The OffsetCrvOnSrf problem

Hello, why does the OffsetCrvOnSrf tool work so strangely?
Untitled.3dm (323.4 KB)


Try CurveOnSurface plugin. This plugin requires at least version 6.3 of Rhino.

Unlike InterpCrvOnSrf, the curve created with CurveOnSurface is independent of the parametrization of the surface. The curve will tend to go in a straight or direct path instead of following the surface isocurves. In fact, when only two points are specified the curve is the shortest possible path (a geodesic) – the same curve made by the ShortPath command. source:

No thanks. I’m just interested in this tool.

@Modeler3D The file you uploaded does not have most of the curves in your illustration. You need to upload a file with the input curve and the output curve(s).

Added - What is the significance of the points?

The points on the surface for the ThroughPoint button in the OffsetCrvOnSrf tool.
But the file with the resulting curves:
Untitled2.3dm (374.7 KB)

The cause of the bad offset curves is not obvious to me. Perhaps someone from McNeel can respond.

There is no problem with the OffsetCrvOnSrf. The outer circle was open - this may have been the source of the problem. Another possible source of the problem is offseting inside with large spacing. When I offset outside (Flip), everything looks OK. I used Reparameterize command on the circles and the surface, but I doubt that it mattered.
Untitled3.3dm (359.9 KB)

Hello - my guess of the moment is that the offsetter does not like the rational surface - it works more cleanly on a rebuilt sphere - say 24 by 12 points, degree 5 - that is a very close approximation of the sphere. Because of the way the curves are extended you’ll need to explode the resulting curves and pick out the pieces that make the expected offset.


Good idea! If I use small offsets, then everything is fine. However, sometimes it may take a lot of clicks for the desired offset.

However, here the original curve is already at the edge, and therefore it can be displaced at any distance.

Here the curve has crossed the edge calmly, but with increasing distance the curve breaks.

So is this Rhino error or is it normal?