'Popping' in the reflection of water during an animation

[Njen]

Hello, I am seeing a number of issues where the reflection in the water 'pops' and changes quite noticeably. I have attached one such example. I have also attached a screenshot of my render settings.

Any idea on what could be causing this?

[Matt]

Sorry for the late reply. Somehow we missed this one.

It looks to me like the pop happens in the reflections of the clouds; is that your belief too?

I see that you're using a GI cache file (or sequence), which is good. I wonder if there is an issue with the cache file sequence, such as running out of frames, or a mix of files from different cache generation renders. If you haven't ruled out that possibility, that might be a place to look.

Did you render the image sequence on one computer? If so, did you pause and resume the sequence at some point? Could this have happened at the frame where the pop occurs? You should be able to pause and resume without problems, but it seems like one of the more likely places that a bug could occur.

I would suggest running another low resolution test (and perhaps with a limited crop region around the lake) on the same frame range that you uploaded here, and see if the pop happens exactly the same as it did the first time. If it's 100% reproducible on the same frame then that's going to make it much easier for us to reproduce the issue here and figure out a bug fix.

[Njen]

You were right about your idea that it had to do with the GI cache. Usually on these long slow env shots I can get away with the GI cache being 32 frames apart every time, but I just did a test for this section of 16 frames apart, and it seems to have fixed the issue.

FYI, to answer your previous questions, the frames were rendered through Deadline on multiple machines, but then when I did a test on the same machine in the same Terragen session I was able to replicate the issue.

Thanks for your help!