Unsupervised Hashing Methods Research Articles

Error Correcting Input and Output Hashing.

Most learning-based hashing algorithms leverage sample-to-sample similarities, such as neighborhood structure, to generate binary codes, which achieve promising results for image retrieval. This type of methods are referred to as instance-level encoding. However, it is nontrivial to define a scalar to represent sample-to-sample similarity encoding the semantic labels and the data structure. To address this issue, in this paper, we seek to use a class-level encoding method, which encodes the class-to-class relationship, to take the semantic information of classes into consideration. Based on these two encodings, we propose a novel framework, error correcting input and output (EC-IO) coding, which does class-level and instance-level encoding under a unified mapping space. Our proposed model contains two major components, which are distribution preservation and error correction. With these two components, our model maps the input feature of samples and the output code of classes into a unified space to encode the intrinsic structure of data and semantic information of classes simultaneously. Under this framework, we present our hashing model, EC-IO hashing (EC-IOH), by approximating the mapping space with the Hamming space. Extensive experiments are conducted to evaluate the retrieval performance, and EC-IOH exhibits superior and competitive performances comparing with popular supervised and unsupervised hashing methods.

Validating User Flows to Protect Software Defined Network Environments

Software Defined Network is a promising network paradigm which has led to several security threats in SDN applications that involve user flows, switches, and controllers in the network. Threats as spoofing, tampering, information disclosure, Denial of Service, flow table overloading, and so on have been addressed by many researchers. In this paper, we present novel SDN design to solve three security threats: flow table overloading is solved by constructing a star topology-based architecture, unsupervised hashing method mitigates link spoofing attack, and fuzzy classifier combined with L1-ELM running on a neural network for isolating anomaly packets from normal packets. For effective flow migration Discrete-Time Finite-State Markov Chain model is applied. Extensive simulations using OMNeT++ demonstrate the performance of our proposed approach, which is better at preserving holding time than are other state-of-the-art works from the literature.

A General Framework for Linear Distance Preserving Hashing.

Binary hashing approaches the approximate nearest neighbor search problem by transferring the data to Hamming space with explicit or implicit distance preserving constraint. With compact data representation, binary hashing identifies the approximate nearest neighbors via very efficient Hamming distance computation. In this paper, we propose a generic hashing framework with a new linear pairwise distance preserving objective and pointwise constraint. In our framework, the direct distance preserving objective aims to keep the linear relationship between the Euclidean distance and the Hamming distance of data points. On the other hand, to impose the pointwise constraint, we instantiate the framework from three different perspectives with pseudo-supervised, unsupervised, and supervised clues and obtain three different hashing methods. The first one is a pseudo-supervised hashing method, which adopts a certain existing unsupervised hashing method to generate binary codes as pseudo-supervised information. For the second one, we get an unsupervised hashing method by considering the quantization loss. The third one, as a supervised hashing method, learns the hash functions in a two-step paradigm. Furthermore, we improve the above-mentioned framework by constraining the global scope of the proposed linear distance preserving objective to a local range. We validate our framework on four large-scale benchmark data sets. The experiments demonstrate that our pseudo-supervised method achieves consistent improvement over the state-of-the-art unsupervised hashing methods, while our unsupervised and supervised methods achieve promising performance compared with the state-of-the-art algorithms.

Comprehensive analysis and evaluation to unsupervised binary hashing method in image similarity measurement

This study proposes an unsupervised binary hashing (UBH) method and provides a comprehensive analysis and evaluation to the UBH method in image similarity. Using the orthogonal locality preserving projection, the proposed UBH method performs the dimensionality reduction (DR). To reduce the quantisation error between low-dimensional vectors and binary hash codes generated in the DR, the proposed UBH method calculates the optimal parameters of rotation and offset. The authors use two test beds to evaluate the preservation of original feature space and the semantic consistency. Experimental results with two test beds show that the proposed UBH method is state of the art.

Adaptive Hashing with Sparse Modification for Scalable Image Retrieval

Approximate Nearest Neighbor (ANN) search is a challenging problem with the explosive high-dimensional large-scale data in recent years. The promising technique for ANN search include hashing methods which generate compact binary codes by designing effective hash functions. However, lack of an optimal regularization is the key limitation of most of the existing hash functions. To this end, a new method called Adaptive Hashing with Sparse Modification (AHSM) is proposed. In AHSM, codes consist of vertices on the hypercube and the projection matrix is divided into two separate matrices. Data is rotated through a orthogonal matrix first and modified by a sparse matrix. Here the sparse matrix needs to be learned as a regularization item of hash function which is used to avoid overfitting and reduce quantization distortion. Totally, AHSM has two advantages: improvement of the accuracy without any time cost increasement. Furthermore, we extend AHSM to a supervised version, called Supervised Adaptive Hashing with Sparse Modification (SAHSM), by introducing Canonical Correlation Analysis (CCA) to the original data. Experiments show that the AHSM method stably surpasses several state-of-the-art hashing methods on four data sets. And at the same time, we compare three unsupervised hashing methods with their corresponding supervised version (including SAHSM) on three data sets with labels known. Similarly, SAHSM outperforms other methods on most of the hash bits.

Multiple hierarchical deep hashing for large scale image retrieval

Learning-based hashing methods are becoming the mainstream for large scale visual search. They consist of two main components: hash codes learning for training data and hash functions learning for encoding new data points. The performance of a content-based image retrieval system crucially depends on the feature representation, and currently Convolutional Neural Networks (CNNs) has been proved effective for extracting high-level visual features for large scale image retrieval. In this paper, we propose a Multiple Hierarchical Deep Hashing (MHDH) approach for large scale image retrieval. Moreover, MHDH seeks to integrate multiple hierarchical non-linear transformations with hidden neural network layer for hashing code generation. The learned binary codes represent potential concepts that connect to class labels. In addition, extensive experiments on two popular datasets demonstrate the superiority of our MHDH over both supervised and unsupervised hashing methods.

Latent Semantic Minimal Hashing for Image Retrieval.

Hashing-based similarity search is an important technique for large-scale query-by-example image retrieval system, since it provides fast search with computation and memory efficiency. However, it is a challenge work to design compact codes to represent original features with good performance. Recently, a lot of unsupervised hashing methods have been proposed to focus on preserving geometric structure similarity of the data in the original feature space, but they have not yet fully refined image features and explored the latent semantic feature embedding in the data simultaneously. To address the problem, in this paper, a novel joint binary codes learning method is proposed to combine image feature to latent semantic feature with minimum encoding loss, which is referred as latent semantic minimal hashing. The latent semantic feature is learned based on matrix decomposition to refine original feature, thereby it makes the learned feature more discriminative. Moreover, a minimum encoding loss is combined with latent semantic feature learning process simultaneously, so as to guarantee the obtained binary codes are discriminative as well. Extensive experiments on several well-known large databases demonstrate that the proposed method outperforms most state-of-the-art hashing methods.

Semi-Paired Discrete Hashing: Learning Latent Hash Codes for Semi-Paired Cross-View Retrieval.

Due to the significant reduction in computational cost and storage, hashing techniques have gained increasing interests in facilitating large-scale cross-view retrieval tasks. Most cross-view hashing methods are developed by assuming that data from different views are well paired, e.g., text-image pairs. In real-world applications, however, this fully-paired multiview setting may not be practical. The more practical yet challenging semi-paired cross-view retrieval problem, where pairwise correspondences are only partially provided, has less been studied. In this paper, we propose an unsupervised hashing method for semi-paired cross-view retrieval, dubbed semi-paired discrete hashing (SPDH). In specific, SPDH explores the underlying structure of the constructed common latent subspace, where both paired and unpaired samples are well aligned. To effectively preserve the similarities of semi-paired data in the latent subspace, we construct the cross-view similarity graph with the help of anchor data pairs. SPDH jointly learns the latent features and hash codes with a factorization-based coding scheme. For the formulated objective function, we devise an efficient alternating optimization algorithm, where the key binary code learning problem is solved in a bit-by-bit manner with each bit generated with a closed-form solution. The proposed method is extensively evaluated on four benchmark datasets with both fully-paired and semi-paired settings and the results demonstrate the superiority of SPDH over several other state-of-the-art methods in term of both accuracy and scalability.

Margin-based two-stage supervised hashing for image retrieval

Similarity-preserving hashing is a widely used method for nearest neighbor search in large-scale image retrieval. Recently, supervised hashing methods are appealing in that they learn compact hash codes with fewer bits by incorporating supervised information. In this paper, we propose a new two-stage supervised hashing methods which decomposes the hash learning process into a stage of learning approximate hash codes followed by a stage of learning hash functions. In the first stage, we propose a margin-based objective to find approximate hash codes such that a pair of hash codes associating to a pair of similar (dissimilar) images has sufficiently small (large) Hamming distance. This objective results in a challenging optimization problem. We develop a coordinate descent algorithm to efficiently solve this optimization problem. In the second stage, we use convolutional neural networks to learn hash functions. We conduct extensive evaluations on several benchmark datasets with different kinds of images. The results show that the proposed margin-based hashing method has substantial improvement upon the state-of-the-art supervised or unsupervised hashing methods.

Instance-Aware Hashing for Multi-Label Image Retrieval.

Similarity-preserving hashing is a commonly used method for nearest neighbor search in large-scale image retrieval. For image retrieval, deep-network-based hashing methods are appealing, since they can simultaneously learn effective image representations and compact hash codes. This paper focuses on deep-network-based hashing for multi-label images, each of which may contain objects of multiple categories. In most existing hashing methods, each image is represented by one piece of hash code, which is referred to as semantic hashing. This setting may be suboptimal for multi-label image retrieval. To solve this problem, we propose a deep architecture that learns instance-aware image representations for multi-label image data, which are organized in multiple groups, with each group containing the features for one category. The instance-aware representations not only bring advantages to semantic hashing but also can be used in category-aware hashing, in which an image is represented by multiple pieces of hash codes and each piece of code corresponds to a category. Extensive evaluations conducted on several benchmark data sets demonstrate that for both the semantic hashing and the category-aware hashing, the proposed method shows substantial improvement over the state-of-the-art supervised and unsupervised hashing methods.

Deep and fast: Deep learning hashing with semi-supervised graph construction

Learning-based hashing methods are becoming the mainstream for approximate scalable multimedia retrieval. They consist of two main components: hash codes learning for training data and hash functions learning for new data points. Tremendous efforts have been devoted to designing novel methods for these two components, i.e., supervised and unsupervised methods for learning hash codes, and different models for inferring hashing functions. However, there is little work integrating supervised and unsupervised hash codes learning into a single framework. Moreover, the hash function learning component is usually based on hand-crafted visual features extracted from the training images. The performance of a content-based image retrieval system crucially depends on the feature representation and such hand-crafted visual features may degrade the accuracy of the hash functions. In this paper, we propose a semi-supervised deep learning hashing (DLH) method for fast multimedia retrieval. More specifically, in the first component, we utilize both visual and label information to learn an optimal similarity graph that can more precisely encode the relationship among training data, and then generate the hash codes based on the graph. In the second stage, we apply a deep convolutional network to simultaneously learn a good multimedia representation and a set of hash functions. Extensive experiments on five popular datasets demonstrate the superiority of our DLH over both supervised and unsupervised hashing methods.

Unsupervised Local Feature Hashing for Image Similarity Search.

The potential value of hashing techniques has led to it becoming one of the most active research areas in computer vision and multimedia. However, most existing hashing methods for image search and retrieval are based on global feature representations, which are susceptible to image variations such as viewpoint changes and background cluttering. Traditional global representations gather local features directly to output a single vector without the analysis of the intrinsic geometric property of local features. In this paper, we propose a novel unsupervised hashing method called unsupervised bilinear local hashing (UBLH) for projecting local feature descriptors from a high-dimensional feature space to a lower-dimensional Hamming space via compact bilinear projections rather than a single large projection matrix. UBLH takes the matrix expression of local features as input and preserves the feature-to-feature and image-to-image structures of local features simultaneously. Experimental results on challenging data sets including Caltech-256, SUN397, and Flickr 1M demonstrate the superiority of UBLH compared with state-of-the-art hashing methods.

Linear unsupervised hashing for ANN search in Euclidean space

Approximate nearest neighbors (ANN) search for large scale data has attracted considerable attention due to the fact that large amounts of data are easily available. Recently, hashing has been widely adopted for similarity search because of its good potential for low storage cost and fast query speed. Among of them, when semantic similarity information is available, supervised hashing methods show better performance than unsupervised ones. However, supervised hashing methods need explicit similarity information which is not available in some scenarios. In addition, they have the problems of difficult optimization and time consuming for training, which make them unpracticable to large scale data. In this paper, we propose an unsupervised hashing method – Unsupervised Euclidean Hashing (USEH), which learns and generates hashing codes to preserve the Euclidean distance relationship between data. Specifically, USEH first utilizes Locality-Sensitive Hashing (LSH) to generate pseudo labels; then, it adopts a sequential learning strategy to learn the hash functions, one bit at a time, which can generate very discriminative codes. Moreover, USEH avoids explicitly computing the similarity matrix by decomposing it into the product of a label matrix and its transposition, which makes the training complexity of USEH linear to the size of training samples when the number of training samples is much greater than the dimension of feature. Thus, it can efficiently work on large scale data. We test USEH on two large scale datasets – SIFT1M and GIST1M. Experimental results show that USEH is comparable to state-of-the-art unsupervised hashing methods.

Semi-supervised constraints preserving hashing

With the ever-increasing amount of multimedia data on the web, hashing-based approximate nearest neighbor search methods have attracted significant attention due to its remarkable efficiency gains and storage reductions. Traditional unsupervised hashing methods are designed for preserving distance metric similarity which may lead to semantic gap among the high-level semantic similarities. Recently, attentions have been paid to semi-supervised hashing methods which can preserve data׳s a few available semantic similarities (usually given in terms of labels, pairwise constraints, tags, etc.). However, these methods often preserve semantic similarities for low-dimensional embeddings. When converting low-dimensional embeddings into binary codes, the quantization error will be accumulated thus resulting in performance deterioration. To this end, we propose a novel semi-supervised hashing method which preserves pairwise constraints for both low-dimensional embeddings and binary codes. It first represents data points by cluster centers to preserve data neighborhood structure and reduce the dimensionality. Then the constraint information is fully utilized to embed the derived data representations into a discriminative low-dimensional space by maximizing discriminative Hamming distance and data variance. After that, optimal binary codes are obtained by further preserving the semantic similarities in the process of quantizing the low-dimensional embeddings. By utilizing constraint information in the quantization process, the proposed method can fully preserve pairwise semantic similarities for binary codes thus leading to better retrieval performance. Thorough experiments on standard databases show the superior performance of the proposed method.

Unsupervised binary hashing method using locality preservation and quantisation error minimisation

An unsupervised binary hashing (UBH) method is proposed. To preserve the local and Euclidean metric structures in the reduced feature space, it performs the dimensionality reduction (DR) by using the orthogonal locality-preserving projection. In addition, it minimises the error between the generated binary hash codes and low-dimensional feature vectors that are obtained in DR. To minimise the quantisation error, the binary hash codes are generated using the optimal rotation and offset. Experimental results show that the proposed UBH method has better performance than other existing methods in terms of the mean average precision and recall–precision curve.

Supervised Hashing for Image Retrieval via Image Representation Learning

Hashing is a popular approximate nearest neighbor search approach for large-scale image retrieval. Supervised hashing, which incorporates similarity/dissimilarity information on entity pairs to improve the quality of hashing function learning, has recently received increasing attention. However, in the existing supervised hashing methods for images, an input image is usually encoded by a vector of hand-crafted visual features. Such hand-crafted feature vectors do not necessarily preserve the accurate semantic similarities of images pairs, which may often degrade the performance of hashing function learning. In this paper, we propose a supervised hashing method for image retrieval, in which we automatically learn a good image representation tailored to hashing as well as a set of hash functions. The proposed method has two stages. In the first stage, given the pairwise similarity matrix $S$ over training images, we propose a scalable coordinate descent method to decompose $S$ into a product of $HH^T$ where $H$ is a matrix with each of its rows being the approximate hash code associated to a training image. In the second stage, we propose to simultaneously learn a good feature representation for the input images as well as a set of hash functions, via a deep convolutional network tailored to the learned hash codes in $H$ and optionally the discrete class labels of the images. Extensive empirical evaluations on three benchmark datasets with different kinds of images show that the proposed method has superior performance gains over several state-of-the-art supervised and unsupervised hashing methods.

Maximum Variance Hashing via Column Generation

With the explosive growth of the data volume in modern applications such as web search and multimedia retrieval, hashing is becoming increasingly important for efficient nearest neighbor (similar item) search. Recently, a number of data-dependent methods have been developed, reflecting the great potential of learning for hashing. Inspired by the classic nonlinear dimensionality reduction algorithm—maximum variance unfolding, we propose a novel unsupervised hashing method, named maximum variance hashing, in this work. The idea is to maximize the total variance of the hash codes while preserving the local structure of the training data. To solve the derived optimization problem, we propose a column generation algorithm, which directly learns the binary-valued hash functions. We then extend it using anchor graphs to reduce the computational cost. Experiments on large-scale image datasets demonstrate that the proposed method outperforms state-of-the-art hashing methods in many cases.