Cartographer源码阅读(5)：PoseGraph位姿图

PoseGraph位姿图

mapping2D::PoseGraph类的注释：

// Implements the loop closure method called Sparse Pose Adjustment (SPA) from
// Konolige, Kurt, et al. "Efficient sparse pose adjustment for 2d mapping."
// Intelligent Robots and Systems (IROS), 2010 IEEE/RSJ International Conference
// on (pp. 22--29). IEEE, 2010.
//
// It is extended for submapping:
// Each node has been matched against one or more submaps (adding a constraint
// for each match), both poses of nodes and of submaps are to be optimized.
// All constraints are between a submap i and a node j.

1.继承关系mapping，mapping2D，mapping3D （以前叫sparse_pose_graph.h，目前名称被修改了）

类中一个很重要的类OptimizationProblem对象optimization_problem_，该类的注释是Implements the SPA loop closure method.即实现了SPA方法的闭环。

类的主要成员变量和成员函数如类图所示，其中重要的方法PoseGraph::RunOptimization()。

pose_graph::OptimizationProblem optimization_problem_;
pose_graph::ConstraintBuilder constraint_builder_ GUARDED_BY(mutex_);
std::vector<Constraint> constraints_ GUARDED_BY(mutex_);

接上一篇在PoseGraph::AddNode方法中调用了 AddTrajectoryIfNeeded() 和 PoseGraph::ComputeConstraintsForNode()方法。

 void PoseGraph::ComputeConstraintsForNode(
     const mapping::NodeId& node_id,
     std::vector<std::shared_ptr<const Submap>> insertion_submaps,
     const bool newly_finished_submap) {
   const auto& constant_data = trajectory_nodes_.at(node_id).constant_data;
   const std::vector<mapping::SubmapId> submap_ids = InitializeGlobalSubmapPoses(
       node_id.trajectory_id, constant_data->time, insertion_submaps);
   CHECK_EQ(submap_ids.size(), insertion_submaps.size());
   const mapping::SubmapId matching_id = submap_ids.front();
   const transform::Rigid2d pose = transform::Project2D(
       constant_data->local_pose *
       transform::Rigid3d::Rotation(constant_data->gravity_alignment.inverse()));
   const transform::Rigid2d optimized_pose =
       optimization_problem_.submap_data().at(matching_id).global_pose *
       pose_graph::ComputeSubmapPose(*insertion_submaps.front()).inverse() *
       pose;
   optimization_problem_.AddTrajectoryNode(
       matching_id.trajectory_id, constant_data->time, pose, optimized_pose,
       constant_data->gravity_alignment);
   for (size_t i = ; i < insertion_submaps.size(); ++i) {
     const mapping::SubmapId submap_id = submap_ids[i];
     // Even if this was the last node added to 'submap_id', the submap will only
     // be marked as finished in 'submap_data_' further below.
     CHECK(submap_data_.at(submap_id).state == SubmapState::kActive);
     submap_data_.at(submap_id).node_ids.emplace(node_id);
     const transform::Rigid2d constraint_transform =
         pose_graph::ComputeSubmapPose(*insertion_submaps[i]).inverse() * pose;
     constraints_.push_back(Constraint{submap_id,
                                       node_id,
                                       {transform::Embed3D(constraint_transform),
                                        options_.matcher_translation_weight(),
                                        options_.matcher_rotation_weight()},
                                       Constraint::INTRA_SUBMAP});
   }

   for (const auto& submap_id_data : submap_data_) {
     if (submap_id_data.data.state == SubmapState::kFinished) {
       CHECK_EQ(submap_id_data.data.node_ids.count(node_id), );
       ComputeConstraint(node_id, submap_id_data.id);
     }
   }

   if (newly_finished_submap) {
     const mapping::SubmapId finished_submap_id = submap_ids.front();
     SubmapData& finished_submap_data = submap_data_.at(finished_submap_id);
     CHECK(finished_submap_data.state == SubmapState::kActive);
     finished_submap_data.state = SubmapState::kFinished;
     // We have a new completed submap, so we look into adding constraints for
     // old nodes.
     ComputeConstraintsForOldNodes(finished_submap_id);
   }
   constraint_builder_.NotifyEndOfNode();
   ++num_nodes_since_last_loop_closure_;
   if (options_.optimize_every_n_nodes() >  &&
       num_nodes_since_last_loop_closure_ > options_.optimize_every_n_nodes()) {
     CHECK(!run_loop_closure_);
     run_loop_closure_ = true;
     // If there is a 'work_queue_' already, some other thread will take care.
     if (work_queue_ == nullptr) {
       work_queue_ = common::make_unique<std::deque<std::function<void()>>>();
       HandleWorkQueue();
     }
   }
 }

可以看到对本类中ComputeConstraint、ComputeConstraintsForOldNodes、HandleWorkQueue方法的调用。这里有个pose_graph::ComputeSubmapPose方法在cartographer/mapping_2d/pose_graph/constraint_builder.h文件中。子图submap位姿？

transform::Rigid2d ComputeSubmapPose(const Submap& submap)
{
      return transform::Project2D(submap.local_pose());
}

注意HandleWorkQueue()方法中调用了RunOptimization方法。

 void PoseGraph::HandleWorkQueue() {
   constraint_builder_.WhenDone(
       [this](const pose_graph::ConstraintBuilder::Result& result) {
         {
           common::MutexLocker locker(&mutex_);
           constraints_.insert(constraints_.end(), result.begin(), result.end());
         }
         RunOptimization();

         common::MutexLocker locker(&mutex_);
         for (const Constraint& constraint : result) {
           UpdateTrajectoryConnectivity(constraint);
         }
         TrimmingHandle trimming_handle(this);
         for (auto& trimmer : trimmers_) {
           trimmer->Trim(&trimming_handle);
         }
         trimmers_.erase(
             std::remove_if(
                 trimmers_.begin(), trimmers_.end(),
                 [](std::unique_ptr<mapping::PoseGraphTrimmer>& trimmer) {
                   return trimmer->IsFinished();
                 }),
             trimmers_.end());

         num_nodes_since_last_loop_closure_ = ;
         run_loop_closure_ = false;
         while (!run_loop_closure_) {
           if (work_queue_->empty()) {
             work_queue_.reset();
             return;
           }
           work_queue_->front()();
           work_queue_->pop_front();
         }
         LOG(INFO) << "Remaining work items in queue: " << work_queue_->size();
         // We have to optimize again.
         HandleWorkQueue();
       });
 }

PoseGraph::HandleWorkQueue

查看PoseGraph::RunOptimization方法，

 void PoseGraph::RunOptimization()
 {
   if (optimization_problem_.submap_data().empty())
   {
     return;
   }

   // No other thread is accessing the optimization_problem_, constraints_ and
   // frozen_trajectories_ when executing the Solve. Solve is time consuming, so
   // not taking the mutex before Solve to avoid blocking foreground processing.
   optimization_problem_.Solve(constraints_, frozen_trajectories_);
   common::MutexLocker locker(&mutex_);

   const auto& submap_data = optimization_problem_.submap_data();
   const auto& node_data = optimization_problem_.node_data();
   for (const int trajectory_id : node_data.trajectory_ids())
   {
     for (const auto& node : node_data.trajectory(trajectory_id))
     {
       auto& mutable_trajectory_node = trajectory_nodes_.at(node.id);
       mutable_trajectory_node.global_pose =
           transform::Embed3D(node.data.pose) *
           transform::Rigid3d::Rotation(mutable_trajectory_node.constant_data->gravity_alignment);
     }

     // Extrapolate all point cloud poses that were not included in the
     // 'optimization_problem_' yet.
     const auto local_to_new_global =
         ComputeLocalToGlobalTransform(submap_data, trajectory_id);
     const auto local_to_old_global =
         ComputeLocalToGlobalTransform(global_submap_poses_, trajectory_id);
     const transform::Rigid3d old_global_to_new_global =
         local_to_new_global * local_to_old_global.inverse();

     const mapping::NodeId last_optimized_node_id =
         std::prev(node_data.EndOfTrajectory(trajectory_id))->id;
     auto node_it = std::next(trajectory_nodes_.find(last_optimized_node_id));
     for (; node_it != trajectory_nodes_.EndOfTrajectory(trajectory_id); ++node_it)
     {
       auto& mutable_trajectory_node = trajectory_nodes_.at(node_it->id);
       mutable_trajectory_node.global_pose =
           old_global_to_new_global * mutable_trajectory_node.global_pose;
     }
   }
   global_submap_poses_ = submap_data;
 }

查看Constraint 结构体，该结构体在PoseGraphInterface中定义。

   // A "constraint" as in the paper by Konolige, Kurt, et al. "Efficient sparse
   // pose adjustment for 2d mapping." Intelligent Robots and Systems (IROS),
   // 2010 IEEE/RSJ International Conference on (pp. 22--29). IEEE, 2010.
   struct Constraint {
     struct Pose {
       transform::Rigid3d zbar_ij;
       double translation_weight;
       double rotation_weight;
     };

     SubmapId submap_id;  // 'i' in the paper.
     NodeId node_id;      // 'j' in the paper.

     // Pose of the node 'j' relative to submap 'i'.
     Pose pose;

     // Differentiates between intra-submap (where node 'j' was inserted into
     // submap 'i') and inter-submap constraints (where node 'j' was not inserted
     // into submap 'i').
     enum Tag { INTRA_SUBMAP, INTER_SUBMAP } tag;
   };