Rhino represents truncated cones as capped surfaces of revolution. For more information on the ON_BrepRevSurface, see it's definition in opennurbs_brep.h.

C++

  CRhinoCommand::result CCommandTest::RunCommand(
          const CRhinoCommandContext& context 
          )
  {
    ON_3dPoint bottom_pt( 0.0, 0.0, 0.0 );
    double bottom_radius = 2;
    ON_Circle bottom_circle( bottom_pt, bottom_radius );


    ON_3dPoint top_pt( 0.0, 0.0, 10.0 );
    double top_radius = 6;
    ON_Circle top_circle( top_pt, top_radius );


    ON_RevSurface* revsrf = new ON_RevSurface;
    ON_LineCurve* pShapeCurve = new ON_LineCurve;
    revsrf->m_curve = pShapeCurve;
    pShapeCurve->m_dim = 3;
    pShapeCurve->m_line.from = bottom_circle.PointAt(0);
    pShapeCurve->m_line.to = top_circle.PointAt(0);
    pShapeCurve->m_t.Set(0, pShapeCurve->m_line.from.DistanceTo(pShapeCurve->m_line.to));
    revsrf->m_axis.from = bottom_circle.Center();
    revsrf->m_axis.to = top_circle.Center();
    revsrf->m_angle[0] = revsrf->m_t[0] = 0.0;
    revsrf->m_angle[1] = revsrf->m_t[1] = 2.0*ON_PI;


    ON_Brep* tcone_brep = ON_BrepRevSurface(revsrf, TRUE, TRUE );
    if( tcone_brep )
    {
      CRhinoBrepObject* tcone_object = new CRhinoBrepObject();
      tcone_object->SetBrep( tcone_brep );
      if( context.m_doc.AddObject(tcone_object) )
        context.m_doc.Redraw();
      else
        delete tcone_object;
    }


    return CRhinoCommand::success;
  }

VB.NET (Rhino 4)

  Public Overrides Function RunCommand(ByVal context As IRhinoCommandContext) _
    As IRhinoCommand.result


    Dim bottom_pt As New On3dPoint(0, 0, 0)
    Dim bottom_radius As Double = 2
    Dim bottom_circle As New OnCircle(bottom_pt, bottom_radius)


    Dim top_pt As New On3dPoint(0, 0, 10)
    Dim top_radius As Double = 6
    Dim top_circle As New OnCircle(top_pt, top_radius)


    Dim revsrf As New OnRevSurface()
    Dim pShapeCurve As New OnLineCurve()
    revsrf.m_curve = pShapeCurve
    pShapeCurve.m_dim = 3
    pShapeCurve.m_line.from = bottom_circle.PointAt(0)
    pShapeCurve.m_line.to = top_circle.PointAt(0)
    pShapeCurve.m_t.Set(0, pShapeCurve.m_line.from.DistanceTo(pShapeCurve.m_line.to))
    revsrf.m_axis.from = New On3dPoint(bottom_circle.Center())
    revsrf.m_axis.to = New On3dPoint(top_circle.Center())
    revsrf.m_angle(0) = 0
    revsrf.m_t(0) = 0
    revsrf.m_angle(1) = 2.0 * OnUtil.On_PI
    revsrf.m_t(1) = 2.0 * OnUtil.On_PI


    Dim tcone_brep As OnBrep = OnUtil.ON_BrepRevSurface(revsrf, True, True)
    If (tcone_brep IsNot Nothing) Then
      If (context.m_doc.AddBrepObject(tcone_brep) IsNot Nothing) Then
        context.m_doc.Redraw()
      End If
    End If


    Return IRhinoCommand.result.success
  End Function

C# (Rhino 4)

  public override IRhinoCommand.result RunCommand(IRhinoCommandContext context)
  {
    On3dPoint bottom_pt = new On3dPoint(0, 0, 0);
    double bottom_radius = 2;
    OnCircle bottom_circle = new OnCircle(bottom_pt, bottom_radius);


    On3dPoint top_pt = new On3dPoint(0, 0, 10);
    double top_radius = 6;
    OnCircle top_circle = new OnCircle(top_pt, top_radius);


    OnRevSurface revsrf = new OnRevSurface();
    OnLineCurve pShapeCurve = new OnLineCurve();
    revsrf.m_curve = pShapeCurve;
    pShapeCurve.m_dim = 3;
    pShapeCurve.m_line.from = bottom_circle.PointAt(0);
    pShapeCurve.m_line.to = top_circle.PointAt(0);
    pShapeCurve.m_t.Set(0, pShapeCurve.m_line.from.DistanceTo(pShapeCurve.m_line.to));
    revsrf.m_axis.from = new On3dPoint(bottom_circle.Center());
    revsrf.m_axis.to = new On3dPoint(top_circle.Center());
    revsrf.m_angle[0] = revsrf.m_t[0] = 0.0;
    revsrf.m_angle[1] = revsrf.m_t[1] = 2.0 * OnUtil.On_PI;


    OnBrep tcone_brep = OnUtil.ON_BrepRevSurface(ref revsrf, true, true);
    if (tcone_brep != null)
    {
      if (context.m_doc.AddBrepObject(tcone_brep) != null)
        context.m_doc.Redraw();
    }
    return IRhinoCommand.result.success;
  }