Variable QR_ERROR_CORRECTION_LEVEL`Const`

QR_ERROR_CORRECTION_LEVEL: "L" = ...

Recommended QR code error correction level

Level L (7% recovery) is optimal for QWBP because:

Screen-displayed QRs have perfect contrast and no physical damage
Higher levels (M=15%, Q=25%, H=30%) increase QR size/version
Using H would push v4 codes back to v5-6, defeating optimization work