Split gives 4 results instead of 2, and the shapes are changed

Rhino Rhino for Windows
1

Hello,
I’m using the split command to cut a surface by a NURBS closed contour:

2025-02-28_12h26_35
2025-02-28_12h26_35908×470 8.73 KB

and instead of having 2 surfaces, I get 4 surfaces which partially don’t fit the cutting curve:

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2025-02-28_12h26_44895×461 13.9 KB

However if I use the command SimplifyCrv beforehand on the cutting curve, I get the expected result:

2025-02-28_12h27_08
2025-02-28_12h27_08932×459 12.2 KB

Additionally,

  • I’ve tested with multiple tolerances, the result is the same.
  • The behavior is the same using Brep.Split() in Python.
  • It happens on Rhino 8, but not Rhino 7.
2

Can you post the file with the closed curve and the surface?

3

Here it is,

Unexpected_Spli_result.3dm (718.8 KB)

4

Strange. If the splitting curve/polyline is split and then joined back together the surface splits normally.

How was the splitting curve/polyline created.

5

Yeah, that’s not supposed to happen. I’ve logged the issue here.

The difference before and after explode+join is that before it is a degree-1 nurbs curve, while after it is a polyline. Of course that shouldn’t make any difference, but it does.

6

It has been created using python rs.AddNurbsCurve with all the control points, knots,…
Same for the surface.