All the technologies in place to rebuild Notre-Dame

From 3D printing to the virtual twin of the cathedral, from the use of drones to digital cartography, this is how we work on the restoration of the monument in Paris

(Photo: Houpline / Sipa / 1904160833) In the aftermath of the fire that destroyed two thirds of the roof of Notre-Dame cathedral (including the nineteenth-century spire), experts and scientists immediately set to work to figure out how to revive the symbolic monument of Paris. An immense job that required over two years of effort just to make the spaces safe and digitized. Because if the real construction site starts this summer, the digital one instead took shape on April 16, 2019, after the flames had been put out.

Livio De Luca, research director at CNRS (Centro for scientific research) and the mixed research unit Map (Models and simulations for architecture and heritage). With his team of 30 experts appointed by CNRS and the French Ministry of Culture, the Italian architect-computer scientist, who has worked in France for 20 years, has been involved in the digital reconstruction of the cathedral, using a 3D model project he proposed. to recreate the immense building from various resources, including point clouds and photographs.

The digital twin of Notre-Dame

One of the key tools of the developed system is called Aïoli, and it is a collaborative and three-dimensional semantic annotation platform that allows to bring together scientific data from more actors. The starting point were the various surveys made before the fire. “Notre-Dame had been documented, in previous years, by various scholars. Among these, Andrew Tallow, professor of art and architecture at Vassar College in Poughkeepsie, in the United States, who had digitized it with a 3D laser scanner. Tallow passed away shortly before the catastrophe, but his wife made the entire archive available and these data, as well as those of other subsequent surveys, were invaluable for the start of our scientific work, ”says De Luca.

It is one of the first times in which such decisive use is made of data for the reconstruction of a historical monument and the final ambition is great: that of reconstructing the cathedral identical to the lost one. But to do this, you need to photograph the past and make it digital and accessible to all. The work groups that revolve around the restoration project are eight, all with specific areas. And the "digital data" team is essential in this first phase of study.

Reconstructing the information

A slide by De Luca depicts a floating church, formed by small blue dots that illuminate a dark background: “This is the cathedral after the fire”. A familiar silhouette takes shape. The hole left in the Gothic nave after the flaming spire fell is in the foreground. As well as the burnt debris; a heap of blackened objects lying in the central nave. "Thousands of photographs have been taken since the fire. Vaults, naves, nuclei, transepts everything has been visually repertory down to the smallest detail. And these data were integrated into a calculation software, designed by the Cnrs team, gathered for this reconstruction work. This has allowed the creation of dedicated indexing algorithms, which refer to all these digital data and allow continuity between the images ", explains the expert.

Violette Abergel © Violette ABERGEL / MAP / Chantier Scientifique Notre -Dame de Paris / Ministère de la culture / CNRS - 2020 Basically, the more data is entered, the more the three-dimensional copy of Notre-Dame takes shape, giving life to a digital puzzle faithfully reproduced to the millimeter. In this way it is possible to trace both the medieval structure of Notre-Dame and the modern one. Part of the roof of the cathedral was rebuilt in the 19th century by the architect Viollet-le-Duc after the riots of the French Revolution.

To photograph every single point of the monument, explains De Luca, “we first used a drone on which we mounted an upward facing camera that allowed us to photograph the intrados of the vaults. And where it was not possible to go with the drone, we used "cable cams", or robots connected to a cable that photographed all the remains of Notre Dame at 360 degrees. Both those that fell and those that remained intact in the vaults ".

Drones, 3D printing and digital maps

Each phase of evacuation was followed and recorded, in order to have a useful archive for the others experts, who will thus be able to better understand where and how the stones, beams and rock fragments fell, and then reposition them properly. The three-dimensional cartographies were made every week, as the debris was removed. The remains of the fire will then be transferred, within a few weeks, to a hangar near Paris and made available to the many researchers involved in the restoration project. De Luca points out a peculiarity of the cathedral's keystones which, in some cases, have remained "almost intact despite the fall and which can therefore be reused for reconstruction".

To understand their state of conservation , these blocks of stones were placed on a rotating plate and photographed 400 times by a very special 3D scanner, created by a start-up of Cnrs to reconstruct their composition and their origin and remodel them thanks to a 3D printer. The technologies used were conceived exclusively for this construction site. This is a world first that brought together various experts from all over the world, including the CNR of Pisa and the Bruno Kessler Foundation of Trento.

A digital archive within everyone's reach

But the work of the digital team will not stop with the end of the restoration. Because in De Luca's dreams there is also the sharing of the secrets of Notre-Dame, not only for experts but also for the general public. “Thanks to these in-depth works, it will be possible to compare the various states of the cathedral and make it possible to make choices regarding the materials and techniques to be used for the restoration. It is important for us to understand how the cathedral was built and how it was transformed, to provide maximum information to those involved in the restoration, "he says. The common goal is to return this immense cultural heritage to the world as soon as possible (to follow the progress of the works: https://www.notre-dame.science). In recent weeks, the French president, Emmanuel Macron, has foreseen a reopening to worship by April 16, 2024.


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Topics

Drones Europe Artificial intelligence Notre Dame Robots 3D printing globalData.fldTopic = "Drones, Europe, Artificial intelligence, Notre Dame, Robots, 3D printing "

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