JP2001186364A - Color information processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To estimate expression color of high precision from a few color chips. SOLUTION: The color chips of sixteen colors by the combination of the solids of CMYK and a color chart including one or more color chips on the primary color of YMCK are printed. The printed color chart is measured and color data corresponding to a set area rate and a color value is generated. A set network area rate and a valid area rate on paper are obtained from color data at every XYZ component of three stimulus values. A conversion formula for obtaining the valid area rate corresponding to the set network area rate of the arbitrary onedimensional color at every XYZ component is generated. Color data as the colors of sixteen color points being the elements of additive color mixture is used. A print color is estimated by using the area rate of XYZ obtained by applying the function of Demichel to the valid area rate of XYZ, which is obtained by the conversion formula as the area rate of the sixteen color points.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the entire process of producing a color print from the design stage to output by a printing press. In particular, it is strongly involved in the management of colors in color prints and in a process called color proofing.

[0002]

2. Description of the Related Art In recent years, a technique called color management or color matching has been emphasized. For example, in the December 1999 issue of the Printers Circle (published by the Japan Printing Technology Association), a magazine on printing technology, there was a special article entitled “Keywords for the 2000 Printing Business”. Explanations are given.

[0003] Color management is to manage colors in order to realize the same colors between different devices. ---- In order to reproduce monitor colors in a printer in the same way, color conversion from RGB to CMYK is required. This is a standard color space (CIE XYZ CIE L ^* a ^* b ^* ). At that time, each device such as a monitor, a printer, and a printed matter has what color reproduction range, and to reproduce a certain CIE L ^* a ^* b ^* value, what kind of RGB or CMY (K) What describes the goodness is ICC (International Color Consort
ium) profile.

[0004] As is widely known today, and as is clear from the cited articles, it is essential for color management to create a profile that accurately describes the color characteristics of a device. (It is not always the ICC profile.) Also, before actually printing the printed matter, the demand to check the color by outputting it with a color printer or to check it on a monitor is increasing along with the demand for streamlining the printing process. . In order to fulfill this requirement, a profile describing the color characteristics of printing is created, and the color characteristics of color printing, that is, the relationship between the set halftone area ratio and the colors actually printed (expression colors) are accurately described therein. There is a need to.

In the prior art, in order to accurately describe the relationship between the set halftone area ratio and the expression color, several hundred to 1,000 or more color patches with different values of CMYK are printed and measured. Then, the relationship between the set halftone area ratio and the expression color obtained as a result is approximated by a method of least squares (the same applies to the method of least squares). Suppose CMYK is 0,20,4 respectively
If a color chart is made to have a set screen area ratio of 0,60,80,100%, it is necessary to measure the color chart of 6 × 6 × 6 × 6 = 1296, and the labor and time required for the color measurement are large. Also, the chart containing the number of color charts becomes large naturally, it is not possible to print in the margin of the printed matter, and it is necessary to operate the printing press for the purpose of printing the chart, which means that This will increase the cost of profile creation.

On the other hand, Neugebauer's equation is conventionally known as a method for predicting an expression color from a small amount of information (a small number of color charts). Today, an extension of the equation originally described by Neugebauer is also called Neugebauer's equation, an example of which is disclosed in Japanese Unexamined Patent Publication No. Hei 3-175452 (Sayanagi et al.).
Are described at the beginning of the examples.

In the example shown by Sayanagi et al., Tristimulus values XYZ of expression colors obtained when halftone printing is performed using four YMCK inks are obtained, and 16 kinds of color points (first color elements) serving as elements of additive color mixing are obtained.
It is stated that, by actually measuring only the values shown in the figure, the XYZ of the expression color can be obtained by the following simple calculation using the values.

X = AwXw + AyXy + AmXm + AcXc + AkXk + A
ymXym + AycXyc + AmcXmc + AykXyk + AmkXmk + AckXck
+ AymkXymk + AyckXyck + AmckXmck + AymcXymc + Aymc
kXymck

[0009] Y = AwYw + AyYy + AmYm + AcYc + AkYk + A
ymYym + AycYyc + AmcYmc + AykYyk + AmkYmk + AckYck
+ AymkYymk + AyckYyck + AmckYmck + AymcYymc + Aymc
kYymck

Z = AwZw + AyZy + AmZm + AcZc + AkZk + A
ymZym + AycZyc + AmcZmc + AykZyk + AmkZmk + AckZck
+ AymkZymk + AyckZyck + AmckZmck + AymcZymc + Aym
ckZymck

In the formula, Ay, Am, Ac,...
The area ratio of each type of color point, Xw, Yw, Zw, Xy, Yy, Zy, ..., etc. are the tristimulus values of each of the above color points, and are YMCK used for printing.
It is obtained by actually measuring from ink and paper. Further, the area ratios of the 16 types of color points are determined from the area ratios of the YMCK inks using the De Michel's relationship.

As described above, the Neugebauer equation is well known, but it is not actually used to create a print profile. The reason is that the error is large. Regarding this error, for example, on page 274 of the Printing Engineering Handbook (published by The Printing Society of Japan, published May 1, 1983), "The causes of this error are internal scattering of paper, ink trapping, dot gain, etc. It is possible. "

As described above, as a problem of the prior art, in order to create a print profile with sufficient accuracy, it is necessary to accurately describe the relationship between the set screen area ratio and the expression color. It is necessary to print a chart containing the color chart and measure the color, and the time required is long and the cost is high. On the other hand, based on Neugebauer's equation, it can be said that expression colors can be predicted from a small number of color charts, but those with sufficient accuracy cannot be created yet.

[0014]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and is based on the concept of Neugebauer's equation, which combines (Demichel) 's relation and additive color mixing. Measured data of the printed matter is used as the color (XYZ value) of the 16 color points (shown in FIG. 1), which are the elements of the additive color mixture, and the area ratio of the 16 color points is calculated as the area ratio of the 16 color points. By using the area ratio of each of XYZ obtained by applying the Michell's relation, the color (expression color) actually printed is accurately predicted from a small number of color charts.

[0015]

In the present invention, according to Neugebauer's equation,
Trapping, dot gain, and internal scattering of paper, which are considered as error factors when predicting the expression color from the set halftone area ratio, are corrected by the following method.

Regarding trapping, the following description can be found in the section on printing order and color reproduction on page 287 of the Printing Engineering Handbook (supra), page 287, and what kind of phenomenon it is.

"In wet printing with a multi-color machine, the ink is not always completely applied from the next cylinder onto the preprinted ink. In particular, the higher the printing speed, the more easily the trapping situation changes. "

In the present invention, this trapping
16 possible with a combination of YK solids (100% net area ratio)
A color chart including all color charts of colors is printed, and correction is performed by calculating the color chart using actual measured values.

Further, according to the present invention, a color chart of a primary color is printed,
From the measured data, the relationship between the set screen area ratio and the effective area ratio is obtained for each of the three stimulus values XYZ based on the measured data, and based on that, the effective area corresponding to the set screen area ratio of any primary color Create a conversion system that calculates the ratio for each component of XYZ,
When performing the actual calculation, XY obtained in this way
By using the effective area ratio for each component of Z, errors due to dot gain and internal scattering of paper are corrected.

The reason why the error can be corrected by the above method will be described below. The dot gain is the difference between the set halftone area ratio of the primary color and the effective area ratio. Correctly obtaining the effective area ratio is the correction of the dot gain. Further, it is considered that the error due to the internal scattering of the paper occurs because the color ink selectively absorbs the light scattered inside the paper according to the color with respect to the color. In addition, since the absorption distance of the light scattered inside the paper is short, the absorption occurs strongly at the edge of the ink net, and therefore, it is expected that the correlation with the area ratio is strong. Therefore, if the effective area ratio is calculated by focusing on each of XYZ, the influence of this absorption can be included therein. In the present invention, based on the actual measurement values of the color chips of the primary colors, a conversion system for obtaining the effective area ratio corresponding to the set halftone area ratio of an arbitrary primary color for each of the XYZ components is created, and the actual calculation is performed. At the time, the error due to the dot gain and the internal scattering of the paper is corrected by using the effective area ratio of each component of XYZ obtained in this way.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is implemented by the following method. A 16-color color chart (W) made from a combination of paper and CMYK solid (100% net area ratio) used as printing ink
(Paper), Y, M, C, K, YM, YC, MC, YK, MK, CK, YMK, Y
CK, YMC, MCK, YMCK) and print the color chart including one or more color charts for each primary color (single color) of YMCK. The colorimetric data corresponding to the tristimulus values XYZ) is created. From the colorimetric data of the color chart of the primary color, the effective area ratio of the ink on the paper corresponding to the set halftone area ratio is determined by the tristimulus value of XYZ. By the least square method or spline interpolation for each component, the effective area ratio obtained by approximation, or the effective area ratio corresponding to the set halftone area ratio of any primary color by interpolation is a tristimulus value. Create a conversion system (calculation formula, look-up table, etc.) for each component of XYZ Based on the idea of Neugebauer's equation, which combines Demichel's relationship and additive color mixing, 16 color point colors (XYZ
Using the above colorimetric data as the value) and applying the Demichel relation to the effective area ratio for each of XYZ obtained by converting the set halftone area ratio by the above conversion system as the area ratio of 16 color points The actual printed color (expression color) using the area ratio for each of XYZ obtained by

Regarding the above, the embodiments will be described in more detail. CMYK is cyan, magenta, yellow, and black used as printing inks. The 16 colors that can be obtained by combining paper and CMYK solids (100% screen area ratio) used as printing ink can be seen more intuitively from FIG. Further, these 16 colors are elements of additive color mixing, as can be seen in the description and formula of the cited published patent publication Hei 3-175452 (Sayanagi et al.). The reason for including one or more color charts for each primary color (single color) of YMCK is to obtain basic data for creating a conversion system that obtains an effective area ratio from a set screen area ratio. Good accuracy is expected, and several or 20 points are good. However, the present invention can be practiced by making only one color chart for each primary color. In this case, a set halftone area ratio of about 50% is appropriate. It is empirically known that the difference between the set net area ratio and the effective area ratio becomes maximum around 50%, and the effective area ratio when the set net area ratio is 0% is 0.0 and the set net area ratio is 0.0. Area ratio 1
It is self-evident that the effective area ratio at the time of 00% is 1.0, so even if only one color chart is made, data of three points can be obtained,
This is because the approximation and interpolation can be performed.

A number of colorimeters for measuring the color of a color chart have already been put on the market, and it is not necessary to supplement the explanation in this section. About To make the explanation concrete and simple, the net area ratio set for each of the primary colors (monochrome) of YMCK is 20, 50, 70%
It is assumed that the color chart has been made. Also, a specific procedure will be described for only C, but the same applies to other colors. C (Cyan) solid (100% net area ratio) with Xw, Yw, Zw for colorimetric data of paper Xc, Yc, Zc Colorimetric values of Xc (20), Yc (20),
AXc (20) = (Xw-Xc (20)) / (Xw, where Zc (20) is the effective area ratio for each of XYZ at that time, AXc (20), AYc (20), AZc (20). -Xc) AYc (20) = (Yw-Yc (20)) / (Yw-Yc) AZc (20) = (Zw-Zc (20)) / (Zw-Zc) The effective area ratio for each of XYZ when the set screen area ratio is 50% is the colorimetric value Xc (5
0), Yc (50), Zc (50) to AXc (50) = (Xw-Xc (50)) / (Xw-Xc) AYc (50) = (Yw-Yc (50)) / (Yw-Yc ) AZc (50) = (Zw-Zc (50)) / (Zw-Zc).

Similarly, when the set screen area ratio is 70%, the effective area ratio for each of XYZ is the colorimetric value Xc (70), Yc (70), From Zc (70) to AXc (70) = (Xw-Xc (70)) / (Xw-Xc) AYc (70) = (Yw-Yc (70)) / (Yw-Yc) AZc (70) = (Zw -Zc (70)) / (Zw-Zc). As shown in, it is assumed that a color chart having a set screen area ratio of 20, 50, and 70% is created, measured, and the effective area ratio is obtained. Again, only C will be described in detail.

Here, the least squares method and the spline interpolation are used, but these algorithms and subroutines are already well known. Therefore, although it is considered that there is no need to specifically describe in detail, as an example, an embodiment using an algorithm subroutine disclosed below will be described. "Scientific and technical calculation subroutine library in C language" (Kiraku Publishing, published on November 30, 1986, Kurose / Matsushima / Matsuo)
(Author) Least square approximation on page 157 (Subroutine name lsts
q) Spline interpolation (subroutine name splin) on page 162
t) As can be seen from the title, the subroutine is written in C. The least squares method is used to calculate the effective area ratio of C in X. AXc (0), AXc (20), AXc (50), AXc (70), the value of AXc (100) has already been determined. Where AXc (0) = 0.0 AXc
(100) = 1.0 is trivial, AXc (20), AXc (50), AXc
(70) asked for. The following five arguments when executing lstsq

The set screen area ratios 0.0, 0.2, 0.5,
AXc (0), which is an array x [] effective area ratio containing 0.7 and 1.0
Array y [] containing the values of AXc (20), AXc (50), AXc (70), AXc (100) Number of data 5, order of least square approximation 2, array c [] containing coefficients of approximated polynomial When lstsq is executed with the above arguments, the coefficient 3
Are placed in the array of c []. This coefficient is calculated as c2, c1, c0
And

An effective area ratio for X corresponding to an arbitrary set screen area ratio of C is obtained as follows. (The above c, c2, c1, c0 are different from C of cyan.) (The following xy is a variable name used only in the calculation below, and is different from XYZ of tristimulus value) Y = c as the set screen area ratio (0 or more and 1.0 or less)
By calculating 2xx + c1x + c0, an effective area ratio y for X (three stimulus values) corresponding to the set net area ratio x of C is obtained. When calculating the effective area ratio of C by Y using the least squares method Of the five arguments when executing lstsq, the array y []
AYc (0), AYc (20), AYc (50), AYc
(70), AYc (100) can be entered, and the other values can be obtained in the same manner as above.

When the effective area ratio of C with respect to Z is determined by the least squares method. Of the five arguments when executing lstsq, the array y []
AZc (0), AZc (20), AZc (50), AZc
(70), the value of AZc (100) is inserted, and the other values can be obtained in the same manner as above. As described above, it is possible to create a conversion system for obtaining the effective area ratio for each of XYZ from the set screen area ratio of an arbitrary C. With this conversion system, if the set network area ratio is appropriately divided from 0.0 to 1.0 and calculated in advance (for example, in 256 steps), a lookup table can be created using the result. The same applies to M, Y, and K, and the description is omitted because there is much overlap. Also,
Even in the case of spline interpolation, based on the explanation in the book,
Since it can be performed in a procedure similar to the case of the least square method,
Detailed examples are omitted.

Regarding CMYK, the set screen area ratio is Cs, Ms, Ys, Ks (%). Using the conversion system obtained in the above, find the effective area ratio of Cs for each of XYZ and AXc (Cs), AYc (Cs), AZc (Cs) .Similarly, find the effective area ratio of Ms for each of XYZ, AXm (Ms), AYm (Ms), AZm (Ms) Calculate the effective area ratio for each, and set it as AXk (Ks), AYk (Ks), AZk (Ks)

By applying the Demichel relation to the twelve numerical values representing the effective area ratios, the area ratios of 16 color points, which are elements of additive color mixture, are obtained for each of XYZ. For the color point w (paper), the following three area ratios of the color point w (paper) for X AXw AXw = (1-AXc (Cs)) (1-AXm (Ms)) (1-AXy (Y
s)) (1-AXk (Ks)) Area ratio of color point w (paper) for Y Ayw AYw = (1-AYc (Cs)) (1-AYm (Ms)) (1-AYy (Y
s)) (1-AYk (Ks)) Area ratio AZw of color point w (paper) for Z

AZw = (1-AZc (Cs)) (1-AZm (Ms))
(1-AZy (Ys)) (1-AZk (Ks)) For the remaining 15 color points, only the formula for calculating the area ratio for X is shown.

Axy AXy = (AXy (Ys)) (1-AXc (Cs)) (1-AXm (Ms)) (
1-AXk (Ks)) Area ratio of color point m to X Axm AXm = (1-AXy (Ys) (1-AXc (Cs)) (AXm (Ms)) (
1-AXk (Ks)) Area ratio of color point c for X Axc AXc = (1-AXy (Ys) (AXc (Cs)) (1-AXm (Ms)) (
1-AXk (Ks)) Area ratio of color point k for X Axk AXk = (1-AXy (Ys) (1-AXc (Cs)) (1-AXm (Ms)
) (AXk (Ks)) Color point ym Area ratio of X to X Axym AXym = (AXy (Ys)) (1-AXc (Cs)) (AXm (Ms)) (1-
AXk (Ks)) Color point yc area ratio for X Axyc AXyc = (AXy (Ys)) (AXc (Cs)) (1-AXm (Ms)) (1-
AXk (Ks)) Area ratio of color point mc for X Axmc AXmc = (1-AXy (Ys)) (AXc (Cs)) (AXm (Ms)) (1-
AXk (Ks)) Area ratio of color point yk to X Axyk AXyk = (AXy (Ys)) (1-AXc (Cs)) (1-AXm (Ms)) (
AXk (Ks)) Area ratio of color point mk for X Axmk AXmk = (1-AXy (Ys)) (1-AXc (Cs)) (AXm (Ms)) (
AXk (Ks)) Area ratio of color point ck for X Axck AXck = (1-AXy (Ys)) (AXc (Cs)) (1-AXm (Ms)) (
AXk (Ks)) Area ratio of color point ymk for X Axymk AXymk = (AXy (Ys)) (1-AXc (Cs)) (AXm (Ms)) (AXk
(Ks)) Area ratio of color point yck to X Axyck AXyck = (AXy (Ys)) (AXc (Cs)) (1-AXm (Ms)) (AX
k (Ks)) Area ratio of color point mck to X Axmck AXmck = (1-AXy (Ys)) (AXc (Cs)) (AXm (Ms)) (A
Xk (Ks)) Area ratio of color point ymc for X Axymc AXymc = (AXy (Ys)) (AXc (Cs)) (AXm (Ms)) (1-A
Xk (Ks)) Area ratio of color point ymck for X Axymck AXymck = (AXy (Ys)) (AXc (Cs)) (AXm (Ms)) (AXk
(Ks))

(As described above, the formula for calculating the area ratio for YZ is omitted.) The area ratios of each of 16 color points for each of XYZ, 48 numerical values, and the actually measured values of 16 color points Can be used to determine the expression color tristimulus values, XYZ, as follows. (× in the formula means the product) X = AXw × Xw + AXy × Xy + AXm × Xm + AXc × Xc + AXk × Xk
+ AXym x Xym + AXyc x Xyc + AXmc x Xmc + AXyk x Xyk +
AXmk × Xmk + AXck × Xck + AXymk × Xymk + AXyck × Xyck +
AXmck × Xmck + AXymc × Xymc + AXymck × Xymck Y = AYw × Yw + AYy × Yy + AYm × Ym + AYc × Yc + AYk × Yk
+ AYym x Yym + AYyc x Yyc + AYmc x Ymc + AYyk x Yyk +
AYmk × Ymk + AYck × Yck + AYymk × Yymk + AYyck × Yyck +
AYmck × Ymck + AYymc × Yymc + AYymck × Yymck Z = AZw × Zw + AZy × Zy + AZm × Zm + AZc × Zc + AZk × Zk
+ AZym x Zym + AZyc x Zyc + AZmc x Zmc + AZyk x Zyk +
AZmk × Zmk + AZck × Zck + AZymk × Zymk + AZyck × Zyck +
AZmck × Zmck + AZymc × Zymc + AZymck × Zymck

AXy, AXm, AXc, AZymck in the above formula are the area ratios of each of the 16 types of color points shown in FIG.
w, Zw, Xy, Yy, Zy, etc. are the tristimulus values of each of the above color points, and are obtained by actually measuring from YMCK ink and paper used for printing. With the above calculations, the expression color can be accurately predicted from the set halftone area ratio only by printing and measuring a small number of color patches.

[0035]

According to the present invention, a color chart including 840 color chips was actually prepared, printed, and measured. When printing this color chart, the most common color printing conditions, 17 lines per inch of screen
Squares were selected for the shapes of the five lines and halftone dots. This color chart is configured to include the entire color gamut by CMYK, and at the same time, the CMYK
There are 16 color patches for each color of the YMCK and 18 primary color patches for each of the YMCK colors.

In the method of the present invention, the expression color is determined from the set halftone area ratio for 840 colors and is used as a calculated value. The calculated value is compared with the actually measured value, and the color difference defined by CIE-L ^* a ^* b ^* is obtained.
An average value of about 1.8 was obtained in (ΔE ^* ab). The evaluation of the color difference by ΔE ^* ab does not completely match the visual sense, but usually, if ΔE ^* ab is 2 or less, it is evaluated as a slight difference, and the effectiveness of the present invention is clear.

[Brief description of the drawings]

FIG. 1 is a diagram showing 16 types of color points made from YMCK four color inks and paper.

FIG. 2 is a flowchart showing a processing procedure according to the embodiment of the present invention.

Continued on the front page F-term (reference) 5B057 BA02 CA01 CA08 CA12 CA16 CB01 CB08 CB12 CB16 CC01 CE18 CH01 5C077 LL11 MP02 MP08 PP37 PQ12 5C079 HA19 HB03 HB08 HB11 LB02 LB06 NA03 PA03

Claims (1)

[Claims] 1. A method for predicting a relationship between a set halftone area ratio, which is a color characteristic of color printing by halftone dots, and a color (expression color) actually printed by the following process. A 16-color color chart (W) made from a combination of paper and CMYK solid (100% net area ratio) used as printing ink
(Paper), Y, M, C, K, YM, YC, MC, YK, MK, CK, YMK, YC
K, YMC, MCK, YMCK), and the process of printing a color chart containing one or more color patches for each of the primary colors (monochrome) of YMCK. The process of creating the corresponding colorimetric data From the colorimetric data of the color chart of the primary color, the effective area ratio of ink on paper corresponding to the set halftone area ratio is calculated for each XYZ component, which is a tristimulus value. The process of obtaining the least squares method, or by spline interpolation, approximates the effective area ratio obtained by, or interpolates and calculates the effective area ratio corresponding to the set halftone area ratio of any primary color as a tristimulus value. The process of creating the conversion system (calculation formula, lookup table, etc.) required for each component Based on the idea of Neugebauer's equation, which combines the Demichel's relationship and additive color mixing, 16 color points (XYZ
Using the above colorimetric data as the value), applying the Demichel relation to the effective area ratio for each of XYZ obtained by converting the set screen area ratio by the conversion system as the area ratio of 16 color points. Process of predicting the actual printed color (expression color) using the area ratio of each of XYZ obtained by