code: def sparse_surface(self):
          
          if self._method == 'ray':
              func = voxelize_ray
          elif self._method == 'subdivide':
              func = voxelize_subdivide
          else:
              raise ValueError('voxelization method incorrect')
  
          voxels, origin = func(
              mesh=self._data['mesh'],
              pitch=self._data['pitch'],
              max_iter=self._data['max_iter'][0])
          self._cache['origin'] = origin
  
          return voxels
docs: Filled cells on the surface of the mesh.
  
          Returns
          ----------------
          voxels: (n, 3) int, filled cells on mesh surface