Mental Ray Parti Volume Shader
Again in Matt Hartle’s lesson, I found an alternative way to do light fogs – using volumetric shaders in mental ray.
What we do is to define the region in which we want our light fog to be in our scene. We put a simple polygon cube to define that region, assign a lambert shader to it (can be anything), then under it’s shading group, go to the mental ray custom shaders tab. Plug a transmat shader (under materials) in material shader, which allows light to affect the render but not actually render the surface.Then plug the parti_volume shader (under volumetric materials) into volume shader.
We probably need to tweak the scatter for a start. Check out what each attribute means from this wiki.
Now we just need to have a light that is within the region and is light-linked to the lambert. We can also define which light illuminate the fog within parti volume shader or just by light linking.
However after some googling I found that this is actually an expensive way to do light fogs, albeit being a more accurate one. So it depends on your render budget and scene. I personally still prefer to use maya light fog with procedural animated textures, then tweak in post-production.
Go play around with light fogs!