Databases Reference
In-Depth Information
T A B L E 13 . 7
Sample quantization table.
16
11
10
16
24
40
51
61
12
12
14
19
26
58
60
55
14
13
16
24
40
57
69
56
14
17
22
29
51
87
80
62
18
22
37
56
68
109
103
77
24
35
55
64
81
104
113
92
49
64
78
87
103
121
120
101
72
92
95
98
112
100
103
99
T A B L E 13 . 8
The quantizer labels obtained
by using the quantization table
on the coefficients.
2
1
0
0
0
0
0
0
−
9
0
0
0
0
0
0
0
3
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
0
be perceived by the human visual system. Different coefficients in the transform have widely
different perceptual importance. Quantization errors in the DC and lower AC coefficients are
more easily detectable than quantization errors in the higher AC coefficients. Therefore, we
use larger step sizes for perceptually less important coefficients.
Because the quantizers are all midtread quantizers (that is, they all have a zero output
level), the quantization process also functions as the thresholding operation. All coefficients
with magnitudes less than half the corresponding step size will be set to zero. Because the
step sizes at the tail end of the zigzag scan are larger, the probability of finding a long run of
zeros increases at the end of the scan. This is the case for the 8
8 block of labels shown in
Table
13.8
. The entire run of zeros at the tail end of the scan can be coded with an EOB code
after the last nonzero label, resulting in substantial compression.
Furthermore, this effect also provides us with a method to vary the rate. By making the
step sizes larger, we can reduce the number of nonzero values that need to be transmitted,
which translates to a reduction in the number of bits that need to be transmitted.
×
13.6.3 Coding
Chen and Pratt [
196
] used separate Huffman codes for encoding the label for each coefficient
and the number of coefficients since the last nonzero label. The JPEG approach is somewhat
more complex but results in higher compression. In the JPEG approach, the labels for the DC
and AC coefficients are coded differently.
