Image courtesy of Cosmo AV
In April 2019, a devastating fire broke out at the iconic Notre-Dame Cathedral in Paris. Starting under the eaves of the building, the blaze soon engulfed the spire and most of the wooden roof, burning all through the night and into the following morning.
What followed was a truly epic restoration project, with more than 1,200 people working together over five painstaking years to repair the damage.
Skilled artisans, including quarry workers, carpenters, mortar makers, and master stonecutters, were hired to rebuild the cathedral with the same materials and techniques used when it was constructed in the 12th century.
In December 2024, the cathedral rose from the ashes in a grand reopening ceremony broadcast on primetime TV.
To showcase the architecture in all its restored glory, France Télévisions enlisted the services of video mapping company Cosmo AV, who in turn asked projection mapping expert Antoine Bourgouin to help produce spectacular architectural lighting for the event.
Image courtesy of Cosmo AV
Projection mapping with Unreal Engine
Antoine Bourgouin first became interested in using buildings as canvases for gigantic trompe-l'œil visuals back in 2010.
Trompe-l'œil—French for “deceive the eye”—is an artistic term for the highly realistic optical illusion of three-dimensional space and objects on a two-dimensional surface.
It’s most often associated with a technique in painting whereby the viewer is tricked into perceiving painted objects or spaces as real.
In the early days, there were no tools on the market for achieving this kind of work. If you wanted to create visuals that could scale and match the contours of a building, you’d have to build your own computer program.
Today, Bourgouin can rely on specialists using Modulo Player—software that acts like a video projector controller—to play complex video projections on surfaces like walls or buildings, precisely warping and adjusting the video to fit each surface.
He’s pushing the art of projection mapping even further by introducing real-time technology into the process.
While traditional videomapping entails projecting prerecorded video, Bourgouin often uses Unreal Engine to develop his visuals and project them in real time onto buildings.
It was this innovation that led him to apply for a MegaGrant with the idea of producing monumental video games in which players’ smartphones act as a gamepad. That’s how he met Pierre-Yves Toulot, CEO of Cosmo Av and Co-Founder of IntensCity.
Image courtesy of Cosmo AV
Mapping the lighting onto a 3D model
Cosmo Av had been invited to create projection-mapped visuals for the reopening of Notre Dame by France’s national public television broadcaster, France Télévisions.
One of the requests was to produce a beautiful light display to showcase the facade of the cathedral.
Toulot and Bourgouin had previously collaborated on similar projects, drawing on the skills of Jean-François Touchard—a talented traditional architectural light designer—when the work required a particular elegance.
It made sense for Toulot to bring Bourgouin and Touchard on board to tackle the Notre Dame project.
To start off, Bourgouin brought a 3D scanned model of the Notre Dame cathedral into Unreal Engine—a process as simple as importing an FBX file.
“With Unreal and Nanite technology, I no longer pay attention to the polygon density of an imported mesh,” says Bourgouin. “The Notre Dame model was a four-million-triangle mesh—it’s so easy to deal with such a polygon number in Unreal today!”
Nanite is Unreal Engine 5's virtualized geometry system that enables you to create highly detailed 3D models consisting of a massive amount of polygons with minimal impact on performance. This system was used to render an incredibly precise mesh of the cathedral, accurate down to the smallest details.
The team would place 500 lights—omni, spot, and rectangular—on the 3D model to highlight all the details of the monument. These lights needed to be harmonized in intensity, temperature, and color.
“The number of lights to manipulate was my biggest challenge on this project,” explains Bourgouin. “But by using Blueprints made on the fly and by tagging light actors to make different groups, I was able to work very smoothly.”
Pierre-Yves acted as the artistic director, while Jean-François brought the expertise when it came to the practical implementation of lighting the detail of the cathedral.
For each statue, the team placed two or three spotlights, accentuating the shape and dimensions of the sculptures by carefully manipulating the shadows.
They used Illuminating Engineering Society (IES) light profiles to ensure their 3D lighting accurately matched the way real-world lights would pick out the details of each arch, balcony, and other architectural elements, and they played with the light temperature to emphasize depth.
The lighting setup was rendered as an image and then projected onto Notre Dame using 30 high-luminosity Panasonic video projectors paired with the Modulo Player system.
Leveraging MegaLights and Lumen
For the Notre Dame project, the team would be projecting pre-recorded visuals onto the cathedral, but using real-time technology brought many advantages when it came to designing the lighting.
To test how the lighting would look on the real-life building, the team were able to update the 3D model in Unreal Engine on the fly while at the location, seeing the results instantly on the cathedral and making adjustments as required.
Bourgouin calls out the power of the lighting system in conjunction with Nanite as a key reason for using Unreal Engine to complete the job.
“It was the ability to work smoothly on a high-polygon mesh with a high number of lights in a what-you-see-is-what-you-get way,” says Bourgouin. “There was no need to imagine the result of the render like in traditional 3D modeling software.”
He also gives a shout out to MegaLights, a powerful new feature recently released in UE 5.5.
MegaLights is an Experimental tool that enables artists to add hundreds of dynamic shadow-casting lights to their scenes. It can be used in conjunction with Unreal Engine’s dynamic global illumination and reflections feature Lumen to achieve ultra-realistic lighting.
“MegaLights was one of the most useful features,” says Bourgouin. “I was able to work with hundreds of lights without compromise on the shadows in real time. It's a fantastic feature to complement Lumen.”
Restoring the glory of Notre Dame
Bourgouin’s lighting work alongside Toulot and Touchard played an integral part in the grand reopening of one of Paris’ most celebrated monuments.
With the help of Unreal Engine, the team shone a light on the incredible work of the restorers and showcased Notre Dame in all her glory.
“Notre-Dame de Paris is one of the buildings you dream of working on when you do projection mapping,” says Bourgouin. “Being able to contribute was a huge honor.”
How to install Unreal Engine
Download the launcher
Before you can install and run Unreal Editor, you’ll need to download and install the Epic Games launcher.
Install Epic Games launcher
Once downloaded and installed, open the launcher and create or log in to your Epic Games account.
Get support, or restart your Epic Games launcher download in Step 1.
Install Unreal Engine
Once logged in, navigate to the Unreal Engine tab and click the Install button to download the most recent version.
Watch how to install