U.S. patent application number 12/505562 was filed with the patent office on 2011-01-20 for printing method for reducing stitch error between overlapping jetting modules.
Invention is credited to James M. Enge.
Application Number | 20110012949 12/505562 |
Document ID | / |
Family ID | 42949715 |
Filed Date | 2011-01-20 |
United States Patent
Application |
20110012949 |
Kind Code |
A1 |
Enge; James M. |
January 20, 2011 |
PRINTING METHOD FOR REDUCING STITCH ERROR BETWEEN OVERLAPPING
JETTING MODULES
Abstract
A method of printing is described for a page-width printer
including a line head having a plurality of jetting modules that
are staggered in a page-width direction. In a set up procedure, a
test pattern is printed using the nozzles of adjacent jetting
modules, and the pattern is analyzed to detect a stitch error in
the overlap regions. The results of this analysis is used to
calculate a set of correction values to be applied to print data
subsequently sent to nozzles of the adjacent jetting modules to
make a correction for the stitch error. During a subsequent
production run, the print data sent to the nozzles of the adjacent
jetting modules is analyzed to sense an image content attribute,
such as gray or density level, of the print data. These results of
the analysis of the print data is then used to calculate a dynamic
adjustment that is used to adjust the set of correction values
calculated during the set up procedure. The line head is then used
to print the corrected print data by applying the set of adjusted
correction values to production print data subsequently sent to the
jetting modules.
Inventors: |
Enge; James M.;
(Spencerport, NY) |
Correspondence
Address: |
EASTMAN KODAK COMPANY;PATENT LEGAL STAFF
343 STATE STREET
ROCHESTER
NY
14650-2201
US
|
Family ID: |
42949715 |
Appl. No.: |
12/505562 |
Filed: |
July 20, 2009 |
Current U.S.
Class: |
347/12 |
Current CPC
Class: |
G06K 15/107 20130101;
G06K 2215/111 20130101; B41J 2/2135 20130101; G06K 15/105
20130101 |
Class at
Publication: |
347/12 |
International
Class: |
B41J 29/393 20060101
B41J029/393 |
Claims
1. A method of printing comprising the steps of: providing a line
head including a plurality of jetting modules that are staggered in
a page-width direction such that adjacent jetting modules partially
overlap each other, each of the plurality of jetting modules
including a plurality of nozzles wherein some of the nozzles of
adjacent jetting modules overlap each other; applying test pattern
print data to the line head for printing a test pattern on a print
media using the nozzles of adjacent jetting modules; analyzing the
test pattern to detect a stitch error; using the results of the
analysis of the test pattern to calculate a set of correction
values to be applied to print data subsequently sent to nozzles of
the adjacent jetting modules to make a correction for the stitch
error; analyzing the print data subsequently sent to the nozzles of
the adjacent jetting modules to sense an attribute of an image
content of the print data; using the results of the analysis of the
image content attribute to calculate a dynamic adjustment that is
used to adjust the set of correction values; and using the line
head to print corrected print data by applying the set of adjusted
correction values to production print data subsequently sent to the
jetting modules.
2. The method as claimed in claim 1 wherein the set of correction
values is a printing mask that is to be applied to print data
subsequently sent to nozzles of the adjacent jetting modules to
make the correction for the stitch error, and the dynamic
adjustment is applied to the printing mask when production print
data is subsequently sent to the jetting modules.
3. The method as claimed in claim 2 wherein the image content
attribute is a density value.
4. The method as claimed in claim 3 wherein the image content
attribute is a gray level of regions of the image represented by
the print data.
5. The method as claimed in claim 1 wherein the step of using the
results of the analysis of the test pattern to calculate a set of
correction values comprises the steps of: using the results of the
analysis of the test pattern to calculate a first set of correction
values to be applied to print data subsequently sent to nozzles of
one of the adjacent jetting modules to make a coarse correction for
the stitch error; and using the results of the analysis of the test
pattern to calculate a second set of correction values to be
applied to print data subsequently sent to one or more overlapping
nozzles of one of the adjacent jetting modules to correct for
sub-pixel stitch error.
6. The method as claimed in claim 5 wherein the step of using the
results of the analysis of the image content attribute to calculate
a dynamic adjustment that is used to adjust the set of correction
values further comprises using the dynamic adjustment to adjust the
second set of correction values.
7. The method as claimed in claim 1 wherein the adjusted correction
values are applied to two or more nozzles of the adjacent jetting
modules.
8. The method as claimed in claim 1 wherein the step of analyzing
the print data subsequently sent to the nozzles of the adjacent
jetting modules to sense an image content attribute of the print
data further comprises the steps of: accessing one or more rasters
of print data applied to one or more of the overlapping nozzles in
adjacent jetting modules, wherein each raster comprises a column of
print data printed by one printing nozzle in the page-length
direction, that is, perpendicular to the page-width direction.
calculating a pseudo-gray level for individual pixels in said one
or more rasters by processing the print data from a pixel
neighborhood within said one or more rasters including said
individual pixel; using the pseudo-gray level to determine a stitch
error correction amount related to print density; and using the
stitch error correction amount related to print density to modify
the set of correction values.
9. A method of printing comprising the steps of: providing a line
head including a plurality of jetting modules that are staggered in
a page-width direction such that adjacent jetting modules partially
overlap each other, each of the plurality of jetting modules
including a plurality of nozzles wherein some of the nozzles of
adjacent jetting modules overlap each other; applying test pattern
print data to the line head for printing a test pattern on a print
media using the nozzles of adjacent jetting modules; analyzing the
test pattern at least in the vicinity of the overlapping nozzles to
detect a stitch error; using the results of the analysis of the
test pattern to calculate a first set of correction values to be
applied to print data subsequently sent to nozzles of one of the
adjacent jetting modules to make a coarse correction for the stitch
joint error; using the results of the analysis of the test pattern
to calculate a second set of correction values to be applied to
print data subsequently sent to one or more overlapping nozzles of
one of the adjacent jetting modules to correct for sub-pixel stitch
error, whereby the second set of correction values is proportional
to print data applied to one or more of the overlapping nozzles;
and using the line head to print the corrected print data by
applying the first set and the second set of correction values to
production print data subsequently sent to the jetting modules.
10. The method as claimed in claim 9 wherein the first set of
correction values comprises instructions for shifting print data
sent to nozzles of one of the adjacent jetting modules in the
page-width direction to make the coarse correction for the stitch
joint error.
11. The method as claimed in claim 10 wherein the print data sent
to nozzles of one of the adjacent jetting modules comprises
instructions for shifting the print data in one direction to
coarsely correct for a dark line artifact and in the opposite
direction to coarsely correct for a white line artifact.
12. The method as claimed in claim 9 wherein the step of analyzing
the test pattern further comprises the step of automatically
processing the test pattern to obtain measurements representative
of the misalignment between adjacent jetting modules.
13. The method as claimed in claim 9 wherein the step of using the
results of the analysis of the test pattern to calculate a second
set of correction values comprises the step of applying the second
set of correction values as a print mask to print data subsequently
sent to one or more overlapping nozzles of one of the adjacent
jetting modules to correct for sub-pixel stitch error, wherein the
print mask selectively masks certain nozzles preventing such
nozzles from printing even if print data for those nozzles should
include an instruction to fire.
14. The method as claimed in claim 9 for use with printed output
having variable levels of density, wherein the second set of
correction values to be applied as a print mask to print data is
adjusted based upon the density of an image represented by the
print data.
15. A method of printing comprising the steps of: providing a line
head including a plurality of jetting modules that are staggered in
a page-width direction such that adjacent jetting modules partially
overlap each other, each of the plurality of jetting modules
including a plurality of nozzles wherein some of the nozzles of
adjacent jetting modules overlap each other; printing a test
pattern on a print media using the nozzles of adjacent jetting
modules; analyzing the test pattern at least in the vicinity of the
overlapping nozzles to detect a stitch error; using the results of
the analysis of the test pattern to calculate a first set of
correction values to be applied to print data subsequently sent to
the nozzles of one of the adjacent jetting modules to make a coarse
correction for the stitch joint error, whereby the coarse
correction includes a shifting in print data in one of the adjacent
jetting modules in the page-width direction; using the results of
the analysis of the test pattern to calculate a second set of
correction values to be applied as a print mask to print data
subsequently sent to two or more of the overlapping nozzles of one
of the adjacent jetting modules to correct for sub-pixel stitch
error, wherein the print mask further includes a correction related
to an image density of the print data; and using the line head to
print the corrected print data by applying the first and second set
of correction values to production print data subsequently sent to
the jetting modules.
16. In a page-width inkjet printer including a plurality of jetting
modules that are staggered in the page-width direction such that
adjacent jetting modules partially overlap in an overlap region,
each of the plurality of jetting modules including a plurality of
nozzles wherein some of the nozzles are nominally adjacent and
aligned in the overlap region and nominally designated either as
printing nozzles or nonprinting nozzles, a method of reducing
stitch error resulting from horizontal misalignment of the jetting
modules comprising the steps of: determining the horizontal
misalignment of the jetting modules, and whether any stitch error
resulting from said horizontal misalignment produces a white line
artifact or a dark line artifact at a stitch juncture within the
overlap region of adjacent jetting modules; for a misalignment
where the artifact is a white line, duplicating existing data from
a printing nozzle in the overlap region of one of the adjacent
jetting modules wherein the white line artifact occurs, and
applying a proportion of the duplicated data to at least one of the
nonprinting nozzles in the overlap region of the other adjacent
jetting module; and for a misalignment where the artifact is a dark
line, shifting the data through one of the adjacent jetting modules
in the page-width direction to convert at least one printing nozzle
to an additional nonprinting nozzle in the overlap region of the
jetting module wherein the dark line artifact occurs, and then
duplicating existing data from a printing nozzle in the overlap
region of the other adjacent jetting module and applying a
proportion of the duplicated data to the additional nonprinting
nozzle in the overlap region; wherein the method controls which
jetting module prints data in the overlap region and whether any
shifting of data needs to occur.
17. The method as claimed in claim 16 further comprising the step
of using the nominally designated nozzles as printing and
nonprinting nozzles in the overlap region when the step of
determining the horizontal misalignment of the jetting modules
shows a substantially perfect alignment within the overlap region
of adjacent jetting modules.
18. The method as claimed in claim 16 further comprising the step
of shifting the data in the page-width direction for a misalignment
where the artifact is a white line extending over more than one
nozzle.
19. The method as claimed in claim 18 further comprising the step
of printing a target to determine the amount of shift that is
needed and what proportion of the duplicated data causes the least
visible artifact.
20. The method as claimed in claim 19 further comprising the step
of visually inspecting the target to determine the amount of shift
that is needed and what proportion of the duplicated data causes
the least visible artifact.
21. The method as claimed in claim 19 further comprising the step
of automatically determining gaps or bands in the target to
determine the amount of shift that is needed and what proportion of
the duplicated data causes the least visible artifact.
22. The method as claimed in claim 19 further comprising the step
of using the target to correlate a correction factor with a known
degree of jetting module misalignment to determine the amount of
shift that is needed and what proportion of the duplicated data
causes the least visible artifact.
23. The method as claimed in claim 16 for use with printed output
having variable density, wherein the proportion of the duplicated
data applied to at least one of the nonprinting nozzles in the
overlap region is further adjusted according to the variable
density.
24. The method as claimed in claim 16 wherein the desired stitch
error correction comprises a mask applied to a raster of print
data, wherein each raster comprises a column of print data printed
by one printing nozzle in the page-length direction, that is,
perpendicular to the page-width direction.
25. The method as claimed in claim 24 wherein the mask comprises a
threshold applied to individual pixels within the raster of print
data.
26. The method as claimed in claim 24 for use with printed output
having variable density, said method further comprising the steps
of: accessing one or more rasters of print data pertaining to
pixels in the overlap region near a stitch juncture; calculating a
pseudo-gray level for individual pixels in said one or more rasters
by processing the print data from a pixel neighborhood within said
one or more rasters including said individual pixel; using the
pseudo-gray level to determine a stitch error correction amount
related to print density; and using the stitch error correction
amount related to print density to further modify the mask applied
to individual pixels.
27. The method as claimed in claim 26 wherein the mask comprises a
threshold applied to individual pixels within the raster of print
data and wherein the stitch error correction amount related to
print density is used to further threshold the threshold applied to
individual pixels.
28. The method as claimed in claim 16 for use with printed output
run through the printer at a variable web speed, wherein the
proportion of the duplicated data applied to at least one of the
nonprinting nozzles in the overlap region is adjusted according to
the web speed.
29. The method as claimed in claim 23 for use with printed output
run through the printer at a variable web speed, wherein the
proportion of the duplicated data applied to at least one of the
nonprinting nozzles in the overlap region is further adjusted
according to the web speed.
30. The method as claimed in claim 24 for use with printed output
run through the printer at a variable web speed, wherein the mask
applied to a raster of print data is adjusted according to the web
speed.
Description
FIELD OF THE INVENTION
[0001] The field of the invention relates to page-width printing
systems using a line head including a plurality of jetting modules
that are staggered in the page-width direction, and more
particularly to methods and algorithms for joining the data from
the jetting modules in the page-width direction.
BACKGROUND OF THE INVENTION
[0002] Stitching refers to the alignment of the print from multiple
jetting modules for the purpose of creating the appearance of a
single page-width line head. For example, as shown in FIG. 1, seven
jetting modules 2--each three inches in length--can be stitched
together at junctures 2a to form a twenty-one inch page-width line
head 4. The page-width image data is processed and segmented into
separate segments for each jetting module 2, and then a segment is
sent--with an appropriate module-to-module delay to account for the
staggered separation of the modules--to the print nozzles 6 of each
jetting module for printing.
[0003] However, though it may be anticipated that the
module-to-module alignment may be very good, mechanical tolerances
may be difficult to consistently maintain and alignment will often
not be perfect. Moreover, even if the jetting modules are perfectly
aligned, differences in the nozzle aim between jetting modules may
make them appear to be misaligned in the printed output.
Consequently, this type of conventional, multi-segment jetting
module configuration suffers from the drawback that the pitch of
the output lines along the juncture 2a of adjacent jetting modules
is irregular and thereby causes lines of lower (if too far apart)
or higher (if too close together) density to appear at the juncture
2a of each jetting module segment and thus impairs the quality of
the printed pattern of the output. In the output medium, such
misalignment typically produces a gap or "white line" artifact 8a
(as shown in FIG. 2A) or an overlap or "dark line" artifact 8b (as
shown in FIG. 2B).
[0004] With a view to overcoming the presence of visible gaps or
bands in the printed image, U.S. Pat. No. 7,118,188 deliberately
sets the print dies of an inkjet printer with a small overlap,
specifically no more than a few times the nozzle spacing. As a
result of the redundancy of nozzles in the region where adjacent
dies overlap, this gives flexibility for compensating for gaps or
bands produced by inaccuracies in locating the dies and thus in
setting the overlap dimension. Although, in an ideal case, 100% of
the required amount of ink (maximum) would be printed by only 50%
of the nozzles of each die in the overlap region, in practice more
or fewer of the nozzles may be fired to compensate for
imperfections. For example, if the overlap is less than intended,
the production of a gap is avoided by firing some of the nozzles
which would not be fired in the ideal case.
[0005] A printing mask is a means for selectively masking off
certain nozzles, i.e., preventing these nozzles from firing even if
printing instructions for those nozzles should include an
instruction to fire. U.S. Pat. No. 7,118,188 further discloses a
method of adding stitching masks to the printed image content,
where artifacts in the printed image caused by the printing nozzles
in the overlapping region are removed, either by (a) measuring the
width of the band produced in the overlapping region and selecting
an appropriate stitching mask for subsequent printing operations,
or by (b) printing out a test pattern in which areas corresponding
to a range of stitching masks are printed out and the optimal mask
is selected for subsequent printing operations. The stitching mask
is then added to, or superimposed on, the printing masks to ensure
that the required correction is made independently of the content
to be printed.
[0006] U.S. Pat. No. 7,118,188 further discloses that the target
may comprise a array of target patches overlapping the die-to-die
boundaries and including a range of stitching masks. The magnitudes
of the die-to-die boundary artifacts are then assessed either by a
user of the machine or automatically by an optical sensor/scanner
system. In the first option, a user visually examines the patches
in each row and selects the one with the better area fill
uniformity at the printed region corresponding to the die-to-die
boundary. The corresponding stitching mask is then applied to that
die-to-die boundary in subsequent normal printing operations. In
the second option, an optical sensor moves over all the patches
detecting the boundary artifact level and supplies the most
appropriate stitching mask for each die pair to a printer control
system, where the masks will then be used in subsequent normal
printing operations.
[0007] In relation to page-width thermal printers, U.S. Pat. Nos.
4,977,410 and 5,450,099 each disclose a thermal line printer
including a plurality of staggered linear head segments arranged in
a pair of parallel rows such that the head segments partly overlap
with each other in overlapping regions near the ends of each
segment. In U.S. Pat. No. 5,450,099, the print data in the
overlapping region is interleaved to eliminate boundary artifacts
at the juncture between segments. In U.S. Pat. No. 4,977,410, the
initial assignment of image bit data to a segment in the
overlapping region is shifted lengthwise to accommodate for
boundary artifacts at the juncture between segments.
[0008] In relation to a carriage-type printer wherein a printhead
is attached to a carriage that is reciprocated to print one swath
of information at a time on a stationary receiving medium, U.S.
Pat. No. 6,663,206 discloses methods for masking stitch errors
between adjacent swaths laid down by operation of such a printer.
In contrast with the afore-mentioned examples of page-width
printers that utilizes a line head including an array of stationary
printheads, after each swath is printed by the carriage-type
printer the receiving medium is stepped a distance equal to the
height of the swath so that the next printed swath overlaps the
pixels from the last line of the previously printed swath. When a
controller determines that a stitch joint error will occur based on
the current relative location between the printhead and the medium
and the location of the previous swath, the location of the next
swath is adjusted relative to the position of the previous swath to
eliminate the stitch joint error.
[0009] According to U.S. Pat. No. 6,663,206, the data is shifted in
the printhead so that the data for the next swath is aligned within
a predetermined pixel accuracy to the measured paper position,
e.g., by having a later nozzle fire the pixel data originally set
to be fired by the first nozzle of the printhead. In addition, the
remaining stitch joint error is covered up by modifying the pixels
at the stitch interface. In one example, the pixels created in the
region between the last line of the previous swath and the first
line of the next swath can be a duplicate line of either the last
line of the previous swath or the first line of the next swath,
where the size and/or density of the pixels can be changed. In
another example, for situations where the stitch error is less than
a pixel, in addition to shifting the data and firing the
information set to be printed, the controller will also fire a line
of fill pixels from the nozzle prior to and immediately adjacent to
the first-fired nozzle. The purpose of a fill pixel is to bridge
the gap between a printed pixel from the last fired nozzle of the
previous swath and a corresponding adjacent printer pixel that will
be formed when the first line of pixels is formed by the nozzle
that will be used for the first line of pixels for the next swath.
According to U.S. Pat. No. 6,663,206, the fill pixels create a
printed image having more uniform continuity and density. The fill
pixels are not produced for all of the pixels located in the last
line of the previous swath. Instead, the fill pixels are produced
when a printed pixel is located in the same position in both the
previous swath and the next swath. The fill pixels can also be at a
reduced size and/or density.
[0010] As thus understood in the prior art, stitch joint error in a
drop-on-demand carriage-type system can be the result of a gap
between the drop of one swath adjacent the stitch joint and the
drop of an adjoining swath adjacent the same stitch joint. As
explained in U.S. Pat. No. 6,663,206, the gap is usually caused by
difficulties in producing adjacent swaths close enough together to
mask this apparent error, and the correction must be produced
on-the-fly during a production run. In contrast, as also explained
in the '206 patent, a page-width printer includes a stationary
printhead having a length sufficient to print across the width or
length of the sheet of receiving medium. The receiving medium is
continually moved past the page-width printhead in a direction
substantially normal to the printhead length and at a constant or
varying speed during the printing process. Thus, it would be
understood that a page-width printer would avoid the need for
on-the-fly corrections between swaths during a production run.
SUMMARY OF THE INVENTION
[0011] The applicant has observed that the correction amount
obtained during a set-up procedure may not be suitable for all
image content in a page-width printer, despite the same degree of
misalignment of the jetting modules. In particular, the applicant
has observed that the correction amount obtained during a set-up
procedure may not be suitable for all density levels in the image.
The applicant has observed that a test pattern, in which an area
corresponding to a range of printing masks is printed out for the
same stitch juncture, sometimes shows highlights corrected but dark
regions misaligned, or other times shows dark regions corrected but
highlights misaligned.
[0012] From such observations the applicant has discovered that
corrections for nozzle alignments are not independent of image
conditions and that it may be necessary to make the correction
amount dynamic, meaning that it should dynamically change during a
production run, that is, on-the-fly, based on an attribute of the
image data. For example, the applicant has discovered that darker
regions should receive more correction than highlight regions, and
in some cases this may require more than one pixel or nozzle for
correction. The applicant has further discovered that the problem
can be solved by sensing an image attribute, such as the gray level
of an image region, and using that attribute to calculate a dynamic
correction that is applied to the printing mask during the
production run.
[0013] The present invention is directed to overcoming one or more
of the problems set forth above. Briefly summarized, according to
one aspect of the invention, the invention resides in a method of
printing involving a line head including a plurality of jetting
modules that are staggered in a page-width direction such that
adjacent jetting modules partially overlap each other, where each
of the plurality of jetting modules include a plurality of nozzles
wherein some of the nozzles of adjacent jetting modules overlap
each other. During a setup or calibration procedure, test pattern
print data is applied to the line head for printing a test pattern
on a print media using the nozzles of adjacent jetting modules, and
the test pattern is analyzed to detect a stitch error. Using the
results of the analysis of the test pattern, a set of correction
values are calculated, which are to be applied to print data
subsequently sent to nozzles of the adjacent jetting modules to
make a correction for the stitch error.
[0014] During a production run, the print data subsequently sent to
the nozzles of the adjacent jetting modules is analyzed to sense an
attribute of an image content of the print data, and the results of
the analysis of the image content attribute are used to calculate a
dynamic adjustment that is used to adjust the set of correction
values. Then, the line head is used to print corrected print data
by applying the set of adjusted correction values to production
print data subsequently sent to the jetting modules.
[0015] The applicant has also observed that, for a page-width
printer, an improved result is obtained if the correction is
separated into a coarse correction, e.g., involving a shift of
data, for whole numbers of pixels and a sub-pixel correction for a
stitch gap of less that a pixel. From such observations, the
applicant has discovered that an improved distribution of data in
the highlights and a more uniform correction throughout the tone
scale of the image being printed can be obtained if the sub-pixel
correction is a proportion of the data raster printed by another
nearby nozzle.
[0016] Accordingly, the present invention is further directed to
overcoming one or more of the problems set forth above in
connection with sub-pixel corrections. Briefly summarized,
according to one aspect of the invention, the invention further
resides in method of printing involving a line head including a
plurality of jetting modules that are staggered in a page-width
direction such that adjacent jetting modules partially overlap each
other, where each of the plurality of jetting modules include a
plurality of nozzles and wherein some of the nozzles of adjacent
jetting modules overlap each other. Test pattern print data is
applied to the line head for printing a test pattern on a print
media using the nozzles of adjacent jetting modules, and the test
pattern is analyzed to detect a stitch error.
[0017] Using the results of the analysis of the test pattern, a
first set of correction values is calculated, which is to be
applied to print data subsequently sent to nozzles of one of the
adjacent jetting modules to make a coarse correction for the stitch
error. Using the results of the analysis of the test pattern, a
second set of correction values is calculated, which is to be
applied to print data subsequently sent to one or more overlapping
nozzles of one of the adjacent jetting modules to correct for
sub-pixel stitch error, whereby the second set of correction values
is proportional to print data applied to one or more of the
overlapping nozzles. Then, the line head is used to print the
corrected print data by applying the first set and the second set
of correction values to production print data subsequently sent to
the jetting modules.
[0018] The technical advantage of the invention is that it enables
stitch error corrections to dynamically change during a production
run based on an attribute, such as density, of the image data.
Stitch artifacts corrected across a stitch joint according to prior
procedures sometimes show highlights corrected but dark regions
misaligned, or other times show dark regions corrected but
highlights misaligned. The invention advantageously provides a
uniform correction across the stitch joint where darker regions may
need more correction than highlight regions, and vice versa.
Furthermore, by making the sub-pixel correction a proportion of the
data raster printed by another nearby nozzle, the distribution of
data in the highlights is improved and a more uniform correction
throughout the tone scale of the image being printed can be
obtained.
[0019] These and other aspects, objects, features and advantages of
the present invention will be more clearly understood and
appreciated from a review of the following detailed description of
the preferred embodiments and appended claims, and by reference to
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a prior art example of a plurality of jetting
modules that are stitched to form a page-width line head.
[0021] FIG. 2A shows a "white line" artifact due to a misalignment
of the jetting modules shown in FIG. 1.
[0022] FIG. 2B shows a "dark line" artifact due to a misalignment
of the jetting modules shown in FIG. 1.
[0023] FIG. 3 is a diagram of a printer, including a line head
having an array of staggered, overlapping jetting modules,
incorporating a stitching algorithm according to the invention.
[0024] FIG. 4 is a flow diagram showing the correction steps
involved in the overall stitching algorithm according to the
invention.
[0025] FIG. 5 is a diagram of the line head shown in FIG. 3,
showing a portion of two overlapping jetting modules that are
substantially perfectly aligned.
[0026] FIG. 6A is a diagram of a portion of the line head shown in
FIG. 3, showing a portion of two overlapping jetting modules that
are misaligned in the horizontal direction, causing the "white
line" artifact.
[0027] FIG. 6B is a diagram of the two overlapping jetting modules
that are misaligned in the horizontal direction as shown in FIG.
6A, further illustrating a duplication correction applied to the
raster generated by one of the non-printing nozzles in one of the
adjacent jetting modules.
[0028] FIG. 7A is a diagram of a portion of the line head shown in
FIG. 3, showing a portion of two overlapping jetting modules that
are misaligned in the horizontal direction, causing the "dark line"
artifact.
[0029] FIG. 7B is a diagram of the two overlapping jetting modules
that are misaligned in the horizontal direction as shown in FIG.
7A, further illustrating a shifting correction applied to the
rasters generated by a row of the printing nozzles in one of the
jetting modules.
[0030] FIG. 7C is a diagram of the two overlapping jetting modules
that are misaligned in the horizontal direction as shown in FIG.
7A, further illustrating a duplication correction applied to the
raster generated by one of the nozzles after the shifting
correction.
[0031] FIG. 8A is one embodiment of a proportional duplication
correction applied to the raster generated by one of the nozzles in
one of the jetting modules, where a mask is used to determine which
data in the raster will be printed.
[0032] FIG. 8B is an example of an 80% mask applied to the
duplicated raster data according to the embodiment described in
FIG. 8A.
[0033] FIG. 9A is a flow diagram for applying a threshold dither
algorithm only to the pixels that contain print drop data.
[0034] FIG. 9B shows an example of the method shown in FIG. 9A for
a 60% correction.
[0035] FIG. 10 shows a target for determining an optimum correction
that is substantially consistent across several density levels.
[0036] FIG. 11 shows a flow diagram of a method for generating a
stitch error correction that is dependent on the print density of
the image data.
[0037] FIG. 12 is a diagram of a portion of two adjacent,
overlapping jetting modules showing several rasters of print data
in the overlap region near a stitch juncture that are obtained for
processing according to the method shown in FIG. 11.
[0038] FIG. 13 is a diagram showing the calculation of pseudo-gray
levels at the stitch joint, according to the method shown in FIG.
11, using print data from a neighborhood of pixels within the
rasters of print data illustrated in FIG. 12.
[0039] FIGS. 14 and 15 are diagrams that jointly illustrate an
indexing process, according to the method shown in FIG. 11, using
the pseudo-gray level associated with each printed pixel from FIG.
13 and the desired stitch correction to determine a further
variable adjustment amount related to print density.
[0040] FIG. 16 is a diagram showing how the variable adjustment
amount derived from FIGS. 14 and 15 is used, according to the
method shown in FIG. 11, to modify a threshold dither matrix
associated with the desired stitch correction in order to provide a
stitch error correction that is dependent on the print density of
the image data.
[0041] FIG. 17 shows a portion of a carriage-type, drop-on-demand
inkjet printer that could incorporate the systems and methods of
the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0042] Because printing systems employing stitching methods are
well known, the present description will be directed in particular
to elements forming part of, or cooperating more directly with,
methods in accordance with the present invention. Elements useful
in practicing the methods but not specifically shown or described
herein may be selected from those known in the art. Certain aspects
of the embodiments to be described may be provided in software.
Given an understanding of the system as shown and described
according to the invention in the following materials, software not
specifically shown, described or suggested herein that is useful
for implementation of the invention is conventional and within the
ordinary skill in such arts.
[0043] FIG. 3 shows an embodiment of a printer 10 incorporating the
invention. In the embodiment of FIG. 3, the printer 10 comprises a
housing 12 having a line head 14 that applies markings or otherwise
forms an image on a receiver medium 16. The line head 14 includes a
fixed array of overlapping jetting modules 18, where adjacent
jetting modules 18 are staggered such that the nozzles 20 of
adjacent jetting modules 18 overlap in an overlap region 24. (The
overlap region 24 is shown for illustration in FIG. 3 between two
of the jetting modules 18, but it should be understood that such
overlap regions also exist between the other jetting modules.) The
line head 14 can record images on the receiver medium 16 using a
variety of known digital marking technologies including, but not
limited to, drop on demand ink jet technology and continuous inkjet
technology. For the purpose of illustrating the following
discussions, the line head 14 will be described as being useful
with continuous inkjet technology that generates monotone images
such as black and white, grayscale or sepia toned images. However,
it will be appreciated that these limitations are not necessary
attributes of the invention and that the claimed methods herein
described can be practiced, e.g., with a line head 14 that
generates color images, or with known digital marking technologies
including drop on demand ink jet technology.
[0044] FIG. 3 shows in part a schematic top plan view of a fixed,
page-width line head 14 including six staggered jetting modules 18,
each comprising at least one row of nozzles 20 which are arranged,
in the preferred embodiment, to fire ink drops onto the receiver
medium 16 as it is advanced through the printer 10 in a medium
transport direction indicated by the arrow 22. The overlaps 24 due
to the staggering between adjacent jetting modules 18 are shown on
an exaggerated scale for the purposes of explanation, and
effectively provide two page-width rows of nozzles. In practice,
in-track timing delays for each jetting module are used to obtain
in-track registration, that is, registration in the medium
transport direction 22, for the printed output from the staggered
modules 18. In the presently preferred embodiment, six
approximately 4.25 inch width jetting modules 18 are staggered at 6
inches in the in-track (web) direction to provide a 24.5 inch
print-width line head 14 in the cross-track (nozzle) direction. As
shown in FIG. 3, jetting modules 18 are oriented such that the rows
of nozzles 20 of jetting modules 18 are aligned along the cross
track direction and positioned perpendicular to the direction 22 of
medium transport. Other orientations of the rows of nozzles 22 are
also permitted. For example, the rows of nozzles 20 can be
positioned at a non-perpendicular, non-parallel angle relative to
both the medium transport direction 22 and the cross track
direction. Additionally, while two rows of jetting modules 18 are
shown in FIG. 3, it is contemplated that more than two rows of
jetting modules can be used with the present invention. For
example, three rows, four rows, or more then four rows of jetting
modules 18 can be implemented in a printing system incorporating
the present invention.
[0045] A medium advance 30 is used to position the receiver medium
16 relative to the line head 14 to facilitate recording of an image
on the receiver medium 16. The medium advance 30 can comprise any
number of well-known systems for moving the receiver medium 16
within the printer 10, including a motor 32 driving pinch rollers
34, a motorized platen roller (not shown) or other well-known
systems for the movement of paper or other types of receiver medium
16.
[0046] The line head 14 and the medium advance 30 are operated by a
processor 36. The processor 36 can include but is not limited to a
programmable digital computer, a programmable microprocessor, a
programmable logic processor, a series of electronic circuits, a
series of electronic circuits reduced to the form of an integrated
circuit, or a series of discrete components. The processor 36
operates the printer 10 based in part upon input signals from one
or more of a user input system 38, sensors 40, a memory 42, a
stitching algorithm 58, and (when connected) a remote computer 50.
A display 44 can provide to a user, without limitation, displays
indicating information, images and operating data useful in
implementing the stitching algorithm of the invention.
[0047] The user input system 38 (which, in certain applications,
can be used to select masks for implementing the stitching
algorithm of the invention) can comprise any form of transducer or
other device capable of receiving an input from a user and
converting this input into a form that can be used by the processor
36.
[0048] The memory 42 can include conventional memory devices
including solid state, magnetic, optical or other data storage
devices. The memory 42 can be fixed within the printer 10 or it can
be removable. For instance, although not shown, the printer 10 may
include a hard drive, a disk drive for a removable disk such as an
optical, magnetic or other disk memory, or a memory card slot that
holds a removable memory such as a removable memory card and has a
removable memory interface for communicating with removable memory.
Data including but not limited to control programs, digital images
and metadata can also be stored external to the printer 10 in the
remote computer system 50, such as a personal computer, a computer
network or other digital system.
[0049] The sensors 40 can optionally include image capture devices
or other light sensors known in the art that can be used to capture
images of targets to determine, e.g., optimal correction amounts
for the stitching algorithm according to the invention. This
information can be captured and processed automatically and
converted into a form that can be used by the processor 36 in
governing operation of the line head 14 and jetting modules 18
and/or other systems of the printer 10. Alternatively, the images
of the targets can be visually examined by an operator and
correction amounts can be entered through the user input 48. The
sensors 40 can also include positioning and other sensors used
internally to sense operating conditions, such as web speed, and
thereby control printer operations.
[0050] According to a preferred embodiment, the sensors 40 are used
in a stitching calibration process and further include a plurality
of stitching cameras 52 oriented along the stitch joints 55 between
the jetting modules 18 to capture a stitching calibration target
54. The captured target data is applied to a stitching camera
processing system 56, which generates stitching parameters that are
applied to the processor 56 and stored in the stitching algorithm
58. FIG. 4 is a flow diagram showing the steps involved in the
overall stitching calibration process, and a subsequent production
run using the results of the calibration process. The diagram
generally shows setting up the module-to-module registration (with
a first set of correction values) so that the modules within a line
head are coarsely aligned to within a pixel and masking the
remaining stitch artifacts from errors (with a second set of
correction values) due to misalignment of less than one pixel (a
sub-pixel correction).
[0051] Referring to FIG. 4, during the setup process, the stitching
calibration target 54 is printed (in printing stage 60) at a speed
suitable for capturing the target in capture stage 62 with the
stitching cameras 52. The stitching camera processing system 56,
which is driven by the processor 36, receives the captured data
from the target 54 and analyzes the elements in an analytic stage
64. Following the analytic stage 64, the stitching camera
processing system 56 calculates the start printing nozzle and the
end printing nozzle for each jetting module in the coarse
correction stage 66, which sets up the module-to-module
registration so that the modules within a line head are aligned to
within a pixel. The stitching camera processing system 56 also
calculates a stitch masking correction amount for each stitch point
in the sub-pixel correction stage 68, which masks the remaining
stitch artifacts from errors due to misalignment of less than one
pixel (a sub-pixel correction).
[0052] In closing the setup process, the parameters calculated
above are applied in a correction transfer stage 70 to the
stitching algorithm 58, which is operative during a production run.
During the production run, the stitching algorithm 58 is applied to
the print data in an algorithm application stage 72, whereby the
parameters calculated above are used to select the start and end
nozzles for each jetting module 18. The web speed is monitored
during the production run by the sensors 40, and the stitch masking
correction amount is adjusted with a look-up-table (not shown),
with the web speed as input. Moreover, in the dynamic adjustment
stage 74 the masking amount for each stitch point is dynamically
adjusted based on the image content (print density compensation).
(It is helpful to understand that the stitching cameras 52 are
ordinarily not used during a production run.)
[0053] The overall calibration process can be run in a new
calibration mode or in a verification mode. In the verification
stage 76, the stitching calibration target 54 is printed (in
printing stage 60) using previous or new corrections and the
stitching camera processing system 56, which is driven by the
processor 36 in the verification mode, captures the corrected
elements of the target 54 and analyzes the elements in the analytic
stage 64 to verify the corrective effect of corrections previously
being used or to verify the corrective effect of newly calculated
corrections. In the new calibration mode, the stitching calibration
target 54 is printed (in printing stage 60) with no corrections
applied and the stitching camera processing system 56 operates as
described above in connection with FIG. 4 to generate new
corrections.
[0054] A stitching algorithm attempts to optimize the data going to
the nozzles, so that misalignment artifacts in the printed output
are minimized or masked. In particular, it is helpful to explain
the operation of the algorithm according to the present invention
by referring to a pair of jetting modules, and more particularly to
the overlap region where nozzles of the respective jetting modules
overlap. Accordingly, and for the most part, the remaining Figures
refer to a partial plan view of a portion of the line head 14,
showing portions of two overlapping jetting modules 18a and 18b and
their overlapping region 24. However, it should be understood that
the disclosed operation and methodology of the stitching algorithm
pertains to all of the jetting modules and all of the overlap
regions. By overlapping the ends of two staggered jetting modules
18a and 18b by a small amount, it is possible to correct, as taught
by the present invention, for virtually any horizontal misalignment
error perpendicular to the paper transport direction 22. (As
mentioned hereinbefore, it is assumed that vertical misalignment
errors parallel to the paper transport direction can be
conventionally compensated for by means of an in-track (web
direction) timing delay from module-to-module, and this is not part
of the present invention.)
[0055] For example, FIG. 5 shows adjacent jetting modules 18a and
18b having an overlap of eight nozzles within an overlap region 24
across the stitch joint 55. The stitching algorithm then controls
which nozzles of which jetting module 18a or 18b prints the data in
the overlap region 24, and whether any shifting or other processing
of the data needs to occur. In the nominal case illustrated in FIG.
5, where the print heads are substantially perfectly aligned
perpendicular to the medium transport direction 22, nozzles 20a
(shown as solid circles) are designated as printing nozzles and
nozzles 20b (shown as open circles) are designated as nonprinting
nozzles.
[0056] Where there is a horizontal misalignment artifact, that is,
an artifact is produced due to cross-track (nozzle direction)
misalignment between jetting modules 18a and 18b perpendicular to
the medium transport direction 22, the stitching algorithm employs
a data duplication and/or shifting method to compensate for the
resulting artifact. FIG. 6A shows a horizontal misalignment causing
a "white line" artifact, where there is a misregistration causing
the first and last printing nozzles 20a of adjacent jetting modules
18a and 18b in the overlap region to be too far apart. In other
words, due to a cross-track error in the relative positioning of
two of the jetting modules 18a and 18b, a light band or gap is
produced in the image content printed by the jetting modules 18a
and 18b as the print medium 16 advances thereunder. (Such a gap 8a
is shown on a print medium in FIG. 2A resulting, for purpose of
illustration, from two misaligned jetting modules.) On the other
hand, FIG. 7A shows a horizontal misalignment causing a "dark line"
artifact, where there is a misregistration causing the first and
last printing nozzles 20a of adjacent jetting modules in the
overlap region to be too close together--even to the point of
overlapping each other. (Such a dark band or line 8b is shown on a
print medium in FIG. 2B resulting, for purpose of illustration,
from two misaligned jetting modules.) In other words, due to an
error in the relative positioning of the jetting modules 18a and
18b, a dark band is produced in the image content printed by the
jetting modules 18a and 18b as the print medium 16 advances
thereunder. Such overlap is expected to exist not only where the
ink drops actually overlap in the direction perpendicular to the
medium advance 22 but also in the case where the distance between
the end printing nozzle 20a of jetting module 18a and the nearest
beginning printing nozzle 20a of jetting module 18b is less then a
nominal distance associated with a correct registration.
[0057] The type of misalignment and, consequently, the type of
artifact produced by the misalignment, determines how the stitching
algorithm corrects for the artifact. In stage 64 of FIG. 4, the
target is analyzed and a determination is made as to whether there
is any horizontal misalignment of the jetting module perpendicular
to the paper transport direction, and further--if there is a
misalignment--whether a gap resulting from such horizontal
misalignment produces a "white line" artifact within the overlap
region(s) of adjacent jetting modules or whether an overlap
resulting from such horizontal misalignment produces a "dark line"
artifact within the overlap region(s) of adjacent jetting modules.
If there is a gap producing a "white line" artifact, the algorithm
first determines whether the gap is greater than one pixel. If
greater than one pixel, in the algorithm application stage 72 the
algorithm utilizes the coarse correction generated in the stage 66
to shift the data through one of the adjacent jetting modules
backward, that is, leftward in the figures, in the page-width
direction to reduce the gap to less than one pixel. Then, or if the
gap was less than one pixel to begin with, the algorithm utilizes
the sub-pixel correction generated in the stage 68 to duplicate, as
shown in the example of FIG. 6B, the existing data (for a raster)
from the printing nozzle 20a in the overlap region of one of the
adjacent jetting modules (module 18a in FIG. 6B), and applies a
proportion of that duplicated data to at least one of the
nonprinting nozzles (nozzle 20b) in the overlap region of the other
adjacent jetting module (module 18b in FIG. 6B). (The "raster" is
defined herein to refer to the column of data printed by one
printing nozzle in the medium or web length direction, that is, in
the medium transport direction.)
[0058] On the other hand, if the analytic stage 64 determines that
there is an overlap resulting from such horizontal misalignment
that produces a "dark line" artifact within the overlap region(s)
of adjacent jetting modules, the algorithm application stage 72
utilizes the coarse correction generated in the stage 68 to shift
the data through one of the adjacent jetting modules forward, that
is, rightward in the figures, in the page-width direction to
generate at least one more nonprinting nozzle in the overlap region
of the jetting module wherein the "dark line" artifact occurs (thus
creating a "white line" artifact corresponding to a gap of less
than one pixel), and then utilizes the sub-pixel correction
generated in stage 68 to duplicate existing data (raster) from a
printing nozzle in the overlap region of the adjacent jetting
module and print at least some proportion of the duplicated data to
the newly generated nonprinting nozzle in the overlap region. Then,
the processor 36 operates the line head whereby the method controls
which jetting module prints data in the overlap region given that
any shifting of data needs to occur.
[0059] In summary, and recalling that FIG. 6A shows a horizontal
misalignment causing a "white line" artifact, where there is a
misregistration causing the first and last printing nozzles 20a of
adjacent jetting modules in the overlap region to be too far apart,
FIG. 6B shows a nominally non-printing nozzle 20b in jetting module
18b filling in the "white line" artifact by printing a percentage
of a raster that is duplicated from data applied to the last
printing nozzle 20a in jetting module 18a. Recalling that FIG. 7A
shows a horizontal misalignment causing a "dark line" artifact,
where there is a misregistration causing the first and last
printing nozzles 20a of adjacent jetting modules in the overlap
region to be too close together--even to the point of overlapping
each other, FIG. 7B shows all data directed to jetting module 18b
shifted to the right by one nozzle to eliminate the overlap or
closeness of the first and last printing nozzles 20a of adjacent
jetting modules 18a and 18b in the overlap region. (If the "dark
line" artifact is caused by print nozzle overlap greater than two
pixels, then it may be necessary to shift data by more than one
pixel.) As also shown in FIG. 7B, this shifting of data causes the
"dark line" artifact to effectively become a "white line" artifact,
that is, by creating--due to the shift--a non-printing nozzle 20b
in the overlap region of jetting module 18b. As a consequence, and
as shown in FIG. 7C, the newly created non-printing nozzle 20b in
jetting module 18b fills in the "white line" artifact by printing a
percentage of a raster that is duplicated from data directed to the
last printing nozzle 20a in jetting module 18a.
[0060] In each case where there has been a misalignment, and where
the method of correction calls for duplication, the spacing between
the first and last printing nozzles of the adjacent jetting modules
will be some proportion of the nominal spacing of perfectly aligned
nozzles (as shown in FIG. 5). Where the spacing is exactly double
the nominal spacing, that is, where the "white line" corresponds
exactly to a single missing raster, then the nominally non-printing
nozzle 20b in jetting module 18b fills in the "white line" artifact
by printing 100% of a raster that is duplicated from data directed
to the last printing nozzle 20a in jetting module 18a. However,
where the spacing is only a proportion of the nominal spacing, that
is, where the "white line" corresponds to less than all of the data
from a single raster, then the nominally non-printing nozzle 20b in
jetting module 18b fills in the "white line" artifact by printing a
proportion of a raster that is duplicated from data directed to the
last printing nozzle 20a in jetting module 18a.
[0061] A proportional correction is made by printing a percentage
of a duplicated raster. Recalling that a raster is defined herein
to refer to the column of data printed by one printing nozzle along
the medium or web length direction, that is, along the medium
transport direction, one way of printing a percentage of a
duplicated raster is to apply a mask to the data representing the
raster, where the mask determines which data in the raster will be
printed. FIG. 8A shows an example of such a technique using a
1.times.10 ordered dither mask (which would be repeated the length
of the raster). As shown, this technique provides 11 percentage
levels to choose from, that is, 100%, 90%, 80%, 70%, 60%, 50%, 40%,
30%, 20%, 10% and 0%. Where the pixel mask is black for a selected
percentage, the raster data is printed and where the pixel mask is
white for the selected percentage, the raster data is not printed.
In practice, referring to FIG. 4, a mask is selected in the
sub-pixel correction stage 68 based on the measured sub-pixel
distance between the last printing nozzle 20a of the module 18a and
the first printing nozzle of the module 18b (after shift
correction), and the processor 36 applies the mask to the data. An
example of this technique is shown in FIG. 8B for the selection of
an 80% mask, where the mask only allows 8 out of 10 pixels to be
printed. The mask is replicated by the processor 36 for the whole
raster (shown for purpose of illustration only as four replications
in the second column B in FIG. 8B) and applied pixel by pixel to
the duplicated raster data in the first column A, resulting in 80%
of the original raster data in the third column C. The resultant
print data is then applied to an appropriate jetting module 18 in
the line head 14.
[0062] For simplicity and clarity, the proportional correction is
described herein in terms of the 1.times.10 ordered dither mask,
shown in FIG. 8A. The invention is not limited to the use of that
particular ordered dither mask. For example, masks having different
pixel arrangements with similar coverage levels can be employed.
Similarly, masks of different sizes, such as 1.times.15 or
1.times.20 can be employed. Further, a random masking process can
be employed, where, for example, on a pixel by pixel basis the
output of a random number generator is compared to the desired
percentage value to determine whether the duplicated pixel data is
to be printed.
[0063] In the technique employed in FIGS. 8A and 8B, a selected
mask is replicated and applied pixel by pixel by the processor 36
to the duplicated raster data. In an alternative application of
this concept, a threshold dither algorithm is instead applied by
the processor 36 to the duplicated raster data. For example, the
effect obtained by the individual 1.times.10 ordered masks shown in
FIG. 8A can be combined into a threshold dither kernel, as
follows,
[0064] 1.times.10 dithered kernel=[10 80 40 100 20 70 50 90 30
60]
where the threshold value is defined by the amount of correction
desired. Each value in the kernel represents a threshold in terms
of a percentage, and a given pixel in the raster will print if its
pixel position in the raster corresponds to a kernel percentage
less than or equal to the selected percentage. In effect, this
represents all the masks of FIG. 8A collapsed into a single
threshold application. For example, for an 80% threshold all
positions would print except the 4.sup.th (100%) and 8.sup.th (90%)
positions--which is equivalent to the 80% mask illustrated in FIGS.
8A and 8B. As before, the usage of the threshold dither kernal is
then repeated for the whole raster.
[0065] In the technique employed in FIGS. 8A and 8B, the selected
mask is applied pixel by pixel by the processor 36 to the
duplicated raster data whether or not a given pixel in the raster
represents print drop data or non-print data. Since the purpose of
the mask is to determine which data in the raster will be printed,
in the alternative embodiment of proportional correction described
above, the threshold dither algorithm is applied only to the pixels
that actually contain print drop data. This alternative method is
expressed in the flow diagram shown in FIG. 9A and in the example
shown in FIG. 9B for a 60% correction. The method shown in FIG. 9A
involves (as shown in counting stage 80) the processor 36 scanning
down the duplicated raster (column A of FIG. 9B) and counting only
the pixels that contain print data (cycling in blocks of 10). In
the example shown in FIG. 9B, the outcome of the counting stage 80
is shown in the second column B for duplicated raster data
equivalent to 19 potential ink drop locations. As shown, only 14 of
these locations contain ink drop data and are therefore counted.
Next, the output of the counting stage 80 is used as an index into
a dither kernel (which occurs in index stage 82, and in the second
column B in FIG. 9B, where each arrow 82a indicates a counted
pixel). With the pixels thus identified, a correction threshold is
applied (as shown in threshold stage 84) based on the values in the
dither kernel. If the value from the dither kernel is greater than
the desired correction amount, then the print data is removed for
that pixel location (as shown in elimination stage 86). This is
shown in the third column C of FIG. 9B for a 60% example, where the
arrows 86a indicate remaining data that is not eliminated.
[0066] In the correction process illustrated in FIGS. 9A and 9B, as
well as FIGS. 8A and 8B, there remains the issue of determining the
correct dither matrix, or the correct mask, that is, the desired
amount of stitch error correction. The desired stitch error
correction is determined either in an process involving the
application of different masks or dither kernels together with the
visual or automatic evaluation of a target image or, as will be
explained, a correlation process of correlating actual misalignment
to an optimum correction level. The first way of determining a
desired stitch error correction is to have the printer 10 print a
target in the overlap region 24 for each pair of staggered,
overlapped jetting modules 18. The processor 36 processes the data
such that the target contains varying degrees of correction by
using different percentage masks, or different threshold kernel
values.
[0067] An exemplary target print is shown in FIG. 10 for masks, or
thresholds, producing a range of correction amounts--including 20%,
40%, 60%, 80% and 100% print drop. Then, the correction level is
picked where the stitching artifact is least noticeable, and this
will constitute the desired stitch error correction. For example,
in FIG. 10 the stitching artifact appears as a "white line"
artifact 2a for 20, 40, and 60% corrections or a dark line artifact
2b for a 100% correction, and the artifact--for this example--is
least noticeable for a correction corresponding to an 80% print
drop. Accordingly, the mask or kernal value providing the least
noticeable rendition--80% in this example--is selected for the
stitching algorithm 58 (FIG. 3). The appropriate correction level
can be detected by a variety of methods, including without
limitation visual inspection of the target; and an automated streak
(band or gap) detection system.
[0068] In addition, The appropriate correction level can be
detected by correlation of a correction factor with a known degree
of jetting module misalignment, where the physical distance
measurements determines the correction amount based on historical
data. In practicing this method of detection, and in reference to
FIG. 4, groups of nozzles in or near the overlap regions in each of
the adjacent, overlapping jetting modules are designated to print a
pattern of patches 53a and 53b that are separated by a measurable
gap 54a overlying the stitch joint 55. (The historical data
predetermines a nominal size for this gap that would indicate
substantially perfect alignment between the jetting modules 18a and
18b, thus not requiring a stitch error correction.) Next, the
target 54 is printed on the print medium 16 by activating the
designated nozzles in the predetermined groups for each of the
overlap regions 24. Then, after images of the patches 53a and 53b
on the print medium 16 are captured by the stitching cameras 52,
the gap 54a is measured by the stitching camera processing system
56. The actual gap measurement--which typically will deviate from
the nominal gap length for perfect module-to-module alignment--can
then be referenced to a table (not shown, but it may be stored in
the memory 42 for use by the stitching camera processing system 56)
that will indicate the amount of correction, based on knowing the
nominal size for this gap, needed to eliminate the stitch error. As
described earlier in this application, this correction may involve
(a) shifting of print data, either forward or backward, and/or (b)
masking of duplicated print data.
[0069] It should be apparent that correlation of a correction
factor with a known degree of jetting module misalignment may take
a number of variations from the procedure described above. For
instance, it has been found useful to activate the nozzles in the
predetermined groups for each of the overlap regions 24 so as to
produce separated blocks of printed output, e.g., 5.times.5 sized
blocks of print (pixel) data, that are strung out in the web length
direction. Then, groups of blocks can be examined at one time. In
one embodiment, the centroid of each block, or averaged sets of
blocks, is calculated for blocks on either side of the stitch joint
55 and the gap between the centroids is compared with a centroid
spacing predetermined from historical data for a nominal spacing
that would produce perfect alignment between the jetting modules
18a.
[0070] The advantage of the technique illustrated in FIGS. 9A and
9B include more uniform distribution of data in the highlights and
more uniform correction throughout the tonescale of the image being
printed. However, the target illustrated in FIG. 10 shows an
optimum correction that is substantially consistent across several
(4) density levels, that is, where the same correction is equally
effective for each of the density levels in the printed output.
However, the same correction level may not be suitable for all
density levels, e.g., darker regions may need more correction than
highlight regions. Thus it may be desirable to make the correction
amount dynamic, meaning that the correction amount should be
capable of changing dependent on certain aspects of the image data,
such as (without limitation) the print density of the image data.
In the preferred embodiment, a dynamic correction amount is
provided by applying an additional threshold or modification of the
printing mask during the production run. For example, as shown in
FIG. 4, in the dynamic adjustment stage 74 the masking amount for
each stitch point is dynamically adjusted based on the image
content (print density compensation).
[0071] FIG. 11 shows a flow diagram for generating a desired stitch
error correction amount that is dependent on (at least one of) the
print density of the image data and the web speed of the printer.
The process begins (in target measurement and nominal masking stage
100) with the measurement of the stitching target 54 and the
determination of a nominal stitch masking amount (as calculated
according to FIGS. 9A and 9B, or as just described in relation to
the target patches 53a and 53b) based on target measurements.
Although the nominal stitch masking amount provides a nominal
correction at each of the density levels based on measurements of
the target, e.g., the gap 54a between the target patches 53a and
53b, the nominal stitch masking amount may need to be adjusted
based on the web speed of the printer 10 because the gap will
become larger at higher speeds. Accordingly, in the masking
adjustment stage 101 (which may take the form of a look up table)
the web speed 101a and the nominal mask from the previous stage 100
are taken as inputs and an adjusted mask is provided as an output.
In one example, which is provided as an illustration without
limitation, a slow speed may not require any adjustment while a
medium speed would require a 10% adjustment to the nominal mask and
a high speed would require a 20% adjustment to the nominal mask. In
each case, the adjustment is an addition to the nominal masking
amount such that, e.g., a 60% nominal mask should become a 70% mask
for medium speeds and an 80% mask for high speeds--where more of
the raster data is printed as the percentage increases.
[0072] In raster access stage 102, one or more rasters of print
data are obtained pertaining to pixels in the overlap region(s)
near a stitch juncture. (As set forth earlier in this application,
each raster of print data comprises a column of data printed by one
printing nozzle in the page-length direction, that is,
perpendicular to the page-width direction.) In some cases, such as
darker regions that require more correction than highlight regions,
more than one raster may be required for the correction. Then (in
gray level stage 104), a pseudo-gray level is calculated for
individual pixels in the one or more rasters by processing the
print data from a neighborhood of pixels within the one or more
rasters including the individual pixel. Using the pseudo-gray level
and the desired stitch error correction, a determination is made
(in stitch error stage 106) as to a proper stitch error correction
modification related to print density. Finally, the stitch error
correction amount related to print density is used (in modification
stage 108) to further modify, e.g., by thresholding, the masking
amount applied to individual pixels.
[0073] In effect, the output of the processing stages shown, e.g.,
in FIG. 9A, which is produced in a set up procedure, is modified by
the output of the processing stages shown in FIG. 11, which is
produced during a production run. Thus, the offline set-up
correction is dynamically modified by (at least one of) the density
and web speed adjustments.
[0074] FIGS. 12-16 represent a preferred embodiment of a process
for obtaining and applying a stitch error correction amount related
to print density. The process identified in raster access stage 102
of FIG. 11 is accomplished as shown in FIG. 12, where several
rasters of print data are obtained pertaining to pixels in the
overlap region(s) near a stitch juncture. More specifically, raster
R0 in jetting module 18b duplicates data from the second last
nozzle of the jetting module 18a and raster R1 in jetting module
18b duplicates data from the last nozzle of the jetting module 18a.
Raster R2 represents the data from the first nozzle in the jetting
module 18b. As shown, a percentage of the duplicated data from the
second last and the last nozzles of the jetting module 18a is used
to print from the third and fourth nozzles of the jetting module
18b in the overlap region where the stitch juncture occurs. The
data duplicated into the rasters R0 and R1 will be used, as
follows, in the determination of the stitch error correction needed
to account for variation in print density.
[0075] The process identified in gray level stage 104 of FIG. 11 is
accomplished as shown in FIG. 13, wherein print data within a
neighborhood of pixels represented by a window 110 is averaged to
determine, for example, a pseudo-gray level 112 appropriate for
corresponding pixels 114 and 116 in rasters R0 and R1,
respectively. The window is then advanced along the rasters one
pixel at a time in a direction 118 to determine pseudo-gray levels
for all of the individual pixels in the respective rasters R1 and
R2. Effectively, therefore, the pixel values originally assigned to
the rasters spanning the stitch juncture 55, that is, rasters R1
(including print data from the last nozzle of module 18a) and R2
(including print data from the first nozzle of module 18b), are
blurred to obtain the pseudo-gray levels. These pseudo-gray levels
are then used to obtain blurred gray level values corresponding to
the print data in both the duplicated rasters, that is, rasters R0
and R1.
[0076] The process identified in stitch error stage 106 of FIG. 11
is accomplished as shown in FIGS. 14 and 15, in which an indexing
process uses the pseudo-gray level associated with each printed
pixel and the desired stitch correction amount to determine a
variable correction amount related to print density. This is best
understood by considering an example. For instance, and as shown in
FIG. 15, if the desired stitch correction amount is 80%, pixels
with a pseudo-gray level of 70% (e.g., pixels 120 and 122 in FIG.
14) will receive a variable correction amount of 20% if they are in
raster R0 (pixel 120) and 100% if they are in raster R1 (pixels
122). These numbers are obtained by first indexing into the
StitchTableLUT table shown in FIG. 15 using the desired stitch
correction amount (e.g., 80%) and the pseudo-gray level (e.g.,
70%), and then using the StitchIndex result (60, in this example)
from the StitchTableLUT to index into the VariableCorrection table
to obtain 20% for these pixels in raster R0 and 100% for these
pixels in raster R1.
[0077] The StitchTableLUT and VariableCorrection tables are built
based upon historical data, i.e., data showing correlation of a
correction factor with a known degree of jetting module
misalignment, where an alignment target is printed on the print
medium 16 and measured as described earlier in connection with FIG.
3. For example, the desired correction may come from the fractional
pixel alignment that is measured (i.e., an 80% correction would
correspond to 80% of a pixel, so at 600 dpi that would equate to
33.9 microns) The ideal correction amount for each gray level is
recorded in the table for this error amount (e.g., row 122 for an
80% correction).
[0078] The process identified in modification stage 108 of FIG. 11
is accomplished as shown in FIG. 16, in which the variable
correction matrix (amount) 124 derived from FIGS. 14 and 15 is used
to threshold the threshold dither matrix 126 in order to determine
which print data should be removed. More specifically, and as shown
for a desired stitch correction of 80%, a correction threshold is
applied based on the values in the variable correction matrix 124
to the threshold dither matrix 126 for the rasters R0 and R1. If
the value from the threshold dither matrix 126 is greater than the
value from the variable correction matrix 124, then the print data
is removed for that pixel location. For example, for raster R0 only
the pixel location 128 associated with a threshold matrix value of
20% and a variable correction of 20% is preserved as a print pixel
because for all other values the value from the threshold dither
matrix 126 is greater than the value from the variable correction
matrix 124. For raster R1 the opposite condition prevails, because
for all other values the value from the threshold dither matrix 126
is less than the value from the variable correction matrix 124.
[0079] It should be understood that FIGS. 12-16 represent one way,
albeit presently preferred, for obtaining a desired stitch error
correction amount that is dependent on the print density of the
image data. Others techniques for accomplishing these results, and
obtaining a desired stitch error correction amount that is
dependent on the print density of the image data, would be apparent
to one of ordinary skill in this art and are believed to fall
within the claims in this application.
[0080] FIG. 17 shows a portion of a carriage-type, drop-on-demand
inkjet printer that could be adapted to incorporate the systems and
methods of the invention. Such a carriage-type printer typically
has a relatively small printhead 120 containing the ink channels
and nozzles. The printhead 120 can be attached to a disposable ink
supply cartridge (not shown). The combined printhead and cartridge
assembly is attached to a carriage 122 that is reciprocated along a
guide rod 124 in a first direction A and a second direction
opposite the first direction A to print one swath of information at
a time on a stationary receiving medium 126, such as paper or a
transparency, supported on a platen 128. A sensor 130 detects marks
132 located on the platen 128, concerning the amount of rotation of
the platen 128, which is output to the controller 134.
[0081] After a swath is printed, and based on information from the
sensor 130 and the controller 134, the receiving medium 126 is
stepped a distance at most equal to the height of a printed swath
140a so that the next printed swath 140b is contiguous or overlaps
with the previously printed swath. This procedure is repeated until
the entire image is printed. Stitch joint error can be, for
example, the result of a gap 142 between the drop of one die or
swath 140a adjacent the stitch joint and the drop of an adjoining
swath 140b or die adjacent the stitch joint. The gap 142 is usually
caused by difficulties in producing adjacent swaths close enough
together to mask this apparent error. Although intended for
continuous inkjet technology, the systems and methods of the
invention could be adapted by one of ordinary skill in this art to
provide a stitch masking algorithm to correct for such gaps
produced by drop-on-demand printers.
[0082] The stitching algorithm 58 of the present invention has been
described as being applied to stitching between jetting modules 18
that include nozzles 20 having a constant and identical nozzle to
nozzle spacing. However, the stitching algorithm 58 of the present
invention can also be applied with similar results to stitching
between jetting modules 18 that include nozzles 20 having differing
nozzle to nozzle spacing or jetting modules 18 that include nozzles
20 where the nozzle to nozzle spacing is different only in the
overlap region. Likewise, the stitching algorithm 58 of the present
invention can also be applied to stitching between jetting modules
18 where the nozzles 20 have constant and identical nozzle to
nozzle spacing but the drop spacing in the overlap region varies
due to the use of a drop deflection mechanism such as differential
nozzle bore heating or air deflection.
[0083] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the scope of the invention.
PARTS LIST
[0084] Jetting module 2 [0085] Juncture 2a [0086] Line head 4
[0087] Nozzles 6 [0088] "White line" artifact 8a [0089] "Dark line"
artifact 8b [0090] Printer 10 [0091] Housing 12 [0092] Line head 14
[0093] Receiver medium 16 [0094] Jetting modules 18 [0095] Nozzles
20 [0096] Printing nozzles 20a [0097] Nonprinting nozzles 20b
[0098] Medium transport direction 22 [0099] Overlap region 24
[0100] Medium advance 30 [0101] Motor 32 [0102] Pinch rollers 34
[0103] Processor 36 [0104] User input system 38 [0105] Sensors 40
[0106] Memory 42 [0107] Display 44 [0108] Remote memory system 50
[0109] Stitching camera 52 [0110] Patch 53a [0111] Patch 53b [0112]
Stitching target 54 [0113] Gap 54a 54a [0114] Stitch joint 55
[0115] Stitching camera processing system 56 [0116] Stitching
algorithm 58 [0117] Printing stage 60 [0118] Capture stage 62
[0119] Analytic stage 64 [0120] Coarse correction stage 66 [0121]
Sub-pixel correction stage 68 [0122] Correction transfer stage 70
[0123] Algorithm application stage 72 [0124] Dynamic adjustment
stage 74 [0125] Verification stage 76 [0126] Counting stage 80
[0127] Indexing stage 82 [0128] Threshold stage 84 [0129]
Elimination stage 86 [0130] target measurement and nominal masking
stage 100 [0131] masking adjustment stage 101 [0132] web speed 101a
[0133] Raster access stage 102 [0134] Gray level stage 104 [0135]
Stitch error stage 106 [0136] Modification stage 108 [0137]
Printhead 120 [0138] Carriage 122 [0139] Guide rod 124 [0140]
Medium 126 [0141] Platen 128 [0142] Sensor 130 [0143] Marks 132
[0144] Controller 134 [0145] First swath 140a [0146] Second swath
140b [0147] Gap 142
* * * * *