Mega Code Archive
How to check if a point belongs to the interior of a polygon
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Title: How to check if a point belongs to the interior of a polygon
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;
{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 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);
{increments appropriate counter}
if XIntersection .X then Inc(ToTheLeftofPoint);
if XIntersection P.X then Inc(ToTheRightofPoint);
end;
{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 .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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