// create a look-at rotation matrix
public void LookAt(Vector3d dir, Vector3d up)
{
//Vector3d dir = new Vector3d(direction.x, direction.y, direction.z);
//dir.Normalize();
Vector3d vx = up.Cross(dir);
vx.Normalize();
Vector3d vy = dir.Cross(vx);
vy.Normalize();
Matrix[0, 0] = vx.x; Matrix[0, 1] = vx.y; Matrix[0, 2] = vx.z; Matrix[0, 3] = 0;
Matrix[1, 0] = vy.x; Matrix[1, 1] = vy.y; Matrix[1, 2] = vy.z; Matrix[1, 3] = 0;
Matrix[2, 0] = dir.x; Matrix[2, 1] = dir.y; Matrix[2, 2] = dir.z; Matrix[2, 3] = 0;
Matrix[3, 0] = 0; Matrix[3, 1] = 0; Matrix[3, 2] = 0; Matrix[3, 3] = 1;
}