Elsevier

Signal Processing

Volume 93, Issue 8, August 2013, Pages 2256-2264
Signal Processing

Dual local consistency hashing with discriminative projections selection

https://doi.org/10.1016/j.sigpro.2012.05.035Get rights and content

Abstract

Semantic hashing is a promising way to accelerate similarity search, which designs compact binary codes for a large number of images so that semantically similar images are mapped to close codes. Retrieving similar neighbors is then simply accomplished by retrieving images that have codes within a small Hamming distance of the code of the query. However, most of the existing hashing approaches, such as spectral hashing (SH), learn the binary codes by preserving the global similarity, which do not have full discriminative power. In this paper, we propose a dual local consistency hashing method which not only makes the similar images have the same codes but also dissimilar images with different codes. Moreover, we propose a PCA projection selecting scheme that choose the most discriminative projection for each bit of the codes. Therefore, the binary codes learned by our approach are more powerful and discriminative for similarity search. Extensive experiments are conducted on publicly available datasets and the comparison results demonstrate that our approach can outperform the state-of-art methods.

Highlights

► We propose a novel hashing approach based on dual local consistency. ► We propose a discriminative projection selecting scheme to learn hash functions. ► Our method outperforms the state-of-the-art methods for fast image search.

Introduction

With the rapid evolution and development of the Internet, the visual contents (e.g. image, video) on web are explosively growing. In order to efficiently exploit such enormous web contents, fast search in large scale has become one of the most critical techniques. It is of great importance to many applications, such as image or video annotation [27], [28], social image retrieval [29]. The traditional content-based image retrieval techniques usually adopt exhaustive comparing the query image with pooled database, which is infeasible because the linear complexity is not scalable in practical situations. For example, the photo sharing website Flickr has over 4 billion images. Another visual content sharing website YouTube receives more than 20 h of uploaded videos per minute. Besides, most large-scale content-based image retrieval applications suffer from the curse of dimensionality since visual descriptors usually have hundreds or even thousands of dimensions. Therefore, beyond the infeasibility of exhaustive search, storage of the original data also becomes a challenging problem.

Over the past decades, several Approximate Nearest Neighbor (ANN) search techniques have been developed for large scale applications. Although there exist many tree-based methods [2], [6], [12], [16], [22] that can perform similarity search effectively in a low-dimensional feature space, for high-dimensional cases and applications with memory constrains, hashing-based ANN techniques have attracted more attention. Hashing-based methods are promising in accelerating similarity search for their capability of generating compact binary codes for a large number of images in the dataset so that similar images will have close binary codes. Retrieving similar neighbors is then accomplished simply by finding the images that have codes with a small Hamming distance from the query. It is extremely fast to perform similarity search over such binary codes [23], because (1) the encoded data are highly compressed and thus can be loaded into the main memory; (2) the hamming distance between two binary codes can be computed efficiently by using bit XOR operation and computing the number of set bits (an ordinary PC today would be able to do millions of Hamming distance computation in just a few milliseconds).

Many hashing algorithms have been proposed to address the fast retrieval issue in recent years. These hashing-based methods for fast image retrieval can be considered as embedding high dimensional feature vectors to a low dimensional Hamming space, while retaining as much as possible the semantic similarity structure of data. In terms of if labeled data is needed, hashing methods can be roughly divided into two categories: unsupervised methods [1], [31], [33] and supervised methods [9], [21], [25], [26]. Unsupervised methods, such as Locality Sensitive Hashing (LSH) [1] and spectral hashing (SH) [31], only use the unlabeled data to generate binary codes for given samples, while supervised methods which incorporate the label information, such as Restricted Boltzmann Machines (RBMs) [9] and Sequential Projection Learning for Hashing (SPLH) [26], are able to preserve the semantic similarity and thus facilitate semantic retrieval and classification.

Although the existing hashing methods have shown success in large-scale image search, there are some shortcomings of them. For example, the traditional spectral hashing (SH) [31] and its extension self-taught hashing (STH) [33] learn the binary codes by making the similar images close to each other in the Hamming space. However, the codes generated in this way may not have the full discriminative power for retrieval and classification task, because a good embedding should not only make the similar images close but also make the dissimilar images far away. In addition, a common step in many binary coding methods [25], [31] is performing principal component analysis (PCA) to get the projection vectors. However, the projections learned in this way do not have the complementary capability, i.e., the errors generated by the previous projection should be corrected as many as possible by the following projection. Therefore, the learned binary codes also lack discriminative power to some extent.

In order to address the above-mentioned problems, we propose a novel discriminative hashing approach based on dual local consistency, which does not only make the similar images have the same codes but also make the dissimilar images have different codes. Moreover, we adopt a more discriminative projection selecting scheme, in which we select the PCA projection for each bit of the codes sequentially so that the current projection can correct the errors generated by the previous projection as many as possible.

The rest of the paper is organized as follows. Section 2 reviews related work. Section 3 describes our dual local consistency hashing approach with discriminative projections selection. In Section 4, we introduce the experiments on two publicly available datasets. Finally, conclusions are given in Section 5.

Section snippets

Fast similarity search

Given a set of data points, the objective in nearest neighbor search is to find a subset that is most similar to the query. Exhaustively comparing the query with each sample in the database is computationally infeasible because linear complexity is not acceptable for large-scale database. To avoid excessive computational and memory costs, an Approximate Nearest Neighbor (ANN) search is more appealing than exhaustive comparing with sublinear query complexity [20].

There has been extensive

The proposed approach

In this section, we will introduce dual local consistency hashing, which do not only make the similar images have similar binary codes but also dissimilar images with different binary codes. In addition, we introduce a more effective projection selecting scheme that can sequentially choose the most discriminative projection for each hash function. Therefore, the binary codes learned by our method perform much better than the other approaches.

Experiments

In this section, we evaluate the proposed approach on the USPS digit and CIFAR-10 datasets. We compare its performance with five state-of-the-art binary coding methods:

  • 1.

    Locality Sensitive Hashing (LSH) [1];

  • 2.

    Restricted Boltzmann Machines (RBMs) [9];

  • 3.

    Spectral Hashing (SH) [31];

  • 4.

    Semi-Supervised Hashing (SSH) [25];

  • 5.

    Sequential Projection Learning for Hashing (SPLH) [26].

Conclusion

In this paper, we propose a novel hashing approach for fast image retrieval. Unlike many traditional hashing methods which only preserve the similarity structure of images in a global manner, our method is based on dual local consistency. In our approach, not only the similar images are projected to the same hash codes, but also the dissimilar images are projected to different hash codes. Moreover, our approach adopts a more discriminative projection selecting scheme, which can choose more

Acknowledgement

This work was supported in part by the 973 Programm under Project 2010CB327905, by the National Natural Science Foundation of China under Grant 61170127, 60975010, 60833006, and 61070104.

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