Issue with OffsetCrvOnSrf on sphere

hi there,

I ran into the following issue today with OffsetCrvOnSrf. The circle is created by OffsetCrvOnSurface for some reason.

20220722_offsetCrvOnSrf_seam.3dm (253.9 KB)

I’m trying to offset the edge of the surface inwards to then split the surface with that curve.

This is done in grasshopper, but seems to be a problem not related to grasshopper. I do have some control over how the initial surface is created, if that would hep avoid this issue.

Appreciate any advise!


Hi Bernd - use ShrinkTrimmedSrf on the surface first. I’ll see if we can tune this up. Note your curve is not closed, you might want to clean that up, if it is meant to be.

RH-69532 OffsetCrvOnSrf: shrink the input


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That was quick, thanks Pascal!
I’ll test it with some other surfaces, but so far it looks very good.


Pascal, is not it bad to automatically shrink the surface since the user will lose the ability to untrim the original surface? What if an invisible copy of the surface is being temporarily created and shrunk, then the offset curve on surface is built on it, then that surface gets deleted automatically and the original surface remains?

Hi Bobi - the shrinking can be done behind the scenes, it need not change the object in the document.


Yes, that’s what I mean, too. Then it should be done. Trimming surfaces also fails sometimes, but shinking the surface helps in most cases.

Right, if the face is a lot smaller than the underlying surface, intersecting can get squirrely - but in that case … thinking… I guess shrinking to get the trimming curves might be OK - I’ll as a developer. (If you happen to have a case that shows this that would be helpful - I’ve seen it but don’t, I think, have a file)


A few years ago I created a topic somewhere here regarding an issue with trimming surfaces at a very small angle along the edge. I will try to find it. The solution I found was to temporarily bump the tolerance 10 or 100 times higher than the file tolerance, then after trimming I reverted to the original file tolerance.

This is the topic in question:

RH-69532 is fixed in the latest WIP.