Rendering takes always more than twice as long as final image really needs (Smoke rendering, CPU) - huge waste of time!

[BeneZ]

Same message here:
I have provided a better clearer file for the render time issue.
It is the same like the green cube, just some more and slightly different settings for opacity. Much better and clearer for comparison!

I have sent the NEW FILE as download link at support@chaosgroup.com
In reply to 298-162-4813

[georgi.zhekov]

Hey,

Just checked your scene.

You're using progressive sampler with 1 subdiv, this is the lowest V-Ray will go. If you switch the sampler type to bucket with 1 subdiv Max, you will get the same result.

You set the Max time to 0.1s - so you expect the render to end after around 6 seconds, though if the progressive pass is not complete by this time - the render will take more time and will finish after the pass is complete.
V-Ray works this way, because if the render finishes before the pass is complete you may get artifacts in your image.

So what happens in your case - even though you're using the progessive sampler with a time limit - V-Ray needs to throw at least 1 subdiv at the scene and this takes more than the specified time limit. Again you can cross check this with the bucket sampler.

On top of that - when using textures to modify the look of your smoke (either for the color or for the opacity) - V-Ray needs to sample these textures as well, so this will add more render time.

At a later point we'll debug the end-of-render issue, but as a workaround for the slow renders, there are a few things you can do to speed things up with your scene.

1. Change the render mode of the Phoenix simulator to Volumetric (instead of Volumetric Geometry). The Volumetric Geometry mode is slower and we advise to use it only if you need to use some specific Render Elements or if you need to blend different atmosphere effects such as Aerial Perspective or Env fog.
2. In the Max Environment settings turn on the Probabilistic shading and set the samples to something like 5 for example (in your scene the GI is off so changing the GI samples won't have any effect). This is what made the biggest impact in my tests. This will render out noisy but will speed up the renders. If you need a cleaner result, just increase the samples. You can find more about the Probabilistic shading here - https://docs.chaosgroup.com/display/...phere+Settings
3. In your scene you're using a dome light and a large V-Ray Rect light. Both of these lights are throwing a lot of rays so they will render slower. If it's an option, try to use a V-Ray sun or a directional light - these will render considerably faster.

We have also made an article on Volumetric Rendering with some tips and tricks that you can check here - https://docs.chaosgroup.com/display/...ering+In-Depth

[BeneZ]

Hi georgi.zhekov
thanks for the detailed answer.

questions:
a) I wonder that bucket could be much faster than progressive. And for bucket it isn't possible to set a time limit.
Do I have to set it on negative values then or what's then the trick?

b) About progressive: I just thought the shortest possible render is a "limited rendertime", progressive with only 1/1 samples and without GI.
Of course I know that the time for precalculations comes always on top. I guess progressive can't be faster than that.

c)
How about the idea, to implement an ability to force to skip to next frame after a certain time for previews?
Or would it maybe be possible by scripting?
You can examine at the scene yourself at which point the quality would have been good enough, to get a rough preview.
This time can of course differ, depending on how large the smoke is becoming at later frames. But maybe there is a percentage slider.

Good tip about the lights, I will test that next time.
I disabled GI to try to minimize all rendertimes and to exclude all possible other reasons from the time issue.

And Geometric mode was recently the only, which worked in IPR. That's why I did not test others. I will test this, too,

Thanks

[georgi.zhekov]

You can read more about the differences between the bucket and the progressive samplers here - https://docs.chaosgroup.com/display/...ortheSituation

If you're using progressive with 1 max subdiv or bucket with 1 max subidv there should be no much difference in render time or noise levels. In the link above, you can find when each sampler works best.

The progressive render will finish after you hit the render time ( with the condition that if you started a render pass, V-Ray needs to finish the whole pass disregarding the time limit) or when it reaches the specified noise threshold. Whichever is reached first, the render will stop.

For the skip after certain time functionality - this is exactly what the time limit is for. In your case the render takes more than the set time for the reasons I listed in my previous post, but using the tips I mentioned should help you to fit in that range.
With the changes I listed above the render took around 5 sec. on my machine.

Cheers!