Understanding Light Cache Subdivs and Retrace

V-Ray V-Ray for 3ds Max Issues
1

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.

2

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.

3

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.

4

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.

5

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!

6

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 .

7

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?

8

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

9

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.

10

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 :roll_eyes:
I think the support team with the files will have better suggestions and hopefully a solution for the problem .

11

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.

12

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

13

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!

14

I have done some further testing on the scene, and while I was reluctant to, I switched to VRay Next ADV (non GPU) and rendered the clip. There were no problems with the render at all, which leads me to believe that something inconsistent is happening during the light calculation when using GPU rendering. I still hope to hear back from someone at Chaos Group about whether or not it is possible to that particular clip and others like it to render on GPU consistently, as it is still much faster.

15

I spoke too soon again. After finishing my clip over night, it seems on VRay Next CPU I still get a few problem frames. Noticeably less than on GPU, (on GPU I had 8-12 problem frames, and on CPU I had only 2), but at this point I have little confidence in the BFLC method. This is the case with Full Lighting evaluation, not adaptive. It works sometimes, and sometimes it doesn’t. My anxiety level is through the roof haha. When I increase the subdivs and retrace values it seems to help these frames, but I’d like to be able to walk away from my render with some confidence knowing the frames are going to turn out fine. I’m still eager to hear back from you guys if I’ve done something janky in my project as well, or if there is indeed a bug here.