Hi curveman,

> It looks like a subD control cage being used to a drive a NURBS engine.

Yup, it's pretty interesting! And only something like $10,000 to buy...

From what I can tell, the modeler itself is all sub-d and then the interesting part is that it has the ability to convert the Subd surface into a set of stitched together NURBS surfaces that can then be imported into their regular NURBS CAD tools.

There is actually a pretty close relationship between the Catmull-clark type subdivision surfaces and NURBS - in fact when you have a very regular quad layout (with each mesh vertex having 4 edges coming out from it), there is a really easy direct conversion possible between Catmull-clark subd and NURBS.

It gets much trickier where there is a non-quad layout. In those areas there isn't an exact conversion and instead something has to be done that calculates a kind of fitted NURBS approximation in there.

Maya actually has some similar conversion tools built in, although they don't seem to have gotten much development for quite a while, and T-splines on Rhino is the other recent thing that is pursuing this kind of Subd + NURBS hybrid approach.

In the future I think this kind of combination will get to be more common.


> I would personally love to see a voxel based freeform foam/gel modeler driving
> a NURBS surface engine or some kind of metaballs -> NURBS modeler for
> organic stuff. One can dream, heh heh. =]

That's a lot more difficult because at least with Sub-d there are a lot of areas that have the exact conversion.

With these other types, there is no exact conversion anywhere, so the entire thing has to go through a kind of fitting / optimization type process.

There are some types of "reverse engineering" type tools that are set up to do this kind of processing, for example check out geomagic.

- Michael