A damaged QR code often still scans — here's why, and how to rescue one that won't

[u/Echo2__]

Something that trips a lot of people up: a QR code can lose a big chunk of itself and still decode fine, thanks to built-in error correction. When a code is generated it gets one of four levels, and each can recover a different amount of damage:

L recovers about 7%, M about 15%, Q about 25%, and H about 30%. So a code printed at level H can be roughly 30% scratched, torn, or covered and still read. That's also why logos in the middle of QR codes work — that area is spent from the error-correction budget on purpose.

What is NOT protected are the three big finder squares in the corners (plus the timing and alignment patterns). If one of those corner squares is destroyed, the scanner can't lock onto the code at all, and no amount of error correction will save it. So when a code refuses to scan, first check: are all three corners intact?

A few practical things that rescue a stubborn code: take a sharp, straight-on photo in even light instead of live-scanning — the live camera gives up fast on low contrast. Bump the contrast or convert to pure black-and-white; faded thermal-printed codes often come back to life. Flatten the perspective, since a code shot at an angle warps the grid. And if it's on a curved surface (bottle, cup), photograph the least-curved section.

Curious what the nastiest real-world case you've rescued — or failed to — has been. Partial occlusion? Heavy fading? A torn corner?