Picturefill

A Responsive Images approach that you can use today, that mimics the proposed picture element using divs, for safety sake.

• Author: Scott Jehl (c) 2012
• License: MIT/GPLv2

Demo URL: http://scottjehl.github.com/picturefill/

Note: Picturefill works best in browsers that support CSS3 media queries. It includes (externally) the matchMedia polyfill which makes matchMedia work in media-query-supporting browsers that don't have matchMedia, or at least allows media types to be tested in most any browser. matchMedia and the matchMedia polyfill are not required for picturefill to work, but they are required to support the media attributes on picture source elements.

Size and delivery

Currently, picturefill.js compresses to around 498bytes (~0.5kb), after minify and gzip. To minify, you might try these online tools: [Uglify]:(http://marijnhaverbeke.nl/uglifyjs), [Yahoo Compressor]:(http://refresh-sf.com/yui/), or Closure Compiler. Serve with gzip compression.

Markup pattern and explanation

Mark up your responsive images like this.

	<div data-picture data-alt="A giant stone face at The Bayon temple in Angkor Thom, Cambodia">
		<div data-src="small.jpg"></div>
		<div data-src="medium.jpg"     data-media="(min-width: 400px)"></div>
		<div data-src="large.jpg"      data-media="(min-width: 800px)"></div>
		<div data-src="extralarge.jpg" data-media="(min-width: 1000px)"></div>

		<!-- Fallback content for non-JS browsers. Same img src as the initial, unqualified source element. -->
		<noscript>
			<img src="external/imgs/small.jpg" alt="A giant stone face at The Bayon temple in Angkor Thom, Cambodia">
		</noscript>
	</div>

Each div[data-src] element’s data-media attribute accepts any and all CSS3 media queries—such as min or max width, or even min-device-pixel-ratio for HD (retina) displays.

Explained...

Notes on the markup above...

• The div[data-picture] element's alt attribute is used as alternate text for the generated img element.
• The div[data-picture] element can have any number of source elements. The above example may contain more than the average situation would call for.
• Each div[data-src] element must have a data-src attribute specifying the image path.
• It's generally a good idea to include one source element with no media qualifier, so it'll apply everywhere.
• Each data-src element can have an optional media attribute to make it apply in different media settings. Both media types and queries can be used, like any media attribute, but support for media queries depends on the browser (unsupporting browsers fail silently).
• The matchMedia polyfill (included in /external) is necessary to support the media attribute across browsers, even in browsers that support media queries, although it is becoming more widely supported in new browsers.
• The noscript element wraps the fallback image for non-JavaScript environments, and including this wrapper prevents browsers from fetching the fallback image during page load (causing unnecessary overhead). Generally, it's a good idea to reference a small image here, as it's likely to be loaded in older/underpowered mobile devices.

HD Media Queries

Picturefill natively supports HD(Retina) image replacement. While numerous other solutions exist, picturefill has the added benefit of performance for the user in only getting served one image.

• The data-media attribute supports compound media queries, allowing for very specific behaviors to emerge. For example, a data-media="(min-width: 400px) and (min-device-pixel-ratio: 2.0) attribute can be used to serve a higher resolution version of the source instead of a standard definition image. Note you currently also need to add the -webkit-min-device-pixel-ratio prefix (e.g. for iOS devices).

	<div data-picture data-alt="A giant stone face at The Bayon temple in Angkor Thom, Cambodia">
		<div data-src="small.jpg"></div>
		<div data-src="small.jpg"         data-media="(min-device-pixel-ratio: 2.0)"></div>
		<div data-src="medium.jpg"        data-media="(min-width: 400px)"></div>
		<div data-src="medium_x2.jpg"     data-media="(min-width: 400px) and (min-device-pixel-ratio: 2.0)"></div>
		<div data-src="large.jpg"         data-media="(min-width: 800px)"></div>
		<div data-src="large_x2.jpg"      data-media="(min-width: 800px) and (min-device-pixel-ratio: 2.0)"></div>	
		<div data-src="extralarge.jpg"    data-media="(min-width: 1000px)"></div>
		<div data-src="extralarge_x2.jpg" data-media="(min-width: 1000px) and (min-device-pixel-ratio: 2.0)"></div>	

		<!-- Fallback content for non-JS browsers. Same img src as the initial, unqualified source element. -->
		<noscript>
			<img src="external/imgs/small.jpg" alt="A giant stone face at The Bayon temple in Angkor Thom, Cambodia">
		</noscript>
	</div>

• Note: Supporting this many breakpoints quickly adds size to the DOM and increases implementation and maintenance time, so use this technique sparingly.

Supporting IE Desktop

Internet Explorer 8 and older have no support for CSS3 Media Queries, so in the examples above, IE will receive the first data-src image reference (or the last one it finds that has no data-media attribute. If you'd like to serve a larger image to IE desktop browsers, you might consider using conditional comments, like this:

	<div data-picture data-alt="A giant stone face at The Bayon temple in Angkor Thom, Cambodia">
		<div data-src="small.jpg"></div>
		<div data-src="medium.jpg" data-media="(min-width: 400px)"></div>

		<!--[if (lt IE 9) & (!IEMobile)]>
		    <div data-src="medium.jpg"></div>
		<![endif]-->

		<!-- Fallback content for non-JS browsers. Same img src as the initial, unqualified source element. -->
		<noscript>
			<img src="small.jpg" alt="A giant stone face at The Bayon temple in Angkor Thom, Cambodia">
		</noscript>
	</div>

Support

Picturefill supports a broad range of browsers and devices (there are currently no known unsupported browsers), provided that you stick with the markup conventions provided.