code: def generate_base_grid(self, vtk_filename=None):
          
          nd, ed, ed_dir = self.gen_grid_fcn(self.data.shape, self.voxelsize)
          self.add_nodes(nd)
          self.add_edges(ed, ed_dir, edge_low_or_high=0)
  
          if vtk_filename is not None:
              self.write_vtk(vtk_filename)
docs: Run first step of algorithm. Next step is split_voxels
          :param vtk_filename:
          :return: