[jillesvangurp]

The qr code scanning algorithm works based on contrast and is designed to work with poor cameras under poor light conditions. Whatever shapes, colors and shades are used are interpreted as either black or white squares.

The process starts by scanning for the 3 markers on the corners. That give you the square containing the qr content and it's orientation. The next jobs are figuring out the resolution of the square and the version of the qr code used. Eventually you end up knowing the grid dimensions. And then you simply scan the grid as the article outlines by simply averaging the pixels in each grid square to be "white" or "black". As long as you have enough contrast, most grid squares will scan correctly.

You can abuse this quite a bit before it starts breaking. There are some interesting QR codes out there that are barely recognizable as QR codes for people. But if you open them in an image editor and play with the levels and contrast a bit, they are obviously qr codes. And there's error correction too so it's OK if some of the qr code is scanned incorrectly.