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Slomo water animation - Mesh flickering

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Dietmar Thal
Slomo water animation - Mesh flickering
on Nov 18, 2010 at 6:15:19 pm

Hi there,

Apologies for cross posting, didn't know which forum fits best...

I'm doing a music video and have a realflow-sim with water being attracted upwards out of a basin. I've uploaded a preview here:

Preview

As you can see, in the upper (and more dense) areas of the mesh, some serious flickering occurs. In the lower part, there is less flicker, because of less refraction traces (my theory).
The scene lighting is set up with a VRayDomeLight / HDRI Texture. The HDRI is used for lighting and as a refraction map for the water.
Can anyone give me a hint what i can try to avoid this flickering?
The realflow sim is simulated with 1200 fps and particle count is around 800000.
I tried to decrease poly size and now have a pretty dense mesh, increasing poly count even more will blow my computer i think smiley-smile
I've come across the clamp color option in VRay, as i've read that overbright colors in HDRI images lead to this kind of flickering. It helped, but it's still there as you can see.
So, any suggestions would help...

Thanks,
Dietmar


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Graham Macfarlane
Re: Slomo water animation - Mesh flickering
on Nov 26, 2010 at 10:07:30 pm

Some things you could try, separately or in combination:

Although unlikely especially since you have tried clamping the output already I would check that your HDRI lighting is not over powerful (compensated by exposure). This can cause longer renders and hotspot flickering.

Also If your using any glossiness in the reflections/refraction of any scene materials, disable it and run a test to see if that is a factor.

Under Global switches change the secondary rays bias from default 0.0 to 0.1 and check if this reduces flickering of any coplanar faces.


Finally, if you havn't already you could try the adaptive DMC Image sampler on default 1/4 or even up to something like 2/6 (min/max image sampler subdivs).
You could also try reducing the DMC noise threshold in combination. I normally try 0.005 from the default 0.01. This will increase render times but can help a lot. You also might find that with a lower DMC noise threshold you don't need to go so high with your adaptive DMC image sampler subdivs.

It's always a bit of trial and error to fix these kinds of problems!
Hope something there helps

Graham Macfarlane
3D animator and VFX specialist
London UK


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Dietmar Thal
Re: Slomo water animation - Mesh flickering
on Nov 29, 2010 at 12:32:20 pm

Thanks Graham,

i will try the options you suggested and post an update...


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Dietmar Thal
Re: Slomo water animation - Mesh flickering
on Nov 29, 2010 at 1:37:39 pm

Hey,

just another question: How do i properly check an HDRI image for correct brightness values? Since the histogram in Photoshop looks ok...

Thank you!


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Graham Macfarlane
Re: Slomo water animation - Mesh flickering
on Nov 29, 2010 at 8:22:31 pm

Sorry I should have been a bit clearer:
When I said that you should check that your HDRI lighting is not over powerful I meant from within Max. It might be that to get enough light in your scene you have boosted the HDRI too much rather than playing with exposure. The result is that Vray has to deal with very high energy levels which both takes longer to process and can cause glitches (hot spots/flickering etc..) in animation.

You should be able to map the highlights when you bring in the HDRI into Max as a bitmap. Just adjust the white point spinner until the pink areas in the thumbnail show up only where you want the hottest areas to be. Then you can adjust the output amount and RGB level for said HDRI to get the right amount of light into your scene.

If using the VrayHDRI material (recommended) you don't get this white point feedback and instead have to play with the "Overall" or "Render" Multipliers and "Gamma" setting to control the HDRI instead.

A good test to start with would be to light your scene with a Vray sun or direct light or Vray area light and turn off the VRayDomeLight. This would be the fastest way to check if the HDRI is the source of your problems.

Graham Macfarlane
3D animator and VFX specialist
London UK


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Dietmar Thal
Re: Slomo water animation - Mesh flickering
on Dec 4, 2010 at 4:04:29 pm

So i think i'll give up on this. I tried everything you suggested.
Adjusted white point for the HDRI, tried another HDRI, switched to VRay area light to see if it is the domelight. Tested different settings for min/max samples in adaptive DMC sampler in combination with secondary rays and/or noise threshold. Nothing helped the flickering... So i think it is in fact the mesh itself, which changes topology between frames, so a realflow related problem.
But anyway, thanks for your suggestions, they'll be useful the next time i hit a similar problem.

Best, Dietmar


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