How to perform color data conversion

Abstract

To provide a method for executing color space conversion with high accuracy by computer support so as to obtain results close to an original.SOLUTION: A source color space includes n colors, a destination color space includes m values, and combinations of m components of the destination color space are assigned to at least part of combinations of n colors of the source color space via a transformation rule. An output value of a combination of unknown inputs is constructed in a stepwise manner from an output value of a combination of known inputs, and a color influence (relative to the output value) of input color added in each step is estimated. An input combination "AB" is decomposed into a contribution degree of partial color "A" and a contribution degree of the remaining color "B" and applied to another base "C" to estimate the unknown combination "AC" with an aim of showing color effect of "A" on a base "B".SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for performing color data conversion from a source color space having n colors to a destination color space having m values using a conversion rule TRV.

Background Art

[0002] In the printing industry, color conversion may be required to create a dataset of a color printing product to be printed.

[0003] The starting point is a digital layout. The digital layout may include, for example, color images captured by a photograph or a scan, illustrations, documents, etc. Such a layout is a template that is reproduced as it is in the form of a printed product. Such a layout is designed or developed in any format and, from the perspective of color, represents what is desired according to the intended purpose. Usually, the layout includes several layout objects. Therefore, each layout object has a uniquely identifiable color appearance. As is well known, in the case of a printed product, for example, by simply printing screen printed inks on each other, each color makes a certain contribution through its effect, and the final result is created by the interaction of different color layers, so that a color appearance can be created by mixing only a small number of colors.

[0004] The color appearance of each layout object is defined by color values ​​(image pixels, outline colors, or fill colors) that represent the data source. In the case of image pixels, there are typically millions of pixels, each with its own color value. These source color values ​​always refer to the supplier's color space, which has a specific number of colors. This means that each object in the layout is assigned to a color space typically associated with the device, which can be a specific RGB color space such as sRGB or AdobeRGB, a CMYK color space, a black-only (grayscale) color space, or other print color spaces. The object's color values ​​are quantified as RGB values, CMYK or grayscale values ​​(0% to 100% color), or other color values, where the total number of device colors in the supplier's color space is denoted by n.

[0005] Color spaces are typically represented by ICC color profiles (ISO 15076-1:2010) and by layouts in PDF and PDF / X files (ISO 15930 series) that contain or reference these ICC color profiles.

[0006] Layouts are reproduced for vastly different reasons and purposes. A wide variety of printing processes and presses can be used for reproduction. This has the potential to apply a specific number of colors to the carrier material in a specific way; that is, the colors used by the devices in the printing system. These colors define or form the basis of the target color space and therefore include all possible combinations of colors used by the devices in the printing system. This is referred to as the m device colors of the target color space. Often, conversion to device-independent values ​​such as Lab or spectral color (see below) is also necessary when verifying post-print measurement results or providing a device-independent database for display on any device. This is why the term "m values ​​of the target color space" is commonly used.

[0007] Standardized color spaces are typically used as the source color space during layout design, but printing is performed using specific output systems with their own unique color behavior. Therefore, for each halftone dot in the layout, a predetermined combination of n device colors from the source color space needs to be converted to a combination of m device colors from the destination color space required for printing. The goal here is to produce a print product that is as close as possible to the original, i.e., the digital layout, and ideally, to reproduce the same appearance to the human eye.

[0008] A halftone point, or pixel, is any point on a layout that results from the specified rasterization of a layout object. This relates, for example, to the so-called image resolution for creating print forms. If a grid with i rows and j columns is placed on the layout, the resolution can be expressed as i × j pixels. Individual pixels can be identified from one another by their color structure.

[0009] Color conversion is performed when converting layout data into printable data with the desired print color set. This results in, for example, uniform and authentic CMYK data for offset printing. Color conversion is also performed to bind and simulate the color appearance of this print data before printing. The color appearance is usually represented either device-independent in the CIELAB color space (abbreviated as Lab) or using spectral reflectance. Print data with the assigned color space is converted into device-independent color data. These are displayed on a calibrated screen or a special proofing printing system (a so-called "proof," usually an inkjet system). In connection with this, there is the task of adapting print data to other printing processes while preserving the impression of the original colors as much as possible; that is, the task of generating new print data for other inks from existing print data through conversion.

[0010] Generally, for typical printing processes with four colors or less, these conversion processes are largely established and standardized by the corresponding data systems.

[0011] However, in packaging printing, for example, four or more printing colors are generally used. Traditionally, brand colors are added as spot colors, and images are constructed with product-specific colors (for example, a light gray printing color is added for dairy products, and various shades of brown are added for chocolate products). In addition, white printing ink is often added to transparent film packaging. Each ink used occupies a printing unit of the printing press and requires its own printing form in ink transfer. Colors are applied sequentially, and in principle, all colors can be combined with each other (overprintable). The wholeness in separate printing forms leads to the process of creating print data, or consequently, the term "separation."

[0012] Machines with 7 to 10 ink units are not uncommon in flexographic, offset lithography, and gravure printing. Designers of branded products can choose printing inks from thousands of spot colors, including over 2,000 hues in the PANTONE (trademarked) system. These inks are manufactured and supplied by ink manufacturers. In so-called digital printing, inkjet and toner-based printing presses are used, often containing 6 to 8 ink units, but operating with fixed ink sets or changing them is too technically complex because inkjet inks and color toners require highly specific, machine-dependent processing characteristics. Typical color sets include CMYK, complemented by orange or red, green, blue or purple, and white.

[0013] Currently, while digital printing processes are slower than traditional printing processes, they significantly reduce setup effort because they eliminate the need to create print forms or change colors. This allows for faster, more economical printing and enables personalized printing. Therefore, even if there is a demand for rapid printing of conventionally produced packages or for personalization as needed (execution 1), the color appearance must be as identical as possible to previously printed products manufactured with individual spot colors. This is to allow different batches of products to be displayed on shelves. For this purpose, print data usable in traditional printing processes is converted from its source color space to the destination color space representing the digital printing process. In the case of extremely modified or personalized printing, different source data needs to be converted for each copy, which becomes a bottleneck.

[0014] Therefore, the color conversion described at the beginning converts from the source color space Q (e.g., having n device colors) to another source color space Z (e.g., having m values, either device colors or three CIELAB values ​​L, a, b). This is done by applying a conversion rule TRV. Such rules refer to calculation formulas, conversion models, or conversion tables already created based on TRV. High throughput is important because it needs to be applied frequently per pixel (millions of pixels per second). To achieve this, interpolation is performed using predefined tables such as ICC industry standards. The table is constructed assuming combinations of n color values. Each of the n input colors can take values ​​between 0% and 100% ink coverage. Not all combinations can be listed on the table, but each input color is sampled at a specified resolution. An n-dimensional grid of squares containing all combinations of sampled levels is obtained. Between them, n-dimensional interpolation is applied. The accuracy depends on this sampling. A typical sample is n=3 (RGB / Lab), which has 33 levels (33 3(approximately 36,000 table entries), and for n=4 (CMYK), there are 17 grayscale levels (6.25% increments, 17 4 (Approximately 84,000 table entries). The number of gradations represents the base in the exponent, and the number of inks n is the exponent. The number of gradations is the base of the power, and the number of inks n is the exponent. The generation of conversion tables is a method familiar to those skilled in the art. They are based, for example, on spectroscopic measurements of printed inks, calculations for estimation and / or interpolation, etc.

[0015] The size of the color table increases exponentially with the number of input colors n. Due to limited memory space, it becomes necessary to coarsely sample for practical reasons. For a typical n=7, only 6 or 7 levels are used. One example is the widely used open-source color management system LittleCMS, which uses 7 levels (16.67% increments). 7 This amounts to approximately 820,000 table entries. Therefore, the table structure has the problem that the larger the number of inks, the larger the table becomes, and the fewer gradients can be stored. Users expect the same level of precision they are used to with CMYK to be achieved with CMYK and additional colors. Therefore, 7 levels is quite coarse.

[0016] A known solution approach is disclosed in Patent Document 1. According to this, an assignment table with four or more input color components is divided into several assignment tables with a maximum of four input color components. This method aims to solve the problem of assignment tables being too large. In the prior art, even with 7-color printing, typically only four or fewer colors are printed overlapping at a single point on the printed sheet. Accordingly, a concrete proposal was made to solve the above problem by essentially reducing it to a 4-component table. For this reason, the reduction to a 4-component table is also being promoted. To make it more reliable, additional processes such as gray component replacement (GCR) and color component replacement (CCR) are applied when creating the separation table. Similar prior art can be found in Non-Patent Document 1 and Patent Document 2.

[0017] The known solution approach uses a model or a combination of a model and a table. A model is typically a mathematically based transformation rule. In this way, the Lab color values ​​of overprints can be estimated from individual print colors. This is done, for example, when displaying layouts or PDF files on a screen, and is fast but lacks accuracy. Further development of ICC, such as iccMAX, proposes either spectrally offsetting monochromatic colors or using a table for the CMYK portion and changing the value for each additional color. In either case, adding individual colors is highly inaccurate because it does not, or does not adequately, consider the interactions of the overprint colors in which these colors are involved.

[0018] While more accurate models exist to better calculate the overprint behavior of color mixing, these models are not suitable for processing at speeds fast enough to be applied to conversions of millions of pixels. Furthermore, the models provide Lab values ​​or spectra, but not device color values, making them unsuitable for direct conversions between devices ("device links"). This is because the color values ​​of the output device often follow special separation rules (see below), and typically require specific processing to improve quality. Nevertheless, conversion to device color has become very common.

[0019] As already detailed, the color characterization of overprinted inks is mapped using a table that converts the input print color components into m non-device-dependent values ​​(e.g., CIELAB color values ​​or spectral reflectance) or m device color values ​​of another output device. These output values ​​are tuples, e.g., 3 Lab components, m ink components, or m spectral value components (for m wavelength bands). The calculation of these tuples is performed component by component.

[0020] These tables are typically based on color combinations from test charts; that is, they only exist for color combinations that were printed and colorimetrically measured in print tests. Such tables may be saved in ICC profile format and used for conversion.

[0021] In many cases, CMYK test charts are used to accurately sample the color gamut of these four key colors. When CMYK is further complemented with process colors to extend the color gamut, orange or red (here, O), green (G), and blue or purple (here, V) are often used. Each of these colors is intended to extend the chromaticity sector (ECG, extended color gamut). A well-known process ("Equinox" (trademark)) uses a CMYK test chart and replaces each color with its complementary color during printing. This results in, for example, OMYK (replacing C with O), CGYK (replacing M with G), and CMVK (replacing Y with V). Thus, test charts of conversion tables for the CMYK, OMYK, CGYK, and CMVK color groups are also obtained. Furthermore, in packaging printing, if there are important and frequently used brand colors, etc., and you want to use them in matching prints, it is common to replace the black printing color in CMYK with another dark color such as dark blue, dark green, or dark brown. This allows the use of established color processing methods such as 4D CMYK.

[0022] However, if there are other additional colors (X, ...) that are not used regularly in the process but only for individual jobs, the number of combinations becomes too large to make sense to print as a test chart (e.g., CMY+X ​​with many different X's). Such colors are usually characterized only on a case-by-case basis, for example, by licensing reference data for Pantone® colors.

[0023] Print data typically uses a maximum of four colors simultaneously, and as a result, color characterization is often limited to a maximum of four colors at a time. Layout objects only require a maximum of four source color spaces, but elements with more than four colors need to be converted. Two common causes are trapping and image scaling due to resampling (see Figure 2).

[0024] Traps are designed to prevent so-called "flashes" caused by registration errors in traditional printing processes by enlarging the contours of vector objects that touch each other within the layout. This enlargement causes objects to overlap to some extent. This overlap may contain more print colors than the individual objects (e.g., merged quantities). Proofing applications, in particular, need to verify these important new edge areas. Obtaining a signal color instead is unacceptable; it indicates a lack of information regarding this overlap.

[0025] When processing pixel data (from images or rasterized vector data), resolution adjustments are often performed. Next, adjacent pixels are proportionally blended together through rescaling (bilinear or bicubic scaling, anti-aliasing). This includes the sum of the ink amounts of the contributing pixels.

[0026] Therefore, in both vector and pixel data, even if the layout contains objects with four or fewer colors, the input data being converted may still have five colors. However, if there is no conversion rule to convert, for example, five colors from the source color space to m values ​​in the destination color space, then traditional conversions using conversion tables are impossible.

[0027] This is generally true when converting color data from n colors in the source color space to m colors in the destination color space, if the combination of color data from the source color space for the n colors includes both a portion or subgroup where the conversion table can be used and a portion or subgroup where the conversion table cannot be used.

[0028] Patent Document 3 discloses a method that enables the conversion of n colors > 4 colors, requiring the availability of n color conversion data with coarse resolution in addition to accurate 4-color conversion. This method improves upon this by using multiple interpolations to first obtain a rough result from a coarse n-dimensional table, combine it with interpolation of an accurate 4-color table, and then combine it with further interpolation of intermediate results. However, this method is not applicable without n color conversion tables. However, generating such tables is based on multiple measurements of printed materials with many colors, as suggested, and its production requires appropriate printing forms and setup times for each color combination, which is very uneconomical when many common spot colors are used, as described above. It is desirable to be able to obtain an ideal result even when tables for n colors are not known. [Prior art documents] [Patent Documents]

[0029] [Patent Document 1] German Patent Application Publication No. 102004003300 Specification [Patent Document 2] U.S. Patent No. 5892891 [Patent Document 3] U.S. Patent Application Publication No. 2011 / 0007332 [Non-patent literature]

[0030] [Non-Patent Document 1] Boll H, “A COLOR TO COLORANT TRANSFORMATION FOR A SEVEN INK PROCESS”, SPIE Proceedings, IEEE, USA, Vol. 2170, February 1, 1994, pp. 108-118. [Non-Patent Document 2] Adobe,PDF Blend Modes,Addendum to PDF Reference 5th edition,version 1.6,2006,p. 2,p. 6 [Overview of the Initiative] [Problems that the invention aims to solve]

[0031] Based on the prior art described above, the present invention addresses the problem of improving a method for performing color space conversion using conversion rules, even in the case of a combination of color data from n colors in a source color space, which includes both components or subgroups for which a conversion table is available and components or subgroups for which a conversion table is not available. This allows for high-precision conversion to m values ​​in the destination color space, resulting in a result as close to the original as possible. [Means for solving the problem]

[0032] A technical solution to this problem is provided by a method having the features described in claim 1 of the claims. Further advantages and features are revealed in the dependent claims.

[0033] This invention provides a fast estimation method for assembling mixtures from known subcomponents. For example, if the input requests a mixed CMYKO, but only CMYK and OMYK interpolation tables or ICC color profiles are available, the entries are cleverly combined.

[0034] According to the present invention, a method is proposed for converting color data from a source color space with n colors to a destination color space with m values ​​using the conversion rule TRV. Here, color i is one of the n colors in the source color space, and for some combinations of the n colors in the source color space, the color components q(1), q(2), ..., q(n) are assigned to the values ​​z(1), z(2), ..., z(m) in the destination color space. This method is a) Color component q KB (1) ~ q KB Among the combinations KB of n colors in the supplier's color space having (n), m components z in the destination's color space KB (1)~z KBFor those combinations of (m) that are not assigned via the TRV, i) Color component q that does not include the component of color i KB (j) ( j is i a different value) for the remaining combinations, if a combination of the color spaces of the suppliers having components z(1) i , z(2) i , ···, z(m) i is assigned, ii) They are identical to each other with one exception. Color components q(1)~q(n) no group combination exists Two more death , Each is assigned a combination of components z(1) to z(m), and the color component q(1) ~q(n) 1 and q(1) <e000013> ~q(n) 2 The two combinations are the color component q(i) of color i. 2 =Print color value FA>0 and q(i) 1 The only difference is that it is equal to 0. 2 re 1) For the combination q(1) 1 =FA having color component q(i) 1 ~q(n) 1 is assigned the corresponding combination of the m values of the color spaces of the suppliers, which form the color dataset Z1, z(1) 1 , z(2) 1 , ···, z(m) 1 be 2) For the combination q(1) 2 =0 having color component q(i) 2 ~q(n) 2 is assigned the corresponding combination of the m values of the color spaces of the suppliers, which form the color dataset Z2, z(1) 2 , z(2) 2 , ···, z(m) 2 If so, ) iii) V(1) i =z(1) 1 / z(1) 2 , V( ) i =z(2) 1 ​ / z(2 ) 2 ,···,V(m) i = z(m) 1 / z(m ) 2 Each component z(1) of the m values ​​of the color dataset Z1 forms a set of factors. 1 , z(2) 1 ,···,z(m) 1 and the corresponding component z(1) of m values ​​in the color dataset Z2 2 , z(2) 2 ,···,z(m) 2 The ratio of each component Calculate this and use it as a factor V(1) i , V(2) i ,···,V(m) i and In that case, The above conditions apply, and the color component FA = q KB (i) A step of selecting a color i of combination KB such that > ​​0, b) Contains color component FA of color i mun When converting a color combination KB from a source color space with m colors to a destination color space with m components, Among the aforementioned combinations KB, the combination in which the color component q(i)=0 is set The z(1) of the color space of the source obtained from the conversion i ,z(2) i ,···z(m) i Factor V(1) i ,V(2) i ,···,V(m) i By multiplying by this, the factor V(1) i ,V(2) i ,···,V(m) i A process to apply the z(m) color space of the supplier. i However, V(1) i ,V(2) i ,···,V(m) i The process is characterized by being multiplied by and

[0035] The basic idea is to stepwise construct the output values ​​of unknown input combinations from the output values ​​of known input combinations, estimate the influence of the input color added at each stage (on the output value), and add this contribution using the method detailed below.

[0036] Given known input color combinations, i.e., output color values ​​available (e.g., through a table or interpolation), the desired contribution of a partial color within this color combination is determined as follows: The input combination "AB" is decomposed into the contribution of partial color "A" and the contribution of the remaining color "B". This can then be applied to another base "C" to show the color effect of "A" on base "B" and estimate an unknown combination "AC".

[0037] In the approach proposed here, Blend AB is considered an overlay of two layers, A and B, and uses "blending modes" established in computer graphics (W3C, PDF, Photoshop). The most important blending mode is "Multiply," which is based on linear color values ​​(in this case, "linear values") that are normalized between 0 and 1. "Multiply" simulates an ink overlay where the colors darken by multiplying the linear values ​​of the two layers in a light-related color space (often RGB in computer graphics). This produces an effect similar to overprinting.

[0038] Next, the linear value of mixture AB = (linear value of A) × (linear value of B) is applied. After determining the contribution of color A, the factor "linear value of A" can be solved. Both the linear values ​​of AB and B are known from a table containing AB. Optically, the linear value of A functions like the transmittance of the additional color, applied to the linear value of base B.

[0039] For color spaces that supply linear light, such as CIEXYZ and spectral reflectance, their output values ​​can be obtained directly as linear values. In the case of CIELAB, they are first converted to CIEXYZ, calculated in CIEXYZ, and then converted again at the end of the calculation.

[0040] For color spaces with nonlinear RGB inputs, calculations are performed in a linearized domain. For example, in a gamma-based RGB color space with values ​​between 0 and 1, the nonlinearity is eliminated by raising it to a power of gamma, and these linearized RGB values ​​are then used to return to the nonlinear RGB space by raising it to a power of 1 / γ.

[0041] In the color space of the ink-related supplier, print color values ​​x between 0 and 100% are inverted to their complementary values ​​(1 to x), linearized as in the case of RGB, and then nonlinearized again after offset. These complementary values ​​are the linear values ​​of the multiplication. Therefore, calculations using print color values ​​correspond, in formula terms, to the blending mode "screen," also called "inverse multiplication." This follows the normal processing in PDFs for RGB or DeviceCMYK (Non-Patent Literature 2). The complementary values ​​again act like transmittance factors, and therefore the contribution of the ink itself acts to absorb.

[0042] This approach is similar to the known concept of ink reception in printing by Preucil, where the linear color values ​​of the first printed ink B and the ink A printed on top, in this case the reflectance values, are measured individually and also measured in the combined print AB. The latter measurement is product approach AB ≈ A fa It is approximated by ×B. Here, the unknown ink acceptance factor "fa" is determined. "fa" is corrected by the film thickness <1, which is mainly A reduced. In contrast, the present invention determines the color effect of A from the AB = A × B approach using known linear values ​​of AB and B.

[0043] This specification proposes a method for determining the color effects of several colors A within a known combination AB, and then applying these effects to other colors C to estimate the output value of combination AC.

[0044] In its simplest form, the effect of each input color on the substrate can be individually determined by comparing the output value of the input color value to the output value of the substrate (input color value zero). The difference in output values ​​is a measure of the color effect of the input color in the quantity described by the value. Then, all these effects are added together.

[0045] However, a further proposal of the present invention significantly improves the estimation of color combinations by adding individual inks to the substrate in stages during the printing sequence. That is, when applying individual inks to the substrate one by one, process-specific ink acceptance characteristics are taken into account, for example, when overprinting solid dark colors wet-on-wet in offset printing, and a transfer factor, i.e., a linear value, is applied exponentially to the ink acceptance by correcting for the process-specific ink acceptance characteristics, which are said to be around 75%.

[0046] On the other hand, if not only the single color but also the color combinations in the subspace table are known, according to the proposal in this invention, it is better to directly read the interactions of as many subcolors of the input combination as possible from the table, which exists as known output values, rather than estimating from the single color. First, select a color group for which the subspace table is available. For example, select one whose input color value covers as much of the overall proportion of the input combination as possible. Next, from the remaining input colors, identify, for example, the one whose input color value has the largest proportion in the combination, and select a subspace table containing this color (ideally including a large overlap with known colors). In this subspace, read the output color values ​​with and without this color and compare them again. The difference is a measure of what happens when this color is added to the given color value. Therefore, apply this difference to the output value of the initial color group, resulting in the added color. This is then a new initial combination until the next remaining color is identified and all the colors of the input combination are included.

[0047] The method according to the present invention is proposed in general form as follows.

[0048] For a given input, a transformation (such as a table) is available, and color groups containing some of the input combinations are searched for. (Complete combinations are inferred and therefore not included.) This set of color groups is processed step by step. For example, one color group is selected as a starting point, and in the first stage, a second color group is selected to be used to add to the input colors. In subsequent stages, more color groups are selected until all the colors that occur in the input are included.

[0049] The following processes occur at each stage: First, common print colors (intersections) are identified from the color groups included in the stage. These are at least two, and at most an entire set. If there are no common inks, the intersection is an unprinted subset, i.e., a zero entry in the table.

[0050] Therefore, on the one hand, each color group in the stage contains not only common printing inks but also additional printing inks. For each color group, these additional, i.e., non-common printing inks are identified.

[0051] For each color group, the portion included in the input is considered a mixture. The mixture is decomposed into the contribution of the general color representing the "base" in the above calculation and the contribution of the additional ("added") color. This gives a measure of the effect of the color (on the base).

[0052] In this way, the color contribution of color component A in mixture AB to base B is determined for each group in the color space to which the output color is supplied. The important point is that this contribution can then be applied to other bases, i.e., the constituent bases. Thus, for each color group involved, the contribution of the common color as a base and the contribution of the additional color as a complementary color are fixed. Now the complementary contribution can be applied to other color groups at this stage. This yields one or more estimates for previously unknown color mixtures.

[0053] If several decompositions occur in a stage, and therefore alternative combination composition methods are possible, these are weighted and averaged to the final result of the stage. Useful weights are the relative certainty of the estimate (such as the distance from a known partial result). The following example illustrates how avoiding jumps improves the quality of the color transition transformation result.

[0054] The averaging process completes the stage. The final result of the stage is offset by further color groups in the next stage, as newly generated color groups through estimation, and more input colors are incorporated until all combinations of input colors are finally estimated. In this way, the mixture is constructed step by step until all inputs are complete.

[0055] The following examples demonstrate different strategies using the CMYK and OMYK color groups, representing different compromises between efficiency and accuracy. Other strategies are possible from a combinatorial perspective of decomposition, and the implementation of these strategies is not limited to those presented here.

[0056] The target input is a combination of CMYK+O colors and the ratios of 30%C, 60%M, 10%Y, 20%K, and 50%O. Tables exist for CMYK and MYKO, but not for CMYKO. For simplicity, it is assumed that the tables already contain linear output values ​​related to light.

[0057] The fastest method is to directly use the first four CMYK color groups and mix only the contribution of the missing color (in this case, only orange). Therefore, the output values ​​of the CMYK color group 30%C, 60%M, 10%Y, and 20%K are read first.

[0058] The contribution of the remaining color in 50%O is determined as follows:

[0059] The common color between CMYK and MYKO is MYK. The additional color for CMYK is C, and the additional color for MYKO is O. MYK is considered to be base B with values ​​of 60% M, 10% Y, and 20% K, and the output value is read. The output value of mixture AB, which has values ​​of 60% M, 10% Y, 20% K, and 50% O, is also read. The output value of mixture AB is read, and the transmittance factor, whose base changes with the addition of color quantity A at 50% O, is the quotient of the output values ​​of AB and B.

[0060] This factor is applied (multiplied) by the above readings of the output values ​​for the CMYK color groups 50%C, 60%M, 10%Y, and 20%K to obtain the desired result for the sum of the CMYKO color values.

[0061] The advantage of this method is that you can move from left to right in order without making any decisions, read the value of the first color listed in the table, and then apply all the colors added to the right as relative factors from your own table (A=AB / B).

[0062] Alternatively, instead of using the common color of B (MYK in this case), only the unprinted subset is used. AB = A = 50%O table value, and the effect of 50%O is the quotient of the 50%O table value and the table value of the unprinted paper.

[0063] However, it can be imagined that orange will have a different effect on white paper than it does in the presence of many other colors. Here, we are trying to explore the effect in the presence of 30+60+10+20% CMYK color amounts. This effect is unknown, and no table exists. For a realistic estimation, it is clear that we should use the possible different effects of 50% O on a 60+10+20% CMYK color set, rather than the possible strong effect of 50% O on paper.

[0064] To improve accuracy, the size of the added contribution is considered. Adding a large amount of 50% O coating may result in lower accuracy than adding a small amount of 30% C coating. In other words, starting with MYK60+10+20+50, the ink coverage of the above CMYK30+60+10+20 is 120%, while the table already provides 140% ink coverage as specific knowledge. This improvement is incorporated in the additional sorting stage.

[0065] However, this procedure, which depends on the magnitude of the contribution, involves a priority order, which becomes problematic when the two values ​​converge on the gradient. For example, suppose cyan increases from 30% to over 50% per pixel. In a sort based on the magnitude of the contribution, MYKO is selected as the base for the first pixel, and the smaller C value is added. When C is equal to or the smallest C is large, the base is changed to CMYK because O's contribution is small. In general, the result of base CMYK + contribution 50% O is not equal to the result of base MYKO + contribution 50% C. Therefore, a jump occurs. This jump is an undesirable result.

[0066] Therefore, to further improve quality, a method is employed that uses a combination of both base and contribution, as described above, and weights and averages them. This ensures that the subsequent transition is stable.

[0067] In a further example, the process flow of the method according to the present invention is demonstrated. The input color data is from a source color space having four color components q(1), q(2), q(3), and q(4). These are converted to a destination color space having three color values ​​z(1), z(2), and z(3).

[0068] The following transformation mappings can be used for color component combinations in the source color space: T(1)[q(1),q(2),q(3)],T(2)[q(1),q(2),q(4)],T(3)[q(1),q(3),q(4)],T(4)[q(2),q(3),q(4)],T(5)[q(3),q(4)].

[0069] The input dataset contains the values ​​q(1), q(2), q(3), and q(4): 20406080.

[0070] First, we select color i. The required assignment conditions are met for all colors 1-4, and color 1 is selected. This is because its color component q(1) is the smallest, and therefore, a smaller impact is estimated and applied to the color combinations of the known large supply destinations of the color components of colors 2-4.

[0071] First, there exists a value Z for which T(4) exists. i The combination (without the color part for color i=1) is, 04060 80, which relates to the maximum value pairing. This gives the value z(1) i ,z(2) i ,z(3) i This can be obtained. 04060 80 22,763 19,558 6,670

[0072] Based on this, we determine the influence of the color component FA=q(1)=20.

[0073] To determine the influence of q(1), the combination of values ​​20060 80 with q(1)=FA, where T(3) exists, is selected. This allows z(1) 1 ,z(2) 1 ,z(3) 1 (Color dataset Z1) is obtained. 20060 80 29,779 22,759 5,153

[0074] Next, the combination 0060 80 is selected, where T(5) exists, q(1)=0, and all other values ​​are equal. This gives z(1) 2 ,z(2) 2 ,z(3) 2 (Color dataset Z2) is obtained. 0060 80 42,435 31,883 6,821

[0075] V is a set of ratios of two values. i A formation is created. 0.701 0.714 0.755

[0076] These factors V i This is applied during the conversion. Therefore, the base color value 04060 80 22,763 19,558 6,670 Then, multiply the obtained results z(1), z(2), and z(3) by the factor. 204060 80 15,948 13,961 5,039

[0077] This invention describes a practical and feasible solution for those skilled in the art that enables sufficiently fast and high-quality color space conversion to a color space, even with very high-resolution layouts and a large number of input colors, in a reliable printing system. [Brief explanation of the drawing]

[0078] Further advantages and features will be explained below with reference to the drawings. The drawings are as follows:

[0079] [Figure 1] Figure 1 is a flowchart illustrating the process steps. [Figure 2] Figure 2 is a flowchart for estimating the missing colors. [Figure 3] Figure 3 shows what happens when color data of four dimensions or more occurs. [Modes for carrying out the invention]

[0080] According to Figure 1, from the color spaces of n color sources, there exists an input combination KB of color data 101 having color components Q={q(1),q(2),···,q(n)}, which is converted to the destination color space having m combinations of values. The color components that do not disappear in this combination form a so-called color group. In step 102, the color i is searched from a color group to which FA=q(i)>0 and specific conditions apply. 1) Color component q(i) i Subset Q of color data 101, where =0 is set. i (103) Output Z from the color space of the supplier i (107) must be assigned by conversion rule TRV106. Therefore, it shows the conversion result for a color group where color i does not exist, and will later be complemented by the influence of color i. 2) q(i) 1 =FA, or any other input Q1(104) must exist which is assigned output Z1(108) by conversion rule TRV106, i.e., the conversion result of another input having the same color component FA of color i. 3) The part with color i should be q(i) 2 An input Q2(105) identical to Q1 is formed, except that it is equal to 0. Output Z2(109) is assigned to this input by the transformation rule TRV106, and therefore shows the transformation result of this other input without the influence of color i.

[0081] If this color i is present, the influence of its color component FA is the ratio V of output pair Z1 and Z2, regardless of the presence or absence of color component FA. i This is estimated from (110). This effect can be applied to all combinations where there is no assignment of color share q(i) = FA, and of course, especially to input combination KB. For this purpose, in step 111, output Z without the contribution of color i is made i The ratio of each component V i Multiply by (110). The resulting value 112 is formed, which is the estimated target color value for the entire color group input combination KB.

[0082] In other words, given that the output is assigned to this smaller color group, the color group of the input combination is reduced in size by removing color i, and then color i is added by estimation so that the original color group is completed again. According to the present invention, this reduction operation can be performed multiple times, and an empty color group, i.e., an unprinted subset, can be reached in a maximum of n steps. From there, each individual color can be added one by one as needed to obtain a complete color group. This is shown in Figure 2.

[0083] As shown in Figure 2, there are n color data 201 from the color space of the source color, which are transformed to represent the input. In step 202, a color group is searched for which source color data 203 is available in the transformation rule. This color group is a subset of the color data 201 and is added to the full set step by step, with the source color data 203 being added to the full output. In step 204, if the current color group is already complete, the output 205 is ready. Otherwise, one of the missing colors, i, is selected in step 206. For this one, a pair of two color datasets Q1 and Q2 is selected. That is, in step 207, m values ​​z(1) from the source color space are selected, which include a given color portion FA=q(i) of color i and form a color dataset Z1(208). 1 , z(2) 1 ,···,z(m) 1 A color dataset Q1 is a combination of these, and in step 209, m values ​​z(1) of the destination color space contain the same color components as Q1 for all colors except color i, with the component of color i being equal to 0, forming a color dataset Z2(210). 2 , z(2) 2 , z(m) 2A pair is selected from color dataset Q2, which contains combinations of i and i. Both selected datasets, a color group with a proportion of i and a color group without a proportion of i, are color data for n colors in the source color space. In step 211, the ratio of m values ​​in color dataset Z1(108) to the m values ​​in color dataset Z2(210) is calculated for each component. These ratios V i As an estimated effect of color i, it is applied component by component to the current output in step 212, and color i is added to the current color group (213). This step is repeated as needed until the color group is complete, i.e., until the result of conversion 205 is available.

[0084] Figure 3, in addition to the explanation on page 7, shows the occurrence of overprinting with more than four colors, even though individual objects do not use more than four colors. A stylized package design 201 is shown, having an image area 202 constructed with four CMYK colors and a logo area 203 composed of spot colors. A magnified view of the design 204 shows that the contours of the CMYK image and the spot color rectangle do not overlap, but merely touch. Therefore, there are no five-color areas in the design. Only so-called trapping 205 in printmaking is used as described above to avoid registration-related flashing. Magnifying one of the contours creates an overprint area 206 of the spot color in CMYK, where five colors appear simultaneously. Independently, halftoning with anti-aliasing 207 can also mix halftone dots from portions of the areas belonging to them. Thus, an additional five-color pixel 208 with CMYK and spot color portions is created.

Claims

1. A method for performing computer-assisted conversion of color data from the source color space to the destination color space using conversion rule TRV, The color space of the supplier consists of n colors, and the n colors are included in the combination of color components q(1) to q(n) of each represented point. The color space of the supply destination is composed of m values, and the m values ​​can be combined to form a combination of components z(1) to z(m). - The m combinations of the components in the color space of the supplier are assigned to at least some combinations of the n colors in the color space of the supplier via the conversion rule TRV. The method is, a) Color component q ＫＢ (1) to q ＫＢ Among the combination KB of the n colors in the color space of the supplier having (n), the m components z of the color space of the destination ＫＢ (1) to z ＫＢ For combinations of (m) that have not been assigned via the TRV, i) Color component q that does not contain color i ＫＢ The remaining combinations of (j) (where j is a different value from i) include component z(1) ｉ , z(2) ｉ , ..., z(m) ｉ When a combination of the color spaces of the supplier having the above is assigned, ii) With one exception, there are two more combinations of color components q(1) to q(n) that are identical to each other. A combination of components z(1) to z(m) is assigned to each, and color components q(1)1 to q(n) １ and q(1) ２ to q(n) ２ The two combinations of and are different only at the points where the color component q(i) of color i １ = printing color value FA > 0 and q(i) ２ = 0 1) Color component q(i) １ = Combination q(1) with print color value FA １ ~q(n) １ This includes the component z(1) of m values ​​in the color space of the supply destination, which forms the color dataset Z1. １ , z(2) １ , ..., z(m) １ The corresponding combination is assigned, 2) Color component q(i) ２ Combinations q(1) that have = 0 ２ ~q(n) ２ This includes the component z(1) of m values ​​in the color space of the supply destination, which forms the color dataset Z2. ２ , z(2) ２ , ..., z(m) ２ If a corresponding combination is assigned, iii) V (1) ｉ = z(1) １ / z(1) ２ , V(2) ｉ = z(2) １ / z(2) ２ , ..., V(m) ｉ = z(m) １ / z(m) ２ Each component z(1) of the m values ​​of the color dataset Z1 forms a set of factors. １ , z(2) １ , ..., z(m) １ and the corresponding component z(1) of m values ​​in the color dataset Z2 ２ , z(2) ２ , ..., z(m) ２ The ratio of each component is calculated and this is factor V(1) ｉ , V(2) ｉ , ..., V(m) ｉ In that case, The conditions i) to iii) apply, and the color component q ＫＢ (i) A step of selecting the color i of combination KB having a print color value FA > 0, b) When converting the color combination KB of the supplier's color space, which includes the color component q(i) of color i, to the destination's color space, the z(1) of the destination's color space obtained from the conversion of the combinations of the supply source KB in which the color component q(i) = 0 is set. ｉ , z(2) ｉ , ..., z(m) ｉ Factor V(1) ｉ , V(2) ｉ , ..., V(m) ｉ By multiplying by this, the factor V(1) ｉ , V(2) ｉ , ..., V(m) ｉ A step of applying the color space of the supplier z(1) ｉ , z(2) ｉ , ..., z(m) ｉ However, V(1) ｉ , V(2) ｉ , ..., V(m) ｉ The process that is multiplied by, including, A method characterized by the following features.

2. The above method is applied to partial combinations of k < n colors for a given combination of n colors having color ratios q(1) to q(n). The method according to claim 1, characterized in that

3. The above method is applied to a series of partial combinations of k < n colors up to k = n, and the resulting factors are multiplied sequentially. The method according to claim 2, characterized in that

4. Colors with smaller color portions than other colors are preferentially selected from candidate colors i, and the size of a color's color portion is considered to achieve higher accuracy than when selecting a color with a larger color portion than the selected color. The method according to any one of claims 1 to 3, characterized in that

5. For several different candidate colors i1, i2, ..., different estimated values ​​z(1) ｉ１ , ..., z(m) ｉ１ , z(1) ｉ２ , ..., z(m) ｉ２ The results, such as those above, are weighted and averaged, selecting weights such that the weight of each estimate increases as the ratio of each color q(i1), q(i2), ... decreases. The method according to any one of claims 1 to 3, characterized in that

6. The components z(1) to z(m) of the color space of the aforementioned supplier are device-dependent values. The method according to any one of claims 1 to 3, characterized in that

7. The components z(1) to z(m) of the color space of the aforementioned supplier are device-independent values. The method according to any one of claims 1 to 3, characterized in that

8. The aforementioned conversion rule TRV is in the form of a conversion table. The method according to any one of claims 1 to 3, characterized in that

9. The method is performed on a computer unit by control software, the computer unit comprising an input unit that provides digital color data for a project, an input unit that provides digital color data for the color space of the source of the project, an output unit that outputs a converted value for the color space of the destination, and a memory that stores a conversion table, and by applying the method using the conversion table, the control software generates values ​​for the color data of the color space of the source of the project and provides them as a dataset. The method according to any one of claims 1 to 3, characterized in that

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