Cloth sim: Sewing and Self Collision issue #65568

System Information
Operating system: Windows-10-10.0.17763 64 Bits
Graphics card: GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 398.36

Blender Version
Broken: version: 2.80 (sub 74), branch: blender2.7, commit date: 2019-06-06 00:27, hash: 8b2b79c210
Worked: (optional)

Short description of error
When using sewing feature of the cloth simulator, it leaves a gap between sewn vertices. This only happens when Self Collision of the cloth is also on. In Blender 2.79 this is working correctly, i.e. there are no gaps.

Exact steps for others to reproduce the error
With the default cube, delete four side faces ("Only Faces" option, leaving the edges intact). The apply the Cloth modified in the Physics tab and use Sewing with Force 100. You can play the simulation to verify now it works correctly. After checking "Self Collision" and playing the simulation again, there are now gaps.

Added subscriber: @Blackx

I have attached a file to demonstrate the issue. Just open the .blend and run the animation/simulation, two frames are enough. There are two clothes with Sewing on and Sewing Force 100 (and Gravity = 0 for easier tests).

The left one is sewn perfectly. The right one has Self Collision set to On with Distance = 0.1, which breaks the sewing process. The vertices on the right are repulsed and are never sewn together, there is a gap between them. The bigger the Self Collision distance, the bigger the gap.

sewing.blend

Added subscribers: @ishbosamiya, @ZedDB

@ishbosamiya is this something that might have been fixed in your GSoC branch?

No, the GSoC project will not tackle/fix this bug.

Added subscriber: @FrancoisRimasson

This bug is there for at least 6 months.
Is anybody still in charge of this task ?

Added subscriber: @LucaRood-3

Not really, I might take a look at it when I have time, but that might take a while.

Added subscriber: @mano-wii

Apparently the strength of the self collision always surpasses that of sewing, regardless of the value.

This issue is caused by the new collision system.

The old system collided vertices with each other, treating them as spheres, and excluded collisions between vertices connected by sewing springs, so they would not collide.
The new system actually computes the collisions between triangles, and it is correct that the collision impulse should always surpass the sewing force, however the mistake is actually computing collisions between triangles connected by sewing springs at all. The new system already excludes collisions between adjacent triangles (those sharing an edge), and fixing this would require also excluding triangles connected by sewing springs.

This is fairly simple to fix, and I can look into it when I have some time, but if someone else can look into this sooner, I think the tips above should help.

This issue was referenced by 5afa4b1dc8

Changed status from 'Confirmed' to: 'Resolved'

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Cloth sim: Sewing and Self Collision issue #65568