DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 38.1 A JPEG 4:2:0
minimum coded unit (MCU)
comprises six 8×8 blocks:
a luma block, a block of C B , and
a block of C R . The six constituent
blocks result from
nonlinear R’G’B’ data being
matrixed to Y’C B C R , then
subsampled according to the
4:2:0 scheme; chroma subsampling
is effectively the first stage
of compression. The blocks are
processed independently.
In MPEG, a macroblock is the area
covered by a 16×16 array of luma
samples. In DV, a macroblock
comprises the Y’, C B , and C R
blocks covered by an 8×8 array
(block) of chroma samples. In
JPEG, an MCU comprises those
blocks covered by the minimumsized
tiling of Y’, C B , and C R blocks.
For 4:2:0 subsampling, all of these
definitions are equivalent; they
differ for 4:1:1 and 4:2:2 (or for
JPEG’s other rarely used patterns).
In desktop graphics, saving JPEG at
high quality may cause R’G’B’ to be
compressed without subsampling.
Quantizer matrices and VLE tables
will be described in the example
starting on page 452.
I use zero-origin array indexing.
Four 8×8 Luma (Y’) blocks
8×8 C B block
8×8 C R block
JPEG blocks and MCUs
An 8×8 array of sample data is known in JPEG terminology
as a block. Prior to JPEG compression of a color
image, normally the nonlinear R’G’B’ data is matrixed
to Y’CBCR , then subsampled 4:2:0. According to the
JPEG standard (and the JFIF standard, to be described),
other color subsampling schemes are possible;
strangely, different subsampling ratios are permitted for
CB and CR . However, only 4:2:0 is widely deployed, and
the remainder of this discussion assumes 4:2:0. Four
8×8 luma blocks, an 8×8 block of CB, and an 8×8
block of CR are known in JPEG terminology as
a minimum coded unit (MCU); this corresponds to
a macroblock in DV or MPEG terminology. The 4:2:0
macroblock arrangement is shown in Figure 38.1 above.
The luma and color difference blocks are processed
independently by JPEG, using virtually the identical
algorithm. The only significant difference is that the
quantizer matrix and the VLE tables used for chroma
blocks are usually different from the quantizer matrix
and VLE tables used for luma blocks.
As I explained in Spatial frequency domain on page 188,
typical images are dominated by power at low spatial
frequencies. In Figure 38.4, on page 452, I present an
example 8×8 array of luma samples from an image. In
Figure 38.2 at the top of the facing page, I show an
8×8 array the spatial frequencies computed from this
luma array through the DCT. The [0, 0] entry (the DC
term), at the upper left-hand corner of that array
448 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES