图像的阈值分割。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，选取一个比较合理的阈值，以确定图像中每个像素点应该属于目标区域还是背景区域，从而产生相应的二值图像。 -Image segmentation. Used to extract the target image and its background in the grayscale characteristics of the regional differences, the images have different gray level as the two types of area (target area and background area) of the portfolio, select a reasonable threshold to to determine each pixel in the image should belong to the target area or background area, resulting in a corresponding binary image.