class Middleman::Extensions::RelativeAssets

def rewrite_url(asset_path, dirpath, request_path)
  uri = ::Middleman::Util.parse_uri(asset_path)
  return if uri.path[0..0] != '/'
  relative_path = uri.host.nil?
  full_asset_path = if relative_path
    dirpath.join(asset_path).to_s
  else
    asset_path
  end
  current_dir = Pathname(request_path).dirname
  result = Pathname(full_asset_path).relative_path_from(current_dir).to_s
  result
end