Sparse Binary Features for Image Classification

The past decade has seen the rise of cheap and ubiquitous access to cameras, especially by its systematic embedding into mobile phones. In conjunction with the increase of network bandwidth and a large worldwide commitment to online social media, we face today a tremendous amount of multimedia data. The rate at which the quantity of data increases is sufficiently high to prohibit the idea of the traditional but cumbersome and lengthy process of human-driven data organization and labelling. This induces a need of automatic processing of multimedia data. Image classification is a good example of such a task. In this work a new method for automatic image classification is proposed. It relies on a compact representation of images using sets of sparse binary features. This work first evaluates the Fast Retina Keypoint binary descriptor and proposes improvements based on an efficient descriptor representation. The efficient representation is created using dimensionality reduction techniques, entropy analysis and decorrelated sampling. In a second part, the problem of image classification is tackled. The traditional approach uses machine learning algorithms to create classifiers, and some works already propose to use a compact image representation using feature extraction as preprocessing. The second contribution of this work is to show that binary features, while being very compact and low dimensional (compared to traditional representation of images), still provide a very high discriminant power. This is shown using various learning algorithms and binary descriptors. These years a scheme has been widely used to perform object recognition on images, or equivalently image classification. It is based on the concept of Bag of Visual Words. More precisely, an image is described using an unordered set of visual words, that are generally represented by feature descriptions. The last contribution of this work is to use binary features with a simple Bag of Visual Words classifier. Tests of performance for the image classification are performed on a large database of images.