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Add a curve-simplification stage (--simplify), paper.js style

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A new pipeline slot between curve fitting and composition: CurvePasses
rewrite each fitted contour, so mosaic mode transforms every shared
boundary segment exactly once and the tessellation stays seam-free by
construction. SimplifyCurves re-fits each smooth run between corners
with the fewest cubics within the tolerance (Schneider's algorithm via
a current flo_curves — visioncortex's copy is pinned to an old one and
block-splits at 200 points), with tangents from the chain's own ends,
corners kept in place, junction endpoints pinned bit-for-bit, and rings
seamed at their sharpest junction. Off by default; polylines pass
through untouched. Cityscape at tolerance 1: 229 -> 138 KB stacked,
103 -> 36 KB watershed cutout, with render diffs under golden noise.

CurveFitter now returns Vec<FittedGeom> (promoted from mosaic::fit) so
stacked contours flow through the same pass machinery; the optimizer's
SimplifyPass is renamed CleanupPass to free the word.
This commit is contained in:
Chris Tsang
2026-07-27 22:30:05 +01:00
parent d585984e78
commit ef9496f792
18 changed files with 742 additions and 80 deletions
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CHANGELOG.md
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@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
* Cutout mode merges neighbouring mosaic regions whose colors are within one gradient step — the flattened tessellation no longer keeps the near-identical faces that stacked gradient layering splits a smooth area into.
* Watershed clustering (`--clustering watershed`): an alternative region-forming frontend — a hierarchical watershed by volume on the pixel graph (Cousty et al., TPAMI 2009; Najman, Cousty & Perret, ISMM 2013), cut at a single `--watershed-detail` dial (0..=255, each +25.5 roughly doubles the region count). Content-adaptive regions with no watershed-line pixels; antialiased boundary pixels snap to the color-midpoint iso-line, so edges come out as calm as the color-cluster frontend's instead of meandering with the pixel noise inside the ramp. With `cutout` the partition reaches the mosaic natively, and near-identical neighbouring faces merge within a detail-derived tolerance (`max(2, (255 − detail) / 8)`: the color-cluster default gradient step at the default detail, and never less than a just-noticeable difference — faces a human cannot tell apart never survive as separate patches); with `stacked` the merge tree itself is the stack — coarse ancestors below, refined regions on top, the same principle as color clustering — so sub-pixel gaps show ancestor colors and overdraw stays seam-free.
* `WatershedHierarchy` is public and split into `build` (expensive, depends only on the image) and `cut` (near-instant): `Session` builds it once and re-cuts on every `watershed_detail`/`filter_speckle` change, making the detail slider fully interactive (~25 ms re-cut vs ~40 ms rebuild on a 1400×775 photo).
* Curve simplification (`--simplify <tolerance>`, `Config::simplify`; off by default): a paper.js-style Schneider re-fit of the fitted splines — each smooth run between corners is re-fitted with the fewest cubics that stay within the tolerance (px), cutting anchor counts and file size (the 1400×775 sample photo: 229 → 138 KB stacked, 103 → 36 KB watershed cutout at tolerance 1). Implemented as a new pipeline stage (`CurvePass`) that runs on fitted geometry *before* composition, so mosaic mode simplifies each shared boundary exactly once and the tessellation stays seam-free; corners survive in place, junction endpoints are pinned bit-for-bit, and pixel/polygon polylines pass through untouched.
### Changed