Hi Martin,

> My question is whether this hugely degrading 'feature' is entirely down to ACIS, or is likely to be some
> unfortunate error in how ACIS is being implemented?

Well, the "error" part is just that the actual ACIS "fit NURBS surface through n-sided boundary" mechanism seems to have a bunch of the problems that are just pretty typical to that class of operation which is a tendency to have bumps and wiggles in the generated result. Wiggles are pretty much a natural byproduct of constraint based fitting mechanisms.

But they are able to mask that problem in many cases because they do have a nicely performing "analytic n-sided patch" surface which is able to be used instead of the NURBS surface for quite a few operations. Any operation within ACIS that is able to use that analytic surface instead of the NURBS surface will behave really nicely. Any operation that actually needs to operate only on a NURBS surface rather than a more abstract analytic surface (export being a prime example) will then have the less nice fitted result come into play.

Whether this is good or not I suppose is open to some interpretation and dependent on what in particular is being done - I guess if you only end up doing the particular things where the analytic surface is able to be used then the lesser quality one is totally masked and never comes into play and you could say that is a good result for that particular case. If you do end up doing something that needs the NURBS result then this sudden quality shift is probably pretty surprising though. And it's bad news for NURBS data exchange certainly.

- Michael