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112 Chapter 4. Basic Motion Estimation Techniques

Foreman @ 8.33 f.p.s.
32
Unrestricted
Restricted
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30

PSNR Y (dB) 29

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25
1 5 10 15 20 25 30
Frame
Figure 4.7: Performance of the BMA with restricted and unrestricted motion vectors

blocks are included in the motion estimation and compensation process. In this
case, a referenced pel outside the frame is usually approximated by the closest
border pel. This unrestricted method can improve the prediction quality along
frame borders, especially in cases of camera or background movement. This
is particularly useful in small frame formats, where border blocks represent a
high percentage of the frame area. Figure 4.7 illustrates this improvement for
part of the FOREMAN sequence. The method is included in the H.263 optional
unrestricted motion vector mode and also in the advanced prediction mode.

4.6.6 Overlapped Motion Compensation

As already discussed, the BMA assumes that each block of pels moves with
a uniform translational motion. Because this assumption does not always hold
true, the method is known to produce blocking artefacts in the reconstructed
frames. One method that reduces this e ect is overlapped motion compensation
(OMC). The method was rst proposed by Watanabe and Singhal in 1991
[101]. In BMA, the estimated block motion vector is used to copy a displaced