# Off-Cloud Anchor Sharing Framework for Multi-User and Multi-Platform Mixed Reality Applications
## 2 State of the Art  
The development of XR applications is a topic that has been explored for years [8]. However, enabling the same XR application to be executed seamlessly by multiple users and on devices with different characteristics remains a significant challenge [9; 10]. On one hand, designing and implementing applications in which several users can interact collaboratively in real-time is a challenging task [26]. In relation to UX, especially in XR environments, one of the most important aspects in multi-user applications is the communications speed among the participating devices [27; 28]. This is required to be fast and fluid enough so that the user does not perceive it and has a pleasant, seamless and uninterrupted experience. This is a topic covered by [29], where an edge-assisted framework was designed for multi-user mobile AR environments, enhancing real-time collaboration among users. On the other hand, when it comes to multi-platform environments, multi-user cooperation and real-time interaction among users is considered to be very relevant. Several works emphasize the need for cross-platform compatibility to enable broader user participation in XR environments [30; 31; 32]. For example, in [30], the authors introduce a system for cross-platform immersive visualization, using sensors and image markers for real-time collaboration within smart buildings. Similarly, in [31], the authors explored privacy-preserving Application Programming Interfaces (APIs) for XR applications to improve interoperability and security between devices, demonstrating the importance of flexible, platform-independent solutions for multi-user interactions. In [32], a cross-platform VR system designed for real-time construction safety training is proposed. Such a system ensures accessibility across devices without requiring specialized VR hardware or software, by simplifying the development of VR training scenarios and improving user engagement. Privative or cloud-dependent solutions can derive into discontinuity or loss of functionality due to service deprecations or occasional system failures. For example, as it was previously mentioned, Azure Spatial Anchors (Microsoft's technology for creating shared experiences and exchanging MR anchors between users) were announced to be -deprecated [18], from November 20 th 2024. Situations like these result in a lack of documentation or tools for developers and highlight the need for independent solutions that ensure continuity, adaptability and control over anchor exchange processes. This dependency has also been detected by other authors [33], which advocate for self-contained systems that do not depend on third parties. Specifically, in [33], the authors describe a blockchain-assisted framework for secure data sharing in MR applications within military contexts. The proposed decentralized approach leverages blockchain and edge computing to secure multi-user data exchanges without relying on third-party services. In addition to the academic works mentioned above, Hubs Foundation [34], a commercial application based on WebXR technology, has also been found. This solution is aimed at hosting virtual rooms for meetings or social gatherings where other users can join, similar to the popular VRChat application [35]. This project was originally known as Mozilla Hubs, which ceased support and passed its maintenance to the Hubs Foundation in mid-2024 [36]. While Hubs supports shared WebXR environments and is accessible via VR headsets and browsers (e.g., Oculus Quest, Vive, Pico Neo), it is primarily focused on virtual-only spaces and does not provide native support for MR scenarios involving the integration of virtual content with the physical environment. Moreover, Hubs appears to be oriented towards social interaction in static virtual rooms rather than supporting complex application logic or spatially anchored digital content in real-world environments.