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.

Rich, informative and realistic haptic feedback is key to enhancing Virtual Reality (VR) manipulation. Tangible objects provide convincing grasping and manipulation interactions with haptic feedback of e.g., shape, mass and texture properties. But these properties are static, and cannot respond to interactions in the virtual environment. On the other hand, vibrotactile feedback provides the opportunity for delivering dynamic cues rendering many different contact properties, such as impacts, object vibrations or textures. Handheld objects or controllers in VR are usually restricted to vibrating in a monolithic fashion. In this article, we investigate how spatialiazing vibrotactile cues within handheld tangibles could enable a wider range of sensations and interactions. We conduct a set of perception studies, investigating the extent to which spatialization of vibrotactile feedback within tangible objects is possible as well as the benefits of proposed rendering schemes leveraging multiple actuators in VR. Results show that vibrotactile cues from localized actuators can be discriminated and are beneficial for certain rendering schemes.

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.