Workflow: Upload or paste an image in the left column; the decoder uses the primary detected symbol. Edit Data in the middle column if needed. Generate refreshes the preview in the right column whenever you want, as long as Data is not empty. Re-decode sits on the upload area (bottom-right): it runs decode again on the current image without opening the file picker.

Code types: Compact buttons list symbologies. QR Code is rendered with Python qrcode. QR Code (ZXing) uses zxing-cpp. Code 93, UPC-E, PDF417, Data Matrix, Aztec are also generated with zxing-cpp. The decoder often reports QRCODE for both QR engines; choose the encoder you want when regenerating. 1D linear types take width from Code size and bar height from Bar height.

Code size (right): Output size in pixels. For QR and other 2D codes this sets the bitmap side. For 1D (vertical-bar) codes it mainly controls width.

Bar height (1D only): Shown only for linear barcodes (Code 128, I25, Code 39, EAN/UPC, Codabar, …). It sets how tall the bars are in the preview box and in the downloaded PNG. QR, PDF417, Data Matrix, and Aztec do not use this control.

Density: QR Model 2 version V → (21+4(V-1)) modules per side. Auto picks the smallest version that fits the payload.

ECC level: L / M / Q / H — stronger correction reserves more modules and changes the pattern.

Mask: Patterns 0–7 (or Auto) alter module placement for the same bytes; try different values to match another printed QR.

Linear barcodes: By default the PNG has no text under the bars. Check Show under text in image to draw it. Text font uses real .ttf files on the server (not browser/CSS names).

Auto-refresh: When Data is not empty, changing options refreshes the preview the same way as Generate.