gistfile1.js

var fs = require('fs');
var im = require('imagemagick');
var async = require('async');

var widths = ['270', '590', '910', '1230'];
var source_dir = process.cwd()+'/source/';
var output_dir = process.cwd()+'/output/';

fs.readdir(source_dir, function(err, files){
	if (err) throw err;
	console.log(files);

	var parallelResizes = 1;
	async.mapSeries(files, function (file, cb) {
		im.identify(source_dir + file, cb);
	},
	
	function (err, identifiers) {
		var queue = async.queue(function (options, callback) {
			im.resize(options, callback);
		}, parallelResizes);
		identifiers.forEach(function (id, file) {
				filename = files[file].split('.');
				for(i = 0; i < widths.length; i++){
					aspect = id.width / id.height;
					width = widths[i];
					height = Math.round(widths[i] / aspect);
					current = {
						srcPath: source_dir + files[file],
						width: width,
						height: height,
						dstPath: output_dir + filename[0] + '_w' + width + '_h' + height + '.' + filename[1],
						quality: 1,
						progressive: true
					};
					queue.push(current);
				}
		});
		queue.drain = function () {
			console.log("resizing complete");
		}
	});
});