The CurveOnSurface plugin is now available for Rhino 6 onFood4Rhino.com16. This plugin requires at least version 6.3 of Rhino.
The CurveOnSurface plug-in for Rhino provides two commands. The CurveOnSurface command is an alternative for the InterpCrvOnSrf command, while the CurveOnSurfacePtOn command turns on control points to edit this curve. The CurveOnSurface plugin is for Rhino for Windows only. There are different versions of the plugin for Rhino 6 and Rhino 5 (both 32-bit and 64-bit).
CurveOnSurfacePtOn will turn on control points at the interpolation points that defined the curve. These can then be edited individually, or in larger selection sets, to directly edit the curve. The points will be constrained to stay on the surface. Control point editing commands such as Nudge may also be used. The PtOff command turns off all control points.
The Rhino 6 version of CurveOnSurface will allow one to draw over the seam of a smooth closed surface. source: CurveOnSurface plugin for Rhino 6
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. CurveOnSurfacePtOn will turn on control points at the interpolation points that defined the curve. These can then be edited individually, or in larger selection sets, to directly edit the curve. The points will be constrained to stay on the surface. Control point editing commands such as Nudge may also be used. The PtOff command turns off all control points. source: http://www.food4rhino.com/app/curveonsurface-0