The MPEG-1 Codec - A Practical Guide to Video and Audio Compression - page 197

Sequence

GOP 2

GOP 1

Figure 10-2 GOP structures within a sequence.

The GOP structure is repeated as often as necessary to produce the sequence. Figure
10-2 illustrates the GOP sequence idea.

10.5.1 Inside the GOP Structures

The structure of a GOP is quite complicated because it is not just a collection of video
frames. The frames have a very special relationship to one another within the GOP.
A GOP will always start with an I-frame because otherwise any successive difference
information stored in the P-frames has no basis on which to build the deltas.
Note that the frames are organized at regular intervals with a constant number of
B-frames between each I- or P-frame. Irregularly spaced key frames are permitted but this
is not common practice.

10.5.2 Closed GOP Structure

A GOP file containing an I-frame, some related P-frames based just on that I-frame, and
some B-frames coded only from pictures within that GOP is called a closed GOP structure.
Using the letter notation, a closed GOP structure starts with an I-frame and ends with a
P-frame.
In Figure 10-3, the arrows show the source of the difference data over several adjoin-
ing GOPs. Note that not all the connections are shown, otherwise it would resemble a ball
of wool.

10.5.3 Open GOP Structure

An alternative structure dispenses with P-frames altogether and just alternates between
I-frames and B-frames. Figure 10-4 shows several GOP structures and where the difference
data is derived.

P I B B B B P B B B B P I

Closed GOP with no external dependencies

Figure 10-3 A closed GOP sequence.