Checkpoint 4

Last Updated: Oct 23, 2025

This checkpoint still under editing

The MBot has a camera that can be used to perform vision tasks like identifying Apriltags and other obstacles in the environment.

Contents

Computer Vision

Task 4.1 Camera Calibration

  1. Find a checkerboard in the lab

    The size of the checkerboard is determined by the number of intersections, not squares. For instance, a notation of [8, 6] means the board has 8 intersections along one dimension and 6 along the other, corresponding to a grid of 9 by 7 squares. You need this information.

  2. Run the camera node in the VSCode Terminal: 
     ros2 run camera_ros camera_node --ros-args \
 -p orientation:=180 \
 -p width:=640 -p height:=480 \
 -p format:=BGR888
  3. On NoMachine, run the following command, the GUI will pop up. 
     ros2 run camera_calibration cameracalibrator \
 --size 6x8 --square 0.025 --ros-args -r image:=/camera/image_raw -p camera:=/camera
    • --size is the num of intersections we introduced at the step 1.
    • --square is the size of the block in meter. Here 0.025 means the block is 25 mm.
    • Adjust the values based on your checkerboard.
  4. Use this link for reference if you don’t know how to use the calibrator.
    • Read from: Tutorial Steps -> 5- Start the camera calibration node
  5. Once you get the calibration result, move it to the mbot workspace: 
     cp /tmp/calibrationdata.tar.gz ~/mbot_ws/src/mbot_vision
 cd ~/mbot_ws/src/mbot_vision
 tar -xvf calibrationdata.tar.gz -C calibration/
    • This will unzip the calibration results to /calibration.
  6. Rebuild the packages so the calibration file is accessible anywhere: 
     cd ~/mbot_ws
 colcon build
 source install/setup.bash

Launch files

To save time on ros2 run all the node individually, we also provide launch files.

  • To launch camera node only, run the following in the VSCode Terminal: 
      ros2 launch mbot_vision camera.launch.py
    • This will publish topics /camera/image_raw and /camera/camera_info
    • You can view the camera feed in rqt after launching this node.
  • To launch the camera with image rectification using the calibration results (camera node + rectification node), run the following in the VSCode Terminal: 
      ros2 launch mbot_vision camera_rectify.launch.py
    • You need to do camera calibration first to run the rectification node. It needs the camera info.
    • This will also publish the topic /image_rect
    • You can view the camera feed in both raw and rectified in rqt after launching these 2 nodes.
  • To launch full vision pipeline (camera + rectification + AprilTag detection), run the following in the VSCode Terminal: 
      ros2 launch mbot_vision apriltag.launch.py
    • This will also publish AprilTag detection results on /detections

Checkpoint Submission

No submission is required for this checkpoint.