Digital Survey of Ancient Mosaics of Ravenna

The aim of this paper is to show a methodology to ease problems due to the richness and heaviness of files of meshes derived from the digital 3D survey of mosaics, in order to store them in databases that can be accessible via Internet. In particular, the paper illustrates a comparative application of mesh simplification algorithms in order to simplify meshes derived from laser scanners without loss of the required details in 3D reconstructions. Despite the apparent two-dimensionality of these artefacts, mosaics have generally different levels of geometric complexity. In addition, problems due to occlusions and to the semi-transparency or reflectance of tesserae can effect metric measures obtained from laser scanners, requiring long post-processing operations of the derived meshes. The amount of these operations mainly depends on mosaics characteristics and on the aim of the survey. In order to allow scholars to access via web to 3D data collected in information systems, for example, meshes have to be simplified in order to show different levels of detail according to different and changing communication aims. This paper shows and compares two different approaches to the problem. In order to obtain different models corresponding to pre-defined LODs, a global and a local incremental simplification of triangular meshes are applied to the Master Model. The purpose of preserving feature edges and varying tessellation density is reached testing different algorithms that are available in common modeling packages.