Using Quaternions Efficiently in Real-time Applications

#ifndef _SLOW_GAME_ENTITY_H
#define _SLOW_GAME_ENTITY_H

#include "GameEntityBase.h"

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

        UpdateRotation();
    }

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

        //transform into my local space
        //transform vector with orthonormal basis - 3 dot products - 9 mults 6 adds
        D3DXVECTOR3 localToTarget;
        localToTarget.x = D3DXVec3Dot( &side_, &worldToTarget );
        localToTarget.y = D3DXVec3Dot( &up_, &worldToTarget );
        localToTarget.z = D3DXVec3Dot( &dir_, &worldToTarget );

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

    virtual void transformManyVertices( unsigned int numVerts )
    {
        D3DXVECTOR3 *vertices = new D3DXVECTOR3[ numVerts ];

        D3DXVECTOR3 transformedVertex;
        D3DXQUATERNION conjQ;
        D3DXQuaternionConjugate( &conjQ, &ori_ );
        for ( unsigned int i=0; i< numVerts; i++)
        {
            //transform into my local space - 15 mults 8 adds
            transformedVertex = vertices[i];
            rotateVectorWithQuat( conjQ, transformedVertex );
        }

        delete [] vertices;
    }

    virtual void calculateOrientation( float dt )
    {
        //calculate new euler angles
        //...

        //update current orientation with new quaternion formed from euler angles
        D3DXQUATERNION tempQ;
        //D3DXQuaternionRotationYawPitchRoll( &tempQ, yaw_, pitch_, roll_ );
        quatFromEuler( tempQ, yaw_, pitch_, roll_ );
        ori_ = tempQ * ori_;

        /*
        //alternative method
        //create 3 new quats one for each angle
        D3DXQUATERNION yawQ, pitchQ, rollQ;
        D3DXQuaternionRotationAxis( &yawQ, up_, yaw_ );
        D3DXQuaternionRotationAxis( &pitchQ, side_, pitch_ );
        D3DXQuaternionRotationAxis( &rollQ, dir_, roll_ );
        //multiply all 3 together to create new orientation
        D3DXQuaternionMultiply( &ori_, &yawQ, &pitchQ );
        D3DXQuaternionMultiply( &ori_, &ori, &rollQ );
        */
    }

};


#endif