Why Can a QR Code Still Scan After Losing a Corner? It Has Backups, Not Magic

Wed, 29 Jul 2026 08:00:00 +0800

The short answer

A QR code can sometimes survive a missing corner because the encoder stores more than the original message. It also adds redundant information that lets a decoder detect and repair a bounded amount of damage. If the missing part is mainly in the data area, the remaining modules and the error-correction codewords may be enough to reconstruct the payload.

But “one missing corner” is not a precise engineering measurement. A corner that contains ordinary data may be recoverable. A small cut through one of the three large finder patterns can be much worse: the scanner may fail to locate and orient the symbol before it even reaches the error-correction stage.

Figure 1: Original artwork made for this article. A QR code behaves less like a single copy of an answer and more like a dictation exercise with extra clues.

You have probably seen this in real life: a poster is torn, a shipping label is covered by tape, or someone puts a logo in the middle of a QR code, yet a phone still opens the destination. Where is the missing information coming from? How can the camera know what the covered black and white modules should have been?

This article starts with a reproducible local experiment and then explains the algorithm through a familiar analogy: a dictation sentence with a few missing characters. We will separate the QR layout, finder patterns, codewords, Reed–Solomon error correction, the meaning of L/M/Q/H, and the common mistake of treating “30% correction” as “30% of the picture can be painted over.”

科普 on Margrop Blog

Why Can a QR Code Still Scan After Losing a Corner? It Has Backups, Not Magic