Summary: Demonstrates how to offset a curve using RhinoScript.

I am looking for a function that can offset a closed curve, inside or outside, a specific distance. How can I do this using RhinoScript?

Yes, you can do this. First of all, make sure you are using the latest Labs version of RHINOSCRIPT as it contains a modified version of the OffsetCurve method that allows for negative distances.

The following function will offset a closed planar curve, either inside or outside, a specified distance.

 ' Description:
 '   Offsets a closed planar curve inside or outside.
 ' Parameters:
 '   strSurface  - String, The identifier of the closed planar curve to offset.
 '   dblDistance - Number, The distance to offset.
 '   blnOutside  - Boolean, Offset outside (true) or inside (false).
 ' Returns:
 '   Array, An array containing the identifiers of the new objects if successful.
 '   Null on error

 Option Explicit
 
 Function RhinoOffsetClosedPlanarCurve(ByVal strCurve, ByVal dblDistance, ByVal blnOutside)
 
   ' Local variables
   Dim arrPlane, arrOldPlane, strView, arrBox, arrPoint
 
   ' Default return value
   RhinoOffsetClosedPlanarCurve = Null
 
   ' Quick parameter validation
   If (VarType(strCurve) <> vbString) Then Exit Function
   If Not IsNumeric(dblDistance) Then Exit Function
   If (VarType(blnOutside) <> vbBoolean) Then Exit Function
 
   ' Curve validation
   If (Rhino.IsCurve(strCurve) = False) Then Exit Function
   If (Rhino.IsCurveClosed(strCurve) = False) Then Exit Function
   arrPlane = Rhino.CurvePlane(strCurve)
   If Not IsArray(arrPlane) Then Exit Function
 
   ' Calculate plane-based bounding box
   strView = Rhino.CurrentView()
   arrOldPlane = Rhino.ViewCPlane(strView, arrPlane)
   arrBox = Rhino.BoundingBox(strCurve, strView)
   Call Rhino.ViewCPlane(strView, arrOldPlane)
 
   ' Offset point so its outside of bounding box
   arrPoint = Rhino.PointAdd(arrBox(0), Rhino.VectorCreate(arrBox(0), arrBox(2)))
 
   ' Offset the curve 
   If (blnOutside = False) Then dblDistance = -dblDistance
   RhinoOffsetClosedPlanarCurve = Rhino.OffsetCurve(strCurve, arrPoint, dblDistance, arrPlane(3))
 
 End Function