j = boundary(x,y,0.1); %这个函数是求一堆(平面)点的边界，它不是凸包，这里的边界可以凹陷，第三个参数0.1代表以松散的形式画出边界，1是以最紧凑的形式画出边界。

 下面这个是官网的例子：(http://www.mathworks.com/help/matlab/ref/boundary.html)

Create and plot a set of random 2-D points.

x = gallery('uniformdata',30,1,1);y = gallery('uniformdata',30,1,10);plot(x,y,'.')xlim([-0.2 1.2])ylim([-0.2 1.2])

k = boundary(x,y);hold on;plot(x(k),y(k));

j = boundary(x,y,0.1);hold on;plot(x(j),y(j));