Responsive Images for Jekyll Sites

jekyll-responsive-magick is a Jekyll plugin for responsive images. It adds filters for setting the srcset, width and height attributes of img elements, while automatically generating image variants of configured sizes using the ImageMagick command-line tools. Resized images are cached to minimize build times, regenerated only when the original source image changes.

The plugin has no dependencies besides the ImageMagick command line tools. This is by design, to make it easy to deploy on services such as Cloudflare Pages. For an example of its use, see indii.org. For a walkthrough of the implementation, see Responsive Images with Jekyll and ImageMagick.

jekyll-responsive-magick is open source software. It is licensed under the Apache License, Version 2.0 (the "License"); you may not use it except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0.

The plugin requires ImageMagick. It is standard in Linux distributions and available through Homebrew on macOS. If you happen to be hosting with CloudFlare Pages, it is already installed.

1. Install the plugin If you are using a Gemfile, add the following:

group :jekyll_plugins do
  gem 'jekyll-responsive-magick', '~> 1.2'

then bundle install. Alternatively, gem install jekyll-responsive-magick or simply copy the lib/jekyll-responsive-magick.rb file into your site's _plugins directory.

2. Enable the plugin Add the following to your site's _config.yml file:

plugins: 
- jekyll-responsive-magick

The plugin provides three filters: srcset, width and height. Each consumes an absolute path to an image (as it would appear in src) and generates a value for the corresponding attribute. The intended usage is:

<img
    src="{{ src }}"
    srcset="{{ src | srcset }}"
    width="{{ src | width }}"
    height="{{ src | height }}"
>

src must be an absolute path, i.e. beginning with /, such as /assets/example.jpg. This is necessary for the plugin to find the right file in your project.

With the plugin installed and one or more uses of the srcset, width or height filters, you can build your site as normal, now with responsive images.

There may be cases where you cannot use responsive images but still want to resize to an explicit width, for example in a background-image style property. A fourth filter, size, can be used for this purpose:

<div style="background-image: url({{ src | size: 480 }});">...</div>

You can configure the size variants and quality of resized images by adding configuration options such as the following to your _config.yml:

responsive:
  widths: [400,500,700,900]
  quality: 30
  format: webp
  verbose: true

The choice of widths should cover the typical sizes of images as they appear on your site. The choice of quality (between 0 and 100) is a trade-off between file size (lower at lower quality) and clarity (higher at higher quality). It could be tuned by eye. Low quality can be satisfactory with the prevalence of high definition displays.

The format option can be used to specify the output format of the resized images. The default is the original format but you can force conversion to webp, for example, which is a modern format with good compression. Simply omit the option to preserve the original format of each image.

Set verbose to true for debugging purposes. It will echo the commands being executed under the hood. Simply omit the option if not debugging.

The srcset, width and height filters call ImageMagick's convert and identity on demand, rather than for all image assets in a project. Caching is used to call identity at most once per image per build, and convert once per image per width, storing resized images in a subdirectory _responsive/ for reuse in subsequent builds. Resized images in _responsive/ are updated only if their original source image changes (detected using last modified times on files).

If you experience any issues with outdated images, or simply wish to clean up, remove the whole _responsive/ directory and rebuild. You may also wish to do this if you change the widths option in _config.yml.

When using width and height, expect additional build time of about one second per 100 images due to the overhead of launching identity processes.

When using srcset for the first time, expect additional build time on the order of minutes as resized images are generated with convert. Performance will improve drastically on subsequent builds because of the _responsive/ subdirectory.

For a walkthrough of the implementation, see Responsive Images with Jekyll and ImageMagick.

To build and install, update the version number in jekyll-responsive-magick.gemspec, then:

gem build jekyll-responsive-magick.gemspec
gem install --user-install jekyll-responsive-magick-VERSION.gem

To release a new version to RubyGems:

gem signin
gem push jekyll-responsive-magick-VERSION.gem

For development and testing purposes, it is often easier to copy the lib/jekyll-responsive-magick.rb file into a site's _plugins directory and make modifications there.