Relative Distribution Entropy Loss Function in CNN Image Retrieval.

Convolutional neural networks (CNN) is the most mainstream solution in the field of image retrieval. Deep metric learning is introduced into the field of image retrieval, focusing on the construction of pair-based loss function. However, most pair-based loss functions of metric learning merely take common vector similarity (such as Euclidean distance) of the final image descriptors into consideration, while neglecting other distribution characters of these descriptors. In this work, we propose relative distribution entropy (RDE) to describe the internal distribution attributes of image descriptors. We combine relative distribution entropy with the Euclidean distance to obtain the relative distribution entropy weighted distance (RDE-distance). Moreover, the RDE-distance is fused with the contrastive loss and triplet loss to build the relative distributed entropy loss functions. The experimental results demonstrate that our method attains the state-of-the-art performance on most image retrieval benchmarks.

1. Introduction

In recent years, the newly proposed image retrieval algorithms based on convolutional neural networks [1,2,3,4,5] (Cn class="Chemical">Npan> class="Chemical">N) have greatly improved the accuracy and efficiency. This has been the mainstream direction of academic research of image retrieval. In the beginning, CNN could only be applied to image classification tasks [6,7,8]. However, image classification is different from the image retrieval. Krizhevsky [9] flexibly applied a convolutional neural network to image retrieval. AlexNet [9] is designed for image classification and retrieval. Subsequently, Noh [10] proposed a Local Feature descriptor, called DELF (Deep Local Feature), which is suitable for large-scale image retrieval. A large number of studies have illustrated that the output features of the convolutional layer of the neural network have excellent discrimination and scalability. More recently, image retrieval algorithms based on convolutional neural networks emerged one after another. These methods are mainly summarized into three categories: fine-tuned networks, pre-trained networks, and hybrid networks. Among them, hybrid networks are less efficient in image retrieval tasks, and pre-trained networks are widely used. The fine-tuned network initializes the network architecture through the pre-trained image classification model and then adjusts the parameters according to different retrieval tasks. The fine-tuned network usually optimizes the network parameters by training the network architecture of metric learning. Metric learning aims to learn an embedding space, where the embedded vectors of positive samples are encouraged to be closer, while negative samples are pushed apart from each other [11,12,13]. Recently, a lot of deep metric learning methods have been based on pairs of samples such as contrastive loss [14], triplet loss [15], quadruplet loss [16], lifted structured loss [17], N-pairs loss [18], binomial deviance loss [19], histogram loss [20], angular loss [21], distance weighted margin-based loss [22], and hierarchical triplet loss (HTL) [23]. Most of the above-mentioned loss functions take common vector similarity (such as Euclidean distance) as the final image descriptor into consideration. However, it is not accurate enough to measure the similarity between features only by Euclidean distance, which lacks the difference in the internal spatial distribution [24] of the image pair. As illustrated in Figure 1, each rectangle represents a feature descriptor obtained after the convolution of the neural network. The value of Euclidean distance between descriptors for different images may be the equal or small, but the spatial distribution of every descriptor may be greatly different. In the information processing field, entropy [25] is an effective measurement to reflect the distribution information. Relative entropy [26,27,28] is a measure of the distance between two random distributions, which is equivalent to the difference of information entropy between two distributions. Inspired by this, we introduce the idea of relative entropy into image retrieval.

Figure 1

Each rectangle represents a feature descriptor obtained after the convolution of the neural network. Different colors of small squares represent different feature intensities in the descriptor. The Euclidean distance () between descriptor n and descriptor q is , while the Euclidean distance () between descriptor p and descriptor q is . D1 is approximately equal to D2, but the internal spatial distribution of p and n is obviously different.

To solve the key problem mentioned above, we propose relative distribution entropy (RDE) to describe the distribution attributes of image descriptors. We combine the relative distribution entropy with the Euclidean distance [29] to build the relative distribution entropy weighted distance (RDE-distance). We n class="Gene">fuse the RDE-distanpan>ce into the conpan>trastive loss anpan>d triplet loss to obtain the relative distributed enpan>tropy conpan>trastive loss anpan>d the relative distributed enpan>tropy triplet loss. We call them the relative distributed pan> class="Disease">entropy loss functions. To be more specific, we make three contributions, as follows:

Firstly, we propose a loss function modified by relative distribution entropy, called the relative distribution n class="Disease">entropy loss function. Furthermore, the relative distributionpan> pan> class="Disease">entropy loss function contains the following two aspects: (1) Euclidean distance between the descriptors. (2) The differences in internal distribution state between descriptors. The core idea of the algorithm is illustrated in Figure 2. We combine the Euclidean distance with the relative distribution entropy to obtain the relative distribution entropy weighted distance (RDE-distance), which increases the discrimination between the image pair. We replace the Euclidean distance in the original contrastive loss and triplet loss with the relative distribution entropy weighted distance (RDE-distance) to obtain the relative distribution entropy loss function.

Figure 2

The core idea of relative distribution entropy (RDE)-loss. D represents the Euclidean distance between two image descriptors. RDE represents the relative distribution entropy between two image descriptors. RDE-distance is a new metric that combines Euclidean distance with relative distributed entropy. We called it the relative distribution entropy weighted distance, which can enhance the discrimination of image descriptors.

Secondly, during the experiment, we use n class="Chemical">GeM pooling [30] anpan>d whitening post-processing [31].

Thirdly, we employ the fine-tuned network [30] to perform our experiments on different datasets to verify the effectiveness of our proposed method.

The organization of this work is as follows. Section 2 introduces the related work. Section 3 mainly introduces our proposed relative distribution n class="Disease">entropy loss function. Specific experimental results anpan>d anpan>alyses will be presented in Sectionpan> 4. Sectionpan> 5 is a summary of the conpan>tributionpan>s anpan>d methods of this paper.

2. Related Work

2.1. Deep Metric Learning

Deep metric learning (DML) has become one of the most interesting research areas in n class="Chemical">machine learnpan>ing. Metric learnpan>ing aims to learnpan> anpan> embedding space. Inpan> this space, images are conpan>verted into embedding vectors. The distanpan>ce betweenpan> embedding vectors of positive samples is small, anpan>d the distanpan>ce betweenpan> embedding vectors of negative samples is large [11,12,13]. pan> class="Chemical">Now, deep metric learning has played a vital role in many areas, such as face recognition [32], pedestrian recognition [33], fine-grained retrieval [34], image retrieval [35], target tracking [36], and multimedia retrieval [37]. We will summarize the recent emergence of metric learning methods in the next section. Here, we introduce deep metric learning into image retrieval. Deep metric learning aims to learn a discriminative feature embedding for input image . In other words, is the descriptor of the image. Formally, we define the Euclidean distance between two descriptors as .

2.1.1. Contrastive Loss

The siamese network [14] is a typical pair-based method. Its embedding is obtained through contrastive loss. The purpose of contrastive loss is to reduce the Euclidean distance between positive samples and increase the Euclidean distance between negative samples. The equation of the contrastive loss function is illustrated in (1). if the sample is a positive sample, and if the sample is a negative sample. is the margin. This keeps negative distances above a certain threshold.

2.1.2. Triplet Loss

However, the above models only focus on the similarity of intra-class of the samples. To solve this problem, the triplet loss function [15] is proposed. Each triplet comprises a positive sample and a negative sample sharing the query. Triplet loss aims to learn an embedding space. In this space, the distance between the query and the negative sample is greater than the distance between the query and the positive sample. The triplet loss function is illustrated in (2).

Here, represents the Euclidean distance between the descriptors of the positive sample and the query. Similarly, represents the Euclidean distance between the descriptors of the negative sample and the query. is the violate margin that requires that the negative distances to be larger than the positive distances.

2.1.3. N-pair Loss

Triplet loss function only compares one negative sample and ignores the negative samples of other classes during the learning stage. As a result, the embedding vector of the query can only be promoted to maintain a large distance between the selected negative samples, but it cannot guarantee to maintain a large distance between the embedding vector and other non-selected negative samples.

N-pair loss [18] has improved the above problems. Unlike triplet loss functionpan>, pan> class="Chemical">N-pair loss considers the relationship between query samples and other negative samples of different classes within a mini-batch. The equation of the N-pair loss function is illustrated in (3).

Each training tuple of n class="Chemical">N-pair loss is composed of samples: . is the positive sample. are the negative samples.

2.1.4. Lifted Structured Loss

The existing triplet loss methods cannot take full advantage of mini-batch SGD training. Lifted structured n class="Disease">loss calculates loss [17] based onpan> all positive anpan>d negative sample pairs in the training set (mini-batch). The lifted structured loss functionpan> is illustrated in Equationpan> (4). where is the set of positive samples in the training set, anpan>d is the set of positive samples in the training set. is the violate margin.

Although these loss functions calculate the distance of descriptors between images, they neglect the difference in internal distribution between the image pair. In this work, we propose a new loss function, which is called the relative distribution n class="Disease">entropy loss function. We use the relative distributionpan> enpan>tropy to reflect the differenpan>ce in the descriptor distributionpan>, anpan>d add this differenpan>ce to the loss functionpan>. Inpan> this way, the new loss functionpan> canpan> combine the Euclideanpan> distanpan>ce with the internpan>al relative differenpan>ce in the distributionpan> state betweenpan> the descriptors of the image pair.

2.2. Application of Spatial Information in Image Retrieval

The performance of image retrieval has been greatly improved in recent years through the use of deep feature representations. However, most existing methods aim to retrieve images that are visually similar or semantically relevant to the query, without considering the spatial information. Before that, some researchers attempted to add spatial information into image retrieval algorithms to improve retrieval performance. Mehmood [38] proposed adding a local region and a global histogram to the BoW algorithm and combining them as the final descriptor. Krapac [39] used the BoW descriptor to encode the spatial part of the image, which improved the performance of the image classification. Koniusz [40] used spatial coordinate coding to represent and simplify the spatial pyramid to provide more compact image features. Sanchezet [41] added the spatial position information of features into the descriptor, which overcame the change in the proportion of retrieved objects and the change in the local area of the image. Liu [42] introduced the concept of spatial distribution entropy and combined it with the VLAD algorithm. These methods have fully introduced the spatial information into the image retrieval algorithm, and great effects are obtained. However, these methods all aimed at the improvement of traditional image descriptors. In this work, we attempt to add spatial information to the descriptor by optimizing the loss function of a deep convolutional network.

2.3. Pooling and Normalization

The feature map generated by the deep retrieval framework reflects the color, texture, shape, and other characteristics of the image. Since the convolutional neural network needs to be integrated into a multi-dimensional feature descriptor before retrieval, the feature map of the convolutional layer needs to be further processed. It is required that the processed result retains the main features of the image while reducing the parameters and calculations of the next layer. Babenko [43] proposed the mean-pooling method, which sums the pixel values of the feature map to obtain an n class="Chemical">N-dimenpan>sionpan>al feature vector. Razavianpan> [44] used max-pooling or meanpan>-pooling onpan> the feature descriptors anpan>d the result reduced pan> class="Chemical">MAC [44] descriptors in dimension with a series of normalization and PCA [45] whitening operations. Finally, the region feature vectors are summed to obtain a single image representation. In this work, we used the generalized mean-pooling [30]. We used to represent the input to the pooling layer and to represent the pooling layer output. The mentioned pooling can be expressed as follows:

Max pooling (n class="Chemical">MAC vector):

Average pooling (n class="Chemical">SPoC vector [43]):

Generalized mean pooling (n class="Chemical">GeM [30]): where is the number of feature maps, meanpan>s the chanpan>nel of features anpan>d is the number of feature values in the -th chanpan>nel feature map. The descriptor finally conpan>sists of a single value per feature map. is the pooling parameter, which canpan> be manpan>ually set or learned. The superscript of is the pooling method. represent the max pooling, average pooling anpan>d generalized meanpan> pooling, respectively.

The max-pooling and mean-pooling are special cases of generalized mean-pooling. When, it is the mean-pooling. On the contrary, when it is positive infinity, it is the max-pooling. Generalized mean-pooling with parameters can better adapt to the network and improve retrieval performance.

In this work, we use normalization to balance the effect of the range of pixel values as follows: where represents a vector, represents the norm of the vector, and represents the value of the dimension on the vector.

2.4. Whitening

In large-scale image retrieval applications, high-dimensional global images typically require the use of PCA to reduce the dimensions of features for the next step. Jegou and Chum [45] studied the influence of PCA on the BoW and VLAD descriptor representations, and highlighted the use of multiple visual dictionaries for dimensionality reduction, thereby reducing the information loss of the dimensionality reduction process. In this work, whitening is used as a post-processing step. In this paper, the method of whitening is the linear discriminant projection proposed by Mikolajczyk and Matas [31].

The processing steps are divided into two parts. In the first part, the intra-class image feature vector is whitened. The whitening part is the reciprocal of the square root of the intra-class image pair (matched image pair) covariance matrix . where and are the descriptors of the image after pooling, represents the image matched pair, and represents the covariance matrix of the matched image pair.

In the second part, the inter-class image feature is rotated. The rotating part is the eigenvector of the covariance matrix between inter-class image pair (the non-matched image pair) where and are the descriptors of the image after pooling, represents the non-matched image pair, and represents the covariance matrix of the non-matched image pair.

Then, we apply the projection to , where is the mean n class="Chemical">GeM [30] vector to perform centering.

3. Method Overview

3.1. Calculation of Relative Distribution Entropy

We firstly introduce the concept of relative distribution entropy. Relative distribution entropy can better represent the distribution difference between two descriptors of image samples. The relative distribution entropy is derived from the relative entropy. Relative entropy can be computed as follows: and are the two probability distributions on the random variable . From this, we can get the equation of relative distribution entropy (RDE). where represent the images. represents the descriptor of image . It is a normalized vector. is the relative distribution entropy of two images. In this work, we use histograms to describe the distribution of image descriptors. is the number of bins, and is an adjustable parameter. is the probability distribution in the bin. The equation for is the same as for. is the dimension of descriptors.

3.2. Relative Distribution Entropy Loss Function

From the above, we introduce the calculation of relative distributed entropy. n class="Chemical">Next, we show how to add the relative distributionpan> entropy into the loss functionpan>. We add the relative distributionpan> entropy to conpan>trastive loss [14] anpan>d triplet loss [15] to build relative distributionpan> pan> class="Disease">entropy loss functions.

Firstly, we introduce the fusion process of contrastive loss and relative distribution entropy. The equation of the contrastive loss function is shown in (14). where represents the Euclidean distance between the descriptors of the query and the sample . if the sample is a positive sample, and if the sample is a negative sample. is the margin.

Then, we add the relative distribution entropy to to get the : where is the weighting parameter of the relative distribution entropy. As illustrated in Equation (16), we get the new distance metric. We call it the relative distribution entropy weighted distance (RDE-distance). We substitute into (14) to get the new contrastive loss function, as is shown in Equation (17):

Similarly, we introduce the fusion process of triplet loss and relative distribution entropy. The equation of the contrastive loss function is shown in (18). is the Euclidean distance between the descriptors of the positive sample and the query. Similarly, is the Euclidean distance between the descriptors of the negative sample and the query.

Then, we add the relative distribution entropy to to get the.

We substitute the new Euclidean distances into Equation (18) and get the new triplet loss.

3.3. Network Architecture for Relative Distribution Entropy Loss Function

3.3.1. CNN Network Architecture

We construct a CNpan> class="Chemical">N neural network to obtain the descriptor of the image. We only use the convolutional layers, discarding the fully connected layer. Convolution layers can extract features of images. The feature map obtained by the convolution layer is vectorized by the GeM pooling operation [30]. If the whitening operation is performed, the whitening is processed following the pooling layer. Whitening can reduce the correlation between features, and it can make the features share the same variance (covariance matrix is 1), which can greatly improve the image retrieval performance. Here, we use the whitening [31] method. The last step is the normalization operation. The purpose of normalization is to make the preprocessed data limited to a certain range (such as [0,1] or [−1,1]), thereby eliminating the adverse effects caused by the singular sample data. The network architecture is pre-trained in ImageNet [46] network architecture.

Furthermore, we adopt network architectures such as ResNet [47] anpan>d pan> class="Chemical">AlexNet [9], and these two networks are also pre-trained on ImageNet [46].

The Cn class="Chemical">Npan> class="Chemical">N network architecture is shown in Figure 3.

Figure 3

Convolutional neural network (CNN) network architecture.

3.3.2. Architecture of Training

The training procedure consists of multiple networks sharing the same weight. The architecture of Cn class="Chemical">Npan> class="Chemical">N is introduced in the previous part. We added our newly proposed relative distribution entropy to the previous loss function, as shown in Figure 4 and Figure 5:

Figure 4

Training process using relative distribution entropy contrastive loss.

Figure 5

Training process using relative distribution entropy triplet loss.

In Figure 4 and Figure 5, D represents the Euclidean distance between the two descriptors, and represents the relative distribution entropy between the two descriptors. RDE-distance is the relative distribution entropy weighted distance obtained after the fusion of D and RDE.

4. Experiments and Evaluation

In this section, we discuss the implementation details of training and testing. Also, we analyze the experimental results and compare them with previous work.

4.1. Training Datasets

In this work, experimental training data are distilled from Retrieval-SFM-120K [48], which contains 7.4 million images. After clustering [49], we get about 20,000 images as the query seed. The structure-from-motion (SfM) algorithm constructs 1474 3D models from the training datasets. We removed the duplications and retained 713 of them, which contained more than 163,000 different images.

There are 91,642 training images in the dataset, and 98 cluster images that are the same or almost the same as the test dataset. Through the minimum hash and spatial verification methods mentioned in the clustering process, about 20,000 images are selected as query images, 18,1697 pairs of positive images and 551 training clusters, including more than 163,000 clusters [50] from the original dataset. The dataset contains all images from the Oxford 5k [51] and Paris 6k [52] datasets.

4.2. Training Configurations

In the experiments, we use the Pytorch deep learning framework to train the deep network model. We use ResNet [47], VGG [53] anpan>d pan> class="Chemical">AlexNet [9], which are all pre-trained on ImageNet [46].

In the experiment of relative distribution entropy contrastive loss, ResNet [47] anpan>d VGG [53] are trained using Adam learning strategy [54], while pan> class="Chemical">AlexNet [9] is trained using SGD. Our initial learning rate for Adam is , and the margin for ResNet and VGG are 0.95 and 0.9. We use an initial learning rate equal to for SGD, and the margin for AlexNet is 0.75.

In the experiment of triplet loss, we also use ResNet [47], VGG [53], anpan>d pan> class="Chemical">AlexNet [9] to initialize the network. They are trained using the Adam learning strategy [54]. Our initial learning rate for Adam is . The margin for ResNet and VGG are 0.5, and the margin for AlexNet is 0.3. The size of the training image is not more than 362 * 362 while maintaining the aspect ratio of the original image.

The experimental environment is an intel(R) i7-8700 processor, GPU with 12GB of memory, n class="Chemical">NVIDIA(R) 2080Ti graphics card, driver versionpan> 419.**. Operating system is Ubuntu 18.04 LTS, PyTorch versionpan> v1.0.0, CUDA versionpan> 10.0, CUDpan> class="Chemical">NN version 7.5. The time spent in each training cycle trained on our method on VGG, ResNet, and AlexNet is 0.48, 0.72, and 0.22 hours, respectively. During the testing phase, testing VGG, ResNet, and AlexNet networks takes 620, 990, and 277 seconds, respectively. There are subtle differences between different test sets. With the same computing power, the training time of our method is almost the same as that of other methods [30].

4.3. Datasets and Evaluation of Image Retrieval

We conduct our testing experiments on the following benchmark datasets frequently. Herein, we give the details of these datasets.

Oxford5k [51] is a widely used landmark dataset consisting of 5062 building images from the Flickr dataset. It contains 11 famous landmarks in the Oxford area, and each landmark building has 55 query images.

Paris6k [52] contains 6392 images and is also one of the widely used datasets in the field of image retrieval. It collects many landmark buildings in Paris, and most of these images are from tourists. Similar to the Oxford 5k dataset, it also has 55 query images.

In addition, we use 100k interference images to n class="Gene">fuse with the Oxford5k anpan>d Paris6k datasets to obtain Oxford105k [51] anpan>d Paris106k [52].

In the experiments, we use mean average precision (mAP) to measure the performance of image retrieval.

4.4. Results and Analysis

4.4.1. The Adjustment Process of Hyperparameter

In this experiment, two hyperparameters and are adjusted to obtain best performance. is the weight of the relative distribution entropy. As mentioned in Section 3.2, when fusing the Euclidean distance with RDE, the weight will affect the ratio of Euclidean distance and relative distribution entropy to the finally generated RDE-distance. As mentioned in Section 3.1, is the total bin amount in the histogram during the calculation of RDE. will affect the degree of differentiation of internal spatial distribution differences between two descriptors. However, a large also increases the computational burden. The ability of RDE-distance to distinguish between two descriptors is determined by two factors and . The values of these two hyperparameters have a great impact on our experimental results. During our experiments, we adjust them to get the best performance. We use n class="Chemical">AlexNet anpan>d VGG networks for tuning anpan>d the pan> class="Chemical">GeM pooling [30]. The partially representative results are shown in Table 1.

Table 1

Experimental results of hyperparameter comparison in the relative distribution entropy contrastive loss function. The best results would be highlighted in bold.

Network	α	β	Oxford5k	Oxford5k(W)	Pairs6k	Pairs6k(W)
AlexNet	0.50	10	58.10	67.60	71.64	79.60
	0.75	20	60.87	67.19	75.33	79.43
	0.85	25	60.79	67.93	75.60	79.59
	0.90	30	61.32	68.22	75.29	80.07
	1.00	50	60.27	67.72	74.88	80.10
VGG	0.85	30	84.62	87.83	82.40	88.01
	0.85	100	76.18	83.04	81.71	87.11
	0.90	30	85.09	88.00	82.69	88.12

Here, we take some representative results. In the relative distribution entropy contrastive loss, we take the value of within 0.5–1, and the results show that the performance is the best when . Additionally, we take the value of within 10–100. When the value of is large (), the effect of adding relative distribution entropy is not obvious. After a large number of experiments, we can make the following conclusions. When and , our performance achieve the best results on Oxford5k and Pairs6k. The performance achieves 88.00% and 88.12% on VGG. On n class="Chemical">AlexNet, the performanpan>ce achieves 68.22% anpan>d 80.07%. Therefore, we set to 30 anpan>d to 0.9 in Respan> class="Chemical">Net and VGG as our final hyperparameters.

4.4.2. Comparison of MAC, SPoC, and GeM

In this section, we combine the relative distribution entropy contrastive loss function with the current most advanced pooling methods, n class="Chemical">GeM [30], pan> class="Chemical">MAC [44] and SPoc [43], for end-to-end training. In this experiment, we use the AlexNet for training. The experimental results are shown in Table 2.

Table 2

Comparative results of different pooling methods on AlexNet. The best results would be highlighted in bold.

Net	Oxford5k	Oxford5k(W)	Pairs6k	Pairs6k(W)
SPoC [43]	41.83	55.34	55.49	68.61
MAC [44]	47.50	55.95	62.16	71.30
GeM [30]	60.79	68.22	75.29	80.07

The conclusions drawn from Table 2 are as follows. The results in the table indicate that the experiment results using n class="Chemical">GeM pooling [30] onpan> pan> class="Chemical">AlexNet are superior to the other two pooling methods. We get the results of 60.79%, 68.22%, 75.29% and 80.07%, which are the maximum values on the different datasets. In the next experiments, we will use the GeM pooling [30] method for training.

4.4.3. Comparison of Relative Distribution Entropy Triplet Loss and Triplet Loss

In this section, we present the experimental results of our method in triplet loss and compare them with the previous method [30]. We perform comparison tests on VGG and Resn class="Chemical">Net. Using the same pooling method, experimental steps, anpan>d network model, we compare the relative distributionpan> entropy triplet loss with the traditionpan>al triplet loss. The comparisonpan> results are shown in Table 3.

Table 3

Performance comparison of relative distribution entropy triplet loss and triplet loss. The best results would be highlighted in bold.

Loss	Network	Oxford5k	Oxford5k(W)	Pairs6k	Pairs6k(W)
Triplet loss [30]	VGG	81.48	82.80	82.79	84.78
Ours		82.39	83.07	83.61	85.45
Triplet loss [30]	ResNet	81.49	85.33	87.70	91.11
Ours		82.88	86.54	89.33	91.97

From Table 3, the result indicates that when we experiment on VGG, our proposed method obtains the best performance on all these datasets, with 82.39%, 83.07%, 83.61%, and 85.45%. The same conclusion is obtained when we perform the experiments on Resn class="Chemical">Net. We get the results of 82.88%, 86.54%, 89.33%, anpan>d 91.97%, which is the best performanpan>ce amonpan>gst the datasets. In this experience, we combine the relative entropy with the Euclideanpan> distanpan>ce into relative distributed entropy weighted distanpan>ce, which is a new metric. We put this new metric into the triplet loss functionpan>, anpan>d experiments have proven that our method is greatly effective.

4.4.4. Comparison with State-of-Art

In this section, we compare the relative distribution entropy contrastive loss with the latest methods. The performance comparison is shown in Table 4. The results of other methods are given by referring to the results in their papers. From Table 4, it can be learned that our proposed method attains better performance on multiple datasets. As shown in the table, we divide the existing networks into two categories: (1) using fine-tuning networks (yes) and (2) not using fine-tuning networks (no). When using the VGG network, compared with the Rn class="Chemical">MAC [55], relative distributionpan> enpan>tropy conpan>trastive loss provides a significanpan>t improvemenpan>t of +4.9% anpan>d +1.0% onpan> the Oxford5k anpan>d Paris6k datasets, respectively. Compared to the latest release, our method also has performanpan>ce improvemenpan>ts. Whenpan> using Respan> class="Chemical">Net, our experimental results achieve +0.6% growth compared to GeM [30] on Oxford 5k. Our method also shows superior performance on large-scale datasets. When using the VGG network, our experimental results achieved +0.2% growth compared to GeM [30] on Oxford 105k. When using the ResNet network, our experimental results achieve +0.3% growth compared to GeM [30] on Oxford105k. Our method shows more obvious performance improvements after adding re-ranking and query expansion. Under the VGG, the gain over GeM + αQE [30] is +0.1% and +0.5% on the Paris 6k dataset, respectively. Under the ResNet, the our method achieved mAP of 91.7%, 89.7%, 96.0%, and 92.1% and offered over 91.0%, 89.5%, 96.7%, and 91.9% gain over the GeM+αQE [37] on Oxford 5k, Oxford 105k, Paris 6k and Paris 106k datasets, respectively.

Table 4

Comparison of our method with the state-of-art image retrieval methods. The best results would be highlighted in bold.

Net	Method	F-tuned	Oxford5k	Oxford105k	Pairs6k	Pairs106k
VGG	MAC [44]	no	56.4	47.8	72.3	58.0
	SPoC [43]	no	68.1	61.1	78.2	68.4
	Crow [56]	no	70.8	65.3	79.7	72.2
	R-MAC [52]	no	66.9	61.6	83.0	75.7
	BoW-CNN [57]	yes	73.9	59.3	82.0	64.8
	NetVLAD [58]	yes	71.6	-	79.7	-
	Fisher [59]	yes	81.5	76.6	82.4	-
	R-MAC [55]	yes	83.1	78.6	87.1	79.7
	GeM [30]	yes	87.9	83.3	87.7	81.3
	ours	yes	88.0	83.5	88.1	79.9
Res	R-MAC [52]	no	69.4	63.7	85.2	77.8
	GeM [30]	yes	87.8	84.6	92.7	86.9
	ours	yes	88.4	84.9	92.7	86.3
Re-ranking(R) and Query Expansion(QE)
VGG	Crow + QE [56]	no	74.9	70.6	84.8	79.4
	R-MAC+R+QE [52]	no	77.3	73.2	86.5	79.8
	BoW-CNN+R+QE [57]	no	78.8	65.1	84.8	64.1
	R-MAC+QE [55]	yes	89.1	87.3	91.2	86.8
	GeM+αQE [30]	yes	91.9	89.6	91.9	87.6
	ours	yes	92.0	89.6	92.4	87.3
Res	R-MAC+QE [52]	no	78.9	75.5	89.7	85.3
	R-MAC+QE [60]	yes	90.6	89.4	96.0	93.2
	GeM+QE [30]	yes	91.0	89.5	95.5	91.9
	ours	yes	91.7	89.7	96.0	92.1

5. Conclusions

In this paper, we discuss the deficiency of traditional loss functions in spatial distribution differences. To make up for the lack of spatial distribution differences in the descriptors of image pair, the concept of relative distribution entropy (RDE) is presented. The calculation process of the relative distribution entropy is introduced in Section 3. n class="Chemical">Next, we combine Euclideanpan> distanpan>ce anpan>d relative distributionpan> enpan>tropy to obtain a new similarity measuremenpan>t, called relative distributionpan> enpan>tropy weighted distanpan>ce (RDE-distanpan>ce). We combine RDE-distanpan>ce with conpan>trastive loss anpan>d triplet loss to obtain relative distributionpan> enpan>tropy conpan>trastive loss anpan>d relative distributionpan> enpan>tropy triplet loss. We train the enpan>tire framework in anpan> enpan>d-to-enpan>d manpan>ner, anpan>d the results of extenpan>sive experimenpan>ts prove that our new method achieves the state-of-the-art performanpan>ce.

Our method mainly focuses on how to n class="Gene">fuse Euclideanpan> distanpan>ce anpan>d spatial informationpan>. Here, we introduce relative distributionpan> entropy to describe spatial informationpan>. We would like to focus onpan> other fusionpan> methods instead of the existing linear fusionpan>. In additionpan>, we would conpan>centrate onpan> adding our relative distributionpan> entropy to other loss functionpan>s in future work.