Fallback logic for image alt text

WP_image_alt_fallback.php

<?php
    /**
     * Fallback logic for image alt tags
     *  Ref: https://wordpress.stackexchange.com/a/175179/10269
     *  Filter each get_post_meta request - if it's a request for image alt tag, we make sure it exists or supply a fallback.
     *  Fallback is saved to the image to reduce overhead on future requests
     *  To get really fancy, consider future use of AWS Rekognition APIs or https://cloud.google.com/vision/ for otehr SEO-specific image APIs
     */

        function air_image_alt_tag_fallback($metadata, $object_id, $meta_key, $single){
            if(!$meta_key || $meta_key !== "_wp_attachment_image_alt")
                return $metadata;

                //avoid a loop
                    remove_filter( 'get_post_metadata', 'air_image_alt_tag_fallback', 100 );

                $alt_text = get_post_meta( $object_id, '_wp_attachment_image_alt', TRUE );
                if(!$alt_text || empty($alt_text)){
                    //fallback to other data
                    $image = get_post($object_id);
                    /**
                    *   Fallback steps:
                    *       Alt Text
                    *       Caption
                    *       Description
                    *       Title
                    */
                    if(!empty($image->post_excerpt)){
                        //Caption
                        $alt_text = $image->post_excerpt;
                    }elseif(!empty($image->post_content)){
                        //Description
                        $alt_text = $image->post_content;
                    }elseif(!empty($image->post_title)){
                        //Title -- will always have a value because WP assigns on upload based on filename
                        $alt_text = $image->post_title;
                    }    

                    //save back to the image
                    update_post_meta( $object_id, '_wp_attachment_image_alt',$alt_text);
                }

                add_filter('get_post_metadata', 'air_image_alt_tag_fallback', 100, 4);


            // Return original if the check does not pass
            return $alt_text;

        }

        add_filter( 'get_post_metadata', 'air_image_alt_tag_fallback', 100, 4 );
?>