## 1 Introduction
The traditional method, called Highlight-RTI (H-RTI), consists of manually moving a hand-held strobe at different heights to produce highlights on a black sphere placed near the subject [17,18]. In recent years, Dome RTI (D-RTI) has been introduced, in which data are collected using a dome at the center of which the object is located and then lit by automatically switching different light bulbs on and off [17,19, 20, 21]. Lately, also robotic arm-based systems have been presented in order to acquire images of large surfaces [17,22]. A significant advancement of RTI application in the field of cultural heritage relies on its integration with other imaging techniques, leading to the development of new approaches. Some examples are High Dynamic Range RTI, Multi-Spectral RTI, Fluorescence RTI, and Focus Variation RTI [17]. In all of the above-mentioned approaches, RTI is combined with other techniques to enrich the results that can be obtained. However, they have not overcome RTI's limitations in the acquisition process, such as the requirement for a controlled environment for data acquisition, or the need to work with small to medium-sized objects. In this context, the integration of advanced imaging techniques continues to evolve, enhancing the researcher's ability to study cultural heritage artifacts in innovative ways. One such cutting-edge approach is the application of Virtual RTI (V-RTI) for the study and investigation of historical artifacts [23,24]. Indeed, this technique can bridge the gap between 2D and 3D data outputs and expand the possibilities of traditional photographic documentation [24], offering a versatile and dynamic multi-dimensional method for surface data digitization. V-RTI is based on the combination of RTI with Structure from Motion (SfM), which reconstructs three-dimensional information from overlapping 2D images [25; 26; 27], providing valuable insights into an object's geometry and surface. Integrating these techniques within a single framework enables researchers to capture, visualize, and document the morphology of an artifact within particularly confined contexts, in which traditional RTI cannot be applied. The process of collecting RTI data is heavily reliant on the location of the object under study, since the need to place the light source at a significant distance (equaling three or four times the object diameter) to ensure a uniform illumination. This condition is not always feasible, especially in cases when objects are wider than 1 m or placed within obstructed or confined locations. This circumstance often occurs in close proximity to archaeological sites, where artifacts may be occluded by environmental elements, or in museum galleries, storages, and archives, where large, fixed objects are confined by their built environment. This poses a significant challenge limiting the feasibility of the technique and therefore its availability to heritage professionals in the study of artifacts. V-RTI can help overcome some of these limitations imposed by the acquisition requirement of traditional RTI, due to its non-invasive nature, versatility, and cost-effectiveness, and since it can be deployed in many contexts. In a recent study, a hybrid approach for the creation of V-RTI adopting photogrammetry is proposed as an alternative to the current standard H-RTI method, including the rendering of the textured model in order to produce a V-RTI that can be easily navigated using a free viewer, like RTI Viewer [24]. This technique can be performed using both conventional (Digital Single Lens Reflex or mirrorless) and mobile phone cameras, establishing itself as an accessible and lightweight research tool for archaeologists, conservation scientists, and non-specialist users. Therefore, the use of V-RTI proves to be a successful approach to achieve correct illumination for large or inaccessible subjects for which a raking light would have been necessary, but not applicable in practice [24]. While V-RTI using SfM has shown to be a promising technique, it is not yet widely adopted as a common practice.