Two-stage optimal component analysis

Linear representations are widely used to reduce dimension in applications involving high dimensional data. While specialized procedures exist for certain optimality criteria, such as principle component analysis (PCA) and Fisher discriminant analysis (FDA), they can not be generalized for more general criteria. To overcome this fundamental limitation, optimal component analysis (OCA) uses a stochastic gradient optimization procedure intrinsic to the manifold giving by the constraints of applications and therefore gives a procedure for finding optimal representations for general criteria. However, due to its generality nature, OCA often requires extensive computation for gradient estimation and updating. To significantly reduce the required computation, in this paper, we propose a two-stage method by first reducing the dimension of input to a smaller one (but larger than the final resulting dimension) using a computationally efficient method and then performing OCA in the reduced space. This reduces the computation time from days to minutes on widely used databases, making OCA learning feasible for many applications. Additionally, since the reduced space is much smaller, the stochastic gradient optimization tends to be more efficient. We illustrate the effectiveness of the proposed method on face classification.