Full Version: Thinking about Curved Surfaces made from Bezier Curves

Hi all,

this is no feature request for MoI, but i was thinking why NURBS modelers do not work also
in a way like Shade 3D (since 1986) or if i'm not mistaken like Animation Master. I ask because
i recently downloaded the newest version of Shade 3D and figured out that one can convert
easily Curved Surfaces models, made of Bezier Splines into NURBS Surfaces models, by simply
loading such a model and then saving it as .iges or .step file. Sure NURBS modelers like MoI
are used for it's precision in making hard surface models and it's ease of use, but maybe such
Curved Surfaces workflow* for organic models would allow one to model without the usage of
poly sds modelers and expensive conversion software.

Here are three sample images of Shade 3D Curved Surfaces models loaded into MoI, after
saving as NURBS surfaces.







P.S. Shade 3D is used mostly in Japan and used for all kinds of 3D. Besides it's Curved Surfaces modeler it has
also a poly/sds modeler and since version 17 also a NURBS Surfaces modeler.

*A Curved Surfaces workflow allows also precise hard surface modeling!

First picture shows the famous FeiFei model from E-Frontier and the second and third images are Shade 3D
content models.

Regards
Stefan

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Hi Stefan, the main problem with Bezier patch modeling is the same as with Bezier curve modeling which is that it's difficult to get G2 continuity across patches or curve segments without constraining everything to be in the symmetrical handle editing mode.

That's basically the fundamental reason why NURBS curves and surfaces were invented, to have a way to generate multiple bezier patch spans from a longer sequence of control points with guaranteed continuity between the internal spans of a single surface.

Even aside from strict G2 continuity, it's just difficult to avoid general surface shaping defects like unwanted ridges and and smooth curvature distribution using that type of patch modeling (with editing points right on the patch) method compared to sub-d.

- Michael

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Hi Michael,

thank you for the detailed explanation! Now i understand why this modeling method
is not used in (more) (NURBS) software.

Regards
Stefan

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about translation (language) what exactly "span" in the text above ?

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Hi Pilou,

check this illustration for a definition of a span:

https://knowledge.autodesk.com/support/alias-products/getting-started/caas/CloudHelp/cloudhelp/2018/ENU/Alias-Tutorials/files/GUID-44C90759-1812-4051-85BF-072140EE6623-htm.html

Regards
Stefan

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Hi Pilou,

re:
> about translation (language) what exactly "span" in the text above ?

It's basically a section of a curve that can be directly evaluated as a polynomial function (as a Bezier).

When you have a curve or surface that has a lot of points in it, it's not practical to use a single polynomial to evaluate the whole thing so there needs to be a way to chain together several of them. Part of how NURBS curves work is that it defines how these spans share points with each other so they can automatically stay smooth to each other.

- Michael

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Ok very clear of what it is! :) Seems 2 different things !

But have you a word in this list https://www.wordreference.com/enfr/span
because
I have not yet satifying word in French!

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Hi Pilou, well some other words that could probably be used would be "section", "portion", "segment", or "piece".

- Michael

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Bonjour Pilou,

Les mots qui me sembleraient les plus proches du sens de span dans ce contexte seraient : la "portée", "l'étendue" ou la "couverture".
Lequel serait le plus précis ?

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Le plus poétique serait "l'empan" pour le second, et "lamelle" pour le premier... :)

Mais c'est toujours étonnant comme l'anglais donne à un même mot une quantité de significations suivant un contexte différent!

The most poetic would be "span" for the second, and "lamella" for the first... :)

But it's always amazing how English gives the same word a number of meanings in a different context!

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