How to detect AprilGrid?

[wfxcqsc]

hello,I am confused with the AprilGrid, when i detect the following picture（Apriltag board） using the opencv_demo, it can be successfully detected.

however,when detect the AprilGrid, it can not detect, i guess the reason is the the border width of Apriltag board is one ，while AprilGrid with border width the size of two

so，i want to figure out whether can set the borderbits, and how to detect AprilGrid?

Looking forward to your reply!

[christian-rauch]

The apriltag library needs a white border around the tag to do the edge detection properly. This border is interrupted by the black squares on the corner of the tags.

[wfxcqsc]

thanks for your reply! i am sorry about my issue，i put the wrong image in the first picture. the Apriltag board that can be Apriltag library successfully detect is the following

however, i have tried to cover up the black squares in the AprilGrid，and detect again. but it still doesn't work.

then, i detect the same picture get from the above using OpenCV aruco , there is a"DICT_APRILTAG_36h11", if i don't set the "markBorderBits"(default value is 1), it also can't detect. while i set the parameter "markBorderBits=2", it can be successfully detect

Therefore, in addition to black square interruption, is the border width also a part of the impact on detection? and whether the apriltag library can detect the APRILGRID?

[mkrogius]

The apriltag library only supports a black border width of 1. It looks like in your image the border width is 2, which is why it is not detected

[christian-rauch]

@wfxcqsc I tried your AprilGrid image with apriltag_demo and opencv_demo and I got a couple of detections. Do you not get any detections?

Alternatively, you can also try the "AprilTags C++ Library" from https://people.csail.mit.edu/kaess/apriltags/ (https://bitbucket.org/kaess/apriltags/src/master/) which generally supports tag families with multiple black border sizes: https://bitbucket.org/kaess/apriltags/src/3aea96d3239bb83870cb5e0a85f142a2b0589563/AprilTags/TagFamily.h#lines-64.

[wfxcqsc]

Thanks for your reply！in fact, when I using opencv_demo to detect AprilGrid which have black squares on the corner of tags, I can not detect any detections. but I have detected AprilGrid successfully by using Kalibr，and I will also try the “AprilTags C++ Library”.