Page 83 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

60 Chapter 3. Video Coding: Standards

component. There are three options for prediction:
(a) DC only: where the DC coe,cient is predicted as the average of
the corresponding coe,cients from the block above and the block
to the left.
(b) Vertical DC and AC: where the DC coe,cient and the /rst row
of AC coe,cients are vertically predicted from the corresponding
coe,cients from the block above.

(c) Horizontal DC and AC: where the DC coe,cient and the /rst
column of AC coe,cients are horizontally predicted from the
corresponding coe,cients from the block to the left.

Special cases are de/ned for situations in which the neighboring blocks
are not INTRA coded or are not in the same video picture segment.
The option that gives the best prediction for the whole macroblock is
chosen.

2. The quantization of INTRA coe,cients is modi/ed. INTRA DC coef-
/cients are quantized using a varying quantization step size, unlike the
/xed quantization step size of 8 utilized when this mode is not in use. In
addition, the quantization of all INTRA coe,cients is performed without
a dead-zone.
3. The scanning of DCT coe,cients is adapted to the prediction method of
the INTRA macroblock. For macroblocks predicted using the DC-only
option, the normal zigzag scanning is utilized; for macroblocks predicted
using the vertical DC and AC option, a new alternate horizontal scanning
pattern (Figure 3.5(a)) is utilized; whereas for macroblocks predicted

0 1 2 3 10 11 12 13 0 4 6 20 22 36 38 52
4 5 8 9 17 16 15 14 1 5 7 21 23 37 39 53
6 7 19 18 26 27 28 29 2 8 19 24 34 40 50 54
20 21 24 25 30 31 32 33 3 9 18 25 35 41 51 55
22 23 34 35 42 43 44 45 10 17 26 30 42 46 56 60
36 37 40 41 46 47 48 49 11 16 27 31 43 47 57 61
38 39 50 51 56 57 58 59 12 15 28 32 44 48 58 62
52 53 54 55 60 61 62 63 13 14 29 33 45 49 59 63

(a) Alternate horizontal scan (b) Alternate vertical scan
Figure 3.5: Alternate scans for the advanced INTRA mode of H.263+