Eye On The Ball
A shot of Andre Agassi slamming a tennis ball across court in a recent Nike commercial may serve to reinforce the maxim that the old ways of doing things are sometimes still the best. For the two second scene, created by Reel EFX, a tennis ball traveling 100 miles per hour across a net and back was captured in slow motion using a method that predates the motion picture camera.
Director Tony Kaye of Tony Kaye Productions approached the Hollywood effects house to brainstorm ideas for the Olympics-related spot. "Tony doesn't like to use optical tricks if he can accomplish the same thing with real photography," stated Reel EFX vice-president Jim Gill, "so shooting the ball on a rod and matting it into tennis court footage was out of the question." Kaye was intrigued with the notion of tracking a tennis ball across the net and back - on a parallel course - to create an unusual dolly shot. The approach proved to be unworkable however. "We calculated the G-forces required to accelerate a camera to 100 miles and hour, then - in a few tenths of a second - reverse it; and we realized that no camera could survive that."
In searching for an alternative strategy, Gill and his team found inspiration in the most unlikely of sources. "A century ago, Eadweard Muybridge set out to prove that all four legs of a horse came off the ground when it galloped, " Gill explained. "He arranged a series of cameras with trip wires alongside a racetrack and, in essence, created the first motion pictures by firing the cameras in sequence as the horse ran by."
Recognizing that the technique might work for the tennis ball shot, Gill proposed a similar setup. "We thought, 'Why try to move a motion picture camera when we can just use still cameras to capture the image at preset intervals?' We suggested setting up an array of 35mm still cameras, all positioned in a line alongside the tennis court, and rigging them to fire at precisely the right moment. Then, in postproduction, the still frames could be arranged in proper order to make a motion picture a'la Muybridge. Since 35mm still frames are about twice the size of motion picture frames, we figured we could shoot with wide-angle lenses and crop out a movie-film-sized portion of the frame, moving the crop location to account for the inevitable variations in movement and shutter timing."
With the commercial shoot just two weeks away, the most pressing requirement was to locate the necessary still cameras. "We needed high-quality cameras that could be electrically triggered and were equipped with automatic film advance," related Reel EFX software programmer and electrical hardware designer Jim Lux. A connection between Andre Agassi and Canon USA - for whom the tennis star was a spokesperson - would prove fortuitous. "On very short notice, they air-freighted 150 top-of-the-line cameras and lenses to us."
Next, the Reel EFX team began viewing videotapes of Agassi at past U.S. Open tournaments to study the aerodynamics of the tennis ball, frame by frame. An air mortar was set up in the parking lot to test-launch tennis balls at a consistent serve speed so that their trajectories could be observed and measured. "By calculating how fast the ball was moving every thirtieth of a second," said Gill, "we could determine how far apart the cameras needed to be positioned so that the ball would be directly in front of them when they fired. When we launched the ball at 100 miles per hour, we discovered that it was only doing seventy by the time it went across the net. After the bounce, it slowed down even more, to fifty or sixty."
The stationary cameras presented inherent limitations. "With a still camera, the moving ball would appear as a streak, about six inches long, against a sharp background," explained Gill, "But we wanted the backgrounds to be blurred and the ball sharp." To compensate, the cameras would have to pan to match the motion - an especially tricky maneuver, since the panning speed needed to change during the flight of the ball in relation to its changing aerodynamics.
|For a Nike commercial, Reel EFX recorded the serve of tennis star Andre Agassi using an array of computer-controlled still cameras.|
While testing continued, Gill and mechanical engineer Dylan Hixon set about designing the construct on which the cameras would be mounted. Built in just four days, the assembly consisted of a welded steel frame - 100 feet long by six feet high - with two racks designed to hold fifty cameras apiece for photographing the ball as it traveled in opposite directions over the net. Telescoping legs allowed for adjustments in the height of the cameras, while laser-aided alignment ensured that the camera rows would be queued up perfectly.
Based on the data obtained during testing, the cameras were configured two feet, ten inches apart at the service end of the court to allow for the faster ball speed, and one foot apart at the return end. Panning movements, ranging to 270 degrees, were achieved by mounting the cameras on urethane skateboard wheels attached to slots in the frame. A hydraulic cylinder, driven by a five horsepower pump, turned the wheels at the required speed.
An industrial pentium-based PC, running software designed by Lux, was employed to control the hydraulics and trigger the shutters on the cameras. "We had about two miles of wire connecting the computer to all of the cameras," commented Lux. "The software was responsible for determining the precise moment to trigger the cameras, corresponding with the desired frame rate of 24 to 100 frames per second. It also regulated the pan, making it possible for the cameras to go from a standing start to a smooth pan at 100 rpm in less than a tenth of a second. The panning movement had to be carefully controlled to avoid jerking, and to match the ball's speed throughout."
Once the rack assembly was completed, it was set up in the Reel EFX parking lot for two days of testing. Unfortunately, the 'dry run' was anything but dry. "We found ourselves right in the middle of a freak rainstorm," recalled Lux. "There we were, holding a plastic tarp over all our electronic equipment while we worked." Processed film from the tests was scanned into a computer and analyzed. "I laid the film out on a big light table, and selected frames for processing. Susan Milliken spent hours scanning the selected negatives to create image files on a PC. Then, using software that I wrote to crop the desired pieces from each frame, we generated a file that could be played back on a PC and recorded to half-inch VHS videotape."
For the day-long shoot at the L.A. Tennis Club, six members of the Reel EFX team joined the production company, Agassi and nearly one thousand extras. Positioned at the baseline so he could observe the tennis pro's serve, Gill was responsible for triggering the button that would set the cameras in motion. "We considered using a optical trigger, " observed Lux, "but found that with a little practice an observer could really do it better." Before film was exposed, a number of practice runs were conducted. "The tests helped Jim anticipate Andre's timing, and fine-tune the calibration of the cameras and the panning motion. Of course, by the time we got around to doing an actual take, the sun had shifted, necessitating a reversal in the shot direction. With everyone looking on, I had to do a frantic software rewrite to run the cameras in the opposite direction."
The crew experimented with a variety of speeds and exposures as approximately twenty takes were captured on film. "We bracketed our shots around our estimations of Andre's serve speed," stated Gill. "We said, 'Okay, we estimate the initial speed of the ball at about 95 miles per hour, so we'll do passes at 90, 95, and 100 miles per hour.' Still, the shots didn't always go as planned. In one instance, we were calibrated for a 100-mile-per-hour server, and ball went off-speed at only 70 miles per hour. So the cameras got way ahead of the ball. In another instance, Tony wanted to shoot a Nike tennis shoe being tossed over the net, which required a radical adjustment of our frame rates and pan speed to accommodate the slower moving object. Fortunately, we had already demonstrated our ability to redo the rig on the fly."
During postproduction, the film was telecined onto D-1 videotape by Pacific Ocean Post and delivered to Mad River Post where it was digitized at low resolution. Reel EFX continued to guide the effort, advising on the final processing of the three takes selected. "Usually, we hand the film to postproduction and our work is done," remarked Gill." In this instance, a lot of the effect happens in the framing, arranging and timing of the shot; so we needed to work with them, making sure it all fit together in the correct sequence."
Despite the low-tech nature of the approach - and the limited testing time afforded it - the end result proved remarkably effective. "We didn't have a chance to iron out all the bugs," allowed Gill, "so we just stunned them enough that they wouldn't come up and bite us at the wrong time. The important thing was that we were able to remain true to the director's vision."
Reproduced from Cinefex, Volume 67, by permission. Copyright 1996, Don Shay.