AprilTag 2 ROS2 Node

This ROS2 node uses the AprilTag 2 library to detect AprilTags in images and publish their pose, id and additional metadata.

For more information on AprilTag 2, the paper and the reference implementation: https://april.eecs.umich.edu/software/apriltag.html

Topics

Subscriptions:

The node subscribes via a image_transport::CameraSubscriber to /apriltag/image. The set of topic names depends on the type of image transport (parameter image_transport) selected (raw or compressed): - /apriltag/image (raw, type: sensor_msgs/Image) - /apriltag/image/compressed (compressed, type: sensor_msgs/CompressedImage) - /apriltag/camera_info (type: sensor_msgs/CameraInfo)

Publisher:

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

The camera intrinsics K 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. "36h11:0") 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.

Configuration

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 file has the format:

apriltag:                           # namespace
    apriltag2:                      # node name
        ros__parameters:
            # required
            image_transport: raw    # image format: "raw" or "compressed" (default: raw)
            family: <tag family>    # tag family name: 16h5, 25h9, 36h11 (default: 36h11)
            size: <tag edge size>   # tag edge size in meter (default: 2.0)
            z_up: true              # rotate about x-axis to have Z pointing upwards (default: false)

            # (optional) tuning of detection
            max_hamming: 0          # maximum allowed hamming distance (corrected bits)
            decimate: 1.0           # decimate resolution for quad detection
            blur: 0.0               # sigma of Gaussian blur for quad detection
            refine-edges: 1         # snap to strong gradients
            threads: 1              # number of threads
            debug: 0                # write additional debugging images to current working directory

            # (optional) list of tags
            tag_lists:
                <frame name>: <id>  # tag frame name and ID
                <frame name>: <id>
                [...]

The parameters family and size are required. family (string) defines the tag family for the detector and must be one of 16h5, 25h7, 25h9, 36h10, 36h11, 36artoolkit. size (float) is the tag edge size in meters, assuming square markers.

tag_lists is an optional list to map detected tag IDs to frame names. If it is provided, only the the listed tag IDs will be published, with child_frame_id set to the frame name. If not provided, the pose of all detected tags is published with a generic child_frame_id.

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_16h5_all.yaml for an example configuration that publishes all markers in the 16h5 family and tags_16h5_filtered.yaml for filtering tags.

To run the node and load the configuration, pass the configuration file to __params:

ros2 run apriltag2_node apriltag2_node \
    /apriltag/image:=/camera/image \
    /apriltag/camera_info:=/camera/camera_info \
    __params:=`ros2 pkg prefix apriltag2_node`/share/apriltag2_node/cfg/tags_16h5_all.yaml