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c:/home/kevn/src/animaniac/ani/Dynamics/operators/ViscousDragOperator.h

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#ifndef VISCOUS_DRAG_OPERATOR
#define VISCOUS_DRAG_OPERATOR

#include <boost/smart_ptr.hpp>
#include <gmtl/Vec.h>
#include "ani/Dynamics/Operator.h"
#include "ani/Dynamics/DynamicSystem.h"
#include <ani/Dynamics/operators/GlobalForceOperator.h>

namespace ani
{
   template<class __EntityType>
   class ViscousDragOperator : public GlobalForceOperator<__EntityType>
   {
   public:
      ViscousDragOperator(){}
      ViscousDragOperator( float coef ) : _dragCoef( coef ) {}
      virtual ~ViscousDragOperator(){}
      void setDrag( const float& coeficient )
      {
         _dragCoef = coeficient;
      }

      //: apply this force function to the particle
      virtual void exec( DynamicSystem<__EntityType>& ps, float timeDelta );

   private:
      float _dragCoef;
   };

   //: apply this force function to the particle
   template< class __EntityType >
   void ViscousDragOperator<__EntityType>::exec( ani::DynamicSystem<__EntityType>& ps, float timeDelta )
   {
      std::vector<EntityTypePtr>::iterator it;
      for ( it = ps.entities().begin(); it != ps.entities().end(); ++it)
      {
         EntityTypePtr p = *it;
         if (this->isIgnored( p ) == false)
         {
            // Calculate the force nesesary to knock of kV velocity

            // get the velocity we need to reduce entities speed
            // eqn: V(new) = V - kV, where k is a coeficient between 0..1
            gmtl::Vec3f antiVelocity = p->linearVelocity() * (-_dragCoef);

            // how much do we have to decelerate it to achieve 'newDesiredVelocity'
            // decelerateAmount = antiVelocity / timeChange;
            // NOTE: timeChange is 1.0, so deceleration is just antiVelocity
            gmtl::Vec3f force = antiVelocity * p->mass();
            p->applyForce( force );
         }
      }
   }

} // end of namespace

#endif

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