topic Re: Convert trimmed NURBS to watertight TSplines in Fusion Design, Validate & Document Forum

Convert trimmed NURBS to watertight TSplines

Anonymous — Sun, 19 Aug 2018 12:07:11 GMT

Hello, I would like to know if there is a way to convert iges files, made of multiple trimmed NURBS patches, to watertight gap-free TSplines. In the end, I need to read the TSpline file and extract geometric data such as control point's coordinate and weights, local knot vectors and their orders in order to use in an isogeometric solver that I have implemented. Any help, enabling me to achieve this task in Fusion 360, would be much appreciated .

Thank you!

Hadi

Re: Convert trimmed NURBS to watertight TSplines

TrippyLighting — Mon, 20 Aug 2018 13:07:32 GMT

Short answer is No, not with Fusion 360,

Each surface patch - has to be converted individually Into it's own T-Spline. The conversion into a T-Spline in Fusion 360 does not use the NURBS control point data, but approximates the surface.

The only tool that I know of that creates a .obj mesh based on the NURBS data is Solid Thinking Evolve and I've only tested this on a simple object (the helm of a boat designed originally in Rhino).

That mesh could then be inserted into Fusion 360 converted into T-SPline and then Surface. That T-spline derived surface perfectly matched the direct import of the Rhino surface in Fusion 360. However, I did not investigate this beyond my trial version of ST Evolve expired.

Re: Convert trimmed NURBS to watertight TSplines

Anonymous — Mon, 20 Aug 2018 13:47:26 GMT

Thank you Peter, this is heartbreaking news. I would then start implementing the method, published by Sederberg in 2008, to do the conversion.
Hadi

Re: Convert trimmed NURBS to watertight TSplines

TrippyLighting — Mon, 20 Aug 2018 13:57:47 GMT

@brianrepp

@jeff_strater

Could either of you two connect this young man to any of the T-Spline gurus within Autodesk ?

Re: Convert trimmed NURBS to watertight TSplines

TrippyLighting — Mon, 20 Aug 2018 19:19:46 GMT

@Anonymous I am assuming you are familiar with this paper ?

Re: Convert trimmed NURBS to watertight TSplines

brianrepp — Tue, 21 Aug 2018 00:22:44 GMT

Thx for the nudge, Peter

@Anonymous i'm tracking down someone with a bit of expertise in this area, stay tuned!

Re: Convert trimmed NURBS to watertight TSplines

Anonymous — Tue, 21 Aug 2018 05:17:45 GMT

Hi Peter, thanks. Yes I know the paper. Could be an option. Also, a work around it would be to convert trimmed NURBS to Bezier surfaces. This is possible in Rhino. Maybe Fusion has a similar functionality. I have to dig it out.

Re: Convert trimmed NURBS to watertight TSplines

colin.smith — Tue, 21 Aug 2018 16:53:25 GMT

Hi @Anonymous,

There is no method in Fusion for directly getting the result you want. @TrippyLighting was correct (as usual) with the method of brining in a quad mesh and converting that to a T-Spline. So if you have a way of converting a NURB to a quad mesh and bring in back in to Fusion - that workflow while clunky - would get you closer. The only issue there would be that if you end with a very dense quad mesh, your result will be a very dense T-Spline surface which is difficult to modify.

Colin

Re: Convert trimmed NURBS to watertight TSplines

Anonymous — Thu, 30 Aug 2018 08:58:48 GMT

@colin.smithThanks Colin. I would not convert my geometry to mesh because the whole point in isogeometric analysis is to keep the original design throughout the analysis. However, one can perhaps design their part in Fusion using Fusion's integrated TSpline module from the beginning till the end. So, I think I would go this way and perhaps can encourage more designers/engineers to design in TSpline environment of Fusion. This will then enable us to do a design-through-analysis loop. One more question, would it be then possible to extract geometrical information of TSplines out of the saved Fusion file? As far as I know the Fusion original file is not ascii file.

Cheers,

Hadi

Re: Convert trimmed NURBS to watertight TSplines

TrippyLighting — Thu, 30 Aug 2018 10:15:58 GMT

@Anonymous

Just as a NURBS surface, a T-Spline describes a surface controlled by a mesh of control points.

So a quad mesh designed in a Sub-D modeling package such as 3DS Max can be converted directly into a T-Spline and from there into a NURBS surface, without loss of fidelity.

NURBS surface patches created by T-Splines however are not a trimmed NURBS surface. So if you have a trimmed NURBS surface you might be able to approximate the trimmed NURBS surface with a T-SPline , but it will only be an approximation. You cannot untrim an T-Spline patch, for example, so for models that involve trimmed NURBS surface there is no direct backwards compatibility to a T-Spline.