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