Canon MultiPASS C530, Service Manual

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MultiPASS C530/C560 Chapter 2: Technical Reference
CIELAB color spacing
In the JPEG compression used in G3FAX, it is necessary to compress/decompress
images with color spacing called L*, a*, b*. CIELAB is defined color spacing by CIE
(Commision Internationale de l'Eclairage). It is the space which holds the optically
near-equivalent and distinguishable difference in the interval between points of equal
space. Here, L* shows the brightness (luminosity index), and a* and b* show the
variation in color (a* shows tones from green to red, b* shows tones from blue to
yellow.)
L*,a*,b* are the color ranges expressed by real numbers determined as shown below:
L*=[0, 100]
a*=[-85, 85]
b*=[-75, 125]
It is inconvenient to digitally process this as-is, so the colors are handled by rounding
off the values to the closest integer. The method used to calculate expressions for 8bit
pixels is as below.
L=(L*)*(255/100)
a=(a*)*(255/170)+128
b=(b*)*(255/200)+96
Sub-sampling
The smallest unit processed by JPEG is called the MCU (minimum coded unit). This is
an image unit of 16
×
16 dots. Also, the MCU is separated into L*/a*/b*, and the
resulting 8
×
8 dots units are called blocks. The sample ratio called 4:1:1 means the
information returned when L* is compared with 4 blocks and a* and b* with one block
sections.This a* and b* get information by taking the average value from the 4 pixels
around them. This makes use of the characteristic of human eyes that they are sensitive
to brightness and insensitive to variations in color.
This process is called sub-sampling.
NOTE
16
16
MCU
L*a*b*Separation
L*
a*
b*
L*
a*
b*
Sampling
16
16
8
8
8
8
4
1
1
Figure 2-43 Sub-Sampling