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IMAGE PROCESSING
20 September 1999
Binary Picture Processing
The 8-bit image data is converted into 1-bit data (black and white image data).
Constant and Dynamic Thresholding
There are two types of threshold:
• Constant threshold: The threshold is fixed
• Dynamic threshold: The threshold value for each pixel depends on the
surrounding pixels.
The type that is used depends on the selected original type (text, text/photo, photo)
and user tool Image Mode Adjustment setting.
However, if the user selects "Service Mode" for either Text or Text/Photo original
type (User Tools - General Features - 09. Image Mode Adjustment), the
thresholding type can be changed with SP4-922, but only for binary picture
processing.
- Constant threshold value -
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, but only for binary picture processing.
Decreasing the threshold value creates a darker image.
- Dynamic threshold value -
Dynamic thresholding is designed to clearly separate text/vector graphic objects
from the background. It prevents low contrast text from disappearing.
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. These adjustments are only for
binary picture processing mode.
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