Page 105 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

82 Chapter 3. Video Coding: Standards

quantizer step size depending on the spatial frequency of the coe,cient. In
MPEG-4, DC coe,cients can also be quantized using a nonlinear quantizer.

3.5.4.4 Prediction of INTRA DCT Coe5cients

To achieve more e,ciency, the quantized coe,cients of an INTRA block can
be predicted from the colocated coe,cients in either the block immediately
to the left of or the block immediately above the current block, as shown in
Figure 3.14. The direction of prediction is adapted depending on the horizon-
tal and vertical DC gradients of neighboring blocks. Thus, if X is the current
INTRA block, QF (0; 0) is the quantized DC coe,cient of block A immedi-
A
ately to the left of the current block, QF (0; 0) is the quantized DC coe,cient
B
of block B above and to the left of X , and QF (0; 0) is the quantized DC
C
coe,cient of block C immediately above X , then the direction of prediction
is chosen as follows: If |QF (0; 0) − QF (0; 0)| ¡ |QF (0; 0) − QF (0; 0)|, then
C
B
B
A
predict from block C; otherwise predict from block A.
Having decided the direction of prediction, there are two types of prediction:
1. DC prediction: Depending on the direction of prediction, the DC coef-
/cient of the current block X is predicted from the DC coe,cient of
either block A or block C. For example, when the horizontal direction is
chosen, the prediction is given by PQF (0; 0)=QF (0; 0) − QF (0; 0).
X
X
A
Top-left Top
neighbor, B
neighbor, C

Left Current
neighbor, A
block, X

DC Coefficient INTRA
AC Coefficient macroblock

Figure 3.14: DC and AC coe,cient adaptive prediction in MPEG-4