    const T& operator()(const unsigned int x, const unsigned int y, const unsigned int z, const unsigned int,
                        const ulongT wh) const {
      return _data[x + y*_width + z*wh];
    }
    const T& operator()(const unsigned int x, const unsigned int y, const unsigned int z, const unsigned int,
                        const ulongT wh) const {
      return _data[x + y*_width + z*wh];
    }