Color correction method for digital printing process

Abstract

The invention relates to a method for color correction in a digital printing process, in which method: a raster image processor (100) generates a screened color separation (104) from at least one color separation (101) of a predetermined digital image (102) by using a screening algorithm (103); the screening algorithm (103) reads color values of the color separation sequentially along a screening path and creates dots for the screened color separation on the basis of the read color values and at least one predetermined threshold value (threshold); and a correction value related to a printing position is taken into account each time the screening algorithm creates a dot.

Description

Technical Field

[0001] This invention relates to a method for color correction in digital printing processes. Background Technology

[0002] Color correction methods for digital printing are known in themselves. These methods identify deviations from the expected print result and take this into account during the printing process to bring the printed material closer to the intended print outcome.

[0003] A major challenge in outputting digital images is obtaining prints with minimal deviation from the reference print, even after extended downtime, when the digital image is reprinted. The causes of deviation are varied; for example, the print substrate may differ in color and flow characteristics, or the printer may have changed over time.

[0004] A particular challenge here is color correction based on position along the print width (i.e., transverse to the feed direction of the printing medium). A major problem when outputting digital images is color and density variations across the printer width, such as those caused by printing artifacts, which are particularly noticeable as stripes in the printing direction for the same color (“monochrome”). The reasons are varied; for example, the print dot size (the droplet size in inkjet printing) is never exactly the same across the entire width of the printhead or from one printhead to another. Furthermore, the positional accuracy of each print dot is subject to varying tolerances, thus altering the hue value initially expected by rasterization, or changing the perceived color value by varying the ratio of additive to subtractive mixing. Moreover, especially in inkjet printing, the spray deposited on the substrate can alter the desired color and density values. Particularly in inkjet printing, varying ink flow behavior (especially on non-absorbent materials) can adversely affect print density due to differences in printhead position in the feed direction and varying distances from the printhead to pre-curing or final drying stages. Furthermore, especially in inkjet printing, broken lines (nozzle errors) can negatively impact the visual appearance of the printed result.

[0005] To determine the location and intensity of these printing artifacts, it is generally known to print a test image and analyze it. The information obtained can then be used for correction in a variety of ways.

[0006] However, one problem with this method of determining correction data is that the results obtained are often insufficient. In particular, the uniformity of the individual colors achieved through correction is sometimes not equally achieved during overprinting. This poses a significant problem when printing essentially monochrome images, as the remaining deviations are particularly noticeable and result in clearly visible stripes in the printing direction. The quality of the determined correction data can have a significant impact on the achievable results.

[0007] For example, EP4113963A1 describes a method for color correction of a digital printer across the print width of a digital printer, the method comprising the steps of: a) providing a predetermined digital image, wherein the digital image includes at least one color channel image, and wherein each color channel image is assigned to a print color channel of the digital printer; b) determining at least one primary color of the predetermined digital image; c) creating a digital test image, wherein the digital test image has at least one monochrome test bar intended to be printed across the print width of the digital printer; d) printing the digital test image by means of the digital printer; e) detecting the color value of the printed test image according to the print width by means of an optical colorimeter; f) generating correction data for the digital printer based on the detected color value; and g) applying the correction data to the digital printer and / or the predetermined digital image.

[0008] In known methods, correction data is taken into account in the printing process in some way.

[0009] To quickly correct current color deviations, color correction can be performed, for example, by adjusting the printhead voltage on inkjet-based systems. Some systems even allow independent adjustment of individual sub-regions of the printhead. However, this alters the printhead's optimal operating point and therefore can only be used for limited correction. Furthermore, color deviations smaller than the smallest adjustable sub-region of the printhead (e.g., color deviations caused by nozzle errors) cannot be corrected, or can only be corrected to a limited extent.

[0010] Another option is to use a color management system associated with the output device to generate new, color-corrected print data in the sub-regions. However, this process can take a considerable amount of time, depending on how the color management system is integrated into the printing press, the printer operator's accessibility to the color management system, the speed of the color management system, and the total size of the digital image files.

[0011] Other methods attempting to avoid reliance on a color management system involve transmitting a correction signal to a printhead controller, which performs correction by adjusting the number and / or size of the print dots. However, a problem with this approach is that it may require specialized hardware. Furthermore, due to the nature of the process, this method cannot apply correction equally along the print feed direction and across the print width. This can lead to defects in the print results. In particular, using this method, very small corrections (distributed across multiple print dots due to their discrete size) cannot be evenly distributed along the print feed direction and across the print width, which can cause artifacts.

[0012] EP4113964A1 describes a color correction method for digital printed matter, wherein the digital printed matter involves outputting a predetermined digital image by means of a digital printer, wherein the predetermined digital image is provided to the digital printer in the form of a color-separated digital image comprising at least one color channel image, wherein each color channel image is assigned to a printing color channel of the digital printer and contains information about a print dot to be set by the printer for the corresponding printing color channel, and wherein the print dot is defined by its print position and print dot size, wherein the method includes the steps of: a) providing correction data for digital printing, wherein the correction data includes at least one correction value for at least one color channel image; b) applying the correction data to the color-separated digital image to obtain a color-corrected color-separated digital image; and c) optionally printing the color-corrected color-separated digital image by means of the digital printer.

[0013] EP3570530A1 describes a method for compensating for position-related density fluctuations in an inkjet printer using a computer, wherein, within the calibration range of the raster image processor of the inkjet printer, specific halftone patterns are assigned to specific grayscale values ​​of a certain color separation of a printed image to be halftoned using a lookup table, these halftone patterns being used by the computer to halftone the printed image, and the halftone printed image being printed on the inkjet printer, characterized in that the lookup table includes the position of each print nozzle of the printhead of the inkjet printer as an additional variable, and for each print nozzle position, the computer enters the complete set of grayscale values ​​along with the associated adapted halftone pattern into the lookup table.

[0014] US2019217629A1 describes a control device for generating a control signal based on an input signal to control a printer. The control device includes: a quantizer configured to generate the control signal by quantizing an intermediate signal to control a correction amount for a print density error, wherein the intermediate signal is generated jointly by: a multiplication of the input signal with a correction gain performed by a first multiplier, and a subtraction operation performed by a first calculator to subtract an error diffusion output signal from the input signal, the error diffusion output signal being output by a diffusion matrix that determines the print density error distribution; a second multiplier configured to multiply the control signal by an adjustment gain determined based on the correction gain; and a second calculator configured to generate an error diffusion input signal to be input into the diffusion matrix based on a signal output by the second multiplier.

[0015] In other words, this document describes the correction of printer control signals in which the input signal is offset using an error diffusion (ED) signal from a diffusion matrix, without involving rasterization, with the aid of a computer. This correction is based on the final input signal and therefore is not theoretically constrained; small corrections cannot be uniformly distributed in the print feed direction and along the print width, which can lead to artifacts.

[0016] US2009051944A1 describes an image processing method that includes converting halftone pixels of an image to be printed from a lower first hue value resolution to a higher second hue value resolution, the halftone pixels being represented at the lower first hue value resolution. Image processing is applied to the pixels at the higher hue resolution to reduce artifacts in the image quality when the pixels are printed at the lower first hue value resolution. In a preferred embodiment, the image processing includes Σ-Δ modulation error diffusion.

[0017] In other words, the correction is performed in a second image processing step using a lookup table with location-related correction values. This second image processing step follows the first image processing step, in which the image is first screened. Therefore, the correction does not occur during rasterization, but only on the screened image.

[0018] Color correction methods used in digital printing processes may still have room for improvement. This potential for improvement may particularly lie in performing color correction more quickly and easily. However, there is still potential for improvement, especially in the quality of the correction results and the avoidance of artifacts. Summary of the Invention

[0019] The purpose of this invention is to provide an improved color correction method for digital printing processes.

[0020] This objective is achieved by the method according to claim 1, the apparatus according to claim 14, and the use according to claim 15. Preferred embodiments of the invention are given in the dependent claims, in the description, or in the drawings, wherein other features described or shown in the dependent claims, description, or drawings may constitute the subject matter of the invention individually or in any combination, unless the context clearly indicates otherwise.

[0021] This invention proposes a color correction method for digital printing processes, wherein, by means of a raster image processor, a halftone image is generated from at least one color separation of a predetermined digital image (continuous tone image) using a halftone algorithm, wherein the halftone algorithm sequentially reads the color values ​​of the color separation along the halftone path, and creates a halftone separation point based on the read color values ​​and at least one predetermined threshold, wherein each time the halftone algorithm creates a point, a correction value related to the printing position is considered.

[0022] Surprisingly, the method according to the invention offers a wide range of advantages. In particular, the method can be integrated into existing processes with exceptional ease. Furthermore, it is particularly efficient compared to other correction methods and has almost no impact on the speed of the raster image processor (considering that such processors are essential in digital printing anyway). Moreover, the method according to the invention enables exceptionally accurate color correction while reducing artifacts. In particular, the method allows for particularly good simultaneous color correction for both small and large color deviations. For example, the method of the invention can simultaneously correct color deviations caused by differences in individual printing nozzles (nozzle errors), especially when printing using multi-row (two-color row) nozzles arranged sequentially along the printing direction.

[0023] In other words, this invention proposes a color correction method for use in digital printing processes. For this purpose, a raster image processor is used. At least one color separation of a predetermined digital image is fed to the raster image processor. This color separation is screened using a screening algorithm, thereby producing a screened color separation, which can be printed, for example, in a digital printing process using a printer. In this process, the screening algorithm sequentially reads the color values ​​of the color separation along the screening path and creates points for the screened color separation based on the read color values ​​and at least one threshold. When the screening algorithm creates points, it considers correction values ​​related to the printing position.

[0024] In the context of this invention, digital printing process can be understood in particular as a process for printing digital images using a digital printer. In the context of this invention, a digital printer can be understood in particular as a device capable of outputting color print dots forming a corresponding image onto a substrate.

[0025] Preferably, it can be specified that the halftone color separation obtained by this method is printed on a digital printer.

[0026] Preferably, the digital printing process can be specified as an inkjet digital printing process. More preferably, the halftone color separation can be specified as being printed on an inkjet printer.

[0027] In the context of this invention, "digital image" can be understood in particular as an image that can be digitally stored, whose image information includes location-dependent color values ​​regarding a color system. In the context of this invention, "predetermined digital image" is particularly a digital image intended for printing. For example, a predetermined digital image can be a pixel-based or vector-based image.

[0028] Accordingly, in the context of this invention, color separation can be understood in particular as a portion of a digital image containing only one color channel information of the color system used to store the digital image. For example, a digital image may be defined by color values ​​in the CMYK color system, and color separation may, for example, contain only the color values ​​of the C color channel.

[0029] Accordingly, in the sense of the present invention, a distinction is made in particular between color separation of a predetermined digital image and color separation by halftone, wherein color separation of the predetermined digital image can be defined in particular by continuous color values ​​(continuous tone image) and color separation by halftone can be defined in particular by monochrome points (halftone image).

[0030] Preferably, the colors of the color separation and the halftone color separation can be specified to correspond to the printing colors of the printer used in the digital printing process. If the method includes multiple color separations and halftone color separations, it can be preferably specified that the colors of these color separations and the halftone color separations respectively correspond to the printing colors of the printer intended for the digital printing process.

[0031] Preferably, an inkjet printer can be provided for the digital printing process, which particularly preferably includes cyan, magenta, yellow, and black (CMYK) as printing colors. In this context, it can be particularly preferably specified that the method creates cyan, magenta, yellow, and black color separations of a predetermined digital image, wherein each color separation is converted into a corresponding halftone image by using a halftone algorithm.

[0032] In the context of this invention, a raster image processor can be understood in particular as software running on a computer (e.g., a printer's computer) for digital printing processes.

[0033] In the context of this invention, the halftone algorithm can be understood in particular as a routine of a raster image processor that can generate corresponding halftone color separations.

[0034] In the context of this invention, the "color value" of color separation can be understood in particular as the color value used for color separation at each location, which can be read by a halftone algorithm. For example, the color of color separation can be defined by 8-bit grayscale values ​​or color saturation (i.e., a gradation of 256 grayscale values).

[0035] In the context of this invention, "screening path" can be understood in particular as a path specified for a screening algorithm, or a path implemented within a screening algorithm, along which color separations can be read partially, substantially completely, or over the entire surface. Therefore, the screening path specifically specifies the order in which color values ​​from different regions of a color separation are read.

[0036] The phrase "considering correction values ​​related to the print position whenever the halftone algorithm creates a dot" can be understood, in the context of this invention, as meaning that when a dot is created, it is modified using correction values ​​related to the print position, such that the resulting halftone color separation takes correction data into account, i.e., when printing with halftone color separation, the color deviation of the printed matter of the predetermined digital image relative to the predetermined digital image is corrected. In other words, this means that the correction data is considered during the creation of each print dot during RIP processing, rather than, for example, before or after RIP processing.

[0037] In a preferred embodiment, the halftone path may be specified to traverse the predetermined digital image with parallel, continuous, and equidistant lines. Preferably, the halftone path may be specified to read these lines in alternating directions. These lines may, for example, extend parallel to, orthogonal to, diagonally to, or at another specific angle relative to the intended print feed direction. Alternatively, these lines may extend parallel to, orthogonal to, diagonally to, or at another specific angle relative to the intended halftone angle of the associated color separation. In a preferred embodiment, the halftone path may be specified to be determined based on an algorithm (e.g., a chaotic algorithm).

[0038] In a preferred embodiment, it can be specified that the raster image processor generates the screened color separation from at least one color separation of the predetermined digital image using more than one screening algorithm, and / or the screening algorithm provides more than one screening path. Specifically, it can be specified that multiple screening algorithms generate the screened color separation at least partially simultaneously (in parallel at the process level), and / or multiple screening paths read the color separation at least partially simultaneously (in parallel at the process level). For example, it can be specified that different screening paths begin at different locations within the color separation, thereby covering corresponding sub-regions of the color separation respectively. Alternatively, for example, it can be specified that the screening paths extend side-by-side or in opposite directions. Parallelization of the process can be particularly simplified by using multiple screening algorithms and / or screening paths.

[0039] In the context of this invention, "predetermined threshold" can be understood in particular as the value at which the halftone algorithm creates points. For example, the threshold can be adapted to the format of the color value read, i.e., it can also be an 8-bit value. Furthermore, the threshold can be adapted in particular to the printer intended for printing and / or the halftone screen used.

[0040] In other words, the color values ​​are preferably summed or integrated by a halftone algorithm during the reading process until they reach a predetermined threshold. Then, points are created in the halftone color separation.

[0041] In the context of this invention, the correction value related to the printing position can be understood in particular as a value describing the color deviation of the printed image relative to a predetermined image (depending on the printing position). In other words, the correction value related to the printing position can therefore be understood as being able to take different values ​​depending on the expected printing position. The term "printing position" can be understood in particular as the position on the printing medium when printing halftone color separations (depending on the printer).

[0042] Preferably, the digital printer may be specified to have a feed direction (y) for the medium to be printed, and the correction value depends on the printing position (x) transverse to the feed direction. For example, the correction value may be specified to form a continuous distribution based on the printing position (x) transverse to the feed direction of the digital printer.

[0043] Preferably, the correction value may be specified to include information about the position of the print nozzles of the digital printer. For example, the correction value may be specified to be a discrete value corresponding to a specific position of the print nozzles of the digital printer, which is transverse to the feed direction of the digital printer.

[0044] If the method includes multiple color separations and halftone color separations, it can preferably be specified that the halftone algorithm considers a correction value related to the print position when generating each halftone color separation. Here, it can be specified that a separate or independent correction value related to the print position is considered for each printing color.

[0045] Alternatively, the halftone algorithm may be specified to not consider print position-related correction values ​​when generating each halftone color separation. For example, the method can create corresponding halftone color separations (halftone images) from the cyan, magenta, yellow, and black color separations of a predetermined digital image using the halftone algorithm, wherein the halftone algorithm does not consider print position-related correction values ​​when generating the halftone yellow color separation.

[0046] Preferably, the halftone algorithm may be specified to use a halftone screen when generating the halftone color separation, wherein the halftone screen is, for example, an amplitude modulation (AM) halftone screen or a frequency modulation (FM) halftone screen.

[0047] Preferably, the halftone algorithm may be specified to use a halftone screen (e.g., an AM halftone screen or an FM halftone screen) when generating the halftone color separation. The method preferably includes multiple color separations and halftone color separations, and the halftone algorithm uses a halftone system (e.g., an AM halftone system or an FM halftone system) when generating the halftone color separations.

[0048] Preferably, the halftone algorithm may be specified to use halftone screens when creating the halftone color separation, wherein the method includes multiple color separations and halftone color separations, and the halftone algorithm uses a halftone system when generating the halftone color separation, wherein, for example, the halftone system is an AM halftone system or an FM halftone system. In the context of this invention, a halftone screen can be understood in particular as a grid of points or cells through which image details can be represented by changing the size of the points and / or the distance between the points. Here, different point shapes can be selected, such as circular, elliptical, or square. In the context of this invention, a halftone system can be understood in particular as a combination of multiple halftone screens that match each other. In particular, the halftone angles and halftone frequencies in the halftone system can be matched to each other to optimize the overlay of the halftone screens.

[0049] Preferably, the predetermined digital image can be processed in an upstream color management system. Specifically, the predetermined digital image can be converted into an image format adapted to the color space of the intended printing process by the upstream color management system, and corresponding color separation images containing each color separation can be created. Here, additional correction values ​​(especially correction values ​​independent of the printing position) can be considered in the color management system.

[0050] Preferably, the correction value related to the print position can be specified as the product of a print position-specific original correction value and at least one scaling factor, wherein the scaling factor is preferably selected from a constant scaling factor, a color value-related scaling factor, and a dot size-related scaling factor, wherein, particularly preferably, the correction value related to the print position is the product of the print position-specific original correction value, the constant scaling factor, and the dot size-related scaling factor.

[0051] Preferably, a constant scaling factor can be understood as a scaling factor that does not depend on any other parameters, i.e., scaling the corrected data linearly (all by the same factor). This inherently allows for a holistic effect on the magnitude of the correction. This can be particularly advantageous when applying this method if the color correction is insufficient or excessive.

[0052] Preferably, the scaling factor associated with the color value can be understood as a scaling factor that depends on the color value being read; that is, it can take different values ​​depending on the color value being read. Therefore, when applying correction values ​​associated with the print position, not only the print position but also the color value used as a reference at that position is considered. In particular, the scaling factor associated with the color value can be expressed as a scaling factor that depends on the color value being read (e.g., in the form of a linear function or polynomial). This allows, for example, particularly bright or dark color values ​​to be corrected more strongly or less strongly accordingly.

[0053] In a preferred embodiment, the scaling factor may be specified to depend on the color of the predetermined digital image. Specifically, this means that the scaling factor depends not only on the color values ​​of the corresponding color separations but also on the color of the predetermined digital image itself. Here, the scaling factor can preferably be interpolated for various colors based on test measurements. This allows, for example, consideration of overprinting different printing colors.

[0054] Preferably, in the context of this invention, the scaling factor related to the dot size can be understood as a scaling factor that depends on the dot size of the generated dot. Therefore, when applying correction values ​​related to the print position, not only the print position but also the expected print dot size at that position is considered. Specifically, the scaling factor related to the dot size can be represented in tabular form, allowing a separate scaling factor to be assigned to each dot size achievable by the printer. Alternatively, the scaling factor can also be specified as a function of the dot size. The conversion between tables and functions can be achieved by interpolating the tabular data or calculating function values ​​for the dot sizes achievable by the printer.

[0055] For example, the original correction value k for an inkjet printer, specific to the print position, can depend on the print position x transverse to the feed direction y, and thus can be represented as k(x). By combining a constant scaling factor s and a scaling factor s' (dot size) related to the dot size, the correction value K(x, dot size) related to the print position can be obtained.

[0056] K(x, point size) = k(x) × s × s'(point size).

[0057] Preferably, the correction value related to the printing position may be specified as being obtained from a color difference, which is the difference between a predetermined test image and a test print based on the predetermined test image, and the correction value related to the printing position is obtained, in particular, from test prints having different color values ​​and / or dot sizes.

[0058] In a preferred embodiment, the correction value is obtained as described in EP4113964A1, similar to correction data. In other words, it can therefore be preferably specified that: a test color of the predetermined digital image is determined and a test image is created, the test image comprising at least one monochrome test strip intended to be printed across the print width of the digital printer; the digital test image is printed using the digital printer; the color value of the printed test image is measured using an optical colorimeter, based on the position of the digital test image in the print width direction; and correction data for the digital printer is generated based on the measured color values.

[0059] Preferably, the test color determined in step b) can be specified as the primary color of the predetermined digital image. In the context of this invention, the primary color of the predetermined digital image can be understood in particular as the color value constituting a significant portion of the digital image.

[0060] Preferably, the printing of the digital test image by means of the digital printer is carried out on a printing substrate, which is the same as the printing substrate on which the predetermined digital image is to be printed.

[0061] Preferably, it can be specified that: At least one monochrome test bar is defined by the print density of all printing colors of the digital printer; and / or At least one monochrome test strip is defined by the printing density of a single printing color of the digital printer; and / or At least one monochrome test bar is defined by the printing density of the digital printer for more than one but fewer than all printing colors.

[0062] In a preferred embodiment, the digital test image may be specified to include at least one of the aforementioned monochrome test bars. The advantage of this approach is that particularly successful corrections can be achieved. Unbound by any theory, it is believed that the combination of these test bars enables the identification of the source of printing artifacts with high certainty, thereby achieving color correction that produces particularly good and stable results.

[0063] Preferably, the printing colors of the digital printer may be specified to have a printing order, wherein a monochrome test bar defined by the printing density of a single printing color of the digital printer is defined by the printing density of the printing color channel to be printed first according to the printing order.

[0064] In this way, color correction can be achieved starting from the color to be printed first, which is not bound by any theory and simplifies further correction in particular by avoiding the superposition of various effects when applying correction.

[0065] Preferably, it can be specified that a monochrome test bar, defined by the print density of the digital printer for more than one but fewer than all color channels, is selected such that: The first test bar in these monochrome test bars is defined by the print density of the first color to be printed in the printing sequence, and additionally by the print density of the next color to be printed in the printing sequence. Optionally, each subsequent test strip in these monochrome test strips is defined by the print density of the corresponding preceding monochrome test strip, and additionally by the print density of the next color to be printed in the printing sequence.

[0066] In this way, the printing colors in the test strips can be combined so that only one color channel is added from the first test strip to the second test strip, thus making color correction calculation particularly simple and reliable.

[0067] For example, it may be particularly preferred to specify that four printing colors (e.g., CMYK) are provided for printing the test color, wherein the color channels are printed in the order of C, M, Y, and K. The digital test image may then particularly preferably include four monochrome test bars, wherein the first monochrome test bar is defined by the printing density of the C color channel intended for printing the test color, the second monochrome test bar is defined by the printing density of the C and M printing color channels intended for printing the test color, the third monochrome test bar is defined by the printing density of the C, M, and Y printing color channels intended for printing the test color, and the fourth monochrome test bar is defined by the printing density of all CMYK printing colors intended for printing the test color. Specifically, for example, it may be specified that several test colors of a predetermined digital image have been determined, and the aforementioned monochrome test bars are determined separately for each test color.

[0068] In this way, it is possible to compensate for artifacts caused by individual printhead rows, as well as artifacts caused by the overprinting of multiple colors, by using a very small number of test strips and through color correction. In particular, this method allows color correction to reliably and accurately account for these artifacts without using complex computational methods to computationally separate these effects.

[0069] Preferably, the test color may be selected from the color that appears most frequently in the predetermined digital image, wherein, optionally, the minimum distance in the color space is maintained between multiple test colors, and / or the test color is selected independently of the frequency of each color in the predetermined digital image, and the maximum possible distance in the color space is maintained between the test colors.

[0070] Preferably, it can be specified that when the test print is created based on the predetermined test image, the screening system used by the screening algorithm is the same as the screening system used by the screening algorithm for creating the screened color separation from the color separation of the predetermined digital image.

[0071] Preferably, the correction value related to the printing position can be specified such that for each point generated by the halftone algorithm, the point size is determined based on the read color value, the predetermined threshold, and the correction value related to the printing position.

[0072] In other words, it is therefore preferable to perform a screening algorithm and read the color value C(x, y). This value is summed or integrated until it reaches the corresponding threshold T. Subsequently, points B(x, y) are generated in the screened color separation. Here, the screening algorithm determines the point size by considering the read color value C(x, y), the threshold T, and the correction value K(x) specified at the corresponding print position and related to the print position.

[0073] Preferably, it can be specified that the correction value K(x) considered in relation to the printing position depends on the size of the dot to be created or the color value read.

[0074] Preferably, the correction value related to the printing position can be specified as described above, such that for each point generated by the halftone algorithm, the dot size is determined based on the read color value, the predetermined threshold, and the correction value related to the printing position, wherein the halftone screen is an AM halftone screen.

[0075] If multiple dot sizes are provided, the threshold T includes multiple thresholds, i.e., separate thresholds for each expected dot size. For example, if the read color value reaches a first threshold T1 but not a second threshold T2, a dot B(x, y) with dot size P1 is created. However, if the halftone algorithm directly triggers a second threshold T2 greater than the first threshold T1, a dot B(x, y) with dot size P2 is created. For all dot sizes that can be implemented by the printer, a corresponding process can preferably be provided up to the maximum threshold T. max and maximum point size P max .

[0076] Preferably, the screening algorithm may include an error allocation algorithm, wherein the error allocation algorithm preferably calculates an error based on the difference between the created point and at least the color value used to create the corresponding point, wherein the error is allocated to the color value of at least one position along the screening path that has not yet been read, and wherein the error preferably takes into account the correction value associated with the printing position.

[0077] In other words, a halftone algorithm is preferably executed and the color value C(x, y) is read. This value is summed or integrated until it reaches the corresponding threshold T. Then, a point B(x, y) is created in the halftone color separation. Here, the error e(x, y) of the error allocation algorithm is determined by the difference between the created point B(x, y) and the summed or integrated color value C(x, y) used to create the point (i.e., the corresponding summed or integrated color value). The error e(x, y) is then canceled out by the subsequently read color value C(x, y). During this process, a correction value K(x) related to the printing position is considered.

[0078] If multiple point sizes and thresholds are provided, the error e(x, y) is preferably determined by the difference between the created point B(x, y) and the summed or integrated color value C(x, y), such that the point size is also taken into account.

[0079] In a preferred embodiment, it may be specified that for each point created by the halftone algorithm, the point size is determined based on the read color value, the predetermined threshold, and the correction value related to the printing position, and the error of the error allocation algorithm is calculated by the difference between the created point and the sum of the color value used to create the corresponding point and the correction value related to the printing position.

[0080] In other words, a screening algorithm is preferably executed, and the color value C(x, y) is read. This value is summed or integrated until it reaches the corresponding threshold T. The screening algorithm determines the dot size by considering the read color value C(x, y), the threshold T, and a correction value K(x) specified at the corresponding print position. The correction value K(x) is added to the summed or integrated color value C(x, y), and based on the resulting amount, a dot B(x, y) is created in the screened color separation. The error e(x, y) of the error allocation algorithm is determined by the difference between the created dot B(x, y) and the reference value used to create the dot, which is the sum of the read color value C(x, y) and the correction value K(x) used to create the dot. The error e(x, y) is then offset against subsequent read color values ​​C(x, y).

[0081] If multiple point sizes and thresholds are provided, the error e(x, y) is preferably determined by the difference between the created point B(x, y) and the sum of the summed or integrated color value C(x, y) and the correction value K(x), such that the point size is also taken into account.

[0082] Preferably, it can be specified that for the points created by the halftone algorithm as described above, the point size is determined based on the read color value, the predetermined threshold, and the correction value related to the printing position, respectively, and the error of the error allocation algorithm is calculated by the difference between the created point and the color value used to create the corresponding point, wherein the halftone screen is an AM halftone screen.

[0083] In a preferred embodiment, it may be specified that the points created by the halftone algorithm are created based on the read color values ​​and the predetermined threshold, and the error of the error allocation algorithm is calculated by the difference between the created point and the sum of the color value used to create the corresponding point and the correction value related to the printing position.

[0084] In other words, a halftone algorithm is preferably executed and the color value C(x, y) is read. This value is summed or integrated until it reaches the corresponding threshold T. Thus, a point B(x, y) is created in the halftone color separation. The error e(x, y) of the error allocation algorithm is determined by the difference between the created point B(x, y) and the summed or integrated color value C(x, y) and the correction value K(x) specified at the corresponding print position, which is related to the print position. The error e(x, y) is then canceled out by subsequent read color values ​​C(x, y).

[0085] If multiple point sizes and thresholds are provided, the error e(x, y) is preferably determined by the difference between the created point B(x, y) and the sum of the summed or integrated color value C(x, y) and the correction value K(x), such that the point size is also taken into account.

[0086] Preferably, the dots created by the halftone algorithm are defined as being created based on the read color values ​​and the predetermined threshold, as described above, and the error of the error allocation algorithm is calculated by the difference between the created dot and the sum of the color value used to create the corresponding dot and the correction value associated with the print position, wherein the halftone screen is an FM halftone screen. In a preferred embodiment, the FM halftone screen may be defined as including more than one print dot size.

[0087] Preferably, the error allocation algorithm may be specified to allocate the error to the color values ​​of at least two locations along the meshing path that have not yet been read, more preferably to at least two to a maximum of ten locations, further preferably to three, four, five, six, seven, eight or nine locations, and particularly preferably to three, four or five locations.

[0088] Preferably, it can be specified that the unread positions along the screen path are arranged near the positions of the read color values, wherein preferably, when allocating errors, the proximity of these positions is used for weighting.

[0089] For example, these positions can be directly adjacent to the position of the read color value. However, these positions can also be only adjacent to the position of the read color value, meaning they are not directly adjacent to it. For example, "adjacent position" can be understood as a position that can be connected to the position of the read color value through one, two, or three other positions. Particularly preferred is that at least one position is directly adjacent to the position of the read color value, and at least one position is not directly adjacent to the position of the read color value. In this case, preferably, each position can be adjacent to at least one other position. Preferably, each position can be adjacent to at least one position of a color value that has already been read.

[0090] Preferably, the error allocation algorithm can be selected from a group consisting of the Floyd-Steinberg algorithm, the Shiau-Fan algorithm, and the Ostromoukhov algorithm.

[0091] As is well known, the Floyd-Steinberg algorithm is described in RW Floyd and L. Steinberg's "Adaptive Algorithms for Spatial Gray Levels", Proc. Soc. Inf. Display, 17:75-77, 1976.

[0092] As is well known, the Shiau-Fan algorithm is described in EP0659012A2.

[0093] As is well known, the Ostromoukhov algorithm is described in V. Ostromoukhov's "A Simple and Efficient Error Diffusion Algorithm", Proceedings of SIGGRAPH 2001, ACM, 567-572.

[0094] In a preferred embodiment, the error allocation algorithm may be specified to depend on the color values ​​read. For example, a threshold-dependent error distribution (TDED) may preferably be used. For instance, if multiple dot sizes and thresholds are provided for an FM screen, the error allocation algorithm may be selected, for example, based on the threshold reached (TDED-FM).

[0095] The present invention also proposes a digital printing apparatus, wherein the digital printing apparatus includes at least one computer configured to perform the above-described method, and wherein the printing apparatus is preferably an inkjet printer.

[0096] The present invention also proposes the use of the above-described method or apparatus for printing decorative objects, particularly for decorative objects with a limited range of colors (e.g., wood and / or stone decorative objects, or monochrome decorative objects).

[0097] Surprisingly, the above method has proven particularly suitable for color correction in such applications. In particular, stripes can be detected very well in such decorative materials, thus requiring especially good color correction for high-quality prints. Attached Figure Description

[0098] Other advantages and advantageous embodiments of the method according to the invention are illustrated by the accompanying drawings and described in the following description. It should be noted that the drawings are for illustrative purposes only and are not intended to limit the invention in any way.

[0099] In the attached diagram: Figure 1 The flowchart of a method according to an embodiment of the present invention is illustrated schematically; Figure 2 The flowchart of the meshing algorithm in one embodiment of the present invention is illustrated schematically; Figure 3 The flowchart of the meshing algorithm in another embodiment of the present invention is illustrated schematically; Figure 4 The flowchart of the meshing algorithm in another embodiment of the present invention is illustrated schematically; Figure 5 The flowchart of the meshing algorithm in another embodiment of the present invention is illustrated schematically; Figure 6 The flowchart of an error allocation algorithm in one embodiment of the present invention is illustrated schematically; Figure 7 The flowchart of an error allocation algorithm according to another embodiment of the present invention is illustrated schematically; and Figure 8 The flowchart of an error allocation algorithm in another embodiment of the present invention is illustrated schematically. Detailed Implementation

[0100] Figure 1The flow of a method according to an embodiment of the present invention is illustrated. In a digital printer 200, a halftone color separation 104 is created from a predetermined digital image 102 color separation 101 using a halftone algorithm 103 by means of a raster image processor 100. The halftone algorithm 103 sequentially reads the color values ​​C(x, y) of the halftone separation along the halftone path S, and creates a halftone color separation point B(x, y) based on the read color values ​​C(x, y) and at least one predetermined threshold T, wherein a correction value K(x) related to the printing position is considered whenever a point is created by the halftone algorithm S. In the illustrated embodiment, the halftone algorithm S includes an error allocation algorithm e(x, y).

[0101] Figure 2 The flowchart of a screening algorithm in one embodiment of the present invention is illustrated schematically, wherein the read color value C(x, y) is summed or integrated until it reaches the corresponding threshold T. Thus, a point B(x, y) is created in the screened color separation. Here, the screening algorithm determines the point size by considering the read color value C(x, y), the threshold T, and a correction value K(x) specified at the corresponding print position and related to the print position.

[0102] Figure 3 The flowchart of a halftone algorithm in another embodiment of the invention is illustrated schematically, wherein the read color value C(x, y) is summed or integrated until it reaches a corresponding threshold T. A point B(x, y) is then created in the halftone color separation. The error e(x, y) of the error allocation algorithm is determined by the difference between the created point B(x, y) and the summed or integrated color value C(x, y) used as the basis for creating that point (i.e., the corresponding summed or integrated color value). The error e(x, y) is then canceled out with subsequently read color values ​​C(x, y). During this process, a correction value K(x) related to the printing position is considered.

[0103] Figure 4The flow of a screening algorithm according to another embodiment of the invention is illustrated schematically, wherein the read color value C(x, y) is summed or integrated until it reaches a corresponding threshold T. The screening algorithm determines the dot size by considering the read color value C(x, y), the threshold T, and a correction value K(x) specified at the corresponding print position. The correction value K(x) is added to the summed or integrated color value C(x, y), and a dot B(x, y) is created in the screened color separation based on the resulting amount. The error e(x, y) of the error allocation algorithm is determined by the difference between the created dot B(x, y) and the sum of the read color value C(x, y) and the correction value K(x) used as the basis for creating the dot. The error e(x, y) is then canceled out with subsequent read color values ​​C(x, y).

[0104] Figure 5 The flowchart of a screening algorithm in another embodiment of the invention is illustrated schematically, wherein the read color value C(x, y) is summed or integrated until it reaches a corresponding threshold T. Thus, a point B(x, y) is created in the screened color separation. The error e(x, y) of the error allocation algorithm is determined by the difference between the created point B(x, y) and the sum of the read color value C(x, y) and the correction value K(x) related to the print position used as the basis for creating that point. The error e(x, y) is then canceled out by subsequent read color values ​​C(x, y).

[0105] Figure 6 The flowchart of an error allocation algorithm as a Floyd-Steinberg algorithm in one embodiment of the present invention is illustrated schematically. Color values ​​are read along the meshing path. When point N is created... 00 At that time, the error is determined. This error is distributed to the four neighboring points N using specified weights. 10 N -11 N 01 and N 11 .

[0106] Figure 7 The flowchart of an error allocation algorithm as the Shiau-Fan algorithm in another embodiment of the present invention is illustrated schematically. Color values ​​are read along the meshing path. When point N is created... 00 At that time, the error is determined. This error is distributed to three adjacent points N. 10 and N -11 And two points N that are not directly adjacent. -31 and N -21 Weighting is also used here, such as... Figure 7 As shown.

[0107] Figure 8 The flowchart of an error allocation algorithm as an Ostromoukhov algorithm in another embodiment of the present invention is illustrated schematically. Color values ​​are read along the meshing path. When point N is generated... 00 At that time, the error is determined and assigned only to three adjacent points N. 10 N -11 and N 01 The weighting is determined by the allocation coefficient d. 10 d -11 and d 01 It is determined that these allocation coefficients vary based on the color values ​​read, and can be stored, for example, in a table.

Claims

1. A color correction method for use in digital printing processes, wherein, The raster image processor (100) generates a halftone color separation (104) from at least one color separation (101) of a predetermined digital image (102) using a halftone algorithm (103). The halftone algorithm (103) sequentially reads the color values ​​of the color separation (101) along the halftone path and creates points for the halftone color separation based on the read color values ​​and at least one predetermined threshold, wherein each time the halftone algorithm creates a point, a correction value related to the printing position is considered.

2. The method according to claim 1, wherein, The halftone color separations obtained by the method are printed on a digital printer (200).

3. The method according to claim 1 or 2, wherein, The halftone algorithm uses a screen, such as an AM screen or an FM screen, when creating the halftone color separation. The method preferably includes multiple color separations and halftone color separations, and the halftone algorithm uses a halftone system, such as an AM halftone system or an FM halftone system, when creating the halftone color separation.

4. The method according to any one of claims 1 to 3, wherein, The correction value related to the print position is the product of a print position-specific original correction value and at least one scaling factor, wherein the scaling factor is preferably selected from a constant scaling factor, a color value-related scaling factor, and a dot size-related scaling factor, and particularly preferably, the print position-specific correction value is the product of the print position-specific original correction value, the constant scaling factor, and the dot size-related scaling factor.

5. The method according to any one of claims 1 to 4, wherein, The correction value related to the printing position is obtained from a color difference, which is the difference between a predetermined test image and a test print based on the predetermined test image. The correction value related to the printing position is obtained, in particular, from test prints with different color values ​​and / or dot sizes.

6. The method according to claim 5, wherein, When the test print is created based on the predetermined test image, the screening system used by the screening algorithm is the same as that used by the screening algorithm for creating the screened color separation from the color separation of the predetermined digital image.

7. The method according to any one of claims 1 to 6, wherein, The correction value related to the printing position is considered such that for each point created by the halftone algorithm, the point size is determined based on the read color value, the predetermined threshold, and the correction value related to the printing position.

8. The method according to any one of claims 1 to 7, wherein, The halftone algorithm includes an error allocation algorithm, wherein, preferably, the error allocation algorithm calculates an error based on the difference between the created point and at least the color value used to create the corresponding point, wherein the error is allocated to the color value of at least one unread position along the halftone path, and wherein the error preferably takes into account the correction value associated with the printing position.

9. The method according to claim 8, wherein, For each point created by the halftone algorithm, the point size is determined based on the read color value, the predetermined threshold, and the correction value related to the printing position, wherein the error of the error allocation algorithm is calculated by the difference between the created point and the sum of the color value used to create the corresponding point and the correction value related to the printing position.

10. The method according to claim 8, wherein, The points created by the halftone algorithm are created based on the read color values ​​and a specified threshold, and the error of the error allocation algorithm is calculated by the difference between the created point and the sum of the color value used to create the corresponding point and the correction value related to the printing position.

11. The method according to any one of claims 8 to 10, wherein, The error allocation algorithm allocates the error to color values ​​at at least two locations along the meshing path that have not yet been read, more preferably to at least two to a maximum of ten locations, further preferably to three, four, five, six, seven, eight or nine locations, and particularly preferably to three, four or five locations.

12. The method according to claim 8 or 11, wherein, The positions along the halftone path that have not yet been read are adjacent to the positions of the read color values, wherein, preferably, the error is weighted based on the proximity of the positions.

13. The method according to any one of claims 8 to 12, wherein, The error allocation algorithm is selected from a group consisting of the Floyd-Steinberg algorithm, the Shiau-Fan algorithm, and the Ostromoukhov algorithm.

14. A digital printing apparatus, wherein, The digital printing apparatus includes at least one computer configured to perform the method according to any one of claims 1 to 13, wherein the printing apparatus is preferably an inkjet printer.

15. Use of the method according to any one of claims 1 to 13 for printing decorative objects, or use of the digital printing apparatus according to claim 14 for printing decorative objects, wherein the decorative object is preferably a decorative object having a limited range of colors, such as a wood and / or stone decorative object or a monochrome decorative object.

Images