Page 190 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 7.6. Hierarchical Search Techniques 167

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Figure 7.4: Hierarchical motion estimation using a mean pyramid of three levels applied to a
QCIF frame

of ± 1 pel around the propagated vector from the lower resolution level. This
gives a )nal vector with a maximum range of ± 15 pels.
There are many variants to hierarchical motion estimation. Some techniques
use the same frame size in all levels of the hierarchy, with larger block sizes
at lower levels. Other techniques use the same block size in all levels of the
hierarchy, with subsampled frames at lower levels. In both cases, any level will
have fewer blocks than the next higher level. Thus, a motion vector estimated
at one level will be propagated to more than one block in the higher level.
In addition to reduced-complexity and robust estimation of high-motion
content, hierarchical motion estimation algorithms are also reported to provide