Canon FACSIMILE BASIC 2000 Service Manual page 164

G3 FACSIMILE COMMUNICATIONS
Processing of First Pixel of Line
The first starting pixel a0 on each coding line is imaginarily placed at a
position just before the first pixel, and is regarded as a white pixel.
The first run length on a line a0a1 is replaced by "a0a1-1".
In other words, if the first run is black and is considered to be coded by the
horizontal mode, then the first code word M(a0a1) corresponds to a white
run of zero run length.
Processing of Last Pixel of Line
Coding of the coding line continues until the position of the imaginary
changing pixel situated just after the last actual pixel has been coded. This
may be coded as a1 or a2. Also, if b1 and/or b2 are not detected at any time
during the coding of the line, they are positioned on the imaginary chang-
ing pixel situated just after the last actual pixel on the reference line.
4.3.5 Coding modes
One of the three coding modes is selected according to the coding proce-
dure.
(1) Pass mode
This mode is stipulated when b2 lies to the left of a1.
When coding has been performed in this mode, a0 is placed on the pixel of
the coding line below b2 in preparation for the next coding operation (i.e.
on a0').
However, the state where b2 occurs just above a1 as shown in Fig. 5-20 is
not considered to be the pass mode.
5–22
Reference line
Coding line
a0
Fig. 5-19 Pass Mode
Reference line
Coding line
a0
Fig. 5-20 Example Not Corresponding to Pass Mode
b1 b2
a0' a1
b1 b2
a1