bassettsj / jekyll-responsive-image

A Jekyll plugin for generating responsive images

Jekyll Responsive Images

Jekyll Responsive Images is a Jekyll plugin and utility for automatically resizing images. Its intended use is for sites which want to display responsive images using something like srcset or Imager.js.

Installation

You can either use the gem and update your _config.yml:

$ gem install jekyll-responsive_image

# _config.yml
gems: [jekyll-responsive_image]

Or you can simply copy responsive_image.rb into your _plugins directory.

Configuration

An example configuration is below.

responsive_image:
  # [Required]
  # Path to the image template.
  template: _includes/responsive-image.html

  # [Optional, Default: 85]
  # Quality to use when resizing images.
  default_quality: 90

  # [Optional, Default: []]
  # An array of resize configuration objects. Each object must contain at least
  # a `width` value.
  sizes:
    - width: 480  # [Required] How wide the resized image will be.
      quality: 80 # [Optional] Overrides default_quality for this size.
    - width: 800
    - width: 1400
      quality: 90

  # [Optional, Default: assets/resized/%{filename}-%{width}x%{height}.%{extension}]
  # The template used when generating filenames for resized images.
  #
  # Parameters available are:
  #   %{basename}    Basename of the file (assets/some-file.jpg => some-file.jpg)
  #   %{filename}    Basename without the extension (assets/some-file.jpg => some-file)
  #   %{extension}   Extension of the file (assets/some-file.jpg => jpg)
  #   %{width}       Width of the resized image
  #   %{height}      Height of the resized image
  #
  output_path_format: assets/resized/%{width}/%{basename}

Usage

Replace your images with the responsive_image tag, specifying the path to the image in the path attribute.

{% responsive_image path: assets/my-file.jpg %}

You can override the template on a per-image basis by specifying the template attribute.

{% responsive_image path: assets/my-file.jpg template: _includes/another-template.html %}

Any extra attributes will be passed straight to the template as variables.

{% responsive_image path: assets/image.jpg alt: "Lorem ipsum..." title: "Lorem ipsum..." %}

Liquid variables as attributes

You can use Liquid variables as attributes with the responsive_image_block tag. This tag works in exactly the same way as the responsive_image tag, but is implemented as a block tag to allow for more complex logic.

Important! The attributes in the responsive_image_block tag are parsed as YAML, so whitespace and indentation are important!

{% assign path = 'assets/test.png' %}
{% assign alt = 'Lorem ipsum...' %}

{% responsive_image_block %}
    path: {{ path }}
    alt: {{ alt }}
    {% if title %}
    title: {{ title }}
    {% endif %}
{% endresponsive_image_block %}

Template

You will need to create a template in order to use the responsive_image tag. Below are some sample templates to get you started.

Responsive images with srcset

{% capture srcset %}
    {% for i in resized %}
        /{{ i.path }} {{ i.width }}w,
    {% endfor %}
{% endcapture %}

<img src="/{{ path }}" srcset="{{ srcset | strip_newlines }} /{{ original.path }} {{ original.width }}w">

Responsive images with <picture>

<picture>
    {% for i in resized %}
        <source media="(min-width: {{ i.width }}px)" srcset="/{{ i.path }}">
    {% endfor %}
    <img src="/{{ path }}">
</picture>

Responsive images using Imager.js

This template assumes an output_path_format of assets/resized/%{width}/%{basename}

{% assign smallest = resized | sort: 'width' | first %}

<div class="responsive-image">
    <img class="responsive-image__placeholder" src="/{{ smallest.path }}">
    <div class="responsive-image__delayed" data-src="/assets/resized/{width}/{{ original.basename }}"></div>
</div>

<script>
    new Imager('.responsive-image__delayed', {
        availableWidths: [{{ resized | map: 'width' | join: ', ' }}]
        onImagesReplaced: function() {
            $('.responsive-image__placeholder').hide();
        }
    });
</script>

Template Variables

The following variables are available in the template:

Variable	Type	Description
path	String	The path of the unmodified image. This is always the same as the path attribute passed to the tag.
resized	Array	An array of all the resized images. Each image is an Image Object.
original	Object	An Image Object containing information about the original image.
*	String	Any other attributes will be passed to the template verbatim as strings.

Image Objects

Image objects (like original and each object in resized) contain the following properties:

Variable	Type	Description
path	String	The path to the image.
width	Integer	The width of the image.
height	Integer	The height of the image.
basename	String	Basename of the file (assets/some-file.jpg => some-file.jpg).
filename	String	Basename without the extension (assets/some-file.jpg => some-file).
extension	String	Extension of the file (assets/some-file.jpg => jpg).