## 3 Materials and Methods
### Technological and Research Areas
#### 3.2.5 Mixed Reality and Space  
MR is the synthesis of a world consisting of real and virtual objects that coexist and interact in real time. As Figure 2 below illustrates, MR occupies the middle ground between the real and the virtual environments as it combines elements from both and encompasses the concepts of augmented reality and augmented virtuality [63].  
Therefore, MR involves representation of digitally generated imagery together with images captured from real life both rendered on a visual display medium. This could be a computer display, a camera or specialized equipment, such as the popular Microsoft Hololens product [64]. Particular importance in terms of final cost and appeal to the public, for a venture like the proposed one, has the final imaging medium, and in the present context, the end-user smart device, which may be the mobile phone itself.  
In order to implement a hybrid representation, the two worlds, the virtual and the real, and their recognized objects, must have a spatial reference to a geographical coordinate system. For this reason, both virtual modeling and real-world imaging in the context of this proposal are based on geoinformatics technologies. These include primarily GIS-based functionalities that support overlaying of multiple vector and raster thematic layers and georeferencing of any type of involved object.  
A particularly critical feature that significantly fosters the functionality of MR applications is the interaction between virtual and real objects [65]. However, this interaction may only be implemented  
Figure 2: Simplified representation of a reality–virtuality continuum.  
as far as the real-world objects are recognized by the MR system in the same way with the virtual ones. In this case, the real objects possess both physical and digital properties and may be termed as mixed objects [66]. Thereby, it will be possible to apply interactivity conditions between virtual and mixed objects merged in a mixed (reality) environment [67]. Some intuitive cases demonstrating MR require immersive technologies and include interaction of the end user with both physical objects of the surrounding real environment and virtual objects. For example, a visitor of a site wearing a head-mounted display is touching a physical or a virtual object and this action enables unfolding of informative digital content. In addition, the physical interaction between the visitor and a virtual object leads to similar results to those occurring on the visitor's interaction with physical objects. Alternatively, as demonstrated later in the present work, the same content unfolds on the screens of the visitors' smart devices when they tap on the mixed or virtual object. The visitor also may dynamically interrupt and alter the behavior of virtual objects. Another example is occlusion [68] where the end user interface \"realizes\" that a moving virtual object is--at a given time in a given user's location--behind a real object (or vice-versa) and renders appropriately the mixed scene by hiding the virtual object (or part of it) behind the real object. Although this capability is not a _sine qua non_ for MR applications [69], it nevertheless represents an interaction between mixed (real) and virtual objects, an invaluable feature for many MR environments, as is the case with \"Mergin' Mode.\"