QR Code는 얼마나 훼손되어도 인식될까?
Real-world damage testing: scratches, tears, stains, fading, and partial obstruction. Visual examples of EC recovery.
How Much Damage Can a QR Code Survive?
One of the QR code's most impressive features is its ability to function despite physical damage. But how much damage is actually tolerable, and what factors determine the survival threshold?
Theoretical Limits
The error correction level defines the theoretical maximum recoverable damage:
| EC Level | Correctable Codewords |
|---|---|
| L | ~7% |
| M | ~15% |
| Q | ~25% |
| H | ~30% |
These percentages refer to codewords, not physical area. Because of codeword interleaving, physical damage is distributed across blocks, often allowing recovery beyond what the raw percentages suggest for localised damage.
Types of Damage
Scratches and Abrasion: Linear scratches across the code typically affect fewer codewords per block (thanks to interleaving) and are often recoverable at Level M or higher.
Stains and Dirt: Opaque stains that cover a contiguous area are the hardest to recover from. A stain covering 15% of the physical area at EC Level M is borderline.
Fading and Low Contrast: Gradual fading reduces the contrast between dark and light modules. Scanners can compensate to some degree with adaptive thresholding, but severe fading causes individual module misreads scattered throughout the code.
Tears and Missing Sections: Missing corners or edges can destroy finder patterns or mask pattern." data-category="QR Code Structure">format information, which may make the code undetectable regardless of EC level. Central damage is generally more survivable.
Critical Regions
Some areas cannot be damaged without catastrophic failure:
- Finder patterns: Loss of two or more finder patterns usually prevents detection entirely
- Format information: Corrupted format data means the decoder cannot determine the EC level or mask pattern (though dual redundancy helps)
- Timing patterns: Damage to timing patterns impairs grid calibration
The data area (the bulk of the QR code) is the most damage-tolerant region thanks to RS error correction.
Practical Testing Results
In controlled experiments:
- A Version 5 QR code at EC-H can survive a 5mm hole punched through its centre
- QR codes on outdoor signs remain readable after months of weather exposure at EC-Q
- Coffee stains covering up to 20% of the area are recoverable at EC-H
- A QR code folded in half and creased remains readable after unfolding at EC-M
Key Takeaways
- EC level determines theoretical damage tolerance (7-30% of codewords)
- Interleaving means localised damage is often more recoverable than scattered damage
- Finder patterns and format information are the most critical (and vulnerable) regions
- Central damage is more survivable than corner/edge damage
- Always test with deliberate damage before deploying in harsh environments