NURBS and Span

 From:  Michael Gibson
5897.3 In reply to 5897.1 
Hi Claas, re: "Single span surfaces" - as far as I know it's just a general attempt to reduce unwanted wiggles in surfaces. It's just a sort of byproduct of single span surfaces that they have the lowest count of control points and so with fewer control points comes fewer chances for them to be arranged chaotically.

That strategy only applies very much to a highly advanced and also finicky form of NURBS modeling that's focused on working on 3D swoopy shapes in a "patch by patch" skinning manner, it doesn't really apply to "Solids modeling" techniques where things are more focused on generating shapes from 2D profile curves rather than working with swoopy organic geometry all the time.

The thing is, that for organic geometry where you're working on a 3D flowing surface that does not have very many 2D elements in it, polygon sub-d modeling can be an overall better fit for that really than NURBS modeling. The "use single span surfaces" advice is roughly the equivalent of "use the fewest cage control points" advice for sub-d modeling. With fewer points it's harder for surfaces to get undulations in them.

But these people who are focused on such things with NURBS are doing pretty specialized work and doing things like manipulating surface control points directly and things like that. MoI is not really focused on such specialized and advanced and super high learning curve type of functions, it's a big focus for MoI to make NURBS modeling more approachable to beginning users and not only to some "high priesthood" of advanced highly trained surface aestheticians.

I've really tried with MoI to very much avoid the need for the user to understand all the underlying mathematical details of NURBS such as curve and surface degrees.

> 1. How is this issue be handled in MOI? Do you have some internal
> mechanism working that adjusts dynamically?

No, it's more that I just don't really focus on that one particular style of "surface by surface patching" modeling and instead try to focus more on techniques for generating models from 2D curves instead which are much easier for people to learn.

But if you do a Blend surface in MoI, that does actually make a single span surface in the blend direction currently.

> 2. Why in 2013 is span still such an issue? I am not familiar with the math - personally
> I just imagined that you can seamlessly blend between two spans as it is a math calculation
> for that curve.

Some of it is just almost superstition I guess, and probably over generalization of advice. It is true that multi-span surfaces can be more difficult to edit directly to morph their end shapes since with lots of control points any manipulation of those control points affects a smaller and more localized region of the surface, so when you go to alter the points with single span surfaces you control more of the surface with any edits that you do. With too much localized effect during editing it will tend to just bend a small area of the surface when you try to match it to other ones rather than making the full surface be affected. A localized warping ends up being more like a small undulation or wiggle in a surface which is not the kind of shape that's usually desired in these cases.

But the benefits are pretty specific to that kind of control point level type manipulations...

Just because single span surfaces are helpful for one particular style of modeling does not mean that they're inherently superior in every circumstance. That's where the over generalization can tend to come into play.

With regards to MoI, it has just never been a particular goal for MoI to be a sort of replacement for a $60,000 highly specialized modeling package like Alias Automotive, MoI is more about the complete opposite of that, trying to make some of the more basic and easy to use areas of NURBS modeling more accessible to people without a massive learning curve.

I actually do expect to improve some areas of organic surface style modeling in the future though too, there are some elements of that in place like Blend and Network already, they just have not been a really major focus so far because that style of modeling is just overall more difficult and a lot of people doing organic style modeling are already using a sub-d modeler to do it anyway.

- Michael