Digital backgrounds may not be as obvious as they once were, but improvements in technology have made background replacement into a safer, cheaper and more flexible substitute for real-life locations that are difficult to shoot, no longer exist or deny access to film crews.

“World Trade Center” had several reasons to re-create its real-world locations. While the towers themselves obviously had to be digitally resurrected, vfx supervisor John Scheele says plans to shoot in lower Manhattan were nixed by sensitivity concerns. “What quickly became clear to everyone is that to have our actors, to introduce any of the effect of what happened that day, would be terribly disruptive to the area and the neighborhood,” he says.

Using more than 500,000 photos taken of the area from a scissor lift on the rear of a truck, as well as accessing news footage and aerial photographs taken by FEMA, the digital backgrounds were combined with footage of the actors shot in Los Angeles.

On “Mission: Impossible III,” ILM’s Roger Guyett says digital technology allowed the filmmakers to “art direct” the city lights and air quality of the Shanghai skyline for an action sequence atop a skyscraper.

“We built downtown Shanghai and then we made it the way we thought it should look,” he says.

While a small crew went to Shanghai, the main unit shot a replica skyscraper roof built atop a parking garage.

That approach also was used for the Chesapeake Bay Bridge sequence after it was decided it was too difficult to shoot over real water and too expensive to close the real bridge for three weeks during tourist and hurricane season.

“It was so much easier to deal with something that is 10 or 12 feet off the ground and you have complete control over the environment,” Guyett says, adding that the process is much easier and quicker to execute than even a few years ago. “Now your turnaround time is so much faster,” he says.

The producers of “The Da Vinci Code” faced a similar problem for a sequence set at Saint-Sulpice, the famous church in Paris that is the site of a key scene in the film. Mark Breakspear, of vfx house Rainmaker, says the problem was solved with a digital background constructed with more than 15,000 digital photos taken by crew members who visited the church as tourists. Vfx supervisor Angus Bickerton paced out the church to provide dimensions for the resulting digital model.

The project took a small crew about four months to complete after assembling the footage, but allowed the film to use the location and film the actors in the controlled environment of a Shepperton greenscreen stage.

“Our method was a much more production-friendly method,” Breakspear says.