This ROS2 node uses the AprilTag library to detect AprilTags in images and publish their pose, id and additional metadata.

For more information on AprilTag, the paper and the reference implementation: https://april.eecs.umich.edu/software/apriltag.html

The node subscribes via a image_transport::CameraSubscriber to rectified images on /apriltag/image_rect. The set of topic names depends on the type of image transport (parameter image_transport) selected (raw or compressed):

• /apriltag/image_rect (raw, type: sensor_msgs/Image)
• /apriltag/image_rect/compressed (compressed, type: sensor_msgs/CompressedImage)
• /apriltag/camera_info (type: sensor_msgs/CameraInfo)

• /tf (type: tf2_msgs/TFMessage)
• /apriltag/detections (type: apriltag_msgs/AprilTagDetectionArray)

The camera intrinsics P in CameraInfo are used to compute the marker tag pose T from the homography H. The image and the camera intrinsics need to have the same timestamp.

The tag poses are published on the standard TF topic /tf with the header set to the image header and child_frame_id set to either tag<family>:<id> (e.g. "tag36h11:0") or the frame name selected via configuration file. Additional information about detected tags is published as AprilTagDetectionArray message, which contains the original homography matrix, the hamming distance and the decision_margin of the detection.

The node is configured via a yaml configurations file. For the complete ROS yaml parameter file syntax, see: https://github.com/ros2/rcl/tree/master/rcl_yaml_param_parser.

The configuration file has the format:

apriltag:                 # node name
  ros__parameters:
    # setup (defaults)
    image_transport: raw  # image format: "raw" or "compressed"
    family: 36h11         # tag family name: 16h5, 25h9, 36h11
    size: 1.0             # default tag edge size in meter
    z_up: true            # rotate about x-axis to have Z pointing upwards
    profile: false        # print profiling information to stdout

    # tuning of detection (defaults)
    max_hamming: 0        # maximum allowed hamming distance (corrected bits)
    detector:
      threads: 1          # number of threads
      decimate: 2.0       # decimate resolution for quad detection
      blur: 0.0           # sigma of Gaussian blur for quad detection
      refine: 1           # snap to strong gradients
      sharpening: 0.25    # sharpening of decoded images
      debug: 0            # write additional debugging images to current working directory

    # (optional) list of tags
    # If defined, 'frames' and 'sizes' must have the same length as 'ids'.
    tag:
      ids:    [<id1>, <id2>, ...]         # tag IDs for which to publish transform
      frames: [<frame1>, <frame2>, ...]   # frame names
      sizes:  [<size1>, <size1>, ...]     # tag-specific edge size, overrides the default 'size'

The family (string) defines the tag family for the detector and must be one of 16h5, 25h9, 36h11, Circle21h7, Circle49h12, Custom48h12, Standard41h12, Standard52h13. size (float) is the tag edge size in meters, assuming square markers.

Instead of publishing all tag poses, the list tag.ids can be used to only publish selected tag IDs. Each tag can have an associated child frame name in tag.frames and a tag specific size in tag.sizes. These lists must either have the same length as tag.ids or may be empty. In this case, a default frame name of the form tag<family>:<id> and the default tag edge size size will be used.

The remaining parameters are set to the their default values from the library. See apriltag.h for a more detailed description of their function.

See tags_36h11.yaml for an example configuration that publishes specific tag poses of the 16h5 family.

The apriltag_node executable can be launched with topic remappings and a configuration file:

ros2 run apriltag_ros apriltag_node --ros-args \
    -r /apriltag/image_rect:=/camera/image \
    -r /apriltag/camera_info:=/camera/camera_info \
    --params-file `ros2 pkg prefix apriltag_ros`/share/apriltag_ros/cfg/tags_36h11.yaml

For more efficient intraprocess communication, a composable node is provided:

$ ros2 component types
apriltag_ros
  AprilTagNode

This AprilTagNode component can be loaded with other nodes into a "container node" process where they used shared-memory communication to prevent unnecessary data copies. The example launch file tag_36h11_all.launch.py loads the AprilTagNode together with the usb_cam::UsbCamNode component from the usb_cam package (sudo apt install ros-$ROS_DISTRO-usb-cam) into one container and enables use_intra_process_comms for both:

ros2 launch apriltag_ros tag_36h11_all.launch.py