Service manual

4.5.7 Gradation Treatment (Dither Processing)
In this machine, 4 x 4 dot dither patterns (4 bits/dot) convert the 8-bit data to
4-bit data before it goes to the printer. This speeds up data processing in the
printer. The result is that for each pixel, the output can have only 16
gradations per color. However, the dither matrixes are designed so that for a
4 x 4 pixel unit on the output, there are
256
possible gradations for each
color. The resulting printout looks very much like a 256-gradation output.
There are 256 dither patterns for each mode (Letter, Photo, OHP). The
elements in the dither matrix are each 4-bit. To generate the output, the
machine converts each 8-bit pixel in the original to the 4-bit pixel in the same
position in one of the dither patterns. The 8-bit value of the pixel in the
original determines the dither pattern (0-255) that is used for that pixel. For
example, if the pixel value is 100, it is replaced with the 4-bit value in the
same position in dither pattern 100.
In Letter mode, the dither patterns are made with 1 x 1 dot units, to obtain
fine resolution.
In Photo mode, the dither patterns are made with 2 x 1 dot units. Using the
larger unit, the resolution is less fine, but the output is softer, which is more
suitable for Photo mode.
This machine has an OHP dot processing feature for OHP mode, which
uses another set of dither patterns made of 4 x 1 dot units. In general,
results from using a 4 x 1 dot unit allow more light to pass through the OHP
sheets.
Rev. 11/96
Detailed
Section
Descriptions
IMAGE PROCESSING
SM 2-61 A166/A187/A189
CÓPIA NÃO CONTROLADA
CÓPIA NÃO CONTROLADA