var canvas, ctx, myImage;
function convertToGray() {
  myImage = document.getElementById("img");
  canvas = document.getElementById("myCanvas");
  canvas.width = img.width;
  canvas.height = img.height;
  if (canvas.getContext) {
    ctx = canvas.getContext("2d");
    myImage.onload = function() {
      ctx.drawImage(myImage, 0, 0);
      getColorData();
      putColorData();
    }
    myImage.src = "images/img8.jpg";
  }
}
function getColorData() {
  var length = canvas.width * canvas.height;
  myImage = ctx.getImageData(0, 0, canvas.width, canvas.height);
  for (var i = 0; i < length * 4; i += 4) {
    var myRed = myImage.data[i];
    var myGreen = myImage.data[i + 1];
    var myBlue = myImage.data[i + 2];
    myGray = parseInt((myRed + myGreen + myBlue) / 3);
    myImage.data[i] = myGray;
    myImage.data[i + 1] = myGray;
    myImage.data[i + 2] = myGray;
  }
}
function putColorData() {
  ctx.putImageData(myImage, 0, 0);
}