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c:/home/kevn/src/animaniac/gator/Collision.h

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#ifndef COLLISION_
#define COLLISION_

#include <gmtl/Vec.h>
#include <gmtl/VecOps.h>

namespace gator
{
   inline void computeCollisionForces( const gmtl::Vec3f& polygonNormal, 
                                const gmtl::Vec3f& incomingVector, 
                                float elastic_coef, 
                                float friction_coef, 
                                float mass, 
                                float timeDelta, 
                                gmtl::Vec3f& frictionForce, 
                                gmtl::Vec3f& normalForce )
   {
      //: split the reflected vector into normal and tangential 
      //  vectors to the plane

      // project the (reverse of the) incoming vector onto the normal (just distance scalar)
      float dot = gmtl::dot(-incomingVector, polygonNormal );

      // project the (reverse of the) incoming vector onto the normal (now the coordinate)
      gmtl::Vec3f normalComponent = polygonNormal * dot;

      // compute perfect reflection using available vectors... 
      // (perfect == don't consider friction/elasticity)
      gmtl::Vec3f reflectionVector = (normalComponent * 2.0f) + incomingVector;

      // compute the tangential component using the available vectors.
      gmtl::Vec3f tangentialComponent = reflectionVector - normalComponent;

      // compute the normal and friction forces
      // 
      // normal: calc the amount of instantaneous force pushing up 
      //         from the ground.  
      // friction: calc the amount of force against the tangential 
      //           direction of motion
      // 
      // NOTE: instantaneous implies we need to calc
      // the amount of force required to halt motion over the 
      // current timestep if the coef is 1 for elastic or 0 for friction
      // (so this is why consider time in the eq)
      normalForce = normalComponent * (1.0f + elastic_coef) * mass / timeDelta;
      frictionForce = -tangentialComponent * friction_coef * mass / timeDelta;
   }
} // end namespace gator

#endif

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