A QR code is not a random mosaic. The phone identifies the structure, corrects perspective, reads the black-and-white modules and rebuilds the data. It does not “photograph a link”; it decodes a two-dimensional message.

Denso Wave designed the system in 1994 for Japan’s car industry, as a quick response to part labels — hence “Quick Response.” The three large corner squares are position-detection patterns: they show where the code is and how it is rotated. A quiet zone around it separates it from the background.

Letters or a web address become bits, then fill the matrix together with format information. Error-correction symbols, of a Reed–Solomon type, are added on top of the data. If some modules are scratched or covered, the reader can recover content from that redundancy.

ISO/IEC 18004 defines several correction levels. At the highest, in good conditions, the equivalent of about 30% of the codewords can be restored. That does not mean any third of the square can be erased: the site of the damage, contrast, focus and whether the corner markers keep their shape all matter.

A logo in the centre is possible because of that reserve, but a crowded design fails easily on small screens or after poor printing. Testing on several phones is more useful than a decorative frame.

The code does not guarantee the destination. It can hide a malicious address as easily as a legitimate one. The preview, the domain and the context are worth checking before opening a page or entering data.

Image: QR-code structure diagram / Wikimedia Commons. A teaching diagram, not a QR that opens a site. Cropped to 16:9.

Source consulted: History of QR Code | DENSO WAVE; Error correction feature | DENSO WAVE.