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

[BeneZ]

Hello,
I am testing a sequence with rendering smoke from Phoenix FD and which render setting gives me the fastest preview result.

I am using VrayNext, update 3.2
I'm Rendering with progressive render without any GI (bucket was too slowly), it is just smoke at grey background, nothing else in the picture.

Now I have the issue, that my image (600x 800px) is visible ready after 18 seconds, but the VFB still renders until 50 seconds were reached.
I have set everything very low: Min/Max 1/2 (Max)Rendertime 0.1 min threshold: 0,5
(It is also taking always exactly same time like progressive with Min/Max 1/100 max. time 0.5 min, threshold 0.1)

- What is happening there, that so much time is wasted without showing any quality difference and I can't switch it off?
No render elements there, I can only see the alpha.
- Could I also disable the alpha to save time?

(When I set Max. transp. levels from 200 down to 50, then I get a bad image in less time, but also here it takes always twice as long to render than it takes my image to appear in the final look in the VFB.)

I was using also TexUVW for Opacity of the smoke ("Volumetric Options"), but I imagine, that this is not calculating anything at rendertime, when the volume is already cached.

EDIT:
Screen-Movie file here: https://www.dropbox.com/sh/hrq5gulyo...S7bRW4qqa?dl=0
I also compared there a GPU rendering. GPU takes less than 10 seconds but the result is much too transparent and not usable. I wonder how the results can differ so much between CPU and GPU.

Thanks

[hermit.crab]

First of all, V-Ray will stop after completing the first pass when a time limit is set and the rendering takes longer than the chosen time period.
No time is wasted, V-Ray simply needs more to calculate such simulations. You can't disable the alpha channel, though if you could, it wouldn't really affect render time.
A value of 500 for the 'Max. transp. levels' is absurd and it is expected to have longer calculation times. Lower this value for a faster preview.
V-Ray GPU is a separate engine and such a direct comparison is not applicable nor accurate.
What's the reason for using TexUVW for opacity?

[BeneZ]

Hi Aleksandar,
thanks,
- as you can see the time limit is set on 0.1 minutes which is 6 seconds. If you look at the screen video, you see that I get the result after 17 seconds adn the render takes 49 seconds. This is 2,88 times longer than needed, especially for a draft.
So how can I limit the time on 18 seconds, to stop these further calculations?
If this is a standard VFB rendering I can press "stop" and use the result. In a sequence it runs from second 18 until 49 without adding any quality.

- I got once a warning, that the max transp. levels need to be increased and for the whole sequence this was just a savety value.
Even if I take only 200 (lower value than 200 gave corrupt results), then the same problem is there: It takes more than 2 times longer to finish the frame than it actually takes to render the result.
(And the difference between transp level 200 and 950 was only 5 seconds in total time.)

- TexUVW is your new phoenix 4.1 tutorial about "thin smoke" and I was testing it here

[BeneZ]

Hi, I think I found an issue/bug with TexUVW and Modulate (Opacity):
- when I disable "Modulate" it stops as it should as soon as the picture is complete
Test with other smoke: 6 seconds

- when I enable "Modulate" to drive opacity by noise, then the render is complete after 6 seconds, too, and you also can see the bar is also finished,
but it runs until 12 seconds were past (it depends a bit on the noise)
so here is the half of all render time wasted

- when I enable COLOR MAP at the noise node, then the same(!) render is finished after 25,7 seconds!
even when the color map curve is not changed.
I had animated the black point of my color map in my scene to dim the effect softly in and out over time.

[BeneZ]

Svetlin.Nikolov Hi Svetlin, please have a short look about what I found out above with TexUVW and "Modulate" Smoke Opacity.
I tested your suggestions, but only disabling TexUVW changed the behaviour.
My further tries:
- optimize big volumetric grids: no difference
- full lights evaluation I did not found (it's also not a scene with many lights)
- "probabilistic shading" slows it more down
- "volume light cache" I could not find. (but the scene is not using GI)
thanks....

[BeneZ]

[Svetlin.Nikolov]

Hey, if you can't find some option, look it up here: https://docs.chaosgroup.com/display/PHX4MAX

Did you also try less or more samples with Probabilistic Shading?

One more question - do you disable TexUVW from the Output rollout and then simulate again?

[BeneZ]

Hi
- I could not find in the documentation, how to enable "advanced mode" of vray in order to get Full Lights Evaluation.
I have searched 4 times now in the doc pages...
(Vray has so many switches at so many places...And there is only an arrow with advanced in the description.)

I made 2 more tests:
- I made now a simulation test only with MODULATE without simulating TexUVW
It is the same result: 9 seconds without Modulate and 27 seconds with Modulate
So Modulate with Noise causes the delay.
[In general I was using informations from this new tutorial https://docs.chaosgroup.com/display/...ing+Thin+Smoke ]


- I testedSmoke based on "Smoke" in comparison to Smoke based on "Simple Smoke" (Volumetric Options)
Render times:
Simple Smoke: 8 seconds
Simple Smoke + Modulate Opacity by Noise 26 seconds
Smoke: 6 Seconds
Smoke + Modulate Opacity by Noise: 10 seconds

So MODULATE Opacity based on "Smoke" takes only 38.5% of the time in comparison to using "Simple Smoke".
(but I have no experience to say, how "Smoke" looks in comparison to "simple smoke")

[BeneZ]

And tested also "probabilistic"
It runs always longer, whatever settings I take.
I'm not wondering, because GI is disabled and porbabilistic wants at least GI 1 sample. Gi is taking lot time.

[Svetlin.Nikolov]

To enable Advanced mode, you just click on the label.

GI Samples don't do anything when GI is off, they don't enable GI, they are just ignored When GI is off, only the Samples option works.

[BeneZ]

Ok, I did not realize that this is a button
I tested, and Lights evaluation makes no difference...

[Svetlin.Nikolov]

Would it be possible to share the scene you are rendering so we can try to reproduce it here?

Also, can you tell me the exact versions of V-Ray and Phoenix you are using?

[BeneZ]

Yes, I will prepare tomorrow...
PHX 4.10.00
Vray Next, update3.2 ( 4.30.02 build 00001 )

[BeneZ]

Svetlin.Nikolov
Hello, I have made you now a simplified PhoenixFD file and screenshots for explanation,
and sent a download-link by email.

Please let me know what you find out.

Many thanks!

[Svetlin.Nikolov]

Awesome, thank you! Will ping you when we know more.