GPU and phx smoke scattering - can't seem to make it work

[Paul Oblomov]

CPU is freaky slow to render, about 3hours.

So, I tried GPU and it is way faster, tho I can't get any scattering from temp at all. I inserted simple sphere light and look at the smoke around - rock solid. Something is not supported or doing wrong here ?

[Svetlin.Nikolov]

Here are some useful tips on how to optimize volume rendering, please make check your settings against these ones: https://docs.chaosgroup.com/display/...ering+In-Depth

... and here is how to report an issue so we would be able to help you: https://docs.chaosgroup.com/display/...o+Report+a+Bug

[Paul Oblomov]

Oh. Ok. Just thought I missed smth since render guide doesn't contain tips to make temp scatter more, than sphere light.

max20

vr4 29960

4.10.02 Nightly, Build ID: 2020041730010


Just press render


https://www.sendspace.com/file/trgh5v

[georgi.zhekov]

Hey,

The Phoenix Fire lights option is not supported on the GPU yet, if you check the V-Ray log window, there should be a message warning for that.

If you disable the option, the renders are pretty similar between CPU and GPU.

I will bump up the priority for the Fire Lights GPU support.

Thanks!

[Paul Oblomov]

So, this is proper GI raytracing scattering I see, right ? Terminology is quite confusing

I have a q - light scatters from the inside of the smoke with ease (as showed on pic), while hotter temp voxels are not. Lights are sampled differently ? And to have each bright voxel emit "light" - you have to write smth in addition ? Am I correct ? It is like GI caustics and photon caustics ?

P.S. So to mimic this I have to scatter lights inside the hot areas f.e. in houdini and then it will kind of work ?

[kostadin.petkov]

Hey,

"Create Fire Lights" option intent was to be able to light up the scene with the fire created by the volume when there is no GI available.
As you know, when V-Ray GI is disabled, The light in the scene comes only from direct illumination by the lights in the scene. Since the Phoenix Volume, is not a light, in this scenario, V-Ray will not be able to illuminate the scene using the fire.
Here is where "Create Fire Lights" helps, as Phoenix would create lights based on the Fire being rendered. If GI is available"Create Fire Lights" is no necessary, as V-Ray would sample everything in the scene to compute the the lighting in the scene.
"Create Fire Lights" with GI enabled, could be used to speed up the rendering, as sampling Lights is faster then sampling fire with GI ray.
You can read more about it here: https://docs.chaosgroup.com/display/PHX4MAX/Render+Fire

Unfortunately V-Ray GPU, as Zhekov said, doesn't support "Create Fire Lights" yet.
So the way to get the Fire to light up the smoke and the scene on V-Ray GPU is to use V-Ray Global Illumination (Brute Force would be a good start, as Volumes do not use Light Cache).
In this case, to achieve realistic results you need to additionally change the scattering type of the Volume Itself. Under Rendering -> Volumetric Options... -> Smoke Color -> Scattering set it to "Ray-traced (GI Only)",
the (GI Only) part means that scattering will happen only if V-Ray has GI enabled.

Regards,
Kostadin

[Paul Oblomov]

The question was mainly about the difference, between GI scattering and simple sphere light inside the smoke. For me it feels odd, that simple light scattters through the smoke with ease, while temperature do not at all. You can even see on the slice, that there's no scattering.

Do you see any GI scattering, not lighting up from the outside ? Smoke is completely solid to everything, except small spherical light. All the settings are there. Everything is raytraced.

To me, it feels like fire lights is the only option, that produces scattering from the temperature on smoke inside. And this is confusing, since you're telling it looks ok. To me - it doesn't look ok.

Light passes through smoke medium, temp not.

So to me it looks like: there's no scatter, without "Create Fire Lights" option, you can only have smoke lit by the bright voxels, not scatter through.

[Paul Oblomov]

Please, show me how to setup scattering through medium.

I have another scene, where we have simple light and simulated light (kind of). Small phx grid, with temp set to pure white with absurd rgb values.

The idea is to have same light through the smoke.

1) press render - you can't see anything
2) turn off second grid and turn on the light - scattering appears

The question - how to see scattering, when second grid lights first grid from inside ?

[kostadin.petkov]

Hi again,

here is a small setup that works.
Keep in mind that, if your fire is covered by really dense smoke then the light from it wont be able to to penetrate it, as all of it's energy is going to get absorbed by the smoke.

As "Create Fire Lights" create light based on on a down-sampled gird (5% by default of the original resolution), resulting in some of the light emitting voxels of the simulation to no longer be covered by smoke, resulting in stronger illumination.As you increase that percentage this error will start to fade away. Currently there is no way to achieve this exact same behavior with V-Ray GPU, so illuminating at this point can be done only with GI.