Developing a low cost 3D imaging solution for inscribed stone surface analysis

Abstract

The article uses a 3D imaging based solution where surface shading is determined by surface geometry. It is applied to inscribed stone surfaces in order to examine lettering and other carvings dated to the 5th - 9th centuries AD. Mid-range terrestrial laser scanning and structure from motion (SfM) based photogrammetry were cross-examined in order to create a low cost, but nevertheless highly accurate solution to 3D imaging that requires a computer, a camera, open source software like CloudCompare and a SfM based service called Photo located at ReCap360.autodesk.com. Ambient occlusion (AO) shading was used to show improvements made to the SfM data, which was achieved by adding known parameters to all photographs used. It simulates the direct light components of a light source so that exposed areas appear lightened and enclosed areas (like crevices and incisions) appear darkened. In the case study, AO was used to differentiate lettering in the inscription from damage and weathering on the granite surface of the Tristan Stone, as well as picking up a previously unnoticed wheel-head cross. This particular inscribed stone is located near Fowey in Cornwall, UK, and was known as the Long Stone before its name was changed on British Ordnance Survey (OS) Maps in 1951. The Tristan Stone was the first artefact to be scanned by the FARO Focus3D laser scanner after its release in 2010.