Cycles renders artifacts on CPU but not on GPU
Open, ConfirmedPublic

Description

During a project I noticed this strange shading effect which I wasn't able to "fix" by just tweaking settings. What's interesting about this is that it only occurs for CPU rendering and not for GPU (usually I would expect it the other way around).

Just load the blend file and render it with CPU and GPU to see the effect.

cycles-cpu-artifacts.blend

Tested with the official Blender build on Windows 7 64.

Details

Type
Bug

I can't reproduce that on my local Linux build (rBa2ebc5), but it wouldn't be the first issue that only happens on one OS.

When you say "the official build", do you mean 2.78a or the latest buildbot?

It can be reproduced with the official 2.78a release and todays buildbot version for Win64.

Scorpion81 has tested it on his Linux system as well and it appeared there too, afaik.

Yes, I have been testing with CPU rendering on FM builds since 2.72.2, since 2.73 the FM builds are based on release builds, and for me this bug happened in every version since 2.73. It does not happen if you render on GPU, or if you disable "Cast Shadows" on the sun lamp (but then you have the same result as in 2.72.2 where no shadow was visible at all)
I use linux mint 17.3 btw, and a I7 2600 CPU, and a GTX 580.

Sergey Sharybin (sergey) triaged this task as "Incomplete" priority.Dec 9 2016, 2:23 PM

Does the issue happen if you apply Subsurf modifier with Simple subdivisions? What's your exact CPU?

1 subdivision:

2 subdivisions:

3 subdivisions:

Tested and confirmed on:
Intel® Core™ i7-980X Processor Extreme Edition
Intel® Core™ i7-3632QM Processor

  1. I do see the artifacts with GPU rendering(GTX 965m), they are just much more faint.
  1. The artifacts disappear when I separate the two pieces into two separate objects.
  1. The artifacts disappear when I change the Z dimension from 0.0001061 to 0.0
  1. The artifacts disappear when I change the dimensions from (1144.902, 1030.813, 0.0001061) to (1144.902, 1030.813, 1.0), this makes it look like a precision issue since the problem goes away as the very small z-dimension approaches the others in relative scale.

For me, the artifacts begin to clear up as the the z-value for 'Dimension' approaches 0.008.

Scaling the model down in size also clears up the artifacts.

I have no idea how to fix this code-wise for Blender, I just know how to avoid the issue. ->Try not to make your models too big with respect to Blenders visible grid and try not to move too far from the origin.

For Sergey :) Here's an article from Siggraph 2012 that goes into some detail about precision problems and solutions when dealing with large scenes.

http://www.humus.name/Articles/Persson_CreatingVastGameWorlds.pdf

Sergey Sharybin (sergey) raised the priority of this task from "Incomplete" to "Confirmed".Dec 12 2016, 9:59 AM
Sergey Sharybin (sergey) claimed this task.

Can reproduce the issue on AVX kernel, the issue does not happen in AVX2 kernel.

Will have a closer look later.

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