The pottery known as African red slip ware (ARS) is a central archaeological object category for the understanding of late antique period worlds of imagination and their change, as well as for ancient economic history. The three-year project, funded by the German Federal Ministry of Education and Research (BMBF), focuses on the digitization of the relief-bearing ARS of the Roman-Germanic Central Museum (RGZM).
One of the most important object categories for the understanding of the late antique period worlds of imagination and their change, as well as for the economic history of this period, is the pottery known as African red slip ware (ARS). The relief-bearing pottery was produced in large quantities in North Africa between the 3rd and 5th centuries A.D. and was traded throughout the Mediterranean, going as far as Britain. Accordingly, the bowls, plates, jugs and pitchers are of outstanding significance.
With regard to its special medial qualities and the great insight potential for questions of cultural history, the category has hardly been studied and has been insufficiently explored for respective analyses. One reason for this is the particular challenge of documenting the objects and their ornamentation. The depictions applied to the vessels using relief appliqués cannot be precisely recorded by traditional 2D documentation methods, given the different curvatures of the vessels. In addition, the relevant literature lacks a standardized approach to the motifs.
The aim of the project is to make a largely unpublished collection accessible for research in an innovative and sustained way with the precise 3D digitization of the relief-bearing ARS collection of the Roman-Germanic Central Museum (RGZM) and the creation of an ontology, not only enabling new research questions, but also establishing documentation standards.
The high-precision 3D data acquisition and processing of the ARS collection at the RGZM, containing about 325 objects, has been completed. It was performed in the laboratories of the i3mainz with the structured-light 3D scanner ATOS TripleScan manufactured by GOM. To give the 3D models a realistic surface, the originals were photographed from all sides with the Nikon D800 system camera using the structure from motion (SfM) method. Post-processing, in other words after the generation of the 3D models and the subsequent combination of the 3D model and the photos, a true-color textured 3D model is now available for each object for further analysis.
During the 3D capture and the subsequent processing steps, technical metadata is generated that provides the user with important information about the origin and quality of the data. A team from the i3mainz and the RGZM is currently working on a cross-project metadata schema to describe the technical process of creating the 3D models.
The open-source framework 3DHOP was chosen for the digital portrayal and processing of the textured 3D models. Because of the high resolution of the objects and the related file sizes, the choice was made to work with the NEXUS data format, which saves the original files in several resolutions. The viewer is automatically provided with the resolution matching the current section. The 3D viewer is being further developed using web technologies such as HTML5 and WebGL. A function that allows the user to interactively mark appliqués in the 3D model has already been implemented. The scientific description and interpretation of the objects by the archaeologists is also performed in this step.
A geometric comparison process aims to help answer archaeological questions that depend on the degree of similarity between two appliqués. First, the area of the appliqué to be compared is digitally cut out to determine its geometry. This is because during the creation process, the geometry of the appliqués was distorted differently by the application to the respective vessel. The 3D model is transformed into a 2.5D representation by projection. After using the projection, residual influences of the deformation are present, which are to be taken into account when evaluating the result in the archaeological similarity analysis. The appliqués are saved as 2.5D images for further processing.
For the geometric comparison of two appliqués, approximate values for the deviations between the two 2.5D images are determined. Using an iterative image comparison procedure, the geometric relationship of the comparison specimens is determined. Based on this, colorized differential images and graphical representations of local displacements are generated. These quantitative and qualitative results will aid archaeologists in their interpretations of possible similarities and consequently, for example, in attributing artifacts to production workshops.
The archaeological observations and interpretations are compiled in a semantic structure called ARS3DOnto. The ontology provides the necessary framework to document meta-, para-, and provenance information of the recorded and processed 2.5D and 3D data from the African red slip ware and its digitized and annotated applications. ARS3DOnto is modeled as far as possible on the concepts of CIDOC-CRM and the CRMDig or CRMSci extensions. In addition to those of CIDOC-CRM, the ontology portrays various axioms required to define ARS and its characteristics. These axioms are related to the axioms defined by CRMs either by classes, subclasses or other relationships. The content semantically structured in this way is stored in an RDF triplestore.
A virtual platform combines all 3D analysis and visualization tools along with the semantic structure ARS3DOnto. It is already available within the project for processing the 3D models. In the future, external users will also have the opportunity to view the 3D models in the viewer and retrieve the results of the analyses.
For referencing and long-term archiving, the digital masters will also be made available through the German Archaeological Institute’s Arachne object database.