How to detect if a Polyline is self-intersecting?

By Philippe Leefsma

Here is a request from one of our ADN developers:

I would like to detect if a certain polyline is self-intersecting and if so I would like to compute those intersection points. How to achieve this using the .Net API?

Solution

The following C# sample prompts the user for selecting an existing Polyline entity and then computes its self-intersecting points, if any:

[CommandMethod("SelfIntersectPline")]

public static void SelfIntersectPline()

{

    Document doc = Application.DocumentManager.MdiActiveDocument;

    Database db = doc.Database;

    Editor ed = doc.Editor;

 

    PromptEntityOptions peo = new PromptEntityOptions(

        "\nSelect Polyline: ");

 

    peo.SetRejectMessage("\nMust be a Polyline…");

    peo.AddAllowedClass(typeof(Polyline), true);

 

    PromptEntityResult per = ed.GetEntity(peo);

 

    if (per.Status != PromptStatus.OK) ret
urn;

 

    using (Transaction Tx =

        db.TransactionManager.StartTransaction())

    {

        Polyline polyline = per.ObjectId.GetObject(

            OpenMode.ForRead)

                as Polyline;

 

        DBObjectCollection entities = new DBObjectCollection();

        polyline.Explode(entities);

 

        for (int i = 0; i < entities.Count; ++i)

        {

            for (int j = i + 1; j < entities.Count; ++j)

            {

                Curve curve1 = entities[i] as Curve;

                Curve curve2 = entities[j] as Curve;

 

                Point3dCollection points = new Point3dCollection();

                curve1.IntersectWith(

                    curve2,

                    Intersect.OnBothOperands,

                    points,

                    IntPtr.Zero,

                    IntPtr.Zero);

 

                foreach (Point3d point in points)

                {

                    // Make a check to skip the start/end points

                    // since they are connected vertices

                    if (point == curve1.StartPoint ||

                        point == curve1.EndPoint)

                    {

                        if (point == curve2.StartPoint ||

                            point == curve2.EndPoint)

                        {

                            // If two consecutive segments, then skip

                            if (j == i + 1)

                            {

                                continue;

                            }

                        }

                    }

 

                    ed.WriteMessage(

                        "\n – Intersection point: " +

                        point.ToString());

                }

  &#1
60;         }

 

            // Need to be disposed explicitely

            // since entities are not DB resident

            entities[i].Dispose();

        }

    }

}

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