colortwist

Automatic convex straps in Houdini

Added 2021-08-17 15:55:27 +0000 UTC

Just a little something I was working on earlier today- partly because I thought it might be useful in the future, but also partly because I wanted to know if it could be done.

This is basically just a simple convex strap solver. It works by intersecting a group of arbitrary input geometry against a plane (though I suppose it could be any shape so long as the intersections resolve cleanly) and essentially calculating an output curve (or band) that is guaranteed to loop around the exterior of those objects. There's some additional voodoo in there to keep the strap angled towards the nearest surface, as well as handling that nice twisty bend effect that you get when the strap is stretched between two objects with different angles- but that's about it in a nutshell.

Surprisingly, the general technique evaluates very fast- far faster than the 60hz refresh rate of my monitor. It runs at realtime in the viewport and is quite fun to play with. The entire setup is totally deterministic, so there's no actual simulation going on here- given the same set of inputs (regardless of what those may be), it will always produce the same set of outputs.

I suspect some of the snapping could probably be animated with additional jiggle deformers or perhaps something like Vellum, but that's another project for another day.