Page 98 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 3.5. The MPEG-4 Standard 75

Input VOP Texture information
Texture
+
_ encoding
Motion-compensated
Decoded
VOP texture Texture
+
decoding
Decoded
Decoded current VOP
reference
Motion Multiplex Bitstream
compensation VOP VOP memory
(MC)
Motion Motion information
Motion encoding
estimation
(ME)
Shape information
Shape encoding

Figure 3.10: An MPEG-4 VOP encoder

motion, and texture. This is illustrated in Figure 3.10. As can be seen, an
MPEG-4 VOP encoder has three main functionalities: shape encoding, mo-
tion encoding (along with motion estimation and compensation), and texture
encoding. Note that the structure of this encoder is very similar to the MC-
DPCM structure utilized by H.263 and most other standards. In fact, for most
cases, the texture encoder is DCT-based and the structure is very similar to
the conventional hybrid MC-DPCM=DCT encoder. The di1erence here is that
the encoded entities can have arbitrary shapes rather than the /xed rectangular
frame shape, and therefore additional shape information needs to be encoded
and transmitted. Note that this object-based representation can be thought of
as a generic representation. When a frame is encoded using a single VOP, this
generic representation degenerates into the special case of rectangular frames
and an MPEG-4 encoder becomes almost identical to an H.263 encoder. In
fact, the MPEG-4 standard provides measures to ensure some level of inter-
operability with MPEG-1=2 and H.263.
A VOP is encoded on a macroblock (MB) basis. MPEG-4 supports a 4:2:0
subsampling format with 4 –12 bits=sample. Thus, an MB consists of six 8 × 8
blocks: four luma blocks and two corresponding chroma blocks. To achieve
e,cient encoding, the arbitrary shaped VOP is /rst encapsulated within a
bounding box. This bounding box is chosen such that it completely contains
the VOP but uses the minimum number of macroblocks. This bounding box
is illustrated for the Balloon VOP in Figure 3.11. Within this bounding box,