JS Canvas Webcam Auto Check If it Is Covered/Respond W Pic

<center>
<body bgcolor="black">
<video id="video" width="640" height="480" autoplay></video>
<b>
<button id="t" hidden><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();
});
}
var cct=getBase64Image(video)
var ctt=getBase64Image(video);
  var doc=document.getElementById('t');
document.addEventListener('mousemove', function() {

  context.drawImage(video, 0, 0, 640, 480);
  ctt=getBase64Image(video);
  var dexrnd=Math
  if(ctt>491990){

    Load_Img('https://api.audiotool.com/track/dj_bru_-_acid_step/2/cover/640x480.jpg?showDefault=true')
    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;
}
  function Load_Img(path){
var h,w;
h=('500');
w=('500')
    var canvas = document.getElementById("canvas");
    var ctx = canvas.getContext("2d");
    var img = new Image();
img.src=(path);
   ctx.drawImage(img, 1, 1,h,w);

}
</script>

</body>
</html>