DeskAlign: Automaticlly Aligning a Tiled Windows Desktop
Grant Wallace, Han Chen and Kai Li
Department of Computer Science, Princeton University, Princeton, NJ 08544.
fgwallace,chenhan,lig@cs.princeton.edu
Abstract
Tiled projector arrays are effective at meeting the needs
for scalable, cost effective, higher resolution displays. In-
creases in PC performance have allowed small tiled dis-
plays to be driven from a single PC with multiple graphics
cards. In this paper we present a system for automatically
aligning the Windows Desktop of a tiled display. This sys-
tem consists of three primary procedures: detecting projec-
tor misalignment, calculating corrective transformations,
and real-time warping of the desktop. This allows users
to run any Windows 2D, 3D or video application without
modifications or special software support. Our experiments
indicate that the system is able to achieve real-time warping
with minimum system performance degradation.
1. Introduction
Large format collaborative displays are increasingly useful
in a variety of application environments including control
rooms, CAD design, education, and business. For collabo-
rative displays to be effective, it is important that they are
easy to use and provide enough resolution and size to read-
ily view the group’s work.
Figure 1: A DeskAlign installation in PPPL Control Room
One common way of building a high-resolution display
is to tile together a set of projectors. Tiled displays have typ-
ically been driven by specialized graphics pipelines such as
the SGI Onyx, or by a cluster of PCs each rendering a sin-
gle tile. These types of rendering systems have performance
advantages, but can be cumbersome for creating effective
collaborative environments. Graphics pipeline computers
are too expensive and specialized for general computing
needs, while a cluster of computers requires special tools
to use and manage. Ideally a collaborative display will be
driven by a system that people find easy to use and are fa-
miliar with. The increasing capabilities of commodity PCs
can make this possible. A single PC with multiple graph-
ics cards can potentially drive upwards of 16 displays with
4 to 8 displays being a more optimal setup. Luckily, Col-
laboration environments often require only small tiled ar-
rays. This makes building a tiled collaborative display from
a single PC an attractive choice because it can present an
intuitive user interface such as the Windows Desktop in an
economical fashion.
An important aspect when tiling together projectors is to
get precise geometric alignment. Even small amounts of
misalignment lead to gaps and double images which make
the display unacceptable. Manual alignment is possible but
tends to be time consuming and less accurate than auto-
mated approaches. Automated approaches are fast and ac-
curate, reducing the maintenance overhead, but must be ap-
plied in real-time to the displayed imageries. Applying the
alignment requires that a projective warp be applied to each
projector’s output. Warping the imagery is typically done
by modifying the rendering application. Unfortunately this
is not easily done with a Desktop environment where the
source code may not be available.
The remainder of this paper presents a system we de-
veloped called DeskAlign which automatically aligns and
warps the windows desktop of a tiled display driven by a
single PC. Some previous work will be discussed in Sec-
tion 2. Section 3 will talk about design choices in creating
such a system. Section 4 will describe our system’s imple-
mentation and section 5 will present some evaluations and
experiences with using DeskAlign. Section 6 will conclude.
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2. Background
The goal of aligning a tiled desktop can draw on the experi-
ences of several categories of research, but is not completely
addressed by any of the current tiling systems. In this sec-
tion we will look at some previous work and compare some
existing systems.
2.1. Automatic Alignment
Automatic alignment of tiled displays is well documented
in the literature. There have been a variety of techniques
that have emerged for different display environments. Sin-
gle camera view alignment of a planar screen is covered by
PixelFlex [11]. Scalable alignment of a planar screen us-
ing multiple camera views is done by H. Chen et al [1], Y.
Chen et al [3]. Aligning displays on an arbitrary surface can
be found in Raskar et al [8]. These techniques all involve
detecting projector feature points with a camera. The rela-
tive position of the feature points with one another is then
used to extract a set of transformations that, when applied to
the displayed imagery, make the projectors appear aligned
from the cameras viewpoint. The techniques typically yield
sub-pixel accurate alignment in a matter of minutes.
2.2. Projector-Camera User Interfaces
Several systems have effectively used projector-camera in-
teractions to create enhanced user interfaces. Smart Projec-
tors [10] uses a camera to correct for projector keystoning,
create a laser-pointer user interface and do shadow elimi-
nation. Also the Everywhere Display [7] uses dual headed
graphics cards to do two pass rendering resulting in one dis-
play output that is automatically keystone corrected on any
surface it projects to. This also has the facility for tracked
user interaction.
2.3. Display Tiling Systems
There are several systems that have been developed to help
bring desktop environments to tiled displays. Most of them
involve running a client-server architecture. Usually one
Display
Display
Display
Virtual Screen
Snooper
Distributor
Primitives
Prim
itiv
es
Figure 2: Virtual Display Driver for Tiled Display
Client
X Proxy
Display
Display
Display
Client
Client
Primitives Prim
itives
Figure 3: Distributed Multiheaded X
computer will act as a proxy. This proxy looks like a sin-
gle display to the applications, but it then divides the dis-
play content and redistributes it to the tile nodes. A win-
dows implementation of such a system is the Virtual Dis-
play Driver (VDD) [4]. VDD creates a virtual Windows
Desktop of arbitrary resolution. When applications running
on that computer make GDI drawing calls, the calls are in-
tercepted, scaled and sent to the appropriate nodes of the
tiled display as shown in Figure 2. Distributed Multiheaded
X (DMX) [6] is a similar proxy for X Windows environ-
ments. A Xserver runs on one PC and accepts display com-
mands. It then redistributes these X rendering commands
to the cluster nodes (see Figure 3). Both VDD and DMX
operate with 2D drawing primitives in order to reduce the
network bandwidth which would be required when sending
pixel information. Another solution is an adaptation of Vir-
tual Network Computing (VNC) [9]. VNC allows a user
to connect to a remote computer and view/interact with the
desktop. It requires the remote computer to run VNC Server
which compresses and ships pixel information to the client
computer. This application was modified in a release called
VNCWall. The VNC Server was modified allowing it to
handle requests for multiple rectangular subsections of the
display. This allows each node in the display to connect to
the server and ask for a different subsection thus creating a
tiled desktop.
One common aspect of the above systems is they are de-
signed to run on a display cluster. The goal of our system
is to run from a single PC. The proxy based systems can
be used in loopback mode where the client and server both
reside on the same PC. This is one potential solution to cre-
L RVNC Server
VNC Client
Host Graphics Card
1) Rendering Primitives
2) Local loopback
3) Warp and Render
Snooper
Figure 4: VNC loopback mode on a single PC
2