Computer screen is 18
feet wide and 8.5 feet high
by Steven Schultz
It's Wednesday afternoon, and the back wall of a 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. 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 said.
Displayed on the vast background, the square seems comically
small. "It's hard to grasp the scale." With the
click of a 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.
Dr. Shedd was demonstrating the "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 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.
The size allows 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 three-dimensional 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," Dr. Shedd
said, who taught a course last semester called Visual &
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 campus center currently under
construction, and the University is working with the computer
chip maker Intel, which has built four display walls of its
The display wall is a collaborative project of six faculty
members and eight graduate students. Project leader Kai
Li, professor of computer science, said the first challenge
was to figure out how to make a display area that could be
In the computer and television markets, bigger always seems
to be better. But Professor 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 high-resolution
screens that are measured in yards rather than inches.
The wall uses 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 avoiding
a border (such as the cabinet around a video monitor). 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," Professor Li said.
The researchers are planning a new wall that will have 15
projectors in a three-by-five configuration. The project is
being aided by Intel, which donated the hardware.
Six million pixels
Despite its size, the display wall has very fine
resolution. The current prototype has a resolution of 6 million
pixels. That compares to about 1 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
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, said Professor of Computer Science
Douglas Clark. "The computing is cheap, and it's
For example, Professor 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.
Professors 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.
Professor Finkelstein also wants to use video cameras to
track the movements of people watching the wall and change
the image to suit what the watchers are doing. One idea is
to invent a system that lets people point instead of using
Photo by Yuqun Chen
The display wall provides a background
to Ben Shedd (center, standing) and students in his course
COS 495: Visual & Audio Design for Large-Scale Computer
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. Professor Funkhouser, who
also is responsible for developing the building walk-through
idea, is working on ways to balance the load so the idle computers
do some of the work.
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?" asked Dr. Shedd, who 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
In his class last semester, Dr. Shedd combined a rigorous
study of visua l 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
"I was just thrilled with the projects," Dr. Shedd
said. "People were pushing the limits of what you could
use this for." He plans to teach the class again this
fall. Such innovative uses for the wall pull in people with
a broad range of interests and talents.
One use for the wall is in displaying complex scientific
data. Professor 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.
Assistant Professor Perry Cook has developed a low-cost
way of adding sound to the display and coordinating it with
what is happening on the screen.
"It's a cool place for anybody to do anything,"
he said. And the credit for that, the researchers said, goes
to Kai Li. It was Li who recruited Dr. Shedd, who originally
came to the University in the Visual Arts Department.
"It's really Kai's imagination that has done this,"
Professor Clark said. "The graduate students call him
For more information about the display wall, visit the Internet
site at: www.cs.princeton.edu/omnimedia.
This story first appeared in the April 12, 1999, Princeton
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