Using Compass to generate normal and retina sprite maps

compass-retina-sprites.scss

/**
 * Setup images sprite having high resolution variant for retina displays.
 *
 * It will create class names for all available icons.
 *
 * This uses custom function to prefix selectors from array. To get it working you will need to include following snippet in your config.rb
 *
 *     module Sass::Script::Functions
 *         def prefix_each(array, prefix)
 *             return Sass::Script::String.new array.to_a.map{|item| prefix.value + item.value}.join(", ")
 *         end
 *         declare :join_prefixed, :args => [:array, :string]
 *     end
 */

@mixin retina-sprite-item($sprite, $sprite-2x, $name) {
	background-position: sprite-position($sprite, $name);
	height: image-height(sprite-file($sprite, $name));
	width: image-width(sprite-file($sprite, $name));

	// Workaround for https://gist.github.com/2140082
	@if (sprite-position($sprite, $name) != sprite-position($sprite-2x, $name)) {
		@media (-webkit-min-device-pixel-ratio: 1.5), (-o-min-device-pixel-ratio: 3/2), (min-device-pixel-ratio: 1.5) {
			background-position: 0 round(nth(sprite-position($sprite-2x, $name), 2) / 2);
			height: round(image-height(sprite-file($sprite-2x, $name)) / 2);
			width: round(image-width(sprite-file($sprite-2x, $name)) /2 );
		}
	}
}


@mixin retina-sprite($glob, $glob-2x, $prefix) {
	$map: sprite-map($glob, $spacing: 1px);
	$map-2x: sprite-map($glob-2x, $spacing: 2px);
	$all-items: sprite-names($map);

	#{prefix-each($all-items, ".#{$prefix}-")} {
		background-image: sprite-url($map);
		background-repeat: no-repeat;
		display: block;
		@media (-webkit-min-device-pixel-ratio: 1.5), (-o-min-device-pixel-ratio: 3/2), (min-device-pixel-ratio: 1.5) {
			background-image: sprite-url($map-2x);
			@include background-size(ceil(image-width(sprite-path($map-2x)) / 2), auto);
		}
	}

	@each $name in $all-items{
		.#{$prefix}-#{$name} {
			@include retina-sprite-item($map, $map-2x, #{$name});
		}
	}
}


@include retina-sprite("icons/*.png", "icons-2x/*.png", "icon");

Thank you for this. Pure genius!

Thanks, that's incredibly useful!

This line doesn't work though:
@if (sprite-position($sprite, $name) != sprite-position($sprite-2x, $name))
because when the y-axis positions of 1x and 2x images in the sprite are equal, the background-position of the 2x will end up half that, because you're "scaling it down" in css terms:
(background-position: 0 round(nth(sprite-position($sprite-2x, $name), 2) / 2))

What about multi sprite? 1 sprite for common, 1 sprite for homepage...