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Maniphest T102689

wrong Interpolation direction for new curve system
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Description

System Information
Operating system: Windows-10-10.0.22621-SP0 64 Bits (windows 11 22H2)
Graphics card: Intel(R) UHD Graphics Intel 4.5.0 - Build 30.0.101.1338

Blender Version
Broken: version: 3.3.1, branch: master, commit date: 2022-10-04 18:35, hash: rBb292cfe5a936
Worked: First Time using this system, so I haven't tested it in other versions.

Short description of error
When using the density or add brush (with interpolation enabled), the newer strands don't follow the direction of other closer strands.

Exact steps for others to reproduce the error
on current scene,
after placing some hair to use as a guide,
truing to use the interpolate feature of the brushes makes the new particle grow in odd directions different from the initial hairs.

bug_report_scene.blend1 MBDownload

scene blend file

Event Timeline

Franklyn Afeso (afeso) created this task.Nov 22 2022, 11:07 AM
Franklyn Afeso (afeso) updated the task description. (Show Details)
Omar Emara (OmarSquircleArt) changed the task status from Needs Triage to Needs Information from Developers.Nov 22 2022, 11:58 AM
Omar Emara (OmarSquircleArt) added projects: Geometry Nodes, Nodes & Physics.
Omar Emara (OmarSquircleArt) added a subscriber: Omar Emara (OmarSquircleArt).

The feature doesn't seem to work as advertised, but not sure if this is a misunderstanding. Tagging developers for more information.

Hans Goudey (HooglyBoogly) added subscribers: Jacques Lucke (JacquesLucke), Hans Goudey (HooglyBoogly).Nov 22 2022, 6:54 PM

Thanks for the report and the video. This is probably expected based on the code, since it finds the five closest neighbors and averages their directions.
But it looks like it should probably take symmetry into account and avoid retrieving neighbors from across the simulation axis.
For implementation, I assume that would build a BVH tree per-symmetry axis combination.

@Jacques Lucke (JacquesLucke), does that make sense to you?

Jacques Lucke (JacquesLucke) closed this task as Archived.Nov 23 2022, 4:28 PM
  • Generally it might make sense to control how many neighbors are taken into account for the interpolation here.
  • I disagree that generally symmetry should be taken into account when finding neighbours.
    • For example, in the video below it makes sense to interpolate between both existing strands.
    • Your use case here is hair parting. Using symmetry for that only makes sense in the special case when the hair is parted exactly on the symmetry axis, which usually is not the case.
  • We should have better support for hair parting in sculpt mode with features like "curve sets" which group hair into sets which belong to the same part.