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Hair Not Rendering in Cycles in 2.81
Closed, ResolvedPublic


System Information
Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: GeForce RTX 2070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 441.20

Blender Version
Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-11-20 14:27, hash: rB26bd5ebd42e3
Worked: (2.80 and earlier versions of 2.81)

In 2.80

In 2.81

Other examples:

Cycles 2.80

Workbench 2.81

Cycles 2.81

Short description of error
When opening a scene with hair created in 2.80 in 2.81, the hair is rendering strangely or not at all (note: this is not happening in every scene - but this is one it is happening in)

Exact steps for others to reproduce the error
Open the attached file in Blender 2.81 (official)
Go to Camera view
Render in Cycles

Event Timeline

@Joe Williamsen (jwilliamsen)
Not Reproducing It. Tested initially as-is, then moved the camera to get more detailed test images of the eye brows and eye lashes.
This machine

I could emulate the effects of your bug by unchecking the hair tick-box on the Properties panel on the Render tab; originally, on file open, it was already set; I trust you made your observations with it set and uploaded the blend file with it still set, but I am noting that the effect of the bug was as if the hair check box was unset.


I'm using a stock Dell laptop with its off-the-shelf graphics stack - nothing like your GeForce RTX 2070 NVIDIA. I noticed while looking over your settings on the Properties panel that you were using the Experimental feature set and the GPU Compute device - your NVIDIA card, of course. Since there is nothing like your card on this laptop, my rendering necessarily fell back to conventional CPU rendering - and that may be the basis of my successful render.

So: does this bug happen to you always - even when restricting yourself to the Supported feature set and CPU rendering, or does it occur specifically when GPU rendering? Since you have an RTX card, were you experimenting with Optix or using CUDA? Have you tried running Blender from a Windows command shell? If you do, do you see any unusual error messages printed in the shell, such as compilation failures in generating GPU-specific rendering code? If you use the --debug-gpu and/or --debug-cycles Blender debug options is anything interesting printed on the console?

Appreciate it if you could post these or any other clarifications on setting up your render. Thank you!

Hello Garry - Thanks for further prompting me to check other settings.

It appears that this is occurring with OptiX enabled in the Preferences. If I go back to just CUDA, the hair renders as expected. I didn't realize that OptiX was enabled in the System Settings. I'm not sure how that happened as the scene was a 2.80 scene just opened in 2.81.

This might explain why in a couple of other scenes that had hair, this was NOT happening (different render/system preferences). However, when I went back and enabled OptiX in those (already working) scenes as a test, the hair still rendered correctly (?) Very weird.

I went back to the original problem scene and disabled OptiX (switched to CUDA) and the scene is rendering as expected in 2.81.

So ... OptiX and hair might not be working correctly at this time(?) I guess I can just disable it....

So, to recap:

  1. CPU Rendering & Cycles - OK
  2. GPU Rendering & Cycles: NVIDIA/CUDA - OK
  3. GPU Rendering & Cycles: NVIDIA/OPTIX - Problems with some files (Specifically hair? Specifically with resized hair? Or are there other kinds of files (non-hair) with issues? [Edit - doubtful])

Your issue bears a strong resemblance to T71255 Particle Hair not showing as expected in Cycles viewport/rendering due to optix rendering which was recently closed by @Patrick Mours (pmoursnv), the lead developer on Blender - Optix integration. The fix lives in the alpha 2.82 Blender, available as an experimental download (daily build), but not in the recently released 2.81 Blender.

Take a look at T71255 and check whether the blender files that are not rendering for you involve resized objects. If so, this issue of yours is probably a recurrance of that bug. You might care to download a daily build of 2.82 alpha and see if the issue goes away in the experimental build.

Optix is a Work In Progress. The tracker task is T69800; you may wish to subscribe to that task and, if time permits, help out in Optix testing, if you care to. There is a benefit to get Optix running: NVIDIA RTX hardware provides very fast, very high quality, renders.

If you confirm that you are a victim of T71255, post the results here. You can actually close this task yourself if you care to. In any case, appreciate any update. Thank you!

Just checked - in the test file you submitted, eyebrows, eyelashes are all scaled at 0.019 in X, Y, and Z, putting your test file in the scope of T71255. Is that the commonality with all the other files you are having trouble with?

Joe Williamsen (jwilliamsen) changed the task status from Unknown Status to Resolved.Nov 24 2019, 5:14 PM
Joe Williamsen (jwilliamsen) claimed this task.

I went ahead and tried the file in 2.82, and it renders as expected - even when scaled - so not exactly the same problem as in T71255, but probably related (I didn't resize my model after applying hair - I did forget to freeze transforms before applying hair, though).

I'm going to do as you suggest and subscribe to the OptiX tracker task and close this bug. Thanks for taking the time to look at this and help me untangle it. If this occurs again, I'll spend more time trying to figure out what's happening.

Thanks again, Garry ;)