The current video coding standards, such as MPEG1 and MPEG2, all use on block based motion compensation to reduce the temporal redundancy. Motion vectors used for the compensation are computed by block matching. The computational load to compute these vectors is very high, and as a result, the range of the motion estimation is limited by hardware constraints. In image sequences where the actual motion exceeds this range, the video quality of the decoded sequences will suffer. By allowing the motion estimation search window to be displaced by a certain offset with respect to the position of the block to be coded, a much larger range of motion can be compensated effectively. We describe a method for computing this offset, the global motion present in a certain frame, and show that this measure can be used as an estimate for the displacement applied to the position of the search window in the next frame. Experimental data is included, and simulation results show the use of global motion can give significant improvements in average number of bits produced per frame using constant quality.
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