它是按图像的灰度特性,将图像分成背景和目标2部分。背景和目标之间的类间方差越大,说明构成图像的2部分的差别越大,当部分目标错分为背景或部分背景错分为目标都会导致2部分差别变小。-It is based on the image of the gray characteristics, the image is divided into 2 parts of the background and objectives. The greater the variance between the background and objectives, the larger the difference between the 2 parts of the image, when the wrong part of the target is divided into the background or part of the background into the target will lead to the 2 part of the difference is small.