IMAGE PROCESSING
2-39
SM A229
Detailed
Descriptions
Error Diffusion and Dithering
The error diffusion process reduces the difference in contrast between light and
dark areas of a halftone image. Each pixel is corrected using the difference
between it and the surrounding pixels. The corrected pixels are then corrected
using an error diffusion matrix.
In the dithering process, each pixel is compared with a pixel in a dither matrix.
Several matrixes are available, to increase or decrease the detail on the copy.
Compared with dithering, error diffusion gives a better resolution, and is more
suitable for “continuous toned” originals. On the other hand, dithering is more
suitable for “screen printed” originals.
In Photo Mode, when the user selects “Normal or Continuous Tone”, error diffusion
is used. However, if the user selects “Screen Printed”, dither processing is used. If
the user selects ‘SP Mode Changed’, then either dithering or one of two types of
error diffusion can be selected with SP4904-024,. When dithering is selected, the
setting of SP4904-002 will decide which dithering matrix is used.
In Text/Photo Mode, the error diffusion process that is used depends on the image
area type (text or photo) as shown below. Therefore, before error diffusion, a
simple text/photo separation process is performed (as mentioned earlier, in the
Text/Photo Separation section).
Area Error Diffusion Type Related SP Mode
Text Area
Photo Area
1 dot error diffusion
2 dot error diffusion
SP4904-007
SP4904-008
Grayscale Processing
The eight-bit data arriving from the gamma correction circuit is passed on
unchanged.
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