Color reconstruction by photometric stereo with unknown lighting of rock art carvings found in two Sardinian Domus de Janas

Artifacts documentation is an important aspect of archaeological studies, not only to preserve these objects but also to learn from them the culture of ancient populations. Hence, the necessity to digitally document and archive artifacts that come from our ancestors. This work can be done with more ease for artifacts as vases, potsherds, or little handmade objects, that can be brought in a laboratory. It is more complicate for rock art carvings, that can not be moved from their original location. The photometric stereo technique allows to obtain the three-dimensional digital reconstruction of an object starting from a set of pictures taken with different lighting conditions. When the method is applied to some experimental datasets, computational problems may occur, due to the fact that some assumptions of the model are not verified: rocks are not Lambertian surfaces in general, and the light sources may not be positioned at a sufficient distance from the object. We deal with these issues by introducing numerical indicators of ideality that allow to figure out if a given dataset is reliable and which images should be selected to better reproduce the object. In this paper, we will show the application of this method to the 3D reconstruction of some engravings found in two Domus de Janas, ancient tombs located in Sardinia, Italy.