IMAGE PROCESSING
10 August, 2001
2-16
Auto Shading
Auto shading does two things.
•
Zeroes the black level for each scan line of data.
•
Corrects for variations in white level across the main scan.
Pre-Filtering
Pre-filter smoothes mainly parallel lines in the main scan direction and extended
lines in the sub-scan direction. This reduces moiré and spurious noise in images.
Main Scan Magnification/Reduction
Changing the scanner speed enables reduction and enlargement in the sub-scan
direction. However, the
IPU-A
chip handles reduction and enlargement in the main
scan direction. The processing for main scan magnification/reduction is the same
as in the previous digital machines.
Mirroring for ADF Mode
When making a copy using the ADF, the magnification circuit creates a mirror
image. This is because the scanning starting position in the main scan direction is
at the other end of the scan line in ADF mode (compared with platen mode). In
platen mode, the original is placed face down on the exposure glass, and the
corner at [A] is at the start of the main scan. The scanner moves down the page. In
ADF mode, the ADF feeds the leading edge of the original to the DF exposure
glass, and the opposite top corner of the original is at the main scan start position.
To create the mirror image, the IPU-A stores each line in a LIFO (Last In First Out)
memory.
B027D564.WMF
[A]