# JRL-YOLO: A Novel Jump-Join Repetitious Learning Structure for Real-Time Dangerous Object Detection

Authors: Yiliang Zeng; Lihao Zhang; Jiahong Zhao; Jinhui Lan; Biao Li
Journal: Computational Intelligence and Neuroscience
 (2021)

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## Abstract

Campus security incidents occur from time to time, which seriously affect the public security. In recent years, the rapid development of artificial intelligence has brought technical support for campus intelligent security. In order to quickly recognize and locate dangerous targets on campus, an improved YOLOv3-Tiny model is proposed for dangerous target detection. Since the biggest advantage of this model is that it can achieve higher precision with very fewer parameters than YOLOv3-Tiny, it is one of the Tinier-YOLO models. In this paper, the dangerous targets include dangerous objects and dangerous actions. The main contributions of this work include the following: firstly, the detection of dangerous objects and dangerous actions is integrated into one model, and the model can achieve higher accuracy with fewer parameters. Secondly, to solve the problem of insufficient YOLOv3-Tiny target detection, a jump-join repetitious learning (JRL) structure is proposed, combined with the spatial pyramid pooling (SPP), which serves as the new backbone network of YOLOv3-Tiny and can accelerate the speed of feature extraction while integrating features of different scales. Finally, the soft-NMS and DIoU-NMS algorithm are combined to effectively reduce the missing detection when two targets are too close. Experimental tests on self-made datasets of dangerous targets show that the average MAP value of the JRL-YOLO algorithm is 85.03%, which increases by 3.22 percent compared with YOLOv3-Tiny. On the VOC2007 dataset, the proposed method has a 9.29 percent increase in detection accuracy compared to that using YOLOv3-Tiny and a 2.38 percent increase compared to that employing YOLOv4-Tiny, respectively. These results all evidence the great improvement in detection accuracy brought by the proposed method. Moreover, when testing the dataset of dangerous targets, the model size of JRL-YOLO is 5.84 M, which is about one-fifth of the size of YOLOv3-Tiny (33.1 M) and one-third of the size of YOLOv4-Tiny (22.4 M), separately.


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## Body

## 1. Introduction

In the past few years, there have been occasional high-profile and horrific acts of violence on campus [1]. Statistical analysis shows that violent incidents on campus are rare [2, 3]; although the incidence is extremely low, these incidents are indeed so terrible and painful that they seriously endanger campus security and social security. Therefore, it is essential to strengthen the management to maintain campus safety.With the development of information technology, computer visual research has become a hotspot in the field of academic and industrial applications [4]. Especially after the introduction of artificial intelligence theory, more and more artificial intelligence products are brought into people’s lives. Although there is a big gap between the visual understanding ability of machines and human beings, machines can be widely used in long-term and extremely harsh working environments [5]. Machines can also help us detect abnormal events in time and make early emergency response.Traditional methods are used to identify specific human activities by detecting gestures, using online alarm system to detect abnormal situations in the campus environment [6, 7]. Although real-time monitoring is possible, traditional methods have limited detection features and cannot simultaneously and accurately distinguish various types of dangerous events. And Wu and Cheng [8] used traditional methods, but there are many parameters that need to be manually determined. The rapid development of computer technology provides a strong support for the development of object detection algorithms with high computational performance. Compared with traditional object detection algorithms, the deep learning algorithm has a great advantage in object detection.Deep learning is now the mainstream method of dangerous object detection. Aiming at the shortcoming of existing feature classifiers with high false detection rate in target detection, a convolutional neural network detection algorithm based on deep learning was proposed to improve the robustness of the algorithm in complex environments [9]. A dangerous substance image detection method based on SSD was proposed in [10]. However, the detection effect of the model for large objects is very good, whereas the detection of small targets is not robust. YOLOv3 has been used to detect dangerous objects such as knives and guns in massive videos, which improves the detection accuracy of small targets, but the large model is difficult to be applied to small devices [11]. Hu et al. [12] had good effect on small targets, but it also cannot guarantee real-time performance.In order to solve the problem of low efficiency of video anomaly detection, an adaptive key frame screening method based on the characteristics of abnormal targets was proposed in [13]. However, the application of this approach of abnormal behavior detection is limited, which is not conducive to its popularization. Xia et al. [14–17] all used deep learning to detect abnormal actions of the human body and achieved good results with strong robustness to noises in the environment. However, due to the large number of parameters in the above model, it does not consider the problem of real-time performance in small devices. Ji et al. [18] proposed an abnormal target detection algorithm based on the Tiny-YOLO network model. At the same time, the network is cut out and transplanted to the embedded hardware platform TX2. However, the final result cannot realize real-time detection. Fan et al. [19] used the spatiotemporal autoencoder and CNN for training, and real-time detection can be achieved at last, but the boundary of dangerous actions is not clearly defined, being prone to result in misidentifications.In 2012, Krizhevsky et al. proposed the AlexNet method based on the convolutional neural network (CNN) [20], which proved that the features extracted by the CNN were more effective than traditional methods and led to higher classification accuracy. Girshick et al. [21] proposed R-CNN to significantly improve the detection performance by combining a large number of convolutional neural networks with the region proposal network (RPN). Fast R-CNN solves the problem of time-consuming and slowness of the traditional R-CNN [22]. Faster R-CNN introduces a fully convoluted RPN to share fully convoluted features, further reducing the amount of computation and improving the detection speed [23].The detection of these two types of targets, dangerous objects and dangerous actions, is usually conducted by separate algorithms, and there is no integrated model yet. Although the deep learning algorithm has high detection accuracy, it has high requirements for hardware devices. In terms of speed, it is difficult to achieve real-time detection for limited hardware facilities such as mobile phones and embedded development boards. Therefore, it is necessary to explore a lightweight and high-precision object detection algorithm model.YOLO (You Only Look Once) [24–27] is a one-stage object detection algorithm. The lightweight network YOLOv3-Tiny, which is a simple version of the YOLOv3 network, can achieve good results of open datasets and can detect targets in real time.In this paper, a new YOLOV3-Tiny backbone network is proposed to design a more lightweight and efficient campus dangerous target detection model. The main work of this paper is summarized as follows:(1)
Considering dangerous objects and dangerous actions, respectively, it is difficult to capture the characteristics of both “shape-unchanged appearance” and “shape-changeable appearance” at the same time. To redefine the campus of the dangerous objects and dangerous behaviors, the two types of targets are integrated into one model, which is lightweight and can achieve higher precision with fewer parameters.(2)
In view of the missing detection phenomenon of objects in the practical application of YOLOv3-Tiny, a jump connection repeatedly learning structure is proposed, which is combined with the SPP structure, as a new backbone network of YOLOv3-Tiny. Its performance is improved compared to YOLOv3-Tiny, especially in the detection of finite-pixel targets.(3)
Inspired by the DIoU-NMS algorithm, the soft-NMS algorithm is improved by adding the measurement of the center distance of the detection box, which has better performances than YOLOv3-Tiny’s own NMS. It can effectively reduce the missing detection phenomenon when two targets are too close.Experimental results show that the jump-join repetitious learning YOLO (JRL-YOLO) not only has high cost performance in practical applications but also can achieve higher precision with fewer parameters than YOLOv3-Tiny.The rest of the paper is organized as follows. In Section2, the main ideas of YOLOv3-Tiny are briefly reviewed. Section 3 introduces our proposed JRL-YOLO, and Section 4 presents experimental results and analysis, with Section 5 concluding our work and giving advices for future work.


## 2. Brief Introduction of YOLOv3-Tiny

YOLOv3-Tiny [26] is a one-stage target detection algorithm, which transforms the target detection problem into a regression problem. It can directly obtain the target position information and category information without the need of the RPN, which is faster than Faster R-CNN. Compared with Faster R-CNN, real-time target detection can be achieved in a lower hardware device.


### 2.1. Principle of YOLO

YOLO will resize the image into a fixed size and feeds it to the network so that each detection layer has a fixed-size feature scale. YOLO uses the convolutional neural network to extract the depth features of the target and then divides the image intoS∗S grids for grid-by-grid detection. It is assumed that there are M detection layers in YOLO, and each detection layer has B anchor boxes. Each detection layer should detect each grid and calculate the confidence category probability of each grid. The end result is that an object might have multiple bounding boxes; however, it is usually expected to see one object corresponding to only one bounding box in the end, so YOLOv3-Tiny uses the NMS algorithm to suppress the redundant bounding boxes.


### 2.2. Target Real-Time Detection Network of YOLOv3-Tiny

YOLOv3-Tiny is lighter than the YOLOv3 network structure and has a good effect on target detection. As shown in Table1, the backbone network of YOLOv3-Tiny mainly consists of 7 convolutional layers of a 3 × 3 size and 6 pooling layers. The first five pooling layers have a stride size of 2, and the last one has a stride size of 1. YOLOv3-Tiny detection layers include shallow network detection and deep network detection. The detection of the shallow network is located after the backbone network, and the detection of the deep network is located behind the shallow network. The feature concat is obtained under different convolution kernel sizes in the shallow and deep layer so as to fuse the shallow and deep features to improve the accuracy of the model.Table 1
The YOLOv3-Tiny network structure.
LayerTypeFiltersSize/strideOutput0Convolutional163 × 3/1416 × 416 × 161Maxpool—2 × 2/2208 × 208 × 162Convolutional323 × 3/1208 × 208 × 323Maxpool—2 × 2/2104 × 104 × 324Convolutional643 × 3/1104 × 104 × 645Maxpool—2 × 2/252 × 52 × 646Convolutional1283 × 3/152 × 52 × 1287Maxpool—2 × 2/226 × 26 × 1288Convolutional2563 × 3/126 × 26 × 2569Maxpool—2 × 2/213 × 13 × 25610Convolutional5123 × 3/113 × 13 × 51211Maxpool—2 × 2/113 × 13 × 51212Convolutional10243 × 3/113 × 13 × 102413Convolutional2561 × 1/113 × 13 × 25614Convolutional5123 × 3/113 × 13 × 51215Convolutional361 × 1/113 × 13 × 3616YOLO———17Route 13———18Convolutional1281 × 1/113 × 13 × 12819Upsample—2 × 2/126 × 26 × 12820Route 19, 8———21Convolutional2563 × 3/126 × 26 × 25622Convolutional361 × 1/126 × 26 × 25623YOLO———YOLOv3-Tiny is an end-to-end detection model whose input image is uniformly resized to a fixed size before entering the network. Generally, the input image is resized to a size of 608 × 608 or 416 × 416. YOLOv3-Tiny contains two YOLO layers to detect different scales, and the detection feature scale of the YOLO layer is related to the size of the input image. When the input image size is 416 × 416, the output feature sizes of the two YOLO layers are 13 × 13 and 26 × 26, respectively.


### 2.3. YOLOv3-Tiny Nonmaximal Suppression Algorithm

YOLOv3-Tiny’s nonmaximal suppression (NMS) algorithm sorts all bounding boxes by the size of the score. First, the bounding box with the highest score is selected, and then a threshold is predetermined. When the intersection over union (IOU) of other bounding boxes and the bounding box of the highest score is larger than this threshold, the bounding box will be inhibited. This method can effectively reduce the number of overlapping boxes. However, when two objects are close to each other, the bounding boxes generated by the two objects are also close to each other. Similarly, the bounding boxes will inhibit each other, leading to one of the bounding boxes being eliminated. At this point, the phenomenon of missing detection of the object occurs.


## 2.1. Principle of YOLO

YOLO will resize the image into a fixed size and feeds it to the network so that each detection layer has a fixed-size feature scale. YOLO uses the convolutional neural network to extract the depth features of the target and then divides the image intoS∗S grids for grid-by-grid detection. It is assumed that there are M detection layers in YOLO, and each detection layer has B anchor boxes. Each detection layer should detect each grid and calculate the confidence category probability of each grid. The end result is that an object might have multiple bounding boxes; however, it is usually expected to see one object corresponding to only one bounding box in the end, so YOLOv3-Tiny uses the NMS algorithm to suppress the redundant bounding boxes.


## 2.2. Target Real-Time Detection Network of YOLOv3-Tiny

YOLOv3-Tiny is lighter than the YOLOv3 network structure and has a good effect on target detection. As shown in Table1, the backbone network of YOLOv3-Tiny mainly consists of 7 convolutional layers of a 3 × 3 size and 6 pooling layers. The first five pooling layers have a stride size of 2, and the last one has a stride size of 1. YOLOv3-Tiny detection layers include shallow network detection and deep network detection. The detection of the shallow network is located after the backbone network, and the detection of the deep network is located behind the shallow network. The feature concat is obtained under different convolution kernel sizes in the shallow and deep layer so as to fuse the shallow and deep features to improve the accuracy of the model.Table 1
The YOLOv3-Tiny network structure.
LayerTypeFiltersSize/strideOutput0Convolutional163 × 3/1416 × 416 × 161Maxpool—2 × 2/2208 × 208 × 162Convolutional323 × 3/1208 × 208 × 323Maxpool—2 × 2/2104 × 104 × 324Convolutional643 × 3/1104 × 104 × 645Maxpool—2 × 2/252 × 52 × 646Convolutional1283 × 3/152 × 52 × 1287Maxpool—2 × 2/226 × 26 × 1288Convolutional2563 × 3/126 × 26 × 2569Maxpool—2 × 2/213 × 13 × 25610Convolutional5123 × 3/113 × 13 × 51211Maxpool—2 × 2/113 × 13 × 51212Convolutional10243 × 3/113 × 13 × 102413Convolutional2561 × 1/113 × 13 × 25614Convolutional5123 × 3/113 × 13 × 51215Convolutional361 × 1/113 × 13 × 3616YOLO———17Route 13———18Convolutional1281 × 1/113 × 13 × 12819Upsample—2 × 2/126 × 26 × 12820Route 19, 8———21Convolutional2563 × 3/126 × 26 × 25622Convolutional361 × 1/126 × 26 × 25623YOLO———YOLOv3-Tiny is an end-to-end detection model whose input image is uniformly resized to a fixed size before entering the network. Generally, the input image is resized to a size of 608 × 608 or 416 × 416. YOLOv3-Tiny contains two YOLO layers to detect different scales, and the detection feature scale of the YOLO layer is related to the size of the input image. When the input image size is 416 × 416, the output feature sizes of the two YOLO layers are 13 × 13 and 26 × 26, respectively.


## 2.3. YOLOv3-Tiny Nonmaximal Suppression Algorithm

YOLOv3-Tiny’s nonmaximal suppression (NMS) algorithm sorts all bounding boxes by the size of the score. First, the bounding box with the highest score is selected, and then a threshold is predetermined. When the intersection over union (IOU) of other bounding boxes and the bounding box of the highest score is larger than this threshold, the bounding box will be inhibited. This method can effectively reduce the number of overlapping boxes. However, when two objects are close to each other, the bounding boxes generated by the two objects are also close to each other. Similarly, the bounding boxes will inhibit each other, leading to one of the bounding boxes being eliminated. At this point, the phenomenon of missing detection of the object occurs.


## 3. Proposed Method

### 3.1. Proposed Structure of the Network

Previous methods, such as SSD and YOLOv3, regard target detection as a regression problem. Although they can achieve accurate and real-time detection effect, they are not good to run on embedded systems or other small-sized equipment. This paper introduces details of the proposed JRL-YOLO network structure that is both micro and fast.Figure1 shows the overall network structure we proposed. The input image is set as 416 × 416, and the output feature sizes of the detection layer are 13 × 13 and 26 × 26, respectively. As shown in Figure 2, the convolution-batch normalization-leaky ReLU (CBL) layer represents a layer that includes the convolution layer, the layer normalization, and the use of leakey ReLU as the activation function. Maxpool indicates that this layer is the largest pooling layer, and upsample indicates that this layer is the upper sampling layer, with SPP being the spatial pyramid pooling structure. Figure 3 shows the proposed JRL structure, which can effectively extract the features of targets. The JRL and SPP structures are used as the backbone network of YOLOv3-Tiny.Figure 1
Structure of the proposed jump-join repetitious learning-YOLO network.Figure 2
Structure of the convolution-batch normalization-leaky ReLU layer.Figure 3
Structure of jump-join repetitious learning.


### 3.2. The Jump-Join Repetitious Learning Structure

The backbone network, as the feature extractor of the deep neural network, plays a very important role in the follow-up target detection. The performance of the backbone network is directly related to the speed and accuracy of target detection and also to the real-time performance and accuracy of the whole model. As the number of layers of the backbone network increases, the feature map extracted from the backbone network usually becomes smaller, which brings a certain amount of difficulty in recognition. Therefore, a more refined backbone network structure is needed to effectively extract the deep-level feature information of the target to enhance the accuracy of target recognition.In this part, a JRL structure is proposed and used as part of the YOLOv3-Tiny backbone network to improve the target detection accuracy. The main advantage of JRL is that the multiscale features of different network levels can be connected by jumping, and the gradient semantic information of different levels can be repeatedly learned in the backpropagation of the network so that the whole network can achieve high accuracy with few parameters. This will be demonstrated in Section4.As shown in Figure4, convolution structures with convolution kernels of 3 × 3 and 5 × 5 are used in this structure. After the convolution operation of the 5 × 5 convolution kernel, a 3 × 3 convolution operation is carried out for the depth features, and then the features obtained from the first 5 × 5 convolution are concatenated after the 5 × 5 convolution.Figure 4
Convolution structure of jump-join repetitious learning.The 5 × 5 convolution kernel can enhance the size of the receptive field of the image, and the larger receptive field information obtained by the 3 × 3 convolution check can be enhanced. Then, the depth features obtained by the 5 × 5 convolution can be combined with the features obtained by the first 5 × 5 convolution, which can enrich the semantic information of the network layer. The network layer with rich semantic information continues to propagate forward to the full connection layer. Compared with the CNN full connection layer which only has the convolutional layer and pooling layer, it can enhance the efficiency of feature extraction for finite pixels. Target features are more discriminable before classification, and the proposed structure can improve the detection accuracy and reduce misidentification compared to the CNN.As shown in Figure4, the base layer is x0, and ∗ is the presentation of the convolution. The backpropagation is used to update the weight, where the function is f, and gi is used to represent the gradient propagated to the ith layer, so the updated equation can be expressed as(1)w1′=fw1,g0,w5′=fw4,g4,g2,w10′=fw9,g7,g5,g0.We can see that, in (1), the gradient information g2, g0, g5, and g7 in w5′ and w10′ is repeatedly used to update different weights. This indicates that different convolutional layers are repeatedly learning the corresponding gradient information, which not only increases the learning ability of the CNN but also simplifies the network structure.


### 3.3. Spatial Pyramid Pooling

In the current research, the detection of dangerous targets and dangerous actions usually faces the following challenges: first, the small targets that are dangerous items occupy few pixels, which brings difficulties to feature extraction. Second, with the movement of the car, the geometric proportion of the same dangerous item or dangerous action in the image usually changes, so the features extracted from the same object at different times may vary, which brings difficulties to the final classification and recognition. In addition, images in real-life situations are often affected by shadows, light, and other external factors. Therefore, we introduce the SPP structure into the backbone network of YOLOV3-Tiny [28].The SPP structure uses multisize pooling for the same set of depth features, which is equivalent to using different levels of semantic information. Finally, concating can produce fixed-sized outputs from images with inputs of any sizes. As shown in Figure5, depth features can be obtained by splicing asynchronously long pooling layers, which can improve the scale invariance of images with different aspect ratios and sizes and reduce overfitting.Figure 5
Structure of spatial pyramid pooling.The SPP structure is placed after the first JRL structure as a further representation of the depth features of small targets can prevent the overfitting phenomenon caused by excessive numbers of convolutional layers in subsequent networks. The output of the SPP structure is connected to the second JRL structure. The addition of the SPP structure can effectively improve the detection accuracy of targets of different sizes.


### 3.4. Soft-NMS considering the Center Distance

Usually, in target detection, the model predicts multiple bounding boxes. The NMS algorithm is used to suppress redundant bounding boxes in postprocessing. The NMS process in YOLOv3-Tiny is described as follows:Step 1: set up the initial set of detection boxesB=b1,...,bNStep 2: set up the corresponding set of detection scoresS=s1,...,sNStep 3: set up the final result setD=d1,...,dNStep 4: select the bounding box with the highest score from setB, denoted by MStep 5: addM to set D, and remove M from set BStep  6: calculate the IOU values of the remaining bounding boxes inB and MStep 7: delete all boxes in setB whose intersection ratio is greater than the overlap threshold NtStep 8: repeat steps 4 to 7 until setB is emptyThe NMS formula of YOLOv3-Tiny is expressed as(2)si=si,iouM,bi<Nt0,iouM,bi≥Nt,where si represents the score of bounding boxes, M represents the highest score of all generated bounding boxes, and iouM,bi represents the IOU of bounding boxes bi and M. When the iouM,bi is greater than or equal to the overlap threshold Nt set by configuring the superparameter, the score of bounding boxes i will be directly set to 0, which is equivalent to direct deletion.The soft-NMS algorithm is an NMS algorithm for accurate target detection and location proposed by Bodla et al. [29]. Compared with the NMS algorithm, soft-NMS not only draws more accurate suppression boxes for detected targets but also improves the average accuracy using public data. Soft-NMS can be expressed as(3)si=si,iouM,bi<Ntsie−iouM,bi2/σ,iouM,bi≥Nt,where si represents the score of bounding boxes, M presents the highest score of all generated bounding boxes, iouM,bi represents the IOU of bounding boxes bi and M, and σ is the superparameter. When iouM,bi is greater than or equal to the overlap threshold Nt, the core of soft-NMS is to add penalty terms to reduce its confidence, and its score will be reduced accordingly.In this paper, inspired by DIoU-NMS [30], we introduce the center distance ratio (CDR) of two detection boxes on the basis of the Gaussian weighted soft-NMS. The formula of CDR is as follows:(4)R=ρ2b,bgtc2,where b and bgt are the center points of the box B and Bgt, separately, and ρ is the Euclidean distance, with c2 representing the diagonal distance of the minimum bounding box Band Bgt. As shown in Figure 6, the square of the ratio between the center distance of the two detection boxes and the diagonal distance of the minimum surrounding box is used as the measurement R in order to measure the distance between two bounding boxes.Figure 6
Center distance model of the detection box.After introducing the CDR, the NMS algorithm in (3) can be improved, which is expressed as(5)si=si,iouM,bi−R<εsie−iouM,bi2/σ,iouM,bi−R≥ε,where si is the score of the bounding box and M represents the maximum score among all generated bounding boxes. iouM,bi is the IOU of the bounding box bi and M, and σ is the superparameter. When the difference value between iouM,bi and R is greater than or equal to the overlap threshold Nt, the penalty terms will be increased to reduce its confidence, and its score will decrease accordingly.The improved NMS algorithm process in JRL-YOLO can be implemented as follows:Step 1: set up the initial set of detection boxesB=b1,...,bNStep 2: set up the corresponding set of detection scoresS=s1,...,sNStep 3: set up the final result setD=d1,...,dNStep 4: select the bounding box with the highest score from setB, denoted by MStep 5: addMto set D, and remove M from set BStep  6: calculate the IOU values of the remaining bounding boxes in setB with MStep 7: calculatec2, the diagonal distance of the minimum bounding box in set B with MStep 8: calculateρ2b,bgt, the Euclidean distance of the bounding box in set B with MStep 9: calculateR, where R=ρ2b,bgt/c2Step 10: calculateNt′=iouM,bi−RStep 11: calculatesi=sie−iouM,bi2/σ in set B whose Nt′ is greater than the overlap threshold εStep 12: if the value ofsi is greater than the IOU threshold displayed by the setting, it is reservedStep 13: repeat steps 4 to 12 until setB is emptyWhen the IOU value of the two boxes is relatively large and the distance between the centers of the two boxes is relatively large, too, the two boxes are objects that are close to each other. This reduces the possibility of two close objects being misjudged as the same object detection box. The following experimental section shows that the phenomenon of missing detection is reduced effectively, and the evaluation result evidences that the proposed algorithm is more practical than state-of-the-art approaches.


## 3.1. Proposed Structure of the Network

Previous methods, such as SSD and YOLOv3, regard target detection as a regression problem. Although they can achieve accurate and real-time detection effect, they are not good to run on embedded systems or other small-sized equipment. This paper introduces details of the proposed JRL-YOLO network structure that is both micro and fast.Figure1 shows the overall network structure we proposed. The input image is set as 416 × 416, and the output feature sizes of the detection layer are 13 × 13 and 26 × 26, respectively. As shown in Figure 2, the convolution-batch normalization-leaky ReLU (CBL) layer represents a layer that includes the convolution layer, the layer normalization, and the use of leakey ReLU as the activation function. Maxpool indicates that this layer is the largest pooling layer, and upsample indicates that this layer is the upper sampling layer, with SPP being the spatial pyramid pooling structure. Figure 3 shows the proposed JRL structure, which can effectively extract the features of targets. The JRL and SPP structures are used as the backbone network of YOLOv3-Tiny.Figure 1
Structure of the proposed jump-join repetitious learning-YOLO network.Figure 2
Structure of the convolution-batch normalization-leaky ReLU layer.Figure 3
Structure of jump-join repetitious learning.


## 3.2. The Jump-Join Repetitious Learning Structure

The backbone network, as the feature extractor of the deep neural network, plays a very important role in the follow-up target detection. The performance of the backbone network is directly related to the speed and accuracy of target detection and also to the real-time performance and accuracy of the whole model. As the number of layers of the backbone network increases, the feature map extracted from the backbone network usually becomes smaller, which brings a certain amount of difficulty in recognition. Therefore, a more refined backbone network structure is needed to effectively extract the deep-level feature information of the target to enhance the accuracy of target recognition.In this part, a JRL structure is proposed and used as part of the YOLOv3-Tiny backbone network to improve the target detection accuracy. The main advantage of JRL is that the multiscale features of different network levels can be connected by jumping, and the gradient semantic information of different levels can be repeatedly learned in the backpropagation of the network so that the whole network can achieve high accuracy with few parameters. This will be demonstrated in Section4.As shown in Figure4, convolution structures with convolution kernels of 3 × 3 and 5 × 5 are used in this structure. After the convolution operation of the 5 × 5 convolution kernel, a 3 × 3 convolution operation is carried out for the depth features, and then the features obtained from the first 5 × 5 convolution are concatenated after the 5 × 5 convolution.Figure 4
Convolution structure of jump-join repetitious learning.The 5 × 5 convolution kernel can enhance the size of the receptive field of the image, and the larger receptive field information obtained by the 3 × 3 convolution check can be enhanced. Then, the depth features obtained by the 5 × 5 convolution can be combined with the features obtained by the first 5 × 5 convolution, which can enrich the semantic information of the network layer. The network layer with rich semantic information continues to propagate forward to the full connection layer. Compared with the CNN full connection layer which only has the convolutional layer and pooling layer, it can enhance the efficiency of feature extraction for finite pixels. Target features are more discriminable before classification, and the proposed structure can improve the detection accuracy and reduce misidentification compared to the CNN.As shown in Figure4, the base layer is x0, and ∗ is the presentation of the convolution. The backpropagation is used to update the weight, where the function is f, and gi is used to represent the gradient propagated to the ith layer, so the updated equation can be expressed as(1)w1′=fw1,g0,w5′=fw4,g4,g2,w10′=fw9,g7,g5,g0.We can see that, in (1), the gradient information g2, g0, g5, and g7 in w5′ and w10′ is repeatedly used to update different weights. This indicates that different convolutional layers are repeatedly learning the corresponding gradient information, which not only increases the learning ability of the CNN but also simplifies the network structure.


## 3.3. Spatial Pyramid Pooling

In the current research, the detection of dangerous targets and dangerous actions usually faces the following challenges: first, the small targets that are dangerous items occupy few pixels, which brings difficulties to feature extraction. Second, with the movement of the car, the geometric proportion of the same dangerous item or dangerous action in the image usually changes, so the features extracted from the same object at different times may vary, which brings difficulties to the final classification and recognition. In addition, images in real-life situations are often affected by shadows, light, and other external factors. Therefore, we introduce the SPP structure into the backbone network of YOLOV3-Tiny [28].The SPP structure uses multisize pooling for the same set of depth features, which is equivalent to using different levels of semantic information. Finally, concating can produce fixed-sized outputs from images with inputs of any sizes. As shown in Figure5, depth features can be obtained by splicing asynchronously long pooling layers, which can improve the scale invariance of images with different aspect ratios and sizes and reduce overfitting.Figure 5
Structure of spatial pyramid pooling.The SPP structure is placed after the first JRL structure as a further representation of the depth features of small targets can prevent the overfitting phenomenon caused by excessive numbers of convolutional layers in subsequent networks. The output of the SPP structure is connected to the second JRL structure. The addition of the SPP structure can effectively improve the detection accuracy of targets of different sizes.


## 3.4. Soft-NMS considering the Center Distance

Usually, in target detection, the model predicts multiple bounding boxes. The NMS algorithm is used to suppress redundant bounding boxes in postprocessing. The NMS process in YOLOv3-Tiny is described as follows:Step 1: set up the initial set of detection boxesB=b1,...,bNStep 2: set up the corresponding set of detection scoresS=s1,...,sNStep 3: set up the final result setD=d1,...,dNStep 4: select the bounding box with the highest score from setB, denoted by MStep 5: addM to set D, and remove M from set BStep  6: calculate the IOU values of the remaining bounding boxes inB and MStep 7: delete all boxes in setB whose intersection ratio is greater than the overlap threshold NtStep 8: repeat steps 4 to 7 until setB is emptyThe NMS formula of YOLOv3-Tiny is expressed as(2)si=si,iouM,bi<Nt0,iouM,bi≥Nt,where si represents the score of bounding boxes, M represents the highest score of all generated bounding boxes, and iouM,bi represents the IOU of bounding boxes bi and M. When the iouM,bi is greater than or equal to the overlap threshold Nt set by configuring the superparameter, the score of bounding boxes i will be directly set to 0, which is equivalent to direct deletion.The soft-NMS algorithm is an NMS algorithm for accurate target detection and location proposed by Bodla et al. [29]. Compared with the NMS algorithm, soft-NMS not only draws more accurate suppression boxes for detected targets but also improves the average accuracy using public data. Soft-NMS can be expressed as(3)si=si,iouM,bi<Ntsie−iouM,bi2/σ,iouM,bi≥Nt,where si represents the score of bounding boxes, M presents the highest score of all generated bounding boxes, iouM,bi represents the IOU of bounding boxes bi and M, and σ is the superparameter. When iouM,bi is greater than or equal to the overlap threshold Nt, the core of soft-NMS is to add penalty terms to reduce its confidence, and its score will be reduced accordingly.In this paper, inspired by DIoU-NMS [30], we introduce the center distance ratio (CDR) of two detection boxes on the basis of the Gaussian weighted soft-NMS. The formula of CDR is as follows:(4)R=ρ2b,bgtc2,where b and bgt are the center points of the box B and Bgt, separately, and ρ is the Euclidean distance, with c2 representing the diagonal distance of the minimum bounding box Band Bgt. As shown in Figure 6, the square of the ratio between the center distance of the two detection boxes and the diagonal distance of the minimum surrounding box is used as the measurement R in order to measure the distance between two bounding boxes.Figure 6
Center distance model of the detection box.After introducing the CDR, the NMS algorithm in (3) can be improved, which is expressed as(5)si=si,iouM,bi−R<εsie−iouM,bi2/σ,iouM,bi−R≥ε,where si is the score of the bounding box and M represents the maximum score among all generated bounding boxes. iouM,bi is the IOU of the bounding box bi and M, and σ is the superparameter. When the difference value between iouM,bi and R is greater than or equal to the overlap threshold Nt, the penalty terms will be increased to reduce its confidence, and its score will decrease accordingly.The improved NMS algorithm process in JRL-YOLO can be implemented as follows:Step 1: set up the initial set of detection boxesB=b1,...,bNStep 2: set up the corresponding set of detection scoresS=s1,...,sNStep 3: set up the final result setD=d1,...,dNStep 4: select the bounding box with the highest score from setB, denoted by MStep 5: addMto set D, and remove M from set BStep  6: calculate the IOU values of the remaining bounding boxes in setB with MStep 7: calculatec2, the diagonal distance of the minimum bounding box in set B with MStep 8: calculateρ2b,bgt, the Euclidean distance of the bounding box in set B with MStep 9: calculateR, where R=ρ2b,bgt/c2Step 10: calculateNt′=iouM,bi−RStep 11: calculatesi=sie−iouM,bi2/σ in set B whose Nt′ is greater than the overlap threshold εStep 12: if the value ofsi is greater than the IOU threshold displayed by the setting, it is reservedStep 13: repeat steps 4 to 12 until setB is emptyWhen the IOU value of the two boxes is relatively large and the distance between the centers of the two boxes is relatively large, too, the two boxes are objects that are close to each other. This reduces the possibility of two close objects being misjudged as the same object detection box. The following experimental section shows that the phenomenon of missing detection is reduced effectively, and the evaluation result evidences that the proposed algorithm is more practical than state-of-the-art approaches.


## 4. Experiments’ Results and Analysis

In this section, the performance of the proposed JRL-YOLO network is evaluated. This section mainly introduces the experimental equipment and parameter setting, the collection of experimental data, and the comparison of experimental results. A large number of experiments were carried out on the self-made dataset and VOC2007 [31] to verify the performance of the proposed model, which was transplanted to an embedded development board to test the speed of running.


### 4.1. Experimental Platform and Parameter Setting

The hardware and software configuration platform of the experiments are shown in Table2, in which the NVIDIA Jetson AGX Xavier board will be embedded into a car for outdoor experiments. To ensure the fairness of the experimental results, all initial algorithm parameters of the experiments are maintained the same, which are given as follows: width = 416, height = 416, learning_rate = 0.001, batch = 64, subdivisions = 8, steps = (16,000, 20,000), and max_batches = 30,000.Table 2
Software and hardware configurations of the experimental platform.
Software and hardwareNVIDIA GeForce RTX 2070NVIDIA Jetson AGX XavierOperating platformWindows 10Ubuntu 18.04CPU/GHzi7-9750H,G5 5590, CPU@2.60 GHz,2.59 GHz8 nuclear ARM 64 8 MB L2 + 4 MB L3GPUCUDA 10.0, cuDNN 7.6CUDA 10.0, cuDNN 7.6RAM16 GB16 GBDeep learning frameworkDarknetDarknetAll models were trained on an NVIDIA GeForce RTX 2070 GPU. The data training in the experiments used the open-source Darknet framework provided by the AlexeyAB team, available in [32]. The version of CUDA is 10.0, and the version of cuDNN is 7.6. In order to verify the speed of the model on embedded system devices, the model was later transplanted to NVIDIA Jetson AGX Xavier to test the speed. NVIDIA Jetson AGX Xavier also uses the Darknet framework, and the operating system is Ubuntu 18.04. The versions of CUDA and cuDNN are also 10.0 and 7.6, respectively.


### 4.2. Details of Experimental Data Collection

We analyzed several common types of targets that may appear in the campus, dividing the dangerous targets into two categories when collecting and making the datasets, dangerous objects and dangerous actions. Dangerous objects mainly include the gun, knife, stick, and short knife, and dangerous actions include fight, kick, and fall. The dataset has overall 6809 pictures, among which each class occupies a relatively balanced proportion of the total dataset. In this dangerous target dataset, we randomly selected 80% of the data for training and the remaining 20% for testing. In fact, there is some uncertainty in such data selection [33], so we also use open datasets for validation, and explicit consideration of these issues may provide better knowledge of the authenticity of the results.The dataset produced in this study includes images from the internet and images taken by ourselves. The data collection follows the following rules: (1) two methods of manual shooting and vehicle collection are used; (2) discrete acquisition of images in the same video is applied, that is, an image is collected at a fixed time interval; (3) dangerous objects and dangerous actions are sampled in multiple outdoor scenes. Figure7 shows examples of the collected dangerous dataset.Figure 7
Self-made dangerous target datasets in different outdoor scenarios.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)In order to verify the robustness and universality of our model, the model was validated on the public dataset Pascal VOC2007. Pascal VOC2007 is an open dataset that is often used for image classification, target detection, and target segmentation. The VOC2007 dataset consists of 20 main classes of 9963 images, of which 5011 are used for training and 4952 are used for testing. Among the 20 categories, the number of pictures of people is relatively large, whereas the number of other categories is relatively small. Thus, there is a phenomenon of data imbalance. However, this makes the performance of our model more prominent on the Pascal VOC2007 dataset.


### 4.3. Comparison of Experimental Results

#### 4.3.1. Comparison of Precision

In this test, 1358 pictures of 7 kinds of objects were randomly selected from our self-made dataset of dangerous objects in each scene, with each kind consisting of 194 pictures. The seven specific categories are named kick, hit, fall, gun, knife, stick, and short knife. The accuracy of JRL-YOLO was evaluated by cross-validation. Six clustering centers of JRL-YOLO were used with sizes of (10,14), (23,27), (37,58), (81,82), (135,169), and (344,319). In the VOC2007 dataset, a common test set containing 4952 images was used for testing.The exact category and prediction category of samples can be divided into four categories [34]: TP (true positive), FP (false positive), TN (true negative), and FN (false negative), which are defined as follows:(i)
TP: a positive sample predicted by the model as a positive class(ii)
TN: a negative sample predicted by the model as a negative class(iii)
FP: a negative sample predicted by the model as a positive class(iv)
FN: a positive sample predicted by the model as a negative classLet us see Figure8 to better understand the meaning of TP, TN, FP, and FN. The “true value” represents the true value of an object, and the “predicted value” is the predicted value obtained by JRL-YOLO after calculation. TP, FP, FN, and TN are the cases where positive values are predicted correctly, negative values are predicted falsely, truth values are predicted falsely, and negative values are predicted correctly, respectively.Figure 8
Confusion matrix: schematic diagram of TP, FP, FN, and TN.Precision refers to the proportion occupied by TP in all positive classes judged by the model (TP + FP). High precision means that most of the results detected by the model are correct. The recall rate is the proportion of TP correctly judged by the model in all positive examples (TP + FN) in the dataset. The higher the recall rate is, the more objects the model can find in the picture. Generally, in order to evaluate the accuracy between different types of targets, MAP [35] is one of the important measures to evaluate test results. The precision and recall rate are defined as follows:(6)precision=TPTP+FP,recall=TPTP+FN.The values of TP, FP, FN, and TN are judged according to the values of IOU. When we calculate the values of precision and recall, we can draw the PR curve to get the values of AP.In order to verify the superiority of the JRL-YOLO model in the lightweight sense, YOLOv2-Tiny and YOLOv3-Tiny were trained and tested on the self-made dangerous target dataset. The statistical comparison experiments of the three groups using AP and MAP as measurements are shown in Table3. According to Postorino and Versaci [36], considering the uncertainty of experimental data results in very few cases during the experimental process, the potential aggregation of objects in the identification process is not as absolutely clear as it is indicated. For example, there are very few misidentification phenomena between sticks and knives. In order to ensure the fairness of the experiments, the confidence of the object category in all the experiments in this paper is set to 0.6. This value can also be adjusted to achieve higher accuracy.Table 3
Comparison of using three different lightweight models on the dangerous target dataset.
PrecisionClassesYOLOv2-TinyYOLOv3-TinyJRL-YOLOAP (%)Gun75.5885.9289.26Knife38.7671.6680.50Hit70.7181.6481.77Kick91.6888.3694.85Fall85.0187.2088.08Stick74.0381.5888.18Short knife75.1477.6681.25MAP (%)—73.2981.8185.03As obviously revealed in Table3, regardless of whether complex targets are included, the AP and MAP of each target in the JRL-YOLO network are significantly higher than those of YOLOv2-Tiny and YOLOv3-Tiny. By replacing the backbone network of YOLOv3-Tiny with the combination of JRL and SPP, the MAP value of the proposed JRL-YOLO network model is increased by 11.74 percent compared with that of YOLOv2-Tiny and by 3.22 percent compared with that of YOLOv3-Tiny.An important advantage of JRL-YOLO is that it can quickly and efficiently extract deep features of objects of different sizes and overcome the problem of low accuracy of target recognition of any size and proportion. In addition, the introduction of the SPP structure can increase the size of the receptive field of the target, effectively overcoming the phenomenon of missing detection and misidentification caused by different geometric deformations of the same object. It also reduces the overfitting of the network. Due to the superior performance of the backbone network, the MAP of JRL-YOLO is 3.94% higher than that of YOLOv3-Tiny and 16.02% higher than that of YOLOv2-Tiny.In the VOC2007 dataset, a total of four groups of comparative experiments were selected. The comparison of AP and MAP results is shown in Table4. It can be seen that only the MAP values of bicycle, car, chair, cow, sheep, and TV monitor in JRL-YOLO are lower than those of YOLOv4-Tiny, whereas the MAP values of other classes are the highest. This may be due to the JRL-YOLO’s smaller number of model parameters. Moreover, the MAP of JRL-YOLO is 23.47% higher than that of YOLOv3-Tiny and 5.12% higher than that of YOLOv4-Tiny. Therefore, it can be concluded that the proposed JRL-YOLO network has high universality and is suitable for the detection of most types of targets.Table 4
Comparison of MAP results tested on the VOC2007 dataset.
PrecisionClassesYOLOv2-TinyYOLOv3-TinyYOLOv4-TinyJRL-YOLOAP (%)Aeroplane39.8346.4754.0659.63Bicycle53.1054.2462.1359.90Bird13.5518.8329.5533.03Boat15.8318.3126.7331.49Bottle3.828.7916.4018.56Bus45.5246.4557.0662.29Car52.5360.6272.8171.21Cat35.1035.7750.1253.93Chair16.5121.1231.1128.66MAP (%)Cow29.1631.6247.7642.56Dining table27.3728.4235.4744.62Dog28.5330.1141.0046.74Horse55.4057.8465.5767.99Motorbike52.1854.3765.3065.52Person39.4748.7963.7364.41Potted plant6.8410.8718.7721.31Sheep31.3640.8546.7042.97Sofa21.1525.7236.5444.79Train46.7551.5659.3268.24TV monitor36.4542.2148.0646.91—35.4339.5946.5048.88The best result is shown in bold.


#### 4.3.2. Comparison of Speed and Model Sizes

Four groups of experiments were selected to compare the model size and MAP on the self-made dangerous target dataset and VOC2007, respectively. As shown in Table5, it is obvious that, on the self-made dangerous target dataset, the size of the JRL-YOLO network model is only 5.84 MB, which is nearly 6 times smaller than that of the YOLOv3-Tiny model and 8 times smaller than that of the YOLOv2-Tiny model, while showing the highest accuracy. Similarly, on the VOC2007 dataset, JRL-YOLO has the highest accuracy but the lowest model size.Table 5
Average MAP of two datasets and comparison of their model sizes.
YOLOv2-TinyYOLOv3-TinyYOLOv4-TinyJRL-YOLODangerous target datasetModel size (MB)42.133.122.45.84MAP (%)73.2981.8184.4985.03Pascal VOC2007 datasetModel size (MB)42.233.222.56.58MAP (%)35.4339.5946.5048.88The best result is shown in bold.In order to verify the real-time performance of JRL-YOLO, input videos of different sizes were used to test the proposed JRL-YOLO on NVIDIA GeForce RTX 2070 and NVIDIA Jetson AGX Xavier. As shown in Table6, with the size of the input video becoming smaller, the frame rate detected increased. The average FPS detection on NVIDIA GeForce RTX 2070 was 98.4 with a video of 1080 × 1920 frame size and 150.0 with a video of 640 × 480 frame size. Thus, the smaller the frame size of the video is, the larger the FPS will be. Real-time detection can also be achieved on NVIDIA Jetson AGX Xavier for videos with frame sizes being up to 1280 × 720.Table 6
FPS comparison of input videos of different sizes.
Video frame sizeNVIDIA GeForce RTX 2070 (FPS)NVIDIA Jetson AGX Xavier (FPS)1080 × 192098.4231280 × 720108.525640 × 480150.034The above results show that the JRL-YOLO model is lightweight, highly precise, and performing in real time.


#### 4.3.3. Qualitative Analysis

In Figure9, we selected 12 detection results of 4 pictures from the test set of our self-made dangerous target dataset to prove the superiority of the JRL-YOLO algorithm. The pictures in the first column are the detection results of YOLOv2-Tiny, the pictures in the second column are the detection results of YOLOv3-Tiny, and the pictures in the third column are the detection results of our proposed JRL-YOLO. It can be clearly seen that target detection is missing in the YOLOv3-Tiny results in Figure 9b(1)–b(4). Although the detection of YOLOv2-Tiny is better than that of YOLOv3-Tiny, the detection results have overlapping frames in Figure 9a(1)–a(4). Additionally, our proposed method can detect small targets, and the detected box is closer to the target in Figure 9c(1)–c(4). The four groups of experiments show that, by improving the backbone network of feature extraction and adding the SPP structure, the proposed JRL-YOLO improves the performance of target detection in real application scenarios.Figure 9
Comparison results of YOLOv2-Tiny (a1–a4), YOLOv3-Tiny (b1–b4), and the proposed JRL-YOLO (c1–c4) tested on the dangerous target dataset.The original image of second line in Figure8 is available online at http://www.sanqin.com/2016/0415/200756.shtml.In order to prove that JRL-YOLO also has superior performance in other types of target detection, eight images were selected from the VOC2007 dataset, tested with utilizing YOLOv3-Tiny and JRL-YOLO, respectively. The test results are shown in Figure10. a(1)–a(8) which are the detection results of YOLOv3-Tiny, and b(1)–b(8) are the detection results of JRL-YOLO. By comparison, it is obvious that YOLOv3-Tiny has missed detection in every picture, while the detection result of JRL-YOLO is much better than that of YOLOv3-Tiny. In b(7) and b(8), JRL-YOLO also has a missing detection phenomenon, which may be caused by too few parameters. Thus, it can be seen that JRL-YOLO has better feature extraction abilities and leads to higher detection accuracy than that of YOLOv3-Tiny.Figure 10
Comparison results of YOLOv3-Tiny (a1–a8) and the proposed JRL-YOLO (b1–b8) tested on VOC2007.


#### 4.3.4. Comparison of Loss Curves

As shown in Figure11, the final avg_loss calculated by JRL-YOLO in Figure 11(b) is 0.4544, while avg_loss of YOLOv3-Tiny is 0.4693. The result of JRL-YOLO is 0.0149 smaller than avg_loss of YOLOv3-Tiny. The convergence trend of both is basically the same, but avg_loss of the JRL-YOLO model is smaller, which indicates that the JRL-YOLO model has better convergence.Figure 11
Comparison of loss curves for two networks: (a) YOLOv3-Tiny network; (b) JRL-YOLO network.
(a)(b)


#### 4.3.5. Comparison of the Improved NMS

Figure12 shows the comparison of two NMS algorithms tested on dangerous targets employing the proposed JRL-YOLO. Obviously, we can see that when two targets are close to each other, two bounding boxes inhibit each other, resulting in one of the detection boxes being inhibited, and the detection missing phenomenon appears in Picture (a1) and Picture (a2). As for the improved NMS algorithm, it effectively reduces the missed detection phenomenon in Picture (b1) and Picture (b2). Thus, the improved NMS algorithm has greatly improved the detection accuracy of the targets.Figure 12
Comparison results of JRL-YOLO using NMS (a1, a2) and JRL-YOLO using the improved NMS (b1, b2).


## 4.1. Experimental Platform and Parameter Setting

The hardware and software configuration platform of the experiments are shown in Table2, in which the NVIDIA Jetson AGX Xavier board will be embedded into a car for outdoor experiments. To ensure the fairness of the experimental results, all initial algorithm parameters of the experiments are maintained the same, which are given as follows: width = 416, height = 416, learning_rate = 0.001, batch = 64, subdivisions = 8, steps = (16,000, 20,000), and max_batches = 30,000.Table 2
Software and hardware configurations of the experimental platform.
Software and hardwareNVIDIA GeForce RTX 2070NVIDIA Jetson AGX XavierOperating platformWindows 10Ubuntu 18.04CPU/GHzi7-9750H,G5 5590, CPU@2.60 GHz,2.59 GHz8 nuclear ARM 64 8 MB L2 + 4 MB L3GPUCUDA 10.0, cuDNN 7.6CUDA 10.0, cuDNN 7.6RAM16 GB16 GBDeep learning frameworkDarknetDarknetAll models were trained on an NVIDIA GeForce RTX 2070 GPU. The data training in the experiments used the open-source Darknet framework provided by the AlexeyAB team, available in [32]. The version of CUDA is 10.0, and the version of cuDNN is 7.6. In order to verify the speed of the model on embedded system devices, the model was later transplanted to NVIDIA Jetson AGX Xavier to test the speed. NVIDIA Jetson AGX Xavier also uses the Darknet framework, and the operating system is Ubuntu 18.04. The versions of CUDA and cuDNN are also 10.0 and 7.6, respectively.


## 4.2. Details of Experimental Data Collection

We analyzed several common types of targets that may appear in the campus, dividing the dangerous targets into two categories when collecting and making the datasets, dangerous objects and dangerous actions. Dangerous objects mainly include the gun, knife, stick, and short knife, and dangerous actions include fight, kick, and fall. The dataset has overall 6809 pictures, among which each class occupies a relatively balanced proportion of the total dataset. In this dangerous target dataset, we randomly selected 80% of the data for training and the remaining 20% for testing. In fact, there is some uncertainty in such data selection [33], so we also use open datasets for validation, and explicit consideration of these issues may provide better knowledge of the authenticity of the results.The dataset produced in this study includes images from the internet and images taken by ourselves. The data collection follows the following rules: (1) two methods of manual shooting and vehicle collection are used; (2) discrete acquisition of images in the same video is applied, that is, an image is collected at a fixed time interval; (3) dangerous objects and dangerous actions are sampled in multiple outdoor scenes. Figure7 shows examples of the collected dangerous dataset.Figure 7
Self-made dangerous target datasets in different outdoor scenarios.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)In order to verify the robustness and universality of our model, the model was validated on the public dataset Pascal VOC2007. Pascal VOC2007 is an open dataset that is often used for image classification, target detection, and target segmentation. The VOC2007 dataset consists of 20 main classes of 9963 images, of which 5011 are used for training and 4952 are used for testing. Among the 20 categories, the number of pictures of people is relatively large, whereas the number of other categories is relatively small. Thus, there is a phenomenon of data imbalance. However, this makes the performance of our model more prominent on the Pascal VOC2007 dataset.


## 4.3. Comparison of Experimental Results

### 4.3.1. Comparison of Precision

In this test, 1358 pictures of 7 kinds of objects were randomly selected from our self-made dataset of dangerous objects in each scene, with each kind consisting of 194 pictures. The seven specific categories are named kick, hit, fall, gun, knife, stick, and short knife. The accuracy of JRL-YOLO was evaluated by cross-validation. Six clustering centers of JRL-YOLO were used with sizes of (10,14), (23,27), (37,58), (81,82), (135,169), and (344,319). In the VOC2007 dataset, a common test set containing 4952 images was used for testing.The exact category and prediction category of samples can be divided into four categories [34]: TP (true positive), FP (false positive), TN (true negative), and FN (false negative), which are defined as follows:(i)
TP: a positive sample predicted by the model as a positive class(ii)
TN: a negative sample predicted by the model as a negative class(iii)
FP: a negative sample predicted by the model as a positive class(iv)
FN: a positive sample predicted by the model as a negative classLet us see Figure8 to better understand the meaning of TP, TN, FP, and FN. The “true value” represents the true value of an object, and the “predicted value” is the predicted value obtained by JRL-YOLO after calculation. TP, FP, FN, and TN are the cases where positive values are predicted correctly, negative values are predicted falsely, truth values are predicted falsely, and negative values are predicted correctly, respectively.Figure 8
Confusion matrix: schematic diagram of TP, FP, FN, and TN.Precision refers to the proportion occupied by TP in all positive classes judged by the model (TP + FP). High precision means that most of the results detected by the model are correct. The recall rate is the proportion of TP correctly judged by the model in all positive examples (TP + FN) in the dataset. The higher the recall rate is, the more objects the model can find in the picture. Generally, in order to evaluate the accuracy between different types of targets, MAP [35] is one of the important measures to evaluate test results. The precision and recall rate are defined as follows:(6)precision=TPTP+FP,recall=TPTP+FN.The values of TP, FP, FN, and TN are judged according to the values of IOU. When we calculate the values of precision and recall, we can draw the PR curve to get the values of AP.In order to verify the superiority of the JRL-YOLO model in the lightweight sense, YOLOv2-Tiny and YOLOv3-Tiny were trained and tested on the self-made dangerous target dataset. The statistical comparison experiments of the three groups using AP and MAP as measurements are shown in Table3. According to Postorino and Versaci [36], considering the uncertainty of experimental data results in very few cases during the experimental process, the potential aggregation of objects in the identification process is not as absolutely clear as it is indicated. For example, there are very few misidentification phenomena between sticks and knives. In order to ensure the fairness of the experiments, the confidence of the object category in all the experiments in this paper is set to 0.6. This value can also be adjusted to achieve higher accuracy.Table 3
Comparison of using three different lightweight models on the dangerous target dataset.
PrecisionClassesYOLOv2-TinyYOLOv3-TinyJRL-YOLOAP (%)Gun75.5885.9289.26Knife38.7671.6680.50Hit70.7181.6481.77Kick91.6888.3694.85Fall85.0187.2088.08Stick74.0381.5888.18Short knife75.1477.6681.25MAP (%)—73.2981.8185.03As obviously revealed in Table3, regardless of whether complex targets are included, the AP and MAP of each target in the JRL-YOLO network are significantly higher than those of YOLOv2-Tiny and YOLOv3-Tiny. By replacing the backbone network of YOLOv3-Tiny with the combination of JRL and SPP, the MAP value of the proposed JRL-YOLO network model is increased by 11.74 percent compared with that of YOLOv2-Tiny and by 3.22 percent compared with that of YOLOv3-Tiny.An important advantage of JRL-YOLO is that it can quickly and efficiently extract deep features of objects of different sizes and overcome the problem of low accuracy of target recognition of any size and proportion. In addition, the introduction of the SPP structure can increase the size of the receptive field of the target, effectively overcoming the phenomenon of missing detection and misidentification caused by different geometric deformations of the same object. It also reduces the overfitting of the network. Due to the superior performance of the backbone network, the MAP of JRL-YOLO is 3.94% higher than that of YOLOv3-Tiny and 16.02% higher than that of YOLOv2-Tiny.In the VOC2007 dataset, a total of four groups of comparative experiments were selected. The comparison of AP and MAP results is shown in Table4. It can be seen that only the MAP values of bicycle, car, chair, cow, sheep, and TV monitor in JRL-YOLO are lower than those of YOLOv4-Tiny, whereas the MAP values of other classes are the highest. This may be due to the JRL-YOLO’s smaller number of model parameters. Moreover, the MAP of JRL-YOLO is 23.47% higher than that of YOLOv3-Tiny and 5.12% higher than that of YOLOv4-Tiny. Therefore, it can be concluded that the proposed JRL-YOLO network has high universality and is suitable for the detection of most types of targets.Table 4
Comparison of MAP results tested on the VOC2007 dataset.
PrecisionClassesYOLOv2-TinyYOLOv3-TinyYOLOv4-TinyJRL-YOLOAP (%)Aeroplane39.8346.4754.0659.63Bicycle53.1054.2462.1359.90Bird13.5518.8329.5533.03Boat15.8318.3126.7331.49Bottle3.828.7916.4018.56Bus45.5246.4557.0662.29Car52.5360.6272.8171.21Cat35.1035.7750.1253.93Chair16.5121.1231.1128.66MAP (%)Cow29.1631.6247.7642.56Dining table27.3728.4235.4744.62Dog28.5330.1141.0046.74Horse55.4057.8465.5767.99Motorbike52.1854.3765.3065.52Person39.4748.7963.7364.41Potted plant6.8410.8718.7721.31Sheep31.3640.8546.7042.97Sofa21.1525.7236.5444.79Train46.7551.5659.3268.24TV monitor36.4542.2148.0646.91—35.4339.5946.5048.88The best result is shown in bold.


### 4.3.2. Comparison of Speed and Model Sizes

Four groups of experiments were selected to compare the model size and MAP on the self-made dangerous target dataset and VOC2007, respectively. As shown in Table5, it is obvious that, on the self-made dangerous target dataset, the size of the JRL-YOLO network model is only 5.84 MB, which is nearly 6 times smaller than that of the YOLOv3-Tiny model and 8 times smaller than that of the YOLOv2-Tiny model, while showing the highest accuracy. Similarly, on the VOC2007 dataset, JRL-YOLO has the highest accuracy but the lowest model size.Table 5
Average MAP of two datasets and comparison of their model sizes.
YOLOv2-TinyYOLOv3-TinyYOLOv4-TinyJRL-YOLODangerous target datasetModel size (MB)42.133.122.45.84MAP (%)73.2981.8184.4985.03Pascal VOC2007 datasetModel size (MB)42.233.222.56.58MAP (%)35.4339.5946.5048.88The best result is shown in bold.In order to verify the real-time performance of JRL-YOLO, input videos of different sizes were used to test the proposed JRL-YOLO on NVIDIA GeForce RTX 2070 and NVIDIA Jetson AGX Xavier. As shown in Table6, with the size of the input video becoming smaller, the frame rate detected increased. The average FPS detection on NVIDIA GeForce RTX 2070 was 98.4 with a video of 1080 × 1920 frame size and 150.0 with a video of 640 × 480 frame size. Thus, the smaller the frame size of the video is, the larger the FPS will be. Real-time detection can also be achieved on NVIDIA Jetson AGX Xavier for videos with frame sizes being up to 1280 × 720.Table 6
FPS comparison of input videos of different sizes.
Video frame sizeNVIDIA GeForce RTX 2070 (FPS)NVIDIA Jetson AGX Xavier (FPS)1080 × 192098.4231280 × 720108.525640 × 480150.034The above results show that the JRL-YOLO model is lightweight, highly precise, and performing in real time.


### 4.3.3. Qualitative Analysis

In Figure9, we selected 12 detection results of 4 pictures from the test set of our self-made dangerous target dataset to prove the superiority of the JRL-YOLO algorithm. The pictures in the first column are the detection results of YOLOv2-Tiny, the pictures in the second column are the detection results of YOLOv3-Tiny, and the pictures in the third column are the detection results of our proposed JRL-YOLO. It can be clearly seen that target detection is missing in the YOLOv3-Tiny results in Figure 9b(1)–b(4). Although the detection of YOLOv2-Tiny is better than that of YOLOv3-Tiny, the detection results have overlapping frames in Figure 9a(1)–a(4). Additionally, our proposed method can detect small targets, and the detected box is closer to the target in Figure 9c(1)–c(4). The four groups of experiments show that, by improving the backbone network of feature extraction and adding the SPP structure, the proposed JRL-YOLO improves the performance of target detection in real application scenarios.Figure 9
Comparison results of YOLOv2-Tiny (a1–a4), YOLOv3-Tiny (b1–b4), and the proposed JRL-YOLO (c1–c4) tested on the dangerous target dataset.The original image of second line in Figure8 is available online at http://www.sanqin.com/2016/0415/200756.shtml.In order to prove that JRL-YOLO also has superior performance in other types of target detection, eight images were selected from the VOC2007 dataset, tested with utilizing YOLOv3-Tiny and JRL-YOLO, respectively. The test results are shown in Figure10. a(1)–a(8) which are the detection results of YOLOv3-Tiny, and b(1)–b(8) are the detection results of JRL-YOLO. By comparison, it is obvious that YOLOv3-Tiny has missed detection in every picture, while the detection result of JRL-YOLO is much better than that of YOLOv3-Tiny. In b(7) and b(8), JRL-YOLO also has a missing detection phenomenon, which may be caused by too few parameters. Thus, it can be seen that JRL-YOLO has better feature extraction abilities and leads to higher detection accuracy than that of YOLOv3-Tiny.Figure 10
Comparison results of YOLOv3-Tiny (a1–a8) and the proposed JRL-YOLO (b1–b8) tested on VOC2007.


### 4.3.4. Comparison of Loss Curves

As shown in Figure11, the final avg_loss calculated by JRL-YOLO in Figure 11(b) is 0.4544, while avg_loss of YOLOv3-Tiny is 0.4693. The result of JRL-YOLO is 0.0149 smaller than avg_loss of YOLOv3-Tiny. The convergence trend of both is basically the same, but avg_loss of the JRL-YOLO model is smaller, which indicates that the JRL-YOLO model has better convergence.Figure 11
Comparison of loss curves for two networks: (a) YOLOv3-Tiny network; (b) JRL-YOLO network.
(a)(b)


### 4.3.5. Comparison of the Improved NMS

Figure12 shows the comparison of two NMS algorithms tested on dangerous targets employing the proposed JRL-YOLO. Obviously, we can see that when two targets are close to each other, two bounding boxes inhibit each other, resulting in one of the detection boxes being inhibited, and the detection missing phenomenon appears in Picture (a1) and Picture (a2). As for the improved NMS algorithm, it effectively reduces the missed detection phenomenon in Picture (b1) and Picture (b2). Thus, the improved NMS algorithm has greatly improved the detection accuracy of the targets.Figure 12
Comparison results of JRL-YOLO using NMS (a1, a2) and JRL-YOLO using the improved NMS (b1, b2).


## 4.3.1. Comparison of Precision

In this test, 1358 pictures of 7 kinds of objects were randomly selected from our self-made dataset of dangerous objects in each scene, with each kind consisting of 194 pictures. The seven specific categories are named kick, hit, fall, gun, knife, stick, and short knife. The accuracy of JRL-YOLO was evaluated by cross-validation. Six clustering centers of JRL-YOLO were used with sizes of (10,14), (23,27), (37,58), (81,82), (135,169), and (344,319). In the VOC2007 dataset, a common test set containing 4952 images was used for testing.The exact category and prediction category of samples can be divided into four categories [34]: TP (true positive), FP (false positive), TN (true negative), and FN (false negative), which are defined as follows:(i)
TP: a positive sample predicted by the model as a positive class(ii)
TN: a negative sample predicted by the model as a negative class(iii)
FP: a negative sample predicted by the model as a positive class(iv)
FN: a positive sample predicted by the model as a negative classLet us see Figure8 to better understand the meaning of TP, TN, FP, and FN. The “true value” represents the true value of an object, and the “predicted value” is the predicted value obtained by JRL-YOLO after calculation. TP, FP, FN, and TN are the cases where positive values are predicted correctly, negative values are predicted falsely, truth values are predicted falsely, and negative values are predicted correctly, respectively.Figure 8
Confusion matrix: schematic diagram of TP, FP, FN, and TN.Precision refers to the proportion occupied by TP in all positive classes judged by the model (TP + FP). High precision means that most of the results detected by the model are correct. The recall rate is the proportion of TP correctly judged by the model in all positive examples (TP + FN) in the dataset. The higher the recall rate is, the more objects the model can find in the picture. Generally, in order to evaluate the accuracy between different types of targets, MAP [35] is one of the important measures to evaluate test results. The precision and recall rate are defined as follows:(6)precision=TPTP+FP,recall=TPTP+FN.The values of TP, FP, FN, and TN are judged according to the values of IOU. When we calculate the values of precision and recall, we can draw the PR curve to get the values of AP.In order to verify the superiority of the JRL-YOLO model in the lightweight sense, YOLOv2-Tiny and YOLOv3-Tiny were trained and tested on the self-made dangerous target dataset. The statistical comparison experiments of the three groups using AP and MAP as measurements are shown in Table3. According to Postorino and Versaci [36], considering the uncertainty of experimental data results in very few cases during the experimental process, the potential aggregation of objects in the identification process is not as absolutely clear as it is indicated. For example, there are very few misidentification phenomena between sticks and knives. In order to ensure the fairness of the experiments, the confidence of the object category in all the experiments in this paper is set to 0.6. This value can also be adjusted to achieve higher accuracy.Table 3
Comparison of using three different lightweight models on the dangerous target dataset.
PrecisionClassesYOLOv2-TinyYOLOv3-TinyJRL-YOLOAP (%)Gun75.5885.9289.26Knife38.7671.6680.50Hit70.7181.6481.77Kick91.6888.3694.85Fall85.0187.2088.08Stick74.0381.5888.18Short knife75.1477.6681.25MAP (%)—73.2981.8185.03As obviously revealed in Table3, regardless of whether complex targets are included, the AP and MAP of each target in the JRL-YOLO network are significantly higher than those of YOLOv2-Tiny and YOLOv3-Tiny. By replacing the backbone network of YOLOv3-Tiny with the combination of JRL and SPP, the MAP value of the proposed JRL-YOLO network model is increased by 11.74 percent compared with that of YOLOv2-Tiny and by 3.22 percent compared with that of YOLOv3-Tiny.An important advantage of JRL-YOLO is that it can quickly and efficiently extract deep features of objects of different sizes and overcome the problem of low accuracy of target recognition of any size and proportion. In addition, the introduction of the SPP structure can increase the size of the receptive field of the target, effectively overcoming the phenomenon of missing detection and misidentification caused by different geometric deformations of the same object. It also reduces the overfitting of the network. Due to the superior performance of the backbone network, the MAP of JRL-YOLO is 3.94% higher than that of YOLOv3-Tiny and 16.02% higher than that of YOLOv2-Tiny.In the VOC2007 dataset, a total of four groups of comparative experiments were selected. The comparison of AP and MAP results is shown in Table4. It can be seen that only the MAP values of bicycle, car, chair, cow, sheep, and TV monitor in JRL-YOLO are lower than those of YOLOv4-Tiny, whereas the MAP values of other classes are the highest. This may be due to the JRL-YOLO’s smaller number of model parameters. Moreover, the MAP of JRL-YOLO is 23.47% higher than that of YOLOv3-Tiny and 5.12% higher than that of YOLOv4-Tiny. Therefore, it can be concluded that the proposed JRL-YOLO network has high universality and is suitable for the detection of most types of targets.Table 4
Comparison of MAP results tested on the VOC2007 dataset.
PrecisionClassesYOLOv2-TinyYOLOv3-TinyYOLOv4-TinyJRL-YOLOAP (%)Aeroplane39.8346.4754.0659.63Bicycle53.1054.2462.1359.90Bird13.5518.8329.5533.03Boat15.8318.3126.7331.49Bottle3.828.7916.4018.56Bus45.5246.4557.0662.29Car52.5360.6272.8171.21Cat35.1035.7750.1253.93Chair16.5121.1231.1128.66MAP (%)Cow29.1631.6247.7642.56Dining table27.3728.4235.4744.62Dog28.5330.1141.0046.74Horse55.4057.8465.5767.99Motorbike52.1854.3765.3065.52Person39.4748.7963.7364.41Potted plant6.8410.8718.7721.31Sheep31.3640.8546.7042.97Sofa21.1525.7236.5444.79Train46.7551.5659.3268.24TV monitor36.4542.2148.0646.91—35.4339.5946.5048.88The best result is shown in bold.


## 4.3.2. Comparison of Speed and Model Sizes

Four groups of experiments were selected to compare the model size and MAP on the self-made dangerous target dataset and VOC2007, respectively. As shown in Table5, it is obvious that, on the self-made dangerous target dataset, the size of the JRL-YOLO network model is only 5.84 MB, which is nearly 6 times smaller than that of the YOLOv3-Tiny model and 8 times smaller than that of the YOLOv2-Tiny model, while showing the highest accuracy. Similarly, on the VOC2007 dataset, JRL-YOLO has the highest accuracy but the lowest model size.Table 5
Average MAP of two datasets and comparison of their model sizes.
YOLOv2-TinyYOLOv3-TinyYOLOv4-TinyJRL-YOLODangerous target datasetModel size (MB)42.133.122.45.84MAP (%)73.2981.8184.4985.03Pascal VOC2007 datasetModel size (MB)42.233.222.56.58MAP (%)35.4339.5946.5048.88The best result is shown in bold.In order to verify the real-time performance of JRL-YOLO, input videos of different sizes were used to test the proposed JRL-YOLO on NVIDIA GeForce RTX 2070 and NVIDIA Jetson AGX Xavier. As shown in Table6, with the size of the input video becoming smaller, the frame rate detected increased. The average FPS detection on NVIDIA GeForce RTX 2070 was 98.4 with a video of 1080 × 1920 frame size and 150.0 with a video of 640 × 480 frame size. Thus, the smaller the frame size of the video is, the larger the FPS will be. Real-time detection can also be achieved on NVIDIA Jetson AGX Xavier for videos with frame sizes being up to 1280 × 720.Table 6
FPS comparison of input videos of different sizes.
Video frame sizeNVIDIA GeForce RTX 2070 (FPS)NVIDIA Jetson AGX Xavier (FPS)1080 × 192098.4231280 × 720108.525640 × 480150.034The above results show that the JRL-YOLO model is lightweight, highly precise, and performing in real time.


## 4.3.3. Qualitative Analysis

In Figure9, we selected 12 detection results of 4 pictures from the test set of our self-made dangerous target dataset to prove the superiority of the JRL-YOLO algorithm. The pictures in the first column are the detection results of YOLOv2-Tiny, the pictures in the second column are the detection results of YOLOv3-Tiny, and the pictures in the third column are the detection results of our proposed JRL-YOLO. It can be clearly seen that target detection is missing in the YOLOv3-Tiny results in Figure 9b(1)–b(4). Although the detection of YOLOv2-Tiny is better than that of YOLOv3-Tiny, the detection results have overlapping frames in Figure 9a(1)–a(4). Additionally, our proposed method can detect small targets, and the detected box is closer to the target in Figure 9c(1)–c(4). The four groups of experiments show that, by improving the backbone network of feature extraction and adding the SPP structure, the proposed JRL-YOLO improves the performance of target detection in real application scenarios.Figure 9
Comparison results of YOLOv2-Tiny (a1–a4), YOLOv3-Tiny (b1–b4), and the proposed JRL-YOLO (c1–c4) tested on the dangerous target dataset.The original image of second line in Figure8 is available online at http://www.sanqin.com/2016/0415/200756.shtml.In order to prove that JRL-YOLO also has superior performance in other types of target detection, eight images were selected from the VOC2007 dataset, tested with utilizing YOLOv3-Tiny and JRL-YOLO, respectively. The test results are shown in Figure10. a(1)–a(8) which are the detection results of YOLOv3-Tiny, and b(1)–b(8) are the detection results of JRL-YOLO. By comparison, it is obvious that YOLOv3-Tiny has missed detection in every picture, while the detection result of JRL-YOLO is much better than that of YOLOv3-Tiny. In b(7) and b(8), JRL-YOLO also has a missing detection phenomenon, which may be caused by too few parameters. Thus, it can be seen that JRL-YOLO has better feature extraction abilities and leads to higher detection accuracy than that of YOLOv3-Tiny.Figure 10
Comparison results of YOLOv3-Tiny (a1–a8) and the proposed JRL-YOLO (b1–b8) tested on VOC2007.


## 4.3.4. Comparison of Loss Curves

As shown in Figure11, the final avg_loss calculated by JRL-YOLO in Figure 11(b) is 0.4544, while avg_loss of YOLOv3-Tiny is 0.4693. The result of JRL-YOLO is 0.0149 smaller than avg_loss of YOLOv3-Tiny. The convergence trend of both is basically the same, but avg_loss of the JRL-YOLO model is smaller, which indicates that the JRL-YOLO model has better convergence.Figure 11
Comparison of loss curves for two networks: (a) YOLOv3-Tiny network; (b) JRL-YOLO network.
(a)(b)


## 4.3.5. Comparison of the Improved NMS

Figure12 shows the comparison of two NMS algorithms tested on dangerous targets employing the proposed JRL-YOLO. Obviously, we can see that when two targets are close to each other, two bounding boxes inhibit each other, resulting in one of the detection boxes being inhibited, and the detection missing phenomenon appears in Picture (a1) and Picture (a2). As for the improved NMS algorithm, it effectively reduces the missed detection phenomenon in Picture (b1) and Picture (b2). Thus, the improved NMS algorithm has greatly improved the detection accuracy of the targets.Figure 12
Comparison results of JRL-YOLO using NMS (a1, a2) and JRL-YOLO using the improved NMS (b1, b2).


## 5. Conclusions

This paper proposes an improved YOLOv3-Tiny real-time detection algorithm for dangerous targets on campus. The improved microtarget detection model is called jump-join repetitious learning YOLO. The main improvement of the JRL-YOLO network lies in a new backbone network composed of the JRL structure and SPP structure. In addition, the proposed method also improves the NMS algorithm by adding the center distance measurement of the two object detection boxes to the soft-NMS algorithm, so as to have high detection accuracy when detecting two objects with a close distance. By extracting multiscale features of images and increasing the ability of repeated learning of the network, the new network structure can improve the accuracy of the original algorithm while reducing the size of the model. To a certain extent, the computational burden is reduced, and the accuracy of target detection is improved. The new network structure is also easy to be transplanted to other algorithm models.Compared with the original YOLOv3-Tiny model, JRL-YOLO designed in this paper has a smaller volume and higher precision. Meanwhile, it can achieve real-time detection on the embedded system NVIDIA Jetson AGX Xavier. The experimental results show that JRL-YOLO has the advantages of few model parameters, high accuracy, and fast detection speed. It can complete the real-time detection task of dangerous targets on campus in complex outdoor scenes, and JRL-YOLO greatly reduces the missing detection phenomenon in the detection process when using YOLOv3-Tiny.In the future work, the polynomial convolution kernel will be used to continue to improve the feature extraction capability of the backbone network, so as to improve the efficiency of feature extraction and the performance of detection. The detection of small targets usually depends on larger network structures, and the future work will also include small-target detection in lightweight networks and building a more efficient feature matching mechanism in the detection layer. The method in this paper can also provide references for abnormal behavior detection in the field of security.



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*Source: 5536152-2021-04-01.xml*