# X-Reality Museums: Unifying the Virtual and Real World Towards Realistic Virtual Museums
## 2 Related Work
### Extended Reality (XR)  
According to the EU, \"technologies such as augmented (AR) and virtual reality (VR) are going to transform the ways in which people interact and share information on the internet and beyond\" [8]. Indeed, the recent proliferation of affordable VR/AR headset gear and smart devices, has enabled both technologies to surpass the Gartner hype cycle [9] peak of inflated expectations (or \"hype\"), and allow the global VR/AR market to grow at an exponential rate, backed by a rapid increase in demand and production [10]. VR technology, as an immersive medium fully occupying (mainly, but not exclusively) the human auditory and visual sensory channels is not new, its roots debatably dating back before the 1960s [11]. Affordable, consumer-grade VR headsets have been around as early as the mid-90s with the release of the Forte VFX1 (retailing at $695) in 1995 [12]. Today, over two decades later, the VR industry can be considered as relatively mature, and in its early stages of adoption [13]. The market however still has not fully responded since VR's second renaissance, arguably signaled by the acquisition of pioneering technology company Oculus by the industrial giant Facebook, in 2014 [14]. Unlike smartphones that exhibitedfast adoption, VR technology mostly depends on enterprise adoption, thus exhibiting slower rates [15]. A 2017 study of PricewaterhouseCoopers with 2216 industry respondents indicated that 7% were currently making substantial investments in VR technology, a percentage that was double-sized (15%) for their forthcoming three years investments [16]. Although the percentages are rather low compared to investments in other technologies (e.g., Internet of Things current investments were 73%, Artificial Intelligence 54%, robotics 15%), they nevertheless confirm that VR is one of the eight technologies that enterprise is interested in, and also that it exhibits an increasing adoption tendency. AR, in its own respect is an emerging technology, focused on integrating virtual (e.g., computer generated) objects into a real-world experience, aligning both real and virtual objects with each other in a complementary manner and creating the illusion that virtual images are seamlessly blended with the real world [17]. Unlike VR, where users are totally immersed in a virtual world, AR aims to combine real and virtual objects, in a way that is beneficial towards accomplishing specific tasks or missions [18]. Popular application domains include education, architecture, industry, health, marketing, and games [17; 19]. Indicative of the popularity of this technology to end-users and of its potential to disrupt the gaming technology industry is the recent 1 billion download [20] milestone reached by the immensely popular Pokemon GO AR game, which was released in the summer of 2016. Apart from the well-established domains of each respective medium, the notion of MR has been defined in the seminal work of [PERSON] and [PERSON] [21], as the merging of real and virtual worlds, in an effort to describe environments not necessarily exhibiting total immersion and complete synthesis, but generally fall in somewhere along a virtuality continuum. More recently, MR has been given the description of a \"broader\" virtual reality of an interdisciplinary nature [22], and a superset containing AR/VR. Hence, on the one hand, MR refers to blurring the boundaries of what is real and what is virtual, enhancing the user's perception of the real world with digital information. In this respect, various MR applications have been proposed in the scientific literature that are closer to the AR end of the virtuality continuum, including among others interactive paper matter [23; 24], augmented restaurant tables [25], sites' reconstruction [26], educational applications [27] and cultural heritage applications [28]. On the other hand, other MR approaches that lie closer to the VR end of the virtuality continuum, refer to the blending of real world physical objects or people into synthetic 3D worlds (augmented virtuality-AV [29]).