Over-complete wavelet approximation of a support vector machine for efficient classification

In this paper, we present a novel algorithm for reducing the runtime computational complexity of a Support Vector Machine classifier. This is achieved by approximating the Support Vector Machine decision function by an overcomplete Haar wavelet transformation. This provides a set of classifiers of increasing complexity that can be used in a cascaded fashion yielding excellent runtime performance. This over-complete transformation finds the optimal approximation of the Support Vectors by a set of rectangles with constant gray-level values (enabling an Integral Image based evaluation). A major feature of our training algorithm is that it is fast, simple and does not require complicated tuning by an expert in contrast to the Viola & Jones classifier. The paradigm of our method is that, instead of trying to estimate a classifier that is jointly accurate and fast (such as the Viola & Jones detector), we first build a classifier that is proven to have optimal generalization capabilities; the focus then becomes runtime efficiency while maintaining the classifier's optimal accuracy. We apply our algorithm to the problem of face detection in images but it can also be used for other image based classifications. We show that our algorithm provides, for a comparable accuracy, a 15 fold speed-up over the Reduced Support Vector Machine and a 530 fold speed-up over the Support Vector Machine, enabling face detection at 25 fps on a standard PC.