NURBS and Span

 From:  Michael Gibson
5897.5 In reply to 5897.4 
Hi Claas,

> Personally I had some bad experiences with Alias where certain blends and surface
> flows were only possible by rebuilding the design with single span curves and while
> I know 3D must be incredible complex I was asking myself if the math code there is
> simply not yet so refined or if this is just the way how Alias works.

It's hard for me to know for sure what the particular issue was for that case without examining the actual geometry used.

It's possible it could have been some kind of limitation or bug in Alias, but also possibly your rebuilt geometry was less wiggly or had some other change in form that avoided some particular kind of shaping problem. But I can't really know without seeing the specific example.

The overall code base for Alias is pretty ancient at this point, and such an old code base just usually has lot of various clunky things about it.

> Out of curiosity how do applications like SolidWorks approach the span issue? I know
> that you can put down the profile curves etc as well, but it seems to me from the short
> exposure that they do not have such a high focus on limiting edit points on a curve like
> Alias Automotive does.

SolidWorks is more similar to MoI's orientation in this regard, they really focus a lot of effort on generating shapes from 2D sketch planes and working more on solids throughout their workflow rather than trying to focus on building surfaces one little small patch surface at a time filling in some complex wireframe outline. They have added more surfacing tools over time but they're more often used for building a cutting surface or something like that, rather than trying to build a big model out of a quilt of individual little small surfaces that have to connect to one another smoothly.

So as far as I know there is no special focus or worry about "single span"-ness in SolidWorks either, it's more focused on mechanical part design and not on modeling broad swoopy forms made of some patchwork of surfaces.

Just in general the SolidWorks type solids modeling approach is much more "mainstream" than the Alias Automotive "try to build a big complex surface patch by patch" type modeling. Alias Automotive is a much more heavily specialized tool and that whole process requires a lot of training and experience to be able to do stuff with it. Meanwhile the SolidWorks type of approach that generates models from 2D curves tends to deliver a lot more productivity for a lower amount of learning curve.

One other general factor though is that the kinds of models that are being worked on with Alias Automotive are also though just fundamentally more difficult models because they are not so well defined only by 2D profile curves, so they're not such a good fit with the traditional 2D-profile driven solids modeling approach. I guess that's why Alias continues to exist at this point at all. But also once you get into the territory where your model is not well defined by 2D profile curves then the whole sub-d modeling toolkit can become a better way to form those kinds of organic models rather than trying to do them with NURBS at all. Usually that's what I steer people towards using instead of MoI if they're trying to build some kind of form that is not very well defined by 2D profile curves.

- Michael