1. A computer-implemented method for recognizing face images, comprising: storing face models or face images of a known person as training faces in a computer storage; joining a group of testing face images with a group of training faces that belong to the known person to form a joint face group; calculating similarity functions, by a computer processor, between pairs of testing face images and training faces in the joint face group; conducting non-negative matrix factorization on values of the similarity functions in the joint face group to test truthfulness of the joint face group; and identifying the testing face images to belong to the known persons if a percentage of the associated similarity functions being true in the joint face group is above a threshold based on the non-negative matrix factorization.