- Abstract:
- Most predictive coding schemes for video-conferencing
and multimedia employ block matching motion compensation. The prediction
quality is dependent on each block representing an area of uniform translational
motion. This is rare for real image sequences, but the assumption becomes
more valid as the block size is reduced. However, if the size of the block
is decreased, then the overhead of computation and transmission of displacement
information is increased. This problem can be alleviated by allowing the
dimensions of blocks to adapt to local activity within the image, larger
blocks being used in large areas of stationary background or uniform motion,
and smaller blocks where the movement is localised or complex.
We describe a variable size block matching motion estimation algorithm which is as computationally efficient as fixed size block matching and yet provides a better quality prediction. The technique starts by matching small square blocks. Displacement information is stored as simple bit vectors for those positions which, when the blocks are matched, result in a mean absolute error being less than a predefined threshold. Blocks are then merged in a quad-tree manner depending on whether they have candidate motion vectors in common. The merging process is trivial, using bit-wise logical AND operations. The threshold, which is found to be proportional to the minimum mean absolute [matched] error of the entire frame, can be adjusted to vary the frame block count. The threshold also ensures that poorly matched blocks are treated as special cases and not merged into inappropriate areas. Good quality motion estimation is achieved regardless of the number of blocks.
The adaptive motion compensation technique has been tested on a number of MPEG-4 image test sequences. The prediction performance is significantly better than fixed size block matching, and yet the computational requirement is comparable.
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