Using Compass to generate normal and retina sprite maps

compass-retina-sprites.scss

$sprites: sprite-map("sprites/*.png");
$sprites-retina: sprite-map("sprites-retina/*.png");

@mixin sprite-background($name) {
  background-image: sprite-url($sprites);
  background-position: sprite-position($sprites, $name);
  background-repeat: no-repeat;
  display: block;
  height: image-height(sprite-file($sprites, $name));
  width: image-width(sprite-file($sprites, $name));
  @media (-webkit-min-device-pixel-ratio: 2), (-o-min-device-pixel-ratio: 3/2), (min-device-pixel-ratio: 2) {
    // Workaround for https://gist.github.com/2140082
    @if (sprite-position($sprites, $name) != sprite-position($sprites-retina, $name)) {
      $ypos: round(nth(sprite-position($sprites-retina, $name), 2) / 2);
      background-position: 0 $ypos;
    }
    // Grab size of non-retina sprite image.
    @include background-size(image-width(sprite-url($sprites)) image-height(sprite-url($sprites)));
    background-image: sprite-url($sprites-retina);
  }
}

// Usage.
.mail-icon {
  @include sprite-background(mail);
}

Getting an error. Seems to be on like 18 (in my scss).

error sass/screen.scss (Line 21 of sass/include/_nav-icons.scss: Unrecognized file type: png'))

Instead of using

@include background-size(image-width(sprite-url($sprites)) image-height(sprite-url($sprites)));

try

@include background-size(image-width(sprite-path($sprites)) image-height(sprite-path($sprites)));

Here is my try to make it little short and for best practice. I wanted to use background-image property for once instead of using it in every class to minimise my code. I make this
https://gist.github.com/mbilalsiddique1/fe0098702e10c27371d1

@import "compass/utilities/sprites";         // Include compass sprite helpers
@import "compass/css3/background-size";      // Include helper to calc background size

// General Sprite Defaults
// You can override them before you import this file.
$icon-sprite-base-class: ".icon-sprite" !default;
$icon-sprite-dimensions: false !default;
$icon-spacing: 10px !default;
$icon-position: 0% !default;
$icon-repeat: no-repeat !default;

$icon-sprites: sprite-map("sprites/*.png", $spacing: $icon-spacing, $repeat: $icon-repeat, $position: $icon-position);
$icon-sprites-retina: sprite-map("sprites-retina/*.png", $spacing: $icon-spacing * 2, $repeat: $icon-repeat, $position: $icon-position);

// All sprites should extend this class
// The icon-sprite mixin will do so for you.
#{$icon-sprite-base-class} {
  background: $icon-sprites $icon-repeat;
}

@media (-webkit-min-device-pixel-ratio: 1.5), (min--moz-device-pixel-ratio: 1.5), (-o-min-device-pixel-ratio: 3 / 2), (min-device-pixel-ratio: 1.5), (min-resolution: 1.5dppx) {
  #{$icon-sprite-base-class} {
    background: $icon-sprites-retina $icon-repeat;
    @include background-size(ceil(image-width(sprite-path($icon-sprites-retina)) / 2) auto);
  }
}

// Extends the sprite base class and set the background position for the desired sprite.
// It will also apply the image dimensions if $dimensions is true.
@mixin icon-sprite($name, $dimensions: $icon-sprite-dimensions, $offset-x: 0, $offset-y: 0) {
  @extend #{$icon-sprite-base-class};
  @include sprite($icon-sprites, $name, $dimensions, $offset-x, $offset-y)
}

// Example Usage.
.icon-analytics {
    @include icon-sprite(main-sprite);
}