Hi PaQ,

Yeah that ability of Maya to transform subd to NURBS does not seem to be very frequently mentioned or used for some reason. But it could definitely be a useful transfer method.


> Here's a other try using maya, still thinking it might be a cool way of
> working ... however there as some areas that are quite
> complex (red lines) and need probably to be rebuild/simplify too.

That kind of smaller patch refinement will generally happen in areas where you have a vertex with a valence of other than 4, meaning more than 4 edges radiating out from it.

Like in this case you seem to have a kind of edge loop that terminates here:



So that created a vertex there that has a valence of 3, and the one right above it has a valence of 5 with 5 edges coming out of it - if you can avoid those where possible then it will probably eliminate that kind of patch refinement.

Like in your case here if you continue that edge loop to the end instead of terminating it, that will probably get fixed up.


There is a common relationship between some of the mechanics of how NURBS surfaces work and how subd surfaces work.

The way that Catmull-Clark subd works, it is actually completely equivalent to a NURBS surface in areas of the subd that have quad polygons with all vertices of valence 4.

Those areas of the subd surface have an exact and natural translation to a NURBS surface.

Areas where that is not the case like at your valence 5 spot, do not have a natural equivalence to NURBS so those areas get processed with a kind of fitting or refinement type scheme to get some NURBS surfaces in there that are close enough to the subd surface. But the refinement will generate additional surfaces, that is what you are seeing there.

- Michael

Hi PaQ,

I see you had another vertex of valence 5 up near the front bottom of the canopy there too, I'm kind of surprised that there was not some additional refinement in that area.

I would be a bit suspect of the surface smoothness in that area, although it seems to look pretty good.

But maybe it just happened to be close enough not to need refinement.

- Michael

Hi Michael,

Maya can only convert what they called 'subdiv surface' into nurbs, so in the image posted, it's the middle model. This model is a conversion from
the polygonal one. The subdiv shape allready shows some complex area in the front of the model, and this seems to be translated into the nurbs model at the end.

I have just no experience about how subdiv actually work, it seems to be way more heavier than catmull scheme. Maybe I should first learn this part to have a better control of the nurbs conversion.

I didn't remember having this kind of result with the t-spline. With the t-spline plugin, the nurbs flow was more looking like the maya subdiv one. (I'm not sure if this sentence is clear enough :P).

Hi PaQ, I see - I'm not really familiar with some of these details on how Maya works, but it appears to me that the middle "subdiv surface" has already been divided down in those valence 5 areas.

So my guess is that a "subdiv surface" in Maya is some kind of hybrid object, which has already been prepared in its structure for converting to NURBS? Something like that anyway.

But if you continue that edge loop that I showed above to the end of the shape in the original polygon model before conversion to "subdiv surface", it will probably simplify that area that you were pointing at with the red arrow above.

You can't always avoid such areas, but in areas that you can it will probably make the result more simplified.

- Michael

Yes I'll give a try with a pure quad model, ... at the moment I'm still learning all the selection and objets organisations tools ... (in other words, I'm nowhere :o)).