Ink dosing for digital printing on reflective substrates

Abstract

A computer-implemented method is provided for dosing ink in a digital printing device having multiple ink channels when printing on a reflective substrate. A digital printing system configured with the method is also provided. Target color data for a target color to be printed on the reflective substrate is captured, the target color data including both color data and spectral reflectance data. The captured color data is processed with a printer model to output a preliminary dosing ratio for each ink channel. A digital opacity value for the target color is calculated from the dosing ratios for each ink channel. A natural opacity value for the target color is calculated from the captured spectral reflectance data, and a difference between the digital opacity value and the natural opacity value is calculated. A predicted dosing ratio for each ink channel is obtained by interpolating the difference for an ink stage of a diffuse ink component of the target color.

Description

[Technical field]

[0001] The present invention relates to color data processing techniques within a colorimetric space for ink dosing in a printing device or system having multiple ink channels when printing on a reflective substrate. [Background technology]

[0002] Digital printers are being rapidly adopted in the field of professional printing because the ease of configuration via computer terminals and associated programs has significantly lowered the economic threshold for print volume, allowing for ever more customization and fewer production runs. Digital printers typically include multiple ink channels, e.g., cyan, magenta, yellow, and black ("CMYK") or more, e.g., additionally orange, green, violet, and red ("CMYKOGVR"), regularly combined with diffusing inks, e.g., white, silver, or additional inks, e.g., gloss and primer.

[0003] In this context, the task of color matching is traditionally performed by digital press operators as an iterative process of attempting to match the printed result with a target color and bridging the device-dependent colorimetric space associated with the printer by transformation through a reference device-independent colorimetric space, a preferred example of which is the International Commission on Illumination ("CIE") Lightness, Green-Red, and Blue-Yellow ("L*a*b") color space ("CIELAB").

[0004] Recent methods have improved the workflow of color matching in the context of digital printing, for example as disclosed in the applicant's European Patent Application Publication No. 20178552.4. However, regardless of the type of printer and whether printing is direct on the substrate or indirect on a film applied to the substrate, color matching, especially opacity matching, remains particularly difficult and tedious when printing on reflective substrates, referred to in the industry as reflective, glossy, and / or mirror-like ("RSM") substrates. Even when achieved, the amount of time and effort to best match is significant.

[0005] A fundamental difficulty arises from the specular reflectivity exhibited by highly reflective surfaces such as metals, where light scattering is significantly greater than that of more traditional printing substrates such as white coated surfaces, paper or cardboard.

[0006] The angular light scattering of colors printed on reflective substrates also depends on the size and shape of the colored pigments from the formulated ink. Conventional ink pigments typically used in analog printing of reflective substrates (e.g., aluminum cans, metal sheets) are more polydisperse and can exhibit larger diameters than inkjet pigments used in digital printers, resulting in significantly more ink particle volume. The resulting mixing of reflectance and scattering exhibited in the printed layer can cause some involuntary selectivity in the orientation of the reflection. Thus, prints on reflective substrates (e.g., flat surfaces or cylindrical objects) produced with conventional ink pigments often exhibit dull reflections that cause increased scattering, which results in a loss of image density, transparency, and sharpness. In contrast, products digitally printed with toner or smaller inkjet pigments, typically UV or latex inks applied directly onto a reflective substrate, or onto a film with a functional inkjet coating layer printed with water-based or eco-solvent inks that can be wrapped or laminated onto a substrate, exhibit more brilliant reflections, sharpness, and color transparency compared to traditional printing processes, resulting in a different color appearance.

[0007] In certain printing applications, especially when printing on white substrates, ink opacity (or density) is represented by a measurement that defines the amount of light that passes through the ink coating applied on the printed surface. Measurements are typically performed with either a reflection or transmission densitometer. However, where other measurable methodologies are disclosed to define the opacity of a given colored ink on a white substrate, these methodologies do not provide satisfactory results because RSM substrate reflectance is a significant component that makes existing methods difficult. It is believed that no method exists to quantify the opacity or transparency of a color on an RSM substrate. Currently, metal decoration ink manufacturers determine the opacity of an ink with their own evaluation, usually based on pigment selection for formulating colored inks with or without a diffusing element such as white ink.

[0008] As printing artwork and effects become increasingly complex, the various inks specifically adapted for use in decorative applications accordingly present additional challenges in matching color and opacity for printing on reflective substrates. Manufacturers formulating inks for reflective substrates have introduced inks with discrete levels of opacity semi-normalized as opaque, semi-opaque and transparent inks, with opaque inks blocking most of the reflectance of the underlying substrate, transparent inks allowing the reflectance of the underlying substrate to substantially pass through the color, and semi-opaque inks providing intermediate levels of opacity.

[0009] Specialist field knowledge highlights an additional source of complexity when color matching for reflective substrate printing. For example, when printing with the wet-on-wet screen process, inks must be matched as semi-opaque variants, and lighter and warmer colors such as yellow, orange and red must be matched with slightly higher opacity. In contrast, colors known as pastel colors, characterized by high lightness and low chroma, should only be matched as opaque variants to obtain the best and cleanest hues.

[0010] The color of diffuse white surfaces is usually measured with a spectrophotometer under 45 / 0° or 0 / 45° geometry, while the color of reflective surfaces is measured with a multi-angle or spherical spectrophotometer under d / 8° geometry, where the spherical spectrophotometer captures the specular reflection in a measurement that either includes ("SPIN") the specular reflection component or excludes ("SPEX"). For example, the SPIN measurement captures the total reflectance without variation regardless of the surface type (matte, gloss or other) and is said to represent "true color", so the target SPIN value should be matched against the printed color to show a satisfactory color rendition. The SPEX measurement usually excludes the gloss component when capturing reflectance variations based on different surfaces and is said to represent "sample appearance", so the SPEX value should be matched against the printed color to show a satisfactory color consistency. Measuring color with SPIN or SPEX results in different respective values ​​for the color, and the difference between the spectral reflectance of the color and its respective converted L*a*b value will vary based on the degree of specular reflectance of the substrate, and non-linearities in the measurement difference in the spectral range can further change the difference in the L*a*b values.

[0011] In a traditional design process to define the final print, colors can be selected from a given printed standard or printed color book such as the Pantone® guide. However, the color selection may or may not be represented on a very different surface reflection such as white paper, while the desired print will be printed on a RSM substrate, resulting in incorrect color selection due to the very different perception when compared to the final print on a RSM substrate.

[0012] Among the various proofing technologies (digital or analog) that attempt to replicate the final print, some have been developed as digital imaging systems with adjustable color density (e.g., Fuji FinalProof® or Kodak® Approval®) that use advanced laser imaging technology that combines the imaging pass of a specific digital donor and transfers a film layer onto a production substrate.

[0013] Improvements have also been presented to optimize color selection decisions in the design process. For example, PantoneLIVE® provides digital color information from colors available in pre-processed Pantone® guides on a variety of printing substrates, including reflective substrates such as metal or aluminum. However, this digital data available from the PantoneLIVE® library defined for metal or aluminum does not provide enough information to reproduce the color accurately and simultaneously with the concept of opacity in external conventional or digital systems. This is mainly because the opacity of such surfaces is arbitrarily defined by a select group of Pantone® or its preferred ink manufacturer partner Sun Chemical, who do not disclose the information, or a given methodology for matching color and opacity.

[0014] Another recent example is the introduction of physical color catalogs in which traditional inks are applied directly to reflective surfaces such as aluminum. For example, the INX Color Perfection® Catalog for two-piece metal decoration, with a range of over 600 colors, claims to be the only color catalog available on metal, not paper. However, this analog process of applying inks on a production substrate requires a lot of labor and it is difficult to maintain tight color shifts when comparing different versions of the catalog. Color samples are available as physical color standards, and the available digital data does not provide enough information to accurately replicate these colors, including opacity, in traditional or digital systems such as flat, roll-to-roll, high-speed or cylindrical printing equipment.

[0015] In the above context, the workflow of color and opacity matching for printing on reflective substrates still broadly corresponds to the conventional workflow disclosed as prior art in EP 20178552.4, where an operator initializes ink values, prints samples, checks color accuracy, and corrects ink values ​​for matching if necessary. WO 2021 / 035105 to Sun Chemical Corporation discloses an example of such prior art in the specific field of reflective substrate printing.

[0016] The color matching workflow begins with an operator obtaining a table of spot or target colors to reproduce on a particular printer. Each printer has its own printer calibration, which depends on the device's ink channels, printhead characteristics, and printing substrate, with a different color profile (e.g., International Color Consortium "ICC" profile) that contains information for converting colors from the L*a*b color space or another color to the device color space, and the resulting L*a*b (or other) values ​​for each color in the table are input into a raster image processor ("RIP") computer program. The RIP program processes the target L*a*b values ​​with the printer's color profile and calculates the output values ​​of each ink as ink doses per color, known as ink separations. For example, if the target colors are L*a*b values ​​[14.191, 18.511, 4.177], the RIP program may suggest that the respective ink dose values ​​for a CMYKOG printer should be [50.196, 100, 100, 50.196, 50.196, 0].

[0017] A printer's device color space is defined by the inks it uses, so in the context of reflective substrate printing this includes opaque, semi-opaque, and transparent variants. Given the nature of the artwork, a great challenge arises for artwork that needs to be digitally printed in the same workflow, where target colors with discrete levels of opacity must be matched. Matching colors in both SPIN and SPEX requires the use of respective color profiles, and operators are faced with the additional challenge that each printer profile calculates separate respective ink separations, each different from the other.

[0018] Therefore, an improved method of ink dosing in a multi-channel printer is desirable when printing artwork on a reflective substrate, and is suitable for providing ink separation that more closely matches the target color, including opacity, and has reduced user input requirements compared to prior art techniques. [Prior art documents] [Patent documents]

[0019] [Patent Document 1] European Patent Application Publication No. 20178552.4 [Patent Document 2] International Publication No. 2021 / 035105 Summary of the Invention

[0020] The present invention provides a method of ink dosing in a multi-channel printer that substantially automates the adjustment workflow inherent in the task of color matching actual and target colorimetry when printing on a reflective substrate, where the opacity of a target color is determined to be printed digitally or conventionally on a reflective substrate.

[0021] According to one aspect of the invention, there is provided a computer-implemented method for dosing ink in a printing device having multiple ink channels when printing on a reflective substrate, the method comprising the steps of: acquiring target color data for at least one target color to be printed on a reflective substrate, the target color data comprising color data and spectral reflectance data including both specular and specular excluded color data; generating a printer model; processing the captured color data with a printer model to output reserve ratios for each ink channel representing each color component of the target color; calculating a curve from each ratio and interpolating it to output a digital opacity value for each ink channel; combining the respective digital opacity values ​​of all the ink channels to output a digital opacity value of at least one target color; calculating a natural opacity value from the captured spectral reflectance data; calculating the difference between the calculated digital opacity value and the calculated natural opacity value; and interpolating the calculated differences for the ink stages of the diffusion ink components of at least one target color to obtain a predicted dosing ratio for each ink channel.

[0022] An embodiment of the method may include the further step of calibrating the printer using both the specular reflection component on the reflective surface and the specular reflection component on the reflective surface having a diffusive ink layer.

[0023] In one embodiment of the method, the step of generating a printer model may comprise a further step of generating a printer calibration model based on a specular reflection component on a reflective surface and a specular reflection component on a reflective surface having a diffuse ink layer.

[0024] An embodiment of the method may further comprise the steps of classifying the or each target color according to a defined target opacity and repeating the processing steps by replacing the captured color data with data representative of the or each classified color, wherein the resulting predicted dosing ratios for each ink channel include a dosing ratio of a diffusive ink, such as a white ink.

[0025] One embodiment of the method may further comprise adding a predicted proportion of the diffuse ink from the target natural opacity value to output a next digital opacity value, and calculating the difference between the natural opacity value and the next digital opacity value, the calculated difference being a value referred to as delta opacity for a reflective substrate.

[0026] A variation of any of the previous embodiments may comprise the further step of correcting the calculated difference before the interpolation step, the correcting step comprising setting the difference to zero if the natural opacity value is below a predetermined threshold or if the calculated difference value is negative. A variation of this further embodiment may comprise the further step of correcting the predicted dosing ratio of each ink channel using a compositing function stored as a look-up table.

[0027] Variations of any of the previous embodiments may further include generating a color chart from the predicted dosing ratios of each ink channel, or generating a color chart from the corrected dosing ratios for each ink channel, and printing the color chart with a printer onto a reflective substrate. The substrate may be an object printed directly by a cylindrical printer, or a flat surface, or an optically clear or transparent layer for placement on the intended reflective substrate.

[0028] Variations of any of the preceding embodiments may include the further step of determining the opacity of the formulated conventional ink, where the colorants or base inks are characterized to specify the opacity at each ink stage with the aim of matching that particular opacity of the final conventional ink formulation.

[0029] An embodiment of the method may comprise the further step of calculating the diffuse ink percentage from the digital opacity values ​​of the target colors to obtain respective versions of digital opacity ranging from transparent to opaque by preserving the saturation and hue of the target color (independent of its opacity level). A variant may comprise the further step of predefining a threshold value predicting the opacity of pastel colors that approximately corresponds to the higher opacity.

[0030] An embodiment of the method may comprise the further step of outputting a predicted dosing ratio, e.g. a Delta E measurement, for each ink channel for each lighter and darker version of the target color at a given distance in the L*a*b color space.

[0031] An embodiment of the method may optionally comprise the further step of generating a digital ink drawdown or color card representing the best possible digital color and opacity match to the target color in lighter and darker versions specified at a predetermined distance Delta-E in the L*a*b color space.

[0032] According to a further aspect of the present invention, there is provided a digital printing system comprising: a digital printer having a plurality of ink channels; means for capturing target color data for at least one target color to be printed on the reflective substrate by the printer; a data processing terminal within or operably interfaced with the digital printer; receiving captured target color data, the target color data comprising both color data and spectral reflectance data including and excluding specular reflectance; generating a digital printer model; processing the captured color data with a printer model to output reserve ratios for each ink channel representing each color component of the target color; calculating a curve from each ratio and interpolating it to output a digital opacity value for each ink channel; combining the respective digital opacity values ​​of all the ink channels to output a digital opacity value of at least one target color; calculating a natural opacity value from the captured spectral reflectance data; calculating a difference between the calculated digital opacity value and the calculated natural opacity value; Interpolating the calculated differences for the ink stages of the at least one target color diffusion ink component to obtain a predicted dosing ratio for each ink channel; a data processing terminal configured with an instruction set for performing A digital printing system is also provided.

[0033] An embodiment of the system may further comprise a network to which the data processing terminal and a remote terminal are operatively interfaced, the captured target color data being received from the remote terminal. Regardless of network connectivity, the capturing means is preferably a spherical or multi-angle spectrophotometer.

[0034] The reflective substrate may be selected from the group including reflective, glossy and / or specular ("RSM") substrates, including metals such as aluminum, plastics such as films, foils and vinyl, and glass.

[0035] In networked embodiments, the system may further comprise bridging means for interfacing the data processing terminal and / or spectrophotometer with a remote storage means, e.g. a cloud-based data storage node, and the color data is optionally encrypted before uploading to the remote storage means.

[0036] In a networked embodiment, the captured target color data may be uploaded from a data processing terminal, stored in a database, and processed by a cloud-based data processing node operatively configured to transform the stored color data in accordance with color data requests of the remote data processing terminal.

[0037] According to another aspect, a method is provided for viewing a target color applied to a reflective substrate according to the present invention in a light booth, where the viewing angle is related to the opacity level of a given color.

[0038] According to another aspect of the present invention there is also provided a set of instructions recorded on a data carrier medium or stored on a network storage medium which, when read and processed by a data processing terminal, configures the terminal to perform the steps of any of the method embodiments described herein.

[0039] Other aspects are as claimed herein.

[0040] For a better understanding of the present invention and to show how the same may be carried into effect, specific embodiments, methods and processes according to the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0041] [Figure 1] 1 is a logical diagram of a digital printing system in a network environment configurable in accordance with the present invention, including a local data processing terminal interfaced with a digital multichannel color printer, a remote data processing terminal, and a table of spot colors. [Diagram 2] 2 is a hardware diagram of a typical hardware architecture of the data processing terminal shown in FIG. 1, including a processor and memory means for storing a set of data processing instructions. [Diagram 3] FIG. 3 is a logic diagram illustrating matching of a target color for printing on a reflective substrate with the digital printing system of FIGS. 1 and 2. [Figure 4] FIG. 4 is a flow chart diagram of the prior art method for matching the target color shown in FIG. 3. [Diagram 5] FIG. 5 is a flow chart diagram of a general method of injecting ink according to the present invention that can be used to improve the targeted color matching methods of FIGS. 3 and 4, including steps of predicting ink separation including diffusive inks and correcting the predicted ink separation. [Figure 6] FIG. 6 is a logic diagram of the contents of the memory means of the terminal shown in FIGS. 1 and 2, including an instruction set embodying the method shown in FIG. 5; [Figure 7] FIG. 6 is a flow chart diagram of a first embodiment of the steps of predicting ink separation in FIG. 5; [Figure 8] 6 is a flow chart diagram of a second embodiment of the steps of predicting ink separation of FIG. 5, where the target color is matched from transparent, to semi-opaque, to opaque. [Figure 9] FIG. 9 is a flow chart diagram of one embodiment of steps for correcting predicted ink separation in FIGS. 5, 7 and / or 8. [Figure 10] FIG. 11 is a flow chart diagram of a method for automatically calculating lighter and darker versions of the target color ink separations output by the methods described herein. [Figure 11]FIG. 1 illustrates a viewing booth where a user can perform a visual check of one or more target colors printed according to the methods described herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] Here, specific embodiments contemplated by the inventors are described by way of example. In the following description, numerous specific details are set forth in order to provide a thorough understanding. However, it will be apparent to those skilled in the art that the present invention can be practiced without being limited to these specific details. In other instances, well-known methods and structures are not described in detail so as not to unnecessarily obscure the description.

[0043] 1 and 2, there is shown an example of a digital printing system 100 that can be configured with embodiments of the data processing method of the present invention. The digital printing system 100 connects, via one or more high bandwidth data connections 112, a digital multi-channel color printer 114, in this example eight color channels 116 including cyan (c), magenta (m), yellow (y), black (k), orange (o), green (g), violet (v), and white (w) as diffusive inks ("CMYKOGV+W"). 1~8 The present invention includes a data processing terminal 110 locally interfaced to an inkjet printer having a spectrophotometer 117.

[0044] Digital printing system 100 is disposed within a network environment in which data processing terminals 110 are personal computing devices that upload and download data encoded as digital signals over high-bandwidth wired or wireless data connections 112, with such signals relayed to and from computers 110, respectively, by local router devices 118 implementing a wired local network operating in accordance with the IEEE 802.3-2008 Gigabit Ethernet transmission protocol, and / or a high-bandwidth wireless local network operating in accordance with the IEEE 802.11 Wi-Fi wireless transmission protocol.

[0045] As a non-limiting example, a typical hardware architecture of a data processing terminal 110, which is a desktop computer, is shown in more detail in Figure 2. The computer consists of a data processing unit 201, data output means such as a video display unit (VDU) 202, data input means such as HID devices, typically a keyboard 203 and a pointing device (mouse) 204, the VDU 202 itself if it is a touch screen display, data input / output means such as a wired or wireless network connection 112 to local and wide area networks via a router 118, and a magnetic data carrier medium reader / writer 206 and an optical data carrier medium reader / writer 207.

[0046] Within the data processing unit 201, a central processing unit (CPU) 208 provides task coordination and data processing functions. Instruction sets and data sets for the CPU 208 are stored in memory means 209 and hard disk storage unit 210, facilitating non-volatile storage of instructions and data. A wireless network interface card (NIC) 211 provides an interface to a network connection 112 with the router 118. One or more universal serial bus (USB) input / output interfaces 212 facilitate connections to keyboards and pointing devices 203, 204. Depending on the presence or absence of network connectivity capabilities in the printer 114 and spectrophotometer 117, data communications between the computer 110, the printer 114 and the spectrophotometer 117 may be routed through the router device 118, or through a wired connection to the computer's USB interface 212, or through a combination thereof.

[0047] All the above components are connected to a data I / O bus 213, as well as a magnetic data carrier medium reader / writer 206 and an optical data carrier medium reader / writer 207. A video adapter 214 receives CPU instructions via the bus 213 to output processed video data to the VDU 202. All components of the data processing unit 201 are powered by a power supply unit 215, which receives power from the local mains power supply and converts it according to the ratings and requirements of the components.

[0048] The router 118 itself is connected via a conventional ADSL or fiber optic connection to a wide area network (one example of which is the Internet 120) via which digital data can be uploaded and downloaded to remote data processing terminals. According to certain embodiments of the methodologies described herein, the network connectivity and interoperable networking protocols of each data processing terminal enable the terminals to connect to each other, communicate data with each other, and receive data from each other.

[0049] The remote data processing terminals may be desktop computers as described with reference to Fig. 2, or portable variants such as laptops and tablet computers. The remote data processing terminals may also include personal communication devices 130, e.g., smartphones, that broadcast and receive data, including voice and / or alphanumeric data, encoded as digital signals via wireless data transmission 132, the signals being relayed to and from each device 130 by a number of geographically closest communication link relays 134, respectively. 1-N uses the remote gateway 136 to communicate with the mobile device 130 1-N Gateway 136 is a communications network switch that couples digital signal traffic between WAN 120 and a wireless telecommunications network, such as the network over which wireless data transmission 132 occurs.

[0050] The digital printing system 100 outputs a printed product 140 having a reflective substrate, a well-known example of which is an aluminum beverage can, and the accuracy of color reproduction by the printer 114 in the output printed product 140 is of paramount importance. Accordingly, a spot or target color table 150 is provided to assist in calibrating the color reproduction by the printer 114 with the data processing terminal 110 through a process known in the art as color matching.

[0051] 3 and 4, color matching on reflective substrates with system 100 has typically been performed as an iterative trial and error process. In addition to switching on computer 110 and loading its operating system and then a raster image processor (RIP) application, the prior art method begins by characterizing printer 114 of digital printing system 100 in step 401, where color and white undergo linearization in steps 401A, 401B, respectively.

[0052] In step 402, a forward printer model is defined at the operator terminal 110 based on a specular component which may consist of a forward transformation function defined by the printer's ICC profile.

[0053] In step 403, at least a first target color 300 is defined in a color library, e.g., table 150 of spot or target colors of the artwork print job that require a match in the digital printing system for color accuracy and color opacity, i.e., to represent the target print color 300. Capture of the target color data is accomplished by measuring the L*a*b values ​​310 of the target colors in table 150 with a spectrophotometer 117, including SPIN or SPEX, and saving the measurements as a set of respective values ​​in terminal 110.

[0054] Each target color 300 corresponds to an ink channel 116 of the printer 114. 1-8 For each target color, a conventional printer model provides a set of input ratios 330 for each of the ink channels, expressed as the percentage of each ink relative to all ink combinations of the target color, known in the art as color separation data.

[0055] Following the color accuracy and opacity matching steps, the forward printer model is initiated, where the captured and stored target colors 310, or the entire library thereof, are read in step 404, the or each target color is classified based on its measured opacity in step 405, and the classified colors are processed with a color prediction algorithm to output respective ink separations 330 in step 406, for example using the ink dosing techniques taught in EP20178552.4.

[0056] In step 407 , a color chart is generated using the output ink separations, where the color separations 330 include opacity, which are then processed through the forward printer model and printed as color chart 150 by printer 114 in step 408 .

[0057] The printed color chart is then measured in step 409 using a spectrophotometer 117 including SPIN or SPEX and the color values ​​in step 406 are typically corrected in step 410 according to the scale of observed differences relative to the values ​​previously measured for the target colors in step 403. Such corrections can be applied using correction algorithms for ink color separation according to tolerance definitions, for example using the ink dosage correction techniques taught in EP20178552.4.

[0058] The iteration of color accuracy and opacity matching steps 404 through 410 continues until the required values ​​of the target color 300 are matched within an acceptable tolerance or until the color gamut limit of the printer is reached. In the example shown in FIG. 3, when output in step 407 and measured in step 409, the color chart 150 -3 3301 of the first set of color separations printed as color chart 150 does not correspond closely enough to target color 300, and this situation is shown in bold type as indicated by the reference numeral 3302. In this example, the color values ​​are triple corrected and output in step 407 and measured in step 409, and a fourth set of color separations 3304 printed as color chart 150 is deemed to be close enough to target color 300.

[0059] The iteration of the printed color chart that is deemed to be the best match to the target color is then exported as a digital library of ink color separations 330 in step 411. The exported digital library can then be used in the digital printing system 100 with a RIP application to print artwork with the matched colors.

[0060] Through experience, the inventors have determined that the non-trivial number of iterations of color accuracy and opacity matching steps 404 to 411 according to the prior art is caused by the scale of difference between the target color characteristics to be matched and the initial set of ink separations 3301 output in step 407. Through expertise and experimentation, the inventors have realized that by using a diffusing ink component, for example white, silver, gloss, primer or another color (white (w) is used throughout this specification to avoid unnecessary complexity, but can be replaced by any other diffusing ink), the differences between the natural opacity and the digital opacity of the target color 300 can be compensated for in order to output a more accurate initial set of ink separations 3301, i.e., to correspond more closely to the separations required to match the target color 300 and its opacity to the prior art.

[0061] The inventors have devised a technique for defining the opacity of a target color when printed conventionally or digitally on a reflective substrate based on measurements using a spherical or multi-angle spectrophotometer 117, where the spectral reflectance with specular inclusion and specular exclusion (or a combination of different measurement angles) defines a target opacity value, for example, by subtracting a value representing the specular exclusion from the target given color data from a value representing the specular inclusion of the target given color data. This technique provides a way to match the natural opacity of a target color to its associated digital opacity by adding a predetermined number of diffusing inks, e.g., white, silver, gloss, primer, or others.

[0062] 5, the color matching process performed by system 100 using one embodiment of an ink dosing method according to the present invention will be described, with like reference numbers indicating like steps as in FIG.

[0063] In addition to switching on the computer 110 and loading its operating system and an instruction set embodying the relevant data processing aspects of the color matching method, the printer 114 is first characterized again in step 401. The color undergoes linearization in step 401A, and in the method of the present invention, the printer undergoes calibration in both the specular reflection component and the specular reflection component on a diffuse ink layer (an exemplary white ink) applied to a substrate in step 501, before the diffuse ink undergoes linearization in step 401B.

[0064] In step 402, a forward printer model is again defined at the operator terminal 110 based on the specular reflection component. In step 502, a further forward printer model is also defined based on the specular reflection component on a diffuse white layer applied to the substrate, and in step 503, the two forward printer models are combined to obtain a single forward model that combines the SPIN and SPEX spectral reflectances of each color.

[0065] In step 403, at least a first target color is defined in a color library, e.g., a table of spot or target colors 150, which requires matching in the digital printing system for color accuracy and color opacity. The capture of the target color data is again accomplished by measuring the target color 150 with a spectrophotometer 117, including SPIN and SPEX, and inputting the measurements as a set of respective values ​​into the terminal 110. The target opacity is also defined in step 504, where the SPEX value is effectively subtracted from the SPIN value to define the opacity therebetween as a percentage.

[0066] Then proceeding to the color accuracy and opacity matching step, in step 404, the forward printer model is started, the stored target color or its entire library is read, and in step 405, the target color or each target color is classified according to its opacity in step 504.

[0067] The classified colors are processed with a color prediction algorithm to output respective ink separations in step 406, for example using the ink dosing techniques taught in EP20178552.4. In step 505, further described with reference to FIG. 8, the classified colors or each classified color is further processed with an opacity prediction algorithm to output respective ink separations including the diffuse ink. The opacity prediction algorithm depends on the natural opacity and digital opacity values ​​of the target colors. The opacity of each target color is calculated using an interpolation on the respective digital opacity curves of each component ink in the separation of step 406, and then summing the digital opacities of all the component inks. The natural opacity of each target color is calculated from the SPIN / SPEX spectral reflectance data obtained in step 403. The opacity prediction algorithm calculates the difference between the natural opacity value and the digital opacity value of the target color and interpolates it to the diffuse ink stage to output a prediction of the diffuse ink (e.g., white) ratio to give the target color the target opacity when printed.

[0068] A color chart is then generated in step 407 using the output ink separations and combining the color separation predictions of step 406 with the color opacity predictions of step 505. The generated color chart is processed by the forward printer model and printed in step 408 by the printer 114 onto a test substrate, which may be the reflective substrate of the intended use or may be an optically clear or transparent substrate that can be overlaid on the reflective substrate of the intended use.

[0069] The printed color chart is then measured in step 410 with a spectrophotometer 117 including SPIN and SPEX, and the color values ​​are optionally corrected according to the observed differences relative to the set of stored values ​​for the target color. In this method, the color values ​​are further corrected in step 506 to adjust for the observed differences relative to the set of stored values ​​for the target opacity using an opacity correction algorithm, which is further described with reference to Figure 9. In one embodiment of the method described below, the opacity correction algorithm relies on multiple interpolations of the curve calculated by the opacity prediction algorithm.

[0070] The iteration of color accuracy and opacity matching steps 404 through 506 continues until the required target color values ​​are matched within an acceptable tolerance or until the printer's gamut limit is reached.

[0071] The iteration of the printed color chart that is found to best match the target color is then exported as a digital library of ink color separations using diffusive inks in step 411, and the digital library generated and exported by the method of the present invention can then be used in the digital printing system 100 using a RIP application to print artwork on a reflective substrate using the matched colors.

[0072] Color data calculation techniques for implementing steps 501 to 506 of the method according to the invention as data processing instructions will now follow, with the following notations being used consistently throughout for ease of understanding: The inks in the example printer 114 include cyan (c), magenta (m), yellow (y), black (k), orange (o), green (g), violet (v), and white (w), collectively designated as color inks 'cmykog-v' μ; the ink phase is designated α and varies from 0 to 1; the color wavelength is designated λ and is, for example, 400 JPEG2025505472000002.jpg718~700 JPEG2025505472000003.jpg718 represents a value in nanometers (nm) within the range.

[0073] The spectral reflectance of the target color μ under the conditions of SPIN including specular reflection and SPEX excluding specular reflection can be expressed as follows, respectively. JPEG2025505472000004.jpg2199

[0074] For example, there are 40 linearly or non-linearly spaced ink steps in the range 0 to 1, and 31 linearly spaced wavelengths in the range 400 to 700 nm, in 10 nm increments. JPEG2025505472000005.jpgFor each of the 69 colors, size JPEG2025505472000006.jpg615 2D array JPEG2025505472000007.jpg811 and Sum of 811 JPEG2025505472000008.jpg The result is JPEG2025505472000009.jpg56.

[0075] The natural opacity of a target color is calculated from its SPIN / SPEX spectral reflectance data, and the areas under the SPIN and SPEX curves of a color ink are given by: JPEG2025505472000010.jpg3741 The natural opacity of the color inks from cyan to violet is given by: JPEG2025505472000011.jpg1725The area is calculated numerically by the midpoint rule and element division using the Hadamard product, although alternative embodiments consider the use of Simpson's 1 / 3 rule, 3 / 8 rule for increased accuracy.

[0076] The digital opacity of the target color is calculated using an interpolation on the respective digital opacity curves of each component ink based on their respective proportions of the target color, and then summing the digital opacities of all the component inks. The digital opacity of the color inks from cyan to violet is given by: JPEG2025505472000012.jpg942 where, JPEG2025505472000013.jpg914 is the substrate opacity, calculated as the average of the opacity at 0% ink stage of all colors excluding the diffuse inks, e.g. white. JPEG2025505472000014.jpg1736

[0077] The ink opacity is curve fitted. The composite curve is constructed from a weighted average of 5th and 6th order polynomials, a Fourier series, and a sine sum curve fit, where each component can be assigned its own weight. The curve fit equations are Polynomial curve fit (5th order) y1, Polynomial curve fit (6th order) y2, Sine sum curve fit (3 terms) y3, and Fourier series curve fit (3 terms) y4, and the composite curve constructed from the above equations is given by: (1) JPEG2025505472000015.jpg1334In formula, JPEG2025505472000016.jpg58 are weights assigned to the individual curve fits; each ink has a different impact, and the formula arrives at an average between the inks, and the best fit is derived from the weighted average of the four.

[0078] The resulting natural opacity curve fit constructed according to Eq. JPEG2025505472000017.jpg716 is given as follows. JPEG2025505472000018.jpg830

[0079] Obtained digital opacity curve fit JPEG2025505472000019.jpg715 has a maximum function max applied to it to avoid negative digital opacity, given by: (2) JPEG2025505472000020.jpg990

[0080] The best fit is obtained by subtracting the opacity of the substrate from the natural opacity of each color, and an opacity of 0 is assigned whenever the calculation outputs a negative value.

[0081] Now considering the diffuse ink components, the spectral reflectance of white “w” under specular reflection inclusive SPIN and specular reflection exclusive SPEX, where the subscripts α and λ represent the ink phase (going from 0 to 1) and wavelength (typically in the range of 400-700 nm), respectively, can be expressed as follows: JPEG2025505472000021.jpg1896

[0082] For example, there are 51 linearly or non-linearly spaced ink steps in the range 0 to 1, with 31 linearly spaced wavelengths in the range 400 to 700 nm, in 10 nm increments. This allows you to choose the size for white. JPEG2025505472000022.jpg515 2D array JPEG2025505472000023.jpg711 and The result is JPEG2025505472000024.jpg711.

[0083] To calculate the natural opacity of a white ink, the area under the SPIN / SPEX curve of the white ink is given by: JPEG2025505472000025.jpg3641

[0084] The natural opacity of the diffuse ink (white in this example) is given by the following formula using the element-by-element Hadamard product: JPEG2025505472000026.jpg1223

[0085] The digital opacity of the diffuse ink is given by subtracting the natural opacity of the white ink at 0% ink from the natural opacity, which is equivalent to: JPEG2025505472000027.jpg777The minimum function min and maximum function max are applied to avoid opacity percentages below 0% and above 100%.

[0086] Digital Opacity JPEG2025505472000028.jpg59 and ink stage The functional relationship between the JPEG2025505472000029.jpg59 is inverted for the diffusive ink and the ink step is fitted to the opacity. Digital Opacity Curve Fit of Diffusive Ink Constructed According to Equation (1) JPEG2025505472000030.jpg620 is given as follows. (3) JPEG2025505472000031.jpg735

[0087] target color JPEG2025505472000032.jpg710 is composed of several colors of color ink μ. The printer model determines the ink fraction of each color ink μ available from the ink separation data. Based on the ink separation, the digital opacity of each target color is calculated on the respective digital opacity curve. JPEG2025505472000034.jpg615 can be found by using an interpolation and then adding up the digital opacity of all the component inks. Alternatively, SPIN / SPEX spectral reflectance data for each target color is also available, from which the natural opacity of the target color can be calculated.

[0088] The principle underlying the solution described herein is to use, for example, white, silver, primer or other diffusing or achromatic inks to compensate for the difference between the natural opacity of the target color and the digital opacity. The opacity difference is therefore interpolated for the diffusing ink stage to obtain a first prediction of the diffusing ink, for example white in the figures herein.

[0089] Based on the above, the prediction of the diffuse ink component ratio is calculated as follows: Assume there are N target colors, which are: JPEG2025505472000035.jpg632. Each target color For JPEG2025505472000036.jpg55, SPIN and SPEX spectral reflectance, JPEG2025505472000037.jpg78 and There is JPEG2025505472000038.jpg78, JPEG2025505472000039.jpg1893 where the array JPEG2025505472000040.jpg78 and JPEG2025505472000041.jpg78 is The file is JPEG2025505472000042.jpg618.

[0090] Each target color i For , there is a color separation calculated from the printer model, shown as follows: JPEG2025505472000043.jpg844This means that for target color N, this is size JPEG2025505472000044.jpg513 array gives JPEG2025505472000045.jpg88.

[0091] The area under the target SPIN / SPEX curve is: JPEG2025505472000046.jpg3638

[0092] The natural opacity of the target color is given by: (4) JPEG2025505472000047.jpg1120

[0093] The target color digital opacity is obtained from the curve fit of equation (2) as follows: (5) JPEG2025505472000048.jpg840

[0094] The difference between the natural opacity and the digital opacity of the target color JPEG2025505472000049.jpg66 has the following white ink ratio: It is compensated for using JPEG2025505472000050.jpg78. JPEG2025505472000051.jpg631

[0095] In one embodiment, the difference between the calculation and the interpolation is determined based on a threshold ε. A preliminary correction may be applied to JPEG2025505472000052.jpg55, and the natural opacity ω i If is less than ε, the opacity difference JPEG2025505472000053.jpg55 is set to 0, and any negative opacity difference values ​​are also set to 0, so (6) JPEG2025505472000054.jpg1677

[0096] Once the opacity difference has been corrected according to the above conditions, to obtain the first white ink prediction, we use the opacity difference function JPEG2025505472000055.jpg815 is interpolated to the white ink step. (7) JPEG2025505472000056.jpg1143To match the notation below, the first white ink prediction for the i-th target color is Represented by JPEG2025505472000057.jpg916.

[0097] The color matching technique of the present invention described above can be used in a variety of color matching processes, and the eight ink channels 116 of the printer 114 1-8 Each of the color data processing algorithms may be implemented as respective color data processing algorithms embodied in corresponding instruction sets processed by computer 110 to dispense ink across one or more network nodes, and may be distributable and scalable across one or more network nodes.

[0098] Thus, referring now to Figure 6, the contents of the memory means 209 of the computer 110 when executed, when configured with an instruction set embodying the color data processing techniques described herein, includes an operating system 601, for example Windows 11™ distributed by Microsoft™ Inc. of Redmond, Wash., USA. The OS 601 includes instructions for managing the basic data handling, interdependencies and interoperability of the computer hardware components as described with reference to Figure 2, and a communications subroutine 602 that configures the computer 110 for bidirectional network communications via the NIC 211 interfaced with a wired connection 112 to the local router 118. The OS 601 also includes an input subroutine for reading and processing various input data consisting of user direct input to the human interface devices, i.e., the keyboard 203 and the computer mouse 204.

[0099] Next, an instruction set is shown at 604, which interfaces with the printer 114 and spectrophotometer 117 through the OS 601 via one or more application programmer interfaces (APIs) 605. The instruction set 604 comprises and coordinates the data processing activities of further function-specific data processing subroutines that embody the various functions and algorithms described herein, including a user interface 606 that is updated in real time and output to the display 202.

[0100] The instruction set 604 further holds various data sets that are processed by the subroutines, including one or more ICC profiles of the printer 114 shown at 607, a printer model based on the diffuse inks generated in step 502 shown at 608, L*a*b data 310 for the or each target color 300 stored in step 403 shown at 609, run-time processed matrices, curves, and interpolation data provided at 610, and ink channel 116 generated in steps 505 and 506 shown at 611. 1-8 Ink separation 330 corresponding to each dose of 1-N and finally, at least one color library generated in step 411, shown at 612, which can be used to print the target color 300 on a reflective substrate with the printer 114.

[0101] Also shown are additional local and network data that may be stored in the memory means 209 during execution, some or all of which may be processed by the color matching application 604 and its subroutines, or by or for other applications that are processed in parallel with the color matching application 604 during execution. Examples of additional local data include spooling data for the printer 114 generated in response to the color chart print command preceding step 408, and / or L*a*b data 310 of the or each target color 300 acquired by the local spectrophotometer 117 and communicated to the terminal 110 via a USB connection to the USB module 212. Examples of network data include L*a*b data 310 of the or each target color 300 received from a remote device 130 over the WAN 120, to which the terminal 110 can respond with one or more sets of ink separations 330 calculated in accordance with the present invention, and / or a remote application or OS updating data communicated by a remote server over the WAN 120.

[0102] 7 and 8, respective embodiments of prediction step 505 are shown, each of which outputs an ink separation that includes diffuse ink components of the or each target color.

[0103] The first embodiment shown in Figure 7 is a straightforward implementation of the inventive technique, where a printed color is set as a target color on a diffusing substrate. The method determines the amount of diffusing ink (e.g., white) required to match the target color when printed on a RSM substrate as described above. The technique then adds (by adding white / diffusing ink) to the natural opacity of the target color in order to make the printed color as transparent (or opaque) as the target color. JPEG2025505472000058.jpg67 with its digital opacity A best match was made to JPEG2025505472000059.jpg67, so that when printed on an RSM substrate, the printed color is perceived to be the same as the target color printed on a white substrate.

[0104] The second embodiment shown in FIG. 8 is called the Transparent-Semi-Opaque-Opaque ("TSO") color matching technique, in which the color printed on the diffusing substrate is again set as the target color. The method again determines the amount of diffusing ink (e.g., white) required to match the target color when printed on the RSM substrate as described above. This embodiment modifies the natural opacity of the target color (by adding white / diffusing ink) according to a specified range to make the printed color more or less transparent (or opaque). JPEG2025505472000060.jpg67 with its digital opacity JPEG2025505472000061.jpg67 is best matched so that when printed on an RSM substrate, the printed color is perceived to be the same as the target color printed on a white substrate, or more or less transparent (or opaque).

[0105] Thus, in the second embodiment, the task is not to match the opacity of a printed color to that of a target color, but to calculate the amount of diffusing ink, e.g., white, needed to produce different versions of a target color given a range of natural opacities. This embodiment advantageously allows the designer to evaluate how transparent or opaque different versions of the target color will appear when printed on an RSM substrate.

[0106] The range of natural opacity can be categorized as follows: The transparent version of JPEG2025505472000062.jpg57 has a transparency range of The target color can be expressed as a value of JPEG2025505472000063.jpg619. The semi-opaque version of JPEG2025505472000064.jpg57 is a semi-transparent range value. It can be represented as JPEG2025505472000065.jpg618. The opaque version of JPEG2025505472000066.jpg57 has the opacity range value It can be represented as JPEG2025505472000067.jpg620. In the case of JPEG2025505472000068.jpg56, TSO is transparent, semi-opaque, and opaque. JPEG2025505472000069.jpg57 version (i.e. from transparent to opaque) JPEG2025505472000070.jpg (610 versions in total).

[0107] The data required to predict the TSO of N number of target colors are given in the previous section. JPEG2025505472000071.jpg811, JPEG2025505472000072.jpg811, JPEG2025505472000073.jpg710, JPEG2025505472000074.jpg711, It can be obtained from JPEG2025505472000075.jpg87.

[0108] The following sequence can be applied to obtain the TSO from the target color: JPEG2025505472000076.jpg821 is loaded first, then the algorithm proceeds as follows: JPEG2025505472000077.jpg49113

[0109] Spectral reflectance If JPEG2025505472000078.jpg722 is next loaded, the algorithm proceeds as follows: JPEG2025505472000079.jpg58100

[0110] Next, the color separation data JPEG2025505472000080.jpg98 is loaded and the target color digital opacity is obtained from the curve fit as defined above in equation (5). The diffuse ink prediction is the sum of the versions of each target color. JPEG2025505472000081.jpg610 numbers (each of T, S and O Therefore, the total number of diffuse ink predictions is The image is JPEG2025505472000083.jpg621. The natural opacity of the JPEG2025505472000084.jpg610 version is The difference between the natural opacity and the digital opacity of the kth version of the target color is The k-th diffusion ink prediction is defined as follows: Calculated as JPEG2025505472000087.jpg810.

[0111] An exception may be made for pastel colors, which are generally characterized by a higher natural opacity. Pastel colors can be defined as a family of pale colors having high luminance value and low chroma. The modifiers "high value" and "low chroma" are intentionally relative terms. For these colors, the pastel flag JPEG2025505472000088.jpg77 is used to classify pastel colors among other colors, and TSO classifies colors ranging from semi-opaque to opaque. JPEG2025505472000089.jpg is represented as a sum of 57 versions, all located at a higher natural opacity, or threshold that may be determined.

[0112] The qualification of a color as a pastel is based on its spectral reflectance and a pastel index ranging from 0 to 1 (or 0% to 100%) under a set of conditions that, taken together, increase confidence in its qualification as a logical output function (true / 1 or false / 0). It can be scaled with JPEG2025505472000090.jpg611 under the following conditions C n are provided as non-limiting examples and are believed to be suitable for providing reliable pastel indications.

[0113] conditions JPEG2025505472000091.jpg57 can be related to the spectral reflectance of a color. One condition is to have a color spectrum that is close to a reference high-luminosity spectrum between the substrate minimum and substrate maximum spectra, JPEG2025505472000092.jpg615 and The color spectrum between the corresponding points in JPEG2025505472000093.jpg515 Some points in JPEG2025505472000094.jpg78 It may be that the image has a resolution of 1280x1024 according to JPEG2025505472000095.jpg45. JPEG2025505472000096.jpg21124This condition is true for at least JPEG2025505472000097.jpg713% JPEG2025505472000098.jpg820 and JPEG2025505472000099.jpg720 should be between 720.

[0114] Another condition JPEG2025505472000100.jpg58 is Color spectrum over the corresponding points in JPEG2025505472000101.jpg79 This condition can be the number of points in the color spectrum at least JPEG2025505472000103.jpg forces 713% to be above the representative light black (or gray) spectrum.

[0115] Other conditions JPEG2025505472000104.jpg711 and JPEG2025505472000105.jpg617 can represent the area between the color spectrum and the white spectrum, and the area between the color spectrum and the bright black spectrum, respectively. These conditions are, JPEG2025505472000106.jpg711 and JPEG2025505472000107.jpg617 is at least JPEG2025505472000108.jpg714 and JPEG2025505472000109.jpg 715 units. Another condition is the ratio JPEG2025505472000110.jpg728 is the largest It may be that it should be JPEG2025505472000111.jpg614.

[0116] A further set of conditions can be related to color space coordinates (L*a*b*, L*c*h*, L*u*v*, HSL, HSV). For example, in the CIE-L*a*b color space: JPEG2025505472000112.jpg56 coordinates are at least JPEG2025505472000113.jpg713 should be used. JPEG2025505472000114.jpg611 coordinates are at least JPEG2025505472000115.jpg713 should be used. JPEG2025505472000116.jpg613 coordinates are at least JPEG2025505472000117.jpg713 should be HSV color based JPEG2025505472000118.jpg612 coordinates are up to JPEG2025505472000119.jpg714. In L*c*h* color space JPEG2025505472000120.jpg611 coordinates are at least It should be JPEG2025505472000121.jpg713. In the Lch color space JPEG2025505472000122.jpg611 coordinates are maximum JPEG2025505472000123.jpg714. Luv color space JPEG2025505472000124.jpg612 coordinates are maximum JPEG2025505472000125.jpg614. Luv color space JPEG2025505472000126.jpg612 coordinates are maximum It should be JPEG2025505472000127.jpg714.

[0117] Another condition may be related to the average RGB ratio. This condition is calculated by dividing the sRGB coordinates of known pastel colors by the The condition is based on the observation that the magnitudes of the images in JPEG2025505472000128.jpg720 are similar. The condition is that the averaged ratio κ is at least JPEG2025505472000129.jpg713 and up To force that the image should be JPEG2025505472000130.jpg714, we can define it as follows: JPEG2025505472000131.jpg30155

[0118] Further conditions may be based on the CMYKOGV or μ color separation, based on the observation that known pastel colors have 0 (or negligibly low) black ink in the μ color separation. JPEG2025505472000132.jpg68 is the black ink % in μ color separation. The conditions are: JPEG2025505472000133.jpg68 is the largest Forces the image to be JPEG2025505472000134.jpg714.

[0119] If the index is 1 or 100%, then all conditions match. However, the pastel index threshold can be defined by the observer. The conditions are not all of equal importance, and each condition parity JPEG2025505472000135.jpg45 with different weights JPEG2025505472000136.jpg58 can be assigned the pastel index according to the following: JPEG2025505472000137.jpg611 contains all the constants used in the criteria to determine the pastel quality of a color. (8) JPEG2025505472000138.jpg1337

[0120] 9, one embodiment of the correction step 506 is shown, which outputs a corrected ink separation that includes a diffuse ink component of the or each target color. In this technique, the predicted diffuse ink proportion is calculated by multiplying the difference δ between the natural opacity and the digital opacity of the target color. i is applied to compensate, the spectral reflectance of the resulting color in SPIN and SPEX is measured, color separation values ​​are again generated, and the new SPIN and SPEX measurements provide the basis for calculating a new Natural Opacity value.

[0121] Therefore, in step 506, based on the procedure outlined in the previous step, the diffusion / white ink percentage needed to compensate between the natural opacity and the digital opacity of the target color is calculated. The calculated target color or each i-th target color of JPEG2025505472000139.jpg916 is corrected. The correction technique is based on the Euclidean distance between the printed color and the target color in any given color space (CIE-XYZ, L*a*b, xyz or other) as a value indicating how close the printed color according to the first predicted diffuse ink composition is to the target color.

[0122] The International Commission on Illumination, commonly referred to as CIE, has defined color difference that takes into account the effects of perceptual non-uniformity. This is typically called Delta E and represents the difference between a printed color and a target color in the L*a*b* color space. However, the perception of a color is also affected by its opacity. Therefore, another useful measure for evaluating color difference is the difference between the natural opacity of the target color and the natural opacity of the printed color, referred to in this disclosure as Delta Opacity ("Delta O"). The mathematical formulas for these two measures are described in detail below.

[0123] JPEG2025505472000140.jpg626 and JPEG2025505472000141.jpg726 is the CIE-L*a*b coordinates of two colors. Color difference between two colors The procedure for calculating JPEG2025505472000142.jpg59 is given according to known Delta E formulas such as Delta E2000. Delta-E JPEG2025505472000143.jpg1017

[0124] Let JPEG2025505472000144.jpg67 be the natural opacity (also called the "target opacity") of the i-th target color according to equation (4) for calculating the natural opacity. Let JPEG2025505472000145.jpg610 be the natural opacity of the jth target color printed on the ith corrected reflective substrate, which is also called the "measured opacity." Delta-O JPEG2025505472000146.jpg1018

[0125] for example, JPEG2025505472000147.jpg614, If JPEG2025505472000148.jpg611 is the natural opacity of the i-th target color after the 0-th correction, i.e., the first prediction, then delta O is defined as follows: JPEG2025505472000149.jpg1047

[0126] Predicted White Ink Correction for JPEG2025505472000150.jpg712 Calculating JPEG2025505472000151.jpg1031 mainly depends on the opacity of the measured and printed colors. Then, we evaluate the performance of the correction through both the opacity difference and the color difference. The proposed formula is as follows: JPEG2025505472000152.jpg1299 where κ is used to improve the formula and is the measured opacity JPEG2025505472000153.jpg610 and target opacity If JPEG2025505472000154.jpg67 and JPEG2025505472000154.jpg67 are very close to each other numerically, the ratio JPEG2025505472000155.jpg119 approaches 1, therefore, JPEG2025505472000156.jpg7The 17th correction is defined to be very close to the jth correction. JPEG2025505472000157.jpg1148

[0127] Thus, the embodiment of step 506 of FIG. 9 optimizes the ink separation including the spread ink prediction 330 output by any of the methods shown and described with reference to FIG. 5, FIG. 7 or FIG.

[0128] Those skilled in the art will recognize that in analog printing systems, a target color is typically formulated using a range of color base inks that, in traditional printing systems, are similar to what μ defines for digital printing systems. c The color bases, represented as β, are also known as colorants or pigments, and the subscript "c" represents the use of each colorant in a typical ink-based system (e.g., 25 colorants). c can be used to define the opacity of c β may be simply replaced by β, which advantageously enhances conventional ink formulation systems. The opacity prediction algorithm described herein can provide the user with opacity information along with color to select the best match recipe. For example, using step 401A, β c can be characterized, and step 401B can be used to characterize conventional diffusive inks. c The natural opacity of can be curve fitted as in equation (1), the resulting digital opacity curve as in equation (2), and the diffuse ink digital opacity as in equation (3). Using equations (4) and (5), further corrections or selection of different recipes can be evaluated based on the differences in the measured Delta E and Delta O.

[0129] Referring now to FIG. 10, an operator operating a conventional printing press may require a physical representation 1000 of a target color, which defines the manufacturing tolerances of a print assignment. The target color is typically a sample of an ink formulation, also known as an ink drawdown 1010, applied directly onto the printed substrate 1000. The target ink drawdown 1010 is often accompanied by a darker version 1020 and a lighter version 1030, which correspond to a lighter and heavier weight, respectively, of the ink film of the target color applied with the same ink formulation onto the same substrate 1000. In this regard, the embodiment of the color matching method shown in FIG. 10 automatically calculates the lighter and darker representations of the target color at a particular distance ΔE in the L*a*b color space from the best match output in step 506.

[0130] The L*a*b values ​​of the target color and the substrate are known, along with the spectral reflectance, ideally along with the spectral reflectance of a black printed on the same substrate on which the target color is measured. By inputting the tolerance thresholds of the lighter and darker versions, e.g., pre-set ΔE values, including potentially different values ​​for each of the lighter and darker versions, the lighter color (with higher luminance at coordinate L) is typically characterized by a spectral curve that is above (positive offset) the spectral curve of the target color. The darker color (with lower luminance at coordinate L) is typically below (negative offset) the spectral reflectance curve of the target color.

[0131] To obtain a lighter color 1030, the spectral reflectance curve of the target color can be increased in positive incremental steps and the resulting ΔE monitored through Spectrum → XYZ, XYZ → L*a*b, and then finally JPEG2025505472000158.jpg864 is calculated. When the difference between the target (desired) ΔE and the theoretical ΔE is within a specified tolerance, the processing loop can end. The calculated ink separation of the lighter color will most likely differ from the target color. Therefore, the opacity of the lighter color usually requires correction according to equations (6) and (7). The darker color 1020 can be obtained by following the same method, but applying a negative offset increment instead.

[0132] However, moving the spectral reflectance curve up or down by a constant offset across the curve can result in perceptually different colors, which deviate significantly not only in the L coordinate but also in the a and b coordinates. To remedy this, the offset itself can be made a function of wavelength. A strategy is proposed in which the offset varies along the spectrum based on the distance of the spectral curve from the corresponding substrate, and ultimately the black spectral reflectance.

[0133] The theoretical L*a*b values ​​of a target color are expected to differ from the theoretical L*a*b values ​​of the target color printed on a substrate due to variations in printing conditions such as ink, printing speed, room humidity and temperature, measurement device tolerance, etc. Most importantly, in digital printing technology, to print a color using a μ-ink process, the target L*a*b coordinates are converted to corresponding μ-ink color separations based on a given printer model as mentioned above or another printer calibration profile, and the color separations themselves are not unique. Therefore, the theoretical target color and the printed target color are expected to have a non-zero ΔE between them.

[0134] This ΔE is automatically carried over to the corresponding lighter and darker versions of the theoretical target color. Therefore, the resulting ΔE between the printed target and the printed lighter version is usually not exactly the same as the desired ΔE. Compensation for this deviation requires an iterative correction procedure, which is provided in this embodiment.

[0135] Target spectrum to target To JPEG2025505472000159.jpg813 To obtain JPEG2025505472000160.jpg840, the theoretical target color JPEG2025505472000161.jpg58 CIE- JPEG2025505472000162.jpg512 Tristimulus coordinates are spectral reflectance JPEG2025505472000163.jpg727 is used to calculate the spectral intensity distribution of the illuminant. JPEG2025505472000164.jpg612 uses the D50 illuminant. The CIE-1931 standard observer is JPEG2025505472000165.jpg49 Tristimulus values ​​to CIE- JPEG2025505472000166.jpg415 coordinates and the observer's color matching function Used for JPEG2025505472000167.jpg742. Theoretical target color JPEG2025505472000168.jpg48 JPEG2025505472000169.jpg416 coordinates are, JPEG2025505472000170.jpg823. Therefore, the operations in this section can be written as follows: JPEG2025505472000171.jpg985

[0136] The spectral reflectances of the lighter and darker colors can be expressed as follows: JPEG2025505472000172.jpg857

[0137] JPEG2025505472000173.jpg819 and JPEG2025505472000174.jpg88 are the target spectral reflectances, which are white and black, respectively. The difference in the spectral curves is defined as follows: JPEG2025505472000175.jpg1954

[0138] P for moving the spectral curve up or down JPEG2025505472000176.jpg78 percent increment (0 to 1). Offset function JPEG2025505472000177.jpg822 is defined as follows: JPEG2025505472000178.jpg2054

[0139] Lighter / Darker Color Spectrum The spectral reflectance of JPEG2025505472000179.jpg718 is obtained by adding the corresponding offset to the target spectrum. JPEG2025505472000180.jpg1839

[0140] The spectral to L*a*b conversion uses the same procedures and utility functions detailed above. The lighter / darker spectral representation to lighter / darker is: JPEG2025505472000181.jpg997

[0141] The library function Delta-E2000 is used to calculate the color difference between the target and the shade of light and dark. The color difference between the target and the shade is expressed as follows: JPEG2025505472000182.jpg966

[0142] Confirmation that the color difference obtained by moving the target spectral curve up or down within a specified tolerance range ε ​​is equal to the desired color difference is performed by JPEG2025505472000183.jpg2152

[0143] JPEG2025505472000184.jpg824, JPEG2025505472000185.jpg825 is stored, then a conversion function such as lab2rgb is used to obtain the RGB coordinates of a light and dark version to plot the color swatch for visual overview on a monitor display. This solution is finally printed with the system 100 containing a darker color 1020 and a lighter color 1030 with the target color 1010 to evaluate color matching.

[0144] The viewing conditions are an important component for a good evaluation of the color matching and opacity matching of printed target colors. The light source has a strong incidence on the angular light scattering of the printed colors on the reflective substrate, whether flat or cylindrical. Remaining under the same lighting conditions, but rotating the colors gives the impression of a change in color when visualized from different viewing angles. In this context, normalized light and viewing systems have been developed and adopted by experts, providing substantially standardized viewing conditions.

[0145] 11 shows an example of a color viewing station 1100 for viewing print colors or color printed objects (flat, cylindrical or shaped) 1110 under normalized direct or indirect light 1120 in a light booth 1130. The light booth typically consists of a shielding wall 1140 to prevent light contamination of the normalized light 1120 with other adjacent light sources such as office lamps, windows, etc.

[0146] A color or object placeholder 1150 that can be placed in a light booth under normalized light 1120 is comprised of a support plane 1152 having a top surface facing the light source 1120, which is rotatable with a manual or automatic crank member 1154 to facilitate observation of colors under different viewing angles, specular or non-specular reflection.

[0147] The observation station 1100 preferably performs the imaging of the present invention (e.g., JPEG2025505472000186.jpg722), where θ is the positive angle of the placeholder support plane 1152 measured from the light booth axial plane, which is ideally perpendicular to the light direction 1122 of the light source 1120. Repeated observation tests have shown that the range of opacity values ​​between transparent and opaque is typically between 15 and 90, respectively. It is therefore preferable to define a range of θ using the same values, which range is substantially less than 15°, for example 15°, for observation conditions suitable for transparent colors evaluated under specular observation conditions. It starts with JPEG2025505472000187.jpg612, whereas For opaque colors viewable under diffuse lighting conditions in JPEG2025505472000188.jpg760, for example, 0° provides similar viewing conditions as 90°.

[0148] When an observer 1160 is positioned at a distance from the placeholder, the color to be evaluated is visually evaluated on the top surface at each viewing angle of that transparency, and then the support plane 1152 can be rotated to evaluate each successive color of the sample.

[0149] The range of opacity for colors can be classified according to the output of the transparent-semi-opaque-opaque embodiment to simplify the procedure of color evaluation within the range. For example, a transparent color is JPEG2025505472000189.jpg718 to opacity range, JPEG2025505472000190.jpg719 to semi-opaque range, and JPEG2025505472000191.jpg524 belongs to the range You can have opaque colors from JPEG2025505472000192.jpg620.

[0150] Thus, the present invention provides a computer-implemented method for dosing ink in a digital multi-channel printer in a single workflow when printing on a reflective substrate, the embodiments of which further provide for easily correcting and adjusting values ​​representing initially calculated diffuse ink dosages.

[0151] The present disclosure advantageously simplifies and accelerates the color matching process for expert users, using a first set of ink separations 330 including diffusion effects that are clearly much closer to matching the target color 300 compared to prior art techniques. The benefits of these techniques extend to optical checking by expert users. That is, a color chart printed with the first set of ink separations 330 calculated according to the present invention, and / or a subsequent set of ink separations corrected according to the present invention, can be printed on an optically transparent or transparent layer substrate, which the user can then overlay on a printable blank of a reflective substrate for evaluation in, for example, a viewing booth 1100.

[0152] It has been observed that overlaying a transparent layer (plastic of a certain thickness) affects the angular light scattering from the incident light. Thus, the transparent layer can be considered to be more or less part of the diffusing element based on the total amount of ink deposited in the printing process. Furthermore, the ink deposited on the transparent layer can be applied to the front or back of the substrate allowing for differential gloss effects typical of transparent layers.

[0153] Network-distributed embodiments of the computer-implemented methods disclosed herein are contemplated, where the predictive aspects of the methods are augmented by machine learning algorithms taught to detect changes in the printing system and compensate for the predicted ink separation 330 according to the detected changes.

[0154] Such machine learning techniques can be developed from a collection of data collected over time that represents color and opacity deviations in a printing system. Such data can be based on variables such as variations in inks or substrates from one production batch to another, variations in spectrophotometer calibration, and wear in digital printing system parts such as printheads, all of which are known to affect the consistency of final printed color from batch to batch over time.

[0155] In a particular example that is particularly relevant to the field of RSM substrate printing, the use of achromatic inks in printing systems often introduces larger and heavier particles compared to the pigment particles of other chromatic inks. White inks use titanium dioxide particles that are difficult to maintain in suspension in the ink formulation vehicle, ultimately causing loss of ink density and / or clogging of the print head, where achromatic inks require permanent or semi-permanent agitation.

[0156] The use of machine learning techniques in accordance with the present method is expected to continuously optimize predictions as the printing system is periodically re-linearized and / or re-calibrated, however, capturing, aggregating and auditing this data for patterns is necessary to continuously improve the predictive model of the color and opacity matching process, for example, by using a gradient descent algorithm, which is a first order iterative optimization for finding local minima of differentiable functions, since the corresponding data is only used at the time of execution of the action.

[0157] In this specification, the terms "comprise", "comprises", "comprised" and "comprising" or any variations thereof, and the terms "include", "includes", "included" and "including" or any variations thereof, are considered to be fully interchangeable and all of them should be given the broadest possible interpretation, and vice versa.

[0158] The invention is not limited to the embodiments described above, which may be modified both in arrangement and detail.

Claims

1. 1. A computer-implemented method for dosing ink in a printing device having multiple ink channels when printing on a reflective substrate, comprising: acquiring target color data for at least one target color to be printed on the reflective substrate, the target color data comprising both color data and spectral reflectance data having different measurement angle combinations; calibrating the printing device using both a specular reflection component and a specular reflection component on a diffusing ink layer applied to the substrate; generating a printer model based on the specular reflection component and the specular reflection component on the diffusing ink layer applied to the substrate; defining a target opacity value by subtracting a value of the target color data representing the exclusion of the specular reflection component from a value of the target color data representing the inclusion of the specular reflection component; processing the captured color data with the printer model to output reserve ratios for each ink channel representing each color component of the target color; calculating a curve from each ratio and interpolating it to output a digital opacity value for each ink channel; combining the respective digital opacity values ​​of all ink channels to output a digital opacity value of said at least one target color; calculating a natural opacity value from the captured spectral reflectance data; calculating the difference between the calculated digital opacity value and the calculated natural opacity value; interpolating the calculated difference for ink stages of the at least one target color diffusion ink component to output a predicted dosing ratio for each ink channel; 1. A computer-implemented method for dispensing ink, comprising:

2. classifying the or each target color according to the defined target opacity; repeating the processing step by replacing the captured color data with data representing the or each classified color; the predicted output input ratio of each ink channel includes an input ratio of the diffusion ink; The method of claim 1.

3. adding said predicted proportion of diffuse ink to said natural opacity value to output a next natural opacity value; calculating a difference between the natural opacity value and the next natural opacity value; The method of claim 2 , wherein the calculated difference is a value referred to as the delta opacity for the reflective substrate.

4. The method of claim 3 further comprising correcting the predicted dosing ratio for each ink channel using a composite function stored as a look-up table.

5. 2. The method of claim 1, further comprising correcting the calculated difference before the interpolating step, wherein the correcting step comprises setting the calculated difference to 0 if the natural opacity value is below a predetermined threshold or if the calculated difference value is negative.

6. 10. The method of claim 1, wherein the target color is formulated by a conventional ink formulation system having characterized colorants, a set of base inks, and conventional diffusion inks, the method further comprising the step of specifying an opacity level for each ink stage.

7. 2. The method of claim 1, further comprising the step of inputting a range of natural opacity values, wherein each output predicted input ratio includes a diffuse ink percentage corresponding to a target color having a digital opacity selected from a range from transparent to opaque, including semi-opaque.

8. defining a set of conditions based on color characteristics associated with pastel colors; filtering the captured target color data with the defined set of conditions; The method of claim 7 , further comprising: classifying the target color data as a pastel color according to the filtering.

9. establishing at least one tolerance threshold representing a distance from the coordinates of the target color in L*a*b color space; 2. The method of claim 1, further comprising: calculating the predicted dosing ratio for each ink channel for each of the lighter and darker versions of the target color according to the tolerance threshold.

10. generating a color chart from the predicted fill ratios for each ink channel; generating a color chart from the corrected input ratios of each ink channel; and printing the color chart onto a test substrate using the printer, and optionally configuring a color viewing station having a normalized light source and a support plane with a variable viewing angle; placing the printed substrate on the flat support surface; The method of claim 1 , further comprising orienting the viewing angle of the support plane according to the calculated opacity level of the target color.

11. 1. A digital printing system comprising: a printing device having a plurality of ink channels; means for capturing target color data for at least one target color to be printed on a reflective substrate by said printing device; a data processing terminal within said printing device or operatively interfaced with said printing device; calibrating the printing device using both a specular reflection component and a specular reflection component on a diffusing ink layer applied to the substrate; receiving the captured target color data, the target color data comprising both color data and spectral reflectance data having different measurement angle combinations; generating a printer model based on the specular reflection component and the specular reflection component on the diffusing ink layer applied to the substrate; defining a target opacity value by subtracting a value of the target color data representing the exclusion of the specular reflection component from a value of the target color data representing the inclusion of the specular reflection component; processing the captured color data with the printer model to output reserve ratios for each ink channel representing each color component of the target color; calculating a curve from each ratio and interpolating it to output a digital opacity value for each ink channel; combining the respective digital opacity values ​​of all ink channels to output a digital opacity value of said at least one target color; calculating a natural opacity value from the captured spectral reflectance data; calculating a difference between the calculated digital opacity value and the calculated natural opacity value; interpolating the calculated difference for ink stages of the at least one target color diffusion ink component to output a predicted dosing ratio for each ink channel; a data processing terminal configured with an instruction set for performing A digital printing system comprising:

12. 12. The system of claim 11, wherein said system further comprises a network to which said data processing terminal and a remote terminal are operatively interfaced, said captured target color data being received from said remote terminal.

13. further comprising bridging means for interfacing said data processing terminal and / or said spectrophotometer with remote storage means; Optionally, said data processing terminal is further configured to encrypt captured color data before uploading it to said remote storage means. The system of claim 12.

14. 14. The system of claim 13, wherein the remote storage means is a data processing node configured to process the stored color data and output a predicted dosing ratio for each ink channel in accordance with a color data request from the remote data processing terminal or another terminal.

15. A set of instructions recorded on a data carrier medium or stored on a network storage medium, which when read and processed by a data processing terminal configures the terminal to carry out the steps of the method of claim 1.

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