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# Extracting a direction from a Quaternion

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OK Here is my problem. I am using a Quaternion to store the rotation of my model. I rotateting using the following method
	/**
* Rotates _currentRotation Quaternion around an axis and an angle
* @param angle
* @param rotationAxis
*/
public void rotate(float angle,Vector3 rotationAxis){

Quaternion	key_quat,
temp_quat;		// temp quaternion

//create new Quat from axis and angle

key_quat = new Quaternion(rotationAxis,angle);

temp_quat = _currentRotation;
_currentRotation = temp_quat.mult(key_quat);

}


I want the direction in a vector.,because I need it as velocity so I can increment my position, in the correct direction.
	public void simulate(float dt) {

_velocity._x += (_force._x/_m) *dt;
_velocity._y += (_force._y/_m) *dt;
_velocity._z += (_force._z/_m) *dt;

_position._x += _velocity._x *dt;
_position._y += _velocity._y *dt;
_position._z += _velocity._z *dt;

}


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simply take a vector that is suppose to be the forward direction of the vehicle, when no orientation is applied (like (0, 0, 1)), and multiply it by the quaternion.

Dir = Vector(0, 0, 1) * Quat;

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humm

Well I have drawn the damn velocity vector just to see it. It works when I DONT rotate :-)

//My quat is init like thisprotected Quaternion _currentRotation = new Quaternion(0,0,0,1);//I create this to make sure its rotated towards forwardthis._currentRotation = Quaternion.mulVectorWithQuaternion(xVector,_currentRotation,null);//this is my multvectorwith Quat code	/**	 *  multiply a vector and a quaternion , if dest is null it creates a new vector	 * @param v	 * @param q	 * @param dest	 * @return	 */	public static final Quaternion mulVectorWithQuaternion(Vector3 v,Quaternion q,Quaternion dest){			if(dest==null)				dest= new Quaternion(0,0,0,1);							dest._w =-(q._x*v._x + q._y*v._y + q._z*v._z);			dest._x = q._w*v._x + q._z*v._y - q._y*v._z;			dest._y = q._w*v._y + q._x*v._z - q._z*v._x;			dest._z = q._w*v._z + q._y*v._x - q._x*v._y;					return dest;			}

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I thought the correct transformation for a rotation was v''=qv(q^-1). And since q is a unit quaternion, q^-1=q*.

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