获取点线面中心

/**
 * 获取点中心点
 * @param {CoordinateType} type - 坐标类型
 * @param {Projection} projection - 坐标投影转换
 * @return {number[]} 中心点坐标
 */
getCenter(type: CoordinateType = CoordinateType.Latlng, projection: Projection = new WebMercator()) {
    if (!this._projected) this.project(projection);
    if (type === CoordinateType.Latlng) {
        return [this._lng, this._lat];
    } else {
        return [this._x, this._y];
    }
}
/**
 * 获取线的中心点
 * @remarks
 * from Leaflet
 * @param {CoordinateType} type - 坐标类型
 * @param {Projection} projection - 坐标投影转换
 * @return {number[]} 中心点坐标
 */
getCenter(type: CoordinateType = CoordinateType.Latlng, projection: Projection = new WebMercator()) {
    if (!this._projected) this.project(projection);
    let i, halfDist, segDist, dist, p1, p2, ratio,
        points = this._coordinates,
        len = points.length;

    if (!len) { return null; }

    // polyline centroid algorithm; only uses the first ring if there are multiple

    for (i = 0, halfDist = 0; i < len - 1; i++) {
        halfDist += Math.sqrt((points[i + 1][0] - points[i][0]) * (points[i + 1][0] - points[i][0]) + (points[i + 1][1] - points[i][1]) * (points[i + 1][1] - points[i][1])) / 2;
    }

    let center;
    // The line is so small in the current view that all points are on the same pixel.
    if (halfDist === 0) {
        center = points[0];
    }

    for (i = 0, dist = 0; i < len - 1; i++) {
        p1 = points[i];
        p2 = points[i + 1];
        segDist = Math.sqrt((p2[0] - p1[0]) * (p2[0] - p1[0]) + (p2[1] - p1[1]) * (p2[1] - p1[1]));
        dist += segDist;

        if (dist > halfDist) {
            ratio = (dist - halfDist) / segDist;
            center = [
                p2[0] - ratio * (p2[0] - p1[0]),
                p2[1] - ratio * (p2[1] - p1[1])
            ];
        }
    }

    if (type === CoordinateType.Latlng) {
        return projection.unproject(center);
    } else {
        return center;
    }
}
/**
 * 获取面的中心点
 * @remarks
 * from Leaflet
 * @param {CoordinateType} type - 坐标类型
 * @param {Projection} projection - 坐标投影转换
 * @return {number[]} 中心点坐标
 */
getCenter(type: CoordinateType = CoordinateType.Latlng, projection: Projection = new WebMercator()) {
    if (!this._projected) this.project(projection);
    let i, j, p1, p2, f, area, x, y, center,
        points = this._coordinates[0],
        len = points.length;

    if (!len) { return null; }

    // polygon centroid algorithm; only uses the first ring if there are multiple

    area = x = y = 0;

    for (i = 0, j = len - 1; i < len; j = i++) {
        p1 = points[i];
        p2 = points[j];

        f = p1[1] * p2[0] - p2[1] * p1[0];
        x += (p1[0] + p2[0]) * f;
        y += (p1[1] + p2[1]) * f;
        area += f * 3;
    }

    if (area === 0) {
        // Polygon is so small that all points are on same pixel.
        center = points[0];
    } else {
        center = [x / area, y / area];
    }

    if (type === CoordinateType.Latlng) {
        return projection.unproject(center);
    } else {
        return center;
    }
}