For inserting a responsive <picture> element in your template, use the get_picture function with the following parameters:

ResponsivePics::get_picture(id, sizes, classes, lazyload, intrinsic);

For inserting a responsive <img> element in your template, use the get_image function with the following parameters:

ResponsivePics::get_image(id, sizes, crop, classes, lazyload);

For inserting a responsive background image in your template, use the get_background function with the following parameters:

ResponsivePics::get_background(id, sizes, classes);

The following image file formats are supported:

Any other image formats, will not be resizes or cropped.

The following syntax is available for each image size in the sizes parameter:

breakpoint:width

Since the <picture> element and background images support art directed images, the following full syntax is available for each image size in the sizes parameter:

breakpoint:width [/factor|height]|crop_x crop_y

1. When visiting a front-end page and a ResponsivePics function call is made, this library will add the resize and/or crop image task as a job to the background process queue using Action Scheduler.
2. On every page load or on the next cron interval, Action Scheduler will run the next batch of jobs in the background process queue. See the Cron section for more information.
3. When a job is up next in the queue and ready to be processed it will execute the resize and/or crop task and save the image in the same location as the original image when successful and it will remove the job from the queue.
4. Once the image variation is created, it will skip the process of that variation on the next page load.
5. When you change one of the image size parameters, it will automatically try and create the new image variation on the next page load.
6. When the original image does not meet the dimension requirements of the requested image size, it will skip that image size variation and proceed to the next image size.
7. Alt text will automatically be added on the picture img element if the original image in the media library has one.

Action Scheduler has a built in administration screen for monitoring, debugging and manually triggering scheduled image resize jobs. The administration interface is accesible via:

Tools > Scheduled Actions

Every resize job will be grouped by it’s wordpress image id.

When you are using the built-in WP-Cron, the background process queue will only process any tasks on every page load.

If you have disabled WP-Cron in your setup and you are using your own cron job on your server, Action Scheduler will use the interval set in that cron job to process the next batch of jobs.

define('DISABLE_WP_CRON', true);

If you’re using Trellis like us ❤️, the default cron interval is set to every 15 mins.
You could override this to for example 1 mins with an environment variable per wordpress site like this:

In for example trellis/group_vars/development/wordpress_sites.yml:

wordpress_sites:
  example.com:
    site_hosts:
      - canonical: example.test
        redirects:
          - www.example.test
    local_path: ../site # path targeting local Bedrock site directory (relative to Ansible root)
    admin_email: admin@example.test
    multisite:
      enabled: false
    ssl:
      enabled: false
      provider: self-signed
    cache:
      enabled: false
    cron:
      interval: 1

In trellis/roles/wordpress-setup/tasks/main.yml:

- name: Setup WP system cron
  cron:
    name: "{{ item.key }} WordPress cron"
    minute: "*/{{ item.value.cron.interval | default(15) }}"
    user: "{{ web_user }}"
    job: "cd {{ www_root }}/{{ item.key }}/{{ item.value.current_path | default('current') }} && wp cron event run --due-now > /dev/null 2>&1"
    cron_file: "wordpress-{{ item.key | replace('.', '_') }}"
    state: "{{ (cron_enabled and not item.value.multisite.enabled) | ternary('present', 'absent') }}"
  with_dict: "{{ wordpress_sites }}"

Don’t forget to re-provision your server after changing this value.

When enabling the lazyload option in the ResponsivePics::get_picture() or ResponsivePics::get_image() function, this library automatically:

• adds a lazyload class to the <img> element.
• swaps the srcset with data-srcset attributes on the picture <source> or the <img> elements.

This will enable you to use a lazy loading plugin such as Lazysizes or Lozad.

You can also set your own lazyload class by passing it to ResponsivePics library in your theme’s functions.php:

if (class_exists('ResponsivePics')) {
	ResponsivePics::setLazyLoadClass('lozad');
}

To install Lazysizes in your wordpress theme as a node module, run the following command from your theme directory:

npm

npm install --save lazysizes

Yarn

yarn add lazysizes

And import the package in your theme’s global javascript file:

import 'lazysizes';

When enabling the intrinsic option in the ResponsivePics::get_picture() function call, this library automatically:

• adds a intrinsic class to the <picture> element and a intrinsic__item class to the picture <img> element.
• adds data-aspectratio attributes on the picture <source> and <img> elements with the calculated source image ratio.

This will enable you to pre-occupy the space needed for an image by calculating the height from the image width or the width from the height with an intrinsic plugin such as the lazysizes aspectratio extension.

To use the Lazysizes aspectratio extension in your wordpress theme, first install lazysizes as a node module as described in the Lazyloading section and import the extension in your theme’s global javascript file:

import 'lazysizes/plugins/aspectratio/ls.aspectratio.js';