During the last years, the problem of Content-Based Image Retrieval (CBIR) was addressed in many different ways, achieving excellent results in small-scale datasets. With growth of the data to evaluate, new issues need to be considered and new techniques are necessary in order to create an efficient yet accurate system. In particular, computational time and memory occupancy need to be kept as low as possible, whilst the retrieval accuracy has to be preserved as much as possible. For this reason, a brute-force approach is no longer feasible, and an Approximate Nearest Neighbor (ANN) search method is preferable. This paper describes the state-of-the-art ANN methods, with a particular focus on indexing systems, and proposes a new ANN technique called Bag of Indexes (BoI). This new technique is compared with the state of the art on several public benchmarks, obtaining 86.09% of accuracy on Holidays+Flickr1M, 99.20% on SIFT1M and 92.4% on GIST1M. Noteworthy, these state-of-the-art accuracy results are obtained by the proposed approach with a very low retrieval time, making it excellent in the trade off between accuracy and efficiency.
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