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MHEG Implementation

Several components may be requires in implementing and MHEG systems:

Runtime Engine (RTE)
-- MHEG-5 runtime engines generally run accross a client-server architecture (See The Armida system (Figure 8.14) referenced below for an example application).

Armedia Client Architecture
Armedia is a client-server based interactive multimedia application retrieval system.

A preceding Start-up Module may be used to perform general initialization etc.:

The MHEG-5 RTE is the kernel of the client's architecture. It performs the pure interpretation of MHEG-5 objects and, as a platform-independent module, issues I/O and data access requests to other components that are optimized for the specific runtime platform.

The RTE performs two main tasks. First, it prepares the presentation and handles accessing, decoding, and managing MHEG-5 objects in their internal format. The second task is the actual presentation, which is based on an event loop where events trigger actions. These actions then become requests to the Presentation layer along with other actions that internally affect the engine.

Presentation layer
-- The presentation layer (PL) manages windowing resources, deals with low-level events, and performs decoding and rendering of contents from different media to the user. This functionality is available to the RTE via an object-oriented API that encapsulates all I/O platform specific aspects. The basic MHEG-5 Presentable classes have counterparts at this API level, which makes provisions for initialization/termination, data access and decoding, setting specific attributes (such as text font, color, and so on), and performing spatial and temporal controls. In addition, an informative flow exists from the PL back to the RTE, which notifies user interaction and stream events.

Access module
-- This module provides a consistent API for accessing information from different sources. It's used by the RTE to get objects and the PL to access content data (either downloaded or streamed). Note that the selection of a particular delivery strategy is out of MHEG-5's scope, and hence remains an implementation issue. Typical applications should support:

The implementation of these mechanisms occurs via the DSMCC interface. The user has full interactive control of the data presentation, including playback of higher quality MPEG-2 streams delivered through an ATM network. Object and content access requests can also be issued to the Web via HTTP. However, this wide-spread protocol still suffers from limitations when high-quality video clips are progressively down-loaded, since the underlying networks may not yet provide adequate quality-of-service (QoS). When accessing the broadcast service, the Access module requires the DVB channel selection component to select the program referred to by a Stream object. To achieve this, you set the Con-tentHook to a value that means Switched DVB and the OriginalContent to the identifier of the channel to select. The MPEG-2 stream transport-ed by the selected channel is displayed in the active Scene's presentation space.

MHEG Authoring Tool: MediaTouch

The availability of an adequate authoring tool is mandatory to create the MHEG applications. The MediaTouch (Figure 8.15) application is one example developed for the Armida Sysytem
( /ArmidaIS/home_en.htm ). It is a visual-based Hierarchical Iconic authoring tool, similar to Authorware in many approaches.

MediaTouch MHEG Authoring Tool (Hierarchy and Links Editor windows)

MediaTouch is based on the native approach, which lets the author operate at the level of MHEG-5 objects.

MediaTouch provides authors with the following functions:

Hierarchy Editor
-- Supports the interactive creation of the structure of Applications and Scenes objects. The author operates by adding, moving, and deleting Ingredients on the tree that represents the hierarchy's current status. Several Scenes from different Applications can be edited at one time and you can copy and move Ingredients between Scenes and Applications.
Properties Editor
-- The author sets the properties of Presentable Ingredients via an input form that is specific to each object class. The author does not need to type a value directly, since the system adopts a menu-based input interface that aims to minimize errors and inconsistencies.

Layout Editor
-- A Scene's layout can be set by interactively adjusting the position and size of the bounding box for every Visible Ingredient. An Ingredient's content data, when available, is also displayed within its bounding box to make the layout match the actual presentation.
Links Editor
-- This function lets authors add, modify, and delete links and actions to Application and Scene objects. The author sets the link conditions, actions, and referenced objects via a menu. Links and actions can also be copied between different scenes and applications.

Figure 8.15) shows a screen shot from MediaTouch where an Application composed of two Scenes and one shared Text is open within the Hierarchy Editor window. One of these Scenes shows its Presentable Ingredients, and the author is editing its Links. The author can then launch the RTE from within MediaTouch to check the outcome of his work.

MHEG Autoring Tool: MHEGDitor
-- MHEGDitor is an MHEG-5 authoring tool based on Macromedia Director. MHEGDitor is composed of an Authoring Xtra to edit applications and of a Converter Xtra to convert resulting movies into MHEG-5 applications. The two MHEGDitor Xtras work separately. With MHEGDitor Authoring Xtra, create and edit your application within Macromedia Director 6. It opens a window to set preferences and links a specific external script castLib to your movie for you to create specific MHEG behaviours rapidly. You can test your application on the spot within Director, as if it were played by MHEGPlayer, the MHEG interpreter companion of MHEGDitor. With MHEGDitor Converter Xtra, convert Macromedia Director 6 movies (edited with MHEGDitor Authoring Xtra) into a folder containing all necessary items for an MHEG-5 application. Associated with Macromedia Director 6, MHEGDitor is the easiest way to author multimedia applications for Interactive TV for both Macintosh and Windows. This tool makes it possible for all system operators or content providers to deliver developed interactive programs to the widest range of potential users.

The Current Release is 1.4 which performs the following:

MHEG writing tool:MHEG Write
-- An editor to create and manipulate MHEG-5 applications by hand This editor is based on the free software "VIM", which is available via Internet from various sites for virtually all operating systems, including DOS, Windows, UNIX, etc. The MHEGWrite extension supports only the MHEG-5 textual notation. MHEGWrite provides macros for object templates, syntax highlighting and syntax error detection. Using MHEGWrite enables you to access every MHEG-5 features to edit your applications, since this editor is able to read native MHEG-5 code. It also gives good support for detailed examination, profile adaptation, and error correction of MHEG-5 applications produced with arbitrary other tools. By purchasing MHEGWrite, the MHEG Centre will also provide you with a set of MHEG-5 application samples, illustrating several MHEG-5 features, such as ListGroup together with cloning, using variables, sharing objects, etc.

Playing MHEG files
-- There are a few ways to play MHEG files:

next up previous
Next: MHEG Future Up: Multimedia and Hypermedia Information Previous: Relationships to Major Standards
Dave Marshall