How to test the winding (turn) of a closed polyline?

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2 responses to “How to test the winding (turn) of a closed polyline?”

1. 

   Dale Bartlett

   October 30, 2013

   Hi Gopinath, Is this possible in .NET? Also many thanks for your very helpful blog. Dale

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2. 

   fixo

   December 18, 2013

   It’s may be too late, anyway:
   //http://adndevblog.typepad.com/autocad/2013/01/how-to-test-the-winding-turn-of-a-closed-polyline.html
   // by Gopinath Taget, ugly converted by me
   static bool polyWindingNumber(Polyline pPoly, out int winding)
   {
   if (!pPoly.Closed)
   {
   winding = 0;
   return false;
   }
   double turn = 0;
   int nSegs = pPoly.NumberOfVertices;
   Vector3dCollection startTans = new Vector3dCollection();
   Vector3dCollection endTans = new Vector3dCollection();
   double PI2 = Math.Round(Math.PI * 2, 12); //6.28318530718;
   for (int i = 0; i < nSegs; i++)
   {
   if (pPoly.GetSegmentType(i) == SegmentType.Arc)
   {
   CircularArc2d arc = pPoly.GetArcSegment2dAt(i);
   Vector2d startTan;
   Vector2d endTan;
   Vector2dCollection startDerivs = new Vector2dCollection();
   startDerivs.Add(arc.EvaluatePoint(arc.StartAngle).GetAsVector());
   startTan = startDerivs[0];
   Vector2dCollection endDerivs = new Vector2dCollection();
   Point2d ap2 = arc.EvaluatePoint(arc.EndAngle);
   endDerivs.Add(arc.EvaluatePoint(arc.StartAngle).GetAsVector());
   endTan = endDerivs[0];
   startTans.Add(new Vector3d(startTan.X, startTan.Y, 0.0));
   endTans.Add(new Vector3d(endTan.X, endTan.Y, 0.0));
   double ang = arc.EndAngle – arc.StartAngle;
   turn += arc.IsClockWise ? -ang : ang;
   }
   else if (pPoly.GetSegmentType(i) == SegmentType.Line)
   {
   LineSegment2d line = pPoly.GetLineSegment2dAt(i);
   Vector2d tan2d = line.EndPoint – line.StartPoint;
   Vector3d tan = new Vector3d(tan2d.X, tan2d.Y, 0.0);
   startTans.Add(tan);
   endTans.Add(tan);
   }
   }
   nSegs = startTans.Count;
   for (int i = 0; i < nSegs; i++)
   {
   double angle = endTans[i].GetAngleTo(startTans[(i + 1) % nSegs]);
   Vector3d norm = endTans[i].CrossProduct(startTans[(i + 1) % nSegs]);
   angle = norm.Z > 0 ? angle : -angle;
   turn += angle;
   }
   turn = turn / PI2;
   double lower = Math.Floor(turn);
   double tol = 1e-6;
   if ((turn – lower) < tol)
   winding = (int)lower;
   else if (((lower + 1) – turn) < tol)
   winding = (int)(lower + 1);
   else
   {
   winding = 0;
   return false;
   }
   return true;
   }
   [CommandMethod(“ISCC”)]
   public static void TestAsdkUtilsPolyWind()
   {
   Document doc = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument;
   Editor ed = doc.Editor;
   Database db = doc.Database;
   SelectionFilter filt = new SelectionFilter(new TypedValue[]{
   new TypedValue(0,”lwpolyline”),
   new TypedValue(70,1)});
   PromptSelectionResult res = ed.GetSelection(filt);
   if (res.Status != PromptStatus.OK) return;
   SelectionSet sset = res.Value;
   SelectedObject obj = sset[0];
   using (Transaction tr = db.TransactionManager.StartTransaction())
   {
   Entity ent = tr.GetObject(obj.ObjectId, OpenMode.ForRead) as Entity;
   if (ent == null)
   return;
   Polyline poly = ent as Polyline;
   if (poly == null) return;
   int winding = 0;
   if (polyWindingNumber(poly, out winding))
   {
   ed.WriteMessage(“\nPolygon has winding number {0}”, winding == 1 ? “CounterClockWise” : “ClockWise”);
   }
   else
   {
   ed.WriteMessage(“\nCannot calculate winding number for polyline”);
   }
   tr.Commit();
   }
   }

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