Inside Montreal duo Atomic3's magical Island of Warmth

At the Montreal en Lumire festival this weekend, not far from the candy-coloured lights of the ferris wheel, the zip line, and the urban slide, a bonfire will be raging at the Place des Festivals. The more people gather to dance around it, the louder the tribal music they'll hear, and the higher the flames will go or seem to go, encased in rods of glass.

People don't know each other, but for a few moments, they are creating something together.-Atomic3's Flix Dagenais on how their installations bring people together

The Island of Warmth, by Atomic3, Montreal's Louis-Xavier Gagnon-Lebrun and Flix Dagenais, is a trompe l'oeil. The "flames" are made by incandescent bulbs, and the music is pre-recorded. Sensors hidden under the fire, beneath the platform where people dance, and under the rocking chairs surrounding them, let the spectators alter what they're seeing and hearing. As they congregate, they create their own warmth.

"At first, they are very curious and shy," says Gagnon-Lebrun. "They come closer, and as soon as they see someone doing something like jumping, they try to jump a little bit."

Dagenais chimes in: "As the fire becomes more musical, they get caught up in the happy spirit."

This playful interactionwith their work, and by extension with the city around it motivates Atomic3. Gagnon-Lebrun and Dagenais met while both were working as lighting designers for Robert Lepage. Inspired by the way Lepage uses technology and innovative sets to tell stories, they decided, Gagnon-Lebrun explains, to "get out of the black box of the theatre and tell those stories in public spaces."

Their first work together, clats de verre (2011), encouraged visitors to bounce two illuminated spring-mounted boxes in the Place milie-Gamelin to control music, animations projected on a nearby building, and lights playing across a snow-covered field full of coloured glass panels.

The next year, they designed Iceberg, a series of aluminum archways running through the Place des Festivals and Place des Arts, whose blue lights turned red as people walked under them suggesting the effects of human-created climate change. Sounds were triggered as well: as icebergs melt, says Gagnon-Lebrun, "they make music. There are crevices in them that work a bit like a pipe organ, with air going through." The farther south one walked along Iceberg's arches, the more traditionally "musical" the sounds became.

Atomic3 built on the musical theme for Maestro (2015), an uncharacteristically summery work where visitors to the Place des Festivals could walk up to a podium, pick up a batonand "conduct" fountains. Sensors on a music stand let them raise and lower water-jets and the volume of surrounding orchestral music. The work was inspired by a YouTube video of a three-year-old's spirited "conducting" of Beethoven; a new version of Maestro this May will let visitors trigger cymbal crashes by jumping up and down.

Upcoming installations work on larger scales, with bigger crowds. Nue de verre, set for later this year, will suspend panes of glass from the ceilings of Trudeau International Airport's boarding gates and arrivals halls, casting coloured shapes that will shift according to how many people are walking through. And for next year's Illuminart Festival, celebrating Montreal's 375th birthday, Atomic3 will work with Moment Factory (who did stage projections for Madonna's 2012 Super Bowl halftime performance) to animate lights on the perennially busy Jacques Cartier bridge across the Saint Lawrence. Their plan is still in progress, but Gagnon-Lebrun says they hope to use mobile phone data "to check the pulse of the city. [The lighting] gets pretty quiet if it's cold outside, people are all inside, and there's no hockey game they're not connecting. If there is a concert at Osheaga in the summer, the bridge will be going crazy."

Atomic3 see all of their work as a means of tapping the potential of a city, of illustrating the connections it creates, and even of creating some themselves.

"There's a challenge to make anonymous people play together and interact," says Dagenais, "and that's one thing we are pretty proud of with The Island of Warmth. People don't know each other, but for a few moments, they are creating something together and are so happy."