DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 40.4 Frame DCT
type involves straightforward
partitioning of luma
samples of each 16×16
macroblock into four 8×8
blocks. This is most efficient
for macroblocks of field
pictures, native progressive
frame pictures, and framestructured
pictures having
little interfield motion.
At first glance it is a paradox that
field-structured pictures must use
frame DCT coding!
Frame and field DCT types
Luma in a macroblock is partitioned into four blocks
according to one of two schemes, frame DCT coding or
field DCT coding. I will describe three cases where frame
DCT coding is appropriate, and then introduce field
DCT coding.
• In a frame-structured picture that originated from
a native-progressive source, every macroblock is best
predicted by a spatially contiguous 16×16 region of an
anchor frame. This is frame DCT coding: Luma samples
of a macroblock are partitioned into 8×8 luma blocks as
depicted in Figure 40.4 above.
• In a field-structured picture, alternate image rows of
each source frame have been unwoven by the encoder
into two fields, each of which is free from interlace
effects. Every macroblock in such a picture is best
predicted from a spatially contiguous 16×16 region of
an anchor field (or, if you prefer to think of it this way,
from alternate lines of a 16×32 region of an anchor
frame). This is also frame DCT coding.
• In a frame-structured picture from an interlaced source,
a macroblock that contains no scene element in motion
is ordinarily best predicted by frame DCT coding.
An alternate approach is necessary in a frame-structured
picture from an interlaced source where a macroblock
contains a scene element in motion. Such a scene
element will take different positions in the first and
second fields: A spatially contiguous 16×16 region of
an anchor picture will form a poor predictor. MPEG-2
486 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES