Digital Signal Processing Reference
In-Depth Information
compress by discarding only data that has little impact on the
image quality.
13.4 Entropy Coding
The next step is to sequence the quantized array values B
j,k
as
in the order shown in
Figure 13.1
. The first value B
0,0
is the
quantized DC coefficient. All the subsequent values are AC values.
The entropy encoding scheme is fairly complex. The AC
coefficients are coded differently than the DC coefficient. The
output of the quantizer often contains many zeros, so special
symbols are provided. One is an EOB (end of block) symbol, used
when the remaining values from the quantizer are all zero. This
allows the encoding to be terminated when the rest of the
quantized values are zero. The coded symbols also allow the zero
run-length following a non-zero symbol to be specified. This
efficiently takes advantage of the zeros present in the quantizer
output. This is known as run length encoding.
The DC coefficients are differentially coded across the image
blocks. There is no relationship between the DC and AC coeffi-
cients. However, DC coefficients in different blocks are likely to
be correlated, as adjacent 8
8 image blocks are likely to have
a similar DC or average luminance and chrominance. So only the
delta, or difference, is coded for the next DC coefficient, relative
to the previous DC coefficient.
Four Huffman code tables are provided in the baseline JPEG
standard:
DC coefficient, luminance.
AC coefficients, luminance.
DC coefficient, chrominance.
AC coefficients, chrominance.
Pixel Coding Sequence
●●●●●●●●
●●●●●●●●
●●●●●●●●
●●●●●●●●
●●●●●●●●
●●●●●●●●
●●●●●●●●
●●●●●●●●
Figure 13.1. Sequencing of pixel coding.
