function BitmapToRgn(Image: TBitmap): HRGN;
var
  TmpRgn: HRGN;
  x, y: integer;
  ConsecutivePixels: integer;
  CurrentPixel: TColor;
  CreatedRgns: integer;
  CurrentColor: TColor;
begin
  CreatedRgns := 0;
  Result := CreateRectRgn(0, 0, Image.Width, Image.Height);
  inc(CreatedRgns);

  if (Image.Width = 0) or (Image.Height = 0) then
  exit;

  for y := 0 to Image.Height - 1 do
  begin
  CurrentColor := Image.Canvas.Pixels[0,y];
  ConsecutivePixels := 1;
  for x := 0 to Image.Width - 1 do
  begin
  CurrentPixel := Image.Canvas.Pixels[x, y];

  if CurrentColor = CurrentPixel then
  inc(ConsecutivePixels)
  else
  begin
  // Входим в новую зону
  if CurrentColor = clWhite then
  begin
  TmpRgn := CreateRectRgn(x - ConsecutivePixels, y, x, y + 1);
  CombineRgn(Result, Result, TmpRgn, RGN_DIFF);
  inc(CreatedRgns);
  DeleteObject(TmpRgn);
  end;
  CurrentColor := CurrentPixel;
  ConsecutivePixels := 1;
  end;
  end;

  if (CurrentColor = clWhite) and (ConsecutivePixels > 0) then
  begin
  TmpRgn := CreateRectRgn(x-ConsecutivePixels, y, x, y+1);
  CombineRgn(Result, Result, TmpRgn, RGN_DIFF);
  inc(CreatedRgns);
  DeleteObject(TmpRgn);
  end;
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
var
  MaskBmp: TBitmap;
begin
  MaskBmp := TBitmap.Create;
  try
  MaskBmp.LoadFromFile('c:\Мои документы\DW.bmp'); //файл-маска
  Height := MaskBmp.Height;
  Width := MaskBmp.Width;
  // ОС владеет регионом, после вызова SetWindowRgn
  SetWindowRgn(Self.Handle, BitmapToRgn(MaskBmp), True);
  finally
  MaskBmp.Free;
  end;
end;