I find rendering volumetrics in Cycles gets really frustrating with noise (and denoising artifacts), and I’m curious how y’all get around those issues. I have a couple of ideas but each has their own issues:
Render more samples
Takes too long
Render volumes and background on different layers, denoise separately, and combine
Not sure how well the layers combine though, given volumetrics affect the spread of light
Depends on the kind of volume. This wouldn’t work as well for a dense smoke simulation that needs to cast a clear shadow on the scene, but a thin mist tends to still look good enough even if you remove its effect on the lighting, as its a small enough effect that the difference usually won’t be significant. I have had pretty good results with this. And it’s not like the audience will have both versions to compare.
If you absolutely need the effect of the fog on the scene, you could use the volume as “indirect only” rather than disabling it completely. This doesn’t sound like it would help the render, but you should be able to override the number of samples for the volume pass to something lower and still get a better denoise result, so it’s still worth it to use separate passes even in that situation.
I find this works really well for fog. It can even be automated in one render if you know the scenes system. Eevee’s volumes don’t have any effect on the lighting, but there again, the volume needs to be quite dense before the effect becomes significant.
A well adjusted depth pass in compositing can look surprisingly good and costs nothing. You don’t get to texture the fog or have shadows in it, but it can be fine if you just need a little bit of atmospheric haze.
I think I have done fake shadows to the ground when rendering uneven volumetric fog with Eevee. I just used the same procedural texture that I used for the fog, but as a mild mask to mix a darkened emissive shader to the ground shader. This way the fake shadows moved at the same speed as the fog, and they had approximately the same pattern.
Sorry for the late reply, but thanks for the suggestions!
If you absolutely need the effect of the fog on the scene, you could use the volume as “indirect only” rather than disabling it completely. This doesn’t sound like it would help the render, but you should be able to override the number of samples for the volume pass to something lower and still get a better denoise result, so it’s still worth it to use separate passes even in that situation.
I will add that currently I already use a multi-pass denoising set-up, which takes care of some artifacts, but not all. However, if I’m understanding correctly, would this essentially be rendering a separate volume indirect pass? Something like:
Scene without volumes + volume indirect + just volumes
In this scene, I am not using the indirect light from the fog, so it’s just disabled in the solid objects pass. If you want to try, you can reactivate the fog collection in the fog view layer, then set it to “indirect only” (in this scene, there is pretty much no visual difference though).
You will see I am not using any fancy setup for denoising. Just the fact it’s separate is enough.
