JP2745717B2 - Color image forming equipment - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus such as a color printer and a color copying machine, and more particularly to a full-color image forming apparatus that performs good color reproduction with four color inks including black. .

2. Description of the Related Art Full color recording in hard copy includes three colors of C (cyan), M (magenta), and Y (yellow), or
This is realized by performing gradation recording for each of the four color inks including K (black). Ideally, full-color color reproduction can be realized with three primary color inks of C, M, and Y. However, since black actually formed by combining three colors is made by the balance of three colors, it is difficult to achieve a perfect balance in all gradations, and the gray scale takes on a tint.
In many cases, the quality of black is not sufficient, for example, the density around black is insufficient due to insufficient density of black, and the color around the pixel is displaced due to the displacement of the three color pixels. Used.

Conventionally, a method called UCR (Under Color Removal) has been performed. This means that the black ink density K
It is obtained by setting an arbitrary amount equal to or less than the amount of the ink having the lowest density among Y, M and C, and subtracting the black ink density K from the density of each ink of Y, M and C.

Equation 1 is an example of a 100% UCR. 3 primary color ink density
Assuming that C, M, Y and the black ink density are K, K is set to the minimum density of C, M, Y, and the C, M, Y ink density used for printing is C1, M
It is represented by 1, Y1.

K = min (Y, M, C), C1 = CK M1 = M−K, Y1 = Y−K (1) In order to perform full-color printing, each of the four primary colors including black is used. Must be recorded in gradation.

The gradation recording includes a density gradation method in which the density can be controlled within a single dot as typified by a sublimation type thermal transfer method and a silver halide photographic method, and a dithering method such as a fusion type thermal transfer method and an electrophotographic method. The density pattern can be broadly classified into an area gradation method in which gradation is expressed by a combination of dots using a visual integration effect.

Both methods use the principle of subtractive color mixture using complementary colors of additive color mixture by R, G, and B, which are the three primary colors of color light. The problem with additive color mixture is only the color gamut of the three primary colors.
While each spectral distribution does not affect color reproduction, in subtractive color mixing, the spectral distribution of the dye has a large effect on color reproduction.

In actual spectral absorption characteristics of ink, the phenomenon that the hue of the recorded image changes and the saturation decreases due to the fact that the center wavelength deviates from the ideal and the absorption characteristics are broad and there are side absorptions. .

Therefore, a technique called masking is used for these problems mainly in the printing field.

Equation 2 is a technique called linear masking, which is widely used because of its simple configuration. The actual main density of the ink is represented by c ', m', y ', and the input spectral density is represented by c, m, y.

c '= a0 * c + a1 * m + a2 * y m' = a3 * c + a4 * m + a5 * y y '= a6 * c + a7 * m + a8 * y (2) Nine correction coefficients a0 to a8 are used for many color charts. Pair (c, m,
For y), a set of actual ink main densities (c ′, m ′, y ′) that realizes that color is obtained, and those colors are obtained to minimize the color difference in an average sense. I have.

In this method, the sum of the increase in concentration due to the three dye amounts is
The addition rule of being equal to the increase of each density component is satisfied, that is, the addition rule of density in subtractive color mixture (Lambert-Bee
It implicitly assumes that r-rule) holds.

However, it is known that when actual ink is overprinted, such linearity of density is not established, and a higher-order masking method capable of expressing a certain degree of nonlinearity has been proposed.

Equation 3 is the simplest second-order masking equation among them.

c '= a0 * c + a1 * m + a2 * y + a3 * c 2 + a4 * m 2 + a5 * y 2 + a6 * m * y + a7 * y * c + a8 * c * m m' = a9 * c + a10 * m + a11 * y + a12 * c 2 + a13 * ^{m 2 + a14 * y 2 +} a15 * m * y + a16 * y * c + a17 * c * m y '= a18 * c + a19 * m + a20 * y + a21 * c 2 + a22 * m 2
^{+ A23 * y 2 + a24 *} m * y + a25 * y * c + a26 * c * m ...... (3) which is spectral density c input, m, with respect to y, the main concentration of real ink representing the color By using a quadratic expression of c, m, and y, the non-linearity of density when ink is overprinted is approximated by a quadratic expression. The 27 correction coefficients a0 to a26 are determined to minimize the color difference in an average sense for many sets of color patches (c, m, y) as in the case of linear masking.

In order to further improve the approximation accuracy, third-order or higher-order ones have been proposed.

In four-color recording, masking was performed by removing the black component.
Performed for residual components of c, m, y.

Problems to be Solved by the Invention The conventional UCR (under color removal) method converts an actual ink having non-linearity in density when overprinted into four colors by linear subtraction, so that accurate Concentration cannot be predicted,
There is a problem that a color recorded by superimposing four color inks does not become a desired color.

Also, the conventional linear masking method for color correction is
Since the number of correction coefficients is nine and the degree of freedom is small, the labor required to determine the correction coefficients is small and the circuit scale is relatively small.
As described above, since the correction is performed by a linear matrix operation on the actual ink having a non-linear density in the overprinting, it is not possible to determine the correction coefficient for reducing the color difference of each color in the entire color space. It is possible and has a problem that the color difference from the target color is large.

The higher-order masking method can reduce the color difference over the entire color space within the color reproduction range by nonlinear approximation, but even if the simplest quadratic method is used, the correction coefficient The number becomes 27, which requires a large number of multipliers, and becomes a huge hardware scale.

In any case, in order to obtain faithful color reproduction with the four-color ink, even if the masking accuracy is improved, an error due to UCR (under color removal) is large, so that a sufficient color reproduction cannot be obtained comprehensively.

In view of these points, the present invention provides a UCR method for compensating for the non-linearity of the superimposed density and a non-linear color correction capable of responding to the non-linearity of the superimposed density with high accuracy, on a small scale that is not much different from the conventional linear UCR and linear masking. It is an object of the present invention to provide a color correction method that can be performed collectively with a simple configuration.

Means for Solving the Problems According to the present invention, in order to solve the above-mentioned problem, a first conversion is performed in which C, M, and Y density information is converted by a non-linear function whose monotone increases and its derivative also increases monotonically. Means, and second conversion means for performing conversion with a non-linear function whose function is monotonically increasing and whose derivative is monotonically decreasing. A black separation means, a subtraction means, and a matrix are provided between the first and second conversion means. A means is provided to control the ink densities of the four colors to perform color printing.

In the present invention, the input density signal is non-linearly converted by the first conversion means into a color space in which linearity corresponding to the amount of dye (dye amount) is established, and linear black separation is performed in a space in which the linearity is established. The means and the subtractor perform four-color conversion and linear conversion using a linear matrix, and further return to the density color space by the second conversion means to perform nonlinear UCR and color correction as a whole.

The correction coefficient is determined by using the relationship between the density of each of the three primary colors and the recorded chromaticity, as in the conventional linear masking method.

Embodiment The configuration of the present invention will be described below based on an embodiment using a sublimation type thermal transfer recording system.

FIG. 1 shows an embodiment of a gradation printer according to the present invention which aims at faithfully recording the colors of input R, G, B signals using ink.

1,2,3 are complementary color conversion means for converting the input luminance information R, G, B into complementary color and converting into density information C, M, Y, 4,5,6 are complementary color conversion means
First conversion means for non-linearly converting each of the main concentration information C, M, and Y of 1, 2, and 3, and 7 is the product of the output C1, M1, Y1 of the first conversion means 4, 5, and 6 and the correction coefficient. A linear matrix means 8 which performs a sum operation and outputs C2, M2, Y2, and 8 is an output C2, M2,
Black separating means for separating the achromatic information from Y2 to generate a black signal k3; 9, 10, 11 calculating means for subtracting the black signal k3 from each of the outputs C2, M2, Y2 of the matrix means 7, 12, 13, 14 , 15 are second conversion means for non-linearly converting the outputs of the subtraction means 9, 10, 11 and the black signal, respectively, and 16 is the second conversion means 12, 13, 14, 15,
The recording control means 17 performs gradation color recording based on the output main density information C4, M4, Y4, k4, and the recording control means 16 controls the amount of dye to be transferred by the heat amount of the thermal head to perform recording.

Complementary color conversion is to convert luminance information R, G, B based on the additive color mixture principle into density information C, M, Y in order to use it as an input of a color printer based on the subtractive color mixture principle.The maximum density of each color is dmax. Then, it is defined by FIG.

When 0 <R, G, B <10- ^dmax C = log (1 / R), M = log (1 / G), Y = log (1 / B) R, G, B> 10- ^dmax In the case of C = dmax, M = dmax, Y = dmax (4) When recording is performed based on the subtractive color mixing principle, the C, M, and Y3 primary colors are superimposed and recorded. The G and C ink layers absorb the R light at the center, so that any color within the color reproduction range is reproduced by the amount of dye. However, for example, the actual C ink absorbs not only R but also G. In other words, the actual C ink contains a considerable amount of the M component in addition to the original C component.
The blue produced by synthesizing the inks of M and M has an excessive amount of the M component and has a purplish color.

Masking means reducing the amount of M ink by the amount of the magenta component contained in C at this time.

In order to do this correctly, it is necessary to add or subtract the density between the magenta component in the C ink and the M component in the M ink.

However, in actual ink, linearity generally does not hold. FIG. 3 illustrates the relationship between the recorded dye amount and the recording density in actual sublimation type thermal transfer recording. It can be seen that the density does not follow the increase in the amount of the dye. FIG. 4 is a plot of the density D1 recorded with a certain amount of dye and the combined density D2 when the amount of dye is recorded twice. The graph shows a shape close to a straight line passing through the origin, and the slope is between 1 and 2. From this graph, it can be seen that the density does not reach twice even when recording is performed twice.

Here, the density at the time of recording with a certain dye amount p is D1, 2p
The density recorded with the amount of the dye is defined as D2.

This is set as D1 = f (p), D2 = f (2p) (5), and assuming that the inclination of the straight line is k, f (2p) = k * f (p), 1 <K <2 ... (6) One of the solutions satisfying the equation 6 is as ^follows : D = f (p) = p ^{1 / a} (7) where a = (log 2 / log k)> 1 Therefore, the equation 7 is expressed in FIG. Is a good approximation. Also, solving Equation 7 conversely gives p = D ^a , a> 1 (8).

Equation 8 represents conversion of the input density into a space in which the linearity of the dye amount is established. Equation 7 means a conversion from the space of the dye amount to the space of the density.

The relationship between Expressions 7 and 8 is slightly different depending on the printing method of the printer and the ink material, but Expression 8 shows that the function and the derivative are monotonically increasing, and Expression 7 is that the function is monotonically increasing and the derivative is monotonically increasing. The thing is common.

In the present embodiment, the first conversion means 4, 5, 6 are represented by Expression 8, and the second conversion means 12, 13, 14, 15 are represented by Expression 7.

Since color correction corresponds to coordinate conversion in a space of the amount of dye in which linearity is established, a linear matrix can be used, and the matrix means 7 can be expressed by Expression 9.

C2 = a0 * C1 + a1 * M1 + a2 * Y1 M2 = a3 * C1 + a4 * M1 + a5 * Y1 Y2 = a6 * C1 + a7 * M1 + a8 * Y1 (9) Also, if the UCR is performed in the space of the amount of dye that holds linearity, it is linear. Can be performed by the following operation. Change the amount of UCR to Q (100
* If UCR of Q% 0 <Q <1), the black separating means 8 is given by the following equation 1.
Expression 0 and subtraction means 9, 10, and 11 can be expressed by expression 11.

K3 = Q * min (C2, M2, Y2) (10) C3 = C2-K3, M3 = M2-K3, Y3 = Y2-K3 (11) By means of the means 12, 13, 14, 15 to convert the actual ink densities of the four colors to C4, M4, Y4, K4, four-color conversion including color correction can be performed.

To summarize the characteristics of this ^{embodiment, C2 = a0 * C a +} a1 * M a + a2 * Y a M2 = a3 * C a + a4 * M a + a5 * Y a Y2 = a6 * C a + a7 * M a + a8 * Y ^a K3 = Q * min (C2, M2, Y2) (where a> 1, 0 <Q <1) C3 = C2-K3, M3 = M2-K3, Y3 = Y2-K3 C4 = C3 ^{1 / a} , M4 = M3 ^{1 / a} Y4 = Y3 ^{1 / a} , K4 = K3 ^{1 / a} (13) As shown in FIG.
A non-linear operation of 256 * (x / 256) ^a (0 ≦ x <255) with accuracy is constituted by a ROM table, and the second conversion means 12, 13, 14, 15
Is, as shown in FIG. 6, 256 * (x / 256) ^{1 / a} (0 ≦ x <2
The calculation of 55) is composed of a ROM table.

FIG. 7 shows one-third of the specific configuration of the matrix means 7, and M2 and Y2 can be operated with the same configuration. Two
Reference numerals 0, 21, and 22 denote multiplication means for performing a product operation of the input C1, M1, and Y1 with the correction coefficients, respectively, and 23 denotes multiplication means 20, 21, and 22.
This is an addition means for calculating the sum of the outputs of.

FIG. 8 shows a specific configuration of the black separating means. 30 and 31 are selection means, 32 and 33 are comparison means, and 34 is a multiplication means.

First, C2 and M2 are compared by the comparing means 32, and when C2 is small, the output S1 is activated. The selecting means 30 selects C2 when S1 is active, and selects M2 at other times, so that min (C2, M2) is output to K1. The comparing means 33 and the selecting means 31 operate in the same manner, and min (C2, M2, Y2) is output to K2. Further, the multiplication means 34 multiplies the coefficient Q for setting the amount of UCR by a coefficient Q of 0 or more and 1 or less, and outputs K3.

With the above configuration, the color image forming apparatus of the present embodiment has a small number of degrees of freedom, which is not much different from the conventional linear masking of a total of ten linear correction coefficients a0 to a8 and one coefficient a representing the nonlinearity of the superimposed density. Therefore, it is possible to perform more flexible and accurate color correction than the higher-order masking by polynomial approximation for the non-linearity of the superimposed density, and it is possible to perform four-color conversion with high accuracy to the non-linearity of the superimposed density.

Next, a method of determining the ten correction coefficients a0 to a8 and the nonlinear conversion coefficient a will be described.

Select tens to hundreds of colors distributed at equal intervals within the color reproduction range defined by the maximum density of ink and the density of paper,
Using the least squares method that minimizes the root mean square of the error between the chromaticity of the color chart actually recording each of these colors and the chromaticity expressed by the input R, G, B for all the color charts You have set.

Equation 14 is an example of a correction coefficient determined by the sublimation type thermal transfer printer.

a = 1.6 (14) Next, another embodiment of the present invention will be described.

FIG. 9 shows an embodiment of a gradation printer which faithfully records the colors of inputted R, G, B signals using ink.

1, 2, and 3 are complementary color conversion means, 4, 5, and 6 are first conversion means, 7 is a linear matrix means, 8 is black separation means, 9, 10, and 11 are subtraction means, 12, 13, 14, and Reference numeral 15 denotes second conversion means, 16 denotes recording control means, and 17 denotes a head. Each means of this embodiment is equivalent to that of the first embodiment.

 The operation of this embodiment will be described.

The input R, G, B signals are converted into spectral densities C, M, and Y by complementary color conversion means 1, 2, and 3, respectively, and dye amounts C1 for which linearity is satisfied by first conversion means 4, 5, and 6 respectively. , M1, Y1. Here, since the amount of dye C1, M1, and Y1 of the spectral density can be linearly calculated, UCR can be performed by linear subtraction. A black signal K3 is obtained by multiplying the smallest one of C1, M1, and Y1 by the UCR coefficient Q by the black separating means, and KCR is performed by removing K3 from C1, M1, and Y1 by subtracting means 9, 10, and 11. The pigment amounts C2, M2 and Y2 are obtained. Further, the actual amount of the pigment of the ink is
C3, M3, Y3 are calculated, and the actual ink densities C4, M4, Y4, K4 are obtained by the second converting means 12, 13, 14, 15.

Both the black separating means 8 and the matrix means 7 in this embodiment are operated in a linear dye amount space.

The above can be expressed by the following equations: C1 = C ^a , M1 = M ^a , Y1 = Y ^a (where a> 1) K3 = Q * min (C1, M1, Y1) (where 0 <Q <1) C2 = C1 -K3, M2 = M1-K3, Y2 = Y1-K3 C4 = (a0 * C2 + a1 * M2 + a2 * Y2) ^{1 / a} M4 = (a3 * C2 + a4 * M2 + a5 * Y2) ^{1 / a} Y4 = (a6 * C2 + a7 * M2 + a8) * Y2) ^{1 / a} K4 = K3 ^{1 / a} (15) With the above configuration, in this embodiment, a total of ten linear correction coefficients a0 to a8 and one coefficient a representing the nonlinearity of the superimposed density are obtained. It can be seen that, with a small degree of freedom that is not much different from that of the conventional linear masking, the non-linearity of the superimposed density can be corrected more flexibly and with higher precision than higher-order masking by polynomial approximation, and that the four-color conversion with high precision can be performed. Further, the correction coefficient can be determined in the same manner as in the first embodiment.

 Next, another embodiment of the present invention will be described.

FIG. 10 shows an embodiment of a gradation printer which faithfully records the colors of inputted R, G, B signals using ink, and performs only UCR.

1, 2, 3 are complementary color conversion means, 4, 5, 6 are first conversion means, 8 is black separation means, 9, 10, 11 are subtraction means, 12, 13, 14, 15 are second conversion means. , 16 are recording control means, and 17 is a head. Each means of this embodiment is equivalent to that of the first embodiment.

The input R, G, B signals are converted into spectral densities C, M, and Y by complementary color conversion means 1, 2, and 3, respectively, and dye amounts C1 for which linearity is satisfied by first conversion means 4, 5, and 6 respectively. , M1, Y1. Here, since the amount of dye C1, M1, and Y1 of the spectral density can be linearly calculated, UCR can be performed by linear subtraction. A black signal K3 is obtained by multiplying the smallest one of C1, M1, and Y1 by the UCR coefficient Q by the black separating means, and KCR is performed by removing K3 from C1, M1, and Y1 by subtracting means 9, 10, and 11. The pigment amounts C2, M2 and Y2 were obtained,
Of the actual ink density C4, M4, Y
4, K4 is obtained. The black separating means 8 of this embodiment is an operation in a linear dye amount space.

If indicated by the equation ^{above, C1 = C a, M1 =} M a, Y1 = Y a ( where a> 1) K3 = Q * min (C1, M1, Y1) ( where 0 <Q <1) C3 = C1 −K3, M3 = M1 − K3, Y3 = Y1 − K3 C4 = C3 ^{1 / a} , M4 = M3 ^{1 / a} Y4 = Y3 ^{1 / a} , K4 = K3 ^{1 / a} ... (16) It can be seen that highly accurate four-color conversion corresponding to the non-linearity of the superimposed density can be performed.

It should be noted that the first conversion means 4,
Although the fifth and sixth conversion means and the second conversion means 12, 13, 14, and 15 are configured as individual tables, they may be included in other continuous conversion tables without any problem.

Further, although the matrix means 7 is configured using a parallel operation of the multiplier and the adder, it is also possible to configure the matrix unit 7 using one multiplier for time division.

As described above, the printer has been described as an embodiment, but it is apparent that the present invention can be used in a color image forming apparatus such as a full-color copying machine.

According to the present invention, a UCR that compensates for non-linearity of overlay density by converting ink density to a linear dye quantity space
By the method, high-precision four-color conversion and high-precision color correction that can cope with the non-linearity of the overlay density can be realized. further,
Four-color conversion and color correction can be performed collectively with a small-scale configuration.

The color correction of the present invention, for higher-order masking that corresponds to nonlinearity by adding several tens of degrees of freedom to linear masking,
By adding one degree of freedom to linear masking, high-precision correction can be performed more flexibly than non-linear masking.

Therefore, a high-precision color correction system equivalent to high-order masking can be realized with a small-scale hardware configuration that is not much different from linear masking, for high-order masking in which the hardware scale becomes enormous.

Further, since the degree of freedom to be determined for realizing color correction is small, the correction coefficient can be determined with a small number of repetitions and with high accuracy.

[Brief description of the drawings]

FIG. 1 is a block diagram of a color image forming apparatus according to an embodiment of the present invention, FIG. 2 is a diagram showing complementary color conversion characteristics, FIG. 3 is a diagram showing the relationship between the amount of ink pigment and density, and FIG. FIG. 5 is a diagram showing the relationship between the densities when the amount of the dye is doubled, FIG. 5 is a diagram showing the conversion characteristics of the first converter, FIG. 6 is a diagram showing the conversion characteristics of the second converter, and FIG. FIG. 8 is a configuration diagram of a matrix unit, FIG. 8 is a configuration diagram of a black separation unit, FIG. 9 is a block configuration diagram of a color image forming apparatus in a second embodiment,
FIG. 10 is a block diagram of a color image forming apparatus according to the third embodiment. 1, 2, 3... Complementary color conversion means, 4, 5, 6... First conversion means, 7... Matrix means, 8.
9, 10, 11 ... subtraction means, 12, 13, 14, 15 ... second conversion means, 16 ... recording control means, 17 ... head, 20, 21, ...
22 multiplication means, 23 addition means, 30, 31 selection means, 32, 33 comparison means, 34 multiplication means.

 ────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-292963 (JP, A) JP-A-3-48575 (JP, A) JP-A-62-175072 (JP, A)

Claims (3)

(57) [Claims] 1. A first conversion means for converting C, M, Y (cyan, magenta, yellow) density information by a non-linear function that is monotonically increasing and its derivative is also monotonically increasing.
Black separating means for separating achromatic information from C1, M1, Y1;
Subtraction means for subtracting the output of the black separation means from C1, M1, Y1; and a second means for converting the output of the subtraction means and the output of the black separation means with a non-linear function whose monotone increases and whose derivative decreases monotonically. A color image forming apparatus, comprising: two conversion means, controlling the ink densities of four colors according to the output of the second conversion means, and performing color printing. 2. A first conversion means for converting the density information C, M, Y by a non-linear function that is monotonically increasing and its derivative is also monotonically increasing, and inputs the conversion outputs C1, M1, Y1. Matrix means for performing a matrix operation, black separation means for separating achromatic information from outputs C2, M2, Y2 of the matrix means,
Subtraction means for subtracting achromatic information by the black separation means from the C2, M2, Y2, and converting the output of the subtraction means and the output of the black separation means into a non-linear function whose monotone increases and whose derivative decreases monotonically; A color image forming apparatus, comprising: a second converting means for performing color printing, and controlling color density of four colors according to an output of the second converting means to perform color printing. 3. A first conversion means for converting the density information C, M, Y by a non-linear function that is monotonically increasing and its derivative is also monotonically increasing, and an achromatic color is obtained from the conversion output C1, M1, Y1. Black separating means for separating information; subtracting means for subtracting achromatic information by the black separating means from the C1, M1, Y1; and matrix means for performing a matrix operation using the outputs C2, M2, Y2 of the subtracting means as inputs. And second conversion means for converting the output of the matrix means and the output of the black separation means with a non-linear function having a monotonically increasing function and a monotonically decreasing derivative thereof. A color image forming apparatus that controls color ink density of four colors and performs color recording.