中文说明：Niblack二值化算法是比较简单的局部阈值方法，阈值的计算公式是T = m + k*v，其中m为以该像素点为中心的区域的平均灰度值，v是该区域的标准差，k是一个系数
English Description：
Niblack binarization algorithm is a relatively simple method of local threshold, threshold formula is t = m + k*v, where m is the average grey scale values of the pixel area centered around the point, and v is the standard deviation of the region, k is a coefficient