"We used digital doubles in ways where we might not have used them prior," said ILM VFX supervisor Russell Earl.

Are you going to see Marvel and Disney's Captain America: The Winter Soldier for the first time -- or maybe the second -- this weekend?

The Hollywood Reporter spoke with Russell Earl, VFX supervisor at Industrial Light + Magic, about some of the most challenging work on the film, which has already brought in $170.6 million domestically.

In this sequel to Captain America: The First Avenger, directors Anthony and Joe Russo pick up after the The Avengers left off, as Captain America (Chris Evans) and Black Widow (Scarlett Johansson) enlist a new ally, the Falcon, played by Anthony Mackie. Below are four things to know about how the VFX were built for the blockbuster.

1. Captain America: The Winter Soldier contains 2500 VFX shots.

Earl related that ILM handled about 900 of a total VFX shot count of roughly 2500, which is considered on the high side for a tentpole picture of that size. The film's overall VFX supervisor was Dan Deleeuw, and the lineup of VFX houses included Scanline, Trixster, Rise and Luma.

2. The film involves a lot of digital doubles -- especially Falcon.

"We used digital doubles in ways where we might not have used it prior," said Earl. "We did a full scan and photo shoot [with the lead actors] in full costume and build detailed digital doubles. [At one point] I was looking at a shot thinking it was real, and it was actually the digital double -- that made us be a little more bold in the use of the doubles. We used them in some of the close-ups."

"The character that we did the most work on was Falcon," he said, adding that it involved a variety of different techniques. "We knew we were going to do CG wings. We also did some shots with wires and some with stunt doubles and head replacement. And we needed a very good digital double."

3. The helicarriers were fully CG.

A big challenge was the creation of the photoreal CG helicarriers. "In the previous film it was more like an aircraft carrier; now it's an aircraft carrier with the addition of battle ship-sized guns," Earl said. "We were all over the carriers [with the virtual camera]. We were on the decks. We were flying next to them. We had a lot of close-ups and different angles. And we didn't just have one -- we had three. On top of that, we had to destroy them all."

This CG environment was also used in close-ups. "The challenge was to get in all of the detail to make it feel like it is a real, working ship," Earl said. "We created details down to the railings and all the human-scale stuff. ... For the shots in which we were destroying them, we had to have the internals as well -- the hallways, the storage areas."

4. Washington, D.C. involved a lot of CG due to air space restrictions.

Created environments were also needed for parts of the film that take place in the nation's Capitol. Washington is, of course, a highly recognizable and well photographed city -- but it also has plenty of flight restrictions, meaning that many locations had to be recreated in the computer.

"You can't fly over the D.C. side of the Potomac River at all. A lot of our camera positions were in the no-fly zone," Earl related, adding that the production shot what it could and the rest was computer generated, with the artists using photos for reference.

E-mail: Carolyn.Giardina@THR.com

Twitter: @CGinLA