Regenerating a single image size (re-)generates auto-scaled big images & auto-rotated images

[kraftner]

Bug Report

• Yes, I reviewed the contribution guidelines.
• Yes, more specifically, I reviewed the guidelines on how to write clear bug reports.

Describe the current, buggy behavior

In WP 5.3 two new features for uploaded images got introduced:

• When you upload an image that is bigger than 2560 in any dimension WP automatically downscales the image
• When you upload an image that has an EXIF rotation WP automatically rotates the image.

Unfortunately for any image affected by the above feature when regenerating any thumbnail size that auto-generated image gets regenerated as well. So even if you use wp media regenerate --image_size=<image_size>.

Under most circumstances this will just recreate an already existing image and won't cause any serious harm.

But it's still not ideal for at least these reasons:

• it updates the timestamp of the image
• it changes the image if between the initial upload and the regeneration the used image library changed from GD to Imagick or vice versa.
• it changes the image if between the initial upload and the regeneration settings have changed, e.g. the JPEG compression
• it newly creates auto-downscaled and auto-rotated images for files that don't have it yet (e.g. uploaded before WordPress 5.3 or while these features were deactivated)

I've noticed these unexpected changes while desperately attempting to understand what happened during a completely botched media regeneration caused by #130, #136 and this very issue.

Describe how other contributors can replicate this bug

1. upload a JPG image that is bigger than 2560 in any dimension
2. ensure that there has been created an image with the postfix -scaled.jpg
3. note the last modified date of this file in the filesystem
4. regenerate just one image size for the image: wp media regenerate <attachment-id> --image_size=medium
5. see that last modified date of the -scaled.jpg file in the filesystem has changed to the time you ran the above command

An alternative approach to test this in a visually noticeable way is to add this snippet in a plugin or themes functions.php between the initial upload and the regeneration. (It reduces the JPEG quality of generated images to "terrible")

add_filter('jpeg_quality', function(){ return 0; });

After the regeneration you should be able to notice that the intentionally regenerated "medium" size thumbnail has terrible compression artifacts, but the -scaled.jpg unexpectedly also has.

Describe what you would expect as the correct outcome

When limiting the media regeneration to a specific image size I expect any other images to be unaffected.

Let us know what environment you are running this on

WordPress 6.4.3

OS:     Linux 6.5.0-26-generic #26~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Tue Mar 12 10:22:43 UTC 2 x86_64
Shell:  
PHP binary:     /usr/local/bin/php
PHP version:    8.1.27
php.ini used:   /usr/local/etc/php/php.ini
MySQL binary:   /usr/bin/mysql
MySQL version:  mysql  Ver 15.1 Distrib 10.11.6-MariaDB, for debian-linux-gnu (x86_64) using  EditLine wrapper
SQL modes:      
WP-CLI root dir:        phar://wp-cli.phar/vendor/wp-cli/wp-cli
WP-CLI vendor dir:      phar://wp-cli.phar/vendor
WP_CLI phar path:       /var/www/html/wp-content/plugins/my-plugin
WP-CLI packages dir:    /home/wordpress/.wp-cli/packages/
WP-CLI cache dir:       /home/wordpress/.wp-cli/cache
WP-CLI global config:   
WP-CLI project config:  
WP-CLI version: 2.10.0

[kraftner]

The problem seems to be that when generating the attachment meta data

media-command/src/Media_Command.php

Line 632 in 210cccf

$metadata = wp_generate_attachment_metadata( $id, $fullsizepath );

this always results in the creation of sub sizes

https://github.com/WordPress/WordPress/blob/1dc3f878703ff788a5dc1b06efda6c665554dcb3/wp-admin/includes/image.php#L559-L560

Looking at the logic in wp_create_image_subsizes()
it seems that this could be disabled by temporarily disabling auto-downscale and auto-rotate with something like this:

add_filter('big_image_size_threshold', '__return_false');
add_filter('wp_image_maybe_exif_rotate', '__return_false');

[danielbachhuber]

Thanks for the great report, @kraftner !

Feel free to submit a pull request, if you'd like. Here is some guidance on our pull request best practices.

[kraftner]

@danielbachhuber Due to my current workload you probably shouldn't hold your breath for that. That is why I did my best to at least document it in case someone else can beat me to it earlier.

In any case I think there needs to be some more thinking about when/how to disable it, how to handle the case that someone wants to regenerate those files etc.

[kraftner]

Today I had some minutes to look at this some more. This particular issue can be solved by adding this

// For auto-downscaled images
$image_size_filters['big_image_size_threshold'] = '__return_false';

// For auto-rotated images
$image_size_filters['wp_image_maybe_exif_rotate'] = '__return_false';

in add_image_size_filters.

But it made me realize that the regeneration of these images is a more general and bigger issue and also can be a problem when regenerating everything and not just one size because then one might run into #136.

So I am leaving this here, but the whole issue needs some more investigation that also considers #136 and has extensive test coverage for all edge cases so I don't deem this ready for a PR yet.