Picturefill

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

• Author: Scott Jehl (c) 2012 (new proposal implemented by Shawn Jansepar)
• License: MIT/GPLv2

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

Draft Specification: http://picture.responsiveimages.org/

Note: Picturefill works best in browsers that support CSS3 media queries. The demo page references (externally) the matchMedia polyfill which makes matchMedia work in media-query-supporting browsers that don't support matchMedia. 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. In non-media query-supporting browsers, the matchMedia polyfill will allow for querying native media types, such as screen, print, etc.

Markup pattern and explanation

Mark up your responsive images like this.

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

		<!-- 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"></img></noscript>
	</span>

Or when using sizes:

    <span data-picture data-alt="Obama with soldiers">
        <span data-sizes="(max-width: 30em) 100%, (max-width: 50em) 75%, 50%"
                 data-srcset="images/pic-small.png 400w, images/pic-medium.png 800w, images/pic-large.png 1200w">
		<noscript><img src="images/pic-small.jpg"></noscript>
    </span>

Explained...

Notes on the markup above...

• The span[data-picture] element's alt attribute is used as alternate text for the img element that picturefill generates upon a successful source match.
• The span[data-picture] element can contain any number of span elements. The above example may contain more than the average situation may call for.
• Each span[data-srcset] element must have a data-srcset 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 - typically a mobile-optimized image is ideal here.
• The span[data-srcset] can take in a single image (like "images/small.jpg"), or an array of images ("images/pic-small.png 0.5x, images/pic-medium.png 1x, images/pic-large.png 1.5x).
• The span[data-sizes] attribute is available to specify the size of the image at different breakpoints. Then, in data-srcset, an array of images at different widths are supplied, and based on the breakpoints defined in data-sizes, the appropriate image will be chosen. So in the example above, "images/pic-small.png 400w, images/pic-medium.png 800w, images/pic-large.png 1200w" will be converted to "images/pic-small.png 0.5x, images/pic-medium.png 1x, images/pic-large.png 1.5x" if the width of the device (in css pixels) was 800px;
• Each [data-srcset] element can have an optional [data-media] attribute to make it apply in specific media settings. Both media types and queries can be used, like a native media attribute, but support for media queries depends on the browser (unsupporting browsers fail silently).
• The matchMedia polyfill (included in the /external folder) is necessary to support the data-media attribute across browsers (such as IE9), 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 mobile optimized image here, as it's likely to be loaded in older/underpowered mobile devices.

How the img is appended

Upon finding a matching span[data-src] element, picturefill will generate an img element referencing that span's data-src attribute value and append the img to the active, matching span[data-src] element. This means you can target CSS styles specific to the active image based on the breakpoint that is in play, perhaps by adding a class to each span. For example, if you have the following markup...

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

...then you could write styles specific to each of the images, which may be handy for certain layout situations.

	.picture .sml img { /* Styles for the small image */ }
	.picture .med img { /* Styles for the medium image */ }
	.picture .lrg img { /* Styles for the large image */ }

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 being 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).

	<span data-picture data-alt="A giant stone face at The Bayon temple in Angkor Thom, Cambodia">
		<span data-src="small.jpg"></span>
		<span data-src="small_x2.jpg"      data-media="(min-device-pixel-ratio: 2.0)"></span>
		<span data-src="medium.jpg"        data-media="(min-width: 400px)"></span>
		<span data-src="medium_x2.jpg"     data-media="(min-width: 400px) and (min-device-pixel-ratio: 2.0)"></span>
		<span data-src="large.jpg"         data-media="(min-width: 800px)"></span>
		<span data-src="large_x2.jpg"      data-media="(min-width: 800px) and (min-device-pixel-ratio: 2.0)"></span>
		<span data-src="extralarge.jpg"    data-media="(min-width: 1000px)"></span>
		<span data-src="extralarge_x2.jpg" data-media="(min-width: 1000px) and (min-device-pixel-ratio: 2.0)"></span>

		<!-- 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>
	</span>

• 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:

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

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

Deferred loading

If picturefill is deferred until after load is fired, images will not load unless the browser window is resized. Picturefill is intentionally exposed to the global space, in the unusual situation where you might want to defer loading of picturefill you can explicitly call window.picturefill().

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.