Page 245 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

222 Chapter 9. Error-Resilience Video Coding Techniques

Concealment Displacement
Displacement
Compensation
Estimation
(a) Stages of temporal concealment

displacement displacement compensation

damaged
block

reference frame current frame
(b) Conventional temporal concealment
Figure 9.8: Temporal error concealment

i.e., displacement estimation, is bypassed and the concealment displacement is
simply set to the original motion vector.
In practice, however, the motion vector of a damaged block is usually lost
or erroneously received. This is due mainly to spatial error propagation. For
example, an erroneous codeword will usually lead to loss of synchronization at
the decoder and all blocks, including their motion information, up to the next
3
synchronization point will be undecodable and completely lost. In such cases,
the displacement estimation stage at the decoder is extremely important. In
fact, the only di erence between the various conventional temporal techniques
reported in the literature is in their displacement estimation algorithm. This
stage is also known as motion information recovery, because it attempts to
recover or provide an approximation to the original motion information.
The simplest and most commonly used technique is to replace the dam-
aged motion vector with (0; 0) [179, 192]. This is based on the center-biased
property of video block-motion elds, which is also equivalent to the temporal
smoothness property of video signals. The technique is usually referred to as

3 As already discussed, RVLCs and data partitioning into motion and texture data are some of
the mechanisms that can be used to reduce this e ect.