How QR Code Scanning Works: From Camera Pixels to Decoded Data

Your phone camera captures an image. Within 100 milliseconds, it identifies a QR code in the frame, corrects for perspective distortion, decodes binary data, applies error correction, and presents you with a URL. The speed makes it seem simple. The algorithm behind it is elegant. Step 1: Finding the QR code A QR code has three finder patterns in three corners. Large concentric squares: dark, light, dark, with a 1:1:3:1:1 ratio of module widths when scanned along any line through the center. This ratio is unique. No natural image produces this pattern accidentally. The scanner sweeps horizontal and vertical scan lines across the image, looking for this 1:1:3:1:1 ratio in pixel brightness transitions. When it finds three instances arranged in the correct geometric relationship (forming a right angle with specific distance ratios), it has located a QR code. The fourth corner (which has no finder pattern) is calculated from the positions of the other three, confirmed by the alignment patte