Subspace Learning for Facial Age Estimation Via Pairwise Age Ranking

Age is one of the important biometric traits for reinforcing the identity authentication. The challenge of facial age estimation mainly comes from two difficulties: (1) the wide diversity of visual appearance existing even within the same age group and (2) the limited number of labeled face images in real cases. Motivated by previous research on human cognition, human beings can confidently rank the relative ages of facial images, we postulate that the age rank plays a more important role in the age estimation than visual appearance attributes. In this paper, we assume that the age ranks can be characterized by a set of ranking features lying on a low-dimensional space. We propose a simple and flexible subspace learning method by solving a sequence of constrained optimization problems. With our formulation, both the aging manifold, which relies on exact age labels, and the implicit age ranks are jointly embedded in the proposed subspace. In addition to supervised age estimation, our method also extends to semi-supervised age estimation via automatically approximating the age ranks of unlabeled data. Therefore, we can successfully include more available data to improve the feature discriminability. In the experiments, we adopt the support vector regression on the proposed ranking features to learn our age estimators. The results on the age estimation demonstrate that our method outperforms classic subspace learning approaches, and the semi-supervised learning successfully incorporates the age ranks from unlabeled data under different scales and sources of data set.