function motion(radius) {
    // curvity: 曲率，值越大越接近圆的曲率，上图的curvity： 0.1
    var i = 0, j = curvity = 0.05;
    
    return function() {
        i = i + j;

        var r = radius*Math.pow(i,0.5),
            ang = 36,
            x = r*Math.sin(i),
            y = r*Math.cos(i);

        i<0 && (j = curvity);
        i>ang && (j = -curvity);

        return [x,y];
    }
}