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Victor Kuznetsov
41f67973ce
The free `visible` path over-subtracted a faint Gemini sparkle on a mid-tone background into a darker-than-background brown diamond instead of removing it (2026-06-18 prod NPS report, "the watermark was not removed, just its color changed"). The existing over-subtraction guard only tripped when reverse-alpha drove a footprint pixel fully negative (the issue #30 dark-background black-pit case); on a mid-tone background the over-subtraction darkens the core well below the background without any pixel crossing zero, so the gate missed it and shipped the dark mark. Add a second over-subtraction signal to `_reverse_alpha_oversubtracts`: predict the reverse-alpha output at the bright core, (core - a*logo)/(1-a), and route to the footprint inpaint when it lands more than `_OVERSUB_DARK_MARGIN` (25) gray levels below the local background ring. Calibrated wide: clean removals predict within ~12 of background (demo_banana ~-1), the prod regression ~-40, the issue #30 dark case ~-82. Corpus-validated on the 479 detected Gemini images: 10 switch reverse-alpha to inpaint, all of them dark-diamond cases that improve or match; the other 469 stay byte-identical. demo_banana stays on the reverse-alpha path (byte-identical). Also crop both reverse-alpha helpers to the region they actually touch, a pure O(image) -> O(mark) win that is byte-identical to the full-frame math (a uint8<->float32 round-trip is exact): - `GeminiEngine._core_and_bg` converts only the footprint+ring crop to gray, not the whole frame (~70 ms -> 0.1 ms on a 12 MP image; it runs for both the alpha-gain estimate and the new gate). Verified identical across 479 images; detector confidence unchanged. - `TextMarkEngine._apply_reverse_alpha` computes the blend on the glyph crop only (`amap` is zero outside it, so the math is a no-op there): ~275 ms -> ~2 ms per placement on a 12 MP frame, up to 2 placements per removal. Verified identical across 142 Doubao/Jimeng placements. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>