Colour-drifted background trapped between hair strands defeats every
single-signal defence: the chroma key reads it as foreground (bgc ~0.07),
the segmenter backs it, ViTMatte rates it opaque, and post-hoc removal is
a proven dead end (it shreds the hair volume the same pixels belong to).
A second, trimap-free matting model (BiRefNet_HR-matting) is the only
tested model that separates this residue from the subject, so its opinion
is fused in as a veto: min-fusion that may only LOWER alpha, restricted to
the background-hued bright suspect zone (proj >= 3, L >= 45, feathered)
and gated by primary-alpha confidence (0.70 -> 0.95 ramp) so soft wisps
and dark hair are exempt by construction.
- settings/config/CLI: cross_check block, --cross-check/--no-cross-check
- alpha_post.cross_check_alpha after clean_alpha; second opinion reuses
BiRefNetSegmenter; saved to debug as cross_check_alpha.png
- chroma.bg_hue_projection extracted and shared with the trimap
- docs: methodology.md (new), hair_gap_artifacts.md (investigation log)
Verified: cross-check ON reproduces the visually-reviewed B1gate
prototype byte-for-byte on TestImage3; --no-cross-check reproduces the
previous baseline byte-for-byte; pink-bg FixImage1 face untouched.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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