JP3492712B2 - Color image forming method and apparatus - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming method and apparatus such as a color printer or a color copying machine for printing out a color image.

[0002]

Color reproduction in a color image forming apparatus in the field of hard copy such as a color printer and a color copying machine is
These are the three primary colors of colored light used in the additive color mixing principle (R, G, B)
The color reproduction is based on the subtractive color mixing principle in which the reflectance of color light is adjusted with three color inks of cyan (C), magenta (M), and yellow (Y) which are complementary colors.

FIG. 9 shows an example of spectral absorption characteristics of ink used in a sublimation thermal transfer recording type printer.

As can be seen from the example of this ink, the actual ink has a spectral absorption characteristic that the center wavelength deviates from the ideal and there is an unnecessary absorption component because the absorption characteristic is broad. In the original color reproduction, the hue is different from the desired color and the color with low saturation is reproduced. Therefore, it is necessary to perform color correction to reproduce a desired color.

Conventionally, in the field of hard copy, a method called masking has been used as this color correction.
The most commonly used masking is (Equation 1)
As shown in, the ink density signal (Y, M, C) is a linear matrix for the three primary color density signals (D _R , D _G , D _B ) which are the complementary colors of the three primary color luminance signals (R, G, B). This is called linear masking that is determined by calculation.

[0006]

[Equation 1]

Linear masking is based on the premise that in color reproduction using actual ink, the density addition law (Lambert-Beer law) is established and color reproduction can be expressed by a linear operation over the entire color space. In color reproduction using, for example, in the case of a sublimation type thermal transfer recording type printer, there are various non-linear factors such as re-sublimation phenomenon of ink, internal reflection of ink, etc. It has been known.

Therefore, there has been proposed a non-linear high-order masking in which the ink density signal (Y, M, C) is determined by a high-order polynomial for the three primary color density signals (D _R , D _G , D _B ). The simplest quadratic masking equation is shown in (Equation 2).

[0009]

[Equation 2]

This is to perform color correction by a quadratic equation including a non-linear factor existing in color reproduction using actual ink, and 27 correction coefficients a0 to a26 are determined by the least squares method regarding the density difference. Are used (for example, "Image processing for color reproduction")
Part1 ").

Further, in color reproduction in hard copy, there is a problem of color reproduction range. The density range that can be recorded by the printer is equal to or lower than the maximum recording density peculiar to the apparatus and is equal to or higher than the surface density of the image receiving paper used for recording. The reproducible color reproduction range is limited by the spectral absorption characteristics of the real ink in which the recordable density range and the unnecessary absorption component are present, and generally C
The color reproduction range of the printer is narrower than that of the RT.

FIG. 10 shows an example of the color reproduction range. FIG. 10 is a three-sided view showing the color reproduction range of the CRT and the color reproduction range of the printer in the CIE L ^* u ^* v ^* system uniform perceptual space. The same figure (a) is the u ^* v ^* plane, the same figure (b) is the L ^* u ^* plane, and the same figure (c) is a figure which projected each color reproduction range on the L ^* v ^* plane. . The color reproduction range of the printer uses the ink having the spectral absorption characteristics shown in FIG. 9, and the color reproduction range of the CRT is that of the NTSC system CRT.

As described above, the color reproduction range of the printer is CR.
Since it is narrower than the color reproduction range of T, a signal requesting a color exceeding the color reproduction range of the printer may be input as an input signal to be recorded. In that case, at least one color signal of the ink density signals (Y, M, C) of the calculation result of the linear masking or the non-linear higher-order masking as described above is a density signal that cannot be recorded by the printer, that is, the paper surface density. There will be lower or higher than maximum concentrations.

In the prior art, when an ink density signal lower than the paper surface density is requested for this non-reproducible ink density signal, an ink density signal exceeding the maximum density is requested for the paper surface density. Recorded with the limiter applied to the highest density.

[0015]

However, since the ink density signal and the color perceived by humans are in a non-linear relationship, the limiter for the ink density signal is not optimal in terms of color reproduction. .

FIG. 11 shows an example of color reproduction when a limiter is applied to the ink density signal of the masking calculation result. In the figure, Pi (i = 1 to 3) is a target color represented by the input signal and is an input signal that cannot be reproduced by the printer. In the figure, Qi (i = 1 to 3) is a color reproduced when a non-recordable ink density signal among the ink density signals of the masking calculation result is subjected to a limiter of the paper surface density and the maximum density. It represents.

As in this example, in the conventional technique, when the target color corresponding to the input signal exceeds the color reproduction range of the printer, there are colors that can be reproduced by the printer that humans feel more preferable. Nevertheless, there is a problem in that the image quality may be deteriorated by reproducing a significantly different color.

In view of the above points, the present invention faithfully reproduces the color of an input signal reproducible by a printer, and optimizes the color reproducible for an input signal not reproducible by the printer. An object of the present invention is to provide a color image forming method and apparatus capable of reproducing colors.

[0019]

In order to solve the above problems, the color image forming method of the present invention is the color image forming method of the present invention, in which an input signal is color-corrected to a color reproducible by a printer. First color correction calculation step for converting to the ink density signal (Y1, M1, C1) of the above, and a judgment step for judging whether the input signal is a color reproducible by the printer or an unreproducible color And a second ink density signal (Y2, M2, C2), which is a color reproducible by the printer, from the input signal determined by the printer to be non-reproducible by the printer.
In the color correction calculation step of
Color reproduction prediction process to predict color reproduction for
And the color reproduction prediction that is the result of the color reproduction prediction process.
Evaluation value calculation step of calculating an evaluation value from the
By repeating the steps and the evaluation value calculation step,
Second color, consisting of a control step to search for ink density signal
A correction calculation step is performed, and if the input signal is a reproducible color, the first ink density signal (Y1, M1, C1) is set to the input signal according to the result of the determination in the determination step. When the color is an unreproducible color, the second ink density signal (Y2, M2, C2) is used to control the ink density to form a color image. Furthermore, this
The bright image forming apparatus uses the upper bits of the input signal to display.
Color the input signal representative point that appears to a color that the printer can reproduce.
By performing the first color correction calculation that performs the correction,
Convert to ink density signal (Y1, M1, C1)
The dot is a color that the printer can reproduce or cannot reproduce.
The input signal representative point
The input signal cost is
And table point was predicted for recordable ink density signal
The evaluation value is calculated from both the color reproduction prediction and
Search for ink density signal by repeating measurement and evaluation value calculation
By performing the second color correction calculation, the printer
Second ink density signal is reproducible color (Y2, M2, C2) to
Convert the input signal representative point according to the result of the determination
Is a reproducible color, the first ink density signal
No. (Y1, M1, C1) is a color that the input signal representative point cannot reproduce.
Said second ink density signal when it is a (Y2, M2, C2)
Ink density signal for the input signal representative point (Y3, M3, C3)
To store the ink density signals (Y3, M3, C3)
Memory means composed of M or RAM and recording
Input each upper bit of the input signal to be
Address generating means for generating an address to be given to the stage,
Ink density signals (Y3, M3, C3) output from the memory means
And the information of each lower bit of the input signal to be recorded is used.
Interpolation calculation is performed on the input signal to be recorded.
Interpolation calculation means to determine the ink density signals (Y, M, C)
Equipped with color recording according to the ink density signals (Y, M, C)
Is to do.

[0020]

[0021]

[0022]

[0023]

[0024]

[0025]

According to the color image forming method of the present invention, the input signal is converted into the first ink density signal (Y1, M1, C1) by the first color correction calculation for optimally correcting the color reproducible by the printer.
Convert to 1) and judge whether the input signal is a color that can be reproduced by the printer or a color that cannot be reproduced by the printer. If the judgment result is a color that can be reproduced, the first color correction calculation The ink density is controlled using the first density signal (Y1, M1, C1) which is the result of the above. Alternatively, if the result of determination is a color that cannot be reproduced, a second color correction calculation that outputs an ink density signal that enables optimum color reproduction of the color that can be reproduced by the printer is applied to the input signal. The second ink density signal (Y2, M2, C2), which is the result of the second color correction calculation, is used to control the ink density to form a color image. In this way, the color image forming method of the present invention uses the optimum color among the colors reproducible by the printer when the input signal is a color reproducible by the printer and also when the input signal is an unreproducible color. It can be reproduced.

Further, in the color image forming method of the present invention, when the input signal is a color that cannot be reproduced by the printer, the first ink density signal (Y1, M1, C1) which is the result of the first color correction calculation is obtained. )
Will include density that cannot be recorded by the printer,
By using the first ink density signal, it is possible to judge whether the input signal is a color reproducible by the printer or a color not reproducible by the printer.

In the color image forming method of the present invention, the color reproduction for the recordable ink density signal is predicted in the color reproduction predicting step, and the evaluation value is calculated from both the input signal and the color reproduction prediction to calculate the color reproduction. By determining the ink density signal with the best evaluation value by repeating the prediction and calculation of the evaluation value, the ink density signal with the best evaluation value is determined for the input signal that cannot be reproduced by the printer. Is possible.

In addition, the color image forming method of the present invention uses an inverse function of the third color correction operation, which is a function that has an inverse function and performs optimum color correction for colors that can be reproduced by the printer. As a result, the ink density signal can be converted into a signal in the color space of the input signal, and color reproduction prediction for the set ink density signal becomes possible.

Alternatively, in the color image forming method of the present invention, since the first color correction calculation is to convert the input signal into the ink density signal, the ink can be obtained by using the inverse calculation of the first ink density calculation. It is possible to convert the density signal into a color space signal of the input signal and perform color reproduction prediction for the set ink density signal.

In the color image forming apparatus of the present invention, the ink density signal with respect to the input signal representative point represented by the upper bits of the input signal is subjected to the first color correction calculation step, the determination step, and the second color correction calculation step. It is obtained and stored in advance in the memory means. When recording, an input operation is performed by performing an interpolation operation using the ink density signal and the lower bit information of the input signal with respect to the input signal representative point stored in the memory means. It is possible to determine the ink density signal with respect to the signal.

[0031]

EXAMPLE A first example relating to the color image forming method of the present invention will be described below using a sublimation type thermal transfer type full color printer using three color inks of yellow, magenta and cyan, and three primary colors for driving a CRT. An example in which the ink density signals (Y, M, C) for the luminance signals (R, G, B) are determined by software will be described.

FIG. 2 shows a block diagram of the experimental apparatus used in the first embodiment. In FIG. 2, 201 is a CPU that executes the color image forming method of the present invention, and 202 is a CPU
201 is a RAM used as a work area when executing a program, 203 is a ROM that stores programs executed by the CPU 201, and 204 is an input signal to be recorded.
I / O means for inputting (R, G, B) and outputting ink density signals (C, M, Y), 205 is CPU 201, RAM 20
2, a bus for connecting the ROM 203 and the I / O means 204 to each other, 206 is a control means for controlling the applied energy according to the ink density signals (C, M, Y) output from the I / O means 204, 207 Is a thermal head that applies heat to an ink sheet (not shown) according to the applied energy controlled by the control unit 206 to form a color image on an image receiving paper (not shown).

In the experimental apparatus configured as described above, the color image forming method of the present invention is executed by software in the first embodiment. The overall flow of processing executed by the CPU 201 is shown in FIG. A more detailed processing flow is shown in FIGS.

First, the overall processing procedure will be described with reference to FIG. In step 101, the input signal (R,
G, B) is input via the I / O means 204.

In step 102, a first color correction calculation is performed on the input signals (R, G, B) for optimal color correction for colors reproducible by the printer, and the first ink Obtain the density signals (Y1, M1, C1).

In step 103, the first ink density signals (Y1, M1, C1) are used to determine whether the input signals (R, G, B) are colors reproducible by the printer or non-reproducible colors. If it is determined that the color is reproducible, the flg signal representing the determination result is set to flg = 0, and if it is determined that the color is not reproducible, flg = 1 is set.

In step 104, in accordance with the result of the judgment in step 103, the colors (f, f) in which the input signals (R, G, B) can be reproduced.
If lg = 0), go to step 106 and input signal (R,
If G, B) is a non-reproducible color (flg = 1), the process branches to step 105.

In step 105, for the input signals (R, G, B) that cannot be reproduced by the printer, the second ink density signal that can perform optimum color reproduction among the colors that can be reproduced by the printer. A second color correction calculation for obtaining (Y2, M2, C2) is performed.

In step 106, the first ink density signal (Y1, M1, C1) or the second ink density signal (Y2, M2, C2) is used for recording in accordance with the flg signal. It is output from the I / O means 204 as M, C).

Then, the control means 206 controls the amount of heat of the thermal head 207 in the order of yellow, magenta, and cyan according to the ink density signals (Y, M, C) output from the I / O means 204. Thus, gradation recording is performed on an image receiving paper (not shown) to form a color image.

Subsequently, the first executed in step 102
The color correction calculation will be described. In this embodiment,
The first operation that combines a linear matrix operation for a luminance signal and a non-linear masking operation for a density signal
It was used as a color correction calculation of. The detailed processing will be described below.

First, with respect to the input signals (R, G, B), the central wavelength of the spectral characteristic of the phosphor of the CRT, which is the target of color reproduction of the printer, and the central wavelength of the spectral absorption characteristic of the ink used in the printer. In order to correct the deviation of the second luminance signal (R ′, G ′,
B ').

[0043]

[Equation 3]

Next, each of the second luminance signals (R ', G', B ') is converted into 3 by the complementary color conversion of (Equation 4) based on the subtractive color mixing principle.
Convert to primary color density signals (D _R , D _G , D _B ).

[0045]

[Equation 4]

Then, a non-linear masking operation is performed for the purpose of correcting color turbidity due to unnecessary absorption components of the ink.

First, the density signal is converted into a signal corresponding to the color material amount of ink. Specifically, if a constant (a>a> 1) that represents the non-linear degree of the relationship between the color material amount and the density of the ink is a (a> 1), the three primary colors obtained by the complementary color conversion by the first non-linear conversion shown in (Equation 5). Each of the density signals (D _R , D _G , D _B ) is non-linearly converted into signals (C ′, M ′, Y ′) corresponding to the color material amount of the ink.

[0048]

[Equation 5]

Subsequently, the output of the first nonlinear conversion is
(C ′, M ′, Y ′) is converted into (C ″, M ″, Y ″) by the linear matrix operation of (Equation 6).

[0050]

[Equation 6]

Further, it is an inverse function of the first non-linear transformation, and by the second non-linear transformation shown in (Equation 7), (C ″,
Each of M ″, Y ″ and the first ink density signal (Y1, M1, C
Convert to 1).

[0052]

[Equation 7]

In the present embodiment, the CRT and the color reproduction of the printer for the color chart of about 100 colors which are averagely positioned in the reproducible area of the printer are performed so that the color correction for the input signal reproducible by the printer works optimally. {B _kl } in the luminance matrix operation of ( _Equation 3) and {a in the linear matrix operation of ( _Equation 6) so that the average value of the error of
_kl } (k = 1 to 3, l = 1 to 3) and the correction coefficient of a in (Equation 5) and (Equation 7) were determined by the convergence calculation. Each correction coefficient used in this embodiment is shown in (Equation 8).

[0054]

[Equation 8]

In the first color correction calculation used in this embodiment, the color reproducible by the printer can be corrected by the color difference Euv = 4.3 in the uniform color space on average. there were.

In this embodiment, the input signals (R, G, B) are converted into the first ink density signals by performing the first color correction calculation described above.
It is to be converted into (Y1, M1, C1).

Next, in step 103, the input signals (R, G,
A detailed description will be given of the process of determining whether B) is a color that can be reproduced by the printer or a color that cannot be reproduced by the printer.

FIG. 4 shows a color space (hereinafter referred to as an ink density space) in which density signals of yellow (Y), magenta (M), and cyan (C) inks used in the printer are represented as axes of the orthogonal coordinate system. Indicates the reproducible area of the printer. 7A is a perspective view of the ink density space, FIG. 8B is a view of the YC plane viewed from above the M axis, and FIG. 7C is a view of the MC plane viewed from above the Y axis. Figure (d) shows YM from above C-axis
It is the figure which looked at the plane. As shown in FIG. 4, in the ink density space, the area that can be reproduced by the printer is represented by the density that can be recorded using each ink. That is, the paper surface densities of yellow, magenta, and cyan are respectively (Y0, M
0, C0), and the maximum printable density is represented by (Ymax, Mmax, Cmax), the reproducible area of the printer in the ink density space is Y =
The area is represented by a rectangular parallelepiped surrounded by six planes Y0, M = M0, C = C0, Y = Ymax, M = Mmax, and C = Cmax.

From this fact, when the first color correction operation is an operation for performing the optimum color correction for the color that can be reproduced by the printer, the input signals (R, G, B) can be reproduced by the printer. In order to determine whether the color is a non-reproducible color or a non-reproducible color, each of the first ink density signals (Y1, M1, C1) of the first color correction calculation result is Y0 ≦ Y1 ≦ Ymax, M0 ≦ M1. The judgment can be made by checking that ≦ Mmax and C0 ≦ C1 ≦ Cmax. Step 103
Then, it is examined whether or not each of the first ink density signals (Y1, M1, C1) is higher than the density on the paper surface and lower than the maximum density.
FIG. 3 shows the detailed processing flow of step 103.

In step 301, the first ink density signal
It is checked whether or not the yellow ink density signal Y1 of (Y1, M1, C1) is in the range of the density on the paper surface and the maximum density. If it is not within the range, it is determined that the input signal is a color that cannot be reproduced, and in step 305 flg
Set = 1.

In steps 302 and 303, the magenta ink density signal M1 of the first ink density signal (Y1, M1, C1),
The same comparison is performed for each of the cyan density signals C1.

Then, the first ink density signal (Y1, M1, C1)
If all are density signals that can be recorded, the input signal
Assuming that (R, G, B) is a reproducible color, flg = 0 is set in step 304.

As described above, in this embodiment, the input signals (R, G, B) are output by the printer using the first ink density signals (Y1, M1, C1) which are the result of the first color correction calculation. Judge whether the color can be reproduced or not.

Next, the second color correction calculation in step 105 will be described. In the second color correction calculation in this embodiment, the color of the input signal (R, G, B), that is, the target color for color reproduction is obtained, and the color reproduction of the printer when the set ink density signal is used is predicted. Then, the ink density signal for which the evaluation value calculated from the target color and the predicted value of the color reproduction is optimum is obtained. FIG. 5 is a diagram showing a detailed processing flow of the second color correction calculation.

First, in step 501, input signals (R, G, B)
Is converted into a color signal of a target color for color reproduction of the printer. In this embodiment, the three primary color luminance signals (R, G, B) that drive the CRT are used as input signals, and the three stimulus values (Xo, Xo, which are reproduced when the three primary color luminance signals are input to the NTSC CRT are used. Yo, Z
o) was calculated by the output equation (Equation 9) of the NTSC CRT.

[0066]

[Equation 9]

Further, taking human visual characteristics into consideration, the tristimulus values (Xo, Yo, Zo) are coordinated in the L ^* u ^* v ^* system uniform color space (Lo ^* uo ^* vo ^* ) by (Equation 10). Converted to. here,
(Equation 10) is L recommended by the International Commission on Illumination CIE.
It is a conversion formula to the ^* u ^* v ^* system uniform color space.

[0068]

[Equation 10]

Next, in step 502, the value of the ink density signal is set in order to search for the optimum ink density signal.

In step 503, color reproduction (Li value) using the ink density signal set in step 502 is used.
Predict ^* ui ^* vi ^* ).

In step 504, the target color (Lo ^* uo ^* vo ^* ) for color reproduction obtained in step 501 and step 50
Using the color reproduction prediction value (Li ^* ui ^* vi ^* ) obtained in step 3, the evaluation value Euv for the set ink density signal is obtained. In this embodiment, the evaluation value Euv is the color difference in the uniform color space between the target color of the input signal represented by (Equation 11) and the predicted value of color reproduction.

[0072]

[Equation 11]

In step 505, it is determined whether the evaluation value Euv obtained in step 504 is the minimum. If it is determined that it is the minimum, the ink density signal set in step 502 is set to the second ink density signal (Y2 , M2, C2).

Then, in step 507, it is judged whether or not the search for the ink density signal is completed. If the search is not completed, the process branches to step 502 to set a new ink density signal, and step 503. ~ Step 50
7 is repeated.

The prediction of color reproduction for the set ink density signal in step 503 will be described next.
The first color correction calculation used in the present embodiment includes a nonlinear conversion such as (Equation 4), (Equation 5), (Equation 7), and is a non-linear operation as a whole. Since it is expressed by the existing function, it is possible to perform the conversion from the ink density signal to the three primary color luminance signals by the inverse operation of the first color correction operation. Therefore, in the present embodiment, the first
By using the inverse operation of the color correction operation of, the set ink density signal is converted into the three primary color luminance signal which is the color space of the input signal,
Similar to the case of obtaining the target color, it was converted into a color signal in the uniform perceptual color space.

First, the set ink density signal (Y2, M2, C2)
Is subjected to the first non-linear transformation of (Equation 12) to obtain (Y2 ″, M2 ″, C2 ″).

[0077]

[Equation 12]

Then, (Y2 ′ ″, M2 ″, C2 ″) is converted to (Y2 ′, M) by (Equation 13) which is the inverse matrix operation of (Equation 6).
2 ', C2').

[0079]

[Equation 13]

Then, by the second non-linear conversion of (Equation 14), three primary color density signals (D2 _R , D2 _G , D2 _B ) corresponding to the set second ink density signal (Y2, M2, C2) are obtained. Convert.

[0081]

[Equation 14]

Further, by the inverse complementary color conversion of (Equation 15), it is converted into a luminance signal (R2 ', B2', C2 ') of additive color mixture.

[0083]

[Equation 15]

Then, the inverse luminance matrix operation of (Equation 16) is performed to convert into the three primary color luminance signals (R2, G2, B2).

[0085]

[Equation 16]

Further, the three primary color luminance signals (R2, G, 2, B2) obtained from the ink density signals are displayed on the CRT according to (Equation 9) as in the case of the input signals (R, G, B). 3 stimulus values (Xi, Y
i, Zi) and the coordinates (Li ^* , ui ^* , vi ^* ) of the uniform color space according to (Equation 10) are used to predict the color reproduction when the set ink density signal is used. To do.

As described above, in the first color correction calculation used in this embodiment, the reproducible colors of the printer are corrected on average with the color difference Euv = 4.3 in the uniform color space. The color reproduction prediction in the present embodiment using the inverse function of the first color correction calculation was also possible with high precision color reproduction prediction with the color difference Euv = 4.3.

Next, the results of color reproduction experiments in the first embodiment will be shown. Figure 6 is of similar to FIG. 11 CIE L ^* u ^* v ^*
The result of the target color and the color reproduction in the system uniform color space is shown. In the figure, Pi (i = 1 to 3) is a target color that cannot be reproduced by the printer, and Qi (i = 1 to 3) is a conventional technique. The ink density signal of the masking calculation result is subjected to the paper surface density and the maximum density limiter. As a result of the color reproduction experiment using the ink density signal, Ri (i = 1 to 3) is the result of the color reproduction experiment when the color image forming method of the first embodiment of the present invention is applied. .

According to this embodiment, the evaluation value Euv for the color that cannot be reproduced by the printer is set to the distance in the uniform color space of (Equation 15) as shown in FIG. It was possible to perform color reproduction using a color that minimizes the distance in the uniform color space among the possible colors, and it was possible to greatly improve the deterioration of image quality.

As described above, the first embodiment of the color image forming method of the present invention
With respect to the embodiment, the configuration and operation of the experimental apparatus, the process of determining the ink density signal with respect to the input signal, and the experimental result have been described.

In the present embodiment, it is first judged whether the input signal is a color reproducible by the printer or an unreproducible color.
This was performed using the result of the color correction calculation of. In order to make the determination without using the result of the first color correction calculation as in the present embodiment, for example, the reproducible area of the printer is expressed in a uniform color space, and the input signal is set in advance as the reproducible area of the printer. It is possible to consider a method of comparing and determining whether the color can be reproduced or not, and using the judgment result as a table. To create this determination table, the reproducible area of the printer has a very complicated shape in the space of the input signal, and therefore a great deal of time and effort is required in making the determination. Also, if the input signal is 8 bits accurate for each color, 2 ²⁴ bi
A memory with a huge capacity of t is required. On the other hand,
The determination method in this embodiment is the first ink density signal (Y1,
The judgment can be made only by the comparison operation with respect to (M1, C1), which is advantageous in terms of processing time when executed by software and in circuit scale when constituted by hardware.

Further, in the present embodiment, the prediction of the color reproduction for the set ink density signal is performed by the inverse operation of the first color correction operation. In order to predict the color reproduction for the ink density signal that is set without using the inverse calculation of the first color correction calculation as in the present embodiment, for example, a color chart corresponding to the boundary of the color reproducible area of the printer, that is, An ink density signal used when creating a large number of color charts using at least one ink color signal of which the density of the paper surface or the maximum density of the ink density signals is the color measurement result of the color chart It is possible to use a table for associating with, and predict the ink density signal other than the created color chart by an interpolation operation. However, in this case, enormous labor is required when creating a large number of color charts and measuring the colors. Also,
Since the reproducible area of the printer has a very complicated shape in the space of the input signal, the error in the prediction by the interpolation operation also becomes large. On the other hand, the method for predicting color reproduction according to the present embodiment does not require labor for creating a table for making correspondence, and the correction accuracy of the first color correction operation for the input signal reproducible by the printer. It is possible to perform color reproduction prediction with the same accuracy as.

Next, a second embodiment of the color image forming apparatus of the invention will be described. In the second embodiment of the present invention, each gamma-corrected luminance signal (r, g, b) with 8-bit precision is input, and three color inks of cyan, magenta, and yellow are used, and yellow, magenta, and cyan are sequentially arranged. An example in which a sublimation-type thermal transfer recording type full-color printer for performing frame-sequential recording on an image receiving paper, which is applied to a video printer for the purpose of color reproduction that is the same color as a color image displayed on a CRT, will be described.

In the second embodiment, 32.times.32 given by the upper 5 bits of each color of the input signal (r, g, b) of 8 bit precision.
Ink density signal corresponding to 32 discrete representative points
(C, M, Y) is stored in advance in a LUT (look-up table) memory, and the output for an input signal located in the middle of the representative points is determined by an 8-point interpolation method which is a three-dimensional linear interpolation method. It is a thing.

Before explaining the configuration and operation of this embodiment,
The interpolation method will be described with reference to the drawings. FIG. 8 shows a unit cube formed by eight representative points Pk (k = 0 to 7) of the representative points of the input signal. For an input signal P located in the middle of the unit cube, the unit cube is divided into eight small rectangular parallelepipeds on a plane that passes through the input signal P and is parallel to each side of the input space, and the volume of this small rectangular parallelepiped is Vk ( k = 0 to 7), the volume of a unit cube is V, and an ink density signal (Yk, Mk, Ck) at a representative point Pk in a diagonal relationship with a k-th small rectangular parallelepiped is used as a main force value ( (Y, M, C) is determined by the interpolation calculation of (Equation 17).

[0096]

[Equation 17]

The configuration of the second embodiment will be described below with reference to the drawings. FIG. 7 is a block diagram of the color image forming apparatus of the second embodiment.

Reference numeral 701 denotes each upper 5 bi of the input signal (r, g, b).
Input t and input signal (r,
Address generating means for outputting addresses of eight representative points of the unit cube including g, b), 702 is an output of the address generating means 701, and 2 bits representing a recording color among yellow, magenta and cyan. COLSE
An LUT memory that outputs an 8-bit ink density signal (Y, M, C) corresponding to an address using the L signal as an address, and in the present embodiment, has a capacity of 1 M bit (128 Kbytes) R
The OM is used, and the area of 96 Kbytes of the total capacity is used.

Reference numeral 703 is a counter which counts up in synchronization with the CLK signal and outputs a 3-bit signal corresponding to k in (Equation 17), and 704 is Vk / in Equation (17).
V is stored in advance as a weighting coefficient, and the input signal (r, g,
Each of the lower 3 bits of b) and the output of the counter 703 are input as an address, and a weighting coefficient table memory 705 that outputs a weighting coefficient corresponding to this address is a LUT memory
A cumulative addition means 706 that multiplies the output of No. 02 and the output of the weighting coefficient table memory 704, further performs cumulative addition in synchronization with the CLK signal, and outputs the ink density signals (Y, M, C) used for printing. Is a recording control unit that performs gradation color recording according to the ink density signals (Y, M, C) output from the cumulative addition unit 705 by controlling the amount of heat applied to an ink film (not shown), and 707 is a recording control unit. 70
The thermal head 6 controls the amount of heat and controls the amount of ink transferred from an ink film (not shown) to an image receiving paper (not shown) for recording.

The operation of recording the first color, that is, the recording of yellow will be described. 3 primary color luminance signals to be reproduced
Assuming that the value represented by each of the upper 5 bits of (r, g, b) is (r0, g0, b0), the address generating means 41 sequentially synchronizes with the CLK signal to (r0, g0, b0), (r0 + 1, g0, b0), (r0, g0 + 1, b0), (r0 + 1, g0
+ 1, b0), (r0, g0, b0 + 1), (r0 + 1, g0, b0 + 1), (r0, g0 + 1, b0 +
1), outputs the address of (r0 + 1, g0 + 1, b0 + 1), and the LUT memory 702 outputs Y0 to Y7 according to the address.

On the other hand, each lower 3 bits of the input signal (r, g, b)
And the output of the counter 703 is the weighting coefficient table 704.
Are sequentially input to the output terminals, and the weighting factors V0 / V to V7 / V are sequentially output.

Then, the cumulative addition means 705 (Equation 17)
Is executed and the yellow ink density signal Y for the input signal (r, g, b) is output.

The recording control means 706 controls the amount of heat of the thermal head 707 according to the value of the ink density signal Y output from the cumulative addition means 705, and gradation recording is performed on the image receiving paper (not shown).

After the above operation is performed for one screen of yellow recording, the same operation is performed for the magenta and cyan inks. Then, the recording of the three colors of ink is completed,
The desired full color image is formed on the receiving paper.

In this embodiment, 32 × 3 of the input signal (r, g, b)
Ink density signals for 2 × 32 representative points are calculated in advance using a computer, and further 1 Mbit
Stored in ROM. In the calculation for obtaining the ink density signal in this embodiment, 32 × 32 × 32 of the input signal (r, g, b) is used.
By performing the CRT inverse γ correction of (Equation 18) using the constant 2.2 on each of the representative points, a linear three-primary luminance signal (R, G, B) without the CRT γ is applied. The ink density signals (Y, M, C) are converted by performing the same processing as that of the first embodiment of the present invention. That is,
Linear linear luminance signals (R,
G, B) is converted to the first ink density signal by performing the first color correction calculation represented by (Equation 3) to (Equation 7), and can be reproduced by the printer by the processing shown in FIG. It is determined whether the color is a non-reproducible color. If the color is a reproducible color, the first ink density signal is used as the ink density signal stored in the LUT memory 702. 5
The second ink density signal determined by the second color correction calculation represented by (Expression 9) to (Expression 16) is used as the ink density signal to be stored in the LUT memory 702.

[0106]

[Equation 18]

The configuration, operation, and method of determining the ink density signal with respect to the input signal stored in the LUT memory regarding the color image forming apparatus according to the second embodiment of the present invention have been described above. The color image forming method of the present invention requires a calculation time for the second color correction calculation for determining the optimum ink density signal for an input signal that cannot be reproduced by the printer. By storing the signal in the LUT memory, it is possible to determine the ink density signal for the input signal that cannot be reproduced by the printer in real time. Further, it is advantageous in terms of memory capacity as compared with the case where the ink density signals for all the input signals are stored in the memory. Moreover, similar to the first embodiment, even for an input signal that cannot be reproduced by the printer, it is possible to reproduce the color using the optimum color among the colors that can be reproduced by the printer, and the deterioration of the image quality is greatly improved. We were able to.

The embodiments of the color image forming method and the color image forming apparatus of the present invention have been described above.

In this embodiment, the input signal is a luminance signal for driving the CRT, and the first color correction operation is an operation in which a linear matrix operation for the luminance signal and a non-linear masking operation for the density signal are combined. The first color correction calculation of the color image forming method of the present invention is not limited to the above calculation. For example, in the case of the color correction calculation expressed by a function having an inverse function like the conventional linear masking, the color reproduction prediction for the ink density signal set in the second color correction calculation is performed by the first color correction calculation. It is possible to use the inverse operation.

Even if the inverse calculation of the first color correction calculation is not used for the color reproduction prediction, for example, the conventional nonlinear high-order masking is used as the first color correction calculation, and the input signal to the ink density signal is changed. It is also possible to express the conversion by conventional linear masking and perform color reproduction prediction for the ink density signal set by using the inverse function of this linear masking. In this case, non-linear high-order masking can be performed with high accuracy in color correction for input signals that can be reproduced by the printer, and linear masking accuracy can be adjusted for input signals that cannot be reproduced by the printer. It is possible to perform optimum color reproduction in the color reproduction prediction.

Further, although the sublimation type thermal transfer recording type color printer was used in this embodiment, it is obvious that the color image forming method and the color image forming apparatus of the present invention are not limited to the recording system of the printer. is there.

Further, the second embodiment of the color image forming apparatus
In the embodiment described above, the 8-point interpolation method using the volume ratio is used for the interpolation calculation when determining the ink density signal, but the color image forming apparatus of the present invention is not limited to the interpolation calculation method.
Obviously, it can be realized by using another interpolation calculation method or without using the interpolation calculation.

[0113]

As described above, according to the color image forming method of the present invention, for the input signal reproducible by the printer, the first ink density signal by the first color correction calculation is reproduced by the printer. By using the second ink density signal by the second color correction calculation for the impossible input signal, the optimum color among the colors reproducible by the printer was used for all the input signals. Color reproduction becomes possible, and deterioration of image quality can be greatly improved.

Further, by using the first ink density signal which is the result of the first color correction calculation, it is possible to judge whether the input signal is a color which can be reproduced by the printer or a color which cannot be reproduced by the printer.
The processing time can be shortened when executed by software, and the circuit scale can be made small when configured by hardware, without the effort required for creating the table for storing the judgment result. It becomes possible.

Further, the color reproduction prediction is performed by using the inverse operation of the first color correction operation, so that the labor for preparing the table for associating the ink density signal and the color reproduction result for predicting the color reproduction can be saved. It is possible to perform the color reproduction prediction with the same correction accuracy as the correction accuracy of the first color correction calculation without the need.

Alternatively, a function that has an inverse function and performs optimum color correction for colors that can be reproduced by the printer is defined as the third color correction operation, and the inverse function of this third color correction operation is used. When predicting color reproduction by converting the ink density signal into a signal in the color space of the input signal, a correction corresponding to the accuracy of the first color correction calculation is applied to the input signal reproducible by the printer. For an input signal that is possible and cannot be reproduced by the printer, it is possible to perform color reproduction with optimality according to the accuracy of the third color correction calculation.

Further, according to the color image forming apparatus of the present invention, it is possible to determine the ink density signal for the input signal which cannot be reproduced by the printer in real time, and the input signal which cannot be reproduced by the printer is included. It is possible to record a full-color image with high image quality.

[Brief description of drawings]

FIG. 1 is a flowchart showing the overall flow of processing of a color image forming method according to a first embodiment of the present invention.

FIG. 2 is a block diagram of an experimental apparatus in the same embodiment.

FIG. 3 is a detailed flowchart for determining whether the input signal in the embodiment is a reproducible color or an unreproducible color.

FIG. 4 is a diagram showing a region that can be reproduced by a printer in an ink density space.

FIG. 5 is a second example of the non-reproducible color in the same embodiment.
Flowchart of color correction calculation of

FIG. 6 is a diagram showing a color reproduction experiment result of the same example in a uniform color space.

FIG. 7 is a block configuration diagram of a color image forming apparatus according to a second embodiment of the present invention.

FIG. 8 is a diagram illustrating an interpolation method in the color image forming apparatus of the embodiment.

FIG. 9 is a diagram showing an example of spectral absorption characteristics of ink used in a sublimation thermal transfer recording type printer.

FIG. 10 is a diagram showing an example of a color reproduction range of a CRT and a printer.

FIG. 11 is a diagram showing color reproduction by a conventional color image forming method when an input signal is a color that cannot be reproduced by a printer.

[Explanation of symbols]

201 CPU 202 RAM 203 ROM 204 I / O means 205 bus 206 control means 207 Thermal head 701 Address generating means 702 LUT memory 703 counter 704 Weighting coefficient table memory 705 cumulative addition means 706 recording control means 707 thermal head

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yasuki Matsumoto               1006 Kadoma, Kadoma-shi, Osaka Matsushitaden               Instrument industry Co., Ltd.                (56) References Japanese Patent Laid-Open No. Sho 63-254889 (JP, A)                 JP-A-4-101566 (JP, A)                 JP-A-4-277978 (JP, A)                 JP-A-61-268662 (JP, A)                 JP-A-61-214892 (JP, A)                 JP-A-5-115000 (JP, A)                 JP-A-63-151263 (JP, A)                 JP-A-3-289874 (JP, A)

Claims (10)

(57) [Claims] 1. A first color correction calculation step of performing color correction of an input signal into a color reproducible by a printer and converting it into a first ink density signal (Y1, M1, C1); A determination step of determining whether the color is reproducible or an unreproducible color, and an input signal determined to be unreproducible by the printer in the determination step is a color reproducible by the printer. A second color correction calculation step for converting into two ink density signals (Y2, M2, C2) , which is a color for predicting color reproduction for a recordable ink density signal.
Reproduction prediction process, input signal and result of the color reproduction prediction process
An evaluation value meter that calculates the evaluation value from both the color reproduction prediction
Calculation step, the color reproduction prediction step and the evaluation value calculation step
By repeating the above procedure, the control for searching the ink density signal is performed.
A second color correction calculation step , which includes a control step, and when the input signal is a reproducible color, the first ink density signal (Y1, M1, C1) and the second ink density signal (Y2, M2, C2) when the input signal is a color that cannot be reproduced, respectively, to control the ink density to form a color image. A color image forming method characterized by the above. 2. The determining step includes the first ink density signal (Y1,
Of the M1 and C1), if the signal of at least one color is the density of the image receiving paper used in the printer or is lower than 0, or higher than the maximum recordable density, the input signal is a color that cannot be reproduced. The color image forming method according to claim 1 , wherein the color image forming method is determined to be present. 3. Reproduction on a printer with a function having an inverse function
A function that performs color correction on possible colors is called the third color correction calculation.
Then, the color reproduction prediction step is an inverse function of the third color correction calculation.
To convert the ink density signal into a color space signal of the input signal.
The color according to claim 1, wherein the color is replaced.
-Image forming method . 4. The color reproduction predicting step comprises a first color correction operation.
Inverse density calculation is used to convert the ink density signal to the color space of the input signal.
3. The method according to claim 1, wherein the number is converted into a number.
Color image forming method . 5. The first color correction calculation step is a step of processing an input signal.
Optimal color correction is performed for colors that can be reproduced by the linter.
Convert to the first ink density signal (Y1, M1, C1) and then perform the second color correction
The calculation process judges that the judgment process cannot be reproduced by the printer.
Of the colors that can be reproduced by the printer for the input signal
Second ink density signal to reproduce a color (Y2, M2, C2) to convert
The color according to claim 1, characterized in that
Image forming method. 6. Represented using the upper bits of the input signal
Correct the input signal representative point to a color that can be reproduced by the printer.
By performing the first color correction calculation, the first ink
Converted to the density signal (Y1, M1, C1) and the input signal representative point
Colors that can be reproduced by the linter or colors that cannot be reproduced
Determine, unreproducible input printer of the input signal representative points
Recordable with this input signal representative point for the signal representative point
Both the color reproduction prediction predicted for the ink density signal
The evaluation value is calculated from, and the color reproduction prediction and evaluation value calculation are performed.
Second color correction, in which the ink density signal is searched by repeating
Colors that can be reproduced by a printer by performing calculations
Convert to the second ink density signal (Y2, M2, C2) , and the input signal representative point can be reproduced according to the result of the judgment.
When the color is an effective color, the first ink density signal (Y1, M
1, C1) , the input signal representative point is a color that cannot be reproduced.
If the second ink density signal (Y2, M2, C2) is input,
Ink density signal (Y3, M3, C3) for the force signal representative point ,
ROM for storing the ink density signals (Y3, M3, C3) , if
Memory means composed of a RAM or RAM, and inputting each upper bit of an input signal to be recorded,
Address generating means for generating an address to be given to the means
And the ink density signals (Y3, M3, C
3) and the information of each lower bit of the input signal to be recorded
Interpolation calculation is performed using the input signal to be recorded.
Interpolation calculation means to determine the ink density signal (Y, M, C)
The color recording is performed according to the ink density signals (Y, M, C).
A color image forming apparatus which is characterized by: 7. A first ink density signal (Y1, M1, C1)
The image-receiving paper for which at least one color signal is used in the printer
Density or less than 0, or the highest recordable
The input signal cannot be reproduced when the density is higher than the density
Claims characterized by being judged to be a different color
Item 7. A color image forming apparatus according to item 6 . 8. A function having an inverse function is reproduced by a printer.
A function that performs color correction on possible colors is called the third color correction calculation.
Then, the ink density is calculated using the inverse function of the third color correction calculation.
Color is converted by converting the signal into a signal in the color space of the input signal.
7. A color image according to claim 6, wherein reproduction prediction is performed.
Image forming device . 9. An inversion operation using an inverse operation of the first color correction operation.
For converting the density signal into the color signal of the input signal
The color reproducibility is predicted more accurately, and the color reproduction is predicted according to claim 6.
Image forming apparatus . 10. The first color correction calculation is an input signal representative point.
The color that can be reproduced by the printer.
Convert to the first ink density signal (Y1, M1, C1)
The correction calculation determines that the judgment process cannot be reproduced by the printer.
The input signal representative point can be reproduced with a printer.
The second ink density signal (Y2, M2, C2) that reproduces the optimum color
7. The method according to claim 6, characterized in that
Color image forming apparatus .