## 6 Evaluation Experiment: Visual Effects
### Results and Discussion  
The results of the experiment are shown in Figure 13. We mapped the comparison results to a unified scale using [PERSON]'s paired comparison method and performed [PERSON]'s test. In the figure, the horizontal axis indicates a psychological scale of the extent to which the transparent virtual object appeared to be real. The yardsticks are \(Y(0.05)=0.2061,Y(0.01)=0.2466\). The following preference differential significance results were obtained for the respective pairs of transparent objects: ( \(|O_{2}-O_{3}|=0.3609>Y(0.01)\), \(|O_{3}-O_{4}|=0.5348>Y(0.01)\), \(|O_{4}-O_{5}|=0.7217>Y(0.01)\), \(|O_{5}-O_{1}|=1.9217>Y(0.01)\) ). Thus, the objects were found convincing in the following descending order: \(O_{1}\), \(O_{5}\), \(O_{4}\), \(O_{3}\), and \(O_{2}\). This in turn suggests that  
Figure 12: Experimental environment.  
a virtual transparent object with either disparity or defocus will be more convincing than one without either visual effect;
2. disparity contributes more than defocus to improving the realism of a virtual transparent object;
3. a virtual transparent object with both disparity and defocus will be more convincing than one with any other combination of characteristics; and
4. even if the proposed method is applied, the realism of a virtual transparent object will not closely approach that of the corresponding real object.  
We obtained comments from all participants. Nineteen participants agreed that the transparent object with both disparity and defocus appeared to be the most convinsing. Two answered that the effects of switching objects were visually uncomfortable. One answered that the pseudo-visual effect appeared to differ slightly from the actual visual effect. One answered that they provided one response without evidence because of an inability to perceive any difference.  
These results confirm that the visual effects obtained using the proposed method improve the realism of virtual transparent objects. Additionally, the observation that disparity is more effective than defocus is in accordance with the research of [PERSON] et al. [21], which reports that disparity in terms of vergence is most effective in representing transparent objects convincingly.  
As noted above, there were some comments indicating that the effects applied by our method are not visually comfortable. Although the specific causes of these comments need to be further explored, there are some possible explanations. First, when users changed focus from one gaze image to another, the switching effect was conspicuous. In the experiments, the gaze image was determined by the experimenter because the participant visual-line detector was insufficiently accurate. This caused unnatural delays in the display of the visual effects that could have induced participant discomfort. Second, the patterns and parallaxes of the refracted images were not accurate. To ensure real-time performance, our method renders only the first refraction based on the assumption that human vision does not accurately recognize the exact pattern of a refracted image. To the best of our knowledge, the actual effect of refracted image accuracy for the realism of rendered transparent objects has not been investigated, and therefore we cannot discard the possibility that some participants felt discomfort with the accuracy of the refracted images. Nevertheless, we can conclude that the use of the proposed method to reproduce visual effects improves the realism of virtual transparent objects despite the inability of the OST-HMD to display at optically correct depths. Further investigation will be necessary to determine the effective range and conditions of our approach.