thi-ng/umbrella

import { centroid } from "@thi.ng/geom-poly-utils/centroid";
import { mixN } from "@thi.ng/vectors/mixn";
import type { Tessellator } from "./api.js";

export const inset =
    (inset = 0.5, keepInterior = false): Tessellator =>
    (tess, faces, pids) => {
        const points = tess.pointsForIDs(pids);
        const c = centroid(points);
        const insets = tess.addPoints(points.map((p) => mixN([], p, c, inset)));
        const n = pids.length - 1;
        for (let i = 0; i <= n; i++) {
            const j = i < n ? i + 1 : 0;
            faces.push([pids[i], pids[j], insets[j], insets[i]]);
        }
        if (keepInterior) faces.push(insets);
        return faces;
    };