鞋带公式，根据顶点计算多边形面积

    // 鞋带公式 计算面积 https://www.zhihu.com/question/58639959/answer/159924222
    getArea(points){
        if(points.length<3) return 0;
        let a =points[points.length-1].x*points[0].y;
        let b =points[0].x*points[points.length-1].y;
        for (let i = 0; i < (points.length-1); i++){
           a+=points[i].x*points[i+1].y;
           b+=points[i+1].x*points[i].y;
        }
        return Math.abs(a-b)/2;
    }