# Natural Virtual Reality User Interface to Define Assembly Sequences for Digital Human Models
## 1 Introduction
### Natural User Interfaces  
Natural user interfaces are interfaces which enable users to interact with the system the same way they do in a physical world [15]. [PERSON] [15] states that \"the ideal user interface would let us perform our tasks without being aware of the interface as the intermediary\" (p. 50). Reality-based interfaces (RBI) are interfaces, which try to mimic the real world and allow direct manipulation. [PERSON] [16] states that \"Direct Manipulation moved interfaces closer to real-world interaction by allowing users to directly manipulate objects rather than instructing the computer to do so by typing commands.\" Therefore, RBI has four different themes: naive physics, body awareness and skills, environment awareness and skills, and social awareness and skills [16].  
* Naive physics (NP) describes simple physical behavior like gravity, friction, speed, and adhesion, as well as scaling.
* Body awareness and skills (BAS) addresses the point that users have a body independent of the environment.  
* Environment awareness and skills (EAS) describe the fact that humans have a physical presence in a 3D environment, which includes objects and landscapes. They increase the spacial orientation.
* Social awareness and skills (SAS) describes that humans are aware of the presence of other humans that can interact with other people socially. These interactions include verbal and non-verbal communication.  
Since not every user interface can address each theme most realistically, designers should \"give up reality only explicitly and only in return for other desired qualities\" [16]. [PERSON] [17] describe approaches in which \"the user's hand becomes the input device--it is not necessary to hold another device in the hand.\" They propose a system, which enables users to navigate through a menu to define actions to manipulate the virtual environment. Furthermore, they introduce a use case that demonstrates how to use a virtual keyboard for text input, as well as a two-handed virtual-navigation technique.  
Human system interaction in Virtual Reality enables users to use a remarkable tool, the human hand. \"It allows us to manipulate physical objects quickly and precisely\" [18] (p. 252). The parameter of quick interaction influences the usability dimension of efficiency [19]. Therefore, it is one focused factor in the interaction development of VR user interfaces. Another influencing parameter for efficient dialogues is learnability [20].\"The interactive system should allow the user to perform the task with minimal learning\" [20] (p. 13). Natural user interfaces, therefore, have the advantage that, if they are used to operate software dialogs, they enable users to learn the application with no effort.  
In the use case of an assembly task, workers use their hands and fingers to grab and manipulate parts to assemble a product. Transferring these actions into VR would enable a more intuitive use of DHMs. Therefore, we propose a system with an interaction method, which eliminates the mouse and keyboard. The system allows users to rely on virtual representations of their hands in VR to instruct the DHM IMMA. This, in turn, eliminates a human system interaction to define an assembly sequence with the DHM and replaces mouse and keyboard. Instead, it tracks the hands, behavior, and interactions of users in VR. The tracking of human behavior and movements in the context of assembly is a well investigated topic. [PERSON] et al. [21], e.g., postulate an on-body sensor network to capture user and workspace characterization of industrial workflows.