Modernizr retina background sass mixin

_background-image-retina.scss

@mixin background-image-retina($file, $type) {

  $image-normal: $file + '.' + $type;
  $image-retina: $file + '@2x.' + $type;

  $height: image-height($image-normal);
  $width: image-width($image-normal);

  background: image-url($image-normal) no-repeat 0 0;
  height: $height;
  width: $width;

  .backgroundsize & {
    background-image: image-url($image-retina);
    background-size: $width $height;
  }
}