This generates 2 css files - sprite image and a css along with a datauri css.

sprite_datauri_writer.rb

#! /usr/local/bin/ruby -w

# This simple program takes a bunch of images and creates a sprite image along with the corrosponding
# css. It also generates the datauri css. So, we can serve performance optimized CSS depending on the 
# Browsers.
#
# Browsers which do not support datauri, like IE7 and below get the sprite image and the sprite css.
# Browsers which support datauri, get the datauri.css
# 
# Usage:
# 
# css = SpriteWriter.new('/path/to/sprite/images', '/path/to/generated/sprite.img', '/path/to/generated/css')
# css.generate
#
# Creator: Jiren Patel.
# 
require 'rubygems'
require 'RMagick'
require 'rvg/rvg'
require 'base64'
require 'pathname'

class SpriteWriter

  attr_reader :dir, :image_path, :image_positions, :image_list, :css_path 
  attr_reader :height, :width

  def initialize(dir = 'public/images/sprites', image_path = 'public/images', css_path = "public/stylesheets")
    @dir = dir
    @image_path = image_path
    @css_path = css_path
    @image_list = nil
    @file_list = []
  end

  def generate
    load_files
    image_writer
    css_writer
    datauri_writer
  end


  private

  def load_files
    dir_path = File.expand_path(dir) + "/*"

    Dir[dir_path].each do |file|
      if file =~ /\.(png|gif|jpg|jpeg)$/
        img = Magick::ImageList.new(file) rescue nil

        if img
          @file_list << file 
          @image_list ? @image_list << img.first : @image_list = img
        end
      end
    end

    @height = @width = 0

    @image_list.each do |img|
      @height = @height + img.page.height + 10
      @width = img.page.width if img.page.width > @width
    end

    @height += 50
    @width += 50
  end

  def image_writer
    start_y = 10
    @image_positions = {}

    rvg = Magick::RVG.new(@width, @height) do |canvas|

      #canvas.background_fill = 'white'
      canvas.background_fill_opacity = 0

      image_list.each do |img|
        canvas.image(img, img.page.width, img.page.height, 10, start_y)
        image_file = File.basename(img.filename).split('.').first
        @image_positions[image_file] = [img.page.width, img.page.height, 10, start_y]
        start_y = start_y + 10 + img.page.height
      end

      canvas.text(10, start_y + 20, "Created by Jiren")

    end
    rvg.draw.write(@image_path + '/sprite.png')
  end

  def css_writer
    f = File.new(css_path + '/sprite.css', 'w')

    img_str = ""
    image_positions.each do |key, value|
      img_str <<  ".#{key},"
    end

    f.puts img_str[0, img_str.length - 1].to_s
    f.puts "{ background: url(/images/sprite.png) no-repeat top left; }"

    image_positions.each do |key, value|
      f.puts ".#{key} {background-position: -#{value[2]}px -#{value[3]}px; width:#{value[0]}px; height:#{value[1]}px;}"
    end

    f.close
  end

  def datauri_writer

    f = File.new(css_path + '/sprite_datauri.css', 'wb')

    @file_list.each do |file|
      image_path = File.basename(file).split('.').first
      ext = File.basename(file).split('.').last

      width_height = "width:#{@image_positions[image_path][0]}px; height:#{@image_positions[image_path][1]}px;"

      if(file.size > 32767)
        f.puts ".#{image_path} { background: url(/images/#{image_path}.#{ext});top left no-repeat; #{width_height}}"
      else
        f.puts  ".#{image_path} { background :url(\"data:image/#{ext};charset=utf-8;base64,#{read_file_and_encode(file)}\");top left no-repeat; #{width_height}}"
      end

    end
  
     f.close
  end

  def read_file_and_encode(file)
    data = File.open(file, 'rb') {|f| f.read }
    Base64.encode64(data).gsub(/\n/, '')
  end

end