Not to be all "I am so smart" but I'm presuming that orienting and figuring out the dimensions of the grid isn't super hard because of the three (as opposed to four) corners have the easily recognizable "target" pattern, which give both orientation and "pixel" size. From there you just do like weighted thresholds.

I know there's some number magic that happens even if you get some of these wrong, but I imagine it's kind of some kind of reverse checksumming thing going on.