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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 video-game industry, putting it on the map in a booming sector of the global electronics economy.

"There are so many start-ups [in Madison] that I haven't kept track of them all," Gerritsen said.

Digital games like "Rune" and "Parallel Kingdom: Age of Thrones" are hardly the first things that come to mind when most think of the Wisconsin economy.

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, Wisconsin is not among the seven states with the greatest number of computer game industry workers: 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).

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 like 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 start-up to national leader in the ­application of game psychology (motivation, decisionmaking and whatever makes games addictive) and adapting them into educational games. 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 such early games 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 start-up, Human Head Studios, which has established itself as a major independent game developer. Today, Human Head is known for such widely sold games as "Rune" and "Prey." (PC Gamer's "Action Game of the Year").

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."