Digitization of Blocks and Virtual Anastylosis of an Antique Facade in Pont-Sainte-Maxence (France)

Abstract

ABSTRACT:This paper is dedicated to the digitization of blocks and virtual anastylosis of an antique façade in Pont-Sainte-Maxence (France). In 2014 during the construction of a shopping center, the National Institute for Preventive Archaeological Research (INRAP) discovered a Gallo-Roman site from the 2 nd century AD. The most interesting part of the site for the study is a façade of 70 meters long by nearly 10 meters high. The state of the conservation of the blocks of the façade makes them exceptional due to the question raised by the collapse. Representative and symbolic blocks of this building have been selected for a virtual anastylosis study. The blocks discovered belong to different types: decorated architectural blocks, monumental statuary elements and details of very fine decorations. The digital reproduction of the façade will facilitate the formulation of hypothesis for the collapse of the structure. The Photogrammetry and Geomatics Group from INSA Strasbourg is in charge of the digitization, the anastylosis and the development of exploratory methods for understanding the ruin of the façade. To develop the three-dimensional model of the facade, approximately 70 blocks of various dimensions were chosen by the archaeologists. The choice of the digitization technique is made according to the following pragmatic criterion: the movable objects are acquired with a scan-arm or a hand-held scanner in the laboratory and the largest blocks are recorded by photogrammetry at the repository near Paris. The expected types of deliverables are multiple: very accurate 3D models with the most faithful representation to document the objects in the best way and with optimized size model allowing easy handling during anastylosis tests. The visual aspect of the models is also a very important issue. Indeed, textures from photos are an excellent way to bring about the realism of the virtual model, but fine details of the object are sometimes blurred by the uniformity of the color of the original material. Acquisition by hand-held scanner does not provide the textures (they must be acquired according to a complementary process). The data types are therefore different depending on the acquisition. The type of rendering of the models depends therefore on precise choices to be defined optimally. After the acquisition, hypothesis for the construction of the façade must be validated and / or adapted by the anastylosis of the digitized blocks. Different cases must be taken into account. First, the reconstruction of broken blocks is done by adjusting the recovered fragments. If all the fragments discovered are close to the initial shape of the block, the process is assimilated to a puzzle of complex surfaces. If the fragments have no contact but are an integral part of the block, the proportion of hypotheses in relation to the contact pieces is changed. And finally, if the blocks are to be assembled together by superposition and thanks to a common plan, as assumed during the construction, the restitution could be based on the posi...

Retraced memories - virtual reconstruction of an architectural landmark

“…• 15 of the 28 isolated decorative fragments have been located into the 3D reconstruction as photogrammetric meshes and have been reassembled in their original probable location by means of virtual anastylosis. In accordance with the considerations of (Alby et al, 2017), different types of anastylosis were distinguished, depending on the relative location of the fragments involved (i.e. the physical distance between the pieces), which determines different modes of assembly.…”

Performance Investigation of a Handheld 3d Scanner To Define Good Practices for Small Artefact 3d Modeling

“…The authors would like to thank the colleagues from the National Institute for Preventive Archeological Research (INRAP, France) for giving us the opportunity to record the stone blocks from the Pont-Sainte-Maxence project (Alby et al, 2017), as well as the "Fondation Groupe EDF" (France) for financing these studies. The authors also would like to thank M.-A.…”

“…For each block, between 100 and 200 pictures have been processed using Photoscan (Agisoft). More information about the acquisition and point cloud computation processes can be found in (Alby et al, 2017).…”

“…The blocks present various dimensions and consist of architectural details and decorative elements, but also human, divine or animal faces that compose remarkable statuary. The digitization of these wellpreserved artefacts using photogrammetry, which was the chosen technique, aims at proposing a virtual reconstruction of the facade (Alby et al, 2017). Thus the point clouds captured with the Freestyle 3D handheld scanner will only be used to assess the quality of the data it provides.…”