Error correction level
Error correction level is how much redundant data a QR code carries so it still scans when damaged, graded L, M, Q, or H from least to most.
QR codes use Reed-Solomon error correction to survive smudges, scratches, and a logo dropped in the middle. The level sets how much of the code can be obscured and still read: L recovers about 7%, M about 15%, Q about 25%, and H about 30%. Higher correction means more modules, so the code looks denser at the same size.
If you're placing a logo over the center of a code, a higher level (Q or H) buys back the room you cover. The trade-off is a busier pattern. See the full error-correction level reference table for the recovery percentages and when to use each.
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