A trustable multispectral photogrammetry pipeline for the reproduction of noncollaborative materials

Many non-invasive techniques are now widely available to experts for the study and understanding of real objects, encouraging the development of new low-cost survey tools. The opportunity to create digital twins opens up many possibilities for in-depth study of art objects. In particular, the increasing adoption of photogrammetry and multispectral reflectography in the field of cultural heritage has provided art historians, conservators and conservation scientists with precise tools for the study of pigments and artistic techniques, facilitating the attribution and dating of art-historical artifacts [1]. Photogrammetry has established itself over time as one of the most common modern survey tools, distinguished by its ease of implementation and ability to obtain metrologically reliable and photo-realistic three-dimensional models in a quick and cost-effective way [2]. Although its use is currently quite popular, certain limits prevent its proper application in the field of cultural heritage, especially in the presence of objects made of shiny, reflective materials such as metals. In recent times, the use of polarized light in the 3D field exploiting the Physically Based Rendering has also provided encouraging insights into the use of photogrammetry even in the presence of non-cooperative materials, expanding the application range and improving the resemblance to the real object [3]. Furthermore, the possibility of producing three-dimensional models integrating data from other spectral bands (UV/IR luminescence and reflectography) and other diagnostic techniques (XRF, RAMAN etc.) opens up new perspectives in the study of constituting materials, allowing information to be combined in a three-dimensional model of more immediate consultation. This work presents a workflow that addresses the various issues and combines them into a single threedimensional model that is correct in size and color, light and easy to consult thanks to the use of free, cross platform visualization tools.The chosen case study, a Chinese gold statuette depicting Buddha, general chronology 6th century BCE, holds many interesting enigmas by addressing the limitations imposed by photogrammetry and integrating data from different techniques into a single three-dimensional model that can be navigated online.