Integrating with the Access Grid: Experiences and
Issues
Chris Greenhalgh a, Alex Irune a, Anthony Steed b, Steve North c
a.
School of Computer Science & IT, University of Nottingham
b.
Department of Computer Science, UCL
c.
Department of Computer Science, University of Glasgow
Abstract
The Access Grid supports collaboration between geographically distributed groups, by
allowing each group’s local “node” to connect to one of several shared “virtual venues”.
We have explored ways of extending the Access Grid Toolkit version 2, specifically: to
integrate a collaborative mobile pollution monitoring and visualisation application; and
to allow immersive virtual reality interfaces to be used as nodes. In this paper we report
our experiences and evaluate the integration approaches available, in particular with
reference to application sharing, based on: new node services interacting with venue
media streams; toolkit-supported shared applications; and external applications
coordinated via service URLs or shared data files.
1.
Introduction
The Access Grid [1] supports collaboration
between geographically distributed groups, by
allowing each group’s local “node” to connect to
one of several shared “virtual venues”. Each node
is typically a meeting room with audio-visual
facilities. All of the nodes connected to a single
virtual venue are inter-connected by a
combination of streaming audio, video, text and
shared applications to facilitate collaboration.
The EPSRC-funded project “Advanced Grid
Interfaces for Environmental e-science in the Lab
and in the Field” has been exploring areas of escience involving significant field-based activity,
including environmental monitoring in the
Antarctic [2] and urban pollution in London [3].
In this project we have also explored ways of
integrating with the Access Grid, in particular the
Access Grid Toolkit version 2 (currently version
2.3). Specifically, we have worked to (1) integrate
a collaborative mobile pollution monitoring and
visualisation application, and (2) allow immersive
virtual reality interfaces (e.g. CAVE™ and
RealityCentre™ type devices) to be used as nodes.
In this paper we report our experiences and
evaluate the integration approaches available, in
particular with reference to application sharing, in
its many possible forms [4].
In section 2 we review the main ways in which
further shared applications or alternative
interfaces can be integrated with the Access Grid
version 2. In section 3 we present examples of our
alternative interfaces to the Access Grid for use in
immersive virtual reality interfaces, integrated
with our collaborative pollution monitoring
application.
2.
Access Grid Toolkit Extension
2.1.
Media streams and node services
Venue Server
Virtual
Venue
Venue
Event
bus
Venue
Data n
1. Join
2. Streams
Node
manager
Service
Manager:
Vic (video)
3. Exec rat
Cameras
Video windows
Service
Manager:
Rat (audio)
3. Exec vic
Venue
client
Local
App n
Shared
App n
User
Data n
Mics
speakers
Figure 1: Joining a venue and its streams
The main software processes and applications
making up a typical node and virtual venue are
shown in figure 1. Associated with the venue is
information about the (by default multicast) audio
and video streams associated with the venue. On
joining the venue this information is used by the
client to start appropriate “node services” to
handle those streams. New node services can also
be added to a node, allowing other applications to
access or contribute to the same media streams.
The Access Grid Toolkit’s node services for
video and audio are simple wrapper applications
for versions of the well-known RAT and VIC
multicast conferencing tools. These wrappers –
like the vast majority of the Toolkit in version 2 –
are implemented in the Python scripting language.
The Toolkit’s use of non-standard SOAP over GSI
(the Globus security “standard”) protocols makes
interoperability with other languages and toolkits
problematic at present.
To allow immersive VR interfaces to be used
as nodes we have created an embeddable library
version of the VIC video tool (see section 3). We
have created a new node service based on the
existing VIC wrapper application so that we can
obtain current stream information for our own
(non-python) applications in the local node,
allowing them to track changes in virtual venue.
We and other projects have also demonstrated
capturing application 2D graphical output (e.g.
rendered visualisations) and generating a video
stream in the appropriate multicast session, e.g.
using a custom Chromium [5] Stream Processing
Unit. Of course, this does in itself allow remote
input to the application. However, it does allow
any node to view the application output without
requiring additional software, and copes
reasonably well with highly dynamic views,
although it is less appropriate for standard 2D
GUIs or high resolutions.
2.2.
Toolkit-supported shared applications
allows almost any application to be shared with no
special support from the application itself.
However, video may not work correctly in some
cases, and applications with very dynamic
displays (e.g. rapid animation) may not distribute
well. Input to the application – at the level of
mouse and keyboard input – may be limited to the
single site hosting the application itself, or open to
all sites, but typically without management or
remote awareness. Although not part of the 2.3
release, a VNC client shared application is present
in the code-base.
Creating other shared applications is
complicated by the same language and
interoperability issues noted in relation to node
services, and in addition the protocol used to
communicate with the event bus is proprietary.
However a tutorial and supporting code makes
developing a new (python) application relatively
simple, providing that appropriate libraries and
facilities are available.
2.3.
Shared data and services
Venue Server
1. create/2. get
Virtual
Venue
1. Get info (Join) /
Save data
2. Get data
Node
manager
Venue
Event
bus
3. start
Venue
client
Venue
Data n
Local
App n
Venue Server
Virtual
Venue
Venue
Event
bus
1. Get info (Join)
Node
manager
Service
Manager:
Vic (video)
Service
Manager:
Rat (audio)
Cameras
Video windows
Mics
speakers
Venue
client
Venue
Data n
4.events
3. get/set
state
Local
App n
2. start
Shared
App n
User
Data n
Figure 2: Running a shared application
The Access Grid Toolkit has direct support for
“shared applications”, which are logically
associated with a particular virtual venue as
shown in figure 2. Standard examples included
with version 2.3 are a shared web browser,
PowerPoint™ presentation viewer and questionqueueing application. An instance of the shared
application runs at each node, and coordinates via
(simple) shared state in the venue service plus
asynchronous events distributed via an associated
event bus.
VNC – which shares the final 2D screen
appearance of any application using a VNC server
– is a common method for sharing applications. It
Service
Manager:
Vic (video)
Service
Manager:
Rat (audio)
Cameras
Video windows
Mics
speakers
Shared
App n
User
Data n
Figure 3: Sharing data
The Access Grid Toolkit also supports sharing of
data in the form of service URLs or files (figure
3). These can be published via a venue or a node,
and downloaded by other nodes which can then
view them, assuming that they have a suitable
local application installed (determined by MIME
type match).
Distributing a data file or document provides a
straight-forward way to independently view the
same information, such as a web page or image.
Custom viewers can also be registering using an
OS-specific MIME-type registration process, e.g.
allowing data-sets or 3D models to be viewed
provided the node has installed the appropriate
viewing application in advance.
Alternatively, a service URL or configuration
file can be used (via a custom MIME-type) to start
up an application which directly supports
distributed collaboration. In this case any
coordination is dependent upon the application
itself, independent of the Access Grid’s normal
facilities. For example, the project’s pollution
monitoring application uses the EQUIP distributed
data-space for coordination between application
instances. Note, however, that this raises
additional complications for firewalls and
security.
3.
3.1.
Immersive Access Grid Use
Supporting Immersive Interfaces
Previously when we have needed to support
immersive interfaces such as CAVEs™ and
RealityCentres™ [6], we have written custom
renderers using the support of libraries such as
VRJuggler or CAVElib. Although designed to be
easy to use, these libraries enforce a particular
architecture on the application developer. On this
occasion, because we had existing client
applications, we decided instead to explore the use
of the Chromium OpenGL streaming package [5].
In particular we wished to explore using a
Java3D1 renderer that could be embedded into the
existing java software and then use Chromium to
share the resulting OpenGL calls over the network
to dedicated display machines.
We developed a relatively simple Java3Dbased client component for the “eGS” (e-Science
George Square) system, which is a synchronous
collaborative pollution monitoring application,
which uses the EQUIP shared dataspace platform
for peer-to-peer coordination (see also [6]). This
3D client maintains a graphical scene including:
simple representations of each user, the area map,
historical pollution data and recent pollution
samples.
We also developed Chromium plug-ins
(Stream Processing Units) to handle issues
specified to our immersive interfaces: geometry
correction and edge blending in a curved-screen
interface such as a RealityCentre, and head
tracking in a CAVE™-like interface.
3.2.
Centre™-like immersive interface with the same
subjective
consistency
from the
user’s
perspective. The current clients display the
incoming video streams in an arc in front of the
user, with the size and position of each image
dynamically adjusted as video sources join and
leave the session. Figure 4 shows the Java3D
Access Grid-VR client in use (our Reality Centre
is currently shared with a driving simulator!).
Access Grid-VR
We have developed two 3D Access Grid video
clients, one in C++ using OpenGL, and one in
Java using Java3D (and hence OpenGL). These
clients use our embedded version of VIC
described in section 2.1 to display the video
streams as live textures on objects within the 3D
scene. Because the video streams are all
consistently positioned within the 3D scene they
can be viewed in a CAVE™ and Reality
1
Java3D uses OpenGL for rendering, except on
Windows where a DirectX renderer is also available.
Figure 4: Access Grid-VR in use
Collaborative audio and video from the node
are provided by a standard Access Grid client
running on another machine. With the normal
Access Grid video client it is necessary to place
and size video windows by hand across the virtual
desktop. Our AG-VR clients, however, will
automatically size and organise the video streams.
Unlike to basic auto-place facility in the normal
client, we make use of information in the RTP
source descriptions to identify video streams
likely to be from the same site (node) and where
possible their spatial order (left, centre, right) and
provide a concise interface to allow users to
selectively hide streams and sites, and adjust their
display order.
It is possible to use these applications directly
with Chromium to drive a multi-projector display.
However the performance is very poor because
the full video texture is typically changing on each
frame. To allow Chromium to be used more
efficiently we have created another SPU which
incorporates the VIC module and replaces a small
fake texture image sent by the client application
(which contains the URI of the appropriate video
stream) with the latest image available in the local
VIC module. The final arrangement of software
components to drive the Reality Centre interface
is shown in figure 5 (excluding eGS’ own
communication and coordination elements).
Field”, the EQUATOR Interdisciplinary Research
Collaboration (EPSRC Grant GR/N15986/01).
Chromium
Array SPU
References
displays
OpenGL API
render
Java3D
OpenGL
video sub
AG-VR/eGS VIC
application
Chromium
servers
warp
VIC
Multicast AG
Video
Tilesort
SPU
Stream IDs
Figure 5: Using Chromium to drive an
immersive display including AG video
3.3.
Immersive Application Sharing
Finally, we have also integrated the Java3D
AG-VR client with the eGS system to show how
this approach allows immersive 3D graphics to be
combined with distributed collaboration on the
Access Grid. As well as the standard audio and
video communication provided by the Access
Grid, the peer-to-peer coordination facilities of the
eGS serve to coordinate the 2D and 3D map and
pollution views of the distributed participants. To
support this over the typical wide-area usage
common in the Access Grid a discovery
rendezvous process is run on an agreed machine
to bootstrap the EQUIP peer discovery process.
Figure 6 shows the eGS system being viewed in
the UCL ReaCTor with integrated Access Grid
video streams (and audio).
Figure 6: A user inside the UCL ReaCTor
facility using the collaborative application
Acknowledgements
This work was supported by EPSRC Grant
GR/R81985/01 “Advanced Grid Interfaces for
Environmental e-science in the Lab and in the
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Stefan Egglestone, Malcom Foster, Chris
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Proceedings of UK e-Science All Hands
Meeting 2003, 2-4th September, Nottingham,
UK
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Croxford, Chris Greenhalgh, “e-Science in
the Streets: Urban Pollution Monitoring”,
Proceedings of UK e-Science All Hands
Meeting 2003, 2-4th September, Nottingham,
UK
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