Retina 対応 mixin

gistfile1.scss

$spriteName: sprite-map("folder/*.png", $spacing: 5px);

@mixin sprite-hoge($name){
  background-image: $spriteName;
  background-repeat: no-repeat;
  width: image-width(sprite-file($spriteName, $name)) / 2;
  height: image-height(sprite-file($spriteName, $name)) / 2;
  $ypos: round(nth(sprite-position($spriteName, $name), 2) / 2);
  background-position: 0 $ypos;
  background-size: round(image-width(sprite-path($spriteName)) / 2) round(image-height(sprite-path($spriteName)) / 2);  
}

.no1{
  @include sprite-hoge($name);
}

round ってのは四捨五入らしい。
http://kanapple.net/study/archives/17

nth について
http://sass-lang.com/docs/yardoc/Sass/Script/Functions.html#nth-instance_method

各関数について
http://develo.org/sass_compass/#/house8