# Effect of Target Size, Location, and Input Method on Interaction in Immersive Virtual Reality
## 3 Method
### Apparatus and Application  
For the VR experiment, a smartphone-based virtual reality head-mounted display (HMD) was used. The smartphone was a Samsung Galaxy S7 (SM-G930K), and the VR headset that holds the smartphone was a Samsung Gear VR (SM-R322) (Figure 1). The liquid crystal display of the smartphone was used as the HMD screen. The VR effect was realized by providing stereoscopic images. The resolution was 2560 \(\times\) 1440, and the maximum viewing angle through the HMD was 96\({}^{\circ}\) on a diagonal basis, which was sufficient for the experiment. The total weight was 470 g (smartphone: 152 g, headset: 318 g), similar to other virtual reality headsets, and the duration of one type of experiment was within 3 min. Therefore, the effect of motion sickness due to the device's weight is expected to be small.  
The applications used to conduct the experiments were created and implemented directly with Unity and C#. In VR, the visual size varies depending on how far away things are spaced. Therefore, when implementing the target, the target size was determined using a viewing angle, which is a unit that takes into account both the absolute size of the target and the absolute distance from the subject's eye to the target, rather than using only the absolute size of the target. Based on the results of [PERSON] et al. (2018), the viewing angle of the target was determined (Figure 2). The distance and the size of the target set here were fixed, so it did not change over the participants. Referring to the experimental design of other studies that analyzed the effect of the target characteristics ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018), a total of 25 square targets, spaced at regular intervals in a 5 \(\times\) 5 array, was placed on a white plane with a viewing angle of about 127\({}^{\circ}\)19\({}^{\prime}\) on a diagonal basis. Therefore, the experiment was performed within a range similar to the mid-peripheral vision of a human, which is not less than the viewing angle provided by the VR HMD. The size of the target was set at two levels. The viewing angle was set at about 3\({}^{\circ}\)50\({}^{\prime}\) for large targets and about 2\({}^{\circ}\)23\({}^{\prime}\) for small targets. Figure 3 shows an example of the experimental area.  
Figure 1: Smartphone-based virtual reality device.  
The experimental conditions comprise a total of four according to the target size and target selection method. The size of the target was two levels, as described above, and the target selection method is also divided into two. Target selection methods included gaze selection and manual selection. Both used a cursor fixed at the center of the screen, reflecting head movement through head tracking, placing the cursor on the point to be selected. In this case, the gaze-selection method was automatically selected after 1000 ms when placing the cursor on the target. This is based on a previous study in which the highest task performance was obtained when the gaze-timing was 1000 ms [57]. For  
Figure 3: Example of the experimental area. In (**a**) left panel: large target size; In (**a**) right panel: small target size; In (**b**): a screen that a subject actually sees with VR HMD.  
Figure 2: The process of converting the length of the target into an angle.  
reference, in the previous study, considering that the average waiting time used for the top 5 applications based on the number of downloads from the Android Play Store was 1.52 (\(\pm\)0.60) seconds, the waiting time was set to 1 s, 1.5 s, and 2 s. However, the manual-selection method was used after placing the cursor on the target and pressing the touchpad on the right side of the HMD (Figure 4).