U.S. patent application number 09/920678 was filed with the patent office on 2003-02-06 for halftoning method and apparatus.
Invention is credited to Krouss, Paul R..
Application Number | 20030025749 09/920678 |
Document ID | / |
Family ID | 25444197 |
Filed Date | 2003-02-06 |
United States Patent
Application |
20030025749 |
Kind Code |
A1 |
Krouss, Paul R. |
February 6, 2003 |
HALFTONING METHOD AND APPARATUS
Abstract
A method of generating with an inkjet printer system a halftone
image from image data comprising a plurality of contone data
values, said system comprising a printhead arranged to print drops
on a print medium whilst moving relative to said print medium in
first and second directions, said method comprising the steps of:
generating a first plurality of halftone data, each of said first
halftone data being generated from a first proportion of a
corresponding contone data value; generating a second plurality of
halftone data, each of said second halftone data being generated
from a second proportion of a corresponding contone data value;
printing drops corresponding to said first plurality of halftone
data in said first direction; and, printing drops corresponding to
said second plurality of halftone data in said second
direction.
Inventors: |
Krouss, Paul R.; (Barcelona,
ES) |
Correspondence
Address: |
HEWLETT-PACKARD COMPANY
Intellectual Property Administration
P.O. Box 272400
Fort Collins
CO
80528-9599
US
|
Family ID: |
25444197 |
Appl. No.: |
09/920678 |
Filed: |
July 31, 2001 |
Current U.S.
Class: |
347/15 |
Current CPC
Class: |
G06K 2215/0094 20130101;
H04N 1/405 20130101; B41J 2/2054 20130101; G06K 15/107 20130101;
H04N 1/40043 20130101; B41J 19/142 20130101 |
Class at
Publication: |
347/15 |
International
Class: |
B41J 002/205 |
Claims
What is claimed is:
1. A method of generating with an inkjet printer system a halftone
image from image data comprising a plurality of contone data
values, said system comprising a printhead arranged to print drops
on a print medium whilst moving relative to said print medium in
first and second directions, said method comprising the steps of:
generating a first plurality of halftone data, each of said first
halftone data being generated from a first proportion of a
corresponding contone data value; generating a second plurality of
halftone data, each of said second halftone data being generated
from a second proportion of a corresponding contone data value;
printing drops corresponding to said first plurality of halftone
data in said first direction; and, printing drops corresponding to
said second plurality of halftone data in said second
direction.
2. A method according to claim 1, wherein the first plurality of
halftone data and the second plurality of halftone data are
generated from the same contone data values.
3. A method according to claim 2, wherein said first and second
proportions of each of said plurality of contone values are
non-equal.
4. A method according to claim 2, wherein the sum of said first and
second proportions of each of said plurality of contone values
substantially equals the value of said corresponding contone
value.
5. A method according to claim 4, wherein said first proportion
lies in the range of 55% to 70% of said corresponding contone
value.
6. A method according to claim 5, wherein said first proportion
substantially equals 60% and said second proportion substantially
equals 40% of said corresponding contone value.
7. A method according to claim 1, wherein said plurality of contone
data values correspond to a color component of said image to be
printed, and said generating and printing steps are repeated in
respect of one or more further pluralities of contone data values,
said one or more further pluralities of contone data values
corresponding to one or more further color components of said image
to be printed.
8. A method according to claim 7, wherein said generating and
printing steps are implemented in respect of at least cyan and
magenta color components.
9. A method according to claim 1, wherein the magnitude of said
first and second proportion of each of said plurality of contone
data values depends upon its respective location in said image.
10. A method according to claim 9, wherein each of said plurality
of contone values is stored in a raster plane having first and
second dimensions, the magnitude of said contone values being
representable in a third dimension, and said first and second
proportions of each of said plurality of contone data values
varying across said raster plane such their relative proportions
may be represented by a three dimensional surface substantially
separating said first and second proportions of said plurality of
contone data values.
11. A method according to claim 10, wherein said three dimensional
surface is undulating or corrugated.
12. A method according to claim 11, wherein number of undulations
or corrugations per unit distance across said image is selected so
that the visual effect of artefacts caused by said three
dimensional surface in said image and noticeable to the human eye
is reduced.
13. A method according to claim 1, wherein said steps of generating
halftone data are carried out using a plane dependent halftone
process.
14. A method according to claim 13, wherein said step of generating
said first plurality of halftone data utilises plane dependent
error diffusion error weightings that are substantially different
from those used to generate said second plurality of halftone
data.
15. A method according to claim 1, further including the steps of
applying first and second printmode masking processes respectively
to said first and second pluralities of halftone data prior to
printing, wherein said first and second printmode masking processes
are substantially different.
16. A method according to claim 1, wherein said drops corresponding
to said first plurality of halftone data and/or said drops
corresponding to said second plurality of halftone data are
distributed over substantially the whole of the image area of said
image to be printed.
17. A method according to claim 1, wherein said steps of printing
are carried out using a 2, 3, or 4 pass printmode.
18. A method according to claim 1, wherein said steps of generating
are carried out in by a processor associated with said inkjet
printer system.
19. A method according to claim 1, wherein said steps of generating
are carried out in by a processor external to said inkjet printer
system, for such as a processor associated with a PC or CAD
system.
20. A method of generating a halftone image from a plurality of
contone data values, with an inkjet printer employing a
bi-directional print mode, the method comprising the steps:
generating first and second sets of halftone data values from said
plurality of contone data values, said first and second sets
comprising substantially different data values, printing elements
of the image corresponding to said first set of halftone data
values in a first print direction; printing elements of the image
corresponding to said second set of halftone data values in a
second print direction.
21. A method of generating a halftone image from contone image data
for printing with an inkjet device having first and second print
directions, comprising the steps of: generating substantially
different first and second sets of halftone image data from said
contone image data; printing a first halftone image corresponding
to said first set of halftone image data in said first print
direction; printing a second halftone image corresponding to said
second set of halftone image data in said second print direction,
superimposed on said first halftone image.
22. A method of generating a halftone print file or halftone print
data from contone image data, for printing with an inkjet device
having first and second print directions, comprising the steps of:
generating substantially different first and second sets of
halftone image data from said contone image data; allocating the
halftone image data of said first set to print operations in the
first direction; allocating the halftone image data of said second
set to print operations in the second direction.
23. A computer program comprising program code means for performing
the method steps of any one of claims 1 to 22 when said program is
run on a computer and/or other processing means associated with
suitable printer device.
24. A printer device arranged to implement the method of any one of
claims 1 to 22.
25. An incremental printer system, comprising a printhead arranged
to print drops on a print medium whilst moving relative to said
print medium in first and second directions, said system being
arranged to generate first and second halftone data from a
plurality of contone data values, each first halftone data being
generated from a first proportion of a corresponding contone value,
each second halftone data being generated from a second proportion
of a corresponding contone value, said system being further
arranged to print drops corresponding to said first plurality of
halftone data in said first direction and to print drops
corresponding to said second plurality of halftone data in said
second direction.
26. A system according to claim 25, comprising an inkjet printer.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to halftone images
and particularly but not exclusively to a method and apparatus for
generating and printing halftone images using ink jet devices
susceptible to bi-directional misalignment errors.
BACKGROUND OF THE INVENTION
[0002] A halftone image consists of a selectively positioned
arrangement of dots of fixed tone levels, which creates the
illusion of a continuous tone, or "contone", image. Thus,
halftoning is a process that allows a continuous tone image to be
modified in order that it may be represented by a device that can
only represent a finite number of tone levels.
[0003] Various halftoning methods have been devised. Initially,
"dithering" or matrix-based techniques were used to halftone
images. In such techniques, a two-dimensional array of values is
mapped over a continuous tone image. The data in each pixel in the
continuous tone image is compared to the matrix value for that
pixel. If the continuous tone value is greater than the matrix
value, then a dot is printed in the corresponding position in the
halftone image.
[0004] Although such halftoning methods are fast, they produce
images that are generally considered to be "grainy". Consequently,
the methods used to halftone images have been refined over the
years to yield images that are more appealing to the eye.
[0005] One such refinement is the so-called "error diffusion"
halftoning process. Error diffusion halftoning is a neighborhood
process. That is to say, when determining whether or not to print a
pixel in a halftone image, the value of the corresponding pixel in
the continuous tone image is considered together with information
pertaining to pixels neighboring that pixel. This process gives
rise to halftone images with reduced grain compared to images
produced by dithering or matrix-based techniques. However, in this
error diffusion process, the different color planes are halftoned
independently. As a consequence, drops in different colors planes
may inadvertently coincide, giving rise to dots of a third,
undesired color. For example, cyan drops that coincide with magenta
drops give rise to dark blue dots.
[0006] This effect creates a degree of graininess, which is
undesirable. Therefore, the basic error diffusion technique has
been further refined to introduce a dependency between the color
planes to overcome this problem. "Plane dependent" error diffusion
utilises an algorithm that ensures that dots in different color
planes do not coincide. Thus, through the precise placement of
drops, plane dependent error diffusion processes generally produce
halftone images that are less grainy and more pleasing to the eye
than those produced by the basic error diffusion process.
[0007] However, drop placement errors in a printing device may
cause a significant decrease in the quality of halftone images
produced using a plane dependent error diffusion process. Errors in
drop placement may be the result of a range of causes, such as:
carriage vibration; paper advance errors; printheads misdirecting
drops; paper deformations; and, bi-directional misalignment. Such
errors disrupt the precise placement of drops that the plane
dependent error diffusion process aims to generate, giving rise to
noticeable "patterning" or "lumpiness". The decrease in image
quality is particularly noticeable if the drop placement errors are
caused by a systematic printer error, such as a print carriage
vibration or a paper advance error, which tend to produce a very
noticeable pattern in the halftone image.
[0008] As is well understood in the art, it is often desirable to
use bi-directional print modes when printing with ink jet printers
in order to increase throughput. By bi-directional print modes, it
is meant that swaths are printed as the printheads travel over the
print media in each direction, as opposed to unidirectional print
modes in which swaths are printed as the printheads travel over the
print media only in one direction. However, bi-directional print
modes not only tend to exacerbate existing drop placement errors
but to cause a further drop placement error problem.
[0009] Generally, when an ink drop is ejected from a nozzle of a
printhead, it is made up of a two or three individual drops. The
first and the largest being generally referred to as the "head" and
the remaining one or more drops, which are significantly smaller,
being generally referred to as the "tail", or "satellite" drops.
The drops making up the tail generally travel slower than the head.
Consequently, the tail of a drop tends to be printed at a different
location on the print media to that of the head. This location is
further along the scan axis in the direction of movement of the
print carriage at the time that the drop was ejected. When printing
in a bi-directional print mode, the effect of satellite drops is to
cause an apparent misalignment between drops printed in either
direction, even if the heads of drops printed in opposite
directions are correctly positioned with respect to each other.
[0010] Satellite drops are particularly noticeable when the overall
drop volume is small, since at small drop volumes the relative
proportion of the tail increases. Currently, the use of drops of
ever decreasing drop volumes is increasing, in order to achiever
higher print resolution. Therefore, when halftone images are
generated using plane dependent error diffusion techniques and
printed with bi-directional print modes, the problem of patterning
increases.
[0011] It would therefore be desirable to provide method and
apparatus for producing halftone images, which overcomes one or
more of the disadvantages associated with the prior art.
SUMMARY OF THE INVENTION
[0012] According to a first aspect of the present invention there
is provided a method of generating with an inkjet printer system a
halftone image from image data comprising a plurality of contone
data values, said system comprising a printhead arranged to print
drops on a print medium whilst moving relative to said print medium
in first and second directions, said method comprising the steps
of: generating a first plurality of halftone data, each of said
first halftone data being generated from a first proportion of a
corresponding contone data value; generating a second plurality of
halftone data, each of said second halftone data being generated
from a second proportion of a corresponding contone data value;
printing drops corresponding to said first plurality of halftone
data in said first direction; and, printing drops corresponding to
said second plurality of halftone data in said second
direction.
[0013] In the present invention, the positions of dots printed in
each direction of a bi-directional printing process are derived
from separate sets of halftone data. Since the separate sets of
halftone data are generated in separate processes, the level of
dependency between the positioning of drops printed in the two
directions may be significantly reduced; i.e. process of
determining the positions of one set of dots may be decoupled from
that determining the positions of one set of dots.
[0014] Thus the present invention provides for a half-toning method
and system that is more robust to misalignments. That is to say
that where misalignment occurs, the "patterning" and/or apparent
change of color resulting from the systematic shift in position of
the drops printed in one print direction relative to those printed
in the other is less noticeable to the human eye.
[0015] The print quality of images generated using the method of
the present invention may have slightly increased grain relative to
certain prior art methods in which there is dependency between the
placement of ink drop printed in each direction and in which no
drop placement error is experienced. However, the expense and
difficulty of producing printing systems that are free from such
errors is in practice often not practicable. Therefore, the present
invention is particularly suited for use with printer devices which
use pens which are either difficult to align, or devices which are
not perfectly aligned. In the first case, printers with a high
number of printheads are generally considered difficult to align.
In the second case, low cost printers generally have a greater
tolerance in pen alignment terms in order to save costs. The
present invention is also particularly suited for use in printing
images with a high proportion of midtones, which are usually most
susceptible to patterning brought about by the combination of
systematic drop placement errors and dependent printing.
[0016] The present invention also extends to the apparatus
corresponding to the claimed methods. Furthermore, the present
invention also extends to a computer program, arranged to implement
the methods of the present invention.
[0017] Other aspects and advantages of the present invention will
be apparent from the following detailed description, together with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] For a better understanding of the invention and to show how
the same may be carried into effect, there will now be described by
way of example only, specific embodiments, methods and processes
according to the present invention with reference to the
accompanying drawings in which:
[0019] FIG. 1 illustrates an ink jet printer arranged to implement
an embodiment of the present invention;
[0020] FIG. 2 shows a flow chart, illustrating the method of an
embodiment of the present invention;
[0021] FIGS. 3a-e illustrate schematically the process by which
direction dependent color data is created and processed in
according to the method illustrated in FIG. 2;
[0022] FIGS. 4a-e illustrate schematically the generation and
content exemplary direction dependent color planes according to the
method illustrated in FIG. 2;
[0023] FIGS. 5 and 6 illustrate the effect of a bi-directional
misalignment on the printed output of the system of an embodiment
of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] There will now be described by way of example only the best
mode contemplated by the inventors for carrying out the invention.
The following description is of an embodiment implemented using two
pass bi-directional printing method and printer. However, the
skilled reader will appreciate that the present invention may also
be used to advantage in bi-directional print modes having a number
of passes that is greater than two.
System of the First Embodiment
[0025] FIG. 1 illustrates an embodiment of an inkjet printing
mechanism, here shown as an inkjet printer 20, which is suitable
for use with the present invention. The printer 20 may be used for
printing conventional engineering and architectural drawings, as
well as high quality poster-sized images, and the like, in an
industrial, office, home or other environment. A variety of inkjet
printing mechanisms are commercially available. Examples of
printing mechanisms that may embody the present invention include
large format printers, office and desk top printers, copiers, and
facsimile machines, to name a few. For convenience the concepts of
the present invention are illustrated in the environment of large
format inkjet printer 20. Commonly assigned U.S. Pat. No.
5,835,108, entitled "Calibration technique for misdirected inkjet
printhead nozzles", describes an exemplary system which can employ
aspects of this invention and the entire contents of which are
incorporated herein by reference.
[0026] While it is apparent that the printer components may vary
from model to model, the typical inkjet printer 20 includes a
chassis 22 surrounded by a housing or casing enclosure 24,
typically of a plastic material, together forming a print assembly
portion 26 of the printer 20. Although the print assembly portion
26 may be supported by a desk or tabletop, it is preferred to
support the print assembly portion 26 with a pair of leg assemblies
28.
[0027] The printer 20 also has a printer controller, illustrated
schematically as a microprocessor 30 that receives instructions
from a host device, which is typically a computer, such as a
personal computer or a computer aided drafting (CAD) computer
system (not shown). The printer controller 30 may also operate in
response to user inputs provided through a key-pad and status
display portion 32, located on the exterior of the casing 24. The
printer controller 30 has associated memory (not shown), which may
include ROM, RAM and a non-volatile data storage module, such as a
high capacity hard disk drive. In this manner, image data to be
printed may be stored when it is downloaded from a host device. The
printer controller 30 may then process the print data as described
in the method of the present embodiment and store the processed
print data once again prior to the printing of the image.
[0028] A monitor coupled to the computer host may also be used to
display visual information to an operator, such as the printer
status or a particular program being run on the host computer.
Personal and drafting computers, their input devices, such as a
keyboard and/or a mouse device, and monitors are all well known to
those skilled in the art.
[0029] A conventional print media handling system (not shown) may
be used to advance a continuous sheet of print media 34 from a roll
through a printzone 35. The print media 34 may be any type of
suitable sheet material, such as paper, poster board, fabric,
transparencies, Mylar.TM., and the like.
[0030] A carriage guide rod 36 is mounted to the chassis 22 to
define a scanning axis 38, with the guide rod 36 slideably
supporting an inkjet carriage 40 for travel back and forth,
reciprocally, across the printzone 35. A conventional carriage
drive motor 41 may be used to propel the carriage 40 in response to
a control signal received from the controller 30. To provide
carriage positional feedback information to the controller 30, a
conventional encoder strip (not shown) may be extended along the
length of the printzone 35 and over the servicing region 42, which
is accessible to the user via access panel 59.
[0031] In the printzone 35, the media sheet receives ink from an
inkjet cartridge, such as a black ink cartridge 50, an enlarged
view of which is shown in FIG. 1, and five monochrome color ink
cartridges 51 to 55. Each of the cartridges, often called "pens" by
those in the art, is mounted on the inkjet carriage 40. The
cartridges 51 to 55 are each arranged to print one of the following
color inks: cyan; magenta; yellow; light cyan; and, light magenta.
In the present embodiment, each of the pens 50 to 55 contains
dye-based ink although pigment based ink could alternatively be
used.
[0032] The illustrated pens 51 to 55 each have a printhead (of
which only printhead 60 of the pen 50 is illustrated in the
figure), which selectively ejects ink to form an image on a sheet
of media 34 in the printzone 35. Each printhead has an orifice
plate with a plurality of nozzles formed therethrough in a manner
well known to those skilled in the art. The nozzles of each orifice
plate are typically formed in at least one, but typically two
generally linear arrays along the orifice plate. These inkjet
printheads have a large print swath (i.e. the height of the band of
a ink that may be printed in one pass of the printhead), for
instance about 20 to 25 millimeters (about one inch) wide or wider,
although smaller inkjet printheads may also be used.
[0033] The illustrated printer 20 uses an "off-axis" ink delivery
system, having main stationary reservoirs (not shown) for each ink
color located in an ink supply region 58. In this off-axis system,
the pens 50-55 may be replenished by ink conveyed through a
conventional flexible tubing system (not shown) from the stationary
main reservoirs. In this manner, only a small ink supply is
propelled by carriage 40 across the printzone 35, which is located
"off-axis" from the path of printhead travel.
[0034] In the present embodiment, the printheads are thermal inkjet
printheads, although other types of printheads may be used, such as
piezoelectric printheads. The thermal printheads typically include
a plurality of resistors associated with the nozzles. Upon
energizing a selected resistor, a bubble of gas is formed which
ejects a droplet of ink from the nozzle and onto a sheet of media
in the printzone 35 under the nozzle. The printhead resistors are
selectively energized in response to firing command control signals
delivered from the controller 30 to the printhead carriage 40.
Method of Operation of the First Embodiment
[0035] Referring to FIG. 2, a flow diagram representing the method
of the present embodiment is illustrated.
[0036] The method is initiated at step 1. At step 3, image data
relating to an image to be printed is generated or processed in a
conventional manner using a suitable image processing device. In
the present embodiment, this is carried out using a personal
computer running an operating system such as Microsoft Window
2000.TM., using a graphics software application such as Adobe
Photoshop.TM.. Once the image has been generated or processed, the
image data may be in any conventional format, such as a bitmap or a
vector format.
[0037] At step 5 it is determined whether the image data is in a
bitmap or another format. If the image data is already in bitmap
form, the method proceeds directly to step 9. However, if the data
is in another format, it is converted into bitmap form in a
conventional rasterization or rendering process at step 7, prior to
proceeding to step 9. Step 5 and the rasterization process of step
7, in the present embodiment, are carried out by the controller 30
of the printer 20. Generally, such rasterization processes are
specific to the printer in question and may be implemented using
hardware, software or firmware. However, the skilled reader will
appreciate that this process may instead by carried out by the CPU
of the personal computer, or indeed a further processing
device.
[0038] At step 9, the controller 30 transforms RGB color pixel data
from the original (or produced) image file to color data specific
to the printing system (including the printer itself, the ink, the
media etc.). Typically, the original (or produced) image file has 8
bits or 256 levels per color. In the present embodiment, this
process is carried out on a conventional manner as is employed by
many of the large format inkjet printers that are currently
commercially available.
[0039] In the present embodiment, the printer 20 is a conventional
six color CMYKcm system, employing 8 bits or 256 levels per color.
As will be appreciated by the skilled reader, in practice any
transformation that is suitable for use with the selected printer
may be used.
[0040] In the present embodiment, the color mapping process of step
9 gives rise to six planes 70a-70f of continuous tone color data
relating to the image to be printed. The color planes 70a-70f,
illustrated schematically in FIG. 3a, respectively store: cyan;
magenta; yellow; black; light cyan; and, light magenta color
information.
[0041] FIG. 4a schematically illustrates an exemplary region of the
continuous tone color data held in the cyan color plane 70a. As can
be seen from the figure, the color data is stored in the form of a
matrix, which corresponds to the dimensions of the image to be
printed. For ease of reference, the columns of the matrix have been
labelled A to E and the rows have been labelled 1 to 4. Each cell
of the matrix stores the continuous tone cyan color value for the
corresponding position in the image to be printed. Furthermore, as
can be seen from the figure, all of the values in the matrix lie
between the values 0 and 255, corresponding to the 256 possible
color levels. It will be appreciated that the structure of each of
the remaining color planes 70b-70f corresponds to that of the cyan
color plane 70a.
[0042] At step 11 the controller 30 divides each color value held
in each of the color planes 70a-70f into two discreet fractions,
which are stored separately in the memory associated with the
controller 30. From this point on in the present embodiment, the
two fractions of each color value are processed and printed
separately. A first fraction of each color value is arranged to be
printed whilst the printer carriage 40 is moving in a first print
direction (left to right as viewed in FIG. 1). The remaining
fraction of each color value is arranged to be printed whilst the
printer carriage 40 is moving in the reverse direction (right to
left as viewed in FIG. 1).
[0043] Although in the present embodiment, the color mapping
process of step 9 is undertaken before the step of splitting the
color planes at step 11, these steps could in practice be
reversed.
[0044] In the present embodiment, the process of step 11 is
implemented in the following manner. For each of the color planes
70a-70f, two direction-dependent color planes are generated; one to
store color data to be printed in the first direction and the other
to store color data to be printed in the reverse direction. Each
direction-dependent color plane is a matrix having the same
dimensions as the color planes 70a-70f and, thus, the same
dimensions as the image to be printed.
[0045] Taking as an example the cyan color plane 70a, a
direction-dependent color plane 72a is generated to store
continuous tone cyan color data that is to be printed in the first
direction. A second direction-dependent color plane 74a is
generated to store continuous tone cyan color data that is to be
printed in the reverse direction.
[0046] Referring to FIGS. 4b and 4c, the process of generating
direction-dependent color planes according to the present
embodiment will now be described.
[0047] FIG. 4b is a matrix that schematically illustrates the
region of the continuous tone color data held in the first
direction dependent cyan color plane 72a. This region corresponds
to the exemplary region of the cyan color plane 70a, shown in FIG.
4a. FIG. 4c is a matrix that similarly illustrates the region of
the continuous tone color data held in the second direction
dependent cyan color plane 74a. This region corresponds also
corresponds to the exemplary region of the cyan color plane 70a,
shown in FIG. 4a. The columns and rows of the matrices shown in
FIGS. 4b and 4c correspond to those of the matrix in FIG. 4a and
are referenced using the same letters and numbers, respectively.
Thus, for example, the cell A1 in the matrices of FIGS. 4b and 4c
correspond to the same location in the image to be printed as does
cell A1 in the matrix of FIG. 4a.
[0048] The color value assigned to each cell of the first direction
dependent color plane 72a is the value held in the corresponding
cell of the color plane 70a multiplied by 0.6. For example, the
value of "44", which is stored in the cell C4 of the matrix of FIG.
4b, is obtained by multiplying the value "72" stored in
corresponding cell C4 of the matrix in FIG. 4a by the factor 0.6.
However, in the present embodiment, where the product of the color
value held in a cell of the color plane 70a and the factor 0.6 is
not an integer value, the product is rounded up to the nearest
integer number prior to being stored in the corresponding cell of
the direction dependent color plane 72a. For example, the value
"57" stored in cell E2 of the matrix in FIG. 4a when multiplied by
the factor 0.6 yields the non-integer value of "34.2". The rounded
up integer value of "35", is stored in the corresponding cell E2 of
the matrix of FIG. 4b.
[0049] The color value assigned to each cell of the second
direction dependent color plane 74a is the value held in the
corresponding cell of the color plane 70a multiplied by 0.4. For
example, the value of "16", which is stored in the cell D4 of the
matrix of FIG. 4c, is obtained by multiplying the value "40" stored
in corresponding cell D4 of the matrix in FIG. 4a by the factor
0.4. However, in the present embodiment, where the product of the
color value held in a cell of the color plane 70a and the factor
0.4 is not an integer value, the product is rounded down to the
nearest integer number prior to being stored in the corresponding
cell of the direction dependent color plane 74a. For example, the
value "193" stored in cell B1 of the matrix in FIG. 4a when
multiplied by the factor 0.4 yields the non-integer value of
"77.2". The rounded down integer value of "77", is stored in the
corresponding cell B1 of the matrix of FIG. 4c.
[0050] In this manner, approximately 60% of the color value
originally held at each pixel location of the color plane 70a is
allocated to the color plane 72a to be printed whilst the printer
carriage 40 is moving from left to right as viewed in FIG. 1.
Approximately 40% of the color value originally held at each pixel
location of the color plane 70a is allocated to the color plane 74a
to be printed whilst the printer carriage 40 is moving from right
to left as viewed in FIG. 1.
[0051] The same process is used to generate two direction-dependent
color planes in respect of each of the six color planes 70a-70f.
Thus, two sets of six direction-dependent color planes are
generated. Thus, color planes 72a-72f are generated to store color
data that is to be printed in the first direction and color planes
74a-74f are generated to store color data that is to be printed in
the reverse direction. Color planes 72a-72f are schematically
illustrated in FIG. 3b, and respectively store: cyan; magenta;
yellow; black; light cyan; and, light magenta continuous tone color
information. Color planes 72a-72f are schematically illustrated in
FIG. 3c, and again, respectively store: cyan; magenta; yellow;
black; light cyan; and, light magenta continuous tone color
information.
[0052] It will be apparent that by rounding the values of the
continuous tone color data up in the case of the values assigned to
color planes 72a-72f and down in the in case values assigned to
color planes 74a-74f, as described in the present embodiment, total
value of continuous tone color data associated with each pixel in
the image to be printed is not substantially changed.
[0053] The skilled reader will also appreciate that in other
embodiments of the invention, the continuous tone color values
assigned to the direction dependent color planes may store
non-integer values, thus ensuring that the degree to which
continuous tone color data associated with each pixel in the image
to be printed is further reduced.
[0054] Returning now to FIG. 2, at step 13, the controller 30
halftones the direction dependent color planes 72a-72f and 74a-74f
of the image and stores the halftone data in the memory associated
with the controller 30. In the present embodiment, this is carried
out using a conventional plane dependent error diffusion halftoning
method. A description of a suitable plane dependent error diffusion
method may be found U.S. Pat. No. 5,949,965, entitled "Correcting
cyan and magenta planes for error diffusion halftoning", in the
name of Hewlett Packard Company.
[0055] In this process, the continuous tone color data to be
printed in the first direction is halftoned separately from the
continuous tone color data to be printed in the second direction.
However, in the present embodiment, the same plane dependent error
diffusion halftoning method is used to halftone the data to be
printed in each direction. Furthermore, in the present embodiment,
the same halftoning weightings are used in halftoning the data to
be printed in each direction.
[0056] Thus, the color planes 72a-72f are halftoned together in a
plane dependent manner, giving rise to six halftone color planes
76a-76f, as are schematically illustrated in FIG. 3d. The halftone
color planes 76a-76f respectively store: cyan; magenta; yellow;
black; light cyan; and, light magenta color data, that is to be
printed in the first direction.
[0057] The color planes 74a-74f are also halftoned together in a
plane dependent manner, giving rise to six further halftone color
planes 78a-78f, as are schematically illustrated in FIG. 3e. The
halftone color planes 76a-76f also respectively store: cyan;
magenta; yellow; black; light cyan; and, light magenta color data,
that is to be printed in the second direction.
[0058] At the conclusion of step 13, each of the halftone planes
76a-76f and 78a-78f, store halftone color data having 1 bit per
color and 2 levels.
[0059] At step 15 of the method of FIG. 2, the controller 30 of the
printer 20 determines which part of the image data stored in the
direction dependent color planes 76a-76f is to be printed in the
first swath; i.e. the first pass by the carriage assembly 40 over
the print media in the first print direction (left to right as
viewed in FIG. 1). This process, generally known as "swath
processing", is well understood in the art of conventional ink jet
printers. For the purposes of illustration, the data 80a, held in
color plane 76a, that contributes to an exemplary swath is
schematically illustrated in FIG. 3d. It will of course be realised
that corresponding "bands" of data held in each of the remaining
planes 76b-76f will also contribute to the same swath as that of
data 80a. The data 80b, held in color plane 76b that contributes to
a similar exemplary swath is schematically illustrated in FIG.
3e.
[0060] The skilled read will appreciate that in the present
embodiment, the printer 20 acts as though it is printing two images
simultaneously, one in the first direction and the other in the
second direction. Swaths are thus generated in the same way as
generated in a traditional printer, however, alternating from on
print file to the other.
[0061] In the present embodiment, as is conventional in many two
pass bi-directional print modes, when each swath is printed the
print media is advanced by a distance equal to half of the swath
height. Generally therefore, half of each swath printed overlaps
the preceding swath and the remaining half of the swath is
overlapped by the subsequent swath. The first swath to be printed
is generally an exception to this arrangement. In this case, the
first swath is only half of the full swath height and is wholly
overlapped by the half of the subsequent swath. A similar case
arises with the last swath to be printed.
[0062] Therefore, the data retrieved from the color planes 76a-76f
in the first swath processing step of step 15 corresponds to a band
of half the full swath height arranged across the leading edge of
the image to be printed.
[0063] At step 17, the retrieved data is processed according to the
selected printmode in a conventional manner. As is known in the
art, the printmode defines various factors that determine how the
image is printed, such as the nature of the printmode mask or masks
used in printing the image and the number of passes to be used.
[0064] When the print mode processing of step 17 is completed, the
processed print data is printed, at step 19, as the carriage
assembly 40 passes over the print media in the first print
direction.
[0065] At step 21, the controller 30 determines whether an even
number of swaths have been completed. As yet, only the first swath
has been printed, therefore, the process proceeds to step 23. The
skilled reader will of course realise that many other methods of
determining the direction of printing exist, and may be used in
conjunction with the present embodiment.
[0066] At step 23, the controller 30 updates an internal register
identifying the data in the color planes 76a-76f which has been
printed, in order to correctly identify the subsequent data in
color planes 76a-76f which is to be included in the next swath to
be printed in the first direction.
[0067] The controller 30 also updates an internal register
indicating that the next swath to be printed is in the reverse
print direction.
[0068] The skilled reader will appreciate that many methods for
determining the print direction and data to be printed in the next
pass are available and may be used with the present invention.
[0069] The process then proceeds to step 15 a further time, in
which the controller 30 again determines which image data is to be
printed in the next swath. This is carried out in a similar manner
to the previous swath processing step. However, as the next swath
is to be printed in the reverse print direction, the data is
retrieved from the corresponding direction dependent color planes;
i.e. planes 78a-78f. Additionally, as explained above, this and
subsequent swaths to be printed are of the full swath height of the
printheads. Thus, the data retrieved from color planes 78a-78f in
step 15 corresponds to a band across the leading edge of the image
to be printed, with a height equal to the swath height of the
printheads.
[0070] At step 17, the controller 30 processes the retrieved
halftone data for printing the reverse direction swath according to
the selected print mode. In the present embodiment, the same
printmode is used for print passes in both the forward and reverse
directions.
[0071] At step 19, the processed print data is printed as the
printer carriage 40 moves in the reverse direction back across the
print media (i.e. left to right as viewed in FIG. 1).
[0072] As has been explained above, in the present print mode the
print media is advanced by a distance equal to half of the swath
height after each swath is printed. Thus, the upper half of this
swath overlaps with the whole of the reduced height swath printed
in the first pass of the carriage assembly 40.
[0073] As has been explained above, the method of the present
embodiment means that the ink drops printed in the first and
reverse directions are positioned in the image in a manner which is
not dependent. Thus, the degree of "patterning" found in images
printed according to the present embodiment remains low even if a
bi-directional error is present in the system.
[0074] Referring to FIGS. 5 and 6, this benefit is illustrated
graphically. FIG. 5a illustrates a portion of an exemplary area
fill, printed using the method of the present embodiment, whilst
the printheads were moving in the first direction. FIG. 5b
illustrates the same portion of the area fill, printed whilst the
printheads were moving in the reverse print direction. In this
figure, no significant bi-directional misalignment is present and
the competed exemplary area fill (i.e. FIG. 5a superimposed on FIG.
5b) is shown in FIG. 5c.
[0075] FIG. 6 illustrates the effect of a bi-directional
misalignment on the printing performance of the system of the
present embodiment when printing the exemplary area fill of FIG. 5.
As was the case in FIG. 5, FIG. 6a illustrates the portion of the
area fill printed whilst the printheads were moving in the reverse
print direction and FIG. 6b illustrates the portion of the area
fill, printed whilst the printheads were moving in the reverse
print direction. The completed area fill (i.e. FIG. 6a superimposed
on FIG. 6b) is shown in FIG. 6c. Due to the lack of dependency
between the portions of the image printed in the different
directions, no appreciable degradation in the print quality is
noticeable in the completed area fill shown in FIG. 6c, relative to
that printed by the system when no bi-directional misalignment is
present, shown in FIG. 5c; i.e. there is little or no increased
"patterning" or "clumpiness".
[0076] The inventor has found that this benefit may be achieved
simply by halftoning the part of the image to be printed in one
direction independently from the remainder of the image. In
practice, the level of dependency between the positioning of drops
printed in either print direction has been found to increase as the
proportion of image data that is printed in each direction
approaches 50%. However, the ratio of 60% of the image data being
printed in one direction and 40% being printed in the other has
been found to adequately reduce this dependency.
[0077] In a deterministic system, 50%/50% split of the color data
will result in identical files to be printed in the first and
second directions. This, however, is not desirable when attempting
to remove the dependencies between the image data printed in each
direction. Thus, unless a further perturbation is introduced into
the system (such as applying different error diffusion weightings
to the two direction-dependent halftoning processes, as discussed
below), using a 50%/50% split of the color data is not desirable in
aiming to achieve the advantages of the present embodiment.
[0078] Returning now to FIG. 2, at step 21, the controller 30
determines, that an even number of swaths have been printed. The
process consequentially moves to step 25, where the controller 30
determines whether the current print job is completed or not; i.e.
whether further swaths remain to be printed. In the event that
further swaths remain to be printed, the process proceeds again to
step 23.
[0079] At step 23, the controller 30 updates a further internal
register identifying the data in color planes 78a-78f which has
been printed in order to correctly identify the subsequent data in
color planes 78a-78f which is to be included in the next swath to
be printed in the reverse direction.
[0080] The controller 30 also updates the internal register
identifying the print direction of the next swath to be printed, to
indicate that the next swath is to be printed in the first print
direction.
[0081] In this manner, the process of the present embodiment
selects direction dependent halftone data from the appropriate
direction dependent color planes at the correct time for processing
and printing.
[0082] The process described with reference to steps 15 to 25
continues until the print job is complete at step 25, whereupon the
printing operation of the present procedure is terminated at step
27.
Further Embodiments
[0083] In the above description numerous specific details are set
forth in order to provide a thorough understanding of the present
invention. It will be apparent however, to one skilled in the art,
that the present invention may be practiced without limitation to
these specific details. In other instances, well known methods and
structures have not been described in detail so as not to
unnecessarily obscure the present invention.
[0084] For example, the skilled reader will appreciate that
dependency between the positioning of drops printed in each print
direction may be reduced or indeed eliminated in a variety of
ways.
[0085] By applying different error diffusion weightings to the two
direction-dependent halftoning processes, the dependency may be
reduced or even substantially eliminated. The extent to which the
different error diffusion weightings should differ from one another
will depend upon the image being printed and the characteristics of
the system, including the printer and data processing techniques
used. However, suitable different error diffusion weightings for
use with the two direction-dependent halftoning processes may be
found by routine experimentation.
[0086] The skilled reader will also appreciate that the direction
dependent halftone data may also be processed using differing print
masks in order to remove the dependency between the positioning of
drops printed in the different print directions.
[0087] The manner in which image is allocated to one or other of
the direction-dependent continuous tone color planes may be varied.
In the above described embodiment, a simple 60%/40% split was used
uniformly across each continuous tone color plane. The skilled
reader will appreciate that this ratio may be varied. As has been
commented above, the level of dependency between the positioning of
drops printed in either print direction has been found to increase
as the proportion of image data that is printed in each direction
approaches 50%. Furthermore, it will be appreciated that if
sufficient image data is printed in one print direction, it will
begin to approximate unidirectional printing and thus lose the
advantages of bi-directional printing. However, the skilled reader
will appreciate that the 60%/40% ratio of the above embodiment may
be varied significantly between these two extremes whilst retaining
the benefits of the invention.
[0088] Furthermore, a method other than a simple uniformly split,
common to each color plane, may be used to allocated continuous
tone color data to one or other of the direction-dependent
continuous tone color planes.
[0089] For example, the method may allocate a varying proportion of
the color data to the respective direction-dependent color planes.
This may be dependent upon the position of the color data in the
image to be printed. For example, for each color plane, a varying
distribution profile may be used. The varying distribution profile
may resemble a corrugated surface, for example, situated within
each color plane. The proportion of each color data value that lies
below the corrugated surface may be allocated to be printed in the
first direction and the proportion of each color data value that
lies above the corrugated surface may be allocated to be printed in
the reverse direction.
[0090] Clearly, the frequency of the corrugation may be varied to
suit the type of image being printed and to avoid artefacts that
are easily perceived by the human eye. Thus, in areas of
comparatively continuous color a high frequency corrugation may be
used since they are less easily noticed by the human eye. However,
in such areas, it would be undesirable to use a low frequency
corrugation may be very noticeable since it may result in a varying
color hues across the area. Furthermore, the orientation of the
corrugation (for example, whether the corrugation run horizontally,
vertically or at another angle to the image to be printed) may also
be varied to suit the image to be printed.
[0091] The skilled reader will appreciate that the characteristics
and positioning of such a varying distribution profile may be
generated by an algorithm or stored in a look up table internal or
external to the printer.
[0092] Although the above-described embodiment was described with
reference to plane dependent error diffusion halftoning, the
skilled reader will appreciate that the present invention may
equally be used to counter print defects that occur in
bi-directional print modes as a result of a bi-directional error,
whether or not plane dependent error diffusion halftoning is used.
Thus it will be apparent that the separate halftoning of dots to be
printed in different print directions may be used address problems
caused by other types of dependency that occur when a
bi-directional error is present in the printing system. For example
dependencies may occur between pens. This may be between pens of
different colors, the same color, or of varying dye levels, such as
normal cyan and light cyan. Furthermore, intra-pen dependencies
exist, in insofar as the position of dots of one color are
determined generally from one source (the raster plane of that
color), regardless of whether they are printed in a bi-directional
print mode or not.
[0093] As will be apparent to the skilled reader, in recent times,
the processes of rasterization, color mapping and halftoning are
commonly carried out by hardware or firmware associated with the
printer, in order to allow the host PC to be freed up for other
tasks as soon as possible. However, it will be appreciated any
combination of these processes may instead be carried out outside
the printer; for example in the printer driver software running on
the host PC. The skilled reader will thus appreciate that the
process of splitting the halftone planes, at step 11 of FIG. 2, or
carrying out the print mode processing at step 11 of FIG. 2 in a
direction dependent manner may also be carried out by a controller
not associated with the printer.
[0094] Furthermore, it will be apparent to the skilled reader that
various process commonly applied to images during the half toning
process may also be applied to the portions of the image that are
printed in both the first print direction and the reverse print
direction; for example: superpixelling and edge enhancement
processes.
[0095] Although the above described embodiment was described with
reference to a six color ink system, it could equally be applied to
a printing system with an increased or decreased range of pens or
colors. For example, the invention could be applied with benefit to
a four color ink system having CMYK inks, or an octachrome system
with CMYK, medium cyan, medium magenta, orange and green inks.
* * * * *