Capturar el fotograma de video actual

// Toolbar action: capture the current frame of the playing video.
(function () {
  var videos = Array.prototype.slice.call(document.querySelectorAll('video'));
  videos.sort(function (a, b) {
    return (b.clientWidth * b.clientHeight) - (a.clientWidth * a.clientHeight);
  });
  var v = videos[0];
  if (!v || !v.videoWidth) { alert('No playable video found.'); return; }
  try {
    var c = document.createElement('canvas');
    c.width = v.videoWidth;
    c.height = v.videoHeight;
    c.getContext('2d').drawImage(v, 0, 0, c.width, c.height);
    var a = document.createElement('a');
    a.href = c.toDataURL('image/png');
    a.download = 'video-frame-' + Math.round(v.currentTime) + 's.png';
    document.body.appendChild(a);
    a.click();
    a.remove();
  } catch (e) {
    alert('Could not capture this frame: ' + e.message);
  }
})();