Render only fog for Composite? help pls.

[rich0550]

I have a sun position system with enviroment in Vray dome. However if I create fog for my lights at night it runs the day sky background. Can I render only the fog out by itself and composite later? How to do that?
Thanks much

[hermit.crab]

Adding a VRayAtmosphere pass should do the trick.

[rich0550]

Thanks I can do that , but how do break it out so its not rendering fog in my main scene? Does that make sense. Thing is the fog is playing tricks with the sun position system. The bacground gets messed up. I have a 24hour scene and really just want the fog on the lights at night? What is the right workflow for such a situation? Do I key the fog? or render twice?

or is there a way just to render fast the fog by itself kind of like would do with a Krakatoa render? that would be quite nice.

[hermit.crab]

You can contain the fog in a gizmo (VRayEnvironmentFog>VRayEnvironmentFog nodes) or alternatively, use a VolumeGrid to generate fog. Otherwise, you may need to render the night fog in a second render.

[rich0550]

Great idea thanks