Optimal motion vector accuracy for block-based motion-compensated video coders

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Abstract
In block-based motion-compensated video coding, a fixed-resolution motion field with one motion vector per image block is used to improve the prediction of the frame to be coded. All motion vectors are encoded with the same fixed accuracy, typically 1 or 1/2 pixel accuracy. In this work, we explore the benefits of encoding the motion vectors with other accuracies, and of encoding different motion vectors with different accuracies within the same frame. To do this, we analytically model the effect of motion vector accuracy and derive expressions for the encoding rates for both motion vectors and difference frames, in terms of the accuracies. Minimizing these expressions leads to simple formulas that indicate how accurately to encode the motion vectors in a classical block-based motion-compensated video coder. These formulas also show that the motion vectors must be encoded more accurately where more texture is present, and less accurately when there is much interframe noise. We implement video coders based on our analysis and present experimental results on real video frames. These results suggest that our equations are accurate, and that significant bit rate savings can be achieved when our optimal motion vector accuracies are used.

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