## 1 Introduction
As such, results from the multispectral analysis of visible and invisible spectrums, ultraviolet fluorescence photography, or infrared reflectography, help researchers to examine paintings and support structures, and select suitable techniques for revealing elements that are invisible to the naked eye [13, 14, 15]. Analytical techniques such as chromatography, mass spectrometry and atomic absorption, X-ray fluorescence, or electron microscopy scanning have made it possible to identify the chemical composition of mortars and polychrome paints [16, 17, 18], but it is also possible to go one step further and establish the extent to which pieces have deteriorated using techniques such as static infrared thermography, spectroscopy and finger-tapping [12, 19, 20, 21]. However, the aforementioned techniques, which are viewed as high-performance techniques in their respective fields of application, are not universally applied, nor are they considered suitable for all types of projects or professionals. The reason for this lies in the fact that equipment is expensive, and extensive specialist training is required in order to know how to use them correctly. What is more, some of these methods are considered too time consuming, e.g., using finger-tapping to detect defects in large format murals in churches [19]. This paper provides details of the process used for the virtual reconstruction of a historic building in a state of advanced deterioration. The 3D reconstruction of the building has been performed using a well-known piece of software and standard topographic surveying and photogrammetry techniques. The software used to recover the deteriorated murals is DStretch\({}^{\circledR}\), which is a piece of software that has not previously been used for this purpose. The processes involved and the use of the aforementioned tool have allowed researchers to recover pictorial elements no longer visible to the naked eye, and then insert these results into a 3D model. DStretch\({}^{\circledR}\) is a digital imaging tool designed to record and document rock art. It works by enhancing different tones using decorrelation algorithms under different colour spaces, and thus generates false-colour composites that reveal pictorial elements that are barely visible [22; 23; 24]. To date, it has proven itself capable of delivering incredible results in this field [25; 26], and researchers have successfully used the tool to enhance pictographs, making them easier to see. As a result of applying DStretch\({}^{\circledR}\) to the historic mural paintings described in this paper, it was possible to appreciate with much greater clarity the compositional aspects and brushstrokes that were virtually impossible to see upon first inspection with the naked eye. This demonstrated that modifying the red-green-blue (RGB) channels of a digital image provides a way to view the pictorial elements of extremely dilapidated murals. In the following sections, the authors describe the process for virtually restoring the St Augustine church, a UNESCO World Heritage Site located in San Cristobal de La Laguna, Tenerife, Spain. Destroyed by fire in 1964, the abandoned church fell into ruin and disrepair. As a consequence, its architectural features and mural paintings have been almost entirely lost. The authors detail the processes and techniques used to (i) obtain a realistic 3D model of the building, (ii) recover dilapidated and destroyed murals, and (iii) create an interactive virtual reality environment that uses a virtual reality headset to grant users access to the majestic structures and paintings that once adorned the ancient ecclesiastical building.