[Sidefx-houdini-list] Building a particle collision system

Rangi Sutton rangi.sutton at gmail.com
Sun Aug 15 20:13:33 EDT 2010

2010/8/16 François Duchesneau <sidefx at trinix.ca>

> I forgot to mention that I want to avoid SDF because even if I agree
> that in many situations it's a great tool, in others it's just too heavy
> to calculate to have a good resolution.
> I started my collision system with this and then I had a shot where I've
> got lots of rocks in a river separated by a great distance. The sdf had
> to be huge. Thus I used many smaller SDF for each rock independently
> merged but the Volume Sample Vop needs to know which primitive.

Yep. you need to loop through the prims, searching for the volume with the
lowest SDF value at that point... then collide against that one if it's
negative. Or.. the first negative one you come across..

We setup optimisations for this by also passing through a point cloud of the
SDF centroids, so we'd only sample the nearest few SDFs to the particle,
pulling the prim numbers off the point cloud. When we had >100 SDFs as
collision sources this sped things up considerably.

This stuff is pretty easy to set up in vops, with some simple sop nets
preparing everytthing... gives you a tonne of control.

That's where I started to think about how fast and reliable Maya can do
> collision on the fly without pre-baking. It should be like that in
> Houdini too.

Yeah, regular houdini particle collisions are slow and buggy compared.
However, getting to a final shot using every trick available, the houdini
build still wins out in my books.

I'd love to see a better straight particle collider working in houdini!


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