Railsアプリでsprite-factoryを使ってRetina対応したい気持ち ref: http://qiita.com/Layzie/items/d4268b8d76d668d3f232

_sprite.scss

.ico-hoge-sp {
  background-image: image-url('sprite@2.png');
  width: 109px;
  height: 29px;
  background-position: 0px -103px;
  background-repeat: no-repeat;
  background-size: 322px auto;
}

assets.rake

require 'sprite_factory'
require 'fastimage'

namespace :assets do
  namespace :sprite do
    desc 'recreate sprite images and css for SP'
    task :sp do
      SpriteFactory.run!(
        'app/assets/images/sprite',
        :layout => 'packed',
        :style => 'scss',
        :cssurl => "image-url('sprite/$IMAGE')",
        :output_style => 'app/assets/stylesheets/_sprite@2.scss',
        :output_image => 'app/assets/images/sprite@2.png'
      ) do |images|
        images.map do |name, data|
          sprite_size = FastImage.size('app/assets/images/sprite@2.png')
          <<EOF
.ico-#{name}-sp {
  background-image: image-url('sprite@2.png');
  width: #{data[:cssw] / 2}px;
  height: #{data[:cssh] / 2}px;
  background-position: #{data[:cssx] / -2}px #{data[:cssy] / -2}px;
  background-repeat: no-repeat;
  background-size: #{sprite_size[0] / 2}px auto;
}
EOF
        end.join("\n")
      end
    end
  end
end