对图像进行逐点计算，并且取最大值和最小值的差值作为该点新的灰度值。
在区别边缘点和非边缘点时，首先确定一个阈值T，若图像中像素点的新
值大于该门限值，则可以初步确定为边缘点，小于T的新值被判断为非边缘点.,Image point by point basis, and taking the maximum and minimum points of difference as the new gray value. Edge in the difference between point and non-edge point, first determine a threshold T, if the image of the new pixel value is greater than the threshold, you can initially identified as the edge point, the new value is less than T have been judged as non-edge point.