## 1 Introduction  
VR and AR techniques can render a virtual environment or superimpose a virtual object onto a physical target; thus, the interaction experience with VR/AR is different from conventional displays that include monitors, tablets, and projectors. VR/AR devices are used for sports training, entertainment, tourism, manufacturing, warehouse work, and medical assistance [1, 2, 3]. They have the potential to change how we learn, recreate, and work [4, 5]. The modes of communication between humans and VR/AR systems have a large impact on their implementation, use, and acceptance. The human interface requirements for VR/AR devices are different from other human-computer systems; the visual demands are greater, the command set is different, and the user may be sitting, standing, or walking. Therefore, researchers are designing tracking, interaction, and display techniques to improve comfort, and efficiency particularly [6], then applying those techniques to the implementation of AR systems. Integrating a human factor approach to the design of gestures with gesture recognition that optimizes latency and accuracy is vital to facilitating effective interactions between humans and VR/AR systems [7, 8, 9, 10, 11]. Users are familiar with touch gestures and voice control due to their pervasive use of smart devices and these input methods have been evaluated by users interacting with VR/AR systems. FaceTouch (FT), a touch-input interface mounted on the backside of a head-mounted display (HMD) [12] was accepted by participants because of its low error rate, short selection time, and high usability. However,it could be only used for short utilitarian purposes because users' arms tend to fatigue easily. The use of pre-programmed voice commands to manipulate objects in VR demonstrated the usefulness of voice control as a practical interface between users and systems [13; 14]. However, the use of voice commands can reveal user actions and disturb others in public. Office workers are accustomed to using a keyboard and mouse. A virtual keyboard to execute commands has been evaluated in an immersive virtual environment by office workers [15; 16]; however, the accurate detection of fingertips and the provision of feedback to users while typing on the virtual keyboard were difficult and limited usability and interaction efficiency. Another common user interface approach when interacting with VR head-mounted displays (HMDs) is the hand-held controller, similar to a game controller, which provides accurate input and feedback while minimizing latency (Controller, HTC VIVE, HTC Inc., New Taipei City, Taiwan, 1997). However, the visual demands to identify buttons on a controller can distract the users and negatively impact their performance. Additionally, the number of buttons that can reasonably fit onto a controller is limited which constrains input efficiency. Furthermore, the use of hand controllers has been shown to increase motion sickness during VR HMD use [17] and the sustained grasp of a hand-held controller may increase the muscle load on the upper extremities. Interaction with VR or AR systems that rely on extra hardware like a controller or touch screen prevents users from performing other tasks with their hands such as manipulating physical objects. Due to these constraints, mid-air, non-contacting, and 3D hand gestures have emerged as an alternative to a controller, touch screen, or voice control while interacting with high-resolution displays, computers, and robots [18; 19; 20; 21]. Importantly, the human proprioception system allows users to perceive the spatial position of the hands relative to the trunk with high precision. The use of VR/AR HMDs is becoming widespread and user experience with HMDs is different from conventional smart devices such as phones, tablets, and TV. In response, the design and use of hand gestures for interacting with VR/AR applications have been investigated [22; 23; 24].