  Deep-networks-based hashing has become a leading approach for large-scale
image retrieval, which learns a similarity-preserving network to map similar
images to nearby hash codes. The pairwise and triplet losses are two widely
used similarity preserving manners for deep hashing. These manners ignore the
fact that hashing is a prediction task on the list of binary codes. However,
learning deep hashing with listwise supervision is challenging in 1) how to
obtain the rank list of whole training set when the batch size of the deep
network is always small and 2) how to utilize the listwise supervision. In this
paper, we present a novel deep policy hashing architecture with two systems are
learned in parallel: a query network and a shared and slowly changing database
network. The following three steps are repeated until convergence: 1) the
database network encodes all training samples into binary codes to obtain a
whole rank list, 2) the query network is trained based on policy learning to
maximize a reward that indicates the performance of the whole ranking list of
binary codes, e.g., mean average precision (MAP), and 3) the database network
is updated as the query network. Extensive evaluations on several benchmark
datasets show that the proposed method brings substantial improvements over
state-of-the-art hashing methods.
