cartographer代码——世界坐标系点和像素坐标系点的转换

构建栅格地图，要弄清楚坐标之间的关系。本篇根据代码，画出了坐标转换的关系。

如下图：

cartographer中的代码如下：

  // Returns the index of the cell containing the 'point' which may be outside
  // the map, i.e., negative or too large indices that will return false for
  // Contains().
  Eigen::Array2i GetCellIndex(const Eigen::Vector2f& point) const {
    // Index values are row major and the top left has Eigen::Array2i::Zero()
    // and contains (centered_max_x, centered_max_y). We need to flip and
    // rotate.
    return Eigen::Array2i(
        common::RoundToInt((max_.y() - point.y()) / resolution_ - 0.5),
        common::RoundToInt((max_.x() - point.x()) / resolution_ - 0.5));
  }

  // Returns the center of the cell at 'cell_index'.
  Eigen::Vector2f GetCellCenter(const Eigen::Array2i cell_index) const {
    return {max_.x() - resolution() * (cell_index[1] + 0.5),
            max_.y() - resolution() * (cell_index[0] + 0.5)};
  }