A simple parallax with DragonRuby

So this is my first try at DragonRuby. I tried to make a parallax, which I already did for a game with CSFML.

The parallax code is in lib/parallax.rb, and the code which controls it in app/main.rb. Other files control the two arrows you see on both sides of the screen; which you don't need if you just want to learn about the parallax.

Show me the simplest working parallax

This code got a little tricky for a beginner, but you can still access one of the first versions of the code. A single file parallax of less than twenty lines which just works.

Credits

Images were found randomly on the web, they were free to use but I think they need to be in the credits.

Screenshot