Offset Curves on Surface creates incorrect Curves, Possible Bug Rhino 8

Rhino
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1

Hi McNeel Team,

after a quick chat with Dale, he told me to post my problem here.

A while ago there was an issue with Offset Curves on Surface that resulted in curves that were really broken. I reported the issue and seemingly the ticket was fixed. But I’m still experiencing the issue (maybe a special case?).

I’ve attached a Rhino File.
voronoi-sphere.3dm (3.6 MB)
It was created in Version 8 (8.19.25132.01002, 2025-05-12).

A quick overview how that geometry was created:

  • Geometry was created in GH,
  • The specific problem occurs in pure Rhino and Grasshopper, too, also with a simplified version of the respective curve
  • Sphere
  • Populate Geometry (random Points on sphere)
  • Voronoi 3D
  • Sphere and scaled (factor 2) voronoi object intersect
  • Curves are offset on the surface of the sphere

Bildschirmfoto 2025-05-21 um 17.11.07
Bildschirmfoto 2025-05-21 um 17.11.071974×634 240 KB

The original issue was tracked here.

Thank you very much for having a look into it.
– Sebastian

2

And comment: It does not always happen on all but on many curves that were created that way!

3

Hi,

The fix I put in when working on that bug only resolves situations where the curve and its offset does not intersect the seam. In this example your curve and its offset intersect the seam. In order to resolve that situation a lot more work is required and not something I’m working on in Rhino 8. I opened RH-87574 so this case be worked on as well.

4

Hi Joshua,

I figured it was some kind of “special” case. Thank you for checking and documenting that. I feared there was no easy fix. I might invest some time and find a workaround for my specific use case.

Thanks!

5

One thing you might try is rotating the seam away from the curve before running the offset