QR Code Format Information: How 15 Bits Control Decoding

1. What the 15 Bits Contain

The format information has 5 data bits and 10 error-check bits. The 5 data bits are 2 bits for the error correction level and 3 bits for the mask pattern (0 to 7). The 10 check bits come from a BCH (15,5) code.

2. Masking the Format String

The final 15-bit string is XORed with the fixed pattern 101010000010010. This prevents an all-zero format string and keeps the area from looking like a blank region.

3. Redundancy

The format string is written twice: once around the top-left finder pattern, and once split between the other two finders. If one copy is damaged, the reader falls back to the other. Version information (Version 7 and above) is a separate 18-bit block also written twice.