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Pseudoholography

Meanwhile, at NYU’s Center for Advanced Technology, the other early leader in the race to produce this new wave of 3-D, Perlin’s group is enlisting a nonholographic technique capable of providing dynamic, angle-adjusted images that look like those produced by holographic systems. Furthermore, the images are not conjured up by using complexly modified laser light. Instead they are displayed on a relatively ordinary monitor in an approach Perlin calls “a holographic interface.” The group pulls this off by taking advantage of the fact that most of the vast and costly processing and display horsepower needed to produce holographic video ultimately goes to waste: a hologram provides more images than those that meet the viewers’ eyes; it also provides dazzling, angle-adjusted images to the many thousands of locations at which there are no eyeballs to appreciate them. Each of these distinct unperceived images have to be computed, transmitted, and displayed, because there is no practical way to limit holographic coverage to an observer’s specific viewing angles. “It’s like wielding an elephant gun to shoot a fly,” says Perlin. His system, therefore, displays images tailored to an observer’s precise position.

Though NYU’s NY3D technology doesn’t enlist holography, it provides an observer with much the same viewing experience as a holographic system: The mechanism is stereoscopic, providing the left and right eyes with different images, and the images change with viewing angle. And of course, no eyewear is needed.

Coaxing hologram-like images from a plain screen requires two tricks. The first comes in the form of a transparent liquid-crystal display (LCD) that alters the view of the image being shown on a monitor. The display sits a half-meter in front of the monitor. On it, black stripes about three centimeters wide flash on and off, blocking vertical swaths of the image-let’s say, a ball-on the monitor behind it. The effect is not obvious to the viewer, because the stripes shift 180 times per second. The speed is too fast for the viewer’s brain to register the location of each stripe and at the same time, gives the monitor a chance to fill in the missing swaths for each eye. The result is that each eye sees a slightly different image through the gaps in the shutter stripes-which produce a stereoscopic sensation of depth (“NYU’s NY3D System,” this page). All this works fine-as long as the viewer’s eyeballs are located exactly where the system expects them to be, each eye lining up with the appropriate image swaths on the monitor. To ensure that this is the case, Perlin’s system employs a second trick, actively tracking the observer’s eyes with two small cameras mounted above the monitor. Moreover, a set of infrared light-emitting diodes (LEDs) next to the cameras give the viewer an unobtrusive case of red-eye-the back-of-the-eye glow that has long been the bane of amateur photographers. The cameras can easily isolate the viewer’s bright pupils, enabling them to track the eyes and adjust the location of the shifting stripes so that they always block the image in a way that sustains the stereoscopic effect.

Of course, a hologram’s realism doesn’t come merely from its stereoscopic properties; holographic images can be inspected from all angles as the viewer’s head moves around them. By virtue of its eye-locating capabilities, the NYU system can readily track head motion and almost immediately alter the images on the monitor as needed. And indeed, a system demo that displays a rotating skeletal foot confirms not only that it provides a clear, fully 3-D image, but also that it allows one person to appraise the image from different angles-including from above or below. (The group is also working on a system that would simultaneously provide 3-D views to multiple observers, such as a team of surgeons debating the best approach to a difficult procedure or a group of video game players competing on a shared monitor.) The result is so realistic, says Joel Kollin, a researcher at the Center for Advanced Technology, that eventual purchasers of the display may want simply to hang it on the wall, where it would present images-say, a Fiji beach or a Paris boulevard-that actually change with respect to the viewer’s angle. “It would be just like looking out a window,” he says. As an MIT Media Lab student in the late 1980s, Kollin was largely responsible for building that group’s first holographic video system.

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