I’m writing simple strategy game (in xna), and would like to have an animated background. In my game all the actions happens inside one screen and thus standard parallax effect does not look appropriate. However, I found a video of a game with suitable background animation for my game http://www.youtube.com/watch?v=Vcxdbjulf90&feature=share&list=PLEEF9ABAB913946E6 (from 3 to 6s, while main character stays at the same place). What is the algorithm to do this stuff? It would be nice if someone can provide a reference for a similar example (language is not important).
It's not the same effect as the video you linked, but one simple possibility is to scroll a texture over the entire screen using the wrap texture addressing mode. Here's one example using a cloud texture:
One effect that I think looks pretty cool for a top-down game taking place in the ground, is to create a texture which contains only the shadows formed by the clouds and wrap that over the scene. I've used that in this demo although the effect is subtle.
As for the video you linked, I believe the effect is probably based on some sort of animated perlin noise so look into that. There are many resources on the subject online.