# A Subjective Study on User Perception Aspects in Virtual Reality
## 1 Introduction  
Thanks to cost decreases and usability increases of virtual reality (VR) devices, 360-degree video (or 360 video for short) has gradually been gaining popularity on streaming platforms such as YouTube and Facebook in recent years. Unlike traditional video, 360 video is capable of providing a 360-degree view of a scene at the same time, and so immersive viewing experience to users. However, 360 videorequires much higher network bandwidth than traditional video [1; 2; 3]. To provide excellent user experience, 360 video should have high quality and high resolution, resulting in a bulky size. Therefore, for effective delivery of 360 video, it is crucial to obtain in-depth understandings of Quality of Experience (QoE) as well as QoE-affecting factors of 360 video. For traditional video, two key QoE aspects are perceptual quality and acceptability, which have been considered in many previous studies [4; 5; 6; 7]. Hence, in this study, we also investigate the two QoE aspects in the context of 360 video. Perceptual quality refers to the degree of user satisfaction of video quality being displayed on rendering devices. Acceptability indicates whether a service or application is acceptable to users. In most previous studies, to obtain the acceptability of a video, viewers in subject tests are asked to give a binary score, i.e., whether a video is acceptable or not [5; 8; 9]. Then, the acceptability rate which is determined as the ratio of the number of viewers accepting a video to the total number of viewers is used as an indicator to represent the acceptability [5; 8; 9]. In the literature, it is well-known that the perceptual quality and the acceptability of a traditional video may be significantly affected by video compression and transmission [4; 10], which could cause video distortions such as blocking and blurring artifacts. In case of 360 video, other QoE aspects such as cybersickness and presence should be considered as well. Presence refers to the sense of \"being there\" in the VR environment with interactions like in the real environment [11]. Cybersickness is the feeling of dizziness or nausea while watching a video. It is expected that users feel more present in a VR environment compared to watching traditional videos. This is one of main different points of user experience between watching 360 videos and traditional videos. Therefore, the presence aspect is also studied in this paper. In a subjective experiment conducted in [12], a large percentage of viewers reported on cybersickness. So, viewers in our experiment are asked to assess their cybersickness as well. In the future, our research will be extended with some other QoE aspects such as enjoyment and usability. So far, existing studies on QoE aspects as well as QoE-affecting factors of 360 video are still very limited [12; 13; 14; 15; 16; 17]. Most previous work focuses on the perceptual quality aspect [15; 16; 17]. There are some studies on the presence aspect [13; 14] and the cybersickness aspect [12; 16]. These studies will be discussed in detail in the next section. So far, no previous study has investigated the acceptability aspect of QoE of 360 video. Regarding QoE-affecting factors, the impact of rendering mode on QoE aspects has not been evaluated in previous studies. Also, the impacts of other factors such as content motion, encoding parameters, and rendering device have not been fully understood. To fill this gap, our study focuses on measuring four QoE aspects of 360 video, namely perceptual quality, presence, cybersickness, and acceptability. In addition, we conduct a subjective experiment to investigate four QoE-affecting factors of encoding parameters, content motion, rendering device, and rendering mode. To the best of our knowledge, this work is the first to cover a large number of factors and QoE aspects of 360 video. The preliminary result of this work has been presented in [18]. In this paper, we evaluate more factors. In particular, the impact of the content motion factor is additionally investigated in detail.