在执行比较之前已加载并绘制图像。尝试此操作：

You are not waiting until your images have loaded and drawn before performing your comparison. Try this:

var img = new Image;
img.onload = function(){
  ctx1.drawImage(img,0,0);
  var img = new Image;
  img.onload = function(){
    ctx2.drawImage(img,0,0);
    // diff them here
  };
  img.src = 'cat.jpg';
};
img.src = 'cat.jpg';

如上所示，您应始终设置 src code> onload 。

As shown above, you should always set your src after your onload.