## 2 Materials and Methods  
### Design of Dynamic Virtual Humans  
The design of the present study was founded on the Facial Action Coding System (FACS) [30]. This system was selected for its widespread utilization and its suitability, as demonstrated through a psychometric evaluation of spontaneous expressions [31]. In addition, it provides more comprehensive information than other systems in terms of facial adjustments. Specifically, FACS categorizes facial movements on the basis of Action Units (AU) and outlines methods for perceiving and scoring them. Each AU represents a group of muscles that work together to produce a change in facial appearance. The different AUs are assembled based on the location of the facial muscles, which are separated into upper and lower facial muscles. The muscles of the upper face comprise the eyebrows, the forehead, the crease covering the eyes, and the upper and lower eyelids. The lower face comprises the muscles surrounding the mouth and lips and is divided into different classifications based on the directions of movement of the muscles. Additionally, other AUs exist based on the muscles that move the neck and the direction of gaze.  
In addition, our approach combines several well-known software packages used in game development to refine the design of the DVFs. The workflow is shown in Figure 1 and described next.  
The process started with the selection and customization of two predefined characters from Adobe Fuse CC, a software tool primarily intended for game developers. The characters obtained through this tool feature a wide range of visual characteristics commonly found in contemporary high-end video games and are fully customizable from facial features to the length of the limbs, torso, and clothing. These characters were then exported to Mixamo, an online platform that enables automated rigging of 3D humanoid models. Rigging, a technique utilized in skeletal animation, is the process of creating a digital bone structure for a 3D model, which allows the model to be manipulated as a puppet for animation. Once rigged, the 3D characters were imported into 3D Studio Max to generate the Action Units (AUs) starting from a neutral facial expression. The blend shapes technique, also known as morph animation targets, was employed to alter the mesh and store the vertex positions for each AU.  
Upon the incorporation of all AUs into the virtual human models, they were subsequently exported to Unity 3D, the real-time engine employed in the reproduction of animations. The facial expressions were further augmented through the utilization of wrinkles, achieved through the generation of a surface shader utilizing normal map textures. Normal maps serve to simulate nuances in the surface of an object by altering the vertex normal, subsequently impacting the calculation of light on the surface. This surface shader described the entirety of the visual appearance of the virtual human's face, utilizing textures for aspects such as skin color, normal mapping, reflections (specularity), and ambient occlusion. The standard lighting model was employed, and shadows were enabled for all light types. A total of seven different normal maps were utilized per virtual human, with one designated for each facial emotion in addition to the neutral expression.  
In conclusion, the fine-tuning of the majority of the AU-related parameters was accomplished through manual adjustments. The virtual humans were initially designed from scratch by two engineers, who subsequently presented a preliminary version. Then, the other engineer and the psychiatrists discussed the similarity of the virtual human's emotions to those of actual humans. The final version of the avatars was achieved through a process of iterative refinement.  
Figure 1: Workflow of the dynamic virtual humans’ design.