如何用image和Bitmap,在窗体显示时实现百叶窗效果(我只能在button1事件中实现)

窗体上放有一个image并设置显示为false,想在窗体显示时实现百叶窗效果
procedure TForm1.Button1Click(Sender: TObject);
var
  newbmp: TBitmap;
  i,j,bmpheight,bmpwidth:integer;
  xgroup,xcount:integer;
begin
 newbmp:= TBitmap.Create;
 newbmp.Width:=image1.Width;
 newbmp.Height:=image1.Height;
 bmpheight:=image1.Height;
 bmpwidth:=image1.Width;
       //百叶窗效果
       xgroup:=16;
       xcount:=bmpheight div xgroup;
       for i:=0  to xcount do
         for j:=0 to xgroup do
           begin
             newbmp.Canvas.CopyRect(Rect(0,xcount*j+i-1,bmpwidth,xcount*j+i),
                image1.Canvas,Rect(0,xcount*j+i-1,bmpwidth,xcount*j+i));
             form1.Canvas.Draw(0,0,newbmp);
           end;
newbmp.free;
end;