How do you traverse space while time stands still? In the real universe, you don’t unless you’re traveling at the speed of light. But BUF Compagnie, an L.A. and Paris-based post-production facility went where no one has gone before, producing "Khakis Swing" for Gap Inc. The spot contained effects that stop time by freezing jumping dancers in mid-air, while the camera appears to arc around them. Called a stereoscopic freeze, it's a remarkable effect that thrusts viewers into a dimension where time and space are disconnected. "Khakis Swing," a spot that appeared last year, was one of the first to use this remarkable technique, with feature films like "The Matrix" following suit.

In this frame the shot is frozen in the middle of a jump.	With the subjects still frozen, the camera appears to move to this position

BUF is not the only company to accomplish this technique, but its approach is unique in that it's post-production intensive. Unlike Dayton Taylor, another time/space separatist who uses arrays of cameras with 80 or more lenses to accomplish this visual miracle, BUF has developed custom Unix software that accomplishes the same task by digitally manipulating shots from just two motion picture or still cameras. "I've seen effects that are shot with arrays of lenses, like what Dayton Taylor is doing, and they look great," said BUF spokesman Gilles de Bonfilhs. "But it's never as perfect as what we can do with post-production software."

For the Gap spot, BUF produced two different effects shots, totaling four seconds of finished video. The idea for the production, depicting a group of khaki-clad young people gleefully jitterbugging to the vintage swing music of Louis Prima, was developed by Gap's in-house creative department. Directed by fashion photographer and music video director Matthew Ralston, the spot was filmed using two 35mm motion picture cameras set up in a triangular arrangement, with the dancers at the top of the triangle and the two cameras at its base.

Later in the spot, another jump is frozen.	The camera appears to quickly swing around to this position (pun intended).

After principle photography was complete, it was time for BUF’s Visual Effects Producer, Francesco Grisi, to work his magic. First, a D1 telecine of each camera's shot was fed into an SGI O2 workstation. Then two effects teams of six artists went to work. The first team busied themselves with the stereoscopic freeze, with the software calculating the differences between the two camera shots and simulating a camera move between them. "Basically the idea is to record different sides of the subject, then extrapolate the missing frames in the middle," de Bonfilhs said. It’s not an effect that requires enormous horsepower, either -- the stereoscopic freeze was rendered on the tiny O2 workstation at a rate of just thirty minutes per frame. Next, the other three artists rotoscoped mattes, cutting out each of the dancers and substituting white for the original background. For this, they used a custom-written software package similar to Photoshop that works specifically with animations.

BUF's custom software has been evolving for a long time. Pierre Buffin, BUF’s founder along with Michel Gondry came up with the idea of software capable of building a stereoscopic freeze about five years ago, in the era when morphing was in its heyday. But the software takes morphing a step further, combining it with in-betweening to create the unique, mind-bending effect. After a year and a half of trial and error, BUF's R&D team of ten developers and artists were finally satisfied with the technique. But don't expect to pick up a copy of this software at your neighborhood software store anytime soon. Understandably, BUF keeps the program's interface and routines a closely-guarded secret.

For more about BUF Compagnie, see http://www.buf.fr

Here's a QuickTime movie of the entire spot, "Khakis Swing" for Gap