Watermarking

How invisible watermarks work in SQR — embedding, scanning, and the full lifecycle.

SQR encodes short messages into images using invisible frequency-domain watermarking. The watermark survives printing, photographing, compression, and moderate cropping.

How it works

1. A unique 5-character ID (e.g. A1B2C) is generated for each record
2. The ID is encoded into the image’s frequency domain using a spread-spectrum technique
3. The watermarked image looks identical to the original — the difference is imperceptible
4. To verify, a photo of the watermarked image is decoded to recover the embedded ID

Embed a watermark

Via record creation (recommended)

When you create a record, the watermark is embedded automatically:

curl -X POST https://yys-sqr-render-bsbe.onrender.com/api/records \
  -H "Authorization: Bearer $SQR_KEY" \
  -F template_id=pharma \
  -F 'metadata={"product_name":"Aspirin"}' \
  -F image=@label.jpg

The response includes watermarked_image (base64) — decode and save it:

import base64

response = create_record(...)
img_bytes = base64.b64decode(response["watermarked_image"])
with open("label_watermarked.jpg", "wb") as f:
    f.write(img_bytes)

Standalone embed

If you just want to embed a message without creating a record:

curl -X POST https://yys-sqr-render-bsbe.onrender.com/api/embed \
  -H "Content-Type: application/json" \
  -d '{
    "image": "base64-encoded-image...",
    "message": "A1B2C"
  }'

The message is limited to 5 characters. This is by design — shorter payloads are more robust against degradation.

Scan a watermark

To decode a watermark from a photograph:

curl -X POST https://yys-sqr-render-bsbe.onrender.com/api/scan \
  -H "Content-Type: application/json" \
  -d '{
    "image": "base64-encoded-photo...",
    "corners": [[10,10], [500,10], [500,500], [10,500]]
  }'

The corners parameter defines the four corners of the label in the photo, allowing SQR to correct for perspective distortion.

Scan via record endpoint

Once you have the decoded watermark ID, look up the record:

curl https://yys-sqr-render-bsbe.onrender.com/api/records/SQR-A1B2C

And log the scan event:

curl -X POST https://yys-sqr-render-bsbe.onrender.com/api/records/SQR-A1B2C/scan \
  -H "Content-Type: application/json" \
  -d '{"scanner_address": "0xabc..."}'

Best practices

• Use high-resolution source images (1000px+ on the shortest side)
• Avoid heavy JPEG compression on the watermarked output
• When printing, use at least 300 DPI
• For scanning, photograph the label straight-on with good lighting