Jordi Ribas-Corbera

Jordi Ribas-Corbera

Department of Electrical Engineering and Computer Science

University of Michigan

November 30, 1995

Optimal Bit Allocations for Motion-Compensated Video Coding

Abstract -
Digital video coding is currently a major topic of research since high levels of compression are necessary to develop a large number of important video products such as video telephone, video conferencing, video databases, and high definition television (HDTV).

In advanced video coders (e.g. MPEG), motion vectors are used to improve the prediction of the frame to be coded. Then, these motion vectors and the prediction error or difference frame must be encoded with bits. It is clear that increasing the number of motion bits decreases the number of bits needed to encode the difference frame, but more analysis is needed to find the optimal motion bit allocations that minimize the total bit rate for real video sequences. In fact, the number of bits allocated to the motion vectors in most video coders is based on heuristics and empirical experiments, and typically is just large enough to encode the motion vectors with 1 or 1/2 pixel accuracy, and all motion vectors are encoded with the same accuracy. In this work, we explore the benefits of encoding the motion vectors with other accuracies, and the benefits of encoding different motion vectors with different accuracies in the same frame. To do this, we analytically model the effect of motion vector accuracy and thereby obtain expressions for the total bit rate in terms of the accuracies, for typical lossless and lossy video coders. Minimizing these expressions leads to simple procedures for determining how accurately to encode the motion vectors. We implement the video coders, present experimental results of their performance on real video sequences, and discuss the benefits of our procedures. Finally, we discuss the generality of our work and its potential applications to other motion-compensated video coders.

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