Hair Dynamics Strange Behaviour #80438
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Reference: blender/blender#80438
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System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce RTX 2080 SUPER/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 451.67
Broken: version: 2.90.0, branch: master, commit date: 2020-08-31 11:26, hash:
0330d1af29I have not seen hair behave this way before. For this example file, I have simply used the default hair dynamics settings. I have a bake in the file attached, and you can try bake yourself to replicate the issue. The hair will fly all over the place, it is not colliding with anything and I cannot figure out what is causing the issue. It was recommended by a developer that I post the issue here for others to have a look, many thanks.
HairProb.blend
Added subscriber: @Discko
HairProb.blend
Interesting test, on the left is a singular strand of the problematic hair - gyrating around and doing it's own thing. In the middle is a brand new UV sphere with a new particle system completely at default settings, behaves normally - On the right is another new UV sphere, with the same particle system as the odd behaving hair - but it behaves fine
Added subscriber: @ZedDB
Changed status from 'Needs Triage' to: 'Confirmed'
I can reproduce this.
It seems like the simulated hair will start freaking out if it is edited into a "C" shape:
recording.mp4
I'm guessing that the cloth/hair springs gets bad initial values somehow perhaps. I haven't looked into it deeply yet though.
Will tag this as a known issue, since particles aren't actively maintained. The explanation by @ZedDB is probably correct. Because the hair sim literally uses a cloth solver i suspect that the spring model runs into an unsupported case here, possibly combined with a buggy implementation.
Hand-wavy explanation: Multiple connected bending springs cause buckling. This shouldn't introduce energy, but that might be a bug or just numerical instability.
I will call this a known issue, since the old cloth/hair sim isn't actively maintained.
It's an interesting demonstration of why a solver made for cloth is not necessarily ideal for a hair. The bending springs on corners leave the rotation around the spring axis totally unconstrained, in other words there is no twist stiffness at all. That means adjacent constraints can be solved by simply rotating around that axis without violating the first constraint. This numerically adds kinetic energy because points are moved without changing equivalent potential energy. The result is the instability observed.
A dedicated elastic rod model takes twist energy into account and is much more suitable for hair simulation, but out of scope for a simple bug fix here.