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File:         Image/ExifTool/README

Description:  ExifTool support modules documentation

Note:         This documentation is a reference to be used by developers when
              adding new tags to ExifTool.

The ExifTool support modules are loaded by ExifTool to allow processing of
various meta information formats.

The tables in these files are used as lookups based on the tag ID values.  The
hash keys are the tag IDs (in decimal or hexadecimal if the ID is numerical as
with EXIF tables, or the tag name if the ID is ASCII as with XMP tables).  In
the case of a BinaryData table, the IDs are numerical and specify offsets into
the binary data block (floating point IDs allow multiple tags for the same
offset, with the integer part being used for the offset).  The corresponding
hash value provides information about the tag (explained later).

Twenty-eight special keys (TABLE_NAME, SHORT_NAME, PROCESS_PROC, WRITE_PROC,
CHECK_PROC, INIT_TABLE, GROUPS, FORMAT, FIRST_ENTRY, TAG_PREFIX, PRINT_CONV,
WRITABLE, TABLE_DESC, NOTES, IS_OFFSET, IS_SUBDIR, EXTRACT_UNKNOWN, NAMESPACE,
PREFERRED, PERMANENT, SRC_TABLE, PRIORITY, AVOID, WRITE_GROUP, LANG_INFO, VARS,
DATAMEMBER and SET_GROUP1) are used to provide additional information about a
table.  The special keys have names that are all capitalized to avoid possible
conflicts with tag keys.  Below is an explanation of the meaning of each special
key:

  TABLE_NAME : Name of this table (set automatically by GetTagTable()).

  SHORT_NAME : Table name with leading "Image::ExifTool::" removed.

  PROCESS_PROC : Reference to a function used to process the directory for this
  table.  If PROCESS_PROC is not given, \&Image::ExifTool::Exif::ProcessExif is
  assumed (except for QuickTime atoms for which
  \&Image::ExifTool::QuickTime::ProcessMOV is the default).  If PROCESS_PROC is
  set to 0, the tags are not added to the lookup. The process proc returns 1 on
  success or 0 on failure, and takes 3 arguments: 0) reference to the ExifTool
  object, 1) reference to a directory information hash (with the following
  entries:), 2) reference to the tag table hash.

    Name        - Tag name for this SubDirectory entry (for verbose messages)
    Base        - Base offset for pointers from start of file
    DataPt      - Reference to data block containing directory (may be undef)
    DataPos     - Position of data block within file (relative to Base)
    DataLen     - Length of data block in bytes
    DirStart    - Offset to start of directory from start of data block
    DirLen      - Length of directory data within block
    DirName     - Name of this directory
    OutFile     - Output file or scalar reference
    Parent      - Name of parent directory
    RAF         - Reference to File::RandomAccess object if available
    NewDataPos  - File position of new data (write proc only)
    Fixup       - Reference to hash of offset fixups (used in EXIF writing only)
    FixBase     - Flag set to attempt to fix base offsets
    FixOffsets  - Evaluated for each value pointer to patch maker note offsets
    LastIFD     - Used by WriteExif() to return offset of last IFD written
    ImageData   - Used by WriteExif() to avoid buffering large image data blocks
    NoRefTest   - Flag to bypass "referenced by previous directory" test

  WRITE_PROC : Function reference or name for writing this directory.  The
  function returns the new directory data or undefined on error.  It takes the
  same arguments as the process proc above except that the second argument
  (reference to directory information hash) is optional, and if specified gives
  information about the source directory for tags to be copied to the output.

  CHECK_PROC : Reference to a function which validates Raw values for writing.
  The function takes 3-4 arguments: 0) ExifTool object reference, 1) tagInfo
  reference, 2) value reference, and 3) [optional] conversion type.  It returns
  undefined (and possibly modifies the input value) if successful, or an error
  message if there was a format problem.  May set ExifTool CHECK_WARN datamember
  for success with a warning.

  INIT_TABLE : Routine called to do any necessary initialization before the
  first time the table is loaded.  Argument is a reference to the table.

  GROUPS : A hash lookup for the default group names for all entries in this
  table.  If not specified, the default Group 0 and 1 names will be set
  automatically according to the name of the module, and the default Group 2
  name will be set to 'Other'.

  FORMAT : Specifies the default tag Format, and corresponding pointer increment
  for entries in a BinaryData table.  Defaults to 'int8u' for BinaryData tables
  if not specified.  The possible values of FORMAT are:

    int8s       - Signed 8-bit integer                    (EXIF 'SBYTE')
    int8u       - Unsigned 8-bit integer                  (EXIF 'BYTE')
    int16s      - Signed 16-bit integer                   (EXIF 'SSHORT')
    int16u      - Unsigned 16-bit integer                 (EXIF 'SHORT')
    int16uRev   - Unsigned 16-bit integer, reversed byte order
    int32s      - Signed 32-bit integer                   (EXIF 'SLONG')
    int32u      - Unsigned 32-bit integer                 (EXIF 'LONG')
    int64s      - Signed 64-bit integer                   (BigTIFF 'SLONG8')
    int64u      - Unsigned 64-bit integer                 (BigTIFF 'LONG8')
    rational32s - Rational consisting of 2 int16s values
    rational32u - Rational consisting of 2 int16u values
    rational64s - Rational consisting of 2 int32s values  (EXIF 'SRATIONAL')
    rational64u - Rational consisting of 2 int32u values  (EXIF 'RATIONAL')
    fixed16s    - Signed 16-bit fixed point value
    fixed16u    - Unsigned 16-bit fixed point value
    fixed32s    - Signed 32-bit fixed point value
    fixed32u    - Unsigned 32-bit fixed point value
    float       - 32-bit IEEE floating point value        (EXIF 'FLOAT')
    double      - 64-bit IEEE floating point value        (EXIF 'DOUBLE')
    extended    - 80-bit extended floating float
    ifd         - Unsigned 32-bit integer sub-IFD pointer (EXIF 'IFD')
    ifd64       - Unsigned 64-bit integer sub-IFD pointer (BigTIFF 'IFD8')
    string      - Series of 8-bit characters              (EXIF 'ASCII')
    undef       - Undefined-format binary data            (EXIF 'UNDEFINED')
    binary      - Binary data (same as 'undef')

  Additionally, the following variable-length Format types may be used in
  individual tag information hashes of a BinaryData table.  The 'var_' formats
  cause subsequent tag indices to be incremented according to the size of the
  data, not including the terminator.  See "Format" below for more details.

    pstring     - Pascal string (1-byte length followed by ASCII string)
    var_string  - variable-length null-terminated ASCII string
    var_ustring - variable-length null-terminated UCS-2 string
    var_pstring - variable-length Pascal string
    var_pstr32  - variable-length Pascal string /w 32-bit len
    var_ustr32  - variable-length UCS-2 string /w 32-bit len
    var_int16u  - variable-length undef data with int6u count
    var_ue7     - variable-length 7-bit-at-a-time integer (BPG)

  And the following format codes are used for string values in XMP metadata.
  (See the XMP specification for more details.)

    boolean     - "True" or "False"
    date        - date/time string
    integer     - integer as a string
    lang-alt    - list of alternate-language strings
    rational    - "N/D" where N=numerator, D=Denominator
    real        - floating point value as a string
    string      - unformatted string
    struct      - structure

  FIRST_ENTRY : Specifies the index for the first tag entry in a binary table.
  This value is only used if the Unknown option is set to 2 or higher, and
  allows the binary data to be scanned for unknown tag entries.

  TAG_PREFIX : Prefix for names of unknown tags.

  PRINT_CONV : Default print conversion for tags where PrintConv isn't
  specified.  PrintConv may be set to undef for individual tags to disable print
  conversion when PRINT_CONV is defined for a table.

  WRITABLE : Indicates that all tags in this table are writable.  This is the
  same as setting the Writable flag for each individual tag in the table, except
  for SubDirectory tags which are not made Writable.

  TABLE_DESC : Short description for this table.  Plain text only.  Used only
  for XML tag database output.

  NOTES : Notes to introduce the table in the TagNames documentation.  Pod
  formatting codes B<>, C<> and L<> may be used in this text.

  IS_OFFSET : Writable BinaryData tables only.  Reference to list of sorted
  TagID's representing offsets.

  IS_SUBDIR : Writable BinaryData tables only.  A reference to a list of sorted
  tag ID's representing subdirectories.

  EXTRACT_UNKNOWN : Used in PDF tables to specify a directory where all unknown
  tags should be extracted.  Set to 0 to extract only unknown numbered tags for
  which the unnumbered tag is known.

  NAMESPACE : Namespace prefix for tags in the XMP table.  If this isn't a
  standard namespace defined in %Image::ExifTool::XMP::nsURI, then the URI must
  be defined as well (however, this is not recommended for pre-defined
  namespaces because then non-standard namespace prefixes won't be recognized).
  To define the URI, the NAMESPACE value is a reference to a hash where the key
  is the namespace prefix and and the value is the URI (alternatively, a
  reference to a 2-element array containing the prefix and URI is also allowed
  for backward compatibility).  The NAMESPACE value may be undef for XMP tables
  where tags have variable namespaces (in this case each tag must have a
  Namespace entry).

  PREFERRED : Set if the tags in this table should always be added when writing
  information.  Higher number are higher precedence.  Overrides the order
  specified by SetNewGroups().  When this feature is used, it may also be
  desirable to specify a preferred group when calling InitWriteDirs() to write
  these tags -- this avoids creating other directories for tags which are
  already being creating in the preferred group.

  PERMANENT : Tags in this table are Permanent, and can't be deleted.  Maker
  notes tags are Permanent by default, without the need to set this property.

  SRC_TABLE : Used internally to store the source table name of a user-defined
  tag table so the appropriate module can be loaded as required.

  PRIORITY : Default Priority for all tags in this table.

  AVOID : Flag set to Avoid writing all tags in this table.

  WRITE_GROUP : Default WriteGroup for all tags in the table.

  LANG_INFO : Code reference to a routine which returns a reference to a
  language-specific tag information hash.  The routine takes two arguments: a
  reference to the non-specific tagInfo hash, and the language code. Used only
  in tables with writable tags which support tag name language extensions (eg.
  MIE and XMP).

  VARS : Hash used to store additional parameters.  Individual modules may use
  this to store any parameters they want.  The following additional parameters
  have been defined, and may be used by any module:

    ID_LABEL      Label to use instead of "Tag ID" for column heading in tag
                  name documentation.  When this is set, numerical TagID's are
                  not converted to hexadecimal notation.  Unless otherwise set,
                  an ID_LABEL of "Index" is assumed for tables which use
                  ProcessBinaryData or set the IS_BINARY flag.

    NO_ID         Avoid printing "Tag ID" column in tag name documentation.

    HEX_ID        Print tag ID in hexadecimal (with 4 hex digits or more).  Set
                  to 0 for string ID's that look like hex numbers to prevent
                  them from being converted when importing an XML file for
                  language translations in BuildLangModules().

    NO_LOOKUP     Do not add tags to TagLookup.pm lookup tables.

    CAPTURE       Used by PDF module to name dictionaries to capture when
                  writing.

    MINOR_ERRORS  [EXIF tables only] Flag to make errors in this IFD minor, or
                  to downgrade already minor errors to warnings while writing.
                  (Errors in MakerNote IFD's are already classified as minor.)
                  Note that for certain types errors, the response is to delete
                  the entire IFD from the image.

    ALPHA_FIRST   Sort alphabetical tags before numerical in documentation.

    LONG_TAGS     Suppress "Long tags" warning when generating documentation.
                  Value is the number of long tag names to expect.

    SORT_PROC     Reference to sort procedure for sorting tag ID's in the
                  documentation.  Takes two arguments and returns -1, 0 or 1
                  if the first argument is less than, equal to, or greater than
                  the second argument.

    ENTRY_SIZE    Used by ProcessPhaseOne to specify size of IFD entry

    MAP_FORMAT    Lookup to map unknown format numbers (eg. Panasonic CameraIFD)

    START_INDEX   [QuickTime tables only] Initial index for indices shown in
                  Verbose output.  Indices are not show otherwise.

    ATOM_COUNT    [QuickTime tables only] Maximum number of atoms contained in
                  within this atom.  Currently used only for Canon CNTH atom,
                  which contains garbage after the first contained atom.

    NIKON_OFFSETS [Nikon Encrypted tables only] Pointer to int32u NumberOffsets
                  for offset-type encrypted Nikon directories.  When set,
                  directory and decryption lengths are calculated automatically.

    ALLOW_REPROCESS Flag to allow reprocessing of another directory at this
                  same location in the file, bypassing recursion avoidance test.

    IS_BINARY     Set if this is a binary table and should show an Index in the
                  first column instead of a Tag ID.

  DATAMEMBER : BinaryData tables only.  A reference to a list of sorted tag ID's
  which must be extracted as data members when writing.  Must also list "var_"
  format tags and tags with Hook so offsets are properly calculated if the table
  is writable.

  SET_GROUP1 : [EXIF tables only] Flag to set group1 name to the directory name
  for all tags in the table.

The remaining entries in a tag table are the tag IDs with their associated
information.  The information may exist in one of three forms:  1) A simple
scalar which is the name of the tag, 2) A reference to a hash of information
describing this tag, or 3) a reference to a list of hashes which contain
Condition expressions allowing the appropriate hash to be selected to suit the
conditions.  The following is a description of possible hash entries.  All
entries are optional, except for the tag Name which is required if the tag ID is
numerical, and generated automatically otherwise.

  Name          : The tag name.  Tag names need not be unique.  If they aren't
                  unique, then duplicate tags will hide the values of previous
                  tags when extracting information unless the Duplicates option
                  is set or the new tag has lower Priority.  With Duplicates
                  set, to allow multiple tags with the same name to exist in the
                  tag information hash, the key of the previous tag is changed
                  to the form "TagName (N)", where N starts at 1 and increments
                  for subsequent duplicate tags.  A tag name should start with
                  an uppercase letter, and contain only the characters in the
                  set [A-Za-z0-9_-].  If not given, the Name is taken from the
                  tag ID with the first character changed to upper case.

  Description   : A more readable description of tag name.  If a tag doesn't
                  specify a Description, then the tag Name is used instead, with
                  spaces inserted between the words.

  Notes         : Notes for this tag in the HTML TagNames documentation.

  Groups        : Hash lookup for group names for this tag.  If not specified,
                  defaults to the GROUPS specified in the table definition.

  Format        : Only valid for BinaryData, EXIF and IPTC tables.  For a Binary
                  or EXIF table, this gives the format that is used to convert
                  the binary data, and is one of the FORMAT types specified
                  above.  If not specified, the Format of an EXIF entry is taken
                  from the EXIF information, and the Format of a BinaryData
                  entry is taken from the FORMAT specified for the table (or
                  int8u if FORMAT is not specified).  Must be specified for
                  Writable tags where the value must be converted for a
                  Condition.  For BinaryData tables, the format may have a size
                  in trailing brackets which is a Perl expression to be
                  evaluated.  The expression may access any of the previous
                  table entries through a %val hash (read-only tags), the data
                  size via $size, or the ExifTool object via $self.  For
                  example, 'string[$val{3}]' defines a string with length given
                  by the table entry with tag index '3'.  An initial "var_" may
                  be added to the Format name of any BinaryData tag with a size
                  in brackets.  In this case, subsequent offsets are adjusted by
                  the value length minus the size of the default table FORMAT
                  (eg. "var_int16u[10]" causes subsequent offsets to be
                  incremented by sizeof(int16u) * 10 - sizeof(int8u) if the
                  default table FORMAT is "int8u").  Note that all "var_" Format
                  tags (as well as tags with values used within "var_" Format
                  expressions) must have corresponding DATAMEMBER entries.

  Count         : Used when writing EXIF information to specify the number
                  values to write, or the number of characters in a fixed-length
                  string.  A value of -1 indicates that the count is variable
                  and should be determined by the number of values provided.
                  Defaults to 1 if not specified for non-string formats.  Note
                  that this count corresponds to the specified Format, so if a
                  different-sized Writable format is defined, the actual count
                  written to the file will be different.

  FixCount      : Flag set to determine correct count from offsets in IFD.  This
                  is necessary for some Kodak tags which write an incorrect
                  Count value.

  Flags         : Flags to specify characteristics for this tag.  May be a
                  simple scalar flag name, a reference to a list of flag names,
                  or a reference to a hash of flag/value pairs.  If not a hash
                  reference, the flag value is set to 1.  Flags are expanded for
                  faster access at run time into members of the tagInfo hash,
                  and may be written directly as members if desired.  The
                  available flag names are:

                  'AutoSplit' - [List tags only] Similar to ListSplit option,
                  but applied automatically to individual tags.  Value specifies
                  pattern for split, or 1 for default pattern ',?\\s+'.

                  'Avoid' - avoid creating this tag when writing if possible. 
                  This is only effective if another tag exists with the same
                  name.  Setting this flag also sets the default Priority to 0
                  for this tag.

                  'Binary' - set to 1 for binary data.  This has the same effect
                  as setting ValueConv to '\$val', but it it a bit cleaner and
                  avoids dummy ValueConvInv entries for writable tags.  Has no
                  effect if ValueConv is defined for the tag.  Some values may
                  be treated as binary data even if this flag is not set.

                  'BlockExtract' - set for writable directories in EXIF
                  information which are extracted by default.  Otherwise
                  writable directories are only extracted as a block if
                  specified explicitly.  Also used for Jpeg2000 and QuickTime
                  XMP-like tags to extract as a block with a name other than
                  'XMP'.

                  'ConvertBinary' - true to apply ValueConv and/or PrintConv to
                  Binary values (ie. SCALAR references).  (By default, these
                  conversions are not done for Binary values.)

                  'DataMember' - name of exiftool data member associated with
                  this tag if it should be stored as a special data member when
                  writing information.  Necessary only if the value of the tag
                  affects other written information.  Currently only used for
                  tags in EXIF tables where it triggers the execution of the
                  RawConv to convert and store the value as an ExifTool data
                  member when writing.

                  'DataTag' - associated tag name containing data for offset or
                  byte count tags.

                  'DelGroup' - set if deleting this tag is the same as deleting
                  the group of the same name.

                  'Drop' - [IFD-format and PhaseOne makernote tags only] set to
                  1 for tags that should be excluded when rebuilding maker notes
                  when copying all tags.  Set to a number larger than 1 to drop
                  only if data is larger than this size. Works for SubDirectory
                  tags too.

                  'EntryBased' - set to 1 if the offset for this value is based
                  on the IFD entry position.  This allows individual values to
                  be entry-based even though some others aren't (as with the
                  Casio PrintIM information).

                  'Flat' - [flattened XMP structure tags only] must be set for
                  all pre-defined flattened tags (including user-defined
                  flattened tags).  This flag is reset to 0 internally after all
                  associated flattened tags in the structure have been
                  generated.

                  'Flattened' - [reserved] used internally to mark Struct tags
                  which have been processed to generate flattened equivalents.

                  'NotFlat' - [XMP tags only] Flag indicates that this tag ID
                  does not represent a flattened tag.  Used to avoid a conflict
                  if the tag ID would be the same as a generated ID for a
                  flattened tag.  The result is that the flattened tag will not
                  be accessible.

                  'Hidden' - set to hide tag from the TagName documentation.
                  Also suppresses verbose output of a BinaryData tag.  The
                  RawConv of a Hidden tag should return undef so the tag value
                  is not seen by the user.

                  'IsComposite' - flag set for Composite tags

                  'IsImageData' - flag set if this is an image data offset to
                  be included in ImageDataHash calculation.  Must have an
                  OffsetPair entry which is the ID of the corresponding size.

                  'IsOffset' - flag set if the tag represents an offset to some
                  data, and causes value will be adjusted to an absolute file
                  offset.  If set to 2, the offset base of the parent directory
                  is used even if the base changes for the current directory
                  (only some maker notes are this messed up).  Set to 3 if
                  absolute file offsets are used.  May be set to an expression
                  to be evaluated.  Expression may access $val and $et,
                  and is evaluated only when reading.

                  'iTXt' - [PNG TextualData tags only] flag to write tag as PNG
                  iTXt chunk even if it contains no special characters.

                  'List' - flag indicating that duplicate entries of this tag
                  are allowed, and will be accumulated in a list.  Note that for
                  XMP information, 3 different types of lists are supported and
                  the List value specifies the type: 'Bag', 'Seq' or 'Alt'.  As
                  well, a value of '1' is used internally in XMP to allow list
                  behaviour for a flattened tag which is itself not a list
                  element (eg. a member of list of structures).  Note that in
                  ExifTool an XMP lang-alt tag (Writable="lang-alt") is NOT a
                  list-type tag (unless it is a list of lang-alt lists, which is
                  uncommon).

                  'MakerNotes' - set if this tag is maker note data.

                  'MakerPreview' - set in the PreviewImageStart tag information
                  if the preview must be stored inside the maker notes.

                  'Mandatory' - set for mandatory tags.  Used only by TagInfoXML
                  and for documentation purposes.  Mandatory tags may be added
                  automatically by ExifTool.

                  'NestedHtmlDump' - flag set if value for this tag is also
                  dumped when processing the SubDirectory.  This flag is implied
                  if the MakerNotes flag is set.  Set to 2 if the dump should
                  only be underlined if nested inside other maker notes.

                  'NotIFD' - set for 'MakerNotes' SubDirectory tags only if the
                  SubDirectory is not EXIF IFD format.  (Note: All SubDirectory
                  tags in the MakerNotes table are 'MakerNotes' type by
                  default.)

                  'OffsetPair' - set if the tag represents half of an offset/
                  byte count pair.  Data for these tags must be handled
                  separately.  Value is the tagID for the paired tag.

                  'Permanent' - flag indicates that a tag is permanent, and
                  can't be added or deleted from the file, although a new value
                  may be written if the tag already exists.  By default, all
                  MakerNotes tags are permanent unless otherwise specified.

                  'PreservePadding' - [QuickTime only] flag to preserve the
                  original size of the QuickTime atom by padding with nulls when
                  writing with the QuickTimePad option.

                  'PrintHex' - specifies that unknown PrintConv values should
                  be printed in hex (eg. 'Unknown (0x1)').  Also causes
                  numerical tag values to be printed in hex in the HTML tag name
                  documentation, padded to the number of digits given by the
                  PrintHex value.

                  'PrintInt' - remove decimal part of tag ID in HTML tag name
                  documentation.  (To avoid confusing ExifTool users because
                  the LensType decimal numbers are for internal use only.)

                  'PrintSort' - causes PrintConv values to be sorted by value
                  rather than key in the HTML tag name documentation.

                  'PrintString' - flag set to force PrintConv values to be
                  printed as strings in the documentation.

                  'Priority' - gives the priority of this tag while reading.  If
                  set to zero, this tag will not override the value of previous
                  tags with the same name.  If the priority is greater than
                  zero, this tag won't be overridden by subsequent tags unless
                  their priority is equal to or greater than this priority.  A
                  special feature is that Priority 0 tags are automatically
                  incremented to Priority 1 if they exist in the IFD of the full
                  resolution image (as determined by SubfileType).  If not
                  defined, the priority defaults to 1 for all tags except except
                  tags in IFD1 of JPEG images which default to priority 0.

                  'Protected' - bit mask to protect tags from writing:
                  Bit 0x01 indicates an 'Unsafe' tag, which is not set via
                  SetNewValuesFromFile() unless specified explicitly.
                  Bit 0x02 indicates a 'Protected' tag, which should not be set
                  directly by the user.

                  'PutFirst' - [EXIF only] flag to place this value before IFD0
                  when writing (ie. immediately after TIFF header).  Only used
                  for values in main IFD's (IFD0, IFD1, etc) and IFD's where
                  SubIFD flag is set to 2 (currently only ExifIFD).

                  'RawJoin' - [List tags only] Joins raw List-type tag values
                  into a single string with a space separator, allowing
                  ValueConv and PrintConv to act on the concatenated string so
                  the tag does not exhibit list-type behaviour.  When writing,
                  the inverse is performed and the value is split at whitespace.

                  'Resource' - [XMP only] flag to write value as an rdf:resource
                  in an empty element instead of as a normal string.

                  'SeparateTable' - set to list PrintConv values in a separate
                  table in the HTML documentation.  Value is 1 for a table name
                  of 'Module TagName', or 'TAG' for 'Module TAG', or 'MODULE
                  TAG' to fully specify the table name.  The table may have a
                  'Notes' entry for a description of the table.

                  'SetResourceName' - [Photoshop tags only] set to 1 to append
                  resource name to the extracted value (eg. 'VALUE/#NAME#/').
                  Also allows resource name to be appended when writing new
                  values.  May be set to any value other than 1 for a default
                  resource name to use when writing if an appended name is not
                  provided.

                  'StructType' - [reserved] used internally by XMP writer for
                  flattened structure tags as a flag to indicate that one or
                  more enclosing structures has a TYPE field.

                  'SubDoc' - [Composite tags only] set to cause this Composite
                  tag to also be generated for each sub-document.  To achieve
                  this, 'Main:' and 'Doc#:' prefixes are added to all Require'd
                  and Desire'd tag names which don't already start with 'Main:'
                  or 'Doc#:', and the Composite tag is built once for each of
                  the main document and all sub-documents.  May be a reference
                  to a list of indices for Desire'd tags, one of which must
                  exist to generate this tag for sub-documents.

                  'SubIFD' - used in writing to determine that the tag specifies
                  an offset to a sub-IFD.  When this flag is set, the Group1
                  name gives the name of the IFD.  Must be set if and only if
                  the tag has a SubDirectory Start that references '$val' (this
                  is validated by BuildTagLookup).  Set to 2 for standard EXIF
                  SubIFD's where the PutFirst flag is valid.

                  'Unknown' - this is an unknown tag (only extracted when the
                  Unknown option is set).  This is set to 2 for Unknown tags in
                  binary tables (extracted when Unknown is 2).

                  'WriteNothing' - flag indicating that nothing is actually
                  written when this tag is set.  It is a fake writable tag that
                  isn't actually written (eg. Geotag).  Used to avoid processing
                  a file unnecessarily if nothing will actually be written.

                  'WritePseudo' - flag indicating that this is a writable pseudo
                  tag.  If set, this tag may be written without actually writing
                  to the file (eg. FileName, FileModifyDate).

                  'WrongBase' - ['IsOffset' tags only] An expression using $self
                  that is evaluated to generate a base shift for 'IsOffset' tags
                  which use a different base than the rest of the tags.

  RawConv       : Used to convert the Raw value at extraction time (while the
                  image file is still open, unlike ValueConv and PrintConv below
                  which are done later only if the value is requested).  May be
                  a scalar expression using $val (the Raw tag value), $self (the
                  current ExifTool object), $tag (the tag key), $tagInfo
                  (reference to the tag information hash), $priority (to
                  dynamically set the priority of a tag), and @grps (to
                  dynamically set the family and/or groups), or a code reference
                  with $val and $self as arguments (in which case $$self{grps}
                  may be used to set the groups).  For Composite tags, $val is a
                  reference to a hash of source ("derived from") tag names, and
                  @val may be used to access the Raw values of these tags.  The
                  returned value may be a scalar which is used as the new Raw
                  value, a scalar reference to a binary data value, a hash
                  reference for Composite tags, an ARRAY reference for a list of
                  values, or undefined to indicate that the tag should be
                  ignored.  If RawConv is specified for a Composite tag, then
                  ValueConv and PrintConv evals will no longer have access to
                  the source @val and @prt values unless the input $val is
                  returned.  RawConv may generate Warning or Error tags, while
                  ValueConv and PrintConv should not (see ValueConv note below).
                  Note: RawConv should only be used if necessary (in general,
                  only if the conversion may return undef to ignore the tag, or
                  if a Warning may be issued) because ValueConv is more
                  efficient since it is only executed if the tag value is
                  requested, while RawConv is executed for all extracted tags.
                  Note that a tag is still writable if it has a RawConv without
                  a RawConvInv (with the exception of QuickTime ItemList and
                  UserData tags, which specifically require this), but this is
                  not true for ValueConv/PrintConv.

  ValueConv     : Used to convert the Raw value to a useable form. May be a hash
                  reference to act as a lookup table, a scalar which is
                  evaluated as a Perl expression, a code reference to a
                  subroutine, or an ARRAY reference (in which case the value is
                  split at whitespace into a list of items and each item is
                  converted by the associated entry in the ValueConv list.  A
                  special value of 'REPEAT' may be used to repeat the previous
                  conversion for all remaining elements).  If a hash reference
                  is used and the Raw value doesn't appear as one of the keys,
                  then the converted value is set to "Unknown (X)", where X is
                  the Raw value (unless either of the special keys exist:
                  'BITMASK', a reference to a hash used to decode individual
                  value bits; or 'OTHER', a reference to a subroutine used to
                  convert unknown values.  The OTHER subroutine takes 3
                  arguments: the value, a flag which is set for the inverse
                  conversion, and a reference to the PrintConv hash, and returns
                  the converted value or undef on error -- it may call warn() to
                  return an error message.  The lookup hash may also contain a
                  'Notes' entry which is used for documentation if the
                  SeparateTable flag is set).  In an expression, $self is a
                  reference to the current ExifTool object, $val is the Raw
                  value, and $tag is the tag key.  The subroutine takes 2
                  arguments: the Raw value and a reference to the current
                  ExifTool object.  The advanced formatting expression (if any)
                  may be accessed via the ExifTool "FMT_EXPR" member variable. 
                  The expression or subroutine is evaluated if and when the tag
                  value is requested (ie. only after all extraction is
                  complete), so if necessary at this time the values of all
                  other tags are available via calls to
                  $self->GetValue("Tag","Raw").  (Note: In theory, types other
                  than "Raw" may be accessed, but they are slower and may lead
                  to cyclical dependencies so they should be avoided).  When
                  evaluated, the expression or subroutine returns a scalar for
                  the converted value, a SCALAR reference to a binary data value
                  (see the 'Binary' flag), or an ARRAY reference for a list of
                  values.  The return value should always be defined -- use
                  RawConv instead to return undef if it is necessary to test the
                  value for validity, otherwise an undef tag may hide a
                  previously defined value when the Duplicates option is not
                  enabled.  If this isn't possible (as with Composite tags where
                  the converted values of the source tags are needed), set the
                  Priority to 0 to avoid taking priority over a valid tag. If
                  ValueConv is not specified, the Raw value is not converted. 
                  Composite tags which Require or Desire other tags may access
                  the ValueConv, PrintConv and Raw values of these tags through
                  the elements of the @val, @prt and @raw lists respectively
                  (only if there was no RawConv or it returned a hash
                  reference).  For these tags, $val may be used in an expression
                  to represent $val[0], and the first argument passed for a code
                  reference is a reference to @val. Note:  Warnings issued by
                  these conversions are intercepted by ExifTool and saved as
                  Warning tags, but since ValueConv and PrintConv conversions
                  are done on demand, the warnings may be generated after the
                  list of extracted tags is returned, so may easily be missed.

  PrintConv     : This entry is similar to ValueConv above, except that it is
                  used to further convert the tag value to a human readable
                  form.  It can be either a hash lookup, a scalar Perl
                  expression, a code reference or a list reference.  In this
                  expression, $self, $val and $tag may be used as with
                  ValueConv, but if ValueConv was defined then $val is the
                  ValueConv value instead of the Raw value.  Composite tags
                  may also use the @val, @prt and @raw lists. The returned value
                  should always be defined.  Note that the print conversion is
                  only done if the PrintConv option is enabled (which it is by
                  default), and if the result of the ValueConv is not a scalar
                  reference.  If it is a list reference, then the converted
                  values are joined by '; ' in the output string.

  RawConvInv    : The inverse of RawConv.  This should only be used in very rare
                  situations when the raw value can not be predetermined.
                  Unlike the other inverse conversions which are applied in
                  SetNewValue(), this conversion is applied in WriteInfo() as
                  the file is being written.  This is important because it means
                  that FILE_TYPE and any DataMember tags ocurring before this
                  tag in the file are available.  Beware that if the return
                  value is not defined, the tag will be deleted unless there is
                  specific logic to avoid this (currently, only EXIF and Binary
                  data directories handle this case).

  ValueConvInv  : The inverse of ValueConv.  Only necessary for Writable tags
                  when ValueConv is specified (except WriteAlso tags).  Note
                  that DataMember tags may NOT be used in the inverse
                  conversions because these conversions are done before the
                  input file is parsed.  Instead, a Condition or RawConvInv must
                  be used.  May return undef on conversion error and call warn()
                  to issue a warning.  If warn() is not called, a generic
                  warning is generated when undef is returned.  An empty warning
                  ("\n") may be issued to suppress warning messages when undef
                  is returned.  If a scalar, the expression may use the
                  variables $val, $self and $wantGroup.  If a code ref, the
                  routine is passed 2 arguments: $val and $self.  Note that this
                  conversion is not applied for deleted tags (ie. $val is
                  undef).

  PrintConvInv  : The inverse of PrintConv.  Necessary for Writable tags
                  when PrintConv is specified (unless WriteAlso is used), but
                  may be used without PrintConv.  See ValueConvInv above for
                  more details.

  PrintConvColumns : Number of columns for PrintConv lookup in HTML docs. If not
                  set, the number of columns is determined automatically
                  according to the maximum width of the entries.

  DelValue      : Raw value to be used when deleting a permanent tag.  (Note all
                  MakerNotes tags are permanent.)  If not specified, an attempt
                  is made to convert an empty string for the raw value.

  Validate      : Scalar expression used to validate the raw value of a tag when
                  the Validate option is set.  The expression may access $val
                  (the Raw tag value), $tag (the tag key) and $self (the current
                  ExifTool object), and should return a warning if any problem
                  is found, or false if OK.  The evaluation is performed in the
                  Image::ExifTool::Validate package namespace, so functions of
                  this package need not be prefixed by the package name.

  Relist        : [Only if ValueConv or PrintConv is a list ref] Reference to a
                  list of original value indices used to reorganize values. Each
                  entry in the list may be a scalar index, or a reference to a
                  list of indices to join values.  (Currently values may be
                  joined, but the order of writable values must not be changed
                  until this ability is added to the writer.)

  Mask          : [BinaryData tags only] Bitmask for this value.  (Multiple tags
                  may have the same index by using floating point indices.  An
                  unknown tag will only be generated for a certain TagID if
                  there is no integral TagID for that tag.)  The Mask and
                  BitShift are applied before evaluating RawConv.

  BitShift      : [Mask tags only] Bit shift for Mask-ed values.  If not
                  specified, set to the number of trailing zero bits in Mask.
                  When reading, the value is shifted right by this number of
                  bits after the Mask is applied.

  Condition     : If given, specifies scalar which is evaluated as a Perl
                  expression at extraction time to decide whether the tag is
                  valid.  If used in a list of alternate tag definitions, the
                  first list entry with a true condition is taken.  If no
                  condition exists, then a 'true' condition is assumed.  The
                  expression may use $self to access the ExifTool object.  The
                  first 128 bytes of the raw data value are accessible through
                  the reference $valPt for EXIF, Jpeg2000, QuickTime, FLAC and
                  BinaryData tags only (note that for BinaryData tags, the raw
                  data of $$valPt is always 'undef' type, and may not be used
                  when writing except for SubDirectory tags).  EXIF tags (and
                  maybe some other types) may also reference the format string
                  and value count through $format and $count.  Note that if the
                  value is writable and $valPt is used, the tag must have a
                  Format (unless 'undef' or 'string'), and a Count (unless 1 or
                  length of the 'undef' or 'string'), so the raw data may be
                  generated for evaluating the Condition.  When writing, $valPt,
                  $format and $count refer to the new value, except for
                  MakerNotes tags where $format and $count refer to the old tag
                  if it existed.

  Require       : [Composite tags only] A hash reference specifying the tags
                  required to calculate the Composite tag value.  The hash
                  values are the names of the required tags, and the keys
                  specify the indices where the tag values are stored in the
                  @val list used in the ValueConv and/or PrintConv expression.
                  The Composite value is only calculated if the values for all
                  Require'd tags are defined.  Require, Desire and Inhibit tag
                  names may be prefixed by an optional group family 0 or 1 name
                  followed by a colon.  Case IS significant.  The keys used by
                  the Require, Desire and Inhibit hashes must not overlap (since
                  they are used as indices into the common @val, @prt and @raw
                  lists), and together the keys must be sequential starting from
                  0.  A special feature allows a scalar tag name to be used
                  instead of the hash reference, in which case the next
                  available index will be used (avoiding collisions with Desire
                  and Inhibit hash entries).  For example, the following two
                  definitions are equivalent:

                        Require => { 0 => 'XMP:Title' },
                        Require => 'XMP:Title',

  Desire        : [Composite tags only] This is the same as Require except that
                  the Composite value is calculated even if the specified tags
                  don't exist.  Beware that the elements of @val, @prt and @raw
                  may be undefined for Desire'd tags.  If no tags are Require'd,
                  at least one of the Desire'd tags must exist for the Composite
                  tag to be generated.  If there are no Require'd or Desire'd
                  tags, then the Composite tag is always generated.  A scalar
                  Desire tag name is given the next available index (after
                  scalar Require tags are allocated).

  Inhibit       : [Composite tags only] Similar to the Require and Desire
                  hashes, except that the Composite tag is NOT built if any of
                  the Inhibit tags exist.  A scalar tag name is given the next
                  available index (after scalar Require and Desire tags are
                  allocated).

  Shift         : [Writable tags only] Specifies type of shift to apply if this
                  value may be shifted.  Set to 'Time' for shifting date/time
                  tags, or '0' to prevent tag from being shifted or
                  conditionally deleted.

  Writable      : Indicates this tag can be written (or not written if Writable
                  is set to zero), and for EXIF and QuickTime tables gives
                  format for writing.  Writable may be set to 1 for MakerNotes
                  information because the existing format is always used,
                  however providing a format is desirable because it is used in
                  validating the value.  Set to 2 for tag to show "yes" in the
                  Writable column of the tag name documentation even when there
                  is no WRITE_PROC defined (eg. if it is written via an Extra
                  tag).  For EXIF and QuickTime tables, the Writable flag may be
                  different than the Format flag, in which case Format is used
                  for converting the binary value and Writable specifies the
                  format code written to the EXIF IFD.  For SubDirectories in
                  EXIF information, this flag is only defined if the
                  SubDirectory is writable as a block, or if the SubDirectory
                  can not be edited (in which case Writable is set to 0).  If
                  non-zero, the SubDirectory is also extracted as a block, so
                  the Binary and Protected flags should usually set as well. 
                  There is currently no way to specify a write format for a
                  SubDirectory that is not writable as a block (the default is
                  'int32u' for IFD-type SubDirectories, and 'undef' for all
                  others).

  WriteAlso     : Used for writable tag to specify other tags to write when this
                  tag is written.  The value is a hash reference.  The hash keys
                  are the names of the tags to write, and the values are
                  evaluated to obtain the ValueConv values of each tag (or undef
                  to delete the tag).  In the eval, $val is the new Raw value of
                  the parent tag (which may be undef if the tag is being
                  deleted, and should not be modified because it will affect
                  subsequent WriteAlso tags), and the %opts hash may be accessed
                  to modify SetNewValue options for each tag.  By default, Type
                  is set to "ValueConv" and the Protected option has bit 0x02
                  set to allow writing of Protected tags that aren't directly
                  writable.  The AddValue, DelValue, Shift and Replace options
                  from the parent tag are also defined, but no other options are
                  set by default. Previous new values of WriteAlso tags have
                  already been removed prior to the eval if the Replace option
                  was used for the parent tag.  If an empty warning is issued
                  ("\n"), the target tag is not written and no error is
                  reported.

  WriteCheck    : If given, specifies a scalar which is evaluated as a Perl
                  expression for a one-time validatation the Raw value being
                  written.  The expression has access to 3 variables: $val is
                  the value to be written, $self is the ExifTool object, and
                  $tagInfo is the tag information hash reference.  It returns an
                  error string, or undef if the value is good.  If the error
                  string is empty, the tag is not written and no warnings are
                  issued, but WriteAlso is still evaluated if it exists.

  WriteHook     : [QuickTime only] Routine to call with these arguments when tag
                  is encountered during writing: 0) tag value, 1) ExifTool ref.

  WriteOnly     : Flag set if tag is write-only.  Used for documentation only.

  DelCheck      : Similar to WriteCheck, but called when the tag is deleted. The
                  expression may access $self, $tagInfo and $wantGroup.  Returns
                  error string, or undef on success, and may set $val to
                  something other than undef.  May return empty string ('') to
                  suppress warning messages but not delete tag (eg. when
                  deleting only associated tags).

  WriteCondition: [Writable EXIF tags only] Specifies a condition to be
                  evaluated before the tag can be written to a specific file.
                  The condition may use $self to reference the ExifTool object,
                  and returns true if it is OK for writing.  Unlike WriteCheck
                  which is done only once when the new value is set, this
                  condition is evaluated just before the tag is written to the
                  file.

  WriteGroup    : [Writable EXIF and Composite tags only] Specifies the IFD
                  where the information gets written by default.  Must be
                  defined for writable EXIF tags if WRITE_GROUP is not specified
                  in table.  A WriteGroup of 'All' is special:  For writable
                  Composite tags this causes any group specified for the
                  Composite tag to be propagated to the WriteAlso tags when
                  writing.  For other tags this allows writing to all EXIF IFD's
                  simultaneously (in which case the tag should have a Condition
                  or WriteCondition to restrict the IFD's to which the tag is
                  actually written).

  IsOverwriting : [Writable EXIF tags only] Specifies reference to subroutine
                  which determines if tag should be overwritten.  Arguments are:
                  0) ExifTool object ref, 1) new value hash ref, 2) old value,
                  3) new value reference.

  Preferred     : This tag is always preferred when writing.

  Override      : [User-defined Composite tags only] Flag to specify that the
                  new tag definition should override the definition of existing
                  Composite tags with the same name.  Default is 1 unless
                  otherwise specified.

  AllowGroup    : [Writable tags only] Regular expression string (case
                  insensitive) to match group names which are allowed when
                  writing this tag.  Only needed if tag can be written to
                  groups other than the normal groups for this tag (very rare).

  Deletable     : [Writable SubDirectory's only] Overrides internal test for
                  metadata types with permanent directories (currently QuickTime
                  and Jpeg2000), allowing the tag containing these directories
                  to be deleted

  OffsetPair    : Used in EXIF table to specify the tagID for the corresponding
                  offset or length tag.

  DataTag       : Used in EXIF table to specify the tag name of the data
                  associated with offset/length tags.

  FixFormat     : [Writable EXIF SubIFD SubDirectory's only] Used to specify a
                  format for writing an IFD pointer other than 'int32u'.

  ChangeBase    : [EXIF tags in JPEG images only] Eval used to return new base
                  for the offset for this tag only.  Eval may use $dirStart and
                  $dataPos.  Note this is geared to the quirky Leica preview
                  image offset, and is only used if the preview is outside the
                  APP1 EXIF segment.

  BitsPerWord   : [BITMASK tags only] Number of bits per decoded word.  Defaults
                  to 32 if not specified.

  BitsTotal     : [Writable BITMASK tags only] Total number of bits in bit mask.
                  Defaults to BitsPerWord if not specified.

  FixedSize     : [EXIF only] Hack to ignore value size and use this instead.
                  Only valid if Format is also defined.

  TruncateOK    : [EXIF only] Hack to demote a truncated value in an IFD to a
                   minor error.

  Struct        : [XMP tags only] Reference to structure hash for structured XMP
                  tags.  See "STRUCTURES" section below for more details.  (For
                  backward compatibility, this may be a name to an entry in
                  %Image::ExifTool::UserDefined::xmpStruct, but this use is
                  deprecated.)  Flattened tags are automatically generated for
                  each field in the structure.  Note that Struct tags can NOT
                  have ValueConv/PrintConv entries, because values that are HASH
                  references are are also used by Composite tags, which use the
                  the Conv entries to generate the Composite value.

  NoSubStruct   : [XMP tags only] Flag set in flattened tag to break cyclical
                  recursion in nested structures.

  Namespace     : [XMP tags only] Gives standard XMP namespace prefix to use for
                  this tag.  If not defined, the tag table NAMESPACE is used.

  FlatName      : [Struct tags and structure fields only] Name used for
                  automatically-generated flattened tag names, defaults to Name
                  if not specified.  In general, this is used to remove the
                  redundant names found in some specifications.  May be empty
                  ('') for a Struct tag but not for a structure field.

  Units         : [MIE tags only] Reference to a list of valid units strings.
                  The default units are the first item in the list.

  Hook          : [BinaryData tags only] Expression to be evaluated when
                  extracting tag to allow dynamic Format, etc for BinaryData
                  tags.  May access $self, $size (the full size of the binary
                  data record), $dataPt (reference to the data block), and $pos
                  (position of this tag in the data block), and assign a new
                  value to $format to dynamically set the tag format, and/or
                  increment $varSize to add a byte offset to subsequent tags. 
                  $varSize may be set to a large number to effectively abort
                  processing of the directory after this tag.  Must have
                  corresponding DATAMEMBER entry in writable tables.  Must not
                  have Unknown set.

  LargeTag      : [BinaryData tags only] Flag to indicate that the data for this
                  tag is large, and that it shouldn't be stored in the %val
                  hash, even if the tag exists as a DATAMEMBER.

  SetBase       : [QuickTime and BinaryData tags only] Sets ExifTool BASE offset
                  for use when ExtractInfo is called with the ReEntry flag from
                  inside the RawConv of a tag.

  TagID         : [reserved] Used internally to save the table key for this tag.
                  Note: For XMP tables this corresponds to the XMP property
                  name, but the table key may have a full XMP namespace prefix
                  added.

  NewTagID      : [reserved] Used internally to save new ID of tag in Composite
                  table if it is different that the original TagID (happens if
                  there was a conflict with an existing entry in the table)

  Index         : [reserved] Used internally to save the index of tagInfo items
                  which are in a conditional list.

  Table         : [reserved] Reference to parent tag table.

  PropertyPath  : [reserved] Used internally by XMP writer to save property path
                  name.  Also used in structure field information hashes, but
                  for these the full property path is not stored.

  SrcTagInfo    : [reserved] Used internally to store reference to default
                  language tagInfo hash for alternate-language tags.

  OtherLang     : [reserved] Used internally by QuickTime module to store a list
                  of tag ID's for alternate-language tags based on this one.

  ParentTagInfo : [reserved] Used internally to store a reference to the tag
                  information hash of the parent structure for flattened
                  structure tags.

  RootTagInfo   : [reserved] Used internally to store a reference to the tag
                  information hash of the top-level structure for flattened
                  structure tags.

  Module        : [reserved] Used internally to store module name for writable
                  Composite tags.

  LangCode      : [reserved] Used internally to indicate language code for
                  alternate language tags (eg. 'fr').  Only used with formats
                  which support alternate languages (eg. XMP, MIE, etc).

  AddedUnknown  : [reserved] Used internally to mark Unknown tags that were
                  added to the table at run time.

  SubDirectory {  If it exists, this specifies the start of a new subdirectory.
                  It contains a collection of variables which specify the type
                  and location of the subdirectory.  Note that ValueConv and
                  PrintConv do not apply to SubDirectory tags, but RawConv is
                  evaluated before the SubDirectory is processed.  The following
                  is a list of SubDirectory variables:

     TagTable   : Specifies the name of the tag table lookup for the new
                  subdirectory.  If not specified, the parent tag table is used.

     Start      : The offset to the start of the subdirectory relative to the
                  current Base.  This is a Perl expression which may use
                  $valuePtr to represent the location of the tag value in the
                  file, or $val for the value itself.  If not specified, a Start
                  of '$valuePtr' is assumed.  Subdirectories in BinaryData may
                  also use $dirStart to represent the offset of the current
                  directory start relative to the start of the data block.

     OffsetPt   : [EXIF directories only] If specified, this is a Perl
                  expression that gives the position of a 32-bit word in the
                  current directory that is added to the Start position to get
                  the position of the new subdirectory.  The expression
                  should use the position of the current tag ($valuePtr).

     Base       : This expression specifies the base offset for all pointers in
                  the subdirectory.  This need not be specified if the offset is
                  the same as the current directory, which is normally the case.
                  May use $start to represent the subdirectory start location
                  relative to the current base, and $base for the value of the
                  current base.  If this is defined, the automatic validation of
                  base offsets is disabled for maker notes directories.  The IFD
                  is flagged as using relative offsets when writing if '$start'
                  is used in this expression.

     EntryBased : [EXIF directories only] Flag indicating that the offsets are
                  based on the individual directory entry position, so offsets
                  are incremented by 12 times the corresponding entry index.

     MaxSubdirs : Maximum number of subdirectories specified by the current tag
                  (if the tag specifies multiple values).  If not specified, the
                  tag value ($val) is used as-is.  If MaxSubdirs is specified,
                  then one subdirectory is parsed for each value found up to the
                  maximum number specified.  Ignored when writing.

     ByteOrder  : Specifies byte ordering if different than than the rest of the
                  file.  Must be either BigEndian, LittleEndian or Unknown.  If
                  Unknown is specified, the byte order will be determined from
                  the directory count (however, this can not be done if OffsetPt
                  is specified).

     Validate   : If given, specifies Perl expression which is used to validate
                  the subdirectory data (regardless of Validate option setting).
                  The following variables may be used in the expression: $val
                  (value of the tag), $dirData (reference to directory data),
                  $subdirStart (offset to subdirectory start) and $size (size of
                  subdirectory).  Returns true if subdirectory is valid.

     ProcessProc: If given, specifies processing procedure used to decode this
                  subdirectory data.  This overrides the default procedure
                  specified by PROCESS_PROC in the tag table.

     WriteProc  : If given, specifies processing procedure used to write this
                  subdirectory data.  This overrides the default procedure
                  specified by WRITE_PROC in the tag table.

     DirName    : Name of this subdirectory.  If not specified, the name is
                  taken from the tag name.  DirName is important because it is
                  used when writing to compare against names in the directory
                  map to determine which directories need to be edited.

     FixBase    : Flag set if base offsets should be fixed.  Used to add a
                  constant to maker notes offsets to fix damage done by some
                  image editing utilities. (maker notes only)  Set to 2 for
                  unknown maker notes to be a bit more flexible in adjusting
                  the base offset.

     AutoFix    : Flag set to patch GE makernote offset quirks and apply FixBase
                  without warnings when writing.

     FixOffsets : Expression to evaluate for each value pointer to patch
                  problems with some EXIF maker note offsets.  May access the
                  following variables: $valuePtr, $valEnd, $size, $tagID and
                  $wFlag.  May return undef when writing to ignore the entry.

     RelativeBase:[EXIF directories only] Flag to adjust all offsets relative
                  to the start of the IFD when writing.

     Multi      : [EXIF directories only] Flag to allow multiple linked IFD's.
                  1 is assumed if DirName is IFD0 or SubIFD unless otherwise
                  defined.

     Magic      : [TiffIFD directories only] Magic number used in TIFF-like
                  header.
  }

STRUCTURES

Structure hashes are very similar to tag tables in that their keys are structure
field ID's and their values are structure field information hashes.  The special
keys in structure hashes are:

  NAMESPACE   : Same as in tag tables, but may be set to undef for a
                variable-namespace structure which holds any top-level tag. This
                is mandatory for built-in tags, but optional for user-defined
                tags (where it defaults to the NAMESPACE of the tag table).

  STRUCT_NAME : Name of structure (used in warning messages and documentation).
                May contain leading module name separated by a space to avoid
                name conflicts with same-named structures in other modules.
                Default module is "XMP" unless otherwise specified.

  TYPE        : rdf:type resource for structure.

  NOTES       : Notes for documentation.

  GROUPS      : Same as in tag table, but only the family 2 group name is used,
                as the default for the flattened tags.

  SORT_ORDER  : Order for sorting fields in documentation.

The contained structure field information hashes are similar to tag information
hashes, except that only the following elements are used:

  Raw/Value/PrintConv (and their inverses), TagID (optional), Groups, List,
  Writable, Struct, Namespace, FlatName, LangCode, PropertyPath, Notes.

But note that for PropertyPath, only the element of the path corresponding to
the specific field is stored (including any necessary list properties).  The
full property path is obtained by walking the structure tree.

Flattened tags corresponding to each field in a structure are automatically
generated (using an on-demand approach to reduce startup overhead).  Pre-defined
flattened tags are allowed, and are used when it is necessary to change the Name
or Description of a flattened tag.  The flattened tag information hash entries
are copied from the corresponding structure field definitions, even for
pre-defined flattened tags.  The exception is that the List property is
generated automatically unless explicitly set to 0 in a pre-defined flattened
tag.

--------------------------------------------------------------------------------
