## 5 Discussion  
This article explores a relatively new approach to RTI through the application of photogrammetry to generate rendered images of three-dimensional models, namely V-RTI. V-RTI has demonstrated its usefulness in scenarios where the RTI acquisition is prevented by spatial constraints. The atypical conditions commonly encountered in museums, archaeological sites, and large-scale artworks (exceeding 1 m) often restrict the feasibility of RTI analysis, thus hindering crucial art historical, archaeological, and scientific research. The current challenge with V-RTI is the achievement of a uniform and shadow-free texture, which frequently results in the final product being constrained to the visualization of the polygonal mesh only [24]. However, in this particular case study, the use of a polarized strobe has enabled the capture of reflection-free images, even with frontal lighting, thereby ensuring accurate texture representation regardless of ambient lighting conditions. This advancement represents a significant improvement compared to the previous study in which V-RTI had been applied [24]. To evaluate the proposed approach, several tests were performed. First, a methodology that integrates photogrammetry and RTI to create a V-RTI was employed in the chapel of [PERSON] in the Museo Egizio of Turin, Italy. The quality of the obtained 3D model by photogrammetry was evaluated by comparison with a ground-truth obtained by a Structured Light Scanner (Artec Leo). Regarding the photogrammetric data acquisition, the use of a frontal polarized on-camera flash offered several advantages during the acquisition phase: the ability to work freehand without additional equipment and the possibility of capturing images free of both reflections and shadows, which is useful for rendering high-quality textures and improving surface definition (Figure 18d-f). Figure 18: (**a**) Museum lighting; (**b**) non-polarized frontal light; (**c**) polarized frontal light; (**d**) V-RTI Diffuse Gain; (**e**) V-RTI Specular Enhancement; (**f**) V-RTI normal map. The implementation of frontal lighting not only expedited the process but also mitigated the impact of shadows (Figure 18b), ensuring accurate documentation of the surface morphology even in the presence of ambient lighting (Figure 18a). By adopting this approach, it was possible to gather detailed information about the materiality of the wall, which would otherwise be difficult to record in the current lighting or in the absence of polarized light (Figure 18c). Regarding the dimensional comparison between the SfM and SLS models, the deviation map (Figure 13a) shows a significantly high discrepancy, mainly located at the edges, between the two models that can be attributed to an incorrect reconstruction of the SfM model. This is a common issue encountered in the acquisition of flat surfaces, occurring when the dataset is not supported by an adequate number of oblique and peripheral images. In addition, as illustrated in the Confidence Map generated by Metashape (Figure 14), the surface bending of the SfM model may have been caused by a poor image overlapping at the edges of the wall. As expected from previous cases reported in the literature [44; 45; 46], the SfM model derived from photographs taken using a 20.2 Megapixel sensor produced less detail and more distortion in the mesh than that obtained using SLS. However, this does not necessarily mean that an image-based method cannot produce similar data density and geometric accuracy. In terms of mesh density, a finer result could be obtained by significantly reducing the operating distance, and implementing a high-resolution sensor while increasing the number of oblique images. In addition, to directly compare RTI and V-RTI outputs, a qualitative comparison of equivalent portions of a small test artifact that has been digitized using both techniques was performed, with the aim of demonstrating the potential of this innovative methodology.