Grayscale Processing; Binary Picture Processing; Error Diffusion; Dithering - Ricoh Russian-C2a Service Manual

10 August, 2001 IMAGE PROCESSING

Grayscale Processing

In this machine, the 8-bit image data is converted into 2-bit data. This produces up
to 4 image density levels for each pixel.
To realize this, this machine uses a form of pulse width modulation. In this
machine, pulse width modulation consists of the following processes:
• Laser diode pulse positioning
• Laser diode power/pulse width modulation
Laser diode power and pulse width modulation is done by the laser diode drive
board (LDD). Briefly, the width of the laser pulse for a pixel depends on the output
level (image density level: from 0 to 255) required for the pixel.
Note that although the LDD can create 256 levels per pixel, the machine only uses
8 of these, and only four are used for any one job. A gamma table determines
which four output levels are used. The gamma table is different for each original
type setting.

Binary Picture Processing

The 8-bit image data is converted into 1-bit data (black and white image data).

Error Diffusion

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 compared with an
error diffusion matrix.
There are two types of error diffusion processing: One is 'normal'. The other is part
of the characteristic detection process, in which the error diffusion method is
determined separately for each pixel. The error diffusion type (normal or
characteristic) depends on the reproduction ratio and the original type (refer to the
SP Modes for Each Image Processing Step tables).

Dithering

Each pixel is compared with the pixel in the same position in a dither matrix.
Several matrixes are available, to increase or decrease the detail on the copy.

Line width correction

This function is effective in all original modes.
Usually, lines will bulge in the main scan direction as a result of the
negative/positive development system that is used in this model. So, pixels on
edges between black and white areas are compared with adjacent pixels, and if the
pixel is on a line, the line thickness will be reduced.
Line width correction is done in the VCU chip on the LDD board.
The line width correction type can be selected with SP2-907.
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