Gestetner A294, Service Manual

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IMAGE PROCESSING
A294/A295 2-32 SM
Four-gradation Processing
This process is used in Text mode and Text/Photo mode.
The image density data after gamma correction is changed into 2-bit data using 3
thresholds. The four gradations are reproduced by laser power control. This
process is suitable for reproducing fine originals which do not include halftone
areas.
Error Diffusion and Dithering
Error diffusion 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.
With dithering, each pixel is compared with a pixel in a dither matrix. Several
matrixes are available, to increase or decrease the detail on the copy.
Comparing 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 Paper” 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 error diffusion or one of three
types dither matrix can be selected with SP4-904-002. There are three types of
dither matrix, 8" x 8", 6" x 6", and 4" x 4". A larger dither matrix causes halftone
areas to become coarser.
2.3.11 LINE WIDTH CORRECTION
This function is effective for Text, Text/Photo and Generation modes.
The software compares each pixel with adjacent pixels. If the differences between
the object pixel and adjacent pixels are more than a threshold, the software judges
that the pixel is on a line edge, and line width correction is applied.
The line width correction setting can be changed separately for Text, Text/Photo
and Generation modes. The widths of vertical and horizontal lines can be corrected
separately.
•
Thinner 1: If the pixel is on the edge of a line, its density is multiplied by 0.75
•
Thinner 2: If the pixel is on the edge of a line its density is multiplied by 0.5
•
Thicker: If the pixel is on the edge of a line, 48 is added to its density
The threshold for detecting edges of lines can also be changed separately for
vertical lines and horizontal lines.
Use SP4-903-75 to 83 to adjust the line width correction settings.
Line width can also be corrected by adjusting the laser output (SP4904-20 to 24).
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