Hatched polygons with https://thi.ng/umbrella

hatching.ts

import {
	asSvg,
	bounds,
	circleFrom2Points,
	group,
	GroupAttribs,
	line,
	Polygon,
	starWithCentroid,
	svgDoc,
} from "@thi.ng/geom";
import { clipLinePoly } from "@thi.ng/geom-clip-line";
import { TAU } from "@thi.ng/math";
import {
	add2,
	cartesian2,
	maddN2,
	normalize2,
	perpendicularCW,
	sub2,
} from "@thi.ng/vectors";
import { writeFileSync } from "fs";

/**
 * Creates a group of hatch lines for given polygon. The direction of the hatch
 * lines is defined by `theta` (in radians) and their density/distance by
 * `step`.
 *
 * @param attribs
 * @param shape
 * @param theta
 * @param step
 */
const hatch = (attribs: GroupAttribs, shape: Polygon, theta = 0, step = 1) => {
	const res = group(attribs);
	const b = bounds(shape)!;
	if (!b) return res;
	// bounding circle of bounding box
	const bc = circleFrom2Points(b.min(), b.max());
	// hatch dir
	const dir = cartesian2(null, [bc.r, theta]);
	// hatch normal (ray direction)
	const norm = normalize2(null, perpendicularCW([], dir));
	// ray start point
	const start = maddN2([], norm, -bc.r, bc.pos);
	for (let d = 0, maxD = bc.r * 2; d <= maxD; d += step) {
		// next point on ray
		const m = maddN2([], norm, d, start);
		// intersect line segment with poly
		const segments = clipLinePoly(
			sub2([], m, dir),
			add2([], m, dir),
			poly.points
		);
		if (!segments) continue;
		// add segments to group
		for (let s of segments) res.add(line(s));
	}
	return res;
};

const poly = starWithCentroid([50, 50], 50, 6, [0.5, 1]);

const geo = group({}, [
	poly,
	hatch({ stroke: "#f00" }, poly, 0),
	hatch({ stroke: "#0f0" }, poly, (1 / 3) * TAU),
	hatch({ stroke: "#09f" }, poly, (2 / 3) * TAU),
]);

writeFileSync("export/hatching.svg", asSvg(svgDoc({ weight: 0.1 }, geo)));