Kschnee: I've also been using Lode and I also had a fisheye problem. But I solved it with an extra pair of (). Try it out. If I can solve it – so can you. Trust me about that one ;)
http://www.permadi.com/tutorial/raycast/
http://student.kuleuven.be/~m0216922/CG/raycasting.html
Here are two tutorials on the subject. I tried making one of these (no
textures), but never did fix the fisheye distortion problem. My version was
also impractically slow. It looks like it's helpful to use matrix-based
math.