Efficient Object Detection Using Cascades of Nearest Convex Model Classifiers

An object detector must detect and localize each instance of the object class of interest in the image. Many recent detectors adopt a sliding window approach, reducing the problem to one of deciding whether the detection window currently contains a valid object instance or background. Machine learning based discriminants such as SVM and boosting are typically used for this, often in the form of classifier cascades to allow more rapid rejection of easy negatives. We argue that "one class" methods - ones that focus mainly on modelling the range of the positive class - are a useful alternative to binary discriminants in such applications, particularly in the early stages of the cascade where one-class approaches may allow simpler classifiers and faster rejection. We implement this in the form of a short cascade of efficient nearest-convex-model one-class classifiers, starting with linear distance-to-affine-hyperplane and interior-of-hypersphere classifiers and finishing with kernelized hypersphere classifiers. We show that our methods have very competitive performance on the Faces in the Wild and ESOGU face detection datasets and state-of-the-art performance on the INRIA Person dataset. As predicted, the one-class formulations provide significant reductions in classifier complexity relative to the corresponding two-class ones.