Understanding Light Cache Subdivs and Retrace

[uforis]

Hello,

I have been following the general methodologies for creating animations in increasing my Light Cache Subdivisions to 3000, and my retrace to 8. In general this seems to work 90% of the time. I'm working with VRay Next GPU, using the Bucket sampler.

I have noticed that at 3000 subdivs and 8.0 retrace, I sometimes still get flickering in a sequence of frames. It would be very nice if someone could explain why in a sequence of frames, such as 1-5 for example, which have a slow camera pan, frames 1, 2, 3 and 5 would render perfectly, but frame 4 seems has calculated the light very differently. I'd just like to know more about what's going on under the hood I guess as I just think it would be good to know why exactly this is happening when the frames are so similar. I was searching my scene endlessly to ensure I did not have anything accidentally keyframed on these "problem" frames, which I do not. Is there some way I can evaluate my scene to determine what LC subdiv or retrace values will never produce this issue?

I'm still testing the animation, but man is it time consuming to do so. Could someone advise, if this occurs, is it better to increase Subdivisions beyond 3000, or the retrace beyond 8, or both? Is there something else going on when the recommended settings aren't cutting it? Interestingly, switching from Buckets to Progressive sampler seems to produce the correct result on these problem frames if I increase the retrace from 8 to 12. The problem frames are still an issue if I don't increase retrace when using Progressive sampler.

Edit: actually my testing with the Progressive Sampler is invalid, as I was using a render region on the problem area of the frame. When i turned off the render region the problem was still there. Testing further...

Edit 2: It might also be worth mentioning that when I am previewing the Light Cache in the Progressive Sampler, the lighting looks correct during the Light Cache process. When the frame starts rendering after the light cache is finished, some additional light seems to come from nowhere making the frame look really different from the one before and after it.

Edit 3: It seems reducing the sample size from 0.01 to 0.005 has resolved the issue on at least one of the problem frames. This was with 3000 Subdivs, 0.005 Sample Size, and 8 Retrace.

So is it safe to say that when using the recommended settings of 3000 subdivs and 8 retrace, that if you still have some flickering, reducing the sample size will likely resolve the issue? Just trying to collect more data from other users whether or not they have encountered similar issues and have devised workarounds. Reducing the sample size did not increase the light cache processing time by very much, but every little bit adds up over a ton of frames.

[matanov]

Ideally the recommended values should work for 100% of the scenes, anything else from that could be considered as a bug. Please send me the scene to check what's going on.

[uforis]

Hi slizer,
How would you recommend sending you my rather large project?

Edit: After further testing, it seems I still have some minor flickering. I upped the resolution of my HDRI dome as well, but that didn't help. The way the light cache computes seems fine, but at render time some weird light comes into the Global Illumination render channel on these problem frames, producing a flicker on that frame. Falling back on BF/BF resolves the issue, so there is definitely something going haywire in the secondary GI pass.

[matanov]

Wetransfer, Google Drive or any other kind of file transfer service. We can also provide you with FTP access to upload it directly, just drop a line to support@chaosgroup.com.

[uforis]

Hi Slizer,
I have created a package and sent it to you via google drive. Let me know if you have any further questions.

Thanks for you help!

[uforis]

Here are the frames that illustrate the issue I was having in a few different sequences during my rendering tests of this clip. You can see that Frame 38 looks noticeably different from the others on the right side of the frame.

[M.Max]

Just a random thought . Do you have Adaptive lights enabled under Global switches? If so, try to disable it and do a test . Few issues can be fixed this way in the past .

[uforis]

Hi M.Max,

Yeah I had adaptive on for the global switches, but also on my HDRI Dome Light. When you say, "turn it off" do you mean turn it to "Full Light Evaluation," or "Uniform Probabilistic"?

Thanks for you thought! Any idea why that might help?

[vlado]

I'm not sure that the problem in this scene is related to any light cache setting or the adaptive (dome) lights. I will look at the scene tomorrow and will let you know. Do you have access to the nightly builds, btw?

Best regards,
Vlado

[uforis]

Hi vlado!

Yes I do have access to the nightlies. Do you recommend trying one of those builds? Thank you a lot for taking some time to help me figure this out. I could be that my workflow and knowledge about the settings has gaps that I need to amend to get a good result, and I'm happy to learn and really appreciate any help.

I'll try out another test render tonight with the Full Light Evaluation. Last night's test had the same result (problem frames), but strangely, this time they were on different frames.

[uforis]

Here is some further testing to provide a bit more data, which I hope is helpful. I rendered the same problem frame at different resolutions, and I was surprised to find out that the outcome was very different. I didn't change any render settings other than resolution between the renders. This was done using VRay Next GPU, Bucket mode, the standard 3000 LC subdivs and 8 retrace, at 0.003 noise threshold. There is a noticeably different lighting produced in the Global Illumination and Reflection render channels, but only in the 1280*720 render. I suspect that if I were to render out the full sequence, that I would encounter similar problem frames in different places at the other resolutions, but I can't say for sure. This is a real head scratcher for me, because some of my other clips in this same project are rendering perfectly at these settings at 1280*720.

Could it have something to do with my camera settings or color mapping? Right now I am using Linear Multiply with "None" as the mode, as it suits my post production workflow with multi-channel EXRs. I am eager to hear back from you guys, and please let me know if you need any more information about the scene or render settings. Could it have something to do with using the CUDA 418 driver, which at render time suggests is not recommended? I'll try updating the driver to 419 next.

EDIT: I upgraded to the recommended 419, and there was no change. Still producing the same error on frame 87 at 1280*720.

[M.Max]

Hi M.Max,

Yeah I had adaptive on for the global switches, but also on my HDRI Dome Light. When you say, "turn it off" do you mean turn it to "Full Light Evaluation," or "Uniform Probabilistic"?

Thanks for you thought! Any idea why that might help?

what I meant is to use "Full Light Evaluation" and disabling adaptive in the dome .
these usually the first suspect options that I check in similar cases so i thought it worth trying
Sorry fr the late reply . I haven't received a notification on this
I think the support team with the files will have better suggestions and hopefully a solution for the problem .

[uforis]

Hey M.Max,

Yeah I figured you meant something like that, so I tried it out last week, and unfortunately i didn't help, I still had the same weird GI and Reflection in the problem frame.

[vlado]

While I'm rendering the scene and trying to reproduce the issue, do you think you can try a nightly build? If you email me to vlado@chaosgroup.com with your login name for our website, we will get you access to the nightlies.

Best regards,
Vlado

[uforis]

Hi Vlado,

I have sent you an e-mail regarding the nightly access.

Any progress on troubleshooting the scene?

Thanks so much for helping us find/fix the issue!