168 lines
4.7 KiB
Markdown
168 lines
4.7 KiB
Markdown
# PostCSS image-set() Function [<img src="https://postcss.github.io/postcss/logo.svg" alt="PostCSS Logo" width="90" height="90" align="right">][postcss]
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[![NPM Version][npm-img]][npm-url]
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[![CSS Standard Status][css-img]][css-url]
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[![Build Status][cli-img]][cli-url]
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[![Support Chat][git-img]][git-url]
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[PostCSS image-set() Function] lets you display resolution-dependent images
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using the `image-set()` function in CSS, following the [CSS Images]
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specification.
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```pcss
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.example {
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background-image: image-set(
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url(img.png) 1x,
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url(img@2x.png) 2x,
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url(img@print.png) 600dpi
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);
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}
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/* becomes */
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.example {
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background-image: url(img.png);
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}
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@media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 192dpi) {
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.example {
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background-image: url(img@2x.png);
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}
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}
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@media (-webkit-min-device-pixel-ratio: 6.25), (min-resolution: 600dpi) {
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.example {
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background-image: url(my@print.png);
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}
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}
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.example {
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background-image: image-set(
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url(img.png) 1x,
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url(img@2x.png) 2x,
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url(img@print.png) 600dpi
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);
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}
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```
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## Usage
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Add [PostCSS image-set() Function] to your project:
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```bash
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npm install postcss-image-set-function --save-dev
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```
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Use [PostCSS image-set() Function] to process your CSS:
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```js
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const postcssImageSetFunction = require('postcss-image-set-function');
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postcssImageSetFunction.process(YOUR_CSS /*, processOptions, pluginOptions */);
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```
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Or use it as a [PostCSS] plugin:
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```js
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const postcss = require('postcss');
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const postcssImageSetFunction = require('postcss-image-set-function');
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postcss([
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postcssImageSetFunction(/* pluginOptions */)
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]).process(YOUR_CSS /*, processOptions */);
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```
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[PostCSS image-set() Function] runs in all Node environments, with special
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instructions for:
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| [Node](INSTALL.md#node) | [PostCSS CLI](INSTALL.md#postcss-cli) | [Webpack](INSTALL.md#webpack) | [Create React App](INSTALL.md#create-react-app) | [Gulp](INSTALL.md#gulp) | [Grunt](INSTALL.md#grunt) |
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| --- | --- | --- | --- | --- | --- |
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## Options
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### preserve
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The `preserve` option determines whether the original declaration using
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`image-set()` is preserved. By default, it is preserved.
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```js
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postcssImageSetFunction({ preserve: false })
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```
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```pcss
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.example {
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background-image: image-set(
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url(img.png) 1x,
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url(img@2x.png) 2x,
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url(img@print.png) 600dpi
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);
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}
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/* becomes */
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.example {
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background-image: url(img.png);
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}
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@media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 192dpi) {
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.example {
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background-image: url(img@2x.png);
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}
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}
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@media (-webkit-min-device-pixel-ratio: 6.25), (min-resolution: 600dpi) {
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.example {
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background-image: url(my@print.png);
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}
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}
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```
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### onvalid
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The `oninvalid` option determines how invalid usage of `image-set()` should be
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handled. By default, invalid usages of `image-set()` are ignored. They can be
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configured to display a `warning` or `throw` an error.
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```js
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postcssImageSetFunction({ oninvalid: 'warning' }) // warn on invalid usages
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```
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```js
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postcssImageSetFunction({ oninvalid: 'throw' }) // throw on invalid usages
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```
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## Image Resolution
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The `image-set()` function allows an author to provide multiple resolutions of
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an image and let the browser decide which is most appropriate in a given
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situation. The `image-set()` also never fails to choose an image; the
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`<resolution>` just helps determine which of the images is chosen.
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Since this plugin is not a browser, the image options are sorted by device
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pixel ratio and the lowest ratio is used as the default, while the remaining
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images are pushed behind media queries.
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Therefore, this plugin can only approximate native browser behavior. While
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images should typically match the resolution as the device they’re being viewed
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in, other factors can affect the chosen image. For example, if the user is on a
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slow mobile connection, the browser may prefer to select a lower-res image
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rather than wait for a larger, resolution-matching image to load.
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[cli-img]: https://img.shields.io/travis/jonathantneal/postcss-image-set-function.svg
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[cli-url]: https://travis-ci.org/jonathantneal/postcss-image-set-function
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[css-img]: https://cssdb.org/badge/image-set-function.svg
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[css-url]: https://cssdb.org/#image-set-function
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[git-img]: https://img.shields.io/badge/support-chat-blue.svg
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[git-url]: https://gitter.im/postcss/postcss
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[npm-img]: https://img.shields.io/npm/v/postcss-image-set-function.svg
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[npm-url]: https://www.npmjs.com/package/postcss-image-set-function
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[CSS Images]: https://drafts.csswg.org/css-images-4/#image-set-notation
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[Gulp PostCSS]: https://github.com/postcss/gulp-postcss
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[Grunt PostCSS]: https://github.com/nDmitry/grunt-postcss
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[PostCSS]: https://github.com/postcss/postcss
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[PostCSS Loader]: https://github.com/postcss/postcss-loader
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[PostCSS image-set() Function]: https://github.com/jonathantneal/postcss-image-set-function
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