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Introduction
The Web is not a Distributed Multimedia System (Yet?)
Erik Duval, Henk Olivié
Departement Computerwetenschappen, Katholieke Universiteit Leuven
Celestijnenlaan 200 A, B-3001 Heverlee, Belgium
Email: {Erik.Duval,Olivie}@cs.kuleuven.ac.be
The Web, although immensely popular, isn't really a distributed multimedia system, because the integration of
different media is very shallow. Rather, the Web can be considered as a 'multiple media' system, where
monomedium objects co-exist.
In order to turn the Web into a true multimedia system, support for spatial and temporal relationships between
objects must be upgraded. In the present state of Web technology, these relationships are barely considered at
all:
• In HTML, the tag indicates that one object, a still image, must be included in another one. Limited
indications with respect to spatial positioning of text around the image can also be included. A
generalisation of is the tag, proposed in a WWW Consortium (http://www.w3.org)
working draft, which also supports inclusion of applets, client-side image maps, etc.
• Conventional hyperlinks also define interrelationships between documents. But these are too limited: one
either 'jumps' to another screen or starts a helper application. In both cases, one switches from one
monomedium object to another.
• Co-ordination in time, i.e. synchronisation, is not considered at all in the WWW. In this context, it is useful
to distinguish between continuous synchronisation (e.g. a video clip where the speaker's lips must remain
synchronized with the sound) and event driven synchronisation where single points of different streams
must coincide (e.g. a slide show with associated audio) Ehley, [ Ilyas & Furht 1995].
‘Web programming languages’ like for instance Java make it possible to define more complex objects with
associated behaviour. However, this additional facility comes at great cost: documents become closed (i.e. their
internal structure is hidden from the Web client), and the Web becomes a platform for application delivery
rather than a distributed multimedia system.
What is needed is a set of higher level constructs for defining the structure of Web documents. Since the
beginning of 1996, we have started a project that investigates appropriate such constructs, to be superimposed
on a flexible, set-based hypermedia data model [Duval, Olivié & Scherbakov 1995]. As explained in the next
section, we are currently concentrating our efforts on time-dependent hyperlinks.
Time-Dependent Hyperlinks
We are investigating the concept of time-dependent hyperlinks in the context of our set-based hypermedia data
model [Duval, Hendrikx & Olivié 1996], that integrates the more important features of earlier data models. In
our model, objects belong to an arbitrary number of sets. The conventional link concept is replaced by a
topology operatorθ: o, s → x : its parameters are a set s (the current context) and an object o (the current
object, an element of s ). The result is a set x , subset of s , with all the elements of s that are accessible
when o is current. In order to model temporal characteristics of hyperlinks, the topology operatorθ will be
extended with time information, so that it becomesθ: o, s, t → x , where t models the time that has elapsed
since the display ofo started.
As a concrete instantiation of the topology operatorθ , we are currently working on an implementation
whereθ is defined by specifying begin and end times for an element to be accessible. This mechanism can be