“…The former approaches can only handle one modality, while the latter approaches support two or more modalities. In addition, they can categorised into unsupervised [6,11,37] and supervised [4,15,20,21,23,27,35,36] depending on the method of learning hash functions. In general, supervised methods can take more advantage of the inner relationships of data from annotation and due to that they perform better than unsupervised methods.…”