JPH10142775A - Method for converting color reproduction data - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for converting color reproduction data capable of obtaining C(cyan), M(magenta), Y(yellow), K(black) color reproduction data which may be actually reproduced by printing from three-dimensional colorimetric data while maintaining color matching conditions under the K plate formation conditions arbitrarily set by a user. SOLUTION: The colorimetric data and color matching ink quantities of CMY which are condition color matching are determined with respect to plural different K plate quantities and achromatic quantities are determined from the ratios of the total ink quantities of CMYK at the respective K plate quantities. The achromatic quantity relations with respect to the K plate quantities are determined in line shape parts. The mapping functions to the ink quantities of CMYK are formed from the colorimetric data and achromatic quantities in the line shape parts. The weighting coeffts. to be reflected in the achromatic quantities are previously determined under the preselected K plate control conditions. The achromatic quantities are determined from the colorimetric data and the ink quantities of CMYK are calculated by using the mapping functions of the CMYK ink quantities from the achromatic quantities and the colorimetric data.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for converting color reproduction data, and more particularly, to a method showing a colorimetric appearance.
From the dimensional data to C (cyan), M (magenta), Y (yellow), and K color reproduction data including K (black) plates for printing and color printers, etc. And a method for conversion while maintaining

[0002]

2. Description of the Related Art Generally, printing is represented by area modulation of CMYK dot%. Arbitrary color reproduction data (for example, R
(Red), G (green), and B (blue)), the conversion of color reproduction data of four or more colors, including the K plate, which is a unique feature of printing, is performed by the user based on several purposes. The conversion conditions such as the decomposition curve and its LUT (look-up table) are determined using a specific method. At this time, it is extremely difficult to determine an arbitrary K-plate generation condition while preserving the colorimetric color reproduction information before the conversion. In general, the mapping relationship from CMYK to a three-dimensional space such as RGB or a colorimetric space such as Lab or XYZ is not a one-to-one mapping, and therefore the inverse mapping from the three-dimensional space to the four-dimensional CMYK is unique and easy. Is not required.

As a solution to this problem, there has been proposed a method in which a certain constraint condition is given to the K-plate, and the K-plate is treated as a mapping to CMY in a three-dimensional space. As a specific example, a method of solving the Neugebauer equation by successive approximation by partial differentiation using a K-version as a constant as in the invention described in JP-A-3-175452,
There is a method of performing printing including a predetermined K plate as in the invention described in Japanese Patent No. 158071, and measuring the color of the printed matter (chart) to achieve 3-3 conversion under specific K plate conditions. .

In the case of these conventional methods, the former method is always possible only with the 3-4 conversion (CMYK generation) under a specific K plate amount condition, and the latter method requires a limited K plate condition (K plate replacement and addition). 3) transformation can be calculated as a limited solution in a pseudo manner, but every time the condition is changed, it is necessary to start over from the beginning and output and measure a similar chart. And there is no freedom to achieve a 3-4 conversion under that condition. Further, when selecting the K plate amount, it is more difficult to set the conditions while being aware of the practical limitation (such as the maximum ink amount) as printing. That is, there is no mention of the key K-version constraint conditions, or only those conditions are fixed or can be regarded as fixed.

In the printing field, at least the user is required to be able to generate arbitrary CMYK as printing conditions for each image for the following printing purposes. In printing, the conditions for determining the K plate amount are as follows:
It is said that it is better to reflect the general printing purpose of typical K-plate generation as follows. 1) The part where CMY is printed in a fixed amount is regarded as gray,
By replacing it with a K plate that is considered to be equivalent in colorimetric reproduction, the stability and accuracy of color reproduction centered on gray are secured (so-called acromatch plate making is realized). 2) Reduce the total amount of ink and consider its balance distribution,
The aim is to stabilize printing. 3) Further increase the maximum density to be reproduced in three colors of CMY, and expand the maximum density range.

However, setting the K-plate conditions for each image in consideration of these conditions requires many years of experience and skill, and takes into account colorimetric reproduction.
Further, as described above, there is a problem that it is more difficult to arbitrarily set the K-plate conditions while being aware of the practical restrictions on printing.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art, and to allow a user to arbitrarily set K-plate generation conditions for the above-mentioned printing purpose.
Regardless of the plate generation conditions, in particular, conversion from three-dimensional data such as colorimetric values to four-color separation data CMYK for printing while maintaining the colorimetric reproduction conditions of the original three-dimensional colorimetric data in the same color In that case, taking into account the non-linear characteristics inherent in printing, the four color separations which can be reproduced in actual printing suitability can be obtained.
An object of the present invention is to provide a color reproduction data conversion method capable of obtaining two-dimensional color reproduction data.

[0008]

In order to solve the above-mentioned problems, the present invention provides C (cyan), M (magenta), Y (yellow) and K including black plate from three-dimensional colorimetric data.
When performing conversion to the four-dimensional color reproduction data of (black ink) while maintaining the color matching conditions, a reverse conversion from a three-dimensional colorimetric space to a three-dimensional CMY space is performed for a plurality of different K plate amounts in advance.
The three-dimensional colorimetric data when the K plate amount is the reference amount and the CMY ink amounts that are the conditionally equal colors are obtained as the equal color ink amounts for the plurality of different K plate amounts, and then the total of CMYK in each K plate amount is calculated. CM when ink amount and K plate amount are reference amount
The ratio of YK to the total ink amount is determined as the achromatic amount at each K plate amount, the portion where the achromatic amount is non-linear with respect to the K plate amount is tested, and the linear portion is compared with the K plate amount. A chromatic amount function is obtained, and a mapping function from the colorimetric data and the achromatic amount to the CMYK ink amount is generated in the linear portion, and the achromatic amount is weighted under a preselected K-plate control condition. For this purpose, the K-plate control condition is determined as a weighting coefficient for sequentially reflecting the achromatic amount, and the constraint condition for inverse conversion from the three-dimensional colorimetric data to the four-dimensional color reproduction data is defined as The achromatic amount is determined from the chromatic amount function and the selected K-plate control condition, and the determined achromatic amount and the third order are determined. There is provided a color reproduction data conversion method characterized by calculating the ink amount of CMYK using the CMYK ink amount mapping function from the colorimetric data.

Here, the K-plate control condition preferably includes at least a K-plate replacement amount, a K-plate maximum limit amount, a saturation weight, and a brightness weight. The inverse conversion from the three-dimensional colorimetric space to the three-dimensional CMY space for the plurality of different K plate amounts is performed by measuring the color of a specific color patch and performing three-dimensional measurement from the four-dimensional CMYK space obtained by interpolation calculation. It is preferably performed based on a forward conversion to a color space.

It is preferable that at least one of the CMYK ink amount mapping function and the achromatic amount limit in the achromatic amount function is stored in advance as a K-plate generation amount profile, and the K-plate generation is performed by being sequentially called. .

When it is necessary to calculate a CMYK ink amount at a point outside the color gamut of printing, the achromatic amount is adjusted so that the K plate outside the color gamut becomes continuous from within the color gamut. Estimation is performed by maximum likelihood using parameters of the same dimension as the constraint condition used for the calculation, and if the estimated K plane is 0% or more and 100% or less, the amount of color matching ink is calculated in the estimated K plane. The CMY ink amount outside the color gamut is obtained as a result, and when the estimated K plane is less than 0% and more than 100%, the color that is continuous from within the color gamut from a regression equation in a specific divided space. C outside reproduction range
Preferably, the MY ink virtual amount is obtained.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the color reproduction data conversion method according to the present invention will be described in more detail. The present invention is intended to generate a K plane when converting three-dimensional colorimetric data into four-dimensional color reproduction data of C (cyan), M (magenta), Y (yellow) and K (black) specific to printing. In order to achieve the above-mentioned printing purpose, it is essential to be able to determine the K-plate conditions arbitrarily while maintaining the colorimetric reproduction conditions. Therefore, the inverse conversion from three-dimensional data to four-dimensional data is required. The achromatic amount based on the amount of ink in the printing color space is used as a constraint condition for performing colorimetrically, and the K-plate amount can be arbitrarily selected by reflecting the K-plate condition that can be arbitrarily selected by the user in the achromatic amount. Can be selected, and CMYK color reproduction data that is colorimetrically equal can be generated.

That is, printing is generally performed in four colors of CMYK, and as a result, CIEXYZ or L
For colorimetric space data such as ab, most of the mappings of a plurality of CMYK values show the same colorimetric value (such as CIELab) instead of a one-to-one mapping relationship, and are not orthogonalized. In this case, it is not easy to obtain an exact solution of a mapping from arbitrary data in the three-dimensional space to the four-dimensional space, and some constraints are required to obtain the mapping. The present invention allows the user to arbitrarily select the K-plate condition under physical restrictions such as the amount of ink in the actual print color space, The mapping from the three-dimensional color space to the four-dimensional color space is obtained under the constraint of the mapping to the color space.

Hereinafter, a process which is a feature of the present invention will be described in detail with reference to the flowchart of the method of the present invention shown in FIG. The most characteristic part of the present invention is to use the following achromatic amount as a constraint in order to achieve a reverse mapping from a three-dimensional colorimetric space to a CMYK four-dimensional device space. As shown in FIG. 1, as an achromatic amount determination method, first, in the first step of the present invention, the equal color ink amount in each K plate amount in the ink amount space is calculated, and as the second step of the present invention, In addition, the total ink amount for each K plate amount is obtained, and further, the K plate replacement ratio (achromatic amount) is obtained from the total ink amount ratio that differs depending on the K plate amount even for the same color.

The term "equivalent color ink amount" means a CMY ink amount which is equal in color to three-dimensional colorimetric data when the K plate amount is a reference amount, for example, 0%. The amount of this color ink is CM
Using forward conversion data from a YK four-color color reproduction system (four-dimensional device space) to a three-dimensional colorimetric space (for example, XYZ), the CM is converted from the three-dimensional colorimetric space to the discrete K plate amount.
It can be obtained by performing 3-3 inverse conversion of Y to a three-dimensional color reproduction system. The 3-3 inverse conversion method required to determine the amount of color matching ink is based on a three-dimensional colorimetric space.
There is no particular limitation as long as the inverse conversion to the three-dimensional color reproduction space is possible, and a known 3-3 inverse conversion method can be used. Preferably, Japanese Patent Application No.
And the color conversion method described in JP-A-156555.

In addition, C such as printing which is the basis of 3-3 inverse conversion
The method for obtaining the forward conversion data from the system that reproduces the four colors of MYK to the colorimetric space is not particularly limited, and may be any conventionally known forward conversion method. That is, the forward conversion data is obtained by a conventionally known method, for example, a specific dot%.
It can be obtained by measuring the color of a color patch or the like printed by the above and performing interpolation calculation. By the way, 3 in the present invention
The three-dimensional colorimetric data is not particularly limited as long as it is three-dimensional colorimetric data represented in a colorimetric space such as the CIEXYZ or CIELab color system, that is, XYZ and Lab.
Any three-dimensional color space data that can be uniquely converted to three-dimensional colorimetric data such as
uv, YIQ, YCC, HSI, and RGB three-dimensional color reproduction data.

Next, as shown in FIG. 1, in the third step of the present invention, the continuity of the achromatic amount obtained in the second step indicates a non-linear portion in printing (there is a problem in print reproducibility). Is tested, and the relationship between the K plate amount and the achromatic amount in the linear portion is converted into a function to obtain an achromatic amount function. Subsequently, as shown in FIG. 1, in the fourth step of the present invention, the achromatic amount function generates a mapping function from the colorimetric data and the achromatic amount to the CMYK ink amount only in the linear part.

On the other hand, as shown in FIG. 1, in the fifth step of the present invention, in order to allow the user to arbitrarily or freely select the K-plate amount, the user selects the K-plate control to be disclosed to the user. A weighting function for sequentially reflecting the conditions on the achromatic amount as a weighting factor is obtained. Here, the selection conditions for the K-version control to be disclosed to the user are as follows:
It is preferable that at least the K-plate replacement amount, the K-plate maximum limit amount, the saturation weight, and the lightness weight. In the present invention, by performing the above steps in advance, an aromatic quantity function, a weighting coefficient for an achromatic quantity based on a user's K plate control condition, and a CMYK ink quantity mapping function are obtained in advance.

Thereafter, as shown in FIG.
In the step, the four-dimensional color space (colorimetric data)
As a constraint condition for 3-4 inverse conversion to a three-dimensional color space (CMYK color reproduction data), a final value is obtained from an achromatic amount function describing only a linear part and a weighting coefficient based on a K-plate control condition arbitrarily selected by a user. Achromatic amount is determined. As shown in FIG. 1, finally, in the seventh step of the present invention, the CMYK ink amount is calculated using the three-dimensional colorimetric data and the mapping function from the determined final achromatic amount to the CMYK ink amount. Ask. Here, it is important that the required CMYK ink amount is Gamut.
(The color gamut of printing). That is, the processing in this step is gamut (Gamu)
t; processing within the color gamut of printing). At this time, it is preferable that the above-described achromatic amount function (achromatic amount limit) and the mapping function from the achromatic amount to the CMYK ink amount are stored as a profile, and sequentially called up to quickly generate the K plate. In this way, if it is within the gamut, the CMYK ink amount (four-dimensional color reproduction data) can be obtained from any three-dimensional colorimetric data under the condition of the printable K plate arbitrarily selected by the user.

When a mapping from an arbitrary colorimetric value (three-dimensional colorimetric data) to a CMYK ink amount is obtained, an interpolation operation or the like is used, but the CMYK ink amount to be obtained is within the gamut, that is, the ink amount space. Even if they exist in the image, a point outside the gamut, that is, a point outside the ink amount space is also required for interpolation calculation or the like for obtaining a mapping from an arbitrary colorimetric value. That is, in the present invention, processing outside the gamut may be required. In the processing outside the gamut, first, in order to keep the K plate outside the gamut continuous from the inside of the gamut, the same dimension as the various constraint conditions (saturation, brightness, etc.) used in obtaining the achromatic amount is used. Maximum likelihood estimation is performed using parameters. Next, if the K plate is 0% or more and 100% or less, the amount of color matching ink is calculated for the K plate to obtain CMY outside the gamut.
Find the amount of ink. Further, when the K version is less than 0% or more than 100%, the virtual CMY ink amount outside the gamut which is continuous from the gamut is estimated from the regression equation in the specific divided space. Thus, according to the method of the present invention, even if the user arbitrarily selects the K-plate condition, the selected K-plate condition is a printable condition, and an arbitrary three-dimensional condition is set under this K-plate condition. The colorimetric data can be converted to CMYK color reproduction data.

Hereinafter, each step of the color reproduction data conversion method of the present invention will be specifically described with reference to the accompanying drawings. FIG.
9 shows an example of a flowchart for calculating an achromatic amount, which is a feature of the present invention. First, as shown in the figure,
A lookup table (hereinafter referred to as an LUT) for performing a forward conversion from CMYK color reproduction data to colorimetric data as a prerequisite to performing the color matching ink amount calculation step of the first step of the present invention.
Must be created. That is, in a system that reproduces four colors of CMYK, such as printing, the color prediction is performed by measuring and interpolating the correlation with the color gamut that can be reproduced using the four colors in advance by measuring a color patch or the like printed with a specific halftone%. It can be obtained as a function or LUT. For convenience of explanation, this correlation function is assumed to be a CMYK 4-dimensional LUT in increments of 10%. For example, after printing a chart such as ANSI-IT8.7 / 3 and measuring the color, an LUT can be created using polynomial approximation, volume interpolation, or the like. Here, the colorimetric data is represented by the CIEXYZ color system.

By the way, using this LUT, 3-4
A method of generating CMYK from XYZ by inverse transformation can be mathematically considered. That is, the successive increment of CMYK with respect to the XYZ partial derivative is obtained from the generalized matrix as in the following equation (1), and thus the root can be obtained using the Newton method or the like. However, the actual color gamut according to CMYK is K K, as shown in FIG.
As described above, it is not possible to obtain the exact solution of CMYK directly from XYZ mathematically because there are overlapping parts in the same color space even if the K plate is different, not independent of plate changes. It is. Hereinafter, an inverse matrix solution method using a 3 × 4 matrix by a generalized matrix will be described.

[0023]

(Equation 1)

Therefore, the K plate is restrained in some form,
It is conceivable to solve the problem by reducing it to a successive approximation in mapping from three-dimensional colorimetric values to CMY three-dimensional color reproduction data or an error maximum likelihood problem under the plate constraint condition. The present invention enables a user to arbitrarily select a K-plate replacement ratio (achromatic amount) or a K-plate intensity from CMYK. When determining a constraint condition from the selection result, the ratio of K to the CMY ink amount or an ink equivalent thereto. By utilizing a linear region having a chromatic ratio, conversion from colorimetric values to CMYK color reproduction values under arbitrarily selective and printable K plate conditions is achieved. That is, in the present invention, this physical quantity space (achromatic quantity) representing actual printability.
The calculation is performed only in the portion where the linearity is guaranteed, and an arbitrary K plate is determined by giving a smooth variable amount. Therefore, a smooth increase / decrease in the K plate amount more consistent with the print reproduction characteristics can be achieved. That is, conversion that can be reproduced in actual printing can be performed.

Here, the achromatic amount characteristic of the present invention can be easily obtained by, for example, the following method. Note that also in this case, the colorimetric values are expressed in the CIEXYZ color system. That is, as shown in FIG. 2, the ratio can be calculated by obtaining the XYZ values at K = 0 and the CMY ink amount (hereinafter referred to as “equal color ink amount”), which is the condition, for 0 <K ≦ 100. That is, first, as the first step of the present invention, the LU obtained by the above-described forward conversion from CMYK to XYZ
Using T, the amount of color matching ink when the K plate amount is set to a certain value is calculated by performing 3-3 inverse conversion. In this example, as described above, the CMYK is changed from the CMYK to the XY
Since the mapping relationship to Z is maintained, the variable amount of the K version is also 1
What is necessary is just to calculate it by changing discretely by 0%. Specifically, CMY when K = 10, 20, 30, 40... 100% is sequentially fixed using the Newton successive approximation shown in Equation 2 below may be inversely calculated from the LUT (FIG. 2). reference). Amount of CMY color matching ink obtained in this way (%)
3 (a) and 4 (a) show the relationship between
Shown in The successive approximation solution by the Newton method is shown below.

[0026]

(Equation 2)

The CMY (K) obtained in the first step as described above
= 0), the ink amount ratio is defined as a physical quantity describing the K plate replacement amount in the second step from the combination of the color matching ink amounts obtained by discretely changing each K plate. And achromatic amount. It should be noted here that, as is often said, the colorimetric equivalent removal amount of CMY with respect to the K plate replacement amount is not constant, and the CMY equivalent amount = K plate as shown in FIG. Generally, a substitution balance such as an amount does not hold. Therefore, the K plate amount is not equal to the achromatic amount. There have been many examples in the past where this non-linear relationship has been described as achromatic theory, but there is still no one that requires an accurate replacement amount that can be used colorimetrically.

In the present invention, this is obtained discretely and comprehensively from the combinations of CMY inks having the same color as obtained above, so that XYZ to C
Makes mapping to MYK achievable. For example, some C _O ,
The colorimetric values at M _O and Y _O (K = 0) are expressed as X _O , Y _O , Z _O ,
When K = N%, the color matching ink amounts are C _N , M _N ,
Represented by Y _N. Therefore, C _N , M _N , Y _N , K _N
Is a substitution ratio of R = f (C _N , M _N , Y _N , K _N ) /
f (C _O , M _O , Y _O ) or the like can be defined as an example. Here, f indicates a weighted average, a summation function, a minimum function, or the like. As described above, the achromatic amount R can be obtained according to the flow shown in FIG.

The achromatic amounts R and K thus obtained are
The relationship with the plate amount (%) is shown in FIGS. 3 (b) and 4 (b). Here, FIG. 3B is obtained from the relationship between the CMY color matching ink amount (%) and the K plate amount (%) shown in FIG. This is an example of a case where the plate replacement ratio (achromatic amount (index)) exceeds 1.0 and consists only of linear components without a non-linear portion which reverses.
That is, in the examples shown in FIGS. 3A and 3B, when the amount of the K plate is increased and replaced by CMY, the total amount of ink decreases linearly. In other words, it can be said that the amount of ink transfer increases almost linearly even if the amount of ink is increased by reducing the amount of the K plate. On the other hand, FIG. 4B is obtained from the relationship between the CMY equivalent color ink amount (%) and the K plate amount (%) shown in FIG. This is an example of a case where a non-linear portion in which the plate replacement ratio R is reversed to exceed 1.0 is included.

Generally, there is an unstable portion in the color space which is almost impossible to reproduce by printing, and the achromatic amount obtained in this manner shows a non-linear characteristic with respect to an increase in the K plate amount. Or For example, FIG.
As shown in (b), when the K plate replacement ratio (achromatic amount) exceeds 1.0 when the K plate amount increases and reverses, it seems that a phenomenon unfavorable in printing characteristics has occurred. It is. As shown in FIG. 4B, the increase / decrease of the K plate replacement ratio with respect to the K plate amount is not always linear. That is, in the case of ideal printing, the replacement amount of CMY ink should increase linearly with an increase in the amount of the K plate. And the ink has not been completely transferred to the paper).
Conversely, the replacement ratio becomes smaller than when the plate amount is large (1.0
Over). When generating the K plate, it is necessary to consider such non-linearity in actual printing.
That is, it is not guaranteed that the converted result can be correctly reproduced on printing.

For this reason, in the third step of the present invention, such an achromatic amount (K-plate replacement ratio) is not linear with respect to the K-plate amount, that is, a non-linear amount in which reproduction on printing is not guaranteed correctly. Excluding the portion, only the linear portion defines the achromatic amount function (correlation function between the achromatic amount and the K plate amount). In other words, the achromatic amount and CMYK in the above-described equal-color ink amount space
Although the correlation function with the quantity can be approximated by a polynomial or the like, it is necessary to avoid this non-linear space. In particular,
The change in the achromatic amount with respect to the increase in the K plate amount in the obtained arbitrary colorimetric values is approximated by a polynomial, and if there is a differential inflection point as shown in FIG. It is sufficient to perform a test that removes from the data the part where there are many.

Thereafter, in a fourth step of the present invention, a mapping function of an achromatic ink replacement ratio and a CMYK ink amount in a colorimetric ink amount space with respect to grid data in a colorimetric space is used as a K-plate generation profile. Constraints such as (polynomial) and its effective range (linear range) are generated. This is preferably used as a K-version generation profile (restriction condition). For example, it is possible to use a polynomial as follows approximating the K-plate amounts at achromatic amount R _{XY Z} at any colorimetric value as achromatic amount function. K _XYZ = a ₁ R _XYZ ⁿ + a ₂ R _XYZ ^n-1 + a ₃ R _XYZ
ⁿ⁻² +... + a _n R _XYZ + a _{n + 1} where ΔR _XYZ / ΔK <0. When the variable amount of the K plate is discretely changed at 10% intervals, there are only 11 values for the K plate amount. Therefore, considering the degree of freedom, if the maximum value of the order n is 8, Well, in this case, the coefficients a ₁ ,
a ₂ ,..., a _n , a _{n + 1} may be determined as nine,
A _1, a _2, which are determined here, ..., of a _8, a _{9 9}
The number of coefficients may be stored as a K-version generation profile.

As described above, when the mapping function between the achromatic amount and the CMYK ink amount for the colorimetric values in the three-dimensional colorimetric space is approximated as a polynomial as described above,
In addition to its linear range as a profile for K version generation,
As described above, if only the above-mentioned coefficients a ₁ , a ₂ ,..., _An , and an _{+ 1} for the K version are stored, the polynomial including the order is Easy to create, C
The MYK ink amount can be determined. Of course, CM
It is also possible to convert the YK ink amount mapping function into an LUT and save it as a K-plate generation profile. By using such polynomials and LUTs, CMYK
A 3-4 inverse conversion to ink volume can be performed. The following points can be expected as advantages of creating such a profile. First, since the achromatic amount after the calculation can be converted into a function as a simple polynomial or LUT, it can be sequentially called to perform 3-4 conversion under arbitrary conditions. In addition, any variable that can be weighted as an achromatic quantity can be used as a selection condition.

A method for actually performing the 3-4 reverse conversion under an arbitrary K-plate generation condition according to the user's selection by using the K-plate generation profile obtained above will be described in detail. First, in the fifth step of the present invention, a K-plate generation constraint condition is created. In order to perform the 3-4 conversion, the K-plate generation condition must be determined. Another feature of the present invention is that the user can arbitrarily determine the K-plate generation conditions. By the way, the K-plate generation constraint condition is determined by the selection condition of the user, and mainly includes a saturation weight (the K-plate replacement is performed more strongly as the color becomes closer to gray) and a lightness weight (the K becomes larger as the overall density becomes larger). Plate replacement is performed), K plate amount intensity (limiter that determines the maximum K plate amount), replacement intensity (total K plate replacement ratio intensity), etc. Any condition can be used as long as it can be reflected, and it can be set for general use.

For example, a GUI (Graphic User Interface) for the entire K-plate replacement intensity, saturation weight, maximum K-plate amount limit, and lightness weight selected by the user as the K-plate generation constraint condition.
Examples of the ace) are shown in FIGS. 5A, 5B, 5C, and 5D, respectively, but the present invention is not limited thereto. FIG. 5A shows a change in the K-plate replacement strength as a whole. For example, when the user increases or decreases the K-plate replacement amount on the CRT, the respective ink amounts of CMY to be replaced increase or decrease at a predetermined replacement ratio. It is shown that the remaining CMY inks decrease and increase. In FIG. 5B, in the a ^* b ^* plane (L ^* = 0) of the L ^* a ^* b ^* space indicating the saturation, the closer to the center, the closer to gray (achromatic color). By moving the circle showing the strong saturation weight toward and away from the center,
It has been shown to increase or decrease the strength of K-plate replacement. FIG.
In (c), the lightness L ^* of the L ^* a ^* b ^* space is plotted on the horizontal axis, and the K plate amount at the rightmost side of the lightness L ^* axis with the darkest lightness is increased / decreased, so that the maximum K plate amount is obtained. It is shown that if the limit value is increased or decreased and the brightest point on the leftmost side of the L ^* axis is fixed, the K plate amount limit value at each lightness increases or decreases. Further, in FIG. 5 (d), in the lightness weight in which the K-plate replacement is performed more strongly as the overall density is higher, the point having the lightest lightness is moved to the left and right along the L ^* axis in the figure to determine which lightness. It is shown that the K plate is inserted up to the brightness (brightness) or the intensity of the K plate amount changes at each brightness.

For example, when the saturation weight is reflected in the achromatic amount, the saturation Ch ^* is calculated from the colorimetric value XYZ ^.
Is defined as follows: Ch ^* = LCH (XYZ) The colorimetric value X for which the inverse mapping of the CMY ink amounts is to be obtained
The saturation of YZ is obtained, and the achromatic amount can be weighted according to a saturation weighting function (for example, a normal distribution) as shown in FIG. By the way, these K-plate generation constraint conditions are ultimately all reflected in the achromatic ink replacement ratio in this way, and the mapping function defined as the K-plate generation profile is used to convert the data in the colorimetric space. The amount of CMYK ink is determined.

Next, the CMYK ink amount is generated in the gamut from the achromatic amount under the K-plate generation constraint condition. In the sixth step of the present invention, the weight of the K-plate generation constraint obtained in this manner is converted into the normalized maximum achromatic amount (calculated from the K-plate generation profile) limited in a linear space. By multiplying, the final achromatic amount under the condition selected by the user is obtained. For example, in this case, the user can use FIG. 5 (a), (b), (c).
As shown in (d) and (d), using the GUI,
K plate generation constraint conditions such as plate replacement intensity, maximum K plate amount limit, saturation weighting and brightness weighting of K plate replacement are reflected in the achromatic amount.

Thereafter, in the final seventh step of the present invention, the colorimetric value under the constraint condition is determined from the colorimetric value by using the correlation function between the achromatic amount of the K plate generation profile and the CMYK ink amount in the linear space. Determine the amount of CMYK ink. Specifically, by reflecting the set weighting factor in the K-plate generation profile (polynomial), the K-plate amount can be restricted in each XYZ from this profile. In this manner, if the K plane is fixed, an inverse mapping (3-3 inverse transformation) from the colorimetric values XYZ to the CMY color reproduction data can be obtained by using the above-described successive approximation method or the like.

The above-mentioned inverse mapping can be reliably obtained within the color reproduction range (gamut) of printing, but cannot be obtained outside the gamut. That is, in the calculation of the CMYK ink amount, the outside of the printing gamut (in the printing ink amount space, CMYK
% Or less than 0%) is not calculated,
Unable to generate CMYK ink amounts outside gamut. However, when mapping from CIE colorimetric data to CMYK, data outside the gamut is often required as an auxiliary point in mathematical prediction such as interpolation calculation. Naturally, even a virtual mapping point must maintain geometric continuity with respect to a point in another gamut.

Therefore, according to the method of the present invention, the increment of the K plate is generated so as to be continuous inside and outside the gamut in the colorimetric space, and if the discrete K plate is in the range of 0% to 100%, the increase is obtained. The amount of color matching ink in the K plate amount is determined. That is, K
In order for the increment of the plate to be likewise continuous in and out of the gamut in the colorimetric space, the maximum likelihood equation is calculated using the data in the gamut with the dimension of the constraint applied to the achromatic amount (eg, saturation, lightness, etc.). Assembling, and thereby, the amount of the K plate outside the gamut may be obtained. Still, K version is less than 0% or 10%
If it deviates from 0%, the colorimetric ink amount cannot be calculated, so the colorimetric space is divided into arbitrary blocks (this time L ^*
It suffices to construct a regression equation based on the data in that space and obtain continuous CMYK virtual quantities.

In this way, by reflecting an arbitrary K-plate weight coefficient on the achromatic amount, the K-plate amount can be fixed, and the exact solution of the inverse mapping to the CMY space can be obtained from the colorimetric values using successive approximation. As a result, the inverse conversion from XYZ to CMYK can be performed under arbitrary K-plate conditions according to the user's selection.

[0042]

As described in detail above, according to the present invention,
Equivalent K for colorimetric reproduction desired by the user for printing
By taking into account the stability and accuracy of color reproduction centering on gray by plate replacement, stabilization of printing by balance distribution of total ink amount, and expansion of the maximum density range, colorimetric The K-plate conditions can be arbitrarily selected while maintaining the reproduction conditions, and the selected K-plate conditions are always the same as the printable K-plate conditions based on the physical quantity of the achromatic amount in the ink amount space. Can be
It is possible to obtain the color reproduction data of the four CMYK colors by reliably performing the inverse conversion from the three-dimensional colorimetric data under the K-plate condition having such optional selectivity and being printable.

[Brief description of the drawings]

FIG. 1 is a flowchart illustrating an example of a color reproduction data conversion method according to the present invention.

FIG. 2 is a flowchart illustrating an example of a calculation process of an achromatic amount in the color reproduction data conversion method of the present invention.

FIGS. 3 (a) and (b) show the relationship between the K plate amount and the CMY ink amount obtained in the equivalent ink amount calculating step and the achromatic amount calculating step in the color reproduction data conversion method of the present invention, respectively. 5 is a graph showing an example of a relationship (of a linear component) between the K plate amount and the achromatic amount obtained in FIG.

FIGS. 4A and 4B show the relationship between the K plate amount and the CMY ink amount obtained in the equivalent ink amount calculating step and the achromatic amount calculating step in the color reproduction data conversion method of the present invention, respectively. 6 is a graph showing another example (including a non-linear component) of the relationship between the K plate amount and the achromatic amount obtained in (1).

FIGS. 5A and 5B show K-plate control conditions in the color reproduction data conversion method of the present invention, wherein FIG.
FIG. 7C is a schematic diagram illustrating an example of a saturation weight, FIG. 7C is a schematic diagram illustrating an example of a maximum limit of a K-plate amount, and FIG.

FIG. 6 is a graph showing an example of a saturation weighting function used in the color reproduction data conversion method of the present invention.

FIG. 7 is a schematic diagram showing a color gamut in an L ^* a ^* b ^* space with different K plate amounts.

FIG. 8 is a schematic diagram showing that the K plate amount is equal to the CMY equivalent removal amount.

Claims (5)

[Claims] 1. C (cyan), M (magenta), Y (yellow) and K (black) including a black plate from three-dimensional colorimetric data.
Before converting to the four-dimensional color reproduction data while maintaining the color matching conditions, inverse conversion from a three-dimensional colorimetric space to a three-dimensional CMY space is performed for a plurality of different K plate amounts in advance, and the K plate amount is set as a reference. The three-dimensional colorimetric data at the time of the amount and the ink amount of the CMY that is the conditionally equal color are obtained as the equal color ink amount for the plurality of different K plate amounts. The ratio of CMYK to the total ink amount when the amount is the reference amount
Determined as the achromatic amount in the plate amount, the portion where the achromatic amount is non-linear with respect to the K plate amount is tested, and the achromatic amount function for the K plate amount is determined in the linear portion. In order to generate a mapping function from the colorimetric data and the achromatic amount to the CMYK ink amount, and to weight the achromatic amount with a preselected K-plate control condition, the K-plate control condition is sequentially changed to the K-plate control condition. Determined as a weighting factor to be reflected on the achromatic amount, as the constraint condition for the inverse conversion from the three-dimensional colorimetric data to the four-dimensional color reproduction data, the achromatic amount function and the selected K-plate control condition From the achromatic amount determined here and the three-dimensional colorimetric data. CMY using K ink mapping function
A color reproduction data conversion method comprising calculating an amount of K ink. 2. The color reproduction data conversion method according to claim 1, wherein the K-plate control condition includes at least a K-plate replacement amount, a K-plate maximum limit amount, a saturation weight, and a lightness weight. 3. The inverse conversion from the three-dimensional colorimetric space to the three-dimensional CMY space for the plurality of different K plate amounts is performed by measuring the color of a specific color patch and calculating the color patch from a four-dimensional CMYK space obtained by an interpolation calculation. 3. The color reproduction data conversion method according to claim 1, wherein the method is performed based on forward conversion to a three-dimensional colorimetric space. 4. The apparatus according to claim 1, wherein at least one of the CMYK ink amount mapping function and the achromatic amount limit in the achromatic amount function is stored in advance as a K-plate generation amount profile, and is sequentially called to generate the K-plate. 4. The method of converting color reproduction data according to any one of claims 1 to 3. 5. When the calculation of the CMYK ink amount requires a point outside the color gamut of printing, the achromatic amount is adjusted so that the K plate outside the color gamut becomes continuous from within the color gamut. Estimate by maximum likelihood using parameters of the same dimension as the constraint condition used for calculation. If the estimated K plane is 0% or more and 100% or less, the amount of color matching ink is calculated in the estimated K plane. In this way, the CMY ink amount outside the color gamut is obtained. If the estimated K plate is less than 0% and more than 100%,
The color reproduction data conversion method according to any one of claims 1 to 4, wherein a CMY ink virtual amount outside the color gamut that is continuous from within the color gamut is obtained from a regression equation in a specific divided space.