New Interpolated Hair Curve node produces flickering hairs between frames #105775
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Reference: blender/blender#105775
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System Information
Operating system: Windows-10-10.0.19045-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 3080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 522.25
Blender Version
Broken: version: 3.5.0 Beta, branch: blender-v3.5-release, commit date: 2023-03-14 01:16, hash:
708e84df9025Worked: N/A
Short description of error
Using the Interpolated Hair node provided in the new asset library produces a hair system with child hairs that disappear and reappear as the mesh is deformed.
On one PC, I had luck applying the scale on both objects to stop the flickering, but the symptoms returned when I added a second bone and started deforming the mesh in more complex ways.
Exact steps for others to reproduce the error
Open attached blend file and start animation.
You may need to subdivide bone -> re-parent with automatic weights -> rotate one of the bones to see a more advanced case if the basic one looks OK on your PC.
This bug and test case were reported by the artist Ruaidri.
This is expected behavior, the
Distribute Points on Facesnode cannot save the result when mesh animating, as it involves many iterations to create points. Because of this, any error/differend accumulates.Further use of these points may not give the same result.
In addition, as I know, hair assets were also created as static, for example, so as not to require a complex structure to get around these kinds of problems.
So I will close this report.
Most of the flickering in there is from the Trim Curve node. Try disabling that and it's almost gone.
Not sure why the colors flicker but they don't if you go to Material Preview shading mode.
@mod_moder thanks for the info! Where can I find out more info about the design decisions for the hair assets? Are these complex structures going to be supported in 3.5? Animated hair has been a mainstay of Blender since Big Buck Bunny, so I hope we get a workable solution in the near future :)
@mod_moder in these kinds of reports I think it's reasonable to view them as "missing documentation" - if something isn't supported, it can be added to a "Known Limitations" section of the user manual. Examples:
Like @ideasman42 says, this sort of hting should be documented before the report is closed.
What exactly needs to be documented?
https://devtalk.blender.org/t/procedural-hair-nodes-nodegroup-assets-for-blender-3-5/27601/39
It's just a complex case in which the wrong things are used.
This and #105774 are the same issue, which is that only the real guide hairs are being deformed and the remaining operations are procedural on top of that.
To some degree this is indeed a mix of focusing on static cases for the initial iteration and missing documentation.
But for most cases this is fixed by doing the deformation last in the stack, whilst using the undeformed base mesh for the procedural operations like the curve interpolation.
The remaining issue is mainly with surface meshes that use a subdivision surface modifier, as the UV smoothing is applied by default, which will mess with the surface attachment UVs. Currently it is unclear what the best way to handle this would be but, it's been decided to be out of scope for 3.5.
The decision in favor of the current behavior was made because it's the most predictable, though it might not be what the user expects.
Reviewing this again now though I do think that it might be better to change the default behavior to work for deformation when no constructive modifiers are used on the surface mesh and document the issue with the Subdivision Surface modifier as a known limitation. Especially as this will be addressed in some way in the future and that would be closer to the wanted behavior.
@HooglyBoogly I chatted about this with Jacques and he agrees that this would be the best way to go about it. It is more sensible that the known issue is with the surface deformation itself, rather than introducing a separate one by avoiding it in the add fur operator.
Could you make the fix in the
Add Furoperator toSurface Deformmodifier to the end of the stackSurface Rest Positionoption in thegenerateandinterpolatemodifiersAnd then we need document the issue for the surface deform node in combination with the subdivision surface modifier? Setting UV smoothing to None fixes the issue.
Could you take care of that as well?