Mirror the left-edge shadow detection for the right side: analyze
brightness gradient in the right 25% to find scanner gray strips
from book spines. Cuts at the last bright column before the shadow
dip. Fixes cropping of book scans where the next page bleeds in.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
The previous algorithm used binary ink projection and found false
splits at normal text column gaps. The spine of a book on a scanner
has a characteristic DARK gray strip (scanner bed) flanked by bright
white paper on both sides.
New approach: column-mean brightness with heavy smoothing, looking for
a dark valley (< 88% of paper brightness) in the center region that
has bright paper on both sides.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Backend: merge gaps within 5% of image width — the spine area may have
thin ink strips splitting one physical gap into multiple detected gaps.
Only use gaps >= 2% width as split points.
Frontend: StepCrop now handles multi_page crop responses without
crashing on missing original_size/cropped_size fields.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
When a book scan (double-page spread) is detected during the crop step,
the system automatically:
1. Detects vertical center gaps (spine area) via ink density projection
2. Splits into N page sub-sessions (reusing existing sub-session mechanism)
3. Individually crops each page (removing its own borders)
4. Returns sub-session IDs for downstream pipeline processing
Detection: landscape images (w > h * 1.15), vertical gap < 15% peak
density in center region (25-75%), gap width >= 0.8% of image width.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
