Why a damaged QR code can sometimes still scan
A QR code includes error-correction information as well as its main data. This redundancy can help a reader recover information when part of the symbol is damaged or obscured. Recovery has limits, and damage to important locating patterns can prevent scanning even when much of the code remains visible.
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Some extra data helps recover missing data
A QR code arranges encoded information in a two-dimensional grid. It also contains structural patterns that help a scanner find the symbol, determine its orientation and interpret the grid.
Error-correction coding adds carefully constructed redundancy. A decoder can use it to recover from a limited amount of missing or incorrect information.
Redundancy is useful information, not simply a duplicate picture. The mathematics links data elements so that some errors can be detected and corrected.
- Locating patterns help the reader recognise the symbol.
- Encoded content carries the message.
- Error correction supports recovery from limited damage.
More error correction uses space that could otherwise carry additional content. A logo placed over a code consumes some of its tolerance, so successful scanning still depends on the design, damage pattern and capture quality.