Spontaneous Facial Action Modeling and Understanding by A Dynamic Bayesian Network
Facial action is one of the most important sources of information for understanding emotional state and intention . Spontaneous facial action is characterized by rigid head movement, nonrigid facial muscular movements, and their interactions. Rigid head movement characterizes the overall 3D head pose including rotation and translation. Nonrigid facial muscular movement results from the contraction of facial muscles and characterizes the local facial action at a finer level. The Facial Action Coding System (FACS) developed by Ekman and Friesen  is the most commonly used system for measuring facial behavior. Based on FACS, nonrigid facial muscular movement can be described by 44 facial action units (AUs), each of which is anatomically related to the contraction of a specific set of facial muscles.
Why we recognize spontaneous facial action?
An objective and noninvasive system for facial action understanding has applications in human behavior science, human-computer interaction, security, interactive games, computer-based learning, entertainment, telecommunication, and psychiatry.
Challenges of recognizing spontaneous facial action
Facial action modeling
Due to the low intensity, nonadditive effect, and individual difference of the spontaneous facial action as well as the image uncertainty, individually recognizing facial action is not accurate and reliable. Hence, understanding spontaneous facial action requires not only improving facial motion measurements, but more importantly, requires exploiting the spatial-temporal interactions among facial motions since it is these coherent, coordinated, and synchronized interactions of facial motions that produce a meaningful facial display. By explicitly modeling and using these relationships, we can improve facial action recognition performance by compensating erroneous or missing facial motion measurements.
Figure 1 presents the complete DBN model for facial action modeling. Specifically, there are three layers in the proposed model:
We employ the first layer as the global constraint for the overall system so that it will guarantee globally meaningful facial action. Meanwhile, the local structural details of the facial components are constrained not only by the local shape parameters, but also are characterized by the related AUs through the interactions between the second layer and the third layer. In addition, the interactions between the rigid head motion and the nonrigid facial actions are indirectly modeled through the 2D global and local facial component shapes. Finally, the facial motion measurements are systematically incorporated into the model through the shaded nodes. This model, therefore, completely characterizes the spatial and temporal dependencies between rigid and nonrigid facial motions and accounts for the uncertainties in facial motion measurements.
Fig. 1. The complete DBN model for facial action understanding. The shaded node indicates the observation for the connected hidden node. The self-arrow at the hidden node represents its temporal evolution from previous time frame to the current time frame. The link from AUi at time t-1 to AUj (j ¹ i) at time t indicates the dynamic dependency between different AUs.
Facial action recognition by exploiting the spatial-temporal interactions among facial motions
Figure 2 shows the proposed facial action recognition system. In the system, various computer vision techniques are used to obtain measurements of both rigid (head pose) and nonrigid facial motions (AUs). These measurements are then used as evidences by the facial action model for inferring the true states of the rigid head pose and the nonrigid AUs simultaneously.
Fig. 2. The flowchart of the facial action recognition system.
(1) Evaluation on ISL multiview facial expression database
In order to demonstrate the robustness of the proposed system, we perform experiments on ISL multiview facial expression database under realistic environment where the face undergoes facial expression and face pose changes. The system performance is reported in Figure 3.
Compared to the AU recognition by using the AdaBoost classifiers only, we can find that: (1) for the frontal-view face, the average relative false-negative rate (error rate of positive samples) decreases by 37.5%, and the average relative false-positive rate (error rate of negative samples) decreases by 44.7%; (2) for the right-view face, the average relative false-negative rate decreases by 40%, and the average relative false-positive rate decreases by 42.2%; and (3) for the left-view face, the average relative false-negative rate decreases by 46.1%, and the average relative false-positive rate decreases by 46.8% by using the proposed model. Here, the relative error rate is defined as the ratio of the error rate of the proposed method to the error rate of the AdaBoost method. Especially for the AUs that are difficult to be recognized, the system performance is greatly improved. For example, for AU23 (lip tighten), its false-negative rate decreases from 52.3% to 20.5%, and its false-positive rate decreases from 7.7% to 3.1% for the left view face; the false-negative rate of AU7 (lid tighten) decreases from 34.8% to 12.5% for the right view face; and the false-negative rate of AU6 (cheek raiser and lid compressor) is decreased from 34% to 14.5% with a significant drop of false-positive rate decreasing from 17.3% to 6.95% for the left view face.
Fig. 3. AU recognition results under realistic circumstance for frontal-view faces (top row), left-view faces (middle row), and right-view faces (bottom row). In each figure, the black bar denotes the result by the AdaBoost classifier, and the white bar represents the result by using the proposed model. (a) The first column demonstrates average false-negative rate. (b) The second column displays average false-positive rate.
(2) Evaluation on a spontaneous facial expression database
Instead of recognizing posed facial activities, it is more important to recognize spontaneous facial actions. Therefore, in the second set of the experiments, the system is trained and tested on a spontaneous facial expression database to demonstrate the system robustness for recognizing spontaneous facial action.
Facial expression image database
 M. Pantic and M. Bartlett, “Machine analysis of facial expressions,” in Face Recognition, K. Delac and M. Grgic, Eds.
 P. Ekman and W. V. Friesen, Facial Action Coding System: A Technique for the Measurement of Facial Movement. Palo
Alto, CA: Consulting Psychologists Press, 1978.