Page 259 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

236 Chapter 10. Error Concealment Using Motion Field Interpolation

improved concealment quality, as will be shown later. A point to note here
is that MFI will be used only for damaged blocks. Thus, provided that the
error rate is relatively low, this will not increase the complexity of the decoder
considerably.

10.3 Temporal Error Concealment Using a
Combined BM-MFI Technique

10.3.1 Motivation
In this section, multihypothesis motion compensation (MHMC) [106] is used
to further improve the second stage, i.e., compensation, of the temporal con-
cealment process. In MHMC, a blockis compensated using a weighted average
of several motion-compensated predictions (hypotheses). This is a general term
that can be used to describe techniques like overlapped motion compensation,
bidirectional motion compensation, and any other technique that compensates
individual pels using more than one motion vector. When applied to temporal
error concealment, this means that each pel of the damaged blockwill be
concealed using more than one concealment displacement. In the described
technique, two concealment displacements are used per pel: one is estimated
using BM, as described in Section 9.7.2, and the other is estimated using
MFI, as described in Section 10.2. The BM technique was chosen because it
is one of the best conventional temporal error concealment techniques. A sim-
ilar combination between BM and overlapped motion compensation has also
been reported in Ref. 198.
In addition to improving the second stage of the temporal concealment pro-
cess, the combination of BM and MFI can provide a more robust performance.
This can be explained as follows. There are many cases where the BM tech-
nique will fail but the MFI technique will not, and vice versa. In such cases, a
combination may be more robust because it may average out the concealment
distortion.

10.3.2 Description of the Technique
Let ˆ m(x; y)=( m x (x; y); m y (x; y)) be the displacement estimated using MFI ˆ ˆ
ˆ ˆ
ˆ
to conceal pel (x; y) of the damaged block D, and let b =(b x ; b y )bethe
displacement estimated using BM to conceal the whole block. Then pel (x; y)
is concealed as follows:
ˆ
m y (x; y))
f(x; y)= w (x n ;y n )f t−:t (x +ˆm x (x; y);y +ˆ
t
ˆ
ˆ
+(1 − w (x n ;y n ))f t−:t (x + b x ;y + b y ): (10.8)