Advice on getting rid of noise in a specific channel?

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I'm rendering this “cornell room” type of scene, and I've got render noise under control for the most part, but there are some specific image planes where I've got some weird things going on.

My scene is a cube room with white walls, an area light up top, and 2 spheres. The spheres are using built-in materials for Glass and Silver.

Composite result (mostly fine). The Indirect Diffuse was also fine, not including it here.


Direct diffuse - what is with the rectangle of noise?


Direct reflect - why is the glass reflection all noisy, when the silver one is fine?


Direct refract - why is this pitch black? Is that normal?


Indirect Reflect - again, the glass just seems noisy for some reason.


Indirect Refract - Oddly, this pass is NOT noisy even though the direct one was.


Direct Samples - Huge rectangular artifacts. Not sure why.


Indirect Samples - Also has some rectangular artifacts.
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Here's the project file.

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t.hipnc (617.9 KB)

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I took a look at your file, the biggest issue I could see is that you have a 0 roughness for your glass material and are using ggx. ggx in my opinion doesnt like a pure 0 for roughness. having a small amount at .001 is better, and renders much cleaner.

I noticed the spheres were crashing through the floor plane and this could also lead to noise and poor shadows.

I modified your file to show a cleaner way of creating the room, and ensuring the spheres dont crash through the flooring.

hope this helps

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t_153_mod.hipnc (642.1 KB)

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Thanks so much! That roughness suggestion was a great fix. And I like the “box” primitive with group-numbers used for material assignment, thanks!

How did you ensure the spheres don't crash through the floor?
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Also, do you have any advice for removing noise? I want to find settings for a multi-hour render I can do overnight with zero noise. I'm already up to 7x7 pixel samples and I don't know what else to do.
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To ensure the spheres dont craash through the ground I did two things

1. in the box object, I used the height of the box divided by 2 to ensure it was at world 0 in Y.

2. I did a similar thing with the two spheres where I used the radius in Y to adjust the height, again keeping the lowest point at world 0 in Y.

These two things will keep them from colliding.

As far as noise, it is just a matter of increasing the Reflection quality slowly so as not to smash your render times.

Also, you could try using the Indirect Sampling controls that I added at the bottom of the Render/Sampling tab on your mantra rop, as it seems the most noise is now in the indirect reflection pass.
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I've got diffuse/reflection/refraction quality at 10, and the indirect lighting node at sampling quality of 10, and I *still* get noise in the “indirect_reflect” channel, any more advice?

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t_153_mod_836.hipnc (641.5 KB)

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I suspect the main thing throwing you off is the ‘preview’ toggle at the top of the render view. It turns on the iterative coarse->fine update, but will give you different results to the final per-bucket render. Try turning it off, should see the artifacts go away.

Have a look at the attached scene, I think you've pushed the quality settings way too hard. I've turned everything down to more sane levels (reduced multipliers everywhere, upped primary samples to 8x8, increased max adaptive samples), renders quickly and looks smooth enough to me.

If you've mainly used non-pathtracers before, it's hard to let go of the notion that all passes should be completely noise free. We had to unlearn that at work where lighters would render millions of passes, then in comp do almost a complete relight by exposing diffuse up by 2 stops, crunch reflection passes to hell, mult on ambocc, saturate and half mix in indirect-diffuse etc… Not only is it incompatible with a pathtracing workflow, its too easy to dial in a look that people look at and say ‘it doesn’t look real and I don't know why'.

You can't expect to get all passes totally clean via pathtracing for crazy regrade possibilities without crazy render times. Instead, aim to get the beauty as close to final look as possible, keep the passes for subtle regrades, but always look at the noise in context. Yes, the indirect reflect pass by itself is a little noisy. In the beauty pass its barely visible. If you expect to grade the reflection pass more visible in comp, then fix it in the shader, so mantra knows where to steer its samples.

Attachments:
cornell.hipnc (647.6 KB)

http://www.tokeru.com/cgwiki [www.tokeru.com]
https://animallogicacademy.uts.edu.au/ [animallogicacademy.uts.edu.au]
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Great advice, thanks!
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