Hi, there
I'm curious about how the frame marker works.

So, I read 'Robust and Unobtrusive Marker Tracking on Mobile Phones' written by Daniel Wagner.

There are some mentions about it.

A 36 bit code including 27 bit redundancy is encoded alongside each of the 4 sides of the frame, in the form of 9 individual black or white squares encoding 1 bit each (9bit = 512 different markers). The code is arranged in clockwise order, and is chosen in a way so that only one of the 4 possible corners yields a valid code without errors. This allows determining the code as well as the correct orientation of the marker.

So, do I have some detailed information about 27 bit redundancy?
Thanks... Happy new year!!