Z-buffered rendering procedure looks like a double-buffered rendering
procedure. You should follow four steps: create a Z-buffer at the beginning
of the program and make the library use it by calling set_zbuffer(). Then,
for each frame, clear the Z-buffer and draw polygons with
POLYTYPE_* | POLYTYPE_ZBUF and finally destroy the Z-buffer when leaving the
program.

Notes on Z-buffered renderers:

Unlike the normal POLYTYPE_FLAT renderers, the Z-buffered ones don't use
the hline() routine. Therefore DRAW_MODE has no effect.

The *LIT* routines work the traditional way - through the set of
blender routines.

All the Z-buffered routines are much slower than their normal counterparts
(they all use the FPU to interpolate and test 1/z values).

ZBUFFER *create_zbuffer(BITMAP *bmp);
   Creates a Z-buffer using the size of the BITMAP you are planning to draw
   on. Several Z-buffers can be defined but only one can be used at the same
   time, so you must call set_zbuffer() to make this Z-buffer active.

   Returns the pointer to the ZBUFFER or NULL if there was an error. Remember
   to destroy the ZBUFFER once you are done with it, to avoid having memory
   leaks.

ZBUFFER *create_sub_zbuffer(ZBUFFER *parent, int x, int y, int width, int height);
   Creates a sub-z-buffer, ie. a z-buffer sharing drawing memory with a
   pre-existing z-buffer, but possibly with a different size. The same rules
   as for sub-bitmaps apply: the sub-z-buffer width and height can extend
   beyond the right and bottom edges of the parent (they will be clipped), 
   but the origin point must lie within the parent region.

   When drawing z-buffered to a bitmap, the top left corner of the bitmap is
   always mapped to the top left corner of the current z-buffer. So this
   function is primarily useful if you want to draw to a sub-bitmap and use
   the corresponding sub-area of the z-buffer. In other cases, eg. if you
   just want to draw to a sub-bitmap of screen (and not to other parts of
   screen), then you would usually want to create a normal z-buffer (not
   sub-z-buffer) the size of the visible screen. You don't need to first
   create a z-buffer the size of the virtual screen and then a sub-z-buffer
   of that.

   Returns the pointer to the sub ZBUFFER or NULL if there was an error.
   Remember to destroy the ZBUFFER once you are done with it, to avoid having
   memory leaks.

void set_zbuffer(ZBUFFER *zbuf);
   Makes the given Z-buffer be the active one. This should have been
   previously created with create_zbuffer().

void clear_zbuffer(ZBUFFER *zbuf, float z);
   Writes z into the given Z-buffer (0 means far away). This function should
   be used to initialize the Z-buffer before each frame. Actually, low-level
   routines compare depth of the current pixel with 1/z: for example, if you
   want to clip polygons farther than 10, you must call
   clear_zbuffer(zbuf, 0.1).

void destroy_zbuffer(ZBUFFER *zbuf);
   Destroys the Z-buffer when you are finished with it. Use this to avoid
   memory leaks in your program.

Scene rendering
---------------

Allegro provides two simple approaches to remove hidden surfaces:

   Z-buffering - (see above)

   Scan-line algorithms - along each scanline on your screen, you keep
   track of what polygons you are "in" and which is the nearest. This
   status changes only where the scanline crosses some polygon edge. So you
   have to juggle an edge list and a polygon list. And you have to sort the
   edges for each scanline (this can be countered by keeping the order of
   the previous scanline - it won't change much). The BIG advantage is that
   you write each pixel only once. If you have a lot of overlapping
   polygons you can get incredible speeds compared to any of the previous
   algorithms. This algorithm is covered by the *_scene routines.


The scene rendering has approximately the following steps:

   Initialize the scene (set the clip area, clear the bitmap, blit a
   background, etc.)

   Call clear_scene().

   Transform all your points to camera space.

   Clip polygons.

   Project with persp_project() or persp_project_f().

   "Draw" polygons with scene_polygon3d() and/or scene_polygon3d_f().
   This doesn't do any actual drawing, only initializes tables.

   Render all the polygons defined previously to the bitmap with
   render_scene().

   Overlay some non-3D graphics.

   Show the bitmap (blit it to screen, flip the page, etc).


For each horizontal line in the viewport an x-sorted edge list is used to
keep track of what polygons are "in" and which is the nearest. Vertical
coherency is used - the edge list for a scanline is sorted starting from
the previous one - it won't change much. The scene rendering routines use
the same low-level asm routines as normal polygon3d().

Notes on scene rendering:

   Unlike polygon3d(), scene_polygon3d() requires valid z coordinates
   for all vertices, regardless of rendering style (unlike
   polygon3d(), which only uses z coordinate for *PTEX*).

   All polygons passed to scene_polygon3d() have to be
   persp_project()'ed.

   After render_scene() the mode is reset to SOLID.

Using a lot of *MASK* polygons drastically reduces performance, because
when a MASKed polygon is the first in line of sight, the polygons
underneath have to be drawn too. The same applies to FLAT polygons drawn
with DRAW_MODE_TRANS.

Z-buffered rendering works also within the scene renderer. It may be
helpful when you have a few intersecting polygons, but most of the
polygons may be safely rendered by the normal scanline sorting algorithm.
Same as before: just OR the POLYTYPE with POLYTYPE_ZBUF. Also, you
have to clear the z-buffer at the start of the frame. Example:

   clear_scene(buffer);
   if (some_polys_are_zbuf) clear_zbuffer(0.);
   while (polygons) {
      ...
      if (this_poly_is_zbuf) type |= POLYTYPE_ZBUF;
      scene_polygon3d(type, tex, vc, vtx);
   }
   render_scene();

int create_scene(int nedge, int npoly);
   Allocates memory for a scene, `nedge' and `npoly' are your estimates of how
   many edges and how many polygons you will render (you cannot get over the
   limit specified here). If you use same values in successive calls, the
   space will be reused (no new malloc()).

   The memory allocated is a little less than 150 * (nedge + npoly) bytes.

   Returns zero on success, or a negative number if allocations fail.

void clear_scene(BITMAP *bmp);
   Initializes a scene. The bitmap is the bitmap you will eventually render
   on.

void destroy_scene();
   Deallocate memory previously allocated by create_scene. Use this to avoid
   memory leaks in your program.

int scene_polygon3d(int type, BITMAP *texture, int vc, V3D *vtx[]);
int scene_polygon3d_f(int type, BITMAP *texture, int vc, V3D_f *vtx[]);
   Puts a polygon in the rendering list. Nothing is really rendered at this
   moment. Should be called between clear_scene() and render_scene().

   Arguments are the same as for polygon3d(), except the bitmap is missing.
   The one passed to clear_scene() will be used.

   Unlike polygon3d(), the polygon may be concave or self-intersecting.
   Shapes that penetrate one another may look OK, but they are not really
   handled by this code.

   Note that the texture is stored as a pointer only, and you should keep
   the actual bitmap around until render_scene(), where it is used.

   Since the FLAT style is implemented with the low-level hline() function,
   the FLAT style is subject to DRAW_MODEs. All these modes are valid. Along
   with the polygon, this mode will be stored for the rendering moment, and
   also all the other related variables (color_map pointer, pattern pointer,
   anchor, blender values).

   The settings of the CPU_MMX and CPU_3DNOW flags of the cpu_capabilities 
   global variable on entry in this routine affect the choice of low-level 
   asm routine that will be used by render_scene() for this polygon.

   Returns zero on success, or a negative number if it won't be rendered for
   lack of a rendering routine.

void render_scene();
   Renders all the specified scene_polygon3d()'s on the bitmap passed to
   clear_scene(). Rendering is done one scanline at a time, with no pixel
   being processed more than once.

   Note that between clear_scene() and render_scene() you shouldn't change
   the clip rectangle of the destination bitmap. For speed reasons, you
   should set the clip rectangle to the minimum.

   Note also that all the textures passed to scene_polygon3d() are stored as
   pointers only and actually used in render_scene().

extern float scene_gap;
   This number (default value = 100.0) controls the behaviour of the
   z-sorting algorithm. When an edge is very close to another's polygon
   plane, there is an interval of uncertainty in which you cannot tell which
   object is visible (which z is smaller). This is due to cumulative
   numerical errors for edges that have undergone a lot of transformations
   and interpolations.

   The default value means that if the 1/z values (in projected space)
   differ by only 1/100 (one percent), they are considered to be equal and
   the x-slopes of the planes are used to find out which plane is getting
   closer when we move to the right.

   Larger values means narrower margins, and increasing the chance of
   missing true adjacent edges/planes. Smaller values means larger margins,
   and increasing the chance of mistaking close polygons for adjacent ones.
   The value of 100 is close to the optimum. However, the optimum shifts
   slightly with resolution, and may be application-dependent. It is here 
   for you to fine-tune.



============================================================
============ Transparency and patterned drawing ============
============================================================

void drawing_mode(int mode, BITMAP *pattern, int x_anchor, int y_anchor);
   Sets the graphics drawing mode. This only affects the geometric routines 
   like putpixel, lines, rectangles, circles, polygons, floodfill, etc, not 
   the text output, blitting, or sprite drawing functions. The mode should 
   be one of the following constants:

      DRAW_MODE_SOLID               - the default, solid color
                                      drawing
      DRAW_MODE_XOR                 - exclusive-or drawing
      DRAW_MODE_COPY_PATTERN        - multicolored pattern fill
      DRAW_MODE_SOLID_PATTERN       - single color pattern fill
      DRAW_MODE_MASKED_PATTERN      - masked pattern fill
      DRAW_MODE_TRANS               - translucent color blending

   In DRAW_MODE_SOLID, pixels of the bitmap being drawn onto are simply 
   replaced by those produced by the drawing function.

   In DRAW_MODE_XOR, pixels are written to the bitmap with an exclusive-or 
   operation rather than a simple copy, so drawing the same shape twice will 
   erase it. Because it involves reading as well as writing the bitmap 
   memory, xor drawing is a lot slower than the normal replace mode.

   With the patterned modes, you provide a pattern bitmap which is tiled 
   across the surface of the shape. Allegro stores a pointer to this bitmap 
   rather than copying it, so you must not destroy the bitmap while it is 
   still selected as the pattern. The width and height of the pattern must 
   be powers of two, but they can be different, eg. a 64x16 pattern is fine, 
   but a 17x3 one is not. The pattern is tiled in a grid starting at point 
   (x_anchor, y_anchor). Normally you should just pass zero for these 
   values, which lets you draw several adjacent shapes and have the patterns 
   meet up exactly along the shared edges. Zero alignment may look peculiar 
   if you are moving a patterned shape around the screen, however, because 
   the shape will move but the pattern alignment will not, so in some 
   situations you may wish to alter the anchor position.

   When you select DRAW_MODE_COPY_PATTERN, pixels are simply copied from the 
   pattern bitmap onto the destination bitmap. This allows the use of 
   multicolored patterns, and means that the color you pass to the drawing 
   routine is ignored. This is the fastest of the patterned modes.

   In DRAW_MODE_SOLID_PATTERN, each pixel in the pattern bitmap is compared 
   with the mask color, which is zero in 256-color modes or bright pink for 
   truecolor data (maximum red and blue, zero green). If the pattern pixel 
   is solid, a pixel of the color you passed to the drawing routine is 
   written to the destination bitmap, otherwise a zero is written. The 
   pattern is thus treated as a monochrome bitmask, which lets you use the 
   same pattern to draw different shapes in different colors, but prevents 
   the use of multicolored patterns.

   DRAW_MODE_MASKED_PATTERN is almost the same as DRAW_MODE_SOLID_PATTERN, 
   but the masked pixels are skipped rather than being written as zeros, so 
   the background shows through the gaps.

   In DRAW_MODE_TRANS, the global color_map table or truecolor blender 
   functions are used to overlay pixels on top of the existing image. This 
   must only be used after you have set up the color mapping table (for 256 
   color modes) or blender functions (for truecolor modes). Because it 
   involves reading as well as writing the bitmap memory, translucent 
   drawing is very slow if you draw directly to video RAM, so wherever 
   possible you should use a memory bitmap instead.

void xor_mode(int on);
   This is a shortcut for toggling xor drawing mode on and off. Calling 
   xor_mode(TRUE) is equivalent to drawing_mode(DRAW_MODE_XOR, NULL, 0, 0).
   Calling xor_mode(FALSE) is equivalent to
   drawing_mode(DRAW_MODE_SOLID, NULL, 0, 0).

void solid_mode();
   This is a shortcut for selecting solid drawing mode. It is equivalent to 
   calling drawing_mode(DRAW_MODE_SOLID, NULL, 0, 0).

256-color transparency
----------------------

In paletted video modes, translucency and lighting are implemented with a 
64k lookup table, which contains the result of combining any two colors c1 
and c2. You must set up this table before you use any of the translucency 
or lighting routines. Depending on how you construct the table, a range of 
different effects are possible. For example, translucency can be implemented 
by using a color halfway between c1 and c2 as the result of the combination. 
Lighting is achieved by treating one of the colors as a light level (0-255) 
rather than a color, and setting up the table appropriately. A range of 
specialised effects are possible, for instance replacing any color with any 
other color and making individual source or destination colors completely 
solid or invisible. Color mapping tables can be precalculated with the
colormap utility, or generated at runtime. Read chapter "Structures and types
defined by Allegro" for an internal description of the COLOR_MAP structure.

extern COLOR_MAP *color_map;
   Global pointer to the color mapping table. You must allocate your own
   COLOR_MAP either statically or dynamically and set color_map to it 
   before using any translucent or lit drawing functions in a 256-color 
   video mode! Example:

      color_map = malloc(sizeof(COLOR_MAP));
      if (!color_map)
         abort_on_error("Not enough memory for color map!");

void create_trans_table(COLOR_MAP *table, const PALETTE pal,
                        int r, g, b, void (*callback)(int pos));
   Fills the specified color mapping table with lookup data for doing 
   translucency effects with the specified palette. When combining the 
   colors c1 and c2 with this table, the result will be a color somewhere 
   between the two. The r, g, and b parameters specify the solidity of each 
   color component, ranging from 0 (totally transparent) to 255 (totally 
   solid). For 50% solidity, pass 128.

   This function treats source color #0 as a special case, leaving the
   destination unchanged whenever a zero source pixel is encountered, so that
   masked sprites will draw correctly. This function will take advantage of
   the global rgb_map variable to speed up color conversions. If the callback
   function is not NULL, it will be called 256 times during the calculation,
   allowing you to display a progress indicator. Example:

      COLOR_MAP trans_table;
      ...
      /* Build a color lookup table for translucent drawing. */
      create_trans_table(&trans_table, pal, 128, 128, 128, NULL);

void create_light_table(COLOR_MAP *table, const PALETTE pal,
                        int r, g, b, void (*callback)(int pos));
   Fills the specified color mapping table with lookup data for doing 
   lighting effects with the specified palette. When combining the colors c1 
   and c2 with this table, c1 is treated as a light level from 0-255. At 
   light level 255 the table will output color c2 unchanged, at light level 
   0 it will output the r, g, b value you specify to this function, and at 
   intermediate light levels it will output a color somewhere between the 
   two extremes. The r, g, and b values are in the range 0-63.

   This function will take advantage of the global rgb_ap variable to speed
   up color conversions. If the callback function is not NULL, it will be
   called 256 times during the calculation, allowing you to display a
   progress indicator. Example:

      COLOR_MAP light_table;
      ...
      /* Build a color lookup table for lighting effects. */
      create_light_table(&light_table, pal, 0, 0, 0, NULL);

void create_color_table(COLOR_MAP *table, const PALETTE pal,
                        void (*blend)(PALETTE pal, int x, int y, RGB *rgb),
                        void (*callback)(int pos));
   Fills the specified color mapping table with lookup data for doing 
   customised effects with the specified palette, calling the blend function 
   to determine the results of each color combination.

   Your blend routine will be passed a pointer to the palette and the two
   indices of the colors which are to be combined, and should fill in the RGB
   structure with the desired result in 0-63 format. Allegro will then search
   the palette for the closest match to the RGB color that you requested, so
   it doesn't matter if the palette has no exact match for this color.

   If the callback function is not NULL, it will be called 256 times during
   the calculation, allowing you to display a progress indicator. Example:

      COLOR_MAP greyscale_table;
      ...
      void return_grey_color(const PALETTE pal,
                             int x, int y, RGB *rgb)
      {
         ...
      }
      ...
         /* Build a color lookup table for greyscale effect. */
         create_color_table(&greyscale_table, pal,
                            return_grey_color, NULL);

void create_blender_table(COLOR_MAP *table, const PALETTE pal,
                          void (*callback)(int pos));
   Fills the specified color mapping table with lookup data for doing a 
   paletted equivalent of whatever truecolor blender mode is currently 
   selected. After calling set_trans_blender(), set_blender_mode(), or any 
   of the other truecolor blender mode routines, you can use this function 
   to create an 8-bit mapping table that will have the same results as 
   whatever 24-bit blending mode you have enabled.

Truecolor transparency
----------------------

In truecolor video modes, translucency and lighting are implemented by a 
blender function of the form:

   unsigned long (*BLENDER_FUNC)(unsigned long x, y, n);

For each pixel to be drawn, this routine is passed two color parameters x 
and y, decomposes them into their red, green and blue components, combines 
them according to some mathematical transformation involving the 
interpolation factor n, and then merges the result back into a single 
return color value, which will be used to draw the pixel onto 
the destination bitmap.

The parameter x represents the blending modifier color and the parameter y 
represents the base color to be modified. The interpolation factor n is in 
the range [0-255] and controls the solidity of the blending.

When a translucent drawing function is used, x is the color of the source, 
y is the color of the bitmap being drawn onto and n is the alpha level   
that was passed to the function that sets the blending mode (the RGB triplet 
that was passed to this function is not taken into account).

When a lit sprite drawing function is used, x is the color represented by 
the RGB triplet that was passed to the function that sets the blending mode 
(the alpha level that was passed to this function is not taken into 
account), y is the color of the sprite and n is the alpha level that was 
passed to the drawing function itself.

Since these routines may be used from various different color depths, there 
are three such callbacks, one for use with 15-bit 5.5.5 pixels, one for 16 
bit 5.6.5 pixels, and one for 24-bit 8.8.8 pixels (this can be shared 
between the 24 and 32-bit code since the bit packing is the same).

void set_trans_blender(int r, int g, int b, int a);
   Enables a linear interpolator blender mode for combining translucent 
   or lit truecolor pixels.

void set_alpha_blender();
   Enables the special alpha-channel blending mode, which is used for 
   drawing 32-bit RGBA sprites. After calling this function, you can use 
   draw_trans_sprite() or draw_trans_rle_sprite() to draw a 32-bit source 
   image onto any hicolor or truecolor destination. The alpha values will be 
   taken directly from the source graphic, so you can vary the solidity of 
   each part of the image. You can't use any of the normal translucency 
   functions while this mode is active, though, so you should reset to one 
   of the normal blender modes (eg. set_trans_blender()) before drawing 
   anything other than 32-bit RGBA sprites.

void set_write_alpha_blender();
   Enables the special alpha-channel editing mode, which is used for drawing 
   alpha channels over the top of an existing 32-bit RGB sprite, to turn it 
   into an RGBA format image. After calling this function, you can set the 
   drawing mode to DRAW_MODE_TRANS and then write draw color values (0-255) 
   onto a 32-bit image. This will leave the color values unchanged, but 
   alter the alpha to whatever values you are writing. After enabling this 
   mode you can also use draw_trans_sprite() to superimpose an 8-bit alpha 
   mask over the top of an existing 32-bit sprite.

void set_add_blender(int r, int g, int b, int a);
   Enables an additive blender mode for combining translucent or lit 
   truecolor pixels.

void set_burn_blender(int r, int g, int b, int a);
   Enables a burn blender mode for combining translucent or lit truecolor 
   pixels. Here the lightness values of the colours of the source image 
   reduce the lightness of the destination image, darkening the image.

void set_color_blender(int r, int g, int b, int a);
   Enables a color blender mode for combining translucent or lit truecolor 
   pixels. Applies only the hue and saturation of the source image to the 
   destination image. The luminance of the destination image is not affected.

void set_difference_blender(int r, int g, int b, int a);
   Enables a difference blender mode for combining translucent or lit 
   truecolor pixels. This makes an image which has colours calculated by the 
   difference between the source and destination colours.

void set_dissolve_blender(int r, int g, int b, int a);
   Enables a dissolve blender mode for combining translucent or lit 
   truecolor pixels. Randomly replaces the colours of some pixels in the 
   destination image with those of the source image. The number of pixels 
   replaced depends on the alpha value (higher value, more pixels replaced; 
   you get the idea :).

void set_dodge_blender(int r, int g, int b, int a);
   Enables a dodge blender mode for combining translucent or lit truecolor 
   pixels. The lightness of colours in the source lighten the colours of the 
   destination. White has the most effect; black has none.

void set_hue_blender(int r, int g, int b, int a);
   Enables a hue blender mode for combining translucent or lit truecolor 
   pixels. This applies the hue of the source to the destination.

void set_invert_blender(int r, int g, int b, int a);
   Enables an invert blender mode for combining translucent or lit truecolor 
   pixels. Blends the inverse (or negative) colour of the source with the 
   destination.

void set_luminance_blender(int r, int g, int b, int a);
   Enables a luminance blender mode for combining translucent or lit 
   truecolor pixels. Applies the luminance of the source to the destination.
   The colour of the destination is not affected.

void set_multiply_blender(int r, int g, int b, int a);
   Enables a multiply blender mode for combining translucent or lit 
   truecolor pixels. Combines the source and destination images, multiplying 
   the colours to produce a darker colour. If a colour is multiplied by 
   white it remains unchanged; when multiplied by black it also becomes 
   black.

void set_saturation_blender(int r, int g, int b, int a);
   Enables a saturation blender mode for combining translucent or lit 
   truecolor pixels. Applies the saturation of the source to the destination 
   image.

void set_screen_blender(int r, int g, int b, int a);
   Enables a screen blender mode for combining translucent or lit truecolor 
   pixels. This blender mode lightens the colour of the destination image by 
   multiplying the inverse of the source and destination colours. Sort of 
   like the opposite of the multiply blender mode.

void set_blender_mode(BLENDER_FUNC b15, b16, b24, int r, g, b, a);
   Specifies a custom set of truecolor blender routines, which can be used 
   to implement whatever special interpolation modes you need. This function 
   shares a single blender between the 24 and 32-bit modes.

void set_blender_mode_ex(BLENDER_FUNC b15, b16, b24, b32, b15x, b16x, b24x,
                         int r, g, b, a);
   Like set_blender_mode(), but allows you to specify a more complete set of 
   blender routines. The b15, b16, b24, and b32 routines are used when 
   drawing pixels onto destinations of the same format, while b15x, b16x, 
   and b24x are used by draw_trans_sprite() and draw_trans_rle_sprite() when 
   drawing RGBA images onto destination bitmaps of another format. These 
   blenders will be passed a 32-bit x parameter, along with a y value of a 
   different color depth, and must try to do something sensible in response.



==========================================================
============ Converting between color formats ============
==========================================================

In general, Allegro is designed to be used in only one color depth at a 
time, so you will call set_color_depth() once and then store all your 
bitmaps in the same format. If you want to mix several different pixel 
formats, you can use create_bitmap_ex() in place of create_bitmap(), and 
call bitmap_color_depth() to query the format of a specific image. Most of 
the graphics routines require all their input parameters to be in the same 
format (eg. you cannot stretch a 15-bit source bitmap onto a 24-bit 
destination), but there are some exceptions:

   blit() and the rotation routines can copy between bitmaps of any format,
   converting the data as required.

   draw_sprite() can draw 256-color source images onto destinations of any
   format.

   draw_character_ex() _always_ uses a 256-color source bitmap, whatever the
   format of the destination.

   The draw_trans_sprite() and draw_trans_rle_sprite() functions are able to
   draw 32-bit RGBA images onto any hicolor or truecolor destination, as long
   as you call set_alpha_blender() first.

   The draw_trans_sprite() function is able to draw an 8-bit alpha channel
   image over the top of an existing 32-bit image, as long as you call
   set_write_alpha_blender() first.

Expanding a 256-color source onto a truecolor destination is fairly fast 
(obviously you must set the correct palette before doing this conversion!). 
Converting between different truecolor formats is slightly slower, and 
reducing truecolor images to a 256-color destination is very slow (it can be 
sped up significantly if you set up the global rgb_map table before doing 
the conversion).

int bestfit_color(const PALETTE pal, int r, int g, int b);
   Searches the specified palette for the closest match to the requested 
   color, which are specified in the VGA hardware 0-63 format. Normally you 
   should call makecol8() instead, but this lower level function may be 
   useful if you need to use a palette other than the currently selected 
   one, or specifically don't want to use the rgb_map lookup table.

   Returns the index of the palette for the closest match to the requested
   color.

extern RGB_MAP *rgb_map;
   To speed up reducing RGB values to 8-bit paletted colors, Allegro uses a 
   32k lookup table (5 bits for each color component). You must set up this 
   table before using the gouraud shading routines, and if present the table 
   will also vastly accelerate the makecol8() and some create_*_table()
   functions. RGB tables can be precalculated with the rgbmap utility, or
   generated at runtime with create_rgb_table().
      
void create_rgb_table(RGB_MAP *table, const PALETTE pal,
                      void (*callback)(int pos));
   Fills the specified RGB mapping table with lookup data for the specified 
   palette. If the callback function is not NULL, it will be called 256 
   times during the calculation, allowing you to display a progress 
   indicator. Example:

      RGB_MAP rgb_table;
      
      create_rgb_table(&rgb_table, palette, NULL);
      rgb_map = &rgb_table;

void hsv_to_rgb(float h, float s, float v, int *r, int *g, int *b);
void rgb_to_hsv(int r, int g, int b, float *h, float *s, float *v);
   Convert color values between the HSV and RGB color spaces. The RGB values 
   range from 0 to 255, hue is from 0 to 360, and saturation and value are 
   from 0 to 1. Example:

      int r, g, b;
      float hue, saturation, value;
      ...
      /* Convert a reddish color to HSV format. */
      rgb_to_hsv(255, 0, 128, &hue, &saturation, &value);
      
      /* Now put our tin foil hat, and verify that. */
      hsv_to_rgb(hue, saturation, value, &r, &g, &b);
      ASSERT(r == 255);
      ASSERT(g == 0);
      ASSERT(b == 128);



=======================================================
============ Direct access to video memory ============
=======================================================

Read chapter "Structures and types defined by Allegro" for an internal
description of the BITMAP structure. There are several ways to get direct
access to the image memory of a bitmap, varying in complexity depending on
what sort of bitmap you are using.

The simplest approach will only work with memory bitmaps (obtained from 
create_bitmap(), grabber datafiles, and image files) and sub-bitmaps of 
memory bitmaps. This uses a table of char pointers, called `line', which is
a part of the bitmap structure and contains pointers to the start of each 
line of the image. For example, a simple memory bitmap putpixel function is:

   void memory_putpixel(BITMAP *bmp, int x, int y, int color)
   {
      bmp->line[y][x] = color;
   }

For truecolor modes you need to cast the line pointer to the appropriate
type, for example:

   void memory_putpixel_15_or_16_bpp(BITMAP *bmp, int x, int y, int color)
   {
      ((short *)bmp->line[y])[x] = color;
   }

   void memory_putpixel_32(BITMAP *bmp, int x, int y, int color)
   {
      ((long *)bmp->line[y])[x] = color;
   }

If you want to write to the screen as well as to memory bitmaps, you need to
use some helper macros, because the video memory may not be part of your 
normal address space. This simple routine will work for any linear screen, 
eg. a VESA linear framebuffers:

   void linear_screen_putpixel(BITMAP *bmp, int x, int y, int color)
   {
      bmp_select(bmp);
      bmp_write8((unsigned long)bmp->line[y]+x, color);
   }

For truecolor modes you should replace the bmp_write8() with bmp_write16(), 
bmp_write24(), or bmp_write32(), and multiply the x offset by the number of 
bytes per pixel. There are of course similar functions to read a pixel value
from a bitmap, namely bmp_read8(), bmp_read16(), bmp_read24() and
bmp_read32().

This still won't work in banked SVGA modes, however, or on platforms like 
Windows that do special processing inside the bank switching functions. For 
more flexible access to bitmap memory, you need to call the following
routines. They are implemented as inline assembler routines, so they are not
as inefficient as they might seem. If the bitmap doesn't require bank
switching (ie. it is a memory bitmap, mode 13h screen, etc), these functions
just return bmp->line[line].

unsigned long bmp_write_line(BITMAP *bmp, int line);
   Selects the line of a bitmap that you are going to draw onto.

   Returns the address of the selected line for writing.

unsigned long bmp_read_line(BITMAP *bmp, int line);
   Selects the line of a bitmap that you are going to read from.

   Returns the address of the selected line for reading.

void bmp_unwrite_line(BITMAP *bmp);
   Releases the bitmap memory after you are finished with it. You only need 
   to call this once at the end of a drawing operation, even if you have 
   called bmp_write_line() or bmp_read_line() several times before it.

More on banked direct memory access
-----------------------------------


Although SVGA bitmaps are banked, Allegro provides linear access to the 
memory within each scanline, so you only need to pass a y coordinate to 
these functions. Various x positions can be obtained by simply adding the x 
coordinate to the returned address. The return value is an unsigned long 
rather than a char pointer because the bitmap memory may not be in your data 
segment, and you need to access it with far pointers. For example, a 
putpixel using the bank switching functions is:

   void banked_putpixel(BITMAP *bmp, int x, int y, int color)
   {
      unsigned long address = bmp_write_line(bmp, y);
      bmp_select(bmp);
      bmp_write8(address+x, color);
      bmp_unwrite_line(bmp);
   }

You will notice that Allegro provides separate functions for setting the 
read and write banks. It is important that you distinguish between these, 
because on some graphics cards the banks can be set individually, and on 
others the video memory is read and written at different addresses. Life is 
never quite as simple as we might wish it to be, though (this is true even 
when we _aren't_ talking about graphics coding :-) and so of course some 
cards only provide a single bank. On these the read and write bank functions 
will behave identically, so you shouldn't assume that you can read from one 
part of video memory and write to another at the same time. You can call 
bmp_read_line(), and read whatever you like from that line, and then call 
bmp_write_line() with the same or a different line number, and write 
whatever you like to this second line, but you mustn't call bmp_read_line() 
and bmp_write_line() together and expect to be able to read one line and 
write the other simultaneously. It would be nice if this was possible, but 
if you do it, your code won't work on single banked SVGA cards.

And then there's mode-X. If you've never done any mode-X graphics coding, 
you probably won't understand this, but for those of you who want to know 
how Allegro sets up the mode-X screen bitmaps, here goes...

The line pointers are still present, and they contain planar addresses, ie. 
the actual location at which you access the first pixel in the line. These 
addresses are guaranteed to be quad aligned, so you can just set the write 
plane, divide your x coordinate by four, and add it to the line pointer. For 
example, a mode-X putpixel is:

   void modex_putpixel(BITMAP *b, int x, int y, int color)
   {
      outportw(0x3C4, (0x100<<(x&3))|2);
      bmp_select(bmp);
      bmp_write8((unsigned long)bmp->line[y]+(x>>2), color);
   }

Oh yeah: the DJGPP nearptr hack. Personally I don't like this very much 
because it disables memory protection and isn't portable to other platforms, 
but a lot of people swear by it because it can give you direct access to the 
screen memory via a normal C pointer. Warning: this method will only work 
with the DJGPP library, when using VGA 13h or a linear framebuffer modes!

In your setup code:

   #include <sys/nearptr.h>

   unsigned char *screenmemory;
   unsigned long screen_base_addr;

   __djgpp_nearptr_enable();

   __dpmi_get_segment_base_address(screen->seg, &screen_base_addr);

   screenmemory = (unsigned char *)(screen_base_addr + 
                                    screen->line[0] -
                                    __djgpp_base_address);

Then:

   void nearptr_putpixel(int x, int y, int color)
   {
      screenmemory[x + y*VIRTUAL_W] = color;
   }



=======================================
============ FLIC routines ============
=======================================

There are two high level functions for playing FLI/FLC animations: 
play_fli(), which reads the data directly from disk, and play_memory_fli(), 
which uses data that has already been loaded into RAM. Apart from the 
different sources of the data, these two functions behave identically. They 
draw the animation onto the specified bitmap, which should normally be the 
screen. Frames will be aligned with the top left corner of the bitmap: if 
you want to position them somewhere else you will need to create a 
sub-bitmap for the FLI player to draw onto.

If the callback function is not NULL it will be called once for each frame,
allowing you to perform background tasks of your own. This callback should
normally return zero: if it returns non-zero the player will terminate (this
is the only way to stop an animation that is playing in looped mode).

The FLI player returns FLI_OK if it reached the end of the file, FLI_ERROR
if something went wrong, and the value returned by the callback function if
that was what stopped it. If you need to distinguish between different return
values, your callback should return positive integers, since FLI_OK is zero
and FLI_ERROR is negative.

Note that the FLI player will only work when the timer module is installed, 
and that it will alter the palette according to whatever palette data is 
present in the animation file.

Occasionally you may need more detailed control over how an FLI is played, 
for example if you want to superimpose a text scroller on top of the 
animation, or to play it back at a different speed. You could do both of 
these with the lower level functions described below.

int play_fli(const char *filename, BITMAP *bmp, int loop, int (*callback)());
   Plays an Autodesk Animator FLI or FLC animation file on the specified
   BITMAP, reading the data from disk as it is required. If `loop' is not
   zero, the player will cycle when it reaches the end of the file, otherwise
   it will play through the animation once and then return. Read the beginning
   of chapter "FLIC routines" for a description of the callback parameter.
   Example:

      /* Let users skip looped animations. */
      int check_escape_key(void)
      {
         if (key[KEY_ESC])
            return 1;
         else
            return 0;
      }
      ...
         int ret = play_fli("animlogo.fli", screen, 1,
                             check_escape_key);
         if (ret == FLI_ERROR)
            abort_on_error("Error playing intro!");

   The FLI player returns FLI_OK if it reached the end of the file, FLI_ERROR
   if something went wrong, and the value returned by the callback function if
   that was what stopped it.

int play_memory_fli(const void *fli_data, BITMAP *bmp, int loop,
                    int (*callback)());
   Plays an Autodesk Animator FLI or FLC animation on the specified BITMAP,
   reading the data from a copy of the file which is held in memory. You can
   obtain the `fli_data' pointer by allocating a block of memory and reading
   an FLI file into it, or by importing an FLI into a grabber datafile. If
   `loop' is not zero, the player will cycle when it reaches the end of the
   file, otherwise it will play through the animation once and then return.
   Read the beginning of chapter "FLIC routines" for a description of the
   callback parameter.

   Playing animations from memory is obviously faster than cuing them
   directly from disk, and is particularly useful with short, looped FLI's.
   Animations can easily get very large, though, so in most cases you will
   probably be better just using play_fli(). You can think of this function
   as a wrapper on top of open_memory_fli(), next_fli_frame() and close_fli().
   Example:

      int ret = play_memory_fli(anim_data, screen, 0, NULL);
      if (ret == FLI_ERROR)
         abort_on_error("Corrupted animation data?");

   The FLI player returns FLI_OK if it reached the end of the file, FLI_ERROR
   if something went wrong, and the value returned by the callback function if
   that was what stopped it.

int open_fli(const char *filename);
int open_memory_fli(const void *fli_data);
   Open FLI files ready for playing, reading the data from disk or memory
   respectively. Information about the current FLI is held in the global
   variables fli_bitmap and fli_palette, which you can use if this function
   succeeds. However, you can only have one animation open at a time.
   Example:

      if (open_fli("intro.fli") == FLI_ERROR)
         abort_on_error("Error playing intro");

   Returns FLI_OK on success, FLI_ERROR if something went wrong, like trying
   to open another FLI file without closing the previous one.

void close_fli();
   Closes an FLI file when you have finished reading from it. Remember to do
   this to avoid having memory leaks in your program.

int next_fli_frame(int loop);
   Reads the next frame of the current animation file. If `loop' is not zero,
   the player will cycle when it reaches the end of the file, otherwise it
   will return FLI_EOF. The frame is read into the global variables
   fli_bitmap and fli_palette. Example:

      while (next_fli_frame(0) == FLI_OK) {
         /* Do stuff, like play audio stream
            or check keys to skip animation. */
         /* Rest some time until next frame... */
      }

   Returns FLI_OK on success, FLI_ERROR or FLI_NOT_OPEN on error, and FLI_EOF
   on reaching the end of the file.

extern BITMAP *fli_bitmap;
   Contains the current frame of the FLI/FLC animation. If there is no open
   animation, its value will be NULL.

extern PALETTE fli_palette;
   Contains the current FLI palette.

extern int fli_bmp_dirty_from;
extern int fli_bmp_dirty_to;
   These variables are set by next_fli_frame() to indicate which part of the 
   fli_bitmap has changed since the last call to reset_fli_variables(). If 
   fli_bmp_dirty_from is greater than fli_bmp_dirty_to, the bitmap has not 
   changed, otherwise lines fli_bmp_dirty_from to fli_bmp_dirty_to 
   (inclusive) have altered. You can use these when copying the fli_bitmap 
   onto the screen, to avoid moving data unnecessarily. Example:

      if (fli_bmp_dirty_from <= fli_bmp_dirty_to)
         blit(fli_bitmap, screen, 0, fli_bmp_dirty_from,
              0, fli_bmp_dirty_from, fli_bitmap->w,
              fli_bmp_dirty_to - fli_bmp_dirty_from + 1);
              
extern int fli_pal_dirty_from;
extern int fli_pal_dirty_to;
   These variables are set by next_fli_frame() to indicate which part of the 
   fli_palette has changed since the last call to reset_fli_variables(). If 
   fli_pal_dirty_from is greater than fli_pal_dirty_to, the palette has not 
   changed, otherwise colors fli_pal_dirty_from to fli_pal_dirty_to 
   (inclusive) have altered. You can use these when updating the hardware 
   palette, to avoid unnecessary calls to set_palette(). Example:

      if (fli_pal_dirty_from <= fli_pal_dirty_to)
         set_palette_range(fli_palette, fli_pal_dirty_from,
                           fli_pal_dirty_to, 1);

void reset_fli_variables();
   Once you have done whatever you are going to do with the fli_bitmap and 
   fli_palette, call this function to reset the fli_bmp_dirty_* and 
   fli_pal_dirty_* variables.

extern int fli_frame;
   Global variable containing the current frame number in the FLI file. This 
   is useful for synchronising other events with the animation, for instance 
   you could check it in a play_fli() callback function and use it to 
   trigger a sample at a particular point. Example:

      while (next_fli_frame(0) == FLI_OK) {
         if (fli_frame == 345)
            play_sample(trumpet_sound, 255, 128, 1000, 0);
         /* Rest some time until next frame... */
      }

extern volatile int fli_timer;
   Global variable for timing FLI playback. When you open an FLI file, a 
   timer interrupt is installed which increments this variable every time a 
   new frame should be displayed. Calling next_fli_frame() decrements it, so 
   you can test it and know that it is time to display a new frame if it is 
   greater than zero. Example:

      while (next_fli_frame(0) == FLI_OK) {
         /* Do stuff, like play audio stream
            or check keys to skip animation. */
         /* Rest some time until next frame... */
         while (fli_timer <= 0)
            rest(0);
      }



=============================================
============ Sound init routines ============
=============================================

Allegro allows you to use the sound hardware in two ways: automatic, or
manual. Usually you should try the automatic version first. This means
calling install_sound() with the autodetection parameters and using the rest
of the sound functions to play samples or music. In this situation, Allegro
will handle the sound devices and mix the samples and/or music the best way
it can.

However, sound hardware has a limitation on the number of samples it may
play all at the same time (from now on, called hardware voices). When you
exceed this limit, Allegro will cut off one of the samples being played and
reproduce the new one. Depending on the type of sounds you are playing, how
many of them you need at the same time and their nature (e.g: vital audio
feedback to the user or useless "ping" when some shrapnel hits a rock in the
scenery) you will want to specify more carefully how hardware voices are
reserved and which samples have priority over others.

The hardware voice reservation phase has to be done before the call to
install_sound(), since it directly affects how Allegro talks to the sound
drivers.

int detect_digi_driver(int driver_id);
   Detects whether the specified digital sound device is available. This
   function must be called _before_ install_sound().

   Returns the maximum number of voices that the driver can provide, or zero
   if the hardware is not present.

int detect_midi_driver(int driver_id);
   Detects whether the specified MIDI sound device is available. This function
   must be called _before_ install_sound().

   Returns the maximum number of voices that the driver can provide, or zero
   if the hardware is not present.

   There are two special-case return values that you should watch out for:
   if this function returns -1 it is a note-stealing driver (eg. DIGMID) that
   shares voices with the current digital sound driver, and if it returns
   0xFFFF it is an external device like an MPU-401 where there is no way to
   determine how many voices are available.

void reserve_voices(int digi_voices, int midi_voices);
   Call this function to specify the number of voices that are to be used by
   the digital and MIDI sound drivers respectively. This must be done
   _before_ calling install_sound(). If you reserve too many voices,
   subsequent calls to install_sound() will fail. How many voices are
   available depends on the driver, and in some cases you will actually get
   more than you reserve (eg. the FM synth drivers will always provide 9
   voices on an OPL2 and 18 on an OPL3, and the SB digital driver will round
   the number of voices up to the nearest power of two). Pass negative
   values to restore the default settings. You should be aware that the
   sound quality is usually inversely related to how many voices you use, so
   don't reserve any more than you really need.

void set_volume_per_voice(int scale);
   By default, Allegro will play a centered sample at half volume on both
   the left and right channel. A sample panned to the far right or left
   will be played at maximum volume on that channel only. This is done so
   you can play a single panned sample without distortion. If you play
   multiple samples at full volume, the mixing process can result in
   clipping, a noticeable form of distortion. The more samples, the more
   likely clipping is to occur, and the more clipping, the worse the output
   will sound.

   If clipping is a problem - or if the output is too quiet - this function
   can be used to adjust the volume of each voice. You should first check
   that your speakers are at a reasonable volume, Allegro's global volume
   is at maximum (see set_volume() below), and any other mixers such as the
   Windows Volume Control are set reasonably. Once you are sure that
   Allegro's output level is unsuitable for your application, use this
   function to adjust it.

   Each time you increase the parameter by one, the volume of each voice
   will halve. For example, if you pass 4, you can play up to 16 centred
   samples at maximum volume without distortion.

   If you pass 0 to this function, each centred sample will play at the
   maximum volume possible without distortion, as will all samples played
   through a mono driver. Samples at the extreme left and right will distort
   if played at full volume. If you wish to play panned samples at full
   volume without distortion, you should pass 1 to this function.
   Note: this is different from the function's behaviour in WIPs 3.9.34,
   3.9.35 and 3.9.36. If you used this function under one of these WIPs,
   you will have to increase your parameter by one to get the same volume.

   Note: The default behaviour has changed as of Allegro 4.1.15. If you
   would like the behaviour of earlier versions of Allegro, pass -1 to this
   function. Allegro will choose a value dependent on the number of voices,
   so that if you reserve n voices, you can play up to n/2 normalised
   samples with centre panning without risking distortion. The exception is
   when you have fewer than 8 voices, where the volume remains the same as
   for 8 voices. Here are the values, dependent on the number of voices:

     1-8 voices - set_volume_per_voice(2)
      16 voices - set_volume_per_voice(3)
      32 voices - set_volume_per_voice(4)
      64 voices - set_volume_per_voice(5)

   Of course this function does not override the volume you specify with
   play_sample() or voice_set_volume(). It simply alters the overall output
   of the program. If you play samples at lower volumes, or if they are not
   normalised, then you can play more of them without distortion.

   It is recommended that you hard-code the parameter into your program,
   rather than offering it to the user. The user can alter the volume with
   the configuration file instead, or you can provide for this with
   set_volume().

   To restore volume per voice to its default behaviour, pass 1.

int install_sound(int digi, int midi, const char *cfg_path);
   Initialises the sound module. You should normally pass DIGI_AUTODETECT 
   and MIDI_AUTODETECT as the driver parameters to this function, in which 
   case Allegro will read hardware settings from the current configuration 
   file. This allows the user to select different values with the setup 
   utility: see the config section for details. Alternatively, see the 
   platform specific documentation for a list of the available drivers. The 
   cfg_path parameter is only present for compatibility with previous 
   versions of Allegro, and has no effect on anything.

   Returns zero if the sound is successfully installed, and -1 on failure.
   If it fails it will store a description of the problem in allegro_error.

void remove_sound();
   Cleans up after you are finished with the sound routines. You don't 
   normally need to call this, because allegro_exit() will do it for you.

void set_volume(int digi_volume, int midi_volume);
   Alters the global sound output volume. Specify volumes for both digital 
   samples and MIDI playback, as integers from 0 to 255, or pass a negative 
   value to leave one of the settings unchanged. Values bigger than 255 will
   be reduced to 255. This routine will not alter the volume of the hardware
   mixer if it exists (i.e. only your application will be affected).

void set_hardware_volume(int digi_volume, int midi_volume);
   Alters the hardware sound output volume. Specify volumes for both digital 
   samples and MIDI playback, as integers from 0 to 255, or pass a negative 
   value to leave one of the settings unchanged. Values bigger than 255 will
   be reduced to 255. This routine will use the hardware mixer to control the
   volume if it exists (i.e. the volume of all the applications on your
   machine will be affected), otherwise do nothing.

void get_volume(int *digi_volume, int *midi_volume);
   Retrieves the global sound output volume, both for digital samples and MIDI
   playback, as integers from 0 to 255. Parameters digi_volume and midi_volume
   must be valid pointers to int, or NULL if not interested in specific value.

void get_hardware_volume(int *digi_volume, int *midi_volume);
   Retrieves the hardware sound output volume, both for digital samples and MIDI
   playback, as integers from 0 to 255, or -1 if the information is not
   available. Parameters digi_volume and midi_volume must be valid pointers to
   int, or NULL if not interested in specific value.

========================================
============ Mixer routines ============
========================================



void set_mixer_quality(int quality);
   Sets the resampling quality of the mixer. Valid values are the same as
   the `quality' config variable. Please read chapter "Standard config
   variables" for details. You can call this function at any point in your
   program, even before allegro_init().

int get_mixer_quality(void);
   Returns the current mixing quality, as specified by the `quality' config
   variable, or a previous call to set_mixer_quality().

int get_mixer_frequency(void);
   Returns the mixer frequency, in Hz.

int get_mixer_bits(void);
   Returns the mixer bit depth (8 or 16).

int get_mixer_channels(void);
   Returns the number of output channels. 2 for stereo, 1 for mono, 0 if
   the mixer isn't active.

int get_mixer_voices(void);
   Returns the number of voices allocated to the mixer.

int get_mixer_buffer_length(void);
   Returns the number of samples per channel in the mixer buffer.



=================================================
============ Digital sample routines ============
=================================================

SAMPLE *load_sample(const char *filename);
   Loads a sample from a file, supporting both mono and stereo WAV and mono
   VOC files, in 8 or 16-bit formats, as well as formats handled by functions
   registered using register_sample_file_type(). Example:

      SAMPLE *sample = load_sample(user_input);
      if (!sample)
         abort_on_error("Couldn't load sample!");

   Returns a pointer to the SAMPLE or NULL on error. Remember to free this
   sample later to avoid memory leaks.

SAMPLE *load_wav(const char *filename);
   Loads a sample from a RIFF WAV file. Example:

      SAMPLE *sample = load_wav("scream.wav");
      if (!sample)
         abort_on_error("Couldn't scare user!");

   Returns a pointer to the SAMPLE or NULL on error. Remember to free this
   sample later to avoid memory leaks.

SAMPLE *load_wav_pf(PACKFILE *f);
   A version of load_wav() which reads from a packfile. Example:

      PACKFILE *packfile;
      SAMPLE *sample;
   
      packfile = pack_fopen("sound.wav", F_READ);
      if (!packfile)
         abort_on_error("Couldn't open sound.wav");
   
      sample = load_wav_pf(packfile);
      if (!sample)
         abort_on_error("Error loading sound.wav");

   Returns a pointer to the SAMPLE or NULL on error. Remember to free this
   sample later to avoid memory leaks.

SAMPLE *load_voc(const char *filename);
   Loads a sample from a Creative Labs VOC file. Example:

      SAMPLE *sample = load_voc("alarm.voc");
      if (!sample)
         abort_on_error("Couldn't alert user!");

   Returns a pointer to the SAMPLE or NULL on error. Remember to free this
   sample later to avoid memory leaks.

SAMPLE *load_voc_pf(PACKFILE *f);
   A version of load_voc() which reads from a packfile. Example:

      PACKFILE *packfile;
      SAMPLE *sample;
   
      packfile = pack_fopen("sound.wav", F_READ);
      if (!packfile)
         abort_on_error("Couldn't open sound.wav");
   
      sample = load_wav_pf(packfile);
      if (!sample)
         abort_on_error("Error loading sound.wav");

   Returns a pointer to the SAMPLE or NULL on error. Remember to free this
   sample later to avoid memory leaks.

int save_sample(const char *filename, SAMPLE *spl);
   Writes a sample into a file. The output format is determined from the
   filename extension. At present Allegro does not natively support the
   writing of any sample formats, so you must register a custom saver routine
   with register_sample_file_type(). Example:

      if (save_sample("sound.wav", sample) != 0)
         abort_on_error("Couldn't save sample!");

   Returns zero on success, non-zero otherwise.

SAMPLE *create_sample(int bits, int stereo, int freq, int len);
   Constructs a new sample structure of the specified type. Read chapter
   "Structures and types defined by Allegro" for an internal description of
   the SAMPLE structure. The `bits' parameter can be 8 or 16, `stereo' can be
   zero for mono samples and non-zero for stereo samples, `freq' is the
   frequency in hertz, and `len' is the number of samples you want to allocate
   for the full sound buffer.

   Returns a pointer to the created sample, or NULL if the sample could not
   be created. Remember to free this sample later to avoid memory leaks.

void destroy_sample(SAMPLE *spl);
   Destroys a sample structure when you are done with it. It is safe to call 
   this even when the sample might be playing, because it checks and will 
   kill it off if it is active. Use this to avoid memory leaks in your
   program.

void lock_sample(SAMPLE *spl);
   Under DOS, locks all the memory used by a sample. You don't normally need 
   to call this function because load_sample() and create_sample() do it for
   you.

void register_sample_file_type(const char *ext,
                          SAMPLE *(*load)(const char *filename),
                          int (*save)(const char *filename, SAMPLE *spl));
   Informs the load_sample() function of a new sample file type,
   providing routines to read and write samples in this format (either
   function may be NULL). Example:

      SAMPLE *load_mp3(const char *filename)
      {
         ...
      }
      
         register_sample_file_type("mp3", load_mp3, NULL);

int play_sample(const SAMPLE *spl, int vol, int pan, int freq, int loop);
   Triggers a sample at the specified volume, pan position, and frequency. 
   The parameters `vol' and `pan' range from 0 (min/left) to 255 (max/right).
   Frequency is relative rather than absolute: 1000 represents the frequency
   that the sample was recorded at, 2000 is twice this, etc. If `loop' is not
   zero, the sample will repeat until you call stop_sample(), and can be
   manipulated while it is playing by calling adjust_sample(). Example:

      /* Scream from the left speaker, twice the freq. */
      int sound = play_sample(scream, 255, 0, 2000, 0);

   Returns the voice number that was allocated for the sample or negative if
   no voices were available.

void adjust_sample(const SAMPLE *spl, int vol, int pan, int freq, int loop);
   Alters the parameters of a sample while it is playing (useful for
   manipulating looped sounds). You can alter the volume, pan, and frequency,
   and can also clear the loop flag, which will stop the sample when it next
   reaches the end of its loop. The values of the parameters are just like
   those of play_sample(). If there are several copies of the same sample
   playing, this will adjust the first one it comes across. If the sample is
   not playing it has no effect.

void stop_sample(const SAMPLE *spl);
   Stop a sample from playing, which is required if you have set a sample
   going in looped mode. If there are several copies of the sample playing,
   it will stop them all. You must still destroy the sample using
   destroy_sample().

Voice control
-------------

If you need more detailed control over how samples are played, you can use 
the lower level voice functions rather than just calling play_sample(). This 
is rather more work, because you have to explicitly allocate and free the 
voices rather than them being automatically released when they finish 
playing, but allows far more precise specification of exactly how you want 
everything to sound. You may also want to modify a couple of fields from the 
SAMPLE structure. Read chapter "Structures and types defined by Allegro" for
its definition.

int allocate_voice(const SAMPLE *spl);
   Allocates a sound card voice and prepares it for playing the specified 
   sample, setting up sensible default parameters (maximum volume, centre 
   pan, no change of pitch, no looping). When you are finished with the 
   voice you must free it by calling deallocate_voice() or release_voice().
   Allegro can manage up to 256 simultaneous voices, but that limit may be
   lower due to hardware reasons.

   Returns the voice number, or -1 if no voices are available.

void deallocate_voice(int voice);
   Frees a sound card voice, stopping it from playing and releasing whatever 
   resources it is using.

void reallocate_voice(int voice, const SAMPLE *spl);
   Switches an already-allocated voice to use a different sample. Calling 
   reallocate_voice(voice, sample) is equivalent to:

      deallocate_voice(voice);
      voice = allocate_voice(sample);

void release_voice(int voice);
   Releases a sound card voice, indicating that you are no longer interested 
   in manipulating it. The sound will continue to play, and any resources 
   that it is using will automatically be freed when it finishes. This is 
   essentially the same as deallocate_voice(), but it waits for the sound to 
   stop playing before taking effect.

void voice_start(int voice);
   Activates a voice, using whatever parameters have been set for it.

void voice_stop(int voice);
   Stops a voice, storing the current position and state so that it may 
   later be resumed by calling voice_start().

void voice_set_priority(int voice, int priority);
   Sets the priority of a voice (range 0-255). This is used to decide which 
   voices should be chopped off, if you attempt to play more than the 
   sound card driver can handle.

SAMPLE *voice_check(int voice);
   Checks whether a voice is currently allocated.

   Returns a pointer to the sample that the voice is using, or NULL if the
   voice is inactive (ie. it has been deallocated, or the release_voice()
   function has been called and the sample has then finished playing).

int voice_get_position(int voice);
   Returns the current position of a voice, in sample units, or -1 if it has 
   finished playing.

void voice_set_position(int voice, int position);
   Sets the position of a voice, in sample units.

void voice_set_playmode(int voice, int playmode);
   Adjusts the loop status of the specified voice. This can be done while 
   the voice is playing, so you can start a sample in looped mode (having 
   set the loop start and end positions to the appropriate values), and then 
   clear the loop flag when you want to end the sound, which will cause it 
   to continue past the loop end, play the subsequent part of the sample, 
   and finish in the normal way. The mode parameter is a bitfield containing 
   the following values:
   
   PLAYMODE_PLAY
      Plays the sample a single time. This is the default if you don't set 
      the loop flag.
   
   PLAYMODE_LOOP
      Loops repeatedly through the sample, jumping back to the loop start 
      position upon reaching the loop end.
   
   PLAYMODE_FORWARD
      Plays the sample from beginning to end. This is the default if you 
      don't set the backward flag.
   
   PLAYMODE_BACKWARD
      Reverses the direction of the sample. If you combine this with the 
      loop flag, the sample jumps to the loop end position upon reaching the 
      loop start (ie. you do not need to reverse the loop start and end 
      values when you play the sample in reverse).
   
   PLAYMODE_BIDIR
      When used in combination with the loop flag, causes the sample to 
      change direction each time it reaches one of the loop points, so it 
      alternates between playing forwards and in reverse.
   
int voice_get_volume(int voice);
   Returns the current volume of the voice, range 0-255.  Otherwise it
   returns -1 if that cannot be determined (because it has finished or
   been preempted by a different sound).

void voice_set_volume(int voice, int volume);
   Sets the volume of the voice, range 0-255.

void voice_ramp_volume(int voice, int time, int endvol);
   Starts a volume ramp (crescendo or diminuendo) from the current volume to 
   the specified ending volume, lasting for time milliseconds. The volume is
   a value in the range 0-255.

void voice_stop_volumeramp(int voice);
   Interrupts a volume ramp operation.

int voice_get_frequency(int voice);
   Returns the current pitch of the voice, in Hz.

void voice_set_frequency(int voice, int frequency);
   Sets the pitch of the voice, in Hz.

void voice_sweep_frequency(int voice, int time, int endfreq);
   Starts a frequency sweep (glissando) from the current pitch to the 
   specified ending pitch, lasting for time milliseconds.

void voice_stop_frequency_sweep(int voice);
   Interrupts a frequency sweep operation.

int voice_get_pan(int voice);
   Returns the current pan position, from 0 (left) to 255 (right).

void voice_set_pan(int voice, int pan);
   Sets the pan position, ranging from 0 (left) to 255 (right).

void voice_sweep_pan(int voice, int time, int endpan);
   Starts a pan sweep (left <-> right movement) from the current position to
   the specified ending position, lasting for time milliseconds.

void voice_stop_pan_sweep(int voice);
   Interrupts a pan sweep operation.

void voice_set_echo(int voice, int strength, int delay);
   Sets the echo parameters for a voice (not currently implemented).

void voice_set_tremolo(int voice, int rate, int depth);
   Sets the tremolo parameters for a voice (not currently implemented).

void voice_set_vibrato(int voice, int rate, int depth);
   Sets the vibrato parameters for a voice (not currently implemented).



===============================================
============ Music routines (MIDI) ============
===============================================

Allegro allows you to play MIDI files. MIDI files basically contain notes and
the type of instrument that is meant to play them, so they are usually very
small in size. However, it's up to the sound card of the end user to play the
notes, and sound cards have been historically known to have poor MIDI
performance (at least those oriented to the consumer market). Few consumer
cards feature decent MIDI playback. Still, as a game creator you can never be
sure if the music of your game will be played as you meant it, because it
totally depends on the hardware of the user.

For this reason Allegro also provides a DIGMID driver. This is a software
implementation of the so called Wavetable synthesis. Sound cards featuring this
store digital samples of real instruments at different pitches, interpolating
those that are not recorded, thus achieving a high sound quality. Implementing
this in software makes you sure that the quality you hear on your computer is
that which will be heard by end users using the same driver.

The disadvantage of the DIGMID driver is that it uses more CPU than simple
MIDI playback, and it steals some hardware voices from the sound card, which
might be more critical for the end user experience than the background music.
At the Allegro homepage (http://alleg.sourceforge.net/) you can find more
information about DIGMID and where to download digital samples for your MIDI
files.

MIDI *load_midi(const char *filename);
   Loads a MIDI file (handles both format 0 and format 1). Example:

      MIDI *music;
      music = load_midi("backmus.mid");
      if (!music)
         abort_on_error("Couldn't load background music!");

   Returns a pointer to a MIDI structure, or NULL on error. Remember to free
   this MIDI file later to avoid memory leaks.

void destroy_midi(MIDI *midi);
   Destroys a MIDI structure when you are done with it. It is safe to call 
   this even when the MIDI file might be playing, because it checks and will 
   kill it off if it is active. Use this to avoid memory leaks in your
   program.

void lock_midi(MIDI *midi);
   Under DOS, locks all the memory used by a MIDI file. You don't normally 
   need to call this function because load_midi() does it for you.

int play_midi(MIDI *midi, int loop);
   Starts playing the specified MIDI file, first stopping whatever music was 
   previously playing. If the loop flag is set to non-zero, the data will be
   repeated until replaced with something else, otherwise it will stop at the
   end of the file. Passing a NULL pointer will stop whatever music is
   currently playing.

   Returns non-zero if an error occurs (this may happen if a patch-caching
   wavetable driver is unable to load the required samples, or at least it
   might in the future when somebody writes some patch-caching wavetable
   drivers :-)

int play_looped_midi(MIDI *midi, int loop_start, int loop_end);
   Starts playing a MIDI file with a user-defined loop position. When the 
   player reaches the loop end position or the end of the file (loop_end may 
   be -1 to only loop at EOF), it will wind back to the loop start point. 
   Both positions are specified in the same beat number format as the 
   midi_pos variable.

   The return value has the same meaning as that of play_midi(): non-zero if
   an error occurs, zero otherwise.

void stop_midi();
   Stops whatever music is currently playing. This is the same thing as 
   calling play_midi(NULL, FALSE).

void midi_pause();
   Pauses the MIDI player.

void midi_resume();
   Resumes playback of a paused MIDI file.

int midi_seek(int target);
   Seeks to the given midi_pos in the current MIDI file. If the target is 
   earlier in the file than the current midi_pos it seeks from the 
   beginning; otherwise it seeks from the current position.

   Returns zero if it could successfully seek to the requested position.
   Otherwise, a return value of 1 means it stopped playing, and midi_pos is
   set to the negative length of the MIDI file (so you can use this function
   to determine the length of a MIDI file). A return value of 2 means the
   MIDI file looped back to the start.

int get_midi_length(MIDI *midi);
   This function will simulate playing the given MIDI, from start to end, to
   determine how long it takes to play. After calling this function, midi_pos
   will contain the negative number of beats, and midi_time the length of the
   midi, in seconds.

   Note that any currently playing midi is stopped when you call this function.
   Usually you would call it before play_midi, to get the length of the midi to
   be played, like in this example:

      length = get_midi_length(my_midi);
      play_midi(my_midi);
      do {
         pos = midi_time;
         textprintf_ex(screen, font, 0, 0, c, -1, "%d:%02d / %d:%02d\n",
            pos / 60, pos % 60, length / 60, length % 60);
         rest(100);
      } while(pos <= length);

   Returns the value of midi_time, the length of the midi.

void midi_out(unsigned char *data, int length);
   Streams a block of MIDI commands into the player in real-time, allowing 
   you to trigger notes, jingles, etc, over the top of whatever MIDI file is 
   currently playing.

int load_midi_patches();
   Forces the MIDI driver to load the entire set of patches ready for use. 
   You will not normally need to call this, because Allegro automatically 
   loads whatever data is required for the current MIDI file, but you must 
   call it before sending any program change messages via the midi_out() 
   command.

   Returns non-zero if an error occurred.

extern volatile long midi_pos;
   Stores the current position (beat number) in the MIDI file, or contains 
   a negative number if no music is currently playing. Useful for 
   synchronising animations with the music, and for checking whether a MIDI 
   file has finished playing.

extern volatile long midi_time;
   Contains the position in seconds in the currently playing midi. This is
   useful if you want to display the current song position in seconds, not as
   beat number.

extern long midi_loop_start;
extern long midi_loop_end;
   The loop start and end points, set by the play_looped_midi() function. 
   These may safely be altered while the music is playing, but you should be 
   sure they are always set to sensible values (start < end). If you are 
   changing them both at the same time, make sure to alter them in the right 
   order in case a MIDI interrupt happens to occur in between your two 
   writes! Setting these values to -1 represents the start and end of the 
   file respectively.

extern void (*midi_msg_callback)(int msg, int byte1, int byte2);
extern void (*midi_meta_callback)(int type, const unsigned char *data, int length);
extern void (*midi_sysex_callback)(const unsigned char *data, int length);
   Hook functions allowing you to intercept MIDI player events. If set to 
   anything other than NULL, these routines will be called for each MIDI 
   message, meta-event, and system exclusive data block respectively. They 
   will execute in an interrupt handler context, so all the code and data 
   they use should be locked, and they must not call any operating system 
   functions. In general you just use these routines to set some flags and 
   respond to them later in your mainline code.

int load_ibk(char *filename, int drums);
   Reads in a .IBK patch definition file for use by the Adlib driver. If 
   drums is set, it will load it as a percussion patch set, otherwise it 
   will use it as a replacement set of General MIDI instruments. You may 
   call this before or after initialising the sound code, or can simply set 
   the ibk_file and ibk_drum_file variables in the configuration file to 
   have the data loaded automatically. Note that this function has no effect 
   on any drivers other than the Adlib one!

   Returns non-zero on error.



===============================================
============ Audio stream routines ============
===============================================

The audio stream functions are for playing digital sounds that are too big 
to fit in a regular SAMPLE structure, either because they are huge files 
that you want to load in pieces as the data is required, or because you are 
doing something clever like generating the waveform on the fly.

You can think of an AUDIOSTREAM structure as a wrapper around two audio
buffers. The first thing you do is fill both buffers with sound data and let
Allegro play them. Once the first buffer has been played, the second starts,
and Allegro lets you know you have to fill the other one (i.e. graphics
double buffering applied to sounds too big to fit into memory).

The implementation of the sound buffers uses normal SAMPLE structures, so you
can use all the voice_*() functions to modify the audio streams. Read chapter
"Digital sample routines", section "Voice control" for a list of additional
functions you can use. Read chapter "Structures and types defined by Allegro"
for the internals of the AUDIOSTREAM structure.

AUDIOSTREAM *play_audio_stream(int len, int bits, int stereo,
                               int freq, int vol, int pan);
   This function creates a new audio stream and starts playing it. The 
   length is the size of each transfer buffer in sample frames (not bytes), 
   where a sample frame is a single sample value for mono data or a pair of 
   interleaved sample values (left first) for stereo data. The length should 
   normally be (but doesn't have to be) a power of 2 somewhere around 1k in 
   size. Larger buffers are more efficient and require fewer updates, but 
   result in more latency between you providing the data and it actually 
   being played.

   The `bits' parameter must be 8 or 16. `freq' is the sample rate of the
   data in Hertz. The `vol' and `pan' values use the same 0-255 ranges as the
   regular sample playing functions. The `stereo' parameter should be set to
   1 for stereo streams, or 0 otherwise.

   If you want to adjust the pitch, volume, or panning of a stream once it is
   playing, you can use the regular voice_*() functions with stream->voice
   as a parameter. The format of the sample data is described in the SAMPLE
   entry of the "Structures and types defined by Allegro" chapter. The formula
   to get the size of the buffers in bytes could be:

      bytes = length * (bits / 8) * (stereo ? 2 : 1)
   Example:

      /* Create a 22KHz 8bit mono audio stream. */
      stream = play_audio_stream(1024, 8, FALSE, 22050, 255, 128);
      if (!stream)
         abort_on_error("Error creating audio stream!\n");

   This function returns a pointer to the audio stream or NULL if it could
   not be created.

void stop_audio_stream(AUDIOSTREAM *stream);
   Destroys an audio stream when it is no longer required.

void *get_audio_stream_buffer(AUDIOSTREAM *stream);
   You must call this function at regular intervals while an audio stream is 
   playing, to provide the next buffer of sample data (the smaller the 
   stream buffer size, the more often it must be called). This function should
   not be called from a timer handler. Example:

      void *mem_chunk;
      ...
      while (TRUE) {
         ...
         mem_chunk = get_audio_stream_buffer(buffer);
         if (mem_chunk != NULL) {
            /* Refill the stream buffer. */
         }
      }

   If it returns NULL, the stream is still playing the previous lot of data,
   so you don't need to do anything. If it returns a value, that is the
   location of the next buffer to be played, and you should load the
   appropriate number of samples (however many you specified when creating the
   stream) to that address, for example using an fread() from a disk file.
   After filling the buffer with data, call free_audio_stream_buffer() to
   indicate that the new data is now valid.

void free_audio_stream_buffer(AUDIOSTREAM *stream);
   Call this function after get_audio_stream_buffer() returns a non-NULL 
   address, to indicate that you have loaded a new block of samples to that 
   location and the data is now ready to be played. Example:

      mem_chunk = get_audio_stream_buffer(buffer);
      if (mem_chunk != NULL) {
         /* Refill the stream buffer. */
         ...
         free_audio_stream_buffer(buffer);
      }



============================================
============ Recording routines ============
============================================

Allegro provides routines to capture sound from the sound card, be it digital
samples or MIDI notes. Ideally this would allow you to create games where
basic speech recognition could be implemented, or voice messages in
multiplayer games over a network. However, many old sound cards are not full
duplex. This means, that the sound device can only be playing or recording,
but not both at the same time.

Any Windows 2000 or better machine comes with a full duplex sound card and
updated drivers. All MacOS X machines allow full duplex recording. Under Unix
your mileage may vary: you can have the right hardware for the task, but the
drivers might not support this feature. Under DOS you should forget about full
duplex altogether.

To find out if your system allows this feature, use the akaitest program,
distributed along with Allegro, in the `tests' directory.

int install_sound_input(int digi, int midi);
   Initialises the sound recorder module. You must install the normal sound
   playback system before calling this routine. The two card parameters should
   use the same constants as install_sound(), including DIGI_NONE and
   MIDI_NONE to disable parts of the module, or DIGI_AUTODETECT and
   MIDI_AUTODETECT to guess the hardware.

   This function returns zero on success, and any other value if the machine
   or driver doesn't support sound recording.

void remove_sound_input();
   Cleans up after you are finished with the sound input routines. You don't 
   normally need to call this, because remove_sound() and/or allegro_exit() 
   will do it for you.

int get_sound_input_cap_bits();
   Checks which sample formats are supported by the current audio input 
   driver, returning one of the bitfield values:

      0 = audio input not supported
      8 = eight bit audio input is supported
      16 = sixteen bit audio input is supported
      24 = both eight and sixteen bit audio input are supported
   Example:

      cap = get_sound_input_cap_bits();
      if (cap == 0) {
         /* Ugh, no audio input supported? */
      } else {
         if (cap & 8) {
            /* We have eight bit audio input. */
         }
         if (cap & 16) {
            /* We have sixteen bit audio input. */
         }
      }
      
int get_sound_input_cap_stereo();
   Checks whether the current audio input driver is capable of stereo 
   recording.

   Returns non-zero if the driver is capable of stereo recording.

int get_sound_input_cap_rate(int bits, int stereo);
   Returns the maximum possible sample frequency for recording in the 
   specified format, or zero if these settings are not supported. The bits
   parameter is the number of bits of the audio, and stereo is a boolean
   parameter. Pass zero for mono, non-zero for stereo input. Example:

      int max_freq;
      ...
      /* What frequency can we record 8 bits mono at? */
      max_freq = get_sound_input_cap_rate(8, 0);
      if (max_freq > 22000) {
         /* Ok, 22KHz and above is good enough. */
      }

int get_sound_input_cap_parm(int rate, int bits, int stereo);
   Checks whether the specified recording frequency, number of bits, and 
   mono/stereo mode are supported (and how) by the current audio driver.

   The function returns one of the following possible values:

      0  = It is impossible to record in this format.
      1  = Recording is possible, but audio output
           will be suspended.
      2  = Recording is possible at the same time as
           playing other sounds (full duplex sound card).
      -n = Sampling rate not supported, but rate 'n'
           would work instead.

int set_sound_input_source(int source);
   Selects the audio input source. The parameter should be one of the values:

      SOUND_INPUT_MIC
      SOUND_INPUT_LINE
      SOUND_INPUT_CD

   The function returns zero on success, or -1 if the hardware does not
   provide an input select register (ie. you have no control over the input
   source).

int start_sound_input(int rate, int bits, int stereo);
   Starts recording in the specified format, suspending audio playback as 
   necessary if the card is not full duplex.

   Returns the buffer size in bytes if successful, or zero on error.

void stop_sound_input();
   Stops audio recording, switching the card back into the normal playback 
   mode.

int read_sound_input(void *buffer);
   Retrieves the most recently recorded audio buffer into the specified 
   location. The buffer size can be obtained by checking the return value
   from start_sound_input(). You must be sure to call this
   function at regular intervals during the recording (typically around 100 
   times a second), or some data will be lost. If you are unable to do this 
   often enough from the mainline code, use the digi_recorder() callback to 
   store the waveform into a larger buffer of your own.

   Note: many cards produce a click or popping sound when switching between
   record and playback modes, so it is often a good idea to discard the
   first buffer after you start a recording. The waveform is always stored
   in unsigned format, with stereo data consisting of alternate left/right
   samples.

   The function will return non-zero if a buffer has been copied or zero if
   no new data is yet available (you were too fast checking the input).

extern void (*digi_recorder)();
   If set, this function is called by the input driver whenever a new sample 
   buffer becomes available, at which point you can use read_sound_input() 
   to copy the data into a more permanent location. It runs in an interrupt
   context, so it must execute very quickly, the code and all memory that it
   touches must be locked, and you cannot call any operating system routines
   or access disk files. This currently works only under DOS.

extern void (*midi_recorder)(unsigned char data);
   If set, this function is called by the MIDI input driver whenever a new 
   byte of MIDI data becomes available. It runs in an interrupt context, so 
   it must execute very quickly and all the code/data must be locked. This
   currently works only under DOS and Windows.



=======================================================
============ File and compression routines ============
=======================================================

The following routines implement a fast buffered file I/O system, which 
supports the reading and writing of compressed files using a ring buffer 
algorithm based on the LZSS compressor by Haruhiko Okumura. This does not 
achieve quite such good compression as programs like zip and lha, but 
unpacking is very fast and it does not require much memory. Packed files 
always begin with the 32-bit value F_PACK_MAGIC, and autodetect files with 
the value F_NOPACK_MAGIC.

The following FA_* flags are guaranteed to work:

      FA_NONE           - Exclude files that have any attribute set
      FA_RDONLY         - Directory entries that are unwritable for current user
      FA_HIDDEN         - Hidden flag
      FA_DIREC          - Directories
      FA_SYSTEM         - Files with system flag set (DOS/Windows only)
      FA_LABEL          - Files with volume label flag set (DOS/Windows only)
      FA_ARCH           - Files with archive flag set (DOS/Windows only)
      FA_ALL            - Match all attributes
Do not use any other flags from DOS/Windows or your code will not compile on 
another platform.
FA_RDONLY is for directory entries with read-only flag on DOS-like systems or 
unwritable by current user on Unix-like systems. Hidden files are directory
entries that have the hidden flag set (DOS/Windows) or have names starting with
'.' (UNIX, excluding '.' and '..').
Flags can be combined using '|' (binary OR operator).

When passed to the functions as the 'attrib' parameter, these flags 
represent an upper set in which the actual flag set of a matching file must 
be included. That is, in order for a file to be matching, its attributes 
may contain any of the specified flags but must not contain any of the 
unspecified flags. In other words, you explicitly exclude the flags that you
do not specify. Thus if you pass 'FA_DIREC | FA_RDONLY', normal files 
and directories will be included as well as read-only files and 
directories, but not hidden files and directories. Similarly, if you pass 
'FA_ARCH' then both archived and non-archived files will be included. If
FA_NONE is passed all attributes are excluded and only files with no attributes
are returned. Conversely, if you pass FA_ALL, no attributes are excluded so all
files are returned (which is what you would usually want).

Functions which accept wildcards as file names support the meta characters
`*' (which means, zero or any quantity of characters) and `?' (which means
any character, but only one).

void get_executable_name(char *buf, int size);
   Fills `buf' with the full path to the current executable, writing at most
   `size' bytes.  This generally comes from `argv[0]' but on Unix systems it
   tries to get the information from the `/proc' filesystem first, searching
   the directories specified in `$PATH' if necessary. If this fails too, it
   tries to find the executable name from the output of the `ps' command,
   using `argv[0]' only as a last resort if all other options fail. Example:

      char name[200];
      ...
      get_executable_name(name, sizeof(name));
      allegro_message("Running `%s'\n", name);

char *fix_filename_case(char *path);
   Converts the filename stored in `path' to a standardised case. On DOS
   platforms, they will be entirely uppercase. On other platforms this
   function doesn't do anything. Example:

      get_executable_name(name, sizeof(name));
      fix_filename_case(name);
      allegro_message("Running `%s'\n", name);

   Returns a copy of the `path' parameter.

char *fix_filename_slashes(char *path);
   Converts all the directory separators in the filename stored in `path' to
   a standard character. On DOS and Windows platforms, this is a backslash.
   On most other platforms this is a slash. Example:

      char buf[200] = "c:/dos\\backup/weirdo\\test";
      ...
      fix_filename_slashes(buf);
      /* Under DOS we would have c:\dos\backup\weirdo\test.
         Under Unix we would have c:/dos/backup/weirdo/test. */

   Returns a copy of the `path' parameter.

char *canonicalize_filename(char *dest, const char *filename, int size);
   Converts any filename into its canonical form, i.e. the minimal absolute
   filename describing the same file and fixing incorrect forward/backward
   slashes for the current platform, storing at most `size' bytes into the
   `dest' buffer. You can use the same buffer both as input and output
   because Allegro internally works on a copy of the input before touching
   `dest'. Example:

      char buf[256];
      ...
      canonicalize_filename(buf, "~/../s22/..\\t3st///hi.c",
                            sizeof(buf));
      /* Running this under Unix would
         return: /home/t3st/hi.c */

   Note that this function won't work as expected if the path to canonicalize
   comes from another platform (eg. a "c:\something" path will canonicalize
   into something really wrong under Unix: "/current/path/c:/something").

   Returns a copy of the `dest' parameter.

char *make_absolute_filename(char *dest, const char *path, const char *filename, int size);
   Makes an absolute filename from an absolute path and a relative filename,
   storing at most `size' bytes into the `dest' buffer. This is like calling
   replace_filename() and then canonicalize_filename(). Example:

      char buf[256];
      ...
      make_absolute_filename(buf, "/usr/games/",
                             "../temp.txt", sizeof(buf));
      /* This would create /usr/temp.txt */

   Returns a copy of the `dest' parameter.

char *make_relative_filename(char *dest, const char *path, const char *filename, int size);
   Attempts to make a relative filename from an absolute path and an absolute
   filename, storing at most `size' bytes into the `dest' buffer. This
   function won't work if the paths are not canonical under the current
   platform (see canonicalize_filename()). Also, `dest' cannot be used as
   input value for `path' or `filename'. Example:

      char base[] = "/long/absolute/path/program.exe";
      char user_input[] = "/nice/and/short.txt";
      ...
      make_relative_filename(buf, base, user_input, sizeof(buf));
      /* Under Unix buf would contain:
         ../../../nice/and/short.txt */

   Returns a copy of the `dest' parameter if it succeeds or NULL if it fails
   (eg. under DOS, one path starts with "C:\" and another with "A:\").

int is_relative_filename(const char *filename);
   Returns TRUE if the filename is relative or FALSE if it is absolute. Note
   that an absolute filename under DOS (with a device separator) will be
   considered as relative under Unix, because there absolute paths always
   start with a slash.

char *replace_filename(char *dest, const char *path, 
                       const char *filename, int size);
   Replaces the specified path+filename with a new filename tail, storing 
   at most `size' bytes into the `dest' buffer. You can use the same buffer
   both as input and output because Allegro internally works on a copy of
   the input before touching `dest'. Example:

      char name[200];
      ...
      get_executable_name(name, sizeof(name));
      replace_filename(name, name, "sound.dat", sizeof(name));

   Returns a copy of the `dest' parameter.

char *replace_extension(char *dest, const char *filename, 
                        const char *ext, int size);
   Replaces the specified filename+extension with a new extension tail,
   storing at most `size' bytes into the `dest' buffer. If the filename
   doesn't have any extension at all, `ext' will be appended to it, adding
   a dot character if needed. You can use the same buffer both as input and
   output because Allegro internally works on a copy of the input before
   touching `dest'. Example:

      replace_extension(buf, "C:\\game\\prog.exe",
                        "dat", sizeof(buf));

   Returns a copy of the `dest' parameter.

char *append_filename(char *dest, const char *path, 
                      const char *filename, int size);
   Concatenates the specified filename onto the end of the specified path, 
   storing at most `size' bytes into the `dest' buffer. If `path' doesn't
   have a trailing path separator, the function will append one if needed.
   You can use the same buffer both as input and output because Allegro
   internally works on a copy of the input before touching `dest'. Example:

      append_filename(buf, "/home/user",
                      "prog.bin", sizeof(buf));

   Returns a copy of the `dest' parameter.

char *get_filename(const char *path);
   Finds out the filename portion of a completely specified file path. Both
   `\' and `/' are recognized as directory separators under DOS and Windows.
   However, only `/' is recognized as directory separator under other
   platforms. Example:

      get_executable_name(name, sizeof(name));
      allegro_message("Running `%s'\n", get_filename(name));

   Note that Allegro won't perform any IO operations during the verification.
   This means that if you have `/a/path/like/this/', which doesn't have a
   filename, the function will return a pointer to the trailing null
   character. However, if you have `/a/path/like/this', Allegro will return
   a pointer to `this', even if it is a valid directory.

   Returns a pointer to the portion of `path' where the filename starts, or
   the beginning of `path' if no valid filename is found (eg. you are
   processing a path with backslashes under Unix).

char *get_extension(const char *filename);
   Finds out the extension of the filename (with or without path
   information). Example:

      get_executable_name(name, sizeof(name));
      allegro_message("The binary has the extension `%s'\n",
                      get_extension(name));

   Returns a pointer to the portion of `filename' where the extension starts,
   or a pointer to the trailing null character if there is no filename or it
   doesn't have extension.

void put_backslash(char *filename);
   If the last character of the filename is not a `\', `/', `#' or a device
   separator (ie. `:' under DOS), this routine will concatenate either a `\'
   or `/' on to it (depending on the platform). Note: ignore the function
   name, it's out of date.

void set_filename_encoding(int encoding)
   Sets the encoding to use for filenames. By default, UTF-8 is assumed.

int get_filename_encoding(void)
   Returns the encoding currently assumed for filenames.

int file_exists(const char *filename, int attrib, int *aret);
   Checks whether a file matching the given name and attributes (see
   beginning of this chapter) exists. If `aret' is not NULL, it will be set
   to the attributes of the matching file. Example:

      /* Check for a normal file. */
      if (file_exists("franken.dat", 0, NULL))
         allegro_message("It is alive!\n");

   Returns non-zero if the file exists, or zero if it doesn't or the
   specified attributes mask it out.

int exists(const char *filename);
   Shortcut version of file_exists(), which checks for normal files, which 
   may have the archive or read-only bits set, but are not hidden, 
   directories, system files, etc.

   Returns non-zero if the file exists, or zero if it doesn't.

uint64_t file_size_ex(const char *filename);
   Returns the size of a file, in bytes. If the file does not exist or an 
   error occurs, it will return zero and store the system error code in 
   errno.

time_t file_time(const char *filename);
   Returns the modification time (number of seconds since 00:00:00 GMT 
   1/1/1970) of a file. If the file does not exist or an error occurs, it 
   will return zero and store the system error code in errno.

int delete_file(const char *filename);
   Removes a file from the disk. You can't delete directories, though.

   Returns zero on success, non-zero on failure.

int for_each_file_ex(const char *name, int in_attrib, int out_attrib,
                     int (*callback)(const char *filename, int attrib,
                     void *param), void *param);
   Finds all the files on disk which match the given wildcard specification
   and file attributes, and executes callback() once for each. Basically, this
   is a convenient wrapper around al_findfirst(), al_findnext() and
   al_findclose(). `in_attrib' is a bitmask specifying the attributes the
   files must carry, `out_attrib' is a bitmask specifying the attributes the
   files must not carry; attributes which are not specified in either bitmasks
   are not taken into account for deciding whether callback() is invoked or
   not.

   The callback function will be passed three arguments: the first is a string
   which contains the completed filename (exactly the same string you passed
   to for_each_file_ex() but with meta characters), the second is the actual
   attributes of the file, and the third is a void pointer which is simply a
   copy of `param' (you can use this for whatever you like). The callback must
   return zero to let the enumeration proceed, or any non-zero value to stop
   it. If an error occurs, the error code will be stored in `errno' but the
   enumeration won't stop. Example:

      int show_name(const char *filename, int attrib, void *param)
      {
         allegro_message("Caught `%s', attribs %d\n",
                         filename, attrib);
         return 0;
      }
      ...
         count = for_each_file_ex("data/level*", FA_DIREC,
                                  0, show_name, 0);
         allegro_message("%d game directories\n", count);

   Returns the number of successful calls made to callback(), that is, the
   number of times callback() was called and returned 0.

int al_findfirst(const char *pattern, struct al_ffblk *info, int attrib);
   Low-level function for searching files. This function finds the first 
   file which matches the given wildcard specification and file attributes 
   (see above). The information about the file (if any) will be put in the 
   al_ffblk structure which you have to provide. The al_ffblk structure 
   looks like:

      struct al_ffblk
      {
         int attrib;       - actual attributes of the file found
         time_t time;      - modification time of file
         char name[512];   - name of file
      };

   There is some other stuff in the structure as well, but it is there for 
   internal use only. Example:

      struct al_ffblk info;

      if (al_findfirst("*.pcx", &info, FA_ALL) != 0) {
         /* Tell user there are no PCX files. */
         return;
      }

   The function returns non-zero if no match is found or if an error occurred
   and, in the latter case, sets `errno' accordingly. It returns zero if a
   match is found, allocating some memory for internal use in the structure.
   Therefore you have to close your search when you are finished to avoid
   memory leaks in your program.

int al_findnext(struct al_ffblk *info);
   This finds the next file in a search started by al_findfirst(). Example:

      if (al_findfirst("*.pcx", &info, 0) != 0)
         return;

      do {
         /* Do something useful here with info.name. */
      } while (al_findnext(&info) == 0);

      al_findclose(&info);

   Returns zero if a match is found, non-zero if none is found or if an
   error occurred and, in the latter case, sets errno accordingly.

void al_findclose(struct al_ffblk *info);
   This closes a previously opened search with al_findfirst(). You need to
   call this on all successfully opened searches to avoid memory leaks in
   your program.

uint64_t al_ffblk_get_size(struct al_ffblk *info);
   This returns the size of the file returned by al_findfirst or al_findnext.

int find_allegro_resource(char *dest, const char *resource, 
                          const char *ext, const char *datafile, 
                          const char *objectname, const char *envvar, 
                          const char *subdir, int size);
   Searches for a support file, eg. `allegro.cfg' or `language.dat'. Passed a
   resource string describing what you are looking for, along with extra 
   optional information such as the default extension, what datafile to look 
   inside, what the datafile object name is likely to be, any special 
   environment variable to check, and any subdirectory that you would like 
   to check as well as the default location, this function looks in a hell 
   of a lot of different places :-). Pass NULL for the parameters you are not
   using.

   Check the documentation chapter specific to your platform for information
   on additional paths this function might search for. Also, don't forget
   about set_allegro_resource_path() to extend the searches. Example:

      char path[256];
      int ret;
      ret = find_allegro_resource(path, "scores.cfg", NULL, NULL,
                                  NULL, NULL, NULL, sizeof(path));
      if (ret == 0) {
         /* Found system wide scores file. */
      } else {
         /* No previous scores, create our own file. */
      }

   Returns zero on success, and stores a full path to the file (at most size
   bytes) into the dest buffer.

int set_allegro_resource_path(int priority, const char *path);
   Sometimes Allegro doesn't look in enough places to find a resource. For
   those special cases, you can call this function before loading your
   resource with additional paths to search for. You set up the priorities,
   higher numbers are searched for first. To modify an already setup path,
   call this function with the same priority and the new path. To remove an
   already setup path, call this function with the priority of the path and
   NULL as the path parameter. Example:

      set_allegro_resource_path(10, "my_game/configs");
      set_allegro_resource_path(0, "users/configs/");
      set_allegro_resource_path(-45, "temp");

   These custom paths will be valid until you call allegro_exit(). You can
   call this function before install_allegro(), but after set_uformat() if
   you want to use a text encoding format other than the default.

   Returns non-zero on success, zero if the path could not be added or you
   wanted to remove a path and the priority used didn't have any associated
   path. Modification of existing paths always succeeds.

void packfile_password(const char *password);
   Sets the encryption password to be used for all read/write operations
   on files opened in future using Allegro's packfile functions (whether
   they are compressed or not), including all the save, load and config
   routines. Files written with an encryption password cannot be read
   unless the same password is selected, so be careful: if you forget the
   key, nobody can make your data come back again! Pass NULL or an empty
   string to return to the normal, non-encrypted mode. If you are using
   this function to prevent people getting access to your datafiles, be
   careful not to store an obvious copy of the password in your executable:
   if there are any strings like "I'm the password for the datafile", it
   would be fairly easy to get access to your data :-)

   Note #1: when writing a packfile, you can change the password to whatever
   you want after opening the file, without affecting the write operation.
   On the contrary, when writing a sub-chunk of a packfile, you must make
   sure that the password that was active at the time the sub-chunk was
   opened is still active before closing the sub-chunk. This is guaranteed
   to be true if you didn't call the packfile_password() routine in the
   meantime. Read operations, either on packfiles or sub-chunks, have no
   such restriction.

   Note #2: as explained above, the password is used for all read/write
   operations on files, including for several functions of the library that
   operate on files without explicitly using packfiles (e.g. load_bitmap()).
   The unencrypted mode is mandatory in order for those functions to work.
   Therefore remember to call packfile_password(NULL) before using them if
   you previously changed the password. As a rule of thumb, always call
   packfile_password(NULL) when you are done with operations on packfiles.
   The only exception to this is custom packfiles created with
   pack_fopen_vtable().

PACKFILE *pack_fopen(const char *filename, const char *mode);
   Opens a file according to mode, which may contain any of the flags:

      `r' - open file for reading.

      `w' - open file for writing, overwriting any existing data.

      `p' - open file in packed mode. Data will be compressed as it is
            written to the file, and automatically uncompressed during read 
            operations. Files created in this mode will produce garbage if 
            they are read without this flag being set. 

      `!' - open file for writing in normal, unpacked mode, but add the
            value F_NOPACK_MAGIC to the start of the file, so that it can 
            later be opened in packed mode and Allegro will automatically 
            detect that the data does not need to be decompressed.

   Instead of these flags, one of the constants F_READ, F_WRITE, 
   F_READ_PACKED, F_WRITE_PACKED or F_WRITE_NOPACK may be used as the mode 
   parameter.

   The packfile functions also understand several "magic" filenames that are 
   used for special purposes. These are:

      `#' - read data that has been appended to your executable file with
      the exedat utility, as if it was a regular independent disk file.

      `filename.dat#object_name' - open a specific object from a datafile,
      and read from it as if it was a regular file. You can treat nested 
      datafiles exactly like a normal directory structure, for example 
      you could open `filename.dat#graphics/level1/mapdata'.

      `#object_name' - combination of the above, reading an object from a
      datafile that has been appended onto your executable.

   With these special filenames, the contents of a datafile object or 
   appended file can be read in an identical way to a normal disk file, so 
   any of the file access functions in Allegro (eg. load_pcx() and 
   set_config_file()) can be used to read from them. Note that you can't 
   write to these special files, though: the fake file is read only. Also, 
   you must save your datafile uncompressed or with per-object compression 
   if you are planning on loading individual objects from it (otherwise 
   there will be an excessive amount of seeking when it is read).

   Finally, be aware that the special Allegro object types aren't the same
   format as the files you import the data from. When you import data like
   bitmaps or samples into the grabber, they are converted into a special
   Allegro-specific format, but the `#' marker file syntax reads the objects
   as raw binary chunks. This means that if, for example, you want to use 
   load_pcx() to read an image from a datafile, you should import it as a
   binary block rather than as a BITMAP object.

   Example:

      PACKFILE *input_file;

      input_file = pack_fopen("scores.dat", "rp");
      if (!input_file)
         abort_on_error("Couldn't read `scores.dat'!");

   On success, pack_fopen() returns a pointer to a PACKFILE structure, and on
   error it returns NULL and stores an error code in `errno'. An attempt to
   read a normal file in packed mode will cause `errno' to be set to EDOM.

PACKFILE *pack_fopen_vtable(const PACKFILE_VTABLE *vtable, void *userdata);
   Creates a new packfile structure that uses the functions specified in
   the vtable instead of the standard functions. The data pointer by `vtable'
   and `userdata' must remain available for the lifetime of the created
   packfile.

   While the created packfile structure can be used with other Allegro
   functions, there are two limitations. First, opening chunks using
   pack_fopen_chunk() on top of the returned packfile is not possible at this
   time. And packfile_password() does not have any effect on packfiles opened
   with pack_fopen_vtable().

   On success, it returns a pointer to a PACKFILE structure, and on error it
   returns NULL and stores an error code in `errno'.

int pack_fclose(PACKFILE *f);
   Closes the stream `f' previously opened with pack_fopen() or
   pack_fopen_vtable(). After you have closed the stream, performing
   operations on it will yield errors in your application (e.g. crash it) or
   even block your OS.

   Returns zero on success. On error, returns an error code which is also
   stored in `errno'. This function can fail only when writing to files: if
   the file was opened in read mode, it will always succeed.

int pack_fseek(PACKFILE *f, int offset);
   Moves the position indicator of the stream `f'. Unlike the standard fseek()
   function, this only supports forward movements relative to the current
   position and in read-only streams, so don't use negative offsets. Note that
   seeking is very slow when reading compressed files, and so should be
   avoided unless you are sure that the file is not compressed. Example:

      input_file = pack_fopen("data.bin", "r");
      if (!input_file)
         abort_on_error("Couldn't open binary data!");
      /* Skip some useless header before reading data. */
      pack_fseek(input_file, 32);

   Returns zero on success or a negative number on error, storing the error
   code in `errno'.

int pack_feof(PACKFILE *f);
   Finds out if you have reached the end of the file. It does not wait for you
   to attempt to read beyond the end of the file, contrary to the ISO C feof()
   function. The only way to know whether you have read beyond the end of the
   file is to check the return value of the read operation you use (and be
   wary of pack_*getl() as EOF is also a valid return value with these
   functions).

   Returns non-zero if you are at the end of the file, zero otherwise.

int pack_ferror(PACKFILE *f);
   Since EOF is used to report errors by some functions, it's often better to
   use the pack_feof() function to check explicitly for end of file and
   pack_ferror() to check for errors. Both functions check indicators that
   are part of the internal state of the stream to detect correctly the
   different situations.

   Returns nonzero if the error indicator for the stream is set, meaning
   that an error has occurred during a previous operation on the stream.

int pack_getc(PACKFILE *f);
   Returns the next character from the stream `f', or EOF if the end of the
   file has been reached.

int pack_ungetc(int c, PACKFILE *f);
   Puts a character back to the file's input buffer. Like with ungetc from
   libc, only a single push back is guaranteed.

   Note: pack_fgets internally uses pack_ungetc, so never use pack_ungetc
   directly after using pack_fgets on a PACKFILE.

   Returns c on success, or EOF on error.

int pack_putc(int c, PACKFILE *f);
   Puts a character in the stream f.

   Returns the character written on success, or EOF on error.

int pack_igetw(PACKFILE *f);
   Like pack_getc, but reads a 16-bit word from a file, using Intel byte
   ordering (least significant byte first, a.k.a. little-endian).

int pack_iputw(int c, PACKFILE *f);
   Like pack_putc, but writes a 16-bit word to a file, using Intel byte
   ordering (least significant byte first, a.k.a. little-endian).

long pack_igetl(PACKFILE *f);
   Like pack_getc, but reads a 32-bit long from a file, using Intel byte
   ordering (least significant byte first, a.k.a. little-endian).

long pack_iputl(long c, PACKFILE *f);
   Like pack_putc, but writes a 32-bit long to a file, using Intel byte
   ordering (least significant byte first, a.k.a. little-endian).

int pack_mgetw(PACKFILE *f);
   Like pack_getc, but reads a 16-bit word from a file, using Motorola byte
   ordering (most significant byte first, a.k.a. big-endian).

int pack_mputw(int c, PACKFILE *f);
   Like pack_putc, but writes a 16-bit word to a file, using Motorola byte
   ordering (most significant byte first, a.k.a. big-endian).

long pack_mgetl(PACKFILE *f);
   Like pack_getc, but reads a 32-bit long from a file, using Motorola byte
   ordering (most significant byte first, a.k.a. big-endian).

long pack_mputl(long c, PACKFILE *f);
   Like pack_putc, but writes a 32-bit long to a file, using Motorola byte
   ordering (most significant byte first, a.k.a. big-endian).

long pack_fread(void *p, long n, PACKFILE *f);
   Reads `n' bytes from the stream `f', storing them at the memory location
   pointed to by `p'. Example:

      unsigned char buf[256];
      ...
      if (pack_fread(buf, 256, input_file) != 256)
         abort_on_error("Truncated input file!");

   Returns the number of bytes read, which will be less than `n' if EOF is
   reached or an error occurs. Error codes are stored in errno.

long pack_fwrite(const void *p, long n, PACKFILE *f);
   Writes `n' bytes to the stream `f' from memory location pointed to by `p'.

   Returns the number of bytes written, which will be less than n if an
   error occurs. Error codes are stored in errno.

char *pack_fgets(char *p, int max, PACKFILE *f);
   Reads a line from the stream `f', storing it at location pointed to by
   `p'. Stops when a linefeed is encountered, or `max' bytes have been read.
   The end of line is handled by detecting the right combination of characters
   for the platform. This supports CR-LF (DOS/Windows), LF (Unix), and CR
   (Mac) formats. However, the trailing carriage return is not included in the
   returned string, in order to provide easy code portability across
   platforms. If you need the carriage return, use pack_fread() and/or
   pack_getc() instead.

   Note: This function internally may make calls to pack_ungetc, so you cannot
   use pack_ungetc directly afterwards.

   Example:

      char buf[256];
      ...
      while (pack_fgets(buf, sizeof(buf), input_file)) {
         /* Process input line. */
      }
      fclose(input_file);

   Returns the pointer `p' on success, or NULL on error.

int pack_fputs(const char *p, PACKFILE *f);
   Writes a string to the stream `f'. The input string is converted from the
   current text encoding format to UTF-8 before writing. Newline characters
   are written as `\r\n' on DOS and Windows platforms. If you don't want this
   behaviour, use pack_fwrite() and/or pack_putc() instead.

   Returns zero on success or a negative number on error.

PACKFILE *pack_fopen_chunk(PACKFILE *f, int pack);
   Opens a sub-chunk of a file. Chunks are primarily intended for use by the 
   datafile code, but they may also be useful for your own file routines. A 
   chunk provides a logical view of part of a file, which can be compressed 
   as an individual entity and will automatically insert and check length 
   counts to prevent reading past the end of the chunk. The PACKFILE
   parameter is a previously opened file, and `pack' is a boolean parameter
   which will turn compression on for the sub-chunk if it is non-zero.
   Example:

      PACKFILE *output = pack_fopen("out.raw", "w!");
      ...
      /* Create a sub-chunk with compression. */
      output = pack_fopen_chunk(output, 1);
      if (!output)
         abort_on_error("Error saving data!");
      /* Write some data to the sub-chunk. */
      ...
      /* Close the sub-chunk, recovering parent file. */
      output = pack_fclose_chunk(output);

   The data written to the chunk will be prefixed with two length counts
   (32-bit, a.k.a. big-endian). For uncompressed chunks these will both be
   set to the size of the data in the chunk. For compressed chunks (created
   by setting the `pack' flag), the first length will be the raw size of the
   chunk, and the second will be the negative size of the uncompressed data.

   To read the chunk, use the following code:

      PACKFILE *input = pack_fopen("out.raw", "rp");
      ...
      input = pack_fopen_chunk(input, 1);
      /* Read data from the sub-chunk and close it. */
      ...
      input = pack_fclose_chunk(input);

   This sequence will read the length counts created when the chunk was 
   written, and automatically decompress the contents of the chunk if it 
   was compressed. The length will also be used to prevent reading past the 
   end of the chunk (Allegro will return EOF if you attempt this), and to 
   automatically skip past any unread chunk data when you call 
   pack_fclose_chunk().

   Chunks can be nested inside each other by making repeated calls to 
   pack_fopen_chunk(). When writing a file, the compression status is 
   inherited from the parent file, so you only need to set the pack flag if 
   the parent is not compressed but you want to pack the chunk data. If the 
   parent file is already open in packed mode, setting the pack flag will 
   result in data being compressed twice: once as it is written to the 
   chunk, and again as the chunk passes it on to the parent file.

   Returns a pointer to the sub-chunked PACKFILE, or NULL if there was some
   error (eg. you are using a custom PACKFILE vtable).

PACKFILE *pack_fclose_chunk(PACKFILE *f);
   Closes a sub-chunk of a file, previously obtained by calling 
   pack_fopen_chunk().

   Returns a pointer to the parent of the sub-chunk you just closed. Returns
   NULL if there was some error (eg. you tried to close a PACKFILE which
   wasn't sub-chunked).

LZSS_PACK_DATA *create_lzss_pack_data(void);
   Creates an LZSS_PACK_DATA structure, which can be used for LZSS
   compression with PACKFILEs.

   Returns a pointer to the structure, or NULL if there was an error.

void free_lzss_pack_data(LZSS_PACK_DATA *dat);
   Frees an LZSS_PACK_DATA structure created with create_lzss_pack_data().

int lzss_write(PACKFILE *file, LZSS_PACK_DATA *dat, int size,
               unsigned char *buf, int last);
   Packs `size' bytes from `buf', using the pack information contained in
   `dat'. The compressed bytes will be stored in `file'.

   Returns 0 on success, or EOF if there was an error.

LZSS_UNPACK_DATA *create_lzss_unpack_data(void);
   Creates an LZSS_UNPACK_DATA structure, which can be used for LZSS
   decompression reading PACKFILEs.

   Returns a pointer to the structure, or NULL if there was an error.

void free_lzss_unpack_data(LZSS_UNPACK_DATA *dat);
   Frees an LZSS_UNPACK_DATA structure created with create_lzss_pack_data.

int lzss_read(PACKFILE *file, LZSS_UNPACK_DATA *dat, int s,
              unsigned char *buf);
   Unpacks from `dat' into `buf', until either EOF is reached or `s' bytes
   have been extracted from `file'.

   Returns the number of bytes added to the buffer `buf'.



===========================================
============ Datafile routines ============
===========================================

Datafiles are created by the grabber utility (see grabber.txt for more
information), and have a `.dat' extension. They can contain bitmaps, palettes,
fonts, samples, MIDI music, FLI/FLC animations, and any other binary data that
you import. You could distribute your bitmaps and samples in a myriad of
separate files, but packing them in a few `.dat' binaries has a few
advantages:

   On some platforms loading a single big datafile at once is faster than
   loading individual resources one by one.

   Instead of several loops for your resources, you can write a single line of
   code with just a single point of failure to take care of.

   You can potentially reduce the size of your data by enabling compression
   on your datafiles. Less download time for your end users, less wait during
   loading screens!

   If you don't need to load the whole datafile at once, you can still enable
   individual file compression. It is slightly worse than global compression,
   but it is very fast with loading times because Allegro can easily seek
   inside the datafile to find a specific object.

   Even without encryption, most end users of your application won't be able
   to look at or modify the resources for your game. A missing sound file or
   a modified bitmap could potentially crash the game if you haven't
   considered this in your loading code!

   It looks much more professional and convenient to distribute levels! For
   example, if you found a bug in a level of your game, just distribute your
   new `level4.dat' and tell users to overwrite their old version.

Allegro allows you to load datafiles once and forget about them. But if you
have many levels it can be wise to load only the resources required for the
current level. You can accomplish the later by separating levels in different
datafiles, or using functions like load_datafile_object() to avoid loading
everything at once. You can even read directly from a specific datafile object
with the pack_fopen() function.

On some platforms you can attach datafiles to your binary, potentially
reducing your game distribution down to a single executable file. Try the
example exexedat on your platform to see if this is possible. However, this
is not recommended for big programs: a single update to your code or binary
data would force end users to download again a potentially big file, no matter
how small your change is. The same warning goes for the tools dat2s or dat2c,
which convert datafiles into assembler and C code respectively, prepared to be
included directly in your binary.

Remember that with Allegro truecolor images can only be loaded after you have
set a graphics mode. This is true for datafiles too. Load all your data after
you have set the graphics mode, otherwise the pixel format (RGB or BGR) will
not be known and the datafile may be converted wrongly. Oh, and read carefully
the warning of fixup_datafile() if you plan on switching resolutions during
runtime.

Note: even though Allegro datafiles provide encryption, you should consider
it weak, so don't plan on hiding there the plans for a Death Star or
something. Determinate knowledgeable users will be able to rip your resources
no matter how hard you try to hide them! Use the encryption only as a slight
deterrent towards unwanted tampering of your data. How to crack an encrypted
datafile is left as an exercise to the reader, though.

Using datafiles once they are loaded is quite simple: you access the elements
of the DATAFILE as a normal array. Read below the section "Using datafiles"
below for several examples on how to access their data.

DATAFILE *load_datafile(const char *filename);
   Loads a datafile into memory in one go. If the datafile has been encrypted,
   you must first call packfile_password() to set the appropriate key. If the
   datafile contains truecolor graphics, you must set the video mode or call
   set_color_conversion() before loading it. Example:

      /* Load the resources for our game. */
      DATAFILE *dat = load_datafile("game.dat");
      if (!dat)
         abort_on_error("Couldn't load sound resources!");
      /* Use resources. */
      ...
      /* Destroy them when we don't need them any more. */
      unload_datafile(dat);

   Returns a pointer to the DATAFILE, or NULL on error. Remember to free this
   DATAFILE later to avoid memory leaks.

DATAFILE *load_datafile_callback(const char *filename,
                                 void (*callback)(DATAFILE *d));
   Loads a datafile into memory, calling the specified hook function once for
   each object in the file, passing it a pointer to the object just read. You
   can use this to implement very simple loading screens where every time the
   hook is called, the screen is updated to let the user know your program is
   still loading from disk:

      void load_callback(DATAFILE *dat_obj)
      {
         static const char indicator[] = "-\\|/-.oOXOo.";
         static int current = 0;
          
         /* Show a different character every time. */
         textprintf_ex(screen, font, 0, 0, makecol(0, 0, 0),
                       makecol(255, 255, 255), "%c Loading %c",
                       indicator[current], indicator[current]);
         /* Increase index and check if we need to reset it. */
         current++;
         if (!indicator[current])
            current = 0;
      }
         ...
         dat = load_datafile_callback("data.dat", load_callback);

   Returns a pointer to the DATAFILE or NULL on error. Remember to free this
   DATAFILE later to avoid memory leaks.

void unload_datafile(DATAFILE *dat);
   Frees all the objects in a datafile. Use this to avoid memory leaks in
   your program.

DATAFILE *load_datafile_object(const char *filename, 
                               const char *objectname);
   Loads a specific object from a datafile. This won't work if you strip the 
   object names from the file, and it will be very slow if you save the file 
   with global compression. Example:

      /* Load only the music from the datafile. */
      music_object = load_datafile_object("datafile.dat",
                                          "MUSIC");
      /* Play it and wait a moment for it. */
      play_midi(music_object->dat);
      ...
      /* Destroy unneeded music. */
      unload_datafile_object(music_object);

   Returns a pointer to a single DATAFILE element whose `dat' member points to
   the object, or NULL if there was an error or there was no object with the
   requested name. Remember to free this DATAFILE later to avoid memory leaks,
   but use the correct unloading function!

void unload_datafile_object(DATAFILE *dat);
   Frees an object previously loaded by load_datafile_object(). Use this to
   avoid memory leaks in your program.

DATAFILE *find_datafile_object(const DATAFILE *dat, const char *objectname);
   Searches an already loaded datafile for an object with the specified 
   name. In the name you can use `/' and `#' separators for nested datafile
   paths. Example:

      char level_name[10];
      DATAFILE *dat, *level;
      ...
      uszprintf(level_name, sizeof(buffer),
                "LEVEL_%02d", level_number);
      level = find_datafile_object(dat, level_name);
      if (!level)
         abort_on_error("That level doesn't exist!");

   Returns a pointer to a single DATAFILE element whose `dat' member points to
   the object, or NULL if the object could not be found.

DATAFILE_INDEX *create_datafile_index(const char *filename);
   Creates an index for a datafile, to speed up loading single objects out of
   it. This is mostly useful for big datafiles, which you don't want to load as
   a whole. The index will store the offset of all objects inside the datafile,
   and then you can load it quickly with "load_datafile_object_indexed" later.
   Use destroy_datafile_index to free the memory used by it again.

   Note: If the datafile uses global compression, there is no performance gain
   from using an index, because seeking to the offset still requires to
   uncompress the whole datafile up to that offset.
   Example:

   DATAFILE_INDEX *index = create_datafile_index("huge.dat");
   DATAFILE *object = load_datafile_object_indexed(index, 1234);
   ...
   unload_datafile_object(object);
   destroy_datafile_index(index);

   A pointer value which you can pass to load_datafile_object_indexed.

DATAFILE *load_datafile_object_indexed(const DATAFILE_INDEX *index, int item)
   This loads a single object, using the index created previously with
   create_datafile_index. See create_datafile_index for an example.

   Returns a pointer to a single DATAFILE element whose "dat" member points to
   the object, or NULL if the object could not be loaded.

void destroy_datafile_index(DATAFILE_INDEX *index)
   This function frees the memory used by a datafile index created with
   create_datafile_index earlier.

const char *get_datafile_property(const DATAFILE *dat, int type);
   Finds the property type of a DATAFILE object. The type parameter must be a
   value created with the DAT_ID() macro. Example:

      const char *name;
      ...
      name = get_datafile_property(game_data,
                                   DAT_ID('N','A','M','E'));
      if (name == empty_string)
         abort_on_error("Object doesn't have a name!");

   Returns a pointer to the text string for the object, or a pointer to the
   variable empty_string if the property isn't present.

void register_datafile_object(int id, void *(*load)(PACKFILE *f, long size),
                              void (*destroy)(void *data));
   Used to add custom object types, specifying functions to load and destroy 
   objects of this type.

void fixup_datafile(DATAFILE *data);
   If you are using compiled datafiles (produced by the dat2s and dat2c
   utilities) on a platform that doesn't support constructors (currently any
   non GCC-based platform), or if the datafiles contain truecolor images, you
   must call this function once after your set the video mode that you will
   be using. This will ensure the datafiles are properly initialised in the
   first case and convert the color values into the appropriate format in
   the second case. It handles flipping between RGB and BGR formats, and
   converting between different color depths whenever that can be done
   without changing the size of the image (ie. changing 15<->16-bit
   hicolor for both bitmaps and RLE sprites, and 24<->32-bit truecolor
   for RLE sprites).

   Note that you can only call this once and expect it to work correctly,
   because after the call the DATAFILE you fixed up is permanently converted
   to whatever is the current component ordering for your screen mode. If you
   call fixup_datafile again, the function assumes you have a freshly loaded
   datafile. It cannot "undo" the previous conversion.

   If your program supports changing resolution and/or color depth during
   runtime, you have two choices: either call fixup_datafile() just once and
   hope that the component ordering and bit depth doesn't change when the
   screen mode changes (unlikely). Or, you can reload your datafiles when the
   screen mode changes.

Macro DAT_ID(a, b, c, d);
   Every object or property in a datafile is identified by a 4 letter ID,
   which can be created with this macro. For example, to access the NAME
   property of a datafile object, you could use:

      get_datafile_property(datob, DAT_ID('N','A','M','E'));



Using datafiles
---------------


In order to access the contents of a datafile, you will need to know where
each object is located. The easiest way to do this is by integer index,
using an automatically generated header file. With the grabber, type a name
into the "Header:" field, and the object indexes will be written to this
file whenever the datafile is saved. With the dat utility, use the '-h'
option, eg. "dat filename.dat -h filename.h". The header will define C
preprocessor symbols for each object in the datafile, for example:

   #define SOME_DATA                        0        /* DATA */
   #define SOME_MORE_DATA                   1        /* DATA */

To prevent name conflicts, you can specify a prefix string for these
definitions by typing it into the "Prefix:" field in the grabber or using 
the '-p' option to dat.

To load a datafile into memory, call the function:

   DATAFILE *load_datafile(char *filename);

This will load the entire file, returning a pointer to it, or NULL on error.
When the data is no longer required, the entire thing can be destroyed by
calling:

   void unload_datafile(DATAFILE *dat);

When you load a datafile, you will obtain a pointer to an array of DATAFILE
structures:

   typedef struct DATAFILE
   {
      void *dat;                    - pointer to the actual data
      int type;                     - object type ID
      long size;                    - size of the data, in bytes
      DATAFILE_PROPERTY *prop;      - list of object properties
   } DATAFILE;

The only really important piece of information here is the `dat' field, which
points to the contents of the object. What type of data this is will depend 
on the type of object: for bitmaps it will be an Allegro BITMAP structure, 
for RLE sprites an RLE_SPRITE, for fonts a FONT structure, etc. If you are 
programming in C you can pass this pointer directly to the relevant Allegro 
library functions, but if you are using C++ you will need to cast it to the 
appropriate type to prevent the compiler giving a warning.

For example, if you have a datafile called `myfile.dat', which contains a
bitmap called COOL_PICTURE, and you have used it to produce a header called 
`myfile.h', you could display the bitmap with the code:

   #include "myfile.h"

   void show_the_bitmap()
   {
      DATAFILE *dat;
      BITMAP *bmp;

      dat = load_datafile("myfile.dat");
      if (!dat) {
         /* report an error! */
         return;
      }

      bmp = (BITMAP *)dat[COOL_PICTURE].dat;
      blit(bmp, screen, 0, 0, 0, 0, bmp->w, bmp->h);
      unload_datafile(dat);
   }

If a datafile contains nested child datafiles, the header will prefix the 
names of objects in the sub-files with the name of their parent datafile. It 
will also define a count of the number of objects in the child file, which 
may be useful if for example the child datafile contains several bitmaps 
which form a 'run' animation, and you want your code to automatically 
adjust to the number of frames in the datafile. 

For example, the following datafile:

   "FILE" - NESTED_FILE
            |- "BMP" - A_BITMAP
            |- "FONT" - A_FONT
   "DATA" - SOME_DATA
   "DATA" - SOME_MORE_DATA

Will produce the header:

   #define NESTED_FILE                      0        /* FILE */

   #define NESTED_FILE_A_BITMAP             0        /* BMP  */
   #define NESTED_FILE_A_FONT               1        /* FONT */
   #define NESTED_FILE_COUNT                2

   #define SOME_DATA                        1        /* DATA */
   #define SOME_MORE_DATA                   2        /* DATA */

The main datafile contains three objects (NESTED_FILE, SOME_DATA, and 
SOME_MORE_DATA) with consecutive indexes, while the child datafile contains 
the two objects A_BITMAP and A_FONT. To access these objects you need to 
reference both the parent and child datafiles, eg:

   DATAFILE *dat = load_datafile("whatever.dat");
   DATAFILE *nested = (DATAFILE *)dat[NESTED_FILE].dat;
   FONT *thefont = (FONT *)nested[NESTED_FILE_A_FONT].dat;

If you need to access object property strings from within your program, you
can use the function:

   char *get_datafile_property(DATAFILE *dat, int type);

This will return a pointer to the property string if it can be found, and an
empty string (not null!) if it does not exist. One possible use of this 
function is to locate objects by name, rather than using the indexes from a 
header file. The datafile array is ended by an object of type DAT_END, so to 
search the datafile dat for the object "my_object" you could use the code:

   const int name_type = DAT_ID('N','A','M','E');
   for (i=0; dat[i].type != DAT_END; i++) {
      if (stricmp(get_datafile_property(dat+i, name_type),
                  "my_object") == 0) {
         /* found the object at index i */
      }
   }
   /* not found... */

If you prefer to access objects by name rather than index number, you can
use the function:

   DATAFILE *find_datafile_object(DATAFILE *dat, char *objectname);

This will search an already loaded datafile for an object with the specified
name, returning a pointer to it, or NULL if the object cannot be found. It 
understands '/' and '#' separators for nested datafile paths.

It is also possible to selectively load individual objects from a datafile, 
with the function:

   DATAFILE *load_datafile_object(char *filename, char *objectname);

This searches the datafile for an object with the specified name, so
obviously it won't work if you strip the name properties out of the file. 
Because this function needs to seek through the data, it will be extremely 
slow if you have saved the file with global compression. If you are planning 
to load objects individually, you should save the file uncompressed or with 
individual compression per-object. Because the returned datafile points to a 
single object rather than an array of objects, you should access it with the 
syntax datafile->dat, rather than datafile[index].dat, and when you are done
you should free the object with the function:

   void unload_datafile_object(DATAFILE *dat);

Example:

   music_object = load_datafile_object("datafile.dat", "MUSIC");
   play_midi(music_object->dat);
   ...
   unload_datafile_object(music_object);

Alternatively, the packfile functions can open and read directly from the
contents of a datafile object. You do this by calling pack_fopen() with a 
fake filename in the form "filename.dat#object_name". The contents of the 
object can then be read in an identical way to a normal disk file, so any of 
the file access functions in Allegro (eg. load_pcx() and set_config_file()) 
can be used to read from datafile objects. Note that you can't write to 
datafiles in this way: the fake file is read only. Also, you should save the 
file uncompressed or with per-object compression if you are planning on 
using this feature. Finally, be aware that the special Allegro object types 
aren't the same format as the files you import the data from, so if for 
example you want to use load_pcx to read an image from a datafile, you 
should import it as a binary data chunk rather than as a BITMAP object.

If you have appended a datafile to the end of your executable with the 
exedat utility, use load_datafile("#") to read the entire thing into memory, 
load_datafile_object("#", "object_name") to load a specific object, and 
pack_fopen("#object_name", F_READ) to read one of the objects directly with 
your own code. Note that unless you use the previous functions to load the
appended data, the OS will not load it into memory just because you are
running the program, so you shouldn't have problems attaching datafiles to
your binary larger than the available system memory.

By default, all graphic objects loaded from a datafile will be converted 
into the current color depth. This conversion may be both lossy and very 
slow, particularly when reducing from truecolor to 256 color formats, so you 
may wish to disable it by calling set_color_conversion(COLORCONV_NONE) or 
set_color_conversion(COLORCONV_PARTIAL) before your call to load_datafile().



Custom datafile objects
-----------------------


Some of the objects in a datafile, for example palettes and FLI animations, 
are simply treated as blocks of binary data, but others are loaded into 
special formats such as bitmap structures or compiled sprites. It is 
possible to extend the datafile system to support your own custom object 
types, eg. map objects for a tile based engine, or level data for a platform 
game. Obviously the grabber has no way of understanding this data, but it 
will allow you to import binary data from external files, so you can grab 
information produced by your own utilities. If you are happy with the data 
being loaded as a simple binary block, that is all you need to do, but if 
you need to load it into a specific structure, read on...

Your custom objects must be given a unique type ID, which is formed from 
four ASCII characters (by convention all uppercase A-Z). If you don't use 
all four characters, the string should be padded with spaces (ASCII 32). You 
should use this ID when creating the objects in the grabber (select 
New/Other and type in the ID string), and in your code you should define an 
identifier for the type, eg:

   #define DAT_MAPDATA  DAT_ID('M','A','P','D')

You then need to write functions for loading and destroying objects of this
type, in the form:

   void *load_mapdata(PACKFILE *f, long size)
   {
      /* Allegro will call this function whenever an object of your custom 
       * type needs to be loaded from a datafile. It will be passed a 
       * pointer to the file from which the data is to be read, and the size 
       * of the object in bytes. It should return a pointer to the loaded 
       * data, which will be stored in the dat field of the datafile object 
       * structure, or NULL if an error occurs. The file will have been 
       * opened as a sub-chunk of the main datafile, so it is safe to read 
       * past the end of the object (if you attempt this, Allegro will 
       * return EOF), and it is also safe to return before reading all the 
       * data in the chunk (if you do this, Allegro will skip any unused 
       * bytes before starting to read the next object). You should _not_ 
       * close the file when you are done: this will be handled by the 
       * calling function. To clarify how all this works, here's an example 
       * implementation of a null-terminated string object:
       */

      #define MAX_LEN  256

      char buf[MAX_LEN];
      char *p;
      int i, c;

      for (i=0; i<;MAX_LEN-1; i++) {
         if ((c = pack_getc(f)) == EOF)
            break;

         buf[i] = c;
      }

      buf[i] = 0;

      p = malloc(i+1);
      strcpy(p, buf);

      return p;
   }

   void destroy_mapdata(void *data)
   {
      /* Allegro will call this function whenever an object of your custom 
       * type needs to be destroyed. It will be passed a pointer to the 
       * object (as returned by the load function), and should free whatever 
       * memory the object is using. For example, the simple string object 
       * returned by the above loader could be destroyed with the code:
       */

      if (data)
         free(data);
   }

Finally, before you load your datafile you must tell Allegro about the
custom format, by calling:

   register_datafile_object(DAT_MAPDATA, load_mapdata, destroy_mapdata);

It is also possible to integrate support for custom object types directly
into the grabber and dat utilities, by copying some special files into the 
tools/plugins directory. This can be used to add whole new object types and 
menu commands, or to provide additional import/export routines for the 
existing formats. See `tools/plugins/plugins.txt' for an overview of how to
write your own grabber plugins.



===================================================
============ Fixed point math routines ============
===================================================

Allegro provides some routines for working with fixed point numbers, and
defines the type `fixed' to be a signed 32-bit integer. The high word is 
used for the integer part and the low word for the fraction, giving a range
of -32768 to 32767 and an accuracy of about four or five decimal places.
Fixed point numbers can be assigned, compared, added, subtracted, negated and
shifted (for multiplying or dividing by powers of two) using the normal
integer operators, but you should take care to use the appropriate conversion
routines when mixing fixed point with integer or floating point values.
Writing `fixed_point_1 + fixed_point_2' is OK, but `fixed_point + integer' is
not.

Unfortunately the only advantage of fixed point math routines is that you
don't require a floating point coprocessor to use them. This was great in the
time period of i386 and i486 machines, but stopped being so useful with the
coming of the Pentium class of processors. From Pentium onwards, CPUs have
increased their strength in floating point operations, equaling or even
surpassing integer math performance.

Depending on the type of operations your program may need, using floating
point types may be faster than fixed types if you are targeting a specific
machine class. Allegro comes with a test program in the `allegro/tests'
directory. Its `Misc' menu contains a basic profile test which can give you
an idea of the speed difference between fixed and float types for a few basic
operations on your machine. However, don't forget to profile your program in
real life conditions, tight loop benchmarks are after all artificial.

Fixed point math is considered "add-on" material and is kept only for
backwards compatibility. Whenever a future release of Allegro breaks backwards
compatibility, fixed point math will likely be moved to a separate add-on
package for the very few users who still find it convenient and useful, and
Allegro functions using fixed point math will use other types.

fixed itofix(int x);
   Converts an integer to fixed point. This is the same thing as x<<16.
   Remember that overflows (trying to convert an integer greater than 32767)
   and underflows (trying to convert an integer lesser than -32768) are not
   detected even in debug builds! The values simply "wrap around". Example:

      fixed number;
      /* This conversion is OK. */
      number = itofix(100);
      ASSERT(fixtoi(number) == 100);
      number = itofix(64000);
      /* This check will fail in debug builds. */
      ASSERT(fixtoi(number) == 64000);

   Returns the value of the integer converted to fixed point ignoring
   overflows.

int fixtoi(fixed x);
   Converts fixed point to integer, rounding as required to the nearest
   integer. Example:

      int result;
      /* This will put 33 into `result'. */
      result = fixtoi(itofix(100) / 3);
      /* But this will round up to 17. */
      result = fixtoi(itofix(100) / 6);

int fixfloor(fixed x);
   Returns the greatest integer not greater than x. That is, it rounds 
   towards negative infinity. Example:

      int result;
      /* This will put 33 into `result'. */
      result = fixfloor(itofix(100) / 3);
      /* And this will round down to 16. */
      result = fixfloor(itofix(100) / 6);

int fixceil(fixed x);
   Returns the smallest integer not less than x. That is, it rounds towards
   positive infinity. Example:

      int result;
      /* This will put 34 into `result'. */
      result = fixceil(itofix(100) / 3);
      /* This will round up to 17. */
      result = fixceil(itofix(100) / 6);

fixed ftofix(double x);
   Converts a floating point value to fixed point. Unlike itofix(), this
   function clamps values which could overflow the type conversion, setting
   `errno' to ERANGE in the process if this happens. Example:

      fixed number;
      number = itofix(-40000);
      ASSERT(fixfloor(number) == -32768);
      number = itofix(64000);
      ASSERT(fixfloor(number) == 32767);
      ASSERT(!errno); /* This will fail. */

   Returns the value of the floating point value converted to fixed point
   clamping overflows (and setting `errno').

double fixtof(fixed x);
   Converts fixed point to floating point. Example:

      float result;
      
      /* This will put 33.33333 into `result'. */
      result = fixtof(itofix(100) / 3);
      /* This will put 16.66666 into `result'. */
      result = fixtof(itofix(100) / 6);

fixed fixmul(fixed x, fixed y);
   A fixed point value can be multiplied or divided by an integer with the 
   normal `*' and `/' operators. To multiply two fixed point values, though,
   you must use this function.

   If an overflow occurs, `errno' will be set and the maximum possible value
   will be returned, but `errno' is not cleared if the operation is
   successful. This means that if you are going to test for overflow you
   should set `errno=0' before calling fixmul(). Example:

      fixed result;

      /* This will put 30000 into `result'. */
      result = fixmul(itofix(10), itofix(3000));
      /* But this overflows, and sets `errno'. */
      result = fixmul(itofix(100), itofix(3000));
      ASSERT(!errno);

   Returns the clamped result of multiplying `x' by `y', setting `errno' to
   ERANGE if there was an overflow.

fixed fixdiv(fixed x, fixed y);
   A fixed point value can be divided by an integer with the normal `/'
   operator. To divide two fixed point values, though, you must use this
   function. If a division by zero occurs, `errno' will be set and the
   maximum possible value will be returned, but `errno' is not cleared if the
   operation is successful. This means that if you are going to test for
   division by zero you should set `errno=0' before calling fixdiv(). Example:

      fixed result;
      /* This will put 0.06060 `result'. */
      result = fixdiv(itofix(2), itofix(33));
      /* This will put 0 into `result'. */
      result = fixdiv(0, itofix(-30));
      /* Sets `errno' and puts -32768 into `result'. */
      result = fixdiv(itofix(-100), itofix(0));
      ASSERT(!errno); /* This will fail. */

   Returns the result of dividing `x' by `y'. If `y' is zero, returns the
   maximum possible fixed point value and sets `errno' to ERANGE.

fixed fixadd(fixed x, fixed y);
   Although fixed point numbers can be added with the normal '+' integer 
   operator, that doesn't provide any protection against overflow. If overflow
   is a problem, you should use this function instead. It is slower than using
   integer operators, but if an overflow occurs it will set `errno' and clamp
   the result, rather than just letting it wrap. Example:

      fixed result;
      /* This will put 5035 into `result'. */
      result = fixadd(itofix(5000), itofix(35));
      /* Sets `errno' and puts -32768 into `result'. */
      result = fixadd(itofix(-31000), itofix(-3000));
      ASSERT(!errno); /* This will fail. */

   Returns the clamped result of adding `x' to `y', setting `errno' to ERANGE
   if there was an overflow.

fixed fixsub(fixed x, fixed y);
   Although fixed point numbers can be subtracted with the normal '-' integer
   operator, that doesn't provide any protection against overflow. If overflow
   is a problem, you should use this function instead. It is slower than using
   integer operators, but if an overflow occurs it will set `errno' and clamp
   the result, rather than just letting it wrap. Example:

      fixed result;
      /* This will put 4965 into `result'. */
      result = fixsub(itofix(5000), itofix(35));
      /* Sets `errno' and puts -32768 into `result'. */
      result = fixsub(itofix(-31000), itofix(3000));
      ASSERT(!errno); /* This will fail. */

   Returns the clamped result of subtracting `y' from `x', setting `errno' to
   ERANGE if there was an overflow.

Fixed point trig
----------------

The fixed point square root, sin, cos, tan, inverse sin, and inverse cos 
functions are implemented using lookup tables, which are very fast but not 
particularly accurate. At the moment the inverse tan uses an iterative 
search on the tan table, so it is a lot slower than the others. Note that on
machines with very good floating point processors using these functions could
be slower in real life code due to cache misses: it may be faster to wait
a few extra cycles for a floating point sine result rather than wait for the
CPU to fetch the precalculated table from main memory. Always profile your
code.

Angles are represented in a binary format with 256 equal to a full circle, 
64 being a right angle and so on. This has the advantage that a simple 
bitwise 'and' can be used to keep the angle within the range zero to a full
circle, eliminating all those tiresome 'if (angle >= 360)' checks.

extern const fixed fixtorad_r;
   This constant gives a ratio which can be used to convert a fixed point
   number in binary angle format to a fixed point number in radians. Example:

      fixed rad_angle, binary_angle;
      /* Set the binary angle to 90 degrees. */
      binary_angle = 64;
      /* Now convert to radians (about 1.57). */
      rad_angle = fixmul(binary_angle, fixtorad_r);

extern const fixed radtofix_r;
   This constant gives a ratio which can be used to convert a fixed point
   number in radians to a fixed point number in binary angle format. Example:

      fixed rad_angle, binary_angle;
      ...
      binary_angle = fixmul(rad_angle, radtofix_r);

fixed fixsin(fixed x);
   This function finds the sine of a value using a lookup table. The input
   value must be a fixed point binary angle. Example:

      fixed angle;
      int result;

      /* Set the binary angle to 90 degrees. */
      angle = itofix(64);
      /* The sine of 90 degrees is one. */
      result = fixtoi(fixsin(angle));
      ASSERT(result == 1);

   Returns the sine of a fixed point binary format angle. The return value
   will be in radians.

fixed fixcos(fixed x);
   This function finds the cosine of a value using a lookup table. The input
   value must be a fixed point binary angle. Example:

      fixed angle;
      float result;

      /* Set the binary angle to 45 degrees. */
      angle = itofix(32);
      /* The cosine of 45 degrees is about 0.7071. */
      result = fixtof(fixcos(angle));
      ASSERT(result > 0.7 && result < 0.71);

   Returns the cosine of a fixed point binary format angle. The return value
   will be in radians.

fixed fixtan(fixed x);
   This function finds the tangent of a value using a lookup table. The input
   value must be a fixed point binary angle. Example:

      fixed angle, res_a, res_b;
      float dif;
      
      angle = itofix(37);
      /* Prove that tan(angle) == sin(angle) / cos(angle). */
      res_a = fixdiv(fixsin(angle), fixcos(angle));
      res_b = fixtan(angle);
      dif = fixtof(fixsub(res_a, res_b));
      allegro_message("Precision error: %f\n", dif);

   Returns the tangent of a fixed point binary format angle. The return value
   will be in radians.

fixed fixasin(fixed x);
   This function finds the inverse sine of a value using a lookup table. The
   input value must be a fixed point value. The inverse sine is defined only
   in the domain from `-1' to `1'. Outside of this input range, the function
   will set `errno' to EDOM and return zero. Example:

      float angle;
      fixed val;

      /* Sets `val' to a right binary angle (`64'). */
      val = fixasin(itofix(1));
      /* Sets `angle' to 0.2405. */
      angle = fixtof(fixmul(fixasin(ftofix(0.238)), fixtorad_r));
      /* This will trigger the assert. */
      val = fixasin(ftofix(-1.09));
      ASSERT(!errno);

   Returns the inverse sine of a fixed point value, measured as fixed point
   binary format angle, or zero if the input was out of the range. All return
   values of this function will be in the range `-64' to `64'.

fixed fixacos(fixed x);
   This function finds the inverse cosine of a value using a lookup table. The
   input value must be a fixed point radian. The inverse cosine is defined
   only in the domain from `-1' to `1'. Outside of this input range, the
   function will set `errno' to EDOM and return zero. Example:

      fixed result;

      /* Sets `result' to binary angle 128. */
      result = fixacos(itofix(-1));

   Returns the inverse sine of a fixed point value, measured as fixed point
   binary format angle, or zero if the input was out of range. All return
   values of this function will be in the range `0' to `128'.

fixed fixatan(fixed x);
   This function finds the inverse tangent of a value using a lookup table.
   The input value must be a fixed point radian. The inverse tangent is the
   value whose tangent is `x'. Example:

      fixed result;

      /* Sets `result' to binary angle 13. */
      result = fixatan(ftofix(0.326));

   Returns the inverse tangent of a fixed point value, measured as a fixed
   point binary format angle.

fixed fixatan2(fixed y, fixed x);
   This is a fixed point version of the libc atan2() routine. It computes the
   arc tangent of `y / x', but the signs of both arguments are used to
   determine the quadrant of the result, and `x' is permitted to be zero. This
   function is useful to convert Cartesian coordinates to polar coordinates.
   Example:

      fixed result;
      
      /* Sets `result' to binary angle 64. */
      result = fixatan2(itofix(1), 0);
      /* Sets `result' to binary angle -109. */
      result = fixatan2(itofix(-1), itofix(-2));
      /* Fails the assert. */
      result = fixatan2(0, 0);
      ASSERT(!errno);

   Returns the arc tangent of `y / x' in fixed point binary format angle,
   from `-128' to `128'. If both `x' and `y' are zero, returns zero and sets
   `errno' to EDOM.

fixed fixsqrt(fixed x);
   This finds out the non negative square root of `x'. If `x' is negative,
   `errno' is set to EDOM and the function returns zero.

fixed fixhypot(fixed x, fixed y);
   Fixed point hypotenuse (returns the square root of `x*x + y*y'). This
   should be better than calculating the formula yourself manually, since
   the error is much smaller.

Fix class
---------

If you are programming in C++ you can ignore all the above and use the fix 
class instead, which overloads a lot of operators to provide automatic 
conversion to and from integer and floating point values, and calls the 
above routines as they are required. You should not mix the fix class with 
the fixed typedef though, because the compiler will mistake the fixed values 
for regular integers and insert unnecessary conversions. For example, if x 
is an object of class fix, calling fixsqrt(x) will return the wrong result. 
You should use the overloaded sqrt(x) or x.sqrt() instead.

On top of that, the Fix class may be slower than using directly the C
functions because of implicit internal conversions from one type to another
which you otherwise could avoid or minimise. Finally, this is the only bit
of C++ in the whole Allegro library, and the developers are certainly going
to move it into add-on space in the next version of Allegro which breaks
source backwards compatibility.



==========================================
============ 3D math routines ============
==========================================

Allegro contains some 3d helper functions for manipulating vectors, 
constructing and using transformation matrices, and doing perspective 
projections from 3d space onto the screen. It is not, and never will be, a 
fully fledged 3d library (the goal is to supply generic support routines,
not shrink-wrapped graphics code :-) but these functions may be useful for 
developing your own 3d code.

Allegro uses a right-handed coordinate system, i.e. if you point the thumb 
of your right hand along the x axis, and the index finger along the y axis, 
your middle finger points in the direction of the z axis.

Allegro's world coordinate system typically has the positive x axis right,
the positive y axis up, and the positive z axis out of the screen.  What
all this means is this: Assume, the viewer is located at the origin (0/0/0)
in world space, looks along the negative z axis (0/0/-1), and is oriented
so up is along the positive y axis (0/1/0). Then something located at
(100/200/-300) will be 100 to the right, 200 above, and 300 in front of the
viewer. Just like in OpenGL. (Of course, both OpenGL and Allegro allow to
use a different system.) Here's a short piece of code demonstrating the
transformation pipeline of a point from world space to the screen.


   /* First, set up the projection viewport. */
   set_projection_viewport (0, 0, SCREEN_W, SCREEN_H);
  
   /* Next, get a camera matrix, depending on the
    * current viewer position and orientation.
    */
   get_camera_matrix_f (&m,
      0, 0, 0,  /* Viewer position, in this case, 0/0/0. */
      0, 0, -1, /* Viewer direction, in this case along negative z. */
      0, 1, 0,  /* Up vector, in this case positive y. */
      32,       /* The FOV, here 45°. */
      (float)SCREEN_W / (float)SCREEN_H)); /* Aspect ratio. */
  
   /* Applying the matrix transforms the point 100/200/-300
    * from world space into camera space. The transformation
    * moves and rotates the point so it is relative to the
    * camera, scales it according to the FOV and aspect
    * parameters, and also flips up and front direction -
    * ready to project the point to the viewport.
    */
   apply_matrix_f (&m, 100, 200, -300, &x, &y, &z);
  
   /* Finally, the point is projected from
    * camera space to the screen.
    */
   persp_project_f (cx, cy, cz, &sx, &sy);

For more details, look at the function descriptions of
set_projection_viewport(), get_camera_matrix(), and persp_project(), as well
as the relevant example programs.

All the 3d math functions are available in two versions: one which uses 
fixed point arithmetic, and another which uses floating point. The syntax 
for these is identical, but the floating point functions and structures are 
postfixed with '_f', eg. the fixed point function cross_product() has a 
floating point equivalent cross_product_f(). If you are programming in C++, 
Allegro also overloads these functions for use with the 'fix' class.

3d transformations are accomplished by the use of a modelling matrix. This 
is a 4x4 array of numbers that can be multiplied with a 3d point to produce 
a different 3d point. By putting the right values into the matrix, it can be 
made to do various operations like translation, rotation, and scaling. The 
clever bit is that you can multiply two matrices together to produce a third 
matrix, and this will have the same effect on points as applying the 
original two matrices one after the other. For example, if you have one 
matrix that rotates a point and another that shifts it sideways, you can 
combine them to produce a matrix that will do the rotation and the shift in 
a single step. You can build up extremely complex transformations in this 
way, while only ever having to multiply each point by a single matrix.

Allegro actually cheats in the way it implements the matrix structure. 
Rotation and scaling of a 3d point can be done with a simple 3x3 matrix, but 
in order to translate it and project it onto the screen, the matrix must be 
extended to 4x4, and the point extended into 4d space by the addition of an 
extra coordinate, w=1. This is a bad thing in terms of efficiency, but 
fortunately an optimisation is possible. Given the 4x4 matrix:

   ( a, b, c, d )
   ( e, f, g, h )
   ( i, j, k, l )
   ( m, n, o, p )

a pattern can be observed in which parts of it do what. The top left 3x3 
grid implements rotation and scaling. The three values in the top right 
column (d, h, and l) implement translation, and as long as the matrix is 
only used for affine transformations, m, n and o will always be zero and p 
will always be 1. If you don't know what affine means, read Foley & Van 
Damme: basically it covers scaling, translation, and rotation, but not 
projection. Since Allegro uses a separate function for projection, the 
matrix functions only need to support affine transformations, which means 
that there is no need to store the bottom row of the matrix. Allegro 
implicitly assumes that it contains (0,0,0,1), and optimises the matrix 
manipulation functions accordingly. Read chapter "Structures and types
defined by Allegro" for an internal view of the MATRIX/_f structures.

extern MATRIX identity_matrix;
extern MATRIX_f identity_matrix_f;
   Global variables containing the 'do nothing' identity matrix. Multiplying 
   by the identity matrix has no effect.

void get_translation_matrix(MATRIX *m, fixed x, fixed y, fixed z);
void get_translation_matrix_f(MATRIX_f *m, float x, float y, float z);
   Constructs a translation matrix, storing it in m. When applied to the 
   point (px, py, pz), this matrix will produce the point (px+x, py+y, 
   pz+z). In other words, it moves things sideways.

void get_scaling_matrix(MATRIX *m, fixed x, fixed y, fixed z);
void get_scaling_matrix_f(MATRIX_f *m, float x, float y, float z);
   Constructs a scaling matrix, storing it in m. When applied to the point 
   (px, py, pz), this matrix will produce the point (px*x, py*y, pz*z). In 
   other words, it stretches or shrinks things.

void get_x_rotate_matrix(MATRIX *m, fixed r);
void get_x_rotate_matrix_f(MATRIX_f *m, float r);
   Construct X axis rotation matrices, storing them in m. When applied to a 
   point, these matrices will rotate it about the X axis by the specified 
   angle (given in binary, 256 degrees to a circle format).

void get_y_rotate_matrix(MATRIX *m, fixed r);
void get_y_rotate_matrix_f(MATRIX_f *m, float r);
   Construct Y axis rotation matrices, storing them in m. When applied to a 
   point, these matrices will rotate it about the Y axis by the specified 
   angle (given in binary, 256 degrees to a circle format).

void get_z_rotate_matrix(MATRIX *m, fixed r);
void get_z_rotate_matrix_f(MATRIX_f *m, float r);
   Construct Z axis rotation matrices, storing them in m. When applied to a 
   point, these matrices will rotate it about the Z axis by the specified 
   angle (given in binary, 256 degrees to a circle format).

void get_rotation_matrix(MATRIX *m, fixed x, fixed y, fixed z);
void get_rotation_matrix_f(MATRIX_f *m, float x, float y, float z);
   Constructs a transformation matrix which will rotate points around all
   three axes by the specified amounts (given in binary, 256 degrees to a
   circle format). The direction of rotation can simply be found out with
   the right-hand rule: Point the dumb of your right hand towards the
   origin along the axis of rotation, and the fingers will curl in the
   positive direction of rotation. E.g. if you rotate around the y axis,
   and look at the scene from above, a positive angle will rotate in
   clockwise direction.

void get_align_matrix(MATRIX *m, fixed xfront, yfront, zfront, 
                                 fixed xup, fixed yup, fixed zup);
   Rotates a matrix so that it is aligned along the specified coordinate 
   vectors (they need not be normalized or perpendicular, but the up and 
   front must not be equal). A front vector of 0,0,-1 and up vector of 0,1,0 
   will return the identity matrix.

void get_align_matrix_f(MATRIX *m, float xfront, yfront, zfront, 
                                   float xup, yup, zup);
   Floating point version of get_align_matrix().

void get_vector_rotation_matrix(MATRIX *m, fixed x, y, z, fixed a);
void get_vector_rotation_matrix_f(MATRIX_f *m, float x, y, z, float a);
   Constructs a transformation matrix which will rotate points around the 
   specified x,y,z vector by the specified angle (given in binary, 256 
   degrees to a circle format).

void get_transformation_matrix(MATRIX *m, fixed scale,
                               fixed xrot, yrot, zrot, x, y, z);
   Constructs a transformation matrix which will rotate points around all 
   three axes by the specified amounts (given in binary, 256 degrees to a 
   circle format), scale the result by the specified amount (pass 1 for no 
   change of scale), and then translate to the requested x, y, z position.

void get_transformation_matrix_f(MATRIX_f *m, float scale,
                                 float xrot, yrot, zrot, x, y, z);
   Floating point version of get_transformation_matrix().

void get_camera_matrix(MATRIX *m, fixed x, y, z, xfront, yfront, zfront,
                       fixed xup, yup, zup, fov, aspect);
   Constructs a camera matrix for translating world-space objects into a
   normalised view space, ready for the perspective projection. The x, y,
   and z parameters specify the camera position, xfront, yfront, and zfront
   are the 'in front' vector specifying which way the camera is facing
   (this can be any length: normalisation is not required), and xup, yup,
   and zup are the 'up' direction vector.

   The fov parameter specifies the field of view (ie. width of the camera
   focus) in binary, 256 degrees to the circle format. For typical
   projections, a field of view in the region 32-48 will work well. 64
   (90°) applies no extra scaling - so something which is one unit away
   from the viewer will be directly scaled to the viewport. A bigger FOV
   moves you closer to the viewing plane, so more objects will appear. A
   smaller FOV moves you away from the viewing plane, which means you see a
   smaller part of the world.

   Finally, the aspect ratio is used to scale the Y dimensions of the image
   relative to the X axis, so you can use it to adjust the proportions of
   the output image (set it to 1 for no scaling - but keep in mind that the
   projection also performs scaling according to the viewport size).
   Typically, you will pass (float)w/(float)h, where w and h are the
   parameters you passed to set_projection_viewport.

   Note that versions prior to 4.1.0 multiplied this aspect ratio by 4/3.

void get_camera_matrix_f(MATRIX_f *m, float x, y, z, xfront, yfront, zfront,
                         float xup, yup, zup, fov, aspect);
   Floating point version of get_camera_matrix().

void qtranslate_matrix(MATRIX *m, fixed x, fixed y, fixed z);
void qtranslate_matrix_f(MATRIX_f *m, float x, float y, float z);
   Optimised routine for translating an already generated matrix: this 
   simply adds in the translation offset, so there is no need to build two 
   temporary matrices and then multiply them together.

void qscale_matrix(MATRIX *m, fixed scale);
void qscale_matrix_f(MATRIX_f *m, float scale);
   Optimised routine for scaling an already generated matrix: this simply 
   adds in the scale factor, so there is no need to build two temporary 
   matrices and then multiply them together.

void matrix_mul(const MATRIX *m1, const MATRIX *m2, MATRIX *out);
void matrix_mul_f(const MATRIX_f *m1, const MATRIX_f *m2, MATRIX_f *out);
   Multiplies two matrices, storing the result in out (this may be a 
   duplicate of one of the input matrices, but it is faster when the inputs 
   and output are all different). The resulting matrix will have the same 
   effect as the combination of m1 and m2, ie. when applied to a point p, (p 
   * out) = ((p * m1) * m2). Any number of transformations can be 
   concatenated in this way. Note that matrix multiplication is not 
   commutative, ie. matrix_mul(m1, m2) != matrix_mul(m2, m1).

fixed vector_length(fixed x, fixed y, fixed z);
float vector_length_f(float x, float y, float z);
   Calculates the length of the vector (x, y, z), using that good 'ole 
   Pythagoras theorem.

void normalize_vector(fixed *x, fixed *y, fixed *z);
void normalize_vector_f(float *x, float *y, float *z);
   Converts the vector (*x, *y, *z) to a unit vector. This points in the 
   same direction as the original vector, but has a length of one.

fixed dot_product(fixed x1, y1, z1, x2, y2, z2);
float dot_product_f(float x1, y1, z1, x2, y2, z2);
   Calculates the dot product (x1, y1, z1) . (x2, y2, z2), returning the 
   result.

void cross_product(fixed x1, y1, z1, x2, y2, z2, *xout, *yout, *zout);
void cross_product_f(float x1, y1, z1, x2, y2, z2, *xout, *yout, *zout);
   Calculates the cross product (x1, y1, z1) x (x2, y2, z2), storing the 
   result in (*xout, *yout, *zout). The cross product is perpendicular to 
   both of the input vectors, so it can be used to generate polygon normals.

fixed polygon_z_normal(const V3D *v1, const V3D *v2, const V3D *v3);
float polygon_z_normal_f(const V3D_f *v1, const V3D_f *v2, const V3D_f *v3);
   Finds the Z component of the normal vector to the specified three 
   vertices (which must be part of a convex polygon). This is used mainly in 
   back-face culling. The back-faces of closed polyhedra are never visible 
   to the viewer, therefore they never need to be drawn. This can cull on 
   average half the polygons from a scene. If the normal is negative the 
   polygon can safely be culled. If it is zero, the polygon is perpendicular 
   to the screen.

   However, this method of culling back-faces must only be used once the X and
   Y coordinates have been projected into screen space using persp_project()
   (or if an orthographic (isometric) projection is being used). Note that
   this function will fail if the three vertices are co-linear (they lie on
   the same line) in 3D space.

void apply_matrix(const MATRIX *m, fixed x, y, z, *xout, *yout, *zout);
void apply_matrix_f(const MATRIX_f *m, float x, y, z, *xout, *yout, *zout);
   Multiplies the point (x, y, z) by the transformation matrix m, storing 
   the result in (*xout, *yout, *zout).

void set_projection_viewport(int x, int y, int w, int h);
   Sets the viewport used to scale the output of the persp_project()
   function. Pass the dimensions of the screen area you want to draw onto,
   which will typically be 0, 0, SCREEN_W, and SCREEN_H. Also don't forget
   to pass an appropriate aspect ratio to get_camera_matrix later. The
   width and height you specify here will determine how big your viewport
   is in 3d space. So if an object in your 3D space is w units wide, it
   will fill the complete screen when you run into it (i.e., if it has a
   distance of 1.0 after the camera matrix was applied. The fov and
   aspect-ratio parameters to get_camera_matrix also apply some scaling
   though, so this isn't always completely true). If you pass -1/-1/2/2 as
   parameters, no extra scaling will be performed by the projection.

void persp_project(fixed x, fixed y, fixed z, fixed *xout, fixed *yout);
void persp_project_f(float x, float y, float z, float *xout, float *yout);
   Projects the 3d point (x, y, z) into 2d screen space, storing the result 
   in (*xout, *yout) and using the scaling parameters previously set by 
   calling set_projection_viewport(). This function projects from the 
   normalized viewing pyramid, which has a camera at the origin and facing 
   along the positive z axis. The x axis runs left/right, y runs up/down, 
   and z increases with depth into the screen. The camera has a 90 degree 
   field of view, ie. points on the planes x=z and -x=z will map onto the 
   left and right edges of the screen, and the planes y=z and -y=z map to 
   the top and bottom of the screen. If you want a different field of view 
   or camera location, you should transform all your objects with an 
   appropriate viewing matrix, eg. to get the effect of panning the camera 
   10 degrees to the left, rotate all your objects 10 degrees to the right.



==================================================
============ Quaternion math routines ============
==================================================

Quaternions are an alternate way to represent the rotation part of a
transformation, and can be easier to manipulate than matrices. As with a 
matrix, you can encode a geometric transformations in one, concatenate 
several of them to merge multiple transformations, and apply them to a 
vector, but they can only store pure rotations. The big advantage is that 
you can accurately interpolate between two quaternions to get a part-way 
rotation, avoiding the gimbal problems of the more conventional Euler angle 
interpolation.

Quaternions only have floating point versions, without any _f suffix. Other 
than that, most of the quaternion functions correspond with a matrix 
function that performs a similar operation.

Quaternion means 'of four parts', and that's exactly what it is. Here is the 
structure:

   typedef struct QUAT
   {
      float w, x, y, z;
   }

You will have lots of fun figuring out what these numbers actually mean, but 
that is beyond the scope of this documentation. Quaternions do work -- trust 
me.

extern QUAT identity_quat;
   Global variable containing the 'do nothing' identity quaternion. 
   Multiplying by the identity quaternion has no effect.

void get_x_rotate_quat(QUAT *q, float r);
void get_y_rotate_quat(QUAT *q, float r);
void get_z_rotate_quat(QUAT *q, float r);
   Construct axis rotation quaternions, storing them in q. When applied to a 
   point, these quaternions will rotate it about the relevant axis by the 
   specified angle (given in binary, 256 degrees to a circle format).

void get_rotation_quat(QUAT *q, float x, float y, float z);
   Constructs a quaternion that will rotate points around all three axes by 
   the specified amounts (given in binary, 256 degrees to a circle format).

void get_vector_rotation_quat(QUAT *q, float x, y, z, float a);
   Constructs a quaternion that will rotate points around the specified 
   x,y,z vector by the specified angle (given in binary, 256 degrees to a 
   circle format).

void quat_to_matrix(const QUAT *q, MATRIX_f *m);
   Constructs a rotation matrix from a quaternion.

void matrix_to_quat(const MATRIX_f *m, QUAT *q);
   Constructs a quaternion from a rotation matrix. Translation is discarded 
   during the conversion. Use get_align_matrix_f() if the matrix is not 
   orthonormalized, because strange things may happen otherwise.

void quat_mul(const QUAT *p, const QUAT *q, QUAT *out);
   Multiplies two quaternions, storing the result in out. The resulting 
   quaternion will have the same effect as the combination of p and q, ie. 
   when applied to a point, (point * out) = ((point * p) * q). Any number of 
   rotations can be concatenated in this way. Note that quaternion 
   multiplication is not commutative, ie. quat_mul(p, q) != quat_mul(q, p). 

void apply_quat(const QUAT *q, float x, y, z, *xout, *yout, *zout);
   Multiplies the point (x, y, z) by the quaternion q, storing the result in 
   (*xout, *yout, *zout). This is quite a bit slower than apply_matrix_f(), 
   so only use it to translate a few points. If you have many points, it is 
   much more efficient to call quat_to_matrix() and then use 
   apply_matrix_f().

void quat_interpolate(const QUAT *from, const QUAT *to, float t, QUAT *out);
   Constructs a quaternion that represents a rotation between from and to. 
   The argument t can be anything between 0 and 1 and represents where 
   between from and to the result will be. 0 returns from, 1 returns to, and 
   0.5 will return a rotation exactly in between. The result is copied to 
   out. This function will create the short rotation (less than 180 degrees) 
   between from and to.

void quat_slerp(const QUAT *from, const QUAT *to, float t, QUAT *out, int how);
   The same as quat_interpolate(), but allows more control over how the 
   rotation is done. The how parameter can be any one of the values:

      QUAT_SHORT  - like quat_interpolate(), use shortest path
      QUAT_LONG   - rotation will be greater than 180 degrees
      QUAT_CW     - rotate clockwise when viewed from above
      QUAT_CCW    - rotate counterclockwise when viewed from above
      QUAT_USER   - the quaternions are interpolated exactly as
                    given



======================================
============ GUI routines ============
======================================

Allegro contains an object-oriented dialog manager, which was originally 
based on the Atari GEM system (form_do(), objc_draw(), etc: old ST 
programmers will know what we are talking about :-) You can use the GUI as-is
to knock out simple interfaces for things like the test program and grabber 
utility, or you can use it as a basis for more complicated systems of your 
own. Allegro lets you define your own object types by writing new dialog 
procedures, so you can take complete control over the visual aspects of the 
interface while still using Allegro to handle input from the mouse, 
keyboard, joystick, etc.

A GUI dialog is stored as an array of DIALOG objects, read chapter
"Structures and types defined by Allegro" for an internal description of the
DIALOG structure. The array should end with an object which has the proc
pointer set to NULL. Each object has a flags field which may contain any
combination of the bit flags:

   D_EXIT          - this object should close the dialog when it is
                     clicked
   D_SELECTED      - this object is selected
   D_GOTFOCUS      - this object has got the input focus
   D_GOTMOUSE      - the mouse is currently on top of this object
   D_HIDDEN        - this object is hidden and inactive
   D_DISABLED      - this object is greyed-out and inactive
   D_DIRTY         - this object needs to be redrawn
   D_INTERNAL      - don't use this! It is for internal use by the
                     library...
   D_USER          - any powers of two above this are free for your
                     own use

Each object is controlled by a dialog procedure, which is stored in the proc 
pointer. This will be called by the dialog manager whenever any action 
concerning the object is required, or you can call it directly with the 
object_message() function. The dialog procedure should follow the form:

   int foo(int msg, DIALOG *d, int c);

It will be passed a flag (msg) indicating what action it should perform, a 
pointer to the object concerned (d), and if msg is MSG_CHAR or MSG_XCHAR, 
the key that was pressed (c). Note that d is a pointer to a specific object, 
and not to the entire dialog.

The dialog procedure should return one of the values:

   D_O_K          - normal return status
   D_CLOSE        - tells the dialog manager to close the dialog
   D_REDRAW       - tells the dialog manager to redraw the entire
                    dialog
   D_REDRAWME     - tells the dialog manager to redraw the current
                    object
   D_WANTFOCUS    - requests that the input focus be given to this
                    object
   D_USED_CHAR    - MSG_CHAR and MSG_XCHAR return this if they used
                    the key

Dialog procedures may be called with any of the messages:

MSG_START:
   Tells the object to initialise itself. The dialog manager sends this to 
   all the objects in a dialog just before it displays the dialog.

MSG_END:
   Sent to all objects when closing a dialog, allowing them to perform 
   whatever cleanup operations they require.

MSG_DRAW:
   Tells the object to draw itself onto the screen. The mouse pointer will 
   be turned off when this message is sent, so the drawing code does not 
   need to worry about it.

MSG_CLICK:
   Informs the object that a mouse button has been clicked while the mouse 
   was on top of the object. Typically an object will perform its own mouse 
   tracking as long as the button is held down, and only return from this 
   message handler when it is released.

   If you process this message, use the functions gui_mouse_*() to read the
   state of the mouse.

MSG_DCLICK:
   Sent when the user double-clicks on an object. A MSG_CLICK will be sent 
   when the button is first pressed, then MSG_DCLICK if it is released and 
   pressed again within a short space of time.

   If you process this message, use the functions gui_mouse_*() to read the
   state of the mouse.

MSG_KEY:
   Sent when the keyboard shortcut for the object is pressed, or if enter, 
   space, or a joystick button is pressed while it has the input focus.

MSG_CHAR:
   When a key is pressed, this message is sent to the object that has the 
   input focus, with a readkey() format character code (ASCII value in the 
   low byte, scancode in the high byte) as the c parameter. If the object 
   deals with the keypress it should return D_USED_CHAR, otherwise it should 
   return D_O_K to allow the default keyboard interface to operate. If you 
   need to access Unicode character input, you should use MSG_UCHAR instead 
   of MSG_CHAR.

MSG_UCHAR:
   If an object ignores the MSG_CHAR input, this message will be sent 
   immediately after it, passed the full Unicode key value as the c 
   parameter. This enables you to read character codes greater than 255, but 
   cannot tell you anything about the scancode: if you need to know that, 
   use MSG_CHAR instead. This handler should return D_USED_CHAR if it 
   processed the input, or D_O_K otherwise.

MSG_XCHAR:
   When a key is pressed, Allegro will send a MSG_CHAR and MSG_UCHAR to the 
   object with the input focus. If this object doesn't process the key (ie. 
   it returns D_O_K rather than D_USED_CHAR), the dialog manager will look 
   for an object with a matching keyboard shortcut in the key field, and 
   send it a MSG_KEY. If this fails, it broadcasts a MSG_XCHAR to all 
   objects in the dialog, allowing them to respond to special keypresses 
   even when they don't have the input focus. Normally you should ignore 
   this message (return D_O_K rather than D_USED_CHAR), in which case 
   Allegro will perform default actions such as moving the focus in response 
   to the arrow keys and closing the dialog if ESC is pressed.

MSG_WANTFOCUS:
   Queries whether an object is willing to accept the input focus. It should 
   return D_WANTFOCUS if it does, or D_O_K if it isn't interested in getting 
   user input.

MSG_GOTFOCUS:
MSG_LOSTFOCUS:
   Sent whenever an object gains or loses the input focus. These messages 
   will always be followed by a MSG_DRAW, to let objects display themselves 
   differently when they have the input focus. If you return D_WANTFOCUS in 
   response to a MSG_LOSTFOCUS event, this will prevent your object from 
   losing the focus when the mouse moves off it onto the screen background 
   or some inert object, so it will only lose the input focus when some 
   other object is ready to take over the focus (this trick is used by the 
   d_edit_proc() object).

MSG_GOTMOUSE:
MSG_LOSTMOUSE:
   Sent when the mouse moves on top of or away from an object. Unlike the 
   focus messages, these are not followed by a MSG_DRAW, so if the object is 
   displayed differently when the mouse is on top of it, it is responsible 
   for redrawing itself in response to these messages.

MSG_IDLE:
   Sent whenever the dialog manager has nothing better to do.

MSG_RADIO:
   Sent by radio button objects to deselect other buttons in the same group 
   when they are clicked. The group number is passed in the c message 
   parameter.

MSG_WHEEL:
   Sent to the focused object whenever the mouse wheel moves. The c message 
   parameter contains the number of clicks.

MSG_LPRESS, MSG_MPRESS, MSG_RPRESS:
   Sent when the corresponding mouse button is pressed.

MSG_LRELEASE, MSG_MRELEASE, MSG_RRELEASE:
   Sent when the corresponding mouse button is released.

MSG_USER:
   The first free message value. Any numbers from here on (MSG_USER, 
   MSG_USER+1, MSG_USER+2, ... MSG_USER+n) are free to use for whatever you 
   like.

Allegro provides several standard dialog procedures. You can use these as 
they are to provide simple user interface objects, or you can call them from 
within your own dialog procedures, resulting in a kind of OOP inheritance. 
For instance, you could make an object which calls d_button_proc to draw 
itself, but handles the click message in a different way, or an object which 
calls d_button_proc for everything except drawing itself, so it would behave 
like a normal button but could look completely different.

Since the release of Allegro version 3.9.33 (CVS), some GUI objects and 
menus are being drawn differently unlike in previous Allegro versions. The 
changes are the following:

   Shadows under d_shadow_box_proc and d_button_proc are always black.

   The most important (and immediately visible) change is, that some objects 
   are being drawn smaller. The difference is exactly one pixel in both 
   height and width, when comparing to previous versions. The reason is, 
   that in previous versions these objects were too large on the screen - 
   their size was d->w+1 and d->h+1 pixels (and not d->w and d->h, as it
   should be). This change affects the following objects :

      d_box_proc,
      d_shadow_box_proc,
      d_button_proc,
      d_check_proc,
      d_radio_proc,
      d_list_proc,
      d_text_list_proc and
      d_textbox_proc.

   When you want to convert old dialogs to look equally when compiling with 
   the new Allegro version, just increase the size of the mentioned objects 
   by one pixel in both width and height fields. 

   When a GUI menu item (not in a bar menu) has a child menu, there is a 
   small arrow next to the child menu name, pointing to the right - so the 
   user can immediately see that this menu item has a child menu - and 
   there is no need to use such menu item names as for example "New...", 
   to show that it has a child menu. The submenu will be drawn to the right 
   of the parent menu, trying not to overlap it.


Menus had been forgotten during the changes for 3.9.33 (CVS), so they were
still drawn too large until version 4.1.0.

int d_clear_proc(int msg, DIALOG *d, int c);
   This just clears the screen when it is drawn. Useful as the first object 
   in a dialog.

int d_box_proc(int msg, DIALOG *d, int c);
int d_shadow_box_proc(int msg, DIALOG *d, int c);
   These draw boxes onto the screen, with or without a shadow.

int d_bitmap_proc(int msg, DIALOG *d, int c);
   This draws a bitmap onto the screen, which should be pointed to by the 
   dp field.

int d_text_proc(int msg, DIALOG *d, int c);
int d_ctext_proc(int msg, DIALOG *d, int c);
int d_rtext_proc(int msg, DIALOG *d, int c);
   These draw text onto the screen. The dp field should point to the string 
   to display. d_ctext_proc() centers the string horizontally, and
   d_rtext_proc() right aligns it. Any '&' characters in the string will 
   be replaced with lines underneath the following character, for displaying 
   keyboard shortcuts (as in MS Windows). To display a single ampersand, put 
   "&&". To draw the text in something other than the default font, set the 
   dp2 field to point to your custom font data.

int d_button_proc(int msg, DIALOG *d, int c);
   A button object (the dp field points to the text string). This object can 
   be selected by clicking on it with the mouse or by pressing its keyboard 
   shortcut. If the D_EXIT flag is set, selecting it will close the dialog, 
   otherwise it will toggle on and off. Like d_text_proc(), ampersands can 
   be used to display the keyboard shortcut of the button.

int d_check_proc(int msg, DIALOG *d, int c);
   This is an example of how you can derive objects from other objects. Most 
   of the functionality comes from d_button_proc(), but it displays itself 
   as a check box. If the d1 field is non-zero, the text will be printed to 
   the right of the check, otherwise it will be on the left.

   Note: the object width should allow space for the text as well as the
   check box (which is square, with sides equal to the object height).

int d_radio_proc(int msg, DIALOG *d, int c);
   A radio button object. A dialog can contain any number of radio button 
   groups: selecting a radio button causes other buttons within the same 
   group to be deselected. The dp field points to the text string, d1 
   specifies the group number, and d2 is the button style (0=circle, 
   1=square).

int d_icon_proc(int msg, DIALOG *d, int c);
   A bitmap button. The fg color is used for the dotted line showing focus, 
   and the bg color for the shadow used to fill in the top and left sides of 
   the button when "pressed". d1 is the "push depth", ie. the number of 
   pixels the icon will be shifted to the right and down when selected 
   (default 2) if there is no "selected" image. d2 is the distance by which 
   the dotted line showing focus is indented (default 2). dp points to a 
   bitmap for the icon, while dp2 and dp3 are the selected and disabled 
   images respectively (optional, may be NULL).

int d_keyboard_proc(int msg, DIALOG *d, int c);
   This is an invisible object for implementing keyboard shortcuts. You can 
   put an ASCII code in the key field of the dialog object (a character such 
   as 'a' to respond to a simple keypress, or a number 1-26 to respond to a 
   control key a-z), or you can put a keyboard scancode in the d1 and/or d2 
   fields. When one of these keys is pressed, the object will call the 
   function pointed to by dp. This should return an int, which will be 
   passed back to the dialog manager, so it can return D_O_K, D_REDRAW, 
   D_CLOSE, etc.

int d_edit_proc(int msg, DIALOG *d, int c);
   An editable text object (the dp field points to the string). When it has 
   the input focus (obtained by clicking on it with the mouse), text can be 
   typed into this object. The d1 field specifies the maximum number of 
   characters that it will accept, and d2 is the text cursor position within 
   the string.
   
   Note: dp must point to a buffer at least (d1 + 1) * 4 bytes long because,
   depending on the encoding format in use, a single character can occupy
   up to 4 bytes and room must be reserved for the terminating null character.

int d_list_proc(int msg, DIALOG *d, int c);
   A list box object. This will allow the user to scroll through a list of 
   items and to select one by clicking or with the arrow keys. If the D_EXIT 
   flag is set, double clicking on a list item will close the dialog. The 
   index of the selected item is held in the d1 field, and d2 is used to 
   store how far it has scrolled through the list. The dp field points to a 
   function which will be called to obtain information about the contents of 
   the list. This should follow the form:

      char *foobar(int index, int *list_size);

   If index is zero or positive, the function should return a pointer to the 
   string which is to be displayed at position index in the list. If index 
   is negative, it should return NULL and list_size should be set to the 
   number of items in the list. 

   To create a multiple selection listbox, set the dp2 field to an array of 
   byte flags indicating the selection state of each list item (non-zero for 
   selected entries). This table must be at least as big as the number of 
   objects in the list!

int d_text_list_proc(int msg, DIALOG *d, int c);
   Like d_list_proc, but allows the user to type in the first few characters 
   of a listbox entry in order to select it. Uses dp3 internally, so you 
   mustn't store anything important there yourself.

int d_textbox_proc(int msg, DIALOG *d, int c);
   A text box object. The dp field points to the text which is to be 
   displayed in the box. If the text is long, there will be a vertical 
   scrollbar on the right hand side of the object which can be used to 
   scroll through the text. The default is to print the text with word 
   wrapping, but if the D_SELECTED flag is set, the text will be printed 
   with character wrapping. The d1 field is used internally to store the 
   number of lines of text, and d2 is used to store how far it has scrolled 
   through the text.

int d_slider_proc(int msg, DIALOG *d, int c);
   A slider control object. This object holds a value in d2, in the range 
   from 0 to d1. It will display as a vertical slider if h is greater than 
   or equal to w, otherwise it will display as a horizontal slider. The dp 
   field can contain an optional bitmap to use for the slider handle, and 
   dp2 can contain an optional callback function, which is called each time 
   d2 changes. The callback function should have the following prototype:

      int function(void *dp3, int d2);

   The d_slider_proc object will return the value of the callback function.

int d_menu_proc(int msg, DIALOG *d, int c);
   This object is a menu bar which will drop down child menus when it is 
   clicked or if an alt+key corresponding to one of the shortcuts in the 
   menu is pressed. It ignores a lot of the fields in the dialog structure, 
   in particular the color is taken from the gui_*_color variables, and the 
   width and height are calculated automatically (the w and h fields from 
   the DIALOG are only used as a minimum size.) The dp field points to an 
   array of menu structures: see do_menu() for more information. The top 
   level menu will be displayed as a horizontal bar, but when child menus 
   drop down from it they will be in the normal vertical format used by 
   do_menu(). When a menu item is selected, the return value from the menu 
   callback function is passed back to the dialog manager, so your callbacks 
   should return D_O_K, D_REDRAW, or D_CLOSE.

int d_yield_proc(int msg, DIALOG *d, int c);
   An invisible helper object that yields time slices for the scheduler (if
   the system supports it) when the GUI has nothing to do but waiting for
   user actions. You should put one instance of this object in each dialog 
   array because it may be needed on systems with an unusual scheduling 
   algorithm (for instance QNX) in order to make the GUI fully responsive.

GUI variables
-------------

The behaviour of the dialog manager can be controlled by the following
global variables.

extern int gui_mouse_focus;
   If set, the input focus follows the mouse pointer around the dialog, 
   otherwise a click is required to move it.

extern int gui_fg_color;
extern int gui_bg_color;
   The foreground and background colors for the standard dialogs (alerts, 
   menus, and the file selector). They default to 255 and 0.

extern int gui_mg_color;
   The color used for displaying greyed-out dialog objects (with the 
   D_DISABLED flag set). Defaults to 8.

extern int gui_font_baseline;
   If set to a non-zero value, adjusts the keyboard shortcut underscores to 
   account for the height of the descenders in your font.

extern int (*gui_mouse_x)();
extern int (*gui_mouse_y)();
extern int (*gui_mouse_z)();
extern int (*gui_mouse_b)();
   Hook functions, used by the GUI routines whenever they need to access the 
   mouse state. By default these just return copies of the mouse_x, mouse_y, 
   mouse_z, and mouse_b variables, but they could be used to offset or scale 
   the mouse position, or read input from a different source entirely.

GUI font
--------

You can change the global 'font' pointer to make the GUI objects use 
something other than the standard 8x8 font. The standard dialog procedures, 
menus, and alert boxes, will work with fonts of any size, but the 
gfx_mode_select() dialog will look wrong with anything other than 8x8 fonts.

int gui_textout_ex(BITMAP *bmp, const char *s, int x, y, color, bg, centre);
   Helper function for use by the GUI routines. Draws a text string onto the 
   screen, interpreting the '&' character as an underbar for displaying 
   keyboard shortcuts. Returns the width of the output string in pixels.

int gui_strlen(const char *s);
   Helper function for use by the GUI routines. Returns the length of a 
   string in pixels, ignoring '&' characters.

void gui_set_screen(BITMAP *bmp);
   This function can be used to change the bitmap surface the GUI routines
   draw to. This can be useful if you are using a double buffering or page 
   flipping system. Passing NULL will cause the default surface (screen) to
   be used again. Example:

      BITMAP *page[2];
      
      /* Allocate two pages of video memory */
      page[0] = create_video_bitmap(SCREEN_W, SCREEN_H);
      page[1] = create_video_bitmap(SCREEN_W, SCREEN_H);
      
      /* Page flip */
      show_video_bitmap(page[0]);
      gui_set_screen(page[0]);


BITMAP *gui_get_screen(void);
   This function returns the current bitmap surface the GUI routines will
   use for drawing. Note that this function will return screen if you have
   called gui_set_screen(NULL) previously, and will never return NULL.

void position_dialog(DIALOG *dialog, int x, int y);
   Moves an array of dialog objects to the specified screen position 
   (specified as the top left corner of the dialog).

void centre_dialog(DIALOG *dialog);
   Moves an array of dialog objects so that it is centered in the screen.

void set_dialog_color(DIALOG *dialog, int fg, int bg);
   Sets the foreground and background colors of an array of dialog objects.

int find_dialog_focus(DIALOG *dialog);
   Searches the dialog for the object which has the input focus, returning 
   an index or -1 if the focus is not set. This is useful if you are calling 
   do_dialog() several times in a row and want to leave the focus in the 
   same place it was when the dialog was last displayed, as you can call 
   do_dialog(dlg, find_dialog_focus(dlg));

int offer_focus(DIALOG *dialog, int obj, int *focus_obj, int force);
   Offers the input focus to a particular object. Normally the function sends
   the MSG_WANTFOCUS message to query whether the object is willing to accept
   the focus. However, passing any non-zero value as force argument instructs
   the function to authoritatively set the focus to the object.

int object_message(DIALOG *dialog, int msg, int c);
   Sends a message to an object and returns the answer it has generated.
   Remember that the first parameter is the dialog object (not a whole
   array) that you wish to send the message to. For example, to make the
   second object in a dialog draw itself, you might write:

      object_message(&dialog[1], MSG_DRAW, 0);

   The function will take care of scaring and unscaring the mouse if the
   message is MSG_DRAW.

int dialog_message(DIALOG *dialog, int msg, int c, int *obj);
   Sends a message to all the objects in an array. If any of the dialog 
   procedures return values other than D_O_K, it returns the value and sets 
   obj to the index of the object which produced it.

int broadcast_dialog_message(int msg, int c);
   Broadcasts a message to all the objects in the active dialog. If any of 
   the dialog procedures return values other than D_O_K, it returns that 
   value.

int do_dialog(DIALOG *dialog, int focus_obj);
   The basic dialog manager function. This displays a dialog (an array of 
   dialog objects, terminated by one with a NULL dialog procedure), and sets 
   the input focus to the focus_obj (-1 if you don't want anything to have 
   the focus). It interprets user input and dispatches messages as they are 
   required, until one of the dialog procedures tells it to close the 
   dialog, at which point it returns the index of the object that caused it 
   to exit, or until ESC is pressed, at which point it returns -1.

int popup_dialog(DIALOG *dialog, int focus_obj);
   Like do_dialog(), but it stores the data on the screen before drawing the 
   dialog and restores it when the dialog is closed. The screen area to be 
   stored is calculated from the dimensions of the first object in the 
   dialog, so all the other objects should lie within this one.

DIALOG_PLAYER *init_dialog(DIALOG *dialog, int focus_obj);
   This function provides lower level access to the same functionality as
   do_dialog(), but allows you to combine a dialog box with your own program 
   control structures. It initialises a dialog, returning a pointer to a 
   player object that can be used with update_dialog() and 
   shutdown_dialog(). With these functions, you could implement your own 
   version of do_dialog() with the lines:

      DIALOG_PLAYER *player = init_dialog(dialog, focus_obj);

      while (update_dialog(player))
         ;

      return shutdown_dialog(player);

   Note that you are responsible for showing and hiding the mouse cursor, which
   do_dialog would otherwise do for you, or saving and restoring the screen
   contents, as popup_dialog would do for you.

int update_dialog(DIALOG_PLAYER *player);
   Updates the status of a dialog object returned by init_dialog(). Returns 
   TRUE if the dialog is still active, or FALSE if it has terminated. Upon a 
   return value of FALSE, it is up to you whether to call shutdown_dialog() 
   or to continue execution. The object that requested the exit can be 
   determined from the player->obj field.

int shutdown_dialog(DIALOG_PLAYER *player);
   Destroys a dialog player object returned by init_dialog(), returning the 
   object that caused it to exit (this is the same as the return value from 
   do_dialog()).

extern DIALOG *active_dialog;
   Global pointer to the most recent activated dialog. This may be useful if 
   an object needs to iterate through a list of all its siblings.

GUI menus
---------

Popup or pulldown menus are created as an array of MENU structures. Read
chapter "Structures and types defined by Allegro" for an internal description
of the MENU structure.

Each menu item contains a text string. This can use the '&' character to 
indicate keyboard shortcuts, or can be an zero-length string to display the 
item as a non-selectable splitter bar. If the string contains a "\t" tab 
character, any text after this will be right-justified, eg. for displaying 
keyboard shortcut information. The proc pointer is a function which will be 
called when the menu item is selected, and child points to another menu, 
allowing you to create nested menus. Both proc and child may be NULL. The 
proc function returns an integer which is ignored if the menu was brought up 
by calling do_menu(), but which is passed back to the dialog manager if it 
was created by a d_menu_proc() object. The array of menu items is terminated 
by an entry with a NULL text pointer.

Menu items can be disabled (greyed-out) by setting the D_DISABLED bit in the 
flags field, and a check mark can be displayed next to them by setting the 
D_SELECTED bit. With the default alignment and font this will usually 
overlap the menu text, so if you are going to use checked menu items it 
would be a good idea to prefix all your options with a space or two, to 
ensure there is room for the check.

int do_menu(MENU *menu, int x, int y);
   Displays and animates a popup menu at the specified screen coordinates 
   (these will be adjusted if the menu does not entirely fit on the screen). 
   Returns the index of the menu item that was selected, or -1 if the menu 
   was cancelled. Note that the return value cannot indicate selection from 
   child menus, so you will have to use the callback functions if you want 
   multi-level menus.

MENU_PLAYER *init_menu(MENU *menu, int x, int y);
   This function provides lower level access to the same functionality as 
   do_menu(), but allows you to combine a popup menu with your own program 
   control structures. It initialises a menu, returning a pointer to a menu
   player object that can be used with update_menu() and shutdown_menu().
   With these functions, you could implement your own version of do_menu()
   with the lines:

      MENU_PLAYER *player = init_menu(menu, x, y);

      while (update_menu(player))
         ;

      return shutdown_menu(player);

int update_menu(MENU_PLAYER *player);
   Updates the status of a menu object returned by init_menu(). Returns TRUE
   if the menu is still active, or FALSE if it has terminated. Upon a return
   value of FALSE, it is up to you to call shutdown_menu() or to continue
   execution.

int shutdown_menu(MENU_PLAYER *player);
   Destroys a menu player object returned by init_menu(), returning the index
   of the menu item that was selected, or -1 if the menu was cancelled (this
   is the same as the return value from do_menu()).
   
extern MENU *active_menu;
   When a menu callback procedure is triggered, this will be set to the menu 
   item that was selected, so your routine can determine where it was called 
   from.

extern void (*gui_menu_draw_menu)(int x, int y, int w, int h);
extern void (*gui_menu_draw_menu_item)(MENU *m, int x, int y, int w,
                                       int h, int bar, int sel);
   If set, these functions will be called whenever a menu needs to be
   drawn, so you can change how menus look.

   gui_menu_draw_menu() is passed the position and size of the
   menu. It should draw the background of the menu onto screen.

   gui_menu_draw_menu_item() is called once for each menu item that is
   to be drawn. bar will be set if the item is part of a top-level
   horizontal menu bar, and sel will be set if the menu item is
   selected. It should also draw onto screen.

int alert(const char *s1, *s2, *s3, const char *b1, *b2, int c1, c2);
   Displays a popup alert box, containing three lines of text (s1-s3), and 
   with either one or two buttons. The text for these buttons is passed in 
   `b1' and `b2' (`b2' may be NULL), and the keyboard shortcuts in `c1' and
   `c2' as ASCII value. Example:

      if (!exists(CONFIG_FILE))
         alert(CONFIG_FILE, "not found.", "Using defaults.",
               "&Continue", NULL, 'c', 0);

   Returns 1 or 2 depending on which button was clicked. If the alert is 
   dismissed by pressing ESC when ESC is not one of the keyboard shortcuts, 
   it treats it as a click on the second button (this is consistent with the 
   common "Ok", "Cancel" alert).

int alert3(const char *s1, *s2, *s3, const char *b1, *b2, *b3, int c1, c2, c3);
   Like alert(), but with three buttons. Returns 1, 2, or 3.

int file_select_ex(const char *message, char *path, const char *ext,
                   int size, int w, int h);
   Displays the Allegro file selector, with the message as caption. The path 
   parameter contains the initial filename to display (this can be used to 
   set the starting directory, or to provide a default filename for a 
   save-as operation). The user selection is returned by altering the path 
   buffer, whose maximum capacity in bytes is specified by the size parameter.
   Note that it should have room for at least 80 characters (not bytes),
   so you should reserve 6x that amount, just to be sure. The list of files
   is filtered according to the file extensions in the ext parameter.
   Passing NULL includes all files; "PCX;BMP" includes only files with
   .PCX or .BMP extensions. If you wish to control files by their attributes,
   one of the fields in the extension list can begin with a slash, followed
   by a set of attribute characters. Any attribute written on its own, or
   with a '+' before it, indicates to include only files which have that
   attribute set. Any attribute with a '-' before it indicates to leave out
   any files with that attribute. The flag characters are 'r' (read-only),
   'h' (hidden), 's' (system), 'd' (directory) and 'a' (archive). For
   example, an extension string of "PCX;BMP;/+r-h" will display only PCX or
   BMP files that are read-only and not hidden. The directories are not
   affected in the same way as the other files by the extension string: the
   extensions are never taken into account for them and the other attributes
   are taken into account only when 'd' is mentioned in the string; in other
   words, all directories are included when 'd' is not mentioned in the
   string. The file selector is stretched to the width and height specified
   in the w and h parameters, and to the size of the standard Allegro font.
   If either the width or height argument is set to zero, it is stretched
   to the corresponding screen dimension. This function returns zero if it
   was closed with the Cancel button or non-zero if it was OK'd.

int gfx_mode_select(int *card, int *w, int *h);
   Displays the Allegro graphics mode selection dialog, which allows the 
   user to select a screen mode and graphics card.
   
   The initial values at the addresses provided by card, w, and h are used as
   the default selections in the dialog if they are found in the driver and
   mode lists. If they are not found then the initial selections will be the
   first in each list. If you wish to ensure that the initial selection is
   always the first entry, then initialize the data at the addresses passed
   to the function to the value of 0 or -1.
   
   If the dialog is OK'd, it stores the selections at the addresses passed to
   the function.

   See the gfx_mode_select_filter function for the return values.

int gfx_mode_select_ex(int *card, int *w, int *h, int *color_depth);
   Extended version of the graphics mode selection dialog, which also allows
   the user to select the color depth.
   
   As with gfx_mode_select, the values stored at the addresses passed to the
   function will be used as suggestions for the initial selections in the
   dialog, defaulting to the first entry in each list if the values are not
   found. Initialize the data stored at the addresses passed to the function
   to the value of 0 or -1 if you want to ensure that the initial selection
   for each list will be the first entry.

   If the dialog is OK'd, it stores the selections at the addresses passed to
   the function.

   See the gfx_mode_select_filter function for the return values.

int gfx_mode_select_filter(int *card, int *w, int *h, int *color_depth,
                            int (*filter)(int, int, int, int));
   Even more extended version of the graphics mode selection dialog, which
   allows the programmer to customize the contents of the dialog and the user
   to select the color depth as well as the resolution and hardware driver.
   `filter' will be passed (card, w, h, color_depth) quadruplets and must
   return 0 to let the specified quadruplet be added to the list of displayed
   modes.

   As with gfx_mode_select, the values stored at the addresses passed to the
   function will be used as suggestions for the initial selections in the
   dialog, defaulting to the first entry in each list if the values are not
   found. Initialize the data stored at the addresses passed to the function
   to the value of 0 or -1 if you want to ensure that the initial selection
   for each list will be the first entry.

   If the dialog is OK'd, it stores the selections at the addresses passed to
   the function.

   Example usage :

   ret = gfx_mode_select_filter(&card, &w, &h, &color_depth, user_filter);
   if (ret) {/* User okayed dialog or user_filter removed all modes */
      if (card == GFX_NONE) {
         // No modes available
         *card = 0;/* Make sure not to leave *card == GFX_NONE */
         return -1;
      }
      /* Handle changing to new mode here... */
      
   } else {/* User cancelled dialog or there was an error (unlikely) */
      if (card == GFX_NONE) {
         /* Error, probably out of memory */
         *card = 0;/* Make sure not to leave *card == GFX_NONE */
         return -2;
      }
      /* Carry on in current graphics mode if that is acceptable */
   }


   Returns zero if the user cancelled the dialog or an error occurred. In the
   case of an error then *card is assigned the value GFX_NONE. The functions
   return non-zero if the user made a selection OR if all the modes were
   filtered out. In the case that all of the modes were filtered out, then
   *card is assigned the value GFX_NONE. This means you should NOT initialize
   the *card to the value of GFX_NONE, as it could interfere with determining
   the proper return value.

extern int (*gui_shadow_box_proc)(int msg, struct DIALOG *d, int c);
extern int (*gui_ctext_proc)(int msg, struct DIALOG *d, int c);
extern int (*gui_button_proc)(int msg, struct DIALOG *d, int c);
extern int (*gui_edit_proc)(int msg, struct DIALOG *d, int c);
extern int (*gui_list_proc)(int msg, struct DIALOG *d, int c);
extern int (*gui_text_list_proc)(int msg, struct DIALOG *d, int c);
   If set, these functions will be used by the standard Allegro dialogs.
   This allows you to customise the look and feel, much like gui_fg_color
   and gui_bg_color, but much more flexibly.



=======================================
============ DOS specifics ============
=======================================

There are four Allegro files which you should redistribute along your program.
These are the files `keyboard.dat', `language.dat', `allegro.cfg' and
`setup.exe'.

The first two contain internationalisation information for keyboard mappings
and system messages to show up localised on the user's computer. The
`setup.exe' program, which comes in Allegro's `setup' directory, is a
standalone tool which you can graphically customise and even embed into your
main binary. The user can generate a configuration file with this tool, to
store special settings or avoid Allegro's autodetection failing on specific
hardware. Even if you distribute `setup.exe', you are recommended to copy too
the empty `allegro.cfg' file, in case the setup program itself is unable to
run and the user has to edit manually the configuration with a text editor.

If you are using get_config_text() in your program to localise text strings,
merge your xxtext.cfg files with the ones provided by Allegro in the
`resource' directory before creating `language.dat', and redistribute this
with your program. This file will contain then both Allegro's system
messages and the strings of your program.

Drivers JOY_TYPE_*/DOS
   The DOS library supports the following type parameters for the 
   install_joystick() function:

   JOY_TYPE_AUTODETECT
      Attempts to autodetect your joystick hardware. It isn't possible to 
      reliably distinguish between all the possible input setups, so this 
      routine can only ever choose the standard joystick, Sidewider, GamePad 
      Pro, or GrIP drivers, but it will use information from the 
      configuration file if one is available (this can be created using the 
      setup utility or by calling the save_joystick_data() function), so you 
      can always use JOY_TYPE_AUTODETECT in your code and then select the 
      exact hardware type from the setup program.

   JOY_TYPE_NONE
      Dummy driver for machines without any joystick.

   JOY_TYPE_STANDARD
      A normal two button stick.

   JOY_TYPE_2PADS
      Dual joystick mode (two sticks, each with two buttons).

   JOY_TYPE_4BUTTON
      Enable the extra buttons on a 4-button joystick.

   JOY_TYPE_6BUTTON
      Enable the extra buttons on a 6-button joystick.

   JOY_TYPE_8BUTTON
      Enable the extra buttons on an 8-button joystick.

   JOY_TYPE_FSPRO
      CH Flightstick Pro or compatible stick, which provides four buttons, 
      an analogue throttle control, and a 4-direction coolie hat.

   JOY_TYPE_WINGEX
      A Logitech Wingman Extreme, which should also work with any 
      Thrustmaster Mk.I compatible joystick. It provides support for four 
      buttons and a coolie hat. This also works with the Wingman Warrior, if 
      you plug in the 15 pin plug (remember to unplug the 9-pin plug!) and 
      set the tiny switch in front to the "H" position (you will not be able 
      to use the throttle or the spinner though).

   JOY_TYPE_SIDEWINDER
      The Microsoft Sidewinder digital pad (supports up to four controllers, 
      each with ten buttons and a digital direction control).

   JOY_TYPE_SIDEWINDER_AG
      An alternative driver to JOY_TYPE_SIDEWINDER.
      Try this if your Sidewinder isn't recognized with JOY_TYPE_SIDEWINDER.

   JOY_TYPE_SIDEWINDER_PP
      The Microsoft Sidewinder 3D/Precision/Force Feedback Pro joysticks.

   JOY_TYPE_GAMEPAD_PRO
      The Gravis GamePad Pro (supports up to two controllers, each with ten 
      buttons and a digital direction control).

   JOY_TYPE_GRIP
      Gravis GrIP driver, using the grip.gll driver file.

   JOY_TYPE_GRIP4
      Version of the Gravis GrIP driver that is constrained to only move 
      along the four main axes.

   JOY_TYPE_SNESPAD_LPT1
   JOY_TYPE_SNESPAD_LPT2
   JOY_TYPE_SNESPAD_LPT3
      SNES joypads connected to LPT1, LPT2, and LPT3 respectively.

   JOY_TYPE_PSXPAD_LPT1
   JOY_TYPE_PSXPAD_LPT2
   JOY_TYPE_PSXPAD_LPT3
      PSX joypads connected to LPT1, LPT2, and LPT3 respectively. See 
      http://www.ziplabel.com/dpadpro/index.html for information 
      about the parallel cable required. The driver automagically detects 
      which types of PSX pads are connected out of digital, analog (red or 
      green mode), NegCon, multi taps, Namco light guns, Jogcons (force 
      feedback steering wheel) and the mouse. If the controller isn't 
      recognised it is treated as an analog controller, meaning the driver 
      should work with just about anything. You can connect controllers in 
      any way you see fit, but only the first 8 will be used.

      The Sony Dual Shock or Namco Jogcon will reset themselves (to digital 
      mode) after not being polled for 5 seconds. This is normal, the same 
      thing happens on a Playstation, it's designed to stop any vibration in 
      case the host machine crashes. Other mode switching controllers may 
      have similar quirks. However, if this happens to a Jogcon controller 
      the mode button is disabled. To reenable the mode button on the Jogcon 
      you need to hold down the Start and Select buttons at the same time.

      The G-con45 needs to be connected to (and pointed at) a TV type monitor
      connected to your computer. The composite video out on my video card 
      works fine for this (a Hercules Stingray 128/3D 8Mb). The TV video 
      modes in Mame should work too.

   JOY_TYPE_N64PAD_LPT1
   JOY_TYPE_N64PAD_LPT2
   JOY_TYPE_N64PAD_LPT3
      N64 joypads connected to LPT1, LPT2, and LPT3 respectively. See 
      http://www.st-hans.de/N64.htm for information about the 
      necessary hardware adapter. It supports up to four controllers on a 
      single parallel port. There is no need to calibrate the analog stick, 
      as this is done by the controller itself when powered up. This means 
      that the stick has to be centred when the controller is initialised. 
      One possible issue people may have with this driver is that it is 
      physically impossible to move the analog stick fully diagonal, but I 
      can't see this causing any major problems. This is because of the 
      shape of the rim that the analog stick rests against. Like the Gravis 
      Game Pad Pro, this driver briefly needs to disable hardware interrupts 
      while polling. This causes a noticeable performance hit on my machine 
      in both drivers, but there is no way around it. At a (very) rough 
      guess I'd say it slows down Mame 5% - 10%.

   JOY_TYPE_DB9_LPT1
   JOY_TYPE_DB9_LPT2
   JOY_TYPE_DB9_LPT3
      A pair of two-button joysticks connected to LPT1, LPT2, and LPT3 
      respectively. Port 1 is compatible with Linux joy-db9 driver 
      (multisystem 2-button), and port 2 is compatible with Atari interface 
      for DirectPad Pro. See the source file (src/dos/multijoy.c) for pinout 
      information.

   JOY_TYPE_TURBOGRAFIX_LPT1
   JOY_TYPE_TURBOGRAFIX_LPT2
   JOY_TYPE_TURBOGRAFIX_LPT3
      These drivers support up to 7 joysticks, each one with up to 5 
      buttons, connected to LPT1, LPT2, and LPT3 respectively. They use the 
      TurboGraFX interface by Steffen Schwenke: see 
      http://www.burg-halle.de/~schwenke/parport.html for details 
      on how to build this.

   JOY_TYPE_WINGWARRIOR
      A Wingman Warrior joystick.

   JOY_TYPE_IFSEGA_ISA
   JOY_TYPE_IFSEGA_PCI
   JOY_TYPE_IFSEGA_PCI_FAST
      Drivers for the IF-SEGA joystick interface cards by the IO-DATA 
      company (these come in PCI, PCI2, and ISA variants).

Drivers GFX_*/DOS
   The DOS library supports the following card parameters for the 
   set_gfx_mode() function:

   GFX_TEXT
      Return to text mode.

   GFX_AUTODETECT
      Let Allegro pick an appropriate graphics driver.

   GFX_AUTODETECT_FULLSCREEN
      Autodetects a graphics driver, but will only use fullscreen drivers,
      failing if these are not available on current platform.

   GFX_AUTODETECT_WINDOWED
      Same as above, but uses only windowed drivers. This will always fail
      under DOS.

   GFX_SAFE
      Special driver for when you want to reliably set a graphics mode and 
      don't really care what resolution or color depth you get. See the
      set_gfx_mode() documentation for details.

   GFX_VGA
      The standard 256-color VGA mode 13h, using the GFX_VGA driver. This is 
      normally sized 320x200, which will work on any VGA but doesn't support 
      large virtual screens and hardware scrolling. Allegro also provides 
      some tweaked variants of the mode which are able to scroll, sized 
      320x100 (with a 200 pixel high virtual screen), 160x120 (with a 409 
      pixel high virtual screen), 256x256 (no scrolling), and 80x80 (with a 
      819 pixel high virtual screen).

   GFX_MODEX
      Mode-X will work on any VGA card, and provides a range of different 
      256-color tweaked resolutions.

      Stable mode-X resolutions:

         Square aspect ratio: 320x240

         Skewed aspect ratio: 256x224, 256x240, 320x200, 320x400,
                              320x480, 320x600, 360x200, 360x240,
                              360x360, 360x400, 360x480

         These have worked on every card/monitor that I've tested.

      Unstable mode-X resolutions:

         Square aspect ratio: 360x270, 376x282, 400x300

         Skewed aspect ratio: 256x200, 256x256, 320x350, 360x600,
                              376x308, 376x564, 400x150, 400x600

         These only work on some monitors. They were fine on my old machine, 
         but don't get on very well with my new monitor. If you are worried 
         about the possibility of damaging your monitor by using these 
         modes, don't be. Of course I'm not providing any warranty with any 
         of this, and if your hardware does blow up that is tough, but I 
         don't think this sort of tweaking can do any damage. From the 
         documentation of Robert Schmidt's TWEAK program:

            "Some time ago, putting illegal or unsupported values or 
            combinations of such into the video card registers might prove 
            hazardous to both your monitor and your health. I have *never* 
            claimed that bad things can't happen if you use TWEAK, although 
            I'm pretty sure it never will. I've never heard of any damage 
            arising from trying out TWEAK, or from general VGA tweaking in 
            any case."

      Most of the mode-X drawing functions are slower than in mode 13h, due 
      to the complexity of the planar bitmap organisation, but solid area 
      fills and plane-aligned blits from one part of video memory to another 
      can be significantly faster, particularly on older hardware. Mode-X 
      can address the full 256k of VGA RAM, so hardware scrolling and page 
      flipping are possible, and it is possible to split the screen in order 
      to scroll the top part of the display but have a static status 
      indicator at the bottom.

   GFX_VESA1
      Use the VESA 1.x driver.

   GFX_VESA2B
      Use the VBE 2.0 banked mode driver.

   GFX_VESA2L
      Use the VBE 2.0 linear framebuffer driver.

   GFX_VESA3
      Use the VBE 3.0 driver. This is the only VESA driver that supports the 
      request_refresh_rate() function.

      The standard VESA modes are 640x480, 800x600, and 1024x768. These 
      ought to work with any SVGA card: if they don't, get a copy of the 
      SciTech Display Doctor and see if that fixes it. What color depths are 
      available will depend on your hardware. Most cards support both 15 and 
      16-bit resolutions, but if at all possible I would advise you to 
      support both (it's not hard...) in case one is not available. Some 
      cards provide both 24 and 32-bit truecolor, in which case it is a 
      choice between 24 (saves memory) or 32 (faster), but many older cards 
      have no 32-bit mode and some newer ones don't support 24-bit 
      resolutions. Use the vesainfo test program to see what modes your VESA 
      driver provides.

      Many cards also support 640x400, 1280x1024, and 1600x1200, but these 
      aren't available on everything, for example the S3 chipset has no 
      640x400 mode. Other weird resolution may be possible, eg. some Tseng 
      boards can do 640x350, and the Avance Logic has a 512x512 mode.

      The SciTech Display Doctor provides several scrollable low resolution 
      modes in a range of different color depths (320x200, 320x240, 320x400, 
      320x480, 360x200, 360x240, 360x400, and 360x480 all work on my ET4000 
      with 8, 15, or 16 bits per pixel). These are lovely, allowing 
      scrolling and page flipping without the complexity of the mode-X 
      planar setup, but unfortunately they aren't standard so you will need 
      Display Doctor in order to use them.

   GFX_VBEAF
      VBE/AF is a superset of the VBE 2.0 standard, which provides an API 
      for accessing hardware accelerator features. VBE/AF drivers are 
      currently only available from the FreeBE/AF project or as part of the 
      SciTech Display Doctor package, but they can give dramatic speed 
      improvements when used with suitable hardware. For a detailed 
      discussion of hardware acceleration issues, refer to the documentation 
      for the gfx_capabilities flag.

      You can use the afinfo test program to check whether you have a VBE/AF 
      driver, and to see what resolutions it supports.

      The SciTech VBE/AF drivers require nearptr access to be enabled, so 
      any stray pointers are likely to crash your machine while their 
      drivers are in use. This means it may be a good idea to use VESA while 
      debugging your program, and only switch to VBE/AF once the code is 
      working correctly. The FreeBE/AF drivers do not have this problem.

   GFX_XTENDED
      An unchained 640x400 mode, as described by Mark Feldman in the PCGPE. 
      This uses VESA to select an SVGA mode (so it will only work on cards 
      supporting the VESA 640x400 resolution), and then unchains the VGA 
      hardware as for mode-X. This allows the entire screen to be addressed 
      without the need for bank switching, but hardware scrolling and page 
      flipping are not possible. This driver will never be autodetected (the 
      normal VESA 640x400 mode will be chosen instead), so if you want to 
      use it you will have to explicitly pass GFX_XTENDED to set_gfx_mode().

   There are a few things you need to be aware of for scrolling:
   most VESA implementations can only handle horizontal scrolling in four 
   pixel increments, so smooth horizontal panning is impossible in SVGA modes. 
   A significant number of VESA implementations seem to be very buggy when it 
   comes to scrolling in truecolor video modes, so you shouldn't depend on 
   this routine working correctly in the truecolor resolutions unless you can
   be sure that SciTech Display Doctor is installed. Hardware scrolling may
   also not work at all under Windows.

   Triple buffering is only possible with certain drivers: it will work in any
   DOS mode-X resolution if the timer retrace simulator is active (but this
   doesn't work correctly under Windows 95), plus it is supported by the
   VBE 3.0 and VBE/AF drivers for a limited number graphics cards.
Drivers DIGI_*/DOS
   The DOS sound functions support the following digital sound cards:

      DIGI_AUTODETECT      - let Allegro pick a digital sound driver
      DIGI_NONE            - no digital sound
      DIGI_SB              - Sound Blaster (autodetect type)
      DIGI_SB10            - SB 1.0 (8-bit mono single shot DMA)
      DIGI_SB15            - SB 1.5 (8-bit mono single shot DMA)
      DIGI_SB20            - SB 2.0 (8-bit mono auto-initialised
                             DMA)
      DIGI_SBPRO           - SB Pro (8-bit stereo)
      DIGI_SB16            - SB16 (16-bit stereo)
      DIGI_AUDIODRIVE      - ESS AudioDrive
      DIGI_SOUNDSCAPE      - Ensoniq Soundscape
      DIGI_WINSOUNDSYS     - Windows Sound System

Drivers MIDI_*/DOS
   The DOS sound functions support the following MIDI sound cards:

      MIDI_AUTODETECT      - let Allegro pick a MIDI sound driver
      MIDI_NONE            - no MIDI sound
      MIDI_ADLIB           - Adlib or SB FM synth (autodetect type)
      MIDI_OPL2            - OPL2 synth (mono, used in Adlib and SB)
      MIDI_2XOPL2          - dual OPL2 synths (stereo, used in
                             SB Pro-I)
      MIDI_OPL3            - OPL3 synth (stereo, SB Pro-II
                             and above)
      MIDI_SB_OUT          - SB MIDI interface
      MIDI_MPU             - MPU-401 MIDI interface
      MIDI_DIGMID          - sample-based software wavetable player
      MIDI_AWE32           - AWE32 (EMU8000 chip)

DOS integration routines
------------------------


extern int i_love_bill;
   When running in clean DOS mode, the timer handler dynamically reprograms 
   the clock chip to generate interrupts at exactly the right times, which 
   gives an extremely high accuracy. Unfortunately, this constant speed 
   adjustment doesn't work under most multitasking systems (notably 
   Windows), so there is an alternative mode that just locks the hardware 
   timer interrupt to a speed of 200 ticks per second. This reduces the 
   accuracy of the timer (for instance, rest() will round the delay time to 
   the nearest 5 milliseconds), and prevents the vertical retrace simulator 
   from working, but on the plus side, it makes Allegro programs work under 
   Windows. This flag is set by allegro_init() if it detects the presence of
   a multitasking OS, and enables the fixed rate timer mode.



===========================================
============ Windows specifics ============
===========================================

In terms of file redistribution, the Windows platform behaves practically the
same as the DOS platform. Read section chapter "Dos specifics" in the manual
to learn more about this.

A Windows program that uses the Allegro library is only required to include
one or more header files from the include/allegro tree, or allegro.h; however,
if it also needs to directly call non portable Win32 API functions, it must
include the Windows-specific header file winalleg.h after the Allegro headers,
and before any Win32 API header file. By default winalleg.h includes the main
Win32 C API header file windows.h. If instead you want to use the C++
interface to the Win32 API (a.k.a. the Microsoft Foundation Classes), define
the preprocessor symbol ALLEGRO_AND_MFC before including any Allegro header
so that afxwin.h will be included. Note that, in this latter case, the Allegro
debugging macros ASSERT() and TRACE() are renamed AL_ASSERT() and AL_TRACE()
respectively.

Windows GUI applications start with a WinMain() entry point, rather than the
standard main() entry point. Allegro is configured to build GUI applications
by default and to do some magic in order to make a regular main() work with
them, but you have to help it out a bit by writing END_OF_MAIN() right after
your main() function. If you don't want to do that, you can just include
winalleg.h and write a WinMain() function. Note that this magic may bring
about conflicts with a few programs using direct calls to Win32 API
functions; for these programs, the regular WinMain() is required and the
magic must be disabled by defining the preprocessor symbol
ALLEGRO_NO_MAGIC_MAIN before including Allegro headers.

If you want to build a console application using Allegro, you have to define
the preprocessor symbol ALLEGRO_USE_CONSOLE before including Allegro headers;
it will instruct the library to use console features and also to disable the
special processing of the main() function described above.

When creating the main window, Allegro searches the executable for an ICON 
resource named "allegro_icon". If it is present, Allegro automatically
loads it and uses it as its application icon; otherwise, Allegro uses the 
default IDI_APPLICATION icon. See the manual of your compiler for a method 
to create an ICON resource, or use the wfixicon utility from the tools/win 
directory.

DirectX requires that system and video bitmaps (including the screen) be 
locked before you can draw onto them. This will be done automatically, but 
you can usually get much better performance by doing it yourself: see the 
acquire_bitmap() function for details.

Due to a major oversight in the design of DirectX, there is no way to 
preserve the contents of video memory when the user switches away from your 
program. You need to be prepared for the fact that your screen contents, and 
the contents of any video memory bitmaps, may be destroyed at any point. You 
can use the set_display_switch_callback() function to find out when this 
happens.

On the Windows platform, the only return values for the desktop_color_depth()
function are 8, 16, 24 and 32. This means that 15-bit and 16-bit desktops 
cannot be differentiated and are both reported as 16-bit desktops. See
below for the consequences for windowed and overlay DirectX drivers.

Drivers JOY_TYPE_*/Windows
   The Windows library supports the following type parameters for the 
   install_joystick() function:

   JOY_TYPE_AUTODETECT
      Attempts to autodetect your joystick hardware. It will use information 
      from the configuration file if one is available (this can be created 
      using the setup utility or by calling the save_joystick_data()
      function), so you can always use JOY_TYPE_AUTODETECT in your code and
      then select the exact hardware type from the setup program.

   JOY_TYPE_NONE
      Dummy driver for machines without any joystick.

   JOY_TYPE_DIRECTX
      Use DirectInput to access the joystick.

   JOY_TYPE_WIN32
      Use the regular Win32 interface rather than DirectInput to access the
      joystick.


Drivers GFX_*/Windows
   The Windows library supports the following card parameters for the 
   set_gfx_mode() function:

   GFX_TEXT
      This closes any graphics mode previously opened with set_gfx_mode.

   GFX_AUTODETECT
      Let Allegro pick an appropriate graphics driver.

   GFX_AUTODETECT_FULLSCREEN
      Autodetects a graphics driver, but will only use fullscreen drivers,
      failing if these are not available on current platform.

   GFX_AUTODETECT_WINDOWED
      Same as above, but uses only windowed drivers.

   GFX_SAFE
      Special driver for when you want to reliably set a graphics mode and 
      don't really care what resolution or color depth you get. See the
      set_gfx_mode() documentation for details.

   GFX_DIRECTX
      Alias for GFX_DIRECTX_ACCEL.

   GFX_DIRECTX_ACCEL
      The regular fullscreen DirectX driver, running with hardware 
      acceleration enabled.

   GFX_DIRECTX_SOFT
      DirectX fullscreen driver that only uses software drawing, rather than 
      any hardware accelerated features.

   GFX_DIRECTX_SAFE
      Simplified fullscreen DirectX driver that doesn't support any hardware 
      acceleration, video or system bitmaps, etc.

   GFX_DIRECTX_WIN
      The regular windowed DirectX driver, running in color conversion mode 
      when the color depth doesn't match that of the Windows desktop. Color 
      conversion is much slower than direct drawing and is not supported 
      between 15-bit and 16-bit color depths. This limitation is needed to 
      work around that of desktop_color_depth() (see above) and allows to 
      select the direct drawing mode in a reliable way on desktops reported 
      as 16-bit:

         if (desktop_color_depth() == 16) {
            set_color_depth(16);
            if (set_gfx_mode(GFX_DIRECTX_WIN, 640, 480, 0, 0)
                != 0) {
               set_color_depth(15);
               if (set_gfx_mode(GFX_DIRECTX_WIN, 640, 480, 0, 0)
                   != 0) {
                  /* 640x480 direct drawing mode not supported */
                  goto Error;
               }
            }
            /* ok, we are in direct drawing mode */
         }

      Note that, mainly for performance reasons, this driver requires the
      width of the screen to be a multiple of 4.
      This driver is capable of displaying a hardware cursor, but there are
      size restrictions. Typically, the cursor image cannot be more than
      32x32 pixels.

   GFX_DIRECTX_OVL
      The DirectX overlay driver. It uses special hardware features to run 
      your program in a windowed mode: it doesn't work on all hardware, but 
      performance is excellent on cards that are capable of it. It requires 
      the color depth to be the same as that of the Windows desktop. In light 
      of the limitation of desktop_color_depth() (see above), the reliable 
      way of setting the overlay driver on desktops reported as 16-bit is:

         if (desktop_color_depth() == 16) {
            set_color_depth(16);
            if (set_gfx_mode(GFX_DIRECTX_OVL, 640, 480, 0, 0)
                != 0) {
               set_color_depth(15);
               if (set_gfx_mode(GFX_DIRECTX_OVL, 640, 480, 0, 0)
                   != 0) {
                  /* 640x480 overlay driver not supported */
                  goto Error;
               }
            }
            /* ok, the 640x480 overlay driver is running */
         }

   GFX_GDI
      The windowed GDI driver. It is extremely slow, but is guaranteed to 
      work on all hardware, so it can be useful for situations where you 
      want to run in a window and don't care about performance. Note that 
      this driver features a hardware mouse cursor emulation in order to 
      speed up basic mouse operations (like GUI operations).

Drivers DIGI_*/Windows
   The Windows sound functions support the following digital sound cards:

      DIGI_AUTODETECT      - let Allegro pick a digital sound driver
      DIGI_NONE            - no digital sound
      DIGI_DIRECTX(n)      - use DirectSound device #n (zero-based)
                             with direct mixing
      DIGI_DIRECTAMX(n)    - use DirectSound device #n (zero-based)
                             with Allegro mixing
      DIGI_WAVOUTID(n)     - high (n=0) or low (n=1) quality WaveOut
                             device

Drivers MIDI_*/Windows
   The Windows sound functions support the following MIDI sound cards:

      MIDI_AUTODETECT      - let Allegro pick a MIDI sound driver
      MIDI_NONE            - no MIDI sound
      MIDI_WIN32MAPPER     - use win32 MIDI mapper
      MIDI_WIN32(n)        - use win32 device #n (zero-based)
      MIDI_DIGMID          - sample-based software wavetable player

Windows integration routines
----------------------------


The following functions provide a platform specific interface to seamlessly 
integrate Allegro into general purpose Win32 programs. To use these routines, 
you must include winalleg.h after other Allegro headers.

HWND win_get_window(void);
   Retrieves a handle to the window used by Allegro. Note that Allegro
   uses an underlying window even though you don't set any graphics mode,
   unless you have installed the neutral system driver (SYSTEM_NONE).

void win_set_window(HWND wnd);
   Registers an user-created window to be used by Allegro. This function is
   meant to be called before initialising the library with allegro_init()
   or installing the autodetected system driver (SYSTEM_AUTODETECT). It
   lets you attach Allegro to any already existing window and prevents the
   library from creating its own, thus leaving you total control over the
   window; in particular, you are responsible for processing the events as
   usual (Allegro will automatically monitor a few of them, but will not
   filter out any of them). You can then use every component of the library
   (graphics, mouse, keyboard, sound, timers and so on), bearing in mind
   that some Allegro functions are blocking (e.g. readkey() if the key buffer
   is empty) and thus must be carefully manipulated by the window thread.

   However you can also call it after the library has been initialised,
   provided that no graphics mode is set. In this case the keyboard, mouse,
   joystick, sound and sound recording modules will be restarted.

   Passing NULL instructs Allegro to switch back to its built-in window if
   an user-created window was registered, or to request a new handle from
   Windows for its built-in window if this was already in use.

void win_set_wnd_create_proc(HWND (*proc)(WNDPROC));
   Registers an user-defined procedure to be used by Allegro for creating
   its window. This function must be called *before* initializing the
   library with allegro_init() or installing the autodetected system
   driver (SYSTEM_AUTODETECT). It lets you customize Allegro's window but
   only by its creation: unlike with win_set_window(), you have no control
   over the window once it has been created (in particular, you are not
   responsible for processing the events). The registered function will be
   passed a window procedure (WNDPROC object) that it must make the 
   procedure of the new window of and it must return a handle to the new 
   window. You can then use the full-featured library in the regular way.

HDC win_get_dc(BITMAP *bmp);
   Retrieves a handle to the device context of a DirectX video or system
   bitmap.

void win_release_dc(BITMAP *bmp, HDC dc);
   Releases a handle to the device context of the bitmap that was
   previously retrieved with win_get_dc().

void win_set_msg_pre_proc(int (*proc)(HWND, UINT, WPARAM, LPARAM, int *retval));
   Set a user callback to be called on window events. This callback function takes
   the same parameters as a normal window callback procedure plus an additional
   pointer to an integer. This pointer should be filled with a return value that
   must be set if the callback has completely processed the window event. If the
   callback has completely processed the window event, it should return 0 and fill
   retval with a proper value (the default is zero), otherwise it should return a
   non-zero value, and event processing will continue in Allegro's default window
   callback procedure.

GDI routines
------------


The following GDI routines are a very platform specific thing, to allow 
drawing Allegro memory bitmaps onto a Windows device context. When you want 
to use this, you'll have to install the neutral system driver (SYSTEM_NONE) 
or attach Allegro to an external window with win_set_window().

There are two ways to draw your Allegro bitmaps to the Windows GDI. When you 
are using static bitmaps (for example just some pictures loaded from a 
datafile), you can convert them to DDB (device-dependent bitmaps) with 
convert_bitmap_to_hbitmap() and then just use Win32's BitBlt() to draw it.

When you are using dynamic bitmaps (for example some things which react to 
user input), it's better to use set_palette_to_hdc() and blit_to_hdc() 
functions, which work with DIB (device-independent bitmaps).

There are also functions to blit from a device context into an Allegro 
BITMAP, so you can do things like screen capture.

All the drawing and conversion functions use the current palette as a color 
conversion table. You can alter the current palette with the 
set_palette_to_hdc() or select_palette() functions. Warning: when the GDI 
system color palette is explicitly changed, (by another application, for 
example) the current Allegro palette is not updated along with it!

To use these routines, you must include winalleg.h after Allegro headers.

void set_gdi_color_format(void);
   Tells Allegro to use the GDI color layout for truecolor images. This is 
   optional, but it will make the conversions work faster. If you are going 
   to call this, you should do it right after initialising Allegro and 
   before creating any graphics.

void set_palette_to_hdc(HDC dc, PALETTE pal);
   Selects and realizes an Allegro palette on the specified device context.

HPALETTE convert_palette_to_hpalette(PALETTE pal);
   Converts an Allegro palette to a Windows palette and returns a handle to 
   it. You should call DeleteObject() when you no longer need it.

void convert_hpalette_to_palette(HPALETTE hpal, PALETTE pal);
   Converts a Windows palette to an Allegro palette.

HBITMAP convert_bitmap_to_hbitmap(BITMAP *bitmap);
   Converts an Allegro memory bitmap to a Windows DDB and returns a handle 
   to it. This bitmap uses its own memory, so you can destroy the original 
   bitmap without affecting the converted one. You should call 
   DeleteObject() when you no longer need this bitmap.

BITMAP *convert_hbitmap_to_bitmap(HBITMAP bitmap);
   Creates an Allegro memory bitmap from a Windows DDB.

void draw_to_hdc(HDC dc, BITMAP *bitmap, int x, int y);
   Draws an entire Allegro bitmap to a Windows device context, using the 
   same parameters as the draw_sprite() function.

void blit_to_hdc(BITMAP *bitmap, HDC dc, int sx, sy, dx, dy, w, h);
   Blits an Allegro memory bitmap to a Windows device context, using the 
   same parameters as the blit() function.

void stretch_blit_to_hdc(BITMAP *bitmap, HDC dc, int sx, sy, sw, sh,
                                                 int dx, dy, dw, dh);
   Blits an Allegro memory bitmap to a Windows device context, using the 
   same parameters as the stretch_blit() function.

void blit_from_hdc(HDC hdc, BITMAP *bitmap, int sx, sy, dx, dy, w, h);
   Blits from a Windows device context to an Allegro memory bitmap, using 
   the same parameters as the blit() function. See stretch_blit_from_hdc() 
   for details.

void stretch_blit_from_hdc(HDC hcd, BITMAP *bitmap, int sx, sy, sw, sh,
                                                    int dx, dy, dw, dh);
   Blits from a Windows device context to an Allegro memory bitmap, using 
   the same parameters as the stretch_blit() function. It uses the current 
   Allegro palette and does conversion to this palette, regardless of the 
   current DC palette. So if you are blitting from 8-bit mode, you should 
   first set the DC palette with the set_palette_to_hdc() function.


========================================
============ Unix specifics ============
========================================

Under Unix you usually have two ways of redistributing your binaries. You
either pack everything in a single directory, even providing Allegro in
binary or source form for the user to compile. Or your program is being
packaged separately from Allegro and stored in different paths. For the
first case of redistribution, read section "Files shared by Allegro" from
the "Dos specifics" chapter to learn more about this.

For the second type, you can ignore redistributing the setup, keyboard
mappings and language datafiles, because they will be already installed in
the system. This, however, is problematic if you are using get_config_text()
to localise your program's text strings.

The problem is that on other platforms you usually mix your program's text
strings with those of Allegro (found in the `resources' directory) to create
a special language.dat. And it is likely that the Allegro library installed
on the user's system already contains a datafile.dat. You can go ahead and
still provide your own language.dat file, but this will mean that if Allegro
is updated, your language.dat file may not contain all the text strings used
by the new version.

Given the slow paced release cycle of Allegro, this might not be a concern.
However, if you want to make it easy on system administrators, instead of
providing your own `language.dat', you should provide the separate
`xxtext.cfg' files it in a separate directory. Then, before showing the
strings to the user you can detect the language setting and use
override_config_file() with the appropriate localisation file and call
reload_config_texts().

In order to locate things like the config and translation files, Allegro
needs to know the path to your executable. Since there is no standard way to 
find that, it needs to capture a copy of your argv[] parameter, and it does 
this with some preprocessor macro trickery. Unfortunately it can't quite 
pull this off without a little bit of your help, so you will have to write 
END_OF_MAIN() right after your main() function. Pretty easy, really, and if 
you forget, you'll get a nice linker error about a missing _mangled_main 
function to remind you :-)

Under Unix resources are searched for in many different paths (see above).
When a configuration resource is looked for, it is usually tried with the
variations `name.cfg' or `.namerc' in multiple paths: the current directory,
the directory pointed to by the ALLEGRO environment variable, the user's home
directory, one or more global system directories which usually only the
root user has access to and any custom paths set up with
set_allegro_resource_path(). Text files, like the main allegro config file or
a language text translation files are looked for in the following places:

   ./allegro.cfg
   $ALLEGRO/allegro.cfg
   ~/allegro.cfg
   ~/.allegrorc
   /etc/allegro.cfg
   /etc/allegrorc

Binary resources like the language translation files packfile (language.dat)
are looked for in:

   ./language.dat
   $ALLEGRO/language.dat
   ~/language.dat
   /etc/language.dat
   /usr/share/allegro/language.dat
   /usr/local/share/allegro/language.dat

Note that if you have installed Allegro from the source distribution with
the typical `make install', global files like `language.dat' and
`allegro.cfg' will not have been installed. As a system administrator you are
required to install them manually wherever you prefer to have them. If you
suspect that an Allegro program is somehow not finding the correct
configuration file, you could try using the following command:

   strace program 2>&1|egrep "(open|stat)"

The strace program traces system calls and signals. By default it outputs
the information to stderr, so that's why we redirect it to stdin with `2>&1'.
Since we are interested only in files being (un)successfully opened, we
restrict the output of the log to stat or open calls with the extended grep
command. You could add another grep to filter only lines with text like
`language' or `allegro'.

Drivers JOY_TYPE_*/Linux
   The Linux library supports the following type parameters for the 
   install_joystick() function:

   JOY_TYPE_AUTODETECT
      Attempts to autodetect your joystick hardware. It will use information 
      from the configuration file if one is available (this can be created 
      using the setup utility or by calling the save_joystick_data()
      function), so you can always use JOY_TYPE_AUTODETECT in your code and
      then select the exact hardware type from the setup program.

   JOY_TYPE_NONE
      Dummy driver for machines without any joystick.

   JOY_TYPE_LINUX_ANALOGUE
      Regular joystick interface. Joystick support needs to be enabled in your
      kernel.


Drivers GFX_*/Linux
   When running in Linux console mode, Allegro supports the following card 
   parameters for the set_gfx_mode() function:

   GFX_TEXT
      Return to text mode.

   GFX_AUTODETECT
      Let Allegro pick an appropriate graphics driver.

   GFX_AUTODETECT_FULLSCREEN
      Autodetects a graphics driver, but will only use fullscreen drivers,
      failing if these are not available on current platform.

   GFX_AUTODETECT_WINDOWED
      Same as above, but uses only windowed drivers. This will always fail
      under Linux console mode.

   GFX_SAFE
      Special driver for when you want to reliably set a graphics mode and 
      don't really care what resolution or color depth you get. See the
      set_gfx_mode() documentation for details.

   GFX_FBCON
      Use the framebuffer device (eg. /dev/fb0). This requires you to have 
      framebuffer support compiled into your kernel, and correctly 
      configured for your hardware. It is currently the only console mode 
      driver that will work without root permissions, unless you are using
      a development version of SVGAlib.

   GFX_VBEAF
      Use a VBE/AF driver (vbeaf.drv), assuming that you have installed one 
      which works under Linux (currently only two of the FreeBE/AF project 
      drivers are capable of this: I don't know about the SciTech ones). 
      VBE/AF requires root permissions, but is currently the only Linux 
      driver which supports hardware accelerated graphics.

   GFX_SVGALIB
      Use the SVGAlib library for graphics output. This requires root
      permissions if your version of SVGAlib requires them.

   GFX_VGA
   GFX_MODEX
      Use direct hardware access to set standard VGA or mode-X resolutions, 
      supporting the same modes as in the DOS versions of these drivers. 
      Requires root permissions.

Drivers GFX_*/X
   When running in X mode, Allegro supports the following card parameters 
   for the set_gfx_mode() function:

   GFX_TEXT
      This closes any graphics mode previously opened with set_gfx_mode.

   GFX_AUTODETECT
      Let Allegro pick an appropriate graphics driver.

   GFX_AUTODETECT_FULLSCREEN
      Autodetects a graphics driver, but will only use fullscreen drivers,
      failing if these are not available on current platform.

   GFX_AUTODETECT_WINDOWED
      Same as above, but uses only windowed drivers.

   GFX_SAFE
      Special driver for when you want to reliably set a graphics mode and 
      don't really care what resolution or color depth you get. See the
      set_gfx_mode() documentation for details.

   GFX_XWINDOWS
      The standard X graphics driver. This should work on any Unix system, 
      and can operate remotely. It does not require root permissions.
      If the ARGB cursor extension is available, this driver is capable
      of displaying a hardware cursor. This needs to be enabled by calling
      enable_hardware_cursor() because it cannot be used reliably alongside
      get_mouse_mickeys().

   GFX_XWINDOWS_FULLSCREEN
      The same as above, but while GFX_XWINDOWS runs windowed, this one uses
      the XF86VidMode extension to make it run in fullscreen mode even
      without root permissions. You're still using the standard X protocol
      though, so expect the same low performances as with the windowed
      driver version.
      If the ARGB cursor extension is available, this driver is capable
      of displaying a hardware cursor. This needs to be enabled by calling
      enable_hardware_cursor() because it cannot be used reliably alongside
      get_mouse_mickeys().

   GFX_XDGA2
      Use new DGA 2.0 extension provided by XFree86 4.0.x. This will work
      in fullscreen mode, and it will support hardware acceleration if
      available. This driver requires root permissions.

   GFX_XDGA2_SOFT
      The same as GFX_XDGA2, but turns off hardware acceleration support.
      This driver requires root permissions.

Drivers DIGI_*/Unix
   The Unix sound functions support the following digital sound cards:

      DIGI_AUTODETECT      - let Allegro pick a digital sound driver
      DIGI_NONE            - no digital sound
      DIGI_OSS             - Open Sound System
      DIGI_ESD             - Enlightened Sound Daemon
      DIGI_ARTS            - aRts (Analog Real-Time Synthesizer)
      DIGI_ALSA            - ALSA sound driver
      DIGI_JACK            - JACK sound driver

Drivers MIDI_*/Unix
   The Unix sound functions support the following MIDI sound cards:

      MIDI_AUTODETECT      - let Allegro pick a MIDI sound driver
      MIDI_NONE            - no MIDI sound
      MIDI_OSS             - Open Sound System
      MIDI_DIGMID          - sample-based software wavetable player
      MIDI_ALSA            - ALSA RawMIDI driver

Unix integration routines
-------------------------


void xwin_set_window_name(const char *name, const char *group);
   This function is only available under X. It lets you to specify the
   window name and group (or class). They are important because they allow
   the window manager to remember the window attributes (position, layer,
   etc). Note that the name and the title of the window are two different
   things: the title is what appears in the title bar of the window, but
   usually has no other effects on the behaviour of the application.

extern void *allegro_icon;
   This is a pointer to the Allegro X11 icon, which is in the format of 
   standard .xpm bitmap data.
   You do not normally have to bother with this at all: you can use the
   xfixicon.sh utility from the tools/x11 directory to convert a true 
   colour bitmap to a C file that you only need to link with your own code
   to set the icon.



========================================
============ BeOS specifics ============
========================================

In terms of file redistribution, the BeOS platform behaves practically the
same as the DOS platform. Read section chapter "Dos specifics" in the manual
to learn more about this.

Drivers GFX_*/BeOS
   BeOS Allegro supports the following card parameters for the
   set_gfx_mode() function:

   GFX_TEXT
      This closes any graphics mode previously opened with set_gfx_mode.

   GFX_AUTODETECT
      Let Allegro pick an appropriate graphics driver.

   GFX_AUTODETECT_FULLSCREEN
      Autodetects a graphics driver, but will only use fullscreen drivers,
      failing if these are not available on current platform.

   GFX_AUTODETECT_WINDOWED
      Same as above, but uses only windowed drivers.

   GFX_SAFE
      Special driver for when you want to reliably set a graphics mode and 
      don't really care what resolution. See the set_gfx_mode() 
      documentation for details.

   GFX_BWINDOWSCREEN_ACCEL
      Fullscreen exclusive mode. Supports only resolutions higher or equal
      to 640x480, and uses hardware acceleration if available.

   GFX_BWINDOWSCREEN
      Works the same as GFX_BWINDOWSCREEN_ACCEL, but disables acceleration.

   GFX_BDIRECTWINDOW
      Fast windowed mode using the BDirectWindow class. Not all graphics
      cards support this.

   GFX_BWINDOW
      Normal windowed mode using the BWindow class. Slow but always works.

   GFX_BWINDOW_OVERLAY
      Fullscreen mode using BWindow with a BBitmap overlay. This mode isn't
      supported by all graphics cards, only supports 15, 16 and 32-bit color
      depths, but allows any fullscreen resolution, even low ones that are
      normally unavailable under BeOS.

Drivers DIGI_*/BeOS
   The BeOS sound functions support the following digital sound cards:

      DIGI_AUTODETECT      - let Allegro pick a digital sound driver
      DIGI_NONE            - no digital sound
      DIGI_BEOS            - BeOS digital output

Drivers MIDI_*/BeOS
   The BeOS sound functions support the following MIDI sound cards:

      MIDI_AUTODETECT      - let Allegro pick a MIDI sound driver
      MIDI_NONE            - no MIDI sound
      MIDI_BEOS            - BeOS MIDI output
      MIDI_DIGMID          - sample-based software wavetable player



=======================================
============ QNX specifics ============
=======================================

In terms of file redistribution, the QNX platform behaves practically the
same as the DOS platform. Read section chapter "Dos specifics" in the manual
to learn more about this.

Drivers GFX_*/QNX
   QNX Allegro supports the following card parameters for the
   set_gfx_mode() function:

   GFX_TEXT
      This closes any graphics mode previously opened with set_gfx_mode.

   GFX_AUTODETECT
      Let Allegro pick an appropriate graphics driver.

   GFX_AUTODETECT_FULLSCREEN
      Autodetects a graphics driver, but will only use fullscreen drivers,
      failing if these are not available on current platform.

   GFX_AUTODETECT_WINDOWED
      Same as above, but uses only windowed drivers.

   GFX_SAFE
      Special driver for when you want to reliably set a graphics mode and 
      don't really care what resolution. See the set_gfx_mode() 
      documentation for details.

   GFX_PHOTON
      Alias for GFX_PHOTON_ACCEL.

   GFX_PHOTON_ACCEL
      Fullscreen exclusive mode through Photon, running with hardware 
      acceleration enabled.

   GFX_PHOTON_SOFT
      Fullscreen exclusive mode that only uses software drawing, rather than
      any hardware accelerated features.

   GFX_PHOTON_SAFE
      Simplified fullscreen exclusive driver that doesn't support any
      hardware acceleration, video or system bitmaps, etc.

   GFX_PHOTON_WIN
      The regular windowed Photon driver, running in color conversion mode 
      when the color depth doesn't match that of the Photon desktop. Color 
      conversion is much slower than direct drawing. Note that, mainly for
      performance reasons, this driver requires the width of the screen to
      be a multiple of 4.

Drivers DIGI_*/QNX
   The QNX sound functions support the following digital sound cards:

      DIGI_AUTODETECT      - let Allegro pick a digital sound driver
      DIGI_NONE            - no digital sound
      DIGI_ALSA            - ALSA sound driver

Drivers MIDI_*/QNX
   The QNX sound functions support the following MIDI sound cards:

      MIDI_AUTODETECT      - let Allegro pick a MIDI sound driver
      MIDI_NONE            - no MIDI sound
      MIDI_ALSA            - ALSA RawMIDI driver
      MIDI_DIGMID          - sample-based software wavetable player


QNX integration routines
------------------------


The following functions provide a platform specific interface to seamlessly 
integrate Allegro into general purpose QNX programs. To use these routines, 
you must include qnxalleg.h after other Allegro headers.

PtWidget_t qnx_get_window(void);
   Retrieves a handle to the window used by Allegro. Note that Allegro
   uses an underlying window even though you don't set any graphics mode,
   unless you have installed the neutral system driver (SYSTEM_NONE).



===========================================
============ MacOS X specifics ============
===========================================

In terms of file redistribution, the MacOS X platform behaves practically the
same as the DOS platform. Read section chapter "Dos specifics" in the manual
to learn more about this.

Allegro programs under MacOS X are Cocoa applications; in order to hide all
the Cocoa interfacing to the end-user, you need to add the END_OF_MAIN()
macro right after your main() function. This is a necessary step: if you omit
it, your program will not compile.

The END_OF_MAIN() macro simply does some magic to make sure your program
executes another function before your main(); this function is defined into
the liballeg-main.a static library, which is automatically linked if you use
the allegro-config script when linking. Otherwise be sure you link against it
unless you want to get undefined symbol errors.

To behave nicely with the MacOS X user interface, Allegro apps will provide a
standard application menu with the "Quit" menu item in it. The default
behaviour when the user hits Command-Q or selects "Quit" is to do nothing.
To override this behaviour you must call the set_close_button_callback() function; under MacOS
X the supplied callback will be used either if the user clicks the window
close button either on Command-Q or "Quit" selection. In this last case the
application will not shutdown, but you are supposed to set some quit flag
in your callback and check for it on a regular basis in your main program
loop.

If you would like to use InterfaceBuilder to design a more
fully-featured menu, Allegro will automatically load it if you create
a bundle for your application. The use of InterfaceBuilder is beyond
the scope of this document. Briefly, you need to create a Nib, called
MainMenu.nib, containing a main menu and a controller. Then, when you
create the bundle, install MainMenu.nib into the Contents/Resources
subdirectory. Note that the actions attached to the menu items will be
called from a different thread than the one in which your Allegro code
is running. Therefore you should not do any work there (especially,
don't call Allegro functions.) Instead, set a variable, and test it in
your main loop.

Drivers GFX_*/MacOSX
   MacOS X Allegro supports the following card parameters for the
   set_gfx_mode() function:

   GFX_TEXT
      This closes any graphics mode previously opened with set_gfx_mode.

   GFX_AUTODETECT
      Let Allegro pick an appropriate graphics driver.

   GFX_AUTODETECT_FULLSCREEN
      Autodetects a graphics driver, but will only use fullscreen drivers,
      failing if these are not available on current platform.

   GFX_AUTODETECT_WINDOWED
      Same as above, but uses only windowed drivers.

   GFX_SAFE
      Special driver for when you want to reliably set a graphics mode and 
      don't really care what resolution. See the set_gfx_mode() 
      documentation for details.

   GFX_QUARTZ_FULLSCREEN
      Fullscreen exclusive mode, using the CGDirectDisplay interface. 
      Supports only resolutions higher or equal to 640x480, and uses 
      hardware acceleration if available.

   GFX_QUARTZ_WINDOW
      Windowed mode using QuickDraw in a Cocoa window.

Drivers DIGI_*/MacOSX
   The MacOS X sound functions support the following digital sound cards:

      DIGI_AUTODETECT      - let Allegro pick a digital sound driver
      DIGI_NONE            - no digital sound
      DIGI_CORE_AUDIO      - CoreAudio digital output (OS >= X.2 required)
      DIGI_SOUND_MANAGER   - Carbon Sound Manager digital output


Drivers MIDI_*/MacOSX
   The MacOS X sound functions support the following MIDI sound cards:

      MIDI_AUTODETECT      - let Allegro pick a MIDI sound driver
      MIDI_NONE            - no MIDI sound
      MIDI_CORE_AUDIO      - CoreAudio MIDI synthesizer (OS >= X.2 required)
      MIDI_QUICKTIME       - QuickTime Music Note Allocator MIDI output
      MIDI_DIGMID          - sample-based software wavetable player



=======================================================
============ Differences between platforms ============
=======================================================

Here's a quick summary of things that may cause problems when moving your 
code from one platform to another (you can find a more detailed version of 
this in the docs section of the Allegro website).

The Windows, Unix and MacOS X versions require you to write END_OF_MAIN() 
after your main() function. This is used to magically turn an ISO C style 
main() into a Windows style WinMain(), or by the Unix code to grab a copy
of your argv[] parameter, or by the MacOS X code to shell the user main() 
inside a Cocoa application.

On many platforms Allegro runs very slowly if you rely on it in order to 
automatically lock bitmaps when drawing onto them. For good performance, 
you need to call acquire_bitmap() and release_bitmap() yourself, and try 
to keep the amount of locking to a minimum.

The Windows version may lose the contents of video memory if the user 
switches away from your program, so you need to deal with that.

None of the currently supported platforms require input polling, but it is 
possible that some future ones might, so if you want to ensure 100% 
portability of your program, you should call poll_mouse() and 
poll_keyboard() in all the relevant places.

On Unix the shared files by Allegro (like `language.dat') may require a
special use due to the nature of distributing the resources in separate paths
instead of putting everything in the same directory. Check the beginning of
your platform's specific chapter to learn more about this.

Allegro defines a number of standard macros that can be used to check 
various attributes of the current platform:

ALLEGRO_PLATFORM_STR
   Text string containing the name of the current platform.

ALLEGRO_DOS
ALLEGRO_DJGPP
ALLEGRO_WATCOM
ALLEGRO_WINDOWS
ALLEGRO_MSVC
ALLEGRO_MINGW32
ALLEGRO_BCC32
ALLEGRO_UNIX
ALLEGRO_LINUX
ALLEGRO_BEOS
ALLEGRO_QNX
ALLEGRO_DARWIN
ALLEGRO_MACOSX
ALLEGRO_GCC
   Defined if you are building for a relevant system. Often several of these 
   will apply, eg. DOS+Watcom, or Windows+GCC+MinGW.

   Note that ALLEGRO_LINUX is a misnomer. It will only be defined if
   Linux console support is enabled. It is not a reliable way to check
   if the program is being built on a Linux system.

ALLEGRO_AMD64
ALLEGRO_I386
ALLEGRO_BIG_ENDIAN
ALLEGRO_LITTLE_ENDIAN
   Defined if you are building for a processor of the relevant type.

ALLEGRO_MULTITHREADED
   Defined if the library is internally multi-threaded on this system.

ALLEGRO_USE_CONSTRUCTOR
   Defined if the compiler supports constructor/destructor functions.

ALLEGRO_VRAM_SINGLE_SURFACE
   Defined if the screen is a single large surface that is then partitioned 
   into multiple video sub-bitmaps (eg. DOS), rather than each video bitmap 
   being a totally unique entity (eg. Windows).

ALLEGRO_CONSOLE_OK
   Defined if when you are not in a graphics mode, there is a text mode 
   console that you can printf() to, and from which the user could 
   potentially redirect stdout to capture it even while you are in a 
   graphics mode. If this define is absent, you are running in an 
   environment like Windows that has no stdout at all.

ALLEGRO_MAGIC_MAIN
   Defined if Allegro uses a magic main, i.e takes over the main() entry 
   point and turns it into a secondary entry point suited to its needs.

ALLEGRO_LFN
   Non-zero if long filenames are supported, or zero if you are limited to 
   8.3 format (in the DJGPP version, this is a variable depending on the 
   runtime environment).

LONG_LONG
   Defined to whatever represents a 64-bit "long long" integer for the 
   current compiler, or not defined if that isn't supported.

OTHER_PATH_SEPARATOR
   Defined to a path separator character other than a forward slash for 
   platforms that use one (eg. a backslash under DOS and Windows), or 
   defined to a forward slash if there is no other separator character.

DEVICE_SEPARATOR
   Defined to the filename device separator character (a colon for DOS and 
   Windows), or to zero if there are no explicit devices in paths (Unix).

Allegro can be customized at compile time to a certain extent with the
following macros:

ALLEGRO_NO_MAGIC_MAIN
   If you define this prior to including Allegro headers, Allegro won't
   touch the main() entry point. This effectively removes the requirement
   on a program to be linked against the Allegro library when it includes
   the allegro.h header file. Note that the configuration and file routines
   are not guaranteed to work on Unix systems when this symbol is defined.
   Moreover, on Darwin/MacOS X systems, this symbol simply prevents the
   program from being linked against the Allegro library! This highly non
   portable feature is primarily intended to be used under Windows.

ALLEGRO_USE_CONSOLE
   If you define this prior to including Allegro headers, Allegro will be 
   set up for building a console application rather than the default GUI 
   program on some platforms (especially Windows).

ALLEGRO_NO_STD_HEADER
   If you define this prior to including Allegro headers, Allegro will not
   automatically include some standard headers (eg <stddef.h>) its own
   headers depend upon.

ALLEGRO_NO_KEY_DEFINES
   If you define this prior to including Allegro headers, Allegro will omit
   the definition of the KEY_* constants, which may clash with other headers.

ALLEGRO_NO_FIX_ALIASES
   The fixed point functions used to be named with an "f" prefix instead of
   "fix", eg. fixsqrt() used to be fsqrt(), but were renamed due to conflicts
   with some libc implementations. So backwards compatibility aliases are
   provided as static inline functions which map the old names to the new
   names, eg. fsqrt() calls fixsqrt(). If you define this symbol prior to
   including Allegro headers, the aliases will be turned off.

ALLEGRO_NO_FIX_CLASS
   If you define this symbol prior to including Allegro headers in a C++
   source file, the 'fix' class will not be made available. This mitigates
   problems with the 'fix' class's overloading getting in the way.

ALLEGRO_NO_VHLINE_ALIAS
   The `curses' API also defines functions called vline() and hline().
   To avoid a linker conflict when both libraries are used, we have
   internally renamed our functions and added inline function aliases which
   remap vline() and hline(). This should not be noticeable to most users.

   If you define ALLEGRO_NO_VHLINE_ALIAS prior to including Allegro headers,
   Allegro will not define the vline() and hline() aliases, e.g. so you can
   include curses.h and allegro.h in the same module.

ALLEGRO_NO_CLEAR_BITMAP_ALIAS
   If you define this prior to including Allegro headers, Allegro will not
   define the clear() backwards compatibility alias to clear_bitmap().

ALLEGRO_NO_COMPATIBILITY
   If you define this prior to including Allegro headers, Allegro will not
   include the backward compatibility layer. It is undefined by default so
   old programs can still be compiled with the minimum amount of issues,
   but you should define this symbol if you intend to maintain your code
   up to date with the latest versions of Allegro. It automatically turns
   off all backwards compatibility aliases.

Allegro also defines a number of standard macros that can be used to 
insulate you from some of the differences between systems:

INLINE
   Use this in place of the regular "inline" function modifier keyword, and 
   your code will work correctly on any of the supported compilers.

RET_VOLATILE
   Use this to declare a function with a volatile return value.

ZERO_SIZE_ARRAY(type, name)
   Use this to declare zero-sized arrays in terminal position inside 
   structures, like in the BITMAP structure. These arrays are effectively 
   equivalent to the flexible array members of ISO C99.

AL_CONST
   Use this in place of the regular "const" object modifier keyword, and 
   your code will work correctly on any of the supported compilers.

AL_RAND()
   On platforms that require it, this macro does a simple shift 
   transformation of the libc rand() function, in order to improve the 
   perceived randomness of the output series in the lower 16 bits.
   Where not required, it directly translates into a rand() call.



=======================================================
============ Reducing your executable size ============
=======================================================

Some people complain that Allegro produces very large executables. This is 
certainly true: with the DJGPP version, a simple "hello world" program will 
be about 200k, although the per-executable overhead is much less for 
platforms that support dynamic linking. But don't worry, Allegro takes up a 
relatively fixed amount of space, and won't increase as your program gets 
larger. As George Foot so succinctly put it, anyone who is concerned about 
the ratio between library and program code should just get to work and write 
more program code to catch up :-)

Having said that, there are several things you can do to make your programs 
smaller:

For all platforms, you can use an executable compressor called UPX, which
is available at http://upx.sourceforge.net/. This usually manages a
compression ratio of about 40%.

When using DJGPP: for starters, read the DJGPP FAQ section 8.14, and take
note of the -s switch. And don't forget to compile your program with
optimisation enabled!

If a DOS program is only going to run in a limited number of graphics modes, 
you can specify which graphics drivers you would like to include with the 
code:

   BEGIN_GFX_DRIVER_LIST
      driver1
      driver2
      etc...
   END_GFX_DRIVER_LIST

where the driver names are any of the defines:

   GFX_DRIVER_VBEAF
   GFX_DRIVER_VGA
   GFX_DRIVER_MODEX
   GFX_DRIVER_VESA3
   GFX_DRIVER_VESA2L
   GFX_DRIVER_VESA2B
   GFX_DRIVER_XTENDED
   GFX_DRIVER_VESA1

This construct must be included in only one of your C source files. The 
ordering of the names is important, because the autodetection routine works 
down from the top of the list until it finds the first driver that is able 
to support the requested mode. I suggest you stick to the default ordering 
given above, and simply delete whatever entries you aren't going to use.

If your DOS program doesn't need to use all the possible color depths, you 
can specify which pixel formats you want to support with the code:

   BEGIN_COLOR_DEPTH_LIST
      depth1
      depth2
      etc...
   END_COLOR_DEPTH_LIST

where the color depth names are any of the defines:

   COLOR_DEPTH_8
   COLOR_DEPTH_15
   COLOR_DEPTH_16
   COLOR_DEPTH_24
   COLOR_DEPTH_32

Removing any of the color depths will save quite a bit of space, with the 
exception of the 15 and 16-bit modes: these share a great deal of code, so 
if you are including one of them, there is no reason not to use both. Be 
warned that if you try to use a color depth which isn't in this list, your 
program will crash horribly!

In the same way as the above, you can specify which DOS sound drivers you 
want to support with the code:

   BEGIN_DIGI_DRIVER_LIST
      driver1
      driver2
      etc...
   END_DIGI_DRIVER_LIST

using the digital sound driver defines:

   DIGI_DRIVER_SOUNDSCAPE
   DIGI_DRIVER_AUDIODRIVE
   DIGI_DRIVER_WINSOUNDSYS
   DIGI_DRIVER_SB

and for the MIDI music:

   BEGIN_MIDI_DRIVER_LIST
      driver1
      driver2
      etc...
   END_MIDI_DRIVER_LIST

using the MIDI driver defines:

   MIDI_DRIVER_AWE32
   MIDI_DRIVER_DIGMID
   MIDI_DRIVER_ADLIB
   MIDI_DRIVER_MPU
   MIDI_DRIVER_SB_OUT

If you are going to use either of these sound driver constructs, you must 
include both.

Likewise for the DOS joystick drivers, you can declare an inclusion list:

   BEGIN_JOYSTICK_DRIVER_LIST
      driver1
      driver2
      etc...
   END_JOYSTICK_DRIVER_LIST

using the joystick driver defines:

   JOYSTICK_DRIVER_WINGWARRIOR
   JOYSTICK_DRIVER_SIDEWINDER
   JOYSTICK_DRIVER_GAMEPAD_PRO
   JOYSTICK_DRIVER_GRIP
   JOYSTICK_DRIVER_STANDARD
   JOYSTICK_DRIVER_SNESPAD
   JOYSTICK_DRIVER_PSXPAD
   JOYSTICK_DRIVER_N64PAD
   JOYSTICK_DRIVER_DB9
   JOYSTICK_DRIVER_TURBOGRAFX
   JOYSTICK_DRIVER_IFSEGA_ISA
   JOYSTICK_DRIVER_IFSEGA_PCI
   JOYSTICK_DRIVER_IFSEGA_PCI_FAST

The standard driver includes support for the dual joysticks, increased 
numbers of buttons, Flightstick Pro, and Wingman Extreme, because these are 
all quite minor variations on the basic code.

If you are _really_ serious about this size, thing, have a look at the top 
of include/allegro/alconfig.h and you will see the lines:

   #define ALLEGRO_COLOR8
   #define ALLEGRO_COLOR16
   #define ALLEGRO_COLOR24
   #define ALLEGRO_COLOR32

If you comment out any of these definitions and then rebuild the library, 
you will get a version without any support for the absent color depths, 
which will be even smaller than using the DECLARE_COLOR_DEPTH_LIST() macro. 
Removing the ALLEGRO_COLOR16 define will get rid of the support for both 15 
and 16-bit hicolor modes, since these share a lot of the same code.

Note: the aforementioned methods for removing unused hardware drivers only 
apply to statically linked versions of the library, eg. DOS. On Windows and 
Unix platforms, you can build Allegro as a DLL or shared library, which 
prevents these methods from working, but saves so much space that you 
probably won't care about that. Removing unused color depths from alconfig.h 
will work on any platform, though.

If you are distributing a copy of the setup program along with your game, 
you may be able to get a dramatic size reduction by merging the setup code 
into your main program, so that only one copy of the Allegro routines will 
need to be linked. See setup.txt for details. In the DJGPP version, after 
compressing the executable, this will probably save you about 200k compared 
to having two separate programs for the setup and the game itself.



===================================
============ Debugging ============
===================================

There are three versions of the Allegro library: the normal optimised code, 
one with extra debugging support, and a profiling version. See the platform 
specific readme files for information about how to install and link with 
these alternative libs. Although you will obviously want to use the 
optimised library for the final version of your program, it can be very 
useful to link with the debug lib while you are working on it, because this 
will make debugging much easier, and includes assert tests that will help to 
locate errors in your code at an earlier stage. Allegro also contains some 
debugging helper functions:

void ASSERT(condition);
   Debugging helper macro. Normally compiles away to nothing, but if you 
   defined the preprocessor symbol DEBUGMODE before including Allegro headers,
   it will check the supplied condition and call al_assert() if it fails,
   whose default action is to stop the program and report the assert. You can
   use this macro even when Allegro has not been initialised. Example:

      #define DEBUGMODE
      #include 
      ...
      void my_blitter(BITMAP *source, int flags)
      {
         int some_variables;
         ASSERT(source != NULL);
         ASSERT(flags & GAME_RUNNING);
         ...
      }

void TRACE(char *msg, ...);
   Debugging helper macro. Normally compiles away to nothing, but if you 
   defined the preprocessor symbol DEBUGMODE before including Allegro headers,
   it passes the supplied message given in ASCII format to al_trace().
   Example:

      #define DEBUGMODE
      #include 
      ...
      void my_blitter(BITMAP *source, int flags)
      {
         static int count_call = 0;
         TRACE("my_blitter() called %d times.\n", count_call++);
         ...
      }

void register_assert_handler(int (*handler)(const char *msg));
   Supplies a custom handler function for dealing with assert failures. Your 
   callback will be passed a formatted error message in ASCII, and should 
   return non-zero if it has processed the error, or zero to continue with 
   the default actions. You could use this to ignore assert failures, or to 
   display the error messages on a graphics mode screen without aborting the 
   program. You can call this function even when Allegro has not been
   initialised. Example:

   int show_but_continue(const char *text)
   {
       alert("Uh oh...", "Fasten your seat belts.", text,
             "&Go on!", NULL, 'g', 0);
       return 1;
   }
   ...
      register_assert(show_but_continue);
      ASSERT(0); /* This won't crash the program now. */

void register_trace_handler(int (*handler)(const char *msg));
   Supplies a custom handler function for dealing with trace output. Your 
   callback will be passed a formatted error message in ASCII, and should 
   return non-zero if it has processed the message, or zero to continue with 
   the default actions. You could use this to ignore trace output, or to 
   display the messages on a second monochrome monitor, etc. You can call
   this function even when Allegro has not been initialised. Example:

   int network_broadcaster(const char *text)
   {
      int f;

      for (int f = 0; f 
         send_msg_to_client(client[f], text);
         
      return 0; /* Let normal tracing occur. */
   }
   ...
      register_trace_handler(network_broadcaster);
      TRACE("Networked tracing activated\n");

void al_assert(const char *file, int line);
   Raises an assert for an error at the specified file and line number. The 
   file parameter is always given in ASCII format. By default, this will call
   the system driver's assert handler. If there is none, the error will be
   sent to stderr and the program will abort. However, if the environment
   variable ALLEGRO_ASSERT is set, this function writes a message into the
   file specified by the environment variable and program execution will
   continue. If none of this behaviours is wanted, you can override them with
   a custom assert handler.

   You will usually want to use the ASSERT() macro instead of calling this
   function directly.

void al_trace(const char *msg, ...);
   Outputs a debugging trace message, using a printf() format string given 
   in ASCII. If you have installed a custom trace handler it uses that, or 
   if the environment variable ALLEGRO_TRACE is set it writes into the file 
   specified by the environment, otherwise it writes the message to 
   "allegro.log" in the current directory. You will usually want to use the 
   TRACE() macro instead of calling this function directly.



====================================================
============ Available Allegro examples ============
====================================================

With Allegro comes quite a bunch of examples, which go from the simple
introductory `Hello world' to more complicated programs featuring truecolor
blending effects. This chapter describes these examples which you can find
in the allegro/examples folder. You don't have to go through them in the same
order as this documentation, but doing so you will learn the basic functions
and avoid missing any important bit of information.

Example exhello
   This is a very simple program showing how to get into graphics
   mode and draw text onto the screen.

Example exmem
   This program demonstrates the use of memory bitmaps. It creates
   a small temporary bitmap in memory, draws some circles onto it,
   and then blits lots of copies of it onto the screen.

Example expal
   This program demonstrates how to manipulate the palette. It draws
   a set of concentric circles onto the screen and animates them by
   cycling the palette.

Example expat
   This program demonstrates the use of patterned drawing and sub-bitmaps.

Example exflame
   This program demonstrates how to write directly to video memory.
   It implements a simple fire effect, first by calling getpixel() and
   putpixel(), then by accessing video memory directly a byte at a
   time, and finally using block memory copy operations.

Example exdbuf
   This program demonstrates the use of double buffering.
   It moves a circle across the screen, first just erasing and
   redrawing directly to the screen, then with a double buffer.

Example exflip
   This program moves a circle across the screen, first with a
   double buffer and then using page flips.

Example exfixed
   This program demonstrates how to use fixed point numbers, which
   are signed 32-bit integers storing the integer part in the
   upper 16 bits and the decimal part in the 16 lower bits. This
   example also uses the unusual approach of communicating with
   the user exclusively via the allegro_message() function.

Example exfont
   This is a very simple program showing how to load and manipulate fonts.

Example exmouse
   This program demonstrates how to get mouse input. The
   first part of the test retrieves the raw mouse input data
   and displays it on the screen without using any mouse
   cursor. When you press a key the standard arrow-like mouse
   cursor appears.  You are not restricted to this shape,
   and a second key press modifies the cursor to be several
   concentric colored circles. They are not joined together,
   so you can still see bits of what's behind when you move the
   cursor over the printed text message.

Example extimer
   This program demonstrates how to use the timer routines.
   These can be a bit of a pain, because you have to be sure
   you lock all the memory that is used inside your interrupt
   handlers.  The first part of the example shows a basic use of
   timing using the blocking function rest(). The second part
   shows how to use three timers with different frequencies in
   a non blocking way.

Example exkeys
   This program demonstrates how to access the keyboard. The
   first part shows the basic use of readkey(). The second part
   shows how to extract the ASCII value. Next come the scan codes.
   The fourth test detects modifier keys like alt or shift. The
   fifth test requires some focus to be passed. The final step
   shows how to use the global key array to read simultaneous
   key presses.
   The last method to detect key presses are keyboard callbacks.
   This is demonstrated by installing a keyboard callback,
   which marks all pressed keys by drawing to a grid.

Example exjoy
   This program uses the Allegro library to detect and read the value
   of a joystick. The output of the program is a small target sight
   on the screen which you can move. At the same time the program will
   tell you what you are doing with the joystick (moving or firing).

Example exsample
   This program demonstrates how to play samples. You have to
   use this example from the command line to specify as first
   parameter a WAV or VOC sound file to play. If the file is
   loaded successfully, the sound will be played in an infinite
   loop. While it is being played, you can use the left and right
   arrow keys to modify the panning of the sound. You can also
   use the up and down arrow keys to modify the pitch.

Example exmidi
   This program demonstrates how to play MIDI files.

Example exgui
   This program demonstrates how to use the GUI routines. From
   the simple dialog controls that display a text or a bitmap to
   more complex multiple choice selection lists, Allegro provides
   a framework which can be customised to suit your needs.

Example excustom
   A follow up of the exgui.c example showing how to customise the
   default Allegro framework. In this case a dialog procedure
   animates a graphical clock without disrupting other GUI
   dialogs.  A more simple option shows how to dynamically change
   the font used by all GUI elements.

Example exunicod
   This program demonstrates the use of the 16-bit Unicode text
   encoding format with Allegro. The example displays a message
   translated to different languages scrolling on the screen
   using an external font containing the required characters to
   display those messages.
   
   Note how the Allegro unicode string functions resemble the
   functions you can find in the standard C library, only these
   handle Unicode on all platforms.

Example exbitmap
   This program demonstrates how to load and display a bitmap
   file.  You have to use this example from the command line to
   specify as first parameter a graphic file in one of Allegro's
   supported formats.  If the file is loaded successfully,
   it will be displayed until you press a key.

Example exscale
   This example demonstrates how to use PCX files, palettes and stretch
   blits. It loads a PCX file, sets its palette and does some random
   stretch_blits. Don't worry - it's VERY slowed down using vsync().

Example exconfig
   This is a very simple program showing how to use the Allegro
   configuration (ini file) routines. A first look at the example
   shows nothing more than a static graphic and the wait for
   a key press. However, the way this graphic is displayed is
   configured through a custom exconfig.ini file which is loaded
   manually. From this file the example obtains parameters like
   fullscreen/windowed mode, a specific graphic resolution to set
   up, which graphic to show, how to blit it on the screen, etc.

Example exdata
   This program demonstrates how to access the contents of an
   Allegro datafile (created by the grabber utility). The example
   loads the file `example.dat', then blits a bitmap and shows
   a font, both from this datafile.

Example exsprite
   This example demonstrates how to use datafiles, various sprite
   drawing routines and flicker-free animation.
   
   Why is the animate() routine coded in that way?  As you
   probably know, VIDEO RAM is much slower than "normal"
   RAM, so it's advisable to reduce VRAM blits to a minimum.
   Drawing sprite on the screen (meaning in VRAM) and then
   clearing a background for it is not very fast. This example
   uses a different method which is much faster, but require a
   bit more memory.
   
   First the buffer is cleared (it's a normal BITMAP), then the
   sprite is drawn on it, and when the drawing is finished this
   buffer is copied directly to the screen. So the end result is
   that there is a single VRAM blit instead of blitting/clearing
   the background and drawing a sprite on it.  It's a good method
   even when you have to restore the background. And of course,
   it completely removes any flickering effect.
   
   When one uses a big (ie. 800x600 background) and draws
   something on it, it's wise to use a copy of background
   somewhere in memory and restore background using this
   "virtual background". When blitting from VRAM in SVGA modes,
   it's probably, that drawing routines have to switch banks on
   video card. I think, I don't have to remind how slow is it.
   
   Note that on modern systems, the above isn't true anymore, and
   you usually get the best performance by caching all your
   animations in video ram and doing only VRAM->VRAM blits, so
   there is no more RAM->VRAM transfer at all anymore. And usually,
   such transfers can run in parallel on the graphics card's
   processor as well, costing virtually no main cpu time at all.
   See the exaccel example for an example of this.

Example exrotscl
   This example demonstrates rotate_scaled_sprite functions.

Example exexedat
   This program demonstrates how to access the contents of an Allegro
   datafile (created by the grabber utility) linked to the executable by the
   exedat tool. It is basically the exdata example with minor
   modifications.
   
   You may ask: how do you compile, append and exec your program?
   
   Answer: like this...
   
   1) Compile your program like normal. Use the magic filenames with '#'
   to load your data where needed.
   
   2) Once you compressed your program, run "exedat foo.exe data.dat"
   
   3) Finally run your program.
   
   Note that appending data to the end of binaries may not be portable
   across all platforms supported by Allegro.

Example extrans
   This program demonstrates how to use the lighting and
   translucency functions. The first part of the example will
   show a dark screen illuminated by a spotlight you can move
   with your mouse. After a key press the example shows the full
   bitmap and the spotlight changes to be a reduced version of
   the background with 50% of translucency.
   
   The translucency effect is easy to do in all color depths.
   However, the lighting effect has to be performed in a different
   way depending on whether the screen is in 8bit mode or another
   color depth. This is because additive drawing mode uses a
   different set of routines for truecolor modes.

Example extrans2
   This program demonstrates how to draw trans and lit sprites and flip them
   at the same time, using draw_sprite_ex() function.
   It displays several images moving around using different drawing modes
   while you can press space key to change the flipping orientation.

Example extruec
   This program shows how to specify colors in the various different
   truecolor pixel formats. The example shows the same screen (a few
   text lines and three coloured gradients) in all the color depth
   modes supported by your video card. The more color depth you have,
   the less banding you will see in the gradients.

Example excolmap
   This program demonstrates how to create custom graphic effects
   with the create_color_table function. Allegro drawing routines
   are affected by any color table you might have set up. In
   the first part of this example, a greyscale color table is
   set. The result is that a simple rectfill call, instead of
   drawing a rectangle with color zero, uses the already drawn
   pixels to determine the pixel to be drawn (read the comment
   of return_grey_color() for a precise description of the
   algorithm). In the second part of the test, the color table
   is changed to be an inverse table, meaning that any pixel
   drawn will be shown as its color values had been inverted.

Example exrgbhsv
   This program shows how to convert colors between the different
   color-space representations. The central area of the screen
   will display the current color. On the top left corner of the
   screen, three sliders allow you to modify the red, green and
   blue value of the color. On the bottom right corner of the
   screen, three sliders allow you to modify the hue, saturation
   and value of the color. The color bars beneath the sliders
   show what the resulting color will look like when the slider
   is dragged to that position.
   
   Additionally this example also shows how to "inherit" the
   behaviour of a GUI object and extend it, here used to create
   the sliders.

Example exshade
   This program demonstrates how to draw Gouraud shaded (lit)
   sprites.  In an apparently black screen, a planet like sprite
   is drawn close to the middle of the screen. In a similar
   way to how the first test of extrans works, you move the
   cursor on the screen with the mouse. Attached to this mouse
   you can imagine a virtual spotlight illuminating the scene
   around. Depending on where the mouse is, the Gouraud shaded
   sprite will show the direction of the light.

Example exblend
   This program demonstrates how to use the translucency functions
   in truecolor video modes. Two image files are loaded from
   disk and displayed moving slowly around the screen. One of
   the images will be tinted to different colors. The other
   image will be faded out with a varying alpha strength, and
   drawn on top of the other image.

Example exxfade
   This program demonstrates how to load and display bitmap files
   in truecolor video modes, and how to crossfade between them.
   You have to use this example from the command line to specify
   as parameters a number of graphic files. Use at least two
   files to see the graphical effect. The example will crossfade
   from one image to another with each key press until you press
   the ESC key.

Example exalpha
   This program demonstrates how to use the 32 bit RGBA
   translucency functions to store an alpha channel along with
   a bitmap graphic.  Two images are loaded from disk. One will
   be used for the background and the other as a sprite. The
   example generates an alpha channel for the sprite image,
   composing the 32 bit RGBA bitmap during runtime, and draws
   it at the position of the mouse cursor.

Example exlights
   This program shows one way to implement colored lighting effects
   in a hicolor video mode. Warning: it is not for the faint of heart!
   This is by no means the simplest or easiest to understand method,
   I just thought it was a cool concept that would be worth
   demonstrating.
   
   The basic approach is to select a 15 or 16 bit screen mode, but
   then draw onto 24 bit memory bitmaps. Since we only need the bottom
   5 bits of each 8 bit color in order to store 15 bit data within a
   24 bit location, we can fit a light level into the top 3 bits.
   The tricky bit is that these aren't actually 24 bit images at all:
   they are implemented as 8 bit memory bitmaps, and we just store the
   red level in one pixel, green in the next, and blue in the next,
   making the total image be three times wider than we really wanted.
   This allows us to use all the normal 256 color graphics routines
   for drawing onto our memory surfaces, most importantly the lookup
   table translucency, which can be used to combine the low 5 bits
   of color and the top 3 bits of light in a single drawing operation.
   Some trickery is needed to load 24 bit data into this fake 8 bit
   format, and of course it needs a custom routine to convert the
   resulting image while copying it across to the hardware screen.
   
   This program chugs slightly on my p133, but not significantly
   worse than any double buffering in what amounts to a 1920x640,
   256 color resolution. The light blending doesn't seem to slow
   it down too badly, so I think this technique would be quite usable
   on faster machines and in lower resolution hicolor modes. The
   biggest problem is that although you keep the full 15 bit color
   resolution, you only get 3 bits of light, ie. 8 light levels.
   You can do some nice colored light patches, but smooth gradients
   aren't going to work too well :-)

Example ex3d
   This program demonstrates how to use the 3d matrix functions.
   It isn't a very elegant or efficient piece of code, but it
   does show the stuff in action. It is left to the reader as
   an exercise to design a proper model structure and rendering
   pipeline: after all, the best way to do that sort of stuff
   varies hugely from one game to another.
   
   The example first shows a screen resolution selection dialog.
   Then, a number of bouncing 3d cubes are animated. Pressing
   a key modifies the rendering of the cubes, which can be
   wireframe, the more complex transparent perspective correct
   texture mapped version, and many other.

Example excamera
   This program demonstrates how to use the get_camera_matrix()
   function to view a 3d world from any position and angle. The
   example draws a checkered floor through a viewport region
   on the screen. You can use the keyboard to move around the
   camera or modify the size of the viewport. The keys that can
   be used with this example are displayed between brackets at
   the top of the screen.

Example exquat
   Euler angles are convenient for storing and creating 3D orientations.
   However, this program demonstrates that they are not good when
   interpolating between two different orientations. The problem is
   solved by using Allegro's quaternion operations.
   
   In this program, two cubes are rotated between random orientations.
   Notice that although they have the same beginning and ending
   orientations, they do not follow the same path between orientations.
   
   One cube is being rotated by directly incrementing or decrementing
   the Euler angles from the starting point to the ending point.
   This is an intuitive notion, but it is incorrect because it does not
   cause the object to turn around a single unchanging axis of rotation.
   The axis of rotation wobbles resulting in the object spinning in
   strange ways. The object will eventually end up in the orientation
   that the user intended, but it gets there in a way that is unattractive.
   Imagine if this method was used to update the position of a camera in a
   game! Sometimes it would swing wildly and disorient the player.
   
   The other cube is animated using quaternions. This results in a much
   more pleasing animation because the cube turns around a single axis
   of rotation.

Example exstars
   This program draws a 3D star field (depth-cued) and a polygon
   starship (controllable with the keyboard cursor keys), using
   the Allegro math functions.

Example exscn3d
   This program demonstrates how to use scanline sorting algorithm
   in Allegro (create_scene, clear_scene, ... functions). It
   also provides an example of how to use the 3D clipping
   function. The example consists of a flyby through a lot of
   rotating 3d cubes.
   

Example exzbuf
   This program demonstrates how to use Z-buffered polygons and
   floating point 3D math routines. It also provides a simple
   way to compute fps (frames per second) using a timer. After
   selecting a screen resolution through the standard GUI dialog,
   the example shows two 3D cubes rotating and intersecting each
   other. Rather than having full polygons incorrectly overlap
   other polygons due to per-polygon sorting, each pixel is drawn
   at the correct depth.

Example exscroll
   This program demonstrates how to use hardware scrolling.
   The scrolling should work on anything that supports virtual
   screens larger than the physical screen.

Example ex3buf
   This program demonstrates the use of triple buffering. Several
   triangles are displayed rotating and bouncing on the screen
   until you press a key. Note that on some platforms you
   can't get real hardware triple buffering.  The Allegro code
   remains the same, but most likely the graphic driver will
   emulate it. Unfortunately, in these cases you can't expect
   the animation to be completely smooth and flicker free.

Example ex12bit
   This program sets up a 12-bit mode on any 8-bit card, by
   setting up a 256-colour palette that will fool the eye into
   grouping two 8-bit pixels into one 12-bit pixel. In order
   to do this, you make your 256-colour palette with all the
   combinations of blue and green, assuming green ranges from 0-15
   and blue from 0-14. This takes up 16x15=240 colours. This
   leaves 16 colours to use as red (red ranges from 0-15).
   Then you put your green/blue in one pixel, and your red in
   the pixel next to it. The eye gets fooled into thinking it's
   all one pixel.
   
   The example starts setting a normal 256 color mode, and
   construct a special palette for it. But then comes the trick:
   you need to write to a set of two adjacent pixels to form a
   single 12 bit dot. Two eight bit pixels is the same as one 16
   bit pixel, so after setting the video mode you need to hack
   the screen bitmap about, halving the width and changing it
   to use the 16 bit drawing code. Then, once you have packed a
   color into the correct format (using the makecol12() function
   below), any of the normal Allegro drawing functions can be
   used with this 12 bit display!
   
   Things to note:
   
   The horizontal width is halved, so you get resolutions
   like 320x480, 400x600, and 512x768.
   
   Because each dot is spread over two actual pixels, the
   display will be darker than in a normal video mode.
   
   Any bitmap data will obviously need converting to the
   correct 12 bit format: regular 15 or 16 bit images won't
   display correctly...
   
   Although this works like a truecolor mode, it is
   actually using a 256 color palette, so palette fades are
   still possible!
   
   This code only works in linear screen modes (don't try
   Mode-X).

Example exaccel
   This program demonstrates how to use an offscreen part of
   the video memory to store source graphics for a hardware
   accelerated graphics driver. The example loads the `mysha.pcx'
   file and then blits it several times on the screen. Depending
   on whether you have enough video memory and Allegro supports
   the hardware acceleration features of your card, your success
   running this example may be none at all, sluggish performance
   due to software emulation, or flicker free smooth hardware
   accelerated animation.

Example exspline
   This program demonstrates the use of spline curves to create smooth
   paths connecting a number of node points. This can be useful for
   constructing realistic motion and animations.
   
   The technique is to connect the series of guide points p1..p(n) with
   spline curves from p1-p2, p2-p3, etc. Each spline must pass though
   both of its guide points, so they must be used as the first and fourth
   of the spline control points. The fun bit is coming up with sensible
   values for the second and third spline control points, such that the
   spline segments will have equal gradients where they meet. I came
   up with the following solution:
   
   For each guide point p(n), calculate the desired tangent to the curve
   at that point. I took this to be the vector p(n-1) -> p(n+1), which
   can easily be calculated with the inverse tangent function, and gives
   decent looking results. One implication of this is that two dummy
   guide points are needed at each end of the curve, which are used in
   the tangent calculations but not connected to the set of splines.
   
   Having got these tangents, it becomes fairly easy to calculate the
   spline control points. For a spline between guide points p(a) and
   p(b), the second control point should lie along the positive tangent
   from p(a), and the third control point should lie along the negative
   tangent from p(b). How far they are placed along these tangents
   controls the shape of the curve: I found that applying a 'curviness'
   scaling factor to the distance between p(a) and p(b) works well.
   
   One thing to note about splines is that the generated points are
   not all equidistant. Instead they tend to bunch up nearer to the
   ends of the spline, which means you will need to apply some fudges
   to get an object to move at a constant speed. On the other hand,
   in situations where the curve has a noticeable change of direction
   at each guide point, the effect can be quite nice because it makes
   the object slow down for the curve.

Example exsyscur
   This program demonstrates the use of hardware accelerated mouse cursors.

Example exupdate
   This program demonstrates how to support double buffering,
   page flipping, and triple buffering as options within a single
   program, and how to make things run at a constant rate no
   matter what the speed of your computer. You have to use this
   example from the command line to specify as first parameter a
   number which represents the type of video update you want: 1
   for double buffering with memory bitmaps, 2 for page flipping,
   3 for triple buffering and 4 for double buffering with system
   bitmaps. After this, a dialog allows you to select a screen
   resolution and finally you will see a kaleidoscopic animation,
   along with a frames per second counter on the top left of
   the screen.

Example exswitch
   This program shows how to control the console switching mode, and
   let your program run in the background. These functions don't apply
   to every platform and driver, for example you can't control the
   switching mode from a DOS program.
   
   Yes, I know the fractal drawing is very slow: that's the point!
   This is so you can easily check whether it goes on working in the
   background after you switch away from the app.
   
   Depending on the type of selected switching mode, you will see
   whether the contents of the screen are preserved or not.

Example exstream
   This program shows how to use the audio stream functions to
   transfer large blocks of sample data to the sound card. In
   this case, the sample data is generated during runtime,
   and the resulting sound reminds of a car engine when you
   are accelerating.

Example expackf
   This program demonstrates the use of the packfile functions, with some
   simple tests.
   
   The first test uses the standard packfile functions to transfer a
   bitmap file into a block of memory, then reads the bitmap out of the
   block of memory, using a custom packfile vtable.
   
   The second test reads in a bitmap with another custom packfile
   vtable, which uses libc's file stream functions.
   
   The third test demonstrates seeking with a custom vtable.
   
   The fourth test reads two bitmaps, and dumps them back into a
   single file, using a custom vtable again.



===================================================================
============ Additional programs included with Allegro ============
===================================================================

Allegro comes with several useful utility programs. They are currently
documented separately. Refer to the HTML documentation (including CHM
and devhelp) of Allegro which includes all the documents, or the single
plain text versions of these documents (makedoc.txt, grabber.txt, dat.txt,
dat2s.txt, dat2c.txt).



===================================
============ Community ============
===================================

Allegro was originally created by Shawn Hargreaves. Published sometime between
1994 and 1995, it was just a simple lib for himself. At that time, many people
were switching from Borland C to DJGPP and looking for a decent graphics
library. Allegro was the first reasonably complete one to show up, so it
attracted enough interest to keep growing, and a little contribution here, and
some more encouragement there made it all light up like fire.

Some time after the latest 3.x stable release, though, Shawn was flooded with
Allegro tasks and Real Life (TM) work, and chose the latter to focus his
energies on. While this somehow stalled Allegro's development, it also
attracted a lot of people who wanted Allegro to live longer. Also, by that
time other people had started to work on Windows and Unix ports of Allegro,
which suggested that Allegro had the potential to survive its only decaying
main platform (DOS).

The current situation is that Shawn still keeps watching Allegro's progress
from time to time, but is not involved with development any more. The
community that grew over the years when Shawn was in charge of everything has
stepped forward to continue improving Allegro. Transformed into a meritocratic
community, users keep sending bug reports to the mailing lists, developers
around the world keep sending patches to fix them, and a few carefully chosen
have write access to the SVN repository, from which releases are built every
now and then.

But, who decides when a build is stable enough? Who decides when somebody is
granted write access to the SVN? Who chooses the lesser of two evils patching
some obscure bug? And more importantly, who decides what's Allegro's mascot?
For all these reasons, the community decided to replace Shawn's position with
the Allegro Dictator.

In republican Rome, political power was with the Senate and the Consuls.
However, if it was necessary that decisions were made very quickly then the
senate could appoint a Dictator. The Dictator was appointed for a specified
duration or charged with a specific task, after which he was expected to
surrender his authority back to the Senate. Nowadays, the Allegro Dictator is
a benevolent figure and rarely has to use his overwhelming fist of iron to put
order into chaos.

The truth is that the Allegro Dictator is usually the person in charge of
doing releases and all that unsexy work inside the community, like pestering
users to test some obscure bugfix or rejecting incomplete patches.

Past Allegro dictators have been: Shawn Hargreaves, George Foot, Peter Wang
and Eric Botcazou. At the moment of writing this, Evert Glebbeek is the
active Allegro Dictator. Should you want to change Allegro in some illogical
way, he's the guy you have to send your bribes too :-)



====================================
============ Conclusion ============
====================================

All good things must come to an end. Writing documentation is not a good 
thing, though, and that means it goes on for ever. There is always something 
we've forgotten to explain, or some essential detail we've left out, but for
now you will have to make do with this. Feel free to ask if you can't figure 
something out.

Enjoy. We hope you find some of this stuff useful.


By Shawn Hargreaves and the Allegro development team.

http://alleg.sourceforge.net/




