str8

Modern straight-skeleton computation for the browser and Node, powered by CGAL's Straight_skeleton_2 package compiled to WebAssembly.

This is a from-scratch rebuild of the idea behind StrandedKitty/straight-skeleton, using CGAL 6.1, Emscripten 4.x (embind), and a modern ESM bundle.

What it computes

The straight skeleton of a simple polygon (with optional holes): the set of edges traced by the polygon's edges as they move inward at constant speed. Each skeleton vertex carries a time value — the distance the wavefront travelled to reach it — which doubles as a roof height.

str8 computes:

• Interior and exterior straight skeletons.
• Offset contours — inset (erode) or outset (dilate) a polygon by a distance, derived from the skeleton.

▶ Live demos

Install

npm install @matthewjacobson/str8

The published package is a single ESM file with the WASM embedded (base64), so there's nothing extra to host or configure.

Usage

import { init, buildFromPolygon } from '@matthewjacobson/str8';

await init(); // load the WASM module once

const skeleton = buildFromPolygon([
  // outer ring (winding order doesn't matter — it's normalized for you)
  [[-1, -1], [1, -1], [1, 1], [-1, 1]],
  // optional holes follow...
]);

if (skeleton) {
  // vertices: flat Float32Array of [x, y, time] triples
  for (let i = 0; i < skeleton.vertices.length; i += 3) {
    const [x, y, time] = skeleton.vertices.subarray(i, i + 3);
  }
  // faces: one entry per skeleton face, each a list of vertex indices
  for (const face of skeleton.faces) {
    // face = [i0, i1, i2, ...] indices into vertices
  }
}

GeoJSON

import { init, buildFromGeoJSON } from '@matthewjacobson/str8';

await init();
const results = buildFromGeoJSON({
  type: 'Polygon',
  coordinates: [[[0, 0], [4, 0], [4, 4], [0, 4]]],
});
// results: (Skeleton | null)[] — one per polygon (MultiPolygon supported)

API

| Export | Description | | --- | --- | | init(): Promise<void> | Loads and instantiates the WASM module. Await once before building. | | isReady(): boolean | Whether the module is loaded. | | buildFromPolygon(rings, opts?): Skeleton \| null | Interior skeleton of a polygon. Ring 0 is the outer boundary; the rest are holes. | | buildFromGeoJSON(geometry, opts?): (Skeleton \| null)[] | Interior skeletons from a GeoJSON Polygon / MultiPolygon. | | buildExteriorSkeleton(rings, { maxOffset }): Skeleton \| null | Exterior skeleton, framed maxOffset beyond the polygon. | | offsetPolygon(rings, distance, opts?): OffsetPolygon[] \| null | Inset (default) or outset ({ exterior: true }) offset contours. |

interface Skeleton {
  vertices: Float32Array; // [x, y, time, ...]
  faces: number[][];      // vertex indices, one list per face
}

interface OffsetPolygon {
  outer: Float32Array;    // [x, y, ...]
  holes: Float32Array[];  // each [x, y, ...]
}

An inset can split into several pieces or vanish past the polygon's max inradius, so offsetPolygon returns an array of contours (possibly empty). All builders accept { forceExact: true } to skip the fast kernel (see below).

Input handling (modernizations over the original)

• Winding order is normalized automatically — outer ring forced CCW, holes CW. You don't have to pre-orient your data.
• Open or closed rings both work — a duplicated closing vertex is dropped.
• Degenerate input (fewer than 3 vertices, empty) returns null instead of aborting the WASM module.

Robustness: automatic exact fallback

CGAL's straight skeleton uses the fast inexact-constructions kernel (EPICK) by default. Highly symmetric inputs — e.g. a grid of identical, evenly-spaced holes — create many simultaneous wavefront events that EPICK's rounded arithmetic can't resolve consistently, and the computation fails.

str8 handles this automatically: if the fast kernel fails, it retries with the exact-constructions kernel (EPECK), which is slower but robust. You normally don't have to think about it. To skip the fast attempt and go straight to exact:

buildFromPolygon(rings, { forceExact: true });
buildFromGeoJSON(geometry, { forceExact: true });

Building from source

Requires the Emscripten SDK and CGAL + Boost headers (e.g. brew install cgal boost on macOS).

# 1. compile the CGAL core to a single-file ES module (src/core/skeleton.js)
source /path/to/emsdk/emsdk_env.sh
npm run build:wasm

# 2. bundle the TS wrapper + emit types -> dist/
npm run build

# or both at once:
npm run build:all

If your headers aren't in /opt/homebrew/include, set DEPS_INCLUDE_DIR:

DEPS_INCLUDE_DIR=/usr/local/include npm run build:wasm

Examples

Static demos live in example/ — serve the repo root over HTTP (e.g. npx serve or python3 -m http.server) and open them:

• example/index.html — a gallery with a dropdown of 28 sample polygons (from LingDong-/interesting-polygon-archive), with pan, zoom, and live vertex/hole/face stats.
• example/geojson.html — paste your own GeoJSON (Polygon, MultiPolygon, Feature, FeatureCollection, or a bare coordinates array) and see its skeleton, with pan/zoom and a Y-flip toggle for screen-coordinate data.
• example/roof.html — 3D straight-skeleton roofs: the same sample polygons lifted into roofs where each vertex's height is its wavefront time, rendered with three.js (orbit, adjustable pitch). three.js is loaded from a CDN, so this page needs network access.
• example/offset.html — offsets & exterior skeletons: inset/outset offset contours (with concentric stepping) and interior/exterior straight skeletons, driven by a distance slider, with pan/zoom.

They import the built dist/str8.js, so run npm run build first.

How it works

• src/core/skeleton.cpp — CGAL create_interior_straight_skeleton_2 over a Polygon_with_holes_2 (EPICK kernel), exposed via embind. Returns a Float32Array of vertices and a number[][] of faces directly as JS objects.
• Built with -sMODULARIZE -sEXPORT_ES6 -sSINGLE_FILE so the WASM is embedded in an ES module. GMP/MPFR are disabled (CGAL_DISABLE_GMP) so nothing native needs cross-compiling.
• src/index.ts — the public API: orientation normalization, GeoJSON helpers, and the WASM lifecycle.

License

MIT. CGAL is used under its own license (GPL/LGPL depending on package); Straight_skeleton_2 is GPL — review CGAL's licensing for your use case.