Single-resource retina sprites

retina_icons.scss

$icons: sprite-map("icons/*.png", $spacing: 3px);

@mixin responsive-sprite($icon) {
    $iconname: "icons/#{$icon}.png";
    $iconwidth: round(image-width($iconname)/2);
    $iconheight: round(image-height($iconname)/2);

    $pos: sprite-position($icons, $icon);
    $posx: round(nth($pos, 1)/2);
    $posy: round(nth($pos, 2)/2);

    width: $iconwidth;
    height: $iconheight;
    background: sprite-url($icons) $posx $posy no-repeat;
    @include background-size(round(image-width(sprite-path($icons)) / 2) round(image-height(sprite-path($icons)) / 2));
}