From: Schoeneck Howell <showell@bop.gov>
Newsgroups: borland.public.delphi.graphics
Subject: Re: point in line?
Date: Fri, 21 Sep 2001 11:55:45 -0400
Message-ID: <34omqtc05j9ds64cr5npc5seum66kli58v@4ax.com>
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I wish I could provide proper attribution for the following, but I
cannot recall where it came from. I used it in an application a few
years back.

t is the point in question, p-q is the line in question


function pointonline(px,py,qx,qy,tx,ty : integer) : integer;
  // return 0 if t not on line p-q
  //        1 if t on open ray ending at p
  //        2 if t on interior p-q
  //        3 if t on open ray ending at q
begin
 result:=2;
 if ( ABS((qy-py)*(tx-px)-(ty-py)*(qx-px)) >=
        (MAX(ABS(qx-px), ABS(qy-py)))) then result:=0;
 if (((qx<px) AND (px<tx)) OR ((qy<py) AND (py<ty))) then  result:=1;
 if (((tx<px) AND (px<qx)) OR ((ty<py) AND (py<qy))) then  result:=1;
 if (((px<qx) AND (qx<tx)) OR ((py<qy) AND (qy<ty))) then  result:=3;
 if (((tx<qx) AND (qx<px)) OR ((ty<qy) AND (qy<py))) then  result:=3;
end;

Nick Howell

On Thu, 20 Sep 2001 11:38:15 +0200, "ADS"
<tech1.yxendis_nospam@wanadoo.fr> wrote:

>hello,
>
>how could I know if a point (x,y) in on a line (x1,y1,x2,y2)?
>
>thanks
>
>Armindo

===================
Schoeneck Howell
Federal Bureau of Prisons
Office of Research and Evaluation
showell@bop.gov