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Chapter 3. Object Description and Synchr... > Object Descriptors: Entry Points to ...

3.1. Object Descriptors: Entry Points to MPEG-4 Content

Part 2 [MPEG-4-2] and Part 3 [MPEG-4-3] of the MPEG-4 standard (corresponding to Chapters 8 to 12 in this book) specify the coding tools to generate compressed audio and visual streams. Each semantic visual or audio object may be coded into one or more such streams. An MPEG-4 presentation that consists of multiple such objects (and, hence, many streams) needs mechanisms to describe all the objects in the presentation. One part of this mechanism is embodied in the object descriptor framework [MPEG-4-1, Clause 8]. All components of an MPEG-4 presentation are discovered recursively through object descriptors (ODs). The details of this approach are explained in this chapter.

Object description enumerates only the streams in a presentation and specifies how they relate to media objects. In order to assemble those media objects into a specific audiovisual scene, most MPEG-4 presentations also include a scene description. The scene description format, called binary format for MPEG-4 scenes (BIFS), is explained in detail in Chapter 4. In order to understand the present chapter, one must understand the general role of scene description and its conceptual difference from object description.


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