投稿サムネイルをretina対応する。

retina-post-thumbnail.php

<?php

add_image_size("my-image-size", 300, 200, true);
add_image_size("my-image-size@2x", 600, 400, true);


add_filter("post_thumbnail_html",function( $html, $post_id, $post_thumbnail_id, $size, $attr ){
    $retina_size = $size."@2x";
    if(!has_image_size($retina_size)) {
        return $html;
    }

    if( $image = image_downsize($post_thumbnail_id, $retina_size) and $image[3] ) {
        $srcset_image = wp_get_attachment_image_src( get_post_thumbnail_id(), $retina_size );
        $html = wp_get_attachment_image( $post_thumbnail_id, $size, false, wp_parse_args($attr, [ "srcset" => $srcset_image[0] . ' 2x']) );
    }

    return $html;

}, 10, 5);