DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 14.6 Example GOP
I0B1B2P3B4B5P6B7B8 Coded B-pictures in a GOP depend upon P- and
I-pictures; coded P-pictures depend upon earlier
P-pictures and I-pictures. Owing to these interdependencies,
an MPEG sequence cannot be edited, except
at GOP boundaries, unless the sequence is decoded,
edited, and subsequently reencoded. MPEG is very suitable
for distribution, but owing to its inability to be
edited without impairment at arbitrary points, MPEG is
unsuitable for production. In the specialization of
MPEG-2 called I-frame only MPEG-2, every GOP is
a single I-frame. This is conceptually equivalent to
Motion-JPEG, but has the great benefit of an international
standard. (Another variant of MPEG-2, the
simple profile, has no B-pictures.)
I have introduced MPEG as if all elements of a P-picture
and all elements of a B-picture are coded similarly. But
a picture that is generally very well predicted by the
past anchor picture may have a few regions that cannot
effectively be predicted. In MPEG, the image is tiled
into macroblocks of 16×16 luma samples, and the
encoder is given the option to code any particular
macroblock in intra mode – that is, independently of
any prediction. A compact code signals that a macroblock
should be skipped, in which case samples from the
anchor picture are used without modification. Also, in
a B-picture, the encoder can decide on a macroblockby-macroblock
basis to code using forward prediction,
backward prediction, or bidirectional prediction.
Reordering
In a sequence without B-pictures, I- and P-pictures are
encoded and transmitted in the obvious order.
However, when B-pictures are used, the decoder typically
needs to access the past anchor picture and the
future anchor picture to reconstruct a B-picture.
Consider an encoder about to compress the sequence
in Figure 14.6 (where anchor pictures I 0, P 3, and P 6 are
written in boldface). The coded B 1 and B 2 pictures may
be backward predicted from P 3, so the encoder must
buffer the uncompressed B 1 and B 2 pictures until P 3 is
coded: Only when coding of P 3 is complete can coding
of B 1 start. Using B-pictures incurs a penalty in
124 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES