DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Whether an encoder actually
searches this extent is not
standardized!
Motion estimation
A motion vector must do more than cover motion from
one frame to the next: With B-frames, a motion vector
must describe motion from one anchor frame to the
next – that is, from an I-frame or P-frame to the picture
immediately before the following I-frame or P-frame. As
page 123’s m value increases, motion vector range must
increase. The cost and complexity of motion estimation
increases dramatically as search range increases.
The burden of motion estimation (ME) falls on the
encoder. Motion estimation is very complex and
computationally intensive. MPEG-2 allows a huge
motion vector range: For MP@ML frame-structured
pictures, the 16×16 prediction region can potentially lie
anywhere within [-1024…+1023 1 ⁄2] luma samples horizontally
and [-128…+127 1 ⁄2] luma samples vertically
from the macroblock being decoded. Elements in the
picture header (f code) specify the motion vector range
used in each picture; this limits the number of bits that
need to be allocated to motion vectors for that picture.
The purpose of the motion estimation in MPEG is not
exactly to estimate motion in regions of the picture –
rather, it is to access a prediction region that minimizes
the amount of prediction error information that needs
to be coded. Usually this goal will be achieved by using
the best estimate of average motion in the 16×16
macroblock, but not always. I make this distinction
because some video processing algorithms need accurate
motion vectors, where the estimated motion is
a good match to motion as perceived by a human
observer. In many video processing algorithms, such as
in temporal resampling used in standards converters, or
in deinterlacing, a motion vector is needed for every
luma sample, or every few samples. In MPEG, only one
or two vectors are needed to predict a macroblock from
a16×16 region in an anchor picture.
If the fraction bit of a motion vector is set, then predictions
are formed by averaging sample values from
neighboring pixels (at integer coordinates). This is
straightforward for a decoder. However, for an encoder
490 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES