webrtc-utilities/src/videoframechecker.js

/*
 *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree.
 */
'use strict';

/* eslint-env node */
/* eslint-env browser */
var Ssim = require('./ssim');

function VideoFrameChecker(videoElement) {
  this.frameStats = {
    numFrozenFrames: 0,
    numBlackFrames: 0,
    numFrames: 0
  };

  this.running_ = true;

  this.nonBlackPixelLumaThreshold = 20;
  this.previousFrame_ = [];
  this.identicalFrameSsimThreshold = 0.985;
  this.frameComparator = new Ssim();

  this.canvas_ = document.createElement('canvas');
  this.videoElement_ = videoElement;
  this.listener_ = this.checkVideoFrame_.bind(this);
  this.videoElement_.addEventListener('play', this.listener_, false);
}

VideoFrameChecker.prototype = {
  stop: function() {
    this.videoElement_.removeEventListener('play' , this.listener_);
    this.running_ = false;
  },

  getCurrentImageData_: function() {
    this.canvas_.width = this.videoElement_.width;
    this.canvas_.height = this.videoElement_.height;

    var context = this.canvas_.getContext('2d');
    context.drawImage(this.videoElement_, 0, 0, this.canvas_.width,
      this.canvas_.height);
    return context.getImageData(0, 0, this.canvas_.width, this.canvas_.height);
  },

  checkVideoFrame_: function() {
    if (!this.running_) {
      return;
    }
    if (this.videoElement_.ended) {
      return;
    }

    var imageData = this.getCurrentImageData_();

    if (this.isBlackFrame_(imageData.data, imageData.data.length)) {
      this.frameStats.numBlackFrames++;
    }

    if (this.frameComparator.calculate(this.previousFrame_, imageData.data) >
        this.identicalFrameSsimThreshold) {
      this.frameStats.numFrozenFrames++;
    }
    this.previousFrame_ = imageData.data;

    this.frameStats.numFrames++;
    setTimeout(this.checkVideoFrame_.bind(this), 20);
  },

  isBlackFrame_: function(data, length) {
    // TODO: Use a statistical, histogram-based detection.
    var thresh = this.nonBlackPixelLumaThreshold;
    var accuLuma = 0;
    for (var i = 4; i < length; i += 4) {
      // Use Luma as in Rec. 709: Y′709 = 0.21R + 0.72G + 0.07B;
      accuLuma += 0.21 * data[i] + 0.72 * data[i + 1] + 0.07 * data[i + 2];
      // Early termination if the average Luma so far is bright enough.
      if (accuLuma > (thresh * i / 4)) {
        return false;
      }
    }
    return true;
  }
};

module.exports = VideoFrameChecker;