This shot shows Volumetric Clouds being rendered. The demo uses 2.0 Pixel shaders under DirectX to render the clouds.
It does this by sampling a 3D texture of Perlin Noise, and using it to build a 5 octave fBm. The fractal is then used to
disturb the point currently being rendered. The point is then fed into an implict equation. If there is more then one
implicit equation being rendered(in other words, more then one cloud), then it is possible to blends between them.
This is similar to the effect described in Texturing and Modeling, except adapted for more limited resources. A
friend of mine had this running at ~20 fps on his graphics card. The thing that would most improve this
effect would be the ability to generate noise on card. I know you can does this if you use PS_2_X, but
then you can not run it on ATI cards. Generating 3D Textures can take quite a long
time(not to mention the texture memory which it eats up). Also, a texture of noise has a much more
limited range then most noise implementations. Baring GPU noise on all pixel profiles however, it would
be nice to have 4D textures, so that the clouds could be animated.