The 256->512 detection-search widening (v0.8) let a large, low-gradient
shape match outrank a genuine mid-size corner sparkle whose raw NCC sits
below the 0.85 corner-promote gate, so `identify` read `unknown` on Gemini
images that v0.7.2 caught (reporter osachub: scale-48 sparkle on light
bedding -- true sparkle spatial 0.775 / grad 0.960 / fusion 0.676, but the
size-weighted argmax locked onto a decoy at spatial 0.628 / grad 0.036).
detect_watermark now keeps the top-K (_SELECT_TOPK=3) size-weighted
candidates (NMS-deduped) plus the corner-promote candidate, scores each by
full fusion (spatial+gradient+variance) via the extracted _grad_var_scores
helper, and selects the highest -- the gradient term lifts the true sparkle
over the decoy. Ranking by the SIZE-WEIGHTED score (not a raw-NCC argmax)
preserves tiny-patch suppression: a raw-NCC argmax re-admitted 16-18px
content false positives (14/65 doubao + 4/11 jimeng visible images). Top-K
adds zero flips on the doubao/jimeng corpora and leaves the 495-image Gemini
set unchanged (479 detected) while recovering the reporter's image at 0.676.
- _grad_var_scores: gradient/variance scoring factored out of detect_watermark
- confidence = best_fused (drop the duplicated fusion recompute)
- tests: rename test_promotion_is_what_rescues_it ->
test_size_weighted_search_alone_traps_on_the_decoy (corner-promote is no
longer the sole rescue path); add a deterministic regression test mirroring
the real spatial/grad signature
- docs: module-internals.md detector section + CLAUDE.md mechanism map
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Victor Kuznetsov