Hi Tom,

> You are right Michael. Seeing what is done with Rhino -and so Moi - like
> http://gallery.mcneel.com/fullsize/46535.jpg
> I did not consider models like this helmet as "organic". So in your defintion and philosophy you
> would recommend to model sth like the helmet with polys ?

Well it kind of falls into a gray area where it requires using a more difficult toolset in NURBS and you are outside of the clearest and strongest areas of NURBS modeling which is when you are able to generate most of the model using 2D curves and booleans rather than trying to work with all 3D swooping shapes only.

But if you can dissect the helmet down to one surface to be generated for each smooth section of the helmet, then that can help a lot to make it work well with NURBS - it's easier to do things with NURBS when each smooth section of the model corresponds to one large sweep or network with possibly some additional trimming done to carve out any irregular boundaries. If you try to approach NURBS modeling in a small "patch by patch" manner as you might with polygon modeling, that does not work very well.


> What about jewelry, vehicles or furniture where curves warping around in 3D space are unavoidabl ?.

Well these all tend to be fundamentally more difficult models to create than mechanical models, ... they are often again in a gray area where they are not necessarily an automatic fit for NURBS modeling unless you are willing to go up a steeper learning curve than what is required for mechanical models.


> A problem I see is to achieve smooth continuity when joining curved surfaces ...

It's this type of "patch by patch" approach trying to build a single smooth skin out of multiple separately constructed surfaces that you really want to stay away from when doing NURBS modeling - if you are getting into a situation where you are worrying about that, it's a big sign that your approach would fit better with sub-d modeling than with NURBS modeling.

If you're going to be doing 3D swoopy surfaces with NURBS modeling you really want each smooth piece of the object to be made of one single large surface that is built in one go, rather than trying to build little pieces at a time that are supposed to be smooth to one another. Often times this means making the surfaces to extend out a ways so that they can follow a simple initial boundary, and then introduce some details by cutting chunks away using trims or booleans, rather than trying to directly construct a bunch of little surfaces "patch by patch" that try to hug complex boundaries directly. Hope that makes sense.... Try to think of NURBS modeling as using surfacing to create large simplified "stock pieces of material" that can then be trimmed to produce the final result. This is very much the case for mechanical objects where the stock pieces are done with simple extrusions. But if your 3D swoopy object can fit within that framework than it can probably be done ok with NURBS too. If your 3D swoopy object has lots of branching in it or little small localized lumpy details, then those are things that don't fit in well with NURBS.

There are some tips here for people coming from a poly modeling background, they may be useful:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4865.2

The key difference between NURBS modeling and polygon modeling is that boolean and trimming operations are a major focus for NURBS modeling and when you see something like a smooth surface that has some zig-zag like overall outline shape, the way you do it in NURBS is to make a big surface out of a few simple boundary curves to generate an extended piece of stock material, and then the zig-zag outline comes from trimming some of that large surface away to get your final result.

- Michael