Apple Compressor 3 - Manual (Page 212)

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
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 such as DVD Studio Pro. 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 DVD Studio Pro.
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.
212 Chapter 18 Creating MPEG-2 Output Files