Open and Closed GOPs
Open GOPs are most efficient because they allow an extra B-frame in the GOP pattern.
Open GOPs start with a B-frame that is able to look at the last P-frame from the preceding
GOP as well as the first I-frame of its own GOP.
:06 :07 :08 :09
:09
:10 :11 :12 :13 :14 :15 :16 :17 :18 :19 :20 :21
:04 :05
Open GOP
(IBBP, 15 frames)
I
B
B
P
B
B
P
B
B
P
B
B
P
B
B
I
B
B
P
:03
:06 :07 :08
:10 :11 :12 :13 :14 :15 :16 :17 :18
:04 :05
Closed GOP
(IBBP, 15 frames)
P
B
B
P
B
P
B
P
B
P
I
B
B
B
B
208
Chapter 18
Creating MPEG-2 Output Files
By definition, closed GOPs cannot contain any frame that refers to a frame in the previous
or next GOP. In contrast, open GOPs begin with one or more B-frames that reference the
last P-frame of the previous GOP. Closed GOPs created by Compressor always begin with
an I-frame.
Open GOPs generally provide slightly better compression than do closed GOPs of the
same structure and size. The illustration above shows that a closed GOP contains one
more P-frame than does an open GOP of the same length. Since P-frames generally require
more bits than do B-frames, the open GOP achieves slightly better compression.
There are limitations to using open GOPs for DVD-Video discs that are created using a
DVD authoring application. One limitation is that only closed GOPs are permitted within
MPEG-2 streams that will be used for mixed-angle or multi-angle DVDs.
The other limitation is that DVD chapter markers can only be set at the beginning of a
closed GOP. The best time to define chapter markers is before doing MPEG-2 transcoding.
For example, if you specify your chapter markers in Final Cut Pro, you can set Compressor
to do MPEG-2 transcoding with open GOPs. Compressor will then force a closed GOP to
begin only at the specified chapter markers and will make all other GOPs open. You can
also accomplish this by specifying “forced I-frames” in the Compressor Preview window
and giving them a chapter name to be used by a DVD authoring application.
However, if you want to specify chapter markers at any GOP boundary after your video
has been transcoded in the MPEG-2 format, you should use only closed GOPs. This freedom
is limited, as it allows you to set chapter markers only at GOP boundaries, rather than at
any video frame.