MPEG video compression - The MPEG - page 152

Typically two B pictures are inserted between P pictures or between I and P pictures. As can be seen, B pictures
are never predicted from one another, only from I or P pictures. A typical GOP for broadcasting purposes might
have the structure IBBPBBPBBPBB . Note that the last B pictures in the GOP require the I picture in the next GOP
for decoding and so the GOPs are not truly independent. Independence can be obtained by creating a closed GOP
which may contain B pictures but which ends with a P picture. It is also legal to have a B picture in which every
macroblock is forward predicted, needing no future picture for decoding.
Bidirectional coding is very powerful. Figure 5.26 is a constant quality curve showing how the bit rate changes with
the type of coding. On the left, only I or spatial coding is used, whereas on the right a IBBP structure is used. This
means that there are two bidirectionally coded pictures in between a spatially coded picture ( I ) and a forward
predicted picture ( P ). Note how for the same quality the system which only uses spatial coding needs two and a
half times the bit rate that the bidirectionally coded system needs.
Figure 5.26: Bidirectional coding is very powerful as it allows the same quality with only 40 per cent of the bit rate
of intra-coding. However, the encoding and decoding delays must increase. Coding over a longer time span is
more efficient but editing is more difficult.
Clearly information in the future has yet to be transmitted and so is not normally available to the decoder. MPEG
gets around the problem by sending pictures in the wrong order. Picture reordering requires delay in the encoder
and a delay in the decoder to put the order right again. Thus the overall codec delay must rise when bidirectional
coding is used. This is quite consistent with Figure 1.5 which showed that as the compression factor rises the
latency must also rise.
Figure 5.27 shows that although the original picture sequence is IBBPBBPBBIBB . . ., this is transmitted as
IPBBPBBIBB . . . so that the future picture is already in the decoder before bidirectional decoding begins. Note that
the I picture of the next GOP is actually sent before the last B pictures of the current GOP.
Figure 5.27: Comparison of pictures before and after compression showing sequence change and varying amount
of data needed by each picture type. I , P , B pictures use unequal amounts of data.
Figure 5.27 also shows that the amount of data required by each picture is dramatically different. I pictures have
only spatial redundancy and so need a lot of data to describe them. P pictures need fewer data because they are
created by shifting the I picture with vectors and then adding a prediction error picture. B pictures need the least
data of all because they can be created from I or P .
With pictures requiring a variable length of time to transmit, arriving in the wrong order, the decoder needs some
help. This takes the form of picture-type flags and time stamps which will be described in section 6.2 .
5.10 Coding applications