James Cameron gets an intense, visceral thrill from being in the vanguard of technological innovation. "It's the same gene that makes me want to explore under the ocean, see things people haven't seen before and bring them back with my tail wagging to show everybody," he says.
Cameron introduced the first "motion capture" characters, in Total Recall, and the first human movements on a CGI (computer-generated imagery) character in Terminator 2; and he has again been working at the boundaries of digital technology to create the imaginary world and characters in Avatar. Cameron and his technical collaborators - in particular Weta Digital, the New Zealand-based visual-effects studio owned by Peter Jackson, director of the Lord of the Rings trilogy - were reinventing the technological wheel as they were working on the film, which took four years to make.
Although about two-thirds of Avatar has been created by a bank of computers - a virtual world - it was also filmed as a live-action shoot. The actors worked both on conventional sets and against "green screen" backgrounds, with the computers transforming them - in real time - into their Avatar characters. Cameron believes that, for the most part, it will be impossible for audiences to tell what is real and what is virtual. "The film is the first true hybrid," he says, "the most complicated thing I have ever done."
So, while the cinematic world of Pandora, the distant moon where most of the film takes place, is both computer-generated and real, the main characters, including Jake's Avatar, and Neytiri, the blue, 10ft tall Na'vi princess he falls in love with, are acted by humans, their performances translated into CGI form.
The challenge, says Jon Landau, Cameron's long-time producer, was to make those CGI characters "engaging and emotive. The real problem, for us, was creating the fidelity of a facial performance for the Avatar and Na'vi characters, creating a window into the soul through the eyes".
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In previous CGI films, facial expression was captured by putting scores of tiny reflective markers on an actor's face, which were reinterpreted by a computer into the facial expressions of the CGI character. Gollum, in The Lord of the Rings, for example, was created in this way. Unfortunately, the results have never been great when CGI characters are meant to express human emotion.
Film-makers call the gulf between normal human expression and the expression of CGI characters the "uncanny valley", which Cameron rightly says "creates this creepiness". What he wanted was a "pure translation of the actor's facial performance" into CGI characters such as Jake's Avatar.
So Cameron and his team spent the first year and a half of the production process perfecting a facial-performance-capture system. The principal actors wore special head rigs fitted with tiny cameras just inches from their faces, which caught every minuscule facial motion, even their dilating pupils. As Cameron likes to say, "Actors don't do motion. They do emotion" - emotion that is consciously and subconsciously read by us human beings.
Those millions of minute facial movements were reinterpreted by Weta Digital into the expressions of the characters. "It's the 21st-century version of prosthetics," Landau says.
The other significant technological advance for Avatar was that Cameron was able to direct and interact with the actors as they were performing. They worked on a large digital stage called the "performance capture volume". As the cameras on their heads captured their facial expressions, the actors wore black bodysuits covered with reflective white dots that allowed dozens of cameras on the ceiling to track their bodily movements. The innovation was that all this visual information was translated by computers, in real time, into the movements of the CGI characters and fed into a virtual camera monitor - called a Simulcam - that Cameron could use on the stage to see exactly what was being shot, as if he were watching an approximation of the finished film. He then had a real-time view of the superimposition of live action into virtual characters, some 10ft tall, in real and CGI settings.
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Beyond that, what audiences are most likely to be aware of are the very latest 3-D effects Avatar will showcase. (The film will also be screened in 2-D in many theatres.) All the live action was shot with a new, lightweight digital 3-D camera system called Fusion, developed by Cameron and his camera whiz, Vince Pace.
"3-D offers an experience which is very inclusive of the audience," says Cameron, who vows to make all his future films in 3-D. "You get to step right into the scene. It takes that big flat screen from being a picture on a wall to being a window into another reality." And he's right. I've seen about 25 minutes of Avatar, and the 3-D effects are both stunning and immersive, without ever being gimmicky.
Despite these incredible technological advances, all of which delight him, Cameron admits that cinemagoers don't really care about technology. "When somebody is sitting in a movie theatre and the lights go down, they couldn't give a rat's butt how the film is made. They just want to be swept along and taken on some magic carpet ride. And really, the less they know about how it's done, the better."