Don't rely on reading the environment within the sprite code. Instead accept variable keyword args.

lib/compass/sass_extensions/functions/sprites.rb

@@ -3,16 +3,32 @@ require 'chunky_png'
 module Compass::SassExtensions::Functions::Sprites
   SASS_NULL = Sass::Script::Number::new(0)
   
-  def generate_sprite_image(uri)
+  # Provides a consistent interface for getting a variable in ruby
+  # from a keyword argument hash that accounts for underscores/dash equivalence
+  # and allows the caller to pass a symbol instead of a string.
+  module VariableReader
+    def get_var(variable_name)
+      self[variable_name.to_s.gsub(/-/,"_")]
+    end
+  end
+
+  def generate_sprite_image(uri, kwargs = {})
+    kwargs.extend VariableReader
     path, name = Compass::Sprites.path_and_name(uri.value)
     last_spacing = 0
     width = 0
     height = 0
+
+    # Get image metadata
+    Compass::Sprites.discover_sprites(uri.value).each do |file|
+      Compass::Sprites.compute_image_metadata! file, path, name
+    end
+
     images = Compass::Sprites.sprites(path, name)
 
     # Calculation
     images.each do |image|
-      current_spacing = number_from_var("#{name}-#{image[:name]}-spacing")
+      current_spacing = number_from_var(kwargs, "#{image[:name]}-spacing", 0)
       if height > 0
         height += [current_spacing, last_spacing].max
       end
@@ -27,14 +43,18 @@ module Compass::SassExtensions::Functions::Sprites
     images.each do |image|
       input_png  = ChunkyPNG::Image.from_file(image[:file])
       
-      position = environment.var("#{name}-#{image[:name]}-position")
+      position = kwargs.get_var("#{image[:name]}-position") || Sass::Script::Number.new(0, ["%"])
       if position.unit_str == "%"
         image[:x] = (width - image[:width]) * (position.value / 100)
       else
         image[:x] = position.value
       end
       
-      repeat = environment.var("#{name}-#{image[:name]}-repeat").to_s
+      repeat = if (var = kwargs.get_var("#{image[:name]}-repeat"))
+        var.value
+      else
+        "no-repeat"
+      end
       if repeat == "no-repeat"
         output_png.replace input_png, image[:x], image[:y]
       else
@@ -49,6 +69,7 @@ module Compass::SassExtensions::Functions::Sprites
     
     sprite_url(uri)
   end
+  Sass::Script::Functions.declare :generate_sprite_image, [:uri], :var_kwargs => true
   
   def sprite_image(uri, x_shift = SASS_NULL, y_shift = SASS_NULL, depricated_1 = nil, depricated_2 = nil)
     check_spacing_deprecation uri, depricated_1, depricated_2
@@ -80,11 +101,12 @@ module Compass::SassExtensions::Functions::Sprites
   
 private
   
-  def number_from_var(var_name)
+  def number_from_var(kwargs, var_name, default_value)
-    if var = environment.var(var_name)
+    if number = kwargs.get_var(var_name)
-      var.value
+      assert_type number, :Number
+      number.value
     else
-      0
+      default_value
     end
   end
   

lib/compass/sprites.rb

@@ -28,6 +28,25 @@ module Compass
           raise Compass::Error, %Q(`@import` statement missing. Please add `@import "#{path}/*.png";`.)
         end
       end
+
+      def discover_sprites(uri)
+        glob = File.join(Compass.configuration.images_path, uri)
+        Dir.glob(glob).sort
+      end
+
+      def compute_image_metadata!(file, path, name)
+        width, height = Compass::SassExtensions::Functions::ImageSize::ImageProperties.new(file).size
+        sprite_name = File.basename(file, '.png');
+        unless sprite_name =~ /\A#{Sass::SCSS::RX::IDENT}\Z/
+          raise Sass::SyntaxError, "#{sprite_name} must be a legal css identifier"
+        end
+        sprites(path, name, true) << {
+          :name => sprite_name,
+          :file => file,
+          :height => height,
+          :width => width
+        }
+      end
     end
 
     def images
@@ -37,68 +56,78 @@ module Compass
     def find(uri, options)
       if uri =~ /\.png$/
         self.path, self.name = Compass::Sprites.path_and_name(uri)
-        glob = File.join(Compass.configuration.images_path, uri)
-        Dir.glob(glob).sort.each do |file|
-          width, height = Compass::SassExtensions::Functions::ImageSize::ImageProperties.new(file).size
-          images << {
-            :name => File.basename(file, '.png'),
-            :file => file,
-            :height => height,
-            :width => width
-          }
-        end
-
-        contents = <<-SCSS
-          $#{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;
-
-          #{images.map do |sprite| 
-            <<-SCSS
-              $#{name}-#{sprite[:name]}-position: $#{name}-position !default;
-              $#{name}-#{sprite[:name]}-spacing: $#{name}-spacing !default;
-              $#{name}-#{sprite[:name]}-repeat: $#{name}-repeat !default;
-            SCSS
-          end.join}
-        
-          \#{$#{name}-sprite-base-class} {
-            background: generate-sprite-image("#{uri}") no-repeat;
-          }
-        
-          @mixin #{name}-sprite-dimensions($sprite) {
-            height: image-height("#{name}/\#{$sprite}.png");
-            width: image-width("#{name}/\#{$sprite}.png");
-          }
-          
-          @mixin #{name}-sprite-position($sprite, $x: 0, $y: 0) {
-            background-position: sprite-position("#{path}/\#{$sprite}.png", $x, $y);  
-          }
-        
-          @mixin #{name}-sprite($sprite, $dimensions: $#{name}-sprite-dimensions, $x: 0, $y: 0) {
-            @extend \#{$#{name}-sprite-base-class};
-            @include #{name}-sprite-position($sprite, $x, $y);
-            @if $dimensions {
-              @include #{name}-sprite-dimensions($sprite);
-            }
-          }
-        
-          @mixin all-#{name}-sprites {
-            #{images.map do |sprite| 
-              <<-SCSS
-                .#{name}-#{sprite[:name]} {
-                  @include #{name}-sprite("#{sprite[:name]}");
-                }
-              SCSS
-            end.join}
-          }
-        SCSS
         options.merge! :filename => name, :syntax => :scss, :importer => self
-        Sass::Engine.new(contents, options)
+        image_names = Compass::Sprites.discover_sprites(uri).map{|i| File.basename(i, '.png')}
+        Sass::Engine.new(content_for_images(uri, name, image_names), options)
       end
     end
 
+    def content_for_images(uri, name, images)
+      <<-SCSS
+// 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;
+$#{name}-repeat: no-repeat !default;
+
+// These variables control the generated sprite output
+// You can override them selectively before you import this file.
+#{images.map do |sprite_name| 
+<<-SCSS
+$#{name}-#{sprite_name}-position: $#{name}-position !default;
+$#{name}-#{sprite_name}-spacing: $#{name}-spacing !default;
+$#{name}-#{sprite_name}-repeat: $#{name}-repeat !default;
+SCSS
+end.join}
+
+// All sprites should extend this class
+// The #{name}-sprite mixin will do so for you.
+\#{$#{name}-sprite-base-class} {
+  background: generate-sprite-image("#{uri}",
+#{images.map do |sprite_name| 
+%Q{    $#{sprite_name}-position: $#{name}-#{sprite_name}-position,
+    $#{sprite_name}-spacing: $#{name}-#{sprite_name}-spacing,
+    $#{sprite_name}-repeat: $#{name}-#{sprite_name}-repeat}
+end.join(",\n")}) no-repeat;
+}
+
+// Use this to set the dimensions of an element
+// based on the size of the original image.
+@mixin #{name}-sprite-dimensions($sprite) {
+  height: image-height("#{name}/\#{$sprite}.png");
+  width: image-width("#{name}/\#{$sprite}.png");
+}
+
+// Move the background position to display the sprite.
+@mixin #{name}-sprite-position($sprite, $x: 0, $y: 0) {
+  background-position: sprite-position("#{path}/\#{$sprite}.png", $x, $y);  
+}
+
+// Extends the sprite base class and set the background position for the desired sprite.
+// It will also apply the image dimensions if $dimensions is true.
+@mixin #{name}-sprite($sprite, $dimensions: $#{name}-sprite-dimensions, $x: 0, $y: 0) {
+  @extend \#{$#{name}-sprite-base-class};
+  @include #{name}-sprite-position($sprite, $x, $y);
+  @if $dimensions {
+    @include #{name}-sprite-dimensions($sprite);
+  }
+}
+
+// Generates a class for each sprited image.
+@mixin all-#{name}-sprites {
+#{images.map do |sprite_name| 
+<<-SCSS
+  .#{name}-#{sprite_name} {
+    @include #{name}-sprite("#{sprite_name}");
+  }
+SCSS
+end.join}
+}
+      SCSS
+    end
+
     def key(uri, options)
       [self.class.name + ":" + File.dirname(File.expand_path(uri)),
         File.basename(uri)]