VG generate wrongly due to polygon_crossing function not accurate

[hansoong]

  below is a convex polygon, each point should only have 2 visible points, but point_3 got 3.
  polygon_crossing function need review, as it return false when checking mid point (208.5, 227)

    self.point_1 = Point(298, 163.0)
    self.point_2 = Point(263, 168.0)
    self.point_3 = Point(199, 194.0)
    self.point_4 = Point(192, 227.0)
    self.point_5 = Point(218, 260.0)
    
    g = vg.VisGraph()
    g.build([[self.point_1, self.point_2, self.point_3, self.point_4, self.point_5]])

[hansoong]

https://stackoverflow.com/questions/217578/how-can-i-determine-whether-a-2d-point-is-within-a-polygon

i dont know how good is this, but seems working..
public bool IsPointInPolygon( Point p, Point[] polygon )
{
double minX = polygon[ 0 ].X;
double maxX = polygon[ 0 ].X;
double minY = polygon[ 0 ].Y;
double maxY = polygon[ 0 ].Y;
for ( int i = 1 ; i < polygon.Length ; i++ )
{
Point q = polygon[ i ];
minX = Math.Min( q.X, minX );
maxX = Math.Max( q.X, maxX );
minY = Math.Min( q.Y, minY );
maxY = Math.Max( q.Y, maxY );
}

if ( p.X < minX || p.X > maxX || p.Y < minY || p.Y > maxY )
{
    return false;
}

// http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html
bool inside = false;
for ( int i = 0, j = polygon.Length - 1 ; i < polygon.Length ; j = i++ )
{
    if ( ( polygon[ i ].Y > p.Y ) != ( polygon[ j ].Y > p.Y ) &&
         p.X < ( polygon[ j ].X - polygon[ i ].X ) * ( p.Y - polygon[ i ].Y ) / ( polygon[ j ].Y - polygon[ i ].Y ) + polygon[ i ].X )
    {
        inside = !inside;
    }
}

return inside;

}