我有一个custom image size hello-image:

add_image_size( \'hello-image\', 700, 300, true );

如果width 原始图像的700px 它工作得很好。但是如果它的宽度小于700px, 压缩到大约0%.

如何测试它下载以下两幅图片：

BIG: 784px × 441px

SMALL: 670px × 377px

然后我创建了一个插件，其中包含所有相关代码：

<?php
/*
Plugin Name: WPSE: custom image size and JPEG compression
Description: custom image size and JPEG compression
Author: WPSE
Version: 1.0
*/

// Set JPEG compression quality
add_filter(\'jpeg_quality\', create_function(\'$quality\', \'return 100;\'));

add_action(\'added_post_meta\', \'ad_update_jpeg_quality\', 10, 4);

function ad_update_jpeg_quality($meta_id, $attach_id, $meta_key, $attach_meta) {

    if ($meta_key == \'_wp_attachment_metadata\') {

        $post = get_post($attach_id);

        if ($post->post_mime_type == \'image/jpeg\' && is_array($attach_meta[\'sizes\'])) {

            $pathinfo = pathinfo($attach_meta[\'file\']);
            $uploads = wp_upload_dir();
            $dir = $uploads[\'basedir\'] . \'/\' . $pathinfo[\'dirname\'];

            foreach ($attach_meta[\'sizes\'] as $size => $value) {

                $image = $dir . \'/\' . $value[\'file\'];
                $resource = imagecreatefromjpeg($image);

                if ($size == \'large\') {

                    // set the jpeg quality for \'large\' size
                    imagejpeg($resource, $image, 35);

                } elseif ($size == \'medium\') {

                    // set the jpeg quality for the \'medium\' size
                    imagejpeg($resource, $image, 35);

                } elseif ($size == \'hello-image\') {

                    // set the jpeg quality for the \'hello-image\' size
                    imagejpeg($resource, $image, 40);

                } else {

                    // set the jpeg quality for the rest of sizes
                    imagejpeg($resource, $image, 35); // 38
                }

                imagedestroy($resource);
            }
        }
    }
}


// add custom image sizes to media uploader
function my_insert_custom_image_sizes( $sizes ) {
    // get the custom image sizes
    global $_wp_additional_image_sizes;
    // if there are none, just return the built-in sizes
    if ( empty( $_wp_additional_image_sizes ) )
        return $sizes;

    // add all the custom sizes to the built-in sizes
    foreach ( $_wp_additional_image_sizes as $id => $data ) {
        // take the size ID (e.g., \'my-name\'), replace hyphens with spaces,
        // and capitalise the first letter of each word
        if ( !isset($sizes[$id]) )
            $sizes[$id] = ucfirst( str_replace( \'-\', \' \', $id ) );
    }

    return $sizes;
}

// Which custom image size selected by default
function my_set_default_image_size () {
    return \'hello-image\';
}

function custom_image_setup () {
    // custom image sizes
    add_image_size( \'medium\', 300, 9999 ); //  medium
    add_image_size( \'hello-image\', 700, 300, true ); // custom
    add_image_size( \'large\', 700, 3000 ); //  large

    add_filter( \'image_size_names_choose\', \'my_insert_custom_image_sizes\' );
    add_filter( \'pre_option_image_default_size\', \'my_set_default_image_size\' );
}

add_action( \'after_setup_theme\', \'custom_image_setup\' );

现在，上传大图像，看看hello-image 图像压缩良好。然后上传小图像，看看同样大小的图像是如何被严重压缩的。为了澄清，以下是结果：

700x300 = 40% compression
670x300 = 0% compression

它们都是通过完全相同的自定义图像大小生成的。原因是什么？如何解决？

我应该强调的是，我的同事发布了question here 询问如果宽度小于700px... 但我是一个WordPress极客，宁愿解决问题的原因，也不愿添加一个黑客补丁。

最后，我在最近的WordPress更新中发现了这个问题，所以这可能是核心的bug，而不是代码中的问题。

Edit: 阅读update.