WPEx Gallery Shortcode - WordPress

/*--------------------------------------*/
/*	WPEX Gallery
/*--------------------------------------*/
function wpex_gallery_shortcode( $atts, $content = null ) {

	extract( shortcode_atts( array(
	  	'columns' => '4',
      	'image_height' => '',
	  	'exclude' => '',
		'lightbox' => ''
      ), $atts ) );

	$output = '';
	global $post;

        //get attachement count
        $get_attachments = get_children( array( 'post_parent' => $post->ID ) );

		//get ID of featured image
        $featured_image_id = get_post_thumbnail_id( $post->ID );

		//generate random number for each gallery
		$rand_gallery = rand();

        //attachement loop
        $args = array(
            'orderby' => 'menu_order',
            'post_type' => 'attachment',
            'post_parent' => get_the_ID(),
            'post_mime_type' => 'image',
            'post_status' => null,
            'posts_per_page' => -1,
			'exclude' => $exclude
        );
        $attachments = get_posts($args);

		//show gallery only if attachments exist
        if ($attachments) {

		$output .= '<article class="gallery-wrap grid-container clearfix">';

		//start attachment loop
		foreach ($attachments as $attachment) :



		//featured image
		($image_height) ? $image_attach_height = 450 * str_replace('%', '', $image_height) / 100 : $image_attach_height = 450;
		$thumb = $attachment->ID;
		$img_url = wp_get_attachment_url($thumb,'full'); //get full URL to image
		$featured_image = aq_resize( $img_url, 450, $image_attach_height, true ); //resize & crop the image

		//output image
		$output .='<div class="gallery-photo grid-'.$columns.'">';
		$output .= '<a href="'. $img_url .'" class="single-gallery-thumb"';
		if($lightbox !='no'){
			$output .= 'rel="prettyPhoto['.$rand_gallery.']"';
		}
		$output .= 'title="'. apply_filters('the_title', $attachment->post_title) .'"><img src="'. $featured_image .'" alt="'. apply_filters('the_title', $attachment->post_title) .'" class="attachment-post-thumbnail" /></a>';
		$output .='</div>';

		//end loop
		endforeach;
		$output .='</article>';

	}
   	return $output;

}

add_shortcode( 'wpex_gallery', 'wpex_gallery_shortcode' );