# Developing Tiled Projection Display Systems

Mark Hereld^1^, Ivan R. Judson^1^, Joseph Paris^1^, Rick L.
Stevens^1,\ 2^

{hereld, judson, paris, stevens}@mcs.anl.gov

Mathematics and Computer Science Division

Argonne National Laboratory^1^

Computer Science Department

University of Chicago^2^

## 

## Motivation

Tiled displays are an emerging technology for constructing
high-resolution semi-immersive visualization environments capable of
presenting high-resolution images from scientific simulation \[EVL,
PowerWall\]. In this way, they complement other technologies such as the
CAVE \[Cruz-Niera92\] or ImmersaDesk, \[Czernuszenko97\], which by
design give up pure resolution in favor of width of view and stereo.
However, the largest impact may well be in using large-format tiled
displays as one of possibly multiple displays in building "information"
or "active" spaces that surround the user with diverse ways of
interacting with data and multimedia information flows \[IPSI,
Childers00, Raskar98, ROME, Stanford, UNC\]. These environments may
prove to be the ultimate successor of the desktop metaphor for
information technology work.

Immersive display systems that can deliver high resolution to a group of
viewers need to cover a large area with lots of pixels. Tiling displays
together to create a single seamless image is one potentially economical
way to do this, provided that the integration methods used to build the
array up from single units scale favorably. It is currently feasible to
deliver 20 Mpixels or so to an area 16 feet by 8 feet with relatively
inexpensive components. Such systems afford the unprecedented and
compelling ability to view huge data sets from a distance of 6 feet or
so, taking in a large swath of rendered representational reality, and in
a few short seconds step closer to examine minute details at
considerable resolution. Our goals when building tiled display systems
are to have:

-   A single seamless large display system, with 5 -- 20 million pixels,

-   Automated or semi-automated setup and maintenance systems,

-   Inexpensive component technologies, utilizing commodity hardware
    wherever possible,

-   Flexibility in input signals, so we can be driven by expensive
    rendering hardware systems (SGI) and by inexpensive clusters of
    commodity computers using game-based video adapters,

-   Scalable solutions to the problems encountered when tiling
    projectors.

How far into the future will tiling techniques be useful? To help us
think about this question, we have tried to imagine a natural limit to
the number of pixels one might want to show in a workspace. Consider
creating a single image using either a modest number of
yet-to-be-developed projectors with extraordinary resolution, or a
difficult-to-underestimate number of modest resolution projectors. The
display area might be 24 feet wide and 8 feet tall -- a human scaled
device that would invite a small group of collaborators to interact with
one another and with the rendered reality. We might naturally limit the
resolution to that which corresponds to high-quality print material or
photographs which today is approximately 1200 dpi. To achieve that level
of resolution across a large-scale display surface during the next
decade is likely to require the use of even more aggressive tiled
display technologies. While we ultimately believe a more integrated
display system (e.g. organic LEDs) or a type of more freeform display
system will supplant tiled display systems, we also believe this
technology to be significant for quite some time into the future.
Current packaging systems for tiled displays make it somewhat difficult
to build compact or wrap around displays that will ultimately be needed
for tiled immersive display systems, we believe the prototype systems
described here are a contribution in that direction.

## Background, Challenges, and Experiments

Building immersive tiled displays involves solving a set of problems
that are not addressed by other immersive system designs. These include
(1) choice of screen materials and support structures, (2) choice of
projectors, projector supports, and optional fine positioners, (3)
techniques for integrating image "tiles" into a seamless whole, (4)
interface devices for interaction with applications, (5) display
generators and interfaces, and (6) display software environment.

Most commercially available large video display systems are not designed
to satisfy the goals of the immersive display system. Rather than
increase the resolution, they make a standard resolution signal viewable
in large area formats by scaling or replicate. Recent work on tiled
display systems by a number of groups is surveyed and summarized in a
topical issue of IEEE Computer Graphics and Applications \[Fox00,
Friesen00, Hereld00, Li00, Schikore00, Wei00\].

In the following we discuss key areas of concern in developing practical
tiled display systems: tile alignment, image overlap blending, color
gamut matching, intensity falloff correction, and distortion correction.
In each area we will give background, discuss major challenges, and
describe experiments that we have been pursuing to characterize the
problems and find solutions.

**Tile Alignment**

In practice, it is difficult to achieve subpixel alignment even with
fine adjustment in each dimension. MIT and UNC have developed software
approaches to automatically "distort" the images prior to projection to
match edges using calibrated cameras \[Surati99, Raskar99\]. Princeton
has extended the ideas to work with uncalibrated cameras \[Chen00a\].
This technique is used in addition to mechanical alignment. University
of North Carolina's Office of the Future Group are exploring physical
"freeform" projector placement and are handling image alignment
completely in software \[Raskar98, Raskar99\].

Projector alignment must be accurate, stable, and inexpensive. Current
low cost commodity projectors can be purchased for about \$5,000.
Supporting hardware such as positioners should cost a fraction of this
in order to keep overall costs low. The pixels on the *Active*Mural are
about 1 mm in size. To preserve the native resolution of the single
tiles we need to control the position and drift to within about a tenth
of that.

We have designed a practical and inexpensive 6D positioner so that
projectors can be aligned quickly and accurately. The adjustment screws
are placed to ensure simple and intuitive relationships between
adjustment and effect. The kinematic design of the contact points
enables repeatable and stable adjustments. Component count and
complexity is kept low in the present design to help keep manufacturing
costs low. The design is available for others to utilize to either
fabricate or refine.

![](media/image1.wmf){width="2.751388888888889in"
height="2.1020833333333333in"}

Figure 1. A projector positioner designed and fabricated by Argonne
National Laboratory.

We have measured the sensitivity of image shape (keystoning) and
alignment on the screen to changes in the positioner screws. Adjustments
as fine as 0.1 mm at a projection distance of 70 or so inches are
possible.

We have also measured the stability of the projected image relative to
the fixed the screen over a range of time scales from 1/15^th^ of a
second up to 5.5 hours. Experiments to extend these measurements to
days, weeks, and months are underway. On these timescales, the
combination of shelving, positioners, and screen frame proves to be very
stable. We place the upper limit to drift at 0.08 mm/hour at the screen
(or less than about 0.1 pixel / hour).

**Image Overlap Blending**

Overlapping image tiles and tapering the brightness of the image from
each projector can result in a smooth intensity transition from tile to
tile. This effect can be achieved in signal electronics \[Panoram,
Trimension\] or in software \[Surati99, Raskar99\]. As has been pointed
out in the literature \[Chen00a\], unvignetted light from the projector
results in a brighter-than-black level in and around the projected
image. Overlapping or even abutting such images results in a bright
region that can only be eliminated by adding baffling to the light path.
One approach, referred to therein as aperture modulation interposes a
baffling window between the projector and screen so as to remove this
stray light while simultaneously grading the light from one projector so
that it's overlap with it's neighbor results in a continuous and smooth
transition.

Residual misalignment, image zoom error, and distortion also drive our
design to include blending (either hardware or software). An added
benefit of overlapping and blending adjacent projectors is that these
problems are somewhat ameliorated by the soft averaging of errors. In
both of our tiled display systems we have incorporated optical blending.
For most purposes we find that the distance from the projector to the
blending mask hardware is not a critical parameter. Accordingly, we have
designed a very inexpensive system of adjustable bars based on
off-the-shelf lightweight extruded aluminum components. The systems are
easy to assemble and easy to adjust. Furthermore, we have experimented
with falloff compensation using alpha masks computed to compensate for
the falloff as viewed from a fixed position.

**Gamut Matching**

Projector color temperature, gamma, and intensity vary not only from
unit to unit, but over time as well. For large-arrays it becomes nearly
impossible to converge color adjustments manually. Several groups have
developed automatic or semiautomatic color-matching techniques that use
colorimeter or digital camera inputs and closed-loop optimization
algorithms to calibrate and correct the illumination reaching the
screen. Groups at UNC \[Majumder00\] and Princeton \[Li00\] have
reported methods for correcting the image computationally before sending
it to the projectors, while we \[Hereld00\] and LLNL \[Schikore00\] are
developing techniques for adjusting the projector characteristics.

Traditionally, color matching has been very important in the publishing
sector, but considerably less so in scientific visualization
applications. When several projectors are arranged to simulate a single
large display the color mismatch is not only noticeable, it reduces the
effect of immersion since it causes the users to perceive the tiling in
the display system. Inexpensive and scalable solutions to problem of
matching color response of tiles are as yet unavailable. Even very
expensive projectors require regular and time-consuming calibration
adjustments.

In the area of color matching and calibration much work has been done in
desktop publishing (and other production) arenas. Well-developed systems
are beginning to mature for characterizing different devices and
matching the image perceived -- all of this under the banner of 'color
management systems'. Corporate consortia have evolved various standards
that look like they are proliferating \[ICC\]. These systems target high
fidelity reproduction of images as they pass through scanning, desktop
displays, and different printing phases. None of the methods are
targeted at displays utilized for real-time visualization applications.
Nor do they address simultaneous calibration of more than one of device,
for high fidelity side-by-side comparison. The basic components might be
available, but will probably require modification to most effectively
address tiled displays.

We have been studying two approaches to solving this problem. First, we
are studying image-based automatic real-time calibration of the entire
tiled display using inexpensive commodity camera, acquisition, and
computing hardware. And, second, we are working on designs for future
commodity projectors that feature modular open design to enable
configurable in-projector frame-buffer processing, internal calibration
sensors, and swappable optical subsystems.

We have surveyed consumer grade digital cameras and video cameras (both
analog and digital) for use as color gamut matching sensors. The digital
still camera market is only just now beginning to supply cameras with
the desired properties: computer controllable, fully 'manual' operation
for simultaneous control over exposure time and aperture setting, high
speed image download (firewire or USB). Until now, such features were
available only at the highest end, the so-called professional models. In
the analog video camera plus frame grabber solution, it has been equally
difficult to find cameras with the right properties.

We worked on methods based on a very inexpensive setup, one that relied
on an unmodified low cost video camera. Automatic gain control and fixed
integration time were serious impediments. By defeated the AGC,
following instructions from the manufacturer in Taiwan, we were able to
extract accurate and precise measurement of R, G, and B components. Out
of the full range of 256 binary codes in each channel, a single pixel in
a single image frame can be read to an accuracy of about 3 digital
units. Modest averaging in time or over several pixels results in very
accurate measurements. Furthermore, we have determined that much of the
noise is correlated in time for each of the three colors, and is
therefore amenable to further reduction.

Using the projector's serial interface we have developed codes to change
some of the projector configuration settings, and used this
infrastructure to experiment with feedback systems involving cameras,
network communications, multi-platform distributed control programs, and
simple algorithms to feedback sensor information to drive projector
matching.

**Intensity Falloff Correction**

The falloff of a projector is the difference in perceived brightness
across the projected image. Since commodity projectors are designed and
sold primarily for single unit use, this is one area that the
manufacturers tend to relax the quality constraints. Different
projection systems have different design considerations that can modify
the light sources natural falloff. Some projectors incorporate optical
elements to flatten the light falloff before it gets to the active
element. In DLP projectors the economical designs of the low cost
projectors rely on off-axis optics, early designs did not incorporate
any elements to flatten the illumination before the active element. One
such projector, the Proxima DX1 is the projector we have in our
*Active*Mural. This off-axis projection optics distort the shape of the
falloff pattern, making it a bit more difficult to correct.

We have investigated image modifications that will alleviate the
problems to a degree. In Figure 2 we show the results of one test where
we have projected a flat field, from that flat field we have generated a
surface that represents the brightness as perceived by a test camera. A
contour of the same data is shown in Figure 3.

![](media/image2.wmf){width="2.75in" height="1.3409722222222222in"}

Figure 2. Three-dimensional plot of the falloff of a Proxima DX1
projector through a Stuart FilmScreen™ material, like that used in the
CAVE™ walls. The projector is showing a flat white field. The view is
from the top middle of the projected image, from in front of the screen.

We have measured the falloff of two different technology projectors (LCD
and DMD) on two different types of screen (Stewart and JenMar) in order
to compare directly the effects of different projector optics (imposed
in part by the underlying light modulation technology) and different
screen materials. We find that the DMD-based projector has a
significantly sharper falloff than the LCD projector tested. See Figure
7 for the comparison measurements.

![](media/image3.wmf){width="2.748611111111111in"
height="2.747916666666667in"}

Figure 3. A contour plot of the falloff of a Proxima DX1 projector
through Stuart FilmScreen™ material. The projector is showing a flat
white field. The view is from in front of the screen surface,
orthographic to the center of projection.

Distortion Correction

Commodity projector optics typically have distortions due to both the
quality of optical components and to cost-saving design decisions. These
distortions cause users to perceive the tiling of the projectors, and
can't be addressed by the projector positioning system. Typical
distortions include barreling and stretching of the projected image as
the zoom is increased.

In order to reduce the distortion we are developing an automated
software system that will measure the distortion of a projector at a
given zoom setting, and create an image transformation that will make
the image appear undistorted. We are interested in measuring how much
data may lost via the transformations needed to correct the projected
images.

-   Studies software solutions for blending, color matching, and
    distortion correction.

-   Open projector design initiative to develop future projectors with
    support for solution to these and other problems facing the
    community.

## ActiveMural and μMural

We have built two rear projected tiled display systems, the
*Active*Mural and the μMural. A quick rundown of each identifies basic
configuration issues and compares these two tiled display form factors.

The *Active*Mural display area is 16 feet wide and 8 feet high, composed
of four, 47.5" wide by 96" long sheets of JenMar Visual Systems
BlackScreen™. The four sheets are suspended vertically side by side with
a custom frame we designed. They are loaded primarily by gravity. The
panel edges are pulled together by springs attached to the bottom of the
frame.

The *Active*Mural uses a 5 x 3 array of Proxima DX1 projectors, which
are native XGA projectors. This endows it with a maximum resolution of
5,120 x 2,304 pixel. The DX1 is based on a single chip micro-mirror
array from Texas Instruments.

We typically drive the *Active*Mural with one of two sources. Connected
to a Linux cluster, with each node in the cluster containing an AGP
graphics adapter, we experiment with distributed rendering techniques.
When driven by our Onyx2 Infinite Reality graphics pipe we run
applications designed to run on the CAVE, and applications that need the
high performance that the shared memory architecture of the Onyx2 can
deliver.

Where possible we have built the *Active*Mural from using commodity
parts. The projector array is arranged on a lightweight, adjustable,
inexpensive, widely available, wire-shelving product known as Metro. We
have constructed an adjustable framework for optical blending of the
images from adjacent projectors. It is made structural aluminum
extrusion, manufactured by Barrington Automation, and sold by the name
FrameWorld.

![](media/image4.wmf)

Figure 4. A drawing showing the construction of the frame of the
ActiveMural. The blue vertical lines show where the seams in the 4' wide
panels appear.

The steel and aluminum frame, which supports the screen panels, was
designed and fabricated by Argonne engineers and shops. The four screen
pieces are hung in the frame as shown in the figure below. We have
designed into our frame a set of adjustment mechanisms, both along the
top and bottom edges of the frame that allow us to minimize the space
between the screen pieces.

![](media/image5.jpeg){width="2.7465277777777777in"
height="2.0597222222222222in"}

Figure 5. A partial model of the μMural, showing the projection path and
the blending hardware. The connections of these parts and the frame that
holds this assembly at viewing height is not shown.

The μMural is a six projector portable version of the *Active*Mural. It
is designed to provide a wide aspect ratio. With overlapped image tiles,
its resolution is 2532 by 1407. The projectors in the μMural are Epson
7500c projectors. These are native XGA, 3 panel LCD projectors. Compared
to the DMD projectors in the *Active*Mural, they have a flatter
illumination pattern.

The structure of the μMural is made entirely of aluminum extrusion. The
projector positioners sit directly onto mounting plates attached to the
extrusion. The screen is framed with extrusion, with this frame it is
simple to attach to the rest of the structure. The blending hardware is
part of the shelving system, which can be raised and lowered both for
shipping and to adjust the height of the screen.

We can drive the μMural projectors from a number of sources: three to
six Infinite Reality graphics pipes, a single eight-channel pipe, or a
cluster of Linux workstations with AGP video adapters. However, it is
normally driven by a single six-headed Linux workstation, using the
latest XFree86 4.0 release, which incorporates the Xinerama extension to
the X server.

The screen material in the μMural is the same BlackScreen™ that we use
in the *Active*Mural. It is a resolution of greater than 200 lines per
inch and a contrast ratio greater than 250 to 1. The ambient light
rejection on the screen is greater than 96%. These factors together make
the screen bright enough to fully usable in a normal room, with the room
set for normal ambient conditions.

## Lessons Learned

In this section we collect our experiences into a discussion of
observations and lessons learned. They are organized into the same
sections that we used to describe our experimental efforts with the
addition of a discussion on projectors themselves.

Alignment

Off-axis optics is not a benefit for many (most) tiled display systems.
In the case of DMD-based, and probably reflective LCD-based, projectors
it is a byproduct of optical needs dictated by the internal layout of
crossed beam paths. It complicates the first pass of gross projector
alignment by augmenting the effects of keystoning. Taking out keystoning
is perhaps the most tedious aspect of projector alignment.

Blending

Optical blending is critical to effective baffling of stray light. We
have found that blending works reasonably well for the LCD projectors in
our μMural which have gradual intensity falloff compare to DMD
projectors we have tested. It has been more difficult for projectors
with a rapid and asymmetric falloff. Accurate adjustment of the distance
between the projector and the blending masks is not necessary. Simple
estimates suffice. Ganged adjustment of the blending edge of several
projectors in a line is convenient and practical. Such arrangements also
lead to less expensive blending hardware.

Color

Even inexpensive cameras seem to have sufficient dynamic range, noise
performance, and repeatability to affect decent (if not excellent) color
matching on a tiled array. Important features include:

-   Controlled exposure time

-   Controlled aperture

-   Adjustable zoom

-   Adjustable focus

-   Fast data path to analyzing computer

It is important to distinguish between two issues: matching versus
absolution calibration. The latter is more restrictive and requires a
truly calibrated camera. Off-the-shelf solutions for absolute
calibration tend to be quite expensive on the one hand, and/or require
manual intervention to place calibration head on each tile area.
Matching is less demanding, and for most purposes completely adequate.

Another simplification is that our measurements can often include large
portions of the tiled display so that comparisons can be made
simultaneously within a single frame. Such measurements are considerably
more tolerant of fluctuations in the camera's behavior.

![](media/image6.wmf){width="2.752083333333333in"
height="1.7027777777777777in"}

Figure 6. This graph shows the Topica 8002-DS camera response to an
alternating, but increasing white signal from the projector. This was
captured at fifteen frames per second, for six seconds. The projector
held each value for 0.6 seconds.

We have looked for inexpensive colorimeters to provide ground truth for
our camera-based colorimetry experiments. For a thousand dollars we
found a fairly nice one (Klein LMX92Q). It is hand-held, sticks by
suction cup to the screen, and includes useful software for configuring
and completing experiments. It is designed for calibrating computer
monitors. We have come across two problems that are worth mentioning.

-   The sensor head is very sensitive to orientation on the screen since
    the light power emanating from the projector and screen are far from
    lambertian. Because of the placement of color sensors in the head,
    we surmise, this results in chromaticity as well as luminosity
    dependence on the orientation angle of the head. Consistently using
    the same orientation is probably sufficient to generate reliable
    enough measurements using this device.

-   The DMD projectors produce fluctuations in intensity and color
    coming from the rapidly multiplexed light of the projector cause
    grievous noise problems. The colorimeter samples too quickly to
    average over these fluctuations appropriately.

Falloff

We have worked with a number of projectors in the laboratory, and a few
in sufficient numbers to work with at least small tiled arrays of them.
As with most unwanted effects in projected images, they become
significantly more perceivable in tiled arrays than when standing alone.

![](media/image7.wmf){width="2.9993055555555554in"
height="1.851388888888889in"}Figure 7. This graph compares the falloff
of the Proxima DX1 and the Proxima 9210 projectors on both JenMar
BlackScreen™ and Stuart FilmScreen™ rear projection surfaces. For each
projector we plot the intensity of the center of the peak (C), the top
right (TR), top left (TL), bottom right (BR), and bottom left (BL) of
the projected image. Values are normalized to the central peak. For a
perfect projector and screen all measured intensities would be 1 in this
graph.

Off-axis projection exacerbates the falloff problem, too. The edge
farthest from the optical axis is very dim, and in symmetrical projector
layouts this places the dark and slowly falling edge at the top of a
projector next to the bright and rapidly changing bottom edge of its
neighbor.

Projectors

Several general observations:

-   Flat illumination is a property of the projector design. It varies
    significantly from model to model in the consumer grade devices. One
    can pick badly.

-   Color is less amenable to selection. In any projector you can expect
    it to vary significantly, with bulb type and bulb age.

-   Bulbs used well past their stated lifetime tend to explode,
    literally.

-   In many ways, off-axis projection is one of the villains for our
    designs.

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-   DMD-based projectors make camera-based measurements somewhat
    difficult because their light is multiplexed by the filter wheel and
    the pulse code modulation.

-   Computer control of projector configuration and internal state is
    extremely useful.

Many of the problems facing development of tiled displays would benefit
from access to one or more of the component subsystems within the
projector itself.

-   Modified optical components might minimize the adverse effects of
    off-axis projection.

-   Access to the digital data stream driving the LCD or DMD could
    enable an inexpensive alpha-buffer-buffer based intensity falloff
    filter with zero impact on rendering applications.

-   It could also enable much better control of gamut matching
    parameters.

-   Modest computing power inserted in the internal stream could be
    employed to digitally correct distortion.

-   Fine adjustment of the image position could help in alignment and
    alignment drift cancellation.

These and other concerns and possibilities have motivated us to
instigate a discussion in the community about devising an open projector
specification \[OpenProjector00\].

## Future work

A Smart Projector is a projection system tightly coupled with enough
computing power to do rendering, image processing, and control of the
projection system. A Smart Projector has a network interface through
which digital information is delivered to the projector for both control
and display. We propose to build a Smart Projector, based upon an open
projector specification.

In two areas we will be continuing and extending our work:

-   continue development of fast, cheap, integrated, and automated color
    calibration techniques;

-   intensity falloff compensation; and

-   camera-based alignment calibration.

We have also begun to work through a design of a folded optical path for
a short throw version of the μMural. With an overall depth of about 40
inches, it would fit into an office.

![](media/image8.jpeg){width="2.75in" height="2.0625in"}

Figure 8. A Smart Projector prototype. The attached NetwinderDM 275
computer drives the Epson 7500c projector. The NetwinderDM reads frames
from the camera to do a variety of measurements on the projector.

## Acknowledgements

The authors acknowledge Mike Papka and Justin Binns at Argonne for their
help with the Futures Lab ActiveMural project and Kai Li, Tom
Funkhouser, and the Scalable Display wall group at Princeton University
for insightful discussions and sharing of ideas and technology. We also
acknowledge the DOE ASCI VIEWS program for providing support and
motivation for some of this work. This work was supported by the
Mathematical, Information, and Computational Sciences Division
subprogram of the Office of Advanced Scientific Computing Research, U.S.
Department of Energy, under Contract W-31-109-ENG-38.

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