Computers, Materials & Continua

A Deep Learning Approach for Crowd Counting in Highly Congested Scene

Akbar Khan1, Kushsairy Abdul Kadir1,*, Jawad Ali Shah2, Waleed Albattah3, Muhammad Saeed4, Haidawati Nasir5, Megat Norulazmi Megat Mohamed Noor5 and Muhammad Haris Kaka Khel1

1Electrical Section, Universiti Kuala Lumpur British Malaysian Institute, Selangor, 53100, Malaysia
2Department of Electrical Engineering, International Islamic University, Islamabad, 44000, Pakistan
3Department of Information Technology, College of Computer, Qassim University, Buraydah, 51452, Saudi Arabia
4Department of Electrical Engineering, National University of Computer and Emerging Sciences, Islamabad, 44000, Pakistan
5Computer Engineering Technology Section, Universiti Kuala Lumpur Malaysian Institute of Information Technology, Kuala Lumpur, 50250, Malaysia
*Corresponding Author: Kushsairy Abdul Kadir. Email: kushsairy@unikl.edu.my
Received: 10 January 2022; Accepted: 04 March 2022

Abstract: With the rapid progress of deep convolutional neural networks, several applications of crowd counting have been proposed and explored in the literature. In congested scene monitoring, a variety of crowd density estimating approaches has been developed. The understanding of highly congested scenes for crowd counting during Muslim gatherings of Hajj and Umrah is a challenging task, as a large number of individuals stand nearby and, it is hard for detection techniques to recognize them, as the crowd can vary from low density to high density. To deal with such highly congested scenes, we have proposed the Congested Scene Crowd Counting Network (CSCC-Net) using VGG-16 as a core network with its first ten layers due to its strong and robust transfer learning rate. A hole dilated convolutional neural network is used at the back end to widen the relevant field to extract a large range of information from the image without losing its original resolution. The dilated convolution neural network is mainly chosen to expand the kernel size without changing other parameters. Moreover, several loss functions have been applied to strengthen the evaluation accuracy of the model. Finally, the entire experiments have been evaluated using prominent data sets namely, ShanghaiTech parts A, B, UCF_CC_50, and UCF_QNRF. Our model has achieved remarkable results i.e., 68.0 and 9.0 MAE on ShanghaiTech parts A, B, 199.1 MAE on UCF_CC_50, and 99.8 on UCF_QNRF data sets respectively.

Keywords: Deep learning; congested scene; crowd counting; fully convolutional neural network; dilated convolution

1  Introduction

Owing to its various applications, crowd analysis in general and counting in the congested scene in particular, has emerged as an effective research area in computer vision. Counting the exact number of people and their density maps estimation in highly congested scenes is a daunting task. Mostly, crowded scenes occur due to panic situations, hazards, and lack of management and control as shown in Fig. 2. It is necessary to understand the risk analysis and safety aspects of crowd dynamics at various important occasions related to sports, cultural and religious activities (specifically Hajj and Umrah) to avoid or minimize the impact of any mishaps. The complexity of monitoring, tracking, and counting increases with the size of the crowd. Usage of drones for crowd detection and counting has achieved popularity due to their ability to travel freely as compared to fixed cameras. In most cases, one drone is enough to provide a fair idea of crowd density and is also effective in capturing and monitoring highly congested scenes with multiple people standing close to each other causing occlusion [1]. Many networks have been proposed to deliver a solution for crowd monitoring during a highly congested scene like assembly controlling, Hajj and umrah, and other such gatherings [25]. In recent years, interest in the visual analysis of crowd scenes has grown due to the widespread deployment of security cameras for crowd surveillance, traffic, and planning management, and even counting cells [6]. The current methodologies for analyzing extremely crowded scenes range from basic crowd counts to density map estimates [7]. Crowd disasters are highly likely to occur in highly congested scenes like religious gatherings (Hajj and Umrah) and rallies which make it important to analyze crowd strength [8]. Deep Neural Network (DNN-based) and Convolutional Neural Network (CNN) approaches have recently been used in congested scene analysis because of the classification, regression, and high accuracy these schemes achieved in semantic segmentation [911]. Three approaches of scene analysis were proposed by the researchers: namely detection-based counting, regression-based counting, and density map-based counting [12]. The first approach specifies the entire body to locate individuals in the scene which results in occlusions of the body or a small number of pixels per person. This approach is not effective for the highly congested scene. The second approach is to generate the density maps of the heads present in the image and further counting is performed on these features [1,8]. The majority of earlier crowded scene analysis research used multi-scale structures and generated excellent results. However, there are many significant flaws in the design they used: a) the appearance of small objects in images, b) the scene has a lot of clutter and occlusion, so much of the human body is hidden, c) non-uniform crowd distribution and scale variation (appearance of heads), d) as the network grows deeper and deeper, it may take a long time to train, and the branch structure may become ineffective [4,1318].

1.1 Motivation

Recently, most researchers are focusing on crowd counting and localization, benefited from crowd monitoring (counting) there are some models which are based on crowd head and face detection. The counting and detection of small heads and occluded faces in crowded images is a challenging task, and the traditional model can not perform well in this scenario. To solve the scale variation and occlusion in the congested scene it is important to design and explore a scientific model which can count the crowd in the congested scene.

1.2 Contributions

In our paper, we have designed a Congested Scene Crowd Counting Network having the ability to count in highly congested scenes and to produce a high-quality density map. The main contribution of this work include:

•   Generation of high-quality density maps and employment of VGG-16 as a front-end with its 10 layers due to its strong transfer learning rate

•   Adaption of Dilated Convolution Neural Network (DCNN) as the back end to get a large range of information from its respective fields

•   Application of different loss functions to minimize the error between predicted and real values

The rest of the paper is divided into sections. Section 2 represents the previous works, Section 3 shows CNN and DCNN frameworks for congested scenes, Section 4 elaborates limitations, while Section 5 explains the architecture of Congested Scene Crowd Counting, Sections 6 and 7 describe the training methods, discussions, and conclusion respectively.

2  Related Works and Understanding the Different Computer Vision Techniques for Crowd Counting

The researchers in the field of crowd monitoring and counting aim to understand the domain and apply various machine learning techniques and methodologies to count the number of individuals in a congested environment. Some of the related works presented here are: Inspired by the idea proposed in [19] for crowd counting, there are three most often used approaches in computer vision namely detection-based method, regression-based method, and density-based estimation method. The taxonomy levels of the computer vision techniques are shown in Fig. 1:


Figure 1: Computer vision techniques

2.1 Detection Based Methods

Most preliminary investigations have focused on the detection framework, in which a sliding window is used as a detector to detect [20] and to count the number of people in the scene [21]. Object detectors have been trained to determine the position of each person in a crowd to count them in detection-based approaches. In both monolithic and parts-based detection, detection-based methods are frequently used. Monolithic detection approaches such as [2225] are often conventional people detection methods that use features (such as Haar wavelets [26], Histogram Oriented Gradients (HOG) [22], edgelet [27], and shapelet [28]) extracted from an entire body of the object to train the classifier. To get low-level information from the entire human body, these methods require well-trained classifiers. The Support Vector Machines (SVM), boosting [29], and random forest [30] are among the different successful learning classifiers used. However, such classifiers are negatively affected by high congested scenes. They have failed to identify the objects in highly congested scenes as the objects are very close to each other and cannot be detected through a moving window. They are also incapable of detecting and counting the number of people [31]. To deal with this challenge, researchers have recommended detecting a specific point in the object rather than the whole object to carry out crowd scene analysis [32]. Correspondingly, a part-based detection approach [3335] has been endorsed. In part-based detection approaches, a specific part of the object (such as the head) is detected for people counting in a specific window [21].


Figure 2: Crowd incidents

2.2 Regression-based Methods

While parts-based algorithms are used to lessen the difficulty of detecting objects in high-density crowd situations, they are less effective in the face of very high-density crowds or noisy background images. To deal with this challenge, researchers have proposed counting objects by performing regression-based methods [3638]. The regression-based method estimates the number of features after performing regression between image features and crowd size. Low-level information has been generated using additional elements such as foreground and texture [39]. Because they acquire generalized density information from crowd images, regression-based methods perform well in the high-density situation. On the other hand, the method has two significant drawbacks: poor performance owing to overestimation and crowd misdistribution [40].

2.3 Density Estimation-based Methods

Both of the preceding techniques were successful in addressing occlusion and clutter concerns. Because they were regressing on a global count, they frequently failed to cope with critical spatial information [41]. The authors of [42] sought to find a linear mapping between local patch features and corresponding object density maps. By that means they incorporate spatial information in the learning process. The tough process of identifying and localizing objects has been avoided as a consequence of adopting this technique of image density estimation that is integral over any region in the density map and offers the count of objects inside that region. The density estimation-based method uses a density map for crowd counting. It initially generates a density map for the objects. Following that, the algorithm learns a linear mapping between the extracted features and their objects’ density maps. When adopting the regression-based method, one key component known as saliency is ignored, resulting in incorrect endings in the local region. This problem was handled in [42] by utilizing linear mapping between local area features and density maps. It adds saliency information throughout the learning phase, since achieving an optimum linear mapping is quite a challenging task. To learn a nonlinear mapping, the author of [43] used random forest regression rather than linear regression.

3  CNN and Deep CNN Frameworks for Highly Congested Scene

Deep Convolutional Neural Networks (DCNN) is an Artificial Neural Network (ANN) that generates hierarchical representations from spatial data in digital images. It was designed to operate with multidimensional (2D and 3D) arrays of high-resolution input data sets such as images and videos [11,44]. Alex Net [11], with seven hidden layers and millions of parameters, was the first DCNN architecture. Image classification [44], crowd counting in congested scenes [45], region-based crowd counting [46], crowd counting [47], and people localization [48] have all demonstrated incredible performance using DCNN. Because of the popularity of CNN and DCNN in a variety of computer vision applications, researchers are being urged to apply their skills to turn crowd images into density maps by learning nonlinear functions [49,50].

4  Limitations of State-of-the-Art Methods

The authors recently presented the Supervised Spatial Divide-and-Conquer Network (SS-DCNet) and the Spatial Divide-and-Conquer Network (S-DCNet), both of which employ spatial divide and conquer networks to count crowds [51,52]. Similarly, in [4] switched CNN is developed, which selects various regressors for input patches using a density level classifier. A contextual pyramid CNN was presented in [13], which employs CNN networks to predict context at multiple levels to get a lower-level error and high-quality density maps. Both techniques employed a Multi-Column Convolutional Neural Network (MCNN)-based architecture and produced good results. However, the main drawbacks of these architectures are taking a large amount of training time, compulsion to use density level classifier rather than of sending images in the MCNN, and implementation of more columns which makes the model more complicated, and it is hard to train properly as described in [15]. Considering all these drawbacks, we propose a unique method of focusing on the deeper features of a crowded scene and producing high-quality density maps. We will focus on crowd counts in congested areas. This model aims to design a deeper convolutional neural network using VGG-16 with its first ten layers as a front end. Further, we increased the dilation rates of the hole convolutional network to get a large level of information from the images which can capture the high-level feature i.e., heads with larger respective fields, as well as the generation of high-quality density maps. Finally, counting the number of individuals in highly congested scenes is done.

5  Understanding the Architecture of Congested Scene Crowd Counting Network (CSCC-NET)

In this model, we first used VGG-16 with its first 10 layers as a front-end with different dilation rates as shown in Fig. 3. VGG-16 has been used in this model because of its robust transfer learning and adaptable architecture. The main idea of dilated convolution is to extend the kernel without increasing the parameters. So, if the dilation rate is “3”, we take the kernel and convolve it on the entire image. Similarly, if we further increase the dilation rate to “4”, once again we can convolve the kernel on the entire image. VGG-16 has been used in different models like in [16] they shaped the first 13-layers of VGG-16 directly and added a 1 × 1 convolutional layer. But the performance was not quite good. Similarly, before sending input images to MCNN, VGG-16 was used as a density level classifier in [4]. CP-CNN has combined the classification results with density map-generating features in [13]. In such cases, VGG-16 works as a supplement to the final accuracy rather than enhancing it. We eliminated the classification component of VGG-16, i.e., fully connected layers, from the model and developed the intended CSCC-Net with the first 10 layers of VGG-16 as a front-end network. Inspired by [5355], a dilated convolutional neural network with different dilation rates has been used as a back end for the extraction of deeper features. If we keep stacking more convolutional and pooling layers (basic components in VGG-16), the output size will shrink even further, making it difficult to create high-quality density maps. It's a good substitute for a pooling layer. Although pooling layers (e.g., max and average pooling) are often used to maintain invariance and minimize overfitting, they substantially reduce spatial resolution, meaning that spatial information from feature maps is lost. As seen in Fig. 3, dilated convolution with sparse kernels is a better choice for now. The sparse kernel is used to increase the size of the corresponding fields while keeping the parameters and computation time the same. The output picture of the front-end network is one-ninth the size of the input image. Because the output result should match the input image, the width and height are reduced by one-ninth. The inter cubic transformation method is applied, and the result is multiplied by “81” to keep the total number of image pixels the same.


Figure 3: The architecture of CSCC-net

5.1 Dilated Convolution

The convolution kernel is a small matrix that is used in the convolution process. The pixel matrix of each layer is continuously tested in steps. The number of tests is multiplied by the number of convolution kernel's corresponding location, and then the sum is calculated. Each test gets a value and generates a new matrix after testing all pixels. The main idea of dilated convolution is to enlarge the kernel while keeping the parameters the same. So, if the dilation rate is 1, we convolve the kernel over the entire image. When the dilation rate is increased to 2, the kernel expands. It is an alternative to pooling layers. Dilated convolution offers the benefit of extending the corresponding field while keeping the parameters and computation time the same. It also maintains the resolution of the feature map more accurately and effectively than other networks.

5.2 Ground Truth Generation

The Gaussian convolution is used to produce the GT map. For each target point x, the nearest k neighbors are found. The average distance is obtained by the correlation calculation di, then a density map through Gaussian convolution is generated. The Gaussian kernel function is normally distributed by a bell-shaped curve. The closer the coordinate is to the center point, the higher the value. The higher the weight, the closer it is to the center point, the farther away from the center, the smaller the weight. Here is the formula:


where xi represents each head marking point, and label with N heads can be expressed as H(x). Here x represents a two-dimensional coordinate, so H (x) is the matrix with only 0 and 1.



It has been seen that the value of variance σ is geometrically adaptive. We utilized di to indicate the average distance of k nearest neighbors for each of the targeted output “xi” in the ground truth δ. To generate the density maps, we convolve δ (x − xi) with the use of a Gaussian kernel and a parameter σi (standard deviation), where x represents the pixel position in the image. For the input with the sparse crowd, we use the configuration of [15] where β = 0.3, k = 3. Further, we applied Gaussian Kernel to blur all head annotations. All the images are cropped into nine patches as indicated by CSRNet, with each patch measuring 1/4 of the original image size. The first four patches are divided into quarters, the remaining five patches are cropped at random, and the training set is eventually doubled [2].

5.3 Generation of Density Maps

In crowd analysis, counting occlusion is the main challenge among crowds. To deal with this issue density maps were proposed in [56]. Density maps are regressing targets per pixel. To regress the density maps is the prediction of dense crowds [9,57,58]. The annotations are most likely provided as the coordinates of human heads in the image in all data sets. These coordinates can then be used to generate a density function [59]. The technique of producing density maps from an annotated file is briefly described here. Create a matrix the same size as the original image and set it to zero, then modify each marked head's position to one. As a result, one may obtain a matrix of 0 and 1, which is then convolved by the Gaussian kernel function to provide a continuous density map. To summarize, the density map is generated via Gaussian convolution on a matrix with only 0 and 1 values [15]. The density map generation can be formulated as:


where Pigt is the image's ground truth head position I. The ground truth density map Dgt of Pigt is obtained by using a Gaussian kernel to convolve annotated points N(p,μ,σ2). Thus, considering the impacts of the Gaussian functions centered at all annotation points, it is possible to compute the density at a certain pixel p of I.


To further assess the quality of density maps, we use the Peak Signal to Noise Ratio (PSNR) and the Structural Similarity Index (SSIM) [60] the results are shown in Tab. 3.



where MAXi signifies the density map's maximum value μx,μy,σxandσy represent the Mean, variance, and covariance of x, y, and c1,c2 are constant values to prevent the division of zeros.

6  Training Methods and its Detail

In the training phase, we use VGG-16 [44] with its first 10 layers for 2D feature extraction as a front-end and hole dilated convolutional neural network has been employed as a back end to switch the pooling layers in the CSCC-Net model. The dilated kernels are used to deliver a large reception field. A well-trained VGG-16 is used to fine-tune the first 10 layers [44]. In the first ten layers of VGG-16, the ReLU has been employed as an activation function. The last three layers in VGG-16 have been removed due to the absence of features they possess. A unified convolutional kernel, size 3 × 3, in each layer is used to reduce the complication of the network. Only three of the five maximum pooling layers of VGG-16 have been used. The basic role of these pooling layers is to suppress the over-fitting. If we use all the five max-pooling layers, then it would affect the output accuracy of the entire model and the image resolution as well, therefore we have minimized the number of max-pooling layers. To maintain the resolution of the image, the hole dilated convolution has been more effective than other networks. The first convolutional layer, such as conv3-64-1, means the size of the convolution kernel is 3 × 3, the number of filters 64 and the dilation rate is 1 as shown in Tab. 1. To quantify the distance between the generated density map and the ground truth map, a fixed learning rate, such as le-7, and Euclidean distance are employed. We have used different loss functions as shown in Tab. 2 during the training phase and found the L1 Loss function quite good as compared to other loss functions. To reduce the possibility of an error, the L1 Loss function is used.



6.1 Loss Functions

The loss function is one of the most significant research topics in machine learning since it is used to build machine learning algorithms and enhance their performance. The loss functions have been concerned and explored by different researchers and it has still a big gap to summarize, analyze, and compare the classical regression loss functions [61]. Some classical loss functions are widely used for regression problems.

6.1.1 L1 Loss Function

The L1 Loss Function is defined as the sum of all absolute differences between the true and predicted values. It is mathematically defined as:


where x,y represent actual and predicted values respectively.

6.1.2 L2 Loss Function

The L2 Loss function is used to measure the mean squared error between actual and predicted values respectively. It is also called mean squared error which can be mathematically represented as:


where x shows the actual value and y predicted value.

6.1.3 Smooth Loss Function

The Smooth L1 Loss function is used in the regression problems when the features have high values. It is also called Huber loss and mathematically expressed as:


6.2 Description of Data Sets

Several publicly accessible crowd data sets are used to validate the experimental outcomes. Tab. 4 lists some of the publicly accessible data sets as well as their description.



6.3 Experimental Results and Discussion

The proposed model was tested on three of the most popular highly congested scene data sets i.e., ShanghaiTech Part A, B [15], UCF_CC_50 [62], and UCF_QNRF [63]. We used these data sets to train the model in phases, the training and validation errors are shown in Fig. 5, Fig. 6, Fig. 7, and Fig. 8 then compared the results to previous state-of-the-art models. Our model has improved in the competition because of its reduced Mean Absolute Error (MAE) and Mean Squared Error (MSE) rates and the counting results are shown in Fig. 4. The learning rate for the entire model has been adjusted at le7 with the inclusion of batch sizes 1 and 2, respectively. The L1 Loss function with Stochastic Gradient Descent (SGD) was utilized as a loss function and an optimizer, respectively. The parameter of the gaussian kernel function varies depending on the data set. For ShanghaiTech part A, UCF_QNRF, and UCF_CC_50, we utilized a geometry adaptive kernel, but for ShanghaiTech part B, we used a fixed kernel size σ = 11. The parameters needed to produce the density map are k, which is the average distance between k adjacent heads, and β, which is a coefficient. When doing congested crowd counting, the kernel can be modified based on the data set. To calculate the errors between estimated values and predicted values we adhere to the existing works [41,56] to employ the MAE and MSE as evaluation metrics. The MAE metric reflects the accuracy of crowd estimation algorithms, whereas the MSE metric reflects the robustness of estimation. They are mathematically defined as:


Figure 4: Counting results


Figure 5: Training and validation losses of part A


Figure 6: Training and validation losses of part B


Figure 7: Training and validation losses of UCF_CC_50


Figure 8: Training and validation losses of UCF_QNRF



where N represents the number of images and Ci,CiGT are the estimated and ground truth values respectively. Estimated count can be mathematically represented as:


where L and W denote the length and width of the density map, respectively. While Zl,w shows the pixel at the length and width of the density map.

6.4 ShanghaiTech Part A, B Data Sets

The ShanghaiTech data set, a typical big data set, contains the most labeled population and is divided into two sections, A and B. In [15] the ShanghaiTech data set for large-scale crowd counting was published. Comprising 1198 images with a total number of 330,165 annotated heads. In terms of head annotation, it is the largest data set. In part A, Images are chosen from the Internet, with a total of 482 annotated images of various resolutions. Each image has a total of 501 people, with 33 being the minimum and 3139 the maximum number of persons. The population density is very high and congested with a total number of people equaling 241,677. This data set has been further split up into two parts i.e., test data and train data. The train data set includes 300 images whereas test data includes 182 images in total. Part A images are highly congested scenes data set in which a lot number of people are standing very close to each other. Part B, on the other hand, has 716 images taken from Shanghai metropolitan street. Part B has relatively sparse images as compared to part A. The resolution of each image is 768 × 1024. Each image has an average of 123 people, with 9 being the minimum and 578 being the maximum. The overall number of people in part B is 88,488 having a low crowd density. It is also split into training and testing, whereas in training the total number of images is 400 while in testing it has 316 images. We compared the model's performance to that of other models. In [5] the authors have designed a single-column structure that can count and generate density maps simultaneously from an image. In [45], a single-column CNN architecture was presented for scaling the image and determining the total number of individuals and then changing the image into different proportions. MCNN, a three-column CNN structure approach, figured out the mapping relationship between image and density maps [15]. Similarly, in [69] a novel CNN end-to-end cascaded network was used to simultaneously learn crowd count classification and density map estimation. The switching convolutional network has been suggested in [4] that influences variation of crowd density within an image enhances the accuracy. These models have used ShanghaiTech part A and part B data set for their experimental evaluation. The Tab. 5 shows the mathematical evaluation of Shanghai Tech part A, B.


6.5 UCF_CC_50 Data Set

The UCF_CC_50 data set [62] is quite challenging to work with since it has a broad range of densities as well as a variety of scenarios. This data was gathered in a variety of places, including concerts, political rallies, stadiums, and the hajj and umrah. There are a total of 50 annotated pictures, with 1279 people per image. This data set's resolution fluctuates, and the number of people ranges from 94 to 4543, suggesting a considerable variation in the image. 5-fold cross-validation was carried out, with the normal setting of [62]. Tab. 6 compares the results of MAE with MSE.


6.6 UCF_QNRF Data Set

The UCF_QNRF data set includes 1,535 high-resolution crowded images obtained from the internet with counts ranging from 49 to 12865. Because this data set is extremely crowded, it is difficult to deal with it. It has the greatest number of crowd images and annotations, as well as a broader range of scenarios with the most diverse collection of perspectives, densities, and illumination changes. In comparison to ShanghaiTech, the resolution is high. The average density, or the number of people per pixel, is also the lowest among all images, suggesting high-quality large images. The existence of background areas, which include both high-density and zero-density regions, leads to reduced per-pixel density. Part A of the Shanghai collection also includes high-count crowd images, although these are heavily cropped to include just crowds. In contrast, the UCF_QNRF data set includes buildings, plants, roads, and the sky as they appear in actual situations. This increases the data set's realism while also making it more difficult [56,63]. The experimental results have been shown in Tab. 7.


7  Conclusions

In this paper, a new framework has been presented, called the Congested Scene Crowd Counting Network (CSCC-Net). This new framework is capable of not only counting the number of people in a highly congested scene but can also generate high-quality density maps. Ground truth values were first created in the context of CSCC-Net to build a high-quality density map for crowd counting in congested scenes. We investigated the concept of congested scene recognition by including a robust VGG-16 network with its first 10 layers as front-end and a hole dilated convolution neural network as the back end. The main benefit of a hole dilated convolution neural network is that it expands the respective fields without losing the resolution of images. We conducted comprehensive experiments on four highly congested data sets to validate our model's performance and value. When the proposed framework's findings were compared to state-of-the-art methodologies, the results of the proposed framework were found to be superior. The CSCC-Net produced impressive results in terms of two significant statistical measurements, namely MAE and MSE. In the future, we intend to explore and modify the network and use it on other computer vision tasks such as crowd localization.

Acknowledgement: The authors are grateful to the Saudi Arabia Ministry of Education's Deputyship for Research and Innovation for financing this study under Project Number QURDO001. The title of the project is Intelligent Real-Time Crowd Monitoring System Using Unmanned Aerial Vehicle (UAV) Video and GPS Data.

Funding Statement: This research is supported by the Ministry of Education Saudi Arabia under Project Number QURDO001.

Conflicts of Interest: The authors declare that they have no conflicts of interest to report regarding the present study.


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