## 2 Literature Review  
Knowledge, understanding and appreciation of cultural heritage is the keystone that supports a society's commitment towards conserving heritage and introducing cultural policies that guarantee its survival [27]. In this sense, the development of new technologies has presented the field of Historic and Artistic Cultural Heritage Management and Conservation with an invaluable resource, one that allows members of the public to familiarise themselves with cultural heritage through virtual experiences that display digital reconstructions of cultural artefacts [28; 29; 30; 31; 32]. Such models improve people's understanding of damaged or missing artefacts and assets via the graphic transposition of the remains of conserved materials and related documentation sources. These virtual reconstructions appeal greatly to modern societies that are becoming increasingly accustomed to virtual reality, and that demand and consume digital experiences [33]. The technical simplicity involved in creating digital reconstructions nowadays means that there is a risk of a tidal wave of pseudo-cultural products that lack scientific rigor and historical accuracy. Such products are capable of generating long lasting misconceptions given just how powerful they are when it comes to disseminating information. For this reason, it is important to establish solid foundations that regulate the production of this type of material and that establish guidelines for generating models truly capable of respecting the history of the cultural assets and materials being represented in the virtual environment [34]. The possibilities offered by 3D modelling tools and virtual reality have brought us closer to a world of information in 3D, which has enabled technological companies, independent professionals and university research teams to explore new developments and conduct ground-breaking research. As a result of their efforts, it has been possible to identify precise methods for digitally documenting cultural heritage sites [35]. One of the most popular techniques for obtaining data and rendering 3D models involves using photogrammetry and digital restitution [10]. Traditionally this technique required specialised instruments and the expert knowledge of photogrammetry specialists. However, certain software can provide simple and straightforward solutions for what are complex photogrammetric calculations, which thereby makes drawing facades in 2D plans much easier [36]. Another common technique used to model historic sites in 3D involves performing scans with high-precision lasers [37]. This method creates extremely accurate models via an automated process [38]. The downside to this technique, however, is that it is a very expensive. Other more economical methods do exist to obtain 3D models. For example, the Microsoft Kinect sensor has been used to scan archaeological artefacts and historical sites, although the results lacked precision when compared against high-precision laser scanning [39]. There are also a large number of mobile apps on the market nowadays that create 3D models from photographs taken in the 3D space around an object. The most noteworthy of these is the ReCap(tm) Photo by Autodesk(r) [40]. The established processes used to recover and restore murals and frescos require specialist techniques and equipment that is extremely expensive, e.g., gas chromatography mass spectrometry, optical microscopy, scanning electron microscopy, infrared spectroscopy, microchemical analysis and X-ray diffraction [41]. When it comes to recovering pictorial art, the digital imaging tool DStretch(r) has proven to be an excellent alternative to such expensive techniques and methods [42]. Although originally designed for rock art researchers, DStretch(r) has also been used to study faded murals [43], frescos [44] and even images obscured by smut and prime [45]. In yet another demonstration of its potential, it has also been used to recover information about polychrome on antique ceramic [46].