Why does VRay not like Nurbs?

[seq]

Hello,

we often have to work with complex CAD-data, mostly as STEP-files and so we have a lot of nurbs patches in the scene.

What I wonder about is why VRay doesn't like nurbs patches? When we try to render the scene VRay exit with a fatal error:

Compiling geometry...
error: Exception (module=1) : Compiling geometry
error: There was a fatal error rendering the scene

The subdivision settings in the RG were at the default values.

I know that Vlado told me last year at a personal meeting, that we should always convert to poly. But why is MR capable rendering the scene and VRay not?

It would be very great if don't need to convert to polys as this is a very time consuming job.

Any tips or tricks?

Thanks
Lars

[vlado]

The reason is simple - we have not put too much effort into the NURBS support. There is certainly more to be desired from our implementation. With that said, if you can send us some examples to vraymaya@chaosgroup.com which produce errors, it will be very helpful.

Best regards,
Vlado

[seq]

Just sent you a Maya 2011 scene with a simple nurbs surface from an imported STEP-file. Really simple, but VRay refuse to render.

Please make VRay more NURBS friendly! This would be a big help for our daily work.

Thanks!

[pechart]

That would be nice, but aren't most people converting those CAD datasets to polys nowadays? You'll never get proper solids into maya if you don't convert it, i guess. Maya can't read those solids, so you get a lot of patches. We send everything we receive from CAD through a good converter and get nice tris, with which you can work much better. Those NURBS-patches in maya are very annoying, especially the fact that you get a huge load of nodes, which make the scene big and slow. (And assigning materials etc. is a pain, of course).
So i'd really say: don't put too much effort in this, it will never work out really nice, because maya does't supports the right NURBS-formats.

[seq]

Which converter do you use? Never found a really good one which does prevent a lot of handwork.

NURBS are not the holy grail, but it would be nice if we could render out images without first converting everything to poly.

[pechart]

I think (as i already said) NURBS are not so easy to implement in the right way, because of the different formats existing. The problem is on the import, so a better rendering would be nice but won't solve the real problems at all. Those are the lack of maya to import and tesselate NURBS correct. If you have solids (as an opposite to surfaces) maya won't know how to import them correctly and you'll get a whole lot of patches. So nearly every studio I know converts them using a appropriate tool that understands NURBS geometry better than maya. There are several ways to get the geo in (import with nPower power nurbs, tesselate with showcase, import+tesselate with Polytrans) and in my opinion this is way better then using the original NURBS in maya. (Polytrans is pretty good, btw.)
Of course it's possible to import with direct connect, but you will have the trouble with patches, continuity, memory, rendering and so on.

[seq]

Yes, you are totally right. Just wonder why VRay can't render a simple nurbs clip. Perhaps it is just a little trick we need.

Do you know if there is a 64 bit version of Polytrans? Found only the 32 bit demo?

[pechart]

I talked to the programmer on that x64 topic. Many people did, he said. The point is, he told me, that Polytrans for Maya is not only a simple plugin that needs to get compiled for 64 bit. It uses original libraries and code from companys like PTC that just don't exist for 64 bit. Polytrans relies on that code, so it can't be done for 64 bit. You have to have a seperate 32 bit maya installation to use the plugins. That's a little bit annoying, but we never ran into problems with memory issues or something like that. We use the 32bit just for importing, then we switch to our regular 64bit maya version.

[GatesB]

The best nurbs-to-poly exporter out there is the one included with Moi3D. You wont be disappointed.

[pechart]

Question is if Moi3d imports stuff like BREP-solids or native Pro/E data correct But i will take a look at it, for sure!

[seq]

Just downloaded and installed demo of NPower Maya Power Translators. Nice thing is that you can change the tesselation after importing for every objects. Can you do this with Polytrans? But it only can import IGES, STEP, SAT and Rhino. For the moment this would be okay for us.

Can't find any info what Moi3D can import? Can you give us some info, GatesB? Thanks.

And pechart, isn't the STEP and IGES a transfer format like in a way FBX? So that you can export a STEP-file from CATIA or Pro/E? I guess not all the data will be translated.

[pechart]

I'm not enough into this topic to give real qualified statements.
But usually you SHOULD be able to transfer the files from package to package. But it depends on how well the formats are implemented in the different packages. Ever tried FBX with more complex datasets from cinema4d<R10? So it goes with CAD data. The best thing is to receive the native format from the CAD application and translate that with native tools (that does Polytrans, nPower doesn't, as far as i know. Someone else said something with reverse engineered libraries and stuff like that some time ago.) This way you get nice and clean results most of the time. Given the engineer did a good job on constructing, of course.
The nPower tools lare nice, but we had some issues with them, so we decided to go for polytrans. But everyone has to decide on his own with that. These are just my experiences over the years.

Oh, and you can't change the resolution after importing with polytrans. But on the other hand you can transfer+render your meshes without having the plugins on other machines. Don't know if that's possible with nPowers tools.

[seq]

Thanks pechart. We are in contact with the Polytrans system architect. Seems that Polytrans has some problems with our STEP-files. Hopefully nothing serious! For this project we only have the STEP-files available.

[pechart]

No problem. Robert is a very helpfull guy. He should be able to solve your problem.

[seq]

Yes, indeed he is very helpfull. It was just a limitation of the demo mode. We just ordered our copy.