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# Three Level Mask Method for Grey Scale Printing

IP.com Disclosure Number: IPCOM000107840D
Original Publication Date: 1992-Mar-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 176K

IBM

## Related People

Dove, DB: AUTHOR [+1]

## Abstract

This disclosure builds upon the basic concept of using a limited number of grey levels or dot sizes, having specific ratios of light absorption, to produce a large number of grey levels in a relatively small super-pixel. If a printer can generate any one of three different optical absorptions at any pixel, and if the first two of these 'levels' absorb 9 percent and 91 percent, respectively, of the light absorbed by the darkest level, then a three-pel by three-pel super- pixel can be used to generate 99 grey levels spaced uniformly between 1 percent and 100 percent of maximum absorption. This is shown in Table 1, where it is assumed that dot A has 9 percent and dot B has 91 percent of the absorption of dot C.

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Three Level Mask Method for Grey Scale Printing

This disclosure builds upon the basic concept of using a
limited number of grey levels or dot sizes, having specific ratios of
light absorption, to produce a large number of grey levels in a
relatively small super-pixel. If a printer can generate any one of
three different optical absorptions at any pixel, and if the first
two of these 'levels' absorb 9 percent and 91 percent, respectively,
of the light absorbed by the darkest level, then a three-pel by
three-pel super- pixel can be used to generate 99 grey levels spaced
uniformly between 1 percent and 100 percent of maximum absorption.
This is shown in Table 1, where it is assumed that dot A has 9
percent and dot B has 91 percent of the absorption of dot C.

This method is in contrast to other known methods where up to
ten different grey levels would be needed at each of the nine pixels
in the super-pixel to achieve the same number of grey levels.
TABLE 1
GREY LEVEL    DOT COMBINATIONS WITHIN TILE
0      0    0    0    0    0    0    0    0 0
1      A    0    0    0    0    0    0    0 0
2      A    A    0    0    0    0    0    0 0
.........
9      A    A    A    A    A    A    A    A A
10     B    0    0    0    0    0    0    0 0
11     B    A    0    0    0    0    0    0 0
.........
18     B    A    A    A    A    A    A    A A
19     C    A    A    A    A    A    A    A A
20     B    B    0    0    0    0    0    0 0
21     B    B    A    0    0    0    0    0 0
.........
98     C    C    C    C    C    C    C    C B
99     C    C    C    C    C    C    C    C C

Since it is only necessary to be able to print three distinct
grey levels at each pixel location to produce 100 grey levels in a
3x3 super-pixel, the main problem is to find the most appropriate
method for generating the three levels in an accurate and
reproducible manner.

For purposes of discussion, consider the electrophotographic
printing process, although the discussion also applies to other
technologies.  There are three basic methods for producing
less-than-maximum absorption at any pixel: partial development ov...