迭代法求阈值的原理：

基于逼近的思想，步骤如下：

1． 求出图象的最大灰度值和最小灰度值，分别记为ZMAX和ZMIN，令初始阈值T0=(ZMAX+ZMIN)/2；

2． 根据阈值TK将图象分割为前景和背景，分别求出两者的平均灰度值ZO和ZB

3． 求出新阈值TK+1=(ZO+ZB)/2；

4． 若TK=TK+1，则所得即为阈值；否则转2，迭代计算。

-　Iteration method threshold principle:　based on the ideological approach, the steps are as follows:　1. Images obtained the maximum and minimum gray value of gray value, denoted ZMAX and ZMIN, so that initial threshold value T0 = (ZMAX+ ZMIN)/2　 2. TK under the threshold will be divided into foreground and background images were obtained between the average gray value ZO and ZB　3. Calculate the new threshold TK+1 = (ZO+ ZB)/2　 4. If TK = TK+1, then the proceeds shall threshold　otherwise turn 2 iteration.