## 4 Features and Issues of Existing Applications
### Visual Representation  
A visual representation in a simulated environment is not a necessary feature of DT for a physical asset [12]. However, there is no doubt that appropriate visual representation, which can be adopted for monitoring, analyzing, and controlling the associated physical asset, makes DT a more attractive and novel tool. This paper focuses primarily on human-focused DT applications that provide a visualized DT for a human worker. Panel (a) of Figure 10 shows how human workers are visually represented in the existing applications. Note that some research papers did not outline their visual representations in detail, and thus are not considered in this section. Panel (a) of Figure 10 shows that whole body avatars are the most popular form of visualization, where various body parts such as the head, hands, arms, and legs are represented [8,14,15,17,24,27,30,31,39,40,41,42]. This form of visualization provides the most realistic representation of the human body and the most impressive visual experiences. However, specialized software tools and libraries are needed to create a whole body avatar and to emulate human workers' motions. Skeletons are a form of visualization that represent the human body by using major joints and by drawing connecting lines between these joints [4,25,26,32,33,34,35,38]. The structure of the skeleton is suitable for visualizing the positions of joints and joint angles between body segments, which are often used in observational methods to calculate ergonomic or safety indices. A point cloud is a set of points indicating the locations occupied by physical assets within a 3D space, and is typically created using depth cameras. Since a point cloud can reflect the actual 3D shape of a human worker, it is suitable for accurately calculating the safety distance between a human worker and a robot. A point cloud can be converted into a full body avatar or skeleton by applying object detection or instance segmentation algorithms. However, several applications have provided visual representations in the form of the point cloud itself [28,29,37]. A symbol is another form of visualization, where the whole body of a human worker is represented as a simple figure. For instance, [PERSON] et al. [13] and [PERSON] et al. [36] indicated the existence and location of human workers within workplaces using dots or simple icons. To provide a visual representation for human workers, modern visualization tools that can process or reflect the collected data are required. Unfortunately, detailed descriptions of these visualization tools are missing in some previous research papers. However, from the papers with detailed descriptions, the visualization tools that provide simulated environments can be grouped into three categories: custom applications, simulation modeling tools, and HW vendor-provided tools. A custom application is an original visualization tool developed by the researchers. Some researchers applied Unity [4,29,30,31] or the Robot Operating System (ROS) [37,40] to develop custom applications. Unity is a game engine widely used to develop 3D games, enabling the convenient creation of a 3D environment and game characters. If a game character can emulate the motions of a human worker, the environment created by applying Unity can be used as a visualization tool for human-focused DT applications for OSH. ROS is an open-source framework for developing applications associated with robots, and it enables the management of a complex network of distributed sensors, devices, and systems. Thus, ROS can help build a visualization environment for DT applications, typically consisting of hardware and software modules. Of note, some custom applications have not been described in detail in associated publications [13,32,38] but, as shown in panel (b) of Figure 10, about half of the existing applications are equipped with custom applications. Commercial software for simulation modeling and experiments of human motion has been gaining more and more attention recently.