Canon MultiPASS C530, Service Manual

Page: 151 / 243

2-72
MultiPASS C530/C560 Chapter 2: Technical Reference
The characteristics of the JPEG method which are used in ITU-T color communications are shown below.
(1) Can effectively compress data for natural images to that the image quality is minimally affected.
(2) Little noise appears on the boundaries of such images as images and text with borders, monochrome
illustrations, etc., and artificial images with clear contrast.
(3) Color spacing adopted from the CIELAB method.
(4) For compatibility with Faxes which do not the width of the scan when starting to scan, a 0 is inserted
in the section which shows the number of lines in the scanning width, which is in the SOF0 marker in
the JPEG header, and transmission begins. The scanning width is conferred to the receiving side by
the addition of a code to the end of the JPEG data which determines the number of lines in the scan,
called the DNL marker, which is added to the compressed image data after transmission.
(5) In order to tell the receiving side that it is an image compressed by the G3 facsimile, information such
as "G3FAX" as an application name, "200dpi" as a resolution, etc. is set in the JPEG header APP1
marker.
JPEG data is composed of a header section which includes several pieces of information which are
necessary for decompression of the image, as well as the section containing the data of the image itself,
which is DCT transformed, quantified, and entropy compressed. The header section is comprised of
several tables called segments, and 2 bytes of encoding, called a marker, which show the contents of the
table, are attached at the head of the segment.
Markers are expressed by "0xFF" and the 1-byte combinations of values peculiar to each marker, so by
reading these, the marker and its connected paramaters can be interpreted. These markers are defined by
ITU-T T.81.
The order of the
segments is
undecided
SOI
DNL
EOI
APP1
DHT
DQT
SOF
SOS
Compressed
Image Data
JPEG header
Data
SOI : (Start of Image)
APP1 : (Reserved for Application Segments)
DHT
: (Define Huffman table)
DQT : (Define Quantization table)
SOF : (Start of frame)
SOS : (Start of Scan)
DNL : (Define Number line)
EOI : (End of Image)
Figure 2-45 JPEG Data Structure