Ricoh A250 Service Manual page 76

IMAGE PROCESSING
In error diffusion or simple binary picture processing, there are two possible types
of threshold: constant threshold, and dynamic threshold.
• The type that is used depends on the selected original type (text, text/photo,
photo) and user tool Image Adjustment setting.
• However, if the user selects "Service Mode" for either Text or Text/Photo original
type (User Tools - General Features - 08. Image Adjustment), the thresholding
type can be changed with SP4-922.
Dithering is only used in Photo mode (except for Glossy Photo, in which error
diffusion is used).
Constant Threshold Value
If the constant threshold method is used, the threshold remains the same all the
time.
The threshold can be adjusted with SP 4-923 when the user selects "Service
Mode" for the Text original type (User Tools - General Features - 08. Image
Adjustment).
Decreasing the threshold value creates a darker image.
Dynamic Threshold Value
Overview
Dynamic thresholding is designed to clearly separate text/vector graphic objects
from the background.
When used with simple binary picture processing (Sharp Text mode)
The software compares each pixel with the pixels immediately surrounding it. It is
tested in four directions: horizontal, vertical, and in the two diagonal directions. If
the image density difference between the object pixel and the surrounding pixels is
more than a certain value in any one of these directions, the pixel is determined to
be on an edge.
Pixels on the edge are treated with dynamic thresholding. The threshold is
calculated by averaging the densities of pixels in the surrounding 7 x 7 area.
However, the calculated threshold cannot exceed maximum and minimum limits; if
it does, the upper or lower limit is used.
Pixels that are not on an edge are treated with a constant threshold value.
As a side-effect of the dynamic threshold process, copies of originals where the
rear side is visible through the paper or the background is dark, may tend to have
dirty background. In this case it is necessary to adjust the image density level with
the image density key on the operation panel.
Instead of sharp text mode, if the user selects "Service Mode" for Text original type
(User Tools - General Features - 08. Image Adjustment), some adjustments can be
made.
A250 2-18 SM