image-2x sass mixin

image-2x.scss

@mixin image-2x($image, $width: 'auto', $height: 'auto') {
  @media (min--moz-device-pixel-ratio: 1.3),
         (-o-min-device-pixel-ratio: 2.6/2),
         (-webkit-min-device-pixel-ratio: 1.3),
         (min-device-pixel-ratio: 1.3),
         (min-resolution: 1.3dppx) {
    background-image: url($image);
    @if $width == 'auto' {
      background-size: contain;
    } @else {
      background-size: $width $height;
    }
  }
}