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Add watershed clustering: hierarchical watershed frontend

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An alternative region-forming frontend selected by --clustering watershed
(the color_mode field is replaced by clustering: color-cluster | bw |
watershed across CLI, Rust, Python, and Node — it selects the algorithm,
not a color space).

The algorithm is the watershed hierarchy by volume on the 4-adjacency
pixel graph, implemented from the papers:

  Cousty, Bertrand, Najman, Couprie, "Watershed Cuts: Minimum Spanning
  Forests and the Drop of Water Principle", IEEE TPAMI 31(8), 2009.
  Najman, Cousty, Perret, "Playing with Kruskal", ISMM 2013.

Edge weights are the max per-channel color difference between adjacent
pixels (no gradient image); a counting-sorted Kruskal pass builds the
binary partition tree as a flat parents array; a leaves-to-root pass
computes subtree area and volume; each merge's persistence (the volume of
the smaller side, plateau-corrected) becomes its MST edge's saliency; and
cutting the hierarchy is single-linkage over MST edges below the cut
level. Watershed cuts label every pixel — no watershed-line pixel class —
so the output is a strict, gapless partition that drops straight into
both stacked and cutout modes. Integer arithmetic and flat u32 arrays
throughout; deterministic across platforms; ~66 ms on a 1400x775 photo.

The one dial, --watershed-detail (0..=255), maps exponentially to a
target region count (each +25.5 doubles it) since the persistence
distribution is far too skewed for a linear threshold. filter_speckle
absorbs undersized basins into their most color-similar neighbour rather
than dropping them, preserving the partition. The largest region is
emitted first as a solid full-canvas background layer so stacked mode
keeps its seam-free overdraw; the mosaic flatten is unaffected.

Tests: partition invariant (disjoint masks tiling the canvas), detail
monotonicity, min-area absorption, determinism, watershed cases in the
pipeline/golden suites, stacked-vs-cutout interior equivalence, a
watershed seam test, and SegmentKey coverage for the new params.
This commit is contained in:
Chris Tsang
2026-07-27 14:45:25 +01:00
parent f4872fcfe3
commit 46a1b90ccd
21 changed files with 854 additions and 67 deletions
Download Patch File Download Diff File Expand all files Collapse all files
nodejs/index.d.ts
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+5 -2
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@@ -2,7 +2,8 @@
export interface Options {
/** Applied before other fields: "bw" | "poster" | "photo". */
preset?: 'bw' | 'poster' | 'photo';
colorMode?: 'color' | 'bw';
/** Region forming: hierarchical color clustering (default), binary threshold, or watershed. */
clustering?: 'color-cluster' | 'bw' | 'watershed';
hierarchical?: 'stacked' | 'cutout';
mode?: 'pixel' | 'polygon' | 'spline';
filterSpeckle?: number;
@@ -19,7 +20,7 @@ export interface Options {
maxColors?: number;
/** 0 = off, 1 = quantize+simplify, 2 = + shorthands/grouping. */
optimize?: number;
/** Binary mode (`colorMode: 'bw'`): fixed threshold 0..=255; foreground when intensity is below it. */
/** Binary mode (`clustering: 'bw'`): fixed threshold 0..=255; foreground when intensity is below it. */
binaryThreshold?: number;
/** Binary mode: use Bradley–Roth adaptive thresholding (handles uneven lighting). */
adaptive?: boolean;
@@ -27,6 +28,8 @@ export interface Options {
adaptiveWindow?: number;
/** Adaptive sensitivity: percent below the local mean (default 15). */
adaptiveT?: number;
/** Watershed clustering: hierarchy cut level 0..=255 (higher = more regions, default 128). */
watershedDetail?: number;
}
/** Vectorize an encoded image (PNG/JPEG/GIF/BMP) buffer to an SVG string. */