Information Technology Reference
In-Depth Information
an 8 x 8 DCT block rather than with a 16 x 16 macroblock. This allows much more precise prediction of complex
motion, especially near the edge of objects, and will result in a reduced residual bit rate. On a macroblock-
bymacroblock basis an MPEG-4 coder can decide whether to code four vectors per macroblock or only one. When
the compressed vector data and residual are combined, one of these approaches will result in the least data.
5.8
I
and
P
coding
Predictive (
P
) coding cannot be used indefinitely, as it is prone to error propagation. A further problem is that it
becomes impossible to decode the transmission if reception begins part-way through. In real video signals, cuts or
edits can be present across which there is little redundancy and which make motion estimators throw up their
hands.
In the absence of redundancy over a cut, there is nothing to be done but to send the new picture information in
absolute form. This is called
I
coding where
I
is an abbreviation of
intra
coding. As
I
coding needs no previous
picture for decoding, then decoding can begin at
I
coded information.
MPEG is effectively a toolkit. The bigger the MPEG-number, the more tools are available. However, there is no
compulsion to use all the tools available. Thus an encoder may choose whether to use
I
or
P
coding, either once
and for all or dynamically on a macroblock-by-macroblock basis. For practical reasons, an entire frame may be
encoded as
I
macroblocks periodically. This creates a place where the bitstream might be edited or where
decoding could begin.
Figure 5.23 shows a typical application of the Simple Profile of MPEG-2. Periodically an
I
picture is created.
Between
I
pictures are
P
pictures which are based on the picture before. These
P
pictures predominantly contain
macroblocks having vectors and prediction errors. However, it is perfectly legal for
P
pictures to contain
I
macroblocks. This might be useful where, for example, a camera pan introduces new material at the edge of the
screen which cannot be created from an earlier picture.
Figure 5.23:
A Simple Profile MPEG-2 signal may contain periodic
I
pictures with a number of
P
pictures between.
Note that although what is sent is called a
P
picture, it is not a picture at all. It is a set of instructions to convert the
previous picture into the current picture. If the previous picture is lost, decoding is impossible.
An
I
picture together with all of the pictures before the next
I
picture form a
group of pictures
(GOP).
5.9 Bidirectional coding
Motion-compensated predictive coding is a useful compression technique, but it does have the drawback that it can
only take data from a previous picture.
Where moving objects reveal a background this is completely unknown in previous pictures and forward prediction
fails. However, more of the background is visible in later pictures. Figure 5.24 shows the concept. In the centre of
the diagram, a moving object has revealed some background. The previous picture can contribute nothing,
whereas the next picture contains all that is required.
