private static function loadFonts()
 {
     if (self::$fonts !== null) {
         return;
     }
     $directory = dirname(__FILE__) . '/font/';
     self::$fonts = array();
     foreach (new Gpf_Io_DirectoryIterator($directory, '.ttf') as $fullName => $file) {
         self::$fonts[$fullName] = $fullName;
     }
 }