The ability to manipulate and physically feel virtual objects without any real object being present and without equipping the user has been a long-standing goal in virtual reality (VR). Emerging ultrasound mid-air haptics (UMH) technology could potentially address this challenge, as it enables remote tactile stimulation of unequipped users. However, to date, UMH has received limited attention in the field of haptic exploration and manipulation in virtual environments. Existing work has primarily focused on interactions requiring a single hand and thus the delivery of unimanual haptic feedback. Despite being fundamental to a large part of haptic interactions with our environments, bimanual tasks have rarely been studied in the field of UMH interaction in VR. In this paper, we propose the use of non-coplanar mid-air haptic devices for providing simultaneous tactile feedback to both hands during bimanual VR manipulation. We discuss coupling schemes and haptic rendering algorithms for providing bimanual haptic feedback in bimanual interactions with virtual environments. We then present two human participant studies, assessing the benefits of bimanual ultrasound haptic feedback in a two-handed grasping and holding task and in a shape exploration task. Results suggest that the use of multiple non-coplanar UMH devices could be an interesting approach for enriching unencumbered haptic manipulation in virtual environments.

Ultrasound mid-air haptic (UMH) devices can remotely render vibrotactile shapes on the skin of unequipped users, e.g., to draw haptic icons or render virtual object shapes. Spatio-temporal modulation (STM), the state-of-the-art UMH shape-rendering method, provides large freedom in shape design and produces the strongest possible stimuli for this technology. Yet, STM shapes are often reported to be blurry, complicating shape identification. Dynamic tactile pointers (DTP) were recently introduced as a technique to overcome this issue. By tracing a contour with an amplitude-modulated focal point, they significantly improve shape identification accuracy over STM, but at the cost of much lower stimulus intensity. Building upon this, we propose spatio-temporally-modulated Tactile Pointers (STP), a novel method for rendering clearer and sharper UMH shapes while at the same time producing strong vibrotactile sensations. We ran two human participant experiments, which show that STP shapes are perceived as significantly stronger than DTP shapes, while shape identification accuracy is significantly improved over STM and on par with that obtained with DTP. Our work has implications for effective shape rendering with UMH and provides insights that could inform future psychophysical investigation into vibrotactile shape perception in UMH.

Ultrasound mid-air haptic interfaces can display highly reconfigurable vibrotactile shapes in mid-air for human-computer interaction (HCI) applications. The choice of stimulus shape, spatial, temporal and modulation parameters yields a complex design space, yet relatively little is known about the impact of these design choices on perceived stimulus properties. We define the combination of a spatial discretization of an abstract shape and a set of rules for the temporal display order and intensity modulation of the resulting points as a sampling strategy. We developed DOLPHIN, an open-source framework to aid in designing mid-air stimuli. DOLPHIN allows the study of the impact of rendering parameters on perceived stimulus properties. This platform-agnostic framework standardizes stimulus descriptions as a step toward more replicability and easier communication in the field. It enables reproduction of stimuli between perceptual experiments and ensures stimuli used in applications correspond to those evaluated in prior perceptual studies. We validated DOLPHIN’s usability by conducting a user study assessing the impact of sampling strategy design on curvature discrimination for dynamic mid-air haptic stimuli. The Weber fractions for just-noticeable differences (JNDs) in curvature were found to range between 1 and 1.4, yet no significant effect of the number of spatial sampling points on curvature discrimination was found. This result shows a practical use-case for DOLPHIN and provides insight into rendering mid-air haptic curvature.