Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2002-06-05
2004-03-09
Nguyen, Lamson D (Department: 2861)
Incremental printing of symbolic information
Ink jet
Controller
C347S043000, C347S041000
Reexamination Certificate
active
06702415
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an ink-jet printing method and apparatus and, more particularly, to an ink-jet printing method and apparatus configured to perform multilevel printing by landing the number of ink droplets corresponding to a gray level value onto each pixel in printing on a printing medium while performing main scanning operation of moving an ink-jet printhead for discharging ink relative to the printing medium.
BACKGROUND OF THE INVENTION
A printing apparatus serving as a printer, copying machine, facsimile apparatus or the like or a printing apparatus used as an output device for a composite electronic device or workstation such as a computer or wordprocessor is designed to print on a printing medium such as a thin plastic plate on the basis of image information including character information and the like.
Such printing apparatuses can be classified into the ink-jet type, wire-dot type, thermal type, laser beam type, and the like. Of the above printing apparatuses, an ink-jet type printing apparatus (ink-jet printing apparatus) is designed to print by discharging ink from a printing means such as a printhead onto a printing medium, and has the following advantages as compared with the other printing schemes. This printing apparatus allows an easy increase in resolution, can operate at high speed, and is very quiet. In addition, the printing apparatus is low in cost.
The need for color prints has increased, and many color ink-jet printing apparatuses have been developed. A general ink-jet printing apparatus uses a printhead formed by integrating pluralities of orifices and liquid channels as ink discharging portions as a printhead formed by integrating an array of a plurality of printing elements in order to attain an increase in printing speed. In addition, in order to realize color printing, such an ink-jet printing apparatus generally has a plurality of printheads.
FIG. 1
is a view showing the schematic arrangement of a general printer portion based on the scheme of printing by scanning a printhead on a printing sheet P. Referring to
FIG. 1
, reference numeral
101
denotes an ink cartridge. These ink cartridges are constituted by ink tanks respectively storing four color inks, i.e., black, cyan, magenta, and yellow inks, and identical printheads
102
provided for the respective inks.
FIG. 2
is a view showing the orifices formed in each printhead when viewed from the z direction. As shown in
FIG. 2
, a plurality of orifices
201
are arranged at predetermined intervals on the printhead
102
.
Referring back to
FIG. 1
, reference numeral
103
denotes a convey roller for a printing medium, which rotates in the direction indicated by the arrow in
FIG. 1
while holding a paper sheet P, together with an auxiliary roller
104
and sequentially feeds the paper sheet P in the y direction;
105
, feed rollers for feeding a printing sheet and also holding the paper sheet P like the rollers
103
and
104
; and
106
, a carriage which supports the four ink cartridges
101
and moves/scans them in printing operation. These ink cartridges are set in the standby state at the home position (h) indicated by the dotted line in
FIG. 1
while no printing is performed, or recovery operation is done for the printheads.
Before printing operation, when receiving a printing start instruction, the carriage
106
at the home position h in
FIG. 1
discharges ink from a plurality of orifices
201
on the printhead
102
while moving in the x direction, thereby printing data. When data is completely printed up to an end portion of a printing sheet surface, the carriage
106
returns to the home position, and prints in the x direction again.
When an image or the like is to be printed, various factors need to be considered including color development characteristics, gray level characteristics, uniformity, and the like. With regard to uniformity, in particular, it is known that slight variations caused on a nozzle basis in a printhead manufacturing process will influence the amount of ink discharged from each nozzle and the discharge direction, resulting in a deterioration in image quality which appears as density irregularity of a printed image.
A specific example of this will be described with reference to
FIGS. 3A
to
3
C and
4
A to
4
C. Referring to
FIG. 3A
, reference numeral
31
denotes a printhead constituted by eight nozzles
32
; and
33
, an ink droplet discharged from the nozzle
32
. In general, it is ideal that ink is discharged with a uniform discharge amount in a uniform direction. If ink is discharged in this manner, dots with a uniform size land on a paper sheet as shown in
FIG. 3B
, and a uniform image without any density irregularity can be obtained as a whole (FIG.
3
C).
In practice, however, each nozzle varies, as described above. If, therefore, printing is done in the above manner without any change, ink droplets discharged from the respective nozzles vary in size and direction as shown in FIG.
4
A and land on a sheet surface in the manner shown in FIG.
4
B. Referring to
FIG. 4B
, blank portions in each of which the area factor cannot be satisfied 100% periodically exist in the head main scanning direction, dots are excessively superimposed in some portions, and white streaks are produced as indicated at a central portion of this drawing.
A set of dots landed in this state exhibits the density distribution shown in
FIG. 4C
in the nozzle array direction. As a consequence, these phenomena are generally perceived as density irregularity by the human eye. In addition, if the convey amount of the printing medium varies, the resultant streaks may become noticeable.
As a countermeasure against density irregularity, the following method is disclosed in Japanese Patent Laid-Open No. 06-143618. This method will be briefly described with reference to
FIGS. 4A
to
4
C and
FIGS. 5A
to
5
C. According to this method, the printhead
31
is scanned three times in the main scanning direction (
FIG. 5A
) to complete the print area shown in
FIG. 5B. A
four-pixel area corresponding to 1/2 each print area is completed by two passes. In this case, the eight nozzles of the printhead are formed into two groups, i.e., four upper nozzles and four lower nozzles. The dot printed by one nozzle upon one main scanning operation corresponds to the data obtained by thinning out specified image data to about 1/2 in accordance with a predetermined image data arrangement (mask pattern). In the second main scanning operation, dots are formed in accordance with the remaining half image data to completely print a four-pixel area. The above printing method will be referred to as a multipass printing method hereinafter.
With the use of such a printing method, even if a printhead like the one shown in
FIG. 4A
is used, since the influences of the variations unique to the respective nozzles on a printed image are reduced to 1/2, an image similar to the one shown in
FIG. 5B
is printed. As a result, black and white streaks like those shown in
FIG. 4B
become less noticeable. As shown in
FIG. 5C
, the density irregularity is considerably reduced as compared with the case shown in FIG.
4
C.
In such multipass printing, image data is divided into complementary data to be used in the first and second main scanning operations according to predetermined mask patterns. In most instances, patterns like staggered patterns in which pixels are vertically and horizontally staggered pixel by pixel as shown in
FIGS. 6A
to
6
C are used as such mask patterns. In a unit print area (four-pixel area in this case), printing is completed by the first main scanning operation of printing a staggered pattern and the second main scanning operation of printing an inverse staggered pattern.
FIGS. 6A
,
6
B, and
6
C show how printing in a predetermined area is done by using these staggered and inverse staggered mask patterns. First of all, in the first main scanning operation, printing is performed by using the four lower nozzles and the staggered mask pattern (FIG.
6
A). In
Kanda Hidehiko
Nakagawa Yoshinori
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