说明：  对图像进行操作，实际上是将图像看成许多个像素点，对每个像素点进行操作。在计算机系统中，灰度图片被看成是许多个由值在［０～２５５］之间的像素点组成的图像，２５５表示白色，０表示黑色，黑白之间存在２５６个灰度级。
(In fact, the image is regarded as many pixels and each pixel is operated. In the computer system, the gray image is regarded as an image composed of many pixels whose values are between 0 and 255. 255 is white, 0 is black, and there are 256 gray levels between black and white.)