Glossary
Mask pattern
A mask pattern is one of eight XOR patterns applied to a QR code's data area to avoid large blocks of one colour and false finder-like shapes. The encoder tries all eight, scores them, and keeps the best.
Why masking exists
Raw encoded data can, by chance, produce a big white region, a big black region, or a 1:1:3:1:1 run that looks like a finder pattern. Any of those confuse a decoder. Masking XORs the data area with a regular pattern to break up such structures.
The eight patterns
Each is a formula over the module's row i and column j:
| Mask | Condition |
|---|---|
| 0 | (i + j) mod 2 = 0 |
| 1 | i mod 2 = 0 |
| 2 | j mod 3 = 0 |
| 3 | (i + j) mod 3 = 0 |
| 4 | (⌊i/2⌋ + ⌊j/3⌋) mod 2 = 0 |
| 5 | (i·j) mod 2 + (i·j) mod 3 = 0 |
| 6 | ((i·j) mod 2 + (i·j) mod 3) mod 2 = 0 |
| 7 | ((i+j) mod 2 + (i·j) mod 3) mod 2 = 0 |
Penalty scoring
Each masked candidate is scored on four rules: runs of five or more same-coloured modules, 2 × 2 same-coloured blocks, finder-like 1:1:3:1:1 runs, and overall dark/light imbalance. Lowest total wins. The chosen mask number is recorded in the format information so the decoder can undo it.
This is why two generators can produce visibly different-looking codes for identical data, if their penalty implementations differ slightly, they pick different masks. Both are valid.
Related
- Format information, Format information is the 15-bit block next to the finder patterns that records the error-correction level and mask pattern. It is stored twice and…
- Finder pattern, The three large concentric squares in the corners of a QR code are finder patterns. A scanner locates them first to work out where the code is, how big it…
- Module, A module is one square cell of a QR code: the smallest unit of the pattern. Codes are measured in modules, not pixels or millimetres, because the module…