Efficient video face recognition based on frame selection and quality assessment

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PeerJ Computer Science

Introduction

Today advanced technologies in the field of biometric identification are becoming increasingly popular in various areas of public life. Existing identification systems use such features as voice, human pose, gait (Ben et al., 2019), etc. However, the best recognition quality is known to be obtained through face recognition techniques. Indeed, a face is one of the most reliable identifier of a person that is impossible to lose or forget. Hence, the face recognition has become widely used in secure companies, for example, banks, in order to prevent violations or to provide targeted advertising to customers. Major airlines are beginning to introduce similar technologies to identify passengers, taking the face image as an entrance ticket to the flight. Several telecommunication companies are equipping the latest gadget models with the face geometry authentication technology to protect personal data (Truong, Graf & Yanushkevich, 2019).

The general goal of a face identification task is to associate an input sequence of video frames {X(n)},n=1,N¯ with one of L subjects (classes) from the reference gallery (Zhao et al., 2003). The frame number here is denoted by n, and the total number of video frames is N. Suppose, classes are defined by using reference facial still images (photos) {Xi}, i = 1,2, …, L with a known label. To simplify the task, we assume that the considered video sequence contains frames of only one person with a previously detected face area (Kharchevnikova & Savchenko, 2018). Thus, the problem of face identification by video is an example of the multi-class classification task. However, an observed object here is not a single image, but a set of images (frames), so that aggregation methods should be used to compute a single descriptor of the whole video or combine the decisions for each frame (Kharchevnikova & Savchenko, 2016, 2018).

As a matter of fact, real-time face recognition systems are based on the analysis of frames received at a given frame rate. Therefore, the reliability and accuracy of these technologies directly depend on the quality of the images coming to the input of the algorithm. However, the environment is not always conducive to obtaining frames of good quality due to lighting conditions, low resolution of the video camera, face positioning, the presence of blur, etc., which leads to unstable work of the system and errors occurring (Chen & Zhao, 2019). Often, just a few key images from the incoming video sequence are enough for the algorithm to guarantee that a person on a video belongs to one of the subjects in the reference database (Savchenko, 2016).

Moreover, face recognition based on traditional deep convolutional neural networks (CNN) is rather slow due to expensive inference operation (Liu et al., 2019). Hence, it usually requires high-performance servers with graphics processors (GPUs) for real-time video recognition. Traditional approach with extraction of facial features (embeddings) from each input frame using very deep CNN significantly slows down the speed of decision-making, especially if the system performs real-time calculations on platforms limited by power and memory resources. Moreover, it is impossible to apply very complex CNN (Hernandez-Ortega et al., 2019) to estimate quality of each frame due to the same restrictions of real-time processing. Therefore, the problem of increasing the robustness and performance of face identification algorithms by video sequence remains an urgent topic in the field of computer vision and machine learning.

The main contribution of this paper is applying knowledge distillation from a cumbersome FaceQnet (Hernandez-Ortega et al., 2019) to a fast CNN in the face identification problem based on selecting K-best frames from an input video sequence. It is experimentally demonstrated that the proposed approach either improves the total running time or demonstrates the high accuracy (up to 10%) compared to the baseline aggregation of all frames with the ResNet-50 (VGGFace2) and InsightFace models. Several lightweight CNN models were trained in order to more effectively implement the face quality assessment. The knowledge distillation is used to reach the performance of slow quality assessment FaceQnet in fully unsupervised manner by using any large facial dataset without need for the training dataset with a given facial quality that is not distributed by Hernandez-Ortega et al. (2019).

Proposed Approach

Frame quality assessment

In this article we use a lightweight CNN for face quality assessment. At first, let us follow the FaceQNet pipeline (Hernandez-Ortega et al., 2019), taking as a basis a similar dataset. Unfortunately, its authors did not provide the dataset on which the FaceQNet model was trained. Therefore, we propose here to consider the learning of an efficient network based on the knowledge distillation from FaceQNet. As student models, it is proposed to consider MobileNet (Howard et al., 2017), as well as a simple network of the LeNet type, consisting of only 4–5 convolutional layers (hereinafter FaceQNet mobile and FaceQNet light, respectively). Since the output of original FaceQNet (Hernandez-Ortega et al., 2019) is regression of the image quality value, the last layer of light networks is also the layer responsible for regression.

Our learning process with the knowledge distillation approach is shown schematically in Fig. 1. A subset of VGGFace2 (Cao et al., 2018), consisting of three hundred first classes, is considered to be a training set. This set is unlabeled in terms of facial quality, so that we train our network in completely unsupervised manner. Hence, at the first stage, it is necessary to obtain predictions of the quality from the teacher model (pre-trained FaceQNet) (Hernandez-Ortega et al., 2019) for each facial image from the training set. These estimates together with corresponding image make up a training set for the student model. As a student model, two architectures are under consideration: the modification of MobileNet (Savchenko, 2019a) and lightweight LeNet-based model. During the training, each image of the training sample is fed to the input of a light network, which returns predicted quality. The value obtained by a student model is used in the loss function, namely, the mean square error (MSE) is calculated based on reference estimates of facial quality provided by a teacher model. Finally, the weights of the student model are updated using the variation of stochastic gradient descent. The Adam optimizer was applied with learning rate 0.001 and decay 1e−5. The student models were trained in 10 epochs with early stopping based on the MSE on validation subset (20% of the whole dataset). Figure 2 demonstrates examples of images with the quality assessments obtained by the FaceQNet teacher model (Q0), as well as by the student models FaceQNet mobile (Q1) and FaceQNet light (Q2).

Distillation training pipeline.

Figure 1: Distillation training pipeline.

Quality scores FaceQNet, FaceQNet mobile, FaceQNet light.

Figure 2: Quality scores FaceQNet, FaceQNet mobile, FaceQNet light.

(A) High quality face image, (B) Low quality face image.

In addition, we train the lightweight CNN using the FIIQA dataset with associated lighting quality metrics (Zhang, Zhang & Li, 2017). Considering the proposed FIIQA-based approach the MobileNet architecture (Savchenko, 2019a), previously trained on VGGFace2 (Cao et al., 2018), is used as the basis for fine-tuning. A fully connected layer with Softmax activation function is added for classification, so that the categorical cross-entropy is considered as a loss function. The model was trained in 20 epochs with early stopping using the Adam optimizer. The output of the trained MobileNet FIIQA model is the likelihood that the image belongs to one of Q = 3 quality classes P*(q|X(n)), q ∈ {1,…,Q}. The final decision on the quality of the frame is interpreted using an estimate of the mathematical expectation (Liu et al., 2019): bn=1Qq=1QP(q|X(n))q

Proposed pipeline

Figure 3 demonstrates the pipeline of the face identification system with additional step of lightweight CNN-based frame quality assessment. Here the proposed block is highlighted in bold. At first, individual frames are extracted in the incoming video sequence with a fixed frame rate and pre-processed (e.g., normalized). Next, the face region is detected using the MTCNN (Zhang et al., 2016). The resulted facial regions are fed to the input of the stage of calculating the quality of the frames, where frame-by-frame evaluation occurs using one of the methods for analyzing the image structure (1)(3) or using deep learning technologies from previous subsection. Next, we select the best frames for supplying to a CNN-based facial feature extractor. Let Q(X(n)) is the frame quality estimate from the input video sequence. We sort the obtained frame quality estimates in descending or ascending order (depending on the algorithm), from “best” to “worst”, so Q(X(1)) ≥ Q(X(n)) ≥ Q(X(N)). From the indices (1)…(N) we take the top-k, which we consider to be the key frames, where the hyper-parameter K ≤ N is determined empirically. The resulting K-best video images are the input to the CNN. The result of the block is the facial feature vectors of the frames. In order to get a single vector that describes the input video, we compute the arithmetic mean of features of each frame (Kharchevnikova & Savchenko, 2016). The final solution is made using the 1-NN descriptor (Nearest Neighbor) with Euclidean distance. This pipeline has been implemented in a publicly available Python application by using Keras framework with TensorFlow backend. Sample screenshot is shown in Fig. 4.

Proposed pipeline of video face identification.

Figure 3: Proposed pipeline of video face identification.

Sample screenshot of the developed implementation of the proposed pipeline.

Figure 4: Sample screenshot of the developed implementation of the proposed pipeline.

Experimental Results

The experiments have been conducted using two datasets:

  1. IJB-C (IARPA Janus Benchmark-C) (Maze et al., 2018) is one of the most popular face recognition datasets. The database contains 3,531 unique objects, namely the faces of celebrities, athletes, political figures for whom individual images and short videos have been collected. In total, the set contains 21,956 photographs of recognized classes, as well as 19,593 videos with pre-selected frames in the amount of 457,512. The average number of frames per video is approximately equal to 33 images. IJB-C contains many images with faces that are truly difficult to recognize.

  2. YTF (YouTube Faces) dataset (Wolf, Hassner & Maoz, 2011) consists of 3,425 videos collected from the famous YouTube platform. Each video clip contains 181.3 frames in average. Recognition classes from YTF have an intersection of 596 subjects with static images from the LFW dataset (Learned-Miller et al., 2016).

Since the standard protocol for the IJB-C dataset (Savchenko, Belova & Savchenko, 2018) contains samples both from video frames and single images, it is not applicable for the current article. Thus, all the results in this article have been obtained from conducted experiments based on the protocol, where the training set contains only still images and the testing set consists of the videos only.

Running time

The efficiency of video-based face recognition largely depends on the speed of CNN together with methods of selecting high-quality frames. The analysis of algorithms performance is conducted on the AMD Ryzen Threadripper 1920X 12-Core Processor server, a 64-bit Ubuntu 16 operating system, RAM 64 GB, with Nvidia GeForce GTX 1080 Ti GPU. Table 1 presents the sizes of the pre-trained CNNs, as well as the average inference time tinference for one frame by CNN. In all tables, the best results are shown in bold and the worst results are marked by italics.

Table 1:
Performance of CNN models for facial feature extraction.
CNN Average inference time per frame tinference, ms Model size, MB
GPU CPU
ResNet-50 (VGGFace2) 9.186 48.917 93
MobileNet-VGG2 6.574 20.246 12.7
InsightFace 15.735 90.407 170
DOI: 10.7717/peerj-cs.391/table-1

Note:

The best results are shown in bold and the worst results are marked by italics.

The MobileNet model here is the fastest of the proposed options in connection with the specifics of this network. The architecture optimized for mobile platforms is capable of processing 3–5 frames per second. MobileNet also has the advantage of the size of a trained balance, the difference is almost 14 times compared to the heavy InsightFace. On average, the GPU gives an acceleration of 5 times.

However, traditional face recognition algorithms involve inference in a CNN for each frame in the incoming video sequence. Following simple arithmetic calculations, one can notice an obvious linear increase in the overall running time T = N · tinference of the face identification system, depending on the growth in the number of incoming frames N. So, a video of N = 1,000 frames in length is processed in 8 s by the fastest of the described CNNs, namely, the multi-output MobileNet (Savchenko, 2019a).

It is worth noting that the key idea of our approach (Fig. 3) is applying effective algorithms for choosing K high-quality frames from the input video sequence. Then, the calculation of the total running time is reduced to: T=Ntframe+Ktinferencewhere tframe is the time to estimate quality of a single frame. To increase the performance of the face recognition system, the running time of the algorithms for selecting high-quality frames should not exceed the time of CNN inference: tframe < tinference. The results of measurements of the frame quality assessment running time tframe per one image are presented in Table 2. The overall running time (5) for the quality assessment methods in dependance on the number K of selected frames are presented in Figs. 5 and 6. The fixed number of quality images K are fed to the input of the CNN (K = N/4 of the total number of all frames in the one video). Here, the traditional approach of facial extraction from each frame without facial quality assessment is marked as Baseline.

Table 2:
Quality assessment methods performance.
Quality assessment Average processing time per frame tframe, ms Model size, MB
CPU GPU
Luminance (1), (2) 0.027 0
Contrast (3) 0.071 0
FaceQNet 12.039 47.897 93.33
FaceQNet mobile 6.351 22.088 12.94
FaceQNet light 3.111 14.213 4.4
FIIQA mobile 6.642 21.409 12.83
DOI: 10.7717/peerj-cs.391/table-2
The inference time (CPU) depending on the number of frames, ResNet-50 (VGGFace2).

Figure 5: The inference time (CPU) depending on the number of frames, ResNet-50 (VGGFace2).

The inference time (GPU) depending on the number of frames, ResNet-50 (VGGFace2).

Figure 6: The inference time (GPU) depending on the number of frames, ResNet-50 (VGGFace2).

Here one can notice that a significant difference in the performance is observed if the number of frames is greater than 150. As soon as the number of frames increasing, simple and fast algorithms for selecting key video frames show their advantage. Thus, the most effective and less resource-intensive algorithms are simple methods for assessing Brightness (Luminance) and Contrast (Contrast). Approaches for selecting quality frames using lightweight CNN models have also shown their effectiveness (Figs. 5 and 6). The original FaceQNet model (Hernandez-Ortega et al., 2019) offers no advantage in recognition speed over pre-trained CNNs for feature extraction.

Accuracy

The previous subsection demonstrated that the proposed approach with distilled CNN (FaceQnet mobile) is able to improve the running time of video face recognition. However, our main goal is to increase the face identification accuracy by selecting only top-k best frames from the video sequence. Accuracy is defined as the ratio of correctly defined classes to the total number of predictions received by the 1-NN descriptor. All considered algorithms for selecting high-quality frames are compared with the traditional approach of identifying each frame (hereinafter referred as Baseline). The main objective of the study is to obtain recognition accuracy not lower than Baseline. Therefore, preference is given to methods with a sufficiently high proportion of correctly predicted classes. In order to compare the various approaches to search for key video images, several clustering algorithms are considered: K-means and MiniBatchKMeans. For the integrity, a predetermined number of images are randomly selected from the incoming sequence of frames and fed to the input of the face identification stage.

Tables 35 demonstrate the dependance of the accuracy on the number K of selected key frames for the IJB-C dataset using ResNet-50 (VGGFace2) (Cao et al., 2018), multi-output MobileNet (Savchenko, 2019a) and InsightFace (ArcFace) (Deng et al., 2019) facial descriptors, respectively.

Table 3:
Recognition accuracy for IJB-C dataset, ResNet-50 (VGGFace2).
Algorithm Number of selected frames K
N/2 N/4 N/8 2 1
Baseline 70.892
Random 70.341 70.178 70.196 63.168 55.767
K-means 71.068 71.193 71.418 69.821 70.601
Luminance less 72.365 77.216 69.917 63.948 56.709
Luminance more 65.775 61.724 58.098 53.766 44.313
Contrast less 71.749 70.679 69.437 62.825 55.455
Contrast more 66.483 62.975 59.895 55.698 46.433
FaceQNet 75.759 77.411 78.499 70.801 66.504
Proposed FaceQNet mobile 75.984 77.948 79.267 72.028 68.082
Proposed FaceQNet light 71.888 71.524 71.649 61.733 59.171
Proposed FIIQA mobile 70.969 69.591 68.268 61.959 54.383
DOI: 10.7717/peerj-cs.391/table-3

Note:

The top three results are shown in bold.

Table 4:
Recognition accuracy for IJB-C dataset, MobileNet-VGG2.
Algorithm Number of selected frames K
N/2 N/4 N/8 2 1
Baseline 61.293
Random 59.378 59.230 58.988 52.257 44.980
K-means 62.102 62.108 61.503 61.759 61.185
Luminance less 61.131 59.760 58.299 51.989 45.092
Luminance more 56.314 51.673 47.937 43.284 35.004
Contrast less 60.740 59.120 56.790 51.128 44.138
Contrast more 56.963 52.726 49.411 44.787 36.482
FaceQNet 65.127 65.631 66.532 59.689 54.532
Proposed FaceQNet mobile 65.411 66.323 66.925 61.011 56.185
Proposed FaceQNet light 61.373 60.537 59.313 50.751 47.525
Proposed FIIQA mobile 59.259 57.516 55.900 50.349 43.552
DOI: 10.7717/peerj-cs.391/table-4

Note:

The top three results are shown in bold.

Table 5:
Recognition accuracy for IJB-C dataset, Insightface.
Algorithm Number of selected frames K
N/2 N/4 N/8 2 1
Baseline 69.834
Random 69.871 70.182 69.835 63.899 62.048
K-means 64.448 64.872 67.846 69.947 70.353
Luminance less 67.449 63.701 62.787 57.771 55.095
Luminance more 62.422 60.785 60.837 58.966 57.254
Contrast less 67.893 63.906 62.682 57.895 55.962
Contrast more 63.525 62.654 62.199 60.349 58.415
FaceQNet 78.853 79.236 80.686 77.604 76.825
Proposed FaceQNet mobile 79.428 79.852 80.706 78.957 77.623
Proposed FaceQNet light 71.984 73.046 73.012 65.752 65.752
Proposed FIIQA mobile 71.750 70.764 70.113 64.511 61.711
DOI: 10.7717/peerj-cs.391/table-5

Note:

The top three results are shown in bold.

The accuracy of face identification using the traditional approach for each frame on the IJB-C data set is 71% (Table 3), which is a high indicator due to the presence of many complex images. It is worth noting that performing keyframe searches using clustering methods works with approximately the same accuracy as Baseline. A number of algorithms for selecting high-quality video images have shown their effectiveness; the best-accuracy methods in the table are highlighted in bold. Based on the results, assessing the quality of frames using the FaceQNet model and its modifications increased the accuracy of facial identification by 7–9% compared to the traditional approach. Here, the maximum value of 79.267% is achieved through the FaceQNet mobile network, which was independently trained by the method of transferring knowledge. The LeNet-based FaceQNet light architecture yielded results slightly worse by 6–7%, however, the accuracy is still not inferior to the traditional approach. For these models, the optimal number of quality frames was revealed in the amount of 1/8 of the overall sequence N, where the average video duration is approximately 33 frames. Moreover, a further decrease in the sample leads to a sharp decrease in the accuracy of the result.

Recognition accuracy using less resource-intensive algorithms for calculating Brightness (1) and Contrast (3) is comparable to the traditional approach. However, the determination of the quality threshold in this case usually occurs empirically, which imposes significant limitations. The CNN model ResNet-50 shows better performance compared to MobileNet-VGG2 and InsightFace. Hover, this architecture is quite “cumbersome”. The accuracy of light MobileNet-VGG2 with FaceQNet mobile as frame quality assessment only 3% inferior to the Baseline of the deeper model ResNet-50.

The same experiments have been also conducted for the YTF dataset (Tables 68). The method of quality assessment using the light “FaceQNet mobile” architecture has shown its effectiveness. The accuracy of this algorithm exceeds the traditional approach by 3.3%. The selection of key frames through FIIQA mobile reaches Baseline in accuracy. Simple algorithms for estimating Brightness (1) and Contrast (2) are less effective for the studied data set.

Table 6:
Recognition accuracy for YTF dataset, ResNet-50 (VGGFace2).
Algorithm Number of selected frames K
N/2 N/4 N/8 2 1
Baseline 83.499
Random 83.346 83.346 83.040 80.748 78.838
K-means 83.499 83.575 83.575 83.728 83.498
Luminance less 82.671 82.213 81.068 78.320 74.809
Luminance more 76.121 75.828 74.803 72.708 70. 956
Contrast less 82.977 82.061 80.611 78.473 75.725
Contrast more 77.711 76.065 75.938 74.870 73.854
FaceQNet 84.141 86.452 85.915 81.770 81.314
Proposed FaceQNet mobile 85.397 86.249 86.790 82.877 80.658
Proposed FaceQNet light 80.803 81.540 80.524 79.188 78.295
Proposed FIIQA mobile 83.435 82.975 82.055 78.834 77.147
DOI: 10.7717/peerj-cs.391/table-6

Note:

The top three results are shown in bold.

Table 7:
Recognition accuracy for YTF dataset, MobileNet-VGG2.
Algorithm Number of selected frames K
N/2 N/4 N/8 2 1
Baseline 68.330
Random 67.759 67.523 67.511 65.791 63.652
K-means 69.440 68.994 68.228 68.101 68.002
Luminance less 67.771 67.762 66.598 65.879 65.463
Luminance more 62.059 61.942 61.779 60.943 59.487
Contrast less 67.742 66.503 66.185 66.022 64.171
Contrast more 63.313 63.217 60.832 60.779 59.050
FaceQNet 70.988 71.882 71.750 69.880 68.706
Proposed FaceQNet mobile 71.912 71.984 72.108 70.178 69.013
Proposed FaceQNet light 69.984 68.649 69.127 67.071 66.921
Proposed FIIQA mobile 68.668 67.786 67.315 66.810 66.882
DOI: 10.7717/peerj-cs.391/table-7

Note:

The top three results are shown in bold.

Table 8:
Recognition accuracy for YTF dataset, Insightface.
Algorithm Number of selected frames K
N/2 N/4 N/8 2 1
Baseline 80.854
Random 79.198 79.082 79.001 77.996 75.823
K-means 79.672 80.135 80.102 80.010 78.890
Luminance less 78.467 77.265 77.245 76.624 74.058
Luminance more 71.120 70.825 71.740 68.985 68.775
Contrast less 79.532 78.984 77.416 77.261 75.379
Contrast more 70.373 70.234 69.707 67.486 66.075
FaceQNet 81.988 82.225 82.824 81.270 80.002
Proposed FaceQNet mobile 81.792 83.323 84.002 80.654 80.325
Proposed FaceQNet light 79.516 80.256 79.937 78.729 77.846
Proposed FIIQA mobile 79.750 78.596 77.065 76.806 74.980
DOI: 10.7717/peerj-cs.391/table-8

Note:

The top three results are shown in bold.

It is worth noting that the best algorithms for the accuracy of the result are deep learning technologies, namely the FaceQNet model and its “light” modifications. In general, it was possible to achieve an increase in identification accuracy by 3-5% for all models for extracting feature vectors. Simple methods for estimating Brightness (1), Contrast (2), which work directly with the matrix of image pixels, turned out to be less effective.

Conclusion

This work is devoted to the study of effective methods for face identification by video based on the selection of high-quality frames. Both traditional methods for assessing the quality of faces based on Brightness (1), Contrast (2), and deep learning technology have been studied. We propose to train the lightweight CNN (“FaceQNet mobile”) for face quality analysis by distilling the knowledge of the FaceQNet ResNet-50 model. It was demonstrated that our key frame selection approach (Fig. 3) is much (up to 8–10%) more accurate when compared to conventional methods. It is important to emphasize that the usage of our distilled model leads to even higher accuracy when compared to original FaceQNet model. It has been experimentally shown that the use of fast methods for evaluating the quality of image leads to a decrease in the time of direct passage through a CNN due to a reduction in the set of frames under consideration.

In future studies, it will be necessary to study the methods for adaptive selection of the number of key frames K. Indeed, complex videos usually needs more frames to reliably identify an observed subject, though even one key frame may be enough for the simplest input videos without noice, occlusion, etc. One possibility here is to apply sequential analysis that was used previously to improve the speed of image recognition (Savchenko, 2019b). Moreover, it is necessary to apply the same techniques for other video-based face processing tasks including age/gender/ethnicity/emotion prediction (Kharchevnikova & Savchenko, 2018; Savchenko, 2019a).

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