Drawing a Form in a defined shape

procedure DrawFormPolygon(Form: TForm; Points: Array Of TPoint);
var HR: HRgn;
pt: array [0..3] of TPoint;
begin
  with Form do begin
    HR := CreatePolygonRgn(Points,Length(Points),ALTERNATE);
    SetWindowRgn(Handle, HR, True);
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
var pt: Array [0..3] of TPoint;
begin
  pt[0].x := 0;
  pt[0].y := 0;
  pt[1].x := 100;
  pt[1].y := 0;
  pt[2].x := 200;
  pt[2].y := 300;
  pt[3].x := 5;
  pt[3].y := 300;
  DrawFormPolygon(Form1,pt);
end