Page 103 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

80 Chapter 3. Video Coding: Standards

based motion estimation and compensation are adapted to the arbitrary-shape
VOP structure of MPEG-4. The standard has three modes for encoding a
given VOP: intra-VOP (I-VOP), predicted-VOP (P-VOP), and bidirectionally-
predicted-VOP (B-VOP).
Since the shape, size, and location of a VOP can change from one instance
to another, the absolute (frame) coordinate system is used for referencing
every VOP. Thus, the motion vector for a particular feature inside a VOP
refers to the displacement of the feature in absolute coordinates. During motion
estimation and compensation, no alignment of VOP bounding boxes at di1erent
time instances is performed.
Motion is estimated only for those MBs within the bounding box of the
current VOP. If the current MB is an internal MB, then motion is estimated
using the usual block-matching method. If, however, the current MB is a
boundary MB, then motion is estimated using a modi/ed block-matching
method called polygon matching. In polygon matching, the distortion measure
is calculated using only those pels in the current macroblock that belong to
the VOP.
The motion estimation and compensation processes may require accessing
pels outside the reference VOP. Padding is used to de/ne the values of such
pels. The luma component is padded per 16 × 16 samples, while the chroma
components are padded per 8 × 8 samples. If the reference MB is a bound-
ary MB, then it is padded using repetitive padding. This process starts by
horizontal repetitive padding, where each sample at the boundary of a refer-
ence VOP is replicated horizontally in the left and=or right direction in order
to /ll the transparent region of the reference MB. If there are two boundary
sample values for /lling a sample, the two boundary samples are averaged.
The remaining un/lled transparent samples are padded by a similar process
as the horizontal repetitive padding but in the vertical direction, i.e., vertical
repetitive padding. The remaining MBs within the reference VOP are exterior
MBs. Such MBs are /lled by extended padding. In this method, samples of
an exterior MB are /lled by replicating the samples at the border of the neigh-
boring boundary MB. If an exterior MB is next to more than one boundary
MB, then one of the boundary MBs is chosen according to a priority criterion
de/ned by the standard. The remaining exterior MBs are /lled with 128 (for
an 8-bit luma component).
Motion vectors are estimated to half-pel accuracy. They are then predic-
tively VLC coded in a similar fashion to the H.263 standard.
Similar to the H.263 standard, MPEG-4 has an advanced prediction mode
(four motion vectors per MB and unrestricted motion vectors) and an over-
lapped motion compensation mode.