cartographer雷达数据转为概率网格

void CastRays(const sensor::RangeData& range_data,
              const std::vector<uint16>& hit_table,
              const std::vector<uint16>& miss_table,
              const bool insert_free_space,
              ProbabilityGrid* const probability_grid) {
  GrowAsNeeded(range_data, probability_grid);

  const MapLimits& limits = probability_grid->limits();
  const double superscaled_resolution = limits.resolution() / kSubpixelScale;
  const MapLimits superscaled_limits(
      superscaled_resolution, limits.max(),
      CellLimits(limits.cell_limits().num_x_cells * kSubpixelScale,
                 limits.cell_limits().num_y_cells * kSubpixelScale));
  const Eigen::Array2i begin =
      superscaled_limits.GetCellIndex(range_data.origin.head<2>());
  // Compute and add the end points.
  std::vector<Eigen::Array2i> ends;
  ends.reserve(range_data.returns.size());
  for (const Eigen::Vector3f& hit : range_data.returns) {
    ends.push_back(superscaled_limits.GetCellIndex(hit.head<2>()));
    probability_grid->ApplyLookupTable(ends.back() / kSubpixelScale, hit_table);
  }

  if (!insert_free_space) {
    return;
  }

  // Now add the misses.
  for (const Eigen::Array2i& end : ends) {
    CastRay(begin, end, miss_table, probability_grid);
  }

  // Finally, compute and add empty rays based on misses in the range data.
  for (const Eigen::Vector3f& missing_echo : range_data.misses) {
    CastRay(begin, superscaled_limits.GetCellIndex(missing_echo.head<2>()),
            miss_table, probability_grid);
  }
}