Show HN: ShadowCat – file transfer through QR Codes in a Browser

Source: Hacker News

A fully offline, single-file HTML page for moving data between two devices via QR codes — intended for old phones whose radios (BLE, NFC, etc.) arevdead but whose cameras and browsers still work.

Tabs

• Generate — encode text into a single QR code.

• Scan — decode a single QR via the camera.

• Send file — pick a file, choose chunk size / FPS / ECC, hit Start. Cycles through [header, chunk1…chunkN] forever at the chosen FPS. Pause / Resume / Stop.

• Start from — begin the loop at a chosen frame index; it then continues forward and wraps back to the header normally.

• Show frame + Show / − / + — display exactly one frame static, for resending a specific missing chunk. The number matches the chunk index shown in the receiver’s missing-chunks grid (0 = header).

• Receive file — start the camera and point at the sender. Header autodetects, progress bar fills in, missing-chunks grid shows which ones haven’t arrived yet. When complete, the file’s CRC is verified and a Download button appears.

Protocol

• Header: QRX1|H||||

• Data: QRX1|D|| (1-indexed)

• Base64 alphabet has no |, so parsing is just split('|').

• Receiver tracks chunks by index, ignores duplicates, dedupes header by CRC.

Practical notes for old phones

• Camera needs HTTPS or localhost — file:// won’t grant getUserMedia permission. Serve with python3 -m http.server 8000 and visit http://:8000/qrcode.html over the local network. iOS Safari additionally requires HTTPS for cross-device access — for a LAN setup, caddy or a self-signed cert helps.

• If render fails on a frame (“code length overflow”), drop chunk size or drop ECC level.

• 500 chars × 3 fps ≈ 1.1 KB/s base64 ≈ 0.83 KB/s raw. A 100 KB file is roughly 2 minutes per loop; receiver typically needs 1-2 loops.

• If old devices struggle to decode: lower FPS, raise ECC to Q, shrink chunk to ~300 chars — produces smaller, less dense QRs.