unless ($white{browser_name => $browser[0]}->compare($blue{browser_name => $browser[1]})) {
    $diff_file = $white->difference($blue);
    print '#The images differ; see ' . $diff_file . ' for details'. "\n";
  

  my $open_cmd;
  if ($^O eq 'darwin') {
      $open_cmd = 'open'
  }
  elsif ($^O eq 'MSWin32') {
      $open_cmd = '';
  }
  else {
      $open_cmd = 'display';
  }
  `$open_cmd $diff_file`;
  }