> Yea so it has to be like normal NURBS, where the mesh object is treated as a NURBS suface...

Except that you would get one of the major benefits of Sub-D over NURBS - your control hull could be set up with a much more arbitrary topology instead of only an M x N grid of points. This would be good for creating things like smooth branching structures. Well, and everything that SubD is good at doing right now.

It would be an alternative method for creating the base NURBS object.


> Anyway, the quad cage construction of a NURBS objects, can't you have any number
> of points on each side of the quad? Like when you make a sweep?

Yes, you can any number on each side, but the points have to be in a grid organization. So that's why you can't take just any polygon mesh and turn it into NURBS by using just the control points of the polygon mesh directly. A random polygon mesh doesn't always have a regular M x N grid layout to it.

For the places in a Sub-D surface that do naturally correspond to a NURBS surface, each quad face of the SubD cage becomes a NURBS surface with a 4x4 control point grid.


> but I thought the big advantage of NURBS was that you could define each
> side of you plane with more points and make it blend with 2-rail sweeps...

That you can define a single patch where each side of the plane with any number of points is a big advantage of NURBS over _Bezier_ surfaces, not over Sub-d surfaces.

Although I guess in a certain sense it does tend to organize things into larger "patch" objects in a natural way which kind of simplifies certain construction-oriented modeling approaches, like there is naturally a longer edge along one side of a sweep instead of a bunch of little tiny edges you would have to chain together for Sub-D. I guess that is kind of a user-interface issue with Sub-Ds - less restriction on topology is not automatically 100% better for all modeling strategies because restrictions on topology have a side effect of creating a type of natural grouping which is suited for certain things.

But for comparison between NURBS and Sub-D, I'd say that the single big advantage to NURBS is the well-defined trim curve mechanism, which is one of the fundamental things that makes stuff like trimming and booleans work well, and having a natural curve-on-surface structure is important to several other operations such as fillets as well.


> so I guess Maya has some special algorithms to handle that.

Yeah, from what I've seen they subdivide a few extra times near the "extraordinary point" (this is what a point shared by other than 4 neighboring faces is called). Each of these subdivisions generates more regular quads and shrinks the difficult area to be smaller and smaller. Then they have some kind of surface fitting mechanism that tries to get a good approximate NURBS surface for the final bit.


> I wonder if Max's NURMS (Non-Uniform Rational MeshSmooth) is catmull-clark sub-D....

I don't know... Probably, it is the most popular subdivision algorithm by far, although there are other methods around.

- Michael