@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;

    }
}
