Extension difference

In the attached are 2 trimmed surfaces, both degree 2x1.
Extension difference.3dm (67.2 KB)

When I sub-select the upper edges, and then move them in Z +2 using gumball, I get different results.
The larger one is still degree 2, the smaller one becomes degree 3.

What causes the different results?
And how to avoid the degree change?

On selection, while Ctrl+Shift is held, I get the ‘Select Object’ dialog.
Hmm, I clearly click the edge.
Also V5 doesn’t ask for what I mean.


No comment?

I’m not a Gumball user but ExtendSrf appears to work as expected.

No. 1 seems related to https://mcneel.myjetbrains.com/youtrack/issue/RH-29411
Notice the surface’s control points when the edge is moved 8 units in +Z.

If all the trim edges were on isocurves, using _ShrinkTrimmedSrf and _Untrim before moving the edge would keep the surface degrees 2 x 1.

I don’t know why it refits one surface and not the other - I guess I’d expect them to behave the same either way - @rajaa - do you have an idea?


ExtendSrf does not change the degree because it works with the existing surface, and only extends surrounding edges. On the other hand, picking an edge and moving it (which you can move in any direction) the command refits.
In the smaller surface in the example, there is a small bend (the surface is not trimmed only from the top, there is another trim along the side). This is causing the calculation of a new surface that fits. ExtendSrf can handle it because it only extends surrounding edges and use the same surface.