Discussion

In order to to duplicate the border curves of surfaces and polysurfaces, you need to search the object for naked edges. A naked edges is one that is not connect to other edges. Once a naked edge has been found, simply duplicate the edge curve and add it to the document.

C++

  CRhinoCommand::result CCommandTest::RunCommand( const CRhinoCommandContext& context )
  {
    CRhinoGetObject go;
    go.SetCommandPrompt( L"Select surface or polysurface" );
    go.SetGeometryFilter( CRhinoGetObject::surface_object |
                          CRhinoGetObject::polysrf_object );
    go.AcceptNothing();
    go.GetObjects( 1, 1 );
    if( go.CommandResult() != success )
      return go.CommandResult();


    const CRhinoObjRef& object_ref = go.Object(0);
    const CRhinoObject* object = object_ref.Object();
    const ON_Brep* brep = object_ref.Brep();
    if( !object || !brep )
      return failure;


    object->Select( false );


    ON_SimpleArray<const ON_Curve*> curve_array( brep->m_E.Count() );


    for( int i = 0; i < brep->m_E.Count(); i++ )
    {
      const ON_BrepEdge& edge = brep->m_E[i];


      // Find only the naked edges 
      if( edge.m_ti.Count() == 1 && edge.m_c3i >= 0 )
      {
        ON_Curve* curve = edge.DuplicateCurve();


        // Make the curve direction go in the natural
        // boundary loop direction so that the curve
        // directions come out consistantly
        if( brep->m_T[edge.m_ti[0]].m_bRev3d )
          curve->Reverse();
        if( brep->m_T[edge.m_ti[0]].Face()->m_bRev)
          curve->Reverse();


        curve_array.Append( curve );
      }
    }


    double tol = 2.1 * RhinoApp().ActiveDoc()->AbsoluteTolerance();
    ON_SimpleArray<ON_Curve*> output_array;


    // Join the curves
    if( RhinoMergeCurves(curve_array, output_array, tol) )
    {
      for( int i = 0; i < output_array.Count(); i++ )
      {
        CRhinoCurveObject* curve_object = new CRhinoCurveObject;
        curve_object->SetCurve( output_array[i]);
        if( context.m_doc.AddObject(curve_object) )
          curve_object->Select();
        else
          delete curve_object;
      }
    }


    // Don't leak memory
    for( int i = 0; i < curve_array.Count(); i++ )
      delete curve_array[i];


    context.m_doc.Redraw();
    return success;
  }

VB.NET (Rhino 4)

  Public Overrides Function RunCommand(ByVal context As IRhinoCommandContext) _
    As IRhinoCommand.result
    Dim go As New MRhinoGetObject()
    go.SetCommandPrompt("Select surface or polysurface")
    go.SetGeometryFilter(IRhinoGetObject.GEOMETRY_TYPE_FILTER.surface_object Or _
                          IRhinoGetObject.GEOMETRY_TYPE_FILTER.polysrf_object)
    go.AcceptNothing()
    go.GetObjects(1, 1)
    If (go.CommandResult() <> IRhinoCommand.result.success) Then
      Return go.CommandResult()
    End If
    Dim object_ref As MRhinoObjRef = go.Object(0)
    Dim obj As IRhinoObject = object_ref.Object()
    Dim brep As IOnBrep = object_ref.Brep()
    If (obj Is Nothing Or brep Is Nothing) Then
      Return IRhinoCommand.result.failure
    End If


    obj.Select(False)


    Dim curve_array As New System.Collections.Generic.List(Of IOnCurve)()


    Dim edges As IOnBrepEdgeArray = brep.m_E
    Dim count As Integer = edges.Count()
    For i As Integer = 0 To count - 1
      Dim edge As IOnBrepEdge = edges(i)
      ' Find only the naked edges 
      If (edge.m_ti.Count() = 1 And edge.m_c3i >= 0) Then
        Dim curve As OnCurve = edge.DuplicateCurve()


        ' Make the curve direction go in the natural
        ' boundary loop direction so that the curve
        ' directions come out consistantly
        If (brep.m_T(edge.m_ti(0)).m_bRev3d) Then
          curve.Reverse()
        End If
        If (brep.m_T(edge.m_ti(0)).Face().m_bRev) Then
          curve.Reverse()
        End If
        curve_array.Add(curve)
      End If
    Next


    Dim tol As Double = 2.1 * context.m_doc.AbsoluteTolerance()
    Dim output_array(0) As OnCurve
    ' Join the curves
    If (RhUtil.RhinoMergeCurves(curve_array.ToArray(), output_array, tol)) Then
      For i As Integer = 0 To output_array.Length - 1
        Dim c As MRhinoCurveObject = context.m_doc.AddCurveObject(output_array(i))
        If (c IsNot Nothing) Then c.Select()
      Next
    End If


    context.m_doc.Redraw()
    Return IRhinoCommand.result.success
  End Function

C# (Rhino 4)

  public override IRhinoCommand.result RunCommand(IRhinoCommandContext context)
  {
    MRhinoGetObject go = new MRhinoGetObject();
    go.SetCommandPrompt( "Select surface or polysurface" );
    go.SetGeometryFilter( IRhinoGetObject.GEOMETRY_TYPE_FILTER.surface_object |
                          IRhinoGetObject.GEOMETRY_TYPE_FILTER.polysrf_object );
    go.AcceptNothing();
    go.GetObjects( 1, 1 );
    if( go.CommandResult() != IRhinoCommand.result.success )
      return go.CommandResult();


    MRhinoObjRef object_ref = go.Object(0);
    IRhinoObject obj = object_ref.Object();
    IOnBrep brep = object_ref.Brep();
    if( obj == null || brep == null )
      return IRhinoCommand.result.failure;


    obj.Select( false );


    System.Collections.Generic.List<IOnCurve> curve_array =
          new System.Collections.Generic.List<IOnCurve>();


    IOnBrepEdgeArray edges = brep.m_E;
    int count = edges.Count();
    for( int i = 0; i < count; i++ )
    {
      IOnBrepEdge edge = edges[i];
      // Find only the naked edges 
      if( edge.m_ti.Count() == 1 && edge.m_c3i >= 0 )
      {
        OnCurve curve = edge.DuplicateCurve();


        // Make the curve direction go in the natural
        // boundary loop direction so that the curve
        // directions come out consistantly
        if( brep.m_T[edge.m_ti[0]].m_bRev3d )
          curve.Reverse();
        if( brep.m_T[edge.m_ti[0]].Face().m_bRev)
          curve.Reverse();
        curve_array.Add( curve );
      }
    }


    double tol = 2.1 * context.m_doc.AbsoluteTolerance();
    OnCurve[] output_array = new OnCurve[0];
    // Join the curves
    if( RhUtil.RhinoMergeCurves(curve_array.ToArray(), ref output_array, tol) )
    {
      for( int i = 0; i < output_array.Length; i++ )
      {
        MRhinoCurveObject c = context.m_doc.AddCurveObject(output_array[i]);
        if( c != null )
          c.Select();
      }
    }


    context.m_doc.Redraw();
    return IRhinoCommand.result.success;
  }