/**
 * return the source html of the featured image
 *
 * @since 1.8
 * @global obj $my_isc isc class
 * @global obj $post current post
 * @param int $post_id id of the post; will use current post if empty
 */
function isc_thumbnail_source($post_id = 0)
{
    global $post;
    $isc_public = ISC_Class::get_instance();
    if (empty($post_id) && isset($post->ID)) {
        $post_id = $post->ID;
    }
    echo $isc_public->get_thumbnail_source_string($post_id);
}
 public function __construct()
 {
     parent::__construct();
     add_action('wp_enqueue_scripts', array($this, 'front_scripts'));
     add_action('wp_head', array($this, 'front_head'));
     add_filter('the_content', array($this, 'content_filter'), 20);
     add_filter('the_excerpt', array($this, 'excerpt_filter'), 20);
     add_shortcode('isc_list', array($this, 'list_post_attachments_with_sources_shortcode'));
     add_shortcode('isc_list_all', array($this, 'list_all_post_attachments_sources_shortcode'));
 }
 /**
  * Setup registers filterts and actions.
  */
 public function __construct()
 {
     // load all plugin options
     $this->_options = get_option('isc_options');
     self::$instance = $this;
 }