A method for calibrating digital printing equipment

Abstract

The present invention discloses a method for calibrating a digital printing device, and belongs to the field of digital printing. The present invention loads the current calibration function to output test data under specific output conditions, uses a measuring device to measure the color value of each output color block in a device-independent color space, calculates the color difference of adjacent color blocks of each colorant, and regenerates a new calibration function. The present invention can realize the calibration of any colorant, and the actual imaging effect of a single colorant is visually close to uniform.

Description

Technical Field

[0001] The invention relates to the field of digital printing, in particular to a method for calibrating digital printing equipment. Background Art

[0002] The graphic information output by digital printing equipment (including digital printing equipment) is realized by imaging one or more colorants on the substrate. Due to the characteristics of the substrate and the colorant, when the digital printing equipment outputs a certain colorant on the substrate alone, it will cause the imaging color value to not match the input color value. For example: when a black and white digital printing equipment prints a gray block with a color value of 50%, the actual color value of the block on the paper is 70% black. The goal of equipment calibration is to ensure that the color value output on the specified substrate for each colorant used by the digital printing equipment is consistent with or close to the input color value under specific output conditions (including but not limited to specified digital printing equipment, colorants, and substrates).

[0003] The traditional equipment calibration method is based on the measurement of density. The general process is as follows: first, output the test file under specific output conditions, and then measure the density value of each color block on the substrate. The density measurement value is a multidimensional vector (D_VIS, D_RED, D_GREEN, D_BLUE), where the measurement values ​​of the black / cyan / magenta / yellow color blocks use the D_VIS, D_RED, D_GREEN, and D_BLUE component values ​​in the measurement value respectively.

[0004] Assume that Den(x) is a density measurement function of a colorant, where 0<=x<=1. Assume that MaxDen is the maximum density measurement value of the color system, and we get NDen(x)=Den(x) / MaxDen.

[0005] Introducing the reference function R(x) = -log 10 (1-(1-10 -1 / yule )*x)*yule, where 0<=x<=1, yule is an empirical value with a positive value. There are other options for reference functions, such as R(x)=x γ , where 0<=x<=1, and γ is an empirical value greater than 0.

[0006] The final calibration function is C(x)=NDen -1 (R(x))

[0007] The traditional method has two significant disadvantages:

[0008] 1. Due to the limitation of density measurement value, the calibration effect of colorants other than cyan, magenta, yellow and black is not good;

[0009] 2. Since the reference function has parameters based on empirical values, the effect of the calibration function is not always visually uniform enough. Summary of the invention

[0010] In view of the shortcomings of the prior art, the object of the present invention is to provide a method for calibrating a digital printing device, by which any colorant can be calibrated, and the actual imaging effect of a single colorant is visually close to uniform.

[0011] The specific technical solutions of the present invention are as follows:

[0012] A method for calibrating a digital printing device, such as Figure 1 As shown, the steps include:

[0013] 1) Obtaining output test data of the calibration function loaded under the current output condition of the digital printing device; wherein there are N color blocks of colorant X in the test data, x n is the color value represented by the nth color block, where x0=0, x N =1, 1<=n<=N, C(x) is the calibration function currently applied to colorant X, 0<=x<=1, 0<=C(x)<=1, and the initial calibration function for any colorant is C(x)=x;

[0014] 2) Use a measuring device to measure the color value of each color block on the specified substrate in the device-independent color space, and obtain the actual measured value of the color block related to colorant X as y n , where 1<=n<=N, y n is a multidimensional vector;

[0015] 3) Select the color difference calculation function Dis(c1,c2) ​​on the measured value, where c1 and c2 are any two color values ​​in the selected device-independent color space. The color difference calculation function must meet the following two conditions:

[0016] Dis(c1,c2)>0, if c1,c2 are not equal

[0017] Dis(c1,c2)=0, if c1=c2

[0018] Calculate the color difference between adjacent color blocks of a colorant, assuming d n =Dis(y n -1,y n ), where 1<=n<=N, supplement y0=y1;

[0019] 4) Calculate the set With the set {(x n ,x n)|1<=n<=N}, given a threshold T, if Diff is less than T, the current device calibration result meets the requirements; if not, a new calibration function C(F(x)) is generated, and step 1) is repeated; wherein, a discrete function f(x) is constructed, Expand the domain of f(x) to 0<=x<=1 to form the function F(x), where F(x n )=f(x n ), 1<=n<=N.

[0020] In the present invention, the color values ​​of each colorant are not less than 3, which must include the color values ​​of 0% and 100% of the colorant.

[0021] The beneficial effect of the present invention is that a calibration function can be generated for any colorant, and the change from light to dark of a single colorant is closer to uniform visually. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the calibration process of the digital printing device of the present invention;

[0023] Figure 2 It is a schematic diagram of test data in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0024] Step S1: Obtaining output test data of the digital printing device under the current output condition with the calibration function loaded

[0025] Under the output conditions of the digital printing device (including but not limited to the specified digital printing device, colorant, and substrate), load the current calibration function to output test data. The test data needs to include the color value of each colorant, and each colorant must have at least 3 different color values, which must include the color value of 0% and 100% of the colorant. Assume that there are N color blocks of colorant X in the test data, x n Represents the color value represented by the nth color block, where x0=0, x N =1, 1<=n<=N, and x n Strictly monotonically increasing. Assuming C(x) is the calibration function currently applied to colorant X, then 0<=x<=1, 0<=C(x)<=1. For any colorant, the initial calibration function is C(x)=x.

[0026] Figure 2 This is a special case of the test data, which embodies four colorants: cyan, magenta, yellow, and black. Each colorant contains 21 color values / color blocks from 0% to 100% with an increment of 5% every time.

[0027] Step S2: Measuring color values ​​in test data

[0028] Use a measuring device to measure the color value of each color block on the specified substrate in a device-independent color space. Commonly used device-independent color spaces include: CIE-XYZ, CIE-L * u * v * 、CIE-L * a * b * The actual measured value of the color block related to the colorant X in step S1 is y n , where 1<=n<=N, y n is a multidimensional vector.

[0029] Step S3: Calculate the color difference between adjacent color blocks of each coloring agent

[0030] Select the color difference calculation function Dis(c1,c2) ​​on the measured value, where c1 and c2 are any two color values ​​in the selected device-independent color space. The color difference calculation function must meet the following two conditions:

[0031] Dis(c1,c2)>0, if c1,c2 are not equal

[0032] Dis(c1,c2)=0, if c1=c2

[0033] Calculate the color difference of the color values ​​of adjacent color blocks of a colorant, assuming d n =Dis(y n -1,y n ), where 1<=n<=N, supplement y0=y1;

[0034] As in CIE-L * a * b * In the color space, let y1 = (L1, a1, b1), y2 = (L2, a2, b2)

[0035] The common color difference calculation function is ⊿E defined by the International Color Consortium (ICC) ab Color difference calculation formula

[0036]

[0037] and ⊿E 00 Color difference calculation formula.

[0038] Step S4: Determine whether the current calibration result meets the requirements

[0039] Calculating Collections With the set {(x n ,x n )|1<=n<=N}. There are many ways to calculate Diff, for example

[0040]

[0041] or

[0042]

[0043] or

[0044]

[0045] Given a threshold value T, 0<=T<=1, if Diff is less than T, then the operation steps are ended; if not, step S5 is executed.

[0046] Step S5: Generate a new current calibration function

[0047] Construct a discrete function f(x) where Expand the domain of f(x) to 0<=x<=1 to form the function F(x), where F(x n )=f(x n ), 1<=n<=N. Then C(F(x)) is the new current calibration function applied to colorant X under the current output conditions.

[0048] There are many ways to extend discrete functions to continuous domains and ranges, including broken lines, quadratic / cubic spline functions, etc.

[0049] Repeat step S1.

[0050] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. A person skilled in the art may modify or make equivalent substitutions for the technical solutions of the present invention without departing from the spirit and scope of the present invention. The protection scope of the present invention shall be subject to the claims.

Claims

1. A method for calibrating a digital printing device, the steps comprising: 1) Obtaining output test data of a calibration function loaded under current output conditions of a digital printing device; There are N color blocks of colorant X in the test data, x n is the color value represented by the nth color block, where x0=0, x N =1, 1<=n<=N, C(x) is the calibration function currently applied to colorant X, 0<=x<=1, 0<=C(x)<=1, and the initial calibration function for any colorant is C(x)=x; 2) Use a measuring device to measure the color value of each color block on the specified substrate in the device-independent color space, and obtain the actual measured value of the color block related to colorant X as y n , where 1<=n<=N, y n is a multidimensional vector; 3) Select the color difference calculation function Dis(c1,c2) ​​on the measured value, where c1 and c2 are any two color values ​​in the selected device-independent color space. The color difference calculation function must meet the following two conditions: Dis(c1,c2)>0, if c1,c2 are not equal; Dis(c1,c2)=0,if c1=c2; Calculate the color difference between adjacent color blocks of a colorant, assuming d n =Dis(y n-1 ,y n ), where 1<=n<=N, supplement y0=y1; 4) Calculate the set With the set {(x n ,x n )|1<=n<=N}, given a threshold T, if Diff is less than T, the current device calibration result meets the requirements; if not, a new calibration function C(F(x)) is generated, and step 1) is repeated; wherein, a discrete function f(x) is constructed, Expand the domain of f(x) to 0<=x<=1 to form the function F(x), where F(x n )=f(x n ), 1<=n<=N; Among them, the color value x of each colorant n There must be at least 3 monotonically increasing color values, including the color values ​​of 0% and 100% of the colorant.

2. The method for calibrating a digital printing device according to claim 1, characterized in that: Device-independent color spaces include: CIE-XYZ, CIE-L * u * v * 、CIE-L * a * b * .

3. The method for calibrating a digital printing device according to claim 1, characterized in that: The calculation formula of the difference Diff is:

4. The method for calibrating a digital printing device according to claim 1, characterized in that: The calculation formula of the difference Diff is:

5. The method for calibrating a digital printing device according to claim 1, characterized in that: The calculation formula of the difference Diff is:

6. The method for calibrating a digital printing device according to claim 1, characterized in that: Methods for forming the function F(x) include broken line, quadratic / cubic spline functions.

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