{
 Check if a Point(X,Y) (e.g a Cursor) is on a Linie (x1,y1) ; (x2,y2)
 d = line width (min. 1)
}
 function CursorOnLinie(X, Y, x1, y1, x2, y2, d: Integer): Boolean;
 var
  sine, cosinus: Double;
  dx, dy, len: Integer;
 begin
  if d = 0 then d := 1;
  asm
  fild(y2)
  fisub(y1) // Y-Difference
  fild(x2)
  fisub(x1) // X-Difference
  fpatan // Angle of the line in st(0)
  fsincos // Cosinus in st(0), Sinus in st(1)
  fstp cosinus
  fstp sine
  end;
  dx := Round(cosinus * (x - x1) + sine * (y - y1));
  dy := Round(cosinus * (y - y1) - sine * (x - x1));
  len := Round(cosinus * (x2 - x1) + sine * (y2 - y1)); // length of line
 if (dy > -d) and (dy < d) and (dx > -d) and (dx < len + d) then Result := True
  else
  Result := False;
 end;
 procedure TForm1.FormPaint(Sender: TObject);
 begin
  Canvas.Pen.Width := 1;
  Canvas.MoveTo(0, 0);
  Canvas.LineTo(Width, Weight);
 end;
 procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
 var
  p: TPoint;
 begin
  GetCursorPos(p);
  p := ScreenToClient(p);
  if CursorOnLinie(p.x, p.y, 0, 0, Width, Height, 1) then
  Caption := 'Mouse on line.'
  else
  Caption := 'Mouse not on line.'
 end;