# X-Reality Museums: Unifying the Virtual and Real World Towards Realistic Virtual Museums
## 3 A Conceptual Architecture for Unifying XR Experiences for Realistic Virtual Museums  
Museums have existed since the third century BC, and until today they have undergone several changes to cope with the sociological, cultural and economic shifts through humankind's history [69]. Nevertheless, one fundamental attribute that has remained intact is their orientation towards education: museums principally aim to share knowledge with their audiences. Contemporary museums have embraced technology and incorporated technological artifacts related to their collections, as a means of creating delightful experiences, increasing their way-factor, entertaining visitors, but also for providing access to collections that are not physically exhibited in the museum and for giving additional information about the exhibited artifacts. Among the wide variety of technological exhibits, AR and VR solutions are increasingly becoming popular, due to the high immersion and presence they offer [70, 71], taking also into account that the hardware involved is now affordable and has achieved important progress in the delivered user experience quality. This is an important accomplishment and constitutes a milestone for further evolving XR experiences in the museum towards becoming unique and unified. Unified XR experiences in realistic virtual museums focus on engaging museum visitors in an interactive and immersive blend of physical and virtual, as if it was a single unified \"world\" [72]. Following this concept, interaction with the XR environment and its agents will be achieved naturally, as when interacting with real world artifacts and counterparts. Embodied virtual agents will interact with museum visitors in order to provide instructions and transfer knowledge in a more direct manner (e.g., historical personalities sharing their stories, infamous artwork becoming \"alive\"). This realistic interplay will introduce passive museum visitors to active partakers, thus endorsing their feeling of presence in the XR environment and achieving better transfer of knowledge and higher enjoyment. From a technical perspective, unified XR experience in realistic virtual museums, requires a distributed Service oriented Architecture (SoA) that will interweave the different technologies in a flexible and scalable manner and promote reusability, interoperability, and loose coupling among its components. Figure 2 illustrates a conceptual model incorporating the fundamental components that such approaches should comprise. Overall, the conceptual model involves two main component categories: (i) elements that directly affect user interaction and are responsible for delivering the XR experience (green area in Figure 2), and (ii) components pertaining to processes unseen by the end user and which are responsible for interpreting user interactions (marked as \"true mediated reality\" in Figure 2--yellow area). In particular, the diminished reality component undertakes the task of removing, in real-time, physical elements (e.g., a museum exhibit) that will be replaced with their virtual counterparts in the user's view. In order to achieve this, several processes have to run. Scene registration and localization processes will identify the user's location in the physical environment and the objects in their field of view. This is an ongoing procedure during the interaction, as the user's location may be modified anytime. Then, physical objects are perceptually removed, by substituting them with the appropriate background.