Regression : cycles motion blur glitch on alembic hair #108415
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Reference: blender/blender#108415
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System Information
Operating system: windows 11
Graphics card: RTX4090
Blender Version
Broken: 3.5 3.51
Worked: 3.4
Caused by
75ad8da1eaShort description of error
alembic hair has way more strength of motion blur than other objects and makes them look transparent in cycles render
Exact steps for others to reproduce the error
just open my blend file and relink the mesh cache in the alembic file then render frames 163 and 178 you will see the glitch
blend file link
https://www.udrop.com/8e8k/MOTION_BLUR_GLITCH.blend
alembic file link
https://www.udrop.com/8e8n/CAOXIN_BONE_HAIR_LAYER_1_0513.abc
motion blur glitch on alembic hairto 3.5 3.5.1 motion blur glitch on alembic hair3.5 3.5.1 motion blur glitch on alembic hairto 3.5 3.5.1 cycles motion blur glitch on alembic hairCan confirm the difference, will check.
Caused by
75ad8da1eaCC @HooglyBoogly
3.5 3.5.1 cycles motion blur glitch on alembic hairto Regression : cycles motion blur glitch on alembic hair@M-M Hi, would you be able to provide a simpler file that also demonstrates the issue? 1.6 GB and 300 MB files make fixing such an issue difficult. If possible, it would help a lot. Thanks.
I could cut the alembic mesh cache into a few frames if it will work for you
And I'm sorry for the delay
Hi, thanks. I don't see a problem on frames 163 and 178 though:
You have to render it in cycles and turn on the motion blur, the alembic hair will have a much stronger motion blur effect which makes it transparent and it only happens in 3.5 and above
And make sure you have applied the hair material before rendering
Here is my guess about this glitch. The alembic hair has two vector data, one is moving with the character globally and the other one is those hair cards tumbling locally, probably those two vector data have a wrong combination algorithm which significantly increases the final vector result. Because I found when the character is standstill and moves her head, the glitch is gone.
and maybe the alpha channel in the hair shader is also related to this glitch, therefore only the hair has this glitch.
My frame 163 looks very different:
If I had to guess based on the bisected commit Philipp mentioned above though, I'd guess that this is the same as #107930, and that changing the auto smooth angle to 180 might work. That does seem to help the render here actually:
My snapshot is not frame 163, it was just for demonstration, this glitch occurred in my whole running scene around 500 frames, not just the two frames I picked. frame 163 is the wrong one and frame 173 is what it was supposed to be.
So this glitch will be fixed in the next update right? I am still using 3.4 for this reason.
I'm still trying to figure out what the problem is and whether it can be fixed. As I mentioned, this might be the same as #107930, which is a known issue.
Can you try setting the auto smooth angle to 180 degrees on the mesh with the particle system?
I've tried several frames with 180 degrees auto smooth and it is fixed, awesome!
But it is still considered a bug and will be fixed right?
Okay, in that case this is the same as #107930. For the reasons described in the last comment there, I don't think this will be fixed in 3.6 unfortunately. I'll close this issue to merge the two.