I understand what Multiple Importance Sampling (or MIS) does, but are there any drawbacks to using it?
When should MIS be used?
Is there any case where one would not want to use it?
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Here's a blog post from the MIS creator's announcement of the addition. He describes a little bit his goals and the purpose behind the feature. Here is also another announcement of the feature. I'll quote from the latter:
"There are generally less samples in the same amount of time compared to vanilla Cycles, but since the samples are smarter, the resultant image is clearer."
Fewer but smarter samples could swing performance either way (as the previous two answers indicate), and the impact on performance will vary widely from scene to scene, but the overall effect is an improvement in render quality. As a general rule, when building a scene performance > quality; when rendering your final scene quality > performance. Therefore, have MIS disabled when building a scene and enable it when you do your final renders. However, since MIS may not noticeably affect performance in some scenes, and it may improve performance in others, you may still want it enabled when building your scene.
There are a number of other settings that will also impact performance and render time and all of these interact with each other in weird and unusual ways in each scene. So go ahead and play with it in different scenes and decide for yourself how and when you want to use it. CGI is a very subjective art, and only you can decide if you like how a particular setting affects your renders.
According to the Wiki: "MIS helps reduce noise for big lamps and sharp glossy reflections, at the cost of samples rendering a bit slower. "
From my personal experience: the rendering time with MIS on is actually shorter because it converges to a smoother image sooner. So keeping is on is probably the way to go.
Multiple Importance Sampling Off
Multiple Importance Sampling On (Default)
I recently rendered a scene with a mesh light that was a very thin loop which had a texture that was partially emissive. Very simple scene with just the lamp and a diffuse ground plane.
When I turned MIS off for that lamp, it cut my render time from 5min to 3min for the same number of samples.
So MIS basically "guides" rays toward the material (for which it is turned on), so my suspicion is that my small, thin mesh light was getting a lot more samples than it really needed. The lamp really only needed about 10 samples to become totally clear (when it's the only thing in the scene), but MIS was giving it a lot more than that.
In short, I'm not sure if my reasoning is correct, but the answer is: turning MIS off for my mesh light saved a lot of time.