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+/**
+ * Responsive images based on 'srcset' and 'window.devicePixelRatio' emulation where needed.
+ *
+ * Call $().hidpi() on a document or part of a document to replace image srcs in that section.
+ *
+ * $.devicePixelRatio() can be used to supplement window.devicePixelRatio with support on
+ * some additional browsers.
+ */
+( function ( $ ) {
+
+/**
+ * Detect reported or approximate device pixel ratio.
+ * 1.0 means 1 CSS pixel is 1 hardware pixel
+ * 2.0 means 1 CSS pixel is 2 hardware pixels
+ * etc
+ *
+ * Uses window.devicePixelRatio if available, or CSS media queries on IE.
+ *
+ * @method
+ * @returns {number} Device pixel ratio
+ */
+$.devicePixelRatio = function () {
+ if ( window.devicePixelRatio !== undefined ) {
+ // Most web browsers:
+ // * WebKit (Safari, Chrome, Android browser, etc)
+ // * Opera
+ // * Firefox 18+
+ return window.devicePixelRatio;
+ } else if ( window.msMatchMedia !== undefined ) {
+ // Windows 8 desktops / tablets, probably Windows Phone 8
+ //
+ // IE 10 doesn't report pixel ratio directly, but we can get the
+ // screen DPI and divide by 96. We'll bracket to [1, 1.5, 2.0] for
+ // simplicity, but you may get different values depending on zoom
+ // factor, size of screen and orientation in Metro IE.
+ if ( window.msMatchMedia( '(min-resolution: 192dpi)' ).matches ) {
+ return 2;
+ } else if ( window.msMatchMedia( '(min-resolution: 144dpi)' ).matches ) {
+ return 1.5;
+ } else {
+ return 1;
+ }
+ } else {
+ // Legacy browsers...
+ // Assume 1 if unknown.
+ return 1;
+ }
+};
+
+/**
+ * Implement responsive images based on srcset attributes, if browser has no
+ * native srcset support.
+ *
+ * @method
+ * @returns {jQuery} This selection
+ */
+$.fn.hidpi = function () {
+ var $target = this,
+ // @todo add support for dpi media query checks on Firefox, IE
+ devicePixelRatio = $.devicePixelRatio(),
+ testImage = new Image();
+
+ if ( devicePixelRatio > 1 && testImage.srcset === undefined ) {
+ // No native srcset support.
+ $target.find( 'img' ).each( function () {
+ var $img = $( this ),
+ srcset = $img.attr( 'srcset' ),
+ match;
+ if ( typeof srcset === 'string' && srcset !== '' ) {
+ match = $.matchSrcSet( devicePixelRatio, srcset );
+ if (match !== null ) {
+ $img.attr( 'src', match );
+ }
+ }
+ });
+ }
+
+ return $target;
+};
+
+/**
+ * Match a srcset entry for the given device pixel ratio
+ *
+ * @param {number} devicePixelRatio
+ * @param {string} srcset
+ * @return {mixed} null or the matching src string
+ *
+ * Exposed for testing.
+ */
+$.matchSrcSet = function ( devicePixelRatio, srcset ) {
+ var candidates,
+ candidate,
+ bits,
+ src,
+ i,
+ ratioStr,
+ ratio,
+ selectedRatio = 1,
+ selectedSrc = null;
+ candidates = srcset.split( / *, */ );
+ for ( i = 0; i < candidates.length; i++ ) {
+ candidate = candidates[i];
+ bits = candidate.split( / +/ );
+ src = bits[0];
+ if ( bits.length > 1 && bits[1].charAt( bits[1].length - 1 ) === 'x' ) {
+ ratioStr = bits[1].substr( 0, bits[1].length - 1 );
+ ratio = parseFloat( ratioStr );
+ if ( ratio <= devicePixelRatio && ratio > selectedRatio ) {
+ selectedRatio = ratio;
+ selectedSrc = src;
+ }
+ }
+ }
+ return selectedSrc;
+};
+
+}( jQuery ) );