Reducing rate/complexity in video coding by motion estimation with block adaptive accuracy

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Abstract
Classical block-based motion-compensated video coders need to find and code a motion field with one motion vector per image block. All motion vectors are computed and encoded with the same fixed accuracy, typically 1 or one-half pixel accuracy. Higher motion vector accuracies have been shown to significantly reduce the total bit rate in some video sequences, but motion estimation at such subpixel accuracies is computationally expensive and is usually not performed in practice. In this paper we show that computing and encoding different motion vectors with different accuracies in the same frame can significantly reduce the total bit rate, and that the complexity of the block adaptive motion estimation procedure can be as low as that of the classical motion estimation at 1 pixel accuracy. Our new block adaptive motion estimator uses a simple technique to decide how accurately to compute the motion vector for each block. This technique results from an analysis on the effect of the motion vector accuracy on the total bit rate, and indicates that motion vectors of higher texture blocks must be computed more accurately and that at higher levels of compression lower motion vector accuracies are needed. We implement two video coders based on our technique, present results on real video frames, and describe the rate/complexity benefits of our procedure.

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