说明：  最大类间方差法原理：最大类间方差法是由日本学者大津于1979年提出的,是一种自适应的阈值确定的方法,又叫大津法
(Otsu Method: Otsu is Otsu scholars from Japan in 1979 to argue that there is an adaptive threshold to determine the method, also known Otsu method)