Person search  aims to localize and recognize a query person from a gallery of unconstrained scene images. Despite tremendous progress achieved by recent works [36, 35, 26, 37, 11, 10, 5, 44], the training process requires strong supervision in terms of bounding boxes and identity labels, shown in Fig. 1(a). However, obtaining such annotations at a large scale can be time-consuming and economically expensive. Even in the most widely used dataset, CUHK-SYSU , almost 72.7% of pedestrian bounding boxes have no identity annotations. It indicates that labeling identities is more difficult than bounding boxes. The infeasible identity annotations largely restrict the scalability of supervised methods.
Instead of relying on the expensive labeling, a lot of researchers have been dedicated to training models with no labels [17, 7] or incomplete labels [27, 45] in the areas of image recognition, object detection, . Nevertheless, relevant explorations are missing in the field of person search. To fill the gap, we investigate the person search modeling in a weakly supervised setting, where only bounding box annotations are required. As shown in Fig. 1(b), the proposed setting alleviates the burden of obtaining manually labeled identities. However, it is more challenging to pursue accurate person search using sole bounding box annotations.
In this paper, we setup a strong weakly supervised baseline called Region Siamese Networks (R-SiamNets). Towards learning meaningful feature representations of each instance, our model minimizes the discrepancy between two encoded features transformed from the same pedestrian region. Specifically, one branch is fed with the whole scene image and extracts the RoI features of the person instance. The other branch extracts features from the cropped person image. The obtained features from the two weight-sharing branches are constrained to be consistent. The motivation of this design is that the context-free instance features extracted from the cropped image regions could help the network to distinguish persons from the irrelevant background content. We formulate a self-instance consistency loss and an inter-instance similarity consistency loss to supervise the learning of context-invariant feature representations.
Further, a cluster-level contrastive learning method is introduced for striking a balance between separation and clustering. The clustering method aggregates the closest instances together and pushes instances from different clusters apart. It is assumed that the closest features have a high probability of being from the same individual. The generated pseudo labels by clustering are used for contrastive learning. We iteratively apply this non-parametric clustering for refinement along with the training process. The cluster-level contrastive learning yields a significant performance gain, with theabsolute improvement of mAP on the CUHK-SYSU dataset.
Our contributions can be summarized in three-folds:
We introduce a weakly-supervised setting for person search. The new setting only requires bounding box annotations, relieving the burden of obtaining manually labeled identities. With this setting, the developed algorithms could be readily utilized for large-scale person search in real-world scenarios.
We propose the R-SiamNet as a baseline under the weakly supervised setting. With the Siamese network, instance-level consistency learning is applied to encourage context-invariant representations. Further, cluster-level contrastive learning is introduced for striking a balance between separation and clustering.
2 Related Work
Recently, person search task has raised a lot of interest in the computer vision community[36, 43, 4, 3, 21, 5, 42]. In the literature, there are two manners to deal with this problem, i.e., two-step and one-step.
For two-step methods, the pedestrian detection and person re-identification are trained with two separated models [43, 4, 3, 21, 15]. Zheng et al.  evaluate various combinations of different detectors and re-ID networks, and develop a Confidence Weighted Similarity (CWS) to assist the pedestrian matching. Chen et al.  enhance the feature representations by introducing a mask-guided two-steam model. Han et al.  develop an RoI transform layer to optimize the two networks end-to-end. With more parameters, these methods guarantee high performance while low efficiency in evaluation.
One-step methods [36, 35, 16, 5] jointly train the detection and re-ID in a unified model, exhibiting high efficiency. Among these methods, [36, 35] take the Faster R-CNN  as a backbone network, and most layers are shared by two tasks. Munjal et al.  first introduce a query-guided end-to-end person search network. With the global context from both query and gallery images, the well-designed framework generates query-relevant proposals and learns query-guided re-ID scores. Yan et al.  explore the contextual information and build a graph learning framework to employ context pairs to update target similarity. Dong et al.  develop a bi-directional interaction network and employ the cropped person patches as the guidance to reduce redundant context influences.
These studies are fully supervised and require precise annotations for each person, including the bounding boxes and the identities. It is impractical to extend these methods in large-scale scenarios due to the expensive labeling process. Thus, we introduce a weakly supervised setting and develop a framework trained solely with bounding boxes.
Siamese Networks. Siamese networks  consist of twin networks which accept distinct inputs, and the comparability is determined by supervision. This architecture is widely used in many fields, including object tracking , one-shot learning , signature  and face verification , . In this paper, we explore the context-invariant embeddings based on a region Siamese network with two forms of inputs, i.e., whole scene images, and cropped images.
Contrastive Learning. Contrastive learning 
aims at attracting the positive sample pairs and repulsing the negative ones, which has been popularized in recent unsupervised learning[19, 34, 39, 17, 7, 8]. Wu et al.  consider each instance as a class, and employ a memory bank to store the instance embeddings. Similar to , Ye et al.  learn the data augmentation invariant and instance spread-out features. MoCo  maintains the dictionary with a queue and a moving-averaged encoder, so the contrastive learning is viewed as the dictionary look-up. SimCLR  deprecates the usage of the memory bank, and directly uses negative samples in the current batch. In this paper, rather than only independently penalizing the incompatibility of each single positive pair at a time, we construct more informative positive pairs by non-parametric clustering.
Unsupervised Person Re-identification. The traditional unsupervised methods can be summarized into three categories, designing hand-craft features, utilizing localized salience statistics, or dictionary learning-based works. However, the performance of these methods is inferior to supervised ones. On the basis of clustering algorithms, recent works exhibit higher performance. Lin et al.  develop a bottom-up clustering framework to iteratively train the network with pseudo labels. Zeng et al. 
combine the hierarchical clustering method with hard-batch triplet loss. Geet al.  design a self-paced contrastive learning strategy with a novel clustering reliability criterion to filer the unstable clusters with DBSCAN 
. Although the cluster-based methods achieve high performance, they require carefully adjusted hyperparameters,e.g., number of clusters [24, 40], or the distance threshold .
In this paper, we employ a non-parametric clustering method , with a filtering mechanism by image information. This manner only relies on the first neighbor of each data point, and requires no hyper-parameters.
3 Proposed Method
In this section, we first introduce the overall region Siamese network in Sec. 3.1. Then we will describe the instance-level consistency learning in Sec. 3.2. At last, the cluster-level contrastive learning is developed in Sec. 3.3.
3.1 Region Siamese Network
Our main target is to locate the person positions and learn representative features for identification. Under the weakly supervised setting, only the bounding box annotations are employed in the training process. Without manually labeled identities, it is essential to design supervision signals for training the network. To achieve this goal, we develop the framework from two aspects: 1) Based on Siamese networks, the comparability is determined by supervision with different augmented inputs. In this paper, both whole scene images and cropped images are taken as inputs. We focus on the instance-level consistency learning to encourage context-invariant embeddings. 2) Pseudo labels generated by clustering permit to model cluster-level supervision. Thus, we apply the cluster-level contrastive learning, reaching a balance between separation and aggregation.
Based on these considerations, we propose a region Siamese network as shown in Fig. 2. There are two branches termed as search path and instance path. With an extra detection head, the search path takes the whole scene images as inputs, training the detection and identification jointly. The feature embedding of each pedestrian is generated by the Region-of-Interest (RoI) align layer. For the instance path, the cropped images are taken as inputs. With less context, the corresponding outputs can focus on the regions of pedestrians. To ensure the context-invariant embeddings, we apply the instance-level consistency learning on the output of two paths. It consists of a self-instance consistency loss and an inter-instance similarity consistency loss. Besides, to model the cluster-level supervision, we employ a non-parametric clustering method based on the nearest neighbor of each sample. A cluster-level contrastive loss is developed to calculate the similarities between the samples in the current batch and the memory bank. During inference, we only utilize the search path.
3.2 Instance-Level Consistency Learning
With no identity annotations, it is observed that the learned instance features are involved with excessive context. As a fine-grained task, the retrieval process is easily affected by interference from surrounding persons and noises. To alleviate this issue, we input the cropped pedestrians which contain less context to build the supervision. The instance-level consistency learning is developed to encourage the context-invariant embeddings from two aspects.
Self-instance consistency loss. Given a mini-batch of scene images, we obtain cropped pedestrian images with bounding box labels. The scene images and cropped ones are processed by the region Siamese network. The output embeddings are denoted as and
for search path and instance path, respectively. To encourage the context-invariant embeddings for a specific instance, we consider to maximize the cosine similarity betweenand . Then the self-instance consistency loss is defined as:
where is the L2 norm, and the loss is averaged over all instances in a mini-batch.
Inter-instance similarity consistency loss. The feature embeddings in each path can be seen as an aggregated feature space. The above self-instance consistency loss only constrains the embedding pairs from the same person to be closer individually. We further apply a constraint to enlarge the similarity distribution between the entire feature spaces. For the search path, the similarity matrix is obtained by , where is produced by a row-wise L2 normalization on . In the same way, is calculated for the instance path. Our goal is to keep the consistency between the two similarity distributions. Based on the divergence, we develop an inter-instance similarity consistency loss as follows:
where denotes the KL divergence. It can be formulated as . Since each distribution can be considered as the target, we adopt the mutual manner.
3.3 Cluster-Level Contrastive Learning
The instance recognition  treats each sample as a category to produce well-separated samples. In person search task, this approach may be inferior. We aim at exploring the similarities among persons and striking a balance between separation and aggregation. A non-parametric clustering method is employed to produce pseudo labels, and a cluster-level contrastive loss is applied between samples in the current batch and the memory bank.
Non-parametric clustering. We build the cluster-level supervision based on an assumption that the most similar feature embeddings have high probabilities of belonging to the same class. This motivates our clustering manner, in which only the nearest neighbor of each sample is aggregated. Meanwhile, there is a prior that the pedestrians in same scene images belong to different identities. Thus, we can filter some false aggregations when clustering. After clustering, all persons are assigned pseudo labels.
Specifically, at the beginning of each epoch, the clustering process is conducted after extracting the embeddings of all instances. Supposing there aresamples, an adjacency matrix can be constructed among all samples, which is initialized with all zeros. To set , two conditions should be satisfied simultaneously:
1) or or , where denotes the first neighbor of sample . Besides the nearest neighbors, the adjacency matrix also links points that have the same neighbor with .
2) The whole scene image of the -th and -th pedestrian should be different. If two persons come from the same scene image, they cannot be clustered.
Different from other clustering algorithms that require carefully adjusted hyperparameters [24, 13, 40], e.g., cluster numbers or distance threshold, we employ the non-parametric clustering approach. It is easily scalable to large-scale data with minimal computational expenses.
Cluster-level contrastive loss. Similar to previous works [36, 5], we maintain a memory bank to store the embeddings of all instances, where denotes the feature dimensions. After clustering, the memory bank is updated by the newly embedded features with pseudo labels. Then, a cluster-level contrastive loss can be computed between the features in the memory bank and current batch.
In a mini-batch, a specific instance feature is denoted as . Assuming that there are positive samples in the memory bank sharing the same pseudo label with . Then, the remaining samples in M are considered as negative samples. The cosine similarities are denoted as and , respectively. Inspired by , we apply the cluster-level contrastive loss to make each greater than :
where is the scale factor. During backward, the memory bank is updated with the samples in current mini-batch: . is the momentum factor and denotes the instance position in the memory bank.
3.4 Training Procedure
Given the input images, we aim to learn a deep convolutional neural network (CNN) modelto realize precise localization and identification. The details of our training procedure are provided in Algorithm 1. In summary, our total training objective is revised as:
where denotes the detection losses, including regression loss and foreground-background classification loss.
In this section, we first introduce two benchmark datasets, followed by the settings under weakly supervised manner and evaluation protocols in Sec. 4.1. Then Sec. 4.2 describes the implementation details and the reproducibility. We conduct a series of ablation studies to analyze the effectiveness of the proposed method in Sec. 4.3. Finally, we discuss the comparison with the state of the arts in Sec. 4.4.
4.1 Datasets and Settings
CUHK-SYSU dataset. CUHK-SYSU  is a large-scale person search dataset, which is composed of urban scene pictures and movie snapshots. There are images with annotated bounding boxes, including labeled identities. The training set consists of images with identities and several unlabeled ones. There are gallery images and probe images in testing set.
PRW dataset. PRW  is captured by six spatially disjoint cameras in the university. It consists of frames with annotated bounding boxes, among which are assigned with identity labels, and the rest are unlabeled ones. The training set contains frames with identities, and the testing set includes gallery images and queries with identities.
Settings. Under the fully supervised setting, the statistics of the training data are shown in Tab. 1. In this paper, we propose a weakly supervised setting for person search, reducing the need of strong supervision during training. Under the weakly supervised setting, our model is trained only using and bounding box annotations for the CUHK-SYSU and PRW datasets, respectively.
|CUHK-SYSU||11206||5532||15080 (27.3%)||40180 (72.7%)|
|PRW||5704||482||14906 (82.6%)||3142 (17.4%)|
Our experiments employ the standard evaluation metrics in person search. One is the cumulative matching cure (CMC), which is inherited from the person re-ID. A candidate is counted if the intersection-over-union (IoU) with ground truth is greater than 0.5. The other is the mean Average Precision (mAP), and it is inspired by the object detection task. For each query, we compute an averaged precision (AP) based on the precision-recall curve. Then, the mAP is calculated by averaging the APs across all the queries.
4.2 Implementation Details
as our backbone network, containing the ImageNet pretrained ResNet-50, the feature pyramid network (FPN) , and a detection head. The search path takes the scene images as inputs, jointly training the detection and recognition. To obtain the pedestrian features , the RoI align is applied on FPN with the ground truth RoIs, followed by a fully connected (fc) layer after flattening. For the instance path, the cropped and resized images are taken as inputs. Similar to , is produced with the same network except the RoI align, and both features are -d.
Training. The scene images are resized to , and cropped images are rescaled to
. The batched Stochastic Gradient Descent (SGD) optimizer is used with a momentum of. The weight decay factor for L2 regularization is set to . We use a mini-batch size of , and an initial learning rate of . The model is trained for epochs with the learning rate multiplied by at and epochs. The scale factor is set to and the momentum is set to
for both datasets. All experiments are implemented on the PyTorch framework, and the network is trained on the NVIDIA Tesla V100.
4.3 Ablation Study
To evaluate the effectiveness of the proposed framework, we conduct detailed ablation studies on the CUHK-SYSU and PRW datasets. Note that all the settings in each experiment are the same as the implementation in Sec. 4.2.
Effectiveness of different components. To verify the effectiveness of each component, we compare the performance under different settings in the training process. The results are shown in Tab. 2.
Instance Recognition (IR)  denotes that each instance is treated as a category in training. A memory bank is maintained to store all the instance features, providing abundant negative samples to compute the contrastive loss. Search path w/ IR means the supervision of IR is only applied on the instance features produced by the search path. This approach can be viewed as the baseline of our method. With the scene images as input, instance features may contain excessive context to disturb the matching, thus the mAP only reaches . Instance path w/ IR indicates the IR is applied on the features generated by the instance path, which takes the cropped persons as inputs. In the search path, only the detection head is trained within the same backbone. Under this setting, the result can reach on mAP. Compared to the search path, the instance path contains less context, exhibiting higher performance.
R-SiamNet w/ takes both scene images and cropped pedestrian images as inputs, and the fused outputs are supervised by IR. To encourage the context-invariant feature embeddings, we apply the self-instance consistency loss between two paths. It maximizes the similarity of pair-wise features from two paths. The mAP is promoted to , surpassing by a large margin. This verifies the importance of keeping consistency. For further restraint, we develop the inter-instance similarity consistency loss , which is applied on the similarity distributions within the mini-batch of two paths. This further guarantees the context-invariant embeddings, and achieves a gain of on rank-1. Moreover, to explore the cluster-level supervision, we implement the non-parametric clustering to produce pseudo labels. Thus, a cluster-level contrastive loss is employed for supervision instead of IR. From Tab. 2, we can see that the performance achieves on mAP and on rank-1. The results show the effectiveness of our framework.
|Search path w/ IR||51.85||59.69||67.31||69.03|
|Instance path w/ IR||63.79||65.55||82.21||86.83|
|R-SiamNet w/ & IR||76.06||78.21||90.28||92.90|
|R-SiamNet w/ & & IR||79.62||82.17||91.69||94.03|
|R-SiamNet w/ & &||85.72||86.86||95.24||96.86|
Scalability with different scales of training data. Our framework is designed to learn discriminative identity embeddings under the weakly supervised setting. The scalability is essential for the methods when giving more training data without identity labeling. To discuss the scalability, we design the experiment from two aspects.
First, we assess the performance with different percentages of training data individually, as shown above the dotted line in Tab. 3. Specifically, we divide the CUHK-SYSU/PRW dataset by 20%, 40%, 60%, 80%, 100% for training. It can be seen that the performance is gradually improved as the training scale increases. Moreover, the growing tendency has not reached saturation on both datasets, indicating that the proposed framework can achieve further improvement with more training data.
Second, to further evaluate the scalability of our method, we expand the training set by combining different datasets. The results are shown below the dotted line in Tab. 3. When training with CUHK-SYSU and PRW datasets together, the performance on both datasets has been considerably improved. Especially for the PRW dataset, the mAP is promoted by a large margin since the added CUHK-SYSU owns a larger data scale. Besides, we also employ a dataset called INRIA  in the pedestrian detection task. There are images that contain pedestrians with bounding boxes labeling. When training with the three datasets together, our performance is further promoted on both CUHK-SYSU and PRW datasets.
All the experiments prove that our framework has the potential to reach promising performance by incorporating more training data.
|CUHK-SYSU & PRW||87.00||85.92||75.06||23.50|
|CUHK-SYSU & PRW & INRIA||87.59||86.19||76.03||25.53|
Visualization and Analysis. To evaluate the effectiveness of the proposed method, we illustrate some qualitative search results on the CHUK-SYSU dataset. As Fig. 3 shows, we present the comparisons on rank-1 of our R-SiamNet and the other two manners. Specifically, the first column shows the query persons with green bounding boxes. Instance path/search path represents that the model employs the cropped images/whole scene images as input within a single path. The last column is the result of our R-SiamNet. The search results are exhibited with different colors, i.e., red boxes denote the wrong matches while the blue boxes show the correct ones.
There are several observations from the visualization. First, it is observed that the wrong matches in the search path are definitely different from the query, but with similar contexts. This verifies the existence of excessive contexts, which disturb the matching process by involving more surrounding persons/backgrounds. Second, we find that most false examples in the instance path have a similar appearance with the query. This indicates the features are hardly disturbed by the contexts. Third, our R-SiamNet promotes the complementary of both paths, maintaining useful context to aid the person search. The results also show the effectiveness of our method, which can successfully localize and match the query person in most cases.
4.4 Comparisons with the State-of-the-arts
In this section, we compare our proposed framework with current state-of-the-art methods on person search in Tab. 4. The results of two-step methods [3, 4, 21, 15, 33] are shown in the upper block while the one-step methods [36, 35, 25, 37, 41, 26, 5] in the lower block.
|Fully supervised setting:|
|Weakly supervised setting:|
Evaluation on CUHK-SYSU. The comparisons between our network and existing supervised methods on the CUHK-SYSU dataset are shown in Tab. 4. When the gallery size is set to 100, our proposed method reaches on mAP and on rank-1. The performance can outperform several supervised methods. Moreover, when using a larger resolution of , our results are further improved and reach on mAP and on rank-1.
To evaluate the performance consistency, we also compare with other competitive methods under varying gallery sizes of . Fig. 4 shows the comparisons with both one-step and two-step methods. It can be seen that the performance of all methods decreases as the gallery size increases. This indicates it is challenging when more distracting people are involved in the identity matching process, which is close to real-world applications. Our method outperforms some supervised methods under different gallery sizes.
Evaluation on PRW. We also evaluate our method on the PRW dataset, as shown in Tab. 4. Following the setting in benchmark , the gallery contains all the testing images. This is challenging since a tremendous number of detected bounding boxes are involved. Compared with the competitive techniques, our performance achieves on rank-1, and it is further promoted with larger resolutions. Our method surpasses most works in both one-step and two-step manners. However, the results exhibit a low mAP due to the minor inter-class variations in this dataset.
In this paper, we introduce a weakly supervised setting for the person search task to alleviate the burden of costly labeling. Under this new setting, no specific identity annotations of pedestrians are required, and we only utilize the accessible bounding boxes for training. Meanwhile, we propose a baseline called R-SiamNet for localizing persons and learning discriminative feature representations. To encourage the context-invariant features, a self-instance consistency loss and an inter-instance similarity consistency loss are developed. We also explore the balance between separation and aggregation by a cluster-level contrastive loss. Extensive experimental results on the widely used benchmarks demonstrate the effectiveness of our framework. The results also show that the gap with supervised state-of-the-arts will be further narrowed with more training data.
This work was partially supported by the Project of the National Natural Science Foundation of China No. 61876210, the Fundamental Research Funds for the Central Universities No.2019kfyXKJC024, and the 111 Project on Computational Intelligence and Intelligent Control under Grant B18024.
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Appendix A Additional Experimental Results
a.1 Paths for Evaluation
In our paper, the instance path is only applied in training, facilitating the identity feature learning of the main search path. During inference, we drop the instance path and the images are only passed through the search path. We compare the results of using different paths for testing, as shown in Tab. 5. It can be seen that using two paths for evaluation cannot bring extra performance gains. This indicates the context-invariant embeddings produced by our framework.
a.2 Different Detection Networks
Following , we choose the RepPoints as the detection network. To show the expandability of our R-SiamNet, different detectors are incorporated into our framework, including Faster R-CNN , RetinaNet  and RepPoints . As reported in Tab. 6, the final performance gaps among different detectors are small, exhibiting the effectiveness and robustness of our framework.
a.3 Comparisons with Two-Step Manner
We combine a well-trained RepPoints detector  and an unsupervised re-ID model called SPCL  as our two-step competitor. As shown in Tab. 7, our method outperforms it by a large margin with higher efficiency. It shows that training detection and identification end-to-end is beneficial for obtaining better representations. It may also imply the importance of instance-level consistency learning.
a.4 Evaluation on filter strategy in the clustering.
To analyze the effectiveness of the filter strategy in clustering, we conduct experiments with/without filtering by image information. As shown in Tab. 8, we observe rank-1 drops on CUHK-SYSU/PRW datasets by removing the filter strategy. This shows that it is beneficial to filter the aggregation of the persons from the same scene images.
|R-SiamNet w/o filter||84.74||85.64||20.31||72.23|
a.5 Different numbers of training epochs
We illustrate the mAP scores with different numbers of training epochs. As Fig. 5 shows, the results of three data scales are exhibited on the CUHK-SYSU dataset. It can be observed that the performance improves steadily to saturation as the epoch increases. With smaller data scales, the mAP reaches saturation earlier.
a.6 Runtime Comparisons
To compare the evaluation efficiency of our framework with other methods, we report the average runtime of the inference stage for a whole scene image, shown in Tab. 9. Since these methods are evaluated with different GPUs, we also exhibit the Tera-Floating Point Operation per-second (TFLOPs) for fair comparisons. Similar to other methods [5, 26, 4], we evaluate the models with an input image size of . As shown in Tab. 9, our R-SiamNet takes milliseconds to process one image, which is faster than the two-step method MGTS  by a large margin. The query-guided method QEEPS  requires to re-compute all the gallery embeddings for each query image. This time-consuming operation reduces the evaluation efficiency. Moreover, our method is faster than NAE . These results clearly demonstrate the efficiency of our R-SiamNet in evaluation.
|Methods||GPU (TFLOPs)||Runtime (ms)|
|MGTS ||K80 (8.7)||1269|
|QEEPS ||P6000 (12.0)||300|
|NAE+ ||V100 (14.1)||98|
|NAE ||V100 (14.1)||83|
Appendix B More Qualitative Analysis
b.1 Feature Visualization
To analyze the discriminative ability of our learned features, we employ the t-SNE  to visualize the feature representations in training. As illustrated in Fig. 6, there are pedestrians with classes, which is a subset of the PRW training set. Different colors represent different classes.
Fig. 6(a) shows the result when training with a single search path. Without applying the instance-level consistency learning and cluster-level contrastive learning, the learned features show large intra-class distances and small inter-class distances. When adding the instance-level consistency learning, including and , the result is shown in Fig. 6(b). It is observed that the feature embeddings of the same category can be aggregated compared with Fig. 6(a). This shows the effectiveness of our instance-level consistency learning. Nevertheless, the features within the class are gathered loosely, and the margins among different classes are not clear. Furthermore, we apply the cluster-level contrastive learning, and the result is shown in Fig. 6(c). It can be seen that both intra-class compactness and inter-class separability are further encouraged. There are obvious margins among most categories. This verifies that our R-SiamNet can generate discriminative embeddings under the weakly supervised settings.