gistfile1.js

var rework = require('rework');
var stdin = require('stdin');
var gm = require('gm');

stdin(function(str){
  rework(str)
  .use(rework.at2x())
  .use(rework.url(retina))
  .toString();
});

function retina(url){
  if (!url.contains('@2x')) return;
  var retinaPath = url;
  var real = remove('@2x', retinaPath);
  var path = resolve(real);

  gm(path)
  .size(realWidth / 2)
  .write(resolve(url), function(){});
}