Page 168 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 6.3. Long-Term Memory Motion-Compensated Prediction 145

frame in memory. This sliding-window technique will be adopted throughout
this chapter.

6.3.1 Properties of Long-Term Block-Motion Fields
This subsection investigates the properties of long-term block-motion )elds
and compares them to those of single-reference block-motion )elds. All il-
lustrations in this subsection were generated using a full-pel full-search long-
term memory block-matching algorithm applied to the luma component of the
FOREMAN sequence with blocks of 16 × 16 pels, a maximum allowed displace-
ment of ±15 pels, SAD as the distortion measure, restricted motion vectors,
and original reference frames.

Property 6.3.1.1 The distribution of the long-term memory spatial displace-
ments (d x ;d y ) is center-biased. This is evident from Figure 6.2, which shows
the distribution of the relative frequency of occurrence of the spatial displace-
ments d x (Figure 6.2(a)) and d y (Figure 6.2(b)). Note that this is similar to
the single-reference case (M =1; skip = 1), although in the case of multiple-
reference (M =50; skip = 1), the distribution is slightly more spread, which
indicates that longer displacements are slightly more probable. This distribu-
tion is even more spread at higher frame skips, (M =50; skip = 4).

Property 6.3.1.2 The distribution of the long-term memory temporal dis-
placement d t is zero-biased. This is evident from Figure 6.3, where the tem-
poral displacement d t = 0 (which refers to the most recent reference frame

QSIF Foreman QSIF Foreman
0.6 0.7
M=1, Skip=1 M=1, Skip=1
M=50, Skip=1 M=50, Skip=1
M=50, Skip=4 M=50, Skip=4
0.6
0.5
0.5
0.4
p(d x ) 0.3 p(d y ) 0.4
0.3
0.2
0.2
0.1
0.1
0 0
−15 −10 −5 0 5 10 15 −15 −10 −5 0 5 10 15
d x d y
(a) Distribution of relative frequency of (b) Distribution of relative frequency of
occurrence of d x occurrence of d y
Figure 6.2: Center-biased distribution of the long-term memory spatial displacements (d x;d y )