Second light messes up textures.

Hi there,
I was doing this background setup with an internal light and a view outside, which looks as expected. But as soon as I add an external light, the internal textures get messed up, even if they are not hit by the new light!

Setup (low-res “sphere” is internal light, square in the background is external light, camera shown)

Only internal light

Added external light

I also tried deactivating the bump of the brick textures, but that does not change anything.
Anyone knows this phenomenon, any ideas?
Thanks a lot for any hint!

Background_demo.blend (11 MB)
Blend file attached.
No idea why it is so big, just because of low-res bricks?

It looks like the classic case of added lights stealing away samples combined with denoising (because regular Cycles path tracing samples a single light for each pixel with each pass).

The solution more or less is to just increase the number of samples (it will take longer to render, but there’s not much else that can be done in this setup).

Thanks a lot for your answer! I never read of light sources stealing samples from each other, would have never guessed something like that.
It seems to go in the right direction, but not as linearly as I hoped. So I was rendering the above images with 30 samples. Double the light source, double the samples, I thought, so I tried 60 samples:

The effect got weaker, but did not disappear. Doubling the samples again to 120 started to make the effect bearable:

On the other hand, I thought that only the internal light with half the samples (15) would produce the same effect. It does, but not as strong as in the two-lights-30-samples setup:

While raising the samples certainly helps, what is really going on here? If I have n light sources, how many more samples do I need? n^2 does not seem to be enough.
If anyone can shed some light on this, I would be really happy. :slight_smile:

Unless I have good reason too (shaped lights or additional shading control), I prefer to light with light objects rather than emissive geometry as they tend to get cleaner results faster. I will also split up my lighting into logical light groups and render separately then simply adding them up in post. I’m considering completely rebuilding my bathroom, and render it out to try different setups. I’ve split my lighting into four separate groups; ceiling&indirect ceiling @ 250 (samples), furniture @ 500, shower @ 500, and (11) spotlights @ 750. 750 is too low, I may have to up it to 2000, but when combined the splotchy noise isn’t that bad. It takes about 6-8 hours for a complete test.

Please arrest me if I’m wrong here, but you could turn off multi importance sampling for any light or material that doesn’t contribute significantly to the scene lighting. White walls and floors should have it on, but maybe not a white washing machine or dark surfaces. Again, not 100% sure how this is supposed to be used.

branched path tracing gets cleaner faster with lots of lights. actually, i found turning OFF multiple importance on lamps helps in some cases.

That, and because you didn’t enable compress from file -> save as dialog. It compresses it from 11MB to 440kB

Wow, I did not know this, thanks a lot!

I hope that someone will still be able to clear the situation with the lights and samples up, otherwise I will close the question in a few days as solved. Thanks for all replies!