remove-ai-watermarks/data/calibration/gemini/README.md

Gemini (Nano Banana) visible sparkle capture

Status (captured 2026-05-31): the required black capture is committed here. The sparkle-on-black assets gemini_bg_{96,48}.png are rebuilt by scripts/visible_alpha_solve.py gemini. They support detection and footprint geometry. Current pixel recovery uses the shared fill backend, not reverse-alpha inversion.

Google Gemini (Nano Banana) stamps a four-point sparkle icon in the bottom-right corner via alpha compositing: watermarked = a*logo + (1-a)*original. Unlike the Doubao/Jimeng text marks, the sparkle is captured over a pure-black background, where watermarked = a*255 (logo is near-white), so the alpha reads directly off the capture (alpha = max(R,G,B)/255) -- no background fit needed. This is the "golden" capture case.

What the captures confirmed (2026-05-31)

• The sparkle at a 2048-wide image is 96x96 px (width_frac ~0.047), bottom-right, margins ~0.031, alpha max ~0.51 -- matching the engine's existing 96px asset.
• Our own controlled capture matches the previously third-party-sourced gemini_bg_96.png to NCC 0.9998, so the bundled asset is validated and now reproducible from our own capture.

How to capture (image-edit path)

For each locally generated black, gray, or white 2048 px seed:

1. Open Gemini image generation, image-edit / reference mode, upload the seed.
2. Prompt: Recreate this image exactly as it is, keep it identical, do not add or change anything
3. Download the ORIGINAL output (not a screenshot). Do not crop / edit / re-save.

Black is the key one (sparkle on black -> exact alpha). Gray/white cross-check.

Committed input

gemini_black_2048.png   # the key capture (sparkle on black)

The gray and white captures were calibration cross-checks and are not needed to rebuild the assets. Synthetic seeds and real-content validation downloads stay local. Rebuild the assets with:

uv run python scripts/visible_alpha_solve.py gemini   # or: all