# A Subjective Study on User Perception Aspects in Virtual Reality
## 4 Subjective Results and Discussions
### Impact of Rendering Mode  
In this subsection, we present a comparison between the non-VR and the VR modes in terms of acceptability and perceptual quality. Because most existing VR streaming platforms (e.g., YouTube and Facebook) are providing video versions at different resolutions, we also give suggestions about which mode should be used for different resolutions.  
Statistical results about the effects of rendering mode on the acceptability and the perceptual quality are shown in Table 8. We can see that there are \"small\" effects of the rendering mode on the perceptual quality and the acceptability (i.e., \(p<0.05\), \(0.06>\eta^{2}>0.01\)). Figures 13 and 14 respectively show the perceptual quality scores and the acceptability rates versus the resolutions, for both the non-VR and VR modes with the QP value of 22. It can be seen that both the perceptual quality scores and the acceptability rates in the non-VR mode are always higher than those in the VR mode. This is because video versions in the VR mode are zoomed up via lenses in HMD. In addition, we can see that, the lower the resolution is, the larger the difference between the two modes becomes.  
Figure 11: Comparison of presence and perceptual quality scores between two different device sets (QP = 22). (a) Content #1; (b) Content #2; (c) Content #3.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Aspect** & **Factor** & \(\chi^{2}\) & \(p\)**-Value** & \(\eta^{2}\) \\ \hline _Acceptability_ & & 86.75 & **<0.0001** & 0.040 \\ _Perceptual quality_ & & 124.03 & **<0.0001** & 0.057 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Statistical results about the effects of rendering mode on QoE aspects based on Kruskal-Wallis test. The bold numbers in column \(p\)-value represent to statistically significant effects of factors. The bold, italic, and underlined numbers in column \(\eta^{2}\) respectively correspond to “large”, “moderate”, and “small” effect sizes.  
Figure 12: Responses to Q6-Q7 for two different device sets.  
Figure 15 shows the number of viewers in percent who prefers the VR mode to the non-VR mode with the QP of 22 and device set D1. It is interesting that, at the resolutions of fHD or higher, more than a half of the viewers prefer the VR mode to the non-VR mode, though the perceptual quality scores and the acceptability rates in the VR mode are lower in the non-VR mode. It can be because that the viewers are excited when watching in the VR mode. At the HD resolution, less than 33% of the viewers prefer the VR mode to the non-VR mode. Therefore, instead of the VR mode, the non-VR mode should be used for watching 360 video versions encoded at the HD resolution. In practice, most current 360 video services such as YouTube and Facebook still provide 360 videos in the HD resolution.  
Also, we can see that, similar to the behavior of the presence scores shown in Section 4.3.1, the number of the viewers who favor the VR mode to the non-VR mode is the highest for Content #2, and lowest for Content #1. This suggests that the higher presence score in the VR mode may result in the higher number of the viewers preferring the VR mode. In addition, the above results show that the higher perceptual quality and acceptability rate do not necessarily mean the higher preference of users. This implies that the perceptual quality and the acceptability rate are only valid in a rendering mode, and so cannot be used to compare different rendering modes.