Data masking
Data masking is a step in QR encoding that XORs the pattern with one of several mask templates to avoid large blank or solid areas that confuse scanners.
If a QR code ended up with big runs of the same color, or patterns that look like the finder squares, scanners would struggle. To prevent that, the encoder tries eight standard masks, scores each for readability, and keeps the best one. The mask used is recorded in the code's format information so a scanner can reverse it.
Masking happens automatically; you never pick it by hand. It's also why two codes for the same URL can look slightly different. And it's part of what makes QR codes reliable across cheap cameras and bad lighting.
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