When Fox Sports aired the first MLB games of the new, shortened season this past weekend, it filled parts of the stadium with digital people, as the pandemic is keeping the real ones at home. These virtual crowds were a first for U.S. sports, but we'll likely see a lot more examples in the coming months. CGI animators have seen a flood of incoming requests to generate crowds for sports events and concerts alike, and insiders say that the trend could provide the industry with new opportunities even after COVID subsides.

Fox Sports only added CGI crowds to some of its camera feeds, leading to a somewhat dissonant mixture of shots of empty seats and shoulder-to-shoulder crowds. The results were nonetheless real enough to confuse some viewers. "Fox fooled me for a second with the digital fans," wrote one viewer on Twitter. Another asked: "Wait there are fans there? Are they all wearing masks?"

Movies, commercials and video games have long used virtual crowds as stand-ins for real humans, but up until now, the craft of generating those digital crowds has gotten very little attention. "There is really no difference between a crowd and a tree," said Dan Warom, who has been producing digital crowds for visual-effects company The Mill around 15 years.

Crowds provide important visual cues to viewers, allowing them to get a sense of scale in a scene. But if CGI artists do their work well, crowds simply blend into the background. "Everyone needs it. But it is not sexy," Warom acknowledged about his line of work.

That changed dramatically over the past several months, as sports leagues and broadcasters tried to figure out how to safely bring sports back to TV. Leagues initially used cardboard cutouts, stuffed animals and even sex dolls to fill the stands, often while augmenting games with prerecorded crowd noise, which itself has come from video games in some cases.

At the same time, the industry began to look for technology that could help make games look and feel more familiar. "Most of the big broadcasters started to seek out a solution to this," Epic Games product manager Andy Blondin said. Epic is best known for Fortnite, but the company also makes the Unreal Engine, a key production tool for video games that is increasingly being embraced by the entertainment industry as well.

Over the past few years, Hollywood has been moving away from adding visual effects in post-processing and toward generating them in real time while actors are being recorded playing in front of green screens. This approach gives directors of feature films a way to watch an approximation of the final shot live on set, an approach that Lucasfilm's visual effects unit ILM used extensively for the latest "Star Wars" movies.

The same technology also makes it possible to add virtual fans to sports games in real time, but there are some unique challenges. For one thing, sports games are being broadcast live, and crowds have to be overlaid over the actual camera feed without noticeable delays. As a result, broadcasters may have to compromise on visual complexity — something that some viewers complained about. The digital assets to create these crowds also have to be customized for each game, "to dress them properly, to put the jerseys in there," Blondin said. "All the nuances that make them feel real."

Adding to the illusion are AI algorithms that allow these virtual fans to respond to cues from the game. Fan behavior in a baseball stadium is very predictable, with viewers sitting on their chairs for much of the game, only to jump up and cheer when someone hits a home run. "The actual AI requirements are very, very low," Warom said, adding that AI can also be used to lighten things up a bit: "You can have the wave propagate through the crowd."

The technology that is now being tested by Fox Sports and others could come in handy even in a post-COVID world, after fans are allowed back into stadiums. For instance, it could give smaller regional sports teams a chance to shine. "For smaller stadiums, you may be able to augment with a second deck," Blondin said.

Broadcasters could potentially also augment and air games that traditionally happen without sizable crowds, like training matches. A team could have its training match on a regular field, and a broadcaster could then add a whole virtual stadium to the TV feed. "It's the difference between playing for the Premier League and playing [on] Sunday morning," Warom said. Adding virtual crowds could also give smaller leagues new revenue opportunities, for instance through commercials. Warom even mused that one day, parents may be able to add virtual crowds to their kids' Little League games.

In the more immediate future, the technology is being adopted by music festivals and others looking to put on believable digital events without real crowds during COVID. Having broadcasters like Fox Sports pilot digital crowds has demonstrated the possibility of the technology, according to business-development manager at Epic Games, Craig Laliberte. "It's opening up a lot of doors," he said.