In the 20-plus years that Tim Gerritsen has been creating video games, working in the realm of imaginary battlefields and mythical kingdoms, the Wisconsin native has found himself in many of the real world's most innovative game development centers: Boston, Seattle, Tokyo, London.

Recently he witnessed a transformation in his home state, where Madison has spawned a industry of its own, putting it on the map in a booming sector of the global electronics economy.

"There are so many startups (in Madison) that I haven't kept track of them all," Gerritsen said.

Digital games with names like "Rune" and "Parallel Kingdom: Age of Thrones" are hardly the first things that come to mind when most think of the Wisconsin economy, which for many still evokes machine shops, paper mills and sausage and cheese.

But when Gerritsen returned to Madison last year from a six-year stint at Irrational Games in Boston, he discovered a lively milieu with a surprising share of bestselling franchises.

"It kind of exploded since I left," he said.

Games that existed as ideas from computer science grads when he left Madison in 2008 had morphed into full-blown development labs such as PerBlue Inc., a pioneer in games played on smartphones with its "Parallel Kingdom" multiplayer game worlds.

In that same time, Filament Games LLC went from startup to national leader in the application of game psychology (motivation, decision-making and whatever makes games addictive) and adapting them into games for educational purposes. Its games - "Lawcraft," "Crisis of Nations" - have been played more than 28 million times in more than 10,000 classrooms.

In a logical extension of the military shooter genre, Filament is developing game simulations for military recruits.

Not all Madison-born games are quite that cerebral. Raven Software Inc., the biggest and oldest of the Madison-area studios, moved from its early games such as "Black Crypt" to work on the blockbuster "Call of Duty" franchise for its parent company, Activision, which acquired it in 1997.

On his return from Boston, Gerritsen, 46, rejoined his original Madison startup, Human Head Studios, which has established itself as a major independent game developer. Today, Human Head is known for widely sold games such as "Rune" and "Prey" (PC Gamer's "Action Game of the Year").

Companies like PerBlue and Filament list openings for engineers and software designers on their websites.

No one pretends that Madison approaches top-tier video-game meccas like Seattle and Tokyo, which have dominated electronic games since the earliest days of bulky consoles, crude graphics and wired joystick controls. According to the Entertainment Software Association, a trade group, Wisconsin is not among the seven states with the greatest number of computer game industry employees: California, Texas, Washington, New York, Massachusetts, Florida and Illinois collectively employ nearly 80 percent of the U.S. industry total.

But there's enough scale in Madison that the Wisconsin Technology Council, an advisory group to state government, wants to promote what it calls a "growing video-game development cluster." The council counts more than a dozen game development studios in the state, most of them clustered around Madison but others sprinkled around Wisconsin, including in Milwaukee (Guild Software Inc.) and Green Bay (Zymo Entertainment).

In addition, the next generation of independent game creators could be dreaming up virtual dragons and medieval castles in dorm rooms across the state, thanks to college game-development programs in Madison, Milwaukee, Stout and Whitewater. Those are mostly young initiatives: The Game Design and Development department at the University of Wisconsin-Stout is five years old, funded by Raven Software, and already has 250 students.

Making it possible for a city like Madison to become an aspiring video-game hub is the rapid transformation within the industry: A pastime that used to be tethered to home computers and consoles such as the PlayStation and Xbox has migrated onto web browsers, tablets and smartphones.

In the process, a trade once dominated by big-budget studios like Sega Corp. and Nintendo Co. now must contend with games that are sold as smartphone apps.

With that shift came a groundswell of independent developers with far lower entry costs, said Kate Edwards, executive director of the International Game Developers Association.

"Many get their funding from Kickstarter or Indigogo," she said, citing two online fundraising sites. "I just met a 17-year-old high school senior in San Francisco who has developed 40 games and is selling four of them on the Apple iTunes store."

The U.S. digital game industry grew more than 9 percent annually from 2009 to 2012, which is roughly four times the pace of the American economy in that time, according to a study last month from the Entertainment Software Association.

Helping drive that growth is the sheer size of the audience: Gaming has become a mainstream phenomenon that spans nations, cultures, genders and age.

The average American gamer is 31, according to the Entertainment Software Association. The Superdata Research firm found that half of all gamers are women, although proportions shift with age and genre: While men outnumber women in "first-person shooter" games, women play more of the other digital games overall than men.

"A large percentage are women who are in their 30s and 40s and multitask while waiting in lines," Edwards said.

"The industry is a lot wider and a lot deeper than people give it credit for," Gerritsen said.

The audience is vast in part because the digital game phenomenon has spanned the globe since its inception.

Japan's Nintendo created the "Legend of Zelda," one of the earliest and most popular adventure games. Finnish developer Rovio unleashed "Angry Birds" on the world, with over 2 billion downloads on all platforms. "Tetris," the popular puzzle game, started as a Cold War creation of the Academy of Science in Soviet Russia.

Gamers explain the popularity with comparisons to traditional media - the key difference being that video games immerse players in the narrative as it unfolds.

"We're creating an entertainment experience that I feel can be equivalent to good movies and a good novel," Gerritsen said. "We set up the universe and immerse you in this experience, and you create the actual story."

The arrival of new platforms for games - phones and tablets - was seized upon by PerBlue shortly after Apple introduced the first iPhone in 2007, when three computer science grads from UW-Madison saw the potential to put games onto the little screen.

They called it the "Parallel Kingdom" series because it uses the GPS system on mobile devices to superimpose a fantasy world on top of a map of the real one. A player walking past Madison's Capitol dome could look at the game and see the same map overlaid with "Dreadknights," virtual castles, dragons, trolls, penguins and weapons.

Because it's a multiplayer game, it also shows other players in the vicinity - blurring the line between social media and games.

It took three years for PerBlue to reach 1 million players with its first "Parallel Kingdom" launch, said Forrest Woolworth, chief operating officer. "We crossed that same 1 million milestone with our newest product, 'Titan Empires,' within three weeks of its global launch."

While Madison might not stand out in the size of its roster of game studios, the city commands national stature for research into the psychology of digital games, said Lee Wilson, chief executive of Filament.

Wilson credits UW-Madison's annual Games Learning & Society conference, now in its 11th year, which makes Wisconsin home to the largest academic -based learning conference in the country.

Filament has developed games for education publishers, National Geographic and the Massachusetts Institute of Technology. One of its biggest clients is iCivics, a nonprofit educational group founded by former Supreme Court Justice Sandra Day O'Connor.

Wilson is a former textbook publisher and academic. He wears a silver goatee and favors tweeds. And that makes him Exhibit A for why gamers defy stereotypes. He has fought zombies.

"I have been a digital gamer since the early '70s," he said. "I met the founders of Filament at the Games Learning & Society conference here in Madison, but really got to know them through playing 'World of Warcraft' together. I was in Austin at the time and they were here."

His fondness for gaming makes it a lot easier to go to the office each day.

"There are times I have to pinch myself and mutter 'work, work, work' as I'm playing," he said.