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b1: The first changing pixel on the reference line to the right of a0 and of
opposite color to a0
b2: The next changing pixel to the right of b1 on the reference line
Reference line
Coding line
Fig. 5-18 Definition of Changing Pixels
The coding mode is used to code each changing pixel on the coding line.
There are three modes: pass mode, vertical mode and horizontal mode.
Details of each coding mode are described in 4.3.5 Coding Modes.
• Coding procedure
The coding procedure detects the coding mode to be used for coding each
of changing pixels on the coding line. When one of the three coding modes
is detected according to Step 1 or Step 2 described below, an appropriate
code word is selected from the code table shown in Table 5-5. This proce-
dure is shown in the flow chart in Fig. 5-22.
[Step 1]
(1) When the pass mode is detected, coding is performed using the word
"0001" (Table 5-5). After this, pixel a0' just under b2 is regarded as
the new starting pixel a0 for the next coding operation.
(2) If the pass mode is not detected, then proceed to Step 2.
[Step 2]
(1) Determine the absolute value of the relative distance a1b1. (|a1b1|)
(2) If |a1b1| 3, as shown in Table 5-5, a1b1 is coded in the vertical mode,
after which position a1 is regarded as the new starting pixel a0 for the
next coding operation.
(3) If |a1b1| > 3, as shown in Table 5-5, following horizontal mode code
"001", a0a1 and a1a2 are respectively coded by MH coding. After this,
position a2 is regarded as the new starting pixel a0 for the next coding.
G3 FACSIMILE COMMUNICATIONS
b1
a0
a1
1
b2
2
a2
3
4
5
6
ït
çi
5–21
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