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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 ) );
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