JPS6335072A - Color masking parameter determining device - Google Patents

Abstract

PURPOSE:To optimize the color reproduction by providing a weight calculating means for calculating the weight to each color patch by comparing a color difference, and a control means for updating a color masking parameter and a black rate so as to minimize an evaluated value, and executing a convergence calculation. CONSTITUTION:In a weight storing means 5, a color distribution is stored as weight Wn. A black quantity calculating means 7 derives Kmax from the three primary color source density and {alJ}, and calculates a black quantity Kn' in accordance with (omega) derived from the three primary color density. An evaluated value calculating means 10 derives net point effective area rates cn', mn', yn' and kn' from four main density signals Cn', Mn', Yn' and Kn', and simulates reproducing colors (L*n', u*n, v*n') and three stimulus values (Xn', Yn' and Zn') of the reproducing colors by a Neugebauer equation. The evaluated value calculating output 10 evaluates and outputs the evaluated value E<2> of color three primary colors to (L*n', u*n', v*n) stored in a color patch storing means 1. A control means 11 varies repeatedly a masking parameter {alJ} in a parameter storing means 2, from the obtained E<2>, and determines {alJ} so that E<2> takes the munimum value.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is applicable to color plate making or color printers, in which the luminance signals of red (R), green (G), and blue (B) of an image are converted into cyan (C), The present invention relates to a device for determining parameters necessary for color conversion into main density signals of magenta (M), yellow (Y), and black (K) inks.

[Conventional technology]

In color plate making or color printers, for example, the three primary color luminance signals (
R, G, 100), and four colors (C) to reproduce this.
, M, Y, K) to obtain the main density signals of the inks, first the three primary color luminance signals (R, G.

B) into the three primary color density signals (D, , DE, Db) using equation (1)
The amount of black is determined within the range of formula (2), and K≦Kmax
-min (Dr, Dg + Db
) (2) Furthermore, conversion into main density signals (C', M', Y') is performed using a matrix (acj) as shown in equation (3). This (aIJ) is called a color masking parameter.

By performing this conversion as accurately as possible, the colors printed with the amount of ink of the main density signals (C', M', Y') will be very similar to the original (R, G, B) colors. In order to make it visible, the color masking parameter (aU) has conventionally been determined by performing the least squares method on the principal densities.

Specifically, no black ink was used, and the known Cn, Ml, Yn (n=1°...
, N), and the three primary color densities (D rn , D gn , D
For example, for cyan ink, e2=X, (Cn an Drn ad2Dgn
a13 Dbn) all that minimizes 2n=1, a12. Find a13. This is obtained by solving simultaneous equations (5).

Similarly, for magenta ink and yellow ink, nine color masking parameters of (apl) can be obtained.

However, in the above method, the color masking parameters are affected by the set of colors of the color patches used, and are not optimal for the image actually converted. In addition, for evaluation where the final color difference is minimal when seen by the human eye, L8u*vx system or L*a*b*
Even though a uniform color space like the system is used, the formula (
In the sense that the minimization of 02 in 4) is a minimization regarding the main density of the three primary color inks, it is insufficient as a method for determining the optimal color masking parameter.

In order to solve this problem, a color masking parameter determination device has been proposed that takes into account the color distribution in the target image and minimizes the color difference seen by the human eye for each image through simulation. 60-122684
). This device will be briefly described in the third drawing.

Color values of virtual color patches uniformly selected in uniform color space (L”n, u*n, V”n) (n = 1,
. . , N) are stored in the color patch storage means 1. Further, the parameter storage means 2 stores an initial value of a masking parameter (a+J). The color conversion means 3 converts the Mihara color (Rp,
Go, Bm) value as (L”fl, u'ff1t
v''Q) value, and the weight calculation means 4 compares the color of each virtual color patch for each pixel with the intercolor distance between the L * u * v cars, and calculates the weight for the color patch of the closest color. A signal is sent to the weight storage means 5 to increase 1 by 1. As a result, after all the pixels of the image have been scanned, the weight storage means 5 stores the distribution of colors present in the image as a weight Wn.

The brightness value calculation means 14 calculates the virtual color patch (L product u
*, v*n) values into the three primary color values (Rn, cn, Bn), and the masking calculation means 15 calculates the equation (1) 1 equation (
3) for each ink main density (Cn', M.', Yn
'). The evaluation value calculation means 16 is (Cn').

Mn', Yn') to ink effective area ratio (cn'+
my1'.

yn') is determined, and the Neugebauer equation is applied to this to predict the color (L"n' + u"n', V*n') that will be reproduced by printing with this combination of inks. The evaluation value is E2 (formula (6)), which is the square of the reproduced color difference for each color patch, multiplied by the weight Wn for each color patch, and added.

E2=RWn<(LXn' -Lmn) 2+("n
'''n) 2+ (V*n'V'n) 21
The control means 17 sets the masking parameter (a+
, +) is updated. By repeating the above steps, the optimal color masking parameters are determined by convergence calculation.

[Problem that the invention seeks to solve]

However, the above method makes the image cyan.

When reproducing with the three primary color inks of magenta and yellow, it is possible to determine the color masking parameters to optimize the color reproduction of the image, but as with normal printing, some of the masking parameters can be replaced with black ink, and four When printing in color, this effect is not taken into account. Therefore, when performing four-color printing, the obtained masking parameters are not necessarily optimal.

SUMMARY OF THE INVENTION An object of the present invention is to provide a color masking parameter determination device that solves the above problem by taking into consideration the process of generating the amount of black ink.

[Means for solving problems]

The color masking parameter determining device of the first invention includes:
Color patch storage means for storing color coordinate values of a plurality of virtual color patches selected within a uniform color space; Parameter storage means for storing color masking parameters of 9 f[la; All or part of the target image color conversion means for converting the three primary color values of the pixels into color coordinate values in a uniform color space; weight calculation means for calculating weights for color patches; weight storage means for storing the calculated weights; density value calculation means for calculating three primary color density values from the color coordinate values of the color patches; black from the three primary color density values. a black amount calculation means for calculating a main density; a UCR calculation means for calculating a three primary color density value after removing the undercolor from the black main density and the three primary color density values; a masking calculation means that performs masking calculation based on parameters and calculates the main densities of the three primary colors of the ink; and a masking calculation means that calculates the reproduced color coordinate values in the uniform color space for this combination of inks from the three primary color main densities of the ink and the black main density, and an evaluation value calculation means for obtaining a color difference from a color coordinate value stored in a storage means and further calculating an evaluation value using a weight for each color patch stored in the weight storage means; and a control means for updating the color masking parameters such that the color masking parameters are updated, and the color masking parameters optimal for the target image are determined by convergence calculation.

The color masking parameter determining device of the second invention includes:
A color patch storage means for storing color coordinate values of a plurality of virtual color patches selected in a uniform color space, a parameter storage means for storing nine color masking parameters, and generation of black ink for other inks. a black ratio storage means for storing a black ratio as a ratio; a color conversion means for converting the three primary color values of all or some pixels of the target image into color coordinate values in a uniform color space; and a color coordinate value of each pixel. , a weight calculating means for calculating a weight for each color patch by comparing the color difference between the color coordinate values of the plurality of color patches, a weight storage means for storing the calculated weight, and a color coordinate value of the color patch. density value calculation means for calculating three primary color density values from coordinate values; black amount calculation means for calculating black main density from the three primary color density values and the black ratio; UCR calculation means for calculating three primary color density values; masking calculation means for performing masking calculation on the three primary color density values after removing the undercolor using the color masking parameters to calculate the three primary color main densities; From the black main density, reproduced color coordinate values in a uniform color space by this combination of inks are determined, and the color bar is calculated.

obtain the color difference from the color coordinate values stored in the storage means;
Further, an evaluation value calculation means for calculating an evaluation value using the weight for each color patch stored in the weight storage means, and updating the color masking parameter and the black rate so as to minimize the evaluation value. and a control means, and is characterized by determining the optimal color masking parameters and black rate for the target image by convergence calculation.

[Effect]

When color printing is performed by dot printing using four-color inks of cyan, magenta, yellow, and black, the effective area ratios of each ink are c' and m', respectively.

y', ni', the main density of each ink C/, M
'.

It is assumed that Y' and N' are connected by the relationship shown in equation (7).

The reproduced color by halftone printing is predicted by the Neugebauer equation of equation (8) using the CIE-1931XYZ system.

Here, α+=(1-c') (1-m') (IM
') (1-to')α2=C'(1-m')
(1-y')(1-ni') (X3 = (1-c
' ) m'(1-y' ) (1-ni'
) α4 = c'm' (I Y')
(1k') α5=(I C') (1-m
')y'(1-to')lrs=c' (1-
m')y'(1-to')α7 = (
1-c') m'V'(1-ni') αB
= c'm'y'(1-to' ) αs
= (1-c') (1-m')
(1-y') k'αIo=C'(1-m'
) (1-1')k'αu= (1-c'
)m'(1-y')k'α+2'!('
m'(1-y')k'α13=(1c'
) (1m')'/'k'α14=c'
(1-m')y'k'α+s= (1-
c')m'(1y')k'α16=c'
m'y'k' However, (X+ , Yt * Z
I) was measured for all combinations of each ink with and without printing (X, Y, Z
) value, normalized to Y=1 for white. i
Table 1 shows the correspondence between the inks that are printed.

Table 1 Normally, it can be considered that in the area where black ink is present, the influence of other inks does not appear on the color, and equation (8) can also be replaced by equation (9).

Here, βl−α1. β2=α2. ..., β8=α8
．． β, = .

At this time, β9=Σα1 holds true.

1+9 In this way, the main ink densities C' of the four colors of each color patch
, M', Y', if ' is given, the color is (
X', Y', Z').

Therefore, the color of the original color patch is virtually (R, G
, B), and the color masking parameter (
aIJ) is given, the main ink densities (C', M', Y') are determined by equations (1) to (3).

To determine whether the halftone-printed reproduced color is actually close to the original (R, G, B) color, convert both to coordinate values in a uniform color space. It can be evaluated by To obtain the optimal masking parameters for a certain target image, the color distribution in the target image is expressed as weights for color patches taken over the golden space, and the sum of squares of color reproduction errors including the weights E2
(Equation (6)) is determined to be minimized. Since this least squares method is not linear, it cannot be easily solved in a form such as equation (5), but it can be solved sequentially using a numerical calculation algorithm called the hill-climbing method.

The formula for converting the color represented by (X, Y, Z) into uniform color space (L*, u*, v*) is formula (10), and (
For example, the formula for converting R, G, B) to (X, Y, Z) is (
If R, G, B) are those of the NTSC color television system, they are given by equation (11).

Here, X X+15Y+32 Y X+15Y+32 u o' =0.201, v o' =0.4
61 Therefore, the virtual color patch is (L*□ u
11n.

v”n), n = 1, 2, .

Bn), and use equations (1), (2), (3),
(7) (cn'1mn', yn', kn')
, and from these, use equation (8) or (9) to calculate the reproduced color (
L Jln I.

u”n', V11n'), and the masking parameters can be optimized based on the results.The virtual color patch set can be created using the previously proposed technology (Japanese Patent Application 122684)
It is appropriate to

Black bonds can be arbitrarily determined within the range of formula (2), but
For example, it can be determined by the constant ω as follows.

K=to°Kmax (12
) Note that KlnaX can be easily calculated using equation (2), but it is important to note that the values of C', M', and Y' obtained by masking after UCR (formula (3)) cannot be negative. Taking this into consideration, it is more accurate to define by equation (13).

a211) r +a22Dg +a23Dba 2+
+ 222 + 223 Usually ω is a constant in the range of 0 to 1. By determining ω, the amount of black changes, and the amount of ink for each color patch changes accordingly as shown in equation (3). Therefore, the optimal masking parameter for each given ω
Find (aI").

Moreover, conversely, ω can also be used as a parameter to be optimized together with [aIJ]. In this case, the optimal masking parameter (aIJ) as well as the optimal ω can be determined.

〔Example〕

FIG. 1 is a block diagram of an embodiment of the first invention constructed based on the principle of the color masking parameter determination method described above.

The L*, ull, ■* values (5% 1
+ u*n, VIIn) (n=1....
+, N) are stored in the color patch storage means l. The parameter storage means 2 stores appropriate color masking parameters (acj) (1=1, 2,
3; J=1°2.3) is stored.

The target image is scanned and the R, G, B values (Ra, G
n, Bz) (1=1. . . .) are converted into L*, u*, v* values (L”, , u”L
v *A). Conversion formula (10), (11
) is executed. The weight calculation means 4 compares the color of each virtual color patch with the intercolor distance in L*u*v8 space for each pixel l, and the weight storage means 5 so as to increase the weight of the color patch with the closest color by 1. send a signal to. As a result, after all pixels of the image have been scanned, the weight storage means 5 stores the distribution of colors present in the image as weights Wn.

At the stage where the above preparations are complete, the density value calculation means 6 calculates each of the virtual color patches (L'H, u'n, v*n).
Quantities are calculated using equations (10) and (11) for the equation (10), and equation (
1) to calculate the three primary color densities (Drn, Dgn,
Dbn). The black amount calculation means 7 calculates KmaX from the three primary color densities and (au) by equation (2) or equation (13), and calculates the black amount K according to the given ω.
Calculate n'.

The UCR calculation means 8 (Drn, Dgn, Dbn
) and K. The three primary color densities after removing the undercolor from '(Drn',
Dgn'.

Dbn') is calculated according to equation (14).

The masking calculation means 9 was obtained here (Drn'
l D button', Dbn'), the current masking parameter (a
Perform the masking calculation according to "c"). The operation is the formula (
3), the main concentration signal <c n'.

Mn', Yn') are obtained.

The evaluation value calculation means 10 calculates four main concentration signals Cn', Mn
', Yn', and Kn', by reversing equation (7), the effective halftone area ratio 'n' + m Q' r
Y n' + kn' is determined, and the tristimulus values of the reproduced color (X
n', Yn', Z. '), and (L"n' + "n'+v*n') is further simulated using equation (10). The evaluation value calculation means 10 is also stored in the color patch storage means 1 (L"n, u'n+v
The evaluation value E2 of the three primary colors with "n" is evaluated and output according to equation (6).

The control means 11 repeatedly changes the masking parameter (a'+j) in the parameter storage means 2 from the obtained E2, performs a hill climbing method by numerical calculation, for example, and obtains E2.
(aIJ) is determined so that 2 takes the minimum value. The execution ends when the control means 11 judges that the improvement of the masking parameter (aIJ) has reached the limit based on a certain criterion, and the optimal masking parameters for the target image are stored in the parameter storage means 2. It will be done. Such a control means can be realized by a well-known electronic computer or the like.

Next, the second invention will be described in detail with reference to FIG. In the second invention, a black rate storage means 13 is added to the configuration of the first invention. Therefore, the control means 12 functions differently from the control means 11 shown in FIG. The other parts are exactly the same as the first invention. That is, the operation is similar until the evaluation value E2 is obtained from the evaluation value calculation means 10. In the second invention, the black rate ω (Equation (12)) is also subject to optimization, similar to the masking parameter (alJl).

The black rate storage means 13 stores an appropriate initial value of ω at the start of operation. The black amount calculation means 7 determines the black amount K n' using this ω.

The control means 12 can change the black rate ω at the same time as the masking parameter fa+j), and the second invention uses a numerical sequential calculation method such as the hill climbing method to change the masking parameter (aIj) that becomes the minimum of the evaluation value E2. The black rate ω is determined at the same time.

In each of the above embodiments, L*u” is used as a uniform color space.
Although the vII system was used, the present invention can be configured in exactly the same manner using the L* ac b* system.

The relationship between the L'a"b" system and the XYZ system is given by equation (15) under the same conditions as equation (10).

In addition, the relationship between ' and ' in equation (7) is stated assuming that the solid density of black ink is infinite, but if this approximation does not hold, the solid density is set as Dk,
Formula (16) is used.

K' --10g (1-k (1-10°Dk)
(16) Furthermore, in each of the above embodiments, the weight calculation means 4 calculates the weight for each color by comparing the colors of all pixels of the target image with each color of the color patch set, but in reality, neighboring pixels are similar. In many cases, the colors are distributed, so instead of scanning all pixels, calculating weights for pixels obtained by thinning the pixels to 1/m (m is a natural number) does not meet the purpose of the present invention. is sufficient.

〔Effect of the invention〕

As described above, by adopting the configuration of the first invention, a virtual color patch and image data are used to determine the optimal color masking parameter (aIj) for color reproduction of a target image during four-color printing. In the second invention, the optimal color masking parameter (aIj) and black rate ω can be determined.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the first invention, FIG. 2 is a block diagram showing an embodiment of the second invention, and FIG. 3 is a block diagram showing a conventional example. 1... Color patch storage means 2... Parameter storage means 3... Color conversion means 4... Weight calculation means 5... Weight storage means 6... ... Density value calculation means 7 ... Black amount calculation means 8 ... UCR calculation means 9.15 ... Masking calculation means 10, 16 ... Evaluation value calculation means 11, 12.17. Control means 13...Black rate storage means 14...Brightness value calculation means

Claims (2)

[Claims] (1) Color patch storage means for storing color coordinate values of a plurality of virtual color patches selected within a uniform color space; parameter storage means for storing nine color masking parameters; and all or part of the target image. color conversion means for converting the three primary color values of the pixels of the part into color coordinate values in a uniform color space; and comparing the color difference between the color coordinate values of each of the pixels and the color coordinate values of the plurality of color patches; weight calculation means for calculating a weight for each color patch; weight storage means for storing the calculated weight; density value calculation means for calculating three primary color density values from the color coordinate values of the color patch; and from the three primary color density values. a black amount calculation means for calculating a black main density; a UCR calculation means for calculating a three primary color density value after removing the undercolor from the black main density and the three primary color density values; a masking calculation means that performs masking calculations using masking parameters and calculates the main densities of the three primary colors of the ink; and a masking calculation means that calculates the reproduced color coordinate values in a uniform color space for this combination of inks from the three primary color main densities of the ink and the black main densities, and an evaluation value calculation means for obtaining a color difference from a color coordinate value stored in a patch storage means and further calculating an evaluation value using a weight for each color patch stored in the weight storage means; and a control means for updating the color masking parameters so as to minimize the color masking parameters, the color masking parameter determination device determining the optimal color masking parameters for a target image by convergence calculation. (2) Color patch storage means for storing color coordinate values of a plurality of virtual color patches selected in a uniform color space, parameter storage means for storing nine color masking parameters, and ink other than black ink. a black ratio storage means for storing a black ratio that is a generation ratio for a target image; a color conversion means for converting the three primary color values of all or some pixels of the target image into color coordinate values in a uniform color space; weight calculation means for calculating a weight for each color patch by comparing the color difference between the value and the color coordinate values of the plurality of color patches; a weight storage means for storing the calculated weight; and a weight storage means for storing the calculated weight; density value calculation means for calculating three primary color density values from the color coordinate values of; black amount calculation means for calculating black main density from the three primary color density values and the black ratio; UCR calculation means for calculating the density values of the three primary colors after removing the under color; masking calculation means for calculating the main densities of the three primary colors of ink by performing masking calculations on the density values of the three primary colors after removal of the under color using the color masking parameters; From the density and the black main density, calculate the reproduced color coordinate values in a uniform color space by this combination of inks, obtain the color difference with the color coordinate values stored in the color patch storage means, and further store it in the weight storage means. and a control means for updating the color masking parameter and the black rate so as to minimize the evaluation value.Convergence calculation A color masking parameter determination device that determines the optimal color masking parameter and black rate for a target image.