埃及的牛人写的.. Imagine that you have a piece of wood and a number of metal pins come out from it randomly (as in the above image), and you picked up an elastic rubber band and open it wide with your hand to contain all the metal pins and let it go. You will observe that the elastic rubber band will shrink and snap around the metal pins (only the outer metal pins). These outer segments of the elastic band form the convex hull.