Composition editing and playback is the mechanism within MADE for developing and viewing multimedia/hypermedia documents, both from the point of view of an author of such documents and also from the point of view of the final user(s) of a MADE application based on the document concept. The composition editing and playback utility is one of the main integrating components of the MADE application environment. It is through the definition of an abstract document structure that a hypermedia document is created and it is the presentation of this hyperdocument which the end user may interact with. During both the authoring and playback modes of operation the composition utility makes direct use of the other MADE utilities for viewing or editing particular media objects, for presenting help information, for navigating the hyperdocument structure, and perhaps also for monitoring the user's actions. The composition utility drives the operation of these other utilities based on a composition graph (ie, the internal representation of the hyperdocument).
An essential aspect of the composition facilities is the ability
to define and manipulate an abstract document structure
The abstract document structure is a
representation of logical components which describes not only the
specific types of media involved in the presentation, but also the
semantic connections between media, the synchronisation constraints
associated with the presentation of the logical components, geometric
and other presentation attributes for each component, and specific
interaction entities to be used in reading the multimedia document.
The authoring and presentation of a hyperdocument is not only determined by the media and the composition utilities. There may be a number of alternative styles (or metaphors) for presenting a particular hyperdocument that are dependent not on the specific document itself but on the application domain in which the MADE application exists.
A specific goal of the composition utilities of MADE as a whole is to separate the presentation metaphor used for authoring and viewing a MADE hyperdocument from the underlying composition graph. The aim is to accommodate different styles of authoring and different forms of visually structuring the hypermedia information. Within the MADE project, a prototype authoring application will be developed, with a specific application area and presentation metaphor. However, this application should be considered merely as a test of the MADE concepts; it is perfectly possible for another application to choose a radically different presentation scheme and implement it on the ``top'' of the MADE composition utilities.
Another important aspect of the composition editing and playback facility is making provision for use of an interchange format that represents the abstract document structure in a more persistent form. An interchange format enables the reuse of existing compositions, either fully or in part, and enables the exchange of documents among MADE applications. This aspect is not straightforward, however, and there are a number of decisions to be made on which specific format should be adopted. The main contenders at the moment appear to be HyTime[13] and MHEG[14]. A third choice would be to develop a MADE specific format (temporarily denoted as MIFF), perhaps based partly on either of the above or some other less well known format. Other possibilities include the Microsoft's AVI format ([21]) and the MOVIE format defines as part of Apple's QuickTime environment ([32]). At the time of writing, the choice of the appropriate format is still to be made.
The composition utilities include some sub-modules with well specified tasks. These include:
An interaction editor, used to create or modify interaction objects (see §3.2.4). This involves defining sensors associated with MADE objects (or with their associated visual metaphor), specifying the objects the interaction object has to control, and editing the corresponding script. The definition and/or the modification of sensors may involve, eg, graphics editing, which means that the interaction editor may also start up a 2D graphics editor internally. In this setting, interaction objects provide a possible internal representation for hyperlinks.
The role of the synchronization editor is to interactively define the synchronization patterns among several synchronizable MADE objects. This may involve the specification of reference points, setting references of other object the synchronizable object has to synchronize with, defining the details of this synchronization, etc. Time objects are also managed by this editor; the user may indeed prefer to use the notions of time, scheduler, and time-constraints for the purpose of synchronization, rather than the concept of reference points. (As described in §3.2.3, both mechanisms are available within the MADE toolkit.)
The choice of the interchange format will greatly influence whether, in the synchronization editor, the emphasis will be placed on reference point on time-based synchronization. HyTime, for example, expresses all synchronizations using an abstract notion of time; quite naturally, if the HyTime format, or a subset of it, is chosen, this will determine the final shape of the synchronization editor, too.
The graph or layout editor gives a visual interface for the direct manipulation and visualization of the composition graph (ie, the hyperdocument structure).
Finally, the composition editor is the most complex composition utility, which combines and controls all other composition utilities as well as the monomedia editors, and MADE toolkit objects. It is this module which lies at the heart of all composition utilities, which is responsible for providing all the general functionalities described above.