Introduction to Distributed Virtual Environment Research
Ten Quick Questions and Answers)
Morgan (page created in 2005)
This page is now quite out of date in places (but I
have not had time to update it). Research has moved on a bit since this was
When project students ask me about
distributed systems research associated to distributed virtual environments I
usually end up answering the following types of questions. The answers I
provide here can be found in the literature (most of it from my papers) but I
have found that students prefer to have a "one stop shop" with the
minimum to read (just getting to the point of the matter). Therefore, I have
put this page together for anyone interested in distributed virtual environment
research who is new to the subject (but mainly for
those project students of mine so they can ask me some more interesting
questions in the future).
1. Why do distributed virtual environment research?
virtual environment (DVE) research at Newcastle
has been ongoing for the last few years (since 2001) and has focused on the
distributed systems type challenges. My background is in distributed systems
(e.g., fault-tolerance, group communication protocols, middleware
technologies) and my initial intention was to see if techniques from
distributed systems research could aid DVE research. The reason for this was
that as I read DVE literature I made two observations: (i)
the majority of academic research had been derived from a number of
non-distributed systems type research groups, such as computer supported
collaborative work (CSCW), human computer interaction (HCI), virtual reality
(VR), and graphics; (ii) for some reason all aspects of DVE research had
become "unpopular" in that little was been published on the
subject anymore and many researchers seemed to have "moved off" into
other domains. The main issue I could see was that although there was quite
substantial middleware technologies (from around twenty years of distributed
systems research) available for distributed application developers to use (much
of it open source), such technologies did not appear to have been incorporated
into DVEs. For example, I fully expected to be able
to download some open source server side solution for
node clustering to allow quick development of scalable server based virtual
worlds (in the same way I would build an auction site for example). Needless to
say, I couldn't. However, with recent commercialisation of online games the
need for DVEs and associated development technologies
is growing. (it really is an interesting problem
·Michael Zyda (USA) is a recognised pioneer in this field, there are
others, but Michael is probably the most prominent person in the field. Worked
on early DVEs related to military simulations in the USA. If you
want to know about DVE research a must read text for beginners is his book
"Networked Virtual Environments - Design and Implementation".
·Steve Benford and Chris Greenhalgh (UK)
wrote many papers related to DVEs in the mid to late
90s. Chris's PhD Thesis won an award for his work in this area and was
published in 1999 ("Large Scale Collaborative Virtual Environments").
·DIVE is one of
the best supported academic implementations of a distributed virtual
environment. An early paper provides a good insight into DIVE (although it has
been an ongoing work for many years now): Carlsson
and Hagsand, "DIVE - A Multi User Virtual
Reality System", IEEE VRAIS, Sept, 1993
work? Well there are many: try Googling the following
to find out about them (add virtual environment for a more narrower search):
SIMNET, NPSNET, SPLINE, RING, MASSIVE, PARADISE,
BrickNet, NetEffect,VNet, Mercury, ATLAS, MOVE.
commercial games? Well Doom was one of the first to provide networking support,
but there are many more now: Quake, Unreal Tournament, Diablo, Ultima, Everquest, Star Wars Galaxies.
·This is an
interesting, but quite obvious (when you think about it), side effect reflected
in user interaction because of network delays and processing overheads (Michael
Zyda and Co. coined the phrase). In fact, this makes
satisfying the research challenge of "consistency between users"
difficult in a DVE. Here is a quick example that will explain what it is:
consistency-throughput tradeoff may be described via
an example involving two virtual world objects, say Obja
and Objb. Obja
is at position (15, 15) in a two dimensional virtual world and sends a message
to Objb describing its coordinates.
Due to message latency Obja has
reached (20, 15) by the time Objb
receives Obja’s message. Objb will have an out of date view of Obja’s position: consistency is reduced,
but the world is dynamic and responsive (object movement is unrestricted). To
improve consistency we may stop Obja
from moving until we are sure all objects view Obja’s
position as (15, 15). This can be done via the use of agreement protocols or a
shared, transactional, central database that maintains virtual world state.
These approaches incur processing overheads that lead to delays in the virtual
world simulation, reducing throughput and making simulations less responsive.
This is not acceptable for real-time simulations; therefore inconsistency is
present to some degree.
·Providing DVEs that may scale to support many hundreds and thousands
of users while satisfying real-time (responsive virtual worlds) and consistency
(participants view mutually consistent events) is a significant research
challenge. The quality of service associated with the underlying messaging
middleware (e.g., latency, jitter, message loss) coupled with finite processing
resources make ensuring all participants in a virtual world view events in a
mutually consistent way difficult. In addition, as message exchange is the only
way to propagate events to geographically dispersed users care must be taken to
prevent exhaustion of bandwidth and available processing resources when
managing message exchange.
management is an accepted
approach to achieving scalability in a DVE: freeing up bandwidth and
processing resources via targeted message passing (not broadcast) ensuring
messages are only sent to recipients that may be interested in them.
Defined areas within a virtual world are used to restrict message passing:
a message and its receivers are associated to an area of a virtual world.
This approach is achieved by attributing the sending and receiving
entities of a message to objects that inhabit a virtual world (i.e.,
objects inform each other of their actions via the exchange of messages).
Areas used for restricting message passing are commonly termed areas of
influence and an object is said to exert influence (send messages) to all
other objects that share its area of influence.
Region - virtual world is divided into well defined
regions that are static in nature (defined at virtual world creation
time). The recipient (object) of a message is limited to only interested
participants (i.e., reside within the same, or neighboring,
region as the sender object). This approach is illustrated by NPSNET,
where the terrain of the virtual world is divided into hexagonal cells.
Aura - each object is associated to an aura that
defines an area of the virtual world over which an object may exert
influence. An aura may be simply modelled as a sphere that shares its
centre with the positional vector of the object it is associated with. An
object may potentially communicate their actions only to objects that fall
within their aura. This approach is illustrated by the Model, Architecture
and System for Spatial Interaction in Virtual Environments projects
(MASSIVE 1 & 2).
consistency-throughput tradeoff, missed
interactions occur because the degree of inconsistency present in a DVE is
sufficient to allow an object’s traversal of an area of influence to go
undetected by an interest management scheme. Considering the example given
earlier when highlighting the consistency-throughput tradeoff,
it is quite conceivable that an area of influence belonging to Objb is identified as between (16,
15) and (19, 15), resulting in Obja
traversing this space with no messages sent to or received by Objb (missed interaction).
9. How is the missed interaction problem reflected in region and aura based interest
Region - When considering an object that represents a
fighter aircraft we can see that the size of a region may be appropriately
measured in kilometres. However, this size of region may not be suitable
for all types of objects. For example, if region size is decided when
considering the top speed of a fighter aircraft then the presence of
soldiers travelling on foot may give rise to unnecessary message exchange
between nodes that host foot soldier objects that are separated by a
distance too great for such objects to influence each other. Furthermore,
if region size is more suited to foot soldier objects then a fighter
aircraft may traverse region boundaries with such frequency that region
membership may not be resolved in a timely fashion. Therefore, when
objects coexist within the same virtual world and can traverse the virtual
world at greatly varying speeds, relying on a region based approach alone
may not be appropriate.
Aura - If the fighter aircraft flies over the
position of the foot soldier and initiates an attack on the soldier’s
position then the DVE must detect when aura collision occurs and enable
message exchange between the appropriate objects. The aura of the fighter
aircraft object may only collide with the aura of the foot soldier object
for such a small period of time that it may not be possible to resolve the
appropriate object group membership in a timely fashion. A solution to
this would be to extend the fighter aircraft’s aura to enable such
interaction. However, extending the aura may result in the fighter
aircraft potentially influencing many more objects than is necessary and
may result in scalability problems as the node hosting the fighter
aircraft would be required to participate in redundant message exchange
with many nodes.