灰度门限化许多情况，图象是由具有不同灰度级的两类区域组成。如文字与纸张、地物与云层（航空照片）。其特点:直方图具有两个峰，分别与两个灰度级范围相对应 。故可选择一个门限，将两个峰分开。-Gray threshold of many cases, the image is gray level of two different regional groupings. If words and paper, surface features and cloud cover (aerial photograph). Its features: histogram has two peak, respectively, with the two gray-scale range corresponding to. So choose a threshold, separating the two peaks.