<title>Simple method to segment motion field for video coding</title>

Abstract

ABSTRACT This paper describes a motion compensated video coding method that employs a simple segmentationscheme to achieve sub-block resolution of the motion field. This increased accuracy of the motion esti-mation reduces the energy of the residue or displaced frame difference by as much as 30%. Including theoverhead to send the side information, the average bit rate is reduced by approximately 10%. 1. INTRODUCTION In block based motion compensated video coding, a frame is divided into non-overlapping blocks ofN * N pixels. For each block, a best match is determined in the previously transmitted frames. The criterion is usually the mean absolute difference (MAD) between the two blocks. The match is subtractedfrom the block to form the displaced frame difference (DFD) or residue which is then compressed by transform coding. The relative position from the block to the matched block in the previous frame is called motion vector. The collection of all the motion vectors forms a motion field.Block based motion estimation assumes that all pixels in a block move by the same amount. When twoor more objects in a block move at different velocities, the motion vector obtained by the conventional blockmatching method may correspond to only one of the objects, or in the worst case, to none of the objects.This can result in large DFD energy. The DFD can be reduced by using smaller block sizes, incurring thecost of sending more motion vectors. Orchard [1] applied segmentation to the previously reconstructedframe by dividing each block into regions corresponding to objects moving at different velocities. Motionestimate of each region is chosen from one of its nearest neighbors, so no extra motion vector needs to besent. However, segmentation adds a nontrivial amount of computation, must be done at both the encoderand decoder, and may not be easy to implement in hardware.In this paper, we present a simple motion field segmentation scheme that achieves sub-block resolution.Simulations show that the DFD energy can be reduced by as much as 30% and the overall bit rate byapproximately 10%.