Ricoh C226 Service Manual page 63

OPTICS/IMAGE PROCESSING
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, for the 1st pixel
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.
C226 2-14 SM