Page 108 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 3.5. The MPEG-4 Standard 85

segment. For example, a background sprite (also referred to in the literature
as a background mosaic) can be constructed by collecting all pels belong-
ing to the background throughout a video segment. Note that in the case of
camera panning, for example, the background sprite can be larger than the
actual frames of the sequence. This still, and possibly large, image needs to
be transmitted only once before transmitting the corresponding video segment.
For each frame of the video segment, there is no need to encode a back-
ground VOP. Instead, a small number of parameters needs to be transmitted
to allow the decoder to warp=crop the sprite and generate an appropriate back-
ground VOP. Thus, in such cases, sprite coding can achieve very high coding
e,ciency.
Sprite coding can operate in three modes: basic sprite coding, low-latency
sprite coding, and scalable sprite coding. In basic sprite coding the whole sprite
is encoded and transmitted to the decoder before transmitting the corresponding
video segment. In low-latency sprite coding only part of the sprite is encoded
and transmitted. This part is su,cient to be used for the /rst few frames of
the video segment. The remaining part of the sprite is transmitted, piecewise,
when required or as the bandwidth allows. In scalable sprite coding the sprite
is encoded and transmitted progressively. In other words, a low-quality version
of the sprite is encoded and transmitted /rst. This is then re/ned gradually by
encoding and transmitting residuals.

3.5.7 Scalability

MPEG-4 supports both temporal and spatial scalability using multiple VOLs.
For example, in the case of two VOLs, one VOL provides the base layer
whereas the other VOL provides the enhancement layer.
MPEG4 uses a generalized scalability framework, as shown in Figure 3.15.
In this framework the functionality of a block depends on the chosen type of
scalability.

I O
1 Enhancement- Enhancement- 1
Layer Encoder Layer Decoder
Demultiplexer Midprocessor
In Out 1
Scalability Scalability
Midprocessor
Preprocessor Postprocessor Out 0
Multiplexer
Base-Layer Base-Layer
Encoder Decoder
I O
0 0
Figure 3.15: MPEG-4 generalized scalability