WebSocket(Socket.IO) で受け取った Data URL 形式の画像をダウンロードさせる

app.js

var express = require('express'),
    io = require('socket.io'),
    fs = require('fs');

var app = module.exports = express.createServer();

// Configuration

app.configure(function(){
  app.set('views', __dirname + '/views');
  app.set('view engine', 'ejs');
  app.use(express.bodyParser());
  app.use(express.methodOverride());
  app.use(app.router);
  app.use(express.static(__dirname + '/public'));
});

app.configure('development', function(){
  app.use(express.errorHandler({ dumpExceptions: true, showStack: true })); 
});

app.configure('production', function(){
  app.use(express.errorHandler()); 
});

// Routes

app.get('/', function(req, res){
  res.render('index', {
    title: 'Express'
  });
});

// Only listen on $ node app.js

if (!module.parent) {
  app.listen(3000);
  console.log("Express server listening on port %d", app.address().port);

  var socket = io.listen(app);
  socket.on('connection', function(client) {
    client.on('message', function(message) {
      fs.readFile('./images/test.jpg', function (err, data) {
        if (err) {
	  throw err;
        }

	var prefix = 'data:image/jpeg;base64,',
	    base64 = new Buffer(data, 'binary').toString('base64'),
	    data = prefix + base64;

	client.send(data);
      });
    });
  });

}

dataurl-download.js

document.addEventListener('DOMContentLoaded', function() {
	var screen, screenContext, socket, downloadLink;

	screen = document.createElement('canvas');
	screen.width = 1600;
	screen.height = 1200;
	screenContext = screen.getContext('2d');

	socket = new io.Socket('localhost');
	socket.connect();

	downloadLink = document.getElementById('download');

	// data は base64 でエンコードされた JPEG 画像データ
	socket.on('message', function(data) {
		var img = new Image();
		img.onload = function() {
			// 縮小して、PNGに変換
			screenContext.drawImage(img, 0, 0, 800, 600);
			img = null;
			downloadLink.href = screen.toDataURL('image/png');
		};
		img.src = data;
	});

	// サーバーから画像取得
	socket.send('get test.jpg');
	
});