DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 38.8 Zigzag scanning
is used to rearrange quantized
coefficients according to
the likely distribution of
image power in the block.
In JPEG and MPEG terminology,
the magnitude (absolute value) of
a coefficient is called its level.
Most of the high-frequency information in this block –
the DCT entries at the right and the bottom of the
matrix – are quantized to zero. Apart from the DC term,
the nonzero coefficients have small magnitudes.
Following quantization, the quantized coefficients are
rearranged according to the likely distribution of image
power in the block. This is accomplished by zigzag scanning,
sketched in Figure 38.8:
F* =
79 0 -1 0 0 0 0 0
-2 -1 0 0 0 0 0 0
-1 -1 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
Once rearranged, the quantized coefficients are represented
in a one-dimensional string; an end of block
(EOB) code marks the location in the string where all
succeeding coefficients are zero, as sketched in
Figure 38.9:
79 0 -2 -1 -1 -1 0 0 -1 EOB
Figure 38.9 Zigzag-scanned coefficient string
In the usual case that just a few high-order quantized
coefficients are nonzero, zigzag reordering tends to
produce strings of repeating zeros. Additional compression
can be accomplished by using variable-length
encoding (VLE, also known as Huffman coding).
Variable-length encoding is a lossless process that takes
advantage of the statistics of the “run length” (the
count of zero codes) and the “level” (absolute value, or
magnitude) of the following transform coefficient.
The DC term is treated specially: It is differentially
coded. The first DC term is coded directly (using a DC
VLE table), but successive DC terms are coded as
CHAPTER 38 JPEG AND MOTION-JPEG (M-JPEG) COMPRESSION 455