The Handheld Virtual Panel (HVP) is the virtual panel attached to the non-dominant hand’s controller in virtual reality (VR). The HVP is the go-to technique for enabling menus and toolboxes in VR devices. In this paper, we investigate target acquisition performance for the HVP as a function of four factors: target width, target distance, the direction of approach with respect to gravity, and the angle of approach. Our results show that all four factors have significant effects on user performance. Based on the results, we propose guidelines towards the ergonomic and performant design of the HVP interfaces. With the increasing popularity of consumer virtual reality (VR), we see more and more VR apps for creativity and productivity. These apps fundamentally require menus and toolboxes for the assortment of options and controls they offer. And the interaction artifact that is quickly becoming the go-to technique for this is the handheld virtual panel (HVP) . The HVP provides the primary toolbox in Google's TiltBrush [15] (Figure 1 (left)) and Blocks [14] , Oculus's Quill [11] and Medium [10] (Figure 1 (right)), and HTC Vive's MakeVR [18] . Szalvari et al. in 1997 [30, 31] proposed the personal interaction panel where the user hold a tracked tablet in the second hand while doing their primary interaction with the dominant hand using a stylus. HVPs extend that concept for virtual panels anchored to the controller in the non-dominant hand and using ray-tracing instead of a stylus. There are multiple advantages to such an interaction [20] . First, handheld windows move along with the user, so they are always within reach. Second, they do not overly clutter the user's view, unless explicitly moved by the user. Third, handheld windows take advantage of the proprioceptive sense because they are attached to the non-dominant hand. However, even with the ubiquity of HVP in products and research literature, we do not have a sense of what factors govern performance of target selection in HVPs. Consequently, there is Unpublished working draft. Not for distribution. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from permissions@acm.org. a need to understand and quantify HVP target selection performance while considering these two factors: 1) hand motion here is governed by the direction of motion in relation to the ground due to the effects of gravity, and (2) since both the target and the pointer can be moved and controlled by the user during acquisition, user's approach will vary depending on the angle of movement in addition to distance and width. We conduct a study to measure HVP target acquisition performance in relation to four factors that relate to the direction of movement with respect to gravity, the angle of movement with respect to the body, distance, and width. The results show that the performance depends significantly on all four factors. Based on the results, we propose guidelines towards the ergonomic design of the HVP interfaces. The results suggest that gravity played a major part even when our experiment design minimized fatigue between conditions. The effect would be much more pronounced with longer, fatigue-inducing tasks. Most current HVPs use a cube-style panel with equal vertical and horizontal sizes. One simple solution to minimize the effect of gravity would be to have HVPs that have larger horizontal widths than vertical. Our distance-based results suggest that minimizing hand motion and instead relying on wrist flicks to move the raycast pointer could help performance (see [26, 27] ). Therefore, as opposed to having smaller panels, panel sizes can be increased (at least horizontally) to encourage the use of coarse wrist flicking. Further, the design needs to minimize motion when the user is performing tasks below the panel (for instance, creating a ground texture) and will need to go against gravity to reach the HVP. One solution here would be arrange targets on the panel such that the high frequency targets are placed at the bottom of the panel, thus making them easier to reach from the bottom, while not overtly affecting the performance from top. Another possibility is to retarget the HVP [2] at a lower position while the non-dominant hand remains at the same position so that the user has to move less against gravity to reach the HVP. Retargeting has not been explored in the context of HVPs and could be a really useful technique to counter such effects. However, the tradeoff of increasing the visuohaptic disconnect in this case would need to be explored. Overall, we suggest three takeaways that should be considered by designers for HVPs depending on the context: 1) Panels with large horizontal widths as opposed to square shaped ones should be considered to counter effects of gravity and encourage wrist flicking, 2) Place high-frequency targets at the bottom of the panel, and 3) investigate retargeting of the HVP given the same non-dominant hand positions to minimize user motion against gravity. The handheld virtual panel is the most popular technique for accessing tools or menus in commercial VR creativity and productivity applications. In this paper, we conduct an evaluation of the target acquisition performance in the HVP as a measure of four variables. Our results show that all four have an effect on user performance. While there are expected effects such as reducing acquisition time with increasing width, the evaluation also suggests that gravity may be a crucial issue even when fatigue is minimized. Based on the results, we list takeaways to help improve the design of HVPs and indicate paths for future explorations. We believe addressing the limitations of HVPs uncovered in our study will go a long way in improving the user experience of HVP-based VR applications.
The paper investigates target acquisition for handheld virtual panels in VR and shows that target width, distance, direction of approach with respect to gravity, and angle of approach, all impact user performance.
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