Chaos Corona for 3ds Max > [Max] I need help!

Animation in render farm flickering

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Deckard:
Hello all,
I have an animation project with camera move only - interior scene.
The scene is lit with an HDRI and some recessed spots (2D disks with corona light material applied).
I have rendered 30 frames as a test in my workstation, with pre-calculated UHD and everything looks fine.
When I send the very same frames to a render farm, each frame comes out with slightly different lighting! I have tried two render farms and this occurs to both (although it was slightly less visible in the second farm). I have also tried sending without pre-calculated GI and the result is the same. When the farm frames are compiled to video, some flickering is visible...
Any ideas why this occurs? It is driving me nuts! Any help would be greatly appreciated. Unfortunately I can't share images or video because it's a confidential project.

Thank you for your help!

Deckard:
Forgot to mention my settings:
All general settings to default.
UHD cache is set to animation (flicker-free).
Noise limit is set to 4.5 and denoising is set to 0.8 (corona high quality). No pass limit or time limit.
This results to about 30 passes being computed in the brighter, sun-lit parts of the video.

GeorgeK:
Hi, is there a possibility that we can investigate an archive of your scene?
Alternatively, I would start by increasing UHD precision to 10.0 and do some tests. You can also try using a different HDRI and see if the issue persists.
Lastly, please make sure all scripts/plugins are the same version across all nodes/server.

Deckard:
Thank you for the tips.
I can't share the file with you, because I'm under an NDA for this project.
I tried submitting some test frames to the farm using PT+PT for the GI and it came out fine, so I guess it must be a problem with the UHD - I must have done something wrong in the first calculation.
Re-calculating the UHD now and will run some more tests.

A weird thing I noticed in the UHD pre-calculation: I have it set to render every 30th frame, but I noticed that the file size of the resulting .uhd file (as saved to disk) gradually decreases slightly - it started from around 7MB, dropped to 5MB and now being 6MB. Shouldn't that file be increasing as the frames progress, since new information from the new frames is added up?

Deckard:
I just wanted to provide an update on this issue with the solution, in case someone found it useful in the future:

After extensive testing and troubleshooting with two render farms (rebus and ranch), it turned out that the problem was in the glass material of the windows!!

Initially the windows had a CoronaPhysicalMtl, with the "glass" preset. For some strange reason, whereas the frames in my workstation were rendered fine, the frames rendered on the render farm nodes came out with flawed GI and caused flickering. This happened with both a pre-calculated UHD and with UHD calculated from scratch for each frame. However, it did not occur when PT+PT was used for the GI.

After changing the glass material to a CoronaLegacyMtl with "standard" glass settings, the Pt+UHD setting for the GI produced correct results on the nodes of both render farms. The same happened when I used a CoronaRaySwitchMtl for the glass, with CoronaPhysicalMtl on all nodes except GI (which was left blank).

Maybe this is some kind of bug for the CoronaPhysicalMtl?

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