sprite placeholder generator still in development, expect performance tweaks and some clean-up

_sprite-placeholder.scss

@mixin sprite-placeholders( $glob, $dimensions: true, $layout: smart ){
	$map  : sprite-map( $glob, $layout : $layout );
	$name : sprite-map-name( $map ) + '-sprite';

	$widths  : ();
	$heights : ();

	$unified-widths  : false;
	$unified-heights : false;

	// check if they share width/height
	@if dimensions {
		@each $sprite in sprite-names( $map ){
			@if length( $widths ) < 2 or length( $heights ) < 2 {
				$width : image-width( sprite-file( $map, $sprite ));
				$height: image-height( sprite-file( $map, $sprite ));

				@if not index( $widths, $width ) {
					$widths  : append( $widths, $width );
				}
				@if not index( $heights, $height ) {
					$heights : append( $heights, $height );
				}
			}
		}

		$unified-widths : ( length( $widths ) < 2 );
		$unified-heights : ( length( $heights ) < 2 );
	}

	%#{$name} {
		background-image: $map;
		@if dimensions {
			@if $unified-widths {
				width: $widths;
			}
			@if $unified-heights {
				height: $heights;
			}
		}
		@content;
	}

	@each $sprite in sprite-names( $map ){
		%#{$name}-#{$sprite} {
			@extend %#{$name};
			background-position: sprite-position( $map, $sprite );
			@if $dimensions {
				@if not $unified-widths {
					width: image-width( sprite-file( $map, $sprite ));
				}
				@if not $unified-heights {
					height: image-height( sprite-file( $map, $sprite ));
				}
			}
		}
	}
}

sample.scss

/*
 * \foo
 *   +- bar.png
 *   +- baz.png
 *   +- foo.png
 */

@include sprite-placeholders( 'foo/*.png' ){
	display: inline-block;
	vertical-align: top;
}

/*
 * this will generate placeholders:
 * %foo-sprite ( general )
 * %foo-sprite-bar
 * %foo-sprite-baz
 * %foo-sprite-foo
 */
 
// ok, now "WTF, no!" moment:
.foo {
	@extend %foo-sprite-foo;
}
.bar {
	@extend %foo-sprite-bar;
}
.baz {
	@extend %foo-sprite-baz;
}

/* generates selector in this order:
 * .bar { ... }
 * .baz { ... }
 * .foo { ... }
 */