Rviz中的地图绘制使用了cairo库。注意gflag的参数include_frozen_submaps 和 include_unfrozen_submaps不能都为false，否则会报错

cartographerrviz的绘制方式，就是`submap_node控制一个子图的绘制，每个子图有一个子图原点，这个是submap_query中的slice_pose。而每个子图之间是有微弱旋转的，而控制旋转的中心也是这个slice_pose。而每个子图之间的旋转和平移可以通过submap_list`的pose得到。

main函数值得看的就一句::cartographer_ros::Node node(FLAGS_resolution, FLAGS_publish_period_sec);，构造函数如下

Node::Node(const double resolution, const double publish_period_sec)
: resolution_(resolution),
	// submap_query 服务的客户端
  client_(node_handle_.serviceClient<::cartographer_ros_msgs::SubmapQuery>(
      kSubmapQueryServiceName)),
  // submap_list话题的订阅者，回调函数 HandleSubmapList
  submap_list_subscriber_(node_handle_.subscribe(
      kSubmapListTopic, kLatestOnlyPublisherQueueSize,
      boost::function<void(
          const cartographer_ros_msgs::SubmapList::ConstPtr&)>(
          [this](const cartographer_ros_msgs::SubmapList::ConstPtr& msg) {
            HandleSubmapList(msg);
          }))),
  // 发布话题 map
  occupancy_grid_publisher_(
      node_handle_.advertise<::nav_msgs::OccupancyGrid>(
          kOccupancyGridTopic, kLatestOnlyPublisherQueueSize,
          true /* latched */)),
  // timer执行DrawAndPublish函数
  occupancy_grid_publisher_timer_(
      node_handle_.createWallTimer(::ros::WallDuration(publish_period_sec),
                                   &Node::DrawAndPublish, this)) {}

Node::DrawAndPublish是画图和发布map，不是重点

HandleSubmapList函数大部分都很简单，终点是下面这句：

auto fetched_textures =
        ::cartographer_ros::FetchSubmapTextures(id, &client_);

内部其实就是submap_query的客户端发起请求，服务端的注册在Node的构造函数里，回调是Node::HandleSubmapQuery

代码调用顺序是： Node::HandleSubmapQuery —— MapBuilderBridge::HandleSubmapQuery —— MapBuilder::SubmapToProto —— PoseGraph2D::GetSubmapData 和 Submap2D::ToResponseProto —— ProbabilityGrid::DrawToSubmapTexture

重点：

// 这里比较简单，最终返回的是后端传来的 global坐标系下的位姿
PoseGraphInterface::SubmapData PoseGraph2D::GetSubmapDataUnderLock(
    const SubmapId& submap_id) const
{
  const auto it = data_.submap_data.find(submap_id);
  if (it == data_.submap_data.end())
      return {};
  auto submap = it->data.submap;
  if (data_.global_submap_poses_2d.Contains(submap_id) )
  {
    // We already have an optimized pose.
    return {submap,
            transform::Embed3D(
                data_.global_submap_poses_2d.at(submap_id).global_pose) };
  }
  // We have to extrapolate.
  return {submap, ComputeLocalToGlobalTransform(data_.global_submap_poses_2d,
                                                submap_id.trajectory_id) *
                      submap->local_pose() };
}

void Submap2D::ToResponseProto(
    const transform::Rigid3d&,
    proto::SubmapQuery::Response* const response) const 
{
  if (!grid_)  	return;
  // 这个 version 是子图插入了多少雷达数据，完成的子图是 2* num_range_data
  response->set_submap_version( num_range_data()  );
  proto::SubmapQuery::Response::SubmapTexture* const texture =
      response->add_textures();
  grid()->DrawToSubmapTexture(texture, local_pose());
}

再看DrawToSubmapTexture

bool ProbabilityGrid::DrawToSubmapTexture(
    proto::SubmapQuery::Response::SubmapTexture* const texture,
    transform::Rigid3d local_pose) const
{
  Eigen::Array2i offset;
  CellLimits cell_limits;
  ComputeCroppedLimits(&offset, &cell_limits);

  std::string cells;
  for (const Eigen::Array2i& xy_index : XYIndexRangeIterator(cell_limits))
  {
    if (!IsKnown(xy_index + offset)) {
      cells.push_back(0 /* unknown log odds value */);
      cells.push_back(0 /* alpha */);
      continue;
    }
    // We would like to add 'delta' but this is not possible using a value and
    // alpha. We use premultiplied alpha, so when 'delta' is positive we can
    // add it by setting 'alpha' to zero. If it is negative, we set 'value' to
    // zero, and use 'alpha' to subtract. This is only correct when the pixel
    // is currently white, so walls will look too gray. This should be hard to
    // detect visually for the user, though.

    // delta范围[-127, 127]
    const int delta =
        128 - ProbabilityToLogOddsInteger(GetProbability(xy_index + offset));
    const uint8 alpha = delta > 0 ? 0 : -delta;
    const uint8 value = delta > 0 ? delta : 0;
    // 存数据有2个值，栅格值value和alpha透明度
    cells.push_back(value);
    cells.push_back((value || alpha) ? alpha : 1);
  }
  // 保存地图栅格数据时进行压缩
  common::FastGzipString(cells, texture->mutable_cells());
  // 地图描述信息赋值，省略
  	......
  *texture->mutable_slice_pose() = transform::ToProto(
      local_pose.inverse() *
      transform::Rigid3d::Translation(Eigen::Vector3d(max_x, max_y, 0.))  );

  return true;
}

// 解压后的栅格数据
struct SubmapTexture {
  struct Pixels {
    std::vector<char> intensity;   // 栅格值
    std::vector<char> alpha;      // 透明度
  };
  Pixels pixels;
  int width;
  int height;
  double resolution;
  ::cartographer::transform::Rigid3d slice_pose;
};

// 压缩后的栅格数据
struct SubmapTextures {
  int version;
  std::vector<SubmapTexture> textures;
};