U.S. patent application number 09/773052 was filed with the patent office on 2002-10-10 for multiple marker inkjet printing for increased print speed.
Invention is credited to Boleda, Miquel, Encrenaz, Michel, Vilanova, Ferran.
Application Number | 20020145643 09/773052 |
Document ID | / |
Family ID | 25097053 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020145643 |
Kind Code |
A1 |
Vilanova, Ferran ; et
al. |
October 10, 2002 |
Multiple marker inkjet printing for increased print speed
Abstract
Techniques for printing ink drops in an inkjet printer, where at
least one color ink can be printed by two (or more) printheads. The
drops to be printed are divided between the two printheads with
purposes of higher printer velocity, higher image quality and
higher print reliability.
Inventors: |
Vilanova, Ferran;
(Barcelona, ES) ; Encrenaz, Michel; (Rubi
Barcelona, ES) ; Boleda, Miquel; (Barcelona,
ES) |
Correspondence
Address: |
HEWLETT-PACKARD COMPANY
Intellectual Property Administration
P.O. Box 272400
Fort Collins
CO
80528-9599
US
|
Family ID: |
25097053 |
Appl. No.: |
09/773052 |
Filed: |
January 31, 2001 |
Current U.S.
Class: |
347/43 ;
347/15 |
Current CPC
Class: |
G06K 15/107 20130101;
G06K 15/102 20130101; H04N 1/4052 20130101; B41J 2/51 20130101;
B41J 2/2139 20130101 |
Class at
Publication: |
347/43 ;
347/15 |
International
Class: |
B41J 002/21; B41J
002/205 |
Claims
What is claimed is:
1. A method for printing ink drops in an inkjet printing system
where image pixels of a given color can be printed by two or more
printheads ejecting droplets of said given color and mounted in a
scanning printhead carriage, the method comprising: receiving print
data from a print job source; producing from the print data a first
plane for said given color; replicating the first plane to produce
a second plane for said given color; using said first plane to
produce a first set of printhead data signals for producing
droplets, and said second plane to produce a second set of
printhead data signal for producing droplets; applying said first
and second sets of printhead data signals to first and second
printheads each for emitting droplets of said given color.
2. The method of claim 1 wherein said step of using said first
plane and using said second plane includes applying a first mask to
said first plane to produce said first set, and applying a second
mask to said second plane to produce said second set.
3. The method of claim 2 wherein said first and second masks are
complementary.
4. The method of claim 1 wherein said applying step results in said
first printhead ejecting 50% of the drops for said given color, and
said second printhead ejecting 50% of the drops for said given
color.
5. A method for printing ink drops in an inkjet printing system
where image pixels of a given color can be printed by two or more
printheads each including a nozzle array for ejecting droplets of
said given color and mounted in a scanning printhead carriage which
moves over a print media in successive passes and swaths, and
wherein a first printhead nozzle array has one or more failed
nozzles, the method comprising: receiving print data from a print
job source; producing from the print data a first plane for said
given color; replicating the first plane to produce a second plane
for said given color; using said first plane to produce a first set
of printhead data signals for producing droplets, and said second
plane to produce a second set of printhead data signals for
producing droplets, wherein pixel data to be printed using said one
or more of said failed nozzles is redirected to be printed by a
corresponding equivalent one or more nozzles on a second printhead
nozzle array; applying said first and second sets of printhead data
signals to first and second printheads each for emitting droplets
of said given color, thereby providing nozzle substitution for said
one or more failed nozzles.
6. The method of claim 5, wherein said step of applying said first
and second sets of printhead data signals to said first and second
printheads is performed during the same print swath of carriage
movement so that said nozzle substitution occurs during a single
swath without requiring a media advance.
7. A method for printing ink drops in a color inkjet printing
system using yellow, cyan, magenta and black inks, and wherein
black image pixels are printed by two or more printheads ejecting
droplets of black ink, comprising: providing print data defining a
print job, said print data in respective yellow, cyan and magenta
data planes and a first black data plane; using said first black
plane to produce a first set of black printhead data signals for
producing black ink droplets, and said second black plane to
produce a second set of black printhead data signals for producing
black ink droplets; using said yellow, cyan and magenta data planes
to respectively produce a set of yellow printhead data signals, a
set of cyan printhead data signals and a set of magenta data
signals; applying said first and second sets of black printhead
data signals to first and second black ink printheads, said set of
yellow printhead data signals to a yellow ink printhead, said set
of cyan printhead data signals to a cyan ink printhead and said set
of magenta printhead data signals to a magenta ink printhead.
8. The method of claim 7 wherein said step of using said first
black plane and using said second black plane includes applying a
first mask to said first black plane to produce said first set, and
applying a second mask to said second black plane to produce said
second set.
9. The method of claim 8 wherein said first and second masks are
complementary.
10. The method of claim 7 wherein said applying step results in
said first printhead ejecting 50% of black ink droplets for said
print data, and said second printhead ejecting 50% of the black ink
droplets for said print data.
11. A method for printing ink drops in an inkjet printing system
where image pixels of a given color can be printed by two or more
printheads ejecting droplets of said given color and mounted in a
scanning printhead carriage, the method comprising: receiving print
data from a print job source; producing from the print data a first
plane for said given color comprising contone values; processing
the contone values to produce two or more different planes of
respective half-tone values corresponding to a number of ink
droplets to be printed to represent respective pixels by said two
or more printheads; using said two or more planes of half-tone
values to produce corresponding two or more sets of printhead data
signals for producing droplets; applying said two or more sets of
printhead data signals to said two or more printheads each for
emitting droplets of said given color.
12. The method of claim 11, wherein said processing step employs an
error diffusion half-toning algorithm.
13. The method of claim 11, wherein said processing step employs a
plane-dependent half-toning algorithm.
14. A method for printing ink droplets in a color inkjet printing
system using yellow, cyan and magenta inks and a secondary color
ink, yellow, cyan and magenta inks being primary colors of ink, and
wherein image pixels of the secondary color are printed by two or
more printheads ejecting droplets of said secondary color ink,
comprising: providing respective yellow, cyan and magenta and said
secondary color data planes, said respective planes defining a
colored image to be printed by the printing system; processing the
secondary color data plane to determine first and second pseudo
primary color planes whose addition results in the secondary color
plane; using said first pseudo primary plane to produce a first set
of secondary color printhead data signals for producing secondary
color ink droplets, and said second pseudo primary plane to produce
a second set of black printhead data signals for producing
secondary color ink droplets; using said yellow, cyan and magenta
data planes to respectively produce a set of yellow printhead data
signals, a set of cyan printhead data signals and a set of magenta
data signals; applying said first and second sets of secondary
color printhead data signals to first and second secondary ink
printheads, said set of yellow printhead data signals to a yellow
ink printhead, said set of cyan printhead data signals to a cyan
ink printhead and said set of magenta printhead data signals to a
magenta ink printhead.
15. The method of claim 14 wherein the first pseudo primary plane
is a pseudo cyan plane, the second pseudo primary plane is a pseudo
yellow plane and the secondary color is black.
16. A method for printing ink droplets in a color inkjet printing
system using yellow, cyan and magenta inks and a secondary color
ink, yellow, cyan and magenta inks being primary colors of ink, and
wherein image pixels of the secondary color are printed by two or
more printheads ejecting droplets of said secondary color ink,
comprising: providing respective yellow, cyan and magenta and said
secondary color data planes, said respective planes defining
respective contone values for pixels of a color image to be printed
by the printing system; converting the contone values of the
respective yellow, cyan and magenta color data planes into
respective values corresponding to a number of ink droplets to be
printed to represent respective pixels; converting the contone
values of the secondary color plane into respective first and
second pseudo primary color plane values corresponding to
respective numbers of ink droplets of the respective first and
second primary colors whose addition results in the secondary color
plane; using said first pseudo primary plane values to produce a
first set of secondary color printhead data signals for producing
secondary color ink droplets, and said second pseudo primary plane
values to produce a second set of secondary color printhead data
signals for producing secondary color ink droplets; using said
yellow, cyan and magenta data planes values to respectively produce
a set of yellow printhead data signals, a set of cyan printhead
data signals and a set of magenta data signals; applying said first
and second sets of secondary color printhead data signals to first
and second secondary ink printheads, said set of yellow printhead
data signals to a yellow ink printhead, said set of cyan printhead
data signals to a cyan ink printhead and said set of magenta
printhead data signals to a magenta ink printhead.
17. The method of claim 16 wherein the converting steps are
performed using an error diffusion halftoning algorithm.
18. The method of claim 16 wherein the first pseudo primary plane
is a pseudo cyan plane, the second pseudo primary plane is a pseudo
yellow plane and the secondary color is black.
19. A method for printing ink drops in an inkjet printing system
where image pixels of a given color can be printed by two or more
printheads ejecting droplets of said given color and mounted in a
scanning printhead carriage, the method comprising: receiving print
data from a print job source; producing from the print data a first
plane for said given color comprising contone values; processing
the contone values by an n-bit half-toning algorithm to produce
respective half-tone values corresponding to a number of ink
droplets to be printed to represent respective pixels, wherein n is
an integer larger than 1, so that the half-tone values can
represent multiple dots per pixel; using said half-tone values to
produce multiple levels of printhead data signals for producing
said dots; dividing said multiple levels of printhead data signals
between the two or more printheads; applying said divided levels of
printhead data signals to said two or more printheads each for
emitting droplets of said given color.
20. The method of claim 19 wherein said given color is black.
21. The method of claim 19 wherein said two or more printheads
consists of a first printhead and a second printhead mounted on
said scanning carriage.
22. The method of claim 21 wherein a lower level is assigned to
said first printhead, and an upper level is assigned to said second
printhead.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] This invention relates to inkjet printing using two or more
printheads of the same color to increase printing speed.
BACKGROUND OF THE INVENTION
[0002] Inkjet printing systems are in common use today. An ink jet
printer forms a printed image by printing a pattern of individual
dots at particular locations of an array defined for the printing
medium. The locations are conveniently visualized as being small
dots in a rectilinear array. The locations are sometimes "dot
locations", "dot positions", or "pixels". Thus, the printing
operation can be viewed as the filling of a pattern of dot
locations with dots of ink.
[0003] Ink jet printers print dots by ejecting very small drops of
ink onto the print medium, and typically include a movable carriage
that supports one or more printheads each having ink ejecting
nozzles. The carriage traverses over the surface of the print
medium, and the nozzles are controlled to eject drops of ink at
appropriate times pursuant to command of a microcomputer or other
controller, wherein the timing of the application of the ink drops
is intended to correspond to the pattern of pixels of the image
being printed.
[0004] Color ink jet printers commonly employ a plurality of
printheads, for example four, mounted in the print carriage to
produce different colors. Each printhead contains ink of a
different color, with the commonly used colors being cyan, magenta,
yellow, and black. These base colors are produced by depositing a
drop of the required color onto a dot location. Secondary or shaded
colors are formed by depositing drops of different colors on
adjacent dot locations; the human eye interprets the color mixing
as the secondary or shading, through well known optical
principles.
SUMMARY OF THE INVENTION
[0005] The present invention relates to a method to print ink drops
in an inkjet printer, and particularly to a printing system and
method where at least one color ink can be printed by two (or more)
printheads. The method divides the drops to be printed between the
two printheads with purposes of higher printer velocity, higher
image quality and higher print reliability.
BRIEF DESCRIPTION OF THE DRAWING
[0006] These and other features and advantages of the present
invention will become more apparent from the following detailed
description of an exemplary embodiment thereof, as illustrated in
the accompanying drawings, in which:
[0007] FIG. 1 is a diagram flow chart that describes an exemplary
print data pipeline for an exemplary inkjet printing system.
[0008] FIG. 2 is a schematic flow diagram of a printing system data
pipeline which provides separation of the black data into two
planes, for two black printheads K1/K2.
[0009] FIG. 3 is an isometric view of an inkjet printing system
employing two printheads of the same color in accordance with an
aspect of this invention.
[0010] FIG. 4 is a schematic block diagram of the control system
for the printing system of FIG. 3.
[0011] FIG. 5 illustrates an exemplary flow diagram for
implementing an alternate embodiment of a technique for printing
black ink droplets using two black print heads.
[0012] FIG. 6 illustrates an exemplary data pipeline for
implementing a embodiment of the invention.
[0013] FIG. 7 is an exemplary print data pipeline with an error
hiding function in accordance with a further aspect of the
invention.
[0014] FIG. 8 is a schematic illustration of exemplary nozzle
arrays for the K1 and K2 printheads.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] An objective of the invention is to provide higher speed
modes by using an extra black print head in an inkjet printing
system. An exemplary application for a printing system using the
invention is in a large format printing system for the CAD market,
i.e the technical user market (engineers, architects, and the like)
who typically print line drawings and image renderings. In an
exemplary embodiment, the use of two black print heads enables a
print speed of 60 inches per second (ips) and allows laying down
two drops per 600 dpi cell (two drops are needed to provide fair
optical density). Thus, a methodology is described to print with
two black print heads in an inkjet printing system, i.e. how the
black channel information is processed through the printing system
data pipeline and gets separated to each print head in order to
print either the same information or complementary information. Of
course, the invention is not limited to increasing speed with two
black printheads, but could be employed in using two or more
printheads of the same color, e.g. two cyan printheads or three
black printheads.
[0016] FIG. 1 is a diagrammatic flow chart that functionally
describes an exemplary data pipeline 50 for an inkjet printing
system. For this embodiment, the inkjet printing system is a color
printing system which includes respective printheads for ejecting
cyan (C), magenta (M), yellow (Y) and black (K) inks. Print data
are provided from the printer driver in RGB (red-green-blue) format
to a color mapping function 52, which maps the RGB data into KCMY
data, at a resolution of 8 bits per pixel for this example. For
this example, the color mapping function converts 8-bit (0-255
values) RGB data into 8-bit KCMY data using an interpolation table.
The KCMY data is then processed by a half-toning function 54, to
provide KCMY data at 1 or 2 bits per pixel. Next, the half-toned
data is processed by a PCL encapsulation function, to produce KCMY
data at 1 or 2 bits per pixel. For this example, the PCL
encapsulation re-formats the information so that the printer can
interpret it; there is no change in resolution or in content for a
given pixel. The PCL print data is then passed through a print mode
masking function 58, and the masked data is employed to print
droplets. This function distributes the print data in different
passes for image quality purposes.
[0017] As used herein, a "channel" or "plane" for a given color
refers to the pixel information to be printed with the given color.
As can be seen in FIG. 1, the black channel or plane propagates all
the way down the pipeline 50 like the other channels/planes (CMY)
from the contone stage (8-bit, coming from the driver) to the
actual drops emitted from the printheads during step 60. The
separation of the black plane into two planes can occur in any of
the steps in FIG. 1, when a fifth plane is added to handle the
second black printhead, since there should preferably be a plane
dependency between the two black printheads.
[0018] An exemplary technique to make the plane separation is that
the data for the two black printheads propagate all the way down
the pipeline as a single black plane, i.e. through color mapping
52, half-toning 54 and PCL encapsulation 56, and at the masking
level (step 58) the separation occurs. Thus, mask level separation
occurs at the print mode/mask level 58. There, five printheads are
defined so that different masks can be laid down for each of the
black printheads, referred to as printheads K1 and K2. In essence,
for this exemplary embodiment, the two masks created are
complementary, so that at the end of the total number of passes,
all the required drops are fired as they were a single unique mask
for black. An example of that is the following. Assume that the
print mode is a 3-pass print mode, 600*600 dpi 2-bits (4 levels),
in which the drops per level are 0, 1, 1, 2. The printing grid for
this example is 2400*600 (half-toned horizontal resolution is
divided by four), and the mask size is 64.times.32. The print mode
has four levels, level 0, level 1, level 2 and level 3. Level 0
contains only 0s, and so all the relevant information is contained
in levels 1, 2, 3. This is part of the architecture of this
exemplary print engine.
[0019] The single or unique mask for the case with only one black
printhead K for this example is the following:
1 Pass 1/Level 1 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 Pass 1/Level 2 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0
0 0 0 0 0 1 0 0 0 0 0 0 0 Pass 1/Level 3 1 1 0 0 0 0 0 0 1 1 0 0 0
0 0 0 1 1 0 0 0 0 0 0 1 1 0 0 0 0 0 0 Pass 2/Level 1 0 0 0 0 1 0 0
0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 Pass 2/Level 2 0
0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 Pass
2/Level 3 0 0 0 0 1 1 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1
1 0 0
[0020] The foregoing example shows a mask of two rows and sixteen
columns. The printer controller tiles down this elemental mask to
the whole height of the printhead, so that facing the tiled mask to
each individual nozzle in the printhead each row of the mask lines
up with one nozzle of the printhead. Each mask row corresponds to
the information to be fired by each nozzle where "1" means drop
being fired and "0" means no drop is fired.
[0021] The example also shows different masks for different passes
and levels. This means that the dots are distributed along
different print passes. The levels refer to the different levels of
half-toning. The half-toning is used to enable multiple firing in
each pixel location. Thus, the print engine is able to, for
example, place one drop for level 1, three drops for level 2 and
six drops for level 3. If there are four levels, level 0, level 1,
level 2 and level 3, level 0 has no information. Thus, there is a
mask for each mask for each level and for each color. This allows
combination of the different firing sequences of drops (single or
multiple) of different color inks and in different instants of time
(print passes).
[0022] The exemplary unique (single printhead) mask shown above
contains many 0s. If all the print masks are summed, all 1s,
corresponding to the completion of all the information to be fired)
would still not result. The reason for this is that a finer
printing grid is used at the half-toning stage. For this exemplary
embodiment, half-toning is done at 600 dpi, and the printing is at
2400 dpi, so that each pixel is divided into four locations. This
finer grid allows for dot locations one next to the other, instead
of one on top of the other in multi-drop printing.
[0023] Multi-drop printing can be achieved by using several print
passes. Alternatively, multi-drop printing can be achieved by using
a finer grid than that of the half-toning (where the levels are
defined), so that dots can be placed in adjacent pixel locations of
the half-toning grid, even in just one pass, although there are
print head firing frequency limitations. In this example, both ways
are used to achieve multi-drop printing. Bits or levels of the
half-toning function correspond to the drops per pixel in different
ways. The following table shows the different possibilities.
2 1-bit 2 levels: level 0, Drops: 0, 1 (level 0, level 1 level 1)
2-bit 4 levels: level 0, Drops: 0, 1, 2, 3 level 1, Drops: 0, 1, 3,
4 level 2, level 3 Drops: 0, 2, 3, 6 etc
[0024] The bits determine the half-toning leveling (1-bit=2 levels,
2-bits=4 levels, . . . ), while the correspondence of levels to
drops per pixel can vary from one print mode to another.
[0025] The unique mask for a single color can be of arbitrary size.
The controller "tiles" the mask to the whole print file (vertically
and horizontally). "Tiling" the mask is a known technique whereby a
defined mask of limited dimensions, i.e. x pixels in width by y
pixels in height, is replicated all through the height and width of
the print file. For this example, there are only six levels, where
six is the maximum number of drops to be fired, but there can be
less or more. The masks in the different levels can be viewed as a
big mask of different layers, one layer per level.
[0026] Now, in accordance with an aspect of the invention, two
black print heads K1, K2 are employed, with two mask sets, as
follows:
3 K1 / Pass 1 / Level 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 1 0 0 0 0 0 0 0 K1 / Pass 1 / Level 2 1 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 K1 / Pass 1 / Level 3 1 0 0 0
0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 K1 / Pass 2
/ Level 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0
0 0 0 K1 / Pass 2 / Level 2 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 1 0 0 0 0 K1 / Pass 2 / Level 3 0 0 0 0 1 0 0 0 0 0 0
0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 K2 / Pass 1 / Level 1 0 0
0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 K2 /
Pass 1 / Level 2 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 K2 / Pass 1 / Level 3 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0
1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 K2 / Pass 2 / Level 1 0 0 0 0 0 0 0
0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 K2 / Pass 2 /
Level 2 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0
0 0 K2 / Pass 2 / Level 3 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 1
0 0 0 0 0 0 0 1 0 0 0
[0027] As can be seen, the two sets above are complementary and the
sum of them is the mask set with a unique K. This is evident from
the following sum operations.
4 K1 + K2/Pass 1/Level 1 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0
0 0 0 1 0 0 0 0 0 0 0 K1 + K2/Pass 1/Level 2 1 0 0 0 0 0 0 0 1 0 0
0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 K1 + K2/Pass 1/Level 3 1
1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1 0 0 0 0 0 0 K1 +
K2/Pass 2/Level 1 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0
0 0 1 0 0 0 0 K1 + K2/Pass 2/Level 2 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0
0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 K1 + K2/Pass 2/Level 3 0 0 0 0 1
1 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1 0 0
[0028] The following is the result of summing Pass 1 and Pass 2
(Pass 1+Pass 2)
5 Level 1 1 0 0 0 1 0 0 1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 0 1 0 0 0 1 0
0 0 0 Level 2 1 0 0 0 1 0 0 1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 0 1 0 0 0
1 0 0 0 0 Level 3 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1
1 0 0 1 1 0 0
[0029] In the foregoing example for level 3, the first 8 mask
locations (first 4 in the top row and first 4 in the bottom row)
represent a 600 dpi cell, while one of the mask locations
represents a 2400 dpi cell. The result of adding the information of
the K1 and K2 masks for this example will result in the following
number of drops per level.
6 2-bit 4 levels: level 0, Drops: 0 (level 0), 2 level 1, level 2,
(level 1), 2 (level level 3 2), 4 (level 3)
[0030] In this exemplary case, the two mask sets (K1 and K2 ) are
each loaded with 50% of the drops, but other patterns could
alternatively be employed, wherein an unequal drop distribution
between the two printheads is used.
[0031] FIG. 2 is a schematic flow diagram of a printing system data
pipeline 100 which provides separation of the black data into two
planes, for two black printheads K1/K2. This pipeline includes the
initial three steps 52, 54, 56 as in the pipeline 50 of FIG. 1, to
perform color mapping (52), half-toning (54) and PCL encapsulation
(56) in the incoming print data. The print data from step 56 is
passed to a plane replication function 102, which replicates the K
data (single plane) into two identical black planes for the K1, K2
printheads, producing K1 K2 CMY data at 1 or 2 bits per pixel for
this embodiment. This data is then passed to the print mode masking
function 104, where the separate K1/K2 masks are applied to the
data, as well as the CMY masks, producing K1K2 CMY 0, 1, 2, . . . 6
. . . drops per pixel. The data from the print mode masking
function is then passed to the print function to produce the actual
drops during printing operations.
[0032] Thus, in accordance with an aspect of the invention, after
the PCL encapsulation at step 56, the black plane is copied to a
new one and thus the K information is exactly the same for this
other K2 plane. There are now two black planes, for K1 and K2.
Complementary print masks are used to split the data between the
two black printheads prior to printing. The mask sets for each
plane are less than 100% coverage (for example, 50% and 50%). If
not, twice the number of drops would be fired. Of course, more than
two black printheads could be employed, e.g. by replicating the
black plane twice, i.e. forming K1, K2, K3 planes, and then
providing complementary print mask sets for the three planes.
[0033] The complexity of the masks will grow with its size and
design requirements but overall it is an easy method and easy to
implement, since the print data pipeline is not modified
drastically.
[0034] FIG. 3 is an isometric view of an inkjet printing system 200
employing two printheads of the same color in accordance with an
aspect of this invention. The printing system 200 is shown without
its outer housing structure to expose the printheads. The printing
system includes a frame structure 202 and a printhead carriage
structure 204. The frame supports a slider bar 206, and the
carriage structure is driven along the slider bar by a carriage
drive system. An output media tray 210 is supported above an input
media tray 212 which provides a source of print media. A pick
system picks the top sheet of media from the tray 212, and a media
drive system transports the picked sheet along a media path to the
print zone generally indicated as 214. The carriage structure 204
has five printhead stalls for holding five removable inkjet
printheads 220, 222, 224, 226, 228, which for this exemplary
embodiment are C/K1/K2/M/Y printheads, respectively. Thus, for this
example, the two black printheads are located between the cyan and
magenta printheads. Other ordering of the respective printheads
could be used. However, this ordering facilitates a technique
placing cyan or magenta drops before and after a black drop to
provide higher optical density for the black areas. These extra
drops are not information in the print file, but are used to obtain
a better ink-media interaction resulting in higher optical density.
In bi-directional print modes, cyan drops are placed ahead of the
two black drops (K1, K2), and in the reverse print direction, the
magenta drops precede the two black drops.
[0035] The printing system 200 further includes five off-carriage
ink supplies 230, 232, 234, 236, 238, each for holding a supply of
ink. Each ink supply is fluidicially coupled to a corresponding
printhead by a fluid conduit, here including a flexible tube
comprising tube set 240. As the carriage traverses its path of
travel during printing operations, the printheads are connected to
corresponding ink supplies to replenish the ink supplies on-board
each printhead. Each ink supply can be replaced when its ink is
exhausted.
[0036] FIG. 4 is a schematic block diagram of the control system
for the printing system 200 of FIG. 3. A controller 250 such as a
microcomputer or ASIC receives print job commands and data from a
print job source 72, which can be a personal computer, digital
camera or other source of print jobs. The controller implements the
functions of the print data pipeline 100 (FIG. 2), acts on the
received commands to activate the pick roller motor 260 to pick a
sheet from the input tray 212, advance the sheet to the media
drive, e.g. to a nip between a drive roller and pinch roller set,
and activate the drive motor system 262 to advance the sheet onto
the belt, and move the belt to advance the sheet to the print zone.
The carriage drive 264 is driven by the controller to position the
carriage 204 for commencement of a print job, and to scan the
carriage along the slider rod 206. As this is done firing pulses
are sent to the printheads 220-228. The controller receives encoder
signals from the carriage encoder 268 to provide position data for
the carriage. The controller is programmed to advance incrementally
the sheet to position the sheet for successive swaths, and to eject
the completed sheet into the output tray 210.
[0037] FIGS. 3 and 4 represent an exemplary embodiment of an inkjet
printing system embodying aspects of the invention. Other inkjet
printing systems could alternatively be employed, including systems
with disposable printhead cartridges and no off-carriage ink
supplies, systems with greater than five printheads, by way of
example only.
[0038] Other techniques for separating the black plane into two
black planes could alternatively be employed. For example, in a
second embodiment, the black print data is a unique plane until
they reach the half-toning step. Two K planes are generated during
half-toning. The data for each K plane is different, and these two
different K planes propagate through the remainder of the pipeline,
i.e. through the PCL ensapsulation and print mode masking steps to
the printing step (FIG. 1). Further advantages can be optionally
provided by using plane dependent half-toning, by using error
diffusion half-toning, and by treating the two K planes as
secondary colors in the half-toning algorithm. Conventional
printmode masking is employed in this embodiment, i.e the masks for
the two K planes are not complementary as in the embodiment of FIG.
2, and in fact are preferably identical.
[0039] Error diffusion algorithms for half-toning are well known.
In general, error diffusion is used to convert from the "continuous
tone" (contone) stage of the information to the "half-toned" stage.
In the contone stage, the information can be encoded with 8 bits,
for example, and therefore the possible values of a channel are
from 0 to 255 (example RGB values on a display or CMYK values sent
from a driver to a printer). Thus, each pixel has the color
information encoded with one byte (one byte is 8 bits) for each
color or plane. If for example, the input format is CMYK, the pixel
has possible values C[0-255], M[0-255], Y[0-255], K[0-255].
[0040] The half-toning step provides a reduction in the amount of
information for each pixel, while preserving overall color
characteristics. After the half-toning, the information for each
color or plane is encoded with generally 1, 2 or 3 bits of
information. Consider the case of two-bits/color output
half-toning. A color channel enters the half-toning step with
possible values [0-255], and output with only four possible values,
namely 0, 1, 2, 3 (two bits of information can only represent four
different values). For the case of one bit output half-toning, also
called binary half-toning, the only possible output values for one
color at the output are 0 or 1, since one bit only has two states.
In the case of three-bit half-toning, possible output values for
the color of a pixel are [0-7], since 3 bits can encode 8 different
values.
[0041] The error diffusion principle is the following. When the
pixel reaches the half-toning step, it is attributed an output
value that minimizes the error, i.e., the mismatch between the
output value that is attributed to the given color for the pixel
and the input value of this given color for this pixel. For
example, consider a pixel of an image reaching the half-toning step
with a value cyan=140, in an 8-bit scale [0-255]. For two-bit
half-toning, the only possible output values are 0, 1, 2 or 3.
Thus, none of the possible output levels or values (0, 1, 2, 3, or
0%, 33.3%, 66.6%, 100%) represents exactly the input value
(140/255, or 54.9%), so an error is committed. If the output level
for this pixel is set to level 2 (66.6%), the error is 11.7%. If
the output level for the pixel is set to level 1 (33.3%) for this
color of this pixel, the error reaches 21.56%. The error would be
even higher if level 0 or level 3 is used. A goal of error
diffusion half-toning techniques is to reduce this error.
[0042] To illustrate error diffusion half-toning, consider the
following example. A pixel enters with a cyan level of 45 (out of
255), and is attributed level 1 out of 4. This pixel is "over
attributed" 7% on the cyan channel, i.e. the error is +7%. This
error is then summed to the next pixel's input value of the same
channel before deciding which output is given to the neighbor. For
this example of a 7% error, this error would be added on the cyan
channel before deciding which output is given to the neighboring
pixel. This is the process of error diffusion.
[0043] Plane dependent error diffusion goes one step further, as it
defines dependencies between specific planes (generally between
cyan and magenta). So in this case, the error committed over one
plane is taken into account, but the sum of errors of each
dependent plane is calculated for a pixel to decide its output
value. This is described in U.S. Pat. No. 6,057,933, the entire
contents of which are incorporated herein by this reference. This
allows the avoidance of pixels accumulating drops from the
dependent planes (say one pixel with cyan+magenta drops) if the
neighboring pixels do not have drops, and therefore leads to a
noticeable reduction of the "grainy" aspect on inkjet print
samples.
[0044] A second technique for separating the black plane into
separate planes for the two black printheads, i.e. for deciding
which ink drops will be printed by each black printhead, is
described. In this case, the separation of the black plane is done
in the half-toning function. An exemplary embodiment of this
technique takes advantage of the "plane dependant error diffusion"
functionality described above, and described in U.S. Pat. No.
6,057,933, although the invention is not limited to error diffusion
or plane dependent half-toning. In this embodiment, then, the black
plane is separated at the half-toning step, and the two black
output planes (which carry different information) are printed using
the classical processes as if they were different colors. The
method is random and the droplet printing order is not
deterministic. The following example will be applied to the example
of a printer with two black printheads, such as printer 200 (FIG.
3).
[0045] This alternate technique includes several steps:
[0046] i) The RGB image data or KCMY data is received by the
printing system from a computer or print job sender device.
[0047] ii)) The image data corresponding to the black color plane
will be processed as if it were a secondary color (for example,
green). This means that the printer controller 250 will calculate
and assign ink drops from two different primary color planes (in
the example cyan and yellow) to achieve the desired secondary
color. The result of this step is two color planes whose addition
would result in the original black image half-toned data. The
droplets have been assigned to each plane using the plane dependant
error diffusion algorithm described above. In order to minimize the
grain aspect due to the black drops, a dependency is established
between the two planes that contain the black information, e.g. the
cyan and yellow planes for the example.
[0048] iii) The resulting print data in 2 color planes from step
(ii) will be directed to respective two black printheads which will
deliver black droplets to the paper to form the desired image.
Thus, the processor calculates the black plane as though it is to
be produced using a composite of two primary colors, e.g. cyan and
yellow, and then sends this pseudo "cyan" and "yellow" print data
to two black print heads instead of a cyan and a yellow print head.
Of course, the print data for these primary colors to make up the
black data is kept separate from the actual print data for the cyan
and yellow channels, by assigning a fifth (black 2) plane for the
second black channel.
[0049] FIG. 5 illustrates an exemplary flow diagram for
implementing this technique for printing black ink droplets using
two black print heads. FIG. 5 shows alternate cases, for an RGB
driver as well as for a CMYK driver. For the case of an RGB driver,
the RGB contone print data 152 is received, and the black (K) data
is extracted at 154 to provide KRGB data at 156. There are various
techniques for extracting black information from RGB data. One
technique is to use a lookup table that associates given values for
K, R, G, B for a given set of input R, G, B values. Another
technique is to use an algorithm, such as the following example:
(i) Calculate the minimum (K.sub.o) of R, G, B values. (ii)
Calculate the difference between K.sub.o and the value of each
channel, i.e. dR is the difference on R, dG is the difference on G,
and dB is the difference on B. (iii) Calculate K=255-K.sub.o. (iv)
Set for an RGB value the KdRdGdB values. These are just
illustrative examples for extracting the black information.
[0050] At step 158, the K plane is copied to a G' color plane,
producing RGBG' print data at 160. At 162, the RGBG' data is first
converted into G'CMY data, and the G' data is split into C'+Y'
data, wherein the G' plane is to be printed by special cyan and
yellow planes, C', Y', i.e. G'=C'+Y'. A half-toning process is then
performed on the data, producing half-toned CMYC"Y" data at
174.
[0051] In the case of a KCYM contone driver, the KCMY contone data
164 is processed at step 166, where the K plane is copied to form
the G' color plane, producing CMYG' print data (168). This data is
half-toned at 170, producing G'=C'+Y', and thus half-toned CMYC'Y'
data at 174.
[0052] At step 174, the data are independent of the particular
driver, and the C' and Y' planes are set to the K1, K2 planes
respectively, at 176, producing print data CMYK1K2 data at 178.
This data are now passed through the classical masking process
(step 58, FIG. 1), where K1, K2 are treated as if they were
different colors, and the drops are ejected. The masks used to
print K1, K2 in this case are not complementary, and in fact can be
identical. The masks with this alternate embodiment will deliver
any pixel information from the color plane K1 or K2 to the print
media, unlike the case with the first embodiment (FIG. 2) where a
drop is fired by either one or the other black printhead.
[0053] Thus, in an exemplary form, the second embodiment (FIG. 5)
takes advantage of the plane dependent error diffusion half-toning
technique. To print a given secondary color, ink drops from primary
colors will be calculated and assigned to pixels, following a
half-toning technique that will randomize the spatial positions of
the drops for each of the primary colors. Plane dependent error
diffusion techniques provides some level of spatial exclusion, so
that drops fired by each of the primary color printheads will tend
to occupy different places on the print media. This provides less
granularity to the printed images, and this is particularly
appropriate for this application where ink drops are to be printed
with the same color (black in this example). For example, when
printing a gray color, it would be desirable to cover as much white
paper as possible for a certain ink density rather than placing
several black drops together in some places and leaving large white
areas on other places. The first procedure delivers a more uniform
color appearance, whereas the second procedure causes the presence
of dark black areas and dull white areas with a perception of
granularity.
[0054] Of course, this second technique for separating the black
plane into two planes could be employed with other techniques for
half-toning, and is not limited to error diffusion half-toning.
[0055] Another technique for separating the black plane into two
planes is that one black print head is mainly printing all the
black pixels that contain low density (1 or 2 drops). The other
black printhead helps out with those pixels demanding more drops so
that these can be printed in less passes. Thus, for this alternate
embodiment, more than one black ink drop is delivered to the pixels
on the print media so that the desired ink density is achieved.
Exemplary multilevel printing techniques are employed in the
Hewlett-Packard PhotoRet system, for example, used in some HP
DeskJet printers such as the HP DeskJet 970 and the Professional
Series 2000. Multi-level printing is illustrated in the following
table.
7 Equivalent in # of drops 2-Bit Encoding (exemplary only) Level 0
00 0 Level 1 01 1 Level 2 10 2 Level 3 11 4
[0056] In accordance with a third embodiment for separating the
black print data into two planes, certain of the levels, e.g.
levels 00, 01, 10, are assigned to the first black printhead (K1),
and the level 11 is assigned to the second black printhead (K2). In
this exemplary case, if a pixel contains a 11, the second black
printhead (K2) will fire 4 drops on that pixel, whereas a pixel
value of 01 or 10 will be printed by the first printhead (K1).
[0057] The following table with exemplary pixel input data further
illustrates the operation of this third embodiment.
8 Position Input Data K1 Drops K2 Drops Pixel #1 01 1 0 Pixel #2 00
0 0 Pixel #3 10 2 0 Pixel #4 11 0 4
[0058] Thus, this third embodiment is particularly adapted to a
printer having the capability to print multiple dots per pixel, for
example 2 bits or four levels. The output pixel data levels or
values are divided between the printheads of the same color, so
that for example one prints the dots for lower levels and another
prints higher levels, or in the case of three printheads of the
same color, a third printhead prints the dots for an intermediate
level (s). The result of a split between lower and higher levels is
that one black printhead does most of the work and is occasionally
helped by the second black printhead for very dark areas. This will
mean that the first printhead will have to be replaced more
frequently, and will lay down more ink, but that the second
printhead will need to be replaced only very infrequently.
[0059] FIG. 6 illustrates an exemplary data pipeline for
implementing this embodiment of the invention. For this exemplary
embodiment, steps 52, 54, 56 and 102 are identical to the
corresponding steps of the pipeline shown in FIG. 2, and result in
two identical black data planes, K1, K2. Step 120 is a plane
filtering step. Here the two identical planes for K1 and K2 are
filtered or divided in such a way that the pixel data for levels
assigned to one printhead are set to 0s in the data plane of the
other printhead, and vice versa. For the example where there are
four levels, and level 0 is all zeros with no pixel information,
the pixel data for level 3 in the K1 data plane is set to all 0's
since level 3 data is to be printed by the K2 printhead, and the
pixel data for levels 1 and 2 in the K2 data plane are set to all
0's, since levels 1 and 2 are to be printed by the K1 printhead.
Standard printmode masking and printing steps 122 and 106
follow.
[0060] As an alternative to the plane filtering step 120, the
levels could be assigned to K1 and K2 respectively, by setting the
print mode mask levels for one black plane which are assigned to
the other plane to all 0's, and vice versa.
[0061] In accordance with a further aspect of the invention, error
hiding (nozzle replacement or substitution) can be performed
between the two printheads of the same color very easily and
effectively. That is, if a nozzle on one printhead fails, the data
that should have been printed by that nozzle can be redirected to
the equivalent nozzle, in the same relative location, on the other
printhead. This in turn means that the error hiding or nozzle
substitution can be performed within the same swath. In contrast,
if error hiding is performed within the same printhead, then, since
each nozzle passes over a unique part of the print media, the
backup nozzle cannot print the failed nozzle's data in the same
pass, but must wait for a media advance. Techniques for error
hiding are described in commonly assigned application Ser. No.
08/810,467, filed Mar. 4, 1997, DYNAMIC MULTI-PASS PRINT MODE
CORRECTIONS TO COMPENSATE FOR MALFUNCTIONING INKJET NOZZLES, and in
commonly assigned application Ser. No. 09/506,740, filed Feb. 18,
2000, METHOD OF PRINTING TO AUTOMATICALLY COMPENSATE FOR
MALFUNCTIONING INKJET NOZZLES, the entire contents of which
applications are incorporated herein by this reference.
[0062] This error hiding function is particularly well suited to
implementation with the embodiment described above regarding FIG.
2, and an exemplary print data pipeline 300 with the error hiding
function is shown in FIG. 7. The steps 52, 54, 56, 102 are
identical to those comprising the pipeline 100 of FIG. 2, and
identical K1 and K2 data planes are established. Step 130 indicates
a step of identifying failed nozzles, e.g. nozzles which do not
fire or which misdirect their ink drops. Typically for purposes of
this aspect of the invention, the nozzle arrays for printheads K1
and K2 will be tested to identify failed nozzles. Techniques for
identifying failed nozzles are known in the art. For example, the
printer can periodically check for failed nozzles by printing a
test pattern on a print medium, and then optically scan the test
image with a carriage-mounted optical sensor to detect nozzles
which have failed. Another technique is to use an optical drop
detector in a service station to detect blocked or misdirected
nozzles. Suitable techniques are described in the above-references
application Ser. Nos. 08/810,467 and 09/506,740. The step 130 can
be performed each time a print job is to be printed, or
periodically and the results saved for use until another test of
the nozzles is performed.
[0063] With the failed nozzles identified (130), the print mode
masking step 132 is performed. Here, the same complimentary masks
as described above with respect to the embodiment of FIG. 2 provide
the initial masks to be applied to the K1 and K2 data planes.
However, to implement error hiding of the identified failed
nozzles, the data that should have been printed by the failed
nozzles on one black printhead are redirected to the equivalent
nozzle, in the same relative location, on the other black
printhead. This is illustrated in FIG. 8, which shows exemplary
nozzle arrays for K1 and K2. Assume for example that nozzle #2 of
K1 has failed. The data which should have been printed by that
nozzle is instead printed by nozzle #2 of K2. This result can be
readily achieved by use of the print mode masks, where the mask
position for nozzle #2 of K1 is changed from 1 to 0, and the mask
position for Nozzle #2 of K2 is changed from 0 to 1. Thus, the
resulting masks are no longer strictly complementary due to the
error hiding. Now the masks are applied to the identical data
planes K1, K2 to print the dots at 106.
[0064] Thus, the present invention relates to a method to print ink
drops in an inkjet printer, and particularly to a printing system
and method where at least one color ink can be printed by two (or
more) printheads. The method divides the drops to be printed
between the two printheads with purposes of higher printer
velocity, higher image quality and higher print reliability.
[0065] It is understood that the above-described embodiments are
merely illustrative of the possible specific embodiments which may
represent principles of the present invention. Other arrangements
may readily be devised in accordance with these principles by those
skilled in the art without departing from the scope and spirit of
the invention.
* * * * *