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Brad Turner discusses how, after the decision was made to set the eighth season in New York City, it was deemed too difficult to extensively film on location, mainly for budgetary reasons but also because, unlike Los Angeles, the seasonal changes in New York would make it impossible to keep a consistent appearance throughout filming. The solution arose when the producers found Stargate Studios, a visual effects company specializing in the "Virtual Backlot" tool that allows scenes to be superimposed upon realistic backdrops via green screens and set extensions. Howard Gordon says that, despite wanting to shoot in New York, the financial realities at the time ruled out that option; nonetheless, the final product truly impressed him.
Visual effects supervisor Sam Nicholson talks about how, in the past, visual effects was a post-production process, but now, the order has been reversed: the background plates are filmed in advance, and the actor later films their scenes with a green-screen to be imposed over the background. The scene of Kevin Wade on the phone near the George Washington Bridge is shown in both its unfinished and final stages. Rodney Charters talks about the difficulties of transitioning from filming on location to filming scenes that are partially or even mostly dependent on visual effects. James Riley says that, with how far the technology has come, it often becomes much easier financially and logistically to film in simpler locations and later finish out the images with computers.
Digital compositing supervisor Chris Martin says that, in addition to compositing the images, it becomes possible to make a wide variety of modifications to a given scene with visual effects, such as changing the lighting conditions and the time of day. Paul Gadd explains that a separate crew went to New York to film various panoramic backdrops, such as the nighttime view of Manhattan over the East River, that are later added to scenes that the 24 crew films in Los Angeles. Brad Turner points out that a particular sequence in the final two episodes, where the CTU Mobile Command Unit is positioned across from the United Nations, would be literally impossible to achieve if it had been filmed on location, as there is no way they could have gotten permission to shut down the real intersection.
Nicholson says that a number of technologies have been developed to better help the actors and crew members visualize how the final scene will look. He shows off a few special camera rig that allows one to see a rough idea of the final product, using real time motion tracking, while filming in the normal location. For the driving scenes in 24, a complex rig of eight cameras are positioned on a vehicle to film the passing exteriors, while the interiors are filmed later using a handheld camera.
Eddie Bonin says that most of the work on 24 is what are called "invisible effects," designed to be as inconspicuous as possible and ranging from adding simple matte backgrounds to placing actors into entire digital worlds. Howard Gordon discusses the scene in "Day 8: 4:00pm-5:00pm" where Jack Bauer and Victor Aruz are walking down an avenue in Manhattan, saying that only a very small window of the frame is actual footage of the two actors while the rest is digitally added. Upon seeing this scene for the first time, he thought it was impossible, and was amazed at how convincing it looked. One of the first - and most complex - visual effects of the season was the lift-off of Cole Ortiz's helicopter from CTU New York.
For the helicopter chase in "Day 8: 10:00am-11:00am," Bonin and Al Lopez explain that Kiefer was filmed on a sound stage in a helicopter shell against a blue screen, while the other helicopters were computer-generated. Both the Apache and Falcon 1 models had to be touched up to look more convincing, as they are on screen for a long duration.
Howard Gordon and Paul Gadd say that they believe the technology used in season 8 is something that a number of television shows are starting to emulate. James Riley says that 24 has done a good job of integrating both the practical and virtual worlds, but says that it will take a few years before the process is perfectly seamless.