Glossary
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 protected by its own BCH code, because a decoder cannot read anything else without it.
Structure
5 data bits — 2 for the error-correction level, 3 for the mask — plus 10 BCH(15,5)
error-correction bits, then XORed with the fixed mask 101010000010010 so an all-zero
format never produces a blank region.
Stored twice
One copy wraps the top-left finder pattern; the other is split between the bottom-left and top-right. If the first copy is unreadable, the decoder uses the second. This redundancy is why damage near one finder pattern is often survivable.
Why it is the most critical region
Everything else depends on it. Get the format bits wrong and the decoder unmasks with the wrong pattern and de-interleaves with the wrong block structure — the payload is irrecoverable even though the data modules are perfect. When styling a code, the area immediately around the finder patterns deserves the most caution.
Related
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