# Parallels of Head-Up Displays and Visual Safety for Driving
## 3 Results  
### Horizontal Parallax  
We evaluated two forms of change in horizontal parallax: the overall parallax uniform change and the adjacent position parallax step change. Two conditions were assessed, and the visual effect presented was different. Overall parallax changes cause a change in the overall depth perception of the image. Stepped parallax changes are depth perceptions that show a step change in the horizontal direction, with the center of the image having the farthest depth perception distance and the two sides becoming progressively closer. In the subjective assessment of visual fatigue, we used the sum of the individual scores of the VAS-F and VFS as the total score for fatigue assessment, with higher scores indicating greater fatigue. Figures 9 and 10 show the results of the visual fatigue analysis of the subjective questionnaire. Comparative analyses of the total scores did not show any significant difference between the uniform parallax change and the stepped parallax  
Figure 8: Schematic diagram of the experimental scenario. change conditions (\(p>0.05\)), but the scores did show a decreasing and then increasing trend as the parallax value increased. Figure 9a shows the trends in the VAS-F assessment scores, while Figure 9b displays the trends in the VFS assessment scores; both had the smallest fatigue score when the overall parallax was 3.31 mrad. Figure 10a shows the trends in the VAS-F assessment scores, and Figure 10b presents the trends in the VFS assessment scores; both had the smallest fatigue score when the stepped parallax was 0.85 mrad. This result suggests that, based on the VAS-F and VFS scores, the highest fatigue was achieved at a lower parallax. The lower parallax is better than the no parallax condition, which indicates that the horizontal parallax under certain conditions can improve the perception of visual comfort, but it does not show a significant difference. We analyzed the D2 performance task under horizontal overall uniform parallax and stepped parallax conditions. The results of the D2 task for the overall parallax variation are shown in Figure 11: (a) presents the trends in TN-E on the D2 performance task, and (b) shows the trends in E on the D2 performance task. The results of the D2 task in the horizontal overall parallax condition did not show significant differences, and the overall scores were more consistent, with only a more pronounced downward trend when the parallax was <20.97 mrad. Figure 10: Trends in VAS-F and VFS questionnaire scores in stepped parallax conditions: (**a**) trend in VAS-F assessment scores had the smallest fatigue score when the stepped parallax was 0.85 mrad; (**b**) trend in VFS assessment scores had the smallest fatigue score when the stepped parallax was 0.85 mrad. Figure 9: Trends in VAS-F and VFS questionnaire scores under overall parallax conditions: (**a**) trend in VAS-F assessment scores had the smallest fatigue score when the overall parallax was 3.31 mrad; (**b**) trend in VFS assessment scores had the smallest fatigue score when the overall parallax was 3.31 mrad. Analyses of stepped horizontal parallax D2 task performance scores using the Kruskal-Wallis test statistic showed that the different parallax samples showed significant differences for TN, E1, E2, TN-E, CP, and FR; the data are presented in Table 3.  
Figures 12 and 13 present a further analysis of the variability in the summary of changes in parallax by step. The analysis of the variability indicated that there were significant differences in the TN-E, CP, E1, and E2 assessment metrics between the 4.05 mrad and 4.65 mrad stepped parallax conditions and conditions with parallax values less than these. Moreover, a tendency for decreased task performance was shown. This result indicates that the two step conditions had a significant effect on the visual task. The condition with a stepped parallax value of 3.31 mrad did not present a significant difference from the condition with a smaller parallax value, and the results remained consistent.