## 2 State of the Art  
In its most basic definition, Augmented Reality (AR) consists of superimposing digital content over a view of the real world. One of the most widespread interpretations today is the one from [PERSON] (2018), who specifies that AR has to meet three basic requirements: (i) to combine the real world with virtual information, (ii) to be real-time interactive and (iii) to be presented in 3D. The concept of AR is closely related to Virtual Reality (VR) and even more to Mixed Reality (MR). In contrast to AR, in which the digital information is superimposed over the real world, in the case of VR the user is immersed within the digital world. [PERSON] and [PERSON] [PERSON] and [PERSON] (2018) introduced the _Virtuality Continuum_ concept to establish a classification based on the user location: real/physical at one end and virtual at the other. The Mixed Reality term appears here as a mix between virtual content and information from the physical world, in such a way that digital content may interact with the real world. Presently, the \(AR\) term is sometimes used in substitution of MR by developers and users. For the purpose of this paper we will do this assimilation. Although we refer to the term AR we also assume that virtual content will be related to the physical world, thus being the proposals directly applicable to MR problems. The concept of collaboration in Augmented Reality context took its first steps shortly after the first AR systems Azuma (2018) emerged. One of the first approaches was the Transvision System from Rekimoto Rekimoto (2018), which allowed multiple users to share virtual content graphics disposed in a table. This system also enabled the users to interact with the content by choosing and action (e.g., selection or manipulation) to be implemented through a physical device. Two more important concepts may be highlighted from this system. First, the ownership of the object has to be transferred among users. Hence, two or more users could not work over the same digital element at the same time. This concept of blocked ownership has been adopted in many of later systems, with its associated advantages and limitations (a user must wait for another user to end-up the interaction in order to do hers). Second, each user had its own 3D model database synchronized with the other ones by propagating modifications. The Studierstube system Studierstube (1961); Studierstube (1961); Studierstube (1961) substituted that process of sharing the same digital content between all the users by the concept of information layers. These layers could be shown or hidden to the users in order to control the information presented to them. The manipulation of the digital content and the interaction with it was performed through a physical device. Once more, the same concept of a master device and multiple slaves from Rekimoto appears here, since the master stores the current state of the virtual content, which is downloaded from the slaves side. Additionally,any changes performed to the state of the virtual content on the slaves side is propagated to the central state maintained by the master. A fact that makes the system interesting is that it can be seen more as a service provider for different applications such as education [9] and gaming [10] than a single application. One of the first systems that explored the idea of combining different platforms to perform the collaboration (mobile devices and stationary computers in this case) was the MARS project [11]. In this system, authors made a distinction between two different contexts where users might be, outdoors and indoors. An indoor user could manipulate virtual annotations of real world objects while the outdoor users would see them properly placed. Subsequently, both users could swap their roles. Despite the limitations imposed by the hardware required at the time of publication, this work stands out as one of the first platforms to integrate different types of devices in collaboration. With the VITA project [12] [PERSON] et al.