Hi Burr,

> "STEP" up to solids

Yeah that can often help, particularly with not having to join things.

Unfortunately STEP has some totally different areas where things can go wrong in - the way trimming boundaries are specified in STEP files, only the 3D curves of the edges are contained in the file, leaving the UV space trim curves to be calculated by the application. That calculation can be fairly difficult in some circumstances, particularly in seam areas of closed surfaces if the trim curve wanders slightly back and forth over the seam.

Also the STEP mechanism allows for things like a single trim curve that crosses over a seam in a closed surface which has to be split for regular NURBS topology. That can again be a difficult calculation if the edge just barely grazes the seam or hugs along it for just a little bit instead of crossing directly over it, stuff like that.

So there are some trim curve calculations and processing that still has to happen with STEP files in order to get it into a normal NURBS topology, and along with any kinds of fairly complex calculations there is potential for things to go wrong in grazing / barely touching / overlapping kinds of situations.

It sure would have been nice if the STEP authors would have made it a more direct container of simple NURBS topology, that would have made it a more ironclad data transfer mechanism.

- Michael