SIFT-based local image description using sparse representations
This paper addresses the problem of efficient SIFT-based image description and searches in large databases within the framework of local querying. A descriptor called the bag-of-features has been introduced in [1] which first vector quantizes SIFT descriptors and then aggregates the set of resulting codeword indices (so-called visual words) into a histogram of occurence of the different visual words in the image. The aim is to make the image search complexity tractable by transforming the set of local image descriptor vectors into a single sparse vector as sparsity particularly permits efficient inner product calculations. However, aggregating local descriptors into a single histogram decreases the discerning power of the system when performing local queries. In this paper, we propose a new approach that aims to enjoy the complexity benefits of sparsity while at the same time retaining the local quality of the input descriptor vectors. This is accomplished by searching for a sparse approximation of the input SIFT descriptors. The sparse approximation yields a sparse vector per local SIFT descriptor, and helps preserving local description properties by using each sparse-transformed descriptor independently in a voting system to retrieve indexed images. Our system is shown experimentally to perform better than histogram based systems under query locality, albeit at an increased complexity.