Gets the data URIs for the images at specific points within a video (MP4 or MKV).

getVideoImages.js

/**
 * Gets one or more images from a video at the given times.  Works for MP4s and
 * at times MKVs.
 * @param {string} path
 *     URL path to the video.
 * @param {Array.<number|Function>|number|Function} times
 *     If a function it will be passed the duration and the return value will be
 *     used as the amount of seconds at which to grab the image.  If a
 *     non-negative number this will be used as the amount of seconds at which
 *     to grab the image.  If a negative number this will be used as the amount
 *     of seconds from the end at which to grab the image.  If an array it
 *     should be an array of numbers and/or functions that will be treated as
 *     previously mentioned.
 * @param {Function} callback
 *     Once done or when an error occurs this function will be called.  The 1st
 *     argument indicate whether the proccess failed.  The 2nd will be an array
 *     of the data URIs representing the captured image(s).  The 3rd will be an
 *     array corresponding to the array of data URIs indicating the times for
 *     each in seconds.
 */
function getVideoImages(path, times, callback) {
  var hasSeeked,
      me = this,
      dataURIs = [],
      captureTimes = [],
      video = document.createElement('video'),
      canvas = document.createElement('canvas'),
      ctx = canvas.getContext('2d');

  video.onloadedmetadata = function () {
    times = Object.prototype.toString.call(times) == '[object Array]' ? times.slice() : [times];
    tryNextFrame();
  };

  video.onseeked = function () {
    if (!hasSeeked) {
      hasSeeked = true;
      canvas.height = video.videoHeight;
      canvas.width = video.videoWidth;
      ctx.drawImage(video, 0, 0, canvas.width, canvas.height);
      dataURIs.push(canvas.toDataURL());
      captureTimes.push(this.currentTime);
      tryNextFrame();
    }
  };

  video.onerror = function () {
    if (!hasSeeked) {
      hasSeeked = true;
      callback(false);
      collectGarbage();
    }
  };

  function tryNextFrame() {
    var time = times.shift();
    if (time) {
      hasSeeked = false;
      if ('function' === typeof time) {
        time = time(video.duration);
      }
      video.currentTime = Math.min(Math.max(0, (time < 0 ? video.duration : 0) + time), video.duration);
    }
    else {
      callback(true, dataURIs, captureTimes);
      collectGarbage();
    }
  }

  // garbage collect video
  function collectGarbage() {
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    delete canvas;
    delete ctx;
    video.removeAttribute('src');
    video.load();
    delete video;
  }

  video.src = path;
}