Why will some joined curves produce polysurfaces and others joined single surfaces?


I’ve made a curve from two curves using BlendCrv with Curvature. When I join these curves and extrude them they produce a polysurface consisting of just two surfaces, instead of expected three. I can’t recall that joined curves produce single surfaces or am I just slightly more confused than normal?

Why is there a single surface from two curves?
Why isn’t the last curve invited to the party?


join-extrude.3dm (277.1 KB)

Here are the files, site wouldn’t let me add them to the post when editing.

If I upped the G on the little left over line and moved the last point, I get a single surface, yay!. However if I leave it straight, join them and then extrude, it’s left out again!
So the conclusion is… if the little line wants to join the fun, it can’t be straight. :slight_smile:

The SplitAtTangents option in the Extrude command is supposed to control whether you get a polysurtace or a single composite surface. However, this option has always been buggy and unreliable. If you want a polysurface the only reliable way to get it is to explode the curve and then extrude and then join the result.

Also be aware that composite surfaces can also be unreliable and will be very unreliable if internally the tangency between spans is not perfect.

ah, thanks for clarifying. I try to be very meticolous with the tangency, or well I let Rhino be, with blend commands, is that enough or do I need to do something more?

Blend command should produce perfect tangency so a composite surface made from a blended curve should work OK most of the time. Ive seen situations where composite surfaces cause problems, but I can’t some up with an example at the moment.