Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1998-12-22
2002-06-25
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
Reexamination Certificate
active
06409299
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a printing apparatus and a printing method and, more particularly, to a printing apparatus and method for printing a binary image.
In an example of processing according to the prior art, four-pass printing is carried out while executing multi-pass print control by a printer using a printhead having eight nozzles for printing image data, which has been obtained by binarizing input multi-valued image data using a dither matrix composed of 4×4 dots, by discharging ink in accordance with the ink-jet method.
FIG. 8
is a diagram showing an example of a binarized image in which pseudo-halftone representation is achieved using a dither matrix of 4×4 dots. This example is illustrative of a case where an original image of a uniform scene has been binarized with comparatively strong highlighted density and shows a halftone image of a repeating pattern of seven black pixels in a dither matrix of 4×4 dots.
FIGS. 9A-9D
are diagrams showing mask patterns when multi-pass control is carried out, particularly for four-pass printing.
In
FIGS. 9A-9D
,
FIG. 9A
illustrates a mask pattern used in a first pass,
FIG. 9B
a mask pattern used in a second pass,
FIG. 9C
a mask pattern used in a third pass and
FIG. 9D
a mask pattern used in a fourth pass.
Reference will now be had to
FIGS. 10A-10G
to describe the progress of processing in a case where the halftone image shown in
FIG. 8
is printed by multi-pass (four-pass) printing by an eight-nozzle printhead using mask patterns of the kind shown in
FIGS. 9A-9D
.
FIGS. 10A-10G
are diagrams showing the manner in which multi-pass printing proceeds.
FIG. 10A
shows the printing result obtained from the first pass,
FIG. 10B
the printing result obtained from the first and second passes,
FIG. 10C
the printing result obtained from the first through third passes, and
FIG. 10D
the printing result obtained from the first through fourth passes. It should be noted that the rectangular shape with the numerals
1
through
8
at the upper right of the drawings of
FIGS. 10A through 10G
represents a printhead and that each of the numerals indicates a nozzle number.
In the example of four-pass printing using the eight-nozzle printhead of the kind shown in
FIGS. 10A through 10G
, a line feed equivalent to two nozzles is performed per scan of the printhead and therefore the printing of the first two lines is completed by the four scans of the printhead that are shown in
FIGS. 10A-10D
. Similarly, two lines composed of the third and fourth lines are printed in their entirety by the four scans of the printhead that are shown in
FIGS. 10B-10E
; two lines composed of the fifth and sixth lines are printed in their entirety by the four scans of the printhead that are shown in
FIGS. 10C-10F
; and two lines composed of the seventh and eighth lines are printed in their entirety by the four scans of the printhead that are shown in
FIGS. 10D-10G
.
In the binary images illustrated in
FIGS. 10A-10G
, the portions in which numerical values are shown indicate the portions printed by the scan of the printhead at that particular time; the numerical value indicates which nozzle performed printing. The hatched portions indicate portions already printed by preceding scans of the printhead.
Thus, printing equivalent to a total of eight lines is completed by the scans of the printhead in
FIGS. 10A through 10G
.
FIG. 11
is a diagram illustrating which nozzles discharged the ink that formed the printed dots obtained by the above-described printing operation.
In accordance with
FIG. 11
, it will be understood that the printing of the first line is performed by the nozzles
1
and
7
; that of the second line by the nozzle
8
alone; that of the third line by the nozzles
1
and
7
; that of the fourth line by the nozzles
2
and
6
; that of the fifth line by the nozzles
3
and
5
; that of the sixth line by the nozzle
4
alone; that of the seventh line by the nozzles
3
and
5
; and that of the eighth line by the nozzles
2
and
6
.
Printing is performed over the entire print medium by subsequently repeating the eight-line printing procedure described above.
The goal of multi-pass printing will be summarized in simple terms.
With a plurality of nozzles provided on a print head, the direction of ink discharge from the nozzles and the amount of ink discharge vary from nozzle to nozzle. Consequently, even when a single horizontal line is printed by a single nozzle, the horizontal line will have a fainter color than other horizontal lines if this nozzle happens to discharge an amount of ink that is small in comparison with the amount of ink discharged by the other nozzles. Such printing results in the appearance of a faint horizontal stripe at the portion printed by the nozzle in the case of a print image obtained by discharging ink over the entire surface of a print medium or an image having a uniform density. The end result is a decline in image quality.
The goal of multi-pass printing is to avoid this situation by printing the same line using a plurality of nozzles, whereby the amount of ink discharged on the line is made uniform overall and the variation in the characteristic of a specific nozzle is rendered inconspicuous.
If such multi-pass printing is controlled in such a manner that the same line is printed by a large number of nozzles, the effects caused by a fluctuation in the amount of ink discharged from a specific nozzle will be dispersed so as not to concentrate on a specific line. The result is that the printed image can be maintained at high quality.
In the example of the prior art described above, however, it should be obvious from the printed results shown in
FIG. 11
that the printing of specific lines (the second and sixth lines in
FIG. 11
) uses only specific nozzles (nozzles
8
and
4
, respectively, in FIG.
11
). This means that the original goal of multi-pass printing is not attained and that a decline in print image quality may result.
The reason for such a degradation of the image is that while the dither matrix has a 4×4 dot configuration, the size of the mask patterns used in multi-pass printing is 8×8 dots. More specifically, since the matrix of the mask patterns for multi-pass printing has a size that is a whole-number multiple of the size of the dither matrix in both the horizontal direction (the scanning direction of the printhead, also referred to as the “main-scan direction”) and vertical direction (the direction in which the printing paper is conveyed, also referred to as the “sub-scan direction”), the printed pattern that is generated periodically by the dither matrix coincides with the periodicity of the nozzles selected by the mask patterns for multi-pass printing and, as a consequence, a situation arises in which in the same line is always printed by the same nozzle.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a printing apparatus and method in which a high-quality image can be printed at all times even in a case where image data is binarized by a binarization method for representing pseudo half tone and the binarized image is printed by multi-pass printing.
According to one aspect of the present invention, the foregoing object is attained by providing a printing apparatus for printing an image on a print medium using a pattern used for representing half tone, comprising: a printhead having a plurality of printing elements; and print control means for scanning a different printing element in the plurality of printing elements of the printhead on the same area of the print medium plural times and performing complementary printing using the pattern used for representing half tone and a mask pattern by the plural times of scans of the printhead; wherein the relationship between a period of a matrix of the pattern used for representing half tone and that of a matrix of the mask pattern is relatively prime.
It is preferred that the print control means includes thinning out means for complementarily t
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Tran Ly T
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