Various volumetric questions

[mitchino]

I'm putting together a scene of an object on the floor of a dark enclosed room with a small window. I have an area light outside shining through the window and backlighting the object. I have set up a volume material and put it in the Scene Environment slot of the render settings.

It's working ok but I have a few questions.

How do I make the light rays/shafts of light brighter but keep the room fairly dark and keep the same exposure on the object?

I have another area light for a bit of front fill on the object, how do I stop that from having light rays too?

Can I render out the rays as a separate render pass?

How do I reduce the noise in the render? Do I just have to let it cook and accept long render times?

[Ealexander]

I'll take a crack at this from my personal experience - others or the developers may have better answers:

I'm putting together a scene of an object on the floor of a dark enclosed room with a small window. I have an area light outside shining through the window and backlighting the object. I have set up a volume material and put it in the Scene Environment slot of the render settings.

It's working ok but I have a few questions.

How do I make the light rays/shafts of light brighter but keep the room fairly dark and keep the same exposure on the object?

--You can use exclude for that light, or better, set up two lights - one for the volumetrics only and one to light the space.

I have another area light for a bit of front fill on the object, how do I stop that from having light rays too?

--No way to do that as far as I know.  This kills me a lot.  Redshift lets you control the volumterics per light which is really nice.  I've only had luck by backing that light further away from the objects.  Hack.

Can I render out the rays as a separate render pass?

--There is a Volumterics option in multipass, though I haven't used it.  I tend to do a separate render where I Material override everything with Matte Pure Black and then comp the rays together in post.  Again - I'm a hack :)

How do I reduce the noise in the render? Do I just have to let it cook and accept long render times?

--Yup....

[Eddoron]

For the last question:

Increase the light samples multiplier

[Ealexander]

For the last question:

Increase the light samples multiplier

It's defaulted to 2.  I've tried played around with increasing this, but not seeing much of a difference.  What do you think a decent setting for this is?  I guess I'm wondering if you mean take it to 4 or if you mean take it to like 512....

[Eddoron]

There was another thread where one had one light(directed) a cube with a volumetric material. The perfect example to test the ray noise.
Bumping up the value to 4 had an obvious visually better result than 2. the render time didn't increase by much. I tried up to 32 which also gave much better results than 4 but the render time was more than doubled.

I'll look for the thread later but meanwhile: increasing the LSM helps a lot to clear volumetric noise but at too high values it takes way too long. A max value of 10 should not be exceeded. (according to my tests)

Found it!

7 passes each. left, LSM 2 / right, LSM 8. 

[Ealexander]

Awesome! Thanks for the info.