The real <picture> element polyfill: picturePolyfill
Let me introduce you picturePolyfill.
This is a responsive images approach that you can use today that uses the real picture
element along with children source
elements with media
and srcset
attributes.
PicturePolyfill is fast and easy to use because:
- markup & go: it uses the picture tag, easy to markup, and futureproof
- loading performance: it serves only one image to your website users, no double HTTP requests are made
-
computing performance: it's designed and coded keeping performance in mind. For example, it doesn't execute while a smooth (animated or manually dragged) browser resize is in progress (avoiding useless DOM parsing and useless HTTP requests to mid-breakpoints images that the user might not need) and it caches the
source
elements data -
support to HD (Retina) displays easily made via the
srcset
attribute ofsource
tags
Differences with picturefill
picturePolyfill is better than picturefill because:
- it uses the real
picture
markup - it gives you the ability to choose a default image that you want to show on Internet Explorer desktop, without the need to add any comment
Markup pattern and explanation
With HD (Retina) images support
To support HD (Retina) images, mark up your responsive images like this.
<picture data-alt="A beautiful responsive image" data-default-src="img/1440x1440.gif">
<source srcset="img/480x480.gif, img/480x480x2.gif 2x"/>
<source srcset="img/768x768.gif, img/768x768x2.gif 2x" media="(min-width: 481px)"/>
<source srcset="img/1440x1440.gif, img/1440x1440x2.gif 2x" media="(min-width: 1025px)"/>
<source srcset="img/1920x1920.gif, img/1920x1920x2.gif 2x" media="(min-width: 1441px)"/>
<noscript>
<img src="img/768x768.gif" alt="A beautiful responsive image"/>
</noscript>
</picture>
Without HD (Retina) support
If you don't need to support HD (Retina) images, you can mark up your responsive images like this.
<picture data-alt="A beautiful responsive image" data-default-src="img/1440x1440.gif">
<source srcset="img/480x480.gif"/>
<source srcset="img/768x768.gif" media="(min-width: 481px)"/>
<source srcset="img/1440x1440.gif" media="(min-width: 1025px)"/>
<source srcset="img/1920x1920.gif" media="(min-width: 1441px)"/>
<noscript>
<img src="img/768x768.gif" alt="A beautiful responsive image"/>
</noscript>
</picture>
Notes about the markup
picture
tag:
* data-default-src
attribute: the image URL that you want to load in IE Desktop < 10.
* data-alt
attribute: the alternative text that will be set in the img
tag
source
tags:
* media
attribute: any media query, but it's adviced to use a min-width
media query to follow the "mobile first" approach.
* srcset
attribute: the image URL or comma separated URLs at the corresponding media
noscript
tag:
* This should wrap the fallback image for non-JavaScript environments and search engines. You could avoid wrapping the img
tag in noscript
, but this will make browsers to fetch the fallback image during page load, causing unnecessary overhead.
How the img
is appended and updated
Upon finding a matching media in the data-picture
array, picturePolyfill will generate an img
element and inside that span.
The img
's src
attribute is then updated at browser resize (see computing performance section above to read about performance at browser resize)
Server-side scaling/cropping tool
Responsive images can be quite complicated to be served on your website if you have to: pre-scale them at many different resolutions; name them; and maybe change their size when developing a new release of your site.
It's then a good practice to have a server-side picture scaling service (like pixtulate) to scale the images for you, just in time, starting from only one big image.
If you want to use an image server, you can code your HTML like the following:
<picture data-alt="A beautiful responsive image" data-default-src="http://demo.api.pixtulate.com/demo/large-2.jpg?w=1440">
<source srcset="http://demo.api.pixtulate.com/demo/large-2.jpg?w=480"/>
<source srcset="http://demo.api.pixtulate.com/demo/large-2.jpg?w=512" media="(min-width: 481px)"/>
<source srcset="http://demo.api.pixtulate.com/demo/large-2.jpg?w=720" media="(min-width: 1025px)"/>
<source srcset="http://demo.api.pixtulate.com/demo/large-2.jpg?w=960" media="(min-width: 1441px)"/>
<noscript>
<img src="http://demo.api.pixtulate.com/demo/large-2.jpg?w=1440" alt="A beautiful responsive image"/>
</noscript>
</picture>
Note that you should serve double resolution images (double width and double height) for HD/retina displays, as you can see in the "With HD (Retina) images support" section of this readme.
Installation
Manually
- Download picturePolyfill from GitHub
- Include the minified file in your project script directory
Using bower
You can install the latest version of picturePolyfill using bower
bower install picturePolyfill
Inclusion
To use picturePolyfill, just include the script tag at the end of your html file, in the head
section of your HTML
pages, OR just before the closure of the body
tag.
Including the defer
attribute in the script
tag will prevent the script download to block page rendering while in progress.
In the head
section
<html>
<head>
Your HEAD content
<script src="picturePolyfill.min.js" defer></script>
</head>
<body>
Your BODY content
</body>
</html>
At the end of the body section
<html>
<head>
Your HEAD content
</head>
<body>
Your BODY content
<script src="picturePolyfill.min.js"></script>
</body>
</html>
Execution
picturePolyfill executes automatically at page load and at browser resizes.
AJAX calls
picturePolyfill is intentionally exposed to the global space, so you can call it as you need it.
For example, if your AJAX call changes a portion of your DOM, after your new DOM has been injected on the page, just call window.picturePolyfill()
or window.picturePolyfill(theChangedElement)
to make picturePolyfill to parse a only the changed portion of the DOM.
Browser support
picturePolyfill supports all modern browsers and down to Internet Explorer 7 (it wasn't tested on IE6).
- On Modern Browsers, Internet Explorer 10 and above: the images will be loaded depending on the matched media query
- On Internet Explorer 7 to 9: the content of the
data-default-src
attribute will be used to reference the image source.
Size and delivery
Currently, picturePolyfill.js
compresses to around 910bytes (~0.88kb) after minify and gzip.