Canon FAX-L1000 - Manual (Page 242)

3-98
FAX-L1000 Chapter 3: Technical Reference
The concept of arithmetic encoding is simply explained below.
The following assumptions are made in order to make the explanation easy to understand.
The probability of accuracy will be 50%, and the probability of inaccuracy will be
50%.
*1
The area of accuracy will be MPS, and the area of inaccuracy will be LPS.
Figure 3-47 Arithmetic Encoding Conceptual Diagram
The special characteristic of this arithmetic encoding is that an additional encoding bit is
not needed because the integer line position is the same as the integer line position of the
previous encoding data in the case of an accurate prediction. It follows that the amount of
encoding will not increase if accurate predictions continue, and the rate of compression
will increase. Conversely, with inaccurate predictions, an additional encoding bit will be
necessary to show the position of the inaccuracy in detail, and thus the amount of
encoding will increase and the rate of compression decrease. In this way, the study table
learns in order to increase the rate of accurate predictions and to reduce the amount of
encoding and raise the compression rate during the encoding process, and then corrects the
table parameters.
NOTE
*1 The actual probability varies with the status because of the extent to which
LSZ occupies in the range (A).
*2 The actual range is hexadecimal 8000~10000.
In the case of an accurate prediction, range A1= hexadecimal 10000-LSZ,
A2=A1-LSZ, and A3=A2-LSZ.
In the case of an inaccurate prediction, range A=LSZ.
*3 Actually, hexadecimal 8000.
*4 Actually, the hexadecimal value will be shifted to the left two times, and
the hexadecimal will be over 8000.
Pixel encoded
Each pixel's code
position according to
arithmetic encoding
LPS
LPS
LPS
LPS
MPS
LPS
LPS
LPS
MPS
MPS
MPS
MPS
MPS
MPS
LPS
MPS
0
0.0011
0.0010
0.001010
0.001001
0.00100010
Position (Binary)
Range
A1
Range
A2 Range
A3
1st 2nd 3rd 4th 5th 6th 7th 8th
Renor
maliz
ed
range
0.001
0.010
0.011
0.100
0.101
0.110
0.111
1.000
Accuracy Inaccuracy
Accuracy Accuracy
Accuracy Inaccuracy
Accuracy Accuracy
0 0 0.001 0.001 0.00100010
0.001 0.00100010