SAN DIEGO (Hollywood Reporter) - To stoke buzz for the sci-fi flick “Avatar,” 20th Century Fox will take over as many Imax and 3D screens around the world as it can on August 21 to show 15 minutes of promotional footage, filmmaker James Cameron announced Thursday at the Comic-Con pop culture convention.

The screenings, in a highly unusual move, will be offered free of charge to pave the way for the film, which opens in the U.S. on December 18.

Cameron and Fox took the wraps off their pricey, visually splashy 3D movie at the San Diego Convention Center’s Hall H on Thursday, to a deeply enthusiastic, if not over-the-top hysterical, crowd Thursday afternoon.

Cameron warmed up the sci-fi-minded crowd by asking, “Who wants to go to another planet?” before rolling nearly 25 minutes of scenes from his otherworldly tale, which is set on the fictional planet of Pandorum.

A few expositional scenes showing the main character, played by Sam Worthington, becoming an avatar (a blue-skinned human-alien hybrid) segued into a series of jungle battle scenes -- many of them of striking color and scope -- in which Worthington and co-star Zoe Saldana fight with prehistoric-looking creatures.

The footage showed a vividly original world complete with its own language and ecology.

“Everybody always asks, where have you been? Well, that’s where I’ve been: Pandorum,” said Cameron, who hasn’t directed a narrative feature since 1997’s “Titanic.”

The footage overlapped with but was not identical to the slightly shorter material that Fox showed at Cinema Expo in Amsterdam last month, the studio said.

The stakes are high for Fox, which has made the December 18 film the cornerstone of its fall slate. The film is important enough to the studio that it was introduced by Fox Filmed Entertainment co-chairman Tom Rothman, who noted that “moments likes these are rare for a movie company.”

MYSTERY BUDGET

At a news conference after the panel, Cameron acknowledged the studio’s support, saying: “They wrote us a big check and were with us right down the line.”

How big a check has become one of the bigger Hollywood mysteries of the season. While some estimates have pegged the budget for “Avatar” in the $300 million range, Fox has stuck to a figure closer to $230 million.

Cameron has faced a lot of talk about budgets ever since “Titanic,” which went on to become the biggest worldwide box-office grosser in history with $1.8 billion, was released.

If the “Avatar” footage didn’t elicit the hysterical reactions from the Comic-Con faithful of the “New Moon” and “Alice in Wonderland” panels earlier in the day, that was understandable. The “Avatar” unspooling marks one of the rare times in recent Comic-Con memory that a film of this scale has not been based on a pre-existing property with a built-in fan base.

That the film is an unknown quantity poses both an advantage and a disadvantage for Fox: While the movie comes without the benefit of prior awareness (and thus makes it a harder sell in the short run), it gives the studio a chance to shape perception in a way that remakes and sequels don’t allow.

The need to build awareness was behind Cameron’s surprise announcement about screening footage for free in theaters in August.

While the unusual move to give moviegoers a deeper look at the upcoming movie is an expensive gambit for the studio’s marketing department, Fox clearly hopes it will show people what cannot simply be translated in a 2D trailer or in TV spots.