Page 106 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 3.5. The MPEG-4 Standard 83

2. AC prediction: Depending on the direction of prediction, the AC co-
e,cients of the /rst row of the current block X are predicted from
the AC coe,cients of the /rst row of block C, or the AC coe,cients
of the /rst column of the current block X are predicted from the AC
coe,cients of the /rst column of block A. To compensate for di1er-
ences in the quantization parameters of adjacent blocks used in AC
prediction, the prediction process is modi/ed so that the predictor is
scaled by the ratio of the current quantization parameter, QP , and the
X
quantization parameter of the predictor block, QP or QP . For exam-
A
C
ple, when the horizontal direction is chosen, the prediction is given by
PQF (0;j)=QF (0;j) − QF A (0;j)QP A . The use of AC prediction can be
X
X
QP X
enabled=disabled at the macroblock level.
If any of the neighboring blocks are outside of the VOP boundary or the
video packet boundary, or if they do not belong to an INTRA coded mac-
roblock, their DC values are assumed to take a value of 2 bits=pel+2 and their
AC values are assumed to take a value of 0. DC and AC predictions are
performed similarly for the luma and each of the two chroma components.
3.5.4.5 Scanning
To prepare the coe,cients for variable-length encoding, a scanning process is
used to convert the 2-D matrix of coe,cients into a 1-D vector.
There are three possible scanning patterns: zigzag, alternate-vertical, and
alternate-horizontal. All non-INTRA blocks use the conventional zigzag scan-
ning pattern. For INTRA blocks, however, the choice of the scanning pattern
depends on the prediction process:
1. If AC prediction is not employed, then the conventional zigzag scanning
pattern is used for all blocks within the macroblock.
2. If, however, AC prediction is employed, then the direction of the DC
prediction is used to select a suitable scanning pattern on a block basis,
as follows:
(a) If the DC prediction employs the horizontal direction, then the
alternate-vertical scanning pattern is used.
(b) If, however, the DC prediction employs the vertical direction, then
the alternate-horizontal scanning pattern is used.

3.5.4.6 Variable-Length Coding
The di1erential (predicted) DC coe,cients in INTRA macroblocks are en-
coded using a concatenation of a VLC codeword and a FLC codeword. The
possible range of encoded di1erential DC coe,cients is divided into subranges