Left-side book fold shadows have a V-shape: brightness dips from the
edge toward a peak at ~5-10% of width, then rises again. The previous
algorithm scanned from the edge inward and immediately found a low
dark fraction (0.13 at x=0), missing the gutter entirely.
Now finds the PEAK of the dark fraction profile first, then scans from
that peak toward the page center to find the transition point. Works
for both V-shaped left gutters and edge-darkening right gutters.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Scanner shadow detection (range > 40, darkest < 180) fails on camera
book scans where the gutter shadow is subtle (range ~25, darkest ~214).
New _detect_gutter_continuity() detects gutters by their unique property:
the shadow runs continuously from top to bottom without interruption.
Divides the image into horizontal strips and checks what fraction of
strips are darker than the page median at each column. A gutter column
has >= 75% of strips darker. The transition point where the smoothed
dark fraction drops below 50% marks the crop boundary.
Integrated as fallback between scanner shadow and binary projection.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
1. page_crop: Score all dark runs by center-proximity × darkness ×
narrowness instead of picking the widest. Fixes ad810209 where a
wide dark area at 35% was chosen over the actual spine at 50%.
2. cv_words_first: Replace x-center-only word→column assignment with
overlap-based three-pass strategy (overlap → midpoint-range → nearest).
Fixes truncated German translations like "Schal" instead of
"Schal - die Schals" in session 079cd0d9.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
The _detect_spine_shadow function was triggering on normal text content
because shadow_range > 20 was too low and convolution edge artifacts
created artificially low values. Now requires: range > 40, darkest < 180,
narrow valley (not text plateau), and brightness rise toward page content.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Refactor left/right shadow detection into shared _detect_spine_shadow()
that finds the darkest column (= book spine center) via argmin of
smoothed brightness. Both sides now cut at the spine center, ensuring
equal page sizes in double-page scans regardless of shadow position.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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>
