Cooperative Use of MHEG and HyTime in Hypermedia Environments

The standards MHEG and HyTime are interchange formats for hypermedia information. While they may seem to compete, they actually can play separate and complementary roles in a complete and open hypermedia environment. MHEG is used for portable final-form hypermedia presentations. HyTime is used for the long-term, presentation-independent storage of hypermedia documents. Given these tasks, MHEG can be used to encode presentations of HyTime documents. This paper explores these two standards, the potential cooperative roles they can play, and their application to the CMIF hypermedia environment architecture. The issues discussed include the semantic overlap between the hypermedia models the standards represent and how the use of each standard affects the cooperative use of the other. 1.0 Introduction The Multimedia and Hypermedia Information Encoding Experts Group (MHEG)8 format and the Hypermedia/ Time-based Structuring Language (HyTime)7,1 are both standards use for hypermedia documentation, but each functions on a different layer of document processing methodology. MHEG is a standard for representing portable hypermedia presentations. HyTime is a standard for representing the hypermedia structure of documents independently of any presentation. Therefore a mapping from this structure to a particular presentation must be defined for the document to be perceived by the user. MHEG represents a potential presentation format for HyTime documents. MHEG encodes portable final-form hypermedia presentations transferred over a distributed network. It assumes minimal capabilities for the user interface system, ensuring that MHEG presentations can be processed for a wide variety of front ends. Presentations are transferred over the network on an as-needed basis. Each step of the presentation, as defined by the user interaction, is fully encapsulated and communicated separately, minimizing network traffic and the transfer of unnecessary information. MHEG takes an object-based approach to defining its constructs. HyTime builds upon and extends into hypermedia the presentation-independent text structuring of Standard Generalized Markup Language (SGML)10,2 . SGML is a widely used ISO standard for representing electronic text documents. It supports and facilitates document exchange, reuse and longevity by representing a document in its permanently stored form without defining its processing into any particular presentation. SGML also provides for the definition of separate document sets, called Document Type Definitions (DTDs), which represent different types of documents used by separate communities. A DTD defines a general structure that a class of documents fit. An individual SGML document is an instance of the DTD associated with it. HyTime’s additional structure includes complex hyperlinking, locating of document objects, and the scheduling of objects within measured coordinate systems such as space and time. HyTime inherits SGML’s ability to define distinct document sets. HyTime, like SGML, requires external mechanisms for specifying the presentation of its document.