Determines whether the vertex is inside the polygon.

How can I determine whether a point is inside a polygon?

(1) area and discriminant method: determine the area of the triangle formed by the target point and each side of the polygon and whether it is equal to the polygon. If it is equal, it is inside the polygon.

(2) angle and discriminant method: Determine the angle between the target point and all sides and whether it is 360 degrees. If it is 360 degrees, it is inside the polygon.

(3) Shooting Method: Start from the target point and draw a Ray to see the number of intersections between the ray and all edges of the polygon. If there is an odd point of intersection, It is internal. If there is an even point of intersection, It is external.

Practice: Compare the Y coordinate of the test point with each point of the polygon to obtain a list of intersection points between the row and the polygon edge of the test point. In this example, eight edges and the row where the test point is located intersect, while six edges do not. If the number of vertices on both sides of the test point is an odd number, the test point is within the polygon; otherwise, it is outside the polygon. In this example, there are five intersections on the left of the test point and three intersections on the right. They are all odd, so they are inside the polygon.

Int pnpoly (int nvert, float * vertx, float * verty, float testx, float testy) {int I, j, c = 0; for (I = 0, j = nvert-1; I <nvert; j = I ++) {if (verty [I]> testy )! = (Verty [j]> testy) & (testx <(vertx [j]-vertx [I]) * (testy-verty [I]) /(verty [j]-verty [I]) + vertx [I]) c =! C;} return c ;}

Internal implementation from a polygon:

      public bool IsInside(PointLatLng p)      {         int count = Points.Count;         if(count < 3)         {            return false;         }         bool result = false;         for(int i = 0, j = count - 1; i < count; i++)         {            var p1 = Points[i];            var p2 = Points[j];            if(p1.Lat < p.Lat && p2.Lat >= p.Lat || p2.Lat < p.Lat && p1.Lat >= p.Lat)            {               if(p1.Lng + (p.Lat - p1.Lat) / (p2.Lat - p1.Lat) * (p2.Lng - p1.Lng) < p.Lng)               {                  result = !result;               }            }            j = i;         }         return result;      }

In special cases: when the point to be detected is on a side of multiple deformation, the results determined by the shooting method are uncertain and need special processing (If the test point is on the border of the polygon, this algorithm will deliver unpredictable results ).

 

 

References:

Http://alienryderflex.com/polygon/

Http://en.wikipedia.org/wiki/Point_in_polygon

Http://www.codeproject.com/Tips/84226/Is-a-Point-inside-a-Polygon