Image Processing Queue is an alternative to on-the-fly (OTF) image processing (e.g. Aqua Resizer) for WordPress themes.
Like OTF image processing, it allows theme developers to define image sizes for specific theme contexts rather than defining a size for all uploaded images. This greatly reduces the number of resized images and hence reduces disk space usage and the wait time when uploading an image.
Image Processing Queue differs from OTF image processing in how it behaves when an image doesn't exist yet. OTF generates the image right away and the end-user has to wait for the image to be generated. With Image Processing Queue there's no waiting. It immediately returns an image that already exists (that is the closest fit to the image size requested) and adds the image size to a queue to be generated in the background as server resources allow. Typically the images are generated right away and will be available for the next request.
Image Processing Queue also accommodates responsive themes much better than OTF. It allows theme developers to define a list of image sizes that will work best for their theme's responsive breakpoints. Images generated by Image Processing Queue are added to the post meta so that WordPress core's responsive functions will automatically add them to the srcset and delete them from the filesystem when the image is deleted from the Media Library.
To install Image Processing Queue as a plugin search for "Image Processing Queue" in your WordPress dashboard and install it from there.
If you're a theme developer and would like to include Image Processing Queue as a library in your theme, you'll need to clone this repository into your theme:
$ cd /path/to/wordpress/wp-content/themes/your-theme/
$ git clone git@github.com:deliciousbrains/wp-image-processing-queue.git image-processing-queue
$ composer install
You'll also need to add the following to your functions.php file:
require_once get_stylesheet_directory() . '/image-processing-queue/image-processing-queue.php';When you want to output an image in your theme, simply call the ipq_get_theme_image() function passing in the ID of the image post, an array of sizes you've decided on for the image in this particular context, and an array of additional HTML attributes for the <img> tag:
echo ipq_get_theme_image( $post_id, array(
array( 600, 400, false ),
array( 1280, 720, false ),
array( 1600, 1067, false ),
),
array(
'class' => 'header-banner'
)
);