server.js

/**
 * @see http://www.tysoncadenhead.com/blog/exporting-canvas-animation-to-mov/
 * @see https://www.ffmpeg.org/
 *
 * ffmpeg -r 30 -i /tmp/frame-%04d.png -vcodec libx264 -vpre lossless_slow -threads 0 output.mp4
 * -r 30                    Framerate 30 FPS.
 * -i /tmp/frame-%04d.png   Input image sequence with four digit index.
 * -vcodec libx264
 * -vpre lossless_slow      Preset file.
 * -threads 0
 * output.mp4               Output file name.
 *
 * These work:
 * ffmpeg -r 30 -i tmp/frame-%04d.png -vcodec libx264 -threads 0 output.mp4
 * ffmpeg -f image2 -framerate 30 -i tmp/frame-%04d.png output.mp4
 */

const express = require('express');
const app = express();
const port = process.env.PORT || 3012;
const server = require('http').createServer(app);
const path = require('path');
const io = require('socket.io').listen(server);
const fs = require('fs');

// listen for HTTP requests on specified port
server.listen(port, () => {
  console.log('listening on http://localhost:%d', port);
});

// allow front-end access to node_modules folder
app.use('/scripts', express.static(`${__dirname}/node_modules/`));

// set public folder as root
app.use(express.static('public'));

app.get('/img/fs-img', function (req, res) {
  const url = `${req.query.dir}${req.query.img}`;
  res.sendFile(path.resolve(url));
});

app.get('/fs-img', function (req, res) {
  const url = `${req.query.dir}${req.query.img}`;
  res.sendFile(path.resolve(url));
});

io.sockets.on('connection', function (socket) {
  socket.on('render-frame', function (data) {

    // pad frame number with zreos so it's four characters in length
    data.frame = (data.frame <= 99999) ? ('0000' + data.frame).slice(-5) : '99999';

    // get rid of the data:image/png;base64 at the beginning of the file data
    data.file = data.file.split(',')[1];
    var buffer = Buffer.from(data.file, 'base64');
    fs.writeFile(__dirname + '/rendered/frame_' + data.frame + '.png',
      buffer.toString('binary'),
      'binary',
      err => {
        if (err) {
          console.log('An error occurred: ', err);
          throw err;
        }
      });
  });
});