function to leverage the RICG WP plugin to deliver responsive images with srcset attr using wp_get_attachment_image

fetch_RICG_thumbnail

/**
* Leverages the RICG WP plugin to deliver responsive images with srcset attr
*
* @param $post_ID
* @param $thumb_id
* @param $size
* @return mixed / wp_get_attached_image markup
*
* @uses PHP 5.2+ due to PATHINFO_FILENAME
* @uses wp_get_attachment_image
*
* @author orionrush
*
*/

function tr_fetch_thumbnail($post_ID, $thumb_id, $thumbnail_size = 'thumbnail', $class = '') {

    $alt_text = get_the_title($post_ID); // title of post, not alt text of image attachment

    // Get the details of the named thumbnail
    $img = wp_get_attachment_image_src( $thumb_id, $thumbnail_size );
    // Get its width
    $img_w = $img[1];
    $srcset_w = ' '. $img_w . 'w';
    $atts[] ='';

// Fetch the array of thumbnail sizes as a string for $srcset
    if (function_exists('tevkori_get_srcset_array') ){
        $sources = tevkori_get_srcset_array( $thumb_id, $thumbnail_size ); //array
        if (is_array($sources)){
            write_log('starting srcset:');
            write_log($sources);
            foreach( $sources as $key => $source ) {
        
                    // Add retina images
                    // PATHINFO_FILENAME requires PHP 5.2.0
                    $retina_file = pathinfo($source, PATHINFO_FILENAME);
                    $retina_file_ext = pathinfo($source, PATHINFO_EXTENSION);

                    // remove width attributes that come as part of the srcset info ie ' 350w'
                    $retina_file_ext = explode(' ', $retina_file_ext);
                    $retina_file_ext = $retina_file_ext[0];
                    $retina_dirnam = pathinfo($source, PATHINFO_DIRNAME);
                    $upload_dir = wp_upload_dir();

                    // Bulid the expected file name and path
                    $retina_candidate = $retina_dirnam . '/' . $retina_file . '@2x.' . $retina_file_ext;
                    $retina_candidate_test = $upload_dir['basedir'] . '/' . $retina_file . '@2x.' . $retina_file_ext;

					// Add the 2x image to the array                    
                    if (file_exists($retina_candidate_test)) {
                        $srcset[] = $retina_candidate . ' 2x';
                    }
                    // Don't add the image that is in the src to the srcset as well
                    // But any potential @x2 version has already been added ;)
                    if (  false === strpos($source, $srcset_w) ) {
                    	$srcset[] = $source;
                    }
            }

        }
    }
    // If we've found any other sizes, other then the target
    if (is_array($srcset)){
        $srcset = array_keys(array_flip($srcset)); // remove any dupes
        $srcset = implode( ', ', $srcset ); // create comma separated list
        $atts = array(
            'srcset' => $srcset,
            'class' => $class,
            'alt'   => $alt_text
        );
    } else {
        $atts = array(
            'class' => $class,
            'alt'   => $alt_text
        );
    }

    return wp_get_attachment_image( $thumb_id, $thumbnail_size, false, $atts );
}

In this example the class isn't filtered for any baddies, in my case that happens earlier.