Working with GOP Settings
A major function of MPEG encoding is to identify redundancy not only within a frame,
but also among a group of frames. A scene with no movement except a car driving by
has a lot of redundancy over time. In this case, a single complete frame could provide
the majority of the video; subsequent frames only need to provide for the areas that
change. Each MPEG stream contains three types of frames that define the video. A Group
of Pictures (GOP) setting defines the pattern of the three frame types used: Intra (I) frames,
Bi-directional (B) frames, and Predicted (P) frames.
• I-frames: Also known as reference or key frames, I-frames contain the complete image
of the current frame, without reference to frames that precede or follow it. The I-frame
is the only type of MPEG frame that can stand by itself, without requiring information
from other frames in the GOP. Every GOP contains one I-frame, although it does not
have to be the first frame of the GOP. I-frames are the least efficient MPEG frame type,
requiring the most disc space. Markers on a DVD track can be placed only at I-frames.
• P-frames: P-frames are encoded from a “predicted” picture based on the closest
preceding I- or P-frame. P-frames are also known as reference frames, because
neighboring B- and P-frames can reference them. P-frames typically require much less
disc space than I-frames.
• B-frames: B-frames are encoded based on an interpolation from I- and P-frames that
come before and after them. B-frames are the most efficient MPEG frame type, requiring
the least amount of disc space. While a GOP can begin with a B-frame, it cannot end
with one.
The goal of encoding video to MPEG is to reduce the video file size as much as possible
while maintaining good quality. The B- and P-frames are what make MPEG so good at
compressing video into such small files. Because they only contain information about
what has changed in the images, they use much less disc space than I-frames.
65
Chapter 4
Preparing Video Assets
There are three aspects to choosing a GOP setting: the GOP pattern, the GOP length, and
whether the GOP is “open” or “closed.”
:09
: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