johner

johner

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H19.5 | Vellum

My Tutorials

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Vellum Constraint Features | H18 Masterclass
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Advanced Vellum Workflows| H17 Masterclass

Recent Forum Posts

weird OpenCl error in vellum April 30, 2021, 2:49 p.m.

This is an issue that arose from recent NVIDIA drivers updating to OpenCL 3.0. It should be fixed in Houdini version 18.5.553:

Houdini 18.5.553: Mon Apr 19 19:00:18 CDT 2021
Update the Vellum Solver to work properly with NVIDIA's recently
released OpenCL 3.0 drivers, particularly the Pressure and Shape Match
constraints.

Is vellum scale dependent? Dec. 15, 2020, 9:10 p.m.

There are options to set the Unit Length and Unit Mass in Preferences | Hip File Options. If you set these *before* creating any Vellum SOPs or DOPs, then physical parameters like gravity should be scaled appropriately.

Unstable Vellum Weld constraints Feb. 11, 2019, 4:31 p.m.

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.)