# A Subjective Study on User Perception Aspects in Virtual Reality
## 3 Study Description and Experiment
### Subjective Experiment  
For the subjective experiment, we use the three videos as mentioned in Section 3.1.1. The duration of each video is 30 s. All the three videos are in Equirectangular projection format [34]. To generate 20 different versions of each video, we use H.264/AVC encoder (libx264) and 20 combinations of QP values and resolutions as mentioned in Section 3.1.2. Note that, the frame rate of all the versions is fixed at 30 fps. Totally, 60 versions are generated from the three videos. It is worth noting that H.264/AVC is still most widely used on the Internet, and the following discussions on the impacts of encoding parameters are specific to this format. In our experiment, we use two rendering device sets, denoted D1 and D2, to display video versions. In particular, device set D1 consists of a Samsung Galaxy S6 phone and a Samsung Gear VR HMD. The Samsung Galaxy S6 has the display size of 5.1 inches and the screen resolution of 1440 \(\times\) 2560 [35]. The Samsung Gear VR has a FoV of 96 degrees [36] covering 80% of human FoV [37]. Device set D2 consists of a Samsung Galaxy S5 phone and a Google Cardboard HMD. The Samsung Galaxy S5 has the display size of 5.1 inches and the screen resolution of 1080 \(\times\) 1920 [35]. The Google Cardboard has a FoV of 90 degrees [38], which is able to cover 75% of human FoV. Table 4 shows the questions employed in our experiment. Questions Q1 and Q2 are respectively used to measure the perceptual quality and presence. The acceptability is evaluated in Question Q3. Question Q4 is a comparative question between the non-VR and VR modes. The degree of cybersickness is measured using Question Q5. Questions Q6 and Q7 are to investigate the effects of wearing HMD and FoV on user experience. Regarding the test processing, viewers are firstly trained to get familiar with the rating procedure and the rendering devices before doing actual subjective tests. Then, with each viewer, the display order of the three videos is randomly determined. For each video, each of the twenty versions is randomly chosen, and then displayed two times in the non-VR and VR modes. In particular, each viewer watches the chosen version in the non-VR mode in the first display time, and then gives answers to questions Q1 and Q3. Then, the viewer watches the same version in the VR mode in the second display time, and then gives answers to questions Q1-Q4. Note that this choice of experiment design is to identify \begin{table}
\begin{tabular}{c p{341.4 pt}} \hline \hline
**Question** & **Description** \\ \hline \multirow{2}{*}{Q1} & How is your assessment about the perceptual quality of the video on the scale from 1 to 5? \\  & (1 means very bad, and 5 means excellent). \\ \hline \multirow{2}{*}{Q2} & How is your assessment about the sense of presence in VR environment on the scale from 1 to 5? \\  & (1 means that you have absolutely no sense of presence, and 5 means that you have a true sense of presence as in a real environment). \\ \hline \multirow{2}{*}{Q3} & Is this viewing acceptable to you? (1 means that you accept and are willing to watch until the end of the session, and 0 means that you do not accept, feel annoying, and want to quit the session). \\ \hline \multirow{2}{*}{Q4} & Which do you prefer, non-VR rendering mode or VR rendering mode? (0 is non-VR and 1 is VR). \\ \hline \multirow{2}{*}{Q5} & How is the level of dizziness or nausea during VR viewing experiment on the scale from 1 to 5? \\  & (1 means very dizzy, and 5 means not dizzy at all). \\ \hline \multirow{2}{*}{Q6} & How much does wearing a VR head-mounted display affect your experience in VR environment on the scale from 1 to 5? (1 means very cubersome and annoying, 5 is absolutely no problem). \\ \hline \multirow{2}{*}{Q7} & How much does the FoV of the device affect your sense of presence in VR environment on the scale from 1 to 5? (1 means very limited, 5 is absolutely no problem). \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Questionnaire and Rating Scale. a viewer's preference between the VR and non-VR modes.