This effect uses OpenGL, but there is no 3D animation. So I thought about trying to implement something that is really 3D. I got the idea to implement something like Windows Vista's Flip 3D (I have never seen this effect properly, so I don't know how close my effect is :)). That is, all windows are displayed on a three dimensional stack and rotated so that all windows can be seen (at least a small part of each). If you switch the window, the next or previous window will appear on top of the stack. In fact the whole stack is moved.
The most difficult part of the implementation was the third dimension. As far as I know, all existing effects are only 2D. So I could not just copy and paste. It was a lot of trial and error. Finally I had some working code. The windows appeared in the frustum, and moved along the z-axis. From that point on I could rely on the methods I learned in the OpenGL course: move every window to the correct position with glTranslatef() and rotate it with glRotatef(). The next step was to animate the whole scene. This is quite simple, as you can just move the whole scene with the help of glTranslatef(). So if you press Alt+Tab each window "flies" to the next position.
Writing this effect was mainly practice for my OpenGL course. But I am also glad to be able to give something to the community after taking for so long! I hope to find time to write some more fancy effects and to help make KDE even better :)