Jekyll Picture Tag NG

This plugin will automatically generate variants of your pictures on build, and change the Kramdown rendering to use the variants with HTML picture tags when including pictures from markdown. Developed for and used on Crocodile Couture

Installation

Install the gem and add to the application's Gemfile by executing:

$ bundle add jekyll-picture-tag-ng

If bundler is not being used to manage dependencies, install the gem by executing:

$ gem install jekyll-picture-tag-ng

Additionally, you will need ImageMagick installed on the system you're using to build your website in order to generate the picture variations. On Debian/Ubuntu systems, you can execute:

$ sudo apt-get install imagemagick

After installing, update your _config.yml to include the plugin :

plugins:   [other-plugins, jekyll-picture-tag-ng]

or

plugins:
  - other-plugins
  - jekyll-picture-tag-ng

Using with GitHub Pages

If you're using GitHub Pages to deploy your site, you'll need to use a custom action to be able to use this plugin and install ImageMagick/libvips. You can create such GitHub action by browsing to

https://github.com/{YOUR/REPO}/new/main?filename=.github%2Fworkflows%2Fjekyll.yml&workflow_template=pages%2Fjekyll

You will need to add the following lines as a step for the build job of your GitHub action (before the jekyll build command) :

- name: Install imagemagick and libvips
  run: sudo apt-get update && sudo apt-get install imagemagick libvips-tools

If you do not intend to use the libvips backend (see Configuration), you should remove libvips-tools from the apt-get install command above, as installing it takes some time (still a lot less than generating all versions with Image Magick)

After adding the custom action to your repository, you'll need to update the repository settings on GitHub : go to the "Pages" section and change the "Source" setting from "Deploy from a branch" to "GitHub Actions".

Usage

By installing the plugin and activating it, jekyll build and jekyll serve commands will perform an additional step to generate several versions of your jpeg and webp files : for each of these files in the source directory, and each version defined, a file will be output in the img/{version} directory of the rendered website.

When using the default markdown syntax for including pictures (![Alt text](PICTURE_URL)) with the Kramdown renderer, <picture> tags with appropriate <source> children tags will be output. This will automatically exclude any picture element with the src attribute starting with "http://" or "https://".

When working locally, you can use the --incremental option to prevent Jekyll from re-generating all the pictures (which can take a long time) when re-launching the jekyll serve command. This is useful when developing a plugin or tweaking the _config.yml file (which both require you to frequently re-launch the jekyll serve command). However, be careful about using this option when changing the plugin's output image formats : the plugin will skip generating the new output formats in --incremental mode for pictures that were previously generated for the old output formats.

Configuration

Configuration is done in the _config.yml of your website, under the picture_tag_ng variable :

picture_tag_ng:
  backend: imagemagick
  parallel: false
  threads: 16
  background_color: FFFFFF
  picture_versions:
    m: 700
    s: 400

The example above is equivalent to the defaults.

• backend can be either imagemagick (more well-tested) or libvips (a lot faster but less tested)
• background_color is the color used to replace transparency when converting from webp to jpeg
• picture_versions in the simplest form, maps version names to target widths in pixels. The default configuration above produces output files 700px wide in img/m/ and 400px wide in img/s/. See below for more complex forms.
• parallel is a boolean indicating if you want to generate the output files in parallel threads. With a website that has a lot of large pictures, I get ~30% speed improvements when generating the site locally.
• threads is the number of concurrent threads for generating the website (only used if parallel is true)

picture_versions option

The picture_versions option must be a map. The keys are the version identifiers, and the values control the output for each version. The values can be defined in one of the following formats :

picture_tag_ng:
  picture_versions:
    s: 400

When the version consists only of an integer, the value is used for both the output width and the corresponding max-width media attribute.

picture_tag_ng:
  picture_versions:
    s:
      out_size: 400

Each version can be defined as a map, with the out_size key being required (must be an integer). This value controls the output width for the version. If out_size is the only defined key, it is also used for the corresponding max-with media attribute.

picture_tag_ng:
  picture_versions:
    m:
      out_size: 700
      media: 1200

Each version that is a map can define the media key. If the value is an integer, produces (max-width: #{media_integer_from_conf}px) for the associated media attribute.

picture_tag_ng:
  picture_versions:
    m:
      out_size: 700
      media: "screen and (max-width: 1200px)"

If media is a string, its value is used as-is for the corresponding media attribute.

Additionally, you can add the default: true property to any version to remove the corresponding media attribute from HTML source elements, and use this version as the src for the default HTML img element. If no version is explicitly set as the default, the largest one will me used.

The following configuration shows one version for each allowed format :

picture_tag_ng:
  picture_versions:
    s:
      out_size: 400
    m:
      out_size: 700
      media: 1200
    l:
      out_size: 1200
      media: "screen and (max-width: 1200px)"
    xl:
      out_size: 2000
      default: true

When using the above configuration, the plugin will convert

![Alt text](/path/to/img/orig.jpg)

to the following HTML :

<picture>
    <source media="(max-width: 400px)" srcset="/img/s/path/to/img/orig.webp" type="image/webp">
    <source media="(max-width: 400px)" srcset="/img/s/path/to/img/orig.jpg" type="image/jpeg">
    <source media="(max-width: 1200px)" srcset="/img/m/path/to/img/orig.webp" type="image/webp">
    <source media="(max-width: 1200px)" srcset="/img/m/path/to/img/orig.jpg" type="image/jpeg">
    <source media="screen and (max-width: 1200px)" srcset="/img/l/path/to/img/orig.webp" type="image/webp">
    <source media="screen and (max-width: 1200px)" srcset="/img/l/path/to/img/orig.jpg" type="image/jpeg">
    <source srcset="/img/xl/path/to/img/orig.webp" type="image/webp">
    <source srcset="/img/xl/path/to/img/orig.jpg" type="image/jpeg">
    <img src="/img/xl/path/to/img/orig.jpg" alt="Alt text" loading="lazy">
</picture>

Extra convert arguments

:warning: The options in this section will do nothing when used with the libvips backend :warning:

The extra_convert_args and pre_extra_convert_args options allow you to add any convert argument to the image conversion processes. The pre_extra_convert_args will be added before the resize operation, and the extra_convert_args after the resize operation. These options must be either arrays of strings or a string that will be split on spaces. This can be used to add a watermark to all your pictures (see example below).

Additionally, you can set different values based on the version for both these options. In the example below, the m version of output pictures will be blurred :

picture_tag_ng:
  pre_extra_convert_args: ["-font", "DejaVu-Sans-Book", "-pointsize", "72",
    "-fill", "black", "-annotate", "+25+75", "Watermark"] # Add watermark to all output pics
  picture_versions:
    m:
      out_size: 700
      extra_convert_args: ["-scale", "20%", "-blur", "0x2.5", "-resize", "500%"] # Blur `m` pics
    s: 400

You can disable the default resize operation by setting replace_convert_args: true either at the plugin level or at the version level. If the, the version-specific value will override the plugin-global value.

Development

After cloning the repo, you can run the following commands in a local Jekyll website's folder to start hacking on the code of jekyll-picture-tag-ng (you'll need to replace the path in the second command) :

$ bundle remove jekyll-picture-tag-ng # if you previously used jekyll-picture-tag-ng from rubygems
$ bundle add --path /absolute/or/relative/path/to/your/local/jekyll-picture-tag-ng/repo jekyll-picture-tag-ng
$ bundle exec jekyll serve # Re-run this when you want to test changes to your local jekyll-picture-tag-ng

To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to the Rubygems repository.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/pcouy/jekyll-picture-tag-ng.

License

The gem is available as open source under the terms of the MIT License.