Hi Burr, yup NURBS circles work in exactly that same way, they have a start and end point that touch each other.

I usually call that the "seam" of the closed curve.

If you extrude a circle into a cylinder, you'll see where the seam was, it will then become the closing seam of the cylinder surface which is closed in a similar way (start and end touching).

There is stuff in MoI that is built in to try and minimize the amount of work that you have to do for manually ensuring that the seam is in the correct place. Like for instance if you loft between 3 circles, MoI will automatically analyze them and re-configure their seam points to match nicely so that the loft does not severely torque or twist.

You can actually adjust those seam points while you are in the final stage of the loft command if you want to purposely introduce twist - before you finish Loft you'll see those points displayed inside the viewport and you can click and drag on one to reposition it.


But I'm not completely sure if that's what you are mentioning - a lot of programs also keep an extra kind of "local axis" for each object that defines a rotation for the object...


Actually in this particular case of the "up" being weird in the Sweep, it isn't really the up of the profiles that is messed up, it is the "up" that is coming from the rail curve - part of the process of sweeping involves making a set of "frames" (like an x/y/z tripod) at each point along the rail.

- Michael