PostCSS image-set() Function

PostCSS image-set() Function lets you display resolution-dependent images using the image-set() function in CSS, following the CSS Images specification.

.example {
  background-image: image-set(
    url(img.png) 1x,
    url([e-mail]) 2x,
    url([e-mail]) 600dpi
  );
}

/* becomes */

.example {
  background-image: url(img.png);
}

@media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 192dpi) {
  .example {
    background-image: url([e-mail]);
  }
}


@media (-webkit-min-device-pixel-ratio: 6.25), (min-resolution: 600dpi) {
  .example {
    background-image: url([e-mail]);
  }
}

.example {
  background-image: image-set(
    url(img.png) 1x,
    url([e-mail]) 2x,
    url([e-mail]) 600dpi
  );
}

Usage

Add PostCSS image-set() Function to your project:

npm install postcss-image-set-function --save-dev

Use PostCSS image-set() Function as a PostCSS plugin:

const postcss = require('postcss');
const postcssImageSetFunction = require('postcss-image-set-function');

postcss([
  postcssImageSetFunction(/* pluginOptions */)
]).process(YOUR_CSS /*, processOptions */);

PostCSS image-set() Function runs in all Node environments, with special instructions for:

Node	PostCSS CLI	Webpack	Gulp	Grunt

Options

preserve

The preserve option determines whether the original declaration using image-set() is preserved. By default, it is preserved.

postcssImageSetFunction({ preserve: false })

.example {
  background-image: image-set(
    url(img.png) 1x,
    url([e-mail]) 2x,
    url([e-mail]) 600dpi
  );
}

/* becomes */

@media (-webkit-min-device-pixel-ratio: 1), (min-resolution: 96dpi) {
  .example {
    background-image: url(img.png);
  }
}

@media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 192dpi) {
  .example {
    background-image: url([e-mail]);
  }
}


@media (-webkit-min-device-pixel-ratio: 6.25), (min-resolution: 600dpi) {
  .example {
    background-image: url([e-mail]);
  }
}

onInvalid

The onInvalid option determines how invalid usage of image-set() should be handled. By default, invalid usages of image-set() are ignored. They can be configured to emit a warning with warn or throw an exception with throw.

postcssImageSetFunction({ onInvalid: 'warn' }) // warn on invalid usages

postcssImageSetFunction({ onInvalid: 'throw' }) // throw on invalid usages

Image Resolution

The image-set() function allows an author to provide multiple resolutions of an image and let the browser decide which is most appropriate in a given situation. The image-set() also never fails to choose an image; the <resolution> just helps determine which of the images is chosen.

Since this plugin is not a browser, the image options are sorted by device pixel ratio and the lowest ratio is used as the default, while the remaining images are pushed behind media queries.

Therefore, this plugin can only approximate native browser behavior. While images should typically match the resolution as the device they’re being viewed in, other factors can affect the chosen image. For example, if the user is on a slow mobile connection, the browser may prefer to select a lower-res image rather than wait for a larger, resolution-matching image to load.