## 6 Evaluation Experiment: Visual Effects
### Experimental Environment  
The virtual transparent objects used in the experiment are shown in Figure 9. In addition, we prepared a transparent object that has either disparity or defocus to confirm the effect of each separately. They were constructed using five types of visual effects: \(O_{1}\)--real object; \(O_{2}\)--no effect; \(O_{3}\)--defocus; \(O_{4}\)--disparity; and \(O_{5}\)--disparity and defocus. The methods used to render the virtual transparent objects and reproduce disparity and defocus are presented in Section 5. The diameter of the observer's pupil was set to 6.0 mm, and no visual-line detector was used because of the individual differences in accuracy that were observed in the preliminary experiments.  
The experimental environment is shown in Figure 12, wherein the Magic Leap One was used as the OST-HMD. The floor patterns placed beneath the transparent objects were the same as the background patterns used in Section 5. The real glass sphere had a diameter  
Figure 11: Fixation point depth data results.  
of 150 mm and a refractive index of 1.51 and had a sticker on its surface that duplicated the virtual sphere surface. To provide a light source, a light (ARRI HMI 575W G22) was placed at the local coordinates (1.0, 1.0, \(-\)1.0) [m] relative to the transparent objects in a room with no other light sources.  
Twenty-one participants (21 men, 2 women; max. age: 26, min. age: 20, avg.: 22.21, SD: 1.56), all in their twenties and having a corrected visual acuity of 1.0 or greater with no eye abnormalities, were involved in the experiment. All had prior experience of AR using OST-HMDs. Before beginning the experiment, each participant was equipped with the Magic Leap One and given an answer sheet. Then, the participants were made aware of the location at which each virtual object would appear by showing them a white dummy cube. After confirming that the participants could perceive the position correctly, two pairs \((O_{i},O_{j})\) of five representations were shown. The stimuli were presented to each participant in a different order to account for the order effect. While observing, the participants were allowed to move only their head freely within the natural range of motion of the sitting posture.  
The following procedures were used to observe and then to compare the observational results. Upon the appearance of the first object, each participant observed it for a total of 20 s by alternately gazing at the surface and refracted images under experimenter instructions. Following this, no image was displayed for 10 s, and then the second virtual transparent object was displayed in a similar manner for 20 s. The visual effects were switched as the experimenter provided instructions.  
After observing, each participant rated the respective objects, \(O_{i}\) and \(O_{j}\), in terms of how convincing they appeared to be. A scale from \(-\)3 to \(+\)3 (\(-\)3: very low, \(-\)2: low, \(-\)1: somewhat low, 0: same, +1: somewhat high, +2: high, +3: very high) was used. Each participant performed a total of 20 sets of observations--each encompassing object observation and question answering--for all combinations. Each set was followed by a 30 s interval that included the answering time. After each experiment, we asked the participants for their comments.