问无法使两个网格在一个OpenVDB文件中工作

EN

void
write_grids_to_vdb(
  const std::vector<
    std::pair<const Grid<Real>&,
              const std::string>
  > & grids_names,
  const std::string filename)
{

  openvdb::initialize();
  openvdb::GridPtrVec grids;

  for (const auto& gn: grids_names) {

    auto vdb_grid = openvdb::FloatGrid::create(0.0);
    vdb_grid->setTransform(
      openvdb::math::Transform::createLinearTransform(1.0));
    vdb_grid->setGridClass(openvdb::GRID_FOG_VOLUME);
    vdb_grid->setName(gn.second);

    openvdb::Coord ijk;
    typename openvdb::FloatGrid::Accessor
      accessor = vdb_grid->getAccessor();
    FOR_IJK(gn.first)
    {
      ijk[0] = i; ijk[1] = j; ijk[2] = k;
      accessor.setValue(ijk, gn.first(i,j,k));
    }

    grids.push_back(vdb_grid);

  }

  openvdb::io::File file(filename);
  file.write(grids);
  file.close();
}