Using Quaternions Efficiently in Real-time Applications

#ifndef _FAST_GAME_ENTITY_H
#define _FAST_GAME_ENTITY_H

#include "GameEntityBase.h"

class FastGameEntity : public GameEntityBase
{
public:
    virtual void updateAI( )
    {
        D3DXVECTOR3 targetPos;
        updatePursuitAction( targetPos );
    }

    virtual void updatePursuitAction( const D3DXVECTOR3 &targetPos )
    {
        //get vector from my pos to the target
        D3DXVECTOR3 toTarget = targetPos - pos_;

        //transform into my local space - 15 mults 8 adds
        D3DXQUATERNION conjQ;
        D3DXQuaternionConjugate( &conjQ, &ori_ );
        rotateVectorWithQuat( conjQ, toTarget );

        //update direction to intercept target
        //...
    }

    virtual void transformManyVertices( unsigned int numVerts )
    {
        UpdateRotation();

        D3DXVECTOR3 *vertices = new D3DXVECTOR3[ numVerts ];

        D3DXVECTOR3 col1( side_.x, up_.x, dir_.x );
        D3DXVECTOR3 col2( side_.y, up_.y, dir_.y );
        D3DXVECTOR3 col3( side_.z, up_.z, dir_.z );

        for ( unsigned int i=0; i< numVerts; i++)
        {
            //transform into my local space
            //multiply vector with rotation matrix - 3 dot products - 9 mults 6 adds
            D3DXVECTOR3 transformedVertex;
            transformedVertex.x = D3DXVec3Dot( &col1, &vertices[i] );
            transformedVertex.y = D3DXVec3Dot( &col2, &vertices[i] );
            transformedVertex.z = D3DXVec3Dot( &col3, &vertices[i] );
        }

        delete [] vertices;
    }

    virtual void calculateOrientation( float dt )
    {
        //calculate angular velocity
        //...

        //update quaternion with angular velocity vector
        ori_ += ( (ori_ * angularVel_) * 0.5f * dt );
    }

};

#endif