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I built a scene to test light portals.

I use a very bright HDRi to light the scene. And used a closed cube with a very tiny hole, for the light to shine through.

enter image description here

The brighter the light, and the tinier the hole, the more noise you get. Material is a basic white diffuse node.

In this test-scene an activated light-portal produces MORE Noise at the same sample-count (1000 samples) enter image description here

What is happening ?

I thought Light-portals ALWAYS reduce the noise at the same sample count?

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    $\begingroup$ Everything is laid out to produce as much noise as possible..... this tells nothing about the scene itself or it's setup. Yes, portals should help reduce noise, but 200 samples is not alot, and may not show much improvement depending on the specific scene and it's setup, lighting, and materials of which we have no information. Please edit your question and add more clarifying information. $\endgroup$ – Timaroberts Apr 21 '18 at 6:11
  • $\begingroup$ Please use the edit link at the bottom of your question (i.stack.imgur.com/lXFuK.png) and add more information on how you set up your project. Add images that might help us understand your scene and settings. Consider sharing your .blend file so that others can inspect it. You can upload it at blend-exchange.giantcowfilms.com and then paste the resulting link as part of your question. $\endgroup$ – cegaton Apr 22 '18 at 18:42
  • $\begingroup$ i edited it. But i dont get how it would improve my question. Its about the fact that i can built a scene that "brakes" the function, a light portal should have. Not that i need a solution for this particular scene. $\endgroup$ – Jens Clar Apr 22 '18 at 19:15
  • $\begingroup$ Light portals don't always reduce the noise of a scene like this. They usually do, but not always. I do not know under what condition they do or don't but I do know that even for interior scenes they don't always help. $\endgroup$ – Brenticus Apr 22 '18 at 19:34
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If you turn diffuse bounces to 0 (disabling indirect light), the source of the mystery reveals itself.

Here's 100 samples with no portal, and no diffuse bounces:

test with no portal, noisy

And the same, but with the portal:

test with the portal, light thrown on the wall is lessy noisy

So why is the full scene noisier when we turn on diffuse indirect light? The portal INCREASES the amount of light on each sample, thus increasing the number of diffuse bounces that return something instead of nothing. So you get more noise there. The combination of the small window and white walls mean the scene is almost entirely indirectly lit.

There's a few things you can do to speed this up. First of all, the portal is much bigger than the window. Shrink it to exactly the size of the window. And while we're at it, throw a solidify modifier on your room cube, that way you don't get the weird light leaks at corners. That's imperfect though, so you'll need to fake a lot more than that. Time to stop world indirect light entirely. Go to the world panel, and change bounces to 0:world bounces removed The portal will still throw sunlight on the wall, but it won't keep bounceing around and making a ton of noise.

Finally, replace the missing lighting by making a copy of the portal (leave it in the exact same place), disable the portal option so it's a regular area light, and give it a sky color. Adjust power to taste. That gets the indirect lighting clean enough that the denoiser can take care of the rest. You can disable desnoising on all passes except diffuse indirect if you like:

denoise indirect

And the result of all that, same 100 samples:

final result

You don't have to use ALL these techniques of course, you use just some to get more realism at the cost of render time. For example, you could set world bounces to 1 instead of 0. Play around to get the performance/quality balance that works for your shot.

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  • $\begingroup$ Thanks so much for the detailed answer. I usally dont care about render time, and go for the realism route and therby ignore reducing bounces or simplifying the scene. Im currently just doing a tutorial in which im also mentioning ways to reduce noise and fireflies, and was gonna mention light portals. And before i put it out there, i wanted to be sure, i knew what was going on with them. Thanks again so much :) $\endgroup$ – Jens Clar Apr 22 '18 at 21:34
  • $\begingroup$ Love it. Great explanation! $\endgroup$ – brockmann Apr 23 '18 at 9:20

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