Using Compass to generate normal and retina sprite maps

compass-retina-sprites.scss

@mixin sprite-background($name, $folder, $sprite-width) {
  $sprites: sprite-map("#{$folder}/*.png");
  $sprites-retina: sprite-map("#{$folder}@2x/*.png");

  background-image: sprite-url($sprites);
  background-position: sprite-position($sprites, $name);
  background-repeat: no-repeat;
  display: inline-block;
  height: image-height(sprite-file($sprites, $name));
  width: image-width(sprite-file($sprites, $name));
  @media (-webkit-min-device-pixel-ratio: 2), (-o-min-device-pixel-ratio: 3/2), (min-device-pixel-ratio: 2) {
    $ypos: round(nth(sprite-position($sprites-retina, $name), 2) / 2);
    background-position: 0 $ypos;
    // There must be a way of detecting this width.
    @include background-size($sprite-width auto);
    background-image: sprite-url($sprites-retina);
  }
}

Why min-device-ration should be 2? If a device supports min-device-ratio it supports background-size so it can scale down the 2x image. I think we can use 2x images for pixel ratios less than 2.