WordPress: Create missing intermediate images

maybe-create-missing-intermediate-images.php

<?php

/*
 * @param int $id Image attachment ID
 * @param string $size_name Name of custom image size as added with add_image_size()
 * return bool True if intermediate image exists or was created. False if failed to create.
 */

function maybe_create_missing_intermediate_images( $id, $size_name ) {

	$sizes = image_get_intermediate_size( $id, $size_name );

	if ( ! $sizes ) { //if size doesn't exist for given image
		//echo "new thumb need for id $id<br />";

		$upload_dir = wp_upload_dir();
		$image_path = str_replace( $upload_dir['baseurl'], $upload_dir['basedir'], wp_get_attachment_url( $id ) );
		$new = wp_generate_attachment_metadata( $id, $image_path );
		wp_update_attachment_metadata( $id, $new );

		if ( image_get_intermediate_size( $id, $size_name ) ) {
			//echo "new image size created for id $id<br />";
			return true;
		}
		else {
			//echo "failed to create new image size for id $id<br />";
			return false;
		}

	} else {
		//echo 'already exists<br />';
		return true;
	}

}