Hi, this is my first post... woot!
I'd like to bump this thread as it has solved my problems with cloud GI in my animations. Currently I'm working on two different cloud renders, both from an aerial perspective of approx. 30,000 feet. I was struggling with flickering GI in the clouds and then came across this thread which solved everything!
In case anyone else is struggling with this same problem I thought I'd share what worked for me.
I've broken the scene into three render passes: one for terrain, one for clouds and atmos, and one for the GI cache process. This setup was mostly to help in the iteration process, so I could iterate on the clouds by themselves to get client buy-off. Anyways, in the GI cache pass I've set the GI prepass to write out every 10th frame, GI cache detail to 2, GI sample quality to 2, GI blur radius to 24, and I've checked on supersample prepass. I've unchecked GI surface details in the Image Pass tab, because as I understand it (PLEASE correct me if I've misunderstood) this isn't handled during the GI prepass phase but during the actual image pixel rendering. I've also turned Cloud GI quality to Still / Low because my Clouds pass will have that set to Sequence Quality x1 which is also handled after the prepass (Again, correct me if I'm wrong). These settings have given me good results, very fast render times (approx 3 mins to generate the GIC file, on very good machines), and a file size of about 150 mb each.
So once I got a decent GI cache, I've setup my Clouds pass to read and interpolate 3 files of the cache, GI surface details turned off, and Cloud GI quality set to Sequence Quality x1. I was ecstatic to find my renders were flickerless!!
Also note that I have my cloud quality set to 1, set the voxels to 60 million, and Atmosphere quality at 32.
I'm rendering quite a lot of clouds at the moment (the radius is set to 170,000 because I can see very far into the distance). Test renders are rendering at between 10 - 15 minutes per frame with the render quality settings at AA 2, 1/16 fist samples, and pixel noise threshold of 0.05 using machines with 12 cores. Granted they are still pretty noisy renders, and only at half res of 1440x540, I'm happy with how things are looking! I'll be honest. After a few weeks of learning Terragen and getting frustrated numerous times, I wasn't convinced it would be able to do what I was hoping. But I think I understand it better now and its all thanks to these forums! Especially this thread.
Cheers, everyone. Terragen 4 is great so far and I'm hoping to use it in more projects.