利用图像中要提取的目标区域与其背景在灰度特性上的差异，把图像看作具有不同灰度级的两类区域(目标区域和背景区域)的组合-Combination of the target area with the background image you want to extract the difference in the grayscale characteristics of the image as the two types of regions with different gray level (the target area and background area)