// Call the invertImage method when this worker receives a message from the calling script.

 // The ‘self’ object contains the only methods a web worker can access apart from those it

 // defines and creates itself

 //当此worker收到自来于发出调用的脚本的消息时，调用invertImage方法。“self”对象所包含的web worker所能访问的方法就只有那些由web worker自己本身进行定义和创建的方法。

 self.addEventListener("message", invertImage, false);

 // Define a function to take an image and invert it, pixel by pixel, using its raw data

 //定义一个函数，用作接收图像的原始数据，按像素来逐粒逐粒地实施反相操作。

 function invertImage(e) {

     // The ‘data’ property of the ‘message’ event contains the pixel data passed from

     // the calling script

     //“message”事件的“data”属性包含着从发出调用的脚本所包含的像素数据。

     var message = e.data,

         // The ‘data’ property of the message passed contains the raw image pixel data

         //所发送过来的消息中的“data”属性包含着原始图像的像素数据。

         imagePixels = message.data,

         x = 0,

         len = imagePixels.length;

     // Loop through each pixel, inverting its value within the original pixel data array.

     // Pixel data is arranged in groups of 4 values, representing the red, green, blue, and

     // opacity values of each visible screen pixel. We therefore loop through in jumps of 4

     // on each iteration

     //循环遍历每一粒像素，使原始像素数据的数组中所保存的值反相。像素数据按4个值进行分组，分别表示屏幕上所见像素的红、绿、蓝和透明度数值。因此，循环的每轮迭代的加数为4。

     for (; x < len; x += 4) {

         // To invert a pixel’s value, subtract it from the maximum possible value, which is 255

         //要对像素值进行反相，可以用最大的可能值（即255）进行相减。

         imagePixels[x] = 255 - imagePixels[x];

         imagePixels[x + 1] = 255 - imagePixels[x + 1];

         imagePixels[x + 2] = 255 - imagePixels[x + 2];

     }

     // Finally, post a message containing the updated pixel data back to the calling script

     //最后，把包含着更新后的像素数据的消息发送回至发出调用的脚本。

     self.postMessage(message);

 }