html5 – context.getImageData()操作不安全

我想为 HTML5画布实现一个简单的灰度过滤器,但我可以将Image数据作为像素抓取.我从FF和Chrome获得安全警告.最后,滤镜不会使图像变灰.

JS FIDLE CODE

JS：

var canvas = document.getElementById('canvas');       
var context = canvas.getContext('2d');  

var image = new Image();
image.onload = function () {
  if (image.width != canvas.width)
    canvas.width = image.width;
  if (image.height != canvas.height)
    canvas.height = image.height;
  context.clearRect(0,canvas.width,canvas.height);
  context.drawImage(image,canvas.height);
  var imageData = context.getImageData(0,canvas.height);
  filter(imageData);
  context.putImageData(imageData,0);
}
image.src = "http://i0.gmx.net/images/302/17520302,pd=2,h=192,mxh=600,mxw=800,w=300.jpg";

function filter(imageData){
 var d = imageData.data;
   for (var i = 0; i < d.length; i += 4) {
     var r = d[i];
     var g = d[i + 1];
     var b = d[i + 2];
     d[i] = d[i + 1] = d[i + 2] = (r+g+b)/3;
   }
return imageData;
}