april_tag

Detects 'AprilTag' 2D fiducial markers in ROS images (sensor_msgs/Image) and produces id, location, and orientation of the tags. Functionality is also provided as a Nodelet.

This package uses the C++ AprilTags library written by Michael Kaess and Hordur Johannson. The AprilTags Visual Fiducial System was developed by Edwin Olson.

More on april tags here: https://april.eecs.umich.edu/software/apriltag.html

April Tags C++ library: http://people.csail.mit.edu/kaess/apriltags/

Launchfiles

• april_tag.launch
• nodelet.launch

Required arguments

• camera_ns: namespace in which camera's image_raw topic exists e.g. /usb_cam
• tag_size_cm: length of the edge of the black frame in cms
• manager: name of nodelet manager (nodelet only)

Optional arguments

• publish_visualization_markers: set to true if you are running RViz and would like to see detected tags
• focal_length_x_px: camera focal length in x dimension in pixels
• focal_length_y_px: camera focal length in y dimension in pixels
• principal_point_x_px: x-coordinate of camera principal point
• principal_point_y_px: y-coordinate of camera principal point

Missing optional arguments are fetched from sensor_msgs/CameraInfo messages published on the $(arg camera_ns)/camera_info topic.

Input

$(arg camera_ns)/image_raw

Output

AprilTagList which is a list of AprilTag:

uint32 	id
uint32 	hamming_distance
float64 distance
geometry_msgs/PoseStamped stamped

where hamming_distance indicates difference between the observed tag and the identified tag, and distance is depth away from camera. x is horizontal with camera right as positive.

Depends on: libeigen3-dev

-- James Giller