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IMAGE PROCESSING
20 September 1999
Auto Shading
There are two auto shading methods. black level correction and white level
correction. Auto shading corrects errors in the signal level for each pixel.
Black Level Correction
The CPU reads the black dummy data from one end of the CCD signal (32 pixels
at the end are blacked off) and takes the average of the black dummy data. Then,
the IPU deletes the black level value from each image pixel.
White Level Correction
Before scanning the original, the machine reads a reference waveform from the
white plate. The average of the white video level for each pixel is stored as the
white shading data in the FIFO memory in the IPU chip.
The video signal information for each pixel obtained during image scanning is
corrected by the IPU chip.
Auto shading for the first original is done before the scanning.
After scanning every page, auto shading is done to prepare for the next page.
If the copy image density or the original mode is changed during copy run, the auto
shading for the next scan is done before the scanning to respond to the mode
changed.
White Line Erase Compensation
During the white level correction, if extremely low CCD output is detected in some
parts of the line, the machine assumes this is due to abnormal black lines on the
white plate. This low output is corrected using neighboring pixels. To switch off this
correction, use SP4-918 (for the original modes known as "Service Mode") and
SP4-942 (other original modes).
Black Line Erase Compensation
In ADF mode, if extremely low CCD output is detected on the scanning line before
the leading edge of original arrives there, this is attributed to abnormal black dots
on the exposure glass. This low output is corrected using neighboring pixels. To
adjust or switch off this correction, use SP4-919 (for the original modes known as
"Service Mode") and SP4-943 (other original modes).
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