From: "John Hutchings" <johnh@datavis.com.au>
Newsgroups: borland.public.delphi.graphics
References: <3bcf3315_2@dnews>
Subject: Re: Detecting whether a point lies within a polygon
Date: Fri, 19 Oct 2001 06:20:44 +1000
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Mike,
This works for me

function PointInPolygonTest( x,y: Integer ;aList: Array of TPoint): Boolean;

// The code below is from Wm. Randolph Franklin <wrf@ecse.rpi.edu>
// with some minor modifications for speed.  It returns 1 for strictly
// interior points, 0 for strictly exterior, and 0 or 1 for points on
// the boundary.

var L, I, J : Integer;

  Function xp(aVal:Integer):Integer;
  Begin
    Result:= PPoint(@aList[aVal]).X;
  end;

  Function yp(aVal:Integer):Integer;
  Begin
    Result:= PPoint(@aList[aVal]).Y;
  end;

begin
  Result:=False;
  L:= Length(aList) ;
  If L=0 then exit;
  J:=L-1;
  for I:=0 to L-1 do
  begin
    if ((((yp(I)<=y) and (y<yp(J))) OR
         ((yp(J)<=y) and (y<yp(I)))) and
        (x < (xp(J)-xp(I))*(y-yp(I))/(yp(J)-yp(I))+xp(I)))
    then Result:=not Result;
    J:=I;
  end;
end;

John
"Mike" <mikedown@*remove_this_bit*totalise.co.uk> wrote in message
news:3bcf3315_2@dnews...
> Hi,
>
> Can anyone help be with detecting whether the screen coord that a user has
> clicked on lies within a defined polygon
> (I`ll know all the screen coords of the points defining the polygon).
> An algorithm would be good, or any links to useful techy sites.
>
> Thanks,
>
> Mike