function grayscale(src){
  var canvas = document.createElement('canvas');
  var ctx = canvas.getContext('2d');
  var imgObj = new Image();
  imgObj.src = src;
  canvas.width = imgObj.width;
  canvas.height = imgObj.height; 
  ctx.drawImage(imgObj, 0, 0); 
  var imgPixels = ctx.getImageData(0, 0, canvas.width, canvas.height);
  for(var y = 0; y < imgPixels.height; y++){
    for(var x = 0; x < imgPixels.width; x++){
      var i = (y * 4) * imgPixels.width + x * 4;
      var avg = (imgPixels.data[i] + imgPixels.data[i + 1] + imgPixels.data[i + 2]) / 3;
      imgPixels.data[i] = avg; 
      imgPixels.data[i + 1] = avg; 
      imgPixels.data[i + 2] = avg;
    }
  }
  ctx.putImageData(imgPixels, 0, 0, 0, 0, imgPixels.width, imgPixels.height);
  return canvas.toDataURL();
}