# Introduction to Building Projection-based Tiled Display Systems

# 

# 

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

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

Mathematics and Computer Science Division

Argonne National Laboratory^1^

Computer Science Department

University of Chicago^2^

Introduction

In this paper we introduce the concepts and technologies used to build
tiled display systems. The motivations for construction of tiled
displays are manyfold. For some applications one is limited by the
resolution of today's single displays. In other applications the desire
is to produce a large-format display with high resolution per unit area.
And in others there is interest in embedding the display into the
working environment in a way that the display becomes an extension of
the traditional desktop display environment.

Tiled displays are an emerging technology for constructing
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.

Several fundamental technological problems needs to be addressed to make
tiled displays practical. 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. We will briefly touch on these topics in
the next sections. Each of the in-depth research papers in this special
issue covers a distinct approach to each of these problems.

Screen Materials

When constructing a tiled display system the first step is to choose a
screen material. Most tiled displays are designed to be rear projected.
This is not a requirement, but the display geometries are greatly
simplified if simple projector throw can be used. Therefore, we limit
our discussion to screens that support rear projection. Various rigid
and nonrigid screen materials are available. The primary criteria for
choosing screens include image performance (e.g., brightness,
resolution, angle of view, contrast ratio), availability of large
seamless sheets, type of mounting method, suitability for touch screen
applications, rigidity (or degree of self-support in large-span
applications), weight, fragility, portability, and cost. As a result of
the popularization of rear-projected large-format home entertainment
systems, there is an increasingly large market for advanced-projection
screen materials \[JenMar\]. However, these materials often are
available in limited sizes and may be optimized for non-overlapping or
single-projector applications (e.g., requiring a Fresnel lens for
increased viewing angle support). In addition to physical constraints,
for example, if a system must be moved, then one needs to use a
lightweight screen and lightweight mounting system (see figure 1). This
would probably indicate using a flexible fabric screen material and a
tension-based mounting system. If the geometry is not planar, then one
may need to build the screen in segments or use flexible screen that can
be bent or stretched \[Stewart\].

For permanent installations more options are available. The best image
quality requires matching the screen with the projectors; in particular,
the color balance and dynamic range of the screen need to be matched
with the projectors. For materials that are not available in arbitrary
sizes, some form of physically tiling the screen is needed. Tiling the
screen may require very rigid mounting structures to ensure that joints
are as seamless as possible and that the screen remains stable over
time. Some of these structures can weigh hundreds of pounds and become
part of the room's physical infrastructure. In some instances screens
can be designed into new construction.

Figure 1. Rendering of a partial model of the Mural, from behind. This
shows the details of the projector support structure and the projector
positioners. These systems are used to align the projected images from
each projector at the screen surface, visible in the rear of the image.

## 

## 

## Projectors

The choice of projectors is perhaps the most difficult decision, partly
because they represent potentially the most expensive subsystem
(depending on the display generator), but also because the number and
variety of manufacturers are great. The international InfoComm
conference is a good source for the latest in projector technology
\[InfoComm\]. For purposes of this paper we categorize the projectors
into three classes: (1) low-cost LCD or DLP projectors designed for the
high-volume portable business-presentation marketplace, (2) multichip
LCD or DLP projectors designed for permanent multimedia installations,
and (3) high-end projectors designed for large-venue professional
imaging marketplace using a variety of high-quality and high-performance
image modulation technologies (CRT, LCD, DLP, Lightvalve, etc.). Another
way to distinguish between these is by price: less than \$8K,
\$8K-\$20K, and greater than \$20K. Since this paper is concerned with
leveraging commodity or near-commodity components, we will focus on the
first case, but we will also address a few issues relating to the other
projectors. Properties important for projectors include resolution,
brightness, color gamut, data interfaces, image quality, optical
quality, noise, remote control interfaces, software controls,
calibration and configuration control, stability (zoom, focus), image
refresh rate and support for stereo, resampling algorithms (e.g.,
automatic resolution conversion), flatness of illumination field,
keystone correction or off-axis projection capability, color
convergence, image alignment or adjustment and power, cooling weight and
size.

Current low-end projectors support native resolutions of 1024x768
pixels, with a move to 1280x1024 expected in the near future. Brightness
is typically in the 500-1000 lumen range, which is sufficiently bright
that an 8- or 15 projector rear-projection array will have no problem
being viewed in a fully lighted room. A critical feature for tiling
applications is the flatness of the illumination field (brightness
falloff from the center of the image to the edges). Ideally the
illumination field would have no falloff; in practice, falloff is one of
the key image quality factors that must be addressed to create seamless
displays.

Color gamut (the range of colors supported by the device) is also
important, as is color uniformity or matching of colors between
projectors. Depending on the application, color gamut testing might need
to be done prior to projector selection. Color calibration and color
gamut matching between projectors are important. Low-end projectors
often have limited color correction capability and a high-degree of
color gamut variability between projectors. As a result, a tiled display
can be limited in its ability to achieve matched colors between tiles.
Higher-end projectors tend to have more color calibration capability and
are more likely to be color matched between projectors.

Data interfaces to the projector are also an important factor in image
quality. Analog RGB inputs can suffer from cable noise and pose problems
in video formatting when switching between different input sources,
often requiring video format adjustments in the projector to preserve
framing. Digital video interfaces are just now becoming available that
offer the potential to address the noise and video format issues. It is
important that the projectors provide a control interface and API that
permits setting and changing projector parameters via a serial interface
or other computer-based control that can be automated.

Low-end projectors often have relatively simple optics and should be
tested for image quality (flatness of focus, sharpness, image
distortion, etc.). A zoom lens can enable flexible throw distances but
also creates the potential for instability if the zoom is power driven
and cannot be manually locked. For multiple-chip projectors, color
convergence (alignment of the subpixel color components into a focused
pixel) is also important for image quality. Higher-end projectors often
support convergence adjustments. Over time, projectors and bulbs age,
thereby affecting both color and brightness.

Projectors also vary in image refresh rate and support for stereo.
Current low-end projectors typically cannot refresh fast enough to
support field sequential stereo (80-100 fps). Often the multiresolution
resampling engine that enables variable input display resolution runs
more slowly than the image modulator, and this limits stereo
capabilities. An effort to identify and prioritize these critical issues
for next generation projector designs was started with the first ASCI
Advanced Projector Workshop \[OpenProjector00\].

## 

Blending, Alignment, and Calibration

Image blending, alignment, and calibration are the key major technical
challenges to producing practical tiled display systems. The goal of a
high-resolution scalable display surface demands that we address the
issues of how to transform a collection of separate tiles into one
seamless display. Work in this area centers on the following primary
problems: (1) image and projector alignment, (2) color and luminosity
matching, (3) edge blending, and (4) calibration techniques.

**Image and Projector Alignment.** When tiling a surface, the tiles must
be accurately aligned so that accumulated alignment errors do not
propagate to the point where the overall geometry is distorted. The
situation is analogous to tiling a floor while maintaining the
rectangular boundary conditions. For displays this means being able to
control the projector and subsequently the image placement and attitude
in all six degrees of freedom, in order to make the tiles uniform in
shape and scale. Such alignment often is accomplished by mounting the
projector on a positioning device. Intel and Princeton have developed a
6 degree of freedom mechanical positioner for mounting portable
projectors (see figure 4). Argonne has improved it's kinematics and made
it more compact (see figure 2). Livermore has used commercial projector
mounting system for its larger CRT projectors.

![](media/image4.wmf){width="3.0520833333333335in"
height="2.3194444444444446in"}

Figure 2. Projector positioner with 6 degrees of freedom, designed at
Argonne. The top purple plate of this positioner has mating holes
drilled to fit a Proxima DX1 projector.

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, the
processing of the images may introduce however a performance penalty.
University of North Carolina's Office of the Future Group are exploring
physical "freeform" projector placement and are handling image alignment
completely in software \[Raskar98, Raskar99\]. One interesting
possibility is video-based closed-loop servos that can automatically a
lign projectors after configuration changes. Current manual approaches
to alignment begin to become difficult at 15 projectors. Displays with
20 or more projectors will probably need some form of automated
alignment capabilities.

**Color and Luminosity Matching.** A major difficulty when tiling with
lower-end projectors is the high degree of color and luminosity
variability inherent in these near-commodity products. One approach is
to carefully sort projectors and bulbs to select a subset of devices
that are as close as possible. This approach is highly recommended. Once
the gross variation has been eliminated, however, considerable color
variability may still exist between neighboring projectors from the
manufacturer and of the same model. Manual adjustment of color using the
built-in color adjustments may be adequate. For large-arrays it becomes
nearly impossible to converge color adjustments manually. Multiple
groups are therefore developing 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 Argonne
\[Hereld00\] and LLNL \[Schikore00\] are developing techniques for
adjusting the projector characteristics. Similar strategies can be
applied to matching brightness across the array of projectors. Moreover,
luminosity falloff within a single tile may need to be addressed by
computing an inverse filter and applying that at rendering time, perhaps
by using an alpha channel.

![](media/image5.wmf){width="2.9715277777777778in"
height="2.252083333333333in"}

Figure 3. Users view the progression of a high-resolution movie on the
Mural. The movie is of a two-dimensional simulation of an X-ray burst on
the surface of a neutron star. The data for the movie was in part
generated by the DOE-supported ASCI Alliance Center for Astrophysical
Thermonuclear Flashes at the University of Chicago.

**Edge Blending.** Merging multiple images together to create a seamless
image requires overlapping image tiles and blending the images to create
a seamless transition from one tile to the next. The issues were first
laid out in work on overlapping projection onto domes \[Lyon85,
Lyon84\]. A number of ways to attack these problems have been further
developed. One can align projectors to create a fixed overlap region
between adjacent images and modify the signal to each projector to blend
the images in the overlap region by applying a virtual shadow mask
\[Inova88\]. This effect can be achieved in signal electronics
\[Panoram, Trimension\] or in software \[Surati99, Raskar99\]. There are
several difficulties with this approach. The human eye is sensitive to
the exact nature of the slope of the falloff curve, and artifacts can be
introduced. Another potential difficulty is the computational cost to
apply this mask unless it can be computed once and applied via hardware.
Furthermore, screen materials will have viewing angle dependencies that
alter the apparent falloff rate in the overlap region.

![](media/image6.jpeg){width="2.9944444444444445in"
height="2.2694444444444444in"}

Figure 4. Intel-Princeton projector positioner with aperture modulation
frame. The magnetically attached vignetting blades are easily adjusted.

One important observation is that some imaging technologies (e.g., LCD)
cannot produce true black. In these cases software-only approaches
cannot address the residual additive effects of stray light, further
reducing the contrast ratios of the image. This has the effect of
graying the overlap region of the images. Some groups have simply used
as accurate an alignment as possible to abut images and have found this
quite satisfactory, particularly where high-end projectors were used
that have superior color matching and flat illumination. An alternative
is to use physical shadow masks \[Li99\] to obscure the projected image
in the overlap region, create a feathered edge to the projected image,
and adjust the masking distance and edge geometry to achieve a blended
image without modifying the source image (see figure 4). This has the
advantage of being fast, but it requires more complex projector light
paths and structures.

**Calibration Techniques.** While it is possible today to manually
align, blend, and balance an array of a dozen or so projectors, this is
a time-consuming process easily taking one or more days of effort.
Changes in configuration require this process to be repeated fairly
often and in addition the systems can drift out of alignment or lose
color settings if powered off or reset through software. For tiled
displays to become a practical, widely deployable technology,
calibration must become an automated process that can be run as often as
necessary depending on the environment and configuration. By
incorporating cameras into the environment and automating control of
projectors and positioners, it will become possible to test a variety of
automated calibration schemes.

## User Interface Devices 

The tiled display is by design an environment that is of larger scale
than the desktop, whether it is a 6-projector super-desktop or a 24
projector wall. In each case it requires reexamination of the user
interface devices that enable interaction with the display system and
software environment. Current work in this area is addressing a number
of approaches: (1) pointing devices including 3D tracking, passive
optical (video) tracking, ultrasonic tracking, mice, and tablet
interfaces, (2) user tracking, for point-of-view rendering or for
gaze-directed interaction, via optical tracking or electromagnetic
tracking, (3) handheld devices providing control interfaces that can be
out of band from the display, (4) voice commands with audio feedback,
and (5) haptics interfaces.

Principal research issues are understanding what types of organizational
metaphors work well on large-format displays, providing user interfaces
for multiple simultaneous users, and enabling new types of interaction
with the displays. Much of this work is leveraged from work in immersive
environments. One advantage that rear-projected tiled displays have is
that the users are free to roam anywhere in front of the display without
occluding the image. This enables a very natural physical zooming and
panning paradigm, where users move around the space alternately walking
closer and further from the screen to get detail view and more global
views. For this reason it is considerably more convenient for the
interface devices to be wireless and the users to be untethered. In
addition, because of the inherent brightness of the tiled displays and
the high levels of ambient illumination possible, high-quality video
input can be available for tracking and interaction.

## Computer Systems

Two types of computer systems are currently being used to drive tiled
displays: shared memory machines (e.g. SGI Oynx2's with multiple
graphics pipes) and PC Clusters. In each case a number of problems need
to be addressed. Data management, computation speed, bandwidth to the
rendering pipelines, display synchronization, and sometimes pixel
read-back speed are among the issues, to say nothing of application
development issues. In the end, the image generator must have sufficient
performance to deliver the 2D pixel draw rates and/or 3D graphics
rendering rates required by the application. For many of the
applications areas we've mentioned this means real-time performance. In
the following paragraphs, we outline the characteristics, advantages,
and disadvantages of these two architectural approaches and cite current
research into how best to exploit their advantages.

**SGI Oynx2**. The SGI Onyx2 is based on the Origin 2000 distributed
shared memory architecture but extended by adding one or more Infinite
Reality graphics accelerators. Since the system is based on a shared
memory model, a single program can have different threads writing OpenGL
primitives into different pipes while reading from a single shared
database and synchronizing display update over shared flags. In addition
SGI distributes with the Onyx2 software that allows X windows based
applications to run across multiple displays (e.g. each display being
driven either by a single pipe or a fraction of pipe) and a window
manager that can handle a simple set of display configurations. The
shared memory of the SGI dramatically simplifies the development of
tiled display applications. At the same time, however, it ultimately
limits the number of projectors that can be supported (due to limits on
the number of graphics subsystems available in an Onyx2). Furthermore,
the high performance shared memory architecture is also relatively
expensive and requires a machine room environment for the computer
system.

Argonne \[Hereld00\], Livermore \[Schikore00\], Sandia \[Friesen00\] and
Stanford \[Humphreys99\] are using multiple pipes of SGI Onyx2's to
drive tiled displays. Each has further extended the as-distributed SGI
capabilities in different directions. Argonne has developed a version of
the CAVElib™ that works with a fifteen projector display (each pipe of a
8 pipe Onyx2 drives two projectors) and uses the shared memory
capability of the SGI for display synchronization, while supporting all
the functionality of the CAVE (except stereo rendering). Livermore has
developed a software package that presents a simple interface to a
virtual display designed for flexibility, performance, and ease-of-use.
Stanford is developing a layered windowing system that manages space on
the virtual display and streams from multiple rendering sources.

In a related approach worth mentioning here, a single PC can host a
modest number (e.g., six) of graphics cards and which can be configured
as a single large virtual desktop. Argonne's Mural uses this approach
with a single Linux workstation with six graphics adapters. It is a very
inexpensive way to support a large number of pixels for some
applications. However, bandwidth to the display may be too limited for
some high-performance applications.

**PC Clusters**. An attractive alternative to high-end computer graphics
systems, is the use of commodity PC clusters to drive the tiled display
systems. There are three principal challenging areas in connection with
these systems. First, PC clusters do not have shared memory, therefore
one needs to partition, distribute, or replicate the display generation
processes needed to create the images and arrange for these images to be
assembled such that each projector is displaying the appropriate image
for each tiled. Second, commodity graphics accelerators until recently
have had significantly less performance (e.g. polygon rendering, pixel
bandwidth) than high-end systems, are often optimized for PC oriented
games (e.g. write bandwidth to the framebuffers typically greatly
exceeds read bandwidth). Finally, the PC cluster must have a
high-performance network to support the data sharing and synchronization
needed to implement the distributed rendering or display model, and to
provide a low-latency display update synchronization needed for smooth
animation. Thus, while the PC cluster approach offers potentially a
dramatic price/performance improvement over the high-end shared memory
systems, this comes at the price of a more complex distributed display
software architecture which is the topic of active research.
Nevertheless, the performance of the PC Clusters is quite encouraging
and should be expected to improve dramatically in the next few years.

Princeton \[Li00\], Argonne \[Hereld00\], AT&T InfoLab \[Wei00\], and
Minnesota \[PowerWall\] are exploring this approach. Princeton and
Minnesota are focusing on enabling NT clusters and are exploring a
variety of techniques for parallel rendering \[Samanta99\]. Argonne and
AT&T are exploring similar concepts on Linux based PC clusters. Both
Princeton and Argonne are using Myrinet \[Li00, Hereld00\] to
interconnect the PCs in their clusters.

Future work in this area will likely investigate networks of
high-performance game consoles such as the Sony Playstation 2 and
Microsoft X-Box as alternative image generation devices due to their
high-performance graphics and low cost. \[Sony, MSX\].

## Display Software Environment

The ultimate usability of tiled display systems will be determined by
the ease with which applications can be developed that achieves adequate
performance. To this end, a number of factors must be addressed by the
software environment. First, existing desktop applications should be
able to be run without changes. Second, the details of how the
environment partitions rendering, display processing, and
synchronization should be hidden from the user. Third, new applications
written specifically for tiled displays should have simple mechanisms
for exploiting the advanced properties of tiled displays, mechanisms
such as new user interfaces or innovative window management features.

**Existing Desktop Applications**. Existing applications can be run on
tiled displays by exploiting the ability of some windowing systems (SGI
X, for example; see figure 5) to support multiple display channels and
appropriately configure the windowing environment. Often, however, these
multiple monitor capabilities have limitations in the number of
channels, or the geometry/topology of the displays and often cannot
address configurations with display space overlap (which is important
for blending). Software virtualization of the display resources is
another way to run existing applications; specifically, one runs them on
a virtual desktop having the appropriate resolution and then redirects
pixels (or other display primitives) to the appropriate display channel
for display. These approaches can be characterized as master-slave
architectures and have been implemented in several different ways. Both
Argonne and the AT&T InfoLab \[Wei00\] have a VNC-based solution \[VNC\]
for distributing pixels from a central server to display nodes.
Princeton has experimented with distributing pixels, 2D primitives, and
3D primitives for Windows NT servers \[Chen00b\]. Stanford distributes
display lists in their MOGL software \[Humphreys99\]. Performance can be
a problem with this approach since it requires running the applications
on a single machine. Chen et al. \[Chen00b\] have developed libraries to
test methods of distributing the workload by synchronizing applications
duplicated on all nodes. Argonne has investigated OpenGL and
Visualization Toolkit (VTk) implementations of distributed rendering
applications using MPI as the communications layer \[Schroeder98,
Gropp94\].

**Figure 5. Users listen to a presentation given on the ActiveMural. The
presentation includes large images, web pages, visualization
applications, and PowerPoint slides.**

**Exploiting Display Abstractions.** Applications that are written to an
abstract presentation or display layer (e.g., OpenGL) can be run on
tiled displays by using a special implementation of the rendering or
display pipeline that intercepts data and routes data to an appropriate
processor. There are multiple approaches to this technique that use
parallel instantiations of the rendering processes and partition image
space, to more complex structures that virtualize the stages of the
pipeline and load balance across available display or rendering
processors. Princeton has exploited this for OpenGL. Depending on the
architecture of the image generator (distributed memory vs. shared
memory), this may require substantial interprocessor communication and
thus require high-performance interconnect. This technique has permitted
very high performance graphics applications to run in real time on tiled
displays and permits the direct comparison of tiled displays with other
multipanel display systems such as the CAVE.

**Specific Applications for Tiled Display.** Work has begun on
developing applications specifically for tiled displays. Stanford,
Princeton, and Livermore are experimenting with approaches to manage
display window resources and abstractions that are appropriate for
large-format and multiple-display channels \[Humphreys99, Li00,
Schikore00\]. Key challenges are portability to multiple configurations
and geometries, decoupling of window and display resource management
from specific devices and interfaces, virtualization of display
resources, and support for new types of user interface mechanisms. Tiled
displays and, in particular collections of tiled displays with arbitrary
geometries and display orientations (e.g., room with walls, tables, etc.
all being active display surfaces) represent a new type of application
interface, and dramatic progress seems possible here in the next few
years.

## Conclusions

Tiled display technologies offer a range of opportunities for exploring
scalable, high-resolution, large-format displays, for applications
ranging from smart information murals to collaboration walls to
high-resolution scientific visualization. Significant progress has been
made in understanding the technological challenges relating to screens,
projectors, distributed graphics environments, and integration into
seamless systems. By exploiting commodity technologies for creating
image tiles (off-the-self projectors and consumer-driven screen
materials) and commodity image-generation technology (PC clusters with
commodity graphics acceleration), these devices should become
increasingly affordable and increasingly useful as additions to the
range of next-generation displays for building active spaces.

## Acknowledgments

The authors acknowledge Mike Papka, Joe Paris, and Justin Binns at
Argonne for their help with the Futures Lab ActiveMural project and Kai
Li, Tom Funkhouser, and the Scalable DisplayWall group at Princeton
University for insightful discussions and sharing of ideas and
technology. We also acknowledge the DOE Office of Science and the DOE
ASCI VIEWS program for providing support and motivation for some of this
work.

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