The render artifacts that can occur with the cloud optimization options vary greatly by cloud type. Indeed sometimes it's necessary to set it to "Optimize none" but I think it's unnecessary to always assume it won't provide good results. I've used it even in animations with no problems. It does depend on the cloud settings, and unfortunately I can't offer a lot more detail than that as I haven't done a thorough comparison to see if there's a consistent pattern (there probably is). My rule of thumb is to leave it set to the default of optimal acceleration, then if I see any problems with the clouds, it's one of the first things I test, turning off acceleration.
1: Defer Atmo/Cloud can be quite useful, but does require some experience and fine-tuning to get best results, with superior speed *and* quality to the non-defer rendering of atmo. When AA needs to be high for objects in the scene (for example), it can be difficult or even impossible to properly balance it and get superior results. In the case you mention, with no landscape, Defer Atmo will probably work quite well and be less tricky to adjust. I tend to keep cloud quality at 1 and just increase AA until the noise is gone in a situation like that.
2: I'm not sure what you mean by "turn off cloud shadows". Where? You mean "Enable Secondary" in the cloud layer(s)? This will turn off self-shadowing too, so no, you don't want to disable it. Turning off the landscape should be enough, I don't think you need to try explicitly disabling any other shadow options (I may be missing something though).
3: I don't know at what specific point a localized cloud would stop being more efficient than a global cloud system, I think it depends much more on visibility overall. If you clouds go completely over the visible horizon, I would guess they may be roughly equivalent, but this doesn't necessarily have to do purely with size; if you're in orbit you could see a very large area of cloud, but localize may still be useful if you only want a particular (large) area. What Localize does is just give the cloud layer a finite boundary for the rendering engine to confine its shading to. This is why it's faster. In the case of clouds all the way to the horizon, the horizon and planet itself provide some occlusion I believe. In the case where you have no terrain being rendered, this may change of course. As you say, use of 2D Shadow Map will probably get memory-prohibitive to maintain equivalent quality when using very large localized clouds. I would basically say the rule of thumb is to use localized clouds when you will *see* the localized boundary area of the cloud, otherwise it's probably not worth bothering.
#4: Voxel Buffer helps to further confine the shading algorithm's work by generating a voxel set from the input noise functions at a coarse resolution and then using this as the shading bounds. Again if the entire screen is taken up by your clouds, it probably won't help, but as long as your clouds are finite in screen space, it will probably help. It's basically a further/more fine-grained "localization" of the clouds. The main effect of increasing resolution is to increase detail of the resulting clouds as it more accurately represents the input noise function with the voxel bounds. This takes longer due to having more area to shade, and probably increased memory use for the voxel buffer itself, so there's a balance. It's easiest to visualize just by testing with it on and off on a relatively detailed cloud layer using default settings, then adjust voxel buffer resolution until you see a minimal difference and you can see what the performance difference is like. As to when voxel buffer helps most, frankly I'm not sure. Matt may have a better answer, but quite honestly *everything* varies scene to scene so much, it may not be possible to provide a really clear rule of thumb. Honestly there is so much tuning involved in creating a scene and multiple test renders as a result, that I think simply checking voxel and/or 2D shadow map to see if they in a given scene (on a lower resolution test obviously) is the best rule of thumb. I would however try to encapsulate a good approach as follows: voxel buffer will save you more time the more finite in area your and the more patchy your cloud function, and/or the more demanding your cloud shading (high samples needed), and having a good sense of the change in detail when using voxel buffer vs. buffer resolution should help inform you as to when to use it (when the trade-off would be acceptable). I can't say why your render was slower with it enabled, but I wold guess you had high coverage clouds or clouds covering a large area of the screen.
#5: (EDITED) Twice the dimensions in the image would give you twice the GI prepass samples in each direction. To produce an equivalent set of prepass samples you would need to halve the GI cache detail, but not the GI sample quality. So as stated in your first example, 4/4 should be equivalent to 2/4 at twice the dimensions. GI sample quality stays the same because that controls the quality of each GI sample point and is independent of resolution.
#6: No, I don't think so, in general leave the jitter where it is. Lowering it can produce banding. You may get away with lower samples if you reduce jitter, and you can test it to see if it does create banding in your situation, but I wouldn't recommend it in general.
Bottom line I see you're trying to squeeze all the performance you can out of things and I get that, but there are always trade-offs. The defaults are setup to give generally good results in a variety of situations. Start with those and test. And, if possible, provide some examples of the kind of scene(s) you're working with and we may be able to give more specific advice.
Last but not least, if you learn anything you feel is more concrete or differs from the above, let us know! Many of these settings are added based on specific situations we or a customer run into in production or on specific projects, and they prove useful when implemented, but they can not always be exhaustively tested in terms of performance impact across a large variety of situations. User feedback is an important part of the process.
- Oshyan
