Next.js plugin for embedding optimized images.

Features

• import png/jpg images
• output to webp format
• resize to multiple screen sizes and densities
• optimize webp and fallback images using sharp
• lazy load in modern browsers with prop forwarding (loading="lazy")
• prevent layout shift with automatic width/height attributes
• streamlined usage with the built in <Picture /> component
• art direction with different images for different breakpoints
• fast deployment and development workflow using persistent cache

By default next-img is configured to use:

• 1 breakpoint at 768px
• 2 pixel densities of 1x, 2x
• to output the original and webp formats

All of these settings and more can be changed in your next.config.js or in the individual image imports.

Developed and used by Humaans.

Motivation

By default Next.js or Webpack doesn't help you much with optimizing images. This means custom configuration or scripting, processing images by hand, using an image CDN or not optimising images at all. next-img provides and alternative streamlined approach for adding images to your Next.js projects. It combines a Next.js plugin, a custom webpack loader and a React component to make serving images in an optimal fashion in a way that is almost as easy as typing <img src='foo.png' />.

In short, it takes the following:

<Picture src={require('./images/jelly.jpg?sizes=375,800')} alt='Jellyfish' />

Imports, resizes, optimizes, caches (persistently in the git repo) and outputs the following HTML:

<picture>
  <source
    type="image/webp"
    srcset="
      /_next/static/images/[email protected]  375w,
      /_next/static/images/[email protected]  750w,
      /_next/static/images/[email protected]  800w,
      /_next/static/images/[email protected] 1600w
    "
    sizes="(max-width: 768px) 375px, 800px"
  />
  <source
    type="image/jpeg"
    srcset="
      /_next/static/images/[email protected]  375w,
      /_next/static/images/[email protected]  750w,
      /_next/static/images/[email protected]  800w,
      /_next/static/images/[email protected] 1600w
    "
    sizes="(max-width: 768px) 375px, 800px"
  />

  <img
    src="/_next/static/images/[email protected]"
    srcset="
      /_next/static/images/[email protected]  375w,
      /_next/static/images/[email protected]  750w,
      /_next/static/images/[email protected]  800w,
      /_next/static/images/[email protected] 1600w
    "
    width="375"
    height="250"
    alt="Jellyfish"
  />
</picture>

View examples.

Usage

Install the package

npm install next-img

Add the plugin to your next.config.js:

const withPlugins = require('next-compose-plugins')
const nextImg = require('next-img/plugin')

module.exports = withPlugins([
  [
    nextImg,
    {
      // specify the default breakpoints
      breakpoints: [768],
    },
  ]
])

In your application, import the images and embed using the <Picture /> component:

import { Picture } from 'next-img'
import jelly from './images/jelly.jpg?sizes=375,800'

export default () => <Picture src={jelly} />

Or inline:

import { Picture } from 'next-img'

export default () => <Picture src={require('./images/jelly.jpg?sizes=375,800')} />

This particular example will generate the following images:

• 375px wide image to show on small screens with low pixel density of 1x
• 750px wide image to show on small screens with high pixel density of 2x or more
• 800px wide image to show on large screens with low pixel density of 1x
• 1600px wide image to show on large screens with high pixel density of 2x or more

The resized and optimized images will be saved to the resources directory in the root of your project during the development. This means, that if you tweak the image import parameters or plugin configuration, you might generate extra images that are no longer used by your project. In that case execute next-img command to remove any unnecessary images and build any missing ones:

npx next-img

Now check in the resources directory to your source control to be reused later for development and production builds. You can turn this feature off by setting persistentCache: false in the plugin configuration, in which case the images will be only stored in a temporary cache inside .next directory.

View more usage examples.

Configuration

Default plugin configuration options:

{
  breakpoints: [768],
  densities: ['1x', '2x'],

  // the directory within Next.js build output
  imagesDir: 'images',
  // the output image name template
  imagesName: '[name]-[size]@[density]-[hash].[ext]',
  // advanced - customise the image public path
  imagesPublicPath: null,
  // advanced - customise the image output path
  imagesOutputPath: null,

  // persistent cache allows for fast deploy and
  // development workflow by avoiding reprocessing
  // images that were previously processed
  persistentCache: true,
  persistentCacheDir: 'resources',

  // this directory within .next is used in case persistent cache is turned off
  cacheDir: path.join('cache', 'next-img'),

  // image quality configuration
  jpeg: {
    quality: 80,
    webp: {
      quality: 90,
      reductionEffort: 6,
    },
  },

  png: {
    quality: 100,
    webp: {
      reductionEffort: 6,
      lossless: true,
    },
  },
}

Refer to sharp documentation for jpeg/png/webp compression options.

Import Params

When importing an image, you can use query parameters to customise the optimisation:

• sizes - a list of comma separated sizes you will be showing images at. Note that you do not need to take into account the pixel densities here. That is, if you're showing an image at 320px wide on your website, simply specify 320 here, the plugin will produce any necessary larger versions based on the densities configuration.
• densities - a list of comma separated densities you need each image size to be produced at. By default 1x and 2x sizes of images will be produced, specify 1x if you want to produce only one image per size, or 1x,2x,3x, etc. if you want more densities.
• jpeg - quality configuration options for jpeg images. Note, the jpeg->webp settings need to be nested under this param, e.g. ?jpeg[webp][quality]=95
• png - quality configuration options for png images. Note, the png->webp settings need to be nested under this param, e.g. ?png[webp][lossless]=false&png[webp][nearLossless]=true

Examples:

import img1 from './images/img.jpg'
import img2 from './images/img.jpg?sizes=375,900'
import img3 from './images/img.jpg?sizes=375,900&densities=1x'
import img4 from './images/img.jpg?sizes=375,900&densities=1x,2x,3x'
import img5 from './images/img.jpg?sizes=375,900&densities=1x,2x,3x&jpeg[quality]=70&jpeg[webp][quality]=70'

Picture Props

next-img comes with a React component making embedding images easier.

Here are the props this component access:

• src the imported image, or an array of imported images.
• breakpoints - a list of breakpoints to override the global configuration.
• sizes - a custom html sizes attribute, by default the sizes attribute is generated based on the available images and breakpoints.
• the rest of the props and ref are forwarded to the img tag. This allows the use of attributes such as alt, loading="lazy", etc..

A note on how sizes/media attributes are generated

When a single image is provided via the src prop, then each size will be configured to show up per each breakpoint available using the html sizes attribute attribute.

For example, with breakpoints [375, 768] and ?sizes=100,400,800 the <Picture> component will apply the following sizes attribute:

(max-width: 375px) 100px,
(max-width: 768px) 400px,
                   800px

When an array of images is provided via the src prop, then each image will be configured to show up per each breakpoint available using the html media attribute.

For example, with breakpoints [375, 768] and src=[img1, img2, img3] the <Picture> component will apply the following media attribute:

<picture>
  <source media="(max-width: 480px)" sizes="{{img1 width}}" />
  <source media="(max-width: 768px)" sizes="{{img2 width}}" />
  <source sizes="{{img3 width}}" />
  <img ... />
</picture>

FAQ

Do I have to use the <Picture /> component?

The Picture component is optional. You can handle the imported image object however you want.

Couldn't the images be optimized further?

Yes, you could probably get ~10%-20% or more compression if you pass the jpg/png through ImageOptim or other tools. Thing is, since this plugin outputs an already well optimized webp and you'll be serving webp to most modern browsers, that removes the need to squeeze that extra file size for jpg/png since they are the fallback images. However, there might be use cases where custom compression algorhithms are needeed and we might add support for arbitrary transformations in this plugin in the future.

Development

To develop this project, you'll need to run a watcher for the <Picture /> component:

npm install
npm run watch

You can use example as the playground:

cd example
npm install
next

To execute the next-img command in the example dir:

node ../bin/next-img

Future

Short term roadmap:

V1:

• Allow different config for jpg->webp and png->webp conversions
• Allow turning webp/jpg/png output off
• Add ?raw query support that doesn’t process the image in any way
• Remove the need for next-img command by plugging directly into next build via webpack plugin

V2:

• Add support for css images, in addition to the html images
• Inline small images

And some ideas for where this project could be taken in the future:

• Allow adding imagemin optimisation plugins into the pipeline via config. This way users can control how to optimise their images more granuarly.
• Translate relative sizes ?sizes=100vw,50vw,900px to pixels based on the breakpoint configuration, this would allow dynamic kind of imports that depend on your design system and relative sizing of images, e.g. if css says "50vw", you will not need to do that calculation manually.
• Debug mode that prints image sizes into images themselves, so you can see what's shown when right in the browser inside images (or overlaying them using js at runtime).
• Babel parser that analyses code for images to avoid the need to require().
• Optimize file read/write/hash operations to the maximum for improved performance.
• A puppeteer script to render the website in all predefined breakpoints and automatically analyses all image sizes required.
• Add support for gif and webp as source images.