Danny mostly covered it, along with others on specific things, but I'll chime in with some cleanup and more in-depth explanation.
First, as Danny indicated, your atmosphere sample settings are just way too high. *Sometimes* you will need settings that high to reduce noise, but really only when there is more shadow in the atmosphere, near sunset or when rays are being cast. In this case, on a relatively sunny day with the sun fairly high in the sky and an atmosphere of fairly normal thickness, you really shouldn't need more than 16 or at most 32 samples. If you're having cloud quality issues remember that the cloud quality settings are separate from atmosphere for a reason (though a noisy atmosphere *can* appear to cause noise on clouds, particularly the undersides, in areas of heavy shadow; that's not the case here).
With Defer Atmosphere your atmosphere samples are influenced by your AA level. With such high atmosphere and cloud Quality/Sample settings you're basically ensuring that the maximum number of samples get taken everywhere, and slowing everything down (your high atmo sample settings require lots of samples to be reached, but your low main AA setting means the maximum number of samples must always be taken to get them). If you use Defer Atmo, you want to use moderate quality/sample settings in your atmo/cloud shaders and low-ish AA (e.g. 4), *or* higher cloud/atmo samples and *more highly adaptive AA* (e.g. aa 8 with 1/64 first sampling level). That being said getting Defer Atmo to work really well (and faster than non-defer) can be a little tricky, so we really don't recommend using it until you're a bit more experienced with TG and how it works and what typical render times are.
So with that in mind I think you may be better off trying to get good results without Defer Atmo. In which case your main Detail level (in the renderer) influences cloud quality (instead of AA), and you'd want to probably raise Detail to 0.5 or 0.65. Turning off Defer and using 0.5 detail already cuts down render time a bit here (although AA8 and 1/64 first samples gives higher quality and is still faster than the settings you were using). Reducing atmosphere samples to 16 reduces that time further and lets you increase to 0.65 detail to improve cloud edge quality a bit. But I do see a little noise in the cloud shadows at 16, so perhaps use 24 Atmosphere Samples in this case. The general policy is to use smaller, incremental increases to get you the quality you want without astronomical render times. It may seem that the settings are "fiddly", but they're really just best used in smaller increments in most cases.
Finally, the cloud Acceleration Cache is often quite effective without noticeable render artifacts, so I'd always recommend leaving it on unless you see an issue in cloud rendering. In this case I turned Acceleration back to Optimal and it halved render time, and the output is pretty much the same.
Also, you really should stop adjusting the Sample Jitter. Really.
Technically you probably won't even notice the effect of Soft Shadows in this scene *as it is*, but that could change if you add vegetation or get closer to the terrain. Basically with a realistic soft shadow diameter, you're far enough away from the terrain and from any shadows actually being cast onto something to not really notice the softness much. So again, render time saved.
In the end, on a representative crop render area, I cut render time down from 1 hour and 10 minutes to only 8 minutes by turning off defer and reducing samples, plus enabling acceleration cache. A huge difference. And the end result looks quite similar, though not identical (attached). There's more noise in the clouds in the optimized version, but you could probably improve that by increasing cloud quality a bit, or maybe increasing atmo samples to 32.
That being said, I just ran a test with Defer Atmo at AA8 and 1/64 first sampling, along with atmo samples 32, and cloud accel cache back to optimal, and it was 8m30s and lower noise than the non-defer 8m render. So... that's probably the winner. It looks like in this case, if you can use highly adaptive AA, then Defer Atmo works well. But be careful in using that kind of setting in other scenes, don't assume it will always be the best combination of quality and speed...
- Oshyan
