Embree shades hair differently from CUDA and OptiX #78152

System Information:
Operating system: Linux-5.4.0-7634-generic-x86_64-with-debian-bullseye
Graphics card: GTX 1050Ti

Blender Version:
Broken: 2.90.0 fdfe85c616 (2020-06-22 15:41)
Worked: Prior to Embree being enabled by default. 99436acde8

Short description of error:
I've found that in some cases hair will render differently between CPU and GPU rendering. The most common difference is shadows are slightly different.
At first glance this doesn't look like much of an issue, however, if you're rendering an animation on a render farm where some frames are work on by GPUs and others on CPUs (or even doing it locally because OptiX doesn't support CPU+GPU), the difference in shadows appear as a kind of flicker in the hair.

Here's a video show casing this issue:

2020-06-23 20-25-59.mp4

This is caused by Embree as switching back to BVH2 on the CPU makes the issue disappear.

The issue seems most apparent when you have a lot of thin hairs close to each other. Also when you're using a fully opaque surface material.
At first I thought this was a noise pattern difference between CPU and GPU. However, increasing sample counts and using denoisers doesn't solve the issue.

Exact steps for others to reproduce the error:
Create some scenes with some hair. Here's two I made:

#78152 - Embree Hair 1.blend
#78152 - Embree Hair 2.blend

Added subscriber: @Alaska

I don't know how feasible a fix for this is, but one "simple" solution jumps to mind:
Expose the "Use Embree" option to the user in a button not hidden behind a debug panel until the Embree for GPU thing is sorted.

Added subscriber: @brecht

Changed status from 'Needs Triage' to: 'Confirmed'

Embree for GPU would not solve this most likely, I would suspect the ribbon intersection code more than the BVH traversal.

Added subscriber: @TomG

@Alaska if you don't use Sky texture and use only sky light, do you have the same render output ?

In #78152#968135, @TomG wrote:
@Alaska if you don't use Sky texture and use only sky light, do you have the same render output ?

Yes, the issue persists if I don't use the sky texture.

Added subscriber: @Raimund58

Added subscriber: @dfelinto

Leaving it tagged as 2.91, but lowering its priority since it won't be tackled immediately.

As a side note for people experiencing this issue, changing the hair shape to 3D curve in the Render Properties panel works around this issue.

This issue was referenced by c60c30205d

Changed status from 'Confirmed' to: 'Resolved'

Testing with the patch seems to greatly reduced the differences between CPU rendering and GPU (CUDA). Thank you @brecht

There are still small differences, but I haven't investigated much.

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Embree shades hair differently from CUDA and OptiX #78152