> Another thing, I believe that for the rendering, Carrara transform the
> 3dm in "poly" in internal for rendering
> It's not a "nurbs renderer" :)

This is not necessarily bad though - the most standard way to render NURBS is to convert it to polygons. There are only very few "true NURBS renderers" out there because rendering NURBS directly tends to be extremely slow.

There are also some other problems with rendering directly other than speed - during the construction of NURBS objects it is normal for there to be very slight gaps (within the modeling tolerance, typically like 0.01 or 0.001) between intersected surfaces. When rendering the NURBS directly, it is possible to have these gaps show up as cracks when zoomed in on spots of the model.

A good quality conversion to polygons will take care of this crack problem because it will generate polygons that are completely sealed together along shared edges of the NURBS model.

- Michael