Hi Vince, in general it is not possible to only get quads, particularly because of trimming curves.

The "underlying surface" of a NURBS model is a 4 sided patch, and that part does tend to convert naturally to quads - that's why you do get plenty of quads in many areas when you export a mesh.

But in any area that has been trimmed away (for example by boolean operations), you get "trim curves" that live on the surface and mark regions as being inactive.

It is not easy to follow these trim curves to make quads.

Here is one example to show you what I mean:



In this case the underlying surface is very simple, it is made up of a plane and it is actually a single quad itself.

However, there are trim curves on this surface that are quite elaborate and they move in all kinds of different directions, they are not restricted to only move along the grid of the quad. So it isn't possible to get quads out of this just by subdividing the starting quad. It would require some kind of stepping and tracing along the trim curves which is a quite difficult procedure to do well automatically.


Just to illustrate it a little more, I have taken that plane and warped it a bit to give it some curvature, then exported it as a mesh:



Here you can see how the meshing works - the underlying surface is subdivided which does create a grid of quads underneath everything, but then when intersected with trimming curves those quads become n-gons.

- Michael

Hi Michael,

Thanks so much for the explanation and visual. Is it possible to export to a different poly editing app to change the ngons to quads? Sorry if this is a dumbe question, I model in Nurbs and am not very familiar to subd modelling. Thanks for your help!

Vince

Hi Vince, some programs like Silo have some "re-topologizer" type tools that could help with that process.

But I think it requires some assistance from you to place points at good areas, I don't think it is like "push one button and convert to quads".

It would be pretty tough to write a fully automatic mechanism for that.

If you only use "untrimmed" NURBS surfaces, like the surfaces you get from lofting or sweeping and never use any boolean operations, then those types of surfaces should convert to nearly all quads, the only place there won't be quads is where the mesh subdivides into smaller levels to add detail.

- Michael