Digital Signal Processing Reference
In-Depth Information
the Cr and Cb planes will be 360 by 480 pixels. Therefore,
a higher degree of compression can be achieved using JPEG on
4:2:2 or 4:2:0 YCrCb images. Intuitively, this makes sense, as
more bits are used to represent the luminance to which the
human eye is more sensitive, and less for the chrominance to
which the human eye is less sensitive.
13.2 DC Scaling
Each color plane of the image is divided up into 8
8 pixel
blocks. Each 8-bit pixel can have a value ranging from 0 to 255.
The next step is to subtract 128 from all 64 pixel values, so the new
range is
8 DCT is next applied to this set of
64 pixels. This DCT output is the frequency domain representa-
tion of the image block.
The upper left DCT output is the DC value, or average of all
the 64 pixels. Since we subtracted 128 prior to the DCT pro-
cessing, the DC value can range from
128 to
127. The 8
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1024 to 1016, which can
be represented by an 11-bit signed number. Without the 128
offset, the DC coefficient would range from 0 to 2040, and the
other 63 of DCT coefficients would be signed (due to the cosine
range). The subtraction of 128 from the pixel block has no
effect upon the 63 AC coefficients (an equivalent method
could be to perform subtraction of
1024 of DC coefficient
after the DCT).
13.3 Quantization Tables
The quantization table used has a great influence upon the
quality of JPEG compression as it influences the degree of
compression. These tables are often developed empirically, to
give the greatest number of bits to the DCT values which are
most noticeable and have the most visible impact.
The quantization table is applied to the output of the
DCT, which is an 8
8 array. The upper left coefficient is
the DC coefficient, and the remaining are the 63 AC coeffi-
cients of increasing horizontal and vertical frequencies as
one moves rightward and downward. As the human eye
is more sensitive to lower frequencies, less quantization
and more bits are used for the upper and leftmost DCT
coefficients.
