You are provided with the abstract of a scientific paper and the abstracts of the papers it references. Your task is to write a Related Work section for the main paper. This section should concisely summarize and integrate the key points from the cited abstracts, reflecting how they relate to the main paper's contributions and context. In your summary, use numerical citation markers like [1], [2], etc., to refer to the corresponding papers.

Main Abstract:
Modeling action units (AUs) on human faces is challenging because various AUs cause subtle facial appearance changes over various regions at different scales. Current works have attempted to recognize AUs by emphasizing important regions. However, the incorporation of prior knowledge into region definition remains under-exploited, and current AU detection systems do not use regional convolutional neural networks (R-CNN) with expert prior knowledge to directly focus on AU-related regions adaptively. By incorporating expert prior knowledge, we propose a novel R-CNN based model named AU R-CNN. The proposed solution offers two main contributions: (1) AU R-CNN directly observes different facial regions, where various AUs are located. Expert prior knowledge is encoded in the region and the RoI-level label definition. This design produces considerably better detection performance than do existing approaches. (2) We also integrate various dynamic models (including convolutional long short-term memory, two stream network, conditional random field, and temporal action localization network) into AU R-CNN and then investigate and analyze the reason behind the performance of dynamic models. Experiment results demonstrate that only static RGB image information and no optical flow-based AU R-CNN surpasses the one fused with dynamic models. AU R-CNN is also superior to traditional CNNs that use the same backbone on varying image resolutions. State-of-the-art recognition performance of AU detection is achieved. The complete network is end-to-end trainable. Experiments on BP4D and DISFA datasets show the effectiveness of our approach. Code will be made available.

Cited Abstract(s):
[1]: Action Unit (AU) detection becomes essential for facial analysis. Many proposed approaches face challenging problems in dealing with the alignments of different face regions, in the effective fusion of temporal information, and in training a model for multiple AU labels. To better address these problems, we propose a deep learning framework for AU detection with region of interest (ROI) adaptation, integrated multi-label learning, and optimal LSTM-based temporal fusing. First, ROI cropping nets (ROI Nets) are designed to make sure specifically interested regions of faces are learned independently; each sub-region has a local convolutional neural network (CNN) - an ROI Net, whose convolutional filters will only be trained for the corresponding region. Second, multi-label learning is employed to integrate the outputs of those individual ROI cropping nets, which learns the inter-relationships of various AUs and acquires global features across sub-regions for AU detection. Finally, the optimal selection of multiple LSTM layers to form the best LSTM Net is carried out to best fuse temporal features, in order to make the AU prediction the most accurate. The proposed approach is evaluated on two popular AU detection datasets, BP4D and DISFA, outperforming the state of the art significantly, with an average improvement of around 13 on BP4D and 25 on DISFA, respectively.

[2]: We propose a novel convolutional neural network approach to address the fine-grained recognition problem of multi-view dynamic facial action unit detection. We leverage recent gains in large-scale object recognition by formulating the task of predicting the presence or absence of a specific action unit in a still image of a human face as holistic classification. We then explore the design space of our approach by considering both shared and independent representations for separate action units, and also different CNN architectures for combining color and motion information. We then move to the novel setup of the FERA 2017 Challenge, in which we propose a multi-view extension of our approach that operates by first predicting the viewpoint from which the video was taken, and then evaluating an ensemble of action unit detectors that were trained for that specific viewpoint. Our approach is holistic, efficient, and modular, since new action units can be easily included in the overall system. Our approach significantly outperforms the baseline of the FERA 2017 Challenge, with an absolute improvement of 14 on the F1-metric. Additionally, it compares favorably against the winner of the FERA 2017 challenge. Code source is available at this https URL.

[3]: This paper presents our work in the FG 2017 Facial Expression Recognition and Analysis challenge (FERA 2017) and we participate in the AU occurrence sub-challenge. Our work of AU occurrence recognition is based on deep learning, and we design convolution neural network (CNN) models for two types of work: facial view recognition and AU occurrence recognition. For facial view recognition, our model could achieve 97.7 accuracy on validation dataset about 9 facial views. For AU occurrence recognition, we use both visual features and temporal information of dataset. We use CNN models to get deep visual feature and then use BLSTM-RNN to learn the high-level feature in the time domain. When training models, we divide dataset into 9 parts based on 9 facial views, and each model is trained in a specific view. When recognizing AUs, we recognize facial view first and then choose the corresponding model for AU occurrence recognition. Finally, our method shows good performance, the F1 score of test data is 0.507 and the accuracy is 0.735.

[4]: Facial action units (AUs) are essential to decode human facial expressions. Researchers have focused on training AU detectors with a variety of features and classifiers. However, several issues remain. These are spatial representation, temporal modeling, and AU correlation. Unlike most studies that tackle these issues separately, we propose a hybrid network architecture to jointly address them. Specifically, spatial representations are extracted by a Convolutional Neural Network (CNN), which, as analyzed in this paper, is able to reduce person-specific biases caused by hand-crafted features (eg, SIFT and Gabor). To model temporal dependencies, Long Short-Term Memory (LSTMs) are stacked on top of these representations, regardless of the lengths of input videos. The outputs of CNNs and LSTMs are further aggregated into a fusion network to produce per-frame predictions of 12 AUs. Our network naturally addresses the three issues, and leads to superior performance compared to existing methods that consider these issues independently. Extensive experiments were conducted on two large spontaneous datasets, GFT and BP4D, containing more than 400,000 frames coded with 12 AUs. On both datasets, we report significant improvement over a standard multi-label CNN and feature-based state-of-the-art. Finally, we provide visualization of the learned AU models, which, to our best knowledge, reveal how machines see facial AUs for the first time.
Researchers have utilized temporal dependencies in video sequences over the last few years. Romero al [2] advocated a two-stream CNN model that combines optical flow and RGB information, and their result was promising. However, they used one binary classification model for each AU, which caused their approach to be time consuming to train and yield numerous model parameters. The CNN and LSTM hybrid network architectures are studied in Chu al [4] , Wei Li al [1] and He al [3] , which feed the CNN-produced features to LSTM to improve performance by capturing the temporal relationship across frames. However, their solutions are inefficient because they are not an end-to-end networks. In our experiments, we also investigate the effects of using temporal feature relationships in the time axis of videos. We use various dynamic models (including two-stream network, ConvLSTM ) that are incorporated into AU R-CNN. Such temporal dependency cannot always improve performance in all cases (Section ).