U.S. patent application number 10/636124 was filed with the patent office on 2004-02-12 for screening method for overlapping sub-images.
Invention is credited to Broddin, Dirk, Tavernier, Serge, Verbruggen, Mario.
Application Number | 20040028291 10/636124 |
Document ID | / |
Family ID | 30773256 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040028291 |
Kind Code |
A1 |
Broddin, Dirk ; et
al. |
February 12, 2004 |
Screening method for overlapping sub-images
Abstract
For the reproduction of originals, images are generated on an
image carrier, for example by printing. The imaging device that
generates the image is usually not capable to cover at once the
complete image area on the carrier. If the device is capable to
cover the full width of the image area, the image may be generated
line by line. Devices not having this capability will generate a
first portion of an image line on the carrier. An adjacent second
portion of the image line is then generated by another imaging
device or after a period of time by the imaging device that
generated the first portion. The region where the first and second
portion meet on the carrier may cause visual artefacts on the final
reproduction due to spatial misregistration of the adjacent line
portions. This problem is solved by dividing the image in adjacent
sub-images having an overlap zone on the carrier. Within this
overlap zone two sub-images will be generated on top of each other
for reproducing the original image in that zone, thereby reducing
or avoiding the artefacts. According to one method, the resulting
optical density of the first and second sub-image is reduced within
the overlap zone as the outer edge of the sub-image in the overlap
zone is approached. The density reduction may be achieved by
reduction of the microscopic density of individual microdots or by
reduction of the dot percentage or by a combination of these
techniques.
Inventors: |
Broddin, Dirk; (Edegem,
BE) ; Verbruggen, Mario; (Baal, BE) ;
Tavernier, Serge; (Lint, BE) |
Correspondence
Address: |
Hoffman, Warnick & D'Alessandro LLC
Three E-Comm Square
Albany
NY
12207
US
|
Family ID: |
30773256 |
Appl. No.: |
10/636124 |
Filed: |
August 7, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10636124 |
Aug 7, 2003 |
|
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|
09427810 |
Oct 27, 1999 |
|
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60112310 |
Dec 14, 1998 |
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Current U.S.
Class: |
382/284 ;
382/254 |
Current CPC
Class: |
H04N 1/1911 20130101;
H04N 1/3876 20130101 |
Class at
Publication: |
382/284 ;
382/254 |
International
Class: |
G06K 009/40; G06K
009/36 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 3, 1998 |
EP |
98203869.7 |
Claims
1. A method for reproducing an original image on an image carrier
comprising the steps of: generating a conjoined first and second
sub-image, each representative for a portion of said original
image; defining an overlap region as a region where both sub-images
give a contribution to the integral optical density of the image
carrier; establishing for each sub-image a peripheral edge in said
overlap region; increasing said contribution by said first
sub-image from said peripheral edge of said first sub-image to said
peripheral edge of said second sub-image.
2. The method according to claim 1, comprising the steps of:
dividing said overlap region in a partition of microdots; assigning
to at least one microdot an intermediate microscopic density
substantially different from a minimum and maximum microscopic
density of said microdots.
3. The method according to claim 2, wherein the step of increasing
said contribution comprises increasing the microscopic density of
said microdots by density steps being smaller than half the
difference between said maximum and minimum microscopic
difference.
4. The method according to claim 1, comprising the steps of;
halftoning said first sub-image by a first frequency-modulated
halftoning method; and, halftoning said second sub-image by a
second frequency-modulated halftoning method, substantially
non-correlated to said first frequency-modulated halftoning
method.
5. The method according to claim 4 comprising the steps of:
generating for a zone in said overlap region by said first
sub-image a first per cent of blank microdots; generating for said
zone by said second sub-image a second per cent of blank microdots,
said second per cent being equal to said first per cent.
6. An imaging system for reproducing an original image by an
imaging device on an image carrier comprising: means for generating
a conjoined first and second sub-image, each representative for a
portion of said original image; means for defining an overlap
region as a region where both sub-images give a contribution to the
integral optical density of the image carrier; means for
establishing for each sub-image a peripheral edge in said overlap
region; means for increasing said contribution by said first
sub-image from said peripheral edge of said first sub-image to said
peripheral edge of said second sub-image.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an image reproduction
method by joining adjacent images, such as two or more image bands,
in an image generating device, such as a printer.
BACKGROUND OF THE INVENTION
[0002] Nowadays many types of image generating devices exist,
including phototypesetters, imagesetters, lithographic printers and
electronic printers for printing electronic (colour) images. The
generated images may be e.g. latent, visible or lithographic and
are generated on a suitable image carrier. An image carrier may be
paper, a transparent PET (polyethyleenterephtalate) material,
photographic material, an electrophotographic drum or a
lithographic printing plate etc. A non-visible image usually
undergoes a process to generate a visible image from it: a latent
image may be developed a lithographic image, comprising ink
accepting and ink repellent zones, may be provided with ink, which
is transferred to a paper image carrier to render a visible
image.
[0003] Some printers use thermal processes to form an image. These
may be direct thermal systems, thermal (wax) transfer system or
thermal systems using dye sublimation to form images on a receiving
material or image carrier. The thermal process can be activated by
using a thermal head or infrared (IR) light sources. An IR light
source commonly used in laser thermal printers is a semiconductor
laser.
[0004] Other popular printing systems use an ink jet printing
technology.
[0005] Droplets of fluid ink are ejected to a receiving layer or
image carrier to form a visible image.
[0006] A very common type of printer in the office environment is a
printer using an electrographic process. According to the
electrophotographic process, which is a specific electrographic
process, a latent electrostatic image is formed by selectively
illuminating or exposing an electrostatically charged
photoconductive drum and developing the latent image by toner,
thereby producing a visual toner image. The toner may thereafter be
transferred to an image carrier or substrate made of e.g.
paper.
[0007] Another electrographic process is referred to as Direct
Electrographic Printing (DEP) and is described in e.g. EP-A-0 675
417. According to this technique, a toner cloud is brought in the
vicinity of a print head structure. That structure has apertures
that may be "opened or closed" by electrostatic action. By
image-wise opening these apertures, toner particles travel
image-wise through the apertures of the print head structure and
impinge on an image substrate such as an intermediate image drum or
a final image substrate such as paper or a transparency
material.
[0008] Most of the above mentioned printers use print heads: these
are units carrying the image-forming or image-generating elements,
and which e.g. provide the heath, emit light or eject ink or toner
is particles in an image wise fashion.
[0009] A print head is generally not capable to generate at once
the complete image on the image carrier. For generating an image,
the area of the image carrier is traditionally (mentally)
partitioned in tiny addressable units, referred to as microdots.
These microdots are disjunctive, i.e. they do not overlap each
other, and all the microdots together fill the complete image area
on the carrier. As such, they form a real partition of the image
carrier. The microdots may be obtained by a grid defined by a first
set of parallel equidistant lines having a first orientation and a
second set of equidistant parallel lines having a second
orientation different from the first orientation. The tiny
parallelogram areas, enclosed by two sets of two parallel line
portions, are referred to as microdots. If the second orientation
is orthogonal to the first orientation, then the microdots have a
rectangular shape. If the distance between two consecutive lines of
the first set equals to such distance of the second set, then the
microdots have a rhombic shape. In most cases the orientation is
orthogonal and the distance is identical, resulting in square
microdots. The multiplicative inverse of the size of the side of a
square microdot is referred to as the spatial resolution of the
imaging device. The microdot size in an electrophotographic device
may be 42 micron (.mu.m). The spatial resolution of the device is
then 1000/42 microdots per mm, i.e. 24 dots per mm or 600 dots per
inch (600 dpi). The notion of an "addressable" microdot refers to
the fact that the imaging device is capable to address the microdot
individually. A binary electrographic device is capable to deposit
either a maximum amount of toner or a minimum amount of toner on
each individual microdot.
[0010] Although the microdots are disjunctive, is may be possible
that some toner particles designated for a first microdot, also
partly cover an adjacent microdot, i.e. a microdot that has a side
or a corner in common with the first microdot. Examples of such
binary devices are the Agfa P400, P3400 and P3400PS devices,
developed and marketed by Agfa-Gevaert N.V. in Mortsel, Belgium and
having a resolution of 400 dpi. A multilevel electrographic device
is capable to deposit on each individual microdot specific variable
amounts of toner, expressed in microgram per square millimetre
(.mu.g/mm.sup.2). The number of such specific amounts may be e.g.
16, such as in the Chromapress system, developed and marketed by
Agfa-Gevaert N.V. in Mortsel. The lowest amount of toner may be
generated by offering to the electrophotographic system a digital
value 0, whereas the highest amount of toner may be generated by
offering to the electrophotographic system a digital value 15. All
values between 0 and 15 may generate on the individually addressed
microdot each a specific amount of toner between said minimum and
maximum amount. Since values from 0 to 15 may be represented by
four bits, this system is referred to as a 4-bit multilevel system.
To achieve the impression of continuous tone for images reproduced
on such system, it may be necessary to introduce some form of
halftoning, as described in EP-A-0 680 195, EP-A-0 634 862 and
EP-A-0 682 438.
[0011] It follows that an A4-sized image carrier (297 mm.times.210
mm) comprises about 35 million microdots in a 600 dpi (ca. 24
microdots per mm) system. An imagesetter for generating a printable
image may have a spatial resolution of 2400 dpi (ca. 95 microdots
per mm). If the film or printing plate has a size of 14".times.17"
(14 inch by 17 inch, i.e. 356 mm by 432 mm), the number of
microdots on the image carrier amounts to 1,371 million. An imaging
device capable to address that large number of microdots at once
and at that resolution would be too costly. Therefore, the image
carrier is rather exposed line by line, e.g. by using a LED array
or even pixel by pixel--i.e. microdot by microdot--by using a
sweeping laser beam. A sweeping laser beam may be generated by an
imaging device comprising a laser source and a light deflection
system such as a rotating polygonal mirror or a rotating
pentaprism. In more complex systems, all microdots arranged on a
plurality of lines may be addressed at once, i.e. at the same
instant. This may be achieved by a plurality of parallel LED arrays
in an electrophotographic system and by a print head structure
having a plurality of parallel lines of printing apertures in a DEP
device. A plurality of sweeping laser beams may give the imaging
device the capability to address a plurality of microdots at once.
According to the above mentioned systems, one imaging device--i.e.
one LED array, one printhead structure, or one laser beam system
comprising a laser source and a deflection means--is capable to
address one line of microdots simultaneously or at least within a
short period of time. With that short period is meant the time to
address all the microdots of one line, without addressing within
that period other lines by the same imaging device.
[0012] Due to the cost of some complex devices, it is sometimes too
expensive to provide a print head having a length equal or larger
than the width of the recording material. For some technologies it
is even impossible to make a good quality print head of a large
size. As such, the shorter print head cannot address instantly all
microdots arranged on one line running from one side to the
opposite side of the image carrier.
[0013] Especially when printing a large size image, e.g. posters,
the print head can print only a portion of the image. A poster may
have a size of 1.5 m width and 2.5 m length. In a 75 dpi system,
state of the art systems have a printing head with a width of 30
cm. To cope with a poster width of 150 cm, the printing head has to
make at least five steps. Therefore the image is printed in several
parallel bands, referred to as sub-images, which are sequentially
printed alongside each other.
[0014] In an inkjet printer having an array of nozzles arranged in
a longitudinal direction parallel to the longer side of the paper
to be printed, the paper is fed stepwise relative to the print head
in a longitudinal direction. The print head has a transversal
shuttle movement relative to the paper for printing image bands by
simultaneous operation of the plurality of nozzles. The bands are
printed one after another. A first image band or sub-image is
printed during a first transversal shuttle movement. Thereafter,
the paper is moved stepwise in a first longitudinal movement. Then
the second sub-image is printed during a second transversal shuttle
movement, followed by a second longitudinal stepwise movement
etc.
[0015] In a thermal laser transfer printer the imaging material can
be mounted on a drum. While the drum is rotated the print head is
stepwise moved along the rotation axis printing the image band
sequentially alongside each other. One such printer is described in
WO 93/04 552 where a thermal print head carrying diode lasers
coupled to fibres is displaced alongside the rotatable drum.
Sequentially printing the image in bands or sub-images may give the
following problems.
[0016] 1. When image bands do not exactly join together it is
possible that a distinct white line in between the printed bands
becomes clearly visible as an image defect. On the other hand, when
the image bands overlap, a clearly visible dark line disturbs the
image.
[0017] 2. Even when print bands join perfectly along the length, a
slight mismatch in the position of the bands along this length can
cause visible artefacts.
[0018] As can be seen in FIG. 2, the mismatch due to displacement
of a first sub-image 21 relative to a second sub-image 22 according
to the size of only one microdot can cause a visible defect when
printing screened (binary) images. This is referred to as a phase
defect of the screened data.
[0019] The artefacts, caused in the image zone 26 where two
sub-images join, may find their origin in the imperfect placement
of the printing head for printing the second sub-image in relation
to the first printed sub-image. This may be due to play of the
mounting and moving system of the printing head.
[0020] The very accurate positioning systems, needed to solve the
above problem, are too expensive to install in the printers
destined to the consumer market.
[0021] The above problems have already been recognised by other
researchers and several solutions have been proposed.
[0022] In EP-A-0 522 980 and EP-A-0 619 188 there is proposed to
make an overlap zone of two printed bands in a thermal sublimation
printer where two fitting stochastic rasters gradually fade towards
the neighbouring band.
[0023] In a laser thermal transfer proofer described in EP-A-0 529
535 and WO 93/4 552 the outermost lines of each band are so called
"dummy" lines. The information recorded in these lines has the
purpose to avoid the occurrence of white side lines due to
incorrect placement.
[0024] In DE-A-4 110 776 the joining of the bands in an ink jet
printer using a shuttling print head is not done along a straight
line but along a curved (random) path.
[0025] Despite of all the proposed measurements hitherto, there is
still a need to obtain a good quality joining of printed bands.
OBJECTS OF THE INVENTION
[0026] It is an object of the invention to provide a method for the
reproduction of an original image including high quality joining of
two sub-images that are printed sequentially by one imaging device
or that are printed by two different imaging devices.
SUMMARY OF THE INVENTION
[0027] The above mentioned objects are realised by a method having
the specific features defined in claim 1. Specific features for
preferred embodiments of the invention are set out in the dependent
claims.
[0028] The original image may be an image of a real scene, captured
e.g. by a photographic camera or a digital camera. The original
image may also be an image on black and white or colour print
material. Such an image may be converted to an electronic image by
an image scanner, such as the Agfa SelectScan.TM. digital scanner.
Such electronic image may also be designated as an original image.
An original image may also originate from a software application
such as PhotoShop (Trademark of Adobe Inc.), for creation or
modification of original or synthetic images. An original
electronic image may represent a black and white image or a colour
image. An electronic image is traditionally represented as one or
more rectangular matrices of image pixels, wherein each pixel is
represented by a digital value. The digital value typically ranges
from 0 to 255, where 0 may represent dark and 255 may represent
light or vice versa. Colour images are usually represented by three
matrices, each matrix representing a colour component such as red,
green, blue; or cyan, magenta, yellow; or hue, intensity,
saturation; etc. Where the current invention refers to an original
image, the following may i.a. be referred to: the electronic
representation of a black and white continuous tone image, of one
colour component of a continuous tone image.
[0029] An image carrier is e.g. paper, transparent or opaque film
material, such as PET, etc. on which the reproduction is made
visible. Before the image is visible, several intermediate
operations may be needed, such as developing a latent image,
applying ink to a lithographic medium and printing on paper,
etc.
[0030] A sub-image is an image that is a portion of the original
image or a derivative thereof. If an electronic image comprises 512
pixels on 512 lines, then a first sub-image may comprise 300 pixels
on 512 lines, e.g. pixels 1 to 300 for each line of the original
image and a second sub-image may also comprise 300 pixels on 512
lines, e.g. pixels 213 to 512 for each line of the original image.
According to the current invention, the first and second sub-image
are conjoined, i.e. they have a common region. According to the
above example, the first and second sub-images have pixels 213 to
300, i.e. 88 pixels, for each line in common, i.e. 88*512=45,056
pixels. The first and second sub-image must be representative for a
portion of the original image, i.e. it is possible to reproduce at
least a portion of the original image on an image carrier by the
first and the second sub-image.
[0031] The overlap region is defined as a region on the final
carrier, i.e. where the reproduction of the original image is
visible. In the overlap region, both the first sub-image and the
second sub-image give a contribution to the integral optical
density of the image carrier. By image carrier is meant the
substrate of the carrier (e.g. paper, PET, . . . ) along with the
toning agent, e.g. toner, ink, black silver (as opposed to e.g.
white silver salt such a silver behenate, which is transformed to
black silver by a thermal reaction), etc. Application of the toning
agent to the substrate, changes the optical density of the image
carrier at the location where the agent is applied. The optical
density may be measured by a densitometer. According to the spot
diameter, two types of densitometers may be distinguished:
traditional densitometers, having a spot diameter of 3 mm
typically, and 2 or 1 mm exceptionally; and, microdensitometers
capable to measure the optical density of a spot having a diameter
between 10 .mu.m and 400 .mu.m. The optical transmission density is
measured by illuminating a transmissive material, e.g. the image
carrier carrying an image, and measuring the transmitted light T.
The 10-logarithm of the ratio of the incident light I and the
transmitted light T is defined as the optical density:
D=log.sub.10(I/T). For the definition of the optical reflection
density, the reflected light R is used instead of the transmitted
light T: D=log.sub.10(I/R). For a large variety of imaging systems,
the microdensitometer is capable to measure the density of one
individual microdot. This density is referred to as microscopic
density. If the microdensitometer is not capable to measure the
microscopic density of one microdot, one may print a matrix of e.g.
20.times.20 identical microdots, and measure the microscopic
density of the patch formed by the matrix. The traditional
densitometer is not capable to differentiate individual microdots.
If all microdots within the spot having a diameter of 3 mm have the
same microscopic density, then the densitometer will read that
density. If the microdots in that spot have different microscopic
densities, the densitometer will read a mean value of these
microscopic densities. This process is referred to as optical
integration, and the measured optical density is referred to as
integral optical density. This process corresponds also to what
happens in the human eye, when it captures an image on an image
carrier. Therefore, the visual interpretation of an image does not
necessarily correspond to the microscopic density, but rather to
the integral optical density. For the reproduction of an original
image, it is more important that the integral optical density of
the reproduction corresponds to the original image, rather than the
microscopic density. To the human observer, a screened reproduction
may look as pretty as a full continuous tone reproduction, although
the microscopic densities of the screened reproduction do not match
the integral optical density as observed. The integral optical
density for an image consisting of a constant grey colour may also
be defined by the mean microscopic density, taken over all
microdots of a screen cell. A screen cell for a contone device
corresponds to one microdot. For a screened image, a screen cell
corresponds to the tile size of the screening method (see e.g.
EP-A-0 682 438 for a definition of tiles).
[0032] A contribution to the integral optical density is defined as
follows. Suppose that the first sub-image is printed alone, without
printing second sub-image, on top of the first one. The integral
optical density D.sub.1 of the image carrier with the first
sub-image printed on it is measured, i.e. the final image carrier
on which the reproduction is visible. This optical density D.sub.1
is the contribution of the first sub-image. By printing the second
sub-image alone, the integral optical density D.sub.2 on the image
carrier may be measured.
[0033] This is the contribution by the second sub-image. The final
optical density D in the overlap region, where both sub-image are
imaged on top of each other, will generally obey the following
inequalities, although exceptions are possible:
D.sub.1, D.sub.2.ltoreq.D.ltoreq.D.sub.1+D.sub.2
[0034] For colour images, these contributions are measured per
component, preferably by a colour densitometer. A colour
densitometer is a densitometer including a specific colour filter,
e.g. a red, green or blue colour filter. If an image is printed as
a cyan, magenta and yellow component, then the contribution by the
first sub-image for the yellow component is measured by printing
the yellow component of the first sub-image alone, without any
other component of the first sub-image, nor any component of the
second sub-image. The integral optical density of the yellow
component is then preferably measured by a colour densitometer
using a blue filter.
[0035] A peripheral edge of a sub-image is one of the edges of the
perimeter of the sub-image. If the sub-image is rectangular, then
the sub-image has four peripheral edges. The peripheral edge in the
overlap region, is the edge of the sub-image bordering the overlap
region. Stricto sensu the first sub-image usually gives no
contribution to microdots on the peripheral edge of the first
sub-image, situated in the overlap region, but that that edge is
also included in the overlap region.
[0036] Increasing the contribution by a sub-image is preferably
realised by electronic image processing. This is set out below,
mainly in conjunction with FIG. 5. If in an overlap region the
contribution by the first sub-image increases, usually the
contribution by the second sub-image decreases. The increase and
decrease are such that the reproduction resembles the original
image. Methods to achieve this are set out below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 shows two parallel image bands that are dislocated by
dx and dy
[0038] FIG. 2 shows the occurrence of a phase defect on the
peripheral edge of two screened image bands.
[0039] FIG. 3 shows a first and second sub-image along with an
overlap region
[0040] FIG. 4 shows the contribution of the first sub-image to the
overlap region
[0041] FIG. 5 shows a block diagram including contribution
processing
[0042] FIG. 6 shows arrangements for test patches generated by a
first and second sub-image, along with an overlap region for
various percentages of halftone dots
[0043] FIG. 6a shows an arrangement for 100% non-blank halftone
dots
[0044] FIG. 6b shows an arrangement for 87.5% non-blank halftone
dots
[0045] FIG. 6c shows an arrangement for 75% non-blank halftone
dots
[0046] FIG. 6d shows an arrangement for 62.5% non-blank halftone
dots
[0047] FIG. 6e shows an arrangement for 50% non-blank halftone
dots
[0048] FIG. 6f shows an arrangement for 37.5% non-blank halftone
dots
[0049] FIG. 6g shows an arrangement for 25% non-blank halftone
dots
[0050] FIG. 6h shows an arrangement for 12.5% non-blank halftone
dots
[0051] FIG. 7 shows the measured optical reflectance of a patch in
an overlay region according to FIG. 6, imaged by specific engine
values E for the first and the second sub-image and having a
specific percentage of non-blank halftone dots
[0052] FIG. 8 shows curves corresponding to a constant density
value, connecting (E.sub.1,E.sub.2)-pairs giving that density shows
patches generated by all possible combinations of engine values
E.sub.1 and E.sub.2.
DETAILED DESCRIPTION OF THE INVENTION
[0053] FIG. 1 shows a first sub-image 21 and a second sub-image 22.
The first sub-image or image band 21 may be generated by linewise
exposure, i.e. parallel to the line direction 24, e.g. by a DEP
device having a single row of apertures or by an inkjet device
having a row of nozzles. After printing the first line, the imaging
device advances according to the printing direction 23 or the image
carrier on which the image is to be printed advances in the
opposite direction. By iteratively printing and advancing, the
sub-image 21 is completely printed. Thereafter, the imaging device
is moved towards the second sub-image 22 or the image carrier is
displaced relative to the imaging device, such that the imaging
device can now print the first line of the second sub-image 22 etc.
The two sub-images 21 and 22, materialised by two parallel bands
according to this example in FIG. 1, must be joined to get a larger
image, e.g. for the production of a poster, larger than the
printing capability of the imaging device. The relative
displacement of the printing device towards the place on the image
carrier where the second sub-image must be printed, may introduce a
first dislocation dx according to the line direction 24 and a
second dislocation dy according to the printing direction 23. The
precision by which dx and dy may be minimised is decisive for the
quality of the reproduced image on the carrier in the neighbourhood
of the junction between the first and second sub-image 21, 22. The
effect of a dislocation dx and dy will be discussed referring to
simple original images.
[0054] If sub-images 21 and 22 are high density regions, e.g.
black, and the dislocation dx equals to the size of one microdot,
then a low density line having a width of dx may appear between the
two high density sub-images 21 and 22. Such line is clearly visible
with a magnifying glass on a 600 dpi system and is even noticed by
the naked eye at a viewing distance between 40 and 50 cm.
[0055] FIG. 2 shows the situation on a 600 dpi system where the
first sub-image 21 is dislocated relative to the second sub-image
22 in the printing direction over a distance dy of 42 .mu.m. Each
square area 25 having a size of 42 .mu.m represents a microdot. The
microdots within both sub-images 21 and 22 have alternately a high
and a low microscopic density according to a chess-board pattern.
Both sub-images represent a halftone pattern with a dot percentage
of 50%.
[0056] When seen at real scale by the naked eye, the halftone
pattern will look as a solid grey region. Due to the supposed
dislocation of 42 .mu.m, corresponding to the size of one microdot,
a disturbing pattern is clearly visible in the neighbourhood of the
junction 26.
[0057] Such pattern will be noticed as a grey disturbance when
observed at real scale. This type of dislocation is referred to as
a phase error. This term refers to the periodicity of halftone
screens. The halftone screen 21, 22 has a specific periodicity,
characterised by a screen angle and a screen ruling. Both
parameters are identical for the sub-images 21 and 22, i.e. the
screen angle is 45.degree. and the screen ruling is 600/{square
root}2 dpi. The halftone screen however has also a "starting
point", that may be situated in the centre of a microdot having a
low density. Due to the periodicity, the starting point may be
freely moved about lines oriented according to the screen angle and
over distances equal to an integer multiple of the pitch of the
screen, where the pitch equals to 1/ruling. It is clear that within
sub-image 21 or 22 of FIG. 2 one always arrives to the centre of
another low density microdot. However, once the junction 26 is
crossed, the starting points do not match. For the example of FIG.
2, the phase shift is maximum and the effect will also be clearly
visible.
[0058] Also periodical screens, such as a screen referred to as an
amplitude modulation (AM) screen, suffer from phase errors even if
small dislocations dx or dy are present between the first and
second sub-image 21, 22. An amplitude modulation screen is a screen
in which halftone dots are arranged on a periodic grid. A periodic
grid may be obtained by two sets of parallel and equidistant lines,
each set having a specific orientation, usually orthogonal to each
other.
[0059] In areas having a low integral optical density, a halftone
dot is a contiguous high density region on a low density
background. In areas having a high integral optical density, a
halftone dot is a contiguous low density region on a high density
background. In low density regions, the area of the (high density)
halftone dots grows as the density increases. In high density
regions, the area of the (low density) halftone dots reduces as the
density further increases. In mid density regions, halftone dots
may start to touch each other. As such, for an amplitude modulated
screen, the number of halftone dots is constant, whereas the size
or area of the halftone dots varies with the tone value of the
original image to be reproduced by an arrangement of the halftone
dots. This screening technique is more extensively described in
e.g. EP-A-0 748 109.
[0060] According to another halftoning technique, referred to as
frequency modulation (FM) halftone screening, the halftone dots
have a fixed size and the number of halftone dots per unit area
varies with the tone value of the original image to be reproduced
by the halftone image. An FM halftone dot may have the size of one
microdot. Also two adjacent microdots, i.e. microdots touching each
other by one side, may constitute one microdot. In some systems, a
halftone dot in a frequency modulation halftone image, is formed by
a cluster of four microdots arranged as a 2.times.2 matrix. Other
arrangements are also used. In low density regions, the high
density halftone dots will be sparsely distributed over the image
carrier. This distribution may be according to a random pattern.
Therefore, this screening technique is also referred to as
stochastic screening. As the density of the region increases, the
number of halftone dots per unit area on the image carrier
increases, rather than the area of the halftone dots. At a certain
density of the image to be reproduced, the halftone dots start to
touch each other at their corner points or start to connect side by
side. Once the density is high enough, no high density halftone
dots on a low density background will be visible anymore, but low
density halftone dots on a high density background will become
visible. At very high densities, the low density microdots will be
sparsely distributed on the high density background. This technique
of halftoning is described in e.g. EP-A-0 642 258 and EP-A-0 682
438.
[0061] The above mentioned techniques of halftoning, i.e. amplitude
modulation and frequency modulation, are mainly known for binary
printing systems, i.e. systems that have the capability to deposit
ink or no ink on a microdot; or to deposit toner or no toner
etc.
[0062] If the printing system, such as the Chromapress system
referred to above, has the capability to deposit a restricted
number (e.g. 16) of intermediate amounts of toner or ink on each
one microdot, then the halftoning techniques may be further refined
in that not only the area of the halftone dots or the number of
halftone dots per unit area is modified but also the microscopic
density of the microdots constituting the halftone dot--or even
constituting the background--may be varied. This feature gives an
extra degree of freedom that may be used in a suitable way to
enhance the reproduction of continuous tone images. Suitable
techniques are described in EP-A-0 634 862 and EP-A-0 682 438.
[0063] The halftone dots used in frequency modulation (FM)
halftoning have usually a size equal to the smallest halftone dots
used in amplitude modulation (AM) halftoning. Therefore, for most
tone levels to be reproduced, the halftone dots in AM are larger
than the halftone dots in FM. On a 600 dpi binary system, the FM
halftone dots preferably have a size of 42.3 .mu.m by 42.3 .mu.m;.
the AM halftone dots require a line ruling of 53 lpi (lines per
inch), in order to be capable to reproduce 128 tone levels. The
size of such AM halftone dots may be as large as 338 .mu.m by 338
.mu.m. Therefore, a dislocation dy as shown in FIG. 1 may be more
visible in AM reproductions than in FM reproductions, having
smaller halftone dots. For that reason, at least in the
neighbourhood of the junction 26 between two sub-images 21, 22, we
have found that it is preferred to use FM halftoning rather than AM
halftoning if halftoning is necessary to achieve a faithful
reproduction.
[0064] The dislocation dx shown in FIG. 1 may either shift the
sub-images 21 and 22 towards each other or away from each other. To
cope with that problem, we have found that a suitable solution is
to define, as shown in FIG. 3, an overlap region 28 around the
junction 26.
[0065] Within that overlap region, the first sub-image 21 gradually
fades out and the second sub-image 22 gradually takes over from
sub-image 22. FIG. 3 shows a reproduced image 27 on an image
carrier. The reproduced image comprises a first sub-image 21 and a
second sub-image 22. Both sub-images extend in an overlap region
28. The first sub-image 21 extends in the overlap region up to the
peripheral edge 29; the second sub-image 22 extends in the overlap
region up to the peripheral edge 30. According to FIG. 3, the first
sub-image has another peripheral edge 31 on the image carrier; the
second sub-image has also another peripheral edge 32. According to
the current invention, between edge 32 and 29, only the second
sub-image 22 contributes to the integral optical density of the
image carrier between edges 29 and 30, i.e. within the overlap
region, both the first sub-image and the second sub-image
contribute in variable degree to the integral optical density of
the image carrier; between edges 30 and 31, only the first
sub-image 21 contributes to the integral optical density of the
image carrier. In order to reproduce the original image, the above
contributions must be in accordance with the image contents of the
original image to be reproduced. To minimise the effect of
dislocation problems, both the first and second sub-image 21 and 22
give variable contributions to the overlap region. Therefore, both
the first and second sub-image 21 and 22 must have information
about the original image to be reproduced in the overlap region 28.
As such, the first sub-image 21 generated from the original image
will be representative for the portion of the original image to be
reproduced between edge 29 and 31; the second sub-image 22
generated from the original image will be representative for the
portion of the original image to be reproduced between edge 30 and
32. Therefore, the first and second sub-image 21 and 22 will both
comprise image information about the overlap region 28. The first
and second sub-image 21, 22 will thus be conjoined, i.e. be joined
together for a common purpose, i.e. to render or reproduce the
portion of the original image within the overlap region 28. During
imaging, the first and second sub-image are brought together so as
to overlap in the overlap region 28.
[0066] The contribution to the integral optical density of the
first sub-image 21 to the overlap region is sketched in FIG. 4.
From edge 32 to edge 29, no contribution or 0% contribution is
given to the image carrier by the first sub-image. As such, the
imaging device for imaging the first sub-image 21 does not need to
be capable to address the microdots in the region between edge 32
and 29. Between edge 30 and 31, the contribution to the density of
the image carrier by the first sub-image 21 will be 100%, i.e. that
region will be imaged by data contained in the first sub-image 21
only. This assertion is true as long as between edge 30 and edge 31
not another overlap region is situated. Within the overlap region
28, i.e. between the peripheral edge 29 of the first sub-image 21
and the peripheral edge 30 of the second sub-image 22, the
contribution to the density of the first sub-image steadily
increases from edge 29 to edge 30. At edge 29, that belongs both to
the first sub-image 21 and to the overlap region 28, the
contribution of the first sub-image is 0%. At edge 30 of the second
sub-image 22, that belongs both to the second sub-image 22 and to
the overlap region 28, and that is situated inside the first
sub-image 21, the contribution of the first sub-image is 100%.
Between these two edges 29 and 30 it is preferred that the
contribution of the first sub-image 21 is in the range [0%, 100%]
and preferably does not decrease as edge 30 is approached, i.e.
there may be a region within the overlap region 28, where the
contribution of the first sub-image 21 is constant as the edge 30
is approached. More preferably, that contribution is increasing as
edge 30 is approached. The increase of that contribution may be
linear, as shown in FIG. 4, but may also be nonlinear. The function
representing the per cent contribution as a function of the
distance d from the edge 29 (or edge 30 for the contribution of the
second sub-image) may have a convex or concave shape or a
combination of both with inflexion point etc. It is clear that as
the contribution of the first sub-image 21 increases, the
contribution of the second sub-image will not increase, e.g. the
contribution of the second image 22 may decrease within the overlap
region 28 as the edge 30 is approached. In a preferred embodiment,
the reproduced image must be such that there is a very close visual
resemblance between:
[0067] the reproduction that could have been obtained by printing
if the imaging device were capable to cover the whole width of the
carrier; and,
[0068] the reproduction that is obtained by printing the two
sub-images according to the method of the current invention and
with the assumption that no dislocation is present.
[0069] How this visual resemblance may be achieved is set out in
more detail further below.
[0070] Depending on the capabilities of the imaging system--i.e.
binary, multilevel or full continuous tone (contone)--and the
preferred options, the increase of contribution may be realised
by:
[0071] a) changing the dot percentage of the halftone screen;
or,
[0072] b) changing the microscopic density of the microdots;
or,
[0073] c) a combination of the above techniques.
[0074] The dot percentage of a region in a binary halftone image is
defined as the number of microdots in that region having a high
microscopic density, divided by the total number of microdots in
that region. In a multilevel halftone image the dot percentage may
be computed by assigning to each microdot in the region a value in
the range [0.0,1.0] by taking a value 0.0 for microdots having the
lowest microscopic density, a value 1.0 for microdots having the
highest microscopic density, and a value between 0 and 1
commensurate to the microscopic density for microdots having an
intermediate microscopic density. The dot percentage is then
obtained by summing the assigned values over all microdots in the
region and dividing that sum by the total number of microdots in
that region. By the above methods, the dot percentage is obtained
as a fraction in the range [0,1]. By multiplication with a factor
100, the dot percentage is expressed as a percentage value in the
range of [0%,100%].
[0075] In case of a binary imaging process, the option of
increasing the contribution by varying the microscopic density of
the microdots automatically results in at least one of the
microdots having the lower density value, getting the higher
density value, i.e. increasing the dot percentage of the binary
system. Because the phase errors due to a dislocation dy (FIG. 1)
are more conspicuous in an AM screen than in an FM screen, it is
preferred to use FM screening techniques with stochastic
distribution of the smaller halftone dots. The stochastic
distribution of the FM halftone dots will smear the phase error due
to dy over the image. In a preferred embodiment, each FM halftone
dot comprises a matrix of 2.times.2 microdots. In a 600 dpi system,
this gives a halftone dot resolution of 300 dpi.
[0076] A continuous tone system, also referred to as contone
device, has the capability to generate multiple density levels with
no perceptible quantization to them. The number of density levels
is typically 256 or more. Agfa's Drystar 2000 is a typical 10-bit
contone device, addressable by 1024 levels applied to a thermal
head, addressing each individual microdot at a spatial resolution
of 300 dpi. This system is manufactured and marketed by
Agfa-Gevaert N.V. in Mortsel, Belgium. The required density of a
microdot may be obtained by time modulation or amplitude modulation
of an electronic signal. Agfa's LR5200.TM. is a 16-bit contone
device, wherein an intensity modulated laser beam exposes a black
and white photographic laser recording film of the type Scopix.TM.
LT2B.TM. at a resolution of 600 dpi. In such contone systems, by
modulation of the amount of heat or light generated by the imaging
device, the contribution of the first and second sub-image may be
decreased or increased in a continuous fashion. An original image
having a constant tone level is reproduced by such systems by a
structure-less image, i.e. the microdots in the reproduction all
have an identical microscopic density level. No screening,
rasterizing or dithering is visible, even not by a magnifying lens.
Increasing the contribution of a sub-image in a contone system may
be done preferably in a quasi continuous way, i.e. by modification
of the value to drive the imaging device. In such system, the
imaging device is operated for 100% in continuous tone mode: the
translation of an image level of the original image into a drive
signal does not depend on the location on the image carrier in
non-overlap regions, as far as spatial corrections inherent to the
imaging device are not considered. As the contribution of the first
sub-image decreases as a function of the location in the overlap
region, typically the contribution of the second sub-image
increases as a function of that location. Within the overlap
region, a first imaging device gives a contribution to the density
of the image carrier and a second imaging device (or the first
device at a later printing stage) gives such contribution at the
same location. The contribution in the overlap region by each
imaging device depends on the image signal from the original image
and on the location within that overlap region. As set out below, a
contribution processed tone value T' may be computed from a tone
value T of the original image, according to T'=T*f(d) or T'=F(T,d).
Instead of imaging the original tone value T, the first imaging
device will image in the overlap zone the tone value T' and the
second imaging device will image there a tone value T", e.g.
computed according to T"=T*f(W-d) or T"=F(T,W-d).
[0077] Binary and multilevel imaging systems require some form of
halftoning, rasterizing or dithering for the faithful reproduction
of continuous tone original images. Binary halftoning may require
modified screening techniques to fade out the first sub-image while
fading in the second sub-image within the overlap region.
Multilevel halftoning has another degree of freedom with respect to
binary halftoning, in that also the microscopic density of
individual microdots may be varied by visible density steps. Most
critical images for testing correctly fading in/out are original
images having one single constant tone level, e.g. grey images.
Screening with FM techniques gives better results in the overlap
region than screening with AM techniques.
[0078] Theoretical simulation and practical experiments have shown
that a stochastic (FM) screen for the first sub-image 21 having a
dot percentage of 50% superposed on a stochastic screen for the
second sub-image 22 having a dot percentage of 50%, does not result
in a 100% dot percentage. There is an exception to this rule, where
the high density halftone dots of the second halftone image exactly
fit in the low density halftone dots of the first halftone image
and vice versa, such as disclosed in EP-A-0 619 188. According to
this arrangement, the screening method for the first sub-image is
thus completely correlated to the screening method for the second
sub-image. If however a dislocation having the size of one halftone
dot occurs, the correlation is completely lost. It is thus
preferred according to the current invention that for FM screening
the screening method for the first sub-image is non-correlated to
the screening method for the second sub-image, at least within the
overlap region. The definition for a set of two non-correlated
screening methods is as follows:
[0079] A first screened image is generated by the first screening
method according to a first constant tone level T.sub.1.
[0080] A second screened image is generated by the second screening
method according to a second constant tone level T.sub.2.
[0081] The second screened image is printed a first time on top of
the first screened image and a first integral optical density
D.sub.1 of the result is measured.
[0082] A second time, the first and second screened images are
generated.
[0083] For a second time, the second screened image is printed on
top of the first screened image, but now with a relative
displacement of one microdot. A second integral optical density
D.sub.2 is measured.
[0084] The above procedure is repeated for all possible relative
displacements and a mean density D.sub.m is computed.
[0085] The two screening methods are now uncorrelated if all
measured densities D.sub.j fall within the range of [0.8 D.sub.m,
1.2 D.sub.m]. The above experiment may be easily simulated by a
computer simulation, for which it is supposed that overprinting
D.sub.1 on D.sub.2 results in a density D=D.sub.1+D.sub.2.
[0086] Overprinting 50% non-correlated dot screens on top of each
other results in a dot percentage of about 75%. This is due to the
fact that on the image carrier a high density microdot for the
first sub-image may coincide either with a high or low density
microdot for the second sub-image. The same applies for a low
density microdot for the first sub-image. There is thus a "density
deficiency" when "adding" two sub-images. For the above reason, in
a preferred embodiment, at least one microdot in the overlap
region, must get a microscopic density that is intermediate the
lowest and highest microscopic density achievable for the
microdots. The lowest achievable microscopic density is the
integral density of the image carrier where it carries no ink,
toner or minimal pigmentation. The highest achievable microscopic
density is the integral density of the image carrier where it
carries the maximum achievable amount of ink, toner or has maximal
pigmentation. The intermediate microscopic density has a value that
is preferably substantially different from the minimum and maximum
integral optical density of the image carrier or of the minimum and
maximum achievable microscopic density of the microdots. By
substantially different is meant that the difference is at least
10% of the difference between the maximum and minimum density of
the image carrier or microdots. This means that, if the minimum
density is D.sub.MIN and the maximum density is D.sub.MAX, then at
least one intermediate microscopic density of a microdot in the
overlap region must be in the interval of:
[0. 9 D.sub.MIN+0.1 D.sub.MAX, 0.1 D.sub.MIN+0.9 D.sub.MAX]
[0087] The more intermediate densities are achievable by the
imaging device, the better a smooth fit of the first sub-image and
the second sub-image may be realised on the image carrier. If the
imaging device is operated by a multilevel halftoning technique,
preferably maximal use of the contone capabilities of the imaging
device is made. This means that for reproducing an original image
having one constant colour (e.g. only a specific grey), the
microdots on the image carrier have a microscopic density within a
narrow density interval, e.g. 20% of D.sub.MAX-D.sub.MIN, more
preferably 10%.
[0088] The gradual transition from the first sub-image 21 to the
second sub-image 22 in the overlap region may be realised for
contone, multilevel and binary devices by the process of the
imaging system 55 as sketched in FIG. 5. According to a specific
embodiment, the original image 33 is an eight bit continuous tone
image. Each grey level or tone value is represented by a value in
the range of 0-255.
[0089] An address generator 34 generates the position co-ordinates
(x,y) for retrieving the original tone value T for the microdot to
be imaged on the location (x,y) of the image carrier by the imaging
device 38. The retrieved eight bit tone value T is transmitted to
the contribution processing module 35. That module gets, apart from
the tone value T, also the location (x,y) from the address
generator 34 and the position of the two edges 29, 30 delimiting
the overlap region 28. As long as the location (x,y) is not within
the zone delimited by the two edges 29, 30, no transformation is
done on the eight bit tone value T, i.e. the tone value T is sent
to the next module 37 as T'=T, as it was received from the previous
module 33.
[0090] If on the other hand the location (x,y) is within the zone
delimited by the two edges 29, 30, the tone value from the original
image 33 undergoes a transformation. In the following it is
supposed that a tone value T=0 results in the lowest optical
density on the image carrier, whereas a tone value T=255 results in
the highest optical density on the image carrier. If the first
sub-image is imaged and the address generation module 34 generates
an address (x,y) for a location situated on the edge 30 of the
second sub-image, then the contribution processing module will
allow the tone value T of the original image 33 to fully contribute
to the density of the image carrier. This is achieved by
transmission of that tone value T, without modification, to the
next module 37 as T', i.e. T'=T. If (x,y) is located on edge 29 of
the first sub-image, then the first sub-image must give no
contribution to the density of the image carrier. According to the
above convention, this is achieved by changing whatever tone value
T from the original image 33 to a tone value of T'=0 for
transmission to the next module 37. If (x,y) is located between
edge 29 and 30, then a tone value T from the original image 33 will
be transformed to a contribution processed tone value T'. With the
above convention, tone value T' will have a value between 0 and T.
According to a linear model, the tone value after contribution
processing T' for the first sub-image 21 may be computed from the
original tone value T by the following equation (see FIG. 3):
T'=d/W*T
[0091] W is the width of the overlap region, expressed e.g. in
.mu.m.
[0092] d is the distance between edge 29 and the point (x,y),
expressed in the same metrics, e.g. .mu.m, as W.
[0093] * means multiplication.
[0094] According to the above equation:
[0095] T'=0 for (x,y) on edge 29; and,
[0096] T'=T for (x,y) on edge 30.
[0097] According to such linear model, the contribution processed
tone value T" for the second sub-image 22 may be computed from the
original tone value T by the following equation:
T"=(W-d)/W*T
[0098] It has been set out before that combination of two
sub-images may result in a density deficiency. Therefore, in a
preferred embodiment the tone value after contribution processing
is a non-linear function f( ) the distance d:
T'=f(d)*T
[0099] Preferably, f(d) is a non-descending function, i.e. as d
grows from 0 to W, f(d) is either increasing or constant. More
preferably, f(d) is an ascending function, e.g. the first
derivative f'(d) of the function f(d) is positive over the interval
[0,W]. Preferably f(0)=0 and f(W)=1 for the first sub-image. For
the second sub-image, the function f(d') may be used, where
d'=W-d.
[0100] According to another embodiment, the per cent attenuation of
the density is not only a function of the distance of (x,y) from
the edge 29, but also a function of the original tone value, i.e.:
T'=F(T,d)
[0101] The equation T'=f(d)*T is a special case of the equation
T'=F(T,d).
[0102] Preferably F(T,0)=0 and F(T,W)=T for all tone values T.
[0103] The design of the functions f(d) and F(T,d) will be set out
below.
[0104] The contribution processed tone values T' are sent to the
bitmap generation module 37. That module converts in the usual way
tone values T' to engine value E, suitable for driving the imaging
device 38. If the imaging device 38 is a contone device, such as
the Agfa Drystar 2000 10-bit colour printer, addressed by a 10-bit
engine value E, the bitmap generation module 37 transforms each
tone value T' to an engine value E independently from the location
(x,y) on the carrier. According to the current example, the 8-bit
tone value T' is thus transformed to a 10-bit engine value E. This
transformation may be realised by a look up table (LUT) having 256
entries and output values in the range of 0-1023. If the imaging
device 38 has multilevel capabilities, such as the Chromapress
system, or is a binary imaging device, such as a graphical
imagesetter or a platesetter for lithographic printing plates, the
bitmap generation module 37 will transform the 8-bit tone value T'
to a specific engine value E, according to the location (x,y) where
the tone value T' belongs to. Such transformation E=H(T',x,y),
referred to as (multilevel) halftoning is described in detail in
EP-A-0 634 862 and EP-A-0 748 109.
[0105] According to an alternative embodiment, the modules 35 and
37 may be integrated for generating a (multilevel) halftone image
for a binary or multilevel imaging device 38. In regions outside
the overlap region, (multilevel) halftoning is performed by making
use of traditional threshold matrices or transformation tiles. For
regions situated within the overlap zone, specific threshold
matrices or transformation tiles may be designed for controlling
the contribution of sub-images to the density of the image carrier.
The engine values E generated by bitmap generation device 37 are
then transmitted to the imaging device 38 for imaging the
sub-image.
[0106] After imaging the complete first sub-image, the second
sub-image may be imaged by the same imaging device. Alternatively,
two systems according to FIG. 5 are present in the imaging system,
both systems having an imaging device, capable to operate in
parallel. The first imaging device will then image the first
sub-image, the second imaging device will image the second
sub-image.
[0107] If the imaging device is a contone device as defined before,
then a smooth fit of two sub-images in the overlap region is
possible.
[0108] If the imaging device is a binary device, such as a
platesetter for a lithographic printing plate, a P3400PS binary
electrophotographic printer, a binary thermographic printer etc.,
then it is preferred, due to the risk of phase errors, that the
bitmap generator 37 generates a binary halftone image according to
a stochastic screening technique (FM screening) for the imaging
device 38.
[0109] In order to determine suitable contribution functions f(d)
or F(T,d), various monochrome experiments have been set up by using
the Chromapress system. This 600 dpi LED-based electrophotographic
system is capable to print duplex colour images on a web
material.
[0110] At each side, the web material sequentially contacts five
photosensitive drums. Each drum carries a toner image, the toner
being supplied from a toner delivery means or toner station. In
normal operation, the toner stations may be filled with colour
toners: cyan, magenta, yellow and black. Each drum comprises means
for adjusting dx and dy as shown in FIG. 1. In the experiment the
toner station for black and that for magenta toner were both filled
with black toner. The drum for black toner was used for imaging the
first sub-image 21, the drum for magenta toner--for the experiments
provided with black toner--was used to image the second sub-image
22. The adjusting means were deregulated to purposively introduce
dislocations dx and dy for simulation experiments. It has been
found that if the overlap region 28 has a size W of 8 to 16 mm,
then dislocations between -200 .mu.m and +200 .mu.m may be
absorbed, when imaged by the method according to the current
invention. Also overlap zones of 2 mm up to 8 mm give suitable
results. For 600 dpi inkjet systems printing in bands, an overlap
region, imaged according to the current invention, and having a
width of 4 to 5 mm gives very acceptable results. Experiments have
shown that in the 600 dpi Chromapress system, dislocations between
-30 and +30 .mu.m or even between -40 .mu.m and +40 .mu.m may be
invisible even without compensation according to the current
invention. The disturbance caused by a clear or dark line having a
width of 40 .mu.m may be visible by the naked eye from a short
viewing distance (20-50 cm).
[0111] Such line having a width of 100 .mu.m (0.1 mm) is acceptable
only from a larger viewing distance (larger than 1 m).
[0112] To evaluate the effect of overprinting sub-images in the
Chromapress system and accordingly to devise suitable contribution
functions f(d).or F(T,d), the following experiments were done. A
300 dpi stochastic screen, referred to as Agfa CristalRaster
(trademark of Agfa-Gevaert N.V.), was programmed in the bitmap
generation module 37 for the imaging device for imaging the first
sub-image, referred to as the first imaging device. Screening was
done according to a 128.times.128 tile, within the meaning as
described in EP-A-0 682 438. The CristalRaster screen was also
programmed for the imaging device for imaging the second sub-image,
referred to as the second imaging device. The Chromapress is a 600
dpi system. Each FM halftone dot was formed by a matrix of
2.times.2 microdots. For the first experiment, schematically shown
in FIG. 6a, 100% halftone dots were printed for both the first
sub-image 21 and the second sub-image 22. By 100% halftone dots is
meant that each halftone dot on the carrier gets the specified
microscopic density. Thus, for 100% halftone dots, no halftone
structure is present, since all microdots within one patch get the
same microscopic density. The first sub-image 21 is divided in 16
patches, numbered from 0 to 15. According to the experiment, each
patch of the first sub-image is imaged with the engine value
displayed in the patch. Therefore, the patch labelled 0 in the
lower right hand corner of FIG. 6a was completely white, since an
engine value E=0 corresponds with the minimum optical density,
obtained by depositing no black toner. The patch labelled 15 in the
upper right hand corner of FIG. 6a was completely black. This is
due to the fact that each microdot within that patch got an engine
value 15, corresponding to the highest amount of toner that can be
deposited by the imaging device. Each individual patch of the first
sub-image labelled with 1-14 also had a homogenous distribution of
toner particles; the patch labelled 1 had small amounts of toner
particles on each microdot, the patch labelled 14 large amounts,
but smaller than the toner amounts in the patch labelled 15. The
second imaging device was used to image the second sub-image 22 in
FIG. 6a.
[0113] The patch labelled with 0 got no toner, each microdot within
the patch labelled 15 got a maximum amount of black toner. The
patches labelled 1-14 got intermediate amounts of toner, equally
distributed over all microdots in the patch, the amounts being
commensurate the label value 1-14. Both the first and second
imaging device were used to image the overlap region 28. Again 16
patches were imaged differently. The patch in FIG. 6a labelled 0/15
got no toner from the second imaging device and got a maximum
amount of toner from the first imaging device. In the patch
labelled 15/0, the situation was reversed. In the patch labelled
3/12, the first sub-image 21 contributed to the overlap region 28
with an engine value E.sub.1=12 and the second sub-image 22
contributed to the overlap region 28 with an engine value
E.sub.2=3. Thus in fact, the first imaging device printed 16
different rectangular homogeneous patches, spanning the overlap
region 28 and the first sub-image 21; the second imaging device
printed 16 different rectangular homogeneous patches, spanning the
overlap region 28 and the second sub-image 22. The 16 patches
0/15-15/0 of the overlap region 28 all had a high optical
density.
[0114] This density was measured by a densitometer. The measured
values may be plotted as shown in FIG. 7. On the horizontal axis
the engine value E.sub.1 for the first sub-image is plotted. From
FIG. 6a it is clear that the engine value E.sub.2 of the patch
plotted by the second imaging device on top of the patch plotted by
the first imaging device within the overlap region 28 fulfils the
equation: E.sub.2=15-E.sub.1. On the vertical axis of FIG. 7, the
optical integral reflectance R of the patch in the overlay region
28 is plotted. The measured values for the overlap region 28 in
FIG. 6a all had a reflectance value R below 20% and were not
plotted in FIG. 7 accordingly.
[0115] The above described experiment is repeated as shown in FIG.
6b.
[0116] There is however an important difference: only 87.5% of the
halftone dots of the first sub-image or the second sub-image take
the engine value E.sub.1 or E.sub.2=15-E.sub.1 respectively. The
other 12.5% of the halftone dots of the first sub-image 21 and the
second sub-image 22 take the engine value E=0, i.e. no toner is
deposited on these halftone dots, referred to as blank halftone
dots. The blank halftone dots are stochastically distributed in the
patches, according to he CristalRaster FM screening technique. The
patch on the lower left hand corner of FIG. 6b has been imaged by
the second imaging device with engine value E.sub.2=15, i.e. each
non-blank halftone dot-gets a maximum amount of toner, whereas each
blank halftone dot, i.e. 12.5% of the halftone dots within that
patch, gets no toner.
[0117] The patch on the lower right hand corner designated with 0
corresponds to E.sub.1=0, i.e. no toner is deposited on any
microdot. The patch annotated with 15/0 in the overlap region 28 of
FIG. 6b results from overprinting the second sub-image 22 having a
dot percentage of 87.5% on top of the first sub-image 21, having a
dot percentage of 0%. That 15/0 patch has the same structure as the
15 patch in the second sub-image 22. The reverse operation is
applied on the patch labelled 0/15 in FIG. 6b. The patch labelled
4/11 in FIG. 6b is the result of overprinting patch 4 of the second
sub-image 22 on patch 11 of the first sub-image 21. In this 4/11
patch four types of microdots are present:
[0118] 1. E.sub.1=0 and E.sub.2=0: neither the first nor the second
imaging device deposit any toner on the microdot. The microdot has
minimal microscopic density.
[0119] 2. E.sub.1=0 and E.sub.2=4: the first imaging device
deposits no toner, the second imaging device deposits an
intermediate amount of toner, e.g. ca. 4/15 of the maximal amount.
The microdot in the patch 4/11 in the overlap region 28 has a low
microscopic density.
[0120] 3.E.sub.1=11 and E.sub.2=0: the second imaging device
deposits no toner, the first imaging device deposits an
intermediate amount of toner, e.g. ca. 11/15 of the maximal amount.
The microdot in the 4/11 patch has a relatively high microscopic
density.
[0121] 4. E.sub.1=11 and E.sub.2=4: the first imaging device
deposits a relatively high intermediate amount of toner, e.g. ca.
11/15 of the maximal amount; the second imaging device deposits a
relatively low intermediate amount of toner, e.g. ca. 4/15 of the
maximal amount.
[0122] The microdot has a high microscopic density, due to
deposition of a small amount of toner on top of a larger amount of
toner.
[0123] Since only 12.5% of the halftone dots of the first sub-image
and the second sub-image are blank halftone dots, and the FM screen
for the first sub-image is non-correlated to the FM screen for the
second sub-image, the situation under point 1 is exceptional: the
chance that the halftone dot is blank by the first sub-image is
12.5% or 1/8; the chance that this dot is blank by the second
sub-image is 12.5% or 1/8; accordingly the chance that a halftone
is blank in the-overlap zone, i.e. case 1, is 0.125*0.125=0.016 or
1/64. Case 2 and 3 have both a chance of 7/64. Case 4 has a chance
of 49/64.
[0124] Again the integral optical density of the patches 0/15 to
15/0 of the overlap zone 28 in FIG. 6b was measured. The results
are plotted on FIG. 7 by the square marks indicated by 40. A smooth
line 39 approximating these measured data was fitted along these
points 40 and represented in FIG. 7 accordingly.
[0125] The same experiment was done according to FIG. 6c, where 75%
of the halftone dots are non-blank, i.e. get the engine value E as
labelled in the patch. As such, 25% of the randomly distributed
halftone dots get no toner, by assigning E=0 to them. Again the
integral density of the overlap zone was measured and the
corresponding optical reflectance was plotted by symbol 41 in FIG.
7.
[0126] The experiment according to FIG. 6d refers to 62.5%
non-blank halftone dots; the reflectance was plotted by symbol 42
in FIG. 7.
[0127] The reflectance indicated by symbol 43 in FIG. 7 corresponds
to 50% non-blank halftone dots in FIG. 6e; symbol 44 corresponds to
37.5% in FIG. 6f; symbols 45 and 46 correspond to 25% and 12.5%
non-blank halftone dots in FIGS. 6g and 6h respectively. For all
the measured points in FIG. 7, a continuous function R=f(E.sub.1),
such as represented by curve 39, was established, for approximating
the measured points for one specific percentage. The curve 39
according to this function for 87.5% CristalRaster is shown in FIG.
7. In an alternative embodiment, not the optical reflectance R is
displayed as a function of the Engine value, with the percentage as
parameter, but the lightness L according to CIE (Commission
Internationale de l'Eclairage) instead of R. The approximating
function L=f(E.sub.1) will give a slightly different approximation,
since lightness L is not a linear function of reflectance R, nor
density D.
[0128] To achieve a certain optical reflectance, e.g. R=42% by
overprinting sub-images, one may select the required reflectance
value R=42% on the vertical axis of FIG. 7 and draw a horizontal
line 52. Vertical lines may be drawn from the engine values 1-15 on
the horizontal axis. Wherever the intersection of the horizontal
line 52 and each one of the vertical lines comes close to one of
the marks 40-46, according to the current example marks 41 and 42,
that combination of engine values E.sub.1 and E.sub.2 along with
the corresponding percentage of non-blank halftone dots may be
selected to achieve the required optical reflectance.
Alternatively, the engine value E.sub.1 may be selected according
to the position d (FIG. 3) within the overlap region 28. If the
engine value has an integer value from 0 to 15, then E.sub.1 may be
selected according to the following equation:
E.sub.1=[16.0*d/W]
[0129] The operation y=[x], where x is a real number and y is an
integer number, means that y is the integer value lower than or
equal to x.
[0130] If E.sub.1 equals 16, E.sub.1 is set to 15.
[0131] In order to make use of the graphs of FIG. 7,
E.sub.2=15-E.sub.1. Suppose that E.sub.1=6. Then a vertical line 53
starting at E.sub.1=6 may be drawn. The intersection 54 of the
horizontal line 52 and the vertical line 53 is situated between a
mark 41 corresponding to 75% and a mark 42 corresponding to 62.5%.
Now a percentage between 62.5% and 75% may be computed based on the
distance between the point 54 and the respective marks or more
exactly the corresponding smoothing curves.
[0132] If the computed per cent is 69%, then a 69% non-blank FM
screen may be used for both the first and second sub-image at the
location where R=42% must be achieved. All these operations may be
automated by a computer system or in hardware, by making use only
of the numerical representations of the curves such as 39.
[0133] A second test was done on the Chromapress system, in order
to define iso-intensity-curves as shown in FIG. 8, i.e. curves in a
two-dimensional (E.sub.1, E.sub.2)-space giving the geometrical
locus for which the intensity, integral optical density or
lightness of the image carrier is constant. To obtain these curves,
169 patches arranged as shown in FIG. 9 were created. Each patch
was an overprint of two sub-images, where each microdot within the
patch received from the first imaging device an amount of toner
commensurate E.sub.1, and each microdot within the patch received
from the second imaging device an amount of toner commensurate
E.sub.2. The level according to the first sub-image is shown on the
horizontal axis E.sub.1, the level for the second sub-image is
shown on the vertical axis E.sub.2. For example, each microdot in
patch 47 got an amount of toner from the first imaging device
dictated by the level E.sub.1=2, and from the second imaging device
by the level E.sub.2=5. According to the above convention, E=0
corresponds to no toner. As such, patch 48 has the lowest optical
density since it received no toner at all, i.e. from none of both
imaging devices; patch 49 has the highest optical density, since it
received from the first and second imaging device a maximum amount
of toner. First, the densities D.sub.1 for the patches with
E.sub.2=0, i.e. the second sub-image depositing no toner, and
E.sub.1 ranging from 0 to 12, were measured. These densities were
set out in FIG. 8 on a linear scale D.sub.1. This gave the
non-linear scale E.sub.1 in FIG. 8. For example, E.sub.1=6 with
E.sub.2=0 resulted in a density of ca. 0.54. The same procedure was
applied for varying engine values for the second imaging engine and
the first imaging engine depositing no toner. This gave the
vertical linear scale D.sub.2 and the non-linear scale E.sub.2 in
FIG. 8.
[0134] Thereafter, the density of all 144 remaining catches was
measured. and those patches having a density between 0.8 and 1.2
were represented by an asterisk 50 in FIG. 8. With each asterisk
50, a measured density is associated. Therefore, a surface in
three-dimensional space (E.sub.1,E.sub.2, D) is defined, D being
the density measured on patch (E.sub.1,E.sub.2). The analytical
form of this surface is obtained by approximating the measured
values D.sub.1 by a fitting surface. Cutting this surface by a
horizontal plane for D=0.8 and projecting the curve of intersection
to the (D.sub.1,D.sub.2)-plane, gives the iso-density curve 51. The
other curves in FIG. 8 representing densities 0.2, 0.4, . . . 1.4
were obtained likewise.
[0135] To obtain a specific density, e.g. D=0.82, in the overlap
region, one may establish the iso-density curve as sketched above
for D=0.82. This curve travels from the horizontal axis D.sub.1 to
the vertical axis D.sub.2. This curve may be divided in equal curve
segments, the number of segments being equal to the number of
microdots in the shortest line from edge 29 to edge 30 in FIG. 3.
Each microdot on that line is assigned consecutively to a curve
segment:
[0136] 1. The microdot situated on edge 29 is assigned to the
segment starting on the vertical D.sub.2 axis, at D.sub.2=0.82. As
such for the microdots situated on edge 29, the contribution to the
density will come from E.sub.2, i.e. the second imaging device
only.
[0137] 2. the microdot situated on edge 30 is assigned to the
segment starting on the horizontal D.sub.1 axis, at D.sub.1=0.82.
As such for the microdots situated on edge 30, the contribution to
the density will come from E.sub.1, i.e. the first imaging device
only.
[0138] The more the microdots are situated closely to edge 29, the
closer the corresponding segment is situated to the D.sub.2 axis.
The required engine values E.sub.1, E.sub.2 to obtain the desired
density of D=0.82 at a specific microdot located at position d
(FIG. 3), are then found on FIG. 8 as follows:
[0139] E.sub.1 by drawing a vertical line
[0140] from the line segment corresponding to the specific
microdot
[0141] to the horizontal E.sub.1 axis, and reading there the
E.sub.1 value;
[0142] E.sub.2 by drawing a horizontal line
[0143] from the line segment corresponding to the specific
microdot
[0144] to the vertical E.sub.2 axis, and reading there the E.sub.2
value.
[0145] Finding E.sub.1 and E.sub.2 are two separate and independent
operations, performed for the same image pixel of the original
image. Ideally E.sub.1 and E.sub.2 are imaged on the same microdot,
but due to dislocations dx, dy, they may be imaged on different
microdots. E.sub.1 and E.sub.2 obtained by the above method may be
non-integer values. The engine however can be driven only by
integer values. For converting the non-integer value to the integer
value, rounding may be used, i.e. a fraction lower than 0.5 is
omitted, other fractions lead to the next higher integer value.
Alternatively, one of the two integer values closest to the
non-integer value may be selected at random. In a preferred
embodiment, a random number r with a constant or homogeneous
distribution is generated in the interval [0,1) (0 included, 1
not).
[0146] The number r is added to the non-integer E-value. From that
sum the fraction is discarded, giving the integer E-value. By this
operation, non-integer E-values close to an integer value have more
chance to be transformed to that closer integer value. Also this
operation is done preferably independently for E.sub.1 and E.sub.2
since due to the uncertainty about the dislocations, it is not
guaranteed that the engine value E.sub.2 will be printed on top of
the engine value E.sub.1.
[0147] The method described herein above may be repeated for all
density values D or their original tone values T and for all
locations d (FIG. 3) within the overlap region 28. As such, a
function E=E(T,d) is obtained for the first and second imaging
device. This function may be incorporated in the contribution
processing module 35 as shown in FIG. 5, to deliver a value T' such
that the bitmap generation module 37 delivers the required engine
value E.
[0148] Alternatively, the tile for halftoning contone values T' to
multilevel values E, and stored in the bitmap generation module 37,
may be different for overlap and non-overlap regions. In such case,
module 36 gives also a signal to the bitmap generation module 37,
such that this module may decide to use either the usual tile or
the tile for overlap regions.
[0149] For contone devices, increasing the contributions by an
overlapping sub-image is preferably done by increasing the
microscopic density of the microdots as for constant tone values of
the original image these microdots get more distance from the
peripheral edge of that sub-image. For contone images, the
microscopic density may be increased by small density steps. These
density steps .DELTA.D are smaller than half the density difference
between the maximum D.sub.MAX and minimum D.sub.MIN microscopic
density achievable on a microdot, i.e.
.DELTA.D<(D.sub.MAX-D.sub.MIN)/2.
[0150] For binary devices, increasing the contributions by an
overlapping sub-image is preferably done by increasing the dot
percentage.
[0151] Preferably, the screening method used is a
frequency-modulated halftone screening and in the overlap region,
the screening method for the first sub-image is preferably
non-correlated to the screening method for the second
sub-image.
[0152] For multilevel devices, a mix of the above two methods for
contone and binary devices may be used.
List of Reference Signs
[0153] dx: dislocation orthogonal to printing direction
[0154] dy: dislocation parallel to printing direction
[0155] T: tone value of a pixel of the original image 33
[0156] T': tone value of a pixel after contribution processing
[0157] 21: first sub-image
[0158] 22: second sub-image
[0159] 23: printing direction
[0160] 24: line direction
[0161] 25: microdot
[0162] 26: junction
[0163] 27: reproduced image
[0164] 28: overlap region
[0165] 29: peripheral edge of first sub-image in overlap region
[0166] 30: peripheral edge of second sub-image in overlap
region
[0167] 31: peripheral edge of first sub-image, outside overlap
region
[0168] 32: peripheral edge of second sub-image, outside overlap
region
[0169] 33: original image
[0170] 34: address generator
[0171] 35: contribution processing
[0172] 36: position of first edge 29 and second edge 30 in overlap
region
[0173] 37: bitmap generation (e.g. multilevel halftoning)
[0174] 38: imaging device
[0175] 40: symbol for measured reflectance of patch in overlap
region according to FIG. 6b (87.5% FM raster)
[0176] 41: symbol for measured reflectance of patch in overlap
region according to FIG. 6c (75% FM raster)
[0177] 42: symbol for measured reflectance of patch in overlap
region according to FIG. 6d (62.5% FM raster)
[0178] 43: symbol for measured reflectance of patch in overlap
region according to FIG. 6e (50% FM raster)
[0179] 44: symbol for measured reflectance of patch in overlap
region according to FIG. 6f (37.5% FM raster)
[0180] 45: symbol for measured reflectance of patch in overlap
region according to FIG. 6g (25% FM raster)
[0181] 46: symbol for measured reflectance of patch in overlap
region according to FIG. 6h (12.5% FM raster)
[0182] 47: patch with E.sub.1=2 and E.sub.2=5
[0183] 48: patch with E.sub.1=0 and E.sub.2=0
[0184] 49: patch with E.sub.1=12 and E.sub.2=12
[0185] 50: asterisk representing a patch E.sub.1, E.sub.2
[0186] 51: iso-density curve for D=0.8
[0187] 52: horizontal line at R=42%
[0188] 53: vertical line at E.sub.1=6
[0189] 54: intersection of horizontal line 52 and vertical line
53
[0190] 55: imaging system
* * * * *