Add more customisation options to Compass' sprite importer

contents.erb

@import "compass/utilities/sprites/base";

@mixin sprites($map, $sprite-names, $base-class: false, $dimensions: false, $prefix: sprite-map-name($map), $offset-x: 0, $offset-y: 0) {
  @each $sprite-name in $sprite-names {
    @if sprite_does_not_have_parent($map, $sprite-name) {
      $full-sprite-name: "#{$prefix}#{$sprite-name}";
      .#{$full-sprite-name} {
        @if $base-class { @extend #{$base-class}; }
        @include sprite($map, $sprite-name, $dimensions, $offset-x, $offset-y);
      }
    }
  }
}

// General Sprite Defaults
// You can override them before you import this file.
$<%= name %>-sprite-base-class: ".<%= name %>-sprite" !default;
$<%= name %>-sprite-dimensions: false !default;
$<%= name %>-position: 0% !default;
$<%= name %>-spacing: 0 !default;
$<%= name %>-repeat: no-repeat !default;
$<%= name %>-prefix: "<%= name %>-" !default;
$<%= name %>-clean-up: true !default;
$<%= name %>-layout:vertical !default;
$<%= name %>-inline: false !default;
$<%= name %>-offset-x: 0 !default;
$<%= name %>-offset-y: 0 !default;

<% if skip_overrides %> 
  $<%= name %>-sprites: sprite-map("<%= uri %>", $layout: $<%= name %>-layout, $cleanup: $<%= name %>-clean-up);
<% else %>  
 // These variables control the generated sprite output
 // You can override them selectively before you import this file.
 <% sprite_names.each do |sprite_name| %>
   $<%= name %>-<%= sprite_name %>-position: $<%= name %>-position !default;
   $<%= name %>-<%= sprite_name %>-spacing:  $<%= name %>-spacing !default;
   $<%= name %>-<%= sprite_name %>-repeat:   $<%= name %>-repeat !default;
 <% end %>

 $<%= name %>-sprites: sprite-map("<%= uri %>",
 <% sprite_names.each do |sprite_name| %>
   $<%= name %>-<%= sprite_name %>-position: $<%= name %>-<%= sprite_name %>-position, 
   $<%= name %>-<%= sprite_name %>-spacing:  $<%= name %>-<%= sprite_name %>-spacing, 
   $<%= name %>-<%= sprite_name %>-repeat:   $<%= name %>-<%= sprite_name %>-repeat,
 <% end %>
 $layout: $<%= name %>-layout,
 $cleanup: $<%= name %>-clean-up
 );
<% end %>

// All sprites should extend this class
// The <%= name %>-sprite mixin will do so for you.
@if $<%= name %>-inline {
  #{$<%= name %>-sprite-base-class} {
    background-image: inline-sprite($<%= name %>-sprites);
  } 
} @else {
  #{$<%= name %>-sprite-base-class} {
    background: $<%= name %>-sprites no-repeat;
  }
}
//sass functions to return the dimensions of a sprite image as units
<% [:width, :height].each do |dimension| %>
  @function <%= name %>-sprite-<%= dimension %>($name) {
    $file: sprite_file($<%= name %>-sprites, $name);
    @return image-<%= dimension %>($file);
  }
<% end %>

// Use this to set the dimensions of an element
// based on the size of the original image.
@mixin <%= name %>-sprite-dimensions($name) {
  @include sprite-dimensions($<%= name %>-sprites, $name)
}

// Move the background position to display the sprite.
@mixin <%= name %>-sprite-position($name, $offset-x: 0, $offset-y: 0) {
  @include sprite-background-position($<%= name %>-sprites, $name, $offset-x, $offset-y)
}

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

@mixin <%= name %>-sprites($sprite-names, $dimensions: $<%= name %>-sprite-dimensions, $prefix: $<%= name %>-prefix, $offset-x: 0, $offset-y: 0) {
  @include sprites($<%= name %>-sprites, $sprite-names, $<%= name %>-sprite-base-class, $dimensions, $prefix, $offset-x, $offset-y)
}

// Generates a class for each sprited image.
@mixin all-<%= name %>-sprites($dimensions: $<%= name %>-sprite-dimensions, $prefix: $<%= name %>-prefix, $offset-x: $<%= name %>-offset-x, $offset-y: $<%= name %>-offset-y) {
  @include <%= name %>-sprites(<%= sprite_names.join(" ") %>, $dimensions, $prefix, $offset-x, $offset-y);
}

example.scss

$icons-prefix: 'icon.';
$icons-spacing: 20px;
$icons-offset-x: 10px;
$icons-offset-y: 10px;

@import "icons/*.png";
@include all-icons-sprites;

/*
.icons-sprite, .icon.delete, .icon.edit, .icon.inbox {
  background: url(/assets/icons-s67b7b50949.png) no-repeat;
}

.icon.delete {
  background-position: 10px 10px;
}

.icon.edit {
  background-position: 10px -25px;
}

.icon.inbox {
  background-position: 10px -61px;
}

*/

initializer.rb

# Redefine CONTENT_TEMPLATE to parse ERB from lib/sprite_importer/contents.erb
Compass::SpriteImporter.send :remove_const, :CONTENT_TEMPLATE
Compass::SpriteImporter.const_set :CONTENT_TEMPLATE, ERB.new(File.read(File.join(Rails.root, 'lib', 'sprite_importer', 'contents.erb')))

Any instructions where to add the content.erb and initializer.rb ?