中文说明：
取一个最优阈值把原图像分为前景色与背景色，两部分的类间方差越大，说明两部分差别越大，便能有效的分割图像。
English Description：
Take an optimal threshold to divide the original image into foreground color and background color. The greater the variance between the two parts, the greater the difference between the two parts, and then the image can be segmented effectively