A new pixel-wise data processing method for reflectance transformation imaging

Reflectance transformation imaging (RTI) is one of the most widely used techniques in order to digitize and analyze material appearance of a surface, finding a great level of utility and applicability in the field of cultural heritage as well as in industrial applications. To date, most of the methods used to process (model and relight) RTI data assume only one light direction for all pixels as well as a single light source-surface distance for the entire image, following the model of a very distant (far) light source. This assumption does not hold in practice. Indeed, the light sources commonly used in RTI acquisitions (spotlight/photo flash) induce to a non-uniform illumination of the surface. This is caused by the variation of incidence angles and per-point distances which directly affect the amount of light received by the surface. We propose a novel pixel-wise methodology for improving lighting based on illumination laws that allows one to correct both loss of energy due to the distance variation as well as the elevation angle. We show the efficiency of the proposed method on RTI acquisitions performed on cultural heritage objects and a manufactured surface. We show that our method corrects the effects of non-uniform illumination and leads to improve the relighting commonly associated with RTI.