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Maniphest T52275

Denoiser will blur sharp reflections
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Description

System Information
Win 10 (latest) 64bit
Alienware 17 GTX 880M

Blender Version
Broken: 2.78.0 702e9c5
Worked: not available

Short description of error
The Denoiser will blur sharp reflections at a special direction. The issue will be described in the attached renderings and scene. Higher sample rates will not fix the issue. It looks like there's a bug in the collected scene data for the Denoiser.

Exact steps for others to reproduce the error
Use the attached .blend file and compare the renderings (with and without Denoising with 128 samples).
Same result of GPU and CPU rendering.
You will notice the white LEGO bricks are blurred in the red bricks while the floor reflection in the black bricks is correct.
For all materials the Roughness is set to 0.

170806_Denoiser_Issue_with_Reflection.zip12 MBDownload

Event Timeline

Michael Klein (Renderbricks) created this task.Aug 6 2017, 1:12 PM
Michael Klein (Renderbricks) updated the task description. (Show Details)Aug 6 2017, 1:31 PM
Michael Klein (Renderbricks) added a project: Render Pipeline.Aug 6 2017, 2:43 PM
Michael Klein (Renderbricks) lowered the priority of this task from 90 to High.Aug 7 2017, 6:32 PM
Brecht Van Lommel (brecht) raised the priority of this task from High to 90.Aug 7 2017, 6:50 PM
Brecht Van Lommel (brecht) added subscribers: Lukas Stockner (lukasstockner97), Brecht Van Lommel (brecht).

Please let us assign the priority, Needs Triage is there to indicate that a developer needs to evaluate the report.

I'll leave it to @Lukas Stockner (lukasstockner97) to decide if this is a bug, not sure if this is a case we expect to be able to handle. Note there's also various denoising parameters that can be tweaked. In general the denoiser is always an approximation and there will be various cases where it can't detect sharp edges. There's a lot of ongoing research in this area and even the state of the art algorithms fail in some cases, we can't always consider those bugs.

The environment map is missing in this file, but I think with a flat background color the problem is visible too. As far as I can tell the reflection does become sharper with high samples, but not as quickly as it should ideally.

Michael Klein (Renderbricks) added a comment.Aug 7 2017, 7:34 PM

Thanks for your detailed answer. I rated this as a bug because the floor reflection is correctly sharp while the wall reflection is sharp in the render progress and will be blurred extremely by the denoising what will falsify the result.

Bastien Montagne (mont29) lowered the priority of this task from 90 to Normal.Aug 8 2017, 10:43 AM
Bastien Montagne (mont29) edited projects, added Cycles; removed Render Pipeline.
Lukas Stockner (lukasstockner97) changed the task status from Unknown Status to Unknown Status.Aug 9 2017, 12:42 AM
Lukas Stockner (lukasstockner97) claimed this task.

The problem here is the diffuse-specular-heuristic. The floor is classified as specular, so the feature passes are written based on the secondary bounce and therefore preserve reflections.
The brick on the other hand is considered diffuse, so the normal/albedo/depth passes contain no information about the white object.

Therefore, there's no simple way to solve this - tweaking the thresholds to make this scene sharper would introduce additional noise in other scenes. Two compromises would be exposing the specularity threshold as a user-tweakable parameter or allowing users to override the selection per-material. Both would be fine with me - of course automatic handling is generally better, but there's no single perfect way to map general BSDFs to a binary choice.

So, I'm going to close this since it's not really a bug, just a case where the heuristic fails. Improving the heuristic definitely is a ToDo for me, though, so this will hopefully improve in the future.

Michael Klein (Renderbricks) added a comment.Aug 9 2017, 1:49 PM

Hello Lukas,

thanks for the explanation.

Additonal info: SSS is affecting the blur extremely. When I set SSS to 0 the reflection is a bit blurred but more as expected.

Michael Klein (Renderbricks) added a comment.Aug 9 2017, 1:56 PM

Metallic and black color (no diffuse) is working fine. Specular and SSS with more diffuse will cause a blurred result.

I am looking forward to improvemts in the future. You guys do all an impressive job on this Autodesk killer. :-)

Michael Klein (Renderbricks) added a comment.Aug 9 2017, 7:09 PM

I found an interim solution by using Branched Path Tracing. This looks much better. I could also set just Glossy higher and turn Indirect off in the Denoiser. But just using Branched works good actually.

Looking forward to your improvements in the future. Keep up the fantastic Autodesk killer work. ;-)