IMAGE PROCESSING1 August 1996
2.4.3 Auto Shading
A193D517.wmf
As with the previous digital copiers, there are two auto shading methods. One
is black level correction and the other is white level correction. Auto shading
corrects errors in the signal level for each pixel.
1) Black Level Correction
The CPU reads the black dummy data from one end of the CCD signal (64
pixels at the end are blacked off) and takes the average of the black dummy
data. Then, the CPU deletes the black level value from each image pixel.
2) 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.
In book mode, auto shading is done at the beginning of each scan.
In ADF mode, auto shading is done at a specific time interval. This interval
can be set with SP4-913 (the default setting is 30 seconds). The machine
waits until the end of the page before doing the auto shading.
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ACEC
