https://typefully.com/DanHollick/qr-codes-T7tLlNi
Second, the mask - what's that? Well, QR readers work best when there are the same amount of white and black areas. But the data might not play ball so a mask is used to even things out. When a mask is applied to the code anything that falls under the dark part of the mask is inverted. A white area becomes black and black area becomes white.
There are 8 standard patterns which are applied one by one. The pattern that achieves the best result is used and that info is stored so the reader can unapply the mask.
