Находится ли точка внутри многоугольника

Оформил: DeeCo

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}