Encoding Method - Canon MultiPASS C530 Service Manual

MultiPASS C530/C560 Chapter 2: Technical Reference

8.2.3 Encoding method

In the JBIG encoding used in the Single Progression Sequential Bi-level Image Compression Method,
uses in the encoder shown below to encode to the original the results of comparison of the line currently
being processed and the previous line, as well as the predicted value of an image pixel (white or black)
used in a model template.
The study table used in the prediction makes the next prediction more accurate by learning and correcting
the study table every time the model template moves to the adjoining pixel. It is characteristic of this
method that if the prediction is accurate the amount of encoding will not increase, and if the prediction is
off the amount of encoding increases, so the increase in prediction accuracy of this study table is very
important.
An outline of the encoding procedure is shown below.
Image data
input
a) In the pattern prediction section, the line currently being processed and the current line are compared,
and judged to match or not match. A flag showing whether or not the lines match (1 bit, 0: match, 1:
don't match) is attached to the head of each line according to this judgment. When the lines match,
only this flag is encoded in the arithmetic encoding section as a suspected pixel, the pixel of the line
being currently processed is not encoded. When the lines do not match, the pixel of the line currently
being processed is encoded in the arithmetic encoding section based on the results of a comparison of
the value of the actual pixel and the pixel (white or black) which is predicted using the model template
and the study table.
NOTE
Comparison of current
and previous lines
Output of match/no match
TYPICAL
PREDICTION
No-match path
Pixel to be encoded
(For comparison against prediction)
Prediction of pixel
to be encoded
MODEL 10-bit pixel
TEMPLATE
Prediction of pixel using
10-bit pixel pattern
Next prediction based on
prediction results
PROBABILITY
ESTIMATION
Figure 2-32 Encoder and Flow of JBIG Encoding
When the lines are judged to match, the line currently being processed is said to be
"typical". When the lines are judged to not match, the line is said to be "not typical".
When the very first line of an image is predicted, the background color is used as the
previous line.
ENCODER
ARITHMETIC
ENCODING
pattern
Prediction and status reference
STUDY and predition results
TABLE
Refer to the next status and pixel prediction
based on the prediction results (Learnig)
Relevant status area reference
TABLE
2-52
Output of image data
encoded by JBIG