66 MPEG Myths p3
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
66 MPEG Myths p3
6. Sequence structure is fixed to a specific I,P,B frame pattern.
A sequence may consist of almost any pattern of I, P, and B pictures
(there are a few minor semantic restrictions on their placement). It
is common in industrial practice to have a fixed pattern (e.g.
IBBPBBPBBPBBPBB), however, more advanced encoders will attempt to
optimize the placement of the three picture types according to local
sequence characteristics in the context of more global
characteristics. Each picture type carries a penalty when coupled with
the statistics of a particular picture (temporal masking, occlusion,
motion activity, etc.).
The variable length codes of the macroblock_type switch provide a
direct clue, but it is the full scope of semantics of each picture type
spell out the costs-benefits. For example, if the image sequence
changes little from frame-to-frame, it is sensible to code more B
pictures than P. Since B pictures by definition are never fed back
into the prediction loop (i.e. not used as prediction for future
pictures), bits spent on the picture are wasted in a sense (B pictures
are like temporal spackle). Application requirements also govern
picture type placement: random access points, mismatch/drift reduction,
channel hopping, program indexing, and error recovery & concealment.
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