Unsupervised feature selection for multi-class object detection using convolutional neural networks

Convolutional Neural Networks (CNN) have proven to be useful tools for object detection and object recognition. They act like feature extractor and classifier at the same time. In this study we present an unsupervised feature selection procedure for constructing a training set for the CNN and analyze in detail the learnt receptive fields. We then introduce, for the first time, a figural alphabet to be used for low-level feature detection with CNN. This alphabet turned out to be useful in detecting a vocabulary set of intermediate level features and considerably reduces the complexity of the CNN. Moreover we propose an optimal high-level feature selection procedure and apply this to the challenging problem of car detection. We demonstrate promising results for multi-class object detection using obtained figural alphabet to detect considerably different categories of objects (e.g., faces and cars).