  Hashing method maps similar data to binary hashcodes with smaller hamming
distance, which has received a broad attention due to its low storage cost and
fast retrieval speed. With the rapid development of deep learning, deep hashing
methods have achieved promising results in efficient information retrieval.
Most of the existing deep hashing methods adopt pairwise or triplet losses to
deal with similarities underlying the data, but the training is difficult and
less efficient because $O(n^2)$ data pairs and $O(n^3)$ triplets are involved.
To address these issues, we propose a novel deep hashing algorithm with unary
loss which can be trained very efficiently. We first of all introduce a Unary
Upper Bound of the traditional triplet loss, thus reducing the complexity to
$O(n)$ and bridging the classification-based unary loss and the triplet loss.
Second, we propose a novel Semantic Cluster Deep Hashing (SCDH) algorithm by
introducing a modified Unary Upper Bound loss, named Semantic Cluster Unary
Loss (SCUL). The resultant hashcodes form several compact clusters, which means
hashcodes in the same cluster have similar semantic information. We also
demonstrate that the proposed SCDH is easy to be extended to semi-supervised
settings by incorporating the state-of-the-art semi-supervised learning
algorithms. Experiments on large-scale datasets show that the proposed method
is superior to state-of-the-art hashing algorithms.
