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procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if button=mbleft then
    begin
      n:=n+1;
      xp[n]:=x;
      yp[n]:=y;
      image1.Canvas.Pen.Color:=clgreen;
      image1.Canvas.rectangle(x-3,y-3,x+3,y+3);
    if n>1 then
       begin
          image1.Canvas. MoveTo(xp[n-1],yp[n-1]);
          image1.Canvas.LineTo(xp[n],yp[n]);
          if ssalt in shift then
          begin
            image1.canvas.LineTo(xp[1],yp[1]);
          end;
       end;
    end;

end;

procedure TForm1.Button1Click(Sender: TObject); 
var 
  i,p: Integer; 
  weitester: Integer; 
  Distanz: Real; 
begin
if n>1 then 
  begin
    weitester:=0;
    Distanz:=0;
    for i:=2 to n do 
      begin 
        if sqrt(sqr(xp[1]-xp[i])+(sqr(yp[1]-yp[i])))>Distanz then 
          begin 
            weitester:=i;
             p:=i;
            Distanz:=sqrt(sqr(xp[1]-xp[i])+(sqr(yp[1]-yp[i]))); 
          end;
          label2.Caption:=inttostr(p);
      end;
    image1.Canvas.Pen.Color:=clRed; 
    image1.Canvas.MoveTo(xp[1],yp[1]); 
    image1.Canvas.LineTo(xp[weitester],yp[weitester]); 
  end

 end;

end.

Gruß Annika					

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if ssalt in shift then					

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image1.canvas.LineTo(xp[1],yp[1]);