Compass mixin that lets you set a background-size in em, even on a magic sprite image.

gistfile1.txt

/** @subsection EM calculator  			*/
/** cf. https://gist.github.com/ijy/1441967	*/

@function em($target, $context: $base-font-size)
{
   @if $target == 0 { @return 0 }
   @return 1em * $target / $context;
}

/** 
 * @subsection Background Size in EM
 * @author Marie Guillaumet
 */
@mixin em-background-size($image, $context: $fs, $sprite-map: false)
{
  
	@if $sprite-map == true
	{
		/** 
		 * Let's retrieve the sprite image's dimensions
		 * @author https://groups.google.com/forum/?fromgroups=#!topic/compass-users/EnGxxzXDZp4
		 */
		$image-width:	image-width(sprite-path($image));
		$image-height:	image-height(sprite-path($image));
		
		@include background-size( em($image-width,$context) em($image-height,$context) );
	}
	@else
	{
		$image-width:	image-width($image);
		$image-height:	image-height($image);
		
		@include background-size( em($image-width,$context) em($image-height,$context) );
	}	
}