Adding a class to an img tag in WordPress depending on image's orientation

This does not look in exif data of the file. This only compares width and height and adds a landscape class or portrait class depending. I'm adding a filter hook using my genesis framework. The hook is genesis_get_image. I'm editing the html for the image. One has to test if $output is FALSE before passing it to simplexml_load_string()

add_filter('genesis_get_image', 'pb_get_image');
function pb_get_image($output){

    if ($output) {
	  $xml = simplexml_load_string($output);
	  $width = $xml["width"];
	  $height = $xml["height"];
	  ($xml["class"]) ? $class = $xml["class"] : $class = "";
      if (intval($width) >= intval($height)){
	       $class .= " landscape";
		   }
	  else {
			$class .= " portrait";
		   }
	  $xml["class"] = trim($class);
	  $output = $xml->asXML();
	  }
	return $output;
}