Water tornado refraction flickering

[The Batchman]

Hello V-Ray users!

I have created a Realflow simulation of a tornado and rendered it with Vray. The main problem is that the tornado is flickering like crazy.

Video here: https://vimeo.com/131527228
Password: stopflickering

Scene
The mesh is a alembic file (38gb) imported into Maya as a Vray proxy.
There is a simple studio setup with top open where my HDRI (vraylightdome) light is coming from. Non animated.
There are 2 lights in the scene. Non animated.
The camera is animated and uses vray physical camera with still image as the camera (doh!). However I also tried movie camera and turning it off which yielded the same results.

All lights (including HDRI) have 8 sampling subdivs.

No sky, no sun.

Shaders
I have an impressive 2 shaders in this scene. Both VrayMtl. One shader for the studio and one for the water.

For the water 2 refraction subdivs seems to be working fine for me.

Obviously the water is the most important one:



Render Settings
Global max depth at 3 and transparency levels at 3. Tried to do this with 10 for both values (also edited the shader to obay), but still got the flickering.

Using the DMC sampler with 1 min 4 max.

Color mapping, linear multiply, gamma 2.2. Don't affect colors, only adaptation. Clamp output at 5.

Irradiance map, very low preset, multiframe incremental. Did several test renders with other presets but hardly any improvement in image quality. Leaving it this low is for render time only.

Light Cache at default and Fly-Though mode.

Render Times
Render times for the first 65 frames are around an hour per frame, after that render times drop down to between 20 and 30 minutes. I still have the backburner log. This render was done with backburner over 6 computers and the render time totaled at 150 hours and 30 minutes.

Have tried rendering on a single computer but that didn't make any difference, still same result.

Problem
The problem is that the flickering seems to appear every 4 frames. The first frame is the lightest which goes to become the darkest over 4 frames. This image illustrates best what I mean:



This goes throughout the entire clip (hence the flickering). For every 4 frames there is a lightest and a darkest.

What did not solve the problem

• Vray physical camera (different cameras and turned it off too)
• Global max depth at 10 (both values)
• Render on a single computer
• Setting Irradiance map to medium animation

If there are any suggestions on how to improve render times in this scene please let me know. I can provide the scene if needed, however the alembic will be a problem.

Thanks in advance for any replies.

[super gnu]

hm, well im a max guy, but id say firstly you should try brute force for primaries just to rule out the imap as the source of the problem (i know it will be slow)

and also, doing a scene like this id use a max depth way above 10.

finally, and i doubt it is the source of your issue, im surprised 2 subdivs for the refraction is sufficient!

[super gnu]

ive just noticed your gi setup is not at all correct for animated geometry. using lightcache in flythrough mode and multiframe incremental irradiance map will result in a static gi solution. this setup is suitable only for animations where all lights and geometry are static, and only the camera is moving.

this is very likely the reason for your issues.

simply setting the imap to "medium animation" will not help if you are still using multiframe incremental and flythrough lightcache.

you -could- try "animation mode" for the imap, which calculates an imap for each frame and stores them, before interpolating them between frames for a less flickery final render, but its not a very effective system in my experience.

if you must have GI (and if your scene is at all indicative of the final one, you really dont) then brute force for primaries and lightcache on "single frame" is really the best (and slow) way to go.

to be honest id light a scene like that with a domelight...with the correct material tweaks, it will look nearly identical.


if you need GI, then be prepared for some horrible rendertimes. motion blurred refractive geometry with glossy refractions is a nightmare rendering scenario.

[Neilg]

It's the irradiance map.

render again with no gi, just use the hdri's with store with ir map checked off.
render a couple frames with bf/lc

i've comped GI in the past - rendering it in a scene with incredibly simplified materials using bf/lc, then comp it over a pass done with gi off and direct light only. it's not correct, but it looks pretty close and renders in a tiny fraction of the time sometimes.

none of the animation modes for the irradiance map will work with this.

[The Batchman]

I just skipped though my render passes in Nuke and found that this flickering only happens inside my refraction pass. Also checked the GI and I don't have issues there.

Seems I didn't fully understand the manual about the IRmap modes. Thanks for the advice on that so far.

[Neilg]

do you still have the issue in refraction when theres no gi?

[The Batchman]

Yes problem is still there when I disable the GI shuffle in Nuke. It's the refraction layer that gives me the flickering.

[super gnu]

id say 99% sure its a gi issue. with those settings there is no way your gi will work properly. its possible its only showing in the refraction pass since the refractive object appears as black in the gi pass? therefore the only bits you can see in the gi pass are the static elements, where the gi would look ok?

[The Batchman]

You are absolutely right, the refractive object does appear as black in the GI. Maybe me sounding silly but I was convinced it was supposed to look like that. Also I thought it was the noise in the GI that you guys where talking about (doh!).

I'm still a student in learning and reading up on the Vray manual. I've been trying to optimize this scene for render times also but I think I'm misinterpreting some of it.



Gonna try the BF/LC version right now and let it render over night. I will also up the global max refractions and reflections and in the shader too. Let's see how many frames it crunches out. I will keep you posted.

[super gnu]

it will be black in the gi pass, as its 100% refractive, so has no diffuse/surface illumination. however this explains why the gi pass looked ok. in the refraction pass, you can see all the gi through the refractive object, hence the errors. thats what i recon anyway.

id really try without gi.. it will render much faster and cleaner, and if you boost the domelight a bit, and turn on "affect shadows" in your refractive material, it should look fine.

[Raven]

Just a question, why are you using gi at all on this? All the GI will be doing is giving you GI caustics, and IR/Lightcache is not really good enough for caustics anyway. Just use caustics instead (if that is what you are after).
It should work well with just a dome light with your HDRi and set your material refraction to affect shadows. Then it is all direct light so no flicker. Plus faster render.

[Neilg]

Yes problem is still there when I disable the GI shuffle in Nuke.

Thats not what I meant, render it with no GI.

[Svetlozar Draganov]

Gonna try the BF/LC version right now and let it render over night. I will also up the global max refractions and reflections and in the shader too.

Did you complete that test?
Let us know if the issue is still there.

[The Batchman]

Render times are astronomical when I set it to BF/LC (see image). I'm using backburner to render over 4 or 5 computers here at the office and over night it got 14 frames done.



I'm definitly going to try this without GI as most of you suggested, right now I'm checking inside Maya how the settings look and will render some frames later today.

However, do you guys recommend using the vray physical camera?

[super gnu]

yes.. it has no disadvantage over a standard camera, but more control.