Glossary
Finder pattern
The three large concentric squares in the corners of a QR code are finder patterns. A scanner locates them first to work out where the code is, how big it is and which way up it is, which is why the missing fourth corner is deliberate.
The 1:1:3:1:1 ratio
Each finder pattern is a 7 × 7 module block: a black 7 × 7 square containing a white 5 × 5 square containing a black 3 × 3 square. Scanned along any line through the centre, the black-white ratio is 1:1:3:1:1; a signature that is rare in natural images and survives rotation, which is what makes QR codes findable at any angle.
Why only three corners
Three points define an orientation; a fourth would be redundant. The empty bottom-right corner is how a scanner knows which way up the code is. This is also why you cannot "balance" a design by adding a fourth square. It breaks detection.
Styling them
Finder patterns tolerate rounding and colour changes provided the 1:1:3:1:1 ratio survives at the sampling resolution and contrast stays high. Filling the centre square with a logo does not survive. UseQR verifies styled output by decoding it before download, which is the only way to know a styled finder still reads.
Related
- Alignment pattern, Alignment patterns are the small 5 × 5 concentric squares scattered through larger QR codes. They let a scanner correct for perspective distortion when…
- Timing pattern, The timing patterns are the alternating black-and-white lines running between the finder patterns, one horizontal and one vertical. They tell the decoder…
- Module, A module is one square cell of a QR code: the smallest unit of the pattern. Codes are measured in modules, not pixels or millimetres, because the module…