Meta Slider - Easy Fancybox Integration

/**
 * INSTALLATION: this code should be pasted into your theme's functions.php file.
 *
 * To Use: Install 'Easy Fancybox'. Leave the URL blank on an image slide in Meta Slider. 
 * The slide will automatically be linked to it's full image in a lightbox
 *
 * @tested on WP - 4.4.11; Meta Slider - 3.5.1
 */
function metaslider_easy_fancybox($attributes, $slide, $slider_id) {
    if (!strlen($attributes['href'])) {
        $attributes['href'] = wp_get_attachment_url( get_post_meta( $slide['id'], '_thumbnail_id', true) );
        $attributes['class'] = 'fancybox';
        $attributes['rel'] = "gallery-{$slider_id}";
    }
    return $attributes;
}
add_filter('metaslider_flex_slider_anchor_attributes', 'metaslider_easy_fancybox', 10, 3);
add_filter('metaslider_nivo_slider_anchor_attributes', 'metaslider_easy_fancybox', 10, 3);
add_filter('metaslider_responsive_slider_anchor_attributes', 'metaslider_easy_fancybox', 10, 3);
add_filter('metaslider_coin_slider_anchor_attributes', 'metaslider_easy_fancybox', 10, 3);
/* END metaslider /simple lightbox integration */