Leading VFX house, Double Negative, has used proprietary technology to create some of the most dynamic visual effects of the year in Oliver Stone new film World Trade Center. The film is a story of human experiences and strength in the aftermath of the September 11th attack and focuses on the two men who were the last survivors to be pulled from the collapsed World Trade Center.

Port Authority policemen John McLoughlin (Nicholas Cage) and Will Jimeno (Michael Pena) were trapped in the rubble during the rescue attempt and kept each other alive for 12 hours by talking about their lives and families. Following critically acclaimed work on films such as Batman Begins and Harry Potter and the Goblet of Fire, Double Negative were asked to create pivotal effects for World Trade Center and worked closely with the production throughout the making of the film, providing on-set supervision at the shoots in New York and L.A. No other single event has been so extensively visually documented, therefore, the onus was on Double Negative to make sure that their recreation of the Lower Manhattan area and the destruction was completely accurate. The endless coverage of the disaster was a double-edged sword, as Double Negative Visual Effects Supervisor, Mike Ellis, pointed out, ‘It was a bonus to have all the footage to refer to, but on the other hand it meant that there was absolutely no room for error’.

Double Negative work covered two main stages; pre-collapse & aftermath which meant building the towers and complex, many of the buildings around them and the surrounding streets, then adding dynamic effects such as smoke, burning falling paper and debris. The second stage was post-collapse which involved building Ground Zero in CG along with all the rubble, smoke and dust. This section included two major shots, a long pullout from within the rubble out to the atmosphere and a helicopter shot of the scene at Ground Zero on the 12th September.

The film is told through the eyes of the people who were there and so does not depict the impact of the planes on the towers, as the men whose story this is, did not see it. McLoughlin and Jimeno were among a team of PAPD first responders who drove from mid-town Manhattan to the World Trade Center to assist in the rescue attempt and so the first time we see the devastation of the towers with billowing smoke streaming from them, is through their eyes.

The film editing and overall supervision was based in LA, while this could have created a logistical nightmare for Double Negative, the team used it to their advantage. They shipped a Nucoda playback system out to L.A., which was situated along side the film Visual Effects Supervisor John Scheele, with a dedicated connection to the London Office. Each day the London team would update the Nucoda with the latest shots, ready for the morning in L.A. Double Negative Visual Effects Producer, Andy Taylor, remembers, “We were effectively able to maximize the time that we had through being split into the different time zones, by producing new work during the day in London and presenting it at the end of our day to the client in L.A. via the Nucoda. During the night this would be dropped into the cut or screened to Oliver, allowing for the most up-to-date information to be ready for our next working day, all of this was key to keeping the large teaming rolling.”

CG Build of World Trade Center Complex
The Double Negative team, led by Mike Ellis, was dispatched to New York to gather reference and materials for the huge job that lay ahead. The extent of the work seemed tremendous, says Ellis, ‘We knew we would have to build the towers and the World Trade Center complex, but we didn’st know how much of the surrounding area we would have to rebuild, including Church Street, Vessey Street etc.’ Unable to film near the Ground Zero site, the Double Negative team instead took over 500,000 stills of the immediate area. These included overall panoramic views of the streets and thousands of images of each individual building face, road surface and street furniture. The stills were used as reference and materials for recreating the buildings surrounding the Twin Towers, a job that had to be perfect. Double Negative CG Supervisor Pete Bebb recalls, ‘ The Director, Oliver Stone and the VFX Supervisor, John Scheele, were both very concerned that this should look photo-real, the buildings needed to blend in completely, with the eye never noticing anything was wrong.’ Using a Scissor lift ?‚àö√ë‚àö¬® a motorised lift which goes up to 50 feet high ?‚àö√ë‚àö¬® they were able to take photographs of the building facades, 3 feet at a time, methodically creating tiles which were then stitched together in Double Negative proprietary software, Stig. Stig removes the distortion where the tiles overlap and flattens the image off. 3D geometry was built of the streets to which the images were applied as a ?‚àö√ë‚àö‚â§texture’s. Each image was exposed several times to create a single high dynamic range image, allowing Double Negative to re-light scenes any way required, they were then painted to remove any shadows or unwanted artefacts such as people, trees or other street objects occluding the building surface.

The buildings were then composited behind many live-action green screen shots that had been filmed in New York and Los Angeles. These shots required the addition of a number of objects which were seen falling from the towers that day, this included fluttering papers and plummeting lumps of concrete and debris. Therefore the greenscreens required intensive rotoscoping as the falling paper and debris were added over and behind the actors. In order to handle the immeasurable numbers of such objects, Double Negative R&D team created a rigid body engine called Dynamite. A rigid body engine generates realistic physical simulations of objects colliding with each other, and simulates real world forces on objects. This system enabled the artists to produce amazingly realistic and diverse motion on the paper which had to bend and flutter (matching on set practical falling paper) and also the rubble and debris raining down on the ground, which had to have more weight and pace as they fell from the towers. With Dynamite the artists were more efficiently able handle high numbers of objects in their shots.

The Twin Towers were a major build of their own. Without similar detailed images of the World Trade Center it had to be completely CG.

The build in itself, was not very complex, but an inherent problem with linear structures, such as the towers, is that they create a ‘moir?¬¨¬©’ effect (a crawling pattern seen when parallel lines are rendered very close together). In order to counteract this, the environment had to be rendered in 4K, and sometimes 8K, creating a huge rendering task. Also, because the buildings were very regular in appearance, it was extremely difficult to make them look real. Significant detail was added to overcome this, such as the specific way the panels reflect the light and seeing the interior of the building through some of the glass windows, all of which added another layer of realism to the image.

Smoke Library
A large part of the work for all the shots was the addition of smoke and dust. Double Negative created the billowing clouds of smoke that were seen issuing from the towers following the impact and at the Ground Zero site in the aftermath of their collapse. In order to prepare for this, Dynamics Supervisor Ryan Cook, led Double Negative T.D.s (Graham Jack, Eugenie Von Tunzelmann & Will Elsdale) in amassing a huge library of smoke simulations. Knowing that some fluid simulations such as those needed to create smoke effects can take up to a week to create at high resolution, the team recognised that preparing as much as possible in advance would pay off.

Double Negative started by building on their knowledge and augmenting a collection that had been created for Batman Begins the year before. The resulting library consisted of all types of simulations from big, heavy, large-scale smoke clouds, to lighter, gossamer wisps of smoke; a thoroughly comprehensive set of fluid simulations that more than merited the amount of work it took. As Cook points out, simulations from the library were used as a starting point in around 95% of the shots. In addition, Senior Technical Director Graham Jack, wrote a script called gto-retime, this tool enabled the artists to post-process (speed up or slow down) any of the simulations without having to re-simulate, which could take up to 7 days, adding even more diversity to the elements available for use. This amount of flexibility was necessary as the smoke also changed during the course of the day, says Ellis, ‘To start with it was thick, black and oily and as the day wore on it seemed more light and creamy and more like dust and particles.’

The work on the smoke didn’st stop there, the R&D department, lead by Jeff Clifford set about re-writing the company proprietary voxel (a kind of 3D pixel) renderer, DNB. DNB was originally devised for Batman Begins, but was enhanced significantly to meet the requirements of the World Trade Center team. Able to handle larger resolutions than other renderers, DNB is much faster and allows the artists to achieve far greater levels of detail and realistic lighting effects on the smoke.

The R&D department developed ways to quickly visualise what smoke would look like, allowing the artists to rapidly layout scenes. This meant that the client could sit with the artist during scene layout and instruct them exactly where to place the plumes and smoke in real time, leading to a much faster sign-off of on the speed and direction of smoke. Previously, this kind of approval could have had a two or three day turn around.

Post Collapse Pull-Out shot
A major sequence Double Negative worked on in the second stage included a long pullout shot which starts underground on McLoughlin and Jimeno trapped beneath the rubble and rises through a fully CG post collapse environment, past the remains of the towers and up into the atmosphere.

This incredibly complex shot was a production in its own right, thanks to the geometry involved in re-building the complex field of debris alone. Says Mike Ellis, ‘On the one hand it was just a big pile of junk and rubble, but we knew it had to be extremely precise, with specific girders and specific beams all with specific positioning.’ With a two-week turnaround on the rendering of the shot, it was also crucial to get the camera move nailed down early and signed off.

Helicopter shot
One of the final shots of the movie is a helicopter shot of the Ground Zero site on the 12th September. Led by Pete Bebb, Double Negative pre-visualised the shot in advance to work out the framing, speed and altitude the shot should be filmed at, this proved essential for the camera crew and the pilot, who used this pre-viz on the day. The shoot had to be cancelled once for bad weather, and indeed on the day the weather was still atrocious, says Bebb, ‘Thanks to the storms we only had a window of about 2 hours to get the shot. The buildings were still wet which wasn’st accurate for the shot, but the majority were too far away for it to be an issue.’ Once the plate was shot Double Negative in house photogrammetry software, dnPhotofit, was used to align the 3D geometry of the surrounding WTC site. The shot was then tracked in a similar fashion to allow for the best 3D track possible. Says Bebb, ‘This allowed us to provide accurate holdout mattes for the additional smoke elements. It also allowed us to ‘enhance’ the buildings with rubble and damage, as they would have looked on Sept 12th.’

A 2 1/2D process was employed to keep the turnaround of the shot to a minimum. 2 1/2D involves painting over a photo/film frame and re-projecting that through the camera on to 3D geometry. This process provides relatively quick and successful results, allowing the artist to retain as much as possible of the ‘photoreal’ look. Consequently the helicopter shot was done in 6 weeks from receiving the plate. Due to the nature of the huge amounts of debris and wreckage at Ground Zero some of the shot also had to be 3D. So specific buildings and ‘iconic’ wreckage pieces (World Trade Center 1&2 remains etc.) were built in 3D, this worked best for the parallax issues associated with multiple vertical objects and a moving camera.

In addition to these major sequences, Double Negative also produced a large number of ?‚àö√ë‚àö‚â§invisible effects’s shots. These included adding Ground Zero set extensions with added NY city buildings in the background and people, cars, signposts and traffic lights to the streets. The company also provided ?‚àö√ë‚àö‚â§set dressing’s for many of the street sets, adding in dust, rubble and debris to what had been shot as a clean environment.

In total Double Negative worked on 80+ shots with a core crew of 40 artists. World Trade Center was released in the U.S. on August 9th, 2006, and in the rest of the world, later in the year.

About Double Negative
Since its formation in 1998, Double Negative has firmly established itself as a leading player in visual effects production worldwide.

Located in the heart of London’s Soho, the company is a pre-eminent visual effects studio with more than 60 feature films to its credit. Led by Managing Director Alex Hope and CEO Matt Holben, Double Negative is capable of handling projects from initial design through on-set supervision and production to post-production. All key post-production technologies are available in-house to allow for maximum flexibility.