说明：  2DPCA,即二维主成分分析，相对于传统的PCA（主成分分析），2DPCA在对二维图像进行降维时不需要转成一维（向量）
(2DPCA, ie two-dimensional principal component analysis, as opposed to the traditional PCA (Principal Component Analysis), 2DPCA in dimensionality reduction of two-dimensional images into one-dimensional is not required (vector))