Dear Fu-Kwun Hwang!
I really interested in phisics generally, and your Billiard sample in particular. I was working on a project about ball collision, but it was quite difficult to find any simple examples, scientific literature or detailed articles about 2D-collision. I found your article very useful and clear for beginners, as well as for phisics/sciense interested people. Thank you for your help, I really moved forward in my work.
With best wishes