Faire un trou dans une Form

procedure TForm1.Button1Click(Sender: TObject);
var
region1, region2 : hrgn;
begin
region1 := CreateRectRgn(0,0,width,height);
region2 := CreateEllipticRgn(30,30,200,200);
CombineRgn(region1, region1, region2, RGN_DIFF);
SetWindowRgn(handle, region1, true);
end;