HRNetO: Human Action Recognition Using Unified Deep Features Optimization Framework

1  Introduction

Human action recognition (HAR) is an extensive research topic in the field of computer vision (CV) [1,2]. HAR applications include e-health, human-computer interaction (HCI), visual information understanding, and video surveillance [3]. Out of these applications, the most critical application is video surveillance [4]. Human action recognition is mainly used to reduce crime rates and security purposes [5]. The primary reason for expanding HAR research is intelligent city cameras for surveillance [6]. In addition, human action recognition is critical in visual administration for detecting human activities in public places [7]. There are different types of human action; these actions are categorized into two classes: involuntary and voluntary [8].

The CV techniques are introduced in the literature for automated HAR in video sequences [9]. Manually processing video frames is difficult and takes more time [10,11]. Primarily the classical techniques are based on the point, texture, shape, and geometric features [12]. A couple of procedures depend on the temporal data of the human; before the extraction of features, few extract the silhouette features of humans [13], so the problems for traditional methods are not suitable for complex activities for better accuracy [14]. Traditional CV methods for feature extraction are less efficient and slower due to the training data and extracted features compared to new deep learning-based techniques [15]. These techniques include Shi-Tomasi Corner Detection (STCD), Harris Corner Detection (HCD), Scale Invariant Feature Transform (SIFT), Speeded-Up Robust Features (SURF), Rotated BRIEF (ORB), and Oriented fast [8,16,17].

The convolutional neural network (CNN) learns about the features directly. The first layer is used to extract deep local features, and the last layer, known as fully connected, is used to extract global features. Most studies show that using different pre-trained CNN models consumes less computational time during the training phase while achieving high accuracy in final image classification and video summarization. In [10], the author utilizes the transfer learning concept and trains seven pre-trained CNN models. The features are extracted using these pre-trained models by implementing the idea of transfer learning.

The researchers designed multiple deep learning techniques for HAR in video sequences as deep learning models became more prevalent [18,19]. Deep learning demonstrated capable results in CV for a variety of tasks, such as action recognition [20,21], gait recognition [22,23], medical [24,25], and a few more [10]. Data representation and learning are made by deep learning at different levels by creating new models [26] by simulating the human brain processing. These models use various preprocessing layers such as the convolutional layer, ReLu layer, pooling layer, F.C. layer, and Softmax layer [27]. Supervised learning, unsupervised learning, hierarchical probabilistic models, and neural network are different models in deep learning [28]. Many HAR deep learning techniques are introduced in the literature. However, there are still several challenges that reduce recognition accuracy. In this work, we effectively proposed an end-to-end deep learning framework for HAR. Our significant contributions are as follows:

•   Performed a frames preprocessing step and trained two fine-tuned deep learning networks through deep transfer learning.

•   Proposed a new Parallel Standard Deviation Padding Max Value (PSPMV) approach for deep features fusion.

•   Proposed an improved tree seed optimization algorithm for best feature selection.

2  Related Work

Human action recognition (HAR) is categorized into two types: (i) machine learning algorithms have been used for the recognition; (ii) hand-crafted features are carried out for the recognition [29]. In [30] to represent relevant activities such as walking, running, and jogging, the Global and Local Zernike Moment (GLZM) is used as a Bag-of-Features (BoF). Furthermore, using the General Linear Model, global features are extracted and fused (GLM). The global features represent the region of the human body where the activities are performed, whereas the local features represent the activity information. The Whitening transformation was used to preprocess fused features. Finally, the final classification is performed using a multiclass support vector machine (SVM). In [31] skeleton-based action recognition scheme is presented using the hierarchical recurrent CNN method. The skeletons are divided into five parts according to physical layouts. The Berkeley mhad, MSRAction3D, and HDM05 are the datasets utilized for experimental results and achieved an accuracy of 100%, 94.49%, and 96.92%, respectively. In [32], convolutional neural network (CNN) based sequential connections are presented with the most extended shortest memory (LSTM) network. The deep fusion framework efficiently exploits the spatial features of CNN models and the temporal features from the longest shortest memory (LSTM). The UCF-Sports, UCF11, and HMDB datasets are utilized for the evaluation and achieved accuracies of 99.1%, 94.6%, and 69.0%, respectively.

In [33], the authors presented a hybrid technique for HAR called HAREDNet. The proposed approach is based on a few essential steps such as (i) The Encoder-Decoder Network (EDNet) is utilized for deep features extraction; (ii) The iSIFT is utilized for the local feature extraction, Local Maximal Occurrence (LOMO), and improved Gabor (iGabor); (iii) the feature redundancy is reduced by using the Cross view Quadratic Discriminant Analysis (CvQDA) and (iv) features are fused using weighted fusion strategy. The presented technique is validated on several datasets such as UCF-101, NTU RGB+D, and HMDB51 with accuracies of 97.48%, 97.45%, and 80.58%. In [10], the authors presented a feature mapping, fusion, and selection method for HAR. Pre_trained CNN models: InceptionV3 and DenseNet201 are fine-tuned and extract features in the first step. The serial-based extended (SbE) fusion method is applied to combine the features. The Kurtosis-controlled Weighted k-nearest neighbor (KNN) is used to select the best features in the third step. Finally, supervised learning methods are used to classify the selected features. Three different datasets were used in the experiments: Hollywood, WVU, KTH, and IXMAS, and they achieved accuracies of 99.8%, 99.3%, 97.4%, and 99.9%, respectively. In [34], the authors presented deep learning and the Spatio-temporal technique for HAR. The training model uses the transfer learning technique to extract deep features. A decoder-encoder (DAE) process is used to learn spatial relationships, and a recurrent neural network (RNN) with an LSTM framework is used. In [8], the authors presented a SNSP method for HAR. The features are extracted by standard normal, slope, and parameter space. Several experiments are performed on KARD-Kinect Activity Recognition Dataset, UTD Multimodal Human Action Dataset, and SBU Kinect Interaction Dataset, achieving improved accuracy. The recent studies above focused on deep learning features and classification using different machine learning classifiers. The gaps in the past studies are the number of features passed to the classifiers for improved accuracy and the presence of redundant features. This article’s proposed framework for HAR is a deep unified learning and an improved optimization algorithm.

3  Proposed Method

The proposed HAR framework is presented in Fig. 1 which represents four core steps: video frames preprocessing, deep learning models training and features extraction, features fusion using a serially extended approach, and finally, best features selection and classification. The main steps of the proposed framework are the fusion of the extracted features and the selection of the best features using an improved tree seed algorithm. The detail of this entire framework is given below subsection.

Figure 1: Proposed deep learning and features optimization based architecture for HAR

3.1 Video Frames Preprocessing

Preprocessing is an essential step in image processing because it allows you to normalize pixel values, resize images, and improve local contrast. In this work, we performed preprocessing step to resize the video frames in dimensions 256×256×3. Initially, the size of each video frame was 512×512×3. These normalized frames are used in the next step, named transfer learning-based training of CNN models.

3.2 Deep Transfer Learning

Transfer learning is reusing previously trained deep learning models for a new task. As shown in Fig. 2, a pre-trained CNN model was initially trained on the ImageNet dataset. This dataset contains 1000 object classes, but in the HAR task, the number of categories is different for each selected dataset; therefore, we utilized the fine-tuned deep models (description is given below) and trained through transfer learning without freezing the weight of any hidden layer. Through this process, a newly introduced deep model is obtained that we finally utilize for the deep features extraction.

Figure 2: Workflow of deep transfer learning for training new deep model for HAR

3.3 Convolutional Neural Network

In the image classification task, CNN is popular due to its improved performance than traditional pattern recognition techniques. The convolutional, pooling and fully connected are the hidden layers used to extract the temporal and spatial features with the help of a filter applied to these layers [30,31]. A simple CNN architecture is the feed-forward artificial neural network (ANN) that is based on the three building blocks such as features are learned by the convolutional layer, the dimensions and the computational time are reduced through max-pooling (subsampling) layer, and classifications are done by the fully connected layer [35]. An architecture of a simple CNN model for the classification of images is shown in Fig. 3.

Figure 3: A simple architecture of CNN for HAR

Convolutional Layer: Local features are extracted through the convolutional layer by finding the local connection among the sample of data coming from the input layer.

FVL=(inputx∗x+weightx∗x)+A(1)

Here, inputx∗x represent the input data, the weightx∗x, represent the weight vector, x represents the filters and kernel size, and A represents the bias. Feature vectors are obtained by adding the pixels together and convolving the filter on the pixel of the images. The features of this layer are then passed to the activation layer called ReLu to resolve the non-linearity among features.

Activation Layer: The ReLu activation layer assigns zero to the nonnegative values obtained from the convolutional layer. The main advantage of this activation layer is faster computational time than the other activation functions, such as leaky ReLu. Mathematically, this layer is described as follows:

δ(R)=max(0,R)(2)

where R represents the element in the input vector.

Max-Pooling Layer: The max-pooling layer divided the feature map into small non-overlapping pooling kernels. It considers the maximum values of every kernel and passes them to the next layer. The max-pooling layer performed two main steps: (1) the data obtained from the previous layer is down-sampled and reduces the dimensions of the data, and (2) improvement in the model parameters for generalizability and less computational time.

Fully Connected Layer: The FC layer performed logical inference, and it converted the 3-D matrix into the 1-D vector by using fully convolutional operations. Mathematically, this layer is defined as:

YzoX1=weightzoXzj.XzjX1.AzoX1(3)

where the input and output vector sizes are represented by Zo and Zj and Y is the output of FC layer.

Softmax Layer: The layer is utilized as a classification layer in the architecture of a CNN and used to find the probability of normalized classes b(x(j)=mj|y(j);X).

(x(j)=mj|y(j);X)=[(x(j)=1|y(j);X(x(j)=2|y(j);X...(x(j)=m|y(j);X]=1∑i=1mvXizy(j)[vX1zy(j)vX2zy(j)...vXmzy(j)](4)

Here, m is the number of samples, j=1…m represents the weights which are replaced by X, and the input for the classifier is vXmzy(j).

3.4 Deep Learning Features Extraction

Fine-Tuned DensNet201 CNN Model: The Dense convolutional network, also called DenseNet, has fewer parameters than several other pre-trained CNN models such as VGG19, VGG16, and many more. This network did not learn the redundant features map [36]. All layers in this network are in narrow styles, such as 12 filters, which add fewer sets of the new feature map. Every layer in this network can directly access the hidden layers. As a result, the computational cost is reduced and made better for image classification. In this work, we fine-tuned this network according to the output classes of selected action recognition datasets. The last fully connected layer is removed, and added a new layer. As illustrated in Fig. 4, the fine-tuned model is trained on the action recognition dataset through T.L. concept. During the training process, several hyperparameters have been utilized, like a learning rate of 0.005, the mini-batch size of 16, epochs of 100, and the optimizer are Adam. After training the fine-tuned DenseNet201 model, the average pooling layer is utilized to extract deep features.

Figure 4: Architecture of DensNet-201pre-trained deep model

Fine-Tuned ResNet50: The residual network is also called (ResNet). This model introduced the residual connections between the layers, which helps preserve the knowledge gain, reduce the loss, and boost the performance during the training process. The ResNet50 model is trained on the 1000-class ImageNet dataset. We fine-tuned this model for HAR in this work. First, the fine-tuning new fully connected layer is added by removing the original fully connected layer. After that built the connections and trained the fine-tuned model through TL. The fine-tuned model is illustrated in Fig. 5. During the training process, several hyper parameters have been utilized mini-batch size is 16, the learning rate of 0.005, epochs are 100, the loss function is cross-entropy, and optimizer is stochastic gradient descent (SGD). Later, the average pooling layer is selected, and the activation function is applied for feature extraction. In the next step, extracted features of both networks are fused in a single vector for better information on subject actions.

Figure 5: The main Architecture of the pre-trained deep model of ResNet-50

3.5 Features Fusion and Optimization

Features Fusion: In this work, we proposed a new features fusion approach named Parallel Standard Deviation Padding Max Value (PSPMV). Consider we have two extracted deep feature vectors of dimension N×K1 and N×K2 denoted by ϕ1 and ϕ2, respectively. Suppose the fused feature vector is denoted by ϕ3 having dimension N×K3. The proposed fusion approach work in three core steps: (1) length finding, (2) equivalent dimension, and (3) maximum selection for fusion. In the first step, maximum and minimum dimensional feature vectors are defined as follows:

ϕmax=max(ϕ1,ϕ2)(5)

ϕmin=min(ϕ1,ϕ2)(6)

Based on the above equations, we consider ϕmax feature vector and find the standard deviation value for the equivalent length of ϕmin. The standard deviation value is utilized for padding instead of zero padding. Mathematically, the standard deviation of ϕmax is computed as follows:

σ=∑(ϕmax(i)−ϕmax¯)n−1(7)

After padding, the features of both vectors ϕ1 and ϕ2~ are fused based on the following criteria, where ϕ2~ is updated vector after padding operation:

Fused={ϕ3{k}forMax(ϕ1(i) and ϕ2~(j))}(8)

The resultant fused vector ϕ3{k} is further optimized through an improved tree seed optimization algorithm. This step’s main objective is to select important features and reduce the classification time.

Improved Tree Seed Algorithm-based Selection: The tree search algorithm is a natural phenomenon that deals with the relationship between the tree and its seeds [37]. It is the natural process by which the trees are spread on the surface by their seeds; over time, these seeds turn to the trees. If we assume the optimization problem’s search space, the surface of a tree, then we can take the location of the seeds and trees to be the ‘problem’s solution. The location of the source is the most important optimization problem because this process strongly impacts the search. This search process is based on two mathematical equations. In the first one, the best location of tree population and the location of a tree which produces seed for the tree are considered to improve the local search. Hence, the new seed can be produced for a tree as follows:

Mab=Nab+βab×(Ob−Nab)(9)

Mab=Nab+βab×(Nab−Ncb)(10)

Here, the Mab is the bth dimension of the ath seed that produces an ath tree, Nab is the bth dimension of the ath tree, Ob is a bth dimension obtain for the best tree position so far, Ncb is a bth dimensions of the cth randomly tree is selected in the range of [1,1], a and c present different indices. In both equations above, the most significant thing is selection of new seed location (produced). In order to control the selection in the range of [0,1], search tendency (ST) has opted. If the ST has a higher mean, it indicates a strong local search and speedy convergence; however, if the ST value is lower than the mean, it indicates slow convergence but a strong global search. The ST parameters control the capability of exploitation and exploration of TSA. Therefore, the optimization problem using the initial position of the seed is defined as follows:

Nab=Pb,min+ca,b×(Db,max−Pb,min)(11)

Here, Pb,min is a search space of lower bound, Db,max is a search space of higher bound, ca,b are the random number produced for each dimension and location in the range of [0,1].

O=min{f(Na)}a=1,2,3,………N(12)

where O shows the best solution from the selected population and N represents the total number of trees in the population. The resultant best-selected features of the tree search algorithm (TSA) are utilized in a maximum function to select the most optimum points as follows:

MO=max(O(k)),k=1,2,3,…N(13)

The selected max optimum features are passed in the fitness function and repeat the above step until the error is reduced to the minimum. Then, the selected features are passed to the supervised learning classifier for the final classification.

4  Results and Discussion

This section discusses the proposed HAR framework’s experimental process with detailed numerical values and visual plots. The framework is tested on five publicly available datasets such as KTH, UT-Interaction, Weizmann, Hollywood, and IXMAS. Each dataset is divided into 50:50, which represent 50% of frames utilized for training and the rest 50% for testing. The K-Fold cross-validation is utilized for the whole testing process. Several classifiers are selected for the classification results, and the performance of every classifier is analyzed based on the following measures: accuracy, F1 Score, precision rate, recall rate, time, and area under the curve (AUC). The detailed numerical results of each dataset are given below subsection.

4.1 UT-Interaction Dataset Results

The numerical results of the proposed framework on the UT-Interaction dataset are given in Table 1. Ten classifiers, such as Narrow Neural Network, Medium Neural Network, and named a few more, are utilized for the classification purpose. The best accuracy achieved is 100% on Wide Neural Network (WNN), where the other performance measures like precision rate, recall rate, F1 Score, and time are 100%, 100%, 100%, and 18.7 (s). The second best-achieved accuracy on Medium Neural Network of 99.9%, with a recall rate of 99.8, a precision rate of 99.8, an F1 Score of 99.8%, and a testing classification time of 12.8 (s). The rest of the classifiers’ accuracies are 99.8%, 99.7%, 99.2%, 99.9%, 98%, 98.8%, and 99.1%. The confusion matrix of WNN is illustrated in Fig. 6 and can be utilized to verify the best-achieved accuracy.

Figure 6: Confusion matrix of wide neural network on U.T. interaction dataset

4.2 Weizmann Dataset Results

The proposed framework results on Weizmann dataset are shown in Table 2. For the experimental process, several classifiers were used, similar to Table 1. The best accuracy achieved of 98.1% by Quadratic SVM, while the precision rate, recall rate, F1 Score, and time are 97.7%, 97.7%, 97.7%, and 85.8% (s). The second-best accuracy achieved on Cubic SVM of 98%, with a precision rate of 97.8, a recall rate of 97.6, an F1 score of 97.7%, and a time of 95 (s). Similarly, the rest of the classifiers attained 96.2%, 97.4%, 96.2%, 94.9%, 89.1%, 92.1%, and 91% accuracies, respectively. Fig. 7 illustrates the confusion matrix of QSVM, which confirms the accuracy of the proposed framework.

Figure 7: Confusion matrix of Quadratic SVM on Weizmann dataset

4.3 KTH Dataset Results

The proposed numerical results on the KTH dataset are shown in Table 3. Several classifiers, such as Cubic SVM, Narrow Neural Network, Medium Neural Network, and named a few more, are utilized for classification purposes. The maximum noted the accuracy of 99.6% on Quadratic SVM, while the other parameter like: 99.6 is the Recall rate, 99.6 is the Precision rate, F1 score is 99.6%, and the testing time is 152.7 (s). The rest of the classifier’s accuracies are 99.4, 99.5, 99.4, 99.5, 98.4, 99.1, and 99.2, as given in the table below. Fig. 8 illustrates the confusion matrix of QSVM, which confirms the performance of the proposed framework.

Figure 8: The confusion matrix of Quadratic SVM on the KTH dataset

4.4 Hollywood Dataset Results

Table 4 shows the result of the proposed framework on the Hollywood dataset. Ten classifiers, such as Quadratic SVM, Narrow Neural Networks, Medium Neural Networks, and named a few more, are utilized for classification purpose. The Medium Neural Network achieves the best accuracy of 99.9%, while the other parameters are: 99.9 is the precision rate, 99.8 is the recall rate, the F1 score is 99.8, and time is 67.2 (s). For the rest of the classifiers, the obtained accuracies are 99.8%, 99.8%, 99.9%, 99.9%, 99.4%, 99.2%, and 99.3%, respectively, as presented in the table below. The confusion matrix of Medium NN is also illustrated in Fig. 9, which confirms the accuracy of the proposed framework on the Hollywood dataset.

Figure 9: Confusion matrix of Medium N.N. for Hollywood dataset

4.5 IXAMS Dataset Results

Table 5 shows the result of the proposed framework on the IXAMS dataset. Ten classifiers, such as Quadratic SVM, Narrow Neural Networks, Medium Neural Networks, and named a few more, are utilized for classification purpose. The highest 97.5% accuracy is achieved on Cubic SVM, while the other parameter like precision rate, recall rate, time, and F1 score are 97.4, 97.4, 1726.5 (s), and 97.4%. The Quadratic SVM achieves the second-best accuracy of 96.9%. The rest of the classifiers achieved accuracies of 90.3%, 93.9%, 88.2%, 87.3%, 87.2%, 86.5%, and 94.3%, respectively. The confusion matrix is also presented in Fig. 10, confirming the proposed framework’s accuracy on Cubic SVM. Finally, the comparison of the proposed method to the state-of-the-art method is made based on accuracy, as represented in Table 6. This table represents that the proposed framework accuracy is improved than the existing techniques on selected datasets.

Figure 10: The confusion matrix of Cubic SVM for IXMAS dataset

5  Conclusion

This article for HAR presents a unified framework based on deep learning and an improved tree seed algorithm. The proposed method’s primary focus is the fusion of deep learning features and selecting the best of them using an improved optimization algorithm. The experiment was carried out on five publicly available datasets, yielding improved accuracies of 100.0%, 98.1%, 99.6%, 99.8%, and 97.5%, respectively. The efficiency of the proposed fusion method is demonstrated based on the obtained results. However, this method requires a significant amount of computational time; as a result, classification accuracy improves, and the improved optimization algorithm alleviates this issue. On the other hand, redundant information in this work increases the computational time and decreases classification accuracy. Future work may investigate the features fusion technique by involving some optimization algorithms. Moreover, the latest dataset shall be considered for the validation of the proposed framework [33]. In the future, multiple angles shall be considered for action recognition. There will be a privacy issue from several angles but some IoT controlled deep learning frameworks may be useful [43].

Funding Statement: This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20204010600090).

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

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