code: def S(self):
          
          S = np.array([[self._plane_width / self._vol_width, 0, 0],
                        [0, self._plane_height / self._vol_height, 0],
                        [0, 0, self._depth_scale / self._vol_depth]])
          return S
docs: :obj:`numpy.ndarray` : The 3x3 scaling matrix for this projection