Digital Signal Processing Reference
In-Depth Information
form the motion estimation and residuals. Restoring or
decompressing the B-frame requires compressed data from
preceding and successive I- or P-frames, plus residual data
and motion estimation corresponding to the current B-frame.
B-frames are inter-coded.
I B B
B
P B B
B
P B B
B
P I
Frame number 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Group of pictures
Figure 14.3. Group of pictures (GOP) example.
By finishing the GOP with a P-frame, the last few B-frames can
be decoded without needing information from the I-frame of the
following GOP.
14.3 Frame Processing Order
The order of frame encode processing is not sequential.
Forward and backward prediction means that each frame has
a specific dependency mandating the processing order, and
requires buffering of the video frames to allow out-of-sequential
order processing. This also introduces multi-frame latency in the
encoding and decoding process.
At the start of the GOP, the I-frame is processed first. Then the
next P-frame is processed, as it needs the current frame plus
information from the previous I- or P-frame. Then the B-frames
in between are processed, as, in addition to the current frame,
information from both previous and post frames are used. Then
the next P-frame, and after that the intervening B-frames, as
shown in the processing order given in
Figure 14.4
.
Note that since this GOP begins with an I-frame and finishes
with a P-frame, it is completely self-contained, which is advan-
tageous when there is data corruption during playback or
decoding. A given corrupted frame can only impact one GOP. The
following GOP, since it begins with an I-frame, is independent of
problems in a previous GOP.
