In a co-located collaborative virtual environment, multiple users share the same physical tracked space and the same virtual workspace. When the virtual workspace is larger than the real workspace, navigation interaction techniques must be deployed to let the users explore the entire virtual environment. When a user navigates in the virtual space while remaining static in the real space, his/her position in the physical workspace and in the virtual workspace are no longer the same. Thus, in the context where each user is immersed in the virtual environment with a Head-Mounted-Display, a user can still perceive where his/her collaborators are in the virtual environment but not where they are in real world. In this paper, we propose and compare three methods to warn users about the position of collaborators in the shared physical workspace to ensure a proper cohabitation and safety of the collaborators. The first one is based on a virtual grid shaped as a cylinder, the second one is based on a ghost representation of the user and the last one displays the physical safe-navigation space on the floor of the virtual environment. We conducted a user-study with two users wearing a Head-Mounted-Display in the context of a collaborative FirstPerson-Shooter game. Our three methods were compared with a condition where the physical tracked space was separated into two zones, one per user, to evaluate the impact of each condition on safety, displacement freedom and global satisfaction of users. Results suggest that the ghost avatar and the cylinder grid can be good alternatives to the separation of the tracked space. CCS CONCEPTS• Human-centered computing → Virtual reality; Collaborative interaction; User studies; KEYWORDSVirtual Reality, Collaborative Virtual Environment Permission to make digital or hard copies of part or all 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 third-party components of this work must be honored. For all other uses, contact the owner/author(s). VRST '17, November 8-10, 2017, Gothenburg, Sweden [Fleury et al. 2010] that results from the virtual navigation of both users in the VE. When these two users wear a HMD, they can still perceive each other in the VE but no longer in the real workspace. In that case, we must avoid any possible physical collision between the two users.

In this paper we propose D3PART (Dynamic 3D Plastic And Redistribuable Technology), a model to handle redistribution for 3D user interfaces. Redistribution consists in changing the components distribution of an interactive system across different dimensions such as platform, display and user. We extend previous plasticity models with redistribution capabilities, which lets developers create applications where 3D content and interaction tasks can be automatically redistributed across the different dimensions at runtime.

Plasticity of 3D user interfaces refers to their capabilities to automatically fit to a set of hardware and environmental constraints. This area of research has already been deeply explored in the domain of traditional 2D user interfaces. Besides, during the last decade, interest in 3D user interfaces has grown. Designers find with 3D user interfaces new ways to promote and interact with data, such as e-commerce websites, scientific data visualization, etc. Because of the wide variety of Virtual Reality (VR) and Augmented Reality (AR) applications in terms of hardware, data and target users, there is a real interest in solutions for automatic adaption in order to improve the user experience in any context while reducing the development costs. An adaptation is performed in reaction to different criteria defining a system such as the targeted hardware platform, the user's context and the structure and the semantic of the manipulated data. This adaptation can then impact the system in different ways, especially content presentation, interaction techniques modifications and eventually the current distribution of the system across a set of available devices. We present the state of the art about plastic 3D user interfaces. Moreover, we present well known methods in the field of 2D user interfaces that could become relevant for 3D user interfaces. With this survey, we show that current solutions do not meet all plasticity requirements. That is why we propose an action plan to meet these requirements.

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