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Chapter 17. MPEG-4 BINARY FORMAT FOR SCE... > Binary Format for Scenes (BIFS)

Binary Format for Scenes (BIFS)

MPEG-4 Systems handles the composition of objects as well as how a user may interact with such objects. The audio, video, and SNHC (synthetic natural hybrid coding) objects provide their own encoding algorithms. In order to combine these media objects into complete presentations, a scene description capability is needed. Binary Format for Scenes is a standard mechanism that provides input data to the presentation layer of the MPEG-4 terminal. Based on VRML97, BIFS includes all the functionalities of this standard (see Part 2 of this book to learn more about VRML). As a text-based 3D scene description language, it describes VRML objects and their actions in plain text format. BIFS code is binary, however, and as such it is capable of representing the same scene in much less space. BIFS content is typically 10 to 15 times smaller in size than when represented in VRML text format. (VRML files can be compressed with GZip, as explained in Chapter 6, "Weaving VRML into Web Pages," although GZip does not compress files as much as BIFS.)

To build interactive applications, BIFS defines commands that can modify, delete, or replace objects in the scene. It is possible to change visual or acoustic properties of an object without changing the object itself; thus, the color alone of a 3D object might be varied. Sending only BIFS-Commands can produce animations, and it is even possible to define the behavior of animated objects in response to user input at the receiver. For streamed animations, BIFS-Anim offers better compression and the ability to synchronize with other media in the scene. A typical example is lip synchronization.


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