Chapter 6. Study 5] Future Mobile Display Devices: Rollable Display
6.4. Discussion
TaskSearch/Video/Mail were 2.0/1.9/2.0 for HeightS, 1.6/1.9/1.7 for HeightM, and 1.6/1.8/1.4 for HeightT, respectively. Through these, it can be seen that a screen aspect ratio of 2.0 or larger seems to be needed only for HeightS, and an especially larger screen aspect ratio (≥ 1.8) is needed during TaskVideo over TaskSearch and TaskMail for HeightM-T.
6.4.3. Device height × Hand length effects
Height × Hand effect was significant, but most of this result can be explained by the Height effect (except HeightT × HandL). HeightT × HandL was in a different group to HeightT × HandM and HeightT × HandS, and it showed a wider preferred screen width. Thus, participants with large hands preferred the same level of screen width for HeightS and HeightM but preferred a larger screen for HeightL. The people with large hands could use a larger device than people with small hands because the people with large hands have a wider thumb range (Xiong & Muraki, 2016).
Across hand lengths and tasks, max (95th percentile) preferred screen aspect ratio ranges for HeightS were 2.0 (1.9-2.0), which was the maximum screen width of a prototype (100 mm, aspect ratio=2.0). Therefore, HeightS appears to be more than an aspect ratio of 2.0. For HeightM and HeightT, the max (95th percentile) preferred screen aspect ratio ranges were 1.98 (1.4-1.8) and 1.79 (1.5-1.7), respectively.
6.4.4. Device height effects
The preferred screen width increased with increasing device height (See Figure 6.7), whereas the preferred screen aspect ratio was the highest with HeightS, which was grouped differently from the other two levels (HeightM and HeightT were in the same group). Its 95th percentile screen aspect ratio was 2.0, which is the maximum possible value of the prototype. This size is the same as released smartphone models in landscape mode. It is expected that the participants unrolled the mock-up as wide as possible owing to the restriction of screen usage (the amount of information or input). When considering user satisfaction, portability, and design attractiveness, HeightM
seems to be the most proper size for a smart device. In the case of user satisfaction, HeightT and HeightM were in the same group (user satisfaction was the highest in HeightT), and that was the lowest in HeightS (Figure 6.8). In the case of portability, HeightM and HeightS were in the same group (portability was the highest in HeightM), and that was the lowest in HeightT (Figure 6.10).
In the case of design attractiveness, HeightM and HeightT were in the same group (design attractiveness was the highest in HeightM), and that was the lowest in HeightS (Figure 6.9).
Meanwhile, the ratio of GripNo became higher as the device height increased (ratio of GripNo for HeightT was approximately 7 times higher than that for HeightS). As the device size increases, the device becomes heavier and two-thumb reach becomes hard to cover the wide screen width.
Therefore, gripping methods such as GripLower or GripLeft, which are one-handed gripping methods, seem to be inconvenient, and participants seem to place the device on the table to use it.
6.4.5. Task type effects
TaskVideo was associated with the widest preferred screen width as well as the highest preferred screen aspect ratio. TaskVideo was the task of watching the video played on the screen without additional input. Among the advantages of the large display mentioned above, a large screen is expected to favor immersion. TaskSearch is a task of reading articles, and the advantages and disadvantages of large screens coexist. The larger the screen, the more information is provided, but the longer the line, and the lower the legibility (Duchnicky & Kolers, 1983; Sanchez & Wiley., 2009). For TaskMail, the screen width is limited by two-thumb reach. For TaskMail, 45/90 (50%) participants used GripBoth among four gripping methods (GripBoth, GripLeft, GripLower, and GripNo).
GripBoth is a dynamic grip with the fingers holding the device and performing additional tasks simultaneously. As mentioned above, unrolling is performed considering two-thumb reach for GripBoth. Therefore, it is expected that the preferred screen width is smaller for TaskVideo, which does not have this constraint. For TaskMail, grip comfort is also low, which was expected because it involves more input motion than other tasks. Compared to TaskSearch, which has both advantages and disadvantages with a large screen and TaskMail, which has limited screen width due to two- thumb reach, TaskVideo appears to have few disadvantages in terms of large screen because it does not require frequent input.
6.4.6. Hand length effects
Although the preferred screen width was wider as the hand size increased, the preferred screen aspect ratio showed no significant difference between the hand length groups. The larger the hand, the larger the screen, but the difference was not so significant as the change the aspect ratio. The larger the hand length, the longer the thumb length (Xiong & Muraki, 2016), so the thumb reach zone is wider and located at the top when using smartphones (Ahn, Kwon, Bahn, Yun, & Yu, 2016; Kim, Choe, Choi, & Park, 2017; Toh, Coenen, Howie, & Straker, 2017).
6.4.7. Gripping method
The grip posture differed according to the smartphone task. Kim et al. (2006) developed a task prediction model by measuring the grip region when performing eight tasks with a smartphone.
Lee et al. (2016) classified the hand posture for the front screen of smartphone usage into three types, and the rear interaction usage into two types based on finger position. However, there are no studies on grip posture when using a tablet PC (the sizes of a typical tablet PC in landscape and portrait modes correspond to HeightM and HeightT). Rollable display devices include the concepts of both smartphones and tablet PCs and also can be mid-sized devices, so an integrated grip posture classification that takes these concepts into account is required.
6.4.8. Limitations and future studies
This study had some limitations. First, the viewing duration of TaskVideo was 10 s. In previous studies, the viewing duration used in display evaluation was very wide, ranging from 10 seconds to four hours (Ardito et al., 1996; Bracken, 2005; Kwon and Lee, 2007; Cho et al., 2010; Fröhlich et al., 2012; Lambooij, Ijsselsteijn, and Heynderickx, 2011; Tam et al., 2011; Lambooij, Ijsselsteijn, and Heynderickx, 2011; Sakamoto et al., 2012; Yand and Chung, 2012; Hou et al., 2012; Zhang, Christou, 2014; liu, et al., 2015; Oh and Lee, 2016). Given that video watching often lasts for a long time, further research on long-term watching is needed. Second, the weights of the three prototype devices were light and different to each other, with the weights of HeightShort/Medium/Tall being 35, 70, and 105 g, respectively. The mean (SD) weight of 286 smartphone models released by the top five smartphone manufacturers worldwide is 140.5 g (37.0 g) (Study 4 (Chapter 5)), and the mean (SD) weight of 170 tablet PC models released by the top five tablet PC manufacturers worldwide is 462.5 g (11.1 g). Hence, an additional study using prototypes with heavier weights is also required. Third, the force of the spring for screen retraction was fixed at 2.5 N. The gripping method, grip regions, or grip comfort could change according to the required pulling force (Kong, Kim, Lee, & Jung, 2012; Dianat, Nedaei, & Nezami, 2015;
Study 4). Fourth, task performance was not considered. Fifth, it is necessary to investigate proper screen sizes for diverse interaction methods (e.g., pinch zoom or drawing with a stylus pen). Sixth, this study did not consider potential usages of a very wide screen (e.g., viewing very wide panoramic pictures). Seventh, only younger individuals were considered. The preferred screen size of older individuals might be different from that of younger individuals considering the aging factors (e.g., decreased visual acuity). Eighth, only Koreans were considered. Each ethnic group has distinct hand anthropometric dimensions in terms of size, proportion, shape, and weight
(Davies, Abada, Benson, Courtney, & Minto, 1980; Courtney, 1984). Therefore, it is necessary to investigate the effect of different ethnic groups on the preferred screen size of rollable display devices. Finally, the gender ratios differed across the three hand-length groups. Although the mean hand length of males is longer than that of females (Tilley, 2002), it is worthwhile to examine gender-related differences in preferred screen size, preferred screen aspect ratio, user satisfaction, grip comfort, portability, and design attractiveness for rollable display devices using two gender groups with comparable hand sizes. Despite these limitations, the fundamental findings of this study will be useful for designing ergonomic rollable display devices.