A NEW METHOD FOR BUILDING LARGE-FORMAT TILED

DISPLAYS SYSTEMS

Sun Hanxu

School of Automation, Beijing University of Posts & Telecommunication

，

Xitucheng Road, Beijing,China

Song Jingzhou, Jia Qingxuan, Gao xin, Yao Fusheng, Cheng Tao

School of Mechanical Engineering & Automation, Beihang University, Beijing, China

Keywords: Multi-projection, Render synchronization, Stereo display, Geometric calibration, Color calibration.

Abstract: Large-format tiled display is a new emerging technology for constructing large scale，high-resolution,

immersive multi-projection virtual environment systems, which can present high-resolution stereo images. A

PC-cluster, five-display-channel tiled display system is built in this paper. It produces a 5120x768 stereo

image. The software infrastructure has been designed using a retained mode sort-first parallel rendering

paradigm. This paper discusses development issues including selecting on projectors and projection surface

to support passive stereo, synchronization tiled displays, geometric correction and color calibration etc.

Finally, overall display performance is given and future work is mentioned.

1 INTRODUCTION

In the past decades, large format displays provide

impressive visuals, high cost and technical expertise

are required to own, operate and maintain these

displays. This restricted their usage only to a few

large institutions and universities and limited the

application of virtual reality technology.

Recently, high-performance PC graphics cards

become available with low cost. This development

enables us to build clusters of high-performance

graphics PCs with reasonable cost. Now, building

Large-format tiled displays based on PC-cluster

become a trend (Michael, 2005. Raij, 2004. Raskar,

2003. Humphreys, 2002 ). It offers a large field of

view and resolution, and affords a strong immersion.

Wall-sized tiled displays are more supportive for

collaboration than regular monitors. Users stay

longer in such displays, move and discuss the

datasets more, and treat such displays as “murals”

that they repeatedly touch, inspect, walk around and

see from different viewpoints.

Following problems come out in designing large-

format tiled displays systems, including the design

of systems infrastructure framework, stereo display

implementation, geometric and color calibration etc.

The existed systems infrastructure framework

mainly had master-slave mode and synchronism

execution mode (Oliver, 2003). Geometric

correction methods mainly include hardware-based

correction, software-based correction and software

automation correction method. How to accomplish

geometric correction automatically and rapidly is

currently a hot researched field (Michael,

2003).With regard to color calibration, the

representative methods are Luminance Attenuation

Map (LAM) method (Aditi, 2002), the gamut-

matching method (Grant, 2003) and perceptually

seamless luminance balancing method (Aditi,

2003).The LAM method is simple but it decreases

the dynamic range of displays. The theoretical

disadvantage of the gamut-matching method lies in

the fact that there is no practical method to find the

common color gamut. The perceptually seamless

luminance balancing method uses expensive light

measuring instruments to address various

photometric issues.

342

Hanxu S., Jingzhou S., Qingxuan J., xin G., Fusheng Y. and Tao C. (2006).

A NEW METHOD FOR BUILDING LARGE-FORMAT TILED DISPLAYS SYSTEMS.

In Proceedings of the First International Conference on Computer Graphics Theory and Applications, pages 342-348

DOI: 10.5220/0001350903420348

 SciTePress

belonging judgement

load balancing

data adjustment

display

equipment

client node

strategy of object

distribution

tiling display

geometry data

rendering node

geometry

data

rendering node

geometry data

rendering node

geometry data

rendering node

Figure1: Diagrammatic representation of the software infrastructure.

By using front-projected passive stereo displays,

commodity Personal Computers (PC), low-end

projectors, 100M Ethernet equipment, and surround

sound system, we build a low-Cost 220

curved

display System, which can provide interactive

graphics application for multi-projector

displays.surround sound system, we build a low-

Cost 220

curved display System, which can provide

interactive graphics application for multi-projector

displays.

The key problems of the presented system including

system infrastructure framework and its parallel

rendering architecture, stereo displays method,

synchronism tiled rendering control, geometry

correction, intensity blending and colour calibration,

etc. are detailedly discussed in this paper.

2 OVERALL FRAMEWORK

Unlike most display wall systems today, which use

high-end graphics workstations and high-end

projectors, our immersive curved display systems

are built with low-cost commodity components: a

cluster of PCs, PC graphic accelerator card,

consumer video and sound equipment, portable

presentation projectors and 100M common network

equipments. It relies on a PC-based distributed

rendering tiled design using a number of light

projectors operating together to form a single logical

display. The screen space is partitioned (virtually)

into a rectangular grid of tiles, each of which is

produced by one projector. The prototype system

consists of projection subsystem, graphics

generation subsystem and human-computer

interaction subsystem. Diagrammatic Representation

of the PC-Driven Curved Display system is as figure

1. The software architecture has been designed using

a retained mode (master-slave) sort-first parallel

rendering architecture (see figure 1). A copy of the

application runs on each display server. The client

handles user-interface events, and sends control

commands, including synchronization events, or

changes in view to each rendering node, i.e.

rendering server. This kind of parallel rendering

system architecture brings lower network bandwidth

load than the immediate-mode systems brings.

We have successfully developed solutions in several

research areas as outlined in this article，including

alignment of projectors, blending of overlapping

projected images, tiling output，synchronization of

displays, etc. we use front projection passive stereo

display technology to produce the virtual

environment economically ， and the tiled wall

integrates a high resolution image “ tiles” into a

seamless whole better.

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343

3 DESIGN AND

IMPLEMENTATION

3.1 Passive Stereo Display

One of the feature of our PC-driven display system

is that it uses front projection passive stereo display

technology to produce the virtual environment

economically. As it is known that there are two

feasible methods in implementing stereo display.

The first uses active shutter glasses synchronized

with the display refresh, which alternately block

each eye, allows successive images to be passed to

each eye in turn. The polarized glasses are

lightweight and cheap, and using passive stereo

method also avoid having to genlock all of the

render machines. The second, passive stereo,

displays two separate images at all times and uses

polarizing filters and glasses to pass the desired

images to the two eyes. To achieve the same

effective frame rate relative to a standard display,

the first method requires doubling the frame rate,

and the second method requires doubling the number

of graphics pipes and projectors. Moreover,

projectors with an extra high refresh rate are very

expensive. So we selected passive stereo instead of

active shutter glasses.

The projector display systems are divided into five

render nodes. Each render node consists of two PCs,

two projectors and polarizing filters (see figure2).

For passive stereo to work, it is important that the

two eyes receive two non-interfering images at the

same time. To achieve this, two arrays of projectors

project the image for the left and right eye

simultaneously to the screen. The light from each

projector is polarized with a polarization filter in

such a way that the image on the screen for the left

eye consists only of linearly polarized light at –45

degrees, whereas the image for the right eye is

polarized at 45 degrees. Inexpensive glasses using

the same polarization filters allow each eye to see

only light that is correctly polarized. The -45 degree

light arrives at the left eye and is blocked at the right

eye and vice versa. The dissimilarities between the

images received by the two eyes create an illusion of

dimension. This technique allows a large audience

due to the low cost of the glasses.

(a) The passive stereo display principle.

(b) the passive stereo display projectors.

Figure 2: The passive stereo display of one render node.

3.2 Synchronization Tiled Displays

The software infrastructure of our system has been

designed using a retained mode (master-slave) sort-

first parallel rendering paradigm. It runs an instance

of the program on every PC in the render clusters.

To control these programs, a client node application

runs on the console PC, which is used to transmit the

user’s inputs and viewpoint information to each

render server node. Every render server node renders

a different part of the screen from its own copy of

the scene database according the global scene view

frustum partition.

Obviously, a synchronization mechanism must be

integrated into the system for getting a logical

coherent display image. It ensures that the geometric,

viewpoint, and other graphics properties are

consistent to all of the tiles over time. It is

mandatory to fulfill two functional requirements

intrinsic to multi-projector tiled environments:

swaplock and datalock. Swaplock is the frame

buffers swaps synchronization and datalock consists

in maintaining coherent views of the scene on all the

render nodes.

Figure 3 shows the synchronization process of frame

buffer swap. The direction network message reply

methods by socket UDP communication is adopted

in our systems. When one of server nodes has

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344

finished rendering task, it notifies client node and

ready for swap buffer. The client node controls

every process of each server node. After all server

nodes have sent their signals to the client, it is the

client node’s turn to broadcast each node to proceed

and swap buffers.

Figure 3: Synchronization process of frame buffer swap.

Data synchronization is performed implicitly

through a mechanism that keeps multiple local

replicas of a scene graph synchronized without

exposing this process to the application programmer

or user. Our own implementation of this conception

is to propagate scene graph changes using reliable

multicast. Using customer defined data structure, the

client node multicast the scene change information

to all render server node when one event arise such

as viewpoint change or mouse motion events. The

server render nodes receive the messages from the

client node and update its local scene immediately.

As a result of synchronization, the program

instances generate logical correct scene image.

3.3 Geometric Correction

Geometric correction is to address the image on

curved screen distortion in the single projection

plane and the geometric continuity of image from

adjacent projection plane. This paper applies two-

pass rendering techniques to do geometric correction.

Figure 4 shows the image distortion calibration

process of single projection plane. Let the projector

plane coordinates be denoted by (x

) and the

screen space coordinates be denoted by (x

). The

image calibration algorithm is as follows:

Step1. Project the regular grid pattern consisted of

sample points（x

）onto the ring screen；

Step2. Compute the map: （ x

）→ （x

）

according to these parameters such as sreen radius,

projection center etc;

Step3. For each frame image from framebuffer,

perform the nonlinear warp by piecewise texturing

in accordance with the map in step;

Step4. The preprocessed image is sent to render

pipeline and appears on screen correctly.

Original

image

Warped

image

Correct

image

Preprocess

according to

map:（x

)→(x

)

Project to

curved

screen

Figure 4: Image distortion warp process.

The image align implement among multiple

projection plane is as follows:

Step1.The regular grid pattern consisted of sample

point are projected onto every channel of screen;

Step2.Adjust the position and shape of each channel

grid pattern by using our special application program,

make all channel grid pattern aligned;

Step3. Triangulate the aligned grid mesh, compute

the triangle vertex coordinates and save them as the

geometric vertex of texturing for next step;

Step4.During the render process of each channel, use

the image from each framebuffer as texture, warp

the image by piecewise texturing and then send it to

pipeline again. Lastly, get the correct aligned project

image.

Our align calibration process adopting the manual

interactively align adjustment makes a more exact

calibration result than the camera based calibration

methods.

3.4 Color Calibration

Color calibration is to blend the luminance between

the abut projection plane and keep the color

continuity of the display. The effect of intensity

blending and color correction decides the equality of

displays. For intensity blending, we adjust it using

software method with handwork interactive input.

This method attenuates the luminance of overlap

area by adjusting its alpha value. Using the special

software, the alpha value is modulated by

controlling the shape of associated NURBS curve.

Recurring to the susceptivity of human eye to

luminance, the method can achieve a preferable

blending effect and its algorithm is as follows:

Step1. Make the two abut projection plane Plane1、

Plane2 project white display simultaneously;

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345

Step2. Attenuate the overlap area luminance of

Plane1 、 Plane2 by manually adjusting control

curve of alpha value until achieve a satisfied

blending effect;

Step3. Save the last results(Luminance attenuation

Map, LAM) adjusted in step 2 as the bitmap files on

PC of Plane1、Plane2；

Step4. Repeat step 1,step 2,step3 to accomplish the

intensity blending of rest of all overlap;

Step5. During the rendering process of each channel,

call its LAM and perform intensity blending

operation using mulit-texturing.

The color continuity adjustment mainly is to solve

the color difference brought by the different gamut

of projectors. The automatic software calibration

method is designed to perform the color calibration

as follows:

Step1. Input projector P1 a series of input, im with

scheduled RGB value, then feedback the response

luminance by a camera and according to it, compute

the gamma response curve of P1;

Step2. For the projector P2 which is abut with P1，

by modifying the input of P2, let the gamma

response curve approach to the one of P1 and

compute the gamma curve amend function of P2;

Step3. Repeating step 1, step 2, accomplish the rest

gamma response curve approach of all abut channel,

and get the corresponding gamma curve amend

functions of each projector;

Step4. During the rendering process of each channel,

load the gamma curve amend functions of each

projector using color look-up table function of

OpenGL. Last, the display attains color continuity

by modifying and adjusting input of each projector.

4 IMPLEMENTATION AND

RESULTS

We conducted experiments on our front-projection

ring screen display systems which have a 220 degree

view field angle. It’s hardware consists of 10 P4

2.4G PC with 512MB RAM, 10 Quadro4 750 XGL

128MB graphic cards, 100 Mbps Ethernet Switch,

10 projectors of NEC MT1065 + etc.(see figure 5).

Figure 5: The structure implement of systems.

The ring screen is 19.19 meter wide and 2.81 meter

high and has a 5 meter radius. It gives a wall

resolution of 5120 x 768 stereo image.

The system runs on Windows 2000 operation system

and its application development environment

includes such as MultiGen Creator, VC++,

OpenGL，Vtree and so on.

During the systems running, users can interact with

the virtual scene in real time and we obtain a

performance over 30 fps. The geometric continuity,

intensity blending and color continuity of the display

is satisfied and present a strong self-presence sense

as shown in Figure 6.

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(a) the effect of a application example running.

(b) the near display effect of another example

Figure 6: Result of systems executing.

Our system adopted the human-machine interactive

software method to adjust the geometry registration

and the intensity blending, which had more exact

geometry correction precision and more consistent

intensity blending effect than the existed tiled

displays systems such as the MSPR (Zefan Jin,

2003, see figure 7, it has a “bright slot”), the system

of UNC (Aditi, 2005, see figure 8).

Figure 7: Result of MSPR systems executing.

Figure 8: The correction effect of UNC’s system.

5 CONCLUSION

The approach using a multiplicity of commodity

parts to construct a scalable display-wall system

works well, it provides a large scale ， high-

resolution immersive visualization environments

capable of presenting high-resolution stereo images.

We have addressed these tradeoffs and developed

solutions in several research areas relating to screens,

projectors, synchronization tiled displays, and

integration into seamless systems.

Tiled display technologies offer a range of

opportunities for exploring scalable, high-resolution,

large-format displays, for applications ranging from

virtual manufacturing to collaboration walls to high-

resolution scientific visualization. These

technologies should increasingly become affordable

and useful in additions to the range of next -

generation displays for building active spaces.

This work suggests several areas for future research:

1. Load balancing: Load balancing algorithms are

designed to optimize the performance of parallel

systems by distributing work as evenly as possible

across each node in the system. In this paper, we

only consider scenarios in which one rendering

processor is dedicated to each display device. But,

then, if the rendering load is not uniformly

distributed over all display devices, or if we have

more graphics processors available than there are

display devices, this simple, static allocation does

not achieve optimal performance. In related work,

we will develop dynamic load balancing algorithms

for PC clusters driving a display wall. Moreover,

further work is required to develop effective load

balancing methods for remote applications.

2. Compression. In order to use this display

environment for remote computing, we need to

provide the accompanying dynamic compression

scheme, and incorporate it into the rendering

pipeline. When the source of the pixels is 2D or 3D

primitive rendering, there may be further

opportunities for efficient compression, either based

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on compression of primitives or based on primitive-

guided pixel compression.

3. Evaluate metric. Currently we get the results from

different algorithms visually, which is subjective.

For more objective evaluation, we need a

sophisticated perceptual metric.

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