Photometric stereo for 3D mapping of carvings and relieves: case studies on prehistorical art in Sardinia

A photogrammetric technique (Photometric Stereo), known since a long time, is here proposed as an agile an cheap tool for 3D recording engravings and relieves. Different from stereoscopic or monoscopic Photogrammetry, the Photometric Stereo uses a standard camera at a fixed point to take typically four snapshots of the same object under different lighting directions. Image processing gives three maps: the “Albedo” (that is the color of the surface without any shading), the x-gradient and the y-gradient. From gradient maps one gets the 3D shape of the original surface, with results comparable with those obtained by the much more expensive and demanding laser scanning machines. The 3D surface, once re-colored with the original Albedo, renders highly natural virtual reconstructions. The key point is the extreme simplicity of the acquisition procedure and the preservation of the 3D information even under non-ideal conditions. Examples will be given on decorations of the sadinian “Domus de Janas”.