How does the light setup affect noise in Cycles?

I’m hoping I can soon use Cycles to make animations. Since GPU rendering doesn’t work on AMD cards and CPU is so slow, I need to keep ray bounces and samples to an absolute minimum… and of course I don’t want to get noise either. I saw many tutorials on how to reduce noise with minimal samples, and disabling bounces entirely is a compromise that helps too. But I noticed something else last night, which wasn’t covered much in those tutorials: The type and positioning of lights heavily affects noise as well. I’m looking to understand this better.

From what I noticed, fewer light sources in the same area reduce noise. This makes sense, since I assume that the more lights you have the more places each ray can jump to. I also noticed that object lights (point, spot, sun, etc) cause less noise than mesh lights. Some artists stated otherwise, but in some cases a single sun light results in absolutely no noise with just 1 sample.

Are there any in-depth tutorials on how the placement of lights must be done to reduce noise to the maximum? What can cause a light to make everything more grainy? How should lights be placed in complex scenes (in holes, between walls, etc) so noise is avoided? Also, when should “Multiple Importance Sample” be used?

I think that a better way to say it might be that there are more places that a ray can jump from. So, there is in effect “more competition” for “what, exactly, is lighting this particular spot,” and this is definitely (part of) where noise comes from. I’ve also noticed more noise … much more … where the lighting effects are intended to be subtle and the overall contrast is low. Perhaps these solutions are more-complicated.

I find that if the render does not “quiet down” very quickly, it’s not going to do so at all.

In general, Cycles is useful to me in the same way that a photographic “soft-box” is useful, and in the same places. It’s the foundation makeup on top of which everything else still has to be applied. Conventional BI renders, used to produce additional “passes,” fill in the rough spots and are used to provide the highlights.

Hi, there was a video about the new Branched Pathtracing in 2.69 from latest Blender Conference.
DingTo also spoke about lights to reduce render times.

I hope some new info for you.

Cheers, mib.

@sundialsvc4: Yes, that sounds correct. The more lights a ray can be related to (especially at the first hit) the more noise will happen as rays decide “where to come from”. So an important way to reduce noise should be to avoid multiple lights affecting the same area. I wonder if this can be improved… for instance by having an equal amount of rays always sent to all light sources in range.

And I got the same feeling about image clarification. If the noise doesn’t mostly disappear after the first 4 or so samples, it will never fully disappear. Except maybe with +1000 samples for those who have the hardware to do that.

@mib2berlin: Thanks, I’ll take a look at that video a bit later.

So you build two complete scenes with identical objects but a different light setup and reassign new materials to every object in the scene?