81 How does MPEG achieve compression? Macroblock:
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
81 How does MPEG achieve compression? Macroblock:
motion vectors are now always represented along a half-pel grid. The
usefulness of an integer-pel grid (option in MPEG-1) diminished with
practice. A intrinsic half-pel accuracy can encourage use by encoders
for the significant coding gain which half-pel interpolation offers.
In both MPEG-1 and MPEG-2, the dynamic range of motion vectors is
specified on a picture basis. A set of pictures corresponding to a
rapid motion scene may need a motion vector range of up to +/- 64
integer pixels. A slower moving interval of pictures may need only a
+/- 16 range. Due to the syntax by which motion vectors are signaled in
a bitstream, pictures with little motion would suffer unnecessary bit
overhead in describing motion vectors in a coordinate system
established for a much wider range. MPEG-1s f_code picture header
element prescribed a radius shared by horizontal and vertical motion
vector components alike. It later became practice in industry to have a
greater horizontal search range (motion vector radius) than vertical,
since motion tends to be more prominent across the screen than up or
down (vertical). Secondly, a decoder has a limited frame buffer size
in which to store both the current picture under decoding and the set
of pictures (forward, backward) used for prediction (reference) by
subsequent pictures. A decoder can write over the pixels of the oldest
reference picture as soon as it no longer is needed by subsequent
pictures for prediction. A restricted vertical motion vector range
creates a sliding window, which starts at the top of the reference
picture and moves down as the macroblocks in the current picture are
decoded in raster order. The moment a strip of pixels passes outside
this window, they have ended their life in the MPEG decoding loop. As
a result of all this, MPEG-2 created separate into horizontal and
vertical range specifiers (f_code[][0] for horizontal, and f_code[][1]
for vertical), and placed greater restrictions on the maximum vertical
range than on the horizontal range. In Main Level frame pictures, this
is range is [- 128,+127.5] vertically, and [-1024,+1023.5]
horizontally. In field pictures, the vertical range is restricted to [-
64,+63.5].
Macroblock stuffing is now illegal in MPEG-2. The original intent
behind stuffing in MPEG-1 was to provide a means for finer rate control
adjustment at the macroblock layer. Since no self-respecting encoder
would waste bits on such an element (it does not contribute to the
refinement of the reconstructed video signal), and since this unlimited
loop of stuffing variable length codes represent a significant headache
for hardware implementations which have a fixed window of time in which
to parse and decode a macroblock in a pipeline, the element was
eliminated in January 1993 from the MPEG-2 syntax. Some feel that
macroblock stuffing was beneficial since it permitted macroblocks to be
coded along byte boundaries. A good compromise could have been a
limited number of stuffs per macroblock. If stuffing is needed for
purposes of rate control, an encoder can pad extra zero bytes before
the start code of the next slice. If stuffing is required in the last
row of macroblocks of the picture, the picture start code of the next
picture can be padded with an arbitrary number of bytes. If the
picture happens to be the last in the sequence, the sequence_end_code
can be stuffed with zero bytes.
The dct_type flag in both Intra and non-Intra coded macroblocks of
frame structured pictures signals that the reconstructed samples output
by the IDCT stage shall be organized in field or frame order. This
flag provides an encoder with a sort of poor mans motion_type by
adapting to the interparity (i.e. interfield) characteristics of the
macroblock without signaling a need for motion vectors via the
macroblock_type variable. dct_type plays an essential role in Intra
frame pictures by organizing lines of a common parity together when
there is significant interfield motion within the macroblock. This
increases the decorrelation efficiency of the DCT stage. For
non-intra macroblocks, dct_type organizes the 16 lines (... luminance,
8 lines chrominance) of the macroblock prediction error. In combination
with motion_type, the meaning....
dct_type
motion_format
interpretation
frame
Intra coded
block data is frame correlated
field
Intra coded
block data is more strongly correlated along lines of
opposite parity
frame
Field predicted
1. a low-cost encoder which only possesses frame
motion estimation may use dct_type to decorrelate
the prediction error of a prediction which is
inherently field by characteristic
2. an intelligent encoder realizes that it is more bit
efficient to signal frame prediction with field
dct_type for the prediction error, than it is to signal
a field prediction.
field
Field predicted
A typical scenario. A field prediction tends to form a
field-correlated prediction error.
frame
Frame predicted
A typical scenario. A frame prediction tends to form a
frame-correlated prediction error.
field
Frame predicted
Makes little sense. If the encoder went through the
trouble of finding a field prediction in the first place,
why select frame organization for the prediction error?
prediction modes now include field, frame, Dual Prime, and 16x8 MC.
The combinations for Main Profile and Simple Profile are shown below.
Frame pictures
motion_type
motion
vectors
per MB
fundamental
prediction block
size (after half-
pel)
interpretation
Frame
1
16x16
same as MPEG-1, with possibly different
treatment of prediction error via dct_type
Field
2
16x8
Two independently coded predictions are
made: one for the 8 lines which correspond
to the top field, another for the 8 bottom
field lines.
Dual Prime
1
16x8
Two independently coded predictions are
made: one for the 8 lines which correspond
to the top field, another for the 8 bottom
field lines. Uses averaging of two 16x8
prediction blocks from fields of opposite
parity to form a prediction for the top and
bottom 8 lines. A second vector is derived
from the first vector coded in the bitstream.
Field pictures
motion_type
motion
vectors
per MB
fundamental
prediction block
size (after half-
pel)
interpretation
Field
1
16x16
same as MPEG-1, with possibly different
treatment of prediction error via dct_type
16x8
2
16x8
Two independently coded predictions are
made: one for the 8 lines which correspond
to the top field, another for the 8 bottom
field lines.
Dual Prime
1
16x16
A single prediction is constructed from the
average of two 16x16 predictions taken from
fields of opposite parity.
concealment motion vectors can be transmitted in the headers of intra
macroblocks to help error recovery. When the macroblock data that the
concealment motion vectors are intended for becomes corrupt, these
vectors can be used to specify a concealment 16x16 area to be extracted
from the previous picture. These vectors do not affect the normal
decoding process, except for motion vector predictions.
Additional chroma_format for 4:2:2 and 4:4:4 pictures. Like MPEG-1,
Main Profile syntax is strictly limited to 4:2:0 format, however, the
4:2:2 format is the basis of the 4:2:2 Profile (aka Studio Profile).
In 4:2:2 mode, all syntax essentially remains the same except where
matters of block count are concerned. A coded_block_pattern extension
was added to handle signaling of the extra two prediction error
blocks. The 4:4:4 format is currently undefined in any Profile.
chroma_format
multiplex order within Macroblock
Application
4:2:0 (6 blocks)
YYYYCbCr
main stream television, consumer entertainment.
4:2:2 (8 blocks)
YYYYCbCrCbCr
studio production environments, professional
editing equipment, distribution and servers
4:4:4 (12 blocks)
YYYYCbCrCbCrCbCrCbCr
computer graphics
Non-linear macroblock quantization was introduced in MPEG-2 to increase
the precision of quantization at high bit rates, while increasing the
dynamic range for low bit rate use where larger step size is needed.
The quantization_scale_code may be selected between a linear (MPEG-1
style) or non-linear scale on a picture (frame or field) basis. The new
non-linear range corresponds to a dynamic range of 0.5 to 54 with
respect to the linear (MPEG-1 style) range of 1 to 31.
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