Converting Nurbs to Polygons

[David Saber]

Hello list!

I need to convert Solidworks IGES nurbs to polygons.
In Softimage, the IGES import fails and creates nothing more than a
null. So I'm testing Maya to achieve this. In Maya, the function is
available in Modify > convert > nurbs to polygon. This works well, but I
don't understand why the resulting object has too many faces, has
corrupted geometry and inverted faces. Nurbs object are always simple,
grid-like surfaces, that are just given a shape, so why aren't they
translated into similar polygonal objects?
After this conversion, I export as FBX and import into Softimage where I
spend too much time cleaning the mesh. Is there a faster way to do this?
Thanks,
David

[Enter Reality]

I strongly suggest you to try Rhino for nurbs conversion and poly optimization/cleaning, since there are some builtin features in order to do that.

Then from there you can export in FBX and have a clean mesh rather then wasting time in Maya or do the cleanup in Softimage.

[David Saber]

Thanks, I will test Rhino as well!

[Leendert A. Hartog]

Haven't tried this myself in a long time, only took their demo "for a spin" way back, 

but MOI ("Moment of Inspiration" - http://moi3d.com/) advertises with the following:

"The icing on the cake is MoI’s unique polygon mesh export that generates exceptionally clean and crisp N-Gon polygon meshes from CAD NURBS models." and is way cheaper than Rhino.

Sent from Mailbird

[toonafish]

Yep, the MOI converter is very good, I used it a lot.  Just give the 30 day trial a test-ride.

-Ronald

[David Saber]

Howdy
Thanks for the tip about MOI. I ran the trial and the nurbs to poly conversion is even better than Rhino's. I didn't try all the export dialog options yet, but with the default settings, the result is very nice.
I never heard about this application before. Its interface is very simple but it seems nice for quick modelling tasks.
Have a great weekend all!
David

[Ponthieux, Joseph G. (LARC-E1A)[LITES II]]

David,

Did you modify any of the Convert NURBS to Polygon Options? Use the option box for the command if you have and try different settings. It will take a little experimentation to get the hang of it. But I doubt that’s your problem.

So..... about Solids to Nurbs to Polys. This has always been a difficult process. Used to do a lot of this kind of stuff from ProE to Soft 3D. This is one of the areas though where Maya is well suited because it imports such a high number of CAD formats natively. The problem as you describe it is likely starting at the Solidworks side. Heres why:

If you created the geometry as a solid, as with any application that generates these kind of objects, ProE, Solidworks, that kind of thing, Solids are not like Nurbs. They are mathematical representations which are geared towards creating watertight assemblies that can be sent to a 3D printer. There can be no unsealed objects in this path to the printer. The closest crude approximation or analogy I can give is that they are more similar to "booleaned" polygon primitives. But that is a severe oversimplification.

When you convert a solid to Nurbs, the solids modeler has to figure out what to do with it. A solid of a sphere doesn’t translate to a nurbs sphere, so when you export it as IGES the software goes through all soft of machinations to try and figure out how to make b-spline surfaces out of it. Generally what happens most often, as was the case with ProE at least it would create surface approximations of a topology it could understand and then trim off the surfaces where they intersect. So for example, a solid sphere might get converted into 8 nurbs surfaces that are trimmed off to form 8 semi quadrants of the sphere. Not very efficient when in maya all we need is a single NURBS surface. Create any complex topology and it will just keep breaking it down further and further until it’s a shape it thinks is reasonable for a spline surface and then trim off any excesses. I converted a solids generated iges of an airplane once into Soft 3D this way it took several hours to load on a massive Onyx SGI. It contained thousands of surfaces and multiple trims per each surface. It was insane.

Dependent upon the detail given to the original solid and whether its been shrinkwrapped or not, results will vary. But suffice it to say, solid to Nurbs to polys is not the best of paths. What you want to do is convert the solid to a mesh inside Solidworks. I don't know Solidworks so I am thinking you'll want to send an STL out of Solidworks. Look into "shrinkwrapping" it can also help the process. We used to have to do that in ProE, but don't know what its equivalent might be in SW. Then read the STL into Maya. There may be some cleanup but it should be mostly deleting stuff you don’t need and sealing edges where necessary. Generally an STL is a "poly mesh" but dependent upon the tolerances and granularity of the original solid they can be very course or very fine, think similar to render tesselation on NURBS, but for a "mesh" (not really a mesh, that’s why its called a solid). Once out of Solidworks though it can't be changed anymore so you have to manage the tolerances from the solids modeler side.

After looking at what is currently supported I see Maya DirectConnect supports something called SW so I'm going to guess it might be able to read native Solidworks in directly? Never tried it so not sure but that would be worth a shot as well. Search Maya help for SW_DC. When dealing with a solid it always best to stay as close to the native formats as possible, but exporting as Nurbs (iges) from a solids modeler is not something I would recommend.



--
Joey Ponthieux
LaRC Information Technology Enhanced Services (LITES II)
Science Systems and Applications Inc. (SSAI)
NASA Langley Research Center
__________________________________________________
Opinions stated here-in are strictly those of the author and do not
represent the opinions of NASA or any other party.

[David Saber]

Hello Joe,

Thanks for your very informative answer, much appreciated. I hope you
don't mind I've forwarded your post to my co-worker who is operating
Solidworks here.

To answer some of your questions :
- In Maya, I have played with the Convert NURBS to Polygon Options but
didn't find any statisfactory settings. Do you know some good settings
to get a clean mesh?
- I didn't know Solidworks was producing these "solid" meshes. I heard
of them before but I didn't get what they were exactly. Now with your
explanations I understand better. I thought Solidworks was just a nurbs
modeller. it would be great to find a way to convert from solid to
polymesh from within Solidworks, we'll try that.
- Is DirectConnect installed with every new Maya installation? Last time
I checked, I didn't find the Solidworks format in the import dialog. But
here is what I read in the Direct Connect PDF at
https://knowledge.autodesk.com/support/maya/learn-explore/caas/CloudHelp/cloudhelp/ENU/123112/files/directconnect-html.html
, in the SolidWorks section:
"Type of data imported : The software imports NURBS for this file format
and maintains the following information on import: Precise geometric
surface and topology information, Data organization, Tolerances and
unit, Colors"
So perhaps DirectConnect translates solid to nurbs at import time, and
in that case, it's the same as exporting an IGES from Solidworks in
order to import it into Maya (our current workflow).
- In another answer in this thread, I said I got nice results with MOI,
but the nurb object I tested was simple, I'll have to test with another,
more complex object.

Thanks again and see you!
David



On 2016-01-29 15:09, Ponthieux, Joseph G. (LARC-E1A)[LITES II] wrote:
> Maya DirectConnect

[Ponthieux, Joseph G. (LARC-E1A)[LITES II]]

David,

Every import will behave differently regarding the nurbs to poly conversion options. It's all dependent on the objects size, its general UV makeup, its topology etc. The options make it easier to adapt to the topology but not easier to convert. Back in the day we would be attempting to convert each surface individually so we could make the UVs of two adjoining surfaces match as close to possible at the NURBS to Poly conversion. But this matching is dependent upon a U or V length. Dragging a model down like this, as I think you already know can be quite time consuming.

My explanation regarding Solids is rather old and traditional I'm afraid. This might prove to give you a better sense of the Solidworks paradigm.

https://books.google.com/books?id=99YzrOr7Hd8C&pg=SA2-PA1&lpg=SA2-PA1&dq=solids+modeling+primer&source=bl&ots=jKS0rLrcUN&sig=PN88V3rDHyCONe0nOKGLzr3rugg&hl=en&sa=X&ved=0ahUKEwj51dLApM_KAhVBlx4KHZU6CCAQ6AEIRzAH#v=onepage&q=solids%20modeling%20primer&f=false

Apparently Solidworks has its own surface/solid methodology that’s not as exclusive to solids as the older applications. As I said, I'm not as familiar with SW as say ProE or Ideas and I don't have access to these apps anymore. I've been wanting to learn SW recently for something totally unrelated to 3D but haven’t had the chance.

Still, my guess is that you're having the difficulty because you are putting IGES into the mix. IGES is an open standard and is notoriously unforgiving sometimes. In fact, we have seen multiple flavors of IGES over the years due to its open nature and it's never quite clear how one app is going to adapt to one flavor of IGES. But to get to IGES you have to reduce the core data to stuff that the IGES standard can understand. My point is I think IGES would be many levels below a Solidworks definition. I've often likened IGES to being similar in theory to VRML, IV, or even OBJ. There are countless ways you can forge any of these files and if you use some of the more obscure stuff in them you can make the files unreadable by some importers. I don't think that’s the case in your situation, but I do think that to get it to IGES its potentially reducing the data to a limited surface subset.

DirectConnect is installed with Maya, though you may have to go into the plugin manager and turn the plugin on.

Reading CAD data has traditionally been one of Maya's strengths due to it close relationship with some of the Alias design products. DirectConnect has been around for almost as long as Maya has. But you have to know the order or precedence of some the conversions dependent upon the source and targets. Not everything can go in a linear one-step conversion.

--
Joey Ponthieux
LaRC Information Technology Enhanced Services (LITES II)
Science Systems and Applications Inc. (SSAI)
NASA Langley Research Center
__________________________________________________
Opinions stated here-in are strictly those of the author and do not
represent the opinions of NASA or any other party.


> -----Original Message-----
> From: softimag...@listproc.autodesk.com [mailto:softimage-
> bou...@listproc.autodesk.com] On Behalf Of David Saber

[David Saber]

Hi Joey
We'll test what you suggested. In the meantime I kept on testing MOI and
even with more complicated mesh, the IGES to polygon object converter
gets me quite good results. The meshes are not clean but they have far
less problems than with the Maya converter.
Thanks again for this load of great info!!
David