Vellum Sim & RBD Sim Interactions

Download Houdini File: Vellum Interactions.hiplc

Phase workflow to achieve more realism between Vellem & RBD dynamics in simulation

Having Vellum simulations work with other types of simulations can require some creativity. My instinct was to use the multi-solver in Houdini to combine different types of solvers together, but it didn’t work out with the Vellum solver. I started tinkering with different ways to mix the Vellum solver with a simple RBD simulation setup and came up with a multiple phase workflow to create a more realistic Vellum cloth dynamics with the RBD simulation.

Vellum SIM to Static Object

I created a very basic Vellum cloth simulation by converting a torus geometry and dropping it onto the ground. Then I converted this whole Vellum Torus simulation into a static object and feed it into the RBD simulation. This worked well as a starting point, but it didn’t generate a realistic simulation that I wanted.

The biggest thing that was bothering me in my initial Vellum & RBD simulation was that the RBD pieces weren’t weighing down the Vellum Torus object. Since the Vellum Torus was converted directly into a static object it doesn’t react to the RBD pieces at all and instead it just acts a static collision against the RBD objects.

Improving Vellum & RBD Sim to Achieve More Realism

3rd Phase in workflow, using initial vellum torus as vellum geometry & constraint for new independent vellum solver

So I added an extra step at the end of this whole setup and referred to each step as a phase. In this case, I would add a 3rd phase and convert the now simulated RBD pieces into a static object. This would be another collision object that can now be fed into the vellum solver. I create a new vellum solver that has a similar animation to the initial Vellum Torus, that I refer to as Phase 1. Therefore I would only use the results of the animated RBD objects from Phase 1 & 2, and create a new Vellum Solver for Phase 3. The animated RBD objects are used in Phase 3.

Rendering Results from Phase 1 to Phase 3 to Achieve more realism between Vellum & RBD dynamics in simulation

Pushing this Further?

I would hook the static RBD pieces to a new Vellum solver on the SOP level and cook the simulation. This resulted in a decent result. This whole setup isn’t a true interaction between the RBD sim and Vellum sim, but it got me a decent result. However I realize that if the scene got more complex it would require a lot more tweaking to improve the simulation and get better results.

If the scene involved more forces and wasn’t simply dropping onto the ground or maybe if it was tossed into the air, anything that was more involved would result in a lot of tweaking if this phase method was used.