Note: Descriptions are shown in the official language in which they were submitted.
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188403 -
INR DROP PLACEMENT FOR IMPROVED IMAf3INf3
TECHNICAL FIELD
The present invention pertains to ink-jet
printing methods for controlling ink drop placement
to
improve the appearance of the printed image.
BACKGROUND INFORMATION
Ink-jet printers include one or more pens for
delivering drops of ink to a printing medium, such
as
to paper. An ink-jet pen typically includes a nozzle
plate
that has formed in it a plurality of nozzles. The
nozzles
are in fluid communication with an ink reservoir.
Any of a number of mechanisms may be employed for
expelling ink through the nozzles of the pen. For
instance, one mechanism, known as thermal-type ink-jet
printing, includes a thin-film resistor mounted
adjacent
. to each nozzle. To expel a drop of ink from a nozzle,
a
current pulse is applied to the resistor for heating
the
resistor. The heated resistor vaporizes a portion
of the
ink near the nozzle. The rapid expansion of the
ink vapor
forces a drop of ink through the nozzle. This ''firing"
of
drops is controlled by a microprocessor in response
to
external data that is provided to the printer and
that
represents part of the desired image to be printed.
The ink-jet printer includes mechanisms for
moving the pen and for advancing the paper relative
to the
_~.~--' pen. Typically, the pen is scanned across the paper
one
or more times, the paper is advanced, and the pen
ie again
scanned across the paper. The microprocessor-controlled
- 30 firing of selected nozzles at selected times during
scanning of the pen produces on the paper an arrangement
of ink dots in a resolution high enough to represent
an
image or textual information.
A measure of the quality of an ink-jet printed
image is the uniformity of the printed ink density
across
the surface of the image. Preferably, individual
ink
drops will penetrate the permeable printing medium
and
diffuse evenly through the medium, joining with
adjacently
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ieeaoa -
printed drops to form a continuous image element
of
substantially uniform ink density.
' Unevenness in ink density causes an undesirable
mottled appearance in the printed image. Uneven
ink
density may result in instances where adjacently
printed
ink drops randomly coalesce prior to penetration
of the
drops into the printing medium. This coalescence
problem
frequently occurs when the printing medium has low
permeability, such as is characteristic of the clear
films
that are used for overhead projection displays.
Various methods have been used in the past to
control ink drop placement for producing uniform,
high-
density images. For example, U.S. Patent No. 4,748,453,
entitled "Spot Deposition for Liquid Ink Printing,"
discloses a method wherein drops of ink printed
in one
3 scan of the pen are placed on the medium in a pattern
that
is intended to prevent overlap of flowable ink drops,
thereby eliminating coalescence. A subsequent scan
of the
pen (which scan is delayed until the previously
deposited
drops sufficiently dry) prints new drops that overlap
the
previously printed drops. While this techniquejmay
be
somewhat effective, the pattern in which the drops
are
. printed during one scan results in diagonally adjacent
drops being in tangential or perimeter contact.
It has
been found that this perimeter contact between
simultaneously printed individual drops will cause
coalescence of at least some of the diagonally adjacent
drops. This uneven or random coalescence of ink
drops
will produce the mottled image mentioned earlier.
Placing drops on the printing medium in a manner
that avoids any contact between simultaneously printed
drops will eliminate the problem of drop coalescencet
however, the overall ink density of the image will
be
reduced because of the corresponding increase in
the
amount of printing medium area that is exposed between
drops, or more scans of the printhead over the printing
medium will be required to achieve adequate ink
density.
CA 02048048 2000-O1-OS
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SUMMARY OF THE INVENTION
According to one aspect of the present
invention there is provided a method for placing ink
drops onto a print medium from an ink-jet printing
device, said ink-jet printing device having an ink-
jet pen for scanning across and directing said ink
drops onto the medium, said print medium having a
surface defined as a matrix of pixels arranged as
adjacent horizontal rows and vertical columns of
pixels, the method comprising the steps of:
during a first scan of said pen across said
medium, directing a series of first clusters of at
least two ink drops in each of said first clusters
into contact with a respective series of first pixel
groups, each of said first pixel groups having at
least two adjacent pixels in a first of said rows
and a first of said columns, the drops of each of
said first clusters being directed to overlap, such
that one of said drops is substantially covering one
of said adjacent pixels each in each of said first
pixel groups, upon contacting each of said first
pixel groups respectively;
directing a series of second clusters of at
least two ink drops in each of said second clusters
into contact with a respective series of second
pixel groups, each of said second pixel groups
having at least two adjacent pixels in a second of
said rows and a second of said columns, the drops of
each of said second clusters being directed to
overlap, such that one of said drops of each of said
second clusters is substantially covering one of
said adjacent pixels each in each of said second
pixel groups, upon contacting each of said second
pixel groups respectively,
CA 02048048 2000-O1-OS
- 3a -
wherein each of said first clusters and each of
said second clusters are directed so that pixel
groups horizontally and vertically adjacent to each
of said first pixel groups and each of said second
pixel groups are substantially uncovered by said
first clusters and said second clusters; and
during at least one subsequent scan across said
medium,
directing subsequent clusters having at least
two ink drops in each of said subsequent clusters
into contact with pixel groups that are horizontally
and vertically adjacent to each of said first pixel
groups and each of said second pixel groups.
By way of added explanation, the present
invention in one embodiment thereof is directed to
an ink drop placement method for applying ink to a
printing medium to produce a high-density image
without mottling. The placement method involves
controlling a conventional ink-jet pen so that drops
of ink are fired in clusters of two or more for
covering selected portions or " pixels" of the
printing medium. As the clusters are printed during
a scan of the pen, pixels that are horizontally and
vertically adjacent to each printed cluster remain
blank. Contact between clusters that are printed
during one pen scan is limited to tangential or
perimeter contact between two drops of diagonally
adjacent printed clusters.
The drops that comprise each cluster overlap in
the center of the cluster. This intentional
overlapping of drops within the central region of
the printed clusters results in a concentration of
forces due to surface tension at the center of the
cluster. Accordingly, during the period immediately
following the instant a cluster is printed, the
cluster-drops tend to coalesce toward the center of
the printed cluster. This internal or central
CA 02048048 2000-O1-OS
- 3b -
coalescence within each printed cluster resists
coalescence force that may arise as a result of
tangential contact between diagonally adjacent
printed clusters.
The coalescence that occurs within the printed
clusters (which coalescence is attributable to the
overlapped arrangement of the ink drops in each
cluster) does not produce observable mottling
because this coalescence is present in all printed
clusters, thereby producing a substantially uniform
appearance across the entire printed image.
Moreover, the amount of the areal overlap of drops
within a printed cluster is established to be
greater than the amount of areal contact that occurs
between tangentially adjacent clusters.
Consequently, the central coalescence force within
each printed cluster (that force being a function of
the surface tension of the ink and the area of
overlap of the cluster drops) is greater than the
coalescence force developed between
20~80s~8
190403
tangentially contacting clusters. As a result, random
coalescence between tangentially contacting clusters is
substantially eliminated.
The method for placing drops in accordance with
the present invention may be employed for printing or
covering 50% of a printing medium region with one scan of
the pen. If desired, 100% coverage may be obtained by
applying the same 50% coverage pattern (albeit offset by
one cluster in both the horizontal and vertical
directions) during a second scan of the pen.
The arrangement of clusters for producing the 50%
coverage pattern mentioned above is such that when a 50%
pattern is converted into a 100% pattern during the second
scan of the pen, substantially no printing medium is
exposed.
As another aspect of this invention, a method for
placing drop clusters to produce a 25% coverage pattern is
disclosed. The 25% coverage pattern is particularly
useful in instances where the ink drop size cannot be
controlled to prevent excessive cluster to cluster overlap
(hence, coalescence) with the 50% coverage patt~rn
mentioned above.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a diagram of an ink-jet printer that is
suitable for carrying out the ink drop placement method of
the present invention.
Fig. 2 is a diagram showing a medium printed with
a preferred arrangement of four-drop clusters covering 50%
of the printing medium surface.
Fig. 3 fs a diagram showing a medium printed with
an arrangement of two-drop clusters covering 50% of the
printing medium surface.
Fig. 4 is a diagram showing a medium printed with
an arrangement of two-drop clusters covering 25% of the
printing medium surface.
. Fig. 5 is a diagram showing a medium printed with
another arrangement of two-drop clusters covering 25% of
the printing medium surface.
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ieavo3 _ 5
Fig. 6 is a diagram showing a medium printed with
another arrangement of four-drop clusters covering 25% of
the printing medium.
DETAILED DESCRIPTION
The diagram of Fig. 1 illustrates an ink-jet
printer 20 having known mechanisms for securing and
advancing relative to the printer a printing medium, such
as a sheet of film 22 used with an overhead projector.
The film 22 is advanced relative to, and in close
l0 proximity with, an ink-jet pen 24: The pen 24 is mounted
to a slide mechanism 26. As is known in the art, the pen
24 is reciprocally driven along the slide mechanism 26
between the side edges 28 of the film 22. The film 22 is
advanced in a direction 30 that is perpendicular to the
direction 32 in which the pen 24 is reciprocated. The
movement of the pen 24 from one edge 28 to another is
hereafter referred to as a scan. All ink drops printed by
the pen 24 during a single scan will be referred to as
being simultaneously printed.
The pen 24 includes a conventional nozzle plate
(not shown) that has formed in it a plurality of nozzles
through which drops of ink are expelled by mechanisms such
as the thermal-type system described above. As the pen 24
is scanned across the film 22, it "covers" a single swath
34. A single swath 34 is illustrated in Fig. 1 as the
space between the dashed reference lines 36 that extend
between the film edges 28. Hy "covered" is meant that the
pen 24 may be controlled during the scan for firing ink
drops through the nozzles to cover any selected area
within the swath 34. A new swath is defined as the film
22 is advanced. Each new swath is immediately adjacent to
the prior swath to ensure printing continuity from the top
to the bottom of the film 22.
It is convenient to consider each swath 34,
hence, the entire film surface, as being defined as a
continuous-matrix of discrete elements, or pixels. As
shown in Fig. 2, the pixels 40 may be considered as four-
sided surface elements arranged in horizontal (that is,
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188403 -
left to right in Fig. 2) rows and vertical (top to bottom
in Fig. 2) columns. Reference lines 42 are shown
for the
purpose of illustrating the boundaries that define
individual pixels 40.
As mentioned earlier, the printer 20 is
controlled for advancing the film 22, and the pen
24 is
scanned across the swath 34 as selected pen nozzles
are
fired so that ink drops may be placed at selected
pixels
40 within the swath 34. In accordance with the present
invention, the printer 20 is controlled to place
over
. selected pixels 40 ink drops that are arranged so
that
there results a substantially uniform-density image
with
substantially no random coalescence of ink.
In accordance with a preferred ink placement
method of the present invention, the ink-jet pen
24 is
fired in a manner such that a cluster 44 of four
ink drops
44a, 44b, 44c, 44d is delivered to the printing
medium 22
to cover four pixels. In the figures, printed ink
drops
are represented by circular solid lines that define
the
outer boundary of the printed drops. Accordingly,
overlapping circular lines represent overlapping
ink
drops.
The cluster 44 is formed by timing the pen firing
so that the four ink drops 44a-d overlap symmetrically
across the common boundaries 42 of the four pixels
40 that
are covered by the cluster 44. In this regard, the
vertical .common boundary of one group of drop-covered
. pixels 40 is indicated by line V in Fig. 2. The
w horizontal common boundary of a group of drop-covered
pixels 40 is indicated by line H. For convenience,
any
four adjacent pixels that are covered (or may be
covered
by) a single drop cluster 44 will be referred to
as a
pixel group 45. One such uncovered group 45 is depicted,
for illustration purposes with cross-hatched lines
in
Fig. 2.
Each drop 44a-d of a cluster 44 is sized to
Y
completely cover the area within boundary lines
42 of a
pixel 40. As shown in Fig. 2, the drops 44a-d also
x extend
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2~~-~~~8
189409 - 7 -
slightly beyond these surface area boundaries 42 in the
horizontal and vertical directions. This extension
permits pixel to pixel overlap where complete ink coverage
of the swath 34 is desired, as described more fully below.
Hest results are achieved where the diagonally adjacent
drop pairs 44a, 44d and 44b, 44c of a four-drop cluster 44
are directed to land immediately adjacent to one another
without overlapping.
Fig. 2 illustrates a 50% coverage pattern. A 50%
coverage pattern means that 50% of the surface area in a
particular region of the printing medium is covered with
ink, and the remaining 50% is not covered with ink.
Accordingly, with single color (e.g., black) printers, a
50% coverage pattern is useful for producing a shade of
grey.
The preferred 50% coverage pattern for printing
the pixels 40 in the swath 34 (Fig. 2) is such that every
other pixel group 45 in a row of pixel groups, and every
other group 45 in a column of pixel groups 45, is printed
with a drop cluster 44. Put another way, pixels 40 that
are horizontally and vertically adjacent to printed
clusters are substantially uncovered by the drop
clusters 44.
y
The printed drop clusters 44 are arranged to be
in tangential contact (that is, contact between ink drop
edges with substantially no overlap) with other clusters
44 that are diagonally adjacent. This tangential contact
between diagonally adjacent clusters ensures complete
pixel coverage in regions that are to be printed with a
100% coverage pattern, as described more fully below.
The above-noted central overlapping of ink drops
near the center of the drop clusters 44 produces a
temporarily uneven distribution of surface tension forces
across the cluster 44. In particular, the surface tension
forces across the cluster 44 (which tension is the primary
force causing coalescence of the cluster) is relatively
high at the overlapped central region of the cluster.
Accordingly, during the period immediately following the
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~eeso3 - g -
instant the cluster 44 contacts the printing medium
22,
the drops 44a-d of the cluster tend to coalesce
generally
toward the center of the printed pixel group 45.
Consequently, this central or internal coalescence
of the
individual cluster 44 resists the relatively lower
coalescence forces that develop between the tangentially
contacting drops, such as occurs at the contact
point
between drops 44d and 44e (Fig. 2) in diagonally
adjacent
clusters 44.
As noted earlier, the coalescence that occurs
within the printed clusters 44 does not produce
observable
mottling because this coalescence is present in
all
printed clusters 44, thereby producing substantially
uniform ink density across the entire printed image.
Moreover, the amount of the areal overlap of the
drops
44a-d of a cluster is established to be greater
than the
amount of areal contact that occurs between tangentially
adjacent clusters. Consequently, the central coalescence
force within each printed cluster 44 (that force
being a
function of the surface tension of the ink and the
area of
overlap of the drops 44a-d) will be greater tha~
the
coalescence force developed between tangentially
. contacting clusters, such as occurs at the contact
point
between drops 44d and 44e (Fig. 2). As a result,
random
coalescence between tangentially contacting clusters
is
substantially eliminated.
It can be appreciated that if a pixel 40 were
covered, for example, with a single ink drop (i.e.,
without overlapping simultaneously printed adjacent
drops), there would be less tendency, as compared
to the
overlapped drops of clusters 44, for the single
drop to
resist coalescence with another tangentially contacting
single drop. More particularly, there would be no
surface
tension gradient (i.e., relatively higher surface
tension
forces away from the edges of the drop) for resisting
coalescence of two tangentially contacting drops.
Accordingly, coalescence of simultaneously printed
adjacent single-drop pixels would occur randomly,
along
IBA~03 -
with the attendant mottled appearance of the printed
i
image.
The ink drop volumes and percentage of ink drop
area overlap for each of the individual drops 44a-d
of the
cluster 44 are selected to ensure that the central
coalescence of the cluster 44 is great enough to resist
coalescence with diagonally adjacent clusters. In this
regard, acceptable results for printing on conventional
s
overhead projection type film are achieved where the
surface area of each drop 44a-d within the cluster 44
overlaps about 20% of a horizontally adjacent drop and
about 20% of a vertically adjacent drop.
As the printed drop cluster 44 penetrates the
film 22, the volume of ink within the cluster 44 flows
outwardly and becomes substantially uniformly distributed
across the area of the printed pixel group 45. The
internal capillarity of the film 22 thereafter prevents
any significant pixel to pixel coalescence between
simultaneously printed pixels.
a
As noted earlier, Fig. 2 represents a 50%
coverage pattern printed during a single scan o~ the
pen 24. The printing medium region shown in Fig. 2 may
be
printed with a 100% coverage pattern (i.e., 100% of
the
pixels in the region being covered with ink). To this
end, the printer 20 is controlled to scan the pen 24
across the swath 34 a second time so that the pen nozzles
may be fired to direct drop clusters 44' to the pixel
groups 45 that are horizontally and vertically adjacent
to
the pixel groups that were printed during the first
scan
described above.
Fig. 2 illustrates in dashed lines the placement
of 4-drop clusters 44' printed during the second scan
of
the pen 24. The second-scan drop clusters 44' overlap
the
first-scan clusters 44 along the common boundaries of
adjacent pixel groups 45. It has been found that a
typical time interval (for example, 0.1 second) between
the first and second scan is sufficient to permit the
first-scan clusters 44 to partly penetrate the printing
I
- to -
medium and to partly dry so that cluster to cluster
coalescence (as would likely occur with two "wet"
simultaneously printed and overlapping clusters) will not
occur.
It can be appreciated that the 100% coverage
pattern depicted in Fig. 2 includes no exposed printing
medium. Accordingly, the overall ink density of
the
printing medium region is very high.
Fig. 3 depicts an alternative drop placement
l0 method wherein two-drop clusters 50 are substituted
for
the four-drop clusters 44 described above. The clusters
50 comprise two ink drops 50a, Sob that overlap
across the
horizontal centerline H of the printed pixel groups
55.
The centers of the drops 50a, 50b are vertically
aligned.
Each pixel group 55 depicted in Fig. 3 comprises
two vertically adjacent pixels 52. The shape of
the pixel
groups 55 are, therefore, rectangular, with central
long
axes being in the vertical direction. One uncovered
group
55 is depicted, for illustration purposes, with
cross-
hatched lines in Fig. 3.
The arrangement of the drop clusters 50 are
formed by timing the pen firing so that the two
ink drops
50a, 50b of the cluster 50 overlap as shown, and
so that
every other pixel group 55 in a row of pixel groups,
and
every other pixel group 55 in a column of pixel
groups, is
printed with a drop cluster 50. As before, the printed
drop clusters 50 are arranged to be in tangential
contact
with other clusters 50 that are diagonally adjacent
to
them.
As was described above with respect to the four-
s
drop cluster 44, each centrally overlapped two-drop
cluster 50 has, immediately after printing, a surface
tension force gradient that results in internal
or central
coalescence of ink within the printed pixel group
55.
This internal coalescence resists random coalescence
between diagonally adjacent simultaneously printed
clusters 50.
a
iBA403 - 11 -
If desired, a second scan of two-drop clusters
(dashed lines 50' in Fig. 3) may be printed over the pixel
groups 55 that were left uncovered after the first scan.
The second scan, therefore, converts the 50% coverage
pattern into a l00% coverage pattern.
Fig. 4 depicts a 25% coverage pattern printed
with two-drop clusters 60. Each two-drop cluster 60
comprises two ink drops 60a, Gob arranged to overlap about
the horizontal centerline H of the pixel group 65. The
l0 centers of drops 60a, 60b are vertically aligned. The 25%
pattern is printed so that every fourth pixel group 65 in
a row of pixel groups, and every other pixel group 65 in a
column of pixel groups, is printed with a two-drop
cluster 60.
Fig. 5 depicts an alternative arrangement of a
two-drop cluster 70 for printing a 25% coverage pattern.
In Fig. 5, the two-drop cluster 70 is arranged so that the
drops 70a, 70b overlap about the vertical centerline V of
the pixel group 75. The centers of the drops 70a, 70b are
horizontally aligned. The pixel groups 75 are oriented to
correspond with the orientation of the drop clu~ter 70.
Accordingly, the rectangular-shaped pixel groups 75, are
oriented with long axes in the horizontal direction. The
25% pattern depicted in Fig. 5 is printed so that every
other pixel group 75 in a row of pixel groups, and every
fourth pixel group in a column of pixel groups is printed
with a two-drop cluster 70.
Fig. 6 depicts another alternative arrangement
for printing a 25% coverage pattern, using four-drop
clusters 80. The four-drop clusters 80 depicted in Fig. 6
are substantially identical to those clusters 44 described
with respect to Fig. 2. Accordingly, the individual drops
80a, 80b, 80c, 80d overlap along the vertical V and
horizontal H centerlines of the pixel groups 85. The 25%
pattern depicted in Fig. 6 is printed so that every other
pixel group 85 in a row of pixel groups 85, and every
other pixel group in a column of pixel groups is
simultaneously printed with the four-drop cluster 80.
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iea~o3 - 12 -
It may be useful to employ the 25% coverage
pattern described with respect to Figs. 4 - 6 in
instances
where the size of the ink drops cannot be sufficiently
controlled to produce 50% coverage patterns that
have no
significant overlap between diagonally adjacent
pixel
groups. The 25% coverage pattern (Figs. 4 - 6) is
advantageous in this regard because the minimum
distance
between any two simultaneously printed clusters
60, 70, 80
is increased compared to clusters 40 and 50 in the
50%
coverage pattern (Figs. 2 - 3). Accordingly, there
is no
tangentiai contact between simultaneously printed
pixel
groups 60, 70, 80, and the distance provided between
the
simultaneously printed pixel groups accommodates
oversize
ink drops that would otherwise overlap with diagonally
adjacent printed clusters.
A 100% coverage pattern may be produced from the
25% patterns of Figs. 4 - 6 by repeating three times
the
pen scan across a single swath with the pen fired
so that
the uncovered pixel groups are covered with the
drop
clusters.
While the present invention has been described in
accordance with preferred embodiments, it is to
be
understood that certain substitutions and alterations
may
be made thereto without departing from the scope
of the
appended claims.
_ .,
