WordPress Featured Image Using Picturefill Responsive Image Solution

wp-picturefill.php

<?php 
//Created using code from http://scottjehl.github.io/picturefill/,
//http://ericportis.com/posts/2014/srcset-sizes/,
//http://www.taupecat.com/2014/05/picturefill-js-wordpress/,
//http://www.smashingmagazine.com/2014/05/12/picturefill-2-0-responsive-images-and-the-perfect-polyfill/

//For homepage where featured image should live
if( has_post_thumbnail() ){
	$thumbnail_id = get_post_thumbnail_id();
	$alt          = get_post_meta( $thumbnail_id, '_wp_attachment_image_alt', true );

	//Each is an array where [0] = url and [1] = width
	$thumb         = wp_get_attachment_image_src( $thumbnail_id, 'thumbnail' );
	$medium        = wp_get_attachment_image_src( $thumbnail_id, 'medium' );
	$large         = wp_get_attachment_image_src( $thumbnail_id, 'large' );
	$homepage_size = wp_get_attachment_image_src( $thumbnail_id, 'NAME_OF_LARGER_IMAGE_SIZE' );

	
	echo '<img srcset="' . $thumb[0] . ' ' . $thumb[1] . 'w,';
			echo $medium[0] . ' ' . $medium[1] . 'w,';
			echo $large[0] . ' ' . $large[1] . 'w,';
			echo $homepage_size[0] . ' ' . $homepage_size[1] . 'w"';
			echo ' sizes="100%" alt="' . $alt . '">';
}


//To add to functions.php where js is enqueued.  Make sure it loads in the head.
//Picturefill 2.1.0 minified can be downloaded from http://scottjehl.github.io/picturefill/#download.
if( is_front_page() ){
	wp_enqueue_script( 'picturefill', get_stylesheet_directory_uri() . '/js/picturefill.min.js' );	
}