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Increased Sampling Noise when Rendering the World Background in Cycles.
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Description

System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 442.19

Blender Version
Broken: version: 2.83.0, branch: master, commit date: 2020-06-03 14:38, hash: rB211b6c29f771
Worked: 2.82a

Short description of error
There seems to be a differing amount of noise when rendering the world background between 2.82 and 2.83 in Cycles. I can't work out whether this noise is actually increased or if it's just different. In any case, it appears that the noise in 2.82 is more uniform and 'correct'. Tweaking the levels for better viewing (see the attached gifs) makes it appear as though in 2.82 the noise was perhaps more spread out and there were both lighter pixels and darker pixels, whereas in 2.83 the noise appears closer together and there are far fewer lighter pixels, seemingly making it more noticeable. Is this intended behaviour?

Exact steps for others to reproduce the error
Both gifs are of default cube renders in each version using identical settings @ 512 samples.


Event Timeline

512 samples don't bring that much noise in my case.
But I confirm that they are different. Although I can't say if it's more or less noiser.

The settings are the same and the release notes do not indicate what may have brought this change:
https://wiki.blender.org/wiki/Reference/Release_Notes/2.83/Cycles

Brecht Van Lommel (brecht) changed the task status from Needs Triage to Needs Information from User.EditedTue, Jun 23, 8:44 PM

Please attach a .blend file to reproduce the problem.

Also ensure you have not enabled adaptive sampling, which does not exist in 2.82.

Sam Walker (Hexbob6) added a comment.EditedSun, Jun 28, 1:55 PM

I have uploaded 2 test files, each are default scenes saved in their respective Blender versions. Adaptive Sampling isn't enabled in the 2.83.0 version. I've also tested the sampling differences between one of the Beta versions of 2.82 (c939b4df18e9), against 2.82a and found that the noise pattern is identical. I can upload references if needed.

Brecht Van Lommel (brecht) claimed this task.

In these default scenes, all the noise is coming from dithering (this can be disabled in Output Properties > Post Processing).

This is an intentional change from rB852cdd476b55: ColorManagement: Dithering Improvement. The goal of dithering is to approximate the color as close as possible, and the previous code was off by 0.5/255 which gives more lighter and darker pixels with the default background color, but with another background color it will be different again.