Parallax Scrolling Demo in Lisp HD

A while ago when I was looking into creating a Defender Game Remake I came across a great tutorial on parallax scrolling. Unfortunately the tutorial was for Flash, so I ported it over to Common Lisp. The port is a simplified version of the tutorial using structs instead of classes. The source code is available for download: https://github.com/sebity/parallax-demo The original tutorial is available here: http://code.tutsplus.com/tutorials/add-depth-to-your-game-with-parallax-scrolling--active-2134