James Cameron has been talking about his sequels to Avatar for many years, and they are finally in production. The director is shooting no fewer than four more movies in the sci-fi series, with the first set to arrive in 2020, and he has spoken recently about some of the technical challenges involved.

　　In an interview with Collider, Cameron revealed that a lot of the first two sequels will be set in and around water. Many the cast are playing Na'vi, the alien inhabitants of the planet Pandora, who are filmed using motion-capture technology. As Cameron explains, mo-cap has never been used underwater before, and his VFX team have had to develop some pioneering methods to make it work.

　　"It's never been done before and it's very tricky," he said. "Our motion capture system, like most motion capture systems, is what they call optical base, meaning that it uses markers that are photographed with hundreds of cameras. The problem with water is not the underwater part, but the interface between the air and the water, which forms a moving mirror. That moving mirror reflects all the dots and markers, and it creates a bunch of false markers.

　　"It's a little bit like a fighter plane dumping a bunch of chaff to confuse the radar system of a missile. It creates thousands of false targets, so we've had to figure out how to get around that problem, which we did. Basically, whenever you add water to any problem, it just gets ten times harder. So, we've thrown a lot of horsepower, innovation, imagination, and new technology at the problem, and it's taken us about a year and a half now to work out how we're going to do it."

　　The first cast image from the Avatar sequel was revealed in September. Although specific plot details about Avatar 2 are scarce, producer Jon Landau did explain that the young cast would help shift the tone of the movie. "We never had this youthful element before, and that brings a different kind of energy to the film," he said. "They represent the future generation of Pandora and play a very significant role--not just in this movie but throughout all the movies."

　　The first Avatar sequel is set to hit theaters on December 18, 2020, with the next ones arriving on December 17, 2021, December 20, 2024, and December 19, 2025.