Invisible watermarking guide
This article explains practical invisible watermarking using DCT/QIM, robustness trade‑offs, and verification.
Why invisible watermarks
- Act as an attribution tag that survives light edits and re‑uploads.
- Complement visible watermarks where aesthetics matter.
How it works (DCT/QIM)
- Convert to luminance; split into 8×8 blocks; run DCT.
- Pick mid‑frequency coefficients with a PRNG seed; quantize to encode bits.
- Inverse DCT and map luminance deltas back to RGB.
Robustness tips
- Redundancy + majority vote across blocks/pages/frames.
- Error‑correcting codes for payload (e.g., Reed‑Solomon/BCH).
- Sync markers to help alignment and thresholding.
Verification
Use the same seed/strength and payload length to decode. For batch media, decode multiple frames/pages and vote.