colortwist

Self colliding body meshes in Houdini

Added 2021-03-29 06:04:52 +0000 UTC

Just an extension to yesterday's post- this is the same system, but implemented with a simple body mesh and a barebones skeleton.

As the legs move around, both the thighs (once inflated) will automatically avoid each other and instead create a sort of rubbing effect where one surface slides across the other.

The usual way to achieve something similar in C4D was to use two invisible cubes in the scene, both parented to the upper leg joints, then constrained to look at each other and automatically resize themselves so the innermost flat edge of the cubes were always touching in-between the legs. You could then use a pair of collision deformers to "collide" each thigh against the opposing cube, forcing that geometry to flatten out at the contact point and simulating the thighs rubbing together. This kinda worked okay, but thighs aren't flat surfaces so it would often create some bizarre artifacts where the legs were almost touching but not quite (as the collider would still catch the edges of the collision cube and land up flattening the thighs when it should not).

So far, the Houdini way seems much simpler. You can easily extract a pair of proxy meshes from the base character mesh (this includes any active corrective morphs, including anything that inflates the mesh), collide those, then transfer the active deformation back to the original body mesh with a bit of clever VEX magic. This seems to work exceptionally well and is quite a bit faster to evaluate than C4D's collision deformer to boot.

Anyways, yeah. Big smooshy inflated thighs in Houdini.