Color conversion method, color conversion device, and program

Abstract

Achieves black plate retention in multi-color profiles. [Solution] The color conversion method includes (c) when an instruction to maintain the black plate is received, the step of performing a black plate retention correction to correct the color conversion table so as to increase the K component and decrease the non-target color component among the output values ​​associated with the target grid point, which is a grid point representing a monochromatic input value in the color conversion table; and (d) a propagation correction to propagate the black plate retention correction to propagation grid points in a predetermined range around the target grid point among the grid points representing the input value in the color conversion table, the step of performing a propagation correction to correct the output values ​​associated with the propagation grid point in the color conversion table using the amount of increase in the K component due to the black plate retention correction and a replacement rate table that defines a color component replacement rate for the non-target color component in order to suppress color fluctuations caused by the propagation correction.

Description

[Technical Field]

[0001] This disclosure relates to a color conversion method, a color conversion apparatus, and a program. [Background technology]

[0002] In printing, features such as black plate retention and GCR (gray component replacement) are sometimes required. Patent Document 1 describes a technique for controlling the amount of black ink in a printing apparatus. Black ink control refers to adjusting the amount of black ink to improve print quality. In the technique described in Patent Document 1, a table is created that converts CMY values ​​to Lab values ​​from a color conversion table called AtoBTag included in the output profile and the calculated amount of black ink, and the CMY values ​​corresponding to the Lab values ​​of the image are searched from this table. In this way, the amount of black ink is controlled by a calculation formula, and the CMY values ​​that reproduce the Lab values ​​are calculated. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-32516 [Overview of the project] [Problems that the invention aims to solve]

[0004] There are multicolor profiles that can handle more color channels than the four process colors. In multicolor profiles, the number of grid points in AtoBTag increases exponentially with the number of color channels. As the number of grid points increases, the data size of AtoBTag becomes very large, making it practically difficult to store AtoBTag in a color profile. Therefore, in the case of multicolor profiles, it is difficult to use the method of using AtoBTag described in Patent Document 1 to adjust the ink amount of a specific color, such as black. [Means for solving the problem]

[0005] This disclosure can be implemented in the following forms:

[0006] A first embodiment of this disclosure provides a color conversion method. This color conversion method includes (a) receiving an instruction to hold the black plate; (b) preparing a color conversion table that defines the correspondence between the input values ​​of the input color space in the image data and the output values ​​of the output color space in a printing device that prints the image data; and (c) when the instruction is received and the color conversion using the color conversion table converts the monochromatic input value, which is the input value containing only the K component in the image data, to the output value containing non-symmetric color components, which are color components other than the K component, among the output values ​​that are associated with the symmetric grid points, which are grid points representing the monochromatic input value in the color conversion table, (d) A step of performing a black plate holding correction to correct the color conversion table so as to increase the K component and decrease the non-asymmetric color component; and a step of performing a propagation correction to propagate the black plate holding correction to propagation grid points in the color conversion table that are within a predetermined range around the target grid point, wherein the output value associated with the propagation grid point in the color conversion table is corrected using the amount of increase in the K component due to the black plate holding correction and a replacement rate table that defines a color component replacement rate for the non-asymmetric color component in order to suppress color fluctuations caused by the propagation correction.

[0007] A second embodiment of this disclosure provides a color conversion device. This color conversion device includes a selection unit that receives an instruction to hold a black plate, a table creation unit that creates a color conversion table that defines the correspondence between the input values ​​of the input color space in the image data and the output values ​​of the output color space in a printing device that prints the image data, and when the instruction is received and the color conversion using the color conversion table converts the monochromatic input value, which is the input value containing only the K component in the image data, to the output value containing non-symmetric color components, which are color components other than the K component, among the output values ​​that are associated with the target grid points, which are grid points representing the monochromatic input value in the color conversion table, The system includes a correction unit which performs a black plate holding correction to correct the color conversion table so as to increase the K component and decrease the non-asymmetric color component, and a propagation correction to propagate the black plate holding correction to propagation grid points in a predetermined range around the target grid point among the grid points representing the input value in the color conversion table, wherein the propagation correction is performed to correct the output value associated with the propagation grid point in the color conversion table using the amount of increase of the K component due to the black plate holding correction and a replacement rate table which defines a color component replacement rate for the non-asymmetric color component in order to suppress color fluctuations caused by the propagation correction, and

[0008] According to a third embodiment of this disclosure, a computer program is provided. This program has a function to receive an instruction to hold the black plate, a function to create a color conversion table that defines the correspondence between the input values ​​of the input color space in the image data and the output values ​​of the output color space in a printing device that prints the image data represented by the image data, and when the instruction is received and the color conversion using the color conversion table converts the monochromatic input value, which is the input value containing only the K component in the image data, to the output value containing non-symmetric color components, which are color components other than the K component, the program increases the K component in the output value that is associated with the symmetric grid point, which is the grid point representing the monochromatic input value in the color conversion table. The computer is provided with the following functions: a function to perform black plate retention correction to correct the color conversion table so as to reduce the non-asymmetric color component; and a function to perform propagation correction to propagate the black plate retention correction to propagation grid points in the color conversion table that are within a predetermined range around the target grid point, among the grid points representing the input value in the color conversion table, wherein the propagation correction is performed to correct the output value associated with the propagation grid point in the color conversion table using the amount of increase in the K component due to the black plate retention correction and a replacement rate table that defines a color component replacement rate for the non-asymmetric color component in order to suppress color fluctuations caused by the propagation correction. [Brief explanation of the drawing]

[0009] [Figure 1] An explanatory diagram showing the printing system. [Figure 2] An explanatory diagram showing an example of the configuration of an image processing device. [Figure 3] An explanatory diagram showing a color cube, which represents the CMYK color space in the form of a cube. [Figure 4] An explanatory diagram showing an example of a typical color conversion. [Figure 5] An explanatory diagram showing an example of color conversion using a color conversion LUT. [Figure 6] A flowchart illustrating the overall processing flow of color correction. [Figure 7] An explanatory diagram showing an example of a selection GUI displayed on a display device. [Figure 8] A flowchart showing a specific process flow (first half) of step S30 in FIG. 6. [Figure 9] A flowchart showing a specific process flow (second half) of step S30 in FIG. 6. [Figure 10] An explanatory diagram of grid points on the KC plane. [Figure 11] An explanatory diagram showing an example of a replacement rate table. [Figure 12] An explanatory diagram showing another example of a replacement rate table. DETAILED DESCRIPTION OF EMBODIMENTS

[0010] A. Embodiment: FIG. 1 is an explanatory diagram showing a printing system 10. The printing system 10 includes an image processing apparatus 100, an input device 200, a display device 300, and a printing apparatus 400.

[0011] The image processing apparatus 100 is a computer including a memory 101, an input / output interface 102, a processor 103, and an internal bus 104. The memory 101, the input / output interface 102, and the processor 103 are bidirectionally communicably connected via the internal bus 104. The memory 101 stores various programs and various data used for various processes executed by the image processing apparatus 100. In the illustrated example, the memory 101 stores input image data IM to be printed, an input profile IPF, an output profile OPF, a program 110, a color conversion LUT (Look Up Table) 120, and a replacement rate table 130. Details of the program 110, the color conversion LUT 120, and the replacement rate table 130 will be described later. The input device 200, the display device 300, and the printing apparatus 400 are connected to the input / output interface 102 via wired communication or wireless communication. The color conversion LUT 120 is also referred to as a "color conversion table".

[0012] The input profile IPF is used for color conversion from an input color space used in image data to a device-independent color space. The input color space is, for example, a CMYK color space or an RGB color space. The device-independent color space is, for example, CIE-L * a * b * color space or a CIE-XYZ color space. Hereinafter, CIE-L * a * b * color space is simply referred to as the Lab color space. L * a * b * is simply referred to as Lab. In color conversion using the input profile IPF, an input value, which is a value representing a color of each pixel included in image data in the input color space, is converted into a colorimetric value, which is a value representing the color of each pixel in the device-independent color space.

[0013] The output profile OPF is used for color conversion from a device-independent color space to an output color space for a printing apparatus 400. The output color space is, for example, a CMYK color space. In color conversion using the output profile OPF, a colorimetric value, which is a value representing a color of each pixel included in image data in the device-independent color space, is converted into an output value representing the color of each pixel in the output color space used by the printing apparatus 400. In the present embodiment, the output profile OPF is created by combining a general output profile and an ink amount conversion LUT. A general output profile is an output profile created by the International Color Consortium (ICC), or an output profile conforming to ICC specifications. The output profile OPF supports multi-color. In color conversion using the output profile OPF, a colorimetric value in the device-independent color space is converted into an ink amount for the printing apparatus.

[0014] The input device 200 is, for example, a keyboard or a mouse. The display device 300 is, for example, a liquid crystal display or an organic EL (Electro-Luminescence) display. The display device 300 displays a GUI (Graphical User Interface), which will be described later. The input device 200 and the display device 300 are used to receive user instructions regarding printing, which will be described later.

[0015] Printing apparatus 400 is an inkjet printing apparatus. Printing apparatus 400 performs printing using CMYKLcLmLk inks. Typical printing apparatuses use process inks, which are process color pigments CMYK, but printing apparatus 400 uses LcLmLk inks in addition to process color CMYK. C is cyan, M is magenta, Y is yellow, K is black, Lc is light cyan, Lm is light magenta, and Lk is light black. Light cyan is a low-density cyan ink. Light magenta is a low-density magenta ink. Light black is a low-density black ink. In this specification, multicolor means including four or more process colors.

[0016] Figure 2 is an explanatory diagram showing an example of the configuration of the image processing device 100. The image processing device 100 includes a color conversion unit 210, a table creation unit 215, a correction unit 220, a selection unit 230, and a print data generation unit 240. The functions of each of these units are realized by the execution of a program 110 pre-stored in the memory 101 by the processor 103.

[0017] The color conversion unit 210 uses the color conversion LUT 120 to perform a color conversion process on the image data, converting the input values ​​in the input color space to the output values ​​in the output color space. In this embodiment, the input color space is the CMYK color space used in the input image data IM. The output color space is the CMYKLcLmLk color space used in the printing device 400.

[0018] The color conversion LUT 120 is a lookup table for converting input values ​​in the input color space to output values ​​in the output color space. The color conversion LUT 120 is created by combining the input profile IPF and the output profile OPF. The color conversion LUT 120 is created when color conversion is performed. By combining the input profile IPF and the output profile OPF, the color conversion process can be executed at high speed. Furthermore, by directly converting from the input color space to the output color space, LUT correction processes such as the black plate retention correction described later can be easily performed.

[0019] Figure 3 is an explanatory diagram showing a color cube, which conceptually represents the CMYK color space in the form of a cube. Here, we explain an example where both the input and output color spaces are CMYK color spaces. This color solid has White (paper white) as its origin and is defined by three coordinate axes: the C axis, M axis, and Y axis, which are mutually orthogonal. Since the CMYK color space is a four-dimensional space, for example, the plane PL1, where the M component is 0, constitutes a hyperplane in four dimensions. Because it is difficult to illustrate such a hyperplane, it is represented as a surface in three dimensions in Figure 3. Note that some grid lines have been omitted from the illustration in Figure 3.

[0020] Multiple grid points are set in the color cube. Each grid point corresponds to an input value in the input color space. More specifically, the grid points indicate coordinate values ​​that represent the component values ​​of each color component when coordinate axes representing each color component are set in the input color space. The coordinates of the grid points represent the input values ​​(C, M, Y, K) in the color conversion LUT 120. Each grid point is associated with the output value in the output color space that corresponds to the input value in the color conversion LUT 120. In this embodiment, the output value of the color conversion LUT 120 represents the amount of ink for each ink. The amount of ink is represented by the ink's duty cycle. The ink's duty cycle represents the amount of ink per unit area ejected onto the medium. The duty cycle can take a range of 0 to 100%. In the illustrated example, for grid point P1, the input values ​​for each CMYK component are (0,0,0,0) and the output values ​​for each CMYK component are (0,0,0,0), and for grid point P2, the input values ​​for each CMYK component are (0,0,80,0) and the output values ​​for each CMYK component are (2,0,78,0).

[0021] For input values ​​where any value other than K is zero, it may be requested that printing be done using only black ink. Therefore, black ink retention may be performed.

[0022] In the color conversion using the initially created color conversion LUT120, input values ​​where all values ​​except K are zero do not necessarily result in output values ​​where all values ​​except K are zero (see grid point P2 in Figure 3). Therefore, for input values ​​where all values ​​except K are zero, it is necessary to correct the color conversion LUT120 using the method described later, in order to enable printing with only black ink.

[0023] Figure 4 is an explanatory diagram illustrating an example of a typical color conversion. Figure 4 shows an example in which color conversion is performed via an instrument-independent color space in the order of A2B conversion and B2A conversion, and then converted to ink volume using an ink volume conversion lookup table. Here, A2B conversion is the conversion of CMYK values ​​to instrument-independent color space color values ​​using AtoBTag. B2A conversion is the conversion of instrument-independent color space color values ​​to CMYK values ​​using BtoATag.

[0024] When A2B and B2A conversions are performed, for example, the input value indicated by the grid point at index (0,0,0,2) is converted to the output value CMYK(20,20,40,10). Subsequently, when the conversion to ink amounts is performed using an ink amount conversion LUT, for example, the input value indicated by the grid point at index (0,0,0,2) is ultimately converted to ink(0,0,0,20,0,0,40), representing the respective ink amounts for CMYKLcLmLk. The index value represents the coordinate value of the grid point. Index (0,0,0,2) indicates that the coordinate values ​​other than the K axis are zero.

[0025] Figure 5 is an explanatory diagram of color conversion using color conversion LUT 120. Color conversion LUT 120 is a LUT that has been corrected to enable black plate retention. In color conversion using color conversion LUT 120, the input CMYK value is converted to the respective ink amounts of CMYKLcLmLk.

[0026] As shown in Figure 2, the table creation unit 215 creates a color conversion LUT 120 by combining the input profile IPF and the output profile OPF, for example, before the first print run by the printing device 400. In the example shown in Figure 2, the replacement rate table 130 is included in the output profile OPF, but the replacement rate table 130 may be located outside the output profile OPF.

[0027] The correction unit 220 corrects the color conversion LUT 120 so that the black plate retention is effective.

[0028] When the selection unit 230 specifies that the black plate should be held, the correction unit 220 performs the following correction process on the color conversion LUT 120. The output values ​​to be corrected in the color conversion LUT 120 are the amounts of each CMYKLcLmLk ink. The specific details of the process will be described later, but in the case where the input value of the color conversion LUT 120 is a single-color input value containing only the K component, and the output value contains non-target color components, the correction unit 220 corrects the color conversion LUT 120 by increasing the K component of the output value and decreasing the non-target color components of the output value. "Non-target color components" are color components other than the K component. Here, decreasing the non-target color components of the output value means decreasing at least one of the color components other than the K component. This correction corrects the amount of ink as an output value associated with the grid points on the K axis. This correction is also called "black plate holding correction".

[0029] Furthermore, the correction unit 220 corrects the output values ​​associated with the grid points surrounding the grid points that represent monochromatic input values ​​containing only the K component. This correction is also called "spread correction." In color conversion processing using a color conversion LUT 120 that corrects only the output values ​​associated with grid points on the K axis, tone jumps may occur. Spread correction is performed to suppress the occurrence of tone jumps caused by holding the black plate.

[0030] The selection unit 230 receives instructions from the user to hold the ink plate.

[0031] The print data generation unit 240 converts the amount of ink represented by the color-converted image data into ink dots (ink droplets) and generates print data to be supplied to the printing device 400.

[0032] The printing device 400 performs printing according to the print data supplied from the print data generation unit 240.

[0033] Figure 6 is a flowchart showing the overall processing flow of the color correction process. Note that the color conversion LUT 120 has been created when the process shown in Figure 6 begins.

[0034] In step S10, the selection unit 230 receives an instruction to hold the ink plate. Figure 7 is an explanatory diagram showing an example of the selection screen W1 displayed on the display device 300. The selection unit 230 displays the selection screen W1 on the display device 300. The user specifies whether or not to perform ink plate holding using the input device 200. Here, it is assumed that the user has specified to perform ink plate holding.

[0035] As shown in Figure 6, in step S30, the correction unit 220 corrects the contents of the color conversion LUT 120. In step S50, the color conversion unit 210 performs color conversion of the image data to be printed using the corrected color conversion LUT 120. In step S60, the print data generation unit 240 generates print data to be supplied to the printing device 400 based on the color-converted image data. The printing device 400 performs printing according to the print data supplied from the print data generation unit 240.

[0036] Figures 8 and 9 are flowcharts showing the specific processing flow of step S30.

[0037] In step S31, the correction unit 220 selects one grid point on the K axis from among multiple grid points distributed in the input CMYK color space. The coordinates of each grid point correspond to the input CMYK values ​​in the color conversion LUT 120, and each grid point is associated with the respective ink amounts CMYKLcLmLk as output values ​​corresponding to the input values ​​in the color conversion LUT 120. The grid point on the K axis is a grid point in the color conversion LUT 120 where all components except the K component of the input value are zero.

[0038] Figure 10 is an explanatory diagram of a grid point on the KC plane. For example, grid point P11 with index (0,0,0,4) is selected. The respective ink amounts for CMYKLcLmLk, which are the output values ​​associated with grid point P11, are (20,20,10,30,0,0,60).

[0039] As shown in Figure 8, in step S32, the correction unit 220 performs correction for ink plate retention. The process in step S32 is performed when it is necessary to perform ink plate retention correction. If it is not necessary to perform ink plate retention correction, the process in step S32 is skipped. The case in which it is necessary to perform ink plate retention correction is when, in the color conversion LUT 120, an output value in which color components other than K are not zero is associated with an input value in which only the K component is included. In the color conversion LUT 120, if an output value in which color components other than K are zero is associated with an input value in which only the K component is included, it is not necessary to perform ink plate retention correction. Here, "color components other than K are zero" includes not only cases in which color components other than K are strictly zero, but also cases in which color components other than K are below a predetermined threshold and can be considered as zero. For ink plate retention correction, the correction unit 220 increases the K component in the output value associated with the grid point on the selected K axis and corrects the color components other than the K component in the output value associated with the grid point to zero. The corrected K value is calculated, for example, as follows: The L value is calculated using the LUT included in the AtoBTag of the output profile OPF, and the K value is calculated using the 1D curve included in AtoBTag that represents the correspondence between the L value and the K value. It is preferable that the LUT included in the AtoBTag of the output profile OPF in memory 101 be about 2 grids. This is because if the LUT included in AtoBTag is too large, the output profile OPF cannot be stored in memory 101.

[0040] In the example shown in Figure 10, the CMYKLcLmLk ink amounts associated with grid point P11 at index (0,0,0,4) are corrected to (0,0,0,50,0,0,0). The correction unit 220 updates the contents of the color conversion LUT 120 with the corrected ink amounts. Hereinafter, the corrected grid point on the K axis will be referred to as the "target grid point". Note that if the color components other than the K component in the output value associated with the grid point are zero, the process in step S32 does not need to be executed.

[0041] As shown in Figure 8, in step S33, the correction unit 220 calculates an increase amount ΔK, which is the difference between the K component of the output value associated with the target grid point after correction and the K component of the output value associated with the target grid point before correction. The increase amount ΔK is also called the "K increase amount".

[0042] In step S34, the correction unit 220 identifies "spreading grid points," which are one or more grid points within a predetermined range around the target grid point. Spreading grid points are grid points that exist within a range that does not exceed a predetermined maximum distance M from the target grid point. For example, the maximum distance M is set to "3". The unit of distance is the interval between grid points. In the example shown in Figure 10, if the target grid point is grid point P11 with index (0,0,0,4), then among the grid points on the KC plane, grid point P12 with index (1,0,0,4) and grid point P13 with index (2,0,0,4) are the spreading grid points. Among the multiple grid points arranged on the KC plane, grid points P12 and P13 are selected.

[0043] Figure 10 illustrates the grid points on the KC plane, but grid points located on the MC, CY, MK, YM, and YK planes that are within a range not exceeding the maximum distance M from the target grid point also qualify as propagation grid points.

[0044] Spreading grid points are not limited to grid points located on any of the KC, MC, CY, MK, YM, or YK planes. Let the index of the target grid point be (C1, M1, Y1, K1). Let the index of a grid point not located on any of the KC, MC, CY, MK, YM, or YK planes be (C2, M2, Y2, K2). Since the spreading grid point is affected by the ink plate retention of the nearest target grid point on the K axis, the K-axis coordinate values ​​of the target grid point and the K-axis coordinate values ​​of the spreading grid point are the same. In this case, the distance D between the target grid point and the grid point with index (C2, M2, Y2, K2) is D = SQRT{(C2-C1) 2 +(M2-M1) 2 +(Y2-Y1)2 +(K2-K1) 2 This can be calculated as follows: Here, SQRT(val) represents the square root of the value val. Note that the target grid point is on the K axis, so C1=M1=Y1=0. Therefore, the distance D between the target grid point and the grid point of index (C2,M2,Y2,K2) is D=SQRT{C2 2 +M2 2 +Y2 2 +(K2-K1) 2 This is expressed as}. Grid points where the distance D is less than the maximum distance M are called propagation grid points. Distance D is also called the "K-axis separation distance".

[0045] As shown in Figure 8, in step S35, the correction unit 220 selects one of the propagation grid points identified in step S34 and calculates the distance D from the target grid point to the selected propagation grid point. For example, in Figure 10, the distance D from the target grid point at index (0,0,0,4) to the grid point P12 at index (1,0,0,4) is "1".

[0046] As shown in Figure 8, in step S36, the correction unit 220 calculates the K correction amount K_CV for the selected propagation grid points based on the following equation (1), using the maximum distance M, the increase amount ΔK, and the distance D. The K correction amount becomes smaller the further it is from the target grid point. The K correction amount K_CV is also called the "K correction value". K_CV = ΔK·(MD) / M···(1)

[0047] For example, in Figure 10, for grid point 12 at index (1,0,0,4), which corresponds to the propagation grid point of the target grid point at index (0,0,0,4), the K correction amount K_CV is calculated as follows. Assume ΔK = 20. K_CV=ΔK·(MD) / M=20·(3-1) / 3=13.33...

[0048] As shown in Figure 8, in step S37, the correction unit 220 selects a record from the replacement rate table 130 that contains a composition ratio close to the composition ratio of the ink amount associated with the propagation grid points.

[0049] Figure 11 is an explanatory diagram showing an example of the replacement rate table 130. The replacement rate table 130 is used to correct the output values ​​associated with propagation grid points. The replacement rate table 130 defines combinations of color components other than the K component that can cancel out the increase in the K component when the K component of the output value associated with propagation grid points is increased. The replacement rate table 130 divides the K axis in the output color space into 256 gradations and defines the replacement rate of the CMYLcLmLk color component that is as close as possible to the ink amount of the K component for each gradation. The replacement rate of the CMYLcLmLk color component is also called the "color component replacement rate". The replacement rate table 130 contains 256 records. The index of the replacement rate table 130 represents the ink amount of the K component in the output color space. The ink amount takes values ​​in the range of 0 to 100%, and the values ​​obtained by dividing this range into 256 gradations are set as the index. For example, the CMYLcLmLk stored in index "127" is calculated as follows. When the CMYKLcLmLk value in the output color space is (0,0,0,127,0,0,0), the CMYLcLmLk value, excluding the K value, is calculated to be as close as possible to the converted lightness L from this output value. The calculated CMYLcLmLk value is then divided by "127". If the device-independent space is the CIE-XYZ color space, first, a conversion from the CIE-XYZ color space to the Lab color space should be performed using a known conversion formula. In black plate retention correction, the K component of the ink is increased, and in propagation correction, the CMYLcLmLk color components are corrected to change the lightness in order to suppress the occurrence of gradation jumps.

[0050] The records in replacement rate table 130 are selected using the following method.

[0051] First, the correction unit 220 calculates the ink composition ratio of color components other than the K component of the output value associated with the selected propagation grid point. Specifically, it divides the ink amounts of the other color components by the ink amount of the color component with the largest ink amount among the color components other than the K component.

[0052] For example, suppose the ink amounts other than the K component of the output value associated with the selected propagation grid point are CMYLcLmLk (30, 20, 10, 0, 0, 60). In this example, the ink amount of the Lk component is the largest. Therefore, if we divide the ink amounts of the other color components by the ink amount of the Lk component, we obtain the ink composition ratio of the propagation grid point (0.5, 0.33, 0.17, 0, 0, 1).

[0053] The correction unit 220 calculates the ink content ratio for each record in the replacement rate table 130 by dividing the ink content of each of the other color components by the ink content of the color component with the maximum ink content.

[0054] The correction unit 220 calculates the absolute difference S[i] between the sum of each element in the ink volume composition ratio of the selected propagation grid point and the sum of each element in the ink volume composition ratio of record R[i] (where i is an integer) in the replacement rate table 130. The subscript i corresponds to the index in the replacement rate table 130. If the ink volume composition ratio of the propagation grid point is (0.5, 0.33, 0.17, 0, 0, 1), the sum of each element in the ink volume composition ratio of the propagation grid point is calculated as 0.5 + 0.33 + 0.17 + 0 + 0 + 1 = 1.01. The sum of each element in the ink volume composition ratio of record R[i] can be calculated similarly. The correction unit 220 calculates the absolute difference S[i] for each of the 256 records in the replacement rate table 130 and selects the record R[i] that takes the smallest absolute difference S[i] among the 256 calculated differences as the record closest to the ink volume composition ratio associated with the propagation grid point. For example, if the ink volume composition ratio (0.5, 0.33, 0.17, 0, 0, 1) is obtained for the propagation grid points, then the corresponding ink volume (0.3, 0.3, 0.15, 0, 0, 1) is found in the record at index "127" of the replacement rate table 130.

[0055] As shown in Figure 9, in step S38, the correction unit 220 multiplies each element of the ink amount (RC, RM, RY, RLc, RLm, RLk) of the record selected in step S37 by the K correction amount K_CV calculated in step S26 to calculate the correction amounts other than the K component (DC, DM, DY, DLc, DLm, DLk). For example, suppose in step S37, the record (0.3, 0.3, 0.15, 0, 0, 1) with index "127" is selected. Assume K_CV = 13.33. In this case, the correction amounts (DC, DM, DY, DLc, DLm, DLk) are calculated as (4, 4, 2, 0, 0, 13.33).

[0056] In step S39, the correction unit 220 calculates the corrected ink amounts (C', M', Y', K', Lc', Lm', Lk') of the propagation grid points based on the following formula (2), using the ink amounts (C, M, Y, K, Lc, Lm, Lk) of the propagation grid points before correction, the K correction amount K_CV, and the correction amounts other than the K component (DC, DM, DY, DLc, DLm, DLk). In the following formula (2), only the K component ink amount is increased, and the ink amounts of the other color components are decreased. (C',M',Y',K',Lc',Lm',Lk')=(C-DC,M-DM,Y-DY,K+K_CV,Lc-DLc,Lm-DLm,Lk-DLk)...(2)

[0057] As shown in Figure 10, for example, suppose that the ink amounts (30, 20, 10, 30, 0, 0, 60) of CMYKLcLmLk are associated with grid point P12. Suppose that the K correction amount K_CV is 13.33 and the correction amounts (DC, DM, DY, DLc, DLm, DLk) are (4, 4, 2, 0, 0, 13.33). In this case, the corrected ink amounts for the propagation grid points are calculated as follows. (C',M',Y',K',Lc',Lm',Lk')=(30-4,20-4,10-2,30+13.33,0-0,0-0,60-13.33)=(26,16,8,43.33,0,0,46.67)

[0058] The ink amounts for CMYKLcLmLk, which are output values ​​associated with grid point P12, are corrected to (26, 16, 8, 43, 0, 0, 47).

[0059] The correction unit 220 updates the contents of the color conversion LUT 120 with the corrected ink volume.

[0060] In the propagation correction, the K component of the ink amount associated with the propagation grid point is increased, while the other components are decreased. This allows the ink plate holding correction to propagate to surrounding propagation grid points on the K axis where the ink plate holding correction was performed.

[0061] As shown in Figure 9, in step S40, it is determined whether the correction process, i.e., the processes in steps S35 to S39, has been completed for all of the propagation grid points identified in step S34. Here, the propagation grid points identified in step S34 are the grid points identified in the process of step S34 that was executed immediately before. If there are any propagation grid points that have not been corrected (step S40; NO), the processes from step S35 onwards are executed again. If it is determined that the correction process has been performed for all of the propagation grid points identified in step S34 (step S40; YES), the process in step S41 is executed.

[0062] In step S41, the correction unit 220 determines whether the correction process, i.e., the processes in steps S31 to S39, has been completed for all grid points on the K axis. If there are any grid points on the K axis that have not been processed (step S41; NO), the processes from step S31 onwards are executed again. If the processing has been completed for all grid points on the K axis (step S41; YES), the correction process for the color conversion LUT 120 in step S30 (see Figure 6) is completed.

[0063] In this embodiment, black plate retention correction is performed in the color conversion LUT 120 by correcting the output values ​​associated with the grid points on the K axis. Furthermore, propagation correction is performed in the color conversion LUT 120 by correcting the output values ​​associated with the propagation grid points around the grid points on the K axis. By performing propagation correction, it is possible to suppress the occurrence of gradation jumps caused by black plate retention when performing color conversion using the color conversion LUT 120.

[0064] If the output values ​​associated with propagation grid points in the color conversion LUT 120 are corrected, the color values ​​of the converted image using the color conversion LUT 120 will fluctuate compared to the image before color conversion. Here, color values ​​refer to the three attribute values ​​of the device-independent color space. In order to suppress fluctuations in color values, in this embodiment, the output values ​​associated with propagation grid points are corrected using the replacement rate table 130.

[0065] To suppress fluctuations in color values, it is conceivable to use AtoBTag, but in multi-color profiles, the data size of AtoBTag is expected to be very large. For example, if AtoBTag converts device color to Lab values, and the number of grids indicating the number of grid points on each axis of the device color space is 17, and the number of color channels is 7 because there are 7 types of colorants in the printing device, then in this case, the total number of grid points is 17^7 = 410,338,673. As the number of grid points increases, the data size of AtoBTag becomes very large. In this embodiment, instead of using AtoBTag, which has a very large data size, a replacement rate table 130 is used. The replacement rate table 130 defines the ink replacement rate for the non-asymmetric color component for each of the 256 gradations. The number of dimensions of the replacement rate table 130 is smaller than the number of dimensions of AtoBTag. Therefore, the size of the data stored in the color profile can be made compact.

[0066] B. Other embodiments: (B1) Figure 12 is an explanatory diagram showing an example of a replacement rate table 131 with a configuration that differs slightly from the embodiment. In addition to the columns included in replacement rate table 130 (see Figure 11), replacement rate table 131 includes a K2L column. The value in the K2L column represents lightness. Lightness L takes values ​​in the range of 0 to 100, and the K2L column is set with values ​​obtained by dividing this range into 256 gradations. As mentioned above, the index value represents the amount of K component ink in the output color space. The amount of K component ink can be obtained by selecting a record from replacement rate table 131 that has an index that matches the lightness L obtained by converting from the CMYK color space to the Lab color space using a profile for the input device, and by referring to the index value of the selected record. The obtained amount of K component ink is also called the "update ink amount".

[0067] In the replacement rate table 131, the amount of K ink corresponding to the brightness in the device-independent color space based on the input value is defined in the replacement rate table, so the amount of K component ink can be easily obtained when making corrections for maintaining the black plate.

[0068] (B2) In the above embodiment, an example was described in which the amount of black ink is increased when performing black plate retention correction. However, when performing black plate retention correction, the amount of black ink and at least one of light black ink may be increased. When performing black plate retention correction, the amount of increase for K and Lk is determined according to the grid point value on the K axis that represents the input value. When performing black plate retention correction, for example, if the value of the K component is close to 100, such as K=90, only K may be increased. Also, for example, if the value of the K component is close to 0, such as K=10, only Lk may be increased. Also, if the value of the K component is an intermediate value in the range of 0 to 100, such as K=45, both K and Lk may be increased. By using K and Lk together, the graininess of the printed image, i.e., the roughness of the image, can be reduced.

[0069] When the printing device 400 uses LLk ink, the amount of at least one of the black, light black, and light-light black inks may be increased during the black plate holding correction. LLk is light-light black. Light-light black is a black ink with a lower density than light black. K, Lk, and LLk are included in the K component ink. During black plate holding correction, the amount of increase for K, Lk, and LLk is determined according to the grid point value on the K axis representing the input value.

[0070] (B3) In the above embodiment, an example was described in which the replacement rate table 130 defines the replacement rate of the CMYLcLmLk color components for each gradation by dividing the K axis in the output color space into 256 gradations. The replacement rate table 130 may also be defined by dividing the K axis into 128 gradations. In this case, the size of the replacement rate table 130 can be made more compact compared to the embodiment in which the K axis is divided into 256 gradations. Alternatively, the replacement rate table 130 may also be defined by dividing the K axis into 512 gradations. In this case, since the replacement rates are defined in a more finely subdivided manner compared to the embodiment in which the K axis is divided into 256 gradations, it is possible to control the suppression of color fluctuations caused by propagation correction more precisely.

[0071] (B4) The methods for ink retention correction and propagation correction according to the above embodiment can also be used when the input color space is the RGB color space. In the color conversion LUT 120 before correction, when the input value is R=G=B, the input value is considered to be a monochromatic input value containing only the K component. When ink retention is specified, and in the color conversion LUT before correction, the input value is R=G=B, and the output value associated with the input value contains color components other than the K component, i.e., non-asymmetric color components, the color conversion LUT is corrected by increasing the K component in the output value and decreasing at least one of the color components other than the K component in the output value. The K value corresponding to the input value in the RGB color space can be obtained by converting the input value to the CMYK color space. For example, when the input value R=G=B=0 is converted to the CMYK color space, it becomes C=M=Y=0, K=100. Furthermore, propagation correction is performed by correcting the output value associated with the propagation grid points in a predetermined range around the grid point where R=G=B.

[0072] The method for ink retention correction and propagation correction according to the above embodiment can also be used when the input color space is the CMY color space. In the color conversion LUT 120 before correction, if the input value is C=M=Y, the input value is considered to be a monochromatic input value containing only the K component. If ink retention is specified, and in the color conversion LUT before correction, the input value is C=M=Y, and the output value associated with the input value contains a non-asymmetric color component, the color conversion LUT is corrected by increasing the K component of the output value and decreasing at least one of the color components other than the K component in the output value. When the input color space is the CMY color space, the K value corresponding to the input value in the CMY color space can be determined by a known method. Furthermore, propagation correction is performed by correcting the output value associated with the propagation grid points in a predetermined range around the grid point where C=M=Y.

[0073] (B5) In the above embodiment, an example of suppressing the occurrence of gradation jumps caused by black plate retention was described. The correction method for the color conversion LUT 120 described in the above embodiment can also be used when pure color retention is performed for color components other than the K component. Pure color retention means that when the input value contains only one of the color components C, M, Y, or K, only the ink of the color corresponding to that color component is output. Black plate retention is also a type of pure color retention. For example, if the input value contains only the C component, the amount of ink for color components other than the C component in the output value is set to zero. Note that the ink for the C component may include not only C but also Lc. For example, suppose pure color retention is performed for the C component. In this case, a replacement rate table 130 corresponding to the target C component is prepared in advance. The output value associated with the target grid point on the C axis is corrected, and the output value associated with the ripple grid points existing in a predetermined range around the target grid point is corrected. As a result, the occurrence of gradation jumps can be suppressed.

[0074] (B6) The printing apparatus 400 may be capable of performing printing using spot color inks. Spot colors are colors other than CMYK process colors, such as Orange, Red, Green, Violet, and Blue. Note that Lc (Light Cyan), Lm (Light Magenta), Lk (Light Black), and LLk (Light Light Black) are process colors, not spot colors.

[0075] The printing device 400 may be capable of performing printing using four CMYK inks. The method of the above embodiment can also be adopted even if the ink used by the printing device 400 is not multicolor. The method of the above embodiment is more effective when the ink used by the printing device 400 is multicolor compared to when it is four colors.

[0076] C. Other forms: This disclosure is not limited to the embodiments described above, and can be implemented in various forms without departing from its spirit. For example, this disclosure can also be implemented in the following forms. The technical features in the embodiments described below that correspond to the technical features in each of the forms described below can be replaced or combined as appropriate in order to solve some or all of the problems of this disclosure, or to achieve some or all of the effects of this disclosure. Furthermore, if such technical features are not described as essential in this specification, they can be deleted as appropriate.

[0077] (1) According to a first embodiment of the present disclosure, a color conversion method is provided. This color conversion method includes (a) receiving an instruction to hold the black plate, (b) preparing a color conversion table that defines the correspondence between the input values ​​of the input color space in the image data and the output values ​​of the output color space in a printing device that prints the image data represented by the image data, and (c) when the instruction is received and the color conversion using the color conversion table converts the monochromatic input value, which is the input value containing only the K component in the image data, to the output value containing the non-symmetric color component, which is the color component other than the K component, among the output values ​​that are associated with the symmetric grid points, which are grid points representing the monochromatic input value in the color conversion table, (d) A step of performing a black plate holding correction to correct the color conversion table so as to increase the K component and decrease the non-asymmetric color component; and a step of performing a propagation correction to propagate the black plate holding correction to propagation grid points in the color conversion table that are within a predetermined range around the target grid point, wherein the output value associated with the propagation grid point in the color conversion table is corrected using the amount of increase in the K component due to the black plate holding correction and a replacement rate table that defines a color component replacement rate for the non-asymmetric color component in order to suppress color fluctuations caused by the propagation correction. According to the above configuration, when black plate retention is performed, propagation correction is applied to the color conversion table to propagate the black plate retention to the output values ​​associated with the surrounding propagation grid points of the target grid point. This suppresses the occurrence of gradation jumps caused by black plate retention during color conversion using the corrected color conversion table. Furthermore, to suppress color fluctuations caused by propagation correction, a replacement rate table is used, which defines the color component replacement rate for non-asymmetric color components other than the K component, to correct the output values ​​converted from input values ​​containing non-asymmetric color components in the color conversion table. Thus, color fluctuations caused by propagation correction can be suppressed.

[0078] (2) In the color conversion method of the above form, the output color space is a color space that includes at least a K component, and the replacement rate table may define the color component replacement rate of the non-asymmetric color component for each grayscale when the K axis in the output color space is divided into multiple grayscales. For example, suppose AtoBTag converts CMYK values ​​to Lab values, and the number of grid points, which indicates the number of grid points on each axis of the CMYK color space, is 17, and the number of color channels is 7 because there are 7 types of colorants in the printing device. In this case, the total number of grid points is 17^7 = 410,338,673. As the number of grid points increases, it is expected that the data size of AtoBTag will become very large. The replacement rate table used in the above configuration defines the color component replacement rate for asymmetric color components for each gradation when the K axis in the output color space is divided into 256 gradations. In the above configuration, instead of using AtoBTag, which has a very large data size, a compact replacement rate table can be used to suppress color fluctuations caused by propagation correction. Also, since the number of dimensions of the replacement rate table is smaller than the number of dimensions of AtoBTag, the amount of data stored in the color profile can be reduced.

[0079] (3) In the color conversion method of the above form, in step (d), a record is selected from the replacement rate table in which a color component composition ratio close to the color component composition ratio for the non-asymmetric color component of the output value associated with the propagation grid point in the color conversion table is defined, and the output value associated with the propagation grid point in the color conversion table is corrected using the amount of increase of the K component due to the black plate holding correction and the selected record. According to the above configuration, by selecting the replacement rate of the color component composition represented by the output value associated with the grid points in the color conversion LUT, fluctuations in color values ​​caused by propagation correction can be suppressed when performing color conversion using the corrected color conversion table.

[0080] (4) In the color conversion method of the above form, step (d) may include: (d1) a step of calculating a K increase amount, which is the increase amount of the K component due to the black plate holding correction; (d2) a step of calculating a K axis separation distance, which is the distance between the target grid point and the propagation grid point in the input color space; (d3) a step of calculating a K correction value as a value obtained by reducing the K increase amount according to the K axis separation distance; (d4) a step of calculating a correction value for each ink color excluding the K component by multiplying the K correction value by each of the color component replacement rates for the non-target color components defined in the record selected from the replacement rate table; (d5) a step of adding the K correction value to the K value among the output values ​​associated with the propagation grid point in the color conversion table; and (d6) a step of subtracting the correction value from the color value representing the color component excluding the K component among the output values ​​associated with the propagation grid point in the color conversion table.

[0081] (5) In the color conversion method of the above form, the output value defined in the color conversion table represents the amount of ink for each ink color, and the replacement rate table further defines the amount of ink for the K component in the output color space for each gradation, and in step (c), the monochromatic input value representing the target grid point in the color conversion table is converted to brightness in the device-independent color space, the amount of ink for the K component corresponding to the brightness is obtained from the replacement rate table as the updated ink amount, the amount of ink for the K component among the output value associated with the target grid point in the color conversion table is updated with the updated ink amount, and the amount of ink for the non-target color component among the output value associated with the target grid point in the color conversion table may be set to zero. According to the above configuration, the amount of K component ink can be easily obtained when correcting the ink plate holding process.

[0082] (6) According to a second embodiment of the present disclosure, a color conversion device is provided. This color conversion device includes a selection unit that receives an instruction to hold a black plate, a table creation unit that creates a color conversion table that defines the correspondence between the input values ​​of the input color space in the image data and the output values ​​of the output color space in a printing device that prints the image data represented by the image data, and when the instruction is received and the color conversion using the color conversion table converts the monochromatic input value, which is the input value containing only the K component in the image data, to the output value containing non-symmetric color components, which are color components other than the K component, among the output values ​​that are associated with the symmetric grid points, which are grid points representing the monochromatic input value in the color conversion table, The system includes a correction unit which performs a black plate holding correction to correct the color conversion table so as to increase the K component and decrease the non-asymmetric color component, and a propagation correction to propagate the black plate holding correction to propagation grid points in a predetermined range around the target grid point among the grid points representing the input value in the color conversion table, wherein the propagation correction is performed to correct the output value associated with the propagation grid point in the color conversion table using the amount of increase of the K component due to the black plate holding correction and a replacement rate table which defines a color component replacement rate for the non-asymmetric color component in order to suppress color fluctuations caused by the propagation correction, and

[0083] (7) According to a third embodiment of the present disclosure, a computer program is provided. This program has a function to receive an instruction to hold the black plate, a function to create a color conversion table that defines the correspondence between the input values ​​of the input color space in the image data and the output values ​​of the output color space in a printing device that prints the image data represented by the image data, and when the instruction is received and the color conversion using the color conversion table converts the monochromatic input value, which is the input value containing only the K component in the image data, to the output value containing non-symmetric color components, which are color components other than the K component, the program increases the K component in the output value that is associated with the symmetric grid point, which is the grid point representing the monochromatic input value in the color conversion table. The computer is provided with the following functions: a function to perform black plate retention correction to correct the color conversion table so as to reduce the non-asymmetric color component; and a function to perform propagation correction to propagate the black plate retention correction to propagation grid points in the color conversion table that are within a predetermined range around the target grid point, among the grid points representing the input value in the color conversion table, wherein the propagation correction is performed to correct the output value associated with the propagation grid point in the color conversion table using the amount of increase in the K component due to the black plate retention correction and a replacement rate table that defines a color component replacement rate for the non-asymmetric color component in order to suppress color fluctuations caused by the propagation correction.

[0084] This disclosure can also be implemented in various forms other than a color conversion method, a color conversion device, and a computer program. For example, it can be implemented in the form of an image processing method or a non-transitory storage medium on which a computer program is recorded. [Explanation of symbols]

[0085] 10…Printing system, 100…Image processing device, 101…Memory, 102…Input / Output interface, 103…Processor, 104…Internal bus, 110…Program, 120…Color conversion LUT, 130…Replacement rate table, 200…Input device, 210…Color conversion unit, 220…Correction unit, 230…Selection unit, 240…Print data generation unit, 300…Display device, 400…Printing device, IM…Input image data, IPF…Input profile, OPF…Output profile, PL1…Plane, P1~P2…Grid points, P11~P13…Grid points, W1…Selection screen

Claims

1. A color conversion method, (a) A process of receiving instructions to hold the ink plate, (b) A step of preparing a color conversion table that defines the correspondence between the input values ​​of the input color space in the image data and the output values ​​of the output color space in a printing device that prints the image data represented by the image data, (c) When the above instruction is received, and the monochromatic input value, which is the input value containing only the K component in the image data, is converted by the color conversion using the color conversion table to the output value, which includes non-asymmetric color components, which are color components other than the K component, The process of performing black plate holding correction is to correct the color conversion table such that the K component is increased and the non-asymmetric color component is decreased among the output values ​​associated with target grid points, which are grid points representing the monochromatic input values ​​in the color conversion table. (d) A step of performing a propagation correction to propagate the ink plate holding correction to propagation grid points in the color conversion table that are within a predetermined range around the target grid point, among the grid points representing the input value in the color conversion table, wherein the propagation correction is performed to correct the output value associated with the propagation grid point in the color conversion table, using the amount of increase of the K component due to the ink plate holding correction and a replacement rate table that defines a color component replacement rate for the non-target color component in order to suppress color fluctuations caused by the propagation correction, A color conversion method, including...

2. A color conversion method according to claim 1, The output color space is a color space that includes at least the K component. The replacement rate table defines the color component replacement rate of the non-asymmetric color component for each grayscale when the K axis in the output color space is divided into multiple grayscales. Color conversion method.

3. A color conversion method according to claim 2, In step (d) above, In the color conversion table, a record is selected from the replacement rate table in which a color component composition ratio close to the color component composition ratio of the non-asymmetric color component of the output value associated with the propagation grid point is defined. The amount of increase in the K component due to the ink plate holding correction and the selected record are used to correct the output value associated with the propagation grid point in the color conversion table. Color conversion method.

4. A color conversion method according to claim 3, The aforementioned step (d) is, (d1) A step of calculating the K increase amount, which is the increase amount of the K component due to the correction of holding the ink plate, (d2) A step of calculating the K-axis separation distance, which is the distance between the target grid point and the propagation grid point in the input color space, (d3) A step of calculating the K correction value as the amount obtained by decreasing the K increase amount according to the K axis separation distance, (d4) A step of calculating a correction value for each ink color excluding the K component by multiplying the K correction value by each of the color component replacement rates for the non-target color components defined in the record selected from the replacement rate table, (d5) A step of adding the K correction value to the K value among the output values ​​associated with the propagation grid points in the color conversion table, (d6) A step of subtracting the correction value from the color value representing the color component excluding the K component among the output values ​​associated with the propagation grid points in the color conversion table, A color conversion method, including...

5. A color conversion method according to claim 4, The output values ​​defined in the aforementioned color conversion table represent the amount of ink for each ink color. The substitution ratio table further defines the amount of ink for the K component in the output color space for each of the grayscale levels. In step (c) above, In the color conversion table, the single-color input value representing the target grid point is converted to brightness in a device-independent color space. From the substitution rate table, the amount of ink of the K component corresponding to the brightness is obtained as the updated ink amount. In the color conversion table, the amount of ink in the K component of the output value associated with the target grid point is updated with the updated ink amount. In the color conversion table, the amount of ink for the non-target color component among the output values ​​associated with the target grid point is set to zero. Color conversion method.

6. It is a color conversion device, A selection section that accepts instructions to retain the ink plate, A table creation unit creates a color conversion table that defines the correspondence between the input values ​​of the input color space in the image data and the output values ​​of the output color space in the printing device that prints the image data, When the above instruction is received, and the color conversion using the color conversion table converts the monochromatic input value, which is the input value containing only the K component in the image data, to the output value, which includes asymmetric color components, which are color components other than the K component, In the color conversion table, a black plate holding correction is performed to correct the color conversion table such that the K component is increased and the non-asymmetric color component is decreased among the output values ​​associated with the target grid points, which are grid points representing the monochromatic input values. A propagation correction is performed to propagate the ink plate retention correction to propagation grid points within a predetermined range surrounding the target grid point among the grid points representing the input value in the color conversion table, wherein the propagation correction is performed to correct the output value associated with the propagation grid point in the color conversion table, using the amount of increase of the K component due to the ink plate retention correction and a replacement rate table that defines a color component replacement rate for the non-target color component in order to suppress color fluctuations caused by the propagation correction. Correction unit, A color conversion device equipped with the following features.

7. A program that is executed by a computer, A function to receive instructions to hold the ink plate, A function to create a color conversion table that defines the correspondence between the input values ​​of the input color space in the image data and the output values ​​of the output color space in the printing device that prints the image data, When the above instruction is received, and the color conversion using the color conversion table converts the monochromatic input value, which is the input value containing only the K component in the image data, to the output value, which includes asymmetric color components, which are color components other than the K component, A function to perform black plate retention correction, which corrects the color conversion table such that the K component is increased and the non-asymmetric color component is decreased among the output values ​​associated with target grid points, which are grid points representing the monochromatic input values ​​in the color conversion table, A function to perform a propagation correction for propagating the ink plate retention correction to propagation grid points within a predetermined range surrounding the target grid point among the grid points representing the input value in the color conversion table, wherein the propagation correction is performed by correcting the output value associated with the propagation grid point in the color conversion table using the amount of increase of the K component due to the ink plate retention correction and a replacement rate table that defines a color component replacement rate for the non-target color component in order to suppress color fluctuations caused by the propagation correction, A program to make a computer realize this.

Images