U.S. patent application number 14/447680 was filed with the patent office on 2016-02-04 for controlling an electrophotographic printer using an image region database.
The applicant listed for this patent is James A. Katerberg, Cumar Sreekumar. Invention is credited to James A. Katerberg, Cumar Sreekumar.
Application Number | 20160033916 14/447680 |
Document ID | / |
Family ID | 55174911 |
Filed Date | 2016-02-04 |
United States Patent
Application |
20160033916 |
Kind Code |
A1 |
Sreekumar; Cumar ; et
al. |
February 4, 2016 |
CONTROLLING AN ELECTROPHOTOGRAPHIC PRINTER USING AN IMAGE REGION
DATABASE
Abstract
A method for controlling an electrophotographic printing system
to print digital image data for an image region. An image region
database stores data characterizing a plurality of reference image
regions, each reference image region having one or more associated
system control parameters that are appropriate for use in printing
the reference image region. The system control parameters can
include parameters related to correcting color registration errors.
The image region to be printed is compared with the reference image
regions in the image region database, and a similar reference image
region is selected. The image region is printed using the system
control parameters associated with the selected similar reference
image region.
Inventors: |
Sreekumar; Cumar; (Penfield,
NY) ; Katerberg; James A.; (Kettering, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sreekumar; Cumar
Katerberg; James A. |
Penfield
Kettering |
NY
OH |
US
US |
|
|
Family ID: |
55174911 |
Appl. No.: |
14/447680 |
Filed: |
July 31, 2014 |
Current U.S.
Class: |
347/1 |
Current CPC
Class: |
B41J 2/2146 20130101;
H04N 1/603 20130101; G03G 15/238 20130101; G03G 15/55 20130101;
B41J 2/01 20130101; G03G 2215/0161 20130101; G03G 15/5062 20130101;
G03G 2215/0141 20130101; B41J 29/38 20130101; B41J 2/2135 20130101;
G03G 15/6517 20130101; B41J 2002/012 20130101; G03G 2215/0158
20130101; G03G 2215/00455 20130101 |
International
Class: |
B41J 2/01 20060101
B41J002/01 |
Claims
1. A method for controlling a digital printing system, comprising:
receiving digital image data for an image region to be printed by
the digital printing system; designating an image region database
stored in a processor-accessible memory including data
characterizing a plurality of reference image regions, each
reference image region having one or more associated system control
parameters that are appropriate for use in printing the reference
image region on the digital printing system; using a data
processing system to compare the image region to be printed with
the reference image regions in the image region database and select
a reference image region that is sufficiently similar to the image
region to be printed; and using the digital printing system to
print the received digital image data for the image region by
applying one or more colorants onto a print medium, wherein the
digital printing system includes an electrophotographic printing
module that is controlled using the system control parameters
associated with the selected similar reference image region during
the printing of at least a subset of the image region.
2. The method of claim 1 wherein the data characterizing a
reference image region includes a parametric representation of the
reference image region.
3. The method of claim 2 wherein the determination of the
parametric representation includes: determining an ink coverage
profile representing a distribution of ink within the reference
image region; and representing the ink coverage profile using a
series expansion based on a set of orthogonal basis functions;
wherein coefficients determined for a plurality of the basis
functions are designated to be parameters for the parametric
representation.
4. The method of claim 1 wherein the data characterizing a
reference image region includes digital image data for the
reference image region, the digital image data including pixel
values for an array of image pixels.
5. The method of claim 4 wherein the comparison of the image region
to be printed with a particular reference image region includes
computing a difference image representing a difference between
digital image data for the image region to be printed and the
digital image data for the particular reference image region.
6. The method of claim 5 further including determining a difference
metric by performing a statistical analysis on the difference
image.
7. The method of claim 5 wherein the difference image has a lower
spatial resolution than the image region to be printed.
8. The method of claim 7 wherein the reference image regions in the
image region database are stored at the lower spatial resolution,
and wherein the digital image data for the image region to be
printed is down-sample to the lower spatial resolution before the
difference image is computed.
9. The method of claim 1 wherein difference parameters are
determined representing differences between the reference image
regions and the image region to be printed.
10. The method of claim 9 wherein the reference image region having
the smallest corresponding difference parameter is designated to be
the selected similar reference image region.
11. The method of claim 9 wherein the selected reference image
region is designated to be sufficiently similar to the image region
to be printed if the difference parameter is smaller than a
predefined difference threshold.
12. The method of claim 1 wherein one or more system configuration
parameters are associated with the reference image regions in the
image region database, and wherein the selection of the similar
reference image region includes comparing the system configuration
parameters associated with the reference image regions to system
configuration parameters to be used when the received digital image
data for the image region is printed.
13. The method of claim 1 wherein the reference image regions in
the image region database are selected to be representative of
different types of image regions that are commonly printed.
14. The method of claim 1 wherein the image region to be printed is
added to the image region database as a new reference digital
image.
15. The method of claim 14 wherein the image region to be printed
is added to the image region database if it is determined to be
sufficiently different from all of the reference image regions in
the image region database.
16. The method of claim 1 wherein the digital printing system is a
color printing system that prints images having a plurality of
color channels, and wherein the system control parameters include
one or more color registration parameters that are used to control
the registration between two or more different color channels.
17. The method of claim 16 wherein the color registration
parameters include an in-track shift value, a cross-track shift
value, an in-track magnification factor, a cross-track
magnification factor, or an image rotation value.
18. The method of claim 1 wherein the digital printing system is a
color printing system that prints images having a plurality of
color channels, and wherein the system control parameters include a
color adjustment parameter that is used to adjust color
reproduction characteristics.
19. The method of claim 1 wherein the digital printing system has a
plurality of different print modes, and wherein the system control
parameters include a print mode selection parameter that is used to
select between the different print modes.
20. The method of claim 1 wherein the digital printing system
includes a media de-curling subsystem for reducing curl in the
print medium, and wherein the system control parameters include a
de-curling subsystem control parameter that is used to control a
setting on the de-curling subsystem.
21. The method of claim 1 wherein the electrophotographic printing
module includes an electrophotographic print engine, and wherein
the system control parameters include one or more
electrophotographic print engine control parameters that are used
to control a setting on the electrophotographic print engine.
22. The method of claim 1 wherein the processor-accessible memory
used to store the image region database and the data processing
system used to compare the image region to be printed with the
reference image regions in the image region database are at a
different location than the digital printing system.
23. The method of claim 1 wherein the processor-accessible memory
used to store the image region database and the data processing
system used to compare the image region to be printed with the
reference image regions in the image region database are components
of the digital printing system.
24. The method of claim 1 wherein the digital printing system
prints on individual sheets of print medium, and wherein the image
region to be printed corresponds to a single sheet of print
medium.
25. The method of claim 1 wherein the digital printing system
prints on a continuous web of print medium.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Reference is made to commonly assigned, co-pending U.S.
patent application Ser. No. ______ (Docket K001459), entitled:
"Improving document registration using registration error model",
by R. Armbruster et al.; to commonly assigned, co-pending U.S.
patent application Ser. No. ______ (Docket K001460), entitled:
"Reducing registration errors using registration error model", by
T. Wozniak et al.; to commonly assigned, co-pending U.S. patent
application Seri. No. ______ (K001512), entitled: "Controlling a
printer using an image region database", by C. Sreekumar et al.; to
commonly assigned, co-pending U.S. patent application Ser. No.
______ (K001529), entitled: "Controlling a web-fed printer using an
image region database", by C. Sreekumar et al., each of which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention generally relates to a digital inkjet
printing system, and more particularly to performing color-to-color
registration correction.
BACKGROUND OF THE INVENTION
[0003] In a digitally controlled printing system, a print medium is
directed through a series of components. The print medium can be in
the form of cut sheets or a continuous web. As the print medium
moves through the printing system, colorant, for example, ink, is
applied to the print medium by one or more printing stations. In
the case of an ink jet printer, the colorant is a liquid ink, and
the printing process is commonly referred to as jetting of the
ink.
[0004] In commercial inkjet printing systems, the print medium is
physically transported through the printing system at a high rate
of speed. For example, the print medium can travel 650 to 1000 feet
per minute. Inkjet lineheads in commercial inkjet printing systems
typically include multiple printheads that jet ink onto the print
medium as the print medium is being physically moved through the
printing system. A reservoir containing ink or some other material
is usually behind each nozzle plate in a linehead. The ink streams
through the nozzles in the nozzle plates when the reservoirs are
pressurized.
[0005] The printheads in each linehead in commercial printing
systems typically jet only one color. Thus, there is a linehead for
each colored ink when different colored inks are used to print
content. For example, there are four lineheads in printing systems
using cyan, magenta, yellow and black colored inks. The content is
printed by jetting the colored inks sequentially, and each colored
ink deposited on the print medium is known as a color plane. The
color planes need to be aligned (i.e., "registered" with each
other) so that the overlapping ink colors produce a quality single
image.
[0006] Color registration errors can be classified into different
types. Examples of color registration errors include, but are not
limited to, a color plane having a linear translation with respect
to another color plane, a color plane being rotated with respect to
another color plane, and a color plane being stretched, contracted,
or both stretched and contracted in different regions or in
different directions with respect to another color plane.
[0007] There are several variables that contribute to the
registration errors in color plane alignment including physical
properties of the print medium, conveyance of print medium, ink
application system, ink coverage, and drying of ink. Color
registration errors are typically managed by controlling these
variables. However, controlling these variables can often restrict
the range of desired print applications. For example,
color-to-color registration errors will typically become larger as
paper weight for the print application is reduced, when ink
coverage is increased, or when the amount of ink coverage is more
variable for successive documents. These limitations compromise the
range of suitable applications for inkjet printing systems.
[0008] There remains a need for improved methods to reduce color
registration errors in digital printing systems.
SUMMARY OF THE INVENTION
[0009] The present invention represents a method for controlling a
digital printing system, comprising:
[0010] receiving digital image data for an image region to be
printed by the digital printing system;
[0011] designating an image region database stored in a
processor-accessible memory including data characterizing a
plurality of reference image regions, each reference image region
having one or more associated system control parameters that are
appropriate for use in printing the reference image region on the
digital printing system;
[0012] using a data processing system to compare the image region
to be printed with the reference image regions in the image region
database and select a reference image region that is sufficiently
similar to the image region to be printed; and
[0013] using the digital printing system to print the received
digital image data for the image region by applying one or more
colorants onto a print medium, wherein the digital printing system
includes an electrophotographic printing module that is controlled
using the system control parameters associated with the selected
similar reference image region during the printing of at least a
subset of the image region.
[0014] This invention has the advantage that performance of an
electrophotographic printing system can be improved by creating an
image region database containing a library of previously printed
image regions for which appropriate system control parameters have
been determined. The system control parameters can include
registration correction parameters for correcting color-to-color
registration errors.
[0015] It has the additional advantage that color registration
errors can be reduced the first time that a print job is printed
without needing to measure actual registration errors in a first
copy of a print job so that they can be corrected in subsequent
copies.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic of a continuous-web inkjet printing
system;
[0017] FIG. 2 is a schematic showing additional details for a
portion of the printing system of FIG. 1;
[0018] FIG. 3 illustrates a print job including a number of
documents;
[0019] FIG. 4 illustrates a color registration error produced by
the translation of one color plane relative to another color
plane;
[0020] FIG. 5 illustrates a color registration error produced by
the contraction or expansion of one color plane relative to another
color plane;
[0021] FIG. 6 illustrates a color registration error produced by
the rotation of one color plane relative to another color
plane;
[0022] FIG. 7 is a flowchart illustrating a method for correction
color registration errors in accordance with the present
invention;
[0023] FIG. 8A shows a portion of an exemplary print job including
two documents;
[0024] FIGS. 8B and 8C show ink coverage profiles corresponding to
the documents of FIG. 8A;
[0025] FIG. 9A shows a portion of an exemplary print job including
two documents;
[0026] FIGS. 9B and 9C show ink coverage profiles corresponding to
the documents of FIG. 9A;
[0027] FIG. 10 illustrates the first several Legendre
polynomials;
[0028] FIG. 11 is a flowchart illustrating a method for correction
color registration errors in accordance with an alternate
embodiment;
[0029] FIGS. 12A and 12B illustrate exemplary image regions for use
with the method of FIG. 11.
[0030] FIG. 13 is a flowchart illustrating a method for determining
system control parameters for a printing system using an image
region database;
[0031] FIG. 14 is a schematic of an electrophotographic printer
suitable for use with various embodiments; and
[0032] FIG. 15 is a schematic showing additional details for one
image forming module of the electrophotographic printer of FIG.
14;
[0033] It is to be understood that the attached drawings are for
purposes of illustrating the concepts of the invention and may not
be to scale. Identical reference numerals have been used, where
possible, to designate identical features that are common to the
figures.
DETAILED DESCRIPTION OF THE INVENTION
[0034] The present description will be directed in particular to
elements forming part of, or cooperating more directly with,
apparatus in accordance with the present invention. It is to be
understood that elements not specifically shown or described may
take various forms well known to those skilled in the art.
[0035] The invention is inclusive of combinations of the
embodiments described herein. References to "a particular
embodiment" and the like refer to features that are present in at
least one embodiment of the invention. Separate references to "an
embodiment" or "particular embodiments" or the like do not
necessarily refer to the same embodiment or embodiments; however,
such embodiments are not mutually exclusive, unless so indicated or
as are readily apparent to one of skill in the art. The use of
singular or plural in referring to the "method" or "methods" and
the like is not limiting. It should be noted that, unless otherwise
explicitly noted or required by context, the word "or" is used in
this disclosure in a non-exclusive sense.
[0036] Where they are used, terms such as "first", "second", and so
on, do not necessarily denote any ordinal or priority relation, but
are simply used to more clearly distinguish one element from
another.
[0037] The present invention is well-suited for use in roll-fed
inkjet printing systems that apply colorant (e.g., ink) to a web of
continuously moving print media. In such systems a printhead
selectively moistens at least some portion of the media as it moves
through the printing system, but without the need to make contact
with the print media. While the present invention will be described
within the context of a roll-fed inkjet printing system, it will be
obvious to one skilled in the art that it could also be used for
other types of printing systems as well.
[0038] In the context of the present invention, the terms "web
media" or "continuous web of media" are interchangeable and relate
to a media that is in the form of a continuous strip of media as it
passes through the web media transport system from an entrance to
an exit thereof. The continuous web media serves as the receiver
medium (e.g., print media) to which one or more colorants (e.g.,
inks or toners), or other coating liquids are applied. This is
distinguished from various types of "continuous webs" or "belts"
that are transport system components (as compared to the image
receiving media) which are typically used to transport a cut sheet
medium in an electrophotographic or other printing system.
[0039] The example aspects of the present invention are illustrated
schematically and not necessarily to scale for the sake of clarity.
One of ordinary skill in the art will be able to readily determine
the specific size and interconnections of the elements of the
example aspects of the present invention.
[0040] As described herein, exemplary aspects of the present
invention are applied to color plane registration in inkjet
printing systems. The example aspects of the present invention are
also applied to the registration or stitching of print swaths of
the individual printheads that are aligned relative to each other
in a linehead. For simplicity, the term registration shall be
applied both the registration of print swaths printed by the
printheads within a linehead and to the registration of color
planes printed by different lineheads.
[0041] Inkjet printing is commonly used for printing on paper.
However, printing can occur on any substrate or receiving medium.
For example, vinyl sheets, plastic sheets, glass plates, textiles,
paperboard, and corrugated cardboard can comprise the print medium.
In addition to conventional inkjet printing, many applications are
emerging which use inkjet printheads or similar nozzle arrays to
emit fluids (other than inks) that need to be finely metered and
deposited with high spatial precision. Such liquids include inks,
both water based and solvent based, that include one or more dyes
or pigments. These liquids also include various substrate coatings
and treatments, various medicinal materials, and functional
materials useful for forming, for example, various circuitry
components or structural components. In addition, a nozzle array
can jet out gaseous material or other fluids. As such, as described
herein, the terms "liquid", "ink" and "inkjet" refer to any
material that is ejected by a nozzle array. While the invention
will be described in terms of a multi-color printer, it should be
understood that the invention similarly applies to other
applications such as the printing of multiple layers of an
electronic circuit where the individual circuit layers would
correspond to an image plane in the color printer. In such
applications, registration of the individual layers must be
maintained for proper operation of the electronic circuit in a
similar manner to the registration of the color image planes in the
color prints. It is anticipated that many other applications may be
developed in which the invention may be employed to enhance the
registration of the image planes.
[0042] Inkjet printing is a non-contact application of an ink to a
print medium. Typically, one of two types of inkjetting mechanisms
are used and are categorized by technology as either "drop on
demand" inkjet or "continuous inkjet". The first technology,
drop-on-demand inkjet printing, provides ink drops that impact upon
a recording surface using a pressurization actuator, for example, a
thermal, piezoelectric, or electrostatic actuator. One commonly
practiced drop-on-demand technology uses thermal actuation to eject
ink drops from a nozzle. A heater, located at or near the nozzle,
heats the ink sufficiently to boil, forming a vapor bubble that
creates enough internal pressure to eject an ink drop. This form of
inkjet is commonly termed "thermal inkjet."
[0043] The second technology, commonly referred to as continuous
inkjet printing, uses a pressurized ink source to produce a
continuous liquid jet stream of ink by forcing ink, under pressure,
through a nozzle. The stream of ink is perturbed using a drop
forming mechanism such that the liquid jet breaks up into drops of
ink in a predictable manner. One continuous printing technology
uses thermal stimulation of the liquid jet with a heater to form
drops that eventually become print drops and non-print drops.
Printing occurs by selectively deflecting drops so that print drops
reach the print medium and non-print drops are caught by a
collection mechanism. Various approaches for selectively deflecting
drops have been developed including electrostatic deflection, air
deflection, and thermal deflection.
[0044] Additionally, there are typically two types of print media
used with inkjet printing systems. The first type is commonly
referred to as a continuous web of print media, while the second
type is commonly referred to as cut sheets of print media. The
continuous web of print media refers to a continuous strip of print
media, generally originating from a source roll. The continuous web
of print media is moved relative to the inkjet printing system
components via a web transport system, which typically includes
drive rollers, web guide rollers, and web tension sensors. Cut
sheets refer to individual sheets of print media that are moved
relative to the inkjet printing system components via a support
mechanism (e.g., rollers and drive wheels or a conveyor belt
system) that is routed through the inkjet printing system.
[0045] The invention described herein is generally applicable to
both types of printing technologies. As such, the terms linehead
and printhead, as used herein, are intended to be generic and not
specific to either technology. Additionally, the invention
described herein is applicable to both types of print medium. As
such, the terms print medium and web, as used herein, are intended
to be generic and not as specific to one type of print medium or
web or the way in which the print medium or web is moved through
the printing system. Additionally, the terms linehead, printhead,
print medium, and web can be applied to other nontraditional inkjet
applications, such as printing conductors on plastic sheets.
[0046] The terms "color plane" and "image plane" are used
generically and interchangeably herein to refer to a portion of the
data that is used to specify the location of features that are made
by a particular printing station of a digitally controlled printing
system on the print medium. Similarly, "color-to-color
registration" is used generically herein to refer to the
registration of such features that are made by different printing
stations on the print medium. For the color printing of images, the
patterns of ink printed by different printheads in printing the
same or different colors must be registered with each other to
provide a high quality image. An example of a non-color printing
application is functional printing of a circuit. The patterns of
material printed by different printheads (i.e., the image planes),
form directly or serve as catalysts or masks for the formation of
different layers of deposited conductive materials, semiconductor
materials, resistive materials, insulating materials of various
dielectric constants, high permeability magnetic materials, or
other types of materials, must also be registered to provide a
properly functioning circuit. The terms color plane and
color-to-color registration can also be used herein to refer to the
mapping and registration of pre-print or finishing operations, such
as the mapping of where the folds or cutting or slitting lines are,
or the placement of vias in an electrical circuit.
[0047] The terms "upstream" and "downstream" are terms of art
referring to relative positions along the transport path of the
print medium; points on the transport path move from upstream to
downstream. In FIGS. 1-6, a print medium 112 moves along a
transport path from upstream to downstream in a transport direction
114.
[0048] The schematic side view of FIG. 1 shows one example of a
continuous web inkjet printing system 100. Printing system 100
includes a first print module 102 and a second print module 104,
each of which includes lineheads 106-1, 106-2, 106-3, 106-4, dryers
108, and a quality control sensor 110. Each linehead 106-1, 106-2,
106-3, 106-4 typically includes multiple printheads (not shown)
that apply ink or another fluid (gas or liquid) to the surface of
the print medium 112 that is adjacent to the printheads. In the
illustrated aspect, each linehead 106-1, 106-2, 106-3, 106-4
applies a different colored ink to the surface of the print medium
112 that is adjacent to the lineheads 106-1, 106-2, 106-3, 106-4.
By way of example only, linehead 106-1 applies cyan colored ink,
linehead 106-2 magenta colored ink, linehead 106-3 yellow colored
ink, and linehead 106-4 black colored ink. The portion of the
transport path in each print module 102, 104 from the first
linehead 106-1 through the last linehead 106-4 is called a "print
zone."
[0049] The printing system 100 also include a web tension system
111 (portions of which are shown in FIG. 1) that serves to move the
print medium 112 through the printing system 100 in a controlled
fashion along the transport path in the transport direction 114
(generally left-to-right as in FIG. 1). The print medium 112 enters
the first print module 102 from a source roll (not shown) and the
lineheads 106-1, 106-2, 106-3, 106-4 of the first print module 102
apply ink to one side of the print medium 112. As the print medium
112 feeds into the second print module 104, a turnover module 116
is adapted to invert or turn over the print medium 112 so that the
lineheads 106-1, 106-2, 106-3, 106-4 of the second print module 104
can apply ink to the other side of the print medium 112. The print
medium 112 then exits the second print module 104 and is collected
by a print medium receiving unit (not shown).
[0050] A processing system 118 can be connected to various
components in the web tension system 111 and used to control the
positions of the components, such as gimbaled or caster rollers.
Processing system 118 can also be connected to the quality control
sensors 110 and used to process images or data received from the
quality control sensors 110. The processing system 118 can also be
connected to components in printing system 100 using any known
wired or wireless communication connection. Processing system 118
can be separate from printing system 100 or integrated within
printing system 100 or within a component in printing system 100.
In various embodiments, the processing system 118 can include a
single processor, or can include a plurality of processors. Each of
the one or more processors can be separate from the printing system
100 or integrated within the printing system 100.
[0051] A storage system 120 is connected to the processing system
118. The storage system 120 can store color plane correction values
in an aspect of the invention. The storage system 120 can include
one or more external storage devices; one or more storage devices
included within the processing system 118; or a combination
thereof. In some embodiments, the storage system 120 can include
its own processor, and can have memory accessible by the one or
more processors in the processing system 118. As will be discussed
in more detail later, in accordance with embodiments of the
invention, the storage system 120 can be used to store data useful
for determining appropriate registration corrections for documents
in a print job.
[0052] FIG. 2 illustrates a portion of the printing system 100 in
greater detail. As the print medium 112 is moved through printing
system 100, the lineheads 106-1, 106-2, 106-3, 106-4, which
typically include a plurality of individual printheads 200, apply
ink or another fluid onto the print medium 112 via nozzle arrays
202 of the printheads 200. The printheads 200 within each linehead
106-1, 106-2, 106-3, 106-4 are located and aligned by a support
structure 204 in the illustrated aspect. After the ink is jetted
onto the print medium 112, the print medium 112 passes beneath the
one or more dryers 108 which apply heat 206 or air to the ink on
the print medium 112 to remove at least a portion of the moisture.
For example, inks typically include colorant particles in a carrier
liquid. In this case, the dryer 108 is used to remove carrier
liquid from the print medium 112.
[0053] Referring now to FIG. 3, there is shown one example of a
print job 300 including a number of documents D.sub.1, D.sub.2,
D.sub.3, . . . D.sub.N to be printed in sequence. As used herein,
the term "print job" refers to a collection of documents to be
printed in sequence. A "document" can include any printed output
such as, for example, text, graphics, or photos, individually or in
various combinations. The printed output can be disposed anywhere
on the print medium 112, and the printed output in each document
can differ from the printed content in the other documents in a
print job 300. The print job 300 can include a subset of the
collection of documents that are printed multiple times with the
documents in the subset being printed in the same sequential order
each time the subset is printed, but is not limited to such
repeated sequence of documents. A print job 300 comprising the
printing of multiple copies of a book is an example of a print job
300 that includes a subset of the collection of documents that are
printed multiple times with the documents in the subset being
printed in the same sequential order each time the subset is
printed. Each document can be made up of one or more pages of
information across the width of the print medium 112, each document
in FIG. 3 comprises two pages printed side by side. In this
example, document D.sub.1 includes pages 302, 304, document D.sub.2
includes pages 306, 308, document D.sub.3 includes pages 310, 312,
and document D.sub.N includes pages 314, 316.
[0054] When the print job 300 is printed, the total amount of ink
printed on the print medium 112 can vary significantly from
document to document within the print job 300. Furthermore, the
spatial distribution of the ink applied to the print medium 112
within the documents can vary significantly within each document.
In turn, the aqueous component of the ink is absorbed into the
print medium 112 and can cause the print medium 112 to swell and
stretch, especially with water-based ink or in high ink laydown
regions of the printed content (e.g., an image with a lot of dense
black background). Stretch can be higher in the in-track direction
404 (i.e., the transport direction 114) than in the cross-track
direction 406. Non uniform swell or stretch of the print medium 112
can cause the print medium 112 to drift laterally as it moves
through the printing system. As the image content changes from
document to document, different portions along the length of the
print medium 112 can drift back and forth in the cross-track
direction 406.
[0055] Additionally, drying of the print medium 112 can cause the
print medium 112 to shrink. When the print medium 112 is heated in
between lineheads 106-1, 106-2, 106-3, 106-4 (FIG. 1), regions of
the print medium 112 can be stretched and shrunk one or more times
as the print medium 112 moves through the printing system 100 (FIG.
1).
[0056] Printing with several color planes, in which each color
record is printed sequentially, requires color laydown
registration. Unanticipated or unaccounted for stretch or shrink in
the print medium 112 can produce a loss of color registration and
can lead to blurry content or hue degradation. Additionally,
printing on both sides of the print medium 112 usually requires
front-to-back registration, and the second side of the print medium
112 is usually printed significantly later than the first side.
[0057] Translation is one type of color registration error. FIG. 4
depicts one example of cross-track and in-track color registration
errors produced by the translation of one color plane relative to
another color plane. Typically, one color plane (e.g., black) is
used as a reference color plane 400 from which the color
registration errors can be measured. In various embodiments, the
reference color plane 400 can be the first color plane to be
printed, the last color plane to be printed, or it can be printed
at any other point in the sequence of printed color planes. Errors
in registration for the remaining color planes can be determined by
comparing each color plane to the reference color plane. In this
example, the image content in color plane 402 is translated (i.e.,
shifted) with respect to the reference color plane 400. In the
illustrated example, color plane 402 has color registration errors
in both the in-track direction 404 and the cross-track direction
406.
[0058] Stretch and contraction represent another type of color
registration error. FIG. 5 depicts an example color registration
errors caused by the stretch or contraction of one color plane
relative to another color plane. The different color planes can be
stretched or contracted by different amounts in the in-track
direction 404 and the cross-track direction 406. In this example,
color plane 410 is contracted in both the in-track direction 404
and the cross-track direction 406 with respect to the reference
color plane 400.
[0059] Rotation or skew is another type of color registration
error. FIG. 6 depicts an example of registration errors resulting
from the rotation of one color plane relative to another color
plane. In this example, the color plane 408 is rotated with respect
to the reference color plane 400. Rotation errors result in
registration errors in both the in-track direction 404 and the
cross-track direction 406.
[0060] The stretching or shrinking can occur in the in-track
direction 404, the cross-track direction 406, or both the in-track
direction 404 and the cross-track direction 406. In some cases, one
color plane can contract in one direction (e.g., the cross-track
direction 406) and stretch in the other direction (e.g., the
in-track direction 404). These shifts and distortions need not be
uniform across the document. As a result, certain regions of a
document may exhibit expansion while other regions may exhibit no
expansion, or may even show contraction. In some cases, the
registration errors can include combinations of the types of color
registration errors shown in FIGS. 4-6, or can include different
types of color registration errors.
[0061] As disclosed in commonly-assigned U.S. Patent Application
Publication 2013/0286071 to Armbruster et al., entitled
"Color-to-color correction in a printing system," it has been found
that when printing multiple copies of a sequence of documents, the
registration errors can vary widely from document to document
within the sequence, but for any given document within the sequence
registration errors are generally consistent from one copy of the
sequence of documents to the next. Armbruster et al. make use of
this consistency to improve color-to-color registration. To provide
good color-to-color registration the registration errors for each
individual document within the repeated set are measured while the
first copy of the repeated set is being printed. Registration
corrections are then determined and applied to the color planes of
each individual document of the repeated set when subsequent copies
are printed based on the measured registration errors for the
corresponding documents in the first copy. This can provide
significant reductions in the registration errors for each copy
after the initial copy.
[0062] In commonly assigned U.S. patent application Ser. No.
14/063,406 entitled "Color-to-color correction in a printing
system," which is incorporated herein by reference, Armbruster et
al. extended the invention described in the aforementioned U.S.
Patent Application Publication 2013/0286071 by recognizing that
small changes in the printed content of an individual document in a
repeated set have little effect on the registration, provided that
the changes in printed content doesn't significantly affect the
amount and distribution of ink on the printed document.
Accordingly, the same registration corrections can be applied to
the sequence documents in the subsequent print jobs, even if the
documents are slightly different than the corresponding documents
in the first print job. This approach is well-suited for
applications where the same basic print job may be printed
repeatedly with only small changes, such as the name and address on
a form letter to be distributed using a bulk mailing. Related
inventions are described in commonly-assigned U.S. patent
application Ser. No. 14/063,276, U.S. patent application Ser. No.
14/063,331, U.S. patent application Ser. No. 14/063,351, and U.S.
patent application Ser. No. 14/063,374, all to Armbruster et al.,
each of which is incorporated herein by reference.
[0063] The present invention addresses the more general problem of
determining appropriate registration corrections for non-repeating
print jobs where a particular print job can include documents that
are significantly different than those in the previous print job.
To solve this problem, the inventors have recognized that the
registration errors observed for previous print jobs can be used to
predict the registration errors for image regions in future print
jobs that have similar image characteristics.
[0064] FIG. 7 shows a flowchart of an exemplary method for
determining image plane correction values 455, such as registration
correction parameters, appropriate for printing a document 420
using a digital printing system (such as the printing system 100 in
FIG. 1) in accordance with an exemplary embodiment. In summary,
this approach involves forming a color registration model 435 that
predicts color registration errors as a function of ink coverage
characteristics for a document 420. An analyze document step 425 is
used to determine ink coverage characteristics for a particular
document 420. A determine predicted color registration errors step
440 determines predicted color registration errors 445 using the
color registration model 435. A determine image plane correction
values step 450 is used to determine corresponding image plane
correction values 455 for at least one color plane of the document
420. (Note that the term "color plane" is used generally within the
context of the present invention, and can include image planes
printed using any type of printable "ink," such as black, gray or
colorless inks) A print document step 460 prints the document 420
using the image plane correction values 455, thereby forming a
printed document 465.
[0065] The document 420 will generally be represented by an array
of image pixels having pixel values which specify the ink coverage
for a plurality of color planes. The analyze document step 425
analyzes the pixel values for each color plane to determine
corresponding ink coverage characteristics 430. The ink coverage
characteristics 430 can be determined and represented using any
method known in the art. It is generally preferable the ink
coverage characteristics be represented using a relatively small
number of variables that summarize the distribution of ink within
each color channel of the document 420.
[0066] In some embodiments, the ink coverage characteristics 430
can be determined by dividing the digital image data for the
document 420 into a lattice of tiles and determining the average
ink coverage within each tile for each color channel. For example,
the document 420 can be broken into a 3.times.3 array of tiles, and
the average code value (representing the average ink coverage) for
each color channel can be computed as a measure of the ink coverage
distribution for the document 420. In this example, if the document
420 has 4 color channels, the ink coverage characteristics 430
would be represented by 3.times.3.times.4=36 numbers.
[0067] In other embodiments, the ink coverage characteristics 430
can be represented in terms of a set of statistics determined by
analyzing the document. For example, the statistics can include an
average and a standard deviation of the ink coverage for each color
channel across the entire document. The statistics can also include
other quantities such as a centroid of the ink distribution. The
statistics can also include various quantities that characterize
the asymmetry of the ink coverage. For example, one statistic can
be a difference between the average ink coverage for the left and
right sides for each color channel of the document.
[0068] In some embodiments, the ink coverage characteristics 430
can be characterized by determining ink coverage profiles
representing the distribution of ink across the width of the print
medium 112. For example, FIG. 8A shows a portion of a web of print
medium 112 that includes two documents. Document 420a includes a
text region 414 having fairly low coverage and two object regions
416 containing objects such as pictures or graphs with much higher
ink coverage that the text region 414. Document 420b also includes
a text region 414 having fairly low ink coverage and two higher
coverage object regions 416.
[0069] FIG. 8B shows an ink coverage plot 418a corresponding to a
particular color channel for document 420a in FIG. 8A. The ink
coverage plot 418a represents the distribution of ink in the
cross-track direction 406 from one edge of the print medium 112 to
the other, and is determined by averaging (or alternatively by
integrating) the image data for document 402a in the in-track
direction 404. The ink coverage plot 418a show two broad peaks
produced by the heavy ink coverage of the two object regions 416.
Similarly, FIG. 8C shows an ink coverage plot 418b determined in a
similar fashion for document 420b. It can be seen that the
locations of the peaks in the ink coverage plot 418b reflects the
positions of the text region 414 and the object regions 416 in the
document 420b.
[0070] It can be seen that the ink coverage plots 418a, 418b for
the documents 420a, 420b are not symmetric around the centerline
422 of the print medium 112. As a result, the registration errors
resulting from the deformation of the print medium 112 due to the
ink coverage will also typically not be symmetric.
[0071] FIG. 9A shows two documents 420c and 420d in a print job.
Corresponding ink coverage plots 418c, 418d are shown in FIGS. 9B
and 9C, respectively. The ink coverages of the documents 420c, 420d
in FIG. 9A are mirror symmetric to the ink coverages of the
documents 420a, 420b in FIG. 8A. In general, the registration
errors produced by the ink coverages of the FIG. 9A documents 420c,
420d will be mirror symmetric to the registration errors produced
by the ink coverages of the documents 420a, 420b in FIG. 8A. The
symmetric behavior of the registration errors can be exploited in
various embodiments to reduce the amount of data that needs to be
evaluated and stored.
[0072] The ink coverage profiles given by the ink coverage plots
418a, 418b, 418c, 418d can be used as relatively compact
representations of the ink coverage characteristics 430 for a
document 420. In some embodiments, the amount of data needed to
represent the ink coverage characteristics 430 can be further
reduced using various analysis techniques. For example, the x-axis
of the ink coverage plots 418a, 418b, 418c, 418d can be divided
into a series of bins (for example, between 4 and 20 bins), and the
average value of the ink coverage within each bin can be computed.
The average ink coverages within each bin can be used as a compact
representation of the ink coverage characteristics 430.
[0073] In other embodiments, the ink coverage profile F.sub.j(x)
for the j.sup.th color plane can be approximated by a series
expansion based on a set of orthogonal basis functions
P.sub.i(x).
F.sub.j(x).apprxeq..SIGMA..sub.i=0.sup.nc.sub.ijP.sub.i(x) (1)
where x is the position across the width of the print medium 112
(FIG. 8A), and c.sub.ij is a coefficient used to scale the i.sup.th
basis function for the j.sup.th color plane. In this example, the
ink coverage profile F.sub.j(x) is approximated using (n+1) basis
functions. An orthogonal set of basis functions is a set of
functions in which any two basis functions P.sub.m(x) and
P.sub.n(x) from the set are orthogonal to each other, that is:
.intg.P.sub.m(x)P.sub.n(x)dx=0; for m.noteq.n (2)
Any appropriate set of basis functions known in the art can be used
in accordance with the present invention, such as the well-known
Fourier basis functions. In an exemplary embodiment, the set of
basis functions is a set of Legendre polynomials that have been
normalized such that for any order m:
.intg..sub.-1.sup.1P.sub.m.sup.2(x)dx=1 (3)
The coefficients c.sub.ij for the basis functions in the series
expansion can be determined by:
c.sub.ij=.intg..sub.-1.sup.1F.sub.j(x)P.sub.i(x)dx (4)
A variety of numerical analysis algorithms are well-known in the
art that can be used to evaluate these integrals.
[0074] FIG. 10 shows a plot 390 of the first six Legendre
polynomials P.sub.i(x) where i=0 to 5. As the Legendre polynomials
are defined for x values ranging from -1 to 1, the cross-track
position variable x, for a print medium width of w needs to be
scaled to range from -1 to 1, with the centerline 422 (FIG. 9A) of
the print medium 112 corresponding to x=0. The ink coverage level
for each color plane are preferably scaled so that no applied ink
corresponds to a coverage level of 0 and complete coverage
corresponds to a coverage level of +1. Typically a tenth order
series expansion or less is sufficient to adequately approximate
the ink coverage characteristics 430 because the more closely
spaced fluctuations in ink coverage of the higher order Legendre
polynomials tend to have less of an effect on registration.
[0075] Returning to a discussion of FIG. 7, the color registration
model 435 can take on a wide variety of forms in accordance with
the present invention. In a preferred embodiment, the color
registration model 435 is a parametric model having one or more
parameters. The inputs to the parametric model can be the variables
that summarize the ink coverage characteristics (e.g., the
coefficients c.sub.ij of the Legendre polynomials determined for
the document 420), and the outputs of the parametric model can be a
set of variables that represent corresponding predicted color
registration errors 445. The parameters of the parametric model can
be determined by printing a plurality of test documents and
measuring the resulting color registration errors (for example,
using the quality control sensor 110 in FIG. 1). In some
embodiments, a mathematical fitting process can then be used to
determine the parameters for the parametric model that provide the
best fit to the measured data.
[0076] In some embodiments, the set of test documents can be
representative of a population of documents that are typically
printed by the printing system 100 (FIG. 1). In other embodiments,
the set of test documents can be chosen that have a wide range
different ink coverage characteristics. For example, a set of
documents can be designed where each document has ink coverage
characteristics 430 corresponding to one of the members of the set
of basis functions P.sub.i(x) (with an appropriate amount of the
uniform zero-order Legendre polynomial added to keep the ink
coverage levels of the document from being negative). This enables
the registration errors associated with each of the basis functions
to be independently characterized. In some embodiments, the set of
test documents can be designed to include documents having
characteristics that are likely to produce registration errors of
one or more of the registration error types which shown in FIGS.
4-6.
[0077] The printing of the set of test documents that are used to
determine the color registration model 435 can be performed during
a system calibration process. The system calibration process can be
performed during an initial system configuration process (e.g., at
the factory or when the printer is installed), or on an as-needed
basis. For example, the system calibration process can be initiated
when an operator observes significant color registration errors, or
when a new print medium is installed in the printing system 100
(FIG. 1). In some embodiments, the system calibration process can
be performed at regular intervals, for example at the start of each
shift.
[0078] In an exemplary embodiment, the color registration model 435
can be a simple linear model of the form:
E=a.sub.0+.SIGMA..sub.n=1.sup.Na.sub.nC.sub.n (5)
where N different variables C.sub.n are used to characterize the
ink coverage characteristics 430 for the document 420 (e.g., the
coefficients c.sub.ij of the Legendre polynomials determined for
the ink coverage profiles F.sub.j(x)), E is a vector of predicted
color registration errors 445, and a.sub.n is a vector of weighting
coefficients determined from the measured registration errors for a
set of test documents using a fitting process. In the case where
the variables C.sub.n are the coefficients for a set of basis
functions, the weighting coefficient a.sub.n would effectively
correspond to the amount of color registration errors that would
result from a document 420 having image content including a unit
amount of the corresponding basis function. In other embodiments,
the color registration model 435 can take different forms besides
the simple linear model given in Eq. (5). For example, the model
can include terms for higher-order polynomial functions such as
higher-power terms (e.g., C.sub.n.sup.2) or cross-terms (e.g.,
C.sub.1C.sub.2). Alternately, the color registration model 435 can
use any other functional form known in the art that is found to
predict the color registration errors with adequate accuracy.
[0079] In an exemplary embodiment, the vector E of predicted color
registration errors 445 includes a set of K different variables
E.sub.k which characterize different aspects of the color
registration errors. For example, the registration error variables
E.sub.k can include color plane translation values in the
cross-track and in-track directions (i.e., .DELTA.x, .DELTA.y) for
each color plane to characterize the translation errors illustrated
in FIG. 4. The registration error variables E.sub.k can also
include color plane magnification values in the cross-track and
in-track directions (i.e., M.sub.x, M.sub.y) for each color plane
to characterize the stretch/contraction errors illustrated in FIG.
5 and color plane rotation values (.theta.) for each color plane to
characterize the rotation errors illustrated in FIG. 6. Other types
of registration error variables E.sub.k can include image plane
skew parameters. Preferably, the registration error variables are
specified in such a way that they are independent of each other.
The registration errors as a function of position within the
document 420 can then be estimated by combining the effects of each
of the different registration error components.
[0080] The above examples of registration error variables are
global in nature in that they specify registration errors across
the entire document 420. In some embodiments, more complex color
registration error functions can be used to characterize
registration errors which may vary from one local region to
another. For example, the registration error variables E.sub.k can
specify the registration errors in terms of translations for a
lattice of positions within the document 420. The registration
errors for intermediate positions can then be estimated by using an
interpolation process. This approach can be useful for cases where
the image content in one portion of the image causes the print
medium 112 to swell in a local region without affecting other
portions of the print medium 112 which received a lower ink
load.
[0081] In some embodiments, the predicted color registration errors
445 for each color plane can be specified with respect to an
absolute position on the print medium 112. In other embodiments,
one of the color planes (e.g., the first color plane that is
printed) can be specified to be a reference color plane, and the
registration errors for the other color planes can specify position
differences relative to the reference color plane.
[0082] The color registration errors that are produced for a
particular document 420 will generally be dependent on the
configuration of the printing system 100. For example, the color
registration errors can vary significantly depending on the
characteristics of the print medium 112. For example, a lighter
weight print medium will be more susceptible to distortions, and
therefore to registration errors, than a heavier weight print
medium. The color registration errors can also be affected by other
factors such as ink formulation and printer operating conditions
(e.g., printing speed, dryer settings and environmental
conditions). In some embodiments, different color registration
models 435 are predetermined for a series of different types of
print media 112, or different combinations of other factors. When a
print job is received, an operator can identify the appropriate
type of print medium 112 and other system configuration factors,
and an appropriate color registration model 435 can then be
selected accordingly.
[0083] In some embodiments, one or more system configuration
parameters can be used as inputs to the color registration model
435. For example, the weight of the print medium 112 as an
additional input to the color registration model 435 rather than
providing a plurality of different color registration models
435.
[0084] The determine predicted color registration errors step 440
is used to determine the predicted color registration errors 445
for a particular document 420 by using the ink coverage
characteristics 430 as inputs to the color registration model 435.
Once the predicted color registration errors 445 are determined, a
determine image plane correction values step 450 is used to
determine a set of corresponding image plane correction values 455
that are appropriate to compensate for the predicted color
registration errors 445. Print document step 460 is then used to
print the document 420 using the image plane correction values
455.
[0085] The determine image plane correction values step 450 can
determine the image plane correction values 455 using any method
known in the art. For example, if the predicted color registration
errors 445 include in-track and cross-track color plane translation
values for a particular color plane (i.e., .DELTA.x, .DELTA.y), the
image plane correction values 455 can include corresponding
in-track and cross-track color plane shifts in the opposite
direction (i.e., .DELTA.x.sub.c=-.DELTA.x,
.DELTA.y.sub.c=-.DELTA.y). Similarly, if the predicted color
registration errors 445 include color plane magnification values in
the in-track and cross-track directions (i.e., M.sub.x, M.sub.y),
the image plane correction values 455 can include compensating
in-track and cross-track color magnification factors (i.e.,
M.sub.xc 1/M.sub.x, M.sub.yc=1/M.sub.y). Likewise, if the predicted
color registration errors 445 include color plane rotation values
(.theta.), the image plane correction values 455 can include
compensating rotations (i.e., .theta..sub.c=-.theta.). If the
predicted color registration errors 445 include more complex
functions that describe the registration errors as a function of
location within the document 420, the image plane correction values
455 can include parameters describing corresponding correction
functions that compensate for the complex registration errors.
[0086] The print document step 460 can apply the image plane
correction values 455 to the document 420 in a variety of different
ways. In some embodiments, digital image data for each color plane
of the document 420 can be modified to incorporate the desired
image plane corrections (e.g., in-track and cross-track color plane
shifts, in-track and cross-track color plane magnification
adjustments, and color plane rotations or skew adjustments). The
modified digital image data can then be printed normally. In other
embodiments, some or all of the image plane corrections can be
applied by adjusting the image data as it is being printed, or by
adjusting the printing process.
[0087] In some embodiments, the image plane correction values 455
can be used to control a web-transport system that moves the
continuous web of print medium 112 through the printing system 100.
For example, the web-transport system can be controlled to steer
the print medium 112, or adjust the speed that the print medium 112
moves through the printing system 100. For example, the print
medium 112 can be steered using the media transport system
described in commonly-assigned, co-pending U.S. Patent Application
Publication 2013/0113857 to Armbruster et al., entitled "Media
transport system including active media steering," which is
incorporated herein by reference. This approach uses structures
such as steered caster rollers to steer the web of media. In other
embodiments, the print medium 112 can be steered using the media
transport system described in commonly-assigned, co-pending U.S.
patent application Ser. No. 14/190,125, to Muir et al., entitled
"media guiding system using Bernoulli force roller," which is
incorporated herein by reference. This approach uses one or more
media-guiding rollers having grooves formed around the exterior
surface. An air source is controlled to provide an air flow into
the grooves, thereby producing a Bernoulli force to draw the web of
media into contact with the media-guiding rollers. An axis of the
media-guiding rollers can be positioned to steer the web of media,
or to perform other functions such providing a stretching force in
the cross-track direction to prevent the formation of wrinkles.
[0088] In some embodiments, in-track color plane shifts can be
applied by adjusting the timing at which lines of image data are
printed using the printheads 200 (FIG. 2). For example, to shift
the image forward along the print medium 112, the lines of image
data can be printed at a slightly earlier time than they would be
nominally, and to shift the image backward along the print medium
112, the lines of image data can be printed at a slightly later
time than they would be nominally.
[0089] In some embodiments, cross-track color plane shifts can be
applied by adjusting which inkjet nozzles are used to print the
image data. For example, the image data supplied to the printheads
can be shifted left or right to use different subsets of the
nozzles in the printheads 200. In other embodiments, a servo-system
can be used to adjust a cross-track position of the print medium
112 to apply the cross-track color plane shifts.
[0090] In some embodiments, cross-track magnification changes can
be applied out using the methods described in commonly assigned,
co-pending U.S. patent application Ser. No. 13/599,067, entitled:
"Aligning print data using matching pixel patterns", by Enge et
al.; and commonly assigned, co-pending U.S. patent application Ser.
No. 13/599,129, entitled: "Modifying image data using matching
pixel patterns", by Enge et al., each of which is incorporated
herein by reference. This method involves inserting or deleting
image pixels across the width of the printhead 200 to adjust the
size of the printed image in the cross-track direction 406.
[0091] In some embodiments, in-track magnification changes can be
applied by adjusting the timing at which lines of image data are
printed by the printheads 200. For example, to increase the
in-track image size, the timing between the printing of successive
lines of image data can be increased slightly, and to decrease the
in-track image size, the timing between the printing of successive
lines of image data can be decreased slightly.
[0092] An optional determine residual registration errors step 470
can be used to analyze the printed document 465 to determine
residual registration errors 475. This enables the system to
monitor whether the determined image plane correction values 455
provided accurate compensation for the color registration errors.
In some embodiments, the determine residual registration errors
step 470 uses the quality control sensor 110 (FIG. 1) to evaluate
the positions of alignment marks printed in the margin of the
document 420. In other embodiments, the printed image data in the
printed document 465 can be analyzed as described in
commonly-assigned, co-pending U.S. patent application Ser. No.
14/061,833 to J. Howard et al., entitled "Printer with image plane
alignment correction," which is incorporated herein by
reference.
[0093] In some embodiments, the color registration of subsequently
printed images can be adjusted accordingly if it is determined that
the residual registration errors 475 are significant and
consistent. In some embodiments, an update color registration model
step 480 can be used to update the color registration model 435
based on the ink coverage characteristics 430 and the corresponding
residual registration errors 475 associated with the document 420.
For example, the set of test documents that are used to determine
the parameters of the color registration model 435 can be updated
to include the document 420, and an updated set of parameters can
be determined.
[0094] The distortions in the print medium 112 that result from
printing a particular document can impact the registration errors
for documents both upstream and downstream of the document. For
example, if an asymmetric ink coverage in a document causes the
print medium 112 to shift laterally, some or all of the lateral
shift may still be present when the next document in the print job
is printed. It can therefore be advantageous to generalize the
methods described earlier with respect to FIG. 7 to account for
this effect.
[0095] FIG. 11 shows a flowchart of a method for correcting color
registration errors at a particular in-track position which
represents a variation of the approach shown in FIG. 7. In this
example, an image region 490, which can be of any size is analyzed
by an analyze image region step 495 to determine corresponding ink
coverage characteristics. In some exemplary embodiments, the image
region 490 can include a plurality of individual documents. For
example, FIG. 12A shows a portion of a web of print medium 112
which includes three documents 420a, 420b, 420c. In accordance with
the method of FIG. 11, it is desired to correct for color
registration errors at a particular in-track position 494. An image
region 490 can be defined, which in this example includes the
document 420b including the in-track position 494, together with
the documents 420a and 420c which are positioned downstream and
upstream of document 420b, respectively. Downstream document 420a
includes previously printed image data, and upstream document 420c
includes yet-to-be printed image data. In other embodiments, the
image region 490 will only include previously-printed image data.
(Note that upstream portions of the print medium 112 may include
previously-printed image data from printheads 200 that are located
upstream of the current printhead 200.) In some embodiments, the
length of the image region 490 is approximately equal to the length
of the print zone from the first linehead 106-1 through the last
linehead 106-4 within a print module 102 or 104 in FIG. 1. For a
particular linehead 106-1, 106-2, 106-3, 106-4, the relative length
of the portion of the image region 490 upstream of the in-track
position 494 to the portion of the image region 490 downstream of
the in-track position 494 can vary depending on the position of the
linehead 106-1, 106-2, 106-3, 106-4 within the print zone. In other
cases, the image region 490 can include only a portion of document
420b, or can include portions of a plurality of documents (for
example, the downstream portion of document 420b and the second
half of downstream document 420a as illustrated in FIG. 12B.
[0096] Returning to a discussion of FIG. 11, the ink coverage
characteristics 430 determined for the image region 490 are used to
determine predicted color registration errors 445 using a method
analogous to that described earlier with respect to FIG. 7. The
only difference being that the color registration model 435 is
adapted to predict the color registration errors at a particular
in-track position 494 as a function of the ink coverage
characteristics 430 for the larger image region 490. In some
embodiments, the image region 490 is divided into a series of
sub-regions, and the inputs to the color registration model can be
a set of variables C.sub.n determined for each sub-region using
methods such as those described earlier. For example, in the
example, of FIG. 12A, the sub-regions can be the areas
corresponding to the three documents 420a, 420b, 420c and a set of
variables C.sub.n is determined for each of the documents 420a,
420b, 420c.
[0097] A determine image data correction values step 496 is used to
determine image data correction values 497 responsive to the
predicted color registration errors 445. In a preferred embodiment,
the image data correction values 497 are used to correct the color
registration errors for a set of in-track positions 492 (FIG. 12A)
that includes the current in-track position 494. In some
embodiments, the set of in-track positions 492 can include the
entire document 420b. In other embodiments, the set of in-track
positions 492 can include only a single image line, or a small set
of image lines including the in-track position 494.
[0098] In some embodiments, image data correction values 497 are
determined at a series of in-track positions 494 which are spaced
apart along the in-track direction 404, and an interpolation
process is used to determine image data correction values 497 to be
applied for intermediate positions to provide smooth transitions in
the corrected image.
[0099] A print image data step 498 is used to print the image data
for the set of in-track positions 492 (FIG. 12A) using the image
data correction values 497, thereby providing printed image data
499. To print the image data for the next set of in-track
positions, the process is repeated using a new image region 490
which contains the next set of in-track positions. The remaining
steps in FIG. 11 are analogous to the corresponding steps in FIG.
7, which were discussed earlier.
[0100] FIG. 13 shows a flowchart of a method for determining
various system control parameters 530, such as registration
correction parameters, appropriate for printing an image region 500
using a digital printing system (such as the printing system 100 in
FIG. 1) in accordance with an alternate embodiment. In summary,
this approach involves forming an image region database 505
including a library of reference image regions 510 for which
appropriate system control parameters 515 (e.g., registration
corrections) have been previously determined. When a new print job
is being printed, the controller analyzes each image region 500
within the print job to find a similar reference image region 525.
In print image region step 535, the controller then prints the
image region 500 using system control parameters 530 associated
with the similar reference image region 525.
[0101] In some embodiments, the image region 500 can be a
"document" such as the documents D.sub.1, D.sub.2, D.sub.3, D.sub.N
in FIG. 3. In other embodiments, the image region 500 can
correspond to a portion of a document, or to an image area of a
fixed length in the in-track direction 404, spanning the width of
the print medium 112 in the cross-track direction 406, such as the
image region 490 in FIG. 12B. In some embodiments, the method can
be applied to printers that print on individual sheets of print
medium 112. In this case, it can be appropriate for the image
regions 500 to correspond to a single sheet of print medium
112.
[0102] In some embodiments, the image region 500 can include at
least a portion of a plurality of individual document pages that
are proximal to a current page since the proximal pages can affect
the registration errors of the current page. For example, the
proximal pages can include one or more downstream pages that have
been printed before the current page is printed. The proximal pages
can also include one or more upstream pages, such as pages where at
least one color channel has been printed before the current color
channel of the current page is printed.
[0103] Image region database 505 stores data characterizing a
plurality of reference image regions 510. Each reference image
region 510 has one or more associated system control parameters 515
that have been previously determined to be appropriate for use in
printing the reference image region 510 with the printing system
100.
[0104] The image region database 505 is stored in a memory which is
accessible by the processor used to perform relevant steps in the
method of FIG. 13. In some embodiments, the method is performed
using a processing system 118 which is a component of the printing
system 100. In this case, the image region database 505 is
preferably stored in storage system 120 associated with the
processing system 118 (FIG. 1). In other embodiments, the analysis
steps, such as compare image regions step 520, are performed using
a processing system at a different location from the printing
system 100. For example, the print job can be ripped and prepared
for printing at a remote location, and the ripped print job can be
transmitted to the printing system 100 over a network. As will be
well-known to those skilled in the art, the term "ripped" refers to
using a Raster Image Processor (RIP) to process an input image to
determine image data appropriate for printing. The process of
"ripping" the image data can include various operations such as
color processing, rasterization, resizing and halftoning. For cases
where the ripping occurs at a remote location, the processing
system 118 used to perform the analysis steps in FIG. 13 can also
be located at the remote location, and the determined system
control parameters 530 can be transmitted to the printing system
100 together with the ripped print job. Accordingly, the
processor-accessible memory used to store the image region database
505 can also be located at the remote location.
[0105] In a preferred embodiment, the system control parameters 515
include registration correction parameters that can be used to
correct for registration errors that have been determined to be
characteristic of the associated reference image region 510. The
registration correction parameters are used to control various
aspects of the printing process in order to control the
registration between two or more different color channels. As was
discussed earlier with respect to the image plane correction values
455 in FIG. 7, registration correction parameters can include
in-track and cross-track color plane shifts, in-track and
cross-track color plane magnification adjustments, and color plane
rotations or skew adjustments.
[0106] The system control parameters 515 can also include other
types of parameters that can be used to control the printing system
in accordance with the characteristics of the image region 500. In
some embodiments, the system control parameters 515 can include a
dryer control parameter that controls a setting on the dryer 108
(FIG. 1). For example, the dryer control parameter can control the
amount of heat or airflow provided by the dryer depending on the
amount of ink printed in the image region 500.
[0107] The system control parameters 515 can also include a color
adjustment parameter that is used to adjust color reproduction
characteristics of the image region 500. In some embodiments, the
color reproduction characteristics are adjusted by adjusting a
color transformation applied to the image data of the image region
500. Depending on the type of printer, the color reproduction
characteristics can also be controlled by adjusting appropriate
printing system parameters. For example, in an electrophotographic
printing system the color reproduction can be controlled by
adjusting an exposure level used to expose the photoconductor or a
development voltage used in the toner development process.
[0108] In some embodiments, the printing system 100 is capable of
printing images using a plurality of different print modes. In this
case, the system control parameters 515 can also include a print
mode parameter that is used to select a print mode that should be
used to print the image region 500. For example, the print mode can
be a color print mode, or a black and white print mode. The
print-mode can also include specifying various attributes such as a
printing resolution, a print speed, or a halftoning algorithm to be
used to process the image data. Depending on the printing
technology the print mode parameter can be used to specify various
printing attributes such as selecting a number of printing passes
(e.g., for a desktop inkjet printer), or a bias voltage setting
(e.g., for an electrophotographic printer).
[0109] In some embodiments, the printer may include a de-curling
subsystem for reducing curl in the print medium. An exemplary
de-curling subsystem is disclosed in U.S. Pat. No. 5,084,731 to
Baruch, entitled "Sheet decurling mechanism and method," which is
incorporated herein by reference. In this case, the system control
parameters 515 can include a setting on the de-curling subsystem.
For example, image regions 500 that require larger amounts of ink
will typically be more susceptible to curl. Therefore, it can be
advantageous to make corresponding adjustments to the de-curling
subsystem, such as adjustments to an amount of force applied at
de-curling nip rollers, to compensate for the larger curl
levels.
[0110] The set of reference image regions 510 can be selected in a
variety of different ways in accordance with the present invention.
In some embodiments, the reference image regions 510 in the image
region database 505 are selected to be representative of different
types of image regions 500 that are commonly printed by the
printing system 100. The selection of the representative reference
image regions 510 can be manually done by a user based on knowledge
of the typical document types printed using a particular printing
system 100. Alternately, the selection of the representative
reference image regions 510 can be done automatically by analyzing
a population of print jobs that are printed by the printing system
100. The population of print jobs can be generic to a particular
type of printer, or can be tailored to the print jobs printed by a
particular customer. In some embodiments, new reference image
regions 510 can be added to the image region database 505 during
the operation of the printing system 100. For example, if an image
region 500 is printed that is sufficiently different from any of
the reference image regions 510 in the image region database 505, a
new entry can be added to the image region database 505.
[0111] In some embodiments, the data characterizing the reference
image regions 510 can be digital image data including an array of
image pixels, each image pixel having a pixel value. To facilitate
both the analysis of the image regions 500, and the storage of
information in the image region database 505, the digital image
data should be stored at a spatial resolution sufficient for
analysis. However, this spatial resolution can be different from
the spatial resolution required for printing the documents. In a
preferred embodiment, the spatial resolution of the image region
500 and the reference image regions 510 used for analysis by the
compare image regions step 520 can be much lower than the spatial
resolution required for printing the documents.
[0112] In some embodiments, the documents that have been ripped for
printing are resized to a lower spatial resolution to provide the
image regions 500 required for analysis. The reference image
regions 510 are preferably stored at the same spatial resolution
for easy comparison with the image regions 500. In other
embodiments, the input image can be ripped a first time to obtain
the image data for printing, and can be ripped a second time to a
lower spatial resolution to obtain the image data needed for
analysis. For example, the documents can be ripped at a low spatial
resolution (e.g., 5-10 pixels/inch) for analysis purposes, whereas
the documents need to be ripped to 600 pixels/inch or higher for
printing. Ripping the documents at a lower spatial resolution
requires less time than is required for ripping them at higher
spatial resolutions for printing. At spatial resolutions of 5-10
pixels/inch, the image data has sufficient resolution to identify
ink coverage patterns that can lead to issues such as
color-to-color registration errors.
[0113] Using image regions 500 having a lower spatial resolution,
and storing the reference image regions 510 at the lower spatial
resolution, has a number of advantages. First, storing the
reference image regions 510 at the lower spatial resolution
requires less storage space for the image region database 505.
Additionally, the analysis of the image region 500 can be faster
since a smaller amount of image data must be analyzed. A further
benefit of storing the reference image regions 510 at a lower
spatial resolution is that this tends to remove any image content
that may be considered confidential. This enables storing the
reference image regions 510 without risking the confidentiality of
the document content.
[0114] In other embodiments, the data characterizing the reference
image regions 510 can be parametric representations of the
reference image regions 510. For example, the parametric
representations can be ink coverage profiles such as the ink
coverage plots 418a in FIG. 8B, or any of the other representations
of the ink coverage characteristics 430 that were discussed
relative to the method of FIG. 7. For example, the data
characterizing the reference image regions 510 can be a set of N
different variables C.sub.n that are used to characterize the ink
coverage of the reference image regions 510. In an exemplary
embodiment, the variables C.sub.n used to characterize the
reference image regions 510 are coefficients c.sub.ij of Legendre
polynomials that are determined for ink coverage profiles
F.sub.j(x) determined for each color channel of the reference image
region 510 as described earlier with respect to Eq. (4).
Preferably, the image region 500 is processed in an equivalent
manner as the reference image regions 510 to determine
corresponding data characterizing the image region 500.
[0115] Compare image regions step 520 compares the image region 500
to the reference image regions 510 to identify a similar reference
image region 525. In an exemplary embodiment, a difference metric
is computed between the image region 500 and each of the reference
image regions 510, and the reference image region 510 having the
smallest difference metric is designated to be the similar
reference image region 525.
[0116] If the data characterizing the image region 500 and the
reference image regions 510 are image data including an array of
image pixels, the difference metric can be computed by determining
a difference image d.sub.i(x,y):
d.sub.i(x,y)=|I(x,y)-R.sub.i(x,y)| (6)
where I(x,y) is the image data for the image region 500 and
R.sub.i(x,y) is the image data for the i.sup.th reference image
regions 510. Statistical analysis can then be applied to the
difference image d.sub.i(x,y) to determine the difference metric.
In some embodiment, the difference metric can be determined by
computing the mean of the pixel values in the difference image
d.sub.i(x,y):
D i = 1 N x N y x = 1 N x y = 1 N y d i ( x , y ) ( 7 )
##EQU00001##
where D.sub.i is the difference metric for the i.sup.th reference
image region 510, and the size of the i.sup.th difference image
d.sub.i(x,y) is N.sub.x.times.N.sub.y. In other embodiments, the
difference metric can take other forms such as the median of the
pixel values, or RMS pixel value.
[0117] If the data characterizing the image region 500 and the
reference image regions 510 are a set of variables C.sub.n that are
used to characterize the ink coverage, the difference metric can be
computed by computing differences between the set of variables for
the image region 500 (C.sub.n,I) and the set variables for the
i.sup.th reference image region 510 (C.sub.n,i). In an exemplary
embodiment, the difference metric D.sub.i for the i.sup.th
reference image region 510 can be determined using the following
equation:
D.sub.i=.SIGMA..sub.n=1.sup.Nw.sub.n|C.sub.n,I-C.sub.n,i|.sup.2
(8)
where N is the number of variables, and w.sub.n is a weighting
coefficient for the n.sup.th variable which can be used to adjust
the relative importance of the set of variables.
[0118] The weighting coefficients w.sub.n can be determined
empirically, or by analyzing a set of training image regions 500
where optimal system control parameters have been predetermined. In
this case, the weighting coefficients can be optimized to maximize
the probability the reference image region 510 having the
associated system control parameters 515 which are closest to the
optimal system control parameters is selected by the compare image
regions step 520. For example, if the variables C.sub.n are
coefficients c.sub.ij of Legendre polynomials, then the
higher-order polynomials will generally be associated with finer
detail features of the ink coverage profile. Therefore, the
variables associated with the higher-order polynomials will
generally make a smaller contribution to the prediction of color
registration corrections than the variables associated with
lower-order polynomials. Consequently, the weighting factors for
the variables associated with the higher-order polynomials can
typically be smaller than for the variables associated with the
lower-order polynomials.
[0119] In some embodiments, the image region 500 can be compared
with both the reference image regions 510, as well as mirror images
of the reference image regions. This can effectively increase the
number of reference image regions 510 stored in the image region
database 505 without requiring an increase in the associated
storage memory. For cases where the stored data characterizing the
reference image regions are coefficients c.sub.ij of Legendre
polynomials, then the mirror image of the reference image region
can be conveniently determined by noting that all of the even
orders of the Legendre polynomials are symmetric about the
centerline, while all the odd orders are anti-symmetric about the
centerline. Therefore, the Legendre polynomial series expansion of
a mirror image ink coverage profile, will have the same
coefficients for the even orders of the expansion and while the odd
orders will differ by a change in sign, that is
c'.sub.ij=(-1).sup.ic.sub.ij. Therefore, the image region 500 can
be compared by the mirror image of a reference image region 510 by
simply applying this modification to the Legendre polynomial
coefficients. If one of the mirror image reference image regions is
found to be the similar reference image region 525, then the system
control parameters can be adjusted accordingly to account for the
fact that the registration errors will also generally be reversed.
However, some types of print media 112 may not exhibit the typical
mirror symmetry response. Therefore it can be desirable to provide
the operator with the option of disabling the mirror symmetry
comparison of the image region 500 to the reference image regions
510.
[0120] In some embodiments, the image region 500 can also be
compared with scaled versions of reference image regions 510. If a
scaled reference image region is found to be the best match to the
image region 500, then the associated system control parameters 530
can be adjusted accordingly. In some cases, it can be assumed that
the registration errors will be proportional to the amount of ink
applied to the print medium 112. In this case, it can be assumed
that the registration corrections can be scaled according to the
scale factor used to scale the reference image region 510. In other
cases, the registration corrections can be scaled using a scale
factor which is a function of the scale factor used to scale the
reference image region 510.
[0121] In some embodiments, one or more system configuration
parameters are associated with the image region 500 and the
reference image regions 510. Examples of system configuration
parameters can include print media type, print media size, print
speed or print mode. In some embodiments, the compare image regions
step 520 compares the system configuration parameters associated
with the image region 500 to the system configuration parameters
associated with the reference image regions 510, and only considers
reference image regions 510 having associated system configuration
parameters that are sufficiently similar to those associated with
the image region 500. For example, since performance attributes
such as registration errors will typically depend on the print
media type, it can be appropriate to limit the reference image
regions that are considered to those having associated system
control parameters 515 that were determined for similar print media
types (e.g., having a similar media weight and composition). In
some cases, a plurality of sets of system control parameters 515
can be associated with a particular reference image region 510,
each one associated with a different set of system configuration
parameters. In some cases, the system configuration parameters can
be included in the computation of the difference metric used to
compare the image region 500 to the reference image regions
510.
[0122] In some embodiments, different image region databases 505
can be provided for different system configurations. For example,
different image region databases 505 can be provided for different
print media types or different print speeds. Before printing a
print job, an operator can configure the printer for the print job,
and an image region database 505 can be automatically or manually
selected which is consistent with the selected system
configuration. In some embodiments, a single image region database
505 can be used which includes a single set of reference image
regions 510, but includes multiple sets of system control
parameters 515 corresponding to different system configurations.
This can have the same effect as defining a plurality of image
region databases 505, but will require less storage memory.
[0123] Print image region step 535 is then used to print the image
region 500 responsive to the system control parameters 530
associated with the selected similar reference image region 525. In
some embodiments, the print image region step 535 prints the entire
image region 500 using the system control parameters 530. In other
embodiments, only a subset of the image region 500 is printed using
the system control parameters 530. For example, in some
embodiments, system control parameters 530 are determined for a
sequence of image regions 500 to be printed on a web of print
medium 112. In this case, the selected system control parameters
530 can be applied for a line of image data in the center of each
image region 500, and interpolation can be used to smoothly
transition the system control parameters from one image region 500
to the next.
[0124] As was discussed earlier, the system control parameters 530
can include registration correction parameters such as in-track and
cross-track color plane shifts, in-track and cross-track color
plane magnification adjustments, and color plane rotations or skew
adjustments. As was discussed earlier with respect to FIG. 7, these
parameters can be used to modify the image data for the image
region, or to control various aspects of the printing system, such
as media-steering components or timing components. The print image
region step 535 in FIG. 13 can apply the registration correction
parameters using any of the methods that were discussed earlier
with respect to the print document step 460 in FIG. 7 and the print
image data step 498 in FIG. 11.
[0125] The performance of the printing system 100 can be monitored
(for example, using the quality control sensor 110) to detect
performance degradations such as registration errors. In some
embodiments, any such degradations that are detected can be used to
update the image region database 505. For example, if it is
observed that the registration errors which result when image
regions 500 are printed using the system control parameters 515
associated with a particular reference image region 510 have a
systematic bias, then the associated system control parameters 515
can be updated to remove that systematic bias.
[0126] In some embodiments, the image region 500 can be added to
the image region database 505 as a new reference image region 510.
This can be particularly desirable if the compare image regions
step 520 determines that the image region 500 is significantly
different than all of the reference image regions 510 in the image
region database 505. For example, if the compare image regions step
520 computes a difference metric D.sub.i as described above, a
sufficiently different test 555 can be used to compare the
difference metric D.sub.i determined for the most similar reference
image region 510 (i.e., the similar reference image region 525) to
a predetermined threshold. If the difference metric D.sub.i exceeds
the threshold, then an add image region to database step 560 is
used to add the image region 500 to the image region database 505.
In this case, a corresponding set of system control parameters 550
can be determined by analyzing the printed image region 540 using a
determine system control parameters step 545. For example, the
determine system control parameters step 545 can determine any
residual registration errors and can modify the system control
parameters 530 that were used to print the image region 500
accordingly. The modified system control parameters 550 can then be
associated with the image region 500 in the image region database
505. In this way, the content of the image region database 505 can
automatically adapt to the content of the print jobs that are
printed by the printing system 100.
[0127] An optional determine system control parameters step 545 can
be used to analyze the printed image region 540 to determine an
updated set of system control parameters 550 that would produce
improved system performance if they had been used to print the
image region 500.
[0128] In some embodiments, the system control parameters 515
associated with the reference image regions 510 can be fixed. In
other embodiments, performance of the printing system 100 can be
monitored (for example, using the quality control sensor 110) and
the system control parameters 515 can be updated to reflect any
residual errors that are observed. For example, if it is observed
that the registration errors which result when the image region 500
is printed using the system control parameters 515 associated with
a particular reference image region 510 exceed a predefined
threshold or exhibit a systematic bias, then the system control
parameters 515 can be updated accordingly.
[0129] In some embodiments, the image region database 505 resides
in components of the printing system 100, while in other
embodiments the image region database 505 is stored at a different
location remote from the printing system 100. The remotely stored
image region database enables multiple digital printing systems 100
to access a common image region database 505. As additional
reference image regions 510 are added to the image region database
505 by any of the connected printing systems 100, each of the other
printing systems 100 can benefit from the expanded image region
database 505. Local storage of the image region database 505 may be
preferred in some embodiments, for example to retain more
confidentiality concerning the print jobs.
[0130] In some embodiments, the data processing system used to form
various steps in the method of FIG. 13 is a component of the
printing system 100 such as the processing system 118 (FIG. 1). In
other cases, the data processing system can be at a different
location remote from the printing system 100. In this case, the
analysis of a print job can be performed at the remote location,
and the resulting system control parameters 530 can be transmitted
to the printing system 100 together with the print job.
[0131] While the above embodiments have been described with
reference to a web-fed inkjet printing system 100 (FIG. 1), the
disclosed methods are also applicable to other types of printing
systems such as sheet-fed inkjet printers and electrophotographic
printers. FIG. 14 shows an example, of an electrophotographic
printing system
[0132] FIG. 14 shows a simplified side elevational view of an
electrophotographic color printer apparatus 600 including five
tandemly arranged image-forming modules M1, M2, M3, M4, M5. Each of
the image-forming modules M1, M2, M3, M4, M5 generates single-color
toner images for transfer to receiver media R successively moved
through the image-forming modules M1, M2, M3, M4, M5. Each receiver
medium R can have transferred in registration thereto up to five
single-color toner images. In a particular embodiment,
image-forming module M1 forms black toner images, image-forming
module M2 forms yellow toner images, image-forming module M3 forms
magenta toner images, and image-forming module M4 forms cyan toner
images. Image-forming module M5 can be used optionally to deposit a
clear or colorless toner image, or alternatively to deposit a
specialty color toner image such as for making proprietary logos or
for expanding the color gamut of a resulting print.
[0133] Receiver media R are delivered from a supply (not shown) and
transported through the image-forming modules M1, M2, M3, M4, M5.
The receiver media R are adhered (e.g., electrostatically via
coupled corona chargers 624, 625) to an endless transport web 601
entrained around and driven by rollers 602, 603. Each of the
image-forming modules M1, M2, M3, M4, M5 includes a respective
photoconductive imaging roller PC1, PC2, PC3, PC4, PC5, an
intermediate transfer roller ITR1, ITR2, ITR3, ITR4, ITR5, and a
transfer backup roller TR1, TR2, TR3, TR4, TR5. Thus in
image-forming module M1, a black toner image can be created on
photoconductive imaging roller PC1, transferred to intermediate
transfer roller ITR1, and transferred again to a sheet of receiver
medium R.sub.(n-1) moving through a transfer station, which
includes a pressure nip formed between the intermediate transfer
roller ITR1 and the transfer backup roller TR1. Similar processes
occur in the other image-forming modules M2, M3, M4, M5.
[0134] A receiver medium R.sub.n, arriving from the supply, is
shown passing over roller 602 for subsequent entry into the
transfer station of the first image-forming module, M1, in which
the preceding receiver medium R.sub.(n-1) is shown. Similarly,
sheets of receiver media R.sub.(n-2), R.sub.(n-3), R.sub.(n-4), and
R.sub.(n-5) are shown moving respectively through the transfer
stations of image-forming modules M2, M3, M4, and M5, respectively.
An unfused print formed on receiver medium R.sub.(n-6) is moving as
shown toward a fuser (not shown) for fusing the unfused print.
[0135] The transport web 601 is reconditioned for reuse at cleaning
station 686 by cleaning and neutralizing the charges on the opposed
surfaces of the transport web 601. A mechanical cleaning station
(not shown) for scraping or vacuuming toner off transport web 601
can also be used independently or with cleaning station 686. The
mechanical cleaning station can be disposed along the transport web
601 before or after cleaning station 686 in the direction of
rotation of transport web 601.
[0136] A power supply unit 605 provides individual transfer
currents to the transfer backup rollers TR1, TR2, TR3, TR4 and TR5,
respectively. A densitometer module, preferably positioned in a
location between the last image-forming module M5 and roller 603,
includes a densitometer module 604 (utilizing one or more light
beams 610 and sensors 606). The densitometer module 604 measures
optical densities of a set of process control patches. In some
embodiments, the process control patches are printed onto a sheet
of the receiver medium R. In other embodiments, the process control
patches can be transferred directly onto the transport web 601, for
example in an inter-frame area 609 between sheets of the receiver
medium R.
[0137] FIG. 15 shows additional details of image forming module M1,
which is representative of image forming modules M2, M3, M4 and M5
(FIG. 14). The components of the image forming module M1 are
sometimes referred to as an "electrophotographic print engine."
Photoreceptor 706 of imaging roller PC1 includes a photoconductive
layer formed on an electrically conductive substrate. The
photoconductive layer is an insulator in the substantial absence of
light so that electric charges are retained on its surface. Upon
exposure to light, the charge is dissipated. In various
embodiments, photoreceptor 706 is part of, or disposed over, the
surface of imaging roller PC1. Photoreceptors can include a
homogeneous layer of a single material such as vitreous selenium or
a composite layer containing a photoconductor and another material.
Photoreceptor 706 can also contain multiple layers.
[0138] Charging subsystem 710 uniformly electrostatically charges
photoreceptor 706 of imaging roller PC1. Charging subsystem 710
includes a grid 713 having a selected voltage. Additional necessary
components provided for control can be assembled about the various
process elements of the respective image forming modules. Meter 711
measures the uniform electrostatic charge provided by charging
subsystem 710.
[0139] An exposure subsystem 720 is provided for selectively
modulating the uniform electrostatic charge on photoreceptor 706 in
an image-wise fashion by exposing photoreceptor 706 to
electromagnetic radiation to form a latent electrostatic image. The
uniformly-charged photoreceptor 706 is typically exposed to actinic
radiation provided by selectively activating particular light
sources in an LED array or a laser device outputting light directed
onto photoreceptor 706. In embodiments using laser devices, a
rotating polygon (not shown) is used to scan one or more laser
beam(s) across the photoreceptor in the fast-scan direction. One
pixel site is exposed at a time, and the intensity or duty cycle of
the laser beam is varied at each dot site. In embodiments using an
LED array, the array can include a plurality of LEDs arranged next
to each other in a line, all dot sites in one row of dot sites on
the photoreceptor can be selectively exposed simultaneously, and
the intensity or duty cycle of each LED can be varied within a line
exposure time to expose each pixel site in the row during that line
exposure time.
[0140] As used herein, an "engine pixel" is the smallest
addressable unit on photoreceptor 706 which the exposure subsystem
720 (e.g., the laser or the LED) can expose with a selected
exposure different from the exposure of another engine pixel.
Engine pixels can overlap (e.g., to increase addressability in the
slow-scan direction S). Each engine pixel has a corresponding
engine pixel location, and the exposure applied to the engine pixel
location is described by an engine pixel level.
[0141] The exposure subsystem 720 can be a write-white or
write-black system. In a write-white or charged-area-development
(CAD) system, the exposure dissipates charge on areas of
photoreceptor 706 to which toner should not adhere. Toner particles
are charged to be attracted to the charge remaining on
photoreceptor 706. The exposed areas therefore correspond to white
areas of a printed page. In a write-black or discharged-area
development (DAD) system, the toner is charged to be attracted to a
bias voltage applied to photoreceptor 706 and repelled from the
charge on photoreceptor 706. Therefore, toner adheres to areas
where the charge on photoreceptor 706 has been dissipated by
exposure. The exposed areas therefore correspond to black areas of
a printed page.
[0142] In a preferred embodiment, a meter 712 is provided to
measure the post-exposure surface potential within a patch area of
a latent image formed from time to time in a non-image area on
photoreceptor 706. Other meters and components can also be included
(not shown).
[0143] A development station 725 includes toning shell 726, which
can be rotating or stationary, for applying toner of a selected
color to the latent image on photoreceptor 706 to produce a visible
image on photoreceptor 706 (e.g., of a separation corresponding to
the color of toner deposited at this image forming module).
Development station 725 is electrically biased by a suitable
respective voltage to develop the respective latent image, which
voltage can be supplied by a power supply (not shown). Developer
734 is provided to toning shell 726 by a developer supply 730,
which can include components such as a supply roller, auger, or
belt. Toner is transferred by electrostatic forces from development
station 725 to photoreceptor 706. These forces can include Coulomb
forces between charged toner particles and the charged
electrostatic latent image, and Lorentz forces on the charged toner
particles due to the electric field produced by the bias
voltages.
[0144] In some embodiments, the development station 725 employs a
two-component developer that includes toner particles and magnetic
carrier particles. The exemplary development station 725 includes a
magnetic core 727 to cause the magnetic carrier particles near
toning shell 726 to form a "magnetic brush," as known in the
electrophotographic art. Magnetic core 727 can be stationary or
rotating, and can rotate with a speed and direction the same as or
different than the speed and direction of toning shell 726.
Magnetic core 727 can be cylindrical or non-cylindrical, and can
include a single magnet or a plurality of magnets or magnetic poles
disposed around the circumference of magnetic core 727.
Alternatively, magnetic core 727 can include an array of solenoids
driven to provide a magnetic field of alternating direction.
Magnetic core 727 preferably provides a magnetic field of varying
magnitude and direction around the outer circumference of toning
shell 726. Further details of magnetic core 727 can be found in
U.S. Pat. No. 7,120,379 to Eck et al., and in U.S. Pat. No.
6,728,503 to Stelter et al., the disclosures of which are
incorporated herein by reference. Development station 725 can also
employ a mono-component developer comprising toner, either magnetic
or non-magnetic, without separate magnetic carrier particles.
[0145] Transfer subsystem 650 includes intermediate transfer roller
ITR1 and transfer backup roller TR1 for transferring the respective
print image from photoreceptor 706 of imaging roller PC1 through a
first transfer nip 701 to surface 716 of intermediate transfer
roller ITR1, and thence to a receiver medium R at a second transfer
nip 702. The receiver medium R receives a respective toned print
image 638 from each image forming module in superposition to form a
composite image thereon. The print image 638 is, for example, a
separation of one color, such as black. Receiver medium R is
transported by transport web 601. Transfer to the receiver medium R
is effected by an electrical field provided to transfer backup
roller TR1 by power supply unit 605, which is controlled by control
unit 620 (FIG. 3). Receiver medium R can be any object or surface
onto which toner can be transferred by application of the electric
field.
[0146] In the illustrated embodiment, the toner image is
transferred from the photoreceptor 706 to the intermediate transfer
roller ITR1, and from there to the receiver medium R. Registration
of the separate toner images is achieved by registering the
separate toner images on the receiver medium R, as is done with the
NexPress 2100. In some embodiments, a single transfer member is
used to sequentially transfer toner images from each color channel
to the receiver medium R. In other embodiments, the separate toner
images can be transferred in register directly from the
photoreceptor 706 in the respective image forming module M1, M2,
M3, M4, M5 to the receiver medium R without using an intermediate
transfer roller. Either transfer process is suitable when
practicing this invention. An alternative method of transferring
toner images involves transferring the separate toner images, in
register, to a transfer member and then transferring the registered
image to a receiver.
[0147] A control system sends control signals to the charging
subsystem 710, the exposure subsystem 720, and the respective
development station 725 of each image forming module M1, M2, M3,
M4, M5 (FIG. 14), among other components. Each image forming module
M1, M2, M3, M4, M5 can also have its own respective controller
system.
[0148] Further details regarding exemplary printer apparatus 600
are provided in U.S. Pat. No. 6,608,641 to Alexandrovich et al.,
and in U.S. Patent Application Publication 2006/0133870, to Ng et
al., the disclosures of which are incorporated herein by
reference.
[0149] In accordance with the present invention, color registration
in the electrophotographic color printer apparatus 600 can be
corrected by using a color registration model to determine
predicted color registration errors 445 as a function of ink
coverage characteristics 430 as has been discussed with respect to
FIGS. 7 and 11. In this case, the "ink" is an electrophotographic
toner.
[0150] In other embodiments, color registration errors can be
reduced by using an image region database 505 which stores
reference image regions 510 together with associated system control
parameters 515 as has been discussed with reference to FIG. 13. In
this case, the system control parameters 515 can include parameters
for correcting the color registration of the printed image. The
color registration can be controlled in various ways such as by
adjusting the image data sent to the exposure subsystem 720, by
adjusting the timing for the exposure subsystem 720 writes the
image data onto the photoreceptor 706, or by adjusting which
exposure elements (e.g., LEDs) in the exposure subsystem 720 are
used to print image pixels of the image data.
[0151] The system control parameters 515 can also include one or
more electrophotographic print engine control parameters that are
used to control a setting on the electrophotographic color printer
apparatus 600. For example, the system control parameters 515 can
include charging level settings for the charging subsystem 710,
exposure level settings for the exposure subsystem 720, bias
voltage settings for the development station 725, bias settings for
the transfer subsystem 650 or fuser settings for the fusing
subsystem. The system control parameters 515 can also control
system attributes such as printing speed.
[0152] 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 spirit and scope of the invention.
PARTS LIST
[0153] 100 printing system [0154] 102 print module [0155] 104 print
module [0156] 106-1 linehead [0157] 106-2 linehead [0158] 106-3
linehead [0159] 106-4 linehead [0160] 108 dryer [0161] 110 quality
control sensor [0162] 111 web tension system [0163] 112 print
medium [0164] 114 transport direction [0165] 116 turnover module
[0166] 118 processing system [0167] 120 storage system [0168] 200
printhead [0169] 202 nozzle array [0170] 204 support structure
[0171] 206 heat [0172] 300 print job [0173] 302 page [0174] 304
page [0175] 306 page [0176] 308 page [0177] 310 page [0178] 312
page [0179] 314 page [0180] 316 page [0181] 390 plot [0182] 400
reference color plane [0183] 402 color plane [0184] 404 in-track
direction [0185] 406 cross-track direction [0186] 408 color plane
[0187] 410 color plane [0188] 414 text region [0189] 416 object
region [0190] 418a ink coverage plot [0191] 418b ink coverage plot
[0192] 418c ink coverage plot [0193] 418d ink coverage plot [0194]
420 document [0195] 420a document [0196] 420b document [0197] 420c
document [0198] 420d document [0199] 422 centerline [0200] 425
analyze document step [0201] 430 ink coverage characteristics
[0202] 435 color registration model [0203] 440 determine predicted
color registration errors step [0204] 445 predicted color
registration errors [0205] 450 determine image plane correction
values step [0206] 455 image plane correction values [0207] 460
print document step [0208] 465 printed document [0209] 470
determine residual registration errors step [0210] 475 residual
registration errors [0211] 480 update color registration model step
[0212] 490 image region [0213] 492 set of in-track positions [0214]
494 in-track position [0215] 495 analyze image region step [0216]
496 determine image data correction values [0217] 497 image data
correction values [0218] 498 print image data step [0219] 499
printed image data [0220] 500 image region [0221] 505 image region
database [0222] 510 reference image region [0223] 515 system
control parameters [0224] 520 compare image regions step [0225] 525
similar reference image region [0226] 530 system control parameters
[0227] 535 print image region step [0228] 540 printed image region
[0229] 545 determine system control parameters step [0230] 550
system control parameters [0231] 555 sufficiently different?
decision step [0232] 560 add image region to database step [0233]
600 printer apparatus [0234] 601 transport web [0235] 602 roller
[0236] 603 roller [0237] 604 densitometer module [0238] 605 power
supply unit [0239] 606 sensor [0240] 609 inter-frame area [0241]
610 light beam [0242] 620 control unit [0243] 624 corona charger
[0244] 625 corona charger [0245] 638 print image [0246] 650
transfer subsystem [0247] 686 cleaning station [0248] 701 first
transfer nip [0249] 702 second transfer nip [0250] 706
photoreceptor [0251] 710 charging subsystem [0252] 711 meter [0253]
712 meter [0254] 713 grid [0255] 716 surface [0256] 720 exposure
subsystem [0257] 725 development station [0258] 726 toning shell
[0259] 727 magnetic core [0260] 730 developer supply [0261] 734
developer [0262] D1 document [0263] D2 document [0264] D3 document
[0265] DN document [0266] ITR1 intermediate transfer roller [0267]
ITR2 intermediate transfer roller [0268] ITR3 intermediate transfer
roller [0269] ITR4 intermediate transfer roller [0270] ITR5
intermediate transfer roller [0271] M1 image-forming module [0272]
M2 image-forming module [0273] M3 image-forming module [0274] M4
image-forming module [0275] M5 image-forming module [0276] PC1
imaging roller [0277] PC2 imaging roller [0278] PC3 imaging roller
[0279] PC4 imaging roller [0280] PC5 imaging roller [0281] R
receiver medium [0282] S slow-scan direction [0283] TR1 transfer
backup roller [0284] TR2 transfer backup roller [0285] TR3 transfer
backup roller [0286] TR4 transfer backup roller [0287] TR5 transfer
backup roller
* * * * *