Face frontalization refers to predicting the frontal view image from a given profile. It is an effective preprocessing method for pose-invariant face recognition. Frontalized profile faces can be directly used by general face recognition methods without retraining the recognition models. Recent studies have shown that frontalization is a promising approach to address long-standing problems caused by pose variation in face recognition system. Additionally, generating photographic frontal faces are beneficial for a series of face-related tasks, including face reconstruction, face attribute analysis, facial animation, etc.
Due to the appealing prospect in theories and applications, research interest has been lasting for years. In the early stage, most traditional face frontalization methods Dovgard & Basri (2004); Hassner (2013); Hassner et al. (2015); Ferrari et al. (2016); Zhu et al. (2015) are 3D-based. These methods mainly leverage theories in monocular face reconstruction to recover 3D faces, and then render frontal view images. The well-known 3D Morphable Model (3DMM) Blanz & Vetter (1999) has been widely employed to express facial shape and appearance information. Recently, great breakthroughs have been made by the methods based on generative adversarial networks (GAN) Goodfellow et al. (2014)
. Those methods frontalize faces from the perspective of 2D image-to-image translation and build deep networks with novel architectures. The visual realism has been improved significantly, for instance, in Multi-PIEGross et al. (2010), some synthesized results Huang et al. (2017b); Zhao et al. (2018a) from small pose profiles are so photographic that it is difficult for human observers to distinguish them from the real ones. Furthermore, frontalized results have been proved to be effective to tackle the pose discrepancy in face recognition. Through the “recognition via generation” framework, i.e., rotating the profiles to the frontal views, which can be directly used by general face recognition methods, frontalization methods Zhao et al. (2017, 2018a) achieve state-of-the-art pose-invariant face recognition performance on multiple datasets, including Multi-PIE and IJB-A Klare et al. (2015).
Even though much progress has been made, there are still some ongoing issues for in-the-wild face frontalization. For traditional 3D-based approaches, due to the shortage of 3D data and the limited representation power of backbone 3D model, their performances are commonly less competitive compared with GAN-based methods albeit some improvements Cole et al. (2017); Tran & Liu (2018) have been made. However, GAN-based methods heavily rely on minimizing pixel-wise losses to deal with the noisy data for in the wild settings. As discussed in many other image restoration tasks Huang et al. (2017a); Johnson et al. (2016), the consequence is that the outputs lack variations and tend to keep close to the statistical meaning of the training data. The results will be over-smoothed with little high-level texture information. Hence, current frontalization results are less appealing in a high-resolution and the output size is often no larger than .
To address the above issues, this paper proposes a High Fidelity Pose Invariant Model (HF-PIM) that combines the advantages of 3D and GAN based methods. In HF-PIM, we frontalize the profiles via a novel texture warping procedure. Inspired by recent progress in 3D face analysis Güler et al. (2017, 2018), we introduce a dense correspondence field to bind the 2D and 3D surface spaces. Thus, the prerequisite of our warping procedure is decomposed into two well-constrained problems: dense correspondence field estimation and facial texture map recovering. We build a deep network to address the two problems and benefit from its greater representation power than traditional 3D-based methods. Furthermore, we propose Adversarial Residual Dictionary Learning (ARDL) to get rid of the heavy reliance on 3D data. Thanks to the 3D-based deep framework and the capacity of ARDL for fine-grained texture representation Dana (2017), high-resolution results with faithful texture details can be obtained. We make extensive comparisons with state-of-the-art methods on the IJB-A, LFW Huang et al. (2007) and Multi-PIE datasets. We also frontalize images from CelebA-HQ Karras et al. (2018) to push forward the advance in high-resolution face frontalization. Quantitative and qualitative results demonstrate our HF-PIM dramatically improves pose-invariant face recognition and produces photographic high-resolution results potentially benefitting many real-world applications.
To summarize, our main contributions are listed as follows:
A novel High Fidelity Pose Invariant Model (HF-PIM) is proposed to produce more realistic and identity-preserving frontalized face images with a higher resolution.
Through dense correspondence field estimation and facial texture map recovering, our warping procedure can frontalize profile images with large poses and preserves abundant latent 3D shape information.
Without the need of 3D data, we propose ARDL to supervise the process of facial texture map recovering, effectively compensating the texture representation capacity for 3D-based framework.
A unified end-to-end deep network is built to integrate all algorithmic components, which makes the training process elegant and flexible.
Extensive experiments on four face frontalization databases demonstrate that HF-PIM not only boosts pose-invariant face recognition in the wild, but also dramatically improves the visual quality of high-resolution images.
2 Related Works
In recent years, GAN, proposed by Goodfellow et al. Goodfellow et al. (2014)
, has been successfully introduced into the field of computer vision. GAN can be regarded as a two-player non-cooperative game model. The main components, generator and discriminator, are rivals of each other. The generator tries to map a given input distribution to a target data distribution. Whereas the discriminator tries to distinguish the data produced by the generator from the real one. Recently, deep convolutional generative adversarial network (DCGAN)Radford et al. (2016) has demonstrated the superior performance of image generation. Info-GAN Chen et al. (2016) applies information regularization to optimization. Furthermore, Wasserstein GAN Arjovsky et al. (2017)
improves the learning stability of GAN and provides solutions for debugging and hyperparameter searching for GAN. These successful theoretical analyses of GAN show the effectiveness and possibility of photorealistic face image generation and synthesis.
GAN has dominated the field of face frontalization since it is firstly used by DR-GAN Tran et al. (2017). Later, TP-GAN Huang et al. (2017b) is proposed with a two-pathway structure and perceptual supervision. CAPG-GAN Hu et al. (2018) introduce pose guidance through inserting conditional information carried by five-point heatmaps. PIM Zhao et al. (2018a) aims to generate high-quality results through adding regularization items to learn face representations more robust to hard examples. CR-GAN Tian et al. (2018) introduces a generation sideway to maintain the completeness of the learned embedding space and utilizes both labeled and unlabeled data to further enrich the embedding space for realistic generations. All those methods treat face frontalization as a 2D image-to-image translation problem without considering the intrinsic 3D properties of human face. They indeed perform well in the situation where training data is sufficient and captured well controlled. However, in-the-wild setting often leads to inferior performance, as we discussed in Sec. 1.
The attempt to combine prior knowledge of 3D face has been made by FF-GAN Yin et al. (2017), 3D-PIM Zhao et al. (2018b) and UV-GAN Deng et al. (2018). Their and our methods are all 3D-based but there are many differences. In FF-GAN, a CNN is trained to regress the 3DMM coefficients of the input. Those coefficients are integrated as a supplement of low-frequency information. 3D-PIM incorporates a simulator with the aid of a 3DMM to obtain prior information to accelerate the training process and reduce the amount of required training data. In contrast, we do not employ 3DMM to present shape or texture information. We introduce a novel dense correspondence field and frontalize the profiles through warping. UV-GAN leverages an out-of-the-box method to project a 2D face to a 3D surface space. Their network can be regarded as a 2D image-to-image translation model in the facial texture space. In contrast, once the training procedure is finished, our model can estimate the latent 3D information from the profiles without the need for any additional out-of-the-box methods.
It is also notable that some face frontalization methods tend to improve the performance of face recognition by data augmentation. Inspired by Shrivastava et al. (2017), DA-GAN Zhao et al. (2017), which acts as a 2D face image refiner, can be employed for pose-invariant face recognition. In brief, the refiner improves the quality of data augmented by ordinary methods. The training processes for face recognition methods benefit from these refined data and the performances are boosted. Thus, DA-GAN is a method for augmenting training data. Note that UV-GAN mentioned above can be used to benefit face recognition in the same manner with DA-GAN, so it is also a data augmentation method. In contrast, our HF-PIM is trained to directly rotate the given profile to the frontal face, which can be directly used for face recognition.
3 High Fidelity Pose Invariant Model
Given a profile face image , our goal is to produce the frontal face image as close to the ground truth as possible (). As a reminder, we use to denote the value of the pixel with coordinate in an image . To learn the mapping, image pairs are employed for model training. Inspired by recent progress in 3D face analysis Güler et al. (2017, 2018), we propose a brand-new framework which frontalizes given profile face through recovering geometry and texture information of the 3D face without explicitly building it. Concretely, the facial texture map and a novel dense correspondence field are leveraged to produce through warping. The facial texture map lies in UV space - a space in which the manifold of the face is flattened into a contiguous 2D atlas. Thus, represents the surface of the 3D face. The dense correspondence field is proposed to establish the connections between 2D and 3D surface spaces. is specified by the following statement: assuming that the coordinate of a point in is , the corresponding coordinate in is after the warping operation. The right side on the top of Fig. 1 provides an intuitionistic illustration. Formally, given and with respect to , can be warped into through the following formulation:
where is the coordinate set of those pixels standing for the facial part of . Our proposed warping procedure inherits the virtue of morphable model construction: geometry and texture are well disentangled whereas bound by dense correspondence. However, there are also some limitations, e.g., neglect of image background. Thus, additional process is necessary to produce the non-facial regions for Eq. 1. To overcome these constraints, we employ a CNN-based wrapper to take as well as as the input and map them to . Our can produce the non-fical parts simultaneously with the facial parts and keep the overall visual realism consistent with . Concretely, is trained via optimizing the reconstruction loss:
where denotes calculating the mean of the element-wise absolute value summation of a matrix. Note that for our , is not limited to the RGB color space. In our experiment, we increase the number of feature channel to 32 and find that better performance is obtained.
In the following, we describe how to estimate the dense correspondence field of the frontal view in Sec. 3.1. Then, the recovering procedure of the facial texture map via ARDL is illustrated in Sec. 3.2
. Regularization items and the overall loss function are introduced in Sec.3.3.
3.1 Dense Correspondence Field Estimation
To obtain the ground truth dense correspondence field of monocular frontal face images for training, we employ face reconstruction method for 3D shape estimation. Concretely, we employ BFM Paysan et al. (2009) as the 3D face model. Through the model fitting method proposed by Zhu et al. (2016), we get estimated shape parameters containing coordinates of vertices. To build , we map those vertices to UV space via the cylindrical unwrapping described in Booth & Zafeiriou (2014). Those non-visible vertices are culled via z-buffering.
To infer the dense correspondence field of the frontal view from the profile image, we build a transformative autoencoder,, with U-Net architecture. Given the input, first encodes it into pose-invariant shape representations and then recover dense correspondence field of frontal view. Further, those shortcuts in U-Net guarantee the preservation of spatial information in the output. To supervise during training, we minimize the pixel-wise error between the estimated map and the ground truth , namely:
3.2 Facial Texture Map Recovery
We employ a transformative autoencoder consisting of the encoder and the decoder for facial texture map recovering. However, the ground truth facial texture map of monocular face image captured in the wild is absent. To sidestep the demand for , we introduce Adversarial Residual Dictionary Learning (ARDL) which provides supervision for learning. During the training procedure, only is required instead of .
The learning dictionary is set as: given a set of texture feature embeddings and a learnable codebook containing codewords with
dimension, the corresponding residual vector is denoted as, for and . Through dictionary encoding, a fixed length representation can be calculated as follows:
where is the corresponding weight for . Inspired by Van Gemert et al. (2008) that assigns a descriptor to each codeword, we make those weights learnable. Concretely, the assigning weight is given by:
where is the smoothing factor, which is also learnable. We denote the mapping from feature embeddings to dictionary representation as . The encoder is also employed to extract features for the dictionary learning.
We combine dictionary representation with adversarial learning, i.e., propose ARDL based on such an observation: when the identity label is fixed, for across different poses, the recovered texture map should be invariant. To this end, should eliminate those discrepancies caused by different views and encode the input into pose-invariant facial texture representation. We introduce adversarial learning mechanism to supervise by making as its rival. Formally, the adversarial loss introduced by ARDL is formulated as:
Accordingly, is optimized to minimize:
where we add a fully connected (FC) layer upon to make binary predictions standing for real and fake. Through optimizing Eq. 6 and 7 alternatively, manages to make the encodings of the profile and the frontal view as similar as possible. In the meantime, tries to find the clues standing for pose information, which provides the adversarial supervision information for .
3.3 Overall Training Method
Following previous work Johnson et al. (2016); Huang et al. (2017b), we also add the perceptual loss to integrate domain knowledge of identities. An identity preserving network, e.g., VGG-Face Parkhi et al. (2015) or Light CNN He et al. (2017), can be employed to supervise the frontalized results to be as close to the ground truth as possible in feature-level. Formally, the perceptual loss is formulated as:
where denotes the extracted identity representation obtained by the second last fully connected layer within the identity preserving network and denotes the vector 2-norm.
We also introduce the adversarial loss in the RGB color image space following those GAN-based methods Tran et al. (2017); Huang et al. (2017b); Hu et al. (2018); Zhao et al. (2018a); Yin et al. (2017). A CNN named is employed to give adversarial supervision in color space. Note that our method can be easily extended to those advanced versions Arjovsky et al. (2017); Mao et al. (2017) of GAN. But in this paper, we simply use the original form of adversarial loss function Goodfellow et al. (2014) to prove that the effectiveness comes from our own contributions.
4.1 Experimental Settings
Datasets. To demonstrate the superiority of our method in both controlled and unconstrained environments and produce high-resolution face frontalization results, we conduct our experiment on four datasets: Multi-PIE Gross et al. (2010), LFW Huang et al. (2007), IJB-A Klare et al. (2015), and CelebA-HQ Karras et al. (2018). Multi-PIE is established for studying on PIE (pose, illumination and expression) invariant face recognition. 20 illumination conditions, 13 poses within 90 yaw angles and 6 expressions of 337 subjects were captured in controlled environments. LFW is a benchmark database for face recognition. Over 13,000 face images are captured in unconstrained environments. IJB-A is the most challenging unconstrained face recognition dataset at present. It has 5, 396 images and 20, 412 video frames of 500 subjects with large pose variations. CelebA Liu et al. (2015) is a large-scale face attributes dataset. Contained images cover large pose variations and background clutter. CelebA-HQ is a high-resolution subset established by Karras et al. (2018). Since Multi-PIE, LFW and IJB-A consist of images with relatively low resolutions, we use CelebA-HQ for high-resolution () face frontalization.
Implementation Details. The training set is drawn from Multi-PIE and CelebA-HQ. We follow the protocol in Tran et al. (2017) to split the Multi-PIE dataset. The first 200 subjects are used for training and the rest 137 ones for testing. Each testing identity has one gallery image from his/her first appearance. Hence, there are 72,000 and 137 images in the probe and gallery sets, respectively. For CelebA-HQ, we apply head pose estimation Zhu et al. (2016) to find those frontal faces and employ them (19, 203 images) for training. We choose those images with large poses (5, 998 ones) for testing. Apparently, there are no overlap between our training and testing sets. LFW and IJB-A are only used for testing. Note that the images selected for training in CelebA-HQ are all frontal view, and we employ the face profiling method in Zhu et al. (2015) to make corresponding profiles. We adapt the model architecture in Zhu et al. (2017) to build our networks. We use Adam optimizer with a learning rate of 1e-4 and2017). Two NVIDIA Titan X GPUs with 12GB GDDR5X RAM is employed for the training and testing process.
Evaluation Metrics. To measure the quality of frontalized faces, the most common method is to evaluate the face recognition/verification performances via “recognition via generation”, which means profiles are frontalized first, and then the performance is evaluated on these processed face images. This evaluation manner prefers frontalization results that preserve more identity information and directly reflect the contributions of frontalization methods on face recognition. Thus, “recognition via generation” has been adopted by a series of existing methods Huang et al. (2017b); Hu et al. (2018); Yin et al. (2017); Zhao et al. (2018a). Besides, since photographic results also indicate the performances qualitatively, visual quality is also compared in our experiment, as most GAN-based methods do.
|DR-GAN Tran et al. (2017)||94.9||91.1||87.2||84.6||-||-|
|FF-GAN Yin et al. (2017)||94.6||92.5||89.7||85.2||77.2||61.2|
|Light CNN He et al. (2017)||98.6||97.4||92.1||62.1||24.2||5.5|
|TP-GAN Huang et al. (2017b)||98.7||98.1||95.4||87.7||77.4||64.6|
|CAPG-GAN Hu et al. (2018)||99.8||99.6||97.3||90.3||83.1||66.1|
|PIM Zhao et al. (2018a)||99.3||99.0||98.5||98.1||95.0||86.5|
4.2 Frontalization Results in Controlled Situations
In this subsection, we systematically compare our method with DR-GAN, TP-GAN, FF-GAN, CAPG-GAN and PIM on the Multi-PIE dataset. Those profiles with extreme poses ( and ) are very challenging cases. Our performances are tested following the protocol of the setting 2 provided by Multi-PIE. Remind that our performance is evaluated by the “recognition via generation” framework. Concretely, when evaluating on Multi-PIE, profiles are first frontalized by our model and then used directly for verification and recognition. As for evaluating on those in-the-wild datasets (discussed in the next subsection), all the faces are frontalized by our model since their yaw angles are not known in advance. After the frontalization preprocessing, Light CNN He et al. (2017) is employed as the feature extractor. We compute the cosine distance of extracted feature vectors for verification and recognition. The results are reported across different poses in Table 1. Note that the manners for evaluating TP-GAN, FF-GAN, CAPG-GAN, and PIM are the same with our model. Light CNN is used for these methods except FF-GAN (their feature extractor is not publicly available). DR-GAN is evaluated in a different manner: the feature vectors are directly extracted by their model. Thus, no extra feature extractor is needed for DR-GAN. Besides frontalization methods, the performance of Light CNN is also included as the baseline. The results are reported across different poses in Table 1. For those poses less than , the performances of most methods are quite good whereas our method performs better. We infer that the performance has almost saturated in this case. For those extreme poses, our methods can still produce visually convincing results and achieve state-of-the-art recognition performance. In general, when testing on Multi-PIE, due to its balanced data distribution and highly controlled environment, most methods perform relatively well (except those extreme poses).
4.3 Frontalization Results in the Wild
|TP-GAN Huang et al. (2017b)||96.13||99.42||DR-GAN Tran et al. (2017)||77.42.7||53.94.3||85.51.5||94.71.1|
|FF-GAN Yin et al. (2017)||96.42||99.45||FF-GAN Yin et al. (2017)||85.21.0||66.33.3||90.20.6||95.40.5|
|Light CNN He et al. (2017)||99.39||99.87||Light CNN He et al. (2017)||91.51.0||84.32.4||93.01.0||-|
|CAPG-GAN Hu et al. (2018)||99.37||99.90||PIM Zhao et al. (2018a)||93.31.1||87.51.8||94.41.1||-|
Extending face frontalization to in-the-wild setting is a very challenging problem with significant importance. We focus on testing on IJB-A and LFW in this subsection. For LFW, we evaluate face verification performance on the frontalized results of the 6000 face pairs provided by the dataset. For IJB-A, both verification and identification are tested in 10-fold cross-validation. The results are summarized in Table 2. All the methods are tested with the same setting. Note that the training set of IJB-A is not been used by any involved method for comparison.
We can see that face frontalization methods only marginally improve the performance on LFW because most faces in this dataset are (near) frontal view. Besides, the baseline model Light CNN has already achieved a relatively high performance. But our method still outperforms existing frontalization methods in this case. When testing on IJB-A which contains lots of images with large and even extreme poses, our method shows a significant improvement for face verification and recognition. The visual comparison111Visual results produced by other methods are released by their authors. Different methods usually report visual examples of different identities. We try our best to find those identities reported by most methods., which is shown in Fig 2, also proves our superiority of preserving identity information and texture details. Thanks to the 3D-based framework and powerful adversarial residual dictionary learning, our HF-PIM produces results with very high fidelity. For other methods, they indeed produce reasonable images but redundant manipulations can be observed. For instance, DR-GAN make the eyes of the subject in the middle in IJB-A open; TP-GAN and CAPR-GAN tend to change the skin color and background.
4.4 High-Resolution Face Frontalization
Generating high-resolution results has great importance on extending the application of face frontalization. However, due to its difficulty, few methods consider producing images with size larger than . To further demonstrate our superiority, frontalized results on CelebA-HQ are proposed in this paper. Some samples are shown in Fig 3. We also make comparisons with TP-GAN and CAPG-GAN. Note that since results on CelebA-HQ have not been reported by previous methods, we contact the authors to get their model and produce results through carefully following their instructions. The images in CelebA-HQ contain rich textures that are difficult for the generator to reproduce faithfully. Even in such a challenging situation, HF-PIM is still able to produce plausible results. The results of Hu et al. (2018) and Huang et al. (2017b) look less appealing.
Existing methods Huang et al. (2017b); Tran et al. (2017); Zhao et al. (2018a); Yin et al. (2017); Hu et al. (2018) measure the performance of face recognition to reflect the quality of frontalized results. This measurement cannot be applied to those datasets without identity labels (like CelebA-HQ) and neglects texture information that are not sensitive to identity. However, the neglected textures also play an import role on the visual quality and should be preserved faithfully. For face attribute analysis, data augmentation and many other practical applications, recovering high-resolution frontal view with detailed texture information has great potential for making progress. Finding new applications for face frontalization and putting forward new metrics need further research.
This paper has proposed High Fidelity Pose Invariant Model (HF-PIM) to produce realistic and identity-preserving frontalization results with a higher resolution. HF-PIM combines the advantages of 3D and GAN based methods and frontalizes profile images via a novel texture warping procedure. Through leveraging a novel dense correspondence field, the prerequisite of warping is decomposed into dense correspondence field estimation and facial texture map recovering, which are well addressed by a unified end-to-end deep network. We also have introduced Adversarial Residual Dictionary Learning (ARDL) to supervise facial texture map recovering without the need of 3D data. Exhaustive experiments have shown proposed method can preserve more identity information as well as texture details, which make the high-resolution results far more realistic.
This work is funded by the National Key Research and Development Program of China (Grant No. 2017YFC0821602, 2016YFB1001000) and the National Natural Science Foundation of China (Grant No. 61427811, 61573360).
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