Cheers, all.
Martin. I played with many different AA settings with this, the most reasonable results(considering render times) turned out to be AA=4. Although the atmosphere is very thick, it's the displacements and the annoying lack of back face culling that raised the render times for this quite drastically, due to the micro polygon rendering of the surfaces that is required.
I ended up masking out the area to be rendered with a simple shape shader, in the opacity channel of a default shader, to only render as far as was in view of the camera and the visibility decay of the clouds. Even then, any frame where the canyons were in front of the camera(about 3/4 of the entire sequence) required the equivalent of rendering the same area twice, due to the canyons in the distance rendering before anything else did.
The atmosphere in the animation version is still a little grainy because I used 4(1/4 first samples) simply for quickness, or I'd probably still be rendering.
In the larger single frames, AA=6(also at default 1/4) gave the best results, quicker than AA=4(max) by about a third with RTA, and
almost, but not quite, as noise-free.
The pixel sampler is great, I liked your experiments with clouds the other day, although, I've never managed to be able to argue '0.05' quality out of the clouds! Good job. I never usually use max AA samples if I go higher than AA=4, it's just too time consuming for me here in most cases. 16 samples all round is normally enough, any more and the RT renderer goes too slow.
In most cases, for me, AA=6(default sampling), gives nearly the same quality as AA=4(max), and it's quicker. It's entirely scene-dependant, though. If there's only sky, you can even squeeze nice enough clouds with AA=1! The ground will be horrid, though!
