Even though I have been taking it as a fact until now, I realised that NURBS and SubD are not back and forth compatible. For example, when a SubD surface (with extraordinary points) is converted to NURBS, the final surface is an approximation of the first, not an identical.
I guess this is because of the underlying maths of SubD, which -I guess again- is not compatible with NURBS (or maybe it is up to a point). Why is that? Do McNeel plan to make it 100% compatible, meaning that when one converts the SubD surface to NURBS they get an identical surface or object?
in general even if you rebuild a curve you have a deviation, so never exactly the same curve.
SubD is a mixform between mesh and nurbs and therefor it is mathematically impossible to âswitchâ without slight deviation.
The more controlpoints you allow to be created, the more precise it will end up. however, SubD is meant to be the less complex possible to obtain the smoothest surfaces. So if you choose to have very little deviation by using high complexity, youâll end up with a very difficult to modify subD model.
Alternatively you can try âquadremeshâ with your model to obtain an ordered mesh which can be converted more easily than Nurbs, IF your model contains Nurbs with high isocurve density.
I hope I havenât written too much garbage from technical view and somehow answered your question
Thank you both. That was the good thing with the T-splines plugin: low-DOF modelling, 100% compatible with NURBS. I guess I have to read more about SubD to understand the advantages they provide in modelling, provided they are not 100% compatible with NURBS.
The article has some info, but it is not absolutely true. For example, Scott claims that all SubD have a NURBS equivalent, which is not the case.
*For example, SubD surfaces with extraordinary points (EOPs) of valence 6 do not have NURBS equivalent. As far as I can see, SubD have a NURBS equivalent if they contain EOPs of valence up to 5. Valence-3 EOPS are behaving well too.
EDIT: My mistake. I checked it again and SubD with EOPS do not behave well, i.e., they do not have a NURBS equivalent (at least in my case).
Just underscoring what has been said.
NURBS, SubD, and Meshes can all be used to describe 3D geometry.
They are not interchangeable.
Because of this, approximations and fitting are required to âconvertâ one to the other.
This will degrade the quality and accuracy of the model.
Reading a marketing article that quotes Scott Davidson should not be interpreted as an accurate and complete technical discussion. Thatâs not was the interview was about.
I donât think we disagree. I was just trying to understand up to which point Scottâs claims were true, thatâs why I started experimenting. You might have been ironic before, but believe me when I say I am a Rhino fan and I try to make the best out of it, as well as to contribute towards being the best product.
Looks like Iâm a little late to the show and JB handled it below.
The salient technical point is that the shape of a Catmull - Clark subdivision surface near some extraordinary points cannot be exactly duplicated using a NURBS surface.
This is a mathematical fact and thus there is nothing to âfix.â (Just like we canât âfixâ the fact that lines and circles have different mathematical properties.)
At these extraordinary points, Rhinoâs ToNurbs function creates a NURBS surface that closely approximates the shape of the Catmull Clark surface.
If a person felt the need to understand the mathematics of the situation, I would begin by reading this paper by Doo and Sabin.
D. Doo and M. Sabin: Behavior of recursive division surfaces near extraordinary points, Computer-Aided Design, 10 (6) 356â360 (1978)
Then I would look for text books and papers discussing Catmull Clark subdivision surfaces that cited this paper and read them.
I donât see any links to or direct quotes from the marketing material mentioned above and cannot comment on what was said.
Thanks for the detailed reply Dale. Since the underlying mathematical representation of SubD depends on Catmull -Clark, it makes sense that there can be no fix. I wasnât aware of the fact that SubD solely depend on the Catmull-Clark algorithm; I mistakenly made the assumption that there would be an effort to develop something close to T-splines. My hypothesis was wrong, from which my question about back-and-forth compatibility initiated. Thank you for clearing things up. Thank you all.
We chose Catmull Clark subdivison surfaces because that representation is completely in the public domain, is used by a wide variety of modeling applications, has been used in a wide variety of applications for decades, and is well documented in readily available books and technical papers.
Sure, if one wants to be anal retentive about it. Or just draw and donât worry about any .002 deviation no one can see or feel, while basking in the glory of all things SubD, where approriate. (Or NURBS when your manufacturing tolerances are .002⌠)
none of T-splines, subdivision surfaces, or NURBS surfaces can always accurately represent the (exact, algebraic) intersection of two surfaces within the same surface representation.
also if somebody wants to make a T-Spline plugin for Rhino -
there you go, an open source Kernel
Thatâs another thing, but it is true. An intersection, is not part of the surface representation itself, therefore it can only be approximated. This is the case for every trimmed surface as well. The curve of trimming cannot be represented accurately by using any mathematical representation.
I am aware of that project. I have been contributing in developing something similar, but in C#, as a Rhino plugin. Iâll check again SourceForgeâs to see if thereâs anything interesting.
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