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Victor Kuznetsov
33fddbc6fa
Port the Gemini sparkle dark-pit guard (commit 41f6797) to the shared
TextMarkEngine reverse-alpha base (roadmap P0#8): on a dark or mid-tone
background the captured alpha can over-estimate this image's mark opacity, and
reverse-alpha leaves a darker-than-background glyph ghost instead of recovering
the true pixels. The sparkle-only fix left the text marks unhandled.
_reverse_alpha_oversubtracts predicts the reverse-alpha output PER PIXEL over the
glyph body from the INPUT ((obs - a*logo)/(1-a), the remover's own math); when
the predicted body lands more than _OVERSUB_DARK_MARGIN (25) gray levels below
the local background ring it abandons the reverse-alpha output for the footprint
and inpaints it from the original surroundings (_inpaint_footprint, wider dilate/
radius than the thin residual pass). Predicting per-pixel from the input (not the
produced output, which depends on which placement the remover picked) keeps a
cleanly captured full-strength mark byte-identical -- it predicts back to the
background everywhere, so the guard never trips on it (verified across all three
engines on white/mid/dark/midgray backgrounds).
Regression-guarded by tests/test_text_mark_oversubtraction.py: predicate True on
faint / False on clean, end-to-end no-dark-pit acceptance, clean-mark byte
identity, and textured-background footprint recovery.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>