Cryptography Reference
In-Depth Information
Display order
I
1
I
1
I
1
P
4
P
4
P
4
B
2
B
2
B
2
B
3
B
3
B
3
I
1
B
2
B
3
P
4
B
5
B
6
P
7
B
8
P
7
P
7
P
7
B
5
B
5
B
5
B
6
B
6
B
6
B
8
B
8
B
B
(a) MPEG picture structure (b) Encoding ord
er and display order
Fig. 2.16.
Picture types and their encoding/display order.
P-Picture: These are forward predicted pictures and are coded using motion
compensation from previous I- or P-pictures. MBs in P-picture are mostly
coded using forward predictive motion compensation. However if a good
match can not be found in reference frame, it will be Intra coded.
B-Picture: These are the bi-directional predicted pictures, which are coded
by interpolating between a previous and a future I- or P-picture. In a
B-picture, there could be three different types of MBs, including B-block,
P-block, and I-block. The default is B-block. If an MB can not find good
reference in previous and future frames simultaneously, it will be coded
using forward or backward predictive coding only. An Intra coding will be
used if good match can not be found in either previous or future frames.
Fig. 2.16(a) shows the structure of the three picture types in an MPEG
bitstream. Since B-pictures depend on the prediction based on the preceding
and future I- or P-pictures, the future frame must be available before a current
B-picture can be (de)coded. For this reason, the display order and coding order
in an MPEG video sequence must be different, as is shown in Fig. 2.16(b).
According to Fig. 2.16(b), the display order and coding order of a typical
IBBPBBcoding can be listed in the following,
Display order: I
1
, B
2
, B
3
, P
4
, B
5
, B
6
, P
7
, B
8
,
Coding order: I
1
, P
4
, B
2
, B
3
, P
7
, B
5
, B
6
, B
10
,
2.4.3 Variable Data Rate and Rate Control
The number of bits produced for each frame vary with the type of pictures
it is used. For example, I-frame requires most number of bits while B-frame
requires the least. On the other hand, the complexity of the scene in the video
signal can change dramatically over time, which results in significant changes
in the resulting data bits. Therefore the bit rate of a video stream will not be
constant but fluctuating. However, in some applications, such as broadcasting,
the transmission bandwidth is fixed. Therefore it is necessary to control the
