A quick method for the texture mapping of meshes acquired by laser scanner

The methodology described in this article was developed in connection with two different projects and entails texture mapping by time-of-flight laser scanner. In order to verify its operational effectiveness and applicability to other contexts, sites with extremely different morphological characteristics were studied. The first case study refers to the three-dimensional mapping of the rupestrian church of Santo Spirito in Monopoli (near Bari, Italy), whose objective was to provide a three-dimensional restitution for restoration purposes and to prepare two-dimensional drawings for the mapping of diagnostic surveys of various kinds, including observation via IR images and Georadar analyses. The church is characterised by a typically irregular underground space, entirely excavated from the rock. The survey looked at a total area of about 458 m2 and produced about 40 million polygons, with a mesh resolution of 4 mm. The second case study refers to the church of Spirito Santo in Lecce, near the ex-hospital of the same name. The layout of the building, with a single nave, has four arches and six niches, richly decorated in stone and baroque stuccoes, each holding an altar. In this case too, the surveys were designed to support diagnostics, conducted as part of the AITECH project (Applied Innovation Technologies for Diagnosis and Conservation of Built Heritage). The project is linked in turn to the "Rete dei Laboratori" (Laboratory Network) initiative, promoted by the Italian Ministry of Economic development, Ministry of Universities and Research and Puglia Regional administration. The basic rationale of this simple method derives from the need to obtain different types of mapping - including RGB real colour images, infra-red images, false colour images from georadar scans, etc. - from the same scanned surface. To resolve this problem, we felt that the most appropriate step was to obtain a UVW mapping based on the high resolution real colour images and then use the same coordinates to rapidly map the false colour images as well. Thus we fitted a device to the camera to determine its trajectory (similar to a gun-sight); when scanned by the laser scanner in the same context as the monument, it makes it possible to know the exact coordinates of the viewpoint. A series of photographs was taken, each time pointing the camera at a target or at an easily recognisable element on the scanned surface. For each photograph, 3D software was used to place the virtual camera in the same position as the viewpoint detected with the laser scanner and to orient it towards the same target. Camera Projection was used to obtain a correct mapping of the textures for the different viewpoints. This rapid method has been experimentally shown to be operationally effective, with a level of precision in the order of 2-3 mm.