## 2 State of the Art
offered a system for off-site visualization of an archaeological excavation. They combined Head Mounted Displays (HMD), a tracked hand-held device, a high resolution display and a multi-touch projected table surface to enable the collaboration. From a high-level point of view, the collaboration was thought as the navigation, manipulation and browsing of digital data. Thus, each user had their own representation of the digital content and they did not share the visualization component. In particular, this system did not enable multiple users to manipulate the same digital element. [PERSON] et al. [13] proposed the use of Augmented Reality to support collaboration between different organization in crisis management scenarios. In this case, AR content sharing between several users is explored, but adapting the visualization to each participant. The authors faced the process of sharing common organization-specific information on a map. The proposed solution consisted of decoupling the data (e.g., the position of a car on the map) from its representation. Hence, the associated AR component for a piece of information was generated based on the organization of the participant. In this system, each client device was notified when another user made any change in its own virtual representation (e.g., if a vehicle's position was changed), and the client internal AR representation was updated. Regarding the interaction, it was performed by means of a physical device (a joystick). Thus, only the user with that device was able to interact with the AR content. On the other hand, the DARCC system (Distributed AR for Collaborative Construction simulation) from [PERSON] et al. [14] offered a system for operating and locate digital construction elements (in particular cranes) in a collaborative manner using AR. The collaboration in this case was performed within the construction site, with all the participants visualizing the AR content. In particular, the digital content was aligned by combining GPS and device sensors to convey the idea of the assets being located in the physical world. Although the AR content was shared among all users, their interaction was limited to control a single associated element (the crane). Since the content alignment was done on GPS, the use of this system was limited to outdoor contexts. Apart from these works, there are many other examples of collaborative systems that can be divided according to their area of application: _Industrial_[15; 16], _Architecture and Construction_[14; 17], _Education and training_[18; 19] and _Entertaining_[20; 21]. An exhaustive review of collaborative systems based on AR and MR to date has been recently proposed by [PERSON] et al. [22]. The creation of frameworks and architectures to facilitate the development of new cross-platform applications was already raised previously in the literature, as previously stated. Some of these works focused on wearable devices [23] or smartwatches [24], infrastructure devices such as the Kinect [25] or the generation of adaptive web applications [26] and user interfaces [27]. Following the same approach of facilitating the development of cross-platform application but focusing on the Augmented Reality Context, [PERSON] et al. [28] developed the XD-AR framework, created for addressing the issues found when developing AR applications in a multi-platform context. More specifically, they focused on the more recent devices and frameworks used in that moment (e.g., Tango, ARCore and ARKit). In industry context, Microsoft has presented their _Spatial Andors_ system [29], which is a cross-platform developer service to deliver multi-user Mixed Reality experiences. It enables storing and afterwards retrieving the position of the users' devices and their relation with the real world. Microsoft has also recently launched their _Spectator View_ system [30]. Spectator View allows streaming the MRdigital content from a HoloLens device (in the form of _holograms_) to be shared with other users who are carrying 3D hand held devices.