var
  px: pbyte;
  y, x, l: integer;
  alphaCount, denom: Integer;
  bmp: TIEBitmap;
begin
  bmp := ImageEnView1.IEBitmap;

  alphaCount := 0;
  if bmp.HasAlphaChannel() then
  begin
    l := IEBitmapRowLen( bmp.AlphaChannel.Width, bmp.AlphaChannel.BitCount, 8);
    for y := 0 to bmp.AlphaChannel.Height - 1 do
    begin
      px := bmp.AlphaChannel.Scanline[y];
      for x := 0 to l - 1 do
      begin
        if px^ < 255 then
          Inc( alphaCount );
        inc(px);
      end;
    end;
  end;

  denom := bmp.Width * bmp.Height;
  ShowMessage( format( 'Image has %d%% transparent pixels', [ Round( alphaCount / denom * 100 )]));
end;