terriajs/lib/Map/computeRingWindingOrder.js

Geospatial data visualization platform.

'use strict';

const WindingOrder = require('terriajs-cesium/Source/Core/WindingOrder');
const DeveloperError = require('terriajs-cesium/Source/Core/DeveloperError');

/**
 * Determine the winding order of a polygon ring.
 * See https://github.com/mapbox/vector-tile-spec/tree/master/2.0#4344-polygon-geometry-type && https://en.wikipedia.org/wiki/Shoelace_formula
 * @param  {Point[]} ring The polygon ring as an array of '@mapbox/point-geometry' Points (or any points conforming to {x: number, y: number}).
 * @return {WindingOrder} The winding order of the polygon ring.
 */
function computeRingWindingOrder(ring) {
    const n = ring.length;
    let twiceArea = ring[n-1].x * (ring[0].y - ring[n-2].y) + ring[0].x * (ring[1].y - ring[n-1].y);
    for (let i = 1; i <= n-2; i++) {
        twiceArea += ring[i].x * (ring[i+1].y - ring[i-1].y);
    }
    if (isNaN(twiceArea)) {
        throw new DeveloperError('Expected points of type {x:number, y:number}');
    }
    return twiceArea > 0.0 ? WindingOrder.COUNTER_CLOCKWISE : WindingOrder.CLOCKWISE;
}

module.exports = computeRingWindingOrder;