Maru, you wrote: "Pass limit means exactly the same quality for each frame. "
But, in my opinion, same Pass = same AA, but it's doesn't mean "same quality"
for example:
(*) in an interior scene, you have a 50 Passes (in 30 minutes), perfect AA, but the GI isn't very good (NL= 10%)
(*) in an exterior scene (easier for GI), with the same 50 Passes (in 5 minutes because you have less GI), you have obviously the same AA, and perfect GI(NL: 3%).
So, if you want the "same quality" (read as same noise level) , Pass limit isn't the road.
You also wrote " if the noise limit does not bring you consistent results for example the visible noise changes across different frames"
But, Noise limit is a choice to have consistent result and it helps to have the same NL across the frames =)
Or am I wrong? =)
Well, in theory the noise limit should give you the same noise level for all frames. But sometimes it is not the case. Then I think using pass limit is the best solution.
I think in both cases it is the safest to find out which frame is the most problematic (has most noise for most time) and then set the pass or noise limit based on that. In worst case you will end up with some frames being unnecessarily over-sampled.
But with the noise limit it can easily go wild. I am attaching two examples. The scene is basically a noisy object, and the camera is moving along it so that on frame one there is sky visible only, and on frame 100 the frame is covered with the noisy object. If we set noise level to some high ("draft quality") value like 10, the first frames where only a small triangle is visible will render super quickly (5 passes only are enough), and then the rendering will gradually become slower (more than 5 passes will be needed), but it will be pretty fast, because the quality is low. This is expected. But if we set some "production" noise level like 2, then at the beginning we will render just 5 passes, which will render the object noisy (even though the reported noise level is <2 !), and then gradually the render times will become higher (as we need 5 passes, 10 passes, 15 passes, etc....) until the render time will become really huge. (I tried rendering the last frame where the whole view is covered and stopped after 30 passes because it did not make much sense to render further with the noise level still over 10).
Of course this is an extreme case, but it is similar to what I described before (camera moves from interior to exterior). With the pass limit you will get consistent quality per each frame in this case - as exactly the same GI and direct light samples will be cast (more or less - as we also have adaptivity, but that's a different story ;) ).
Also, I am absolutely NOT saying that the noise limit is useless! It is perfectly usable in cases where the scene doesn't change much throughout the animation - e.g. if the camera moves inside the interior only, or if the moving objects do not change their positions that much.
The scene + animation frames are here if anyone wants to play with it:
https://www.dropbox.com/s/bbpvm3e5kip8caa/noiselimit.zip?dl=0
