This paper presents an approach to alleviating the server-client communication bottleneck in collocated multi-user virtual reality (VR). Instead of transmitting unique packets to each client, the system multicasts the same packets to all users, combining packetization, visibility-aware prioritization, and lightweight acknowledgments to ensure efficiency and fairness. To enable rapid scene completion, the environment is partitioned into independent fixed-size packets that can be decoded immediately upon arrival, and transmission order is guided by precomputed visibility footprints. Scalability is achieved through repeated multicast until compact bitmap acknowledgments confirm reception, keeping communication cost mostly independent of the number of clients. A controlled study with 23 participants in groups of 7 and 16 showed that this method reconstructs environments several times faster and with fewer missing parts than a conventional unicast TCP-based approach. These findings demonstrate that multicast transmission over commodity Wi-Fi can support large collocated VR groups.

Redirection in virtual reality (VR) enhances haptic feedback versatility by relaxing the need for precise alignment between virtual and physical objects. In mixed reality (MR), where users see the real world and their own hands, haptic redirection enables a physical interaction with virtual objects but poses greater challenges due to altering real-world perception. This paper investigates the effect of the realism of the user's surroundings and of the user's hand on haptic redirection. The user's familiarity with their actual physical surroundings and their actual hand could make the redirection manipulations easier-or harder-to detect. In a user study (N = 30) participants saw either a virtual environment or their actual physical surroundings, and saw their hand rendered either with a generic 3D model or with a live 2D video sprite of their actual hand. The study used a two-alternative forced choice (2AFC) design asking participants to detect hand redirections that bridged physical to virtual offsets of varying magnitudes. The results show that participants were not more sensitive to 2D video sprite hand redirection than to VR hand redirection, which supports the use of haptic redirection in MR.

Abstract An encountered-type haptic device (ETHD) moves a physical object to align it with the virtual object with which the user of a virtual reality application makes contact, providing haptic feedback. One limitation of ETHDs is their limited reachability due to mechanical constraints. One approach to extending the reachability of an ETHD involves redirection, achieved by altering the pose of the user’s body, hand, or handheld prop in the virtual world. While previous studies have quantified the detection thresholds of redirection in the context of stationary objects, ETHD’s present the opportunity and the need to study redirection in the context of dynamic objects. This paper presents a user study $(\mathrm{N}=25)$ with two experiments aimed at investigating whether dynamic object properties, such as direction and speed, significantly affect redirection detection thresholds. The first experiment finds that the mere presence of dynamic objects does not decrease the detection threshold. Consequently, previously measured detection thresholds remain applicable for ETHD reachability extension. However, the second experiment unveils a crucial relationship between the speed of dynamic objects and the original reachability of the ETHD. Although the virtual object can move $15 \mathrm{~cm} / \mathrm{s}$ faster than the ETHD, this increase in speed is insufficient to compensate for the extended reachability enabled by the detection threshold.

VR headsets have limited rendering capability, which limits the size and detail of the virtual environment (VE) that can be used in VR applications. One solution is cloud VR, where the "thin" VR clients are assisted by a server. This paper describes Cio VR, a cloud VR system that provides fast loading times, as needed to let users see and interact with the VE quickly at session startup or after teleportation. The server reduces the original VE to a compact representation through near-far partitioning. The server renders the far region to an environment map which it sends to the client together with the near region geometry, from which the client renders quality frames locally, with low latency. The near region starts out small and grows progressively, with strict visual continuity, minimizing startup time. The low-latency and fast-startup advantages of CloVR have been validated in a user study where groups of 8 participants wearing all-in-one VR headsets (Quest 2's) were supported by a laptop server to run a collaborative VR application with a 25 million triangle VE.

An important application of augmented reality (AR) is the design of interfaces that reveal parts of the real world to which the user does not have line of sight. The design space for such interfaces is vast, with many options for integrating the visualization of the occluded parts of the scene into the user's main view. This paper compares four AR interfaces for disocclusion: X-ray, Cutaway, Picture-in-picture, and Multiperspective. The interfaces are compared in a within-subjects study (N = 33) over four tasks: counting dynamic spheres, pointing to the direction of an occluded person, finding the closest object to a given object, and finding pairs of matching numbers. The results show that Cutaway leads to poor performance in tasks where the user needs to see both the occluder and the occludee; that Picture-in-picture and Multiperspective have a visualization comprehensiveness advantage over Cutaway and X-ray, but a disadvantage in terms of directional guidance; that X-ray has a task completion time disadvantage due to the visualization complexity; and that participants gave Cutaway and Picture-in-picture high, and Multiperspective and X-ray low usability scores.

This paper proposes a general handheld stick haptic redirection method that allows the user to experience complex shapes with haptic feedback through both tapping and extended contact, such as in contour tracing. As the user extends the stick to make contact with a virtual object, the contact point with the virtual object and the targeted contact point with the physical object are continually updated, and the virtual stick is redirected to synchronize the virtual and real contacts. Redirection is applied either just to the virtual stick, or to both the virtual stick and hand. A user study (N = 26) confirms the effectiveness of the proposed redirection method. A first experiment following a two-interval forced-choice design reveals that the offset detection thresholds are [-15cm, +15cm]. A second experiment asks participants to guess the shape of an invisible virtual object by tapping it and by tracing its contour with the handheld stick, using a real world disk as a source of passive haptic feedback. The experiment reveals that using our haptic redirection method participants can identify the invisible object with 78% accuracy.

This paper investigates providing grounded passive haptic feedback to a user of a VR application through a handheld stick with which the user taps virtual objects. Such an investigation benefits VR applications beyond those where the stick interaction is actually an integral part of the narrative. Providing passive haptic feedback through a handheld stick as opposed to directly through the user’s hand has the potential for more believable and more frequent feedback opportunities. The stick is likely to dull the user’s haptics perception and proprioception, potentially avoiding a haptics perception uncanny valley and increasing the redirection detection thresholds. Two haptics redirection methods are proposed: the DriftingHand method, which alters the position of the user’s virtual hand, and the Vari-Stick method, which alters the length of the virtual stick. Detection thresholds were measured in a user study (N = 60) by testing the two methods for a range of offsets between the virtual and the real object, for multiple stick lengths, and multiple distances from the user to the real object. Overall, the study reveals that VariStick and DriftingHand provide an undetectable range of offsets of [-20cm, +13cm] and [-11cm, +11cm], respectively.