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Regarding the timing and coordination of the hyperdocument or hypergraph as a whole, different approaches have been proposed. One approach is the one suggested by Stotts and Furuta [STOTTS and FURUTA 1989] who consider the use of Petri-Nets in order to control and coordinate the presentation of hyperdocuments. They model the hyperdocument in a bipartite graph consisting of nodes and transitions. Transitions are associated with buttons at the presentation level. When this buttons are press, they may fire a transition. For a transition to be fired, it is required to mark all the previous nodes. An initial marking is required. Depending on this initial marking, access control is provided and the presentation coordinated. The approach is proposed by OGAWA et al [OGAWA et al. 1990, OGAWA et al. 1992] who use four modes or activities to follow when designing a presentation, fir
One important research area is related with the definition of conventions and specifications for presentation of hypermedia. Also it is important to provide integration between the new technology and the current technology. Meta-languages like Hy-Time define position and times for anchors and links provide an idea for a standard.
Focusing on time-based media, an interesting work is Aron's work, Hyperspeech [ARONS 1991]. Aron explores a hypermedia space base that uses only audio. This work presents an interesting perspective, since it is not possible to escape the sequential nature of speech. Nevertheless some parallel sound production is possible and also is interesting to node how to chunk the information into smaller nodes. Also is interesting to note when and how interruptions are allowed.
One kind of system that has received more attention than hyperspeech is the hypervideo. Research of the semantics and rhetoric of linking in hypervideo is done by Hardman et al. [HARDMAN et al. 1993]. This work deals with the representation of anchors for video and audio, and with the fact that when traversing a link to a video or audio, different behaviors may be expected by the user. For instance, a link could replace or insert some video, after which the previous video may keep running, or be restarted. There could be different situations were the system should behave differently. Therefore this behavior should also be specified with the links. The representation they used in this system was simpler that the one presented by Ogawa et al. [OGAWA et al. 1990, OGAWA et al. 1992] . In this work a 2 dimensional representation of the different components present at one time (they impl
The next section presents briefly some of the possible future research directions associated with time-based hypermedia.
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