Characterizing Humans on Riemannian manifolds

In surveillance applications, head and body orientation of people is of primary importance for assessing many behavioural traits. Unfortunately, in this context people is often encoded by few, noisy pixels, so that their characterization is difficult. We face this issue, proposing a computational framework which is based on an expressive descriptor, the covariance of features. Covariances have been employed for pedestrian detection purposes, actually, a binary classification problem on Riemannian manifolds. In this paper, we show how to extend to the multi-classification case, presenting a novel descriptor, named Weighted ARray of COvariances, WARCO, especially suited for dealing with tiny image representations. The extension requires a novel differential geometry approach, in which covariances are projected on a unique tangent space, where standard machine learning techniques can be applied. In particular, we adopt the Campbell-Baker-Hausdorff expansion as a means to approximate on the tangent space the genuine (geodesic) distances on the manifold, in a very efficient way. We test our methodology on multiple benchmark datasets, and also propose new testing sets, getting convincing results in all the cases.