Ricoh Aficio SP C231, Service Manual, Page: 55 / 247

IMAGE-PROCESSING
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SM C231
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Error Diffusion
Error diffusion is used to reproduce halftone images in photo mode.
Before a 6-bit image signal is converted into a single-bit signal based on the
threshold level, there is a difference between the image signal value and the
complete black value (63 for a 6-bit signal) or white value (0). With the error
diffusion process, the difference is distributed among the surrounding pixels. (The
MTF process simply erases these differences.)
When considering error diffusion in one dimension only (across the page), the 6-bit
data shown in the example below produces white and black data output as shown
below. In practice, this one-dimensional error diffusion is done in all directions on
each pixel (across the page, down the page, etc.).
Image data from one scan line
7 11 13 21 30 38 41 44
In each dimension, the difference between the pixel value and the nearest extreme
(0 or 63) is transferred to the next pixel. The 1st pixel in the row becomes either
black or white, whichever is closest. Then, in the example above, the difference
between 7 and 0 is added to the 2nd pixel. The value of the 2nd pixel, which is now
18, is then added to the 3rd pixel. The 4th pixel becomes 52, which is closer to 63
than 0. In such cases, the difference is subtracted (not added) to get the next pixel
value. In this example, the difference is 63-52=11, and the next pixel value (30-11)
becomes 19.
C231D518.WMF