I work with curves a lot going back and forth between Rhino and Blender. I tend to use OBJ, but it’s hacky. On importing, the scene has to have a mesh object in order for the curves to import. They do match the Rhino output. When exporting objs, i export polylines and need to rebuild nurbs curves based on the vertices. The workflow is not completely accurate though.
When i did need completely accurate result, I had to write a small script because the weight wasn’t getting picked up properly by the OBJ importer. This was a few years back at v 2.78, so not sure if there’s been any change with the current OBJ importer.
I am able to re-import objs (exported from blender) containing nurbs without any extant mesh object, but the paramaterisation is destroyed; the curve is approximated by a dense degree 1 curve. This is 2.83.
This is unrelated to NURBS but just a heads up for FBX in general.
I saw a Developer Update for Godot where Gordon McPherson said he did a lot of work on the FBX import and export compatibilities in Godot which indirectly Blender’s exporter was also going to benefit a lot from (he mentioned that it could eliminate problems with Maya exchanges).
Don’t know how much of this is already done or went into Blender already but he mentioned that he is glad for anyone helping or asking him about this because it was just so much work and he wants anyone to benefit from it.
Just posting this here, for the chance that it maybe is something that could impact your work.
If not - just ignore this post.
I use both obj and dxf to import curves from Rhino.
It depends how the curves are build in cad, but often the curves have missing parts (i guess higher degree which is not supported)?
As you can see all the vertices are imported but, if the spline is open, the end vertices of each spline are ignored in defining the spline.
The round part of the “d”, which is closed, it is drawn properly