EP3530471A1

EP3530471A1 - Method and apparatus for compensating printing element shooting position deviation

Info

Publication number: EP3530471A1
Application number: EP18158878.1A
Authority: EP
Inventors: Koen J. Klein Koerkamp; Catharinus Van Acquoij
Original assignee: Oce Holding BV
Current assignee: Canon Production Printing Holding BV
Priority date: 2018-02-27
Filing date: 2018-02-27
Publication date: 2019-08-28

Abstract

The present invention relates to a method of controlling an inkjet printing device comprising a print head. In the method of controlling an inkjet printing device of the present invention an inkjet printing device comprises a print head, wherein the print head comprises an array of printing elements arranged to eject a number of droplets of a liquid onto a recording medium resulting in an array of dots on the recording medium. Depending upon the magnitude of the shooting position deviation of the printing elements of the array, in order to avoid the appearance of artefacts in the printed image, the present invention creates a second raster image derived from a first raster image created from the image to be printed, in which a redistribution of the pixel coverage values is performed based on said magnitude of the shooting position deviation of the printing elements of the array.

Description

FIELD OF THE INVENTION

The present invention generally relates to a printing device and, more particularly, the invention relates to an ink jet printer. Further, the present invention relates to a method and apparatus for controlling the disturbing effects of printing element shooting position deviations in inkjet printing.

BACKGROUND OF THE INVENTION

Know inkjet printers comprise a print head which in turn comprises a plurality of printing elements, each of which being arranged to eject a droplet of a liquid, for example ink, through said printing element. In these devices, each printing element is associated a print location, e.g. a pixel.
It is a well-known problem in inkjet printing that at least some of the plurality of printing elements do not print on the exact location corresponding to the printing element position, but print shifted perpendicularly with respect to the movement direction of the carriage (in scanning inkjet printing), or perpendicularly with respect to the paper feed direction (in page wide array single pass printing), thereby incurring in a printing element shooting position deviation with respect to the intended printing location of said printing elements. This printing element shooting position deviation leads to coverage of a printing element that differs slightly from the initial coverage intended for said printing element, wherein coverage indicates an amount of ink to be jetted by a printing element in a predetermined area of a recording medium, as for example paper. When this printing element shooting position deviation is large enough, resulting in coverage very different than the intended coverage, it results in a visual image quality artefact on the print. The printing element shooting position deviations of all printing elements in the system can be measured with special test prints in combination with a scanner. Said measurement can be performed from the distance between dots on a recording medium, as the printing element shooting position deviations lead to distance variations between dots. The acceptable deviation before a print quality artefact becomes visible is dependent upon many aspects, such as dot size, print resolution, ink properties and the local ink coverage. For a certain print process, a printing element shooting position deviation threshold can be determined at which the visual artefact becomes too large. When the printing element shooting position deviation of a printing element becomes larger than this threshold, a process of compensating said printing element shooting position deviation is triggered.
In order to solve the aforementioned problem, US patent 7,289,248 proposes a method of image correction in which a nozzle, or printing element, producing a an ejection failure, or an ejection abnormality, is identified on the basis of image information obtained from the print determination unit, and dot data is generated by correcting the image data in such a manner that dots which were originally to have been formed by the nozzle producing an ejection failure are formed instead by using other nozzles which are operating normally. The method and apparatus of this patent uses a dot pattern for compensating for omissions, called DCO, in order to prevent the appearance of artefacts in the image. DCO is a fixed pixel pattern used for the pixels corresponding to the ejection failure nozzle and the pixels peripheral to the pixels corresponding to the ejection failure nozzle, and the on/off switching of the nozzles peripheral to the ejection failure nozzle is set according to the DCO. In order to be able to remedy the different abnormalities possible in the ejection of the nozzles, this patent proposes calculating and storing a plurality of DCO's specified depending upon the input value of the pixels corresponding to an abnormally shooting nozzle and the relative coordinates of the pixel to have been ejected by the ejection failure nozzle and those pixels peripheral to it.
The method presented in US 7,289,248 must therefore calculate and store a plurality of patterns, amongst which a selection must be made to compensate each of the nozzles showing an abnormality in its ejection.
The present invention has been developed as an alternative method for compensating the shooting deviation of printing elements, which reduces the complexity of the calculations performed and can accurately adapt to the exact shooting deviations of the printing elements as detected in a test print.
It is an object of the invention to provide a jetting device which overcomes the previously described deficiencies of the prior art.

SUMMARY OF THE INVENTION

In an aspect of the present invention, a method of controlling an inkjet printing device comprising a print head according to claim 1 is provided. In the method of controlling an inkjet printing device of the present invention an inkjet printing device comprises a print head, wherein the print head comprises an array of printing elements arranged to eject a number of droplets of a liquid onto a recording medium resulting in an array of dots on the recording medium.
As understood in the present invention, a raster image, also commonly known in the art as a bitmap image, is a dot matrix data structure, representing a generally rectangular grid of pixels, or points of colour, viewable via a monitor, paper, or other display medium. Raster images are known in the art to be stored in image files with varying formats.
When a raster image is created from an image to be printed, said raster image comprises a plurality of even spaced pixels defined by a coverage value indicating an amount on ink to be jetted in a predetermined area. The shooting position deviations of the printing elements cause a deviation from this mentioned even spacing that leads to variations in ink density on the recording medium from those values stablished in the image. As a consequence, visible artefacts may appear in the printed image.
The method of the present invention provides a method for compensating the shooting deviation of printing elements which can accurately adapt to the exact shooting deviations of the printing elements as detected in a test print, and at the same time is computationally inexpensive and does not need the storage of compensation patterns in memory.
In all the embodiments of the present invention the print head and the recording medium are arranged to be moved relative to one another in a transport direction perpendicular to a page width direction.
As a consequence, when the present invention is applied in scanning inkjet printing it will result in a row of dots in a direction perpendicular to the movement direction of the carriage. On the other hand, when the present invention is applied in page wide array printing it will result in a row of dots in a direction perpendicular to the paper feed direction.
In the method of the present invention, a first step consists in the creation a first raster image from an image to be printed, wherein the first raster image comprises first pixels defined by a first pixel coverage value indicating an amount of ink to be applied to the recording medium by one of printing elements of the array of printing elements associated with said first pixel, the array of printing elements being arranged and controlled such that a row of pixels in the raster image corresponds to a row of dots ejected by the array of printing elements on the recording medium. When there is no shooting position deviation in the printing element associated with a particular pixel, said printing element shoots in a position centred in the pixel to which it is associated.
Subsequently, a shooting position deviation for each of the printing elements of the array from a distance between dots on the recording medium in a test pattern is determined. As explained in further detail below with reference to the Figures, from the distance between dots in the page width direction, calculated from a test print, it is possible to determine the shooting position deviation of each of the printing elements with respect to their shooting position when no deviation occurs, namely with respect to the position centred on the pixel with which they are originally associated.
As a next step, a second raster image is derived, comprising second pixels in which the first pixel coverage is distributed to two second pixel coverage values, each second pixel coverage value being associated with a printing element of the array of printing elements, wherein the distribution is based on the shooting position deviations of the printing elements of the array associated with the two second pixels.
Before printing, a halftoning process is performed in the previously mentioned second raster image, in which dot patterns are used to represent the image to be printed. Halftoning makes possible to produce varying shades of grey using only black dots, or a nearly infinite array of colours using only a few colours of dots. As a final step, the halftoned second raster image is printed.
The method previously described takes into account the individual shooting position deviations of the printing elements of the array of printing elements in the inkjet printing machine to adapt the pixel coverage values that each of those printing elements is in charge of printing. In this manner, a method that can adapt instantaneously to the shooting position deviations of the printing elements of the array is executed, thereby allowing a more exact redistribution of said pixel coverage values leading to a reduced likelihood of the appearance of image artefacts in the printed image, mainly streaks in the image caused by misalignment of the ejections of the printing elements.
In some embodiments of the present invention, namely when the present invention is applied in scanning inkjet printing, the shooting position deviation of the printing elements in a first direction that the present invention aims at correcting will result in a shooting position deviation shifted perpendicularly with respect to the movement direction of the carriage. On the other hand, when the present invention is applied in page wide array printing said shooting position deviation will result in a shooting position deviation in a first direction shifted perpendicularly with respect to the paper feed direction.
Further, in other embodiments, namely when the present invention is applied in scanning inkjet printing, the shooting position deviation of the printing elements in a second direction that the present invention also aims at correcting will result in a shooting position deviation shifted in parallel to the movement direction of the carriage. On the other hand, when this other embodiment is applied in page wide array printing said shooting position deviation will result in a shooting position deviation in a second direction shifted in parallel to the paper feed direction.
In an embodiment, the two printing elements associated with a second pixel coverage value to which the first pixel coverage value is distributed are the two printing elements of the array of printing elements that eject a dot to the recording medium closest in both sides to an intended dot position of the first pixel in a first direction. In this manner, taking into account their shooting position deviations the two printing elements closest to each pixel will distribute between them the first pixel coverage value of each first pixel that was originally assigned to the printing element associated with said first pixel, when performing the distribution step of a second pixel coverage value from the pixel coverage values of said two first pixels, such that the original first pixel coverage value is covered by the two printing elements. This distribution to two printing elements taking into account their respective shooting position deviations has the advantage of permitting a distribution of the pixel coverage value of each pixel of the first raster image that takes into account the exact position of the shooting of the printing elements and that selects those printing elements that can more accurately and reliably eject the coverage of those pixels, thereby avoiding the presence of areas with a lower ink coverage resulting in possibly visible artefacts in the printed image.
In another embodiment, as explained in more detail with respect to the Figures below, the distribution to two second pixel coverage values is proportional to the ratio of the shooting position deviation of the printing elements of the array that eject the droplet of a liquid onto the recording medium resulting in the two dots on the recording medium closest in both sides in a first direction to an intended dot position corresponding to the first pixel. In this way, a more accurate distribution can be performed, when deriving a second raster image to be subsequently halftoned and printed, that allows compensating the coverage profile of all printing elements, and that simultaneously allows compensating failing printing heads automatically. This more accurate distribution in the derivation of a second raster image further reduces the chance that visible artefacts appear in the printed image because the resulting coverage profile is more uniform that in the prior art solutions.
In alternative embodiments, the present invention aims to correct shooting position deviations of the printing elements in a second direction. Said second direction corresponds parallel to movement direction of the carriage when the inkjet printing device is an inkjet scanning printing apparatus, and corresponds to a direction parallel to paper feed direction when the inkjet printing device is a page wide array orienting apparatus. In these embodiments the two printing elements associated with a second pixel coverage value to which the first pixel coverage value is distributed are the two printing elements of the array of printing elements that eject a dot to the recording medium closest in both sides to an intended dot position of the first pixel in a second direction perpendicular to the first direction. In this manner, taking into account their shooting position deviations the two printing elements closest to each first pixel will distribute between them the pixel coverage value of each pixel that was originally assigned to the printing element associated with said pixel, when performing the distribution step of a first pixel coverage value into two second pixel coverage values, such that the original pixel coverage value is covered by the two printing elements. As there are two printing elements each being associated with each of the two second pixel coverage values, this distribution to two printing elements taking into account their respective shooting position deviations has the advantage of permitting a distribution of the pixel coverage value of each pixel that takes into account the exact position of the shooting of the printing elements and that selects those printing elements that can more accurately and reliably eject the coverage of those pixels, thereby avoiding the presence of areas with a lower ink coverage resulting in possibly visible artefacts in the printed image.
In another embodiment, as explained in more detail with respect to the Figures below, the step of deriving a second raster image in which a first pixel coverage value is distributed into two second pixel values based on shooting position deviations of the printing elements of the array with respect to an intended dot position associated with said two first pixels comprises that the distribution to two second pixel coverage values is proportional to the ratio of the shooting position deviation of the printing elements of the array that eject the droplet of a liquid onto the recording medium resulting in the two dots on the recording medium closest in both sides in a second direction to an intended dot position corresponding to the second pixel. In this way, a more accurate distribution can be performed, when deriving a second raster image to be subsequently halftoned and printed, that allows compensating the coverage profile of all printing elements, and that simultaneously allows compensating failing printing heads automatically. This more accurate distribution in the derivation of a second raster image further reduces the chance that visible artefacts appear in the printed image because the resulting coverage profile is more uniform that in the prior art solutions.
In another embodiment, the step of deriving a second raster image comprises that for those printing elements of the array of printing elements with a shooting position deviation smaller than a shooting position negligibility threshold, said printing elements of the array of printing elements ejecting the dot covering the first pixel coverage value with no distribution between two printing elements. Consequently, for those pixels of the first raster image for which the associated printing element of the array of printing elements has a shooting position deviation with a magnitude smaller than a shooting position deviation negligibility threshold, the originally associated printing element will remain responsible for printing its pixel coverage value. In this way, when the shooting position deviation of a printing element is small (below a threshold), unnecessary compensation is avoided when deriving a second raster image, said compensation being related to printing elements that do not have shooting position deviations likely to produce visible artefacts. As a consequence, the overall computational complexity of the compensation method is significantly reduced.
In an embodiment, the above mentioned shooting position deviation negligibility threshold is between one fourth and the whole value of the pitch between printing elements without taking into account their shooting position deviation, i.e. of the distance between dots when the shooting position deviations are null.
In an embodiment, when the shooting position deviation of a printing element of the array of printing elements exceeds a shooting position deviation reliability threshold, said printing element is deactivated such that it is not taken into account in the step of distributing the first pixel coverage value of each pixel to two second pixel coverage values, each associatedwith a printing element of the array of printing elements based on the shooting position deviation of each of the associated printing elements with respect to the pixel with which each of the printing elements is associated. As a consequence thereof, a printing element with a significant shooting position deviation will not be taken into account in the distribution and printing steps, which is equivalent to considering said printing element as a non-working printing element. In other words, the inherent unreliability of those printing elements with a significant shooting position deviation is eliminated when a second raster image is derived, allowing the compensation method to perform pixel coverage distribution and printing steps that more accurately match the actual shooting positions of the printing elements, thereby further reducing the likelihood of appearance of visual artefacts in the printed image. The shooting position deviation reliability is typically between one and two times the pitch between printing elements without taking into accounts their shooting position deviation.
In order to achieve this object, according to the invention, a new method and apparatus are provided for performing a better compensation of the position deviation of printing elements with a position deviation over a threshold. The method and apparatus proposed herein can be used to compensate the coverage profile for all printing elements at the same time.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiment examples of the invention will now be described in conjunction with the drawings, wherein:

Fig. 1 is a perspective view of a first embodiment of an inkjet scanning printing apparatus.
Fig. 2 is a perspective view of a first embodiment of a page wide array printing apparatus.
Fig. 3 is a schematic view of the shooting of two neighbouring printing elements in one of the embodiments of the present invention.
Fig. 4 is a graph indicating the coverage part of a printing element with a shooting position deviation depending upon the magnitude of said shooting position deviation.
Fig. 5 is a graph displaying the coverage profile for different compensation techniques of the shooting position deviation of printing elements.
Fig. 6 is a table showing the empiric results of one of the embodiments of the present invention.
Fig. 7 is a table showing the empiric results of another of the embodiments of the present invention.

The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate particular embodiments of the invention and together with the description serve to explain the principles of the invention. Other embodiments of the invention and many of the attendant advantages of the invention will be readily appreciated as they become better understood with reference to the following detailed description.
It will be appreciated that common and/or well understood elements that may be useful or necessary in a commercially feasible embodiment are not necessarily depicted in order to facilitate a more abstracted view of the embodiments. The elements of the drawings are not necessarily illustrated to scale relative to each other. It will further be appreciated that certain actions and/or steps in an embodiment of a method may be described or depicted in a particular order of occurrences while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used in the present specification have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study, except where specific meanings have otherwise been set forth herein.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an ink jet printing assembly 3. The ink jet printing assembly 3 comprises supporting means for supporting an image receiving member 2. The supporting means are shown in FIG. 1 as a platen 1, but alternatively, the supporting means may be a flat surface. The platen 1, as depicted in FIG. 1, is a rotatable drum, which is rotatable about its axis as indicated by arrow A. The supporting means may be optionally provided with suction holes for holding the image receiving member in a fixed position with respect to the supporting means. The ink jet printing assembly 3 comprises print heads 4a-4d, mounted on a scanning print head carriage 5. The scanning print head carriage 5 is guided by suitable guiding means 6, 7 to move in reciprocation in the main scanning direction B. Each print head 4a-4d comprises an orifice surface 9, which orifice surface 9 is provided with at least one orifice 8. The print heads 4a-4d are configured to eject droplets of marking material onto the image receiving member 2.
The platen 1, the carriage 5 and the print heads 4a-4d are controlled by suitable controlling means 10a, 10b and 10c, respectively. The image receiving member 2 may be a medium in web or in sheet form and may be composed of e.g. paper, cardboard, label stock, coated paper, plastic or textile.
Alternatively, the image receiving member 2 may also be an intermediate member, endless or not. Examples of endless members, which may be moved cyclically, are a belt or a drum. The image receiving member 2 is moved in the sub-scanning direction A by the platen 1 along four print heads 4a-4d provided with a fluid marking material. The scanning print head carriage 5 carries the four print heads 4a-4d and may be moved in reciprocation in the main scanning direction B parallel to the platen 1, such as to enable scanning of the image receiving member 2 in the main scanning direction B.
Only four print heads 4a-4d are depicted for demonstrating the invention. In practice an arbitrary number of print heads may be employed. In any case, at least one print head 4a-4d per colour of marking material is placed on the scanning print head carriage 5. For example, for a black-and-white printer, at least one print head 4a-4d, usually containing black marking material is present. Alternatively, a black-and-white printer may comprise a white marking material, which is to be applied on a black image-receiving member 2. For a full-colour printer, containing multiple colours, at least one print head 4a-4d for each of the colours, usually black, cyan, magenta and yellow is present.
The print head carriage 5 is guided by guiding means 6, 7. These guiding means 6, 7 may be rods as depicted in FIG. 1. The rods may be driven by suitable driving means (not shown). Alternatively, the print head carriage 5 may be guided by other guiding means, such as an arm being able to move the print head carriage 5. Another alternative is to move the image receiving material 2 in the main scanning direction B.
The present invention may be applied to the ink jet printing assembly of FIG. 1. Accordingly, in the ink jet printing assembly of FIG. 1 the print head and the image receiving member 2, which is a recording medium, may be arranged to be moved relative to one another in a sub-scanning direction, which may be referred to as transport direction. Further the droplets of liquid ejected by a print head result in a row of dots in the previously described main scanning direction B, which may be referred to as page width direction.
FIG. 2 shows schematically an embodiment of a page wide printing system 100 according to the present invention. The page wide printing system 100, for purposes of explanation, is divided into an output section 500, a print engine and control section 300, a local user interface 700 and an input section 400.
The paper path comprises a plurality of paper path sections 302, 303, 304, 305 for transporting the image receiving material from an entry point 306 of the print engine and control section 300 along the print head or print assembly 301 to the inlet 503 of the output section 500. The paper path sections 302, 303, 304, and 305 form a loop according to the present invention. The loop enables the printing of a duplex print job and/or a mix-plex job, i.e. a print job comprising a mix of sheets intended to be printed partially in a simplex mode and partially in a duplex mode.
The print head or print assembly 301 is suitable for ejecting and/or fixing marking material to image receiving material. The print head or print assembly 301 is positioned near the paper path section 304. The print head or print assembly 301 may be an inkjet print head, a direct imaging toner assembly or an indirect imaging toner assembly.
While an image receiving material is transported along the paper path section 304 in a first pass in the loop, the image receiving material receives the marking material through the print head or print assembly 301. A next paper path section 302 is a flip unit 302 for selecting a different subsequent paper path for simplex or duplex printing of the image receiving material. The flip unit 302 may be also used to flip a sheet of image receiving material after printing in simplex mode before the sheet leaves the print engine and control section 300 via a curved section 308 of the flip unit 302 and via the inlet 503 to the output section 500. The curved section 308 of the flip unit 302 may not be present and the turning of a simplex page has to be done via another paper path section 305.
In case of duplex printing on a sheet or when the curved section 308 is not present, the sheet is transported along the loop via paper path section 305A in order to turn the sheet for enabling printing on the other side of the sheet. The sheet is transported along the paper path section 305 until it reaches a merging point 304A at which sheets entering the paper path section 304 from the entry point 306 interweave with the sheets coming from the paper path section 305. The sheets entering the paper path section 304 from the entry point 306 are starting their first pass along the print head or print assembly 301 in the loop. The sheets coming from the paper path section 305 are starting their second pass along the print head or print assembly 301 in the loop. When a sheet has passed the print head or print assembly 301 for the second time in the second pass, the sheet is transported to the inlet 503 of the output section 500.
The present invention may also be applied to the inkjet printing assembly of FIG. 2. Accordingly, in the inkjet printing assembly of FIG. 2 the print head and the image receiving material, which is a recording medium, may be arranged to be moved relative to one another in a transport direction perpendicular to a page width direction.
Fig. 3 shows an example of two neighbouring pixels of an image to be printed.
In these figure it can be observed the shooting position of the printing elements associated with both pixels when the position deviation of said printing elements is zero. Said shooting position is centred in each of the pixels. Further, it can be observed the shooting position of said two printing elements including their respective position deviations. In reality, almost none of the printing elements are able to shoot in the centred position, but with a position deviation in the page width direction.
In a particular embodiment, the coverage value of the second pixel of the derived second raster image is proportional to the ratio of the shooting position deviation of the printing elements of the array that eject the droplet of a liquid onto the recording medium resulting in the two dots on the recording medium closest in both sides in a first direction to an intended dot position corresponding to the second pixel. The following equations reflect the distribution that makes place in which, for each pixel, the coverage is distributed over the closest printing element to the one of the sides in a first or second direction with respect to the pixel. The coverage ratio between the printing elements is proportional in distance from the printing element to the pixel, as expressed in the following equation: $\begin{matrix} covR = cov * disL / (dirR + dirL) \\ covL = cov * disR / (dirR + dirL) \end{matrix}$
Elements 31 represent the position of the dots related to two neighbouring pixels of the first raster image when the position deviation of the printing elements associated with them is zero. It can be observed in Fig. 3 that the position of said dots is centred in the respective pixels. Elements 32 and 33 represent respectively the shooting position of the printing elements associated with neighbouring pixels n, n + 1, including their respective shooting position deviations 34 and 35. In relation with Equation 1 above, when deriving a second raster image composed from two pixel coverage values, element 36 represents disL which is the distance in one of the directions from the shooting position when the position deviation of the associated printing element is zero, and the shooting position including the shooting position deviation. Further, element 37 represents disR, which is the distance in the opposite direction than element 36 from the shooting position when the position deviation of the associated printing element is zero, and the shooting position including the shooting position deviation. Depending upon both the distance reflected by elements 36 and 37 it is performed a distribution of the first pixel coverage value to two second pixel coverage values, each second pixel coverage value being assigned with a printing element of the array fo printing elements, such that the coverage ratio assigned to each pixel of the second raster image is proportional of the shooting position deviation of the two printing elements closest in each of the sides in a first or second direction to each pixel of the first raster image taking into account their respective shooting position deviations. The above equation allows deriving with simple calculations the coverage value of second pixels to be assigned to the respective printing elements.
Fig. 4 shows the distribution of the coverage in an embodiment of the present invention. Fig. 4 shows how much of the original coverage assigned to a printing element remains assigned to a printing element depending upon the shooting position deviation of said printing element. It can be observed that when a printing element has no position deviation, said printing element remains responsible of printing the complete pixel coverage value of the pixel to which said printing element was originally associated. On the other hand, as the position deviation of the printing element grows, the coverage of the pixel coverage value by the pixel to which said printing element was originally associated diminishes. In this case, the remaining pixel coverage value will be taken care of by other neighbouring printing element according to the distribution performed when deriving a second raster image. Namely, when a printing element, which is originally associated with the printing of one pixel, has a shooting position deviation to one side in a first or second direction, the printing element with a shooting position closest to the other side in that first or second direction in the step of deriving a second raster image is responsible for printing the part of the pixel coverage that the printing element originally associated with said pixel is not printing according to the graph of Fig. 4.
Fig. 5 shows a graph representing the relative ink coverage value for different compensation schemes. Fig. 5 illustrates a particular case of a printing element (associated with pixel position 5) with a particular printing element shooting position deviation (printing element shooting position deviation 0.5 pixels to the right). The relative ink coverage for different compensation techniques is shown, compared to the relative ink coverage that results when a printing element is shooting with a shooting position deviation of zero.
When the printing elements shoot with no position deviation, line 51 represents a constant unitary relative ink coverage value.
Further, line 52 represents the case in which a printing element shoots with a position deviation and no compensation by distribution to the neighbouring printing elements is performed. Additionally, line 53 represents the previously described prior art in which when one printing element shoots with a position deviation, said printing element is treated as if it were a completely non-working printing element. It can be observed that the relative ink coverage remains closer to the case in which there is no deviation than in the case of line 52, in which no compensation is performed in the step of deriving a second raster image. Lastly, line 54 represents the case of the compensation by distributing the pixel coverage value of each pixel to two printing elements (neighbouring pixels), performed when deriving a second raster image, based on the shooting position deviation of each of the associated printing elements with respect to the pixel with which each of the printing elements is associated (its shooting position with no deviation is centred on said pixel) shown in the above Equation 1. It can be observed, when comparing lines 52 and 53 that the relative ink coverage of line 53 is closer to the ideal case in which there is no position deviation of line 51 than in the cases represented by lines 52 and 53.
Figs. 6 and 7 represent particular examples of applying the above Equation 1 to a plurality of printing elements that are printing with initial pixel coverage of 50%.
In the table in Figure 6, the distribution of the first pixel coverage value to two printing elements is performed for all pixels in a raster image independently of the absolute value of the shooting position deviation of the printing element associated with that pixel. It can be observed that a distribution that takes advantage of the positioning of all the printing elements takes place, resulting in a printed image with further reduced likelihood of presence of image artefacts, and that takes into account the exact shooting position deviation of each element when composing a second pixel coverage value in the step of deriving a second raster image. Further, the process of deriving a second raster image is computationally simple.
As it can be observed in the example shown in Figure 6, different pixel coverage of the second pixel coverage values is assigned to the different printing elements:

Printing element #1: 47%
Printing element #2: 3% + 39%
Printing element #3: 11% + 49%
Printing element #4: 1% + 49% + 7%
Printing element #5: 1% + 43% + 0%
Printing element #6: 50% + 16%
Printing element #7: 34% + 9%
Printing element #8: 41%
Printing element #9: 47%
Printing element #10: 3% + 50%

When assigning the first pixel coverage value originally associated with printing element #3, it is first determined which are the pixels whose shooting position lies closer in both sides in the direction for which the effects of the shooting position deviations of the printing elements are to be reduced. It can be seen that for said first pixel, the printing elements closest in both sides are printing element #3 and printing element #4. When a distribution to second pixel coverage values is performed, printing element #3 is assigned 49% of the first pixel coverage value, while printing element #4 is assigned 1% of the first pixel coverage value. In this way, the original first pixel coverage value of 50%, which was associated with printing element #3, has been distributed, when deriving a second raster image, between printing elements #3 and #4, as those were the printing elements which are jetting closer in both directions from the position of pixel #3 in the first raster image. Further, the original first pixel coverage value of 50% assigned to pixel #4 in the first raster image is distributed in a 49% to printing element #4 (the one it was originally associated with) and in a 1% to printing element #5. Also, the original first pixel coverage value of 50% assigned to pixel #5 in the first raster image is distributed in a 7% to printing element #4 and in a 43% to printing element #5.
As it can be deducted, printing element #4, which was originally associated with pixel #4 of the first raster image, is assigned to take care after the distribution of 1% of pixel #3 of the fisrt raster image, 49% of pixel #4 of the first raster image, and 7% of pixel #5 of the said first raster image. Equivalent distribution has been performed for all the printing elements, as it can be seen in Figure 7.
In the table in Figure 7, the distribution of the first pixel coverage value to two second pixel coverage values is performed exclusively for those pixels in a raster image for which the absolute value of the shooting position deviation of the printing element associated with that first pixel is bigger than a position deviation negligibility threshold. Therefore, when the position deviation is below a threshold, namely a position deviation negligibility threshold, the printing elements that were originally associated with a pixel remain responsible for their coverage while a distribution of coverage between the originally associated printing element and a neighbouring printing element takes place when composing a second pixel coverage value in the step of deriving a second raster image. This method and related apparatus further reduce the computations involved.
The invention also relates to an apparatus, namely an inkjet printer, comprising a print head, wherein the print head comprises an array of printing elements arranged to eject a droplet of a liquid onto a recording medium, and the print head and the recording medium being arranged to be moved relative to one another in a transport direction perpendicular to a page width direction, and further comprising control means configured to apply the method according to any of the described embodiments of the present invention.
The invention also relates to a computer-program product embodied on a non-transitory computer readable medium and configured to execute any of the methods of the different embodiments according to the invention when executed on a processor.
The various blocks and modules described in connection with the disclosure herein may be implemented or performed with a general-purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other components, or any combination designed to perform the functions described in this disclosure. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine.
The steps of a method or algorithm described in connection with the disclosure herein may be embodied directly in hardware, in a software/firmware module executed by a processor, or in a combination thereof. A software/firmware module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, phase change memory (PCM), registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known.
The invention also relates to a non-transitory data carrier having stored thereon the computer-program product according to any of the embodiments of the invention.
The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Different modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the scope of the disclosure. As a consequence, the disclosure is not intended to be limited to the examples and designs described herein, but it is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

A method of controlling an inkjet printing device comprising a print head, wherein the print head comprises an array of printing elements arranged to eject a number of droplets of a liquid onto a recording medium resulting in an array of dots on the recording medium, and the print head and the recording medium being arranged to be moved relative to one another in a transport direction perpendicular to a page width direction, the method comprising the steps of:
a) creating a first raster image from an image to be printed, wherein the first raster image comprises first pixels defined by a first pixel coverage value indicating an amount of ink to be applied to the recording medium by one of the printing elements of the array of printing elements associated with said first pixel, the array of printing elements being arranged and controlled such that a row of pixels in the raster image corresponds to a row of dots ejected by the array of printing elements on the recording medium;

b) determining a shooting position deviation for each of the printing elements of the array from a distance between dots on the recording medium in a test pattern;

c) deriving a second raster image comprising second pixels in which the first pixel coverage is distributed to two second pixel coverage values, each second pixel coverage value being associated with a printing element of the array of printing elements, wherein the distribution is based on the shooting position deviations of the printing elements of the array associated with the two second pixels; and

d) halftoning and printing the second raster image.
The method of claim 1,wherein said two printing elements of step c) associated with the two second pixel coverage values are the two printing elements of the array of printing elements that eject a dot to the recording medium closest in both sides to an intended dot position of the first pixel in a first direction.
The method of claim 2, wherein the distribution in step c) to two second pixel coverage values is proportional to the ratio of the shooting position deviation of the printing elements of the array that eject the droplet of a liquid onto the recording medium resulting in the two dots on the recording medium closest in both sides in a first direction to an intended dot position corresponding to the first pixel.
The method of claim 1,wherein said two printing elements of step c) associated with the two second pixel coverage values are the two printing elements of the array of printing elements that eject a dot to the recording medium closest in both sides to an intended dot position of the first pixel in a second direction perpendicular to the first direction.
The method of claim 2, wherein the distribution in step c) to two second pixel coverage values is proportional to the ratio of the shooting position deviation of the printing elements of the array that eject the droplet of a liquid onto the recording medium resulting in the two dots on the recording medium closest in both sides to an intended dot position corresponding to the first pixel in a second direction perpendicular to the first direction.
The method of any preceding claim, wherein step c) comprises, for the printing elements of the array of printing elements with a shooting position deviation with respect to an intended dot position associated with said first pixel smaller than a shooting position negligibility threshold, said printing elements of the array of printing elements ejecting the dot covering the first pixel coverage value with no distribution between two printing elements.
The method of claim 4, wherein the shooting position deviation negligibility threshold is between one fourth and the whole value of the distance between dots without taking into account the shooting position deviation of the printing elements of the array.
The method of any preceding claim, wherein step c) comprises for the printing elements of the array of printing elements with a shooting position deviation bigger than a shooting position reliability threshold, not taking said printing elements of the array of printing elements into account when distributing the first pixel coverage value to two second pixel coverage values.
The method of claim 8, wherein the shooting position deviation reliability threshold is the distance between dots without taking into account the shooting position deviation of the printing elements of the array.
The method of any preceding claim, wherein inkjet printing device is an inkjet scanning printing apparatus.
The method of any preceding claim, wherein inkjet printing device is a page wide array printing apparatus.
The method of claim 10, wherein the first direction is perpendicular to the movement direction of a carriage.
The method of claim 11, wherein the first direction is perpendicular to a paper feed direction.
An inkjet printer comprising a print head, wherein the print head comprises an array of printing elements arranged to eject a droplet of a liquid onto a recording medium, and the print head and the recording medium being arranged to be moved relative to one another in a transport direction perpendicular to a page width direction, and further comprising control means configured to apply the method according to any of the claims 1 - 13.