retina_image_tag.rb

# image_uploader.rb
version :small_3x                          do process resize_to_limit: [900, 900] end
version :small_2x, from_version: :small_3x do process resize_to_limit: [600, 600] end
version :small,    from_version: :small_2x do process resize_to_limit: [300, 300] end

version :thumb_3x, from_version: :small    do process resize_to_limit: [150, 150] end
version :thumb_2x, from_version: :thumb_3x do process resize_to_limit: [100, 100] end
version :thumb,    from_version: :thumb_2x do process resize_to_limit: [50, 50]   end

# application_helper.rb
def retina_image_tag(uploader, version, options={})
  options.symbolize_keys!
  fallback = options.delete(:fallback)

  return image_tag(fallback, options) unless uploader

  options[:srcset] ||=  (2..3).map do |multiplier|
                          name = "#{version}_#{multiplier}x"
                          if uploader.version_exists?(name) && source = uploader.url(name).presence
                            "#{source} #{multiplier}x"
                          else
                            nil
                          end
                        end.compact.join(', ')

  image_tag(uploader.url(version).presence || fallback, options)
end