中文说明：迭代法阈值分割，迭代法自动计算最优阈值。已经写成函数形式，输入灰度图，输出分割好后的阈值图。
English Description：
Iterative threshold segmentation, iterative method automatically calculate the optimal threshold. It has been written in the form of function, input gray image, output threshold image after segmentation.