code: def unmasked_sparse_to_sparse(self):
          
  
          return mapping_util.unmasked_sparse_to_sparse_from_mask_and_pixel_centres(
              mask=self.regular_grid.mask,
              unmasked_sparse_grid_pixel_centres=self.unmasked_sparse_grid_pixel_centres,
              total_sparse_pixels=self.total_sparse_pixels).astype(
              'int')
docs: The 1D index mappings between the unmasked sparse-grid and masked sparse grid.