CODEWITHSUNDEEP
c#geometry
World TV
World TV

Reputation: 17

C# Trouble with detection is point in polygon?

I have polygon with these points:

A= (-8598.07,7513.37)

B= (-8598.07,5169.17)

C= (-8496.47,5271.77)

D= (-4735.47,5271.77)

E= (-4736.47,7411.77)

F= (-8497.47,7411.77)

G= (-4838.07,7310.17)

H= (-4838.07,5372.37)

So my polygon has 8 segments.

And I have point 'I' inside the polygon

(-6616,6802.6537)

enter image description here

Using this popular algorithm:

public bool IsPointInPolygon(Point2D[] polygon, Point2D testPoint)
        {
               var result = false;
            var j = polygon.Count() - 1;
            for (var i = 0; i < polygon.Count(); i++)
            {
                if (polygon[i].Y < testPoint.Y && polygon[j].Y >= testPoint.Y || polygon[j].Y < testPoint.Y && polygon[i].Y >= testPoint.Y)
                {
                    if (polygon[i].X + (testPoint.Y - polygon[i].Y) / (polygon[j].Y - polygon[i].Y) * (polygon[j].X - polygon[i].X) < testPoint.X)
                    {
                        result = !result;
                    }
                }
                j = i;
            }
            return result;
        }

But it return false.

My List with points:

(-8598.07; 7513.37 -> -8598.07;5169.17)
(-8496.47; 5271.77 -> -4735.47;5271.77)
(-4736.47; 7411.77 -> -8497.47;7411.77)
(-4838.07; 7310.17 -> -4838.07;5372.37)
(-8598.07; 7513.37 -> -8497.47;7411.77)
(-8496.47; 5271.77 -> -8598.07;5169.17)
(-4736.47; 7411.77 -> -4838.07;7310.17)
(-4735.47; 5271.77 -> -4838.07;5372.37)

The known working C code is:

bool pointInPolygon() {

  int   i, j=polyCorners-1 ;
  bool  oddNodes=NO      ;

  for (i=0; i<polyCorners; i++) {
    if ((polyY[i]< y && polyY[j]>=y
    ||   polyY[j]< y && polyY[i]>=y)
    &&  (polyX[i]<=x || polyX[j]<=x)) {
      oddNodes^=(polyX[i]+(y-polyY[i])/(polyY[j]-polyY[i])*(polyX[j]-polyX[i])<x); }
    j=i; }

  return oddNodes; }

Upvotes: 0

Views: 170

Answers (1)

Adam Hodovanets
Adam Hodovanets

Reputation: 223

Order them in that way:

(-8598.07, 7513.37, -8598.07, 5169.17)

(-8598.07, 5169.17,-8496.47, 5271.77)

(-8496.47, 5271.77, -4735.47, 5271.77)

(-4735.47, 5271.77, -4838.07, 5372.37)

(-4838.07, 5372.37, -4838.07, 7310.17)

(-4838.07, 7310.17, -4736.47, 7411.77)

(-4736.47, 7411.77, -8497.47, 7411.77)

(-8497.47, 7411.77,-8598.07, 7513.37)

You just need to "close" the polygon.

Upvotes: 1

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