## 1 Introduction  
Ever since virtual reality (VR) and augmented reality (AR) technologies became feasible options, there has been great interest in using these technologies to represent, restore and reconstruct cultural heritage [1]. These technologies provide us with an interesting and attractive channel through which it is possible to educate a wide audience about historic artefacts or sites [2]; or equally, to assist audiences in visiting sites that are difficult, if not impossible, to access in real life. Inserting information into a virtual environment has created entertaining and interactive experiences for users [3,4], and consequently, these technologies are becoming increasingly common, given their appeal to users and the endless possibilities on offer. Logically, the field of Architectural Heritage Conservation and Restoration could benefit immensely from immersive VR apps offering realistic interactive images; in fact, such apps are already being used by many museums and tourist attractions. For example, [PERSON], [PERSON] and [PERSON] [5] have used CAVE(r) to create virtual tours of historic sites, and more recently, museums have begun to offer immersive virtual to help familiarise visitors with sites and scenes from the past, using headsets (HMD) such as HTC Vive [6]. The recent advances in VR and AR technologies now enable these technologies to be used on smartphones or tablets, a fact that has provided cultural heritage organisations with a wealth of new opportunities for attracting and engaging with users [7]. The creation of interactive experiences has provided a more efficient and effective way to enrich creative processes, learning, collaboration and also training in the education sector [8]. In fact, the personalised interactive experiences offered by VR help different stakeholders (conservator-restorers, tourists, teachers, museums, etc.) create and enjoy visual products that embody scientific excellence. In the professional world, VR technologies have proven apt for cultural heritage preservation and have become a powerful tool for professional conservator-restorers, archaeologists and the suchlike. Such professionals, having completed the necessary groundwork and research, can use VR to restore elements found to be in a state of ruin or disrepair [9]. As such, VR can be used to assist in conserving, reconstructing, documenting, researching and promoting cultural heritage. Different methods can be used to obtain 3D models [10], and graphic engines for videogames can be used to add factors to virtual environments, such as interactivity, visualisation and realism [11]. If one of the important challenges faced in the architectural conservation of historic buildings and archaeology is establishing how to correctly interpret structural and architectural features, so too is establishing how to reconstruct murals correctly. As mural paintings and frescos are extremely delicate and highly susceptible to wear and tear, this task is labour intensive, and requires expensive technical expertise [12]. Deterioration mechanisms include: unstable ambient conditions; structural problems affecting the building that can lead to delamination and plaster stratigraphy, which cause artworks to lose their artistic qualities; dampness; saline efflorescence; biodeterioration; and poor repairs that do not respect the building's antiquity or its original materials. All of these mechanisms can impact the quality of such artworks by causing them to be partially obscured, i.e., whitewash, or completely destroyed. Prior to any intervention, researchers must possess a thorough knowledge of the piece in question in order to ensure that conservator-restorers have of all the necessary techniques and instruments needed to recover as much information as possible for any damaged or missing items.