JPH04157979A - Method and device for color image formation and correction coefficient determining method - Google Patents

Abstract

PURPOSE:To perform correction of high precision by using color correction operation with a density signal in a subtractive mixture system and luminance matrix operation with a luminance signal in an additive mixture to perform correction of color turbidity and correction of hue deviation from a target color independently of each other. CONSTITUTION:When three primary color luminance signals (R, G, B) of a picture to be reproduced are inputted, a luminance matrix operation means 1 converts them to second luminance signals (R', G', B') corresponding to the center wavelength of the spectral absorption characteristic of ink. Complementary color converting means 2, 3, and 4 convert; signals (R', G', B') to three primary color density signals (DR, DG, DB) as respective complementary colors. A color correcting means 5 determines an yellow ink density Y in accordance with Y=b6.DR+b7.DG+b8.DB and outputs this density Y. A recording control means 6 controls the calorific value of a thermal head 7 in accordance with the value of the signal Y outputted from the color correcting means 5 to record gradations on image receiving paper. The same operation is performed for magenta ink and cyan ink to form a desired full color picture on the image receiving paper.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a color image forming apparatus such as a color copying machine for printing out color images.

Conventional TechnologyColor Printing In color image forming devices such as color copying machines, the three primary colors of colored light, red, green,
CRT,
Color Original The purpose of color reproduction is to achieve color matching with the target color of each color image forming device.

However, the spectral absorption characteristics of actually existing inks are broad and do not act as ideal absorption filters for each color of light. For example, the spectral absorption characteristics of commonly used sublimable dyes are shown in Figure 7.As shown in Figure 7, the spectral absorption characteristics of actually existing inks are broad, and are shown in the shaded area in the figure. As shown in the figure, color light of wavelengths that should normally be completely transmitted is not absorbed. 4 There are so-called unnecessary absorption components. When Lfi color is used, a density different from the density that should be reproduced by each ink is reproduced, and the saturation is decreased, so-called color turbidity. arise.

In addition, 11cR is used to achieve color matching with the target color.
There is a problem with the difference between the center wavelength of the spectral characteristics of the coloring material and the center wavelength of the spectral absorption characteristics of the ink used in color mixing systems that reproduce the target color, such as T, and color separation systems for the target color, such as color scanners. .

For example, Figure 8 shows the spectral characteristics of a typical CRT phosphor. From Figure 8, the center wavelength of the spectral characteristics of each color phosphor,
It can be seen that the center wavelength power of the spectral absorption characteristics of the ink shown in FIG. 7 does not match.

From this, even if color turbidity due to unnecessary absorption components of the ink is prevented, the center wavelength of the spectral absorption characteristics of the ink and the center wavelength of the spectral characteristics of the three primary colors of the mixed color system, which is the goal of color reproduction, are misaligned. In this case, the hue of the reproduced color ζ is different from the target color.

Conventionally, to solve these problems, a correction method called masking has been used mainly in the printing field.

The most commonly used method is called linear masking shown in equation (1). Linear masking (ink density signals (C, M) that control the density of ink used
, Y) as the three primary color luminance signals (R, G
, B) are the complementary colors of the three primary color density signals (DR, DG, D
o) is determined by linear matrix calculation.

Linear masking is based on the premise that the additive law (Lambert-Beer law) and the proportional law of density hold in subtractive color mixing using real inks. In the case of the recording method, there are various nonlinear factors such as the re-sublimation phenomenon of the ink and the internal fouling of the ink, so strictly speaking, the law of additive proportions does not hold.

Therefore, nonlinear high-order masking has been proposed in which the ink density signals (C, M, Y) are determined by high-order polynomials for the three primary color density signals (DIl, DO, Dll). The simplest quadratic masking equation is shown in equation (2): C= ao・D R+al・l)o+a2・Ds+
a3-pn2+a4-De2+a5・De2+a6・D
R-Do10a7・DO・DBBaB4DB・DRM=8
9・D+++alLDo+all・De+a12・D1
12+a13・D02+a14・DB2+a15・DR
-DG+816・DO・DB1317・DB・DRY=
a18・Dp+a19-Do+a20・De+a21・
DR2+a22・D02+a23・DB2+a24・D
p-])G+a25・Do −De・+a26・l')
e-I)11 This approximates the nonlinearity that exists in color reproduction using actual ink using a quadratic equation.

Nine correction coefficients (a
k+) (k=1-3.1=1-3), it is difficult to analytically determine the 27 correction coefficients aO to a26 used for secondary masking, and conventionally, density signals of subtractive color mixture system are used. It is determined by the least squares method for .

This method will be explained using FIG. 9.

FIG. 9 is a model of a color reproduction system in which this method is used. X is a known density signal, and a sufficiently large number of X's (n) are used to create a color sample using a target printer, and the sample is separated into colors using a scanner to obtain three primary color density signals. Considering that this process is affected by the transfer function φ, the color correction system has this inverse characteristic. -1 is determined. That is, the color correction coefficients (a and I) used for linear masking are set such that the error regarding the density signal is minimized on average (k=1 to 3, l=1 to 3).
) or aO to a26 used for secondary masking.

(For example, "Image processing for color reproduction", Photo industry special edition "
Problems to be Solved by the Invention However, conventional color correction techniques such as linear masking and nonlinear high-order masking suffer from color turbidity due to unnecessary absorption components of actual ink and problems with the center wavelength of the spectral absorption characteristics of the ink used. This method attempts to correct the hue shift caused by the shift by calculating the subtractive color mixture density signal.

Equipment Linear masking uses linear calculations to approximate the nonlinearity that exists in color reproduction using real ink.
The correction accuracy is said to be insufficient for applications that require high-fidelity color reproduction.

Furthermore, nonlinear high-order masking does not analytically express the color reproduction of a recording system, but rather approximates the nonlinearity of color reproduction by adding a nonlinear term. Therefore, there is still a problem that the correction accuracy is insufficient.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a color image forming method and apparatus that can reproduce colors more faithfully to a target color.

Means for Solving the Problems In order to solve the above problems, the color image forming method of the present invention converts the three primary color luminance signals (R, G, B) of octa-additive color mixture into second luminance signals (R'', G', B'), and each color of this second luminance signal (R'', G', B') is converted into a subtractive three-primary color density signal (DR, Da) by complementary color conversion.
, Do) and convert these three primary color density signals (DR, Do, De) into ink density signals (C, M, De) used for printing by color correction calculation.
Y) and controls the ink density according to this ink density signal (C, M, Y) to perform color recording.

action The effects of the color image forming method of the present invention are shown below.

Luminance matrix calculation is performed on the input three primary color luminance signals (R, G, B) of additive color mixture, and the center wavelength of the color material used to output the target color, for example, the spectral characteristics of each color phosphor of a CRT, is calculated. The deviation between the center wavelength of the spectral absorption characteristics of the ink and the center wavelength of the spectral absorption characteristics of the ink is corrected, and the difference is converted into a second luminance signal (R', G', B'') corresponding to the center wavelength of the spectral absorption characteristics of the ink.

Subsequently, the second luminance signals (R', G', B') are subjected to complementary color conversion into three primary color density signals (DR, DG, DB) of subtractive color mixture.

Furthermore, a color correction calculation is performed to correct color turbidity due to unnecessary absorption components of the ink used for recording, and the signal is converted into an ink density signal (C, M, Y) used for recording.

Then, the ink density is controlled according to the ink density signals (C, M, Y) to perform color recording.

In other words, in the color image forming method of the present invention, the shift in the center wavelength of the spectral absorption characteristics of the ink is independently resolved by brightness matrix calculation in an additive color system, and the color turbidity due to unnecessary absorption components of the ink is independently resolved by a color correction calculation in a subtractive color system. Since the deviation of the center wavelength is corrected by linear matrix calculation in the additive color mixing system where the law of proportionality holds, it is possible to record colors with high fidelity to the target color. becomes possible.

Embodiment As a first embodiment of the present invention, in a video printer that prints out a color image to be output to a CRT, a matrix coefficient used for brightness matrix calculation and a color correction coefficient used for color correction calculation will be used. An example of determining a correction coefficient will be explained.

In this example, the color correction calculation is performed using the linear masking formula (1), and the inverse brightness matrix operator uses the matrix coefficient and the inverse matrix of the color correction coefficient.
The correction coefficient is determined according to the color difference minimization condition in the homogeneous color space. Here, the brightness matrix operation converts the three primary color brightness signals of additive color mixture into second brightness signals (R', G'', B'') It is expressed by equation (3).

In addition, the inverse brightness matrix operator and inverse color correction calculation are equations (4) and (5), which are inverse functions of equations (3) and (1), respectively.

However, Fig. 3 is a flowchart showing the process of determining correction coefficients, and shows the matrix system number (bk+), color correction coefficient (a
The determination of k+ ) (k=1 to 3.1=1-3) will be explained in order according to each step.

First, in the color patch signal generation process of step Sl, n sets of ink density signals (Cj, Mj, Yj) (j=1-n,
n is a natural number), and in the color chart creation process of step S2, an ink density signal (
Cj, Mj, Yj) to control the ink density of each color to create n sets of color patches, and in the color measurement process of step S3, a colorimeter is used to calculate the color signal of the color chart created in step 2 ( X+j, Yll
, 'Z+j), and in step S4, the color signal (X+j,
Y + j, Z Ij ) as the coordinates of the uniform color space (L”
+j, u”+j, v”Ij).

L″+=116 (Y+/ Yll) −(1/3)
16(Y+/Yo>0.008856)9
03.29 (Yl/Y,) (Y+/Yo≦0.008856)U″+=1
3L"+ (u +'-〇n')V"+=13L"
I (Vl"t/I+')u+' = 4X
+/ (X++1 5Y++32I)v+' =
9Y+/ (X++15Y++3ZI) However, when the standard light source used for illumination is a C light source and has a 2-degree field of view, Y, -100 u -' = 0.2009 Vn'' = 0.4609.

On the other hand, in step S5, the initial value (a
'kl) and the initial value of the inverse matrix coefficient (+ for b') are set, and the inverse electricity correction coefficient (1 for a') is used in the inverse electricity correction calculation step of step S6. By calculation,
The ink density signals (Cj, Mj, Yj) are converted into three primary color density signals (DRj, Do j, [) a j), and in the inverse complementary color conversion step of step S7, the three primary color densities are converted by the inverse complementary color conversion of equation (7). Signal (DRj, DQj, De
j) as the second luminance signal (R' j, Gj, B'
j).

R'j = 10 31 G'j = 10-"I B"j = 1O-Doj - (7) In the inverse brightness matrix calculation step of step S8, the inverse matrix coefficient (b'b1) was used (4) Reverse brightness of formula 7
The second luminance signal (R' j, G'j
, B'j) as three primary color luminance signals (Rj, Gj, Bj
).

In step S9, the three primary color luminance signals (R, G, B) are converted into CRT output color signals (Xoj, Yoj, Zoj) using the color conversion formula (8) of the NTSC system.

=19- In step 310, the CRT output color signals (Xoj, Yoj, Zoj) are converted to coordinates (L'oj, uooj, v''oj) in the uniform color space in the same manner as in equation (6), and the color difference calculation process of step Sll is performed. In step S
4 and found in step S10 ( L”+ j, u
“l j, v”+j) and (L”oj, u+0.
The color difference E expressed by equation (9) using L v”oj)
Calculate uv.

E u v −(1/n)Σ((L'oj-I-IJ
)20(U o j −u ' ■ j
) 2+(v ”o j −v ″, j
)211'2 Next, in step S12, it is determined whether Euv found in step S11 is the minimum, and if it is not the minimum, in step 813, the inverse matrix coefficient (b'+=1) and the inverse electric correction coefficient (a' Then, steps 86 to Sll are executed using the newly set inverse matrix coefficient and inverse electric correction coefficient.

Furthermore, in step S12, a convergence calculation process is performed in which the above calculation loop is repeated until it is determined that Euv is the minimum. If it is determined that LEuv is the minimum, the calculation loop is exited and the inverse matrix coefficient ( b '11+), and the reverse electric correction coefficient coefficient (a'10
kl) is determined.

Finally, in the inverse function calculation step of step 814, the inverse matrix of the inverse matrix coefficient (b'1・kl) obtained above and the inverse electric correction coefficient (a'・1nkl) is found and used for brightness matrix calculation. It determines the matrix coefficient (bkl) and the color correction coefficient (akl) used for color correction calculation.

In this embodiment, the reverse electric current correction calculation process in step S6, the inverse complementary color conversion process in step S7, the inverse brightness matrix calculation process in step S8, and the color difference calculation process in step Sll correspond to the evaluation value calculation process.
The coordinate transformation of IO to the uniform color space is a coordinate transformation for converting the color difference as an evaluation value into a two-color distance in the uniform color space. Further, step S12 and step S13 correspond to a convergence calculation step.

It is well known that the least squares method for the color difference Euv in this example can be solved by a numerical sequential solution method using a nonlinear mathematical programming called a non-linear optimization method.In this example, the optimization method using the Fletcher Powell method is Examples of the brightness matrix coefficient and color correction coefficient obtained in this example using the equations (10) and (11) are shown below.

As the first embodiment of the present invention, a method for determining a correction coefficient will be explained, and nine embodiments will be explained to explain the minimum color difference in a uniform color space, including the influence of nonlinearity that exists in color reproduction using actual ink. The correction coefficients are determined based on a number of factors, and calculations using these correction coefficients make optimal corrections for human visual characteristics (faithful color reproduction is possible).

Next, examples related to the color image forming apparatus of the present invention will be described.
Uses 3 colors of ink: Jias Magenta Yellow~
This is a full-color printer using a dye sublimation thermal transfer recording method that records yellow, magenta, and cyan in a field-sequential manner on receiving paper.
An example in which the present invention is applied to a video printer for the purpose of color reproduction that is the same as the color image output by T is explained.

FIG. 1 is a block diagram of a color image forming apparatus according to a second embodiment.

1 inputs the three primary color luminance signals (R, G, B) of the pixel to be recorded, performs the luminance matrix calculation of equation (3), and outputs the second luminance signal (R', G", B") This is a brightness matrix calculation means.

2, 3, and 4 are second luminance signals (R', G', B')
This is a complementary color conversion means that inputs each color and performs complementary color conversion to convert it into three primary color density signals (Dll, Do, DB) of subtractive color mixture.

Reference numeral 5 denotes color correction means for inputting three primary color density signals (DR, Do, DB), performing color correction calculations in row IIX, and outputting ink density signals (C, M, Y) used for recording.

Reference numeral 6 denotes a recording control means which performs gradation color recording according to ink density signals (C, M, Y) output from the color correction means 5 by controlling the amount of heat applied to an ink film (not shown).

Reference numeral 7 denotes a thermal printing head whose heat amount is controlled by the recording control means 6 to control the amount of ink transferred from an ink film (not shown) to an image receiving paper (not shown) for recording.

Brightness matrix calculation means 1 (Yo) This is for correcting the deviation between the center wavelength of the spectral characteristics of the phosphor of the CRT and the center wavelength of the spectral absorption characteristics of the ink, and is based on the additive proportionality law in the additive color system. The 7-trix operation of the formula is executed.The specific configuration diagram is shown in FIG.

11 to 19 are matrix coefficients (b +) for the three primary color luminance signals (R, G, B) (k = 1 to 3, l = 1 to 3)
20, 21 and 22 are the output generators of the multiplication means 11, 12 and 13, respectively; 14, 1
This is an addition means that calculates the sum of the outputs of 1117, 18, and 19 as % of the output of 5.16, respectively, and outputs the calculation results (R', G', B'').

Complementary color conversion means 2, 3, and 4 are for converting a luminance signal based on the additive color mixing principle into a density signal based on the subtractive color mixing principle, and in this embodiment, a memory that stores the results of the conversion equation defined by equation (12) in a table is used. is used.

D R= log(1/R') DG= log(1/G') (12)D
e = log(1/B') - Yoiichi The color correction means 5 performs a color correction calculation to remove the influence of unnecessary absorption components of the ink used and correct color turbidity, and in this embodiment, the color correction calculation is performed. The operation of Ikenji's second embodiment will be explained using the linear masking shown in equation (1).

First, the operation for recording the first color, that is, yellow, will be explained.

When the three primary color luminance signals (R, G, B) of the image to be reproduced are input, the luminance matrix calculation means 1 performs the luminance matrix calculation expressed by equation (3) (X A second luminance signal (R',
G', B').

Next, the complementary color conversion means 2, 3, and 4 convert the second luminance signal (R''
G'B') as three primary color density signals (DR,
Da, Ds).

The color correction means 5 generates three primary color density signals (DR, Do, Dl
Among the linear masking calculations in equation (1) for l), the equation that determines the ink repellency Y for yellow is Y=b6・D+++b7・l:)G+b8・])e
-(13) is applied and a yellow ink density signal Y is output.

- The recording control means 6 controls the amount of heat of the thermal head 7 in accordance with the value of the ink density signal Y outputted from the milk monochromatic correction means 5, and performs gradation recording on an image receiving paper (not shown).

After performing the above operation for one yellow recording screen,
A similar operation is performed for mazenkyu cyan ink. Then, recording with the three colors of ink is completed, and a desired full-color image is formed on the receiving paper.

In this embodiment, the matrix coefficient (bkl) (k=1 to 3.1=1 to 3) used for brightness matrix calculation, and the color correction coefficient (ab+
) (k = 1 to 3, l = 1 to 3) (y) Using the correction coefficient of formula (10)t (11) determined in the first embodiment of the present invention, the second embodiment of the present invention In the second embodiment, which describes the configuration and operation of a color image forming apparatus, the difference between the center wavelength of the spectral characteristics of the phosphor of the CRT and the center wavelength of the spectral absorption characteristics of the ink is calculated using a brightness matrix calculation means. Color turbidity due to color mixing caused by partial ink absorption components is corrected independently using a color correction calculation means using linear masking, resulting in higher accuracy compared to conventional color correction using density signals only. A third embodiment of the color image forming apparatus according to the present invention will now be described.

The third embodiment of the present invention is an ink density signal (C, M , Y) are stored in a LOT (look-up table) memory in advance, and the output for the three primary color luminance signals located between these representative points is determined by an eight-point interpolation method, which is a three-dimensional linear interpolation method.

Before explaining the configuration and operation of this embodiment, this interpolation method will be explained using FIG. FIG. 4 shows a unit cube formed by eight input representative points Pk (k=o to 7). c'll- for an input signal P located in the middle of the unit cube.

-%- Divide the unit cube into 8 small rectangular parallelepipeds along a plane that passes through the input signal P and is parallel to each side of the input space, and let the volume of this small rectangular parallelepiped be V k (k=o~7), and the volume of the unit cube Assuming that the ink density signal at the representative point Pk, which is in a diagonal relationship with the V, k-th small rectangular parallelepiped, is (Ck, Mk, Yk), the output value (C, M, Y) at the point P is expressed by equation (13). It is calculated as follows.

C=Σ Ck-Vk/V Y-Σ Yk-Vk/V The configuration of the third embodiment will be described below with reference to the drawings. FIG. 5 is a block diagram of a color image forming apparatus according to a third embodiment.

31 is a unit cube 8 which inputs the upper 5 bits of each of the input three primary color luminance signals (R, G, B) and contains the (R, G, B) signals in synchronization with the input CLK signal. This is an address generating means that outputs addresses of representative points.

32 is the output of the address generating means 31 and yellow;
Mazenkyu This is a LUT memory that uses a 2-bit C0LSEL signal representing the color being recorded among cyan as an address and outputs an 8-bit ink density signal according to the address.

33 counts up in synchronization with the CLK signal 13)
This is a counter that outputs 3 bits corresponding to k in the equation.

34 stores Vk/V in equation (13) in advance as a weighting coefficient, and inputs the lower 3 bits of each of the three primary color luminance signals (R, G, B) and the output of the counter 33 as an address, and then calculates the value according to this address. This is a weighting coefficient table memory that outputs weighting coefficients.

351; t, , Multiply the output of the LUT memory 32 and the output of the weighting coefficient table memory 34 ＼ Furthermore, perform cumulative addition in synchronization with the CLK signal ＼ Ink density signal (C, M, Y) used for recording A cumulative addition means 36 performs gradation color recording according to the ink density signal (C, M, Y) output from the cumulative addition means 35 by controlling the amount of heat applied to an ink film (not shown). It is a recording control means.

Reference numeral 37 denotes a thermal head whose amount of heat is controlled by the recording control means 36 to control the amount of ink transferred from an ink film (not shown) to an image receiving paper (not shown) for recording.

The operation for printing the first color, that is, yellow, will be explained.

Top 5 of each of the three primary color luminance signals (R, G, B) to be reproduced
If the value represented by bit is (RO, Go, BO),
The address generating means 31 sequentially (R
O,GOlBO), (RO+1.GO,BO), (
RO, GO+l, BO), (RO+1.GO+1, B
O), (RO, Go, BO+1), (RO+I,
Go, BO+1), (RO, GO+1, BO+1)
, (RO+1.GO+1.BO+1), and the LUT memory 32 changes to YO according to that address.
~ Outputs ¥7.

On the other hand, the lower 3 bits of each of the three primary color luminance signals (R, G, B)
The output of the counter 33 is input to the weighting coefficient table 34, and the weighting coefficients VO/V to V7/V are sequentially output.

Then, the cumulative addition means 35 executes equation (13) and outputs a yellow ink density signal Y corresponding to the input three primary color luminance signals (R, G, B).

The recording control means 36 controls the amount of heat of the thermal head 37 according to the value of the ink density signal Y outputted from the cumulative addition means 35, and performs gradation recording on an image receiving paper (not shown).

After performing the above operation for one yellow recording screen,
A similar operation is performed for mazenkyu cyan ink. Then, recording with the three colors of ink is completed, and a desired full-color image is formed on the receiving paper.

Next, 32
Creation of ink density signal (C, M, Y) data corresponding to x32 x 32 discrete representative points will be explained.

First: Three primary color luminance signals (]?, G, B of 32x32-wings-x32 representative points stored in the LUT memory 32
), the matrix coefficient (1)k+) of equation (10)
(k = 1 to 3, ■ = 1 to 3) is applied to the luminance matrix calculation of equation (3) to obtain the second luminance signal (R', G'
, B'').

Next, each color of this second luminance signal (R', G', B') is subjected to the complementary color conversion of equation (12) and converted into three primary color density signals (DR, Do, Da).

Then, for the three primary color density signals (DIl, Dll, DB), the color correction coefficient (ak+) of equation (11) (k=1
-3, l = 1 to 3) is converted into an ink density signal (C, M, Y) which should be stored in the LUT memory 32. These calculations can be performed using a computer. When determining the ink density signal (C, M, '/) to be stored in the LUT memory 32, it is converted to an integer with 8-bit precision. By adopting a configuration using an interpolation circuit, it is possible to realize each bird with a small-scale circuit configuration compared to a configuration in which brightness matrix calculation, complementary color conversion, and color correction calculation are each configured using hardware. = By performing each calculation when determining the ink density signal to be stored in the LUT memory using floating point calculations, recording can be performed using more accurately determined ink density signals without deteriorating the calculation accuracy. Next, a fourth embodiment of the color image forming apparatus of the present invention will be described.

In the fourth embodiment, the CRT's γ-corrected luminance signal (
The present invention is applied to a video printer to which input signals (R", G", B") are input.

In the fourth embodiment, a circuit configuration similar to that using the LUT memory and interpolation circuit in the third embodiment shown in FIG. Explain the creation.

First: CRT's γ-corrected luminance signal (R"', G"
, B"°), apply CRT inverse T correction using equation (14) using a constant of 2.2 for each color. Linear three primary colors that are not multiplied by CRT's r.
Convert to Sanichi luminance signal (R, G, B).

k=R""G=G"" 2-(14) B=B"" For linear three primary color luminance signals (R, G, B)
) matrix coefficient (bb3) (k=1~3, l=
1 to 3) is applied to the brightness matrix calculation of equation (3).
It is converted into a second luminance signal (R', G', B').

Next, each color of this second luminance signal (R'', G', B') is subjected to complementary color conversion according to equation (12) and converted into three primary color density signals (D++, DG, DB).

Then, for the three primary color density signals (DR, Dll, DB), the color correction coefficient (a b3) (k=1
3.1 = 1-3) is applied to the ink density signal (C, M, '/) to be stored in the LUT memory 32. As in the third embodiment, the calculation is performed by floating point arithmetic using a computer (X). Now, the difference when the input signal is a linear three-primary color luminance signal or a CRT T-corrected luminance signal will be explained.

Figure 6 shows linear three primary color luminance signals (R, G, B) and CR
This figure shows the relationship between the brightness index L° of the uniform color space and the gray scale of the T-corrected luminance signal (R″, G″, B″) of T. As can be seen from Figure 6, the relationship is linear. It can be seen that compared to the luminance signal, the CRT's T-corrected luminance signal has a more linear relationship with the brightness perceived by humans.

In this example, by inputting the CRT's T-corrected luminance signal, it is possible to reduce the deterioration in accuracy due to interpolation, and in particular, the gradation reproducibility for low-luminance input signals is significantly improved, exceeding 9. An example of applying the invention to a video printer will be explained. When processing a color original (a), the difference between the center wavelength of the spectral transmission characteristics of the color separation filter used in a color scanner that separates color originals and the center wavelength of the spectral absorption characteristics of the ink must be corrected.In that case, In determining the correction coefficient of the present invention, the NTSC color conversion formula (8
) When a color scanner reads a document, it converts the color signals (XO, Yo, ZO) into three primary color luminance signals (R,
G, B) may be used.

Furthermore, it is clear that the difference in the recording principle of the printer described in this embodiment using high energy is irrelevant to the present invention.

In the third and fourth embodiments, the data stored in the LUT memory is used as an ink density signal.The relationship between the ink density, which is called the T characteristic of the ink sheet, and the pulse width for applying voltage to the thermal head is not determined in advance. Pulse width data for reproducing the ink density may be stored in the LUT memory.

Further details In the third and fourth embodiments, linear masking is used as a color correction calculation when determining the ink density signal to be stored in the LUT memory.According to the configuration of these embodiments, linear masking is limited. It is also possible to realize color correction using non-linear masking, in which the density signal of each color is carved before and after linear masking and non-linear calculations are performed. It is possible to respond more flexibly to nonlinearity in reproduction, and even more faithful color reproduction is possible.

Effects of the Invention According to the present invention, correction of color turbidity due to unnecessary absorption components of ink and correction of hue deviation from the target color due to deviation of the center wavelength of the spectral absorption characteristics of the ink are performed using density signals of a subtractive color mixture system. By independently performing the color correction performance and brightness matrix calculation using the brightness signal of the additive color mixture system, it is possible to perform correction with higher precision than when both are corrected by only the color correction calculation.

Furthermore, by determining the matrix coefficients used in the brightness matrix calculation and the color correction coefficients used in the color correction calculation using the method according to the present invention, it becomes possible to perform optimal correction for human visual characteristics.

Ma? , :, A circuit configuration using LUT memory and an interpolation circuit can realize brightness 7-trix performance, complementary color conversion, and color correction calculations with a smaller circuit configuration compared to a circuit configuration using hardware. It is possible to perform accurate correction without causing deterioration in accuracy in each calculation when determining the ink density signal to be stored in the LUT memory.

Furthermore, when the input signal is a CRT T-corrected luminance signal with a circuit configuration using LOT memory and an interpolation circuit, 1
Since there is a linear relationship with the brightness index, tone reproducibility, especially for low-luminance input signals, is significantly improved.

[Brief explanation of drawings]

FIG. 1 shows a block configuration of a color image forming apparatus according to an embodiment of the present invention. FIG. 2 shows the configuration of a luminance matrix calculation means. FIG. 3 is a flowchart for determining a correction coefficient in an embodiment of the present invention. Fig. 4 shows the interpolation method used in the embodiment. Fig. 5 shows another embodiment of the present invention. Fig. 6 shows the luminance signal and the brightness index L°. Figure 7 shows the spectral absorption characteristics of common sublimable dyes. Figure 8 shows the spectral characteristics of typical CRT phosphors. Figure 9 shows conventional color correction. FIG. 3 is a diagram showing a method for determining coefficients. 1... Brightness matrix calculation means, 2, 3, 4...
Complementary color conversion means, 5...Color correction means, 6...-Recording control means, 7...Thermal heating 11-19 multiplication means, 20, 21, 22...Addition and half-immersion 31.--
Address generation means, 32...LUT memory, 33.
...Karan Hisashi 34...Weighting coefficient to Tableno+ 35
・−・Cumulative addition multiplication hand Name of agent Patent attorney Osamu Oka
Akira Haga 2 people Figure 7 Shi Kocho (basket → yellow ink

Claims (6)

[Claims] (1) Matrix convert the three primary color luminance signals (R, G, B) of additive color mixture into a second luminance signal (R', G', B'), and convert the second luminance signal (R', G', ', B') into three primary color density signals (D_R, D_G, D_B) of subtractive color mixture, and these three primary color density signals (D_R, D_G, D
_B) is converted into an ink density signal (C, M, Y) used for recording by color correction calculation, and this ink density signal (C, M,
A color image forming method characterized by controlling ink density according to Y) and performing color recording. (2) The three primary color luminance signals (R, G, B) of additive color mixture are converted into second luminance signals (R', G', B'
); and complementary color conversion means that converts each color of the second luminance signal (R', G', B') into three primary color density signals (D_R, D_G, D_B) of subtractive color mixture. a color correction means for converting the three primary color density signals (D_R, D_G, D_B) into ink density signals (C, M, Y) used for recording; A color image forming apparatus characterized by controlling density and performing color recording. (3) A brightness matrix calculation step for converting the three primary color brightness signals into a second brightness signal, a complementary color conversion step for converting each color of this second brightness signal into three primary color density signals of subtractive color mixture, and the three primary color concentrations. A memory means consisting of a ROM or RAM that stores ink control signals obtained through a color correction calculation process that converts the signals into ink density signals used for recording, and three primary color luminance signals (R, G, B) to be recorded. ), and generates an address to be given to the memory means, an ink control signal output from the memory means, and the three primary color luminance signals (R, G, B) to be recorded. An interpolation operation is performed using the information of each lower bit of the 3 to be recorded.
Ink control signal (
1. A color image forming apparatus, comprising: interpolation calculation means for determining ink control signals (I_C, I_M, I_Y), and performs color recording in accordance with the ink control signals (I_C, I_M, I_Y). (4) The three primary color luminance signals (R, G, B) of additive color mixture are
The matrix coefficients used in the luminance matrix calculation to convert to the luminance signals (R', G', B') and the three primary color density signals (
D_R, D_G, D_B) are converted into ink density signals (C, M, Y) used for recording. ) (j=1~
a color chart signal generation step of generating a color chart signal (n, n is a natural number); a color chart creation step of controlling the ink density using the ink density signal (Cj, Mj, Yj) and creating n sets of color charts; a colorimetric step of measuring the color chart, an evaluation value calculation step of calculating an evaluation value by calculation using the correction coefficient, and a determination as to whether the evaluation value that is the output of this evaluation value calculation step is the minimum. A correction coefficient determining method comprising a convergence calculation step of updating the correction coefficient according to a determination result, repeating the evaluation value calculation step, and performing a convergence calculation to minimize the evaluation value. (5) The evaluation value calculation step converts the ink density signals (Cj, Mj, Yj) (j = 1 to n, n is a natural number) into three primary color density signals (D_Rj) by an inverse function of color correction calculation using color correction coefficients. , D_Gj, D_Bj)
a reverse color correction calculation step for converting the three primary color density signals (D_Rj, D_Gj, D_Bj)
The second luminance signal (R
The second luminance signal (R'j, G'j, B'j) is converted to an inverse brightness matrix calculation step for converting the color into three primary color brightness signals (Rj, Gj, Bj), and a color difference calculation step for calculating color difference using the colorimetric color signal obtained as a result of the colorimetry step and the output of the inverse brightness matrix calculation step. 5. The correction coefficient determining method according to claim 4, further comprising a calculation step. (6) a CRT inverse γ correction calculation step for converting each color of the γ-corrected luminance signal into a linear three-primary-color luminance signal; a brightness matrix calculation step for converting the three-primary-color luminance signals into a second luminance signal; Each color of the second luminance signal is subtractively mixed into 3
An RO that stores ink control signals obtained through a complementary color conversion step for converting into primary color density signals and a color correction calculation step for converting these three primary color density signals into ink density signals used for recording.
memory means constituted by M or RAM, and address generation means for inputting each upper bit of the gamma-corrected luminance signal (R'', G'', B'') to be recorded and generating an address to be given to the memory means. Then, an interpolation operation is performed using the ink control signal outputted from the memory means and the information of each lower bit of the gamma-corrected luminance signal (R'', G'', B'') of the CRT to be recorded. , the C to be recorded
and interpolation calculation means for determining ink control signals (I_C, I_M, I_Y) for the γ-corrected luminance signals (R″, G″, B″) of RT.
, I_M, I_Y).