Responsive Images

Functions for statically generating responsive image markup. These currently exist as a convenience wrapper around the Eleventy Image plugin, though it can be used with any site that has a static build step in its markup generation.

It determines which image sizes you need to generate by parsing a sizes query string and comparing it with a range of target devices. This means that, as long as you write a good sizes query, you don't need to specify the widths for each image you want to generate.

For all methods and options please refer to the full documentation.

Basic Usage

npm install @dawaltconley/responsive-images

The basic API is flexible, but designed around the idea of chaining methods, which generate the necessary images and return appropriate markup.

import ResponsiveImages from '@dawaltconley/responsive-images'

// set global config options
const { responsive } = new ResponsiveImages({ scalingFactor: 0.5 })

// resize an image based on a sizes query string
// this generates the required images and returns an object representing them
const metadata = await responsive('./path/to/image.png').fromSizes(
  '(max-width: 1280px) 800px, 100vw',
)

// call one of the available methods to get the required markup
const html = `<picture>${metadata.toSources({
  class: 'responsive-image',
  alt: 'image properties go here',
})}</picture>`

This library uses a proxied Promise object, so you can continue to chain through the asynchronous methods without awaiting each one.

const hast = await responsive('./path/to/image.png')
  .fromSizes('(max-width: 1280px) 800px, 100vw')
  .toHast({ alt: 'image properties go here' })

If you want to generate the images and markup separately, you can use the getWidthsFromSizes function instead.

Configure

Most configuration is handled globally in the default class constructor. This can be exported as a singleton for use in multiple files.

import ResponsiveImages from '@dawaltconley/responsive-images'

const config = new ResponsiveImages({
  scalingFactor: 0.6,
  disable: false,
  defaults: {
    formats: ['webp', null],
    urlPath: '/assets/generated/',
    outputDir: './dist/assets/generated/',
  },
})

export default config
export const { responsive } = config

Configuration Options

All config options are optional.

defaults<ImageOptions> = {}

A set of options that gets passed to Eleventy Image whenever it is called. You probably want to specify at least a urlPath and outputDir for the generated images. See the Eleventy Image docs for all available options.

scalingFactor<number> = 0.8

The maximum difference in size between any two images created when downsizing. This uses pixels to approximate a file size. At a scaling factor of 0.5, for example, each image created will have no more than half the total pixels of the previous image.

The number should be a proportional float between 0 and 1. A higher scaling factor creates more images with smaller gaps in their sizes. A lower scaling factor creates fewer images with larger gaps in their sizes. At very low scaling factors, some devices may need to load larger images than necessary. Meanwhile, at a scaling factor of 1, a new image will be created and optimized for each device supported, no matter how close it is to other images.

devices

A set of devices to support when calculating image sizes from a sizes query string. Defaults to an internal device list that can be imported from @dawaltconley/responsive-images/devices.

const ipadsOnly = new ResponsiveImages({
  devices: [
    {
      w: 1366,
      h: 1024,
      dppx: [2, 1],
      flip: true,
    },
    {
      w: 1024,
      h: 768,
      dppx: [2, 1],
      flip: true,
    },
  ],
})

sassPrefix<string> = 'image'

A string used to prefix the Sass background-image functions, which can be edited to avoid global namespace collisions. Defaults to image, which creates functions named image-resize and image-queries.

import ResponsiveImages from '@dawaltconley/responsive-images'
import { getSassFunctions } from '@dawaltconley/responsive-images/sass'
import sass from 'sass'

const config = new ResponsiveImages({
  sassPrefix: 'custom-prefix',
})

sass.compileAsync('./styles/images.scss', {
  functions: getSassFunctions(config),
})

If changed, you must also pass this prefix when importing the mixins.

@use '@dawaltconley/responsive-images' with (
  $prefix: 'custom-prefix'
);

.example {
  @include responsive-images.bg('./assets/some-background.png');
}

disable<boolean> = false

If true, disables new image generation so that all methods output image elements at their original size. Useful for avoiding repeated rebuilds on development environments.

Plugins

This package includes a few plugins for compatible static site generators.

Eleventy

The Eleventy plugin provides shortcodes for the Nunjucks and Liquid templating languages. It relies on their async methods to generate new images together with the responsive image markup.

// .eleventy.js
const responsiveImagesPlugin = require('@dawaltconley/responsive-images/eleventy')

module.exports = eleventyConfig => {
  eleventyConfig.addPlugin(responsiveImagesPlugin, {
    scalingFactor: 0.5,
    defaults: {
      urlPath: '/assets/generated/',
      outputDir: './_site/assets/generated/',
    },
  })
}

You can also pass a configured ResponsiveImages class instance to the plugin, if you are using its methods elsewhere.

Then, call the functions within your templates:

<div>
  {% picture "assets/example.jpg",
    alt="A responsive picture element",
    sizes="(min-width: 800px) 400px, 100vw", %}

  {% set picHtml = "assets/example.jpg" | picture({ alt: 'used as a filter', sizes: '100vw' }) #}
  {{ picHtml | safe }}

  <picture class="picture-class">
    {% img "assets/example.jpg",
      alt="Use the img shortcode to include image sources without a picture element",
      class="passed-to-image-element",
      sizes="(min-width: 1000px) 50vw, 100vw" %}
  </picture>

  {# The resize filter just calls EleventyImage and returns a metadata object #}
  {% set metadata = "assets/example.jpg" | resize({ widths: [1200, null] }) %}
</div>

Astro

There is not currently a plugin for Astro projects, but integration is fairly straightforward. Refer to this guide for more information.

Sass

This package includes Sass mixins and custom functions for generating responsive images using media queries and the CSS background-image property.

@use './node_modules/@dawaltconley/responsive-images';

.example {
  background-position: center;
  background-size: cover;
  background-repeat: no-repeat;

  @include responsive-images.bg(
    $src: './assets/bg.jpg',
    // can be passed as the first positional argument
    $sizes: '(min-width: 1000px) 50vw, 100vw',
    // optional; defaults to 100vw
    $widths: 1000 800 600,
    // optional list of manual widths. doesn't change media queries.
    $formats: webp null,
    // optional; defaults to null (the original image format is preserved).
    $orientations: portrait landscape,
    // optionally target a single viewport orientation
  );
}

These mixins rely on custom functions, which can be imported from @dawaltconley/responsive-images/sass and passed to Sass's compileAsync method.

In addition to the above keyword arguments, you can pass additional background-image backgrounds (such as linear-gradient functions) using the $prepend-backgrounds and $append-backgrounds arguments.

This currently generates a fair amount of media queries. If you are creating lots of responsive background images, you may want to use a post-processor like postcss-sort-media-queries.

This mixin makes use of the image-set function to serve multiple image formats. Formats will be used (as available) in the order provided: the last format in the list will be used as a fallback for browsers that don't support image-set.