White Paper: Stereoscopic Imaging

Posted 03/22/2010 at 9:45am | by Michael Brown

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3D is threatening to go mainstream in games, movies, Blu-ray discs, and even cable/satellite content. Here's how it works.

You can’t swing a dead Na’vi without hitting a new 3D display product these days. Three-dimensional imaging was actually invented in the 1800s, and has been used sporadically in movies since the 1920s, but James Cameron’s sci-fi epic Avatar is bringing it into the mainstream.

Now that 3D is less of a gimmick, TV manufacturers are beginning to incorporate the technology into their products. Panasonic, Samsung, and Sony all announced new 3D TVs at CES this past January. And Avatar could be the best thing to happen to Nvidia and Zalman in their efforts to sell PC gamers on their respective videocards and 3D displays. Market research firm DisplaySearch projects that annual sales of 3D-ready monitors will grow from 40,000 units in 2009 to 10 million by 2018.

So, given that at least some early adopters will buy a 3D display in due time, it’s worth knowing how this visual trickery works. Knowledge is power in the world of upgrading.

Competing technologies may use different implementations, but all 3D video is based on stereoscopic imaging: An illusion of depth is created by presenting a slightly different image to each eye. Each image is of the same object or scene but from a faintly different perspective. Your brain then synthesizes the two images into a spatial representation. The most common 3D applications depend on the viewer wearing either active eyewear (e.g., liquid-crystal shutter glasses) or passive eyewear (e.g., linearly or circularly polarized 3D glasses).

Liquid-Crystal Shutter Glasses

Liquid-crystal shutter glasses, the technology Nvidia uses for its GeForce 3D Vision product, interact with a display or a projector (we’ll use an LCD monitor for the purposes of this example, since that’s the most common solution for PC users). The lenses in these glasses contain liquid crystal and a polarizing filter that is transparent in the absence of voltage, but darkens when voltage is applied.

All 3D display technologies create the illusion of depth by simultaneously presenting the viewer’s left and right eyes with a different perspective of the same object or scene. Polarized glasses, as shown in this example, passively block light waves. The lenses in LCD shutter glasses darken when voltage is applied.

The monitor, meanwhile, rapidly alternates between showing an image from one perspective and then another. The shutter glasses and the display are synchronized so that the left lens darkens when the display is showing an image intended for the right eye, and the right lens darkens when the display is showing an image intended for the left eye. Nvidia uses an infrared wireless connection to sync its glasses to the display, but Bluetooth, RF, or any type of wired connection can also do the job.

Since the display’s refresh rate is effectively halved, the viewer’s eyes will detect noticeable flicker unless the monitor uses a very high refresh rate. Nvidia’s technology requires a monitor with a 120Hz refresh rate in order to deliver an effective refresh rate of 60Hz. Shutter glasses also deliver a darker picture than you would see without them. This is because they block half the light from reaching your eyes, and their polarizing filters are slightly dark even when they’re allowing light to pass through.

Polarized 3D Glasses

Polarized 3D glasses cost much less than shutter glasses because they produce the illusion of a 3D image without active electronics. In this “passive” scheme, polarized filters in each lens restrict the light that reaches each eye. The glasses are paired with a monitor or a projector that uses two polarized filters to display two different perspectives of the same object or scene superimposed over one another.

Light emanating from natural sources, or even common light bulbs, is randomly polarized—its wavelengths “spray” in various directions. When one of these light waves passes through a polarizing filter—say, your polarized sunglasses—only those waves parallel to the direction of the filter emerge from the other side. Glare reflected off road surfaces and water tends to emanate horizontally, and this is why polarized sunglasses with horizontally oriented filters are popular among drivers and fishermen.

In a pair of eyeglasses set up for 3D polarization, the two discrete lenses are imbued with different polarization filters. This scheme ensures that the viewer will see one “set” of light waves through one eye, and a second “set” through the other. Linearly polarized glasses use a horizontal polarization filter for one eye and a vertical polarization filter for the other. The drawback to linear polarization is that if the viewer tilts his or her head, images from the left and right channels can leak into each other. When circular polarization technology is used, a clockwise filter is used for one eye and a counterclockwise filter is used for the other. With this technology, the viewer can tilt his or her head without destroying the left/right image separation, but tilting the head still causes vertical misalignment. Zalman utilizes circular polarization in its Trimon 3D monitor.

Stereovision in Games and Movies

Stereovision can make games look fantastic, but you’ll take a significant hit in frame rate because the videocard must render twice as many images to produce the effect. For example, when we benchmarked Batman: Arkham Asylum with a GeForce GTX 285 at 1680x1050 with stereovision turned off, we achieved an average frame rate of 55fps. When we turned stereovision on, frame rate dropped to 36fps.

Movie theaters typically rely on passively polarized glasses because they’re cheaper to manufacture. Sony’s 4K system, for instance, places a circularly polarizing filter in front of a pair of projection lenses. One lens projects the right-eye image and the other projects the left-eye image. The two images are then superimposed on the screen, and the glasses filter the light bouncing off the screen so that the audience’s right eyes see only the right-eye image and their left eyes see only the left-eye image.

After spending many hours comparing 3D-enabled games on a high-refresh-rate monitor and Nvidia’s LCD shutter glasses versus Zalman’s Trimon monitor and circularly polarized passive spectacles, we developed a strong preference for Nvidia’s active solution. Ultimately, however, the quality of the gaming experience depends on how much effort developers invest in implementing the technology. Some games, like the aforementioned Batman: Arkham Asylum, look stunning; others will leave you wondering why the developer even bothered.

As for watching 3D movies at home, it’s too early to pick a winning technology. The Blu-ray Disc Association published its specification for 3D-enhanced Blu-ray video only last December, and none of the 3D-capable televisions announced at CES were shipping as we went to press. But one thing is certain: It will be far more economical to outfit the entire family with passive 3D eyewear than LCD shutter glasses.