Noise in Shiny Gold Metal Material

[slice]

I have noise appearing on (the edges of) a shiny metal object. I have been through all the forums and advice I can locate on this topic. The noise is restrained to this gold metal object only. No problems anywhere else in the scene. The noise does eventually go with more passes (like many more) which is fine for my high res stills, however I will be animating these scenes so I need to be able to get my scenes to render in far fewer passes.

My scene is reasonably simple, a few objects, a couple of corona lights, a simple studio warehouse HDRI for extra reflections, rendering with path/UHD. What is unusual about the scene is that it is very dark. Very low lighting and other than the light fitting all materials are very dark or black with the detail in the image coming out through the reflections/glossies of that scene geometry.

I have adjusted my material to reflect Corona shiny metal advice (diffuse 0.0, Fresnal controlling lightness 20-100, keeping reflection colour lower than 200 and controlling colour of the material through reflection colour slot). (in the process darkening my metal more than I want it - refer test old for the brighter colour that I preferred). I have no maps in the material.

The problem repeats on simpler geometry with the same material, although again just on the edges.

I have tested GI, AA, LSM, MSI, Albedo as per Corona guidelines to reduce noise in scenes.

The problem does not occur if the reflection is turned down or off. The problem is reduced or resolved faster if I turn off the rest of the scene geometry (less reflections I guess). The HDRI I am using is not high quality, but the noise is there without the HDRI in the environment slot as well. It is worse with the HDRI in, but only because there are a few more highlight reflections generated which is where the noise occurs.

The problem seems very specifically to do with the reflection component of this material. But I don't know what I can adjust to resolve it.

And denoising to remove these areas of noise degrades the rest of the image too much.

I hope someone has some suggestions that may assist!

Thanks

[pokoy]

Are you using the RoundedEdges map in that material? If not are you sure the model's edges/chamfers are clean?

[Njen]

Glossy metals take a lot longer to clean up, trust me, I have scenes that have a lot of metallic surfaces, and those renders take a lot longer to clean up than scenes with mostly dielectric surfaces.

[slice]

Hi Pokoy, I am not using the Corona RoundEdge map. Are you suggesting I should?
I think the geometry is clean. It is modelled in max and is just a simple champhered edge. I have tested the material on a champhered box - native max geometry and I still get the noise.

[pokoy]

Could you share a simple scene that shows the noise?

[slice]

Sure Pokoy. Here is a stripped back scene with the light fitting with the revised/advised gold material (and my preferred brighter material in the material slot above it). Single light source, default render settings. HDRI imagery also attached as only the external files required.
I did notice when packaging this scene that I was using only the jpg version of the HDR file because I only wanted some extra reflections, not the full effect of the lighting. Possibly this is causing some issue. I ran some tests with the proper HDR and it is improved, but of course it changes my lighting. I will have to run some more comprehensive tests across my full scenes to see if this impacts on the noise problem. I had previously noticed that removing the HDRI from the environment slot didn't reduce the noise so I suspect this isn't the source of the problem.
Anyway would be great to know if anything jumps out in the scene for you.
Thanks!

[maru]

I made some tests. Here are my results:

1) reset-settings-2min.jpg - reset all render settings (scene tab > reset settings), then render for 2 minutes - it looks quite ok to me, but I can imagine some flickering in animation, so let's continue:
2) reset-settings-20s.jpg - reset all render settings, then render for 20 seconds. It is easier to see the quality issues now.
3) 20s-givsaa4-lsm1.jpg - lowered gi vs aa to 4, lowered light samples multiplier to 1, then rendered for 20 seconds - as you can see the noise/fireflies are more acceptable now, because we are rendering AA with better quality - there is slight improvement
4) 20s-givsaa4-lsm1-POST.jpg - same as above, but enabled bloom and glare (default settings) and blur+sharpen (default settings) in the vfb - the pixels are additionally blurred
5) 20s-givsaa4-lsm1--CLAMP-POST.jpg - same as above, but enabled highlight clamping in system settings (note: it is NOT the same as "highlight compression" in the vfb!) - all highlights are clamped now, but the quality is much better. It is almost identical to the 2minute rendering. Keep in mind that this will kill the dynamic range of your image, so if you are planning to save to some 32-bit format, then you should not touch highlight clamping. Otherwise, it is perfectly fine to use it, and it can cut down render times dramatically.

Another idea would be to render the image in higher resolution, and then reduce it to the target size. This way the bright highlights would not be smaller than one pixel any more, and it would be easier to sample them more efficiently.

Note: I was using the newest daily build of Corona, where adaptivity is working slightly different, and there is a fix for oversampling dark areas, so results in 1.7 may be a little different. Trying the newest build may be an interesting idea too: https://coronarenderer.freshdesk.com/support/solutions/articles/5000570015

[slice]

Thankyou Maru. I will review your feedback with the full scenes and see how it works.

[pokoy]

Maru beat me to it but I second his advice. I gave it a go and highlight clamping will help most, setting it to 20 already helped, 10 was quite good. Keep in mind this has an impact on how bloom&glare is calculated, you'll probably have to adjust.

The 'problem' is that sampling works randomly and it won't know about your thin object edges and that these need more samples to look clean. Lowering AA will help, too, as this one will force more samples on AA, but it may still miss the object's edges. Setting Highlight Clamping to something around 10-20 will make sampling more efficient everywhere, and will help specifically with hot pixels.

[slice]

Thank you for those suggestions Pokoy and Maru. These have certainly really assisted.
I should reset those settings when I do the same scenes as high res stills I assume?

[maru]

I should reset those settings when I do the same scenes as high res stills I assume?

Yeah, but only if they are causing some issues. If not, then it is safe to keep them.