DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Each nonintra macroblock in an interlaced sequence
can be predicted either by frame prediction (typically
chosen by the encoder when there is little motion
between the fields), or by field prediction (typically
chosen by the encoder when there is significant interfield
motion). This is comparable to field/frame coding
in DV, which I described on page 463. Predictors for
a field picture must be field predictors. However,
predictors for a frame picture may be chosen on
a macroblock-by-macroblock basis to be either field
predictors or frame predictors. MPEG-2 defines several
additional prediction modes, which can be selected on
a macroblock-by-macroblock basis. MPEG-2’s prediction
modes are summarized in Table 40.9 overleaf.
Motion vectors (MVs)
A motion vector identifies a region of 16×16 luma
samples that are to be used for prediction. A motion
vector refers to a prediction region that is potentially
quite distant (spatially) from the region being coded –
that is, the motion vector range can be quite large.
Even in field pictures, motion vectors are specified in
units of frame luma samples. A motion vector can be
specified to half-sample precision: If the fractional bit of
a motion vector is set, then the prediction is formed by
averaging sample values at the neighboring integer
coordinates – that is, by linear interpolation. Transmitted
motion vector values are halved for use with
subsampled chroma. All defined profiles require that no
motion vector refers to any sample outside the bounds
of the anchor frame.
Each macroblock’s header contains a count of motion
vectors. Motion vectors are themselves predicted! An
initial MV is established at the start of a slice (see
page 494); the motion vector for each successive
nonintra macroblock is differentially coded with respect
to the previous macroblock in raster-scan order.
Motion vectors are variable-length encoded (VLE), so
that short vectors – the most likely ones in large areas
of translational motion or no motion – are coded
compactly. Zero-valued motion vectors are quite likely,
so provision is made for compact coding of them.
484 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES