In virtual reality (VR), users typically control one virtual body — their avatar. Previous works enable users to act through multiple avatars simultaneously. These works, though, lack a systematic account of why users want to interact with multiple avatars and do not explain how users can manipulate and generate avatars. To address this, we run six workshops with 12 VR experts and develop a design space that captures four fundamental dimensions for acting through multiple avatars in VR: Appearance , Context , User-Avatar Mapping , and Motion Control . Researchers can use the design space to generate novel interaction opportunities involving multiple avatars or analyze existing work. We then run a usability study with 17 participants to understand the practicalities of an interface that integrates parts of the design space, which reveals conceptual and technical challenges that we address through design recommendations. • Our design space explains how users can interact through multiple avatars in VR. • Users want multiple avatars to stand in for themselves, other persons, or things. • Users use multiple avatars to perceive and act in multiple contexts simultaneously. • Challenges with multiple avatars include clutter, syncing, and switching between them.

Body ownership illusions (BOIs) occur when participants experience that their actual body is replaced by a body shown in virtual reality (VR). Based on a systematic review of the cumulative evidence on BOIs from 111 research articles published in 2010 to 2021, this article summarizes the findings of empirical studies of BOIs. Following the PRISMA guidelines, the review points to diverse experimental practices for inducing and measuring body ownership. The two major components of embodiment measurement, body ownership and agency, are examined. The embodiment of virtual avatars generally leads to modest body ownership and slightly higher agency. We also find that BOI research lacks statistical power and standardization across tasks, measurement instruments, and analysis approaches. Furthermore, the reviewed studies showed a lack of clarity in fundamental terminology, constructs, and theoretical underpinnings. These issues restrict scientific advances on the major components of BOIs, and together impede scientific rigor and theory-building.

In virtual reality (VR), interactions may fail when users encounter new, unknown, or unexpected objects. We propose using feedforward in VR to help users interact with objects by revealing how such objects work. Feedforward lets users know what to do and how to do it by showing the available actions and outcomes before an interaction. In this article, we first chart the design space of feedforward in VR and illustrate how to design feedforward for specific VR interactions. We discuss starting the feedforward, previewing actions and outcomes, and returning the virtual world to its state before the feedforward. Second, we implement three real-world VR applications to show how feedforward can be applied to multistep interactions, perceived interactivity, and discoverability. Third, we conduct an evaluation of the design space with 14 VR experts to understand its usefulness. Finally, we summarize the findings of our work on VR feedforward in 15 guidelines.

Virtual reality (VR) is increasingly used in complex social and physical settings outside of the lab. However, not much is known about how these settings influence use, nor how to design for them. We analyse 233 YouTube videos of VR Fails to: (1) understand when breakdowns occur, and (2) reveal how the seams between VR use and the social and physical setting emerge. The videos show a variety of fails, including users flailing, colliding with surroundings, and hitting spectators. They also suggest causes of the fails, including fear, sensorimotor mismatches, and spectator participation. We use the videos as inspiration to generate design ideas. For example, we discuss more flexible boundaries between the real and virtual world, ways of involving spectators, and interaction designs to help overcome fear. Based on the findings, we further discuss the ‘moment of breakdown’ as an opportunity for designing engaging and enhanced VR experiences.

Tool use extends people's representations of the immediately actionable space around them. Physical tools thereby become integrated in people's body schemas. We introduce a measure for tool extension in HCI by using a visual-tactile interference paradigm. In this paradigm, an index of tool extension is given by response time differences between crossmodally congruent and incongruent stimuli; tactile on the hand and visual on the tool. We use this measure to examine if and how findings on tool extension apply to interaction with computer-based tools. Our first experiment shows that touchpad and mouse both provide tool extension over a baseline condition without a tool. A second experiment shows a higher degree of tool extension for a realistic avatar hand compared to an abstract pointer for interaction in virtual reality. In sum, our measure can detect tool extension with computer-based tools and differentiate interfaces by their degree of extension.

Human-computer interaction is replete with ways of talking about qualities of interaction or interfaces, including if they are expressive, rich, fluid, or playful. An example of such a quality is subtle. While this word is frequently used in the literature, we lack a coherent account of what it means to be subtle, how to achieve subtleness in an interface, and what theoretical backing subtleness has. To create such an account, we analyze a sample of 55 publications that use the word subtle. We describe the variants of subtle interaction in the literature, including claimed benefits, empirical approaches, and ethical considerations. Not only does this create a basis for thinking about subtleness as a quality of interaction, it also works to show how to solidify varieties of quality in HCI. We conclude by outlining some open empirical and conceptual questions about subtleness.