# Off-Cloud Anchor Sharing Framework for Multi-User and Multi-Platform Mixed Reality Applications
## 1 Introduction  
Nowadays, a variety of commercial applications that use Extended Reality (XR) technologies are being developed across industries such as healthcare [1, 2], marketing [3], entertainment [4], construction [5], or robotics [6]. Recently, the release of new XR devices, including Augmented Reality (AR), Mixed Reality (MR) and Virtual Reality (VR) technologies, expands the variety of platforms on which an application can operate. Moreover, the adoption of multi-user XR elevates the immersion of the developed applications to a higher level, greatly enhancing User Experience (UX) and enabling smooth communications among participants [7]. The development of XR applications has been the focus of numerous studies [8]. However, ensuring that a single XR application can be executed on devices with different specifications remains a significant challenge [9]. In addition, enabling multiple devices on different platforms to share a collaborative experience, especially when one of those is an XR platform, becomes even more challenging [10]. This is mainly due to the unique characteristics of each visualization technology and the development frameworks associated with each device. Fortunately, initiatives such as the OpenXR framework [11] are making significant progress towards unifying and simplifying the development process, promoting interoperability among XR platforms. Regarding multi-user and collaborative experiences, the market is still emerging and offers limited solutions. One of the reasons is the need for tracking the environment, which is fundamental in MR applications that need to position digital content accurately within the physical environment [12; 13; 14]. A key technology in MR systems for tracking the environment is the use of spatial anchors. These anchors represent a specific point in the real world that the system tracks over time, providing a stable coordinate system to ensure that virtual content remains aligned with the physical space [15]. This allows users to move freely throughout the room while keeping virtual objects in place, avoiding dragging or flickering of the 3D models [16; 17]. Moreover, spatial anchors are fundamental for creating persistent and consistent MR experiences across sessions and shared experiences among multiple devices. For example, until recently, the only option provided by Microsoft for sharing, transferring and synchronizing anchors between XR devices was the use of its proprietary tool Azure Spatial Anchors. This service is no longer available as it was announced that this Microsoft product would be deprecated and retired from the market by the end of 2024 [18]. Furthermore, to the knowledge of the authors of this article, Microsoft does not offer any official substitute with similar capabilities. This leaves developers without alternatives for a highly useful functionality in their applications, as there is no simple and straightforward approach for sharing and synchronizing spatial anchors within Microsoft's official documentation. This article, which is an extension of the preliminary work developed in [19], introduces an innovative framework for anchor sharing and for building multi-user and multi-platform XR applications. The proposed solution uses the latest standards, open-source tools and libraries to simplify development. Moreover, this article provides guidelines for future application development. Furthermore, to illustrate the potential of the proposed solution, the development and evaluation of a proof-of-concept application is provided. Such an application allows multiple users to collaborate in real-time to solve the challenges presented in a collaborative game. The application utilizes Unity [20] and Mirror [21] for building the game and for handling all the network communications among all the participating XR devices (both clients and servers).