Pi-Tag Fiducial Marker Detector

Recently, I’ve been looking at various fiducial markers for computer vision applications. While some of these markers, such as AprilTags, have readily available reference implementations, others have no published code. While one of the markers I was looking at, Pi-Tags, does not have a reference implementation, it does have a third party implementation in the form of a Robot Operating System (ROS) module, cob_fiducials; this is great for robotics applications using ROS, but problematic otherwise. Since I wasn’t interested in using the detector with ROS, I separated it out into a standalone library. Additionally, I modified the detector to add a function that just returns the image pixel coordinates of the detected markers instead of calculating their poses, and added the ellipse refinement step1 that was mentioned in the Pi-Tag paper.2

Detected Pi-Tag

  1. Ouellet, Jean-Nicolas, and Patrick H├ębert. “Precise ellipse estimation without contour point extraction.” Machine Vision and Applications 21, no. 1 (2009): 59.  

  2. Bergamasco, Filippo, Andrea Albarelli, and Andrea Torsello. “Pi-tag: a fast image-space marker design based on projective invariants.” Machine vision and applications 24, no. 6 (2013): 1295-1310.  

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