使用matlab自带的函数princomp()计算主成分，当维数很高时通常会出现内存耗尽的错误，即使内存足够也非常耗时。快速pca能加快计算进程且减小内存占用，更容易计算较大尺寸的图像主成分。(Use MATLAB's own function princomp () to calculate the principal component. When the dimension is very high, the memory exhaustion error usually occurs, even if the memory is enough, it is very time-consuming. Fast PCA can speed up the computation process and reduce memory footprint, making it easier to calculate the larger image principal components.)