Illumination variation is a traditional challenge in real-world face recognition systems. Near infrared (NIR) imaging provides a low-cost and effective solution to acquire high-quality images in low lighting or complete darkness conditions. Hence, it has been widely adopted in mobile device, video surveillance and user authentication applications. However, many applications require that the enrollment of face templates is based on visible (VIS) images, such as online registration and pre-enrollment using passport or ID card. That is, NIR images are face images captured under near infrared lighting, and VIS images are face images captured under visible lighting. Therefore, face matching between NIR and VIS images has drawn much attention in computer vision and machine learning. It also has been the most studied research topic in heterogeneous face recognition (HFR) that refers to matching faces across different spectral (or sensing) domains and is different from conventional VIS face recognition under homogeneous conditions.
Since it is expensive and time-consuming to obtain a large-scale pair-wised face images from different domains, current deep HFR methods mainly resort to the convolutional neural network (CNN) trained on a web-scale VIS face dataset, and then fine-tune it on a NIR-VIS dataset to obtain better HFR performance. Recently, to extend VIS face recognition methods to other spectral domains, face synthesis methods have gained much attention . Lezama et al.  proposed a cross-spectral hallucination and low-rank embedding to synthesize a heterogeneous image in a patch way. Song et al.  employed generative adversarial networks (GAN)  with a two-path model to synthesize VIS images from NIR images.  synthesize visible face images from thermal face images. One major advantage of these synthesis methods is that given the synthesized visible face images, any VIS face recognition method trained on VIS face data can be used to match the synthesized image to the enrolled VIS images .
The use of these synthesis methods poses opportunities as well as new challenges. Current synthesis results are less appealing in high-resolution and their output size is often no larger than . One possible reason lies in sensing gap. That is, VIS and NIR face images of the same subject are captured by using different sensory devices with different settings so that their visual appearances are significantly different. Both geometric and textural details of NIR faces are different from those of VIS faces. This gap results in high intra-class variations and makes the high-resolution synthesis of one spectrum from another very difficult. Particularly, as shown in Fig. 1, some visual appearances or contents are often always missed or corrupted in NIR images (e.g., the pixels around cheek, hair or eyes) so that the synthesis of visible images is also a challenging inpainting problem.
Another possible reason making the synthesis challenging lies in the pose difference between VIS faces and NIR faces . Pose variations often result in self-occlusion so that the texture of a NIR face image may be incomplete . Since VIS faces and NIR faces are often captured under different distances and environments, it is difficult to simultaneously capture the VIS faces and NIR faces under the same pose. For some real-world applications on mobile devices, there are often various poses in NIR face images . On contrast, poses of the pre-enrolled VIS images using passport or ID card are often frontal. Moreover, compared to the dataset for the synthesis in VIS domain, the NIR-VIS dataset is often small-scale. Since a small-scale training dataset will lead to the over-fitting problem 
, cross-spectral face rotation is more challenging than face rotation in VIS domain. Both sensing gap and pose difference make the synthesis from NIR to VIS be an one-to-many unsupervised image-to-image translation problem as shown in Fig.1.
To address the above two issues, this paper proposes an end-to-end generative framework, named Cross-spectral Face Completion (CFC), by performing generative adversarial networks. CFC presents a deep framework for generating a frontal VIS image of a person’s face given an input NIR face image. It decomposes the unsupervised heterogeneous synthesis problem into two complementary problems, generating a texture inpainting component and a pose correction component that are addressed by deep networks. The inpainting component synthesizes and inpaints VIS image textures from incomplete NIR image textures. The correction component transforms any pose in NIR images to a frontal pose in VIS images. Once the face pose of a VIS face image is given, the texture synthesis and inpainting become one-to-one supervised image-to-image translation problem, which results in paired NIR and VIS textures and facilitates pixel-level losses. Second, a warping procedure is developed to integrate the texture and pose procedures into an end-to-end deep network. A fine-grained discriminator is employed to supervise the disentanglement process by guiding the generator to minimize the intra-class variance in an adversarial manner; a wavelet-based discriminator is designed to supervise the visual quality in a multi-scale manner. One UV loss, two adversarial losses and one pixel loss are imposed to ensure high-quality synthesis results.
We train our proposed CFC approach only on CASIA NIR-VIS 2.0 , and evaluate it on CASIA NIR-VIS 2.0, BUAA-VisNir , and Oulu-CASIA . A new benchmark ’recognition via generation’ protocol on CASIA NIR-VIS 2.0 is established for systematically evaluating NIR-VIS cross-spectral synthesis. Experimental results verify that by attaching the pose correction procedure, we can simplify cross-spectral face completion from one-to-many unpaired image translation to one-to-one paired image translation, and minimize spectral and pose discrepancy during heterogeneous recognition. Extensive cross-database experiments show that our approach not only generates photo-realistic and identity-preserving VIS face images and but also facilitates HFR performance.
In summary, the main contributions of this work are as follows,
A novel GAN-based end-to-end deep framework is proposed for cross-spectral face synthesis without assembling multiple image patches. It contains encoder-decoder structured generators and two novel discriminators to fully consider variations of NIR and VIS images.
It is the first time that the unsupervised heterogeneous face synthesis problem is simplified to a one-to-one image translation problem. The decomposition of texture inpainting and pose correction enables the generation of realistic identity preserving VIS face images possible.
This is the first approach simultaneously transforming face pose and cross-spectrum appearance for heterogeneous face recognition. A new benchmark on CASIA NIR-VIS 2.0 is also established to quantitatively evaluate the performance of ’recognition via generation’.
We achieve state-of-the-art face synthesis and face recognition performance on multiple HFR benchmark datasets, including CASIA NIR-VIS 2.0, BUAA-VisNir, and Oulu-CASIA. We synthesize visible faces to push forward the advance in cross-spectral face synthesis.
An early version of this work was first proposed in . Although  and this work are both based on GANs, they adopt different strategies to address the unsupervised heterogeneous face synthesis problem.  employed the cycle-GAN architecture  to handle the unsupervised synthesis problem and a two-path network structure to enhance local textures. On contrast, this paper simplifies the unsupervised synthesis problem to a supervised one by decomposing the synthesis into two complementary components. This decomposition has significantly extended our previous work 
in network structure and loss function, resulting in a high-resolution synthesis image. Particularly, different from that trained a feature representation on synthesized VIS images, this paper directly uses synthesized visible images for HFR without fine-tuning VIS face recognition models on synthesized images.
2 Background and Related Work
The heterogeneous problem of matching people across different domains has received increasing attention in biometrics (e.g., face  and iris ). NIR-VIS HFR has been one of the most extensively studied topics in heterogeneous biometrics. In this section, we mainly review some recent advances related to the heterogeneous matching problem from three aspects : image synthesis, latent subspace, and domain-invariant features.
Image synthesis methods aim to synthesize face images from one domain into another so that heterogeneous images can be directly compared in the same spectral domain. These methods bridge the domain discrepancy at the image preprocessing stage. They transform face images from one domain to another, and thereby perform face matching . Image synthesis was firstly studied to synthesize and recognize a sketch image from a face photo .  proposed an analysis-by-synthesis framework to synthesize a face image from one domain to another before face matching.  applied Markov random fields to transform pseudo-sketch to face photo in a multi-scale way.  resorted to statistical learning to synthesize a 3D face from a single NIR face image using canonical correlation analysis (CCA). In , coupled or joint dictionary learning was used to reconstruct face images and then perform face matching. These three methods constrain the representation of heterogeneous images in each dictionary to be the same.  novelly decomposed sketch-photo face synthesis into an inter-domain transfer process and an intra-domain transfer process and hence proposed a dual-transfer synthesis framework.
Based on the recent advances in deep learning,
proposed a two-step procedure (VIS feature estimation and VIS image reconstruction) to synthesize VIS faces from polarimetric thermal faces. developed a fusion technique to concatenate different stokes images for VIS face synthesis.  proposed a cross-spectral synthesis and low-rank embedding to synthesize a heterogeneous image in a patch way. To achieve better rank-1 accuracy,  used a new testing protocol rather than the standard 10-fold testing protocol .  proposed a global and local perception GAN to reduce the pose discrepancy between a face profile and a frontal face. Inspired by ,  employed generative adversarial networks to perform cross-spectral image synthesis. A two-path model was introduced to alleviate the lack of paired images. Although deep learning methods have significantly improved synthesis results, synthesizing a heterogeneous image from another domain is still challenging and the output size is often no larger than . As shown in Fig. 4 and Fig. 6, this synthesis is also an inpainting or completion problem because background contents of NIR images are corrupted during sensing.
methods project two different domains to a common latent space, in which the relevance of heterogeneous data can be measured. Dimension reduction techniques such as Principal Component Analysis (PCA), Canonical Correlation Analysis (CCA) and Partial Least Squares (PLS) are often used.
proposed a method called Common Discriminant Feature Extraction (CDFE) to incorporate both discriminative and locality information. considered the locality information in kernel space and proposed a coupled discriminant analysis. Then,  developed a regularized discriminative spectral regression method to seek a common spectral space. A common subspace learning method was proposed in 
by introducing feature selection, and then was applied as a baseline method in NIR-VIS HFR. proposed a prototype random subspace method with kernel similarities for HFR. 
proposed a domain adaptive self-taught learning approach to derive a common subspace. By using transfer learning, projected both NIR and VIS data to a generalized subspace where each NIR sample can be represented by some combination of VIS samples. 
employed Restricted Boltzmann Machines (RBMs) to learn a shared representation between different domains, and then suggested to apply PCA to remove the redundancy and heterogeneity. Multi-view discriminant analysis and mutual component analysis  were further developed to reduce the domain discrepancy.  integrated multi-task clustering with extreme learning machine to learn coupled mapping for NIR-VIS HFR.  treated HFR as a multi-view discriminant analysis problem and projected the examples from different modalities (or views) to one discriminant common space.
Domain-invariant feature methods seek discriminative features that are only related to face identity and disregard domain information. Traditional methods in this category are almost based on handcrafted local features, such as Local Binary Patterns (LBP), Histograms of Oriented Gradients (HOG) and Difference of Gaussian (DoG) . To capture high-level semantics across different domains,  constructed a hierarchical hyper lingual-words based on bag of visual words.  converted facial pixels into an encoded face space with a trained common encoding model. Recently, deep neural networks show great potential to learn domain-invariant features of heterogeneous images. These deep methods are often pre-trained on a large-scale VIS face dataset, and then are fine-tuned on NIR face images to learn domain-invariant features.
Based on a pre-trained VIS CNN,  explored different metric learning strategies to improve HFR performance.  designed two types of NIR-VIS triplet loss to reduce the domain discrepancy meanwhile to augment training sample pairs.  gave two new network structures (named VisNet and NIRNet) with small convolutional filters, and used a Siamese network to couple the learnt features from the two networks.  employed deep neural networks to learn a non-linear mapping to bridge the domain gap between thermal and VIS face images.  divided the high-level layer of CNN into two orthogonal subspaces so that domain-invariant identity information and domain-related spectrum information can be presented independently.  explored a disentangled latent variable space to optimize the approximate posterior for NIR and VIS features. 
designed a Wasserstein CNN to reduce the discrepancy between NIR and VIS feature distributions. By performing deep features, these methods significantly improve HFR results against traditional methods.
Although some HFR methods have been developed to improve recognition performance, HRF is more challenging than VIS face recognition. The high performance of VIS recognition benefits from deep learning techniques and large amounts of VIS face images. Due to limited samples and sensing gap, HFR is still a challenging recognition research topic. Moreover, the pose difference between NIR and VIS faces also affects HRF performance and draws less attentions. Deep learning based image synthesis methods pose opportunities as well as new challenges.
3 Cross-spectral Face Completion
The NIR-VIS face completion problem can be formulated as learning a mapping from face images in a visual domain to another visual domain . For each face, there is an identity label whose corresponding identity information should be well preserved in the completion procedure. Based on practical applications, we assume that each identity is included in both and during the training process. However, as we pointed out in Sec. 1, data captured by the devices without synchronization settings will introduce large variations in pose, expressions, background, and etc. Therefore, the training data are not simply regarded as strictly paired.
Our network is established in an adversarial learning framework as shown in Fig. 2. The generation modules consist of a pose correction network , a texture inpainting network , and a backend network named fusion warping net. Given an input face, is trained to estimate the normalized shape information with the aid of the dense UV correspondence field. aims to learn to produce pose-invariant facial texture representation. The fusion warping net combines the corrected shape and completed texture information, and then produces the final results. There are two discriminators in our network, a multi-scale discriminator and a fine-grained discriminator . The former is designed to supervise the visual quality by discriminating between real images in domain and translated images. The latter aims to supervise the disentanglement process by guiding the generator to minimize the intra-class variance of the input in an adversarial manner. In the following, we will describe the details of the pose correction, texture inpainting, and fusion warping processes in Sections 3.1, 3.2, and 3.3, respectively. The overall loss function is summarized in Sec. 3.4.
3.1 Pose Correction via UV Field Estimation
The pose correction network is based on the estimation of the dense correspondence field that denoted as the field in the following part. The field is employed to bind the UV facial texture space and the RGB color image space. The facial texture space refers to the space where the manifold of the face is flattened into a contiguous 2D atlas. Compared with shape guidance information applied widely in face manipulation, e.g., landmarks and facial parsing maps, the field has two significant advantages: 1) the pixel-wise relation between the pose-invariant facial texture map and the 2D image is specified by the UV field. Therefore, the field contains complete shape information for a given face. 2) 3D supervision is subtly integrated by the UV field. Since a facial texture map in the space represents the flattened surface of a 3D human face, the field makes our approach 3D-aware.
To obtain the ground truth field during the training process, we fit the 3DMM (provided by ) through the Multi-Features Framework  to get the estimated 3D shape information. Then we map those vertices to the space via the cylindrical unwrapping method . The non-visible vertices are culled via z-buffering. As highlighted above, the training data are not perfectly paired due to the difficulties in data acquisition, and our goal is to produce transferred VIS face images with normalized pose. To this end, we calculate the mean VIS faces for each identity and render the ground truth fields on these mean faces. We find it is a simple yet effective manner for guiding our approach to estimate the normalized shape information. Concretely, our loss item takes the following form:
where denotes the mean field and denotes calculating the mean of the element-wise absolute value summation of a matrix. The norm can be treated as a robust estimator to ensure that the majority parts of and are close and similar.
3.2 Transformative Adversarial Texture Inpainting
The texture inpainting network aims to encode a given face texture into a compact identity representation, and then to decode the representation into the facial texture map in the VIS domain. Note that in our experiments, the input faces are sampled from both the NIR and VIS domains. On the one hand, this modification augments the training data for to learn the pose correction; on the other hand, we find that making our be aware of the spectrum of the input also slightly improves the performance. For the input faces in the VIS domain, will learn the identity mapping. In order to disentangle the texture representation, should learn to filter out the other irrelevant information. We accomplish this goal by introducing as the rival to supervise during the training process. Concretely, takes a couple of representations that have the same identity label and predicts whether the input is a real representation pair. In our experiment, the representations of the real pair are all from VIS training data. The fake pair consists of one representation from our synthesized faces and another one from the VIS training data. tries to deceive into believing that the fake pair is real. In this adversarial training scheme, the improvements of are two-fold: 1) will learn to make synthesized results as similar to the real data in feature space as possible. 2) can eliminate the intra-class variations of the real data in pose, expression, etc., ensuring that the learned representations are closely related to identity. Formally, the loss item introduced by the adversarial learning scheme above is as follows:
In the meantime, the loss item for our discriminator is defined as:
3.3 Fusion Warping Net
A fusion warping network is designed to combine the output of and , producing final transformed VIS results. It is inspired by the classical warping operation applied in face manipulation. Recall that once the field is specified, the facial texture map can be warped into the corresponding 2D face image. However, the values of pixels standing for background parts, e.g., non-facial areas, ears and hair, are undefined in this case. Hence, the post process is necessary to complete the missing parts. To produce the background parts simultaneously with synthesizing the facial one, we build the fusion warping net with several convolution layers. The final output of , i.e., the predicted facial texture map, remains to be warped into the facial region. In the meantime, the output of the second last layer of , which can be regarded as the facial texture feature map, is fed into the fusion warping net along with the warped facial part. Besides, the predicted facial texture map is not limited to being in the RGB color space for our fusion warping net. In our experiment, we increase the number of feature channel of the facial texture map to 32 and find that better performance is obtained.
We introduce the adversarial learning in RGB color space to supervise the fusion warping net on producing realistic transferred faces. To achieve high-resolution NIR-VIS face completion, we employ a multi-scale discriminator, . Specifically, we apply wavelet decomposition on the full-size input data by a factor of 2, yielding a series of wavelet coefficients. We choose Haar wavelet because it is enough to depict different-frequency facial information. We use 2-D fast wavelet transform (FWT)  to compute Haar wavelets. and aim at discriminating the difference between the real data and the synthetic results in the low-frequency and the high-frequency coefficients, respectively. Compared with the single-scale discriminator, our effectively supervises the generator to produce both globally and locally consistent results. We find that when dealing with high-resolution (larger than ), the single-scale discriminator has limited power in generating plausible local textures. To address this problem, we assign a larger weight for minimizing the high-frequency adversarial loss. Formally, the multi-scale adversarial loss for our generator is formulated as:
where we denote the output of our fusion warping net as . and denote the decomposed high-frequency and low-frequency wavelet coefficients, respectively. We set in our experiment to emphasize generating plausible high-frequency information.
Correspondingly, the multi-scale adversarial loss for our discriminator takes the form:
3.4 Regularization Items and the Overall Loss Function
Most identity-preserving face generation tasks resort to a pre-trained face recognizer, e.g., VGG-Face  and Light CNN , to provide guidance on keeping the identity information. Specifically, the goal of identity-preserving is achieved by minimizing the distance between the extracted identity representations of the real and the synthetic face pair. Some methods also optimize the distance between the median outputs provided by the face recognizer, e.g., the output of the last pooling layer. This regularization item is referred to as the perceptual loss. We also include this item to improve the identity-preserving ability of our network, which is formulated as:
where denotes the extracted identity representation obtained by the second last fully connected layer within the identity preserving network and
means the vector-norm.
The pixel-wise losses like and losses have been proved effective in keeping low-frequency information. Since the training set of our NIR-VIS face completion does not have perfectly matched data pair, heavy reliance on the pixel-wise loss will lead to over-smooth results. When the resolution increases, this limitation will be exaggerated and pose significant challenges of producing plausible texture information. Considering that, we assign a relatively small factor for pixel-wise loss to help the learning of global structure. Our pixel-wise loss item takes the following form:
where we assign in our experiment since heavy reliance on the pixel-wise loss significantly harms the visual quality, especially for the high-resolution face completion.
In summary, the overall loss functions for our generator are given as:
The loss functions for the multi-scale discriminator and the fine-grained discriminator are and , respectively. The generator and the discriminators are iteratively optimized as suggested by . We use the perceptual loss as the indicator and stop the training process when it converges.
3.5 Implementation Details
Our end-to-end network is implemented based on the deep learning library Pytorch. An NVIDIA Titan XP GPU with 12GB GDDR5X RAM is employed for the training and testing processes. We buildwith a U-Net structure  network and adopt the network structures in  to build our and discriminators. We optimize the parameters of our model by Adam optimizer  with a learning rate of 2e-4 and momentum of 0.5. We use the perceptual loss as the indicator and stop training when it no longer decreases. The training processes of our models producing and outputs last for 3 and 11 hours, respectively.
We employ a pre-trained Light CNN  as the baseline recognizer111available at https://github.com/AlfredXiangWu/LightCNN. We also use it to calculate the perceptual loss during the training process. The network is composed of 29 convolution layers with a variation of maxout operations, i.e., we use LightCNN-29v2. We train the Light CNN on MS-Celeb-1M , which consists of 10K identities with 8.5M VIS face images. Note that faces in the NIR domain do not appear in the training process of Light CNN. The performance of face verification is evaluated by ’recognition via generation’: NIR faces are first processed by our NIR-VIS face completion method and then fed to the Light CNN for matching. We also evaluate the verification performance by directly using the NIR faces as a reference.
In this section, the proposed cross-spectral face completion framework is systemically evaluated against state-of-the-art HFR methods and deep learning methods on three widely used HFR face databases. New benchmark protocols for evaluating ’recognition via generation’ are proposed. Both quantitative and qualitative results are reported.
4.1 Experimental Settings
The CASIA NIR-VIS 2.0 Face Database  is the mostly used HFR database (or cross-modal database ) because it is the largest public and most challenging HFR database. It is collected in four recording sessions from 2007 to 2010. There are large variations of the same identity, including lighting, expression, pose, and distance. Moreover, wearing glasses or not is also considered to increase variations. The age distribution of the subjects spans from children to old people. The total number of the subjects in this database is 725. Each subject has 1-22 VIS and 5-50 NIR images. Since each image is randomly gathered, NIR and VIS images have no one-to-one correlations (i.e., they are unpaired). Fig. 3 shows some samples of cropped VIS and NIR faces. We observe that NIR-VIS images are unpaired and have large pose variations, which make heterogeneous synthesis and matching on this database challenging. Two views of matching protocols have been used in this database. View 1 is designed to adjust super-parameter, and View 2 can be adopted for training and testing.
The matching protocol in View 2 contains 10-fold experiments. In each fold, there is a collection of training and testing lists. The training and testing sets in each fold include nearly equal numbers of identities that are kept disjoint from each other. This means that the identities used for training and testing are entirely different, which facilitates a fair comparison of heterogeneous synthesis and recognition. For each fold, there are about 6,100 NIR images and 2,500 VIS images from about 360 identities. These subjects are exclusive from the 358 identities in the testing set. For the testing of each fold, the gallery contains 358 identities and each identity has one VIS image. The probe has over 6,000 NIR images from the same 358 identities. Each NIR image in the probe set compares against all images in the gallery set, resulting in a similarity matrix of size by around .
The BUAA-VisNir face database  is a standard HFR database and can also be used as a testing platform for domain adaptation . It has 150 subjects with 9 VIS images and 9 NIR images, which are captured simultaneously by using a single multi-spectral camera. It is often used to evaluate domain adaptation methods across imaging sensors. The nine images of each subject are captured under nine distinct poses or expressions, i.e., neutral-frontal, left-rotation, right-rotation, tilt-up, tilt-down, happiness, anger, sorrow and surprise. The training set is composed of 900 images of 50 subjects and the testing set contains 1800 images from the remaining 100 subjects. To avoid that the probe and gallery images are in the same pose and expression, we select only one VIS image of each subject in the gallery set during testing. As a result, there are 100 VIS images and 900 NIR images in the gallery set and the probe set respectively. Since there are pose and illumination variations in the probe set, this testing protocol is still challenging. Fig. 3 (b) lists some samples of cropped VIS and NIR faces.
The Oulu-CASIA NIR-VIS database  is often used to study the effects of illumination variations to facial expressions and heterogeneous face recognition. It contains NIR and VIS images from 80 subjects. 50 subjects are from Oulu University and the other 30 subjects are from CASIA. For each subject, there are six expression variations (anger, disgust, fear, happiness, sadness, and surprise). NIR and VIS images are acquired under three different illumination environments: normal indoor, weak and dark. Fig. 3 (c) shows some cropped VIS and NIR faces. The image resolutions of VIS and NIR images are different. Some NIR images are blurred and low-resolution, which makes HFR more difficult. Forty subjects are selected for evaluation , including 10 subjects from Oulu University and 30 subjects from CASIA 2.0. Eight face images are randomly selected from each expression for each domain. Then, there are totally 48 NIR images and 48 VIS images for each subject. 20 subjects are used as training and the remaining 20 subjects are used as testing. During testing, the gallery set contains all VIS images of the 20 subjects in testing, and the probe set contains all their corresponding NIR images.
Even though there have been some NIR-VIS heterogeneous databases , they are often used for HFR rather than face synthesis. Since heterogeneous databases are often small-scale, different synthesis protocols were adopted to enrich training set . To the best of our knowledge, there is still no a benchmark protocol to evaluate ’recognition via generation’ for HFR. To be consistent with the standard 10-fold protocol in the CASIA NIR-VIS 2.0 database, we define two different protocols in this paper for further research as follows,
Synthesis protocol: Considering that there are only limited training samples, we employ the NIR and VIS images of all 357 identities in the training set of the first fold of CASIA 2.0 to train a generative model and the testing protocol of the first fold to qualitatively evaluate the synthesized results of different methods222There are 10 fold experiments on the CASIA 2.0 database. We have trained our model on each fold and only report the visual results on the first fold.. There are 6,010 NIR images and 2,547 VIS images from the 357 identities. The identities used for training and testing are entirely different.
Recognition protocol: We follow the ’recognition via generation’ framework  to evaluate recognition performance. That is, VIS face recognition methods are directly used to match VIS images and the synthesized images from NIR images. VIS face recognition methods are not fine-tuned on the synthesized images. For the CASIA NIR-VIS 2.0 database, we train the generative model on each fold and use the testing protocol on each fold for evaluation. The training set is only used to train a generative model. For the BUAA-VisNir database and the Oulu-CASIA NIR-VIS database, the training sets in these two databases are not used. We directly employ the generative model trained on the first fold of the CASIA NIR-VIS 2.0 database to translate NIR domain to VIS domain. Then the testing sets in the two databases are used for evaluation.
4.2 Face Image Synthesis
In this subsection, we compare the synthesis results of different methods. The first fold of CASIA 2.0 is used to train and verify performance. Pixel2Pixel  and cycleGAN  are used as baselines. Since Cross-Spectral Hallucination (CSH)  and Adversarial Discriminative Feature Learning (ADFL)  used different training protocols, we only select the four subjects in the testing set of the first fold of CASIA 2.0 for visual comparison. These four subjects are also used in . Hence, the synthesized results of CSH are directly copied form . Note that the face images in CASIA 2.0 are collected in four recording sessions (different environments) that range are from 2007 to 2010 . Hence the VIS face images in CASIA 2.0 have different skin colors and backgrounds, which make synthesis tasks quite challenging.
Fig. 4 shows visualization results of different methods. It is clear that our CFC method significantly outperforms its competitors. Due to large sensing gap and pose difference, Pixel2Pixel and cycleGAN can not obtain satisfactory results on the NIR-VIS heterogeneous problem. Their synthesized results are consistent with the observation in . CSH almost loses all background contents and its synthesized face appearances are quite similar to those of NIR face images. Since the backgrounds of NIR images are corrupted and those of VIS images have large variations, all methods can not perfectly complete background pixels around hair and ears. Our CFC method seems to generate background pixels better. We also observe that the synthesized faces by our CFC method have different skin colors. This is because the face images in CASIA 2.0 are collected in four recording sessions. The ground-truth (denoted by ’GT’) VIS images and input NIR images may be not from the same recording session.
Table I further lists the quantitative comparison results of different synthesis methods. As expected, our CFC method can further improve the matching performance of Light CNN. The performance of pixel CNN is quite low. This is because NIR-VIS heterogeneous face synthesis is an unpaired image translation problem. There is no exact pixel-level correspondence between NIR and VIS images. We also observe that cycleGAN can not improve the performance of Light CNN. Although cycleGAN is developed for unpaired or unsupervised image synthesis, large face variations (pose, expression) make cycleGAN fail to capture all differences between NIR and VIS domains. As a result, the HFR performance of cycle-GAN is not competitive against CFC.
Fig. 5 further shows the synthesized VIS face images from two subjects. There are pose and expression variations in NIR images, which make face image synthesis and completion challenging. Our CFC method translates an input NIR face to a frontal VIS face. Although there are large pose variations, our method can still generate stable results. There are only large variations on the generated areas around ear and hair. This may be due to the fact that the visual background information of a NIR image is often corrupted. However, the generated VIS face areas are almost similar. Since CFC can simultaneously reduce sensing gap and pose difference, its generated VIS images potentially facilitate heterogeneous matching.
4.3 NIR-VIS Face Recognition
Table II lists recognition results on the CASIA NIR-VIS 2.0 Database. Recently proposed state-of-the-art HFR methods are compared, including six traditional methods and nine deep learning methods. The traditional methods include Learning Coupled Feature Spaces (LCFS) , DSIFT , Coupled Discriminant Feature Learning (CDFL) , Gabor+RBM, H2(LBP3) , common encoding feature discriminant (CEFD) . The results of LCFS and CDFL are from , and those of the remaining compared traditional methods are from their published papers. For deep learning methods, we compare the recently proposed TRIVET, HFR-CNNs , IDNet , Invariant Deep Representation (IDR)  and Adversarial Discriminative Feature Learning (ADFL) . Moreover, the results of three VIS CNN methods are also discussed, including VGG , SeetaFace  and CenterLoss . The result of Light CNN is used as the baseline of deep methods. Since most of HFR methods use the standard protocol in View 2 for evaluation, the results of compared methods are directly reported from their published papers.
We observe that some deep learning methods significantly outperform traditional HFR methods. However, some deep learning methods (including VGG, SeetaFace, and HFR-CNNs) even perform worse than the traditional Gabor+RBM method in terms of rank-1 accuracy. This may be because the sensing gap and the over-fitting problem on small-scale datasets make HFR challenging for deep learning methods. Different from other methods, our cross-spectral face completion method does not fine-tune a VIS CNN model on heterogeneous datasets. CFC directly employs Light CNN to match VIS faces against synthesized images. It further improves the rank-1 accuracy of Light CNN from to . Compared to other deep learning, CFC also achieves comparable recognition performance in terms of Rank-1 accuracy and verification rates. The performances of CFC and ADFL are close. However, ADFL needs to fine VIS recognition models on NIR-VIS datasets to improve accuracy. CFC-Fuse further fuses the features of the original NIR image and its synthesized VIS image. Both the two features are extracted by Light CNN. We use the mean value of two features as the feature of CFC-Fuse. It is obvious that the fusion strategy can further improve performance. These results suggest that the sensing gap between NIR and VIS domains can be bridged in a synthesis way.
The proposed method is further evaluated on the BUAA VisNir database. The recognition testing protocol in  is used. We compare CFC with MPL3 , KCSR , KPS , KDSR , KDSR  and H2(LBP3) . The results of MPL3, KCSR, KPS, KDSR and H2(LBP3) are from . TRIVET, IDR and ADFL are used as the baseline of deep learning methods. Table III shows Rank-1 accuracy and verification rates of different HFR methods. We observe that the deep learning methods outperform the traditional methods in terms of both rank accuracy and verification rates. Our CFC significantly outperforms its competitors. These performance improvements of CFC partly benefit from the usage of Light CNN. Light CNN is trained on a large-scale VIS dataset so that it can better capture intra-class variations to facilitate face recognition. Compared to Light CNN, we can observe that CFC improves the rank-1 accuracy from 96.5% to 99.7% and VR@FAR=0.1 from 86.7% to 97.8%. These improvements further suggest that CFC can reduce the sensing gap.
Table IV lists HFR performance of different methods on the Oulu-CASIA NIR-VIS Database. The results of MPL3, KCSR, KPS, KDSR and H2(LBP3) are from . It is observed that the HFR methods can be ordered in ascending rank-1 accuracy as MPL3, KPS, KCSR, KDSR, H2(LBP3), TRIVET, IDR, ADFL and CFC. As expected, CFC achieves the highest rank-1 accuracy (100%). That is each probe NIR image is correctly matched to its corresponding VIS image from the same identity. To the best of our knowledge, it is the first time that such high rank-1 accuracy is achieved by using a image synthesis method. Although CFC achieves high rank-1 accuracy on this database, the verification rates of CFC on the Oulu-CASIA NIR-VIS database are lower than those of CFC on the previous databases. The lower accuracy of CFC can be attributed to the factor that the NIR images in this database are blurred and low-resolution, resulting in large and unpredictable domain difference. This factor also makes TRIVET and IDR only slightly outperform five traditional methods at a low FAR.
4.4 Ablation Study
In order to demonstrate the compelling perceptual results generated by our CFC method as well as the contribution of each component, we visualize high-resolution NIR-VIS results under different configurations. Beyond the light spectrum difference, there are also pose, expression, age and shape variations between NIR and VIS images. Fig. 6 shows visualization results of high-resolution () NIR-VIS face generation under different configurations. The first column and last column contain input NIR images and ground-truth VIS images respectively. Note that the ground-truth image may be not captured under the same time with the NIR input image. It is interesting to find that there are large variations on the generated background areas by different configurations. This may be because background pixels are often corrupted in NIR images and some information (e.g., hair texture) is missed.
We observe that all components of our CFC method contribute to a high-resolution and high-quality result. The notation ’w/o ’ indicates that CFC only uses the pixel loss and does not contain a discriminator on the final synthesized RGB face image. It is clear that the images in the second column are more blur than those in the fifth column. Although CFC has also a discriminator during adversarial texture inpainting, the discriminator w.r.t. plays an important role during face synthesis. ’single-scale ’ indicates that we replace the multi-scale of CFC with a single-scale one. That is, we do not perform a discriminator on Wavelet decomposition. It seems that the images in the third column have fewer local details than the images in the fifth column. These results demonstrate the effectiveness of wavelet based multi-scale discriminators. The notation ’w/o ’ indicates the CFC model without using identity preserving loss . Comparing the images in the fourth column and fifth column, we observe that there are obvious differences between the pixels around ear and hair. It seems that when the identity preserving loss is used, the pixels around ear and hair are better generated. Since the background pixels are corrupted in NIR images, identity preserving loss potentially ensures that CFC can better complete missing information during synthesis.
In this paper, we have proposed a new architecture for NIR-VIS face image synthesis. Different from previous synthesis methods for HFR, we model the heterogeneous synthesis using two complementary components: a texture inpainting component and a pose correction component. The synthesis problem is simplified into two learning problems, facilitating one-to-one supervised texture completion. Then a warping procedure is used to fuse the two components in an end-to-end deep network. A multi-scale discriminator and a fine-grained discriminator have been developed to improve image quality. A new benchmark on CASIA NIR-VIS 2.0 has been established to systematically evaluate the synthesis results for HFR performance. Extensive experimental results on cross-database validation show that our network not only generates high-resolution face images and but also facilitates the accuracy improvements of state-of-the-art heterogeneous face recognition.
This work is funded by State Key Development Program (Grant No. 2016YFB1001001), the National Natural Science Foundation of China (Grants No. 61622310), and Beijing Natural Science Foundation (Grants No. JQ18017).
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