I don’t use Grasshopper. Several methods in Rhino:
ExtendSrf but to in the direction which makes the surface smaller rather than extending it.
OffsetCrvOnSrf selecting the dividing curve to create a curves the desired distance from the edge.
Trim using the new curve.
Pipe with Cap=None around the dividing curve with the radius equal to the desired distance from the dividing curve or the diameter the desired distance between surfaces.
ExtendSrf the pipe so it protrudes beyond the edges of the surface.
Trim using the pipe.
Sometimes instead of using
Trim it is easier to
Split with the Shrink option and then
Delete the parts not needed.
Depending on what you will be doing with the surface you may want untrimmed surfaces. If the trimmed edge is exactly an isocurve you can
ShrinkTrimmedSrf the trimmed surface to the edge and it will become an untrimmed surface. If the trimmed edge is not an isocurve there are several alternatives:
- Use an isocurve obtained using
ExtractIsocurve to trim the surface, and then Shrink. The new edge may not in exactly the same location.
Split with the Isocurve option and Shrink option to split the surface, and then delete the unwanted part. The new edge may not in exactly the same location.
RefitTrim to rebuild the surface with an untrimmed edge. The new edge may deviate from the original trimmed edge, and the the shape of the surface may be altered.