# Natural Virtual Reality User Interface to Define Assembly Sequences for Digital Human Models
## 2 Materials and Methods
### Technical System
#### 2.2.2 User-System-Interaction Module  
The module contains the functions that enable users to navigate themselves in the VE as well as select and manipulate 3D models. By walking around, users can navigate freely through the VE. The Leap Motion sensor transfers the real hand into the VE. Figure 3 shows that the Leap Motion sensor is an optical tracking sensor based on infrared light (IR). It contains LED lights and a stereoscopic camera, that captures the hand and finger movements. It has a 120\({}^{\circ}\) field of view on the long side and 150\({}^{\circ}\) on the short side of the sensor (Figure 3). It is capable of tracking hands with a maximum distance of 80 cm, but the higher the distance, the less accurate the tracking gets [26]. Users do not have to look at their hands due to the vast field of view. Still, their hands need to be in the tracking volume of the leap motion.  
We implemented a simple virtual hand [18], which means that the hand position, in reality, is the same as in VR. According to RBIs, we address EAS, since users can pick up, place, alter, and arrange objects by grasping them with the virtual representation of their hands. In NP, we address the topics: persistence of objects, velocity, and relative scale in the VE. Through the virtual representation of the users' hands, we achieve body awareness in the VE. A higher NP would be achievable by using haptic data gloves. However, as our approach addresses a use case in which users quickly want to use the system, we decided not to use wearable devices.  
The system continually measures the distances between the fingers and palm to the surface of the virtual objects (Figure 4).  
Figure 3: Hardware (left) and the field of view (right) of the Leap Motion Sensor (pictures modified from [27]).  
Figure 2: HTC Vive with a Leap Motion [24].  
We implemented a second-hand grasping metaphor. There is one leading hand and one following hand. As soon as the leading hand or one finger gets in contact with a virtual object, this part of the hand stays on the surface of the object. If users close their hands even more, a second hand is displayed. This hand represents the position of the real hand. Additionally, the user gets a visual feedback on how far away their fingers are from the surface.  
This feedback is provided if the distance between the finger and the surface object is more than 1.5 cm (yellow) or less than 1.5 cm (green) [28]. The virtual representation of the fingers changes color, depending on their distance to the object. After a grip is detected, the users get a user interface with three different grip types. These grip types are the most likely ones of the grip type recognition system (Section 2.2.3). The users select a grip type by moving their hand to the left, to choose the first grip type, to the front to select the second grip type, and to the right to choose the third grip type. After the grasping phase, the object is attached to the hand, and users can move the object freely in the VE. To release the object, the users have to open their hands.