otsu算法确定二值化阀值，最大类间方差是由日本学者大津(OTSU)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,简称OTSU。

-In computer vision and image processing, Otsu s method is used to automatically perform histogram shape-based image thresholding,[1]　or, the reduction of a graylevel image to a binary image. The algorithm assumes that the image to be thresholded contains two classes of pixels (e.g. foreground and background) then calculates the optimum threshold separating those two classes so that their combined spread (intra-class variance) is minimal.[2]　The extension of the original method to multi-level thresholding is referred to as the Multi Otsu method[3].