@mixin retina-image($filename, $background-size, $extension: png, $retina-filename: null, $asset-pipeline: false) {

@if $asset-pipeline {
  background-image: image-url("#{$filename}.#{$extension}");
}
@else {
  background-image: url("#{$filename}.#{$extension}");
}

@include hidpi {

  @if $asset-pipeline {
    @if $retina-filename {
      background-image: image-url("#{$retina-filename}.#{$extension}");
    }
    @else {
      background-image: image-url("#{$filename}@2x.#{$extension}");
    }
  }

  @else {
    @if $retina-filename {
      background-image: url("#{$retina-filename}.#{$extension}");
    }
    @else {
      background-image: url("#{$filename}@2x.#{$extension}");
    }
  }

  background-size: $background-size;

}

}