def S(self):
        """:obj:`numpy.ndarray` : The 3x3 scaling matrix for this projection
        """
        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
:obj:`numpy.ndarray` : The 3x3 scaling matrix for this projection