HTML Canvas中的getPixel？

W3C文档中有关于像素 *** 纵的部分。

这是有关如何反转图像的示例：

var context = document.getElementById('myCanvas').getContext('2d');// Get the CanvasPixelArray from the given coordinates and dimensions.var imgd = context.getImageData(x, y, width, height);var pix = imgd.data;// Loop over each pixel and invert the color.for (var i = 0, n = pix.length; i < n; i += 4) {    pix[i  ] = 255 - pix[i  ]; // red    pix[i+1] = 255 - pix[i+1]; // green    pix[i+2] = 255 - pix[i+2]; // blue    // i+3 is alpha (the fourth element)}// Draw the ImageData at the given (x,y) coordinates.context.putImageData(imgd, x, y);