Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2002-06-11
2003-11-04
Nguyen, Lamson (Department: 2861)
Incremental printing of symbolic information
Ink jet
Controller
C358S534000, C358S533000
Reexamination Certificate
active
06641241
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of generating halftone threshold data to convert continuous tone image data into binary image data or multi-valued image data for a color halftone image output apparatus in printing applications, such as a color printer, an image setter, a CTP (Computer To Plate) apparatus, a CTC (Computer To Cylinder) apparatus, a DDCP (Direct Digital Color Proof) apparatus, or the like.
2. Description of the Related Art
Halftone image output apparatus record a halftone image on a recording medium such as a printing sheet, a film, or the like by comparing continuous tone image data obtained from an original image with halftone threshold data to generate binary or multi-valued halftone image data, and controlling a laser beam or the like based on the generated binary or multi-valued halftone image data.
FIG. 9
of the accompanying drawings shows the corresponding relationship between a single dot cell
2
made of a plurality of halftone threshold data and pixels
4
formed by a laser beam or the like on a recording medium in a main scanning direction which is indicated by the arrow X and an auxiliary scanning direction which is indicated by the arrow Y. The halftone threshold data are established with respect to the respective pixels
4
in the dot cell
2
.
When a plurality of halftone images are superposed to generate a halftone image, a moiré pattern in the halftone image is reduced if each dot cell
2
is established obliquely at a certain angle &thgr; (screen angle &thgr;) to the main scanning direction X or the auxiliary scanning direction Y. The number of tones or gradations of the halftone image is normally determined by the number of pixels
4
which make up each dot cell
2
. The output resolution (dpi) of a halftone image output apparatus is defined as the number of pixels
4
per inch, and the screen ruling (halftone screen period) (lpi) is defined as the number of dot cells
2
per inch.
FIG. 10
of the accompanying drawings shows, by way of example, a halftone image generated using the halftone threshold data of the dot cell
2
shown in FIG.
9
. The halftone image output apparatus compares the magnitude of continuous tone image data with the magnitude of the halftone threshold data established with respect to the respective pixels
4
in the dot cell
2
, thus generating binary image data. Halftone dots
6
, shown hatched, represent image areas where the pixels
4
are blackened by a laser beam, for example, based on the generated binary image data.
In order to generate a color halftone image using a halftone image output apparatus, it is necessary to generate halftone images in different colors C, M, Y, K, for example, and superpose the generated halftone images in those colors. When the halftone images are superposed, the generation of a moiré pattern due to the halftone screen period of the dot cells
2
in the halftone image in each color should be avoided. The halftone screen period of the dot cells
2
occurs in a direction along corners a
1
, a
2
of each dot cell
2
and a direction, perpendicular thereto, along corners a
1
, a
4
of each dot cell
2
. The pitch of the moiré pattern is smaller as the angles between the directions of the halftone screen period of the dot cells
2
in the superposed halftone images in the different colors differ more widely from each other. The screen angles &thgr; of the colors are established such that the difference between the screen angles &thgr; of the colors C, M, K, which are loud colors, is a maximum, i.e., 30°. Traditionally, the screen angles &thgr; of the colors C, M, K are set to 15°, 45°, and 75°, respectively, and the screen angle &thgr; of the color Y is set to 0°. Since the color Y is a visually less intensive color, the difference between its screen angle and the screen angles of the other colors is set to 15°.
FIG. 11
of the accompanying drawings shows vectors representing the halftone screen periods of the colors and the period of a moiré pattern generated thereby. The magnitudes of the vectors are proportional to the screen ruling (halftone screen period). A vector D
1
representing a color image having a screen angle &thgr;
1
and a halftone screen period d
1
and a vector D
2
representing a color image having a screen angle &thgr;
2
and a halftone screen period d
2
make up a vector D
12
representing the direction and period of a primary moiré pattern generated by direct interference between the two halftone screen periods. The vector D
12
of the primary moiré pattern has components represented by (d
2
·cos &thgr;
2
−d
1
cos &thgr;
1
, d
2
·sin &thgr;
2
−d
1
·sin &thgr;l).
As described above, a color halftone image is formed by three or more superposed images in different colors. If the color images are represented by respective vectors D
1
, D
2
, D
3
having respective screen angles &thgr;
1
, &thgr;
2
, &thgr;
3
(&thgr;
1
<&thgr;
3
<&thgr;
2
, see
FIG. 11
) and respective halftone screen periods d
1
, d
2
, d
3
, then since general color images are periodic at equal pitches in two perpendicular directions, the vectors D
1
, D
2
, D
3
are associated with respective vectors D
1
⊥, D
2
⊥, D
3
⊥ which are perpendicular to the vectors D
1
, D
2
, D
3
and have halftone screen periods equal to those of the vectors D
1
, D
2
, D
3
. When the three color images are superposed according to the relationship shown in
FIG. 11
, then because the vector D
12
representing a primary moiré pattern due to the interference between the vectors D
1
, D
2
and the vector D
3
⊥ have similar magnitudes and angles, a secondary moiré pattern that can easily be recognized by human vision is generated if the two vectors D
12
, D
3
⊥ deviate slightly from each other.
In order to eliminate such a secondary moiré pattern, the vector D
12
may be equalized to the vector D
3
⊥. Specifically, if the following conditions are satisfied:
d
3
·cos &thgr;
3
=d
1
·cos &thgr;
1
−d
2
·cos &thgr;
2
(1)
d
3
·sin &thgr;
3
=d
2
·sin &thgr;
2
−d
1
·sin &thgr;
1
(2)
then the period of the secondary moiré pattern becomes infinitely large, making the secondary moiré pattern invisible to human vision. More specifically, when the screen angle &thgr; of the color image of M is set to 45°, the period of the primary moiré pattern generated by the color images of C, K whose screen angles &thgr; are set to 15° and 75°, respectively, and the halftone screen period of the color image of M whose screen angle &thgr; is set to 45° are equalized to each other, avoiding the generation of a secondary moiré pattern (see Japanese Patent Publication No. 2578947 for details).
In order to satisfy the conditions according to the above equations (1), (2), it is necessary to set the screen angles &thgr;
1
through &thgr;
3
and the halftone screen periods d
1
through d
3
of the respective colors to appropriate values.
According to a process of digitally generating the halftone threshold data that make up the dot cell
2
shown in
FIG. 9
, the halftone threshold data are generated according to the condition of a rational tangent. The condition of a rational tangent is a condition in which when a corner al of the square dot cell
2
is placed on a grid of pixels
4
, other corners a
2
through a
4
of the dot cell
2
are also placed on the grid of pixels
4
. With respect to the dot cell
2
having the screen angle &thgr;, there are established integers m, n which are mutually prime, as represented by the following equation (3):
&thgr;=tan
−1
(
n/m
) (3)
If the dot cell
2
has a pitch P, which represents the distance between the corners a
1
, a
2
with pixels
4
serving as a unit, then the coordinates of the corners a
1
through a
4
in the main scanning direction X and the auxiliary scanning direction Y are established as shown in
FIG. 9
using the corner al as the origin.
In order for the dot cell
2
having the screen angle &thgr; and the pit
Fuji Photo Film Co. , Ltd.
Nguyen Lamson
Sughrue & Mion, PLLC
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