retina sprite mixin with Compass

retina.sprite.scss

@mixin icon-sprite-2x($sprite) {
  $sprites: sprite-map("icon/*.png");
  text-indent: -9999px !important;
  @include icon-sprite($sprite);
  background-position: 0 round(nth(sprite-position($sprites, $sprite), 2) / 2);
  height: image-height(sprite-file($sprites, $sprite)) / 2;
  width: image-width(sprite-file($sprites, $sprite)) / 2;
  background-size: (image-width(sprite-path($icon-sprites)) / 2) auto;
  .ie & {
    background-image: image-url("icon-1x/#{$sprite}.png") !important;
    background-position: 0 round(nth(sprite-position($sprites, $sprite), 2));
  }
}

// usage
.github { @include icon-sprite-2x(github) }