(PANORAMA – Université Libre de Bruxelles, Bruxelles, Belgum)

Keywords: 3D scanning, automatic pipeline, archaeological exploitation, multimodal rendering

In order to make its architectural and archaeological study, the roofs, the courtyard and the facades of the Brussels City Hall were digitized with a 3D scanner, coupled with photographic acquisitions. Various elements, such as the tympanum of the main portal and the archangel Michael, an exceptionally preserved 5-metre-high medieval metal wind vane that adorned the top of the 96-metres-high tower, were also digitized in high definition. We present two tools : an automated pipeline to clean colorimetric and geometric defects in 3D survey; and a software to easily manipulate all this data with fluidity in the rendering and the manipulation, designed for architects and archeologists users.
The complete acquisitions consist in 97 scanning positions for the building, and 13 for the Archangel. Due to the changes in natural lighting conditions during the survey, the resulting raw data presents some unpleasant colorimetric defects, coupled with geometric errors (i.e. erroneous points related to the presence of persons and vehicles, flying points or noise inherent to the acquisition device). Manual processing is laborious and very time-consuming. A specific and automated processing pipeline has therefore been developed and applied to correct all these problems.
To create or complete elevations and plans, or to extract details and measurements, a specialized software has been developed to manipulate the large amount of 3D data, inherent of 3D scanner acquisitions of complex building, and to present the data as an enriched 2D representation for architectural and archaeological purpose. This is similar to orthophotos complemented by the possibility to navigate in the depth (e.g. to see an object and behind it) and to vary the rendering mode (including color, intensity, orientation), to highlight elements (like surfaces, edges or small details) hardly visible in simple color mode of rendering.
Its functionalities allow to realize very precise elevations and to generate projection images in high definition of all building parts and of the archangel statue. Indeed, this survey allowed us to draw the statue in its entirety to the real scale and in a completely proportioned way. For the Archangel, we have been able to distinguish the main components of the work and to better understand the articulations between its constituting parts and the various transformations made to the metal statue over the centuries.