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14.9. 2D Compositor

This section describes some of the most important issues regarding the implementation of the 2D MPEG-4 compositor, a software implementation that had as a target the Complete 2D scene graph and graphics profiles (see Chapter 13). Because object composition is not normative in MPEG-4, this module is not included in the reference software as a normative part but as an informative one. Still, the code described here was useful to validate the specification, as only the technology that was implemented in the context of the reference software was included in the standard.

Being a 2D compositor, only a subset of the MPEG-4 BIFS nodes has been implemented (a list of the nodes implemented can be found in [MPEG4-5]). As a consequence, the problems that had to be faced were reduced. In particular, lights, navigation, and, of course, 3D primitives were left out of the scope of this implementation. This section will illustrate how the core framework was used, along with the implementation details of the efficient rendering of 2D media objects and the synchronization of ESs. This description does not cover all of the implementation issues of an MPEG-4 terminal, but illustrates some implementation choices made when implementing a compositor on top of the core reference software classes.


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