docs: def _count_truth_pixels(self):
          """
          Count the pixels belonging to each truth class
          """
          pixel_count = np.array([[np.nonzero(mask)[0].shape[0] for mask in masktype]
                                  for masktype in self.truth_masks])
  
          pixel_count = np.moveaxis(pixel_count, 0, -1)
  
          if self.pixel_truth_counts is None:
              self.pixel_truth_counts = np.copy(pixel_count)
          else:
              self.pixel_truth_counts = np.concatenate((self.pixel_truth_counts, pixel_count))