Purpose

Typical Applications

Features

Hytime architecture and modules

Advantages of Current Specification

Enhancements to the Published Standard

Implementation Issues

Purpose

The Hypermedia Time-Based Document Structuring Language (HyTime) is a standard language for representing the logical structure of documents with requirements for space and time based coordinates and addressing. HyTime is based on SGML (ISO 8879), and uses the grammatical and syntactical conventions of SGML. HyTime provides the capability to package information objects using a standardized markup language whose structure will enable non-sequential access, querying, version control, and long-term maintenance despite system evolution or migration.

By using the SGML/HyTime standards, the application designer can create system independent files that are transferable and interoperable across dissimilar computer applications. HyTime provides architectural forms for the definition of SGML element classes in SGML Document Type Definitions (DTD). HyTime does not provide a DTD, as such, but instead, constitutes a meta-DTD from which conforming application DTDs can be created.

HyTime is not yet a CALS standard. It is perceived as a potential standard supporting future interactive, electronic, hypertext and multimedia CALS applications.

Typical Applications

The HyTime language can be directly applied to hypertext (documents that enable multiple access paths) and multimedia applications. These include the design and encoding of information for Interactive Electronic Technical Manuals and Portable Maintenance Aids (IETM/PMA), online review of existing documents both in and not in neutral formats, and the creation of large interoperable hyperdocument libraries or design data bases.

HyTime has potential applications in the areas of project management, enterprise process design, discrete event simulation, and music.

Features

HyTime is designed for modular application. Features of the language which are not needed for an application need not be supported. Depending on which features are supported, HyTime provides:

• Location addressing: a standard way of encoding a system-neutral address of any information object or any part of an information object within or external to any given document. Addressing may be by name, position, or semantic property.
• Hyperlinking: models for hyperlink classes independent of the number of objects linked to, and the context of the link. One model even provides for attaching properties to information objects that cannot be modified or over-written.
• Scheduling: synchronization and alignment of information objects relative to one another. Information objects are positioned within events on the spatio-temporal axes of a Finite Coordinate Space (FCS). The axes of the FCS can be related and can be named to match the context of the application. For example, the X axis can represent a virtual time line as seen in a project management schedule for project phases, and the Y axis can represent the real clock time as seen by a calendar.
• Object Modification: Object modification is scheduled by HyTime but must be applied by application-specific functions. This enables the scheduling of rendering instructions in other notations, e.g., PostScript.
• Event Projection: Events may be scheduled and projected onto alternative finite coordinate systems and scaled accordingly. For example, if a graphic in a document must be rendered in a smaller area on a display screen, this projection and scaling can be indicated by HyTime notation.
• Parsability: HyTime documents are parsable by SGML applications; parsing checks for correct SGML grammar and syntax as well as conformance of the instance to the DTD.

Hytime architecture and modules

The modules of HyTime are:

• Base Module: includes hyperdocument management facilities, SGML, identification facilities for replacing HyTime-specific identifiers with user-defined identifiers with provisions for name collisions, coordinate addressing for scheduling dimensions, positions of events, and document locations addressing by position. There is optional support for specifying activity tracking policies by an activity tracking attribute that is part of the SGML document, and for other basic utilities used to declare default attribute values and definition tables.
• Location Address Module: includes functionality to provide addressing of information objects without a unique identifier within the current document's name space. Supports addressing by coordinate location (discrete dimensions of arbitrary universe), semantic location (by SGML attribute name or by notation-specific address), or namespace location (SGML entities and SGML elements in external documents).
• Hyperlink Module: uses five metaclasses of hyperlinks to define application-specific hyperlink elements with their own processing semantics. The link classes are:
  1. independent links can have any number of link ends with optional end terms used as text or icons to invoke the link,
  2. property links (two link ends which associate an attribute name and value with an element),
  3. contextual links (two link ends, one of which is a link's own location),
  4. aggregate location links link multiple locations and treat them as a single location,
  5. span links allow contiguous information to appear to be undivided by SGML markup.
• Finite Coordinate Space Module (FCS): provides for scheduling of objects with optional projection and modification modules. Event schedules define the position and occurrence of objects. Objects occur in an FCS as the content of an event. An event is a conceptual bounding box. Each event has a set of dimension specifications for its position and extent on the coordinate axes of the FCS in which the event schedule appears. The FCS coordinates can be expressed in the terms of the application. Finite Coordinate Spaces can be nested. For example, if a project schedule is modeled as processes nested within processes, the FCS can be used to encode this nesting, the relationship of time changes that occur within a process and the effect of these changes on processes within which it is nested.

Advantages of Current Specification

Users of HyTime-compliant systems can incorporate active references within documents and to external online documents. This includes referencing to non-HyTime documents. HyTime can reference documents in multiple notation languages, e.g., IGES, VHDL, ODA, etc. HyTime location addressing includes the capability to reference read-only documents which is crucial to incorporating legacy data.

HyTime provides a standard way to represent abstract time dependencies. HyTime's representation of time and space measurements is the same and can be extended to any measurement domain with any number of axes.

HyTime does not restrict the potential sets of applications nor the application design except in the agreements about how to express hyperlinks. This enables maximum interoperability of hyperdocuments without attempting to standardize the information object notations or modifiers.

Enhancements to the Published Standard

The newest and most significant addition to the HyTime published standard is the HyQ query language. It was added to provide an alternative user interface (sanctioned by ISO) not only to HyTime and SGML documents but non-SGML documents as well by using HyTime features. Some of the recent technical changes to the published standard impact the Content Data Model which must be revised accordingly.

Implementation Issues

Non-HyTime notations used in scaling factors cannot be executed by a HyTime system. Such notations might include the potential for asynchronous interrupts by a user. HyTime has been created by the ISO X3Vl.8M committee with much effort and time spent anticipating conceivable applications. However, at this time, there are no full scale commercial applications that can be examined to determine if the standard is effective.