Impostor

/* -*-c++-*- OpenSceneGraph - Copyright (C) 1998-2006 Robert Osfield 
 *
 * This library is open source and may be redistributed and/or modified under  
 * the terms of the OpenSceneGraph Public License (OSGPL) version 0.0 or 
 * (at your option) any later version.  The full license is in LICENSE file
 * included with this distribution, and on the openscenegraph.org website.
 * 
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the 
 * OpenSceneGraph Public License for more details.
*/

#ifndef OSGSIM_IMPOSTOR
#define OSGSIM_IMPOSTOR 1

#include <osg/LOD>
#include <osg/buffered_value>

#include <osgUtil/CullVisitor>

#include <osgSim/ImpostorSprite>

namespace osgSim {

/** Impostor - is a form of Level Of Detail group node which allows both switching 
  * between children depending on distance from eye point and image caching.
  * 
  * The principle behind Imposters is that they cache an image of real geometry and then the image is drawn 
  * in subsequent frames instead of the real geometry. It's a bit like a
  * Billboard *but* is updated at runtime and w.r.t view point. By drawing
  * just the texture mapped quad you can cut down scene complexity and
  * improve performance.
  * 
  * For more details have a look at:
  * 
  *    http://grail.cs.washington.edu/projects/hic/
  * 
  * The OSG doesn't implement exactly the same technique as above, but its
  * should be a good starting place. The OSG's impostors are much less
  * intrusive since you don't need to restructure your whole scene to use
  * them.
  *
  * All you need to do to use Impostors is to set up the visible
  * range values for each LOD child of the Impostor, as per osg::LOD,
  * and set an Impostor threshold to tell the renderer at what distance
  * the Impostor's image caching should cut in.  The osg::CullVisitor
  * automatically handles all the setting of pre-rendering stages to
  * calculate the required ImpostorSprites (which encapsulates the image
  * cache and quad), and updates them as the view point changes. If you 
  * use osg::SceneView/CullVisitor all the complexity of supporting 
  * Impostor will be nicely hidden away.
  *
  * TODO:
  * Various improvements are planned for the Impostor-
  * 1) Estimation of how many frames an ImpostorSprite will be reused, if
  * it won't be used more often than a minimum threshold then do not create
  * ImpostorSprite - use the real geometry.
  * 2) Sharing of texture memory between ImpostorSprites.
  * 3) Simple 3D geometry for ImpostorSprite's rather than Billboarding.
  * 4) Shrinking of the ImpostorSprite size to more closely fit the underlying
  * geometry.
  */
class OSGSIM_EXPORT Impostor : public osg::LOD
{
    public :
        Impostor();

        Impostor(const Impostor& es, const osg::CopyOp& copyop=osg::CopyOp::SHALLOW_COPY):
            osg::LOD(es,copyop),
            _impostorThreshold(es._impostorThreshold) {}
            
        META_Node(osgSim, Impostor);
        
        virtual void traverse(osg::NodeVisitor& nv);

        typedef std::vector< osg::ref_ptr<ImpostorSprite> > ImpostorSpriteList;

        /** Set the Impostor threshold distance.
          * For eye points further than this threshold the Imposter is used if appropriate,
          * otherwise the LOD children as chosen as per a standard LOD node.
        */
        inline void setImpostorThreshold(float distance) { _impostorThreshold = distance; }
        
        /* Get the Impostor threshold distance. */
        inline float getImpostorThreshold() const { return _impostorThreshold; }

        /** Set the Impostor threshold distance relative to the node's bounding
          * sphere's radius.
          */
        inline void setImpostorThresholdToBound(float ratio=1.0f) { _impostorThreshold = getBound().radius()*ratio; }

        /** Find the ImposterSprite which fits the current eye point best. */
        ImpostorSprite* findBestImpostorSprite(unsigned int contextID, const osg::Vec3& currLocalEyePoint) const;
        
        /** Add an ImpostorSprite to the Impostor. */
        void addImpostorSprite(unsigned int contextID, ImpostorSprite* is);

        /** Get the list of ImpostorSprites attached to this Impostor. */
        inline ImpostorSpriteList& getImpostorSpriteList(unsigned int contexID) { return _impostorSpriteListBuffer[contexID]; }

        /** Get a const list of ImpostorSprites attached to this const Impostor. */
        inline const ImpostorSpriteList& getImpostorSpriteList(unsigned int contexID) const { return _impostorSpriteListBuffer[contexID]; }

        virtual osg::BoundingSphere computeBound() const;
        
    protected :

        virtual ~Impostor() {}

        mutable osg::buffered_object<ImpostorSpriteList>  _impostorSpriteListBuffer;
        
        ImpostorSprite* createImpostorSprite(osgUtil::CullVisitor* cv);

        float _impostorThreshold;

};

}

#endif