unit LineDDAUnit;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, ExtCtrls;

type
  TForm1 = class(TForm)
    Timer1: TTimer;
    procedure Timer1Timer(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

  {the callback function prototype}
  procedure AnimLines(X, Y: Integer; lpData: lParam); stdcall;

var
  Form1: TForm1;
  Offset: Integer;

const
  AL_HORIZONTAL = 1;     // indicates if the line to be drawn is
  AL_VERTICAL = 2;       // horizontal or vertical

implementation

{$R *.DFM}

procedure AnimLines(X, Y: Integer; lpData: lParam);
var
  Coord: Integer;      // holds the coordinate used in the calculation
begin
  {if the line is horizontal, use the X coordinate, otherwise use Y}
  if lpData=AL_HORIZONTAL then
    Coord := X
  else
    Coord := Y;

  {determine if the pixel at this point should be black or white}
  if (Coord mod 5=Offset) then
    SetPixelV(Form1.Canvas.Handle, X, Y, clBlack)
  else
    SetPixelV(Form1.Canvas.Handle, X, Y, clWhite);
end;

procedure TForm1.Timer1Timer(Sender: TObject);
begin
  {increment the offset}
  Inc(Offset);

  {if the offset has gone too far, reset it}
  if Offset>4 then Offset := 0;

  {draw a rectangle with animated lines}
  LineDDA(20, 20, 120, 20, @AnimLines, AL_HORIZONTAL);
  LineDDA(120, 20, 120, 120, @AnimLines, AL_VERTICAL);
  LineDDA(20, 20, 20, 120, @AnimLines, AL_VERTICAL);
  LineDDA(20, 120, 120, 120, @AnimLines, AL_HORIZONTAL);
end;

end.