Находится ли точка внутри многоугольника

function
 PtInRgn(TestPolygon : array
 of
 TPoint; const
 P : TPoint): boolean;
 var

   ToTheLeftofPoint, ToTheRightofPoint : byte;
   np : integer;
   OpenPolygon : boolean;
   XIntersection : real;
 begin

   ToTheLeftofPoint := 0;
   ToTheRightofPoint := 0;
   OpenPolygon := False;

   {Prufen ob das Polygon geschlossen ist}
   {tests if the polygon is closed}

   if
 not
 ((TestPolygon[0].X = TestPolygon[High(TestPolygon)].X) and

     (TestPolygon[0].Y = TestPolygon[High(TestPolygon)].Y)) then

     OpenPolygon := True;

   {Tests fur jedes Paar der Punkte, um zu sehen wenn die Seite zwischen 
   ihnen, die horizontale Linie schneidet, die TestPoint durchlauft}
   {tests for each couple of points to see if the side between them 
   crosses the horizontal line going through TestPoint}

   for
 np := 1 to
 High(TestPolygon) do

     if
 ((TestPolygon[np - 1].Y <= P.Y) and

       (TestPolygon[np].Y > P.Y)) or

       ((TestPolygon[np - 1].Y > P.Y) and

       (TestPolygon[np].Y <= P.Y))
       {Wenn es so ist}    {if it does}
       then

     begin

       {berechnet die x Koordinate des Schnitts}
       {computes the x coordinate of the intersection}

       XIntersection := TestPolygon[np - 1].X +
         ((TestPolygon[np].X - TestPolygon[np - 1].X) /
         (TestPolygon[np].Y - TestPolygon[np - 1].Y)) * (P.Y - TestPolygon[np - 1].Y);

       {Zahler entsprechend verringern}
       {increments appropriate counter}
       if
 XIntersection < P.X then
 Inc(ToTheLeftofPoint);
       if
 XIntersection > P.X then
 Inc(ToTheRightofPoint);
     end
;

   {Falls das Polygon offen ist, die letzte Seite testen}
   {if the polygon is open, test for the last side}

   if
 OpenPolygon then

   begin

     np := High(TestPolygon);  {Thanks to William Boyd - 03/06/2001}
     if
 ((TestPolygon[np].Y <= P.Y) and

       (TestPolygon[0].Y > P.Y)) or

       ((TestPolygon[np].Y > P.Y) and

       (TestPolygon[0].Y <= P.Y)) then

     begin

       XIntersection := TestPolygon[np].X +
         ((TestPolygon[0].X - TestPolygon[np].X) /
         (TestPolygon[0].Y - TestPolygon[np].Y)) * (P.Y - TestPolygon[np].Y);

       if
 XIntersection < P.X then
 Inc(ToTheLeftofPoint);
       if
 XIntersection > P.X then
 Inc(ToTheRightofPoint);
     end
;
   end
;

   if
 (ToTheLeftofPoint mod
 2 = 1) and
 (ToTheRightofPoint mod
 2 = 1) then
 Result := True
   else

     Result := False;
 end
; {PtInRgn}

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