I've been doing some scanning to PNG and have discovered that Piwigo doesn't read IPTC metadata from PNG files.  I'd like to be able to read at least the keywords from these files and it seems like the Piwigo-exiftool_keywords plugin by plg ( http://piwigo.org/ext/extension_view.php?eid=848 ) would be a good starting point.  I extended it to look for the Keywords block coming out of exiftool:

  else if (isset($metadata[0]['Keywords']))
  {
    $exif['Keywords'] = $metadata[0]['Keywords'];
  }

and with some debug logging I can see that the this code is getting hit with the right keywords but...no keywords are added.  I'm getting a bit lost trying to trace up the stack, it seems like this should be pretty easy, does anyone have any hints?