CG artists then turned their attention to cars and vehicles with a dramatic sequence in which every traffic light turns green... Wiseman wanted to depict car crashes from a variety of vantage points -- from car hood level hero crashes to overhead views with multi-intersection pileups. To create the sequence, Digital Dimension used a variety of techniques, including rigid-body dynamics and key frame animation. "We rigged the vehicles so that any animator could easily create realistic suspension behavior, taking into account the center of gravity, ground friction, etc." Roberts notes. "We used image based lighting (which had been acquired on location during principal photography), and selectively modeled portions of the scenes for the cars to more accurately reflect their surroundings, as dictated by proximity to buildings or other obstacles."

Tunnel extensions and flipping cars also were on Digital Dimension's plate. Photo credit: Digital Dimension. 

No Way Out
In the most spectacular sequence tackled by Digital Dimension, McClane and another character are trapped in a car tunnel, trying to evade a sniper in a helicopter. The plates were shot in an underpass that was normally used for deliveries. "Whenever the camera pointed deeper into the tunnel in our shots, we tracked in a CG tunnel rendered image, or an animation where the camera traveled down the tunnel," Bruhwiler explains. "Some of the tunnel extensions had to be accomplished through a cracked windshield, with flickering light as the car races down the tunnel, and with crashed cars, flashing lights and smoke zipping by. Very challenging! Extensive and challenging wire and rig removal was required for all of the stunt action."

The 3D team worked from blueprints and reference photos to create a 250-foot tunnel section. The CG environment was specifically designed to be tiled many times to reach any desired length. The final renders proved so seamless that, in addition to using the tunnel for set extensions, it was also employed as a hero environment for some shots. Depending on the type of camera move, the team would either 3D track the shot and render the tunnel through the moving camera, or for gentler moves, render a still frame for compositors to track in Digital Fusion.

Enhancing Practical Stunts
At one point in the sequence, the two lead characters are nearly crushed by a car that flips over in their direction and miraculously lands on two vehicles that drive past them. Interestingly enough, the car flip was realized as a practical stunt, not via CG animation. The two cars on either side of the characters were pulled along by a cable pulley system. The car flipping through the air was suspended by a cable that was passed through a gap in the roof of the tunnel location, attached to a large crane. All three cars were filmed simultaneously in camera for the hero element.

After the rig and cable removal was complete, the actors were integrated into the shot. "They had been filmed in a separate pass, and needed to be rotoscoped out and then tracked into the background plate," notes lead compositor Ryan Smolarek. "To really immerse the characters in the scene, additional CG debris was added. Because the roof of the location was not actually fully enclosed, the scene was treated to hide areas exposed to the sky and to remove as much light as possible, as to sell the idea that it was really a tunnel."

In the most publicized shot of the movie, McClane finally manages to launch his car at the helicopter and to destroy it. The remarkable stunt was completely captured in camera. The car was propelled into the air by two powerful nitrogen ratchets, and impacted the helicopter that was dangling from a 100-foot crane. Digital Dimension was then asked to paint out the rigs and crewmembers, to add a CG rotor, and to create a split composite with a stunt man jumping from the helicopter at the last second.

Playing with Models
In the film's most shocking moment, the Capitol building gets destroyed. The landmark construction was a practical miniature element that was composited into a plate of the real Capitol building. Two different takes of the model explosions were combined to create the massive blast. The effect was then enhanced with CG smoke elements, practical tree models reacting to the explosion, and matting to keep the debris near the building.

A practical model was also used for a scene in which a power plant is destroyed, but this time, the explosion only required minor enhancements. "To help depict tons of concrete falling from the roof of the collapsing power plant, as seen from the inside, we developed a debris system which included controls for debris speed, direction, rotation and debris content," says Roberts. "This enabled our artists to dial in the right proportion of concrete, wood and pipes to the director's satisfaction. In particular, he wanted to see hero slabs of concrete to tie the shots in closely with live action props seen later in the sequence. These debris elements were meticulously modeled and textured with procedural displacement shaders to ensure they held up close to camera."

A Coordination Feat
At The Orphanage, Hendershot will definitely remember Live Free or Die Hard as a dramatic race against time. "The production team really made a massive effort to make it happen. It took such extensive coordination to allow so many things to happen in parallel, just to get the shots done in time."

Alain Bielik is the founder and editor of renowned effects magazine S.F.X, published in France since 1991. He also contributes to various French publications, both print and online, and occasionally to Cinefex. In 2004, he organized a major special effects exhibition at the Musée International de la Miniature in Lyon, France.