spritesmith retina sprite mixin

_r-sprite.scss

// http://blog.webcreativepark.net/2014/04/06-190024.html
// @include sprite($a);

@mixin r-sprite-width($sprite) {
  width: nth($sprite, 5)/2;
}

@mixin r-sprite-height($sprite) {
  height: nth($sprite, 6)/2;
}

@mixin r-sprite-position($sprite) {
  $sprite-offset-x: nth($sprite, 3)/2;
  $sprite-offset-y: nth($sprite, 4)/2;
  background-position: $sprite-offset-x  $sprite-offset-y;
}

@mixin r-sprite-size($sprite) {
  background-size: nth($sprite, 7)/2 nth($sprite, 8)/2;
}

@mixin r-sprite($sprite) {
  @include r-sprite-image($sprite);
  @include r-sprite-position($sprite);
  @include r-sprite-width($sprite);
  @include r-sprite-height($sprite);
  @include r-sprite-size($sprite);
}