用字体开透明窟窿

procedure TForm1.Button1Click(Sender: TObject);
begin
  canvas.Font.Name := 'Times New Roman';
  canvas.Font.size := 72;
  canvas.Font.style := [fsItalic,fsBold];
  beginpath(canvas.handle);
    SetBkMode(Canvas.Handle, TRANSPARENT ); //试验一下不用这行语句，会得到很有意思的结果
    canvas.Polygon([Point(0,0),Point(Width,0),
                    Point(Width,Height),Point(0,Height),
                    Point(0,0)]); //试验不用该语句看看结果如何
    canvas.TextOut(20,40,'Hello');
  endpath(canvas.handle);

  SetWindowRgn( Handle, PathToRegion(canvas.Handle), true );
end;