Apple Final Cut Pro 6 User Manual
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Interframe Compression
Instead of storing complete frames, temporal compression stores only what has
changed from one frame to the next, which dramatically reduces the amount of data
that needs to be stored while still achieving high-quality images.
Groups of Pictures
MPEG formats use three types of compressed frames, organized in a group of pictures,
or GOP, to achieve interframe compression:
 I-frames: Intra (I) frames, also known as reference or key frames, contain all the necessary
data to re-create a complete image. An I-frame stands by itself without requiring data
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 largest type of MPEG frame, but
they are faster to decompress than other kinds of MPEG frames.
 P-frames: Predicted (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 refer to them. P-frames are typically much smaller
than I-frames.
 B-frames: Bi-directional (B) frames are encoded based on an interpolation from I- and
P-frames that come before and after them. B-frames require very little space, but
they can take longer to decompress because they are reliant on frames that may be
reliant on other frames. A GOP can begin with a B-frame, but it cannot end with one.
GOPs are defined by three factors: their pattern of I-, P-, and B-frames, their length, and
whether the GOP is “open” or “closed.”
GOP Pattern
A GOP pattern is defined by the ratio of P- to B-frames within a GOP. Common
patterns used for DVD are IBP and IBBP. All three frame types do not have to be used
in a pattern. For example, an IP pattern can be used. IBP and IBBP GOP patterns, in
conjunction with longer GOP lengths, encode video very efficiently. Smaller GOP
patterns with shorter GOP lengths work better with video that has quick movements,
but they don’t compress the data rate as much.
Some encoders can force I-frames to be added sporadically throughout a stream’s GOPs.
These I-frames can be placed manually during editing or automatically by an encoder
detecting abrupt visual changes such as cuts, transitions, and fast camera movements.