Princeton Weekly Bulletin April 12, 1999

Wall comes to life

Collaboration produces computer monitor 18 feet wide and 8.5 feet high

By Steven Schultz

It's Wednesday afternoon, and the back wall of the computer lab is coming to life. At first just an expanse of gray plastic, the wall flickers with a warm light, then goes dark except for a smallish white square on the lower left side.

Display wall provides background to Ben Shedd (c) and students in his course Visual and Audio Design (photo by Yuqun Chen)


Ben Shedd, senior visiting scholar in the Computer Science Department, places a tape measure diagonally across the square. "That's the size of a 17-inch monitor," he says. Displayed on the vast background, the square seems comically small.

"It's hard to grasp the scale," says Shedd. With the click of wireless mouse he holds in his hand, the wall changes, progressing through a series of vividly colored, wall-to-wall images--panoramic pictures of campus, murals, text with foot-high letters and giant versions of everyday computer software.

Shedd was demonstrating the Computer Science Department's "display wall," a computer monitor that is 18 feet wide and 8.5 feet high. The project, the result of years of research into high-speed computing and graphics, has shattered conventional limits on the size of computer displays. It has opened up so many new ways of presenting information--at such a low cost--that the researchers believe it could revolutionize the way people interact with computers.

Group, cooperative space

The size allows viewers, or whole groups of viewers, to become immersed in images in ways that are impossible with ordinary computer monitors. For example, an architect might use the screen to "walk" a client through a life-size 3D rendering of a building. Or in a classroom or business meeting, a group of people could stand in front of the wall looking and pointing at the same image--say a spreadsheet--while another group looks at something related--say a graph or a photo--at the other end of the room.

"It becomes a group, cooperative space," says Shedd, who taught a course last semester called Visual and Audio Design for Large Scale Computer Displays, in which he and his students invented ways to use the display wall. Those responsible for developing the display wall are eager to put their technology to more wide-spread use. There is a proposal to put a display wall in the new student center currently under construction, and the University is working with the computer chip maker Intel, which has built four display walls of its own.


A network of eight four-way Pentium-Pro SMPs with E&S graphics accelerators drives eight Proxima 9200 LCD Polysilicon projectors. (photo by John Roemer Photography)


Eight projectors

The display wall is a collaborative project of six faculty members and eight graduate students. Project leader Kai Li, professor of computer science, says the first challenge was to figure out how to make a display area that could be expanded indefinitely.

In the computer and television markets, bigger always seems to be better. But Li explains that extra size comes at two major costs: quality and price. A small increase in screen size often means spending a lot more money to see a fuzzy image that lacks detail. But the display wall eliminates both problems, clearing the way for inexpensive and higresolution screens that are measured in yards rather than inches.

The wall relies on a technique called tiling, in which a large image is composed of blocks, or tiles, each containing some portion of the whole. The problem with tiling is how to avoid having a border (such as the cabinet around a video monitor) that interferes with the picture. The Princeton scientists have figured out how to make a nearly seamless series of tiles.

What allows them to do that--the guts of the display wall--is a network of ordinary desktop computers. Each of these networked computers drives a projector, much like those used in projection television systems. The projectors, placed behind the wall, cast their images on a rear projection screen that makes up the wall. As with the computers themselves, the projectors and screen are inexpensive, off-the-shelf models. The result is that the display can be expanded simply by adding more computers and projectors. In the current prototype, there are eight projectors, two high by four across.

"In our minds, what we have here is a miniature of what we think we will have in the future," Li says. In fact, the researchers are planning a new wall that will have 15 projectors in a three by five configuration. The development of the project is being aided by Intel, which donated the hardware.

Six million pixels

Despite its size, the display wall has very fine resolution. That is, the screen has a high concentration of the minute dots of light, called pixels, that make up a computer screen. The current prototype has a resolution of six million pixels. That compares to about a million for a desktop monitor. The upgraded wall will have 20 million pixels. The result is that the wall can display finely detailed images, like streets on a road map, while also showing several whole maps' worth of area at once.

Making these images look really good, however, means solving a host of technical problems. The researchers prefer to solve these problems by writing computer programs rather than buying more expensive hardware, says Professor of Computer Science Douglas Clark. "The computing is cheap, and it's always better."

For example, Clark and Assistant Professor Adam Finkelstein are working on ways to make the seam between the screen panels even less visible. Each projector overlaps its image a little with the next one, creating bands of brightness at each border. Also, the color never quite matches between the different projectors, and every time the projectors are turned on, they point in a slightly different direction, making the panels misaligned. Clark and Finkelstein are developing a system in which a small video camera mounted in the back of the room monitors the wall, allowing the controlling computers to compensate for these problems.

Finkelstein also wants to use video cameras to track the movements of people watching the wall and change the image to suit what they are doing. One idea is to invent a system that lets people point at the wall with a finger or a simple plastic stick, instead of using a mouse.

Assistant Professor Thomas Funkhouser is trying to increase the wall's versatility by using the computer network more efficiently. If there's an image in only one section of the wall, the computer behind that section does all the work while the others sit idle. Funkhouser, who also is responsible for developing the building walkthrough idea, is working on ways to balance the load so the idle computers do some of the work.

IMAX pioneer

Beyond these technical matters, the final challenge is how to use the wall. "What do we do with this vast amount of visual real estate?" asks Shedd. Shedd is the one to answer that question: He is an academy-award winning director and producer of documentaries and a pioneer of IMAX, the giant format films shown on screens as tall as multistory buildings.

In his class last semester, Shedd combined a rigorous study of visual and graphics arts theory with open-ended exploration of new ways of presenting images and information. The class attracted students with backgrounds from art to economics. "I was just thrilled with the projects," Shedd says. "People were all pushing the limits of what you could use this for." He plans to teach the class again this coming fall.

One use for the wall is in displaying complex scientific data. Finkelstein, for example, is working with faculty from astrophysics to show how the density of hydrogen gas is distributed across a large volume of space, something that is impossible to visualize on a conventional screen.

Such innovative uses pull in people with a broad range of interests and talents from outside the Computer Science Department--something the display wall researchers find gratifying. "It's a cool place for anybody to do anything," says Assistant Professor Perry Cook, who has developed a low-cost way of adding sound to the display and coordinating it with what is happening on the screen. And the credit for that, the researchers say, goes to Kai Li. It was Li who recruited Shedd, who originally came to the University in the Visual Arts Department.

"It's really Kai's imagination that has done this," Clark says. "The graduate students call him 'The Man.'"

For more on the display wall, see