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Nearly a century after D.W. Griffith’s prime, visual f/x artists have finally caught up with the most impressive feat of classic Hollywood epics, the “cast of thousands” — only now, crowd scenes don’t need an actual crowd.

That’s because software pioneered for “The Lord of the Rings” and “Troy” can create swarms of convincing digital extras (or “autonomous agents”) using artificial intelligence systems that make their own decisions.

“We’re not trying to micromanage the exact behavior of thousands of sentient beings,” says Digital Domain vfx supervisor Matthew Butler, who used a program called Massive to re-create the amphibious assault on Iwo Jima by 30,000 Marines, for Paramount’s WWII film “Flags of Our Fathers.”

“What you’re doing,” Butler explains, “is creating a brain that makes lower-level decisions on when to activate specific strategic maneuvers and how to blend between them.”

In “Flags,” the CG soldiers needed to hold their shoulders down, carry packs and run with guns. They needed to be able to drop to their knees or lie flat-bellied on the beach, dodge shrapnel, respond to injuries and pick up wounded compadres to be carried away on stretchers.

“It sounds like a very simple set, but in no time at all, it became this very complicated array of abilities,” Butler says. “…Interestingly enough, it doesn’t take that many different options before the mass appearance looks somewhat random and complex enough to fool the human eye.”

London-based Moving Picture Co. created a similar program called Alice for “Troy,” then used it to simulate a battle between 50,000 soldiers in Chinese epic “Curse of the Golden Flower.”

According to “Curse” vfx supervisor Angela Barson, director Zhang Yimou had hundreds of extras available, but the expense of costuming them all would have been prohibitive. MPC’s challenge was to ensure the CG shots matched the human crowds. “When they’re filming the live action, they try and make it as perfect as possible. When we do our CG, we try and put as many glitches and imperfections in as possible to make it look real,” Barson says.

Massive is also an ideal tool for computer-animated toons. It was so effective on “The Ant Bully” that director John Davis sometimes had trouble distinguishing between Massive agents and hand-animated “hero characters.” On “Happy Feet,” vfx firm Animal Logic used Massive to animate background penguins, who were more naturalistic than the speaking characters.

Warner exec VP of digital production Chris deFaria, who oversaw both “Ant Bully” and “Happy Feet,” says Massive proved its effectiveness a year earlier on “Charlie and the Chocolate Factory,” creating subtle variations in a crowd of dancing Oompa Loompas, all played by the same actor.

“Massive is one of the few products that is changing the game in one fell swoop. In a world of limited resources, it allows us to deliver scope and scale,” says deFaria. “In the past, we would find a way around it. In old movies, someone would say, ‘My God, you should have seen the size of that army.’ Well, now we can see the size of that army.”