.NET

## .NET

Active Contributor
Posts: 42
Registered: ‎07-02-2013
Message 1 of 5 (375 Views)

# Point on polygon

375 Views, 4 Replies
07-30-2013 04:34 AM

Hi,

I want to check if a point lies inside a polygon and i also want to check if that particular point lies on the polygon.

Thanks

Gulzar

Mentor
Posts: 231
Registered: ‎11-09-2012
Message 2 of 5 (367 Views)

# Re: Point on polygon

07-30-2013 05:40 AM in reply to: gulzar25
Valued Contributor
Posts: 95
Registered: ‎04-04-2012
Message 3 of 5 (321 Views)

# Re: Point on polygon

07-30-2013 07:38 PM in reply to: gulzar25

I use the following algorithm. Try this.

```public struct Point
{
public double x;
public double y;

public Point(double _x, double _y)
{
x = _x;
y = _y;
}
}

// wn_PnPoly(): winding number test for a point in a polygon
//      Input:   P = a point,
//               V[] = vertex points of a polygon V[n+1] with V[n]=V[0]
//      Return:  wn = the winding number (=0 only if P is outside V[])
public static bool IsInPolygon(Point[] V, Point P, int n)
{
int wn = 0;    // the winding number counter

// loop through all edges of the polygon
for (int i = 0; i < n; i++)
{   // edge from V[i] to V[i+1]
if (V[i].y <= P.y)
{         // start y <= P.y
if (V[i + 1].y > P.y)      // an upward crossing
if (isLeft(V[i], V[i + 1], P) > 0)  // P left of edge
++wn;            // have a valid up intersect
}
else
{                       // start y > P.y (no test needed)
if (V[i + 1].y <= P.y)     // a downward crossing
if (isLeft(V[i], V[i + 1], P) < 0)  // P right of edge
--wn;            // have a valid down intersect
}
}

if (wn == 0)
return false;
else
return true;
}

// isLeft(): tests if a point is Left|On|Right of an infinite line.
//    Input:  three points P0, P1, and P2
//    Return: >0 for P2 left of the line through P0 and P1
//            =0 for P2 on the line
//            <0 for P2 right of the line
//    See: the January 2001 Algorithm "Area of 2D and 3D Triangles and Polygons"
internal static double isLeft( Point P0, Point P1, Point P2 )
{
return ( (P1.x - P0.x) * (P2.y - P0.y) - (P2.x - P0.x) * (P1.y - P0.y) );
}```

Active Contributor
Posts: 42
Registered: ‎07-02-2013
Message 4 of 5 (307 Views)

# Re: Point on polygon

07-30-2013 10:51 PM in reply to: gulzar25

Hi,

Thanks for the reply.

I tried using the above code but it gives same results for point on polygon and for point inside polygon.Is there any way to differentiate.

Thanks

Gulzar

Valued Contributor
Posts: 95
Registered: ‎04-04-2012
Message 5 of 5 (303 Views)

# Re: Point on polygon

07-31-2013 12:38 AM in reply to: gulzar25
Oh, didn't know you also wanted to check if the point is on polygon.
I guess you can just check that with the linear equation of every side of the polygon.
Or you can use GetDistAtPoint(Point3d point) method of the Curve entity.
It throws an InvalidInput Exception if the point is not on the curve entity.
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