JS Canvas Check Webcam Coverage Status FINAL SRC!!!!!

<center>
<body bgcolor="black">
<video id="video" width="640" height="480" autoplay></video>
  <b>
  <button id="t"><b>Test Now!!</button>
<canvas id="canvas" width="640" height="480"></canvas>
<script>
document.addEventListener("DOMContentLoaded", function() {
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
var video = document.getElementById('video');
var mediaConfig =  { video: true,audio:false};
var errBack = function(e) {
console.log('An error has occurred!', e)
};
if(navigator.mediaDevices && navigator.mediaDevices.getUserMedia) {
navigator.mediaDevices.getUserMedia(mediaConfig).then(function(stream) {
video.src = window.URL.createObjectURL(stream);
video.play();

});
}
else if(navigator.getUserMedia) {
navigator.getUserMedia(mediaConfig, function(stream) {
video.src = stream;
video.play();

}, errBack);
} else if(navigator.webkitGetUserMedia) {
navigator.webkitGetUserMedia(mediaConfig, function(stream){
video.src = window.webkitURL.createObjectURL(stream);
video.play();
}, errBack);
} else if(navigator.mozGetUserMedia) {
navigator.GetUserMedia(mediaConfig, function(stream){
video.src = window.URL.createObjectURL(stream);
video.play();
}, errBack);
}
 var cct=getBase64Image(video)
  var ctt=getBase64Image(video);
  var doc=document.getElementById('t');
doc.addEventListener('click', function() {

  context.drawImage(video, 0, 0, 640, 480);
  ctt=getBase64Image(video);

  if(ctt<=501990){
    alert("Webcam Is Covered!!")
    return 0;
  }else{
    alert('Webcam Is Visible!!')
  }

});
}, false);
  function getBase64Image(img) {

    var canvas = document.createElement("canvas");
    canvas.width = img.width;
    canvas.height = img.height;

    var ctx = canvas.getContext("2d");
    ctx.drawImage(img, 0, 0);

    var dataURL = canvas.toDataURL("image/png");

    return dataURL.length-4000;
}
</script>