This Rhino script allows the user to select two curves to be connected, closing them into a single polyline, using either a rough equivalent of blend curve or straight connections… or not, it’s your call, give it a try you’ll see what I mean. Its a great time saver for closing offset curves.

Rhino Script

Option Explicit
'Script written by <David Mans>
'Script copyrighted by <Neoarchaic Design>
'Script version Monday, September 01, 2008 6:39:25 AM

Call Main()
Sub Main()
	Call connectTwoCurves()
End Sub
Function connectTwoCurves()
	connectTwoCurves = Null
	Dim curve(1)
	curve(0) = Rhino.GetObject("Select Curve 1", 4)
	If isNull(curve(0)) Then Exit Function
	curve(1) = Rhino.GetObject("Select Curve 2", 4)
	If isNull(curve(1)) Then Exit Function
	Call Rhino.EnableRedraw(False)
	Dim dir
	Dim ptA(1),ptB(1),dist(1),cap(1)
	ptA(0) = Rhino.CurveStartPoint(curve(0))
	ptA(1) = Rhino.CurveEndPoint(curve(0))
	ptB(0) = Rhino.CurveStartPoint(curve(1))
	ptB(1) = Rhino.CurveEndPoint(curve(1))
	dist(0) = Rhino.Distance(ptA(0), ptB(0))
	dist(1) = Rhino.Distance(ptA(0), ptB(1))
	
	If dist(0) = dist(1) Then
		dir = True
		cap(0) = Rhino.AddLine(ptA(0), ptB(0))
		cap(1) = Rhino.AddLine(ptA(1), ptB(1))
	Else
		If dist(0) < dist(1) Then
			dir = True
			cap(0) = Rhino.AddLine(ptA(0), ptB(0))
			cap(1) = Rhino.AddLine(ptA(1), ptB(1))
		Else
			dir = False
			cap(0) = Rhino.AddLine(ptA(0), ptB(1))
			cap(1) = Rhino.AddLine(ptA(1), ptB(0))
		End If
	End If
	Call Rhino.EnableRedraw(True)
	Dim arrResults,arrInputs,arrValues
	arrInputs = array("Flip", "Yes", "No", "Type", "Smooth", "Straight", "Join", "Yes", "No")
	arrValues = array(True, True, False)
	arrResults = Rhino.GetBoolean("Connection Options", arrInputs, arrValues)
	
	Call Rhino.EnableRedraw(False)
	Dim tan(3),blend, ptC(3)
	If arrResults(1) = False Then
		Call Rhino.DeleteObjects(cap)
		blend = 0.25
		tan(0) = Rhino.VectorScale(Rhino.VectorUnitize(Rhino.VectorReverse(Rhino.CurveTangent(curve(0), Rhino.CurveDomain(curve(0))(0)))), Rhino.Distance(ptA(0), ptA(1)) * blend)
		tan(1) = Rhino.VectorScale(Rhino.VectorUnitize(Rhino.CurveTangent(curve(0), Rhino.CurveDomain(curve(0))(1))), Rhino.Distance(ptA(0), ptA(1)) * blend)
		tan(2) = Rhino.VectorScale(Rhino.VectorUnitize(Rhino.VectorReverse(Rhino.CurveTangent(curve(1), Rhino.CurveDomain(curve(1))(0)))), Rhino.Distance(ptB(0), ptB(1)) * blend)
		tan(3) = Rhino.VectorScale(Rhino.VectorUnitize(Rhino.CurveTangent(curve(1), Rhino.CurveDomain(curve(1))(1))), Rhino.Distance(ptB(0), ptB(1)) * blend)
		
		ptC(0) = Rhino.PointAdd(Rhino.CurveStartPoint(curve(0)), tan(0))
		ptC(1) = Rhino.PointAdd(Rhino.CurveEndPoint(curve(0)), tan(1))
		ptC(2) = Rhino.PointAdd(Rhino.CurveStartPoint(curve(1)), tan(2))
		ptC(3) = Rhino.PointAdd(Rhino.CurveEndPoint(curve(1)), tan(3))
		If arrResults(0) = True Then
			If dir = False Then
				cap(0) = Rhino.AddCurve(array(ptA(0), ptC(0), ptC(3), ptB(1)), 3)
				cap(0) = Rhino.AddCurve(array(ptA(1), ptC(1), ptC(2), ptB(0)), 3)
			Else
				cap(0) = Rhino.AddCurve(array(ptA(0), ptC(0), ptC(2), ptB(0)), 3)
				cap(0) = Rhino.AddCurve(array(ptA(1), ptC(1), ptC(3), ptB(1)), 3)
			End If
		Else
			If dir = False Then
				cap(0) = Rhino.AddCurve(array(ptA(0), ptC(0), ptC(2), ptB(0)), 3)
				cap(0) = Rhino.AddCurve(array(ptA(1), ptC(1), ptC(3), ptB(1)), 3)
			Else
				cap(0) = Rhino.AddCurve(array(ptA(0), ptC(0), ptC(3), ptB(1)), 3)
				cap(0) = Rhino.AddCurve(array(ptA(1), ptC(1), ptC(2), ptB(0)), 3)
			End If
			
		End If
	Else	
		If arrResults(0) = False Then
			Call Rhino.DeleteObjects(cap)
			If dir = False Then
				cap(0) = Rhino.AddLine(ptA(0), ptB(0))
				cap(1) = Rhino.AddLine(ptA(1), ptB(1))
			Else
				cap(0) = Rhino.AddLine(ptA(0), ptB(1))
				cap(1) = Rhino.AddLine(ptA(1), ptB(0))
			End If
		Else
		
		End If
	End If
	If arrResults(2) = False Then
		Call Rhino.JoinCurves(array(curve(0), cap(0), curve(1), cap(1)), True)
	End If
	Call Rhino.EnableRedraw(True)

	connectTwoCurves = array()
End Function