Like the Perspecta, the LightSpace DepthCube uses a three-chip DLP. But instead of a single projection screen, its casing houses a stack of 20 liquid-crystal projection screens, each about 5 millimeters from the next [see diagram, "Peering into the DepthCube"].

Each screen sandwiches liquid crystals between two panes of glass treated with an antireflective coating. When a voltage is applied to the screen, the liquid crystals line up in the same direction as the light that is being projected onto it, and light passes directly through the now-transparent screen. When the voltage is taken off the panel, the liquid crystals relax into random orientations. In that state, the liquid crystals scatter light shone onto them, creating a voxel that looks as if it emanated from that surface location, and not the DLP projector at the back of the display.

At any given moment, 19 of the screens are transparent, and only one is in a white-scattering state. However, relying on persistence of vision, we sequence the image back and forth across the 20-screen stack. Because the screens at the back of the monitor are physically farther away from you than the screens at the front, your eyes converge and focus naturally on voxels wherever they appear.

With the projector sending out 1200 image slices per second, you'd think that the composite 3-D image, consisting of 20 two-dimensional images, would appear chopped up from one screen to the next. But by taking advantage of another psychological component of 3-D perception, people can be tricked, in effect, into seeing planes between the physical screens, virtually eliminating jitter and jagged edges.

We call this technique depth anti-aliasing, and it works like this: if a viewer is shown two 50 percent bright voxels at the same x, y location on two adjacent DepthCube planes, the viewer will perceive a single voxel at 100 percent brightness halfway between the two planes. In most cases the viewer is completely unable to see the individual image slices. Depth anti-aliasing effectively converts the DepthCube's 15.3 million physical voxels (1024 by 748 by 20) into more than 465 million perceived voxels (1024 by 748 by 608).

As in other volumetric displays, 3-D images within the DepthCube are visible over a wide and continuous field of view and have all of the depth cues of real 3-D objects, except for opacity. Light cannot block other light, and since these 3-D images are made of light, one image cannot block, or occlude, another. In other words, 3-D images are translucent, not opaque. Still, when looking at a DepthCube display you see a different image depending on your perspective—the images possess both vertical and horizontal motion parallax, which lets the viewer see around translucent objects in the foreground to reveal previously obstructed objects.

Unlike swept-volume displays, the DepthCube is entirely solid-state, so it is not affected by vibration. And, because the DepthCube is intended for front viewing rather than 360-degree viewing, its DLP projector can operate at a more modest 1200 frames per second and still provide 15-bit color (or 32 768 mixed colors), which is sufficient for rendering the optical effects of lighting, shading, and texture mapping that make 3-D images really pop.

Another advantage of this approach is its relative compatibility with existing 3-D graphics software. Because DepthCube images are projected onto 2-D planes, they have a Cartesian geometry, making the DepthCube compatible both with software that uses the OpenGL graphics language, common in technical 3-D modeling software, and with standard 3-D graphics cards.

Modern 3-D graphics cards are not completely 3-D. They translate, rotate, and scale the geometry for 3-D images using x, y, and z (or depth) information but ultimately produce 2-D images. So, for instance, when a bird flies in front of a tree, the pixels corresponding to the leaves, twigs, and branches disappear, replaced by the pixels corresponding to the image of the bird. The card accomplishes this feat by storing in one frame-buffer memory the location on the x-y plane where the pixel with its particular color will appear, and storing the z-axis information in a second frame-buffer memory. Using proprietary software we call the GLInterceptor, this depth information can be extracted in real time to create DepthCube images from nearly any OpenGL application.

After a 3-D application has filled the graphics card's frame-buffer memories with its 3-D image, our software extracts the color position and depth information for each pixel—its z location—and passes it onto the DepthCube's 3-D frame buffer, which holds image data for all 20 screens. The color and x-y location are sent to whichever of the 20 screens corresponds to the correct depth, or z location, creating a voxel. This is the key to showing different-colored voxels at different depths at the same x-y locations, to form, say, a multicolored ribbon [see photo, "That's Deep"].

We are now fine-tuning the DepthCube architecture for different markets. For example, there is a modest market for very expensive 3-D displays with diagonals of more than 50 inches. These displays, costing more than $100 000, are useful in air-traffic-control rooms and for collaboration among engineers and scientists in the automotive, aerospace, and oil and gas industries. However, the largest market for volumetric 3-D displays is for single-user desktop displays that cost less than $5000 and can be used continuously for any 3-D visualization you can imagine. We hope to demonstrate such a display later this year.

Success with less-expensive displays for technical applications will open the door to institutional and consumer markets. High school biology students could forgo the queasy trial of hands-on animal dissections by simply gathering around the classroom volumetric display to view the innards of any creature in the lesson plan. Home shopping, or even dating, via the Web might be less fraught with uncertainty. Is that a crack in the vase I want to buy on eBay or merely a scratch in the paint? Are those hair plugs or is that mane really all his? Amateur cosmologists could probe the deepest reaches of space, orbit stars, and pass through nebulae, or survey far-off worlds. Even forbidden ones.