https://www.lemonde.fr/sciences/article/2021/09/15/des-maths-dans-votre-passe-sanitaire_6094702_1650684.html

QR codes are loaded with math. The health pass consists of 7,225 little white or black squares, arranged in 85 rows and 85 columns, that encode vaccination status, or test result, or certificate of recovery. This poses some very interesting mathematical and computer problems.

The first problem is geometric. The optical reader that scans the QR code sees the square in perspective, as any quadrilateral. So the perspective must be straightened: this is fairly easy. You also have to recognize the top and bottom, the right and left. This is also quite easy because three of the four corners are decorated with small 7 × 7 squares that are easily recognizable. Sometimes the QR code is presented on a sheet of paper that has been folded or crumpled and the rows and columns are not straight: you have to rectify them. Thirteen 5 × 5 squares, also recognizable, are spread out in the large square to help the software get it all straight.