Hair Guides not following surface deformations #119763
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Reference: blender/blender#119763
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System Information
Operating system: Windows-10-10.0.19045-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 4090/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 551.23
Blender Version
Broken: version: 4.0.2, branch: blender-v4.0-release, commit date: 2023-12-05 07:41, hash:
9be62e85b727Worked: never?
Short description of error
Hair guides (geometry hair) are not following surface deformations (e.g. shape keys) when hair is interpolated.
Exact steps for others to reproduce the error
At least for me, this is a serious issue, after I can't fine tune hair curves on a deformed mesh. I have always to disable the Interpolate modifier, but then I don't see how my sculpt is affecting the hair.
Can confirm (but also need to double-check if this has been reported before -- i assume it is)
#107856 and #109285 might be related.
I still dont have a clear picture of what should or what shouldnt in therms of modifier deformation and sculpting (or animation as well).
It might absolutely be expected behavior, though having support for crazyspace sculpting of curves in one situation and not in another is quite a bad user experience imo.
@SimonThommes : could you once again share your wisdom here?
I think the issue is in how Blender's Interpolate Hair Curve node is build.
VFX Grace Addon "3D Hair Brush" just released an update (version 4.6) with own Geo Nodes.
One of them is an interpolation node, which is not corrupting the hair guides on a deformed mesh:
Just tested it:
The issue is indeed a consequence of how the interpolate node-group is built. The original guides are not part of the output but replaced with 'identical' children, so the guide overlay falls back to the original data.
The choice to implement it this way, rather than adding the interpolated curves to the original guides was made deliberately, mainly to ensure that surface attributes are available on the guides just like they are on all the interpolated children. I don't remember this issue, I think the consistency with crazy space is something that slipped through.
I agree that this should be resolved, but I'm not sure what the best solution here would be. Going back to an initial (simpler) implementation that would keep the original guides intact would drastically change the behavior and is not a solution imo.
To fix this problem properly we would need to be able to sample and store all surface attributes on the guides with a wildcard, which is not implemented. For the time being it would be preferable if either the original guides could be correlated with the interpolated curves that take their places or somehow skip modifiers that lose the mapping for the overlay.
I don't really think any of these solutions are possible at the moment tbh.
@JacquesLucke any ideas?
@SimonThommes
After we don't get a response from @JacquesLucke I've an idea for an alternative solution.
How about to add a flag/checkbox into the geometry node. Per default it's off with the old/current behavior. If it's on, than sculpting hair will work, but you get the different behavior you described.
I'd rather not create checkboxes to toggle between two suboptimal solutions where the differences and pros/cons are quite subtle and not clear to the user.