3D-based methods.
-
Early 3D-based approaches pre-built 3D models of specific people and then rendered the models. Compared to 2D methods, this method can have better control over motion. However, the construction cost of such a 3D model is relatively high, and the effect of changing a new identity cannot be guaranteed. In synthesizing Barack Obama’s videos, these works
[8,11][15][16] synthesize realistic speaking facial videos by pre-constructing 3D facial models and learning to map audio sequences to video sequences to drive the model. In addition, there are many generative talking-head models based on 3DMM parameters
[10[17][18][19][20],
19,20,23], and models, such as blendshape
[19][18], flame
[36][21], and 3D mesh
[37][22], are used with the audio as model input. Among them, VOCA
[16][13] uses the blendshape of the character’s head to create the model. Meshtalk
[37][22] uses the neutral face template mesh as the basis to generate the talking-head video. However, the model with intermediate parameter 3DMM brings a certain loss of information. Moreover, VOCA is an independent 3D talking-head synthesis model that can capture different speech styles, while Meshtalk can parse out the absolute latent space of audio-related and audio-independent facial movements.
Table 1. The main model of talking-head generation in recent years. ID: The model can be divided into three types: identity-dependent (D), identity-independent(I), and hybrid(H). Driving Data: Audio(A), Text(T), and Video(V).
Most current methods reconstruct 3D models from training videos directly. NVP (neural voice puppetry) has since designed the Audio2ExpressionNet and the 3D model of the independent identity. NeRF (Neural Radiance Fields)
[38[33][34][35][36],
39,40,41], which simulates implicit representation with MLP, can store 3D spatial coordinates and appearance information and be used for large-resolution scenes. To reduce information loss, AD-NeRF
[25][29] trains two NeRFs for head and drive rendering of talking-head synthesis and obtains good visual effects. Many models require unrestricted universal identity and speech as input in practical application scenarios. Prajwal et al.
[22,42][28][37] take any unidentified video and arbitrary speech as input to synthesize unrestricted talking-head video.
4. Datasets and Evaluation Metrics
4.1. Datasets
In the era of artificial intelligence, the dataset is an important part of the deep learning model. At the same time, data sets also promote the solution of complex problems in the field of virtual human synthesis. However, in practical applications, there are few high-quality annotation data sets that cannot meet the training needs of the speech synthesis model. Moreover, many institutions/researchers are affected by the deepfake technical ethics issues, which increase the difficulty of obtaining some data sets. For example, only researchers, teachers, and engineers from universities, research institutions, and enterprises are allowed to apply, and students are prohibited from applying. In
Table 2,
wthe researche
rs briefly highlighted the data sets commonly used by most researchers, including statistics and download links.
Table 2.
Summary of talking-head video datasets.
The GRID
[68][38] dataset was recorded in a laboratory setting with 34 volunteers, which is relatively small in a large dataset, but each volunteer spoke 1000 phrases for a total of 34,000 utterance instances. The phrase composition of the dataset conforms to certain rules. Each phrase contains six words, randomly selected from each of the six types of words to form a random phrase. The six categories of words are “command”, “color”, “preposition”, “letter”, “number”, and “adverb”. Dataset is available at
https://spandh.dcs.shef.ac.uk//gridcorpus/, accessed on 30 December 2022.
LRW
[69][39], known for various speaking styles and head poses, is an English-speaking video dataset collected from the BBC program with over 1000 speakers. The vocabulary size is 500 words, and each video is 1.16 s long (29 frames), involving the target word and a context. Dataset is available at
https://www.robots.ox.ac.uk/~vgg/data/lip_reading/lrw1.html, accessed on 30 December 2022.
LRW-1000
[70][40] is a Mandarin video dataset collected from over 2000 vocabulary subjects. The purpose of the dataset is to cover the natural variation of different speech modalities and imaging conditions to incorporate the challenges encountered in real-world applications. There are large variations in the number of samples in each category, video resolution, lighting conditions, and attributes such as speaker pose, age, gender, and makeup. Note: the official URL (
http://vipl.ict.ac.cn/en/view_database.php?id=13, accessed on 30 December 2022.) is no longer available, you can go to the paper page for details about the data and download the agreement file here (
https://github.com/VIPL-Audio-Visual-Speech-Understanding/AVSU-VIPL, accessed on 30 December 2022.) if you plan to use this dataset for your research.
ObamaSet
[8][15] is a specific audio-visual dataset that focuses on analyzing the visual speech of former US President Barack Obama. All video samples are collected from his weekly address footage. Unlike previous datasets, it only focuses on Barack Obama and does not provide any human annotations. Dataset is available at
https://github.com/supasorn/synthesizing_obama_network_training, accessed on 30 December 2022.
VoxCeleb2
[71][41] is extracted from YouTube videos, including the video URL and discourse timestamp. At the same time, it is currently the largest public audio-visual data set. Although this dataset is used for speaker recognition tasks, it can also be used to train a talking-head generation model. However, the dataset needs to apply to obtain the download permission to prevent misuse of the dataset. The URL for the permit application is
https://www.robots.ox.ac.uk/~vgg/data/voxceleb/, accessed on 30 December 2022. Because the dataset is huge, it requires 300 G+ storage space and supporting download tools. The download method is available at
https://github.com/walkoncross/voxceleb2-download, accessed on 30 December 2022.
VOCASET
[19][18] is a 4D-face dataset with approximately 29 min of 4D face scans, synchronized audio from 12-bit speakers (six women and six men), and recorded 4D-face scans at 60 fps. As a representative high-quality 4D face-to-face audio-visual dataset, Vocaset greatly facilitates the research of 3D VSG. Dataset is available at
https://voca.is.tue.mpg.de/, accessed on 30 December 2022.
MEAD
[44][42], the Multi-View Emotional Audio-Visual Dataset, is a large-scale, high-quality emotional audio-video dataset. Unlike previous datasets, it focuses on facial generation for natural emotional speech and takes into account multiple emotional states (eight different emotions on three intensity levels). Dataset is available at
https://wywu.github.io/projects/MEAD/MEAD.html, accessed on 30 December 2022.
HDTF
[51][43], a large in-the-wild high-resolution audio-visual dataset, stands for the High-definition Talking-Face Dataset. The HDTF dataset consists of approximately 362 different videos of 15.8 h. The resolution of the original video is 720 P or 1080 P. Each cropped video is resized to 512 × 512. Dataset is available at
https://github.com/MRzzm/HDTF, accessed on 30 December 2022.
4.2. Evaluation Metrics
The evaluation task of talking-head video generation is an open problem that requires the evaluation of generation results from both objective and subjective aspects. Chen et al.
[72][44] reviewed several state-of-the-art talking-head generation methods. They designed a unified benchmark based on their properties. Subjective evaluation is often used to compare the generated content’s visual quality and sensory effects, such as whether lip-sync audio is in sync with the picture. Due to the high cost of subjective factors in the evaluation process, many researchers have attempted to quantitatively evaluate generated content using objective metrics
[22,28,29,52][28][31][32][45]. These metrics can be classified into three types: visual quality, audio-visual semantic consistency, and real time based on quantitative model performance evaluations.
Visual Quality. Reconstruction error measures (e.g., mean squared error) are a natural way to evaluate the quality of generated video frames. However, the reconstruction error only focuses on the pixel alignment, ignoring the global visual quality. Therefore, existing works typically employ the peak signal-to-noise ratio (PSNR), structural similarity index metric (SSIM)
[29[32][46],
73], and learned perceptual image patch similarity (LPIPS)
[74][47] to evaluate the global vision of generated image quality. Since metrics, such as PSNR and SSIM, cannot explain human perception well, and LPIPS is closer to human perception in visual similarity judgment, it is recommended to use LPIPS to evaluate the visual quality of generated content quantitatively. More recently, Prajwal et al.
[22][28] introduced the Fréchet inception distance (FID)
[75][48] to measure the distance between synthetic and real data distributions, as FID is more consistent with human perception assessments.
Audio-visual semantic consistency. The semantic consistency of the generated video and the driving source mainly includes audio-visual synchronization and speech consistency. For audio-visual synchronization, the landmark distance (LMD)
[12][23] computes the Euclidean distance of lip region landmarks between the synthetic video frame and the ground truth frame. Another synchronization evaluation metric uses SyncNet
[7][49] to predict the offset of generated frames and ground truth. For phonetic coherence, Chen et al.
[74][47] proposed a synchronization evaluation metric, the Lip-Reading Similarity Distance (LRSD), which can evaluate semantically synchronized lip movements.
Real-time performance. The length of time to generate the talking-head video is an important indicator for existing models. In the practical application of human–computer interaction, people are very sensitive to factors such as waiting time and video quality. Therefore, the model should generate the video as quickly as possible without sacrificing visual quality or the coherence of audio-visual semantics. NVIDIA
[10][17] uses a deep neural network to map low-dimensional speech waveform data to high-dimensional facial 3D vertex coordinates and uses traditional motion capture technology to obtain high-quality video animation data to train the model.
Delayed Talking-Head Synthesis Model. Ye et al.
[6][50] presented a novel, fully convolutional network with DCKs for the multimodal task of audio-driven talking-face video generation. Due to the simple yet effective network architecture and the video pre-processing, there is a significant improvement in the real-time performance of talking-head generation. Lu et al.
[76][51] present a live system that generates personalized photorealistic talking-head animation only driven by audio signals at over 30 fps. However, many studies ignore real-time performance, and we should consider it as a critical evaluation metric in the future.
Human–computer interaction is a method for the future development of virtual humans. Unlike one-way information output, digital human needs to have multimodal information such as natural language, facial expression, and natural human-like gestures. Meanwhile, it also needs to be able to feedback on high-quality video in a short time after a given voice request
[33,77][7][52].
However, in generating high-quality and low-latency digital human video, many researchers do not take real time as the evaluation index of the model. Hence, many models generate videos too slowly to cover the application requirements. For example, to generate 256 × 256 resolution facial video without background, ATVGnet
[17][10] takes 1.07 s, You Said That
[35][12] takes 14.13 s, X2Face
[78][53] takes 15.10 s, DAVS
[16][13] takes 17.05 s, and 1280 × 720 resolution video with background takes longer. Although it only takes 3.83 s for Wav2lip
[22][28] to synthesize a video with a background, the definition of the lower part of the face is lower than that of other areas
[6][50].
Many studies have attempted to establish a new evaluation benchmark and proposed more than a dozen evaluation metrics for virtual human video generation. Therefore, the existing evaluation metrics for virtual human video generation are not uniform. In addition to objective indicators, there are also subjective indicators, such as user research.