In the past days i've been working on atmosphere and scattering effects. I'm using the algorithm described by Lutz Justen in this thread. It normally involves a lot of calculations in a vertex shader, including two costly sphere/ray intersection tests. So i'm now trying to offset this work into lookup tables, and a few operations only in a pixel shader. I haven't succeeded yet, but i hope on the right tracks. Here's a test (on the CPU) with a very thick atmosphere: