Developed at the University of Michigan (https://april.eecs.umich.edu/software/apriltag), AprilTag is like a 2D barcode or a simplified QR Code. It contains a numeric ID code and can be used for location and orientation.
(Figure: AprilTags on Robots. Photo Credit: University of Michigan)
AprilTag is a type of visual fiducial, or fiducial marker, containing information and designed for easy recognition.
(Figure: A sample of different AprilTag families)
The above samples represent different formats, or families. A project typically uses a single AprilTag family.
FIRST Tech Challenge uses a common family called 36h11. A PDF showing the numbers 0 through 20 from the 36h11 family can be downloaded here:
Each number is the ID code of that tag.
Here’s an AprilTag representing ID code 2. The SDK software recognizes and overlays the ID code onto the image (small blue rectangle ID 02).
(Figure: Stream output showing the detected tag ID 02)
The above image shows a camera preview image, called LiveView, from a Robot Controller device (Control Hub or RC phone).
The AprilTag family 36h11 has a capacity of 587 ID codes. To see them all, follow this link:
- https://github.com/rgov/apriltag-pdfs/tree/main/tag36h11/us_letter/100mm
The square AprilTag pattern contains smaller black and white squares, each called a pixel. A 36h11 tag contains 10 x 10 pixels, including an outer border of all white pixels and an inner border of all black pixels.
Tag size is measured across the outside edge of the inner border which comprises the black pixels for 36h11.
(Figure: Figure demonstrating the tag size measurement)
The above image shows a complete AprilTag with outer white border. From the 36h11 family, its ID code is 42.