Canvas Text löschen

[steppo]

Funktioniert nicht bei mir.
Woran liegt es?

Delphi-Quellcode:

unit Main;

interface

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

type
  TForm1 = class(TForm)
    Timer1: TTimer;
    Label1: TLabel;
    OpenDialog1: TOpenDialog;
    procedure Timer1Timer(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    //    procedure FormPaint(Sender: TObject);
  private
    { Private-Deklarationen }
  public
    { Public-Deklarationen }
  end;

var
  Form1: TForm1;
  Nachricht: string;
  Canv: TCanvas;
implementation

{$R *.dfm}

procedure TForm1.Timer1Timer(Sender: TObject);
begin

  Timer1.Enabled := not Timer1.Enabled;

  case Timer1.Enabled of
    True:
      begin
        Canv.Font.Color := clBlack;

        //Position zur Ausgabe der Schrift
        Canv.textout(Screen.Width div 2 - Canv.TextWidth(Nachricht) div 2,
          Screen.Height div 2 - Canv.TextHeight(Nachricht) div 2, Nachricht);

        Application.ProcessMessages;

        sleep(1000);
      end;
    False:
      begin
        Application.ProcessMessages;

        Canv.Font.Color := clWhite;

        //Position zur Ausgabe der Schrift
        Canv.textout(Screen.Width div 2 - Canv.TextWidth(Nachricht) div 2,
          Screen.Height div 2 - Canv.TextHeight(Nachricht) div 2, Nachricht);

        Refresh;

        sleep(1000);
      end;
  end;
  Timer1Timer(Self);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Canv := TCanvas.create;
  Canv.handle := GetWindowDC(0);
  OpenDialog1.Execute;
  Nachricht := ExtractFileName(OpenDialog1.FileName);
  with canv do
  begin
    font.name := 'Arial';
    font.size := 24;
    brush.style := bsclear;
    Pen.Mode := pmNot;
  end;
  Timer1.Enabled := True;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  Canv.Free;
end;

end.