Image Processing Outline - Canon MultiPASS C530 Service Manual

8.5.3 Image processing outline

(1) Images scanned by the color contact sensor are read into memory as RGB data.
(2) The read RGB data undergoes color transformation processing, and is converted to L*a*b* data.
Also, after this processing, it is changed into 16x16dot elements called MCU.
(3) Using the fact that the human eye is sensitive to changes in brightness and insensitive to changes in
color, the resolution of a* and b* is roughened and the amount of information lessened without
affecting the image which is seen by the human eye.
(4) Each block undergoes DCT transformation and the image data is separated into 64 cos waves.
(5) Photos of natural images, etc, have characteristically high numbers of low-frequency elements and
low numbers of high-frequency elements. Because of this, an operation table (quantization
parameter) is used in which low-frequency elements are left and high-frequency elements are
reduced, and unnecessary sections are compressed.
(6) The compressed data is made in a single dimension facing from the upper left to the lower right.,
and this is encoded with Huffman encoding using a special Huffman table.
(7) The data which has finished being encoded is transmitted in ECM mode using a modem.
(8) At the receiving side, the original image is decompresses using the information in the JPEG image
header. First, decoding is done using the Huffman table included in the header. After that, the image
is made in two dimensions, facing from the lower right to the upper left.
(9) Reverse quantization is performed using the quantization parameter in the header. During
quantization, the high-frequency elements which have little effect on the image seen by the eye are
lost, so the original data is not perfectly restored.
(10) Inverse Dispersal Cosine Transformation (IDCT) is performed, transforming into L*' a*' b*' data.
Sub-sampling is done at the time of transmission, so the amount of a*', b*' data is 1/4 that of l*' data.
(11) The L*'a*'b*' data has color transformation processing done, is converted into RGB data (R'G'B'),
and stored in memory.
(12) The converted RGB data is transformed into dual-value CMYK data for printing, and printed out
with the BJ printer assembly.
Transmiting side
(1)
R,G,B
Image
Contact
Buffer
sensor
Receiving Side
(12)
CMYK
Image
Printer
Buffer
MultiPASS C530/C560 Chapter 2: Technical Reference
(2) (3)
R,G,B L*,a*,b*
Sub-Sampling
Color
4:4:1
conversion
ITU-T T.42
(11)
R',G',B' L',a',b'
Color
Conversion
Figure 2-46 Image Processing Out
2-73
JPEG Compression
Single
dimensionality
(4) (5)
L,a,b
DCT
Quantification
Conversion
ITU-T T.81
Double
Dimensionality
(10) (9)
Reverse
IDCT
Quantification
Conversion
JPEG Decompression
(6)
Huffman Image
encoding memory
(7)
Telephone
Line
(8)
Huffman
Image
Decoding
memory