// Returns a quaternion such that q*start = dest
Quaternion Math::RotationBetweenVectors( Vector3 start, Vector3 dest )
{
    start = Normalize( start );
    dest = Normalize( dest );

    Scalar cosTheta = Dot( start, dest );

    Vector3 rotationAxis;

    // cosTheta > 1 no needed
    
    if (cosTheta < -1 + 0.001f) {
        // special case when vectors in opposite directions :
        // there is no "ideal" rotation axis
        // So guess one; any will do as long as it's perpendicular to start
        // This implementation favors a rotation around the Up axis,
        // since it's often what you want to do.
        rotationAxis = Cross( Vector3( 0.0f, 0.0f, 1.0f ), start );
        if (LengthSquare( rotationAxis ) < 0.1f) // bad luck, they were parallel, try again!
            rotationAxis = Cross( Vector3( 1.0f, 0.0f, 0.0f ), start );

        rotationAxis = Normalize( rotationAxis );
        return Quaternion( rotationAxis, XM_PI );
    }

    // Implementation from Stan Melax's Game Programming Gems 1 article
    rotationAxis = Cross( start, dest );

    float s = sqrtf( (1.0f + cosTheta) * 2.f );
    float invs = 1 / s;

    rotationAxis *= Vector3( invs, invs, invs );
    return Quaternion( Vector4( rotationAxis, s * 0.5f ) );
}