DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

This method is sometimes
called “logarithmic,” which
I consider to be a very poor
term in this context.
to produce 1 ⁄2-luma-sample motion vectors in both
horizontal and vertical axes requires quadruple the
computational effort of producing full-sample vectors.
There are three major methods of motion estimation:
• Block matching, also called full search, involves an
exhaustive search for the best match of the target
macroblock through some two-dimensional extent of
the anchor frame. For the large ranges of MPEG-2, full
block matching is impractical.
• Pixel-recursive (or pel-recursive) methods start with
a small number of initial guesses at motion, based upon
motion estimates from previous frames. The corresponding
coordinates in the anchor frame are searched,
and each guess is refined. The best guess is taken as the
final motion vector.
• Pyramidal methods form spatial lowpass-filtered
versions of the target macroblock, and of the anchor
frames; block matches are performed at low resolution.
Surrounding the coordinates of the most promising
candidates at one resolution level, less severely filtered
versions of the anchor picture regions are formed, and
block-matches are performed on those. Successive
refinement produces the final motion vector. This technique
tends to produce smooth motion-vector fields.
Rate control and buffer management
A typical video sequence, encoded by a typical MPEG-2
encoder, produces I-, P-, and B-pictures that consume
bits roughly in the ratio 60:30:10. An I-picture requires
perhaps six times the number of bits as a B-picture.
Many applications of MPEG-2, such as DTV, involve
a transmission channel with a fixed data rate. This calls
for constant bit-rate (CBR) operation. Other applications
of MPEG-2, such as DVD, involve a recording
channel with variable (but limited) data rate. This calls
for variable bit-rate (VBR) operation, where the instantaneous
bit-rate is varied to achieve the desired picture
quality for each frame, maximizing storage utilization.
CHAPTER 40 MPEG-2 VIDEO COMPRESSION 491