JP5278699B2 - Color image processing apparatus, color image processing method, program, and recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To set a one-dimensional correction table so as to be favorably balanced with a target color of each secondary color. <P>SOLUTION: A gamma correction part 1 gamma-corrects image data of a patch by the one-dimensional correction table, and a printer output part 2 converts the image data into an amount of a color material to output the converted result to a paper surface 3. A colorimetry part 4 performs the colorimetry of each patch outputted to the paper surface 3 to obtain a colorimetric value. A setting part 5 prepares an output table of a CMY plain color on the basis of the colorimetric value of each patch, prepares a plain color correction table for R, G, and B using an approximate expression computed with the colorimetric value, performs the weighting synthesis of the plain color correction table for R, G, and B, and thereafter combines the result of the weighting synthesis with the output table of the CMY plain color to prepare the one-dimensional correction table for secondary color calibration. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a color image processing apparatus, a color image processing method, a program, and a recording medium for calibrating a secondary color.

  In color printers and color MFPs (digital color multifunction peripherals), gradation characteristics fluctuate due to changes over time and the like, and calibration is performed to correct gradation so that there is no deviation from the target gradation characteristics. However, even if calibration is performed so that the primary colors (C, M, Y) match the target value, the secondary color (mixed color of two colors) and the tertiary color (mixed color of three colors) are stable in the same color. Therefore, it is necessary to calibrate mixed colors. Natural images include mostly mixed colors, and it is a great advantage that the mixed colors are always output stably in the same color.

  In the mixed color calibration, the first priority is the gray balance of the gray (hereinafter referred to as 3C gray) expressed by the three-color superposition which is the tertiary color, and then the secondary color from the viewpoint of a more accurate calibration. Is considered.

  On the other hand, since calibration requires real-time processing, it is desirable that the processing amount be as small as possible. If only the accuracy is taken into consideration, the mixed color calibration that matches the secondary color and tertiary color to the target color requires a high-order correction table such as 3D, but one-dimensional correction is taken into consideration to reduce the processing load. It is practical to implement color mixing calibration using a table.

  Therefore, a method for setting a one-dimensional correction table so as to match the secondary color to the target color has been proposed for color mixture calibration (see, for example, Patent Document 1).

  However, in the method of Patent Document 1 described above, three secondary colors, that is, R (red), C, and C, which are combinations of M and Y among C, M, and Y due to restrictions realized by a one-dimensional correction table. G (green), which is a combination of Y, B (blue), which is a combination of C and M, one-dimensional so that it matches the target color for all colors, and well-balanced to the target color of each secondary color It is difficult to set a correction table.

The present invention has been made in view of the above problems,
An object of the present invention is to provide a color image processing apparatus, a color image processing method, a program, and a recording medium that set a one-dimensional correction table so as to be well balanced with the target colors of the respective secondary colors.

The present invention provides a color image processing apparatus that calibrates color signals corresponding to three or more color materials, wherein the first secondary color composed of the first color material and the second color material is the first color. A first correction table setting means for setting a first one-dimensional correction table to match a target color of one secondary color, and a second second color comprising the first color color material and the third color color material. Second correction table setting means for setting a second one-dimensional correction table so that the next color matches the target color of the second secondary color, the first one-dimensional correction table, and the second one-dimensional Third correction table setting means for setting a third one-dimensional correction table for correcting a color signal corresponding to the first color color material by weighting and combining the correction tables; and the first color color material; The second color material and the third color material A fourth correction table setting means for setting a fourth one-dimensional correction table so that the tertiary color of the third color matches the gray target color, and the fourth one-dimensional correction for the gradation value corresponding to the low density side In order to apply the third one-dimensional correction table to the gradation value corresponding to the high density side to which the table is applied, the two first-dimensional correction tables are synthesized according to the gradation value to obtain the fifth primary. A fifth correction table setting means for setting an original correction table, and performing calibration based on the fifth one-dimensional correction table is the main feature.

  According to the present invention, the one-dimensional correction table for calibrating the secondary color is set by weighting and synthesizing the one-dimensional correction table set so as to match the target color of each secondary color. The secondary color can be output in a balanced and stable color, and the gray balance and the secondary color stability can both be achieved.

The structure of the Example of this invention is shown. A patch for secondary color calibration is shown. It is a figure explaining 5x5 patch. FIG. 6 is a diagram illustrating processing of a setting unit according to the first embodiment. An example in which a combination of Mr (i), Yr (i), and L * is approximated by a polynomial is shown. It is a figure which converts L * into an input gradation value. The monochrome target correction table for R calibration is shown. FIG. 10 is a diagram illustrating processing of a setting unit according to the second embodiment. It is a figure explaining the application example of the secondary color calibration which does not affect a gray balance.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  According to the present invention, when setting a one-dimensional correction table for calibrating a secondary color, in the setting example of a one-dimensional correction table for C (cyan), two secondary colors G (green) and B (blue) including C are used. It is essential that the first correction table and the second correction table set so as to meet the respective target values are synthesized with optimum weights determined in consideration of the average value of the difference from the target color, etc. To do.

  FIG. 1 shows the configuration of an embodiment of the present invention (configuration of an embodiment in which a one-dimensional correction table for secondary color calibration is set).

  The gamma correction unit 1 performs correction according to the one-dimensional correction table on the CMY image data composed of the secondary color calibration patches, and the printer output unit 2 converts the image data into a color material amount. And output to the paper surface 3. Although the input / output characteristics of the printer output unit 2 are ideally linear, the characteristics fluctuate as described above.

  The color measurement unit 4 measures the color of each patch output on the paper surface 3 using a colorimeter, and obtains an L * a * b * value. The secondary color calibration one-dimensional correction table setting unit 5 (hereinafter, setting unit 5) sets the one-dimensional correction table so that the secondary color matches the target color. The one-dimensional correction table applied in the gamma correction unit 1 at the time of patch output for secondary color calibration is a one-dimensional correction table set during the most recently performed calibration.

  Hereinafter, a gradation value input to the gamma correction unit 1 is referred to as an input gradation value, and a gradation value output from the gamma correction unit 1 is referred to as an output gradation value.

  FIG. 2 shows a patch for secondary color calibration. In FIG. 2, “for R” is a patch composed of gradation values around M = Y for setting a one-dimensional correction table for M and Y so as to match the target color of R. A 5 × 5 patch surrounded by a dotted line is a patch composed of gradation values around the gradation value (M, Y) = (d, d). Here, the gradation value d as the center is assumed to be eight stages of d = 16, 48, 80, 112, 144, 176, 208, 240. The same applies to G and B. A patch composed of gradation values around C = Y for setting a one-dimensional correction table of C and Y to match the target color of G and a target color of B. This is a patch composed of gradation values around C = M for setting a one-dimensional correction table for C and M. Further, as shown on the right side of FIG. 2, C, M, and Y single-color gradation patches (33 patches from 0 to 255 every 8 gradations) are also prepared.

  FIG. 3 is a diagram illustrating the 5 × 5 patch surrounded by the dotted line of the secondary color calibration patch of FIG. Taking the R patch as an example, the center patch is (M, Y) = (d, d), and the M gradation value is assigned to five levels from d-16 to d + 16 in the vertical direction, and the horizontal direction. A patch is prepared in which the gradation value of Y is changed in five stages from d-16 to d + 16.

FIG. 4 is a diagram illustrating processing of the setting unit 5 according to the first embodiment.
(A) Creation of a single color output characteristic table From the L * a * b * colorimetric values of the C, M, and Y single color gradation patches, a color difference ΔE with paper white is calculated, respectively, and the output gradation value, paper white, Is obtained, and a single color output characteristic table is created.

(B) Creation of single-color target correction table for R calibration Expressions 1 and 2 are approximate expressions for obtaining the color difference ΔE between the single color M and the white paper from the a * and b * values of the R target color. .

Ｍｒ（ｉ）、Ｙｒ（ｉ）はＭとＹの紙白との色差ΔＥである。ｉは１〜８の整数値をとり、二次色キャリブレーション用パッチの階調値ｄ＝１６、４８、…、２４０に対応する。ｉ値毎に対応するｄのＲ用５×５パッチの測色値に基づき、パラメータｍｒ１１（ｉ）〜ｍｒ３２（ｉ）を算出する。式１と式２は線形式であるが、全階調を線形式で近似すると精度が不十分になるため、８階調で区分近似する。二次色や三次色ではａ＊、ｂ＊平面での色相ズレや色相ばらつきが特に目に付きやすく、キャリブレーションで厳密に合わせる必要がある。
ＭとＹの紙白との色差ΔＥから、ＭとＹを重ねて出力したときの混色のＬ＊を求める近似式を式３とする。

Mr (i) and Yr (i) are color differences ΔE between M and Y paper white. i takes an integer value of 1 to 8, and corresponds to the gradation values d = 16, 48,..., 240 of the secondary color calibration patch. The parameters mr11 (i) to mr32 (i) are calculated based on the colorimetric values of the R 5 × 5 patch corresponding to each i value. Equations 1 and 2 are in linear form, but if all gradations are approximated in line form, the accuracy becomes insufficient, and therefore, a piecewise approximation is performed with 8 gradations. In secondary and tertiary colors, hue shifts and hue variations on the a * and b * planes are particularly noticeable and must be matched precisely by calibration.
An approximate expression for obtaining L * of the mixed color when M and Y are superimposed and output from the color difference ΔE between M and Y paper white is represented by Expression 3.

ＭｒとＹｒはＭとＹの紙白との色差ΔＥである。Ｒ用の全パッチの測色値に基づき、パラメータｋｒ１〜ｋｒ３を算出する。

Mr and Yr are the color difference ΔE between M and Y paper white. Based on the colorimetric values of all R patches, parameters kr1 to kr3 are calculated.

Using Equation 1 and Equation 2, the combination of Mr (i) and Yr (i) that realizes the a * and b * values of the target color of R is obtained for all of i = 1 to 8, and Equation 3 is used. The mixed color L * corresponding to the combination of Mr (i) and Yr (i) realizing the a * and b * values of the R target color is obtained, and Mr (i), Yr closest to the R target color is obtained. (I) Find a combination of L *.
L * is converted into an input gradation value, the correspondence between the input gradation value and the color difference ΔE between M and Y paper white that realizes the R target color is obtained, and a single-color target correction table for R calibration is obtained. create.
Details of the parameter setting mr11 (i) to mr32 (i) and the calculation method of kr1 to kr3 will be described later.

(C) Creation of single-color target correction table for G calibration The same thing as (b) is performed for G. Expressions 4 and 5 are approximate expressions for obtaining the color difference ΔE between the single color C and the Y paper white from the a * and b * values of the G target color.

Ｃｇ（ｉ）、Ｙｇ（ｉ）はＣとＹの紙白との色差ΔＥである。ｉは１〜８の整数値をとり、二次色キャリブレーション用パッチの階調値ｄ＝１６、４８、…、２４０に対応する。ｉ値毎に対応するｄのＧ用５×５パッチの測色値に基づき、パラメータｍｇ１１（ｉ）〜ｍｇ３２（ｉ）を算出する。

Cg (i) and Yg (i) are color differences ΔE between C and Y paper white. i takes an integer value of 1 to 8, and corresponds to the gradation values d = 16, 48,..., 240 of the secondary color calibration patch. Parameters mg11 (i) to mg32 (i) are calculated based on the colorimetric values of the G 5 × 5 patch corresponding to each i value.

  An approximate expression for obtaining L * of a mixed color when C and Y are superimposed and output from the color difference ΔE between C and Y paper white is represented by Expression 6.

ＣｇとＹｇはＣとＹの紙白との色差ΔＥである。Ｇ用の全パッチの測色値に基づき、パラメータｋｇ１〜ｋｇ３を算出する。

Cg and Yg are the color difference ΔE between C and Y paper white. Parameters kg1 to kg3 are calculated based on the colorimetric values of all the patches for G.

Using Equation 4 and Equation 5, the combination of Cg (i) and Yg (i) that realize the a * and b * values of the target color of G is obtained for all i = 1 to 8, and Equation 6 is used. L * of the mixed color corresponding to the combination of Cg (i) and Yg (i) that realizes the a * and b * values of the G target color is obtained, and Cg (i), Yg closest to the G target color is obtained. (I) Find a combination of L *.
L * is converted into an input tone value, the correspondence between the input tone value and the color difference ΔE between C and Y paper white that realizes the G target color is obtained, and a single-color target correction table for G calibration is obtained. create.

(D) Creation of single-color target correction table for B calibration The same thing as (b) and (c) is performed for B. Expressions 7 and 8 are approximate expressions for obtaining the color difference ΔE between the monochrome C and M paper white from the a * and b * values of the target color B.

Ｃｂ（ｉ）、Ｍｂ（ｉ）はＣとＭの紙白との色差ΔＥである。ｉは１〜８の整数値をとり、二次色キャリブレーション用パッチの階調値ｄ＝１６、４８、…、２４０に対応する。ｉ値毎に対応するｄのＢ用５×５パッチの測色値に基づき、パラメータｍｂ１１（ｉ）〜ｍｂ３２（ｉ）を算出する。

Cb (i) and Mb (i) are color differences ΔE between C and M paper white. i takes an integer value of 1 to 8, and corresponds to the gradation values d = 16, 48,..., 240 of the secondary color calibration patch. Parameters mb11 (i) to mb32 (i) are calculated based on the colorimetric values of the 5 × 5 patches for B corresponding to each i value.

  An approximate expression for obtaining L * of the color mixture when C and M are output in an overlapping manner from the color difference ΔE between C and M paper white is represented by Expression 9.

ＣｂとＭｂはＣとＭの紙白との色差ΔＥである。Ｂ用の全パッチの測色値に基づき、パラメータｋｂ１〜ｋｂ３を算出する。

Cb and Mb are color differences ΔE between C and M paper white. Based on the colorimetric values of all the patches for B, parameters kb1 to kb3 are calculated.

Using Equation 7 and Equation 8, the combination of Cb (i) and Mb (i) that realize the a * and b * values of the target color of B is obtained for all i = 1 to 8, and Equation 9 is used. L * of the mixed color corresponding to the combination of Cr (i) and Mr (i) realizing the a * and b * values of the B target color is obtained, and Cb (i) and Mb closest to the B target color are obtained. (I) Find a combination of L *.
L * is converted into an input gradation value, a correspondence relationship between the input gradation value and the color difference ΔE between C and M paper white that realizes the B target color is obtained, and a monochrome target correction table for B calibration is obtained. create.

(E) Weighting synthesis The monochrome target correction tables for R, G, B calibration created in (b), (c), and (d) are weighted and synthesized. When the weighting synthesis is expressed by an expression, Expressions 10 to 12 are obtained.

入力階調値ｓにおける、Ｍｒｅ（ｓ）とＹｒｅ（ｓ）はＲキャリブレーション用の単色ターゲット補正テーブルのＭとＹに関する出力値であり、Ｃｇｅ（ｓ）とＹｇｅ（ｓ）はＧキャリブレーション用の単色ターゲット補正テーブルのＣとＹに関する出力値であり、Ｃｂｅ（ｓ）とＭｂｅ（ｓ）はＢキャリブレーション用の単色ターゲット補正テーブルのＣとＭに関する出力値であり、Ｃ’ｅ（ｓ）とＭ’ｅ（ｓ）とＹ’ｅ（ｓ）は重み付け合成後の補正テーブルの出力値である。Ｐｃ、Ｐｍ、Ｐｙは０以上、１以下の値をとる重み係数である。最も単純には、Ｐｃ＝Ｐｍ＝Ｐｙ＝０．５とし、１：１の比率で合成する方法がある。それとは異なる重み係数設定方法に関しては後述する。

In the input gradation value s, Mre (s) and Yre (s) are output values related to M and Y in the monochromatic target correction table for R calibration, and Cge (s) and Yge (s) are for G calibration. Cbe (s) and Mbe (s) are output values related to C and M in the monochrome target correction table for B calibration, and C′e (s). , M′e (s), and Y′e (s) are output values of the correction table after weighted synthesis. Pc, Pm, and Py are weighting factors that take values of 0 or more and 1 or less. The simplest method is to synthesize Pc = Pm = Py = 0.5 at a ratio of 1: 1. A different weighting factor setting method will be described later.

(F) Creation of correction table for secondary color calibration From the monochromatic output characteristic table created in (a) and the correction table after weighted synthesis created in (e), a one-dimensional correction table for secondary color calibration Create (f). Both the table in (a) and the table in (e) use the fact that the vertical axis is the color difference ΔE from the monochromatic paper white, and the input tone value in the table in (e) is the color difference from the monochromatic paper white. By converting to ΔE, the table of (a) is reversed and the color difference ΔE from the monochromatic paper white is converted to the output gradation value, so that the correspondence relationship between the input gradation value and the output gradation value can be understood. A correction table (f) for next color calibration can be created.

  The correction table (f) for secondary color calibration created in this way is set in the gamma correction unit 1. Since the set input / output characteristics of the correction table (f) are inverse functions of the input / output characteristics of the printer output unit 2, the change in the gradation characteristics in the printer output unit 2 is canceled and the characteristics become linear.

  Next, details of (b) creation of a single-color target correction table for R calibration will be described. The same applies to (b) and (c), and only R in (b) will be described as a representative.

Procedure 1) Parameters mr11 (i) to mr32 (i) of Equation 1 and Equation 2 are calculated by linear regression analysis.
A set of colorimetric values a * and b * for the R 5 × 5 patch corresponding to i, and a colorimetric value L * a * b * for the monochrome gradation patch corresponding to the M and Y gradation values of the R patch The color difference ΔE from the paper white is calculated from the above, and a set of Mr (i) and Yr (i) is obtained and used as the matrix coefficient of Equation 13.

  By obtaining the matrix coefficients mr11 (i) to mr32 (i) on the right side of the right side by linear regression analysis, Mr (i), Yr obtained by substituting the colorimetric values a * and b * into Equation 1 and Equation 2 (I) Parameters mr11 (i) to mr32 (i) in which the mean square error between the (calculated value) and the measured value is the minimum in 25 sets of data (n in Equation 13 is n = 5 × 5) are calculated. Is done.

Procedure 2) Parameters kr1 to kr3 of Equation 3 are calculated by linear regression analysis.
The set of the color difference ΔE between the colorimetric value L * of the R patch and the paper white obtained from the colorimetric values of the single color gradation patch corresponding to the M and Y gradation values of the R patch is used as the matrix coefficient of Equation 14. By calculating the matrix coefficients kr1 to kr3 on the right side of the right side by linear regression analysis, the mean square error between L * obtained by substituting Mr and Yr into Equation 3 and the measured value is 200 sets of data (Equation The parameters kr1 to kr3 that are minimum when N of 14 is N = 25 × 8) are calculated.

Procedure 3) Using equations 1 to 3, find a combination of Mr (i), Yr (i), and L * that is close to the target value of R, and perform polynomial approximation based on that data.
For example, an example in which a combination of Mr (i), Yr (i), and L * that is close to the target value of R is found using Equations 1, 2 and 3 when i = 3 will be described. . i = 3 corresponds to the gradation value d = 80 of the secondary color calibration patch.

  Step 3-1) Input the a * and b * values of the target color at d = 72, 73,..., 88, which are gradation values around d = 80, into Equation 1 and Equation 2 when i = 3. , Mr (i), Yr (i) are calculated.

  Procedure 3-2) The calculated Mr (i) and Yr (i) are input as Mr and Yr in Equation 3, and L * is calculated.

Procedure 3-3) For each d value, a difference ΔL * between L * of the target color and the calculated L * is obtained, and Mr (i), Yr (i), L * of which the absolute value of ΔL * is minimized. Find a combination.
For i = 1, 2,..., the combination of Mr (i), Yr (i), and L * that is close to the R target color is calculated by the above procedure.

  FIG. 5 shows an example of a result obtained by plotting a combination of Mr (i), Yr (i), and L * close to the target value of R in the procedure 3) and performing polynomial approximation.

  The part where there is no data between plots needs to be filled in using approximation or interpolation. If polynomial approximation is used, it is possible to create a smooth line graph while suppressing the influence of noise components, and it is possible to perform calibration in which the hue does not fluctuate for each gradation.

  Procedure 4) Convert L * to input gradation value.

  FIG. 6 is a diagram for explaining conversion of L * into input gradation values. L * of paper white corresponding to the minimum value 0 of the input gradation value is set as the maximum value L * _max of L *. It is assumed that the gradation value is expressed by 8 bits, and the target color L * and R patch corresponding to the maximum value 255 of the input gradation value (M, Y) = (255, 255) From the colorimetric values L * of included patches (M, Y) = (255, 255), the larger value (brighter) is set as the minimum value L * _min of L *. In consideration of solid density fluctuation, if the patch of (M, Y) = (255, 255) is brighter than the target color, it cannot be corrected to the target value by gamma correction, so the colorimetric value is reflected. It is necessary to determine L * _min.

  Procedure 5) A monochrome target correction table for R calibration is created by combining FIG. 5 and FIG.

  FIG. 7 shows a monochrome target correction table for R calibration created by combining FIG. 5 and FIG.

  Since the horizontal axis in FIG. 5 and the vertical axis in FIG. 6 are both L *, when two graphs are combined, the input grayscale value on the horizontal axis in FIG. 6 is input and the paper white on the vertical axis in FIG. A graph with ΔE as an output can be created. The completed graph represents the M and Y single target colors expressed by ΔE with the paper white to match the R target color.

Next, a weight coefficient setting method in (e) weighted synthesis will be described.
(First Example of Weighting Factor Determination Method)
The weighting coefficients Pc, Pm, and Py are varied to obtain a weighting coefficient that minimizes the average value of the differences between the target colors of R, G, and B and the color calculated by the approximate expression. This will be specifically described below.

Weight coefficient determination procedure 1)
A temporary correction table for converting an input gradation value into a color difference ΔE with a monochromatic paper white is created by performing weighted composition using the weighting coefficients Pc1, Pm1, and Py1. Based on the created temporary correction table, the input gradation values (for example, every 16 gradation values d = (16, 32,..., 255)) are converted, and the color differences Ce ′ and Me from C, M, and Y paper white are converted. Find ', Ye'.

Weighting factor determination procedure 2)
When Formula 1 and Formula 2 that are approximate formulas for R are converted, Formula 15 and Formula 16 are obtained. That is, Expressions 15 and 16 are also approximate expressions.

Ｍｅ’、Ｙｅ’のセットを右辺のＭｒ（ｉ）、Ｙｒ（ｉ）に代入し、ａ＊、ｂ＊を算出する。目標色のａ＊、ｂ＊値と、算出したａ＊、ｂ＊値を、それぞれ彩度ｃ＊に変換し（ｃ＊＝√（ａ＊×ａ＊＋ｂ＊×ｂ＊））、彩度差Δｃ＊を求める。

Substituting the set of Me ′ and Ye ′ into Mr (i) and Yr (i) on the right side, a * and b * are calculated. The a * and b * values of the target color and the calculated a * and b * values are converted into saturation c * (c * = √ (a * × a * + b * × b *)), respectively. The difference Δc * is obtained.

  Similarly for G and B, Equations 4 and 5, and Equations 7 and 8 are converted into approximate equations in which the a * and b * values are on the left side, and then the saturation difference Δc * with the target color is obtained.

Weight coefficient determination procedure 3)
An average value of the saturation difference Δc * of all the colors R, G, and B is obtained.

Weighting determination procedure 4)
The weighting coefficient is changed and the weighting determination procedures 1, 2, and 3 are repeated to obtain a weighting coefficient that minimizes the average value of the saturation difference Δc *.

(Second Example of Weighting Factor Determination Method)
The weighting coefficients Pc, Pm, and Py are varied to obtain a weighting coefficient that minimizes the maximum difference between the target colors of R, G, and B and the color calculated by the approximate expression.
The procedure is almost the same as in the first embodiment, but the weight determination procedures 3 and 4 are changed to the following 3a and 4a.

Weighting factor determination procedure 3a)
The maximum value of the saturation difference Δc * for all R, G, and B colors is obtained.

Weighting determination procedure 4a)
The weighting coefficient is changed and weighting determination procedures 1, 2, and 3a are repeated to obtain a weighting coefficient that minimizes the maximum value of the saturation difference Δc *.

(Third embodiment of weighting factor determination method)
In accordance with the approximation accuracy, the weighting coefficient is adjusted so that the combination ratio of colors having poor approximation accuracy among R, G, and B is low.
The parameters of Equation 1 and Equation 2 for obtaining the combination of Mr (i) and Yr (i) from the R * values of a * and b * are obtained based on the colorimetric value by linear regression analysis in Equation 13. As described above, the parameter that minimizes the mean square error is obtained, and even if the colorimetric values a * and b * used to determine the parameters are substituted into the equations 1 and 2, they match the colorimetric values. Mr (i) and Yr (i) are not calculated, and there are variations. If the variation is large, the approximation accuracy is poor. Combining colors with poor approximation accuracy at a large ratio is not preferable because it only deteriorates the calibration accuracy. Therefore, the colorimetric values a * and b * used to determine the parameters for R are substituted into Equations 1 and 2 to calculate Mr (i) and Yr (i), and the standard deviation of the error from the colorimetric values. Similarly, the standard deviation is obtained for G and B, and the weighting coefficient is adjusted so that the composition ratio of the color having the large standard deviation is lowered. For example, on the basis of Pc = Pm = Py = 0.5, if the standard deviation of R is larger than G and B, the equation is changed to Pc = 0.5, Pm = 0.25, Py = 0.25. The weighting factors Pm and Py applied to Mre (s) and Yre (s), which are values related to R in 11 and Expression 12, are suppressed to small values.

  In the second embodiment, a correction table for M and Y is created so as to match the target value of one of the secondary colors (here, R), and the correction table for C is stored for the remaining two colors (here, G and B). It is an Example set based on a target value. For example, this embodiment is effective for a request that the corporate color is red and only red is output in a particularly stable color.

FIG. 8 is a diagram illustrating the processing of the setting unit 5 according to the second embodiment.
(A) Creation of Monochromatic Output Characteristics Table The same as FIG. 4 of the first embodiment.
(B) Creation of single-color target correction table for R calibration The same as FIG. 4 of the first embodiment.
(C) Creation of single-color target correction table for G calibration Using a combination of Mr (i), Yr (i), and L * that is close to the R target color obtained in (b), Yr (i ), An approximate expression for obtaining Cg (i) for realizing the G target color is represented by Expression 17. Here, the target color of G is represented by a color difference ΔEw from paper white. Parameters mg1 (i) -mg3 (i) are calculated by linear regression analysis using Equation 18. The color difference ΔEw1 (i) to ΔEwn (i) from the paper white calculated from the colorimetric values L * a * b * of the 5 × 5 patch for G corresponding to i, and the C and Y gradation values of the G patch The color differences Cg1 (i) to Cgn (i) and Yg1 (i) to Ygn (i) from the paper white calculated from the colorimetric values L * a * b * of the corresponding monochrome gradation patches are used as the matrix coefficients of Expression 18. , Parameters mg1 (i) to mg3 (i) are obtained.

(D) Creation of single-color target correction table for B calibration Using a combination of Mr (i), Yr (i), and L * that is close to the R target color obtained in (b), Mr (i ), An approximate expression for obtaining Cb (i) for realizing the target color of B is represented by Expression 19. Here, the target color of B is represented by a color difference ΔEw from paper white. Parameters mb1 (i) to mb3 (i) are calculated by linear regression analysis using Equation 20. The color difference ΔEw1 (i) to ΔEwn (i) from the paper white calculated from the colorimetric values L * a * b * of the 5 × 5 patch for B corresponding to i, and the C and M gradation values of the B patch The color differences Cb1 (i) to Cbn (i) and Mb1 (i) to Mbn (i) from the paper white calculated from the colorimetric values L * a * b * of the corresponding monochrome gradation patches are used as the matrix coefficients of Expression 20. , Parameters mb1 (i) to mb3 (i) are obtained.

(E) Weighting synthesis The monochrome target correction table for G and B calibration created in (c) and (d) is weighted and synthesized. The synthesis is only for the correction table of C, and the formula is the same as formula 10 in the first embodiment. The setting of the weighting coefficient Pc is the same as that of the first embodiment, depending on the method of simply setting Pc = 0.5, the minimum average color difference between the G and B target colors, the minimum maximum color difference, or the approximation accuracy. There is a way to set.
(F) Creation of correction table for secondary color calibration Regarding M and Y, the correction table for secondary color calibration (f) is created by combining the table created in (a) and the table created in (b). ). Regarding C, the table created in (a) and the table created in (e) are combined into a secondary color calibration correction table (f).
The correction table (f) for secondary color calibration created in this way is set in the gamma correction unit 1. Since the set input / output characteristics of the correction table (f) are inverse functions of the input / output characteristics of the printer output unit 2, the change in the gradation characteristics in the printer output unit 2 is canceled and the characteristics become linear.

  In pursuing higher accuracy in color mixture calibration, it is important to calibrate so that the secondary color can be output stably in the same color, but the most important is to match the secondary color to the target value. If the gray balance that should be lost is broken, it is a fall at the end. The following is an application example of secondary color calibration that does not affect the gray balance.

Application example 1)
In the technique of Patent Document 2, calibration is performed using a two-dimensional correction table. There is a one-dimensional axis corresponding to the primary color (single color), secondary color, and tertiary color (3C gray) in the two-dimensional plane, and the value of the one-dimensional correction table is applied to the two-dimensional correction table. create. Therefore, it is necessary to set a one-dimensional correction table to be applied to the axis corresponding to the secondary color, and the present invention can be applied.

Application example 2)
The printer performs processing for switching image processing according to object information, such as switching color correction with reference to object information. The one-dimensional correction table applied to the gamma correction unit 1 in FIG. 1 is switched for each object, and for characters and graphic objects, the one-dimensional correction of C, M, and Y created so that the secondary color matches the target color A one-dimensional correction table created so that 3C gray matches the target color can be applied to the image object by applying the table. In the case of characters and graphic objects, if 3C gray is converted to K single color and output, gray balance is not affected.

Application example 3)
FIG. 9 is a diagram illustrating an application example 3 of the secondary color calibration that does not affect the gray balance.
Gray balance is important in color mixture calibration, but high density areas with large input tone values are density areas where stable color output is not guaranteed due to engine characteristics, limiting the total amount of toner overlap. Output after processing the total amount restriction. In the density range where the total amount restriction is applied, even if a calibration patch near 3C gray is output, an unstable patch such as toner peeling occurs, and the calibration is set based on the colorimetric value. The accuracy of the correction table is questionable. Further, when reproducing gray with CMYK, the high density side (near solid) of the C, M, Y single colors is not used for gray output. Therefore, on the low density side used for gray output, a one-dimensional correction table set so that 3C gray matches the target value is applied, and on the high density side, the primary color set so that the secondary color matches the target color. The original correction table should be applied. In the vicinity of the gradation value at which the 3C gray correction table and the secondary color correction table are switched, the two tables are synthesized by smoothly changing the synthesis ratio.

  According to the present invention, a storage medium in which a program code of software for realizing the functions of the above-described embodiments is recorded is supplied to a system or apparatus, and a computer (CPU or MPU) of the system or apparatus is stored in the storage medium. This is also achieved by reading and executing the code. In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments. As a storage medium for supplying the program code, for example, a hard disk, an optical disk, a magneto-optical disk, a nonvolatile memory card, a ROM, or the like can be used. Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an OS (operating system) operating on the computer based on an instruction of the program code. A case where part or all of the actual processing is performed and the functions of the above-described embodiments are realized by the processing is also included. Further, after the program code read from the storage medium is written into a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. This includes the case where the CPU or the like provided in the board or function expansion unit performs part or all of the actual processing, and the functions of the above-described embodiments are realized by the processing. Further, the program for realizing the functions and the like of the embodiments of the present invention may be provided from a server by communication via a network.

1 Gamma correction unit 2 Printer output unit 3 Output (paper)
4 Color measurement unit 5 One-dimensional correction table setting unit for secondary color calibration

JP 2005-176032 A JP 2005-12793 A

Claims (4)

In a color image processing apparatus that calibrates color signals corresponding to three or more color materials, a first secondary color composed of a first color material and a second color material is a first secondary color. A first correction table setting means for setting a first one-dimensional correction table to match a target color of the color, and a second secondary color composed of the first color color material and the third color color material, A second correction table setting means for setting a second one-dimensional correction table to match the target color of the second secondary color, and weighting the first one-dimensional correction table and the second one-dimensional correction table A third correction table setting means for setting a third one-dimensional correction table for combining and correcting a color signal corresponding to the first color material; and the first color material and the second color table . Three color materials and the third color material The fourth correction table setting means for setting the fourth one-dimensional correction table so that the color matches the gray target color, and the fourth one-dimensional correction table is applied to the gradation value corresponding to the low density side. Then, for the gradation value corresponding to the high density side, the fifth one-dimensional correction table is synthesized by combining the two one-dimensional correction tables according to the gradation value so that the third one-dimensional correction table is applied. And a fifth correction table setting means for setting the color image processing apparatus , wherein calibration is performed based on the fifth one-dimensional correction table . In a color image processing method for calibrating color signals corresponding to three or more color materials, a first secondary color composed of a first color material and a second color material is a first secondary color. A first correction table setting step of setting a first one-dimensional correction table to match a target color of the color, and a second secondary color composed of the first color color material and the third color color material, A second correction table setting step for setting a second one-dimensional correction table to match the target color of the second secondary color, and weighting the first one-dimensional correction table and the second one-dimensional correction table A third correction table setting step for combining and setting a third one-dimensional correction table for correcting a color signal corresponding to the first color material; and the first color material and the second color table . Three color materials and the third color material In the fourth correction table setting step of setting the fourth one-dimensional correction table so that the color matches the gray target color, and the gradation value corresponding to the low density side, the fourth one-dimensional correction table is applied. Then, for the gradation value corresponding to the high density side, the fifth one-dimensional correction table is synthesized by combining the two one-dimensional correction tables according to the gradation value so that the third one-dimensional correction table is applied. And a fifth correction table setting step for setting a color image processing method , wherein calibration is performed based on the fifth one-dimensional correction table . A program for causing a computer to implement the color image processing method according to claim 2 . A computer-readable recording medium recording a program for causing a computer to implement the color image processing method according to claim 2 .

Images