Developing displays that give users an intuitive, almost visceral experience of 3-D data is a tall order made all the more difficult by the complex brain functions such displays must trigger. To understand the challenge, you've got to first understand how we see the world. We perceive three dimensions because our brain combines the slightly different images seen by each of our eyes. A subtle interplay of optical illusions, eye-muscle tension, image focus and overlap, and head motion augment those two images with information our brains use to create the perception of 3-D.

Missing from all 2-D displays are the physical cues that guide our brains in processing a 3-D scene. Just take a look around you. That small difference between the images seen by each of our eyes is called binocular disparity. It forces our eyes to perform two other actions that are crucial to seeing in 3-D: our eyes must both converge, or point toward a common viewing location where the images from both eyes overlap, and focus at that depth. Also, movement of the viewer's head, allowing him or her to see previously obstructed parts of the 3-D image—gives the brain vital data for the 3-D image it constructs. That movement-engendered depth sense is called motion parallax.

Conventional stereoscopic 3-D display technologies, like the red-and-green glasses that brought depth to such movie classics as 1954's Creature from the Black Lagoon, provide two images to the viewer, a slightly different one for each eye. The brain resolves these into a 3-D image, but it is necessarily a yellowish monochrome one. More recently, we've advanced to goggles that use liquid-crystal shutters or light polarization to direct different images to the right and the left eye. They're used for some technical visualizations, games, amusement park rides, and in those stereo glasses for movies. But even the best goggles are hard on your eyes and difficult for most people to use for more than a few minutes.

Autostereoscopic displays dispense with the glasses, instead requiring users to position themselves precisely in front of the display. Most of these kinds of displays use special filters placed over the screen's pixels or, in the case of some 3-D liquid-crystal displays, inserted between the backlight and the screen to direct different images to each eye. Commercial systems can produce a reasonably convincing full-color image, but most people can stare at them for only a few minutes before eyestrain ensues, or they shift in their seats and lose sight of the 3-D image. Other displays made by companies such as Dimension Technologies Inc., in Rochester, N.Y., have up to nine different viewing perspectives. But the additional views come at the expense of resolution: the total LCD pixel count is divided by the number of views, resulting in low-resolution images unsuitable for computer-aided-design applications or medical visualization.

Glasses-based stereoscopic and glassless autostereoscopic displays cause physical discomfort, because they force our eyes into unnatural contortions to resolve the image. A viewer's eyes must remain focused at the depth of the display but must converge, or point, to depths either in front of or behind the display to cause the images from the two eyes to overlap. This mismatch in focus and convergence strains the eyes, resulting in significant visual fatigue, headaches, and even nausea in a majority of viewers.

Yet another kind of 3-D display, the hologram, allows viewers to see 3-D images comfortably. It has the advantage of not requiring the brain to combine 2-D images into 3-D, but it isn't electronic. Most holograms are fixed in film, so they can't be manipulated, rendering them useless for interactive technical purposes, at least for now [see sidebar, Merging 3-D Imaging and Holography].

Volumetric displays share holography's ability to create 3-D images that are easy on the eyes and less taxing on the brain than conventional 3-D displays. Their images consist of a set of voxels—volumetric pixels—distributed throughout an enclosed 3-D volume, a space that could look like anything from a half-meter-diameter crystal ball to an unusually blocky monitor. Because voxels appear at different physical depths inside the volume, our eyes converge and focus on them just as they would on any solid object.