I. Introduction
Feature selection for face representation is one of central issues to face recognition (FR) systems. Among various solutions to the problem (see [1], [2] for a survey), the most successful seems to be those appearance-based approaches, which generally operate directly on images or appearances of face objects and process the images as two-dimensional (2-D) holistic patterns, to avoid difficulties associated with three-dimensional (3-D) modeling, and shape or landmark detection [2]. Principle component analysis (PCA) and linear discriminant analysis (LDA) are two powerful tools used for data reduction and feature extraction in the appearance-based approaches. Two state-of-the-art FR methods, Eigenfaces [3] and Fisherfaces [4], built on the two techniques, respectively, have been proved to be very successful.