docs: def _get_bolean_mask(self, mask):
          """Cast to two dimensional boolean mask."""
          if mask.dtype.kind == 'f':
              warnings.warn("mask image should be unsigned byte between 0"
                            " and 255. Got a float array")
          if mask.ndim == 2:
              boolean_mask = mask == 255
          elif mask.ndim == 3:
              # if all channels are white, mask out
              boolean_mask = np.all(mask[:, :, :3] == 255, axis=-1)
          else:
              raise ValueError("Got mask of invalid shape: %s" % str(mask.shape))
          return boolean_mask