Modelling the Memorial
The Memorial for Abolition of Slavery
Located along the Loire riverfront in the center of the city of Nantes, this memorial, designed by Wodiczko + Bonder, is a metaphorical and emotional evocation of the struggle for the abolition of slavery. This exceptional project, embedded into an old wharf, pays homage to all those who have fought and are still fighting against slavery in all its forms throughout the world. All along the green esplanade are 2000 commemorative plaques noting ships leaving Nantes on slave trade expeditions and the main African and American trading posts, revealing the enormity of the abhorrent slave-trafficking. Under the quayside, a 90 meter-long underground passage evokes the captives in the ships’ holds, inciting us to think about fighting for the abolition of slavery and the modern form it takes today. Throughout this thought-provoking walk reached by a decorative staircase, a series of quotations, in every language, from times past and present and from all over; they depict humanity’s long combat against slavery.
An interesting challenge for optical simulation
The commemorative plaques are made of high-tech glazings, featuring three sheets of glass laminated together, and including coatings, serigraphy, acid etch and paints. This gives them a unique set of visual properties, such as text appearing sometimes dark on bright, sometimes bright on dark, depending on light conditions. The crisp letters are surrounded by a soft shadow, and are faded away when viewed under high angle.
This structure is especially difficult to simulate, as most effects are caused by the multiple light bounces between the coatings, paints and acid etch layer. Therefore, this was a very interesting test subject for Ocean.
Modeling the scene
The project CAD and technical specifications were kindly provided by Philippe Bompas from RFR, Paris, glass consultant on the project. The scene was imported and edited with Rhinoceros 3D.
The glazings are a laminate of three glass panes, with coatings, serigraphy, paints and acid etch on the various interfaces:
The three sheets were modelled with their real thickness, and the PVB laminate foils absorptions were added to the glass as an effective medium (PVB refractive index is closely matching the glass).
The interfaces were modeled as described in the project documents:
- The front glass interface is acid etched. As the roughness was unknown and difficult to measure on site, it was adjusted so that the diffusion halo of a point light source matches the real material. This is the only parameters that was fitted to match the project.
- The second interface, between glass 1 and 2, is a metallic coating deposited with a mask and defining the text of the glazings.
- The third interface, between glass 2 and 3, is a pyrolytic solar control reflective coating, whose optical properties were taken from the IGDB database.
- The last back interface is a grey paint specified by its RAL color, converted to L*a*b* values, and modelled by a lambertian material.
The reference picture of the project was taken at around noon on November, 29th. In order to closely match the lighting conditions, an HDR spherical picture of the environment was taken at the same time from the wharf.
Side by side comparison
The image below compares real pictures on the left with the simulation on the right (click to enlarge). The picture was taken with a SLR camera in raw mode. Its color profile was calibrated on-site with a color chart. Both were applied a 5900K white point and a linear response curve.
The following image shows the comparison in the same conditions, but from another view point:
Some small differences may be observed:
- The artificial light in the lower part of the glass was modeled very simply, it does not match the exact light set-up of the real scene.
- The glass on the real picture looks slightly bluer than the simulated image. As colorimetry was precisely calibrated, the camera is most likely not in cause. The product name of the pyrolytic coating on pane 3 was not known with certainty when modelling the glass structure, and we used a rough model for it, matching the performance and color of SGG Antelio Silver at normal incidence. The missing blue tint could be due to the product which is not the same, or to the model, which does not takes into account the specific variation of product color with angle of incidence.
This picture also shows some differences:
- The etched glass surface looks somewhat different, especially the highlight around “Aboli”. It seems that water flowing on the glass has modified its optical properties, by depositing materials, making the material less glossy, more diffuse. On the right picture, the vertical stripes corresponding to water flow are well visible.
- The artificial light source was again modeled using a continuous light bar, whereas it is made of smaller light sources at regular intervals on the real project.
Besides these observations, this shows the potential of Ocean for designing complex glass structures. As physical prototypes are very expensive, they can be replaced by virtual prototypes simulated at early stages of development, for faster and cheaper decision-making.
Architects: Wodiczko + Bonder
Location: Nantes, France
Consultants: Thomas Long, Design Collaboration & Visualization, Graphic Design; Snehal Intwala, Nicholas Capone, James Shen, Ryan McClain, Bill Panasuik, Emmanuelle Chérel (Research),; Patrick Charles (Design & Technical Consultant); Maximo Rohm, Ron Henderson, Michael Blier, (Landscape Consultants – USA)
Glass Consultants: James Carpenter, James Carpenter Design Associates (preliminary phase); RFR Engineurs, Paris – Philippe Bompas & Nicolo Baldassini (development & construction)
Historic Advisors: David Blight, Gilder Lehrman Center on Slavery, Resistance & Abolition, Yale University; Sven Beckert, Harvard University; Kirk Savage, Pittsburgh University; Vince Brown, Duke University
Ville de Nantes – Nantes Métropole: Jean-Marc Ayrault, Former Député-Maire de Nantes / Current French Prime Minister; Yannick Guin, conseiller municipal; Octave Cestor, conseiller municipal; Marie-Hélène Jouzeau, directrice Patrimoine et tourisme; Hervé Guégan, chef de projets, Atelier urbain, Nantes Métropole; Jean-Pierre Brindel, directeur Atelier urbain, Nantes Métropole; Aurélie Roger, directeur projets Atelier urbain, Nantes Métropole
Maitre D’Oeuvre: Arcadis Engineurs, Gildas Legall, Bruno Vasseur, Francois Bailly; Roulleau Architectes, Nantes – Michel Roulleau, Jean Marie Beslou
Construction: General Construction: DLE Ouest, Nicholas Boterf; Glass Wall: Polar, Torino, Paolo Cherasco; Inserts: Atelier Barrois, Brioude, Emmanuel Barrois; Lighting: Citelum, Nantes; Landscape: ISIS Espaces Vertes, Nantes; Steel/Metals: CMR, St. Nazaire, Nantes