Check if the polyline segments are on the same side

[ChuotSieuNhan]

Hello guys, I'm having a bit of a problem determining the direction of a polyline segment, how do I know if the segments have the same direction or in different directions? How to determine if a polyline belongs to case 1 or case 2 ? thanks you  😍

[cuongtk2]

IsClockWise(p1, p2, p3) == IsClockWise(p2, p3, p4)

[ChuotSieuNhan]

anh cho em thêm chút thông tin keyword để tìm hiểu được không anh, em seach theo từ khoá của anh nhưng chưa ra thông tin

[kerry_w_brown]

@ChuotSieuNhan 

Show us how you are determining the segments ?

You could compare the unitVectors of

startVertex => secondVertex

with

secondlastVector => lastVector

[ChuotSieuNhan]

Thank you, I have found the solution according to your instructions @cuongtk2 

[cuongtk2]

public static bool IsClockWise (this Point2d p1, Point2d p2, Point2d p3 )
{
    double num =(p1.Y - p3.Y) * (p3.X - p2.X) - (p3.Y - p2.Y) * (p1.X - p3.X);
    return num < 0;
}

var seg1 = pline.GetLineSegment2dAt(i);
var seg2 = pline.GetLineSegment2dAt(j);

var bool1 = seg2.StartPoint.IsClockWise(seg1.StartPoint, seg1.EndPoint);
var bool2 = seg1.EndPoint.IsClockWise(seg2.StartPoint, seg2.EndPoint);
string str = bool1 == bool2 ? "Same side" : "Other side";

[ChuotSieuNhan]

Dạ vâng anh, em cảm ơn anh, em cũng đã tìm được giải pháp tương tự từ Gille ^^ Cảm ơn anh nhiều 

[kerry_w_brown]

@ChuotSieuNhan 

The code suggested by @cuongtk2 and @_gile is nice.

. . . a bit cleaner than what I'd imagined.

This was my Vector solution

private bool DetermineSameLegDirection(LineSegment2d firstSegment,
                                       LineSegment2d lastSegment,
                                       Tolerance tolerance)
{
   var diff = firstSegment.EndPoint - firstSegment.StartPoint;
   Vector2d v1 = new Vector2d(diff.X, diff.Y);

   diff = lastSegment.StartPoint - lastSegment.EndPoint;
   Vector2d v2 = new Vector2d(diff.X, diff.Y);

   return v1.IsEqualTo(v2, tolerance);
}

Test usage:

[CommandMethod("Answer_11_25_b")]
public void Answer_11_25_b()
{
   Document doc = CadApp.DocumentManager.MdiActiveDocument;
   Database db = doc.Database;
   Editor ed = doc.Editor;

   PromptEntityOptions peo = new PromptEntityOptions("\nSelect a polyline: ");
   peo.SetRejectMessage("Not a polyline.");
   peo.AddAllowedClass(typeof(Polyline), true);
   PromptEntityResult per = ed.GetEntity(peo);
   if (per.Status != PromptStatus.OK)
      return;

   using (Transaction tr = db.TransactionManager.StartTransaction())
   {
      Polyline pline = (Polyline)tr.GetObject(per.ObjectId, OpenMode.ForWrite);
      var firstIndex = 0;
      var lastIndex = 2;
      var firstSegment = pline.GetLineSegment2dAt(firstIndex);
      var lastSegment = pline.GetLineSegment2dAt(lastIndex);
      Tolerance t = new Tolerance(Tolerance.Global.EqualVector, 
                                 Tolerance.Global.EqualPoint);
      ed.WriteMessage($"\nTolerance => {t.EqualVector}\n");
      bool result = DetermineSameLegDirection(firstSegment, lastSegment, t);
      tr.Commit();

      ed.WriteMessage(
         $"{(result ? "Legs same direction" : "Legs Different direction")}");
   }

Regards,

[kerry_w_brown]

The complexity of my last post bothered me a bit, so I thought about it a little more  :

The LineSegment2d derives from LineEntity2d which has a Direction property which is a Vector2d.
and ,
The Vector2d Method   v1.IsEqualTo(v2)  defaults to using  Tolerance.Global.EqualVector  so I didn't need to include the Tolerance parameter in the call.

Not needing to create 2 new Vector2d's and a new Tolerance cleaned up the functionality considerably.

. . . in effect , to a simple one-line statement. 

This means the code actually doing the work can be reduced to :

bool result = pline.GetLineSegment2dAt(firstIndex).Direction.IsEqualTo(
               pline.GetLineSegment2dAt(lastIndex).Direction.Negate());

and the full transaction can now be :

using (Transaction tr = db.TransactionManager.StartTransaction())
{
   Polyline pline = (Polyline)tr.GetObject(per.ObjectId, OpenMode.ForWrite);
   var firstIndex = 0;
   var lastIndex = 2;

   bool result = pline.GetLineSegment2dAt(firstIndex).Direction.IsEqualTo(
                  pline.GetLineSegment2dAt(lastIndex).Direction.Negate());

   ed.WriteMessage(
      $"{(result ? "Legs same direction" : "Legs Different direction")}\n");

   tr.Commit();
}

This, of course, assumes the orientation of the legs is intended to be 'parallel' ie extending in the same direction, equidistant at all points, and never converging or diverging, not just on the same side of the center segment.

Regards,

[ChuotSieuNhan]

Thank you for your interest in my article. Your solution applies well to the case where the angles are 90 degrees. In case the angle error is 1 to 2 degrees more, the result will be wrong. So I still use it the way @cuongtk2 . 

 Thank you very much

[kerry_w_brown]

Yes, that was the conditional . . .

[quote]This, of course, assumes the orientation of the legs is intended to be 'parallel' [/quote]