public function add_filters()
    {
        // check if WYSIWYG is enabled 
        if ( get_user_option('rich_editing') )
        {
            add_filter( 'mce_external_plugins', array( $this, 'register_plugin' ) ); 
            add_filter( 'mce_buttons'.($this->config['row'] > 1 ? '_'.$this->config['row'] : ''), array( $this, 'register_button' ) ); 
        }
    }