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My goal is to make a retina and non-retina image when user uploads a photo. This is doable like so:

add_action('init', function() {
    // Get the original image sizes and duplicate
    $thumb_w = get_option('thumbnail_size_w') * 2;
    $thumb_h = get_option('thumbnail_size_h') * 2;
    $thumb_crop = get_option('thumbnail_crop');
    $medium_w = get_option('medium_size_w') * 2;
    $medium_h = get_option('medium_size_h') * 2;
    $large_w = get_option('large_size_w') * 2;
    $large_h = get_option('large_size_h') * 2;

    // Register the actual sizes
    add_image_size('thumbnail-retina', $thumb_w, $thumb_h, $thumb_crop);
    add_image_size('medium-retina', $medium_w, $medium_h);
    add_image_size('large-retina', $large_w, $large_h);
});

This is good, but the most optimal would be to chop half the full image, then use the original as the retina and the new as non-retina.

Here's my rationale on it:

  1. Intercept the image upload process with a hook, add_attachment
  2. Measure the img and chop half with wp_get_image_editor
  3. Suffix it with "1x"
  4. Save the new image

What I have so far (this works, but it's a bit faulty):

add_filter('add_attachment', function ($attachment_id) {
    $src = get_attached_file($attachment_id);

    if (!$src) {
        return;
    }

    $img_info = pathinfo($src);
    $image_editor = wp_get_image_editor($src);

    // Get image sizes
    $size = $image_editor->get_size($src);
    $width = $size['width'];
    $height = $size['height'];

    // Chop the image in half and save
    $image_editor->resize($width / 2, $height / 2, false);
    $image_editor->save("{$img_info['dirname']}/{$img_info['filename']}-1x.{$img_info['extension']}");

    return $attachment_id;

}, 10, 2);

How does that look to you, am I doing it right?

The fault is: WordPress doesn't know anything about the new 1x image, so when the image is deleted, the 1x copy stays behind. Any ideas how to approach that?

Here's a similar post, but it doesn't really deal with the full image.

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