LIO-SEGMOT

The official implementation of LIO-SEGMOT (LiDAR-Inertial Odometry via Simultaneous Ego-motion Estimation and Multiple Object Tracking), an optimization-based odometry approach targeted for dynamic environments. LIO-SEGMOT can provide continuous object tracking results while preserving the keyframe selection mechanism in the odometry system. This work is accepted for publication in ICRA 2023.

You can check out our video to understand the main idea of LIO-SEGMOT. For more, please refer to our paper:

• Yu-Kai Lin, Wen-Chieh Lin, Chieh-Chih Wang, Asynchronous State Estimation of Simultaneous Ego-motion Estimation and Multiple Object Tracking for LiDAR-Inertial Odometry. 2023 International Conference on Robotics and Automation (ICRA), pp. 1--7, May 2023. (paper) (preprint) (code) (video)

If you use this project in your research, please cite:

@article{lin2023lio-segmot,
  title={Asynchronous State Estimation of Simultaneous Ego-motion Estimation and Multiple Object Tracking for LiDAR-Inertial Odometry},
  author={Lin, Yu-Kai and Lin, Wen-Chieh and Wang, Chieh-Chih},
  booktitle = {2023 International Conference on Robotics and Automation, {ICRA} 2023,
               London, UK, May 2023},
  pages     = {1--7},
  year      = {2023},
}

• ⚙️ Installation
  • Step 1. Preparing the Dependencies
  • Step 2. Building the LIO-SEGMOT Project
  • Step 3. Preparing Object Detection Services
• 🗃️ Sample Datasets
• 🏃‍♂️ Run
• ♿ Services of LIO-SEGMOT
  • /lio_segmot/save_map
  • /lio_segmot/save_estimation_result
• 📝 Remarks
  • Hyperparameters
    • Hierarchical Criterion
    • Factor Graph Optimization
    • High-speed Moving Object Supports in Early Steps
    • Lifecycle Management of Tracking Objects
  • Limitations of LIO-SEGMOT
  • Possible Future Research Directions
• 🎁 Acknowledgement

⚙️ Installation

The project is originally developed in Ubuntu 18.04, and the following instruction supposes that you are using Ubuntu 18.04 as well. I am not sure if it also works with other Ubuntu versions or other Linux distributions, but maybe you can give it a try 👍

Also, please feel free to open an issue if you encounter any problems of the following instruction.

Step 1. Preparing the Dependencies

Please prepare the following packages or libraries used in LIO-SEGMOT:

1. ROS Melodic (w/ Desktop-Full Install) and the following dependencies:

   #!/bin/bash
   sudo apt update
   sudo apt install -y "ros-${ROS_DISTRO}-navigation" \
                       "ros-${ROS_DISTRO}-robot-localization" \
                       "ros-${ROS_DISTRO}-robot-state-publisher" \
                       "ros-${ROS_DISTRO}-jsk-recognition-msgs" \
                       "ros-${ROS_DISTRO}-jsk-rviz-plugins"

2. gtsam 4.0.3

   #!/bin/bash
   cd ~
   git clone -b 4.0.3 https://github.com/borglab/gtsam && cd gtsam
   mkdir build && cd build
   cmake ..
   sudo make install

Step 2. Building the LIO-SEGMOT Project

You can use the following command to build the project:

#!/bin/bash
source "/opt/ros/${ROS_DISTRO}/setup.bash"
mkdir -p ~/catkin_ws/src
cd ~/catkin_ws/src
git clone [email protected]:StephLin/LIO-SEGMOT.git
cd ..
catkin_make

Step 3. Preparing Object Detection Services

We provide two object detection services for LIO-SEGMOT:

• StephLin/SE-SSD-ROS (Apache-2.0 license) (based on Vegeta2020/SE-SSD)
• StephLin/livox_detection_lio_segmot (GPL-3.0 license) (based on Livox-SDK/livox_detection)

Please refer to their installation instructions accordingly.

🗃️ Sample Datasets

We provide the following pre-built bag files for KITTI raw sequences and the Hsinchu dataset (GuangfuRoad sequence):

If you cannot download sample datasets from above links, please refer to this alternative link.

Ground truth robot trajectories (based on GPS data provided by KITTI) are stored as the TUM format. Each row has 8 components containing timestamps (sec), xyz-position (meter), and xyzw-orientation (quaternion):

timestamp x y z qx qy qz qw

🏃‍♂️ Run

Please follow the steps to execute LIO-SEGMOT properly:

1. (Optional) Launch the ROS core:

   roscore

2. Launch the core LIO-SEGMOT service:

   #!/bin/bash
   # Please select one of the following configs to launch the service properly:
   # 1. With KITTI configuration
   roslaunch lio_segmot run_kitti.launch
   
   # 2. With Hsinchu configuration
   roslaunch lio_segmot run_hsinchu.launch
   
   # 3. Undefined (same as KITTI configuration)
   roslaunch lio_segmot run.launch

3. Launch the selected object detection service:

   #!/bin/bash
   # SE-SSD-ROS & livox_detection_lio_segmot
   # Please check their documentation to see how they are launched

4. Start the customized ROS bag player:

   #!/bin/bash
   rosrun lio_segmot lio_segmot_offlineBagPlayer _bag_filename:="path/to/your/sequence.bag"

   The default registered LiDAR and IMU topics are /points_raw and /imu_raw, respectively. If you want to register other LiDAR/IMU topics, please add additional options _lidar_topic and _imu_topic. For example, if you are using the GuangfuRoad sequence (/velodyne_points and /imu/data for LiDAR and IMU topics, respectively):

   rosrun lio_segmot lio_segmot_offlineBagPlayer _bag_filename:="GuangfuRoad-06-13.bag" \
                                                 _lidar_topic:="/velodyne_points" \
                                                 _imu_topic:="/imu/data"

♿ Services of LIO-SEGMOT

/lio_segmot/save_map

Usage: rosservice call /lio_segmot/save_map [RESOLUTION] [OUTPUT_DIR]
Example: rosservice call /lio_segmot/save_map 0.2 /path/to/a/directory/

This service saves LiDAR map to the local machine.

/lio_segmot/save_estimation_result

Usage: rosservice call /lio_segmot/save_estimation_result

This service outputs current estimation results including

• nav_msgs::Path robotTrajectory: The robot trajectory
• $\color{gray}\textsf{(INTERNAL USE)}$ nav_msgs::Path[] objectTrajectories: Trajectories for each object (indexed by the factor graph)
• $\color{gray}\textsf{(INTERNAL USE)}$ nav_msgs::Path[] objectVelocities: Linear and angular velocities for each object (indexed by the factor graph)
• nav_msgs::Path[] trackingObjectTrajectories: Trajectories for each object (indexed by LIO-SEGMOT)
• nav_msgs::Path[] trackingObjectVelocities: Linear and angular velocities for each object (indexed by LIO-SEGMOT)
• lio_segmot::ObjectStateArray[] trackingObjectStates: States for each object during its lifetime (indexed by LIO-SEGMOT)
• $\color{gray}\textsf{(INTERNAL USE)}$ lio_segmot::flags[] objectFlags: Flags for each object during its lifetime (indexed by the factor graph)
• lio_segmot::flags[] trackingObjectFlags: Flags for each object during its lifetime (indexed by LIO-SEGMOT)

in which custom types lio_segmot::ObjectStateArray (underlying lio_segmot::ObjectState) and lio_segmot::flags are given by

• lio_segmot::ObjectStateArray

  Header header
  lio_segmot::ObjectState[] objects

• lio_segmot::ObjectState

  Header header
  
  // The corresponding detection (measurement)
  jsk_recognition_msgs::BoundingBox detection
  
  // States of object pose and velocity in the factor graph
  geometry_msgs::Pose pose
  geometry_msgs::Pose velocity
  
  // Residual and innovation of the tightly-coupled detection factor
  bool hasTightlyCoupledDetectionError
  float64 tightlyCoupledDetectionError         // Residual
  float64 initialTightlyCoupledDetectionError  // Innovation
  
  // Residual and innovation of the loosely-coupled detection factor
  bool hasLooselyCoupledDetectionError
  float64 looselyCoupledDetectionError         // Residual
  float64 initialLooselyCoupledDetectionError  // Innovation
  
  // Residual and innovation of the smooth movement factor
  bool hasMotionError
  float64 motionError         // Residual
  float64 initialMotionError  // Innovation
  
  int32 index            // Object index
  int32 lostCount        // Counter of losing detections
  float64 confidence     // Detection's confidence score (given by detection methods)
  bool isTightlyCoupled  // Is the object tightly-coupled at this moment?
  bool isFirst           // Is the object just initialized at this moment?

• lio_segmot/flags

  // Flags of the object in its lifetime
  int32[] flags  // 1: the object is tightly-coupled
                 // 0: the object is loosely-coupled

📝 Remarks

Hyperparameters

There are various covariance matrices designed for the hirarchical criterion (innovation filtering) and factor graph optimization (increments of LIO-SEGMOT w.r.t. LIO-SAM) in LIO-SEGMOT. This section is going to explain them.

All covariance matrices in settings are expressed as diagonal vectors.

Taking the KITTI configuration for example, we have the following settings:

Hierarchical Criterion

This section collects settings for the hierarchical criterion. It can be viewed as a kind of innovation filtering. In brief, the criterion is designed to progressively make the following decisions when a new detection $\boldsymbol{z}\in SE(3)$ is coming into the system:

The first two questions (Q1) and (Q2) are determined by using the Mahalanobis distance of the error vector,

$$ \Big\Vert\text{ detection error of }\boldsymbol{z}\text{ and the }i\text{-th object } \boldsymbol{x}{t,i} \text{ }\Big\Vert{\Sigma} \leq \varepsilon, $$

with given covariance with given covariance matrices $\Sigma\in{\Sigma_ {\text{Q}_ 1},\Sigma_ {\text{Q}_ 2}}\subsetneq\mathbb{R}^{6\times 6}$ and a threshold $\varepsilon>0$. We assume that $\Sigma_ {\text{Q}_ 2}-\Sigma_ {\text{Q}_ 1}$ is positive semidefinite (PSD), i.e., $\Sigma_ {\text{Q}_ 2}-\Sigma_{\text{Q}_ 1} \succeq 0$, to prevent ambiguity of the hierarchical criterion that (Q2) holds but (Q1) does not hold.

Two spatial information-based tests are conducted to determine (Q3), which are the detection constraint and the velocity constraint:

• (Detection Constraint) The above equation holds with another given covariance matrix $\Sigma_ {\text{Q}_ {3,1}}$ that satisfies $\Sigma_ {\text{Q}_ {3,1}}-\Sigma_ {\text{Q}_ {2}} \succeq 0$.

• (Velocity constraint) The variance of velocities in previous steps is small enough. That is,

  $$\frac{1}{N}\sum_ {s=1}^{N} \Big\Vert \text{Log}(\boldsymbol{v}_ {t-s,i}) - \text{Log}(\bar{\boldsymbol{v}}_ {t,i}) \Big\Vert_ {\Sigma_{Q_ {3,2}}}^2 \leq \varepsilon$$

  with a given covariance matrix $\Sigma_{Q_{3,2}}$, where $N$ is the fixed number of previous velocities of object states and $\bar{\boldsymbol{v}}_{t,i}\in SE(3)$ is the mean of the $N$ previous velocities.

If (Q1) holds for the detection $\boldsymbol{z}$ and the corresponding $i$-th object, the new state of the $i$-th object along with a loosely-coupled detection factor would be added to the factor graph.

Furthermore, if (Q2) holds, a constant velocity factor and a smooth movement factor would be also added to the factor graph.

Finally, if (Q3) holds, the loosely-coupled detection factor would be replaced with a tightly-coupled detection factor. It means that the $i$-th object are regarded as a reliable object that are suitable to refine the odometry.

For engineering purposes, we use two thresholds $\varepsilon$ and $\varepsilon^\prime$ in the implementation. In addition, we decouple the angular part and the linear part of $\Sigma_{\text{Q}_{3,2}}$. The following equations are shown to coincide with the expression used in our paper:

$$ \begin{aligned} \displaystyle\Sigma_{\text{Q}{3,1}} &= \left(\frac{\varepsilon^\prime}{\varepsilon}\right)^2 \cdot \displaystyle\Sigma{\text{Q}{3,1}}^\prime, \ \displaystyle\Sigma{\text{Q}{3,2}}^\prime &= \begin{bmatrix}\sigma{\text{Q}{3,2}}^\text{A} \ & \sigma{\text{Q}{3,2}}^\text{A} \ && \sigma{\text{Q}{3,2}}^\text{A} \ &&& \sigma{\text{Q}{3,2}}^\text{L} \ &&&& \sigma{\text{Q}{3,2}}^\text{L} \ &&&&& \sigma{\text{Q}{3,2}}^\text{L} \end{bmatrix}, \ \displaystyle\Sigma{\text{Q}{3,2}} &= \frac{1}{\varepsilon^2} \cdot \displaystyle\Sigma{\text{Q}_{3,2}}^\prime. \end{aligned} $$

Factor Graph Optimization

Covariance matrices used in factor graph optimization. Different from the above section, they are essential to "balance" different types of measurements in a unified factor graph. Those matrices are relatively rare to be modified.

High-speed Moving Object Supports in Early Steps

As all tracking objects' velocities are initialized with zero-speed, it may be hard to associate detections in different moments for high-speed moving objects in the early stage. To mitigate this issue, a larger covariance matrix for (Q2) in the hierarchical criterion is used in the first few steps to accommodate objects that are moving fast.

Since those steps are used to figure out the inital speed of an object, we do not account them for the velocity constraint in the tightly-coupled detection factor checks. Therefore, we have $N^\prime = N + N^\text{E}$.

Lifecycle Management of Tracking Objects

In real world applications, it's likely to lose detections of tracking objects due to occlusion or other complicate environment circumstances. To mitigate frequent ID-switching in multiple object tracking due to the above issue, we still track each object for a little while, even though they do not have any corresponding detections.

Limitations of LIO-SEGMOT

Currently, most state-of-the-art object detection approaches (e.g., SE-SSD, PointPillars, PointRCNN, PV-RCNN, SPG, and ST3D) are constructed under machine learning-based neural network architectures, while the issue of domain adaptation for data-driven object detection approaches still remains a challenging open problem targeted by recent researches (e.g, SPG and ST3D). That is, the performance of object detection models change varying under different geographic appearances or weather conditions.

The model weight of SE-SSD used in LIO-SEGMOT is trained under a subset of the KITTI dataset, and thus it might work well in other subsets of the KITTI dataset. However, we can observe that there are numerous false positive detections in the Hsinchu dataset. Therefore, it forces us to choose different detection models (i.e., PointPillars w/ Livox's model weights) when experimenting LIO-SEGMOT in different real world datasets. In addition, since the model still cannot perform as good detection results in the Hsinchu dataset as SE-SSD does in the KITTI dataset, we need to use a more strict criterion for the detection constraint in the tightly-coupled detection factor checks (by decreasing $\varepsilon^\prime$ from 26.0 to 19.0). This points out the first limitation of LIO-SEGMOT. That is, covariance matrices and thresholds related to object detections (mainly hyperparameters in the hierarchical criterion) are required to be adjusted according to the stability of object detections. Despite it affects generalization capability of the proposed method, we believe that the problem can be mitigated with the breakthrough of the domain adaptation for 3-D object detection.

The second limitation of LIO-SEGMOT is related to the motion model of tracking objects. If an object does not move at constant velocity, LIO-SEGMOT may miscalculate the object velocity, leading to inaccurate predicted object pose in the subsequent state. The main reason is that objects' velocities are supposed to be steady and constant in LIO-SEGMOT. It is possible to be resolved by introducing multiple motion models to LIO-SEGMOT and adaptively selecting proper models during the factor graph optimization, in which the concept is similar to an interacting multiple model (IMM) in filtering-based object tracking approaches.

Possible Future Research Directions

There are two possible future research directions of LIO-SEGMOT:

• The optimization problem of LIO-SEGMOT produces a multi-robot architecturethat may break the efficiency of maintaining single root Bayes trees in iSAM2. It causes an unignorable computational cost, especially when there are lots of dynamic objects coupled in the system. In forthcoming researches, we would like to overcome the bottleneck by introducing the multi-robot iSAM2 (MR-iSAM2) algorithm.
• In addition, rule-based coupling conditions make the proposed method lack the ability to explore global optimality when considering combinatorial ambiguity as unknown integer variables. It raises a complicated mix-integer programming (MIP) problem, whereas a recent work called multi-hypothesis iSAM (MH-iSAM2) still promotes us to investigate the challenging problem in the future.

🎁 Acknowledgement

The project is mainly developed based on Tixiao Shan's excellent work LIO-SAM, which helps me a lot in constructing LIO-SEGMOT. I would like to express my sincere thanks first. In addition, I would like to thank Wu Zheng and Livox for developing and releasing their awesome object detection modules SE-SSD and Livox Detection respectively.