A lot of people were fooled — including plenty of locals. After Justin Timberlake duetted with a virtual Prince during last night's Super Bowl halftime show, an overhead shot of U.S. Bank Stadium seemed to show purple lights blinking on all around the stadium, culminating in a giant Love Symbol ringing the stadium.

Was it real? No. Many observers with views of the stadium went running to their windows, and were disappointed not to see the Love Symbol out there in real life.

"It was definitely a cool effect," says Eric Riggs, a creative director at Minneapolis agency Crash + Sues. "I was at a Super Bowl party with a bunch of people, and everyone was like, 'Oh! How did they do that?' I was like, well, I'm pretty sure that was not real."

We've asked the NFL (which produces the show) to comment on the effect, but they haven't yet responded. Riggs points out, though, that viewers actually see similar digital effects every time they watch a football game on TV.

"Any time you watch football, the yellow line painted on the field" to mark the yardage goal is virtual, notes Riggs.

"They're tracking that camera live. That line isn't real. It's mathematically placed in to match the perspective. They even now do that with superimposed things that are sitting on the field — stats and things, with a camera that's moving. All that stuff's tracked in."

The overhead shot was previously recorded, then digitally altered to add the Love Symbol. The shot was finished well before the game began, and even appeared on the stadium's interior screens during rehearsals.

"You can see the buildings in the background, and everything in the sky is all purple," observes Riggs. "That's one indicator that something fishy's going on there. Things aren't naturally all that purple. They're totally altering the image."

Although effects like the Love Symbol animation are well within the reach of today's computers, this one took some particular attention to detail. If you wanted to pull an effect like that off without using computers, notes Riggs, "that would take a lot of infrastructure to pull that off and make it look as cool as it did."

Update 2/7: Nick Whitehouse, a member of the team that created visuals for the halftime show, tells Billboard that the giant glyph "came from one of those JT genius ideas. He said to us 'wouldn’t it be amazing if we could light up all of Minneapolis Purple' as we discussed ideas of how to pay a fitting tribute to the city and it evolved from there. As for how we did it, all I'm going to say is it took an amazing amount of effort and co-ordination over many months [...] they pulled it off in a way far better than we had ever imagined."