Glossy surfaces and noise

[boumay]

Hello,
A6 is really better at removing noise quicker.
But sometimes, the noise persists, no matter how much time we let rendering go.
I know there are a lot of topics about noise, and I learned a lot but I must admit that I need some information I didn't find yet.
For example, I have a scene that where the noise is eliminated quickly, but when I added the lights and the assets (furniture etc), it began to render with difficulty to eliminate the noise, even after 30 minutes, a lot of noise is present. So I decided to troubleshoot the problem and isolate the objects, removing materials, checking albedo, everything that I learned so far, and it seems that glossy surfaces is the issue here. Glossy surfaces seem to generate more noise that everything else in my scene.
I have wood tables with glossiness  (refl. 0.75/gloss. 0.35) and desklamps with glossy gold. I have also approx. 160 objects with a mid complexity.

Does anyone has an advice to improve the situation?
help appreciated.

[Ludvik Koutny]

One picture's better than 1000 words :)

[boumay]

That's right.
Here it is.
You can clearly see the persisting noise on the tables even after 38 min. The scene isn't so complex.
Also, it seems that the glossy surfaces of the tables "contaminate" the other parts of the scene as I don't have this noise when the scene is empty (environment only), or perhaps it is the complexity of the tables and chairs geometry (not so complex for me but I'm not an expert) that causes the problem.

[Ludvik Koutny]

It just seems like very difficult light sampling scenario. You got a lot of very small and strong light varying light sources. It may not be the noise of lights, but areas that are very close to light sources, such as those lamp shades. First, i would try to put just gray coronaMTL as material override in render settings, then disable all lights except environment, and start enabling them back one by one to see, which one causes most noise.

Also, what types of light sources do you use?

[boumay]

Yes, you're probably right about the lamp shades, as I noticed a worsening of the noise when I added them if I remember correctly.
My light sources are all spheres (with ies pattern, except for the desklamp=sphere only), there are only the ceiling lamps that are from a corona light mtl. The shades are housing 3 lights each.
I'll try overriding with corona gray mtl.

[Ludvik Koutny]

IES lights are really slow...  so i would be careful with them...  As for spheres...  you could maybe try rectangles facing down from the shades... And also those neon tubes on large rectangular ceiling lights may shuffle the light sampling balance around. Not saying you should remove them, but you should be aware that even those may change the balance of sampling, if it's coronaLightMTL. If it's CoronaMTL with self illumination, then it won't matter that much, but they will also not influence your scene lighting that much.

[boumay]

Thank you for telling me that ies light are slow, I didn't know. I didn't find that they are the cause of the problem of noise btw.
Here are some tests. It seems that the shade and ceiling lights are causing trouble, especially the shade lights (even after 13min30, the noise is quite strong)
The ceiling lights are corona light mtl with emit light on.

[Ludvik Koutny]

Yup, shade lights seems to make most of the problem. Noise may actually be from GI, which samples very bright areas around the light sources.

Several things you could try:

Decrease Max sample intensity - Default is 20...  so you could try 10. It will introduce slight bias but could speed things up.

You could set the lights under shades to be disc shape. Make sure they are facing downwards, and try to cover entire circular end of the shade, to minimize light bounced off the shade and maximize direct light contribution. Of course that would look bad if you looked on the lamps from the bottom, as it would look like one big flat surface. But if you do not plan to render the scene from such angle, it should not matter :)

Then, you could try to replace thin ceiling light material with CoronaMTL with diffuse set to 0, and self illumination enabled. After that, place a large, not too strong CoronaLight in rectangular mode right under them, rectangle shape being about the size of all those ceiling lights together, and make it invisible to camera and reflections and refractions :)

And last, if above does not help, you can try using standard 3ds Max lights with inverse square falloff (physically correct light falloff) and CoronaShadows... It's partially a fake, but it should not make much difference. They are not as physically accurate, but they render almost instantly :)

[boumay]

Thank you very much for these informations, very useful! I'll try these tips.
Btw, on this last render, I turned the materials back on, disabled the shade lights and ceiling, disabled ies on all lights, but even after 26min, there's still so much noise on the tables.
I'll see after applying your tips.

[Ludvik Koutny]

Yeah...  interesting....

Is there any possibility meshes could be co-planar? Meaning that by some modelling accident, there could be two faces in one... ?

It would help if you could post the settings of that material, and also try to apply that material on a simple box roughly the size of a table, and place it next to one of the tables.

[boumay]

I don't know, it's a model I downloaded, I'll check that.

[Ludvik Koutny]

Are those small light sources on the left side very close to walls invisible or turned away? You are creating super bright indirect hotspots that may be quite hard to sample...  But those should be hard to sample in any renderer, not just Corona.

So yeah, try all 3 possibilities :)

[boumay]

Ok, I'll try to move the little lights away from the walls as well. I'm trying to simplify the geometry of the tables and chairs, as there are too many polygons for nothing useful right now.
I'll keep you informed.
Thank you for helping.

[boumay]

Ok, so I tried the majority of the tips you provided (moving the lights away from the left walls, replace shade lights by a disc shape facing down, reducing max samples to 10, turned off ceiling lamps and replaced with a big sphere light to see).
And there is a good improvement of the situation. Nice.
I'll try max lights tomorrow, it's a bit late now.
Here is the last test I did, with disc lights under 2 of the shades. After almost 10 mins, noise is lesser on the tables (except when it's reflecting the shade lighting), so that's good. It remains noise around the shades area and bright dots in the noise as well, so I Wonder how much time it'll need or what to tweak to eliminate that, perhaps the gold high reflective material is causing issues.
But the overall situation is definitely better.
Thank you again.

[Ludvik Koutny]

I am not sure it's better, there's no noise because we removed all the highlights, so theres nothing for noise to be in :D For example that noise from reflection of light shades on the table in front could take over an hour to get to acceptable noise level....

I would try to make lights visible, in case their are invisible... Now i mean those lights above on the left...

It's really not an easy scene to render...  And rays/s aren't exactly high either... Could you maybe upload the packed scene somewhere (or at least a part of it) and then send me a link via personal message? I could take a look if i can get the scene rendering faster without changing lighting much :)