Method for converting color space

Abstract

A method for converting color values of a first device-dependent color space into color values of a second device-dependent color space, such that the visual impression of the colors reproduced in both color spaces is substantially the same, is described, wherein at least one absolute rendering intent is combined with at least one relative rendering intent when converting the color values.

Description

Technical Field

[0001] This invention relates to a method for converting color values ​​in a device-dependent first color space into color values ​​in a device-dependent second color space. For example, this method is used in the printing industry to achieve a balance in visual color impressions. Background Technology

[0002] In the printing industry, a printing template is created for a printed page containing all the elements to be printed, such as text, graphics, and images. In the case of electronically produced printing templates, these elements exist in the form of digital data. For images, this data is generated, for example, by scanning the image point by point and line by line with a scanner, breaking down each image point into color components, and digitizing these color components. Typically, an image is broken down in the scanner into the color components red, green, and blue (R, G, B), that is, into components of a three-dimensional color space (RGB color space). However, for color printing, such as in a printing press, other color components are required. In four-color printing, these are the printing colors cyan, magenta, yellow, and black (C, M, Y, K), that is, components of a four-dimensional color space (CMYK color space).

[0003] During the printing process, image data must be converted from the scanner's RGB color space to the CMYK color space used in the printing process. If the CMYK image data generated for the printing process is to be presented on a screen (e.g., an LCD) as a so-called softproof, or if the image data is to be output as a proof beforehand, for example, on an inkjet printer, additional color conversion is required to ensure that the visual impression of the colors corresponds as closely as possible to subsequent printing processes used for the version printing.

[0004] Such color conversion is necessary in the printing industry because all devices and processes have certain limitations and peculiarities in presenting and reproducing colors. Therefore, different color spaces exist for different devices and processes, such as scanners, monitors, proofing output devices, and printing processes. These color spaces describe the color attributes of the device or process in the best possible way, and are called device-dependent color spaces.

[0005] In addition to device-dependent color spaces, there also exist device-independent color spaces, which are based on the so-called standard observer's human visual attributes. Such color spaces are, for example, those standardized by the CIE (Commission Internationale d'Études en Sciences et al.). The XYZ color space is defined, or the L*a*b* color space derived from it. L*a*b* color components can be converted to XYZ color components, and vice versa. If you want to know whether two colors are perceived as the same or different by the human eye under the same environmental conditions, especially under the same lighting, it is sufficient to measure the XYZ color components or the L*a*b* color components. The L*a*b* color components form a color space with one luminance axis [L*] and two color axes [a*, b*], which can be imagined on a plane of a color circle extending from the center point of the luminance axis.

[0006] US 7,161,710 B1 describes the conversion of color spaces using a rendering intent, for example, for printing applications.

[0007] DE 103 22 378 A1 describes a method for color conversion between color spaces using a color profile, which utilizes rendering intents.

[0008] A device or color processing procedure can be characterized in terms of its color attributes by assigning the XYZ color components, as perceived by humans in colors produced using these value combinations, to all possible combinations of values ​​within the device-dependent color space. In printing, different combinations of CMYK values ​​produce different printed colors. Using a colorimeter, the XYZ components of the printed color can be determined and assigned to combinations of CMYK values. This assignment, which associates device-dependent colors produced by the device or process with a device-independent color space (XYZ or L*a*b*), is called a color profile, or, in the case of a printing process, an output color profile. The definition and data format of color profiles have been standardized by the ICC (International Color Consortium). In an ICC color profile, the color space assignment is stored in two directions, for example, assigning XYZ = f1 (CMYK) and reversing the assignment to CMYK = f2 (XYZ). The assignments determined using the color profile can be implemented using lookup tables. For example, if XYZ color components are to be assigned to CMYK color components in a printing process, the table storage unit must have a storage space for each possible combination of CMYK color component values, storing the assigned XYZ color components in that space. However, this simple assignment method has the disadvantage that the table storage unit can become very large, thus often requiring the use of interpolation methods.

[0009] The allocation between device-dependent and device-independent color spaces given in the color profile can be used for color conversion between device-dependent color spaces, thereby, for example, converting the color values ​​[C1,M1,Y1,K1] of the first printing process here to the color values ​​[C2,M2,Y2,K2] of the second printing process, so that the second print has the same color as the first print based on visual impression.

[0010] Figure 1The diagram schematically illustrates the principle of this color conversion for matching printing processes according to the prior art. A first color conversion 1, from color values ​​[C1, M1, Y1, K1] in the first printing process to XYZ color values, and a second color conversion 2, from XYZ color values ​​to color values ​​[C2, M2, Y2, K2] in the second printing process, are performed sequentially. The two color conversions 1 and 2 can also be combined into an equivalent color conversion 3, which directly assigns color values ​​[C1, M1, Y1, K1] and [C2, M2, Y2, K2] to each other. Since color values ​​[C1, M1, Y1, K1] and [C2, M2, Y2, K2] that yield the same XYZ color values ​​are assigned to each other through a device-independent XYZ intermediate color space, the printed colors assigned in the two printing processes are perceived to be visually identical to a large extent within the printed color range.

[0011] In the ICC specification, the device-independent color space (where device-dependent color spaces are interconnected in color conversions through the device-independent color space) is called the Profile Connection Space (PCS). The Profile Connection Space is the interface between the device and process color profiles. This Profile Connection Space is defined as the ideal reflective stencil color space for an ideal viewing environment. It is based on the standard color spaces CIE 1931 XYZ and CIE 1976 L*a*b* defined by the CIE. The white point of the Profile Connection Space is defined by the standard illuminance D50 commonly used in graphics technology, i.e., illuminance using a 5000 Kelvin illuminant. This white point WPD50 has XYZ color values:

[0012] X WPD50 =0.9642 Y WPD50 =1,0000 Z WPD50 =0,8249

[0013] There are variations in the allocation between the device-dependent color space and the profile connection space described in the ICC profile, which are used according to the reproduction intent (rendering intent). These rendering intents are referred to as "relative colorimetry" or "relative," "absolute colorimetry" or "absolute," "perception," and "saturation." Furthermore, they differ in what is called gamut mapping embedded in the color profile. Gamut mapping represents a method or strategy that matches different color ranges in the device-dependent color space to each other. For example, not all bright and saturated colors that can be rendered on a display are printable, especially when printed on inferior and relatively gray paper (e.g., newsprint). The unprintable display colors must then be converted to similar colors on the edges of the printable color range through the allocation of the color profile, resulting in an overall harmonious color impression without subjectively perceived color distortion.

[0014] By leveraging perceptual rendering intents, the aim is to consider other attributes important for image reproduction, such as contrast, detail rendering, and realistic viewing environment, in addition to visually perceived color equivalence when mapping to the configuration file's connection space. Saturation rendering intents primarily preserve pure and saturated colors and are applied in the reproduction of graphics.

[0015] For example, relative rendering intentions are used to map different printing processes to each other using color conversion, where the color and brightness ranges of the target process are fully utilized. Specifically, this means mapping the white point of the source process, i.e., the paper white, to the white point of the target process. If the white point of the target process is brighter than the white point of the source process, then after color conversion, the colors are reproduced brighter and more vividly when printing using the target process.

[0016] Conversely, with the help of absolute rendering intent, the white point and XYZ color values ​​of the source process are reproduced unchanged when printing using the target process during color conversion. This is on the premise that the printable color gamut and brightness gamut of the target process are greater than those of the source process. Therefore, absolute rendering intent is used to reproduce the source printing process, for example, on a high-quality inkjet printer, as a color-accurate and verbindliche proof using the target printing process.

[0017] In the allocation table created for relative rendering intent, the XYZ color values ​​assigned to the device-dependent color values ​​in the profile join space are scaled such that the possible range of values ​​in the profile join space is fully utilized. Specifically, this means that the measured white point WP1 (media white point) in the device-dependent color space is assigned a white point WPD50 in the profile join space. If the white point WP1 of the source printing process (e.g., newspaper printing) has measured XYZ color values ​​[X...] WP1 ,Y WP1 Z WP1 When creating a color profile, all color values ​​[X1,Y1,Z1] measured for different combinations of values ​​[C1,M1,Y1,K1] are calibrated component-by-component on a test sample using the ratio of white point WPD50 and WP1 to obtain the color values ​​[X1,Y1,Z1] allocated to the profile's connection space. PCS1 ,Y PCS1 Z PCS1 ].

[0018] X PCS1 =X1 x X WPD50 / X WP1

[0019] Y PCS1 =Y1 x Y WPD50 / Y WP1

[0020] Z PCS1 =Z1 x Z WPD50 / Z WP1 (1)

[0022] Similarly, when creating a color profile for a target printing process (e.g., offset printing) with a white point WP2, the color values ​​[X2, Y2, Z2] measured for different value combinations [C2, M2, Y2, K2] are calibrated component-by-component using the ratio of the white point WPD50 and WP2 to obtain the assigned color values ​​[X2, Y2, Z2] in the profile join space. PCS2 Y PCS2 Z PCS2 ].

[0023] X PCS2 =X2 x X WPD50 / X WP2

[0024] Y PCS2 =Y2 x Y WPD50 / Y WP2

[0025] Z PCS2 =Z2 x ZWPD50 / Z WP2 (2)

[0027] Because of the Figure 1 When associating color profiles, group the same values ​​in the profile link space [X] PCS1 Y PCS1 Z PCS1 ] and [X PCS2 Y PCs2 Z PCS2 The colors are mutually assigned, therefore, when converting colors from the source process to the target process, the following relationship is derived based on the relative rendering intent:

[0028] X2 = X1 x X WP2 / X WP1

[0029] Y2 = Y1 x Y WP2 Y WP1

[0030] Z2 = Z1 x Z WP2 / Z WP1 (3)

[0032] The device-related color values ​​[C1, M1, Y1, K1] of the source process are thus converted into device-related color values ​​[C2, M2, Y2, K2] of the target process, such that the corresponding XYZ color values ​​are calibrated component-by-component compared to the white point values. In particular, from relation (3), the white point WP1 of the source process is converted into the white point WP2 of the target process.

[0033] If the white points of the source and target processes are relatively far apart, this simple scaling of the XYZ color values ​​(derived according to the ICC specification for relative rendering intent) is not optimal. In this case, although the XYZ color values ​​in the target process are linearly calibrated, the relative spacing of colors printed on media with different white points is not perceived as equivalent to that in the source process because the human visual system performs white point-related chromatische adaptation when observing the colors in the target printing process.

[0034] However, if no color value conversion occurs between different devices, the color value changes during the output process when the output is on the output device, provided the paper white is different from that used in the input process. Correspondingly, when the same printed image (e.g., in the form of a PDF file) is output on a darker substrate, a bright red that was printed on a light substrate will appear darker.

[0035] Methods known from the prior art for converting color values ​​have drawbacks. Therefore, it is particularly desirable to perform color value conversions without having to simulate the color of the printing material (paper white). Summary of the Invention

[0036] The objective of this invention is to describe a method for efficiently converting color values ​​between two different device-related color spaces and for obtaining the same visual impression in both color spaces.

[0037] Correspondingly, the present invention relates to a method for converting color values ​​in a device-dependent first color space into color values ​​in a device-dependent second color space, such that the visual impression of the colors reproduced in the two color spaces is substantially the same. This method involves using at least one rendering intent to describe the color values ​​when converting the color values ​​in the device-dependent first color space into those in the device-dependent second color space. Furthermore, when converting the color values ​​in the device-dependent first color space into those in the device-dependent second color space, at least one absolute rendering intent describing absolute color values ​​is combined with at least one relative rendering intent describing relative color values. The color values ​​in the device-dependent first color space are converted into those in the device-dependent second color space using color values ​​from an intermediate color space.

[0038] According to the invention, at least one absolute rendering intention is combined with at least one relative rendering intention. In a preferred embodiment, "combined" here means that the absolute rendering intention and the relative rendering intention are blended between the images of the two rendering intentions. In other words, an absolute colorimetric image (absolute rendering intention) is used in certain areas of the color space, and a relative colorimetric image (relative rendering intention) is used in other areas of the color space, especially in the case of the base background color, preferably paper white, and a blending is performed between them. This is because [C1,M1,Y1,K1] => [X,Y,Z] and [X,Y,Z] => [C2,M2,Y2,K2] (see...) Figure 1 The gradient is not linear, and further, the overlay gradient in [X,Y,Z], or more precisely, the color space coordinates based on the linearized Bradford-Transformation, work in conjunction with the exponent (see below), and only approximately applies to [C2,M2,Y2,K2]=a*[C2,M2,Y2,K2]. absolut +(1-a)*[C2,M2,Y2,K2] relativ The gradient parameter 'a' depends on its position in the color space.

[0039] In a particularly preferred embodiment, the combination of absolute rendering intent and relative rendering intent according to the invention means that, when converting color values ​​in a device-dependent first color space to color values ​​in a device-dependent second color space, a portion of the color values ​​in the device-dependent first color space is converted by at least one absolute rendering intent, and a portion of the color values ​​in the device-dependent first color space is converted by at least one relative rendering intent.

[0040] In a preferred embodiment, at least one absolute rendering intent and at least one relative rendering intent are colorimetric rendering intents, respectively.

[0041] In another preferred embodiment, when converting color values ​​in a device-dependent first color space to color values ​​in a device-dependent second color space, exactly one absolute rendering intent is combined with exactly one relative rendering intent.

[0042] According to the present invention, the conversion of color values ​​in a device-dependent first color space to color values ​​in a device-dependent second color space is performed via color values ​​in an intermediate color space. In a preferred embodiment, the intermediate color space is a device-independent color space (profile connection space). In a very particularly preferred embodiment, the color values ​​in the intermediate color space correspond to the D50 2° standard observer.

[0043] In another preferred embodiment, color values ​​in a device-dependent first color space are converted to color values ​​in an intermediate color space, and color values ​​in the intermediate color space are converted to color values ​​in a device-dependent second color space. In a particularly preferred embodiment, when converting color values ​​in the device-dependent first color space to color values ​​in the intermediate color space, at least one absolute rendering intent is combined with at least one relative rendering intent. In a particularly preferred embodiment, when converting color values ​​in the intermediate color space to color values ​​in the device-dependent second color space, at least one absolute rendering intent is combined with at least one relative rendering intent. In a particular embodiment, at least one absolute rendering intent is combined with at least one relative rendering intent not only when converting color values ​​to the intermediate color space, but also when converting color values ​​from the intermediate color space to color values ​​in the device-dependent second color space.

[0044] In the standard ICC profile, there are three conversion tables for four rendering intents: i) one conversion table for perceptual, ii) one conversion table for saturation, and iii) one conversion table for absolute and relative. In a preferred embodiment, rendering intents from two different conversion tables are not combined with each other, but only within the same conversion table. Correspondingly, for example, a combination of perceptual and absolute is not preferred.

[0045] Correspondingly, in a preferred embodiment, color values ​​in a device-dependent first color space are converted to color values ​​in an intermediate color space using at least one profile including at least one conversion table, and color values ​​in the intermediate color space are converted to color values ​​in a device-dependent second color space using at least one profile including at least one conversion table, wherein at least one absolute rendering intent and at least one relative rendering intent use the same conversion table. In a particularly preferred embodiment, the at least one profile is at least one ICC profile. The intermediate color space is then preferably a profile connection space (PCS).

[0046] In a particularly preferred embodiment, color values ​​in a device-dependent first color space are converted to color values ​​in an intermediate color space using at least one profile including at least one conversion table, wherein the absolute rendering intent and the relative rendering intent use the same conversion table. In another particularly preferred embodiment, color values ​​in the intermediate color space are converted to color values ​​in a device-dependent second color space using at least one profile including at least one conversion table, wherein the absolute rendering intent and the relative rendering intent use the same conversion table. It is also possible that the combination of absolute and relative rendering intents according to the invention is used simultaneously, not only when converting into color values ​​in the intermediate color space but also when converting out of color values ​​in the intermediate color space, using at least one profile having at least one conversion table.

[0047] In another preferred embodiment, the device associated with the first color space of the device is a different device from the device associated with the second color space of the device.

[0048] In another preferred embodiment, the conversion of color values ​​in a device-dependent first color space to color values ​​in a device-dependent second color space is performed during the printing method in which a digitally printed image is printed onto a substrate using a printing press, wherein the device-dependent first color space is the screen's color space and the device-dependent second color space is the printing press's color space. In a particularly preferred embodiment, the printing method is selected from: letterpress printing, offset printing, gravure printing, and combinations thereof, especially offset printing, inkjet printing, flexographic printing, screen printing, and gravure printing.

[0049] In another preferred embodiment, when converting color values ​​in a device-related first color space to color values ​​in a device-related second color space, no simulation is performed on the color values ​​of the unprinted substrate.

[0050] In another preferred embodiment, the color value of the unprinted substrate in a device-related first color space is mapped to the color value of the unprinted substrate in a device-related second color space.

[0051] In another preferred embodiment, the color value of the unprinted substrate is paper white.

[0052] In another preferred embodiment, the output is converted to have no printed dots by converting the color values ​​of a device-dependent first color space to color values ​​of a device-dependent second color space, and converting the brightest points of the device-dependent first color space to the brightest points of the device-dependent second color space. In other words, the conversion is performed such that the color values ​​corresponding to the white points in the input color space (e.g., unprinted paper when the printing press color space is the input color space) are also mapped back to the white points in the output color space. Thus, it means that for such color values, no color is printed onto the substrate in the output. In other words, the unprinted areas in the input remain unprinted areas in the output, and no printing of any dots is performed to simulate the darker white of the input paper.

[0053] In another preferred embodiment, at least one relative rendering intention includes black point compensation. This means that the darkest point of the input process is also mapped to the darkest point of the output process. Through this compensation, depth rendering is also preserved. Without this compensation, all colors darker than their corresponding darkest colors in the output color space after considering relative / absolute / combined color transformations will be clipped and trimmed to renderable chromaticity during the output process. The lower limit. Therefore, it is possible to then achieve an interrupted color gradient (Abrisse in...). ).

[0054] The subject of this invention is also a printing press used to perform the method according to the invention. Preferred printing presses for implementing the method according to the invention are offset printing presses, inkjet printing presses, flexographic printing presses, screen printing presses, and gravure printing presses.

[0055] The subject of this invention is still the application of the method according to the invention in a printing press. Preferred printing presses in which the method according to the invention is used are offset printing presses, inkjet printing presses, flexographic printing presses, screen printing presses, and gravure printing presses.

[0056] To date, the goal of rendering intents used in the prior art is an acceptable reproduction (photographic and photographic, saturation preservation = saturation and relative colorimetric rendering intent) or an absolute reproduction (absolute rendering intent) of the device-specific input chromaticity in the device-specific output chromaticity. Rendering intents “relative colorimetric” and “relative colorimetric with depth compensation” are here considered as a simple implementation of an acceptable mapping.

[0057] By definition, when simulating a process with a bright spot on a process with a dark spot, absolute rendering intent simulates the dark spot. This is generally undesirable in the packaging industry. In contrast, other rendering intents are designed such that the entire chromaticity of the input process is distorted into the output chromaticity. While this differs between the two forms of relative rendering intent... The process typically involves a simple multiplication of the white point and the darkest point in the XYZ coordinates, but a strategy using the corresponding ICC profile algorithm is employed to perceive or render the intended saturation as equal to saturation.

[0058] The implementation scheme that combines absolute and relative rendering intentions to form the brightest point of the chromatic body in the target process is unknown.

[0059] A method is known in the prior art in which the white of the paper has a white spacer in the chromaticity of the target process. However, simply replacing the brightest point of the chromaticity of the input process with the white spacer in the chromaticity of the output process is perceived as unattractive in the image. The transitions are particularly problematic. Specifically, transitions near the brightest point of the input chromaticity (e.g., in the case of a wedding dress or white shirt) are discontinuous in the target process's chromaticity at the transition between white and a section that is no longer filled with a white space. To date, no method has been known to achieve this using standard ICC profiles.

[0060] In a preferred embodiment of the invention, proofing data of a substrate having white point W1 should be output to a substrate having significantly different white points W2. ICC profiles describing the processes exist for both. Two colorimetric rendering intentions, "absolute rendering intention" and "relative rendering intention," are known.

[0061] Absolute rendering intent: To accurately reproduce the measured color values ​​(XYZ, L*a*b*) anywhere the target process allows.

[0062]

[0063] The output process simulates the white area of ​​the paper, meaning that in some cases (when the target paper is brighter or a different color than the output paper), white areas are also printed. If the target paper is too dark, the brighter areas are cut off.

[0064] Relative rendering intent: To replicate the measured color values ​​(XYZ, L*a*b*) relative to the white points of the corresponding process.

[0065] The color coordinates (x, y, z) are calibrated using the ratio of the white point's coordinates:

[0066]

[0067] The color coordinates (x, y, z) can be, for example, XYZ, or a coordinate system K that matches human eye sensitivity. In this case, the input values ​​and the color coordinates of the paper white must first be transformed into coordinate system K, and then the calibrated coordinates must be transformed back. In any case, the paper white remains unprinted during the target process. In the case of dark target paper, the hue will also be darkened accordingly.

[0068] However, what is generally desired is the following behavior: under this behavior, the hue is accurately reproduced with a clear, surface-printed quality (e.g., full-tone cyan or red composed of 100% magenta and 100% yellow), as in an absolute rendering intent. For white paper, this behavior should correspond to a relative rendering intent, so that the paper remains unprinted and the image is also preserved in light.

[0069] Without ICC-based color management, such behavior can be achieved on an offset printing press by using the printing plate of the output paper, inserting a sheet with another white spot W2, and attempting to approximate the standard full-tone and hue value increments used for the original paper as closely as possible when setting up the machine. This method is only feasible if the printing plate can be taken over. If the target process uses more or fewer printing colors than the output process, the spot colors in the output process are simulated by the printing colors in the target process, and the two processes differ not only in paper whiteness but also in process technical properties (position of full tones, overprinting behavior, etc.), or if the target process is a different printing technology, then this method will not work.

[0070] In the scheme according to the present invention, the following transformation method of color coordinates is used as a combination of (4) and (5):

[0071] (x2,y2,z2)=F((x1y1,z1)) (6)

[0073] This method demonstrates the desired behavior. The color coordinates x, y, and z can here be, for example, XYZ or a coordinate system that matches human visual perception.

[0074] The two limiting cases of the transformation are possible:

[0075] i) Relative behavior in white paper:

[0076]

[0077] ii) Absolute behavior when at least one color coordinate in the color coordinate system is smaller:

[0078]

[0079] and

[0080] By combining equations (7) and (8), we can provide the following expression for the transformation:

[0081]

[0082] The exponent α here depends on the color position and must apply: for white paper, α = 1, and if the color coordinates approach zero, then... In theory, the exponent α x α y α z They can be different. In the following text, the first option should be limited to the case where the exponent is the same for all components, i.e., α.x =α y =α z =α. Therefore, the transformation equation (10) becomes

[0083]

[0084] The challenge now lies in determining the color coordinates x1, y1, z1 and x... white.1 ,y white.1 ,z white.1 The exponent α of the function. Here, in addition to the boundary conditions derived from (7) and (8), other conditions should also be satisfied:

[0085] 1. For xyz1=xyz white,1 α = 1;

[0086] 2. and and

[0087] 3. For 0 and xyz white For all values ​​between 0 and 1, α should be between 0 and 1;

[0088] 4. For each component of xyz, α should be strictly monotonically increasing;

[0089] 5. α should be continuous;

[0090] 6. To enable the conversion and Precisely inversely related, it must be irrelevant whether α is determined with respect to the original coordinates or the transformed coordinates. That is to say, α(xyz1,xyz) white,1 )=α(xyz2,xyz white,2 );

[0091] Here, we introduce the abbreviation xyz = (x, y, z) for the entire coordinate vector.

[0092] To ensure that condition 6, which is independent of the transformation direction, is guaranteed, we first introduce a coordinate system with xyz coordinates. m =(x m ,y m ,z m An intermediate color space independent of the white point. If for each output white point W1 it is always mapped to this intermediate color space, at least formally, and from there to the coordinates of the target white point W2, then when determining the exponent in this intermediate space, it is guaranteed to be direction-independent in all cases.

[0093] In the following text, for clarity, we will first consider only the components x of the color space, and will initially describe the mapping from x = x1 or x2 to the intermediate space as...

[0094] x=f(x m x white ) := f(x m ) (12)

[0096] Instead of the notation with the exponent α, for the function f, the following conditions must now apply:

[0097] 1. f(0) = 0

[0098] 2.

[0099] 3. f(1) = x white

[0100] 4. Furthermore, in order to guarantee these conditions, the following scheme is used to determine f in the differential equation:

[0101]

[0102] The equation has the following general solution.

[0103]

[0104] And by substituting the boundary conditions, we can obtain...

[0105]

[0106] Equation (15) can now also be applied to the transformation from absolute color coordinates on paper white W1 or W2 to directions in the intermediate color space:

[0107]

[0108] If we now substitute the equation (16) of x = x1 into the equation (15) of x = x2, we obtain the following as a result of having x white,1 x1 to have x white,2 The transformation equation for x²:

[0109]

[0110] In order to be able to Equation (17) is written in formal form, and the result is obtained.

[0111] in,

[0112] Since the relationship between x1 and x2 in equation (17) is independent of the transformation direction through the construction of the intermediate color space and equation (18) is just another representation of this relationship, the independence from the transformation direction must also apply to equation (18).

[0113] Equation (18) can now be applied individually to the three color space coordinates x, y, and z. This yields the relative colorimetric behavior toward white and the absolute colorimetric behavior toward black. However, this method also has two special properties:

[0114] 1. The exponent α is different for x, y, and z, which may cause distracting color shifts.

[0115] 2. Once one of the coordinates x, y, or z has a value that is significantly different from 0, the coordinate is no longer converted in a manner similar to absolute colorimetry.

[0116] Therefore, a representative coordinate q with the following properties is introduced:

[0117] i)q can be transformed in the same way as x, y, and z, that is

[0118]

[0119] ii) q approaches 0 once at least one of the coordinates x, y or z approaches 0.

[0120] The coordinates that satisfy these conditions are the geometric mean of coordinates x, y, and z, etc.

[0121]

[0122]

[0123] Therefore, a common exponent α can now be determined for all coordinates, and the final transformation equation can be summarized as follows:

[0124]

[0125] in, and

[0126] It is important to note here that the xyz coordinates do not necessarily correspond to XYZ. If necessary, a linear transformation must first be applied to the XYZ coordinates, such as a Bradford matrix known in principle from existing technology, and then the inverse Bradford matrix must be applied after the transformation. In this case, the coordinates of the white point should, of course, be handled in a similar manner.

[0127] It's also important to note that the color coordinates (x, y, z) used should always be understood as absolute, meaning they are not calibrated using the coordinates of the white point. Since internal calculations using the ICC profile primarily use color coordinates in units of white points, this calibration must be adhered to. Because it's a simple multiplication and the white point is known at any given time, this does not imply any limitation on applicability.

[0128] For the purpose of considering the limiting case later, it makes sense to represent equations (22) and (23) in a centered color space coordinate system, i.e., using the coordinates of the white point.

[0129]

[0130] By substituting (24) into (22), we obtain

[0131] and Similar. (25)

[0132] For the exponent α, by substituting (24) into (23), we get

[0133]

[0134] If now we are using color space coordinates for centralization Defined as

[0135]

[0136] Then equation (25) becomes

[0137] and Similar. (28)

[0138] If the equation comes from (27) Substituting into equation (28), this can be further simplified by introducing a new abbreviation.

[0139]

[0140] Therefore, the transformation of the centralized coordinates can be compactly written as

[0141]

[0142] in,

[0143] and

[0144] The factor β in equation (31) can be calculated when undergoing the Ausetzen transformation, since these factors depend only on the color space coordinates of the white point.

[0145] Although the transformation F described by equations (30), (31), and (32) has the following property: the transformation from white point W1 to white point W2 and the transformation from white point W2 to white point W1 are exactly opposite to each other, that is...

[0146]

[0147] However, the result of the transformation is usually not independent of the following: whether the transformation from white point W1 to white point W3 is first performed to white point W2, or whether a direct transformation is calculated; that is, it is generally applicable.

[0148]

[0149] To create this property, the transformation can be modified as follows: instead of directly transforming between the output white point and the target white point, the white point of the illumination and viewing conditions D50 light type and 2° observer (D50O02) defined in the ICC specification is always selected as the intermediate point. Therefore, the transformation G is derived from the transformation F described in equations (30), (31), and (32) as follows:

[0150]

[0151] Typically, all centralized color space coordinates and It should lie between 0 and 1. Thus, equation (32) is only valid on this interval. However, in some cases, negative values ​​or values ​​higher than 1 may occur. This may happen, for example, when viewing self-illuminating colors. Another reason for irregularly centered color space coordinates may be, for example, the linear approximation of the Bradford transformation to color space coordinates when transforming extremely colorful colors. In order to continue the transformation of these values, colors with at least one non-positive color space coordinate should be subjected to absolute colorimetric treatment, and colors with a color space coordinate product greater than 1 should be subjected to relative colorimetric treatment.

[0152] Therefore, the transformation equation (30) can be modified as follows:

[0153]

[0154] similar.

[0155] The proposed method is based on determining, with the help of equation (32), a factor for scaling the geometric mean q of the color space coordinates. And based on the exponential βx β x β z This factor is plotted onto a color space coordinate system. This method is initially unsuitable precisely when truly distinct white points W1 and W2 have the same geometric mean.

[0156] x white，1 ·y white，1 ·Z white，1 =x white，2 ·y white，2 ·Z white，2 (37)

[0158] as well as

[0159] (x white，1 ≠x white，2 ∨y white，1 ≠y white，2 ∨Z white，1 ≠Z white，2 (38)

[0160] Exactly at the ratio This situation can occur when a logarithm contains both values ​​greater than 1 and values ​​less than 1. Therefore, the logarithm sign is defined as...

[0161]

[0162] in,

[0163] And the dominant logarithmic symbol is

[0164]

[0165] Therefore, the ratio of the geometric mean of the white points can now be replaced in equations (31) and (32).

[0166]

[0167] Wherein, the weight σ = -1, where 1 is yet to be determined, and the weight indicates which ratios are inverse and which are direct.

[0168] For a direct conversion from white point W1 to W2, the component whose logarithmic sign deviates from the dominant sign should enter in the opposite direction.

[0169] This means that,

[0170]

[0171] And σ y , σ z similar.

[0172] Therefore, the ratio in equation (42) becomes 1 only when the two white points W1 and W2 are exactly equal. In this case, the entire transformation is an identity. And it can be omitted.

[0173] For conversions using intermediate steps via D50O02, it can generally be assumed that the color space coordinates of the white point are lower than those of the D50O02 color space. Therefore, when converting to D50O02, those ratios with the positive logarithmic sign Σ are reversed, and when converting to D50O02, those ratios with the negative logarithmic sign ∑ are reversed.

[0174]

[0175] in,

[0176]

[0177]

[0178] in,

[0179]

[0180] Now, these ratios can also become 1 only if the corresponding white point W1[W2] exactly corresponds to the white point of the lighting condition. In this case, the conversion from W1 to D50O02 (or from D50O02 to W2) is the same mapping and can be omitted. Attached Figure Description

[0181] The attached diagram shows:

[0182] Figure 1 This schematically illustrates the principle of color conversion for matching printing processes according to existing technology; and

[0183] Figure 2 The method according to the invention is illustrated schematically. Detailed Implementation

[0184] Figure 1The diagram schematically illustrates the principle of color conversion for matching printing processes according to the prior art. A first color conversion 1, from color values ​​[C1, M1, Y1, K1] in a first printing process to XYZ color values, and a second color conversion 2, from XYZ color values ​​to color values ​​[C2, M2, Y2, K2] in a second printing process, are performed sequentially. The two color conversions 1 and 2 can also be combined to form an equivalent color conversion 3, which directly assigns color values ​​[C1, M1, Y1, K1] and [C2, M2, Y2, K2] to each other. Because the color values ​​[C1, M1, Y1, K1] and [C2, M2, Y2, K2] are assigned to each other through a device-independent XYZ intermediate color space, this results in identical XYZ color values. Therefore, the printed colors assigned in the two printing processes are perceived to be visually identical to a large extent within the range of printed colors.

[0185] Figure 2 The method according to the invention is illustrated schematically. Color values ​​in a device-dependent first color space are converted to an intermediate color space. The conversion to the intermediate color space is performed using a combination of absolute and relative rendering intentions. The color values ​​converted to the intermediate color space are then converted to color values ​​in a device-dependent second color space. The conversion from the intermediate color space is performed using a combination of absolute and relative rendering intentions.

[0186] List of reference numerals

[0187] 1. First color conversion

[0188] 2 Second color conversion

[0189] 3. Third color conversion

Claims

1. A method for converting color values ​​in a device-dependent first color space into color values ​​in a device-dependent second color space, such that the visual impression of the colors reproduced in the two color spaces is substantially the same, wherein, When converting color values ​​in a device-dependent first color space to color values ​​in a device-dependent second color space, at least one rendering intent is used, the at least one rendering intent being used to describe the color values, characterized in that... When converting color values ​​in the device-dependent first color space to color values ​​in the device-dependent second color space, at least one absolute rendering intent describing absolute color values ​​is combined with at least one relative rendering intent describing relative color values, wherein the conversion of color values ​​in the device-dependent first color space to color values ​​in the device-dependent second color space is performed using color values ​​from an intermediate color space. The color values ​​of the device-related first color space are converted into color values ​​of the intermediate color space, and the color values ​​of the intermediate color space are converted into color values ​​of the device-related second color space. When converting the color values ​​of the device-related first color space to the color values ​​of the intermediate color space, at least one absolute rendering intent is combined with at least one relative rendering intent, and When converting color values ​​from the intermediate color space to color values ​​from the device-dependent second color space, at least one absolute rendering intent is combined with at least one relative rendering intent.

2. The method according to claim 1, wherein, The at least one absolute rendering intent and the at least one relative rendering intent are respectively colorimetric rendering intents.

3. The method according to claim 1 or 2, wherein, The intermediate color space is a device-independent color space.

4. The method according to claim 1 or 2, wherein, The color values ​​in the intermediate color space correspond to the D50 2° standard observer.

5. The method according to claim 1 or 2, wherein, The color values ​​of the device-dependent first color space are converted to color values ​​of the intermediate color space using at least one configuration file including at least one conversion table, and the color values ​​of the intermediate color space are converted to color values ​​of the device-dependent second color space using at least one configuration file including at least one conversion table, wherein the at least one absolute rendering intent and the at least one relative rendering intent use the same conversion table.

6. The method according to claim 5, wherein, The at least one configuration file is at least one ICC configuration file.

7. The method according to claim 1 or 2, wherein, The device referred to in the first color space associated with the device is a different device than the device referred to in the second color space associated with the device.

8. The method according to claim 1 or 2, wherein, The conversion of the color values ​​of the device-related first color space to the color values ​​of the device-related second color space is performed during a printing method in which a digital printed image is printed onto a substrate using a printing press, wherein the device-related first color space is the screen color space and the device-related second color space is the printing press color space.

9. The method according to claim 8, wherein, The printing method is selected from the following: letterpress printing, offset printing, and gravure printing.

10. The method according to claim 8, wherein, The printing method is offset printing, inkjet printing, flexographic printing, screen printing, or gravure printing.

11. The method according to claim 8, wherein, When converting the color values ​​of the device-related first color space to the color values ​​of the device-related second color space, no simulation is performed on the color values ​​of the unprinted substrate.

12. The method according to claim 11, wherein, The color value of the unprinted substrate is paper white.

13. The method according to claim 1 or 2, wherein, By converting the color values ​​of the device-related first color space to the color values ​​of the device-related second color space, and converting the brightest point of the device-related first color space to the brightest point of the device-related second color space, the output is converted into one without printing dots.

14. The method according to claim 1 or 2, wherein, The at least one relative rendering intent includes black point compensation.

15. A printing press by means of which the method according to any one of claims 1 to 14 is performed.

16. An application of the method according to any one of claims 1 to 14, wherein the method is applied in a printing press.

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