[solved] Inside Volume Mode - Extremely slow

[vizform]

Hi All,

Is the new Inside Volume mode for fog supposed to be this slow or am I doing something wrong? I have used the standard fog before and though I experienced a big drop in performance it was still usable and not nearly as substantial is this.

I am experiencing a drop from 8 000 000 rays to just 25 000 when I enable the volume fog with inside mode enabled
That is an insane drop in performance.

My scene is lit using an hdri. I have tried just using sun and sky and experienced the same drop in performance.
My fog is applied to a cube that sits almost right in front of the camera.

My specs are as follows.

AMD Threadripper 1
128 GB DDR4 Ram
GTX 1070.

[maru]

This is mainly dependent on the "step size". It should be set to as high value as possible, until artifacts are visible. You should also always use "single bounce only".
See: https://coronarenderer.freshdesk.com/support/solutions/articles/12000045276#rendering

[TomG]

You have to keep in mind that inside volume mode can result in MUCH more work for the renderer - where it previously just had to evaluate the fog once, when the ray from the camera hits the surface, now it has to evaluate things at every step size; this could mean hundreds or thousands (or even more) of calculations where previously just one was done. And of course this multiplies without "single bounce only" being enabled. As mentioned, Step Size is your key to balancing quality (whether blocky artifacts appear) and speed, and disabling Single Bounce Only will result in much slower calculations (much more so than the On Surface mode) so unchecking that should be used very sparingly, with the understanding of how it will impact render times when it is unchecked.

[vizform]

Ok great! thanks guys. I did not know about the step size.

I will give that a try.

[vizform]

Ok, I played around with the step size. I set it to 2000cm, which sounds like quite a high number to me, but it seems to be rendering fine and I am getting 1 500 000 rays now.
That is a huge improvement!

[TomG]

Awesome news! As a general guide, the larger the volume, and the more "wispy" the effect, the larger a step size you can probably get away with. Small step sizes come into play mostly when dealing with, say, "swiss cheese" where some things are solid and some areas empty and then the blocks (you can think kind of like voxels) can be an issue; for fog and mist a larger step size is usually fine and won't show artifacts. Clouds are probably somewhere in-between, as they are "fairly solid".