计算图中所有灰阶的分布概率

定一阈值使得影像分成两类

计算两类的方差、平均灰度值与总影像平均灰度

选择最佳阈值-calculated all gray-scale map of the probability distribution of a fixed threshold value makes images divided into two categories : computing the variance, Gray and the average total value of the average gray-scale images to choose the optimal threshold