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HTML 

              
                <div class="info">
  <h1>Retina Image Mixin & Media Query</h1>
  <p>Easily replace background images in css with their @2x version.</p>
  <div class="bot">
    <div class="non"></div>
    <div class="retina"></div>
  </div>
  <p class="links"><a href="https://twitter.com/michaelarestad" target="_blank">@michaelarestad</a> // <a href="https://twitter.com/see8ch" target="_blank"/>@see8ch</a>
</div>

<div class="demo non"></div>
<div class="demo retina"></div>
!

CSS 

              
                @import "compass/css3";

//  Retina Image Mixin & Media Query
//  Authors: 
//  Michael Arestad @michaelarestad
//  Chris Holder @see8ch


@mixin retina-background ($url, $file-type, $width: auto, $repeat: repeat, $ratio: 1.5, $suffix: "@2x") {
	
  background: url($url + "." + $file-type);
  background-repeat: $repeat;
  
  /* Media queries from https://github.com/thoughtbot/bourbon/blob/master/app/assets/stylesheets/css3/_hidpi-media-query.scss */
	@media only screen and (-webkit-min-device-pixel-ratio: $ratio),
    only screen and (min--moz-device-pixel-ratio: $ratio),
    only screen and (-o-min-device-pixel-ratio: #{$ratio}/1),
    only screen and (min-resolution: #{round($ratio*96)}dpi),
    only screen and (min-resolution: #{$ratio}dppx) {
		
    background: url($url + $suffix + "." + $file-type);
		background-size: $width auto;
	}
}







//  Demo Stuff
$bkgd-color: #16a085;
$shadow: darken($bkgd-color, 12%);

.info {
  position: relative;
  z-index: 2;
  width: 800px;
  margin: 50px auto;
  background: $bkgd-color;
  padding: 15px 0;
  text-align: center;
  font-family: 'helvetica';
  color: #ecf0f1;
  box-shadow: 7px 6px 0 $shadow;
  
  h1 {
    text-shadow: 2px 3px 0 $shadow;
  }
  p {
    margin-bottom: 35px;
  }
  
  .links {
    color:$shadow; 
    margin-bottom:5px;
    font-size:smaller;
  }
  
  a {  
    color:$shadow;    
    transition: all 400ms ease-in-out; 
    
    &:hover {color:#ecf0f1;}
  }
}
.bot {
  position: relative;
  width: 800px;
  height: 295px;
  border-top: 7px solid darken($shadow, 15%);
  border-bottom: 7px solid darken($shadow, 15%);
  
  .non,
  .retina {
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 70%;
  }
  .non {
    background: url(http://michaelarestad.com/images/misc/bot.jpg);
    z-index: 1;
    border-right: 1px rgba(255,255,255,.8) solid;
    overflow: hidden;
    resize: horizontal;
    max-width: 800px;
  }
  .retina {
    left: 0;
    right: 0;
    @include retina-background('http://michaelarestad.com/images/misc/bot',jpg, 800px);
  }
}
.demo {
  position: fixed;
  top: 0;
  bottom: 0;
  left: 0;
  right: 50%;
}
.demo.non {
  background: url(http://www.see8ch.com/codepen/tiny_grid.png);
  z-index: 1;
  border-right: 2px #2980b9 solid;
  overflow: hidden;
  resize: horizontal;
}
.demo.retina {
  left: 0;
  right: 0;
  @include retina-background('http://www.see8ch.com/codepen/tiny_grid',png, 26px);
}
!

JS
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