light fog in mental ray

[Bitter]

parti-volume would be the most convincing and scatter correctly but it's expensive (naturally) Tried Maya spotlight fog on the lights? That renders in mental ray. If you need the lights to "swim" through the haze then try a 3D procedural on them so when the car turns the fog doesn't seem attached to the car.

[Nothingness]

Depending on your system it could be workable in mental ray.
I did a volume fog in my last render and had to figure it out too.

Basically you best use a spotlight rather than a volume light just
in sake of render time. (I tried it so i know)

Not totally sure how to do it - i'm not at a maya machine - but
you simply create a box where you want the fog to appear ( you could
for example attach it to the headlights if you animate this)
Add a new lambert shader and go to that SG shader node just above.
Then add a transmat shader to the material slot (i think)
and then add a particleVolume to the volume shader slot.
Then you need to attach the light and do some settings.

I learned from this site:
http://www.jhavna.net/main.php?page=scatter/

cheers

[Bitter]

That method uses caustics for participating media, that can be very slow. I'd suggest just direct lighting in this case.

[Nothingness]

One should try it to found out if rendertime is long or short. It all depends on many things and in general i think it's worth it.

Rendering as a different pass is a good idea and just requires some surface shaders and a bit brains. When you use volume lights you get higher rendertimes so avoid them. I find this way much more free and less complecated than maya's volume fog system.

Second, because it's composited, you have an advantage of changing render settings to get a faster rendertime. In some ways you can use a post filer in a compositing software while being able to render for example at a sampling of 0.

The use of Photons doesn't mean it's slow. I hardly noticed it on my 3k render that didn't took too much time at a high sampling rate.

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