geotoolbox/src/concavehull.js

geotoolbox is GIS javascript library. It is based on d3geo, topojson and geos-wasm.

import { geosloader } from "./helpers/geos.js";
import { geojsonToGeosGeom, geosGeomToGeojson } from "geos-wasm/helpers";
import { check } from "./helpers/check.js";

/**
 * @function concavehull
 * @summary Returns a "concave hull" of a geometry. A concave hull is a polygon which contains all the points of the input, but is a better approximation than the convex hull to the area occupied by the input. Frequently used to convert a multi-point into a polygonal area. that contains all the points in the input Geometry.
 * @description Based on `geos.GEOSConcaveHull()`.
 * @async
 * @param {object|array} data - A GeoJSON FeatureCollection, an array of features, an array of geometries, a single feature or a single geometry.
 * @param {object} options - Optional parameters.
 * @param {boolean} [options.ratio = 0] - The edge length ratio value, between 0 and 1.
 * @param {boolean} [options.holes = true] - When non-zero, the polygonal output may contain holes.
 * @returns {object|array} - A GeoJSON FeatureCollection, an array of features, an array of geometries, a single feature or a single geometry (it depends on what you've set as `data`).
 * @example
 * await geotoolbox.concavehull(*a geojson*, {ratio: 0.5})
 */

export async function concavehull(data, { ratio = 0, holes = true } = {}) {
  const geos = await geosloader();
  const handle = check(data);
  let x = handle.import(data);
  const geosgeom = geojsonToGeosGeom(x, geos);
  const output = geos.GEOSConcaveHull(geosgeom, ratio, holes ? 1 : 0);
  let result = geosGeomToGeojson(output, geos);
  geos.GEOSFree(geosgeom);
  geos.GEOSFree(output);

  return handle.export({
    type: "FeatureCollection",
    name: "concavehull",
    features: [{ type: "Feature", properties: {}, geometry: result }],
  });
}