## 3 Analysis of the Collaboration in AR Context  
### Definition of Collaboration in AR Context and Its Main Characteristics  
A possible meaning for \"collaborate\" is \"to work jointly with others or together\" [31]. Thus, from a high point of view, several users in a group may do some actions to achieve a certain goal. However, in addition to those common goals, it may be possible that each user has personal goals. Hence, _competition_ can be seen as a sub-case of the collaboration. In this Section, we aim at analyzing the concept of collaboration in AR context to define a set of features or characteristics that have to be managed in an AR-oriented collaborative architecture. Three are the main characteristics of the collaboration:  
**Number of participants in the collaboration**: First and most obvious, to perform a collaboration multiple users are required. Thus, the number of these participants may affect the design of collaborative AR systems. **Individual vs. common service goals**: Depending on the application, there may be one or several objectives common to all users. However, it is also possible that each user has one or more personal objectives, which may not coincide with the common ones. As an example, let us suppose a multiplayer game based on AR. Players in a team may be willing to win, while each player individually may be targeting to improve his personal score. Another example could be a collaborative BIM-based (Building Information Modeling) tool in the architecture context. Several participants may work together to design a building, being each user focused on different components (e.g., one may arrange the electricity system while another may place the piping system). In general, each of them can work independently in their own copy of the building. However, in specific occasions both may have to work together and even reach agreements. **Operational features to take into account when implementing collaboration**: to achieve the collaboration goal, some actions have to be performed by the users. Thus, there may be interaction with the AR content as well as communication between the participants. The implementation of courses of action in collaborative AR requires to determine aspects such as time and space management,interaction methods, visualization and, all in all, the collaboration model used to orchestrate the collaboration. we following comment on all these issues:  
* _Real time vs. shift collaboration_: Time management restrictions are important when defining collaborative AR applications. It is possible for example that all the users are working with the AR content at the same time, which could be seen in _real time_. However, it is also possible that they work _in shifts_ and that they do not even coincide in time. In this case, the state in which a user left the digital information must be able to be retrieved by a second user in order to continue working. * _Shared physical space vs. remote_: If all participants of the AR collaboration experiences are located in the same physical space, it is usually said that they are co-located and they are working \"locally\". On the contrary, if the participants do not share the same space, it is usually said that they work \"remetely\". It is also possible to have a mixed situation in which some users are co-located and they are working remotely with another one. * _Interaction methods_: According to the target AR platform, interaction with the virtual content is implemented in a different manner. For example, in hand-held devices such as smartphones, the main interaction method is touch-based. However, in case AR Head Mounted Displays (HMDs), usually the interaction is carried out by hand gestures and/or voice commands. Thus, the type of interaction will condition the design of the means to implement the course of action. * _Information exchange channel_: in order to collaborate, exchange of information between the participants is required. This can be done by visual means, including annotations or check lists, speech or combined strategies.