I'm new to Processing, but I've been experimenting a bit and I've ran into somewhat of a strange problem. I was trying to build a simple house with windows, but as I continued building I noticed that only certain things are visible trough the window. I made a video to demonstrate my problem:

http://www.youtube.com/watch?v=KIXHWZF9bw8

Obviously, the black cube should be visible trough the window. I can't share the source code, since I'll be using this code for a school project and I don't want my classmates to copy it. 

Thanks for taking the time to read/watch this. If you have any idea how to solve this, I'd love to hear it!