How to render a dark room properly?

What is the best way to render a completely dark room which is only lit by light coming under the door?

The picture below is result of over an hour of rendering. I used denoising, added shadow, AO, emission and environment passes, used full global illumination preset and the result still looks like crap.

Light is right behind the door and there’s also a narrow corridor going past the door.

How much does it improve if you add a portal near the bottom opening?
Does it improve if you use a rectangular area lamp (not geometry) to make it a more direct lamp, or at least reduce the bounces done?
It’s kind of a hard problem, that I don’t really have opportunity to look into right now.

What do you mean by “adding a portal”?

I haven’t tried area lamp (but I will). Somehow it feels like cheating, because the lamp outside the door reflects a real-life setup.

I did some experimenting with this and it seems to me if you put an area light behind the door and switch to Render Properties > Light Paths > Presets > Direct Light, you will speed up your render and get rid of a lot of noise. I don’t know if it is “proper” but I think it will give you the effect. Also I think the Compositor > Shift A > Filter > Denoise may suit you better. I also tried a portal light and it didn’t seem to help. How many samples did you use in that image?


Add an area light and set it to portal. It doesn’t produce any light, but it will “guide the rays looking for a lightsource” in the right direction, improving the probability to find the light source. You can try using Branched Path Tracing and play with the light sample switches, but I have little experience with this one.

Ok, that changes things a bit. Yes, it is cheating, but cheating is what you do if you don’t want long rendertimes from massive amount of samples/bounces. I’m not sure if portals “work” with lamps, might be a thing only in order to “find” the world, I’m not sure.

That will also set number of diffuse bounces to 0, so the room won’t be lit by it. Reason for the noise is because all light lighting the room are from indirect bounces.

For sure, highly recommended. It’s subject to creating horrible artifacts on low lit surfaces though.

This is the result with added area light portal and branched path tracing. Light path preset is stilll full global illumination (all bounces at 128). I also added a sphere on the corner to show that light quality drops tremendously with distance.

Share the scene. I was able to quickly get much better results than this.

Well, I sorta gave up and added a lamp inside the room. Now it’s combination of direct and indirect lighting. This is fast and accurate enough. Still have to fine-tune the balance.

What bothers me a bit is that light is leaking from the lower right corner of the doorway, although the walls are made of one solid object.