椭圆的近似图形：72边形

def update_points(self):
        """ 椭圆的近似图形：72边形 """
        n = max(8, min(72, int(2*sqrt(self.r_x+self.r_y))))
        d = pi * 2 / n
        x, y, r_x, r_y = self.x, self.y, self.r_x, self.r_y

        ps = []
        for i in range(n):
            ps += [(x + r_x * sin(d * i)), (y + r_y * cos(d * i))]
        self.points = tuple(ps)