QR images and Polar

Step 01

Get an invoice from Polar

Open the terminal for a running LND node in Polar. List that node’s invoices:

lncli --network=regtest listinvoices

Copy a complete payment_request value. To create a fresh demo invoice for 1,000 sats:

lncli --network=regtest addinvoice --amt 1000 --memo "Decoder demo"

These commands run inside the Polar node terminal, not the decoder’s terminal.

Step 02

Generate a QR image

Back in your project’s local terminal, replace the placeholder with that invoice. Keep the quotes.

cargo run -p lightning-cli --example sample_qr -- 'PASTE_COMPLETE_INVOICE_HERE' my-invoice.png

This creates a PNG containing the invoice, with a lightning: prefix. Choose a new output filename if the file already exists. Encoding a QR does not create a new invoice or change its expiry.

Step 03

Import the QR

In the web app, choose or drop an image, review the extracted text, and select Decode invoice. In Ratatui, press F3 and enter the local image path.

Both clients support PNG, JPEG, and WebP up to 10 MB. The browser allows up to 16 million pixels; the terminal limits dimensions to 4096 × 4096. Use one QR per image.

A wallet’s BOLT11 invoice QR works too. Lightning address, LNURL, and BOLT12 offer QR codes are not currently supported as invoice input.

Step 04

Use the bundled samples

Find the coffee and donation images in frontend/public/samples/. Regenerate them with:

cargo run -p lightning-cli --example sample_qr

These public sample invoices are expired and intended for inspection.