void Submap2D::Finish()

CHECK(grid_);
CHECK(!insertion_finished());
grid_ = grid_->ComputeCroppedGrid();
set_insertion_finished(true);

这里的set_insertion_finished(true);就是子图结束建图了，可以添加函数isFinished判断，其实就是判断insertion_finished_是否true。 子图建完才会进入后端

std::unique_ptr<Grid2D> ProbabilityGrid::ComputeCroppedGrid() const
{
  Eigen::Array2i offset;
  CellLimits cell_limits;
  ComputeCroppedLimits(&offset, &cell_limits);
  const double resolution = limits().resolution();
  const Eigen::Vector2d max =
      limits().max() - resolution * Eigen::Vector2d(offset.y(), offset.x());
      // conversion_tables_ 是两个表
  std::unique_ptr<ProbabilityGrid> cropped_grid =
      absl::make_unique<ProbabilityGrid>(
          MapLimits(resolution, max, cell_limits), conversion_tables_);
   // 对应的cell设置概率，在SetProbability里又转成了空闲概率
  for (const Eigen::Array2i& xy_index : XYIndexRangeIterator(cell_limits))
  {
    if (!IsKnown(xy_index + offset)) continue;
    cropped_grid->SetProbability(xy_index, GetProbability(xy_index + offset) );
  }
  return std::unique_ptr<Grid2D>(cropped_grid.release());
}

void set_insertion_finished(bool insertion_finished)
{
    insertion_finished_ = insertion_finished;
}