## 3 Analysis of the Collaboration in AR Context
### Conceptual Framework to Define Collaboration
In summary, by defining the rules that shape a CM, it is possible to control the flow of an application without modifying its basic logic. An important fact is that the same set of rules (i.e., the same Collaboration Model) can be applied to completely different applications. In other words, collaboration models are application independent and can be reused. Below we propose five types of high-level collaboration models to illustrate this concept. Figure 1: Collaboration Layers. _Turn-based model_: the interaction with the AR elements is managed by using a token. Only the user who has the token (therefore in her turn) can interact with the virtual objects. The interactions of other users with the AR content are in principle discarded. 2. _Ownership-based model_: in this collaboration model, each digital element has an associated owner. Thus, the visibility and interaction with each element may be restricted only to its owner. It is also possible that the elements are visible for all or a group of users with only the interaction being limited. 3. _Layer-based model_: This collaboration model is characterized by having one or many _layers_, to which the AR objects are linked to. One digital element may be present in several of those layers. Users on their side will have access to certain layers and they will be able to visualize and also interact with its associated digital elements. 4. _Hierarchy-based model_: in this case users are organized in a tree-based hierarchical structure. On one hand, the user at the top of the hierarchy has access to all AR elements, both for interaction and visualization. Changes performed by that user will always prevail over those initiated at a lower level of the hierarchy. On the other hand, users located in the lower branches of the tree will have limited both their visibility and their ability to interact. It is reasonable to assume that it will be the participants themselves who define these restrictions for their subordinates. 5. _Unconstrained model_: in this collaboration model there are no restrictions. All user interactions are processed and no control over the visibility of AR elements is established. In this case a FIFO (First In, First Out) policy will be applied. It is important to note that these five models gathers the main and common collaboration schemes we identified and that there may be more possibilities, since the rules themselves are what define each of the models. Which collaboration model to use entirely depends on the service goal of the application to be developed, i.e., SARA is not imposing any collaboration model but enabling choosing or combine the most adequate ones for the application to be developed. In Table 1 we illustrate how different collaboration models can be applied within two different applications. On the one hand, we propose the voxel-based world editor that will be described in more detail in Section 6. At this point, it is enough to indicate that multiple users need to be able to interact with a portion of land formed by cubes (voxels), so that by adding or removing those cubes they can create landscapes and buildings. The resources to be handled will be (1) the land itself, (2) the voxels (which may be added, removed or change their color) and all players will have identical permissions. On the other hand, we propose a prototype application for visualization and management of SmARt city resources (the SmARt City viewer). In its most basics, this prototype allows users (from the municipality, service providers, operators) to inspect injected data sources on a 3D representation of the terrain. The key concept of this prototype is that AR resources visualization will be based on the roles assigned to the users. Table 1 summarizes how each collaboration model may fit or not for both applications. It is also important to note that an application could also integrate different collaboration models, e.g., depending on the users' role and the interaction level.