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As GilLevi pointed out, FREAK and BRISK are binary descriptors. If you want to use BoW (and I guess this would be the easiest and most accurate way) than you'll need to make some extra effort to work with binary descriptors, see http://answers.opencv.org/question/24835/is-there-a-way-of-using-orb-with-bow/#24858. Note that for the BoW-descriptor (i.e. the histograms) you will also change the nearest neighbor matcher to use Hamming-Distance. However, in most image classification tasks float descriptors like SIFT (typically densely sampled) are used for BoW, so is a large speed really required? What is the actual application?