Ricoh PRIPORT VT 2000 Series Service Manual page 70

2) Dither Processing
This is part of the artificial graduation system. As each pixel can only
be expressed by either black or white at print time, the dither process is
used to transform a graduated original into 4 x 4 arrays containing only
black and white pixels. Each pixel of the original corresponds to a loca-
tion in the 4 x 4 dither matrix. During dither processing, the value of
each four-bit data signal from the A/D converter is compared to the
corresponding fixed threshold level (V
verted to either black or white. This introduces graduations by changing
the ratio of black pixels to white pixels. The 4 x 4 dither matrix is stored
in the image processing ROM.
NOTE:
E > V
E ≤ V
Main Scan
5
5
5
5
5
5
5
5
5
5
5
Sub
5
5
5
5
5
Scan
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
Pixels 4-bit data to be read
Example: All pixels are at 5.
3) Selection of Dither Processing or MTF Correction During Photo Mode
(Edge Emphasis)
If characters are processed using the dither method, they will be dis-
jointed and difficult to read due to the distorting nature of the dither
process. Therefore, when the density difference between a pixel and
the surrounding pixels is greater than the specified level, the data is
processed using MTF instead of the dither method.
The VT2300/2500 uses a 6 x 6 dither matrix that improves
the image quality in Photo mode.
(The VT2100/2130/2150 use a 4 x 4 dither matrix.)
.................... Black
TH
.................... White
TH
5
5
5
5
14
5
5
5
5
5
5
5
5
4
11
5
5
5
5
14
5
5
5
5
14
5
5
5
5
5
5
5
5
11
5
5
5
5
14
Threshold level of the
dither matrix (Number
shows each V
TH
4
7
14
8
4
7
0
9
3
1
0
9
5
11
6
2
5
13
14
12
10
13
7
14
8
4
7
0
9
3
1
0
9
5
11
6
2
5
13
14
12
10
13
4 x 4 dither matrix
).
TH
2-43
) in the dither matrix and is con-
8
4
3
1
6
2
12
10
8
4
3
1
6
2
10 12
Image reappearance
(Shaded pixels are
black.)
28 February '91