Sass mixin for scaling elements to an aspect ratio, useful for responsive background images

sass_scale_aspect_ratio

@function strip-unit($num) {
	@if unitless( $num ) != true {
		@return $num / ($num * 0 + 1);
	}
	@else {
		@return $num;
	}
}

// Shortcut for scaling elements to an aspect ratio, useful for responsive background images
// Takes: the image's actual dimensions (any units as long as they are consistent)
// Outputs: a height: 0 and an appropriate height as a padding-bottom percentage

@mixin scale-aspect-ratio($img_width, $img_height) {
	@if type-of( $img_width ) == number and type-of( $img_height ) == number {
		$img_width: strip-unit( $img_width );
		$img_height: strip-unit( $img_height );
		
		$padding-height: percentage( $img_height / $img_width );

		height: 0;
		padding-bottom: $padding-height;
	}
	@else {
		@warn "Either #{$img_width} or #{$img_height} are not numbers"
	}
}