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8.7. Summary

In this chapter, a general overview of the basic MPEG-4 natural video coding approach and its respective tools was presented. First, the wide scope of the MPEG-4 object-based video coding approach was highlighted. Video is no longer restricted to rectangular images; now it is possible to code images and image sequences of arbitrary shape. This provides a number of new functionalities, going beyond pure video compression. The remainder of the chapter was subdivided into two parts: The first dealt with the coding of video sequences, and the second concentrated on the coding of still images, also referred to as visual texture coding.

The first part started with a description of the MPEG-4 video tools for the coding of rectangular video sequences. As it is based on the well-known motion-compensated hybrid coding, in particular, the new tools that were introduced by MPEG-4 Visual to improve coding efficiency have been described. These are (among others) quarter-pel motion compensation, global motion compensation, bidirectional motion compensation with direct mode, intra AC/DC prediction, and alternative DCT coefficient scans. The combination of these tools—as used, for example, in the Advanced Coding Efficiency (ACE) profile or the Advanced Simple profile (see Chapter 13)—provides a significantly higher coding efficiency compared to pre-MPEG-4 video coding technology, as has been shown in formal verification tests [N2824]. Based on this first part, the additional object-based coding tools were explained: binary and gray-level shape coding, object-based motion compensation, and object-based texture coding tools such as padding and shape adaptive DCT. Although not aimed at coding efficiency, these tools provide many content-based functionalities, including different quality for different video objects, flexible recomposition of scenes at the receiver side, and access to and manipulation of single video objects in a scene. The MPEG-4 video temporal, spatial, and quality scalability features were described. Based on the MPEG-2 Video scalable coding approach, they can be combined with the shape coding tools and thus applied to arbitrarily shaped objects. Finally, a section addressed special coding tools such as interlaced, error resilience, reduced resolution, and sprite coding, as well as the MPEG-4 short video header operational mode and studio-related tools.


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