Enabling Surface Rest Position in the Interpolate Hair Curves Modifier Disconnects Hair from Surface Mesh #109398
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Reference: blender/blender#109398
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System Information
Operating system: Windows-10-10.0.22621-SP0 64 Bits
Graphics card: Intel(R) UHD Graphics 770 Intel 4.5.0 - Build 31.0.101.3959
Blender Version
Broken: version: 3.5.1, branch: blender-v3.5-release, commit date: 2023-04-24 18:11, hash:
e1ccd9d4a1d3Worked: No versions that I am aware of.
Short description of error
Toggling on "Surface Rest Position" in the Interpolate Hair Curves modifier detaches the hairs from the surface mesh in an animation. This is resolved by positioning the Surface Deform modifier at the top of the modifier stack, however this is not recommended (as per #109285)
Exact steps for others to reproduce the error
Likely the same cause as #107856 ?
That was @SimonThommes 's inclination, however since the issue persists when the subsurface modifier is applied, he suggested making a separate report (hence this one).
Can confirm
I believe you are misinterpreting how it works before enabling rest position:
-With rest position disabled, it's just "snapping" the source curves onto the closest surface at the moment, thus it looks like the hair moves with the animated mesh- but it's just re-snapping each frame
-With rest position enabled, it properly snaps to the rest position mesh before posing
-However, this mesh has invalid UVs: so this makes it so they cannot pose at all. I think this is because you are using an UDIM uv map, which causes invalid attachments
Simplest fix is to create a new uv map that is just within the regular bounds, and has no mirrors.
It is also possible to edit the interpolation node to accept a second "attachment" UV map separate from the color UV map, but that is more advanced and you'd have to dive into the interpolation node group, which is what I did for one of my projects that had UDIM uv maps
I'm still somewhat new to UDIMS and might be misunderstanding, but as far as I can tell the UVs are contained within the first UDIM tile, and the only references to UDIMs that I can find is in the Overlays panel, where Tiles X is set to 4 (which is only a visual guide, from what I gather), and the UV Map name being "UVMap UDIM".
Nonetheless, the Invalid surface UVs message remains when setting Tiles X to 1 and creating a new UV map.
If the UVs are all within the first tile then it's fine, yes.
I've found an issue with the file, on the interpolation node modifier you do not have the UV Map attribute set. You need to set it to the UV map's name, in this case, "UVMap UDIM"
After setting this the curves get their valid attachment.
Note: If you alter the uv map, you must re-connect the curves by going into sculpt mode -> snap to nearest surface
This does appear to be the culprit; the UV map attribute in the Interpolate Hair Curves modifier must have been lost at some point while I was trying to isolate the original bug from which this one stemmed (the original issue still stands though).
Thanks for your input, I'll close this report.