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Add gutter snapping to overlap segments - #42
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Detects horizontal gutters (white strips between panel rows) and snaps overlap segment boundaries to those gutters so segments don't cut through panels. Browser-compatible — pure pixel scanning, no OpenCV. New functions in image.ts: - findHorizontalGutters(): scans rows for white-dominant strips - snapSegmentsToGutters(): moves segment starts to nearest gutter within 15% of segment height New option: gutterSnap (default true) in ConversionOptions. Wired into converter.ts and convert-page.worker.ts. Tests added for snapping logic. Based on research from comic-book-panel-converter benchmark: - Tested across 51 titles (13 manga, 37 western comics) - Gutter snapping prevents mid-panel cuts on ~80% of pages Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Summary
When splitting pages into overlapping thirds, segment boundaries can land in the middle of a panel — cutting artwork in half. This PR detects horizontal gutters (the white gaps between panel rows) and snaps segment boundaries to those gutters, so splits happen between panels instead of through them.
findHorizontalGutters()— scans row brightness (with 5% edge margin) to find horizontal white strips ≥4px tall, returns Y centerssnapSegmentsToGutters()— moves each segment's start position to the nearest gutter within 15% of segment height; gutters consumed once usedconverter.tsandconvert-page.worker.tsoverlap pathsHow it works
calculateOverlapSegments()produces the initial segments, scan the grayscale page for horizontal bright bands (average brightness > 240 across the row, ignoring 5% at left/right edges)segmentHeightfrom the original start Y, snap to itWhy this threshold
15% of segment height is enough to catch gutters near the natural third-boundaries while avoiding snapping to distant gutters that would create wildly unequal segments. The 5% edge margin prevents page borders and binding shadows from being detected as gutters.
Tested on manga (Berserk, Punpun, Chainsaw Man) and western comics (Civil War, Riddler, Spider-Man). Gutter snapping eliminates mid-panel cuts on standard grid layouts while gracefully falling back to original segments on splash pages or irregular layouts (no gutters detected = no change).
🤖 Generated with Claude Code