阅读说明：本技术 防故障式颜色变换 (Fail-safe color conversion ) 是由 H·西格里茨 于 2019-07-12 设计创作，主要内容包括：本发明涉及一种用于借助计算机减少在喷墨印刷机的多色印刷工艺过程中缺陷印刷喷嘴的影响的方法,计算机制作颜色变换表格,颜色变换表格针对所述喷墨印刷机的每个工艺色包含距该工艺色的颜色距离最小的相应其它工艺色的工艺色组合,颜色变换表格是通过对具有所有工艺色的测试样张进行印刷及测量而制成,借助计算机采用插入法,将颜色变换表格使用于补偿缺陷印刷喷嘴,所述颜色变换表格由计算机优化,避免所述颜色变换表格的控制点之间所不期望的过渡,并且使得所得到的颜色变换表格相应地包含对缺陷印刷喷嘴而言敏感度最小的工艺色组合,计算机将其用于执行多色印刷工艺过程。(The invention relates to method for reducing the influence of defective printing nozzles in a multicolour printing process of an inkjet printer by means of a computer, which computer produces a colour conversion table which contains for each process colour of the inkjet printer a process colour combination of the respective other process colour which has the smallest colour distance from the process colour, the colour conversion table being produced by printing and measuring test coupons having all process colours, the colour conversion table being used by the computer to compensate for defective printing nozzles by means of interpolation, the colour conversion table being optimized by the computer, avoiding undesired transitions between control points of the colour conversion table and the resulting colour conversion table containing the smallest process colour combination in sensitivity to the defective printing nozzle, the computer being used to carry out the multicolour printing process.)

method for reducing the influence of defective printing nozzles (8) during a multicolour printing process of an ink jet printer (7) by means of a computer (6),

wherein a color conversion table (16,17) is produced by a computer (6), said color conversion table containing, for each process color of the inkjet printer (7), a process color combination (15) of the respective other process color having the smallest color distance (14) from the process color,

wherein the color conversion tables (16,17) are produced by printing and measuring test coupons having all process colors (13) and are inserted by means of a computer (6) and the color conversion tables (16,17) are used to compensate defective printing nozzles (8),

it is characterized in that the preparation method is characterized in that,

the color transformation table (16) is optimized by a computer in such a way that undesired transitions (12a) between control points of the color transformation table (16) are avoided and in such a way that the resulting color transformation table (16) accordingly contains process color combinations (15) that are least sensitive to defective printing nozzles (8) and is used by a computer (6) for carrying out a multicolor printing process of the inkjet printer (7).

2. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

in order to produce the color conversion tables (16,17) by means of a computer (6), the proportional share of the process colors to be replaced is equal to zero for each process color combination (15).

3. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

in order to produce the color transformation tables (16,17) by means of the computer (6), the proportional share of the process colors to be replaced is greater than zero for each process color combination (15).

4. The method of any preceding claim ,

it is characterized in that the preparation method is characterized in that,

in the case of six-color or seven-color printing, the number of control points in the color conversion table (17) is reduced as compared with the case of four-color printing.

5. The method of claim 4, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

the way in which the number of control points is reduced in six-color or seven-color printing is:

discarding control points in which the value for the process color black is not less than of the other process color proportion fractions, or

Discarding control points of complementary process colors which are not allowed to jointly constitute the two largest proportional shares of the process points.

6. The method of any preceding claim ,

it is characterized in that the preparation method is characterized in that,

as test coupons, CMYK test coupons with all existing process colors (13) with all combinations of process color scale fractions of 0%, 30%, 60% and 100% were used, and

the following combinations of process color scale fractions were excluded from the CMYK test swatches: the combination is higher than the sum of the permissible area coverage in terms of its process color proportion.

7. The method of any preceding claim ,

it is characterized in that the preparation method is characterized in that,

the control points outside the gamut are individually processed by means of a computer (6) by gamut clipping or gamut mapping and mapped onto control points that are accessible within the gamut.

8. The method of any preceding claim ,

it is characterized in that the preparation method is characterized in that,

in order to reduce the computation time and the memory space required by the computer (6), firstly control points of the gray axis are determined solely by means of fitting calculations, and then the color transformation table (17) is filled radially outward on the basis of the gray axis, wherein the existing adjacent control points and the sensitivity to defective printing nozzles (8) are used to evaluate the suitability of the other control points.

Technical Field

The present invention relates to a method for reducing the effects of defective printing nozzles during a multi-color printing process of an inkjet printer.

The technical field to which the invention belongs is digital printing.

Background

In many of the ink jet printers known from the prior art, the printing material is guided through or more rows of printing nozzles which are stationary with respect to the machine during printing and which eject drops of ink of the order of a few picoliters onto the moving printing material.

If deposition occurs in these printing nozzles, then may result in a reduction in the volume of ink drops released, and may result in a change in the direction of the ink drop flight path, for example, if a uniform color plane is to be printed with printing colors, then there may be the effect that a bright line appears below the disturbed nozzle and possibly a darker line is formed next to it.

The color impression of the printed side by a human observer is described in terms of colorimetry by three values (for example XYZ or Lab). This is in line with the presence of three types of color receptors (Farbrezeptoren) in the eye of a standard observer of the tristimulus (trichmatisehen). If more than three process colors are used for printing, there are usually a plurality of possibilities for producing a certain color impression. The determination of the respective degrees of freedom for transforming the image data from the instrument-independent color space into the instrument-dependent color space, that is to say for example from XYZ or Lab to CMYK or CMYKOGV, is referred to below as color construction (Farbaufbau).

For the purposes of the method described here, it is important that the color formed by the different ink proportions (Tintenanterilen) leads to different responses to fluctuations in the involved inkjet printing nozzles, even if the inkjet printing nozzles are operated with different colors.

The nozzle disturbance caused by the deposits can in the extreme case lead to a total failure, i.e. no more ink reaches the print substrate at this point, but more often the drop size is reduced to a greater or lesser extent, and this effect usually also fluctuates in time, so that, for example, a functionally restricted nozzle ejects deposits again after hours of constant operation and again achieves the original drop size and the original flight trajectory.

In order to reduce the problem of deposits in these printing nozzles, it is then necessary, for example, to provide regular maintenance cycles with a powerful actuation of all the printing nozzles. Such a strong flush can in most cases avoid drying out of the ink even if certain defined printing nozzles are used less or even not used in a given print job. In addition, maintenance may be performed by pressing ink through the printing nozzles at elevated pressure. By using a solvent-containing cleaning liquid for the printing nozzle from the outside, the deposits at the opening of the printing nozzle can be dissolved.

Exemplary scattering of the production-related printing nozzles (Exemplar-Streuungen) can be equalized by individual calibration for each piezoelectric element by means of an adapted electrical pulse shape, the method can also be used to compensate for temporal variations to a certain extent at .

However, what the above measures cannot change is: fluctuations in print nozzle permeability between two maintenance cycles can affect print quality.

In the field of color building technology when printing with more than three inks, methods have been established which make the ink consumption particularly small, such as in particular GCR (Gray component replacement). This, although reducing the color proportion by favoring black, reduces the printing costs, facilitates drying and increases the image sharpness, it leads to an image impression which is very sensitive to fluctuations in the printing nozzles for black ink.

The limited use of GCR for color structuring, as is customary in offset printing (for example 50% instead of CMY proportional share), although it also reduces the disturbing effects, is by no means optimal, since it is primarily directed at specific proportional relationships between the printing colors and not at visible interferences.

If the interference of the printing nozzles cannot be eliminated in this way at all, it is often attempted to compensate for the interference by other locally adjacent printing nozzles. In addition to compensation by means of directly adjacent printing nozzles which attempt to improve ink ejection to eliminate defects, a familiar approach in multicolor printing is: the interference is masked as good as possible by the remaining printing nozzles of the undisturbed color ink.

For this purpose, german patent application DE 102014219965 a1 discloses a method for compensating defective printing nozzles in an inkjet printer, comprising the steps of calculating all reduced color allocation variables, which are derived on the basis of the process colors used and which respectively lack process colors, detecting a nozzle failure of a printing nozzle with of these process colors during the execution of the printing job, selecting appropriate color allocation variables on the basis of the missing process colors, determining a target color position at the defective printing nozzle position, determining, on the basis of the selected color allocation variables, an alternative color position at the defective printing nozzle position having the smallest color distance from the target color position, continuing the printing job by means of the newly determined combination of the remaining process colors at the defective printing nozzle position.

Disclosure of Invention

The object of the invention is to provide interference-proof systems

Inkjet printing method which is as unaffected as possible by fluctuations in the quality of the printing nozzles.

This object is achieved by methods for reducing the influence of defective printing nozzles in a multicolor printing process of an inkjet printer by means of a computer, wherein a color conversion table (Farbransformationstabelle) is produced by the computer, which color conversion table contains, for each process color (Prozessfarben) of the inkjet printer, a substitute combination of the respective other process colors at a minimum color distance from the process color, and is used for compensating defective printing nozzles, the method being characterized in that the color conversion table is produced by printing and measuring a test proof (Testform) having all process colors, and the interpolation method is used by the computer, the color conversion table being optimized by the computer in such a way that undesired transitions between control points of the color conversion table are generally avoided, and the resulting color conversion table accordingly contains the smallest combination of process colors for the defective printing nozzles, and the computer executes the color conversion table, for example, and the method for carrying out the printing process of the inkjet printer according to the invention, the color conversion table is used for the smallest possible process color combinations of process colors, and the process conversion table is carried out, for the missing nozzle, for which the missing nozzle is usually produced by the process image, the method, which is carried out, using, for the missing nozzle, for example, the missing nozzle, the original process conversion table, the missing nozzle, before the process of the process conversion table, the process of the process, which is carried out of the process of the present invention, and the process of.

Advantageous and therefore preferred developments of the method result from the dependent claims and the description with the figures.

In order to produce the color conversion table by means of a computer, the proportion of process colors to be replaced is equal to zero for each combination of substitute colors, which means that the print nozzle which prints the respective process color is assumed to be defective, which is an extreme case of , since it is assumed here that the respective defective print nozzle is no longer printing at all and thus no longer contributes at all to the print image to be produced by means of the respective process color.

In this case, the fact that the defective printing nozzles do not fail completely, but rather are printed only with reduced power (or are printed askew) means that the method depends on the user-specific compensation strategy, in principle, only those defective nozzles which are printed only weakly or askew are more frequently than those which are completely failed, but the way in which these printing nozzles are frequently switched off is also relevant, i.e. it is only meaningful to calculate a substitute color combination having a proportion of the process color to be replaced which is greater than zero if either such printing nozzles which are not completely failed or, despite a smaller deviation from their expected performance, is still printed within the intended tolerance limits in comparison with compensating printing nozzles which are still printed within the intended tolerance limits.

In this case, a preferred development of the method according to the invention provides that, in the case of six-color or seven-color printing, the number of control points in the color conversion table is reduced compared to four-color printing. The color conversion table becomes correspondingly more complex during printing processes that use more than four colors, typically six or seven colors. Thus, an exponentially increasing number of control points results in a large increase in the memory requirements for such a color conversion table. In order to reduce this storage space requirement, the number of control points in such a six-color or seven-color printing is then also reduced. This must of course be done in such a way that as large a partial area as possible of the color space to be transformed is also covered by the table.

In this case, a preferred refinement of the method according to the invention provides that the reduction of the number of control points in six-color or seven-color printing is achieved by discarding those control points in which the value of the process color black is not less than the proportional share of any of the other process colors, or by discarding those control points of complementary process colors which together do not allow a maximum proportional share of two of the process points.

In this case, a preferred development of the method according to the invention is to use, as test coupons, CMYK test coupons with all the existing process colors, which have all combinations of process color proportion fractions of 0%, 30%, 60% and 100%, and to eliminate those process color proportion fraction combinations in the CMYK test coupons whose sum of the process color proportion fractions is higher than the sum of the permitted area coverage.

To make the color conversion table, test coupons containing all process colors CMYK must be printed and measured. The test specimen contains both CMYK full-tone areas and grid areas with process color proportions of 0, 30% and 60%, and also the individual process colors in combination with one another, which combinations are produced by overprinting. After being printed, these test coupons are measured by means of an image sensor, digitized and processed by means of computer analysis. By means of such measurementsThe color conversion table is produced in that each defined use of a process color (e.g. CMYK) on a test specimen corresponds to corresponding synthesized measured LAB values, whereby a color conversion table in the form of ICC color profile values can be produced accordingly, control point pairs are obtained for each combination pair between a process color combination value and a synthesized LAB value, since here the inkjet printing process is involved, and of course also the special conditions and limitations of the inkjet printing have to be taken into account, of these limitations relates to the maximum overprinting of different process colors (inks in the inkjet case), unlike offset printing, overprinting of different inks only covers the sum of the areas that are specifically allowed

Within the scope of this is feasible. Thus, if the combination of process colors exceeds the allowed sum of area coverage, such a combination of process colors is not allowed to be printed in a CMYK test proof. If the color values in the corresponding CMYK color space (or LAB color space) cannot be covered by such test samples, these color values have to be supplemented later by a computer (e.g. by interpolation).

In this case, a preferred development of the method according to the invention provides that the control points outside the color Gamut are processed individually by means of a computer by color Gamut Clipping or color Gamut Mapping and mapped onto the control points that are achievable within the color Gamut. It may occur that, with a corresponding color space transformation, there are regions in the LAB color space which cannot be mapped by means of the process color space (i.e. the process color by means of the inkjet printer). Therefore, these control points that are outside the color gamut must be brought back to the control points that are achievable within the color gamut. This is correspondingly performed by the computer with the aid of different tools, such as gamut clipping or gamut mapping.

In order to reduce the computation time and memory space required by the computer, the control points of the gray axis are first determined by fitting calculations (ausgleichschrechnnung) only, and the color conversion table is then filled radially outward from the gray axis, wherein the already existing adjacent control points and the sensitivity to defective printing nozzles each contribute to evaluating the suitability of the other control points.

Drawings

Such an invention and structurally and/or functionally advantageous refinements of the invention are described in further steps on the basis of at least preferred embodiments with reference to the drawing, in which elements that correspond to one another are each denoted by the same reference numeral.

The figures show:

FIG. 1: an example of the structure of a page inkjet printer,

FIG. 2: an example of the reduction in drop volume due to deposits on the print nozzles,

FIG. 3: a schematic example of the white line caused by a missing nozzle,

FIGS. 4a-4 b: color transformation between the Lab color space and the process color space,

FIG. 5: exemplary progression of the method according to the invention.

Detailed Description

A preferred application area of the embodiment variant is an inkjet printer 7. An example of the basic structure of such a machine 7 is shown in fig. 1, the machine 7 comprising a feeder 1 as far as a receiver 3 for supplying a print substrate 2 into a printing unit 4, where the print substrate 2 is printed by a print head 5.

In this case, a sheet-fed ink-jet printing machine 7 is controlled by a control computer 6. as already described, in operation of this printing machine 7, it is possible for individual printing nozzles in the printing head 5 of the printing unit 4 to fail, which is shown in fig. 2a and 2b again times schematically, fig. 2a shows a printing nozzle 8 which ejects conventional ink drops 9, whereas in fig. 2b, deposits are formed at the printing nozzle opening 10, which cause the ejected ink drops 9a to be too small.

The result is then a "white line" 19, or in the case of multicolor printing, a distorted color value. Fig. 3 shows an example of such a "white line" 19 in the printed image 18.

Now, the method described herein aims at: by optimizing the color architecture of multicolor inkjet printing, print result 18 is made particularly insensitive to fluctuations in print nozzle permeability in all color channels. Low printing costs or fast drying are not yet the most important in the present method.

Furthermore, it is important for the method that, despite the presence of specific edge conditions, a reproduction of the required color transformation by means of a control point grid (St ü tzpunkt-Gitter) obtained by means of interpolation, for example in the Lab color space 11, is possibleWithout causing uneven color gradations between stored control points

This means that the method excludes sudden changes in the proportion of ink between adjacent control points of the table. Thus, even in seven-color printing, such color conversion can be presented as an ICC color layout parameter of a medium size without a formation (ohne Artefakte).

The procedure of the method according to the invention in its preferred embodiment variant is schematically illustrated in fig. 5 as step , by means of an inkjet printer 7, suitable test sheets are printed for the number of printing colors 13 used, and the corresponding process colors 13 are then measured colorimetrically, the color zones of such test sheets being defined in the instrument-dependent color space (for example CMYK or 7c) and constituting, with their respective color proportion of the inks involved, in general, dot-scan samples (abdastung) of the n-dimensional process space, and then, by means of a suitable interpolation method, an overall mapping from the instrument-dependent color space to the instrument-independent color space (for example XYZ or Lab) can be carried out in a very close manner on the basis of these measured points.

For CMYK four-color printing, such test coupons are preferably standardized according to ISO 12642-2 or IT 8.7/4. In this way, for more than four printing colors, very simple and rough test copies can be formed with all combinations of proportions of 0%, 30%, 60%, 100% of the ink in the color channels. Combinations whose ink proportion sum is higher than the permissible area coverage sum (for example 350%) can be removed. More accurate test patterns are more refined from coarse scan patterns by additional points in process space-dependent regions, thereby allowing more accurate interpolation in these regions.

In a preferred embodiment of the method, only complete failure (or blockage) of the printing nozzles 8 of the involved color channels is taken into account, since failure of all printing nozzles 8 is a minority event with reference to the overall total number of printing nozzles 8, and therefore only failure of a single color channel at the location of the image 18 to be printed is taken into account, and the extremely rare simultaneous failure of two or more printing nozzles 8 of different color channels at the same location is ignored.

The failure of printing nozzles 8 in color channels means that only 0% of the printing is carried out at the location concerned, instead of the predefined ink proportion 9, for a specific ink proportion 9 combination (i.e. for a specific point of the process space), unmodified Lab points are produced in each case when the process model is applied, each ink component can then be set to 0% one after the other, and the color distance 14 of the produced modified Lab point from the Lab point is determined, where, for example, the euclidean distance of DeltaEab or the more precise distance determined according to the DeltaE2000 formula can be used as the color distance 14, the maximum value of the distances 14 of the individual components is used as a measure of the sensitivity of the given ink combination to a printing nozzle failure, so that, of all ink combinations which lead to a specific Lab point, those ink combinations which are very small for a printing nozzle failure are particularly suitable for the color configuration to be determined.

In a further step, in order to map the instrument-dependent color space (i.e. the preferred Lab space 11) into the process space, a color conversion table 16 (also referred to as ICC table/color configuration variable) is then retrieved, which color conversion table 16 contains for each Lab table control point (St ü tzpunkt) the ink combination with the lowest sensitivity to printing nozzle faults 15.

For the practical purposes of the method, it is important to note that this ICC table 16 leads in many cases to unusable printing results, for which it is easily understood that, for example, a five-color printing process with the inks CMYK and green, in which in particular Lab points in the lower luminance region of the achievable colors, process points with a high proportion of C, M, Y may be produced for a particular table point in the Lab space 11, taking into account the maximum permissible sum of the area coverage, however, process points with a high proportion of M, green and K may occur more suitably for adjacent table points, in the case of the application of the color table 16 (or ICC color configuration variables 16) implemented by means of interpolation between these table control points, the transition between two Lab points in the images to be printed may not lead to the desired transition 12 shown in fig. 4a, but rather, due to the strongly non-linear process characteristics, to the fact that the transition 12a shown in fig. 4b would tend to lead to a sudden change in the color configuration from the optimum point to , even if the tables deviate from the optimum point suddenly.

To be able to meet the above requirements, a large number of points are generated in the memory of the computer, which scan sample the process space. The computer may be a computer in the prepress stage or may be the control computer 6 of the ink jet printer 7. In a four-color process (e.g. CMYK) it is possible to: all ink proportion combinations between 0% and 100% are kept available in the computer 6 at a distance of, for example, 5%, and the number of Lab points and the above-mentioned values relating to the susceptibility to printing nozzle failure are correspondingly determined by means of a process model. To do this, 21^4 ^ 194481 points in CMYK and Lab, and a corresponding number of sensitivity values are required in this example. Appropriate coding (such as 8 bits per channel for CMYK, 3 x 10 bits for Lab, and 8 bits for sensitivity) is sufficient for the required accuracy. The five color process also needs to be handled in this way.

In the case of six-and seven-color processes, it is advantageous in terms of the storage space and the computation time required by the computer 6 to limit the number of these points by additional conditions since the process color black generally has the strongest influence on the position of a color in the Lab space, so that all process points in which the black value is not less than any other ink proportion share can be excluded, for example, for a reference quantity (Ausgangs-range).

The point reference quantities can then be classified according to the L value in the process model or indirectly by means of an index array, also in parallel with respect to L, a and b. Thereby, for the table control points given in Lab 11, all the points of the reference quantity located in the periphery can be found quickly. For this purpose, a tolerance that is sufficiently large for a 5% gradation in the reference quantity is used for the values L, a and b. Because of the rough grading of these points of the reference quantity, supplementary iterations (nanocover) are usually required for the found points with the aid of a process model in order to achieve the desired accuracy of the color reproduction; typically less than 0.5 DeltaE.

For those points that lie at the edge of the color volume (such as points with 100% cyan), only candidates for complementary iterations are generally left within the required accuracy, and the sensitivity that is found there is no longer any influence due to the lack of alternatives.

For example, the largest difference in these ink channels can be used as an evaluation criterion for the expected process point neighborhood in a control point grid predefined in Lab 11; small differences are desirable and thus also correspond to the desired small slopes of the values in the separations.

The optimal filling of the entire ICC table 16 is calculated times by means of the additional conditions described above, which is extremely complex with regard to the required storage space and the calculation time, since the candidates of the complementary iteration must remain available simultaneously for all table points, and an extreme number of possible combinations must be taken into account.

In contrast, in another embodiment of the invention, a complete failure of the printing nozzle 8 is no longer taken into account, but rather a reduction in the drop volume 9a is taken into account, for which purpose the reduction in the drop volume 9a is treated in an approximate manner, as is the reduction in the proportion of the respective ink, so that partial failures of the printing nozzle 8 or differential changes in the permeability of the printing nozzle can also be treated.

List of reference numerals

1 feeder

2 currently printed substrate/currently printed sheet

3 material collector

4 ink-jet printing mechanism

5 ink jet print head

6 computer

7 ink jet printer

8 printing nozzle

9 conventional ink drops from the printing nozzle

9a reduced ink drop from a blocked printing nozzle

10 printing deposits at nozzle openings

11 Lab color space

Desired transition between two control points in 12 Lab color space

Transition of distortion between two control points in 12a Lab color space

13 measured Process color

Color distance between 14 nominal color values and color values of defective printing nozzles

15 optimum process color combination/hue value combination for compensation

16 optimal ICC table/color profile

17 complementary modified ICC table/color profile

18 printed image on currently printed sheet

19 white line