Swapped Face Detection using Deep Learning and Subjective Assessment
The tremendous success of deep learning for imaging applications has resulted in numerous beneficial advances. Unfortunately, this success has also been a catalyst for malicious uses such as photo-realistic face swapping of parties without consent. Transferring one person’s face from a source image to a target image of another person, while keeping the image photo-realistic overall has become increasingly easy and automatic, even for individuals without much knowledge of image processing. In this study, we use deep transfer learning for face swapping detection, showing true positive rates 96% with very few false alarms. Distinguished from existing methods that only provide detection accuracy, we also provide uncertainty for each prediction, which is critical for trust in the deployment of such detection systems. Moreover, we provide a comparison to human subjects. To capture human recognition performance, we build a website to collect pairwise comparisons of images from human subjects. Based on these comparisons, images are ranked from most real to most fake. We compare this ranking to the outputs from our automatic model, showing good, but imperfect, correspondence with linear correlations . Overall, the results show the effectiveness of our method. As part of this study, we create a novel, publicly available dataset that is, to the best of our knowledge, the largest public swapped face dataset created using still images. Our goal of this study is to inspire more research in the field of image forensics through the creation of a public dataset and initial analysis.
Face swapping refers to the process of transferring one person’s face from a source image to another person in a target image, while maintaining photo-realism. It has a number of applications in cinematic entertainment and gaming. However, in the wrong hands, this method could also be used for fraudulent or malicious purposes. For example, “DeepFakes” is such a project that uses generative adversarial networks (GANs)  to produce videos in which people are saying or performing actions that never occurred. While some uses without consent might seem benign such as placing Nicolas Cage in classic movie scenes, many sinister purposes have already occurred. For example, a malicious use of this technology involved a number of attackers creating pornographic or otherwise sexually compromising videos of celebrities using face swapping . A detection system could have prevented this type of harassment before it caused any public harm.
Conventional ways of conducting face swapping usually involve several steps. A face detector is first applied to narrow down the facial region of interest (ROI). Then, the head position and facial landmarks are used to build a perspective model. To fit the source image into the target ROI, some adjustments need to be taken. Typically these adjustments are specific to a given algorithm. Finally, a blending happens that fuses the source face into the target area. This process has historically involved a number of mature techniques and careful design, especially if the source and target faces have dramatically different position and angles (the resulting image may not have a natural look).
The impressive progress deep learning has made in recent years is changing how face swapping techniques are applied from at least two perspectives. Firstly, models like convolutional neural networks allow more accurate face landmarks detection, segmentation, and pose estimation. Secondly, generative models like GANs  combined with other techniques like Auto-Encoding  allow automation of facial expression transformation and blending, making large-scale, automated face swapping possible. Individuals that use these techniques require little training to achieve photo-realistic results. In this study, we use two methods to generate swapped faces [21, 9]. Both methods exploit the advantages of deep learning methods using contrasting approaches, discussed further in the next section. We use this dataset of swapped faces to evaluate and inform the design of a face-swap detection classifier.
With enough data, deep learning based classifiers can typically achieve low bias due to their ability to represent complex data transformations. However, in many cases, the confidence levels of these predictions are also important, especially when critical decisions need to be made based on these predictions. The uncertainty of a prediction could indicate when other methods could be more reliable. Bayesian Deep Learning, for example, assumes a prior distribution of its parameters and integrates the posterior distribution when making a prediction, given the dataset . However, it is usually intractable for models like neural networks and must be employed using approximations to judge uncertainty. We propose a much simpler approach by using the raw logits difference of the neural network outputs (i.e., the odds ratio). We assume, in a binary classification task, if the model has low confidence about a prediction the difference of the two logits output should be small compared with high confidence prediction. We also show that the odds ratio of the neural network outputs is correlated with the human perception of “fake” versus “real.”
The end goal of malicious face swapping is to fool a human observer. Therefore, it is important to understand how human subjects perform in recognizing swapped faces. To this end, we not only provide the accuracy of human subjects in detecting fake faces, but we also provide the ranking of these images from most real to most fake using pairwise comparisons. We selected 400 images and designed a custom website to collect human pairwise comparisons of images. Approximate ranking is used  to help reduce the number of needed pairwise comparisons. With this ranking, we compare the odds ratio of our model outputs to the ranking from human subjects, showing good, but not perfect correspondence. We believe future works can improve on this ranking comparison, providing a means to evaluate face swapping detection techniques that more realistically follow human intuition.
We enumerate our contributions as follows:
A public dataset comprising 86 celebrities using 420,053 images. This dataset is created using still images, different from other datasets created using video frames that may contain highly correlated images. In this dataset, each celebrity has approximately 1,000 original images (more than any other celebrity dataset). We believe our dataset is not only useful for swapped face detection, it may also be beneficial for developing facial models.
We investigate the performance of two representative face swapping techniques and discuss limitations of each approach. For each technique, we create thousands of swapped faces for a number of celebrity images.
We build a deep learning model using transfer learning for detected swapped faces. To our best knowledge, it is the first model that provides high accuracy predictions coupled with an analysis of uncertainties.
We build a website that collects pairwise comparisons from human subjects in order to rank images from most real to most fake. Based on these comparisons, we approximately rank these images and compare to our model.
Ii Related Work
There are numerous existing works that target face manipulation and detection. Strictly speaking, face swapping is simply one particular kind of image tampering. Detection techniques designed for general image tampering may or may not work on swapped faces, but we expect specially designed techniques to perform superior to generic methods. Thus, we only discuss related works that directly target or involve face swapping and its detection.
Ii-a Face Swapping
Blanz et al.  use an algorithm that estimates a 3D textured model of a face from one image, applying a new facial “texture” to the estimated 3D model. The estimations also include relevant parameters of the scene, such as the orientation in 3D, camera’s focal length, position, illumination intensity, and direction. The algorithm resembles the Morphable Model such that it optimizes all parameters in the model in conversion from 3D to image. Bitouk et al. , bring the idea of face replacement without the use of 3D reconstruction techniques. The approach involves the finding of a candidate replacement face which has similar appearance attributes to an input face. It is, therefore, necessary to create a large library of images. A ranking algorithm is then used in selecting the image to be replaced from the library. To make the swapped face more realistic, lighting and color properties of the candidate images might be adjusted. Their system is able to create subjectively realistic swapped faces. However, one of the biggest limitations is that it is unable to swap an arbitrary pair of faces. Mahajan et al.  present an algorithm that automatically chooses faces that are facing the front and then replaces them with stock faces in a similar fashion, as Bitouk et al. .
Chen et al.  suggested an algorithm that can be used in the replacement of faces in referenced images that have common features and shape as the input face. A triangulation-based algorithm is used in warping the image by adjusting the reference face and its accompanying background to the input face. A parsing algorithm is used in accurate detection of face-ROIs and then the Poisson image editing algorithm is finally used in the realization of boundaries and colour correction. Poisson editing is explored from its basics by Perez et al. . Once given methods to craft a Laplacian over some domain for an unknown function, a numerical solution of the Poisson equation for seamless domain filling is calculated. This technique can independently be replicated in color image channels.
The empirical success of deep learning in image processing has also resulted in many new face swapping techniques. Korshunova et al. approached face swapping as a style transfer task. They consider pose and facial expression as the content and identity as the style. A convolutional neural network with multi-scale branches working on different resolutions of the image is used for transformation. Before and after the transformation, face alignment is conducted using the facial keypoints. Nirkin et al.  proposed a system that allows face swapping in more challenging conditions (two faces may have very different pose and angle). They applied a multitude of techniques to capture facial landmarks for both the source image and the target image, building 3D face models that allow swapping to occur via transformations. A fully convolutional neural network (FCN) is used for segmentation and for blending technique after transformation.
The popularity of Auto-Encoder and generative adversarial networks (GANs)  makes face swapping more automated, requiring less expert supervision. A variant of the DeepFake project is based on these two techniques . The input and output of an Auto-Encoder is fixed and a joint latent space is discovered. During training, one uses this latent space to recover the original image of two (or more) individuals. Two different auto-encoders are trained on two different people, sharing the same Encoder so that the latent space is learned jointly. This training incentivizes the encoder to capture some common properties of the faces (such as pose and relative expression). The decoders, on the other hand, are separate for each individual so that they can learn to generate realistic images of a given person from the latent space. Face swapping happens when one encodes person A’s face, but then uses person B’s decoder to construct a face from the latent space. The variant of this method in  uses an auto-encoder as a generator and a CNN as the discriminator that checks if the face is real or swapped. Empirical results show that adding this adversarial loss improves the quality of swapped faces.
Natsume et al.  suggests an approach that uses hair and faces in the swapping and replacement of faces in the latent space. The approach applies a generative neural network referred to as an RS-GAN (Region-separative generative adversarial network) in the generation of a single face-swapped image. Dale et al.  bring in the concept of face replacement in a video setting rather than in an image. In their work, they use a simple acquisition process in the replacement of faces in a video using inexpensive hardware and less human intervention.
Ii-B Fake Face Detection
Zhang et al.  created swapped faces using labeled faces in the wild (LFW) dataset . They used sped up robust features (SURF)  and Bag of Words (BoW) to create image features instead of using raw pixels. After that, they tested on different machine learning models like Random Forests, SVM’s, and simple neural networks. They were able to achieve accuracy over 92%, but did not investigate beyond their proprietary swapping techniques. The quality of their swapped faces is not compared to other datasets. Moreover, their dataset only has 10,000 images (half swapped) which is relatively small compared to other work.
Khodabakhsh et al.  examined the generalization ability of previously published methods. They collected a new dataset containing 53,000 images from 150 videos. The swapped faces in their data set were generated using different techniques. Both texture-based and CNN-based fake face detection were evaluated. Smoothing and blending were used to make the swapped face more photo-realistic. However, the use of video frames increases the similarity of images, therefore decreasing the variety of images. Agarwal et al.  proposed a feature encoding method termed as Weighted Local Magnitude Patterns. They targeted videos instead of still images. They also created their own data set. Korshunov et al. also targeted swapped faces detection in video . They evaluated several detection methods of DeepFakes. What’s more, they analyze the vulnerability of VGG and FaceNet based face recognition systems.
A recent work from Rössler et al.  provides an evaluation of various detectors in different scenarios. They also report human performance on these manipulated images as a baseline. Our work shares many similarities with these works. The main difference is that we provide a large scale data set created using still images instead of videos, avoiding image similarity issues. Moreover, we provide around 1,000 different images in the wild for each celebrity. It is useful for models like auto-encoders that require numerous images for proper training. In this aspect, our data set could be used beyond fake face detection. The second difference is that we are not only providing accuracy from human subjects, but also providing the rankings of images from most real to most fake. We compare this ranking to the odds ratio ranking of our classifier showing that human certainty and classifier certainty are relatively (but not identically) correlated.
|Nirkin’s Method ||AE-GAN ||Total|
Face swapping methods based on auto-encoding typically require numerous images from the same identity (usually several hundreds). There was no such dataset that met this requirement when we conducted this study, thus, we decided to create our own. Access to Version 1.0 of this dataset is freely available at the noted link 111https://www.dropbox.com/sh/rq9kcsg3kope235/AABOJGxV6ZsI4-4bmwMGqtgia?dl=0. The statistics of our dataset are shown in Table I.
All our celebrity images are downloaded using the Google image API. After downloading these images, we run scripts to remove images without visible faces and remove duplicate images. Then we perform cropping to remove extra backgrounds. Cropping was performed automatically and inspected visually for consistency. We created two types of cropped images as shown in Figure 1 Left. One method for face swapping we employed involves face detection and lighting detection, allowing the use of images with larger, more varied backgrounds. On the other hand, another method is more sensitive to the background, thus we eliminate as much background as possible. In a real deployment of such a method, a face detection would be run first to obtain a region of interest, then swapping would be performed within the region. In this study, for convenience, we crop the face tightly when a method requires this.
The two face swapping techniques we use in this study are representatives of many algorithms in use today. Nirkin’s method  is a pipeline of several individual techniques. On the other hand, the Auto-Encoder-GAN (AE-GAN) method is completely automatic, using a fully convolutional neural network architecture . In selecting individuals to swap, we randomly pair celebrities within the same sex and skin tone. Each celebrity has around 1,000 original images. For Nirkin’s method, once a pair of celebrities is chosen, we randomly choose one image from these 1,000 images as the source image and randomly choose one from the other celebrity as the target image. We noticed, for Nirkin’s method, when the lighting conditions or head pose of two images differs too dramatically, the resulted swapped face is of low quality. On the other hand, the quality of swapped faces from the AE-GAN method is more consistent.
Existing swapped face detection systems based on deep learning only provide an accuracy metric, which is insufficient for a classifier that is used continuously for detection. Providing an uncertainty level for each prediction is important for the deployment of such systems, especially when critical decisions need to be made based on these predictions.
In this study, we use the odds ratio of the binary classification output (i.e., the raw logits difference of each neural network output) as an uncertainty proxy. For binary classification tasks, the final layer of a deep learning model usually outputs two logits (before sending them to a squashing function). The model picks the large logit as the prediction. We assume if the model is less certain about a prediction, the difference of these two logits should be smaller than that of a more certain prediction. We note that this method is extremely simple as compared to other models that explicitly try to model uncertainty of the neural network, such as Bayesian deep learning methods. The odds ratio, on the other hand, does not explicitly account for model uncertainty of the neural network—especially when images are fed into the network that are highly different from images from the training data. Even so, we find that the odds ratio is an effective measure of uncertainty for our dataset, though more explicit uncertainty models are warranted for future research.
Deep learning methods usually take days or weeks to train. Models like ResNet can easily have tens or hundreds of layers. It is believed with more layers, more accurate hierarchical representation could be learned. Transfer leaning allows us to reuse the learned parameters from one task to another similar task, thus avoiding training from scratch, which can save a tremendous amount of resources. In this study, we apply transfer learning using ResNet-18, which is originally trained to perform object recognition on ImageNet . Since we are performing binary classification in this study, we replace the final layers of ResNet-18 with custom dense layers and then train the model in stages. During the first stage, we constrain the ResNet-18 architecture to be constant while the final layers are trained. After sufficient epochs, we then “fine tune” the ResNet-18 architecture, allowing the weights to be trained via back-propagation for a number of epochs.
Iii-C Human Subjects Comparison
Because face swapping attacks are typically aimed at misleading observers, it is highly important to understand how human beings perform at detecting swapped faces. Thus, a research contribution should also compare classification with human subjects. In this research it is not only our aim to provide the accuracy of human subjects at detecting swapped faces, but also to establish a ranking of images from most real to most fake. For example, if a rater thinks that an image is fake, is it obvious or is that rater not quite sure about their decision? We argue that this uncertainty is important to model. Moreover, we argue that, if humans are somewhat adept at finding fake images, then the machine learning model should have a similar ranking of the images from most real to most fake. We argue this because the human mind can leverage many information sources and prior knowledge not available to a simple machine learning algorithm. Thus, while two machine learning model may perform detection perfectly, if one follows human ranking more closely, it can be judged superior.
However, it is impractical to rank all image pairs in our dataset with multiple human raters. Therefore we apply two techniques to mitigate the ranking burden. First we only rank a subset of the total images, and, second, we perform approximate ranking of image pairs. As a subset of images, we manually select 100 high-quality swapped faces from each method together with 200 real faces (400 images in total). The manual selection of high quality images is justified because badly swapped faces would be easily recognized. Thus, an attacker would likely perform the same manner of “re-selecting” only high quality images before releasing them for a malicious purpose. It is of note that, even with only 400 images, the number of pairwise ratings required for ranking (over 79,000) poses a monumental task.
Iii-C1 Approximate Ranking
To get the ranking, we designed and deployed a website that implements the approximate ranking algorithm in . Users on the website are asked to compare two images and select which image appears most fake, subjectively. We use approximate ranking because even for only 200 images, a full ranking would require more than 19,900 comparisons (with each evaluator ranking every image pair only one time, despite the fact that different subjects may have different opinions). To converge, this method could easily require many more evaluations when two evaluators do not agree (perhaps more than 100,000 pairwise ratings). The approximate ranking algorithm, Hamming-LUCB, helps alleviate this need . This algorithm seeks to actively make identification of two ordered sets of images and , representing the highest and lowest ranked images, respectively. For a set of images, subsets and consist of a range of items of size , where is the “allowed” number of mistakes in each set, and the items perceived as least fake comprise the second set. Between the two sets, there is a high confidence that the items contained in the first set score similarly as compared to those items that are contained in the second set. The remaining items, after finding the two sets, can arbitrarily be distributed in high confidence to the two sets such that an accurate (but approximate) Hamming ranking is obtained. In the algorithm, the two sets are defined on the basis of adaptive definition of estimations of the scores (image rankings) for each .
|Nirkin’s Method ||AE-GAN |
|True Positive||False Positive||Accuracy||True Positive||False Postive||Accuracy|
|Manually Selected 200||ResNet-18||96.00%||0.00%||98.00%||100.00%||0.00%||100.00%|
A non-asymptomatic version of the iterated algorithm forms the basis of the definition of a confidence bound. The law therefore takes the form , where represents the integer that gives the number of the comparisons made and is a fixed risk parameter . The score is for the number of comparisons that are made and is registered together with the associated score’s empirical permutation of in every round such that . Then the following indices can be defined:
Two additional indices, and , defined as
are the standard indices of the upper and the lower ranked images for the Lower-Upper Confidence Bound strategy represented in the work by Kaufman et al. .
The main goal in this comparison is obtaining the two subsets and by making sufficient estimation of scores of the items. At each time instant, the algorithm determines which pair of items to present for comparison based on the outcomes of previous comparisons. The score’s current estimations and the intervals of confidence associated with the scores are the parameters underlying the decision about which images to compare next in this strategy. As a result of comparing two items, Hamming-LUCB receives an independent draw of success in the view point of the comparator in response. The algorithm focuses on the upper ranked items given as and the lower ranked items given as . Nevertheless, it does not disregard the rest of the items in between the two bounds within the sets. The confidence intervals of these items are kept below the confidence intervals for items and .
Pseudo-code for the Hamming-LUCB is shown in Algorithm 1. The algorithm terminates based on the associated stopping condition, . In our case, Hamming-LUCB can select the next two images from the dataset to compare based on the current rankings, thus converging to an ordering with many fewer comparisons than a brute force method.
Iii-C2 Website Ratings Collection
The inspiration of our website comes from that of the GIGGIF project for ranking emotions of GIFs222http://gifgif.media.mit.edu. Figure 2 shows a screenshot of the website. The text ”Which of the following two faces looks MORE FAKE to you” is displayed above two images. When the evaluator moves the mouse above either image, it is highlighted with a bounding box. The evaluator could choose to login using a registered account or stay as an anonymous evaluator. In this website, there are two instances of Hamming-LUCB running independently for two types of swapped faces. The probability of selecting either swapped type is 50%. Over a three month period, we recruited volunteers to rate the images. When a new rater is introduced to the website, they first undergo a tutorial and example rating to ensure they understand the selection process. We collected 36,112 comparisons in total from more than 90 evaluators who created login accounts on the system. We note that anyone using the system anonymously (without logging in) was not tracked so it is impossible to know exactly how many evaluators used the website.
To evaluate the performance of our classifier, we use five-fold cross validation to separate training and testing sets. We don’t distinguish two types of swapped faces during training. In other words, we mix the swapped faces generated using both methods during training, but we report prediction performance on each method separately. Table II gives the overall detection performance of our classifier for the entire dataset and for the 400 images that were ranked by human subjects. We also report the accuracy with which humans were able to select images as real or fake based on the pairwise ranking. That is, any fake images ranked in the top 50% or any real images ranked in the bottom 50% were considered as errors. From the table, we can see that both human subjects and the classifier achieve good accuracy when detecting swapped faces. Our classifier is able to achieve comparable results to human subjects in 200 manually selected representative images (100 fake, 100 real) for each method.
Iv-a Classification Accuracy
As we mentioned above, we created two types of cropped images for each method. The AE-GAN method contains minimal background and Nirkin’s method contains more background. We can see from Table II that our classifier is able to detect face swapping better for the AE-GAN generated images—this holds true regardless of testing upon the entire dataset or using the manually selected 200. As we can see from Figure 1 (Right), the AE-GAN generates swapped faces that are slightly blurry, which we believe our model exploits for detection. On the other hand, Nirkin’s method could generate swapped faces without a decrease in sharpness. Thus, it may require the model to learn more subtle features, such as looking for changes in lighting condition near the cropped face or stretching of facial landmarks to align the perspectives.
For version 1.0 of the dataset, we have collected more than 36,112 pairwise comparisons from more than 90 evaluators (approximately evenly split between each method). Human subjects may have different opinions about a pair of images, thus it requires many pairwise comparisons, especially for these images in the middle area. However, we can see human subjects still give a reasonable accuracy, especially for the AE-GAN method. It is interesting to see that both our classifier and human subjects perform better on the AE-GAN generated images.
Iv-B Classifier Visualization
To elucidate what spatial area our classifiers are concentrating upon to detect an image as real or fake, we employ the Gradient-weighted Class Activation Mapping (Grad-CAM) visualization technique . This analysis helps mitigate the opaqueness of a neural network model and enhance explain-ability for applications in the domain of privacy and security. Grad-CAM starts by calculating the gradients of most dominant logit with respect to the last convolutional layer. The gradients are then pooled channel wise as weights. By inspecting these weighted activation channels, we can see which portions of the image have significant influence in classification. For both types of generated swapped faces, our classifier focuses on the central facial area (i.e., the nose and eyes) rather than the background. This is also the case for real faces as we can see from Figure 3. We hypothesize that the classifier focuses on the nose and eyes because the most visible artifacts are typically contained here. It is interesting that the eyes and nose are focused upon by the classifier because human gaze also tends to focus on the eyes and nose when viewing faces .
Iv-C Images ranking
Rather than reporting only accuracy of detecting swapped faces from human subjects, we also provide a ranking comparison. Ranking gives us more information to compare the models with, such as does the ResNet model similarly rate images that are difficult to rate for humans? Or, on the contrary, is the ranking from the model very different from human ratings?
Figure 4 gives the overall ranking for faces generated using two methods using the Hamming-LUCB ranking from human evaluators. Red boxed points are false negatives, black boxed points are false positives. The alpha in the plot gives a confidence interval based on the Hamming-LUCB. As we can see, Human subjects have more difficulty classifying the faces generated using Nirkin’s method. As mentioned, the AE-GAN generated faces are blurrier compared with Nirkin’s method. Human subjects seemingly are able to learn such a pattern from previous experience. While some mistakes are present for the AE-GAN, these mistakes are very near the middle of the ranking. Swapped faces generated using Nirkin’s method keep the original resolution and are more photo realistic—thus they are also more difficult to discern as fake.
To compare the human ranking to our model, we need to process the outputs of the neural network. During training, the model learns a representation of the input data using convolutions. Instances belonging to different classes usually are pushed away in a high dimensional space. But this distance between two instances is not necessarily meaningful to interpret. Despite this, the output of the activation function can be interpreted as a relative probability that the instance belongs to each class.
We assume for the odds ratio of the last full connected layer (before sent to the squashing function), the wider the margin, the more confident the classifier is that the instance is real or fake. Fig. 5 gives the comparison of log margin of our model and human rating for the 200 faces. For Nirkin’s Method, the linear correlation is 0.7896 and Spearman’s rank order correlation is 0.7579. For the AE-GAN Method, the linear correlation 0.8332 and Spearman’s rank order correlation is 0.7576. This indicates that the uncertainty level of our model and human subjects is consistent, though not perfect. This consistency is encouraging because it shows the model learns not only a binary threshold, but captures similarity in ranking of the images from most fake to most real. We anticipate that future work can further improve upon this ranking similarity.
In this study, we investigated using deep transfer learning for swapped face detection. For this purpose, we created the largest public face swapping detection data set using still images. Moreover, the data set has around 1,000 real images for each individual (publicly known largest), which is beneficial for models like the AE-GAN face swapping method. We use this data set to inform the design and evaluation of a classifier and the results show the effectiveness of the model for detecting swapped faces. More importantly, we compare the performance of our model with human subjects. We designed and deployed a website to collect pairwise comparisons for 400 carefully picked images from our data set. Approximate ranking is calculated based on these comparisons. We compared the ranking of our deep learning model and find that it shows good correspondence to human ranking. We hope this work will assist in the creation and evaluation of future image forensics algorithms.
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