说明：  子空间的出现使模式识别有了突破性的发展，而特征提取则是子空间实现的一个重要环节，提取特征的好坏决定了子空间方法的成功与否
(The emergence of sub-space to make a breakthrough with the development of pattern recognition, and feature extraction is a sub-space realization of an important link, extract the characteristics of good and bad decisions of the sub-space method of the success of)