I’m using Offset with Throughpoint on two different edges on the same surface:
So what’s going on here?
EDIT: So I wasn’t using InCplane, which would use the ortho view, but I’d still would like to know why Rhino considered these two edges from the same surface to have different “plane of the original curve”…
offsetcrv.3dm (56.1 KB)
Hello - the sorter edge is not planar so Rhino has to cook up a plane somehow to offset it - that plane is different then the plane of the longer edge.
-Pascal
Oh dear lord. It’s trim all over again!
Please add explicit options for offset/trim directions, instead of having you guess after every time using these tools. 
Hello - What is the result you expect in this example?
-Pascal
Since I’m offsetting an edge and not a curve, and since the only direction option that exists is InCplane, I guess I’d expect both to offset along at least the tangent of the surface.
(Btw, again with the tool consistency… I didn’t see a ChainEdges option?)
Well, plain Offset as it exists is a 2d operation - it requires a plane - if you want something other than that, you’ll need to be pretty specific - a ‘true’ 3d offset is not really an option - that’s a pipe. There needs to be some reference. So, for now, there is OffsetCurveOfSrf - not a 2d operation, OffsetNormal, which could, I think get a Direction=Normal/Tangent option like Fin has - To get the curve I think you want use Fin, Direction=Tangent and dup edge. If that gives the desired result, then I think OffsetNormal can no doubt be tweaked to work tangent as well - my guess is it is using exactly the same code as Fin.
@eobet - in case it helps, here is an updated ‘FinTools’ that optionally returns curve, surface, or both - maybe it does something useful in this context.
FinTools.py (21.4 KB)
To use the Python script use RunPythonScript, or a macro:
_-RunPythonScript "Full path to py file inside double-quotes"
FinTools.rhp (35 KB)
-Pascal
That’s great, thank you for the update! …although, if I’m allowed to be nitpicky, that tool doesn’t have ChainEdges/SelChain either 
(Yes, I’m stupidly trying to hold on to what little history I can manage with curves in Rhino…) 