/*
-----------------------------------------------------------------------------
This source file is part of OGRE
(Object-oriented Graphics Rendering Engine)
For the latest info, see http://www.ogre3d.org/

Copyright (c) 2000-2014 Torus Knot Software Ltd
Copyright (c) 2006 Matthias Fink, netAllied GmbH <matthias.fink@web.de>                             

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
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furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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*/
#ifndef __ShadowCameraSetupLiSPSM_H__
#define __ShadowCameraSetupLiSPSM_H__

#include "OgrePrerequisites.h"
#include "OgreShadowCameraSetupFocused.h"
#include "OgreHeaderPrefix.h"

namespace Ogre 
{

    /** \addtogroup Core
    *  @{
    */
    /** \addtogroup Scene
    *  @{
    */
    /** Implements the %Light Space Perspective Shadow Mapping Algorithm @cite WSP04

    Shadow mapping was introduced by Williams in 1978. First a depth image is rendered
    from the light's view and compared in a second pass with depth values of the normal 
    camera view. In case the depth camera's depth value is greater than the depth seen
    by the light the fragment lies in the shadow.
    The concept has a major draw back named perspective aliasing. The shadow map distri-
    butes the samples uniformly meaning the position of the viewer is ignored. For the 
    viewer however the perspective projection affects near objects to be displayed 
    bigger than further away objects. The same thing happens with the shadow map texels:
    Near shadows appear very coarse and far away shadows are perfectly sampled.
    PSM @cite stamminger2002perspective battles the perspective aliasing by distributing 50% of the shadow map
    texels for objects in the range of < near clipping plane > to < near clipping plane * 2 >
    which inverts the problem: The shadows near the viewer are perfectly sampled, 
    however far away shadow may contain aliasing artefacts. A near clipping plane may be
    a problem. But this is not the only one. In the post-perspective space the light 
    sources are non-intuitively mapped: Directional lights may become point light and 
    point lights may become directional lights. Also light sinks (opposite of a light 
    source) may appear. Another problem are shadow casters located behind the viewer. 
    In post-projective space objects behind the viewer are mapped in front of him with 
    a flipped up-vector.
    LiSPSM battles the light source problem of the post-projective space by rearranging
    the light space before transformation in such a way that no special cases appear. 
    This is done by converting point/spot lights into directional lights. The light 
    space is arranged in such a way that the light direction equals the inverse UNIT_Y.
    In this combination the directional light will neither change its type nor its 
    direction. Furthermore all visible objects and shadow casters affecting the user's 
    visible area lie in front of the shadow camera: After building the intersection body
    that contains all these objects (body intersection building was introduced with PSM; 
    have a look at the description for the method "calculateB" for further info) a 
    frustum around the body's light space bounding box is created. A parameter (called 
    'n') automatically adjusts the shadow map sample distribution by specifying the 
    frustum's view point - near plane which affects the perspective warp. In case the 
    distance is small the perspecive warp will be strong. As a consequence near objects 
    will gain quality.
    However there are still problems. PSM as well as LiSPSM only devote to minimize
    perspective aliasing. Projection aliasing is still a problem, also 'swimming 
    artefacts' still occur. The LiSPSM quality distribution is very good but not the 
    best available: Some sources say logarithmic shadow mapping @cite lloyd2007practical
    is the non plus ultra, however others reject this thought.

    More information can be found on the webpage of the TU Wien: 
    http://www.cg.tuwien.ac.at/research/vr/lispsm/

    Original implementation by Matthias Fink <matthias.fink@web.de>, 2006.
    */
    class _OgreExport LiSPSMShadowCameraSetup : public FocusedShadowCameraSetup
    {
        /// Warp factor adjustment
        Real mOptAdjustFactor;
        /// Use simple nopt derivation?
        bool mUseSimpleNOpt;
        /// Extra calculated warp factor
        mutable Real mOptAdjustFactorTweak;
        /// Threshold (cos angle) within which to start increasing the opt adjust as camera direction approaches light direction
        Real mCosCamLightDirThreshold;

        /** Calculates the LiSPSM projection matrix P.

        The LiSPSM projection matrix will be built around the axis aligned bounding box 
        of the intersection body B in light space. The distance between the near plane 
        and the projection center is chosen in such a way (distance is set by the para-
        meter n) that the perspective error is the same on the near and far plane. In 
        case P equals the identity matrix the algorithm falls back to a uniform shadow
        mapping matrix.
        @param lightSpace Matrix of the light space transformation
        @param bodyB Intersection body B
        @param bodyLVS Intersection body LVS (relevant space in front of the camera)
        @param sm Scene manager
        @param cam Currently active camera
        @param light Currently active light
        */
        Matrix4 calculateLiSPSM(const Matrix4& lightSpace, const PointListBody& bodyB, 
            const PointListBody& bodyLVS, const SceneManager& sm, 
            const Camera& cam, const Light& light) const;

        /** Calculates the distance between camera position and near clipping plane.

        n_opt determines the distance between light space origin (shadow camera position)
        and the near clipping plane to achieve an optimal perspective foreshortening effect.
        In this way the texel distribution over the shadow map is controlled.

        Formula:
                       d
        n_opt = ---------------
                sqrt(z1/z0) - 1

        Parameters:
        d: distance between the near and the far clipping plane
        z0: located on the near clipping plane of the intersection body b
        z1: located on the far clipping plane with the same x/y values as z0        
        @note
        A positive value is applied as the distance between viewer and near clipping plane.
        In case null is returned uniform shadow mapping will be applied.
        @param lightSpace Matrix of the light space transformation
        @param bodyBABB_ls Bounding box of the transformed (light space) bodyB
        @param bodyLVS Point list of the bodyLVS which describes the scene space which is in
        front of the light and the camera
        @param cam Currently active camera
        */
        Real calculateNOpt(const Matrix4& lightSpace, const AxisAlignedBox& bodyBABB_ls, 
            const PointListBody& bodyLVS, const Camera& cam) const;

        /** Calculates a simpler version than the one above.
        */
        Real calculateNOptSimple(const PointListBody& bodyLVS, 
            const Camera& cam) const;

        /** Calculates the visible point on the near plane for the n_opt calculation

        z0 lies on the parallel plane to the near plane through e and on the near plane of 
        the frustum C (plane z = bodyB_zMax_ls) and on the line x = e.x.
        @param lightSpace Matrix of the light space transformation
        @param e The LiSPSM parameter e is located near or on the near clipping plane of the
        LiSPSM frustum C
        @param bodyB_zMax_ls Maximum z-value of the light space bodyB bounding box
        @param cam Currently active camera
        */
        Vector3 calculateZ0_ls(const Matrix4& lightSpace, const Vector3& e, Real bodyB_zMax_ls, 
            const Camera& cam) const;
    public:
        /// @deprecated use create()
        LiSPSMShadowCameraSetup(Real n = 0.1f, bool useSimpleNOpt = true, Degree angle = Radian(0.451f));

        virtual ~LiSPSMShadowCameraSetup();

        /**
         * @param n The adjustment factor
         * @param useSimpleNOpt
         * @param angle camera Light Direction Threshold
         */
        static ShadowCameraSetupPtr create(Real n = 0.1f, bool useSimpleNOpt = true, Degree angle = Radian(0.451f))
        {
            return std::make_shared<LiSPSMShadowCameraSetup>(n, useSimpleNOpt, angle);
        }

        /** Returns a LiSPSM shadow camera.

        Builds and returns a LiSPSM shadow camera. 
        More information can be found on the webpage of the TU Wien: 
        http://www.cg.tuwien.ac.at/research/vr/lispsm/
        */
        void getShadowCamera(const SceneManager *sm, const Camera *cam,
            const Viewport *vp, const Light *light, Camera *texCam, size_t iteration) const override;

        /** Adjusts the parameter n to produce optimal shadows.

        The smaller the parameter n, the stronger the perspective warping effect.
        The consequence of a stronger warping is that the near shadows will gain 
        quality while the far ones will lose it. Depending on your scene and light
        types you may want to tweak this value - for example directional lights
        tend to benefit from higher values of n than other types of light, 
        especially if you expect to see more distant shadows (say if the viewpoint is
        higher above the ground plane). Remember that you can supply separate
        ShadowCameraSetup instances configured differently per light if you wish.
        @param n The adjustment factor - default is 0.1f. 
        */
        virtual void setOptimalAdjustFactor(Real n) { mOptAdjustFactor = n; }
        /** Get the parameter n used to produce optimal shadows. 
        @see setOptimalAdjustFactor
        */
        virtual Real getOptimalAdjustFactor() const { return mOptAdjustFactor; }
        /** Sets whether or not to use a slightly simpler version of the 
            camera near point derivation (default is true)
        */
        virtual void setUseSimpleOptimalAdjust(bool s) { mUseSimpleNOpt = s; }
        /** Gets whether or not to use a slightly simpler version of the 
        camera near point derivation (default is true)
        */
        virtual bool getUseSimpleOptimalAdjust() const { return mUseSimpleNOpt; }

        /** Sets the threshold between the camera and the light direction below
            which the LiSPSM projection is 'flattened', since coincident light
            and camera projections cause problems with the perspective skew.

            For example, setting this to 20 degrees will mean that as the difference 
            between the light and camera direction reduces from 20 degrees to 0
            degrees, the perspective skew will be proportionately removed.
        */
        void setCameraLightDirectionThreshold(Degree angle);

        /** Sets the threshold between the camera and the light direction below
        which the LiSPSM projection is 'flattened', since coincident light
        and camera projections cause problems with the perspective skew.
        */
        virtual Degree getCameraLightDirectionThreshold() const;


    };
    /** @} */
    /** @} */

}

#include "OgreHeaderSuffix.h"

#endif

