Several cross platform video and audio standards have been established including still and motion JPEG, and a number of different MPEG standards. So far, there has been no standard method of bringing all these formats together to produce multimedia presentations. The MHEG model described in this article aims to solve this by providing a system independent presentation standard for hardware and software engineers and presentation authors to conform to. In this way, a presentation created on one hardware platform should be viewable on others.
MHEG is an abbreviation for the Multimedia and Hypermedia Experts Group. This is another group of specialists, eminent in their field which has been set up by ISO, the International Standards Organisation. This group was set the task of creating a standard method of storage, exchange and display of multimedia presentations. MHEG version 5 is the current work item for the group, and this was created in November 1994. It has been a DIS (Draft International Standard) since December 1995, and is due to be promoted to full international standard in July 1996. Its basic goals are:
In order for products to have maximum commercial success, they must appeal to the largest number of consumers possible. There are various multimedia presentation packages available, but they are proprietary and do not work across different hardware platforms. This means that an author who wants to publish an interactive multimedia book for example, will have to produce several versions that comply with different standards, or risk losing potential customers. The MHEG standard would do away with all this, allowing an author to produce his/her work in one universally acceptable format. It also has advantages for the hardware and software suppliers. They are able to concentrate on producing the one standard MHEG engine rather than an engine for every presentation standard that is available.
The applications to which MHEG may be put are growing all the time, as people dream up more and more applications of multimedia. Here are a few examples:
MHEG defines the abstract syntax through which presentations can be structured. This is the definition of data structures and the fields in those data structures, through which two computers may communicate. In the case of a multimedia book, these two computers would be that of the author and that of the user.
The MHEG model is object orientated, and defines a number of classes from which object instances are created when a presentation is designed. There are several classes, and these are used to describe the way video is displayed, audio is reproduced and how the user can interact with the ongoing presentation. The relationship that is created between instances of these classes forms the structure of the presentation. In addition to just replaying existing multimedia data MHEG also defines some types of its own. e.g. an MHEG compliant system is able to overlay titles onto video scenes. It is also able to display menus and buttons to allow the user to make choices
There are several different types of class in the MHEG model:
There are many more classes defined by MHEG. Some are concerned with the structure of the presentation and grouping of objects, whilst others are involved in the interchange of information between machines.
The text form of MHEG code is written in ASN 1 - abstract syntax notation version 1 - which is another ISO standard. It is quite verbose, and as such is fairly easy to read. The final form of MHEG code is binary, not textual, and this binary form must be common to all hardware platforms for the standard to work. The exact way in which the abstract classes are displayed (e.g. the check boxes and push buttons) are considered an implementation issue and are up to the designer of the MHEG engine for each platform. This is fine since it does not effect the compatibility of different systems, just the style of display.
Example MHEG Code
text: content-hook: #textHook original-box-size: (560 40) original-position: (80 100) content-data: included-content: "1. Return to main menu",cr, "2. Play video footage",cr, "3. Hear sound track"
This short code fragment would display a simple textual menu on the screen.
The outlook for MHEG is good. The ideas are not yet an ISO standard, but there is much interest in the area. Several large companies including IBM are already developing software that conforms to the standard. No longer will publishers have to take a gamble on what media platform to provide their wares on. This standard will allow the interchange and playback of multimedia information on any platform. The fact that a standard is now being agreed is creating new application areas for multimedia technology.
MHEG Explained by Thomas Meyer-Boudnik and Wolfgang Effelsberg - Article from IEEE Multimedia, Spring 1995 edition pages 26-38.
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