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IMAGE PROCESSING
3 December 2004
Scan line correction
The CCD consists of three lines of elements, one for each color (red, green and
blue). Since there is a slight gap between the lines, each line is scanning a slightly
different part of the image. To produce the final image, these three outputs need to
be synchronized. Since the deviation between the lines varies with the scanner
speed, a deviance value must be calculated in proportion to the speed.
White level correction
The video signal information for each pixel obtained during image scanning is
corrected by the image processing circuits. Due to the loss of exposure lamp
brightness, less brightness at the edges of the lens and distortions in the light path,
the machine scans a white plate before scanning each original. The white plate is
uniform in color and in reflection.
Gamma (γ) correction (Input/Output)
There should be a linear relationship between the original image density and the
CCD output.
Gamma correction adjusts for deviations from this ideal. It includes both brightness
and contrast corrections.
Line skipping correction
Ideally, this machine would scan at a constant speed. However, when the data
transfer speed slows down, the scanner optics unit must also slow down to match.
If the scanner optics unit must stop to wait on the client or server, duplicate lines
may be scanned. These can create a visible seam between the scanned sections.
Line skipping correction erases duplicate image lines, ensuring a smooth final
image.
Mirroring for ADF Mode
When scanning an original 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).
Otherwise, a scanned image in book mode differs from ones in ADF mode. This
prevents a different image.
Magnification
The CPU on the MCB enlarges or reduces the image data in the scan direction.
Changing the scanning speed does reduction and enlargement in the sub scan
direction.
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