    const CImg<T>& save_off(const CImgList<tf>& primitives, const CImgList<tc>& colors,
                            const char *const filename) const {
      return _save_off(primitives,colors,0,filename);
    }
    const CImg<T>& save_off(const CImgList<tf>& primitives, const CImgList<tc>& colors,
                            const char *const filename) const {
      return _save_off(primitives,colors,0,filename);
    }