# Effect of Target Size, Location, and Input Method on Interaction in Immersive Virtual Reality
## 1 Introduction  
Recently, as communications and display technologies have evolved, a basis for commercialization has been established, and virtual reality (VR) has emerged as a next-generation platform along with augmented reality and mixed reality. Research on virtual reality is largely divided into two categories: studies to attenuate motion sickness and to analyze input device task performance. Currently, there are many factors obstructing the popularization of VR. One of the main problems is VR sickness [1]. In order for VR-related markets to continue to develop in the future, an environment in which VR can be experienced for a long time without motion sickness must be prepared. Hardware-based research has primarily focused on reducing the gap between motion and visual information [2, 3]. The inconsistency of visual information is the main reason for the occurrence of VR sickness. When a user turns his or her head and senses a change of position through a sensory organ, sickness may occur if the VR motion does not correspond to natural sensations [4, 5, 6]. This difference is referred to as motion-to-photon (MTP) latency. The hardware market has an aim of reducing this phenomenon to less than a 20-ms latency [7, 8, 9]. In addition, studies have been conducted to reduce this MTP latency using machine learning algorithms [10] or IMU sensors [9]. In terms of software, motion sickness has been effectively reduced by adjusting focus [11, 12, 13], viewing angle [13], and visual effect or guide [14, 15, 16, 17]. These studies suggest that motion sickness levels can be adjusted not only by reducing MTP latency but also by the interaction method and interfaceconfiguration of virtual reality. In addition, studies have been conducted mainly to measure VR sickness levels after watching videos [18] or experiencing a first-person roller coaster ride [19], that is, in an environment where there was little interaction by participants. Therefore, it is necessary to study the level of VR sickness when the virtual reality and the user actively interact, not in such a passive environment. Conventional platforms, such as PC, web, and mobile, have been studied thus far, and users have adapted to most interfaces and unique features of each platform. However, because VR is still in the early stages of popularization, related studies have not been carried out sufficiently, and standard input methods have not been set for current commercial products. Therefore, interaction research such as VR input tools and selection methods needs to be carried out for immersive VR. Since the user's head movement is used for interaction of VR, the research analyzing the performance for each input device in virtual reality is mainly focused on head tracking or helmet tracking. The head/helmet tracking method is a method of visually indicating how close the cursor is to a target by calculating the head direction angle at every moment [20] and is also applied to the head-mounted display (HMD) of virtual reality. [PERSON] and [PERSON] [21] compared the time it takes to press a target when using a joystick with when using an infrared light mounted on the helmet to measure the position of the head. Additionally, [PERSON] [22] compared the task completion time of the same task with four methods (mouse, eye-tracking, head-tracking, and mixture of eye-tracking and head-tracking). However, while these studies used head-tracking, most of them are the non-immersive virtual reality that controls the cursors on a computer monitor screen. That is, studies conducted on immersive virtual reality environments are still insufficient. In addition, these studies tend to focus on a front-facing field-of-view (FOV) [23; 24], and little has been carried out to analyze the differences in task performance according to the characteristics of the target. Unlike other displays (e.g., monitor or touchscreen), where the available space is limited to a single screen, in VR, objects can be placed anywhere where the user turns his or her head in any direction, similar to the real world.