No continuity options

Why don’t I get any edge continuity options here when using sweep2? Rhino 8 Beta.
sweep2 no continuity options.3dm (94.2 KB)

Hi @phcreates
You need to enable “Rebuild cross sections” (or “Refit cross sections”) so that Rhino is allowed to change the shape of the sweep shape to match the edges. Note that a minimum of 6 control points are needed if you want curvature continuity on both edges.

HTH, Jakob

Also notice that these two edges are G0, so it’s not going to be a clean transition.

I guess I should clarify: In my head it should work without rebuild, because the sweep shape is degree-3 already. But, I’m seeing that it’s rational (I’m not used to running into rational 3-degree edges because I rarely build those. I wonder how I ended up with this one?)

So, I have a different question: why can’t a rational 3-degree sweep shape have continuity options? I think the bottom line is that I don’t understand why rational curves are different. After all, in this case I can build the surface, then use matchsrf to create tangency on both edges, and the result is better than if I had clicked ‘rebuild’ during the sweep. So why can’t Rhino do that match behind the scenes?
rational match.3dm (156.6 KB)