Unstable Vellum Weld constraints

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I am having strange behavior where weld constraints tend to exhibit high velocity at the seems during various circumstances.
It seems like the seems tend to either 1) all blow up at the same time and then iron themselves out or 2) they protrude quickly at the seam before smoothing themselves back down. I have tried adjusting the iterations and even had this problem with defaults on basic welded cloth. Anyone run into this issue?
Edited by Noboru Garcia - Feb. 4, 2019 16:41:49
- “spooky action at a distance”. Albert Einstein
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Any ideas?
- “spooky action at a distance”. Albert Einstein
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attach your broken setup, you will have much higher chance of an answer since

Vellum welds are used heavily and in my experience without issues you are mentioning so without a file its difficult to imagine what you are talking about

unless your welded points are not at the same position at the start of the sim, then they will obviously snap together straight away which may be what you are talking about and in that case use stitch constraints first to bring the points together nicely and when they are close enough, weld them, similarly to what Vellum Drape does
Tomas Slancik
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As the weld threshold is reached and the constraint is broken it seems like the geometry tears away with much more force than I expect. I'm not sure what attributes contribute to that effect.

Attachments:
Exploding constraints.hipnc (524.5 KB)

- “spooky action at a distance”. Albert Einstein
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What's happening is the welds are breaking, which re-enables collision between the points, but they're still close enough to each other to self-collide. This gives a fairly explosive tear as the points immediately collide in the next timestep. There are a few different ways to handle this, but an effective one for this case is to disable self-collisions on any unwelded points just for the timestep when they break.

You can do this in a GeometryWrangle into the third input of the Vellum Solver, with the group set to “broken”, and code of i@disableself = 2;. Setting that flag to 2 will disable self collisions for a point until it is no longer self-colliding, at which time it gets reset to zero. (You can set it to 1 to permanently disable self-collisions.) This gives a much smoother tear in this case, as does setting Breaking Frequency to Per Substep on the Advanced tab of the Vellum Solver.

See attached.

(Note this only works for DOPs-based Vellum setups, since the third input is not exposed in the Vellum Solver SOP, and the “broken” group is only set for that third input.)
Edited by johner - Feb. 11, 2019 16:34:33

Attachments:
Exploding_constraints_disableself.hipnc (525.1 KB)

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Ahhhhhhhhhhhhhhhhh. Thank you for that, what an interesting thing to discover. Now i'm wondering where might this be I did search the docs? Did i miss something… seems kind of crucial.
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Noboru Garcia
Now i'm wondering where might this be I did search the docs? Did i miss something… seems kind of crucial.

Hi, I ask myself the same question.
Thanks
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The peculiar behaviour of disableself may be found in the vellum attributes docs:
http://www.sidefx.com/docs/houdini/dyno/vellumattributes.html [www.sidefx.com]

The idea of how to use this to solve this case isn't in the docs. We're exploring how to make as a proper built-in, and will probably be adding an option to the vellum solver to auto-disable freshly broken welds.

One issue is that if your tear is pulling apart quickly, you don't want to auto-disable as it may cause layered cloth to fail.
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Thank you Jlait !
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