JP3362281B2 - Color reproduction device and color reproduction method - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates, when the color coordinates are given, preferred color reproduction device and the color reproduction direction applied to a case of accurately estimating the input color separation image signals corresponding to the color coordinate
Related to the law, and in particular for Improving the gradation of gray color.

[0002]

2. Description of the Related Art In the case of full color printing by printing, thermal transfer, ink jet, electrophotography, etc., the output colors of the color print are generally Y, M, C and K.
Color is often used. There are various methods for how to combine the Y, M, C, and K in order to express the colors with four colors. For example, as one of the methods,
There is a so-called UCR (Under Color Removal).

As a concrete method, a method called 100% UCR method is the original Y, M, C as shown in FIG.
A method of replacing the minimum density of (Y _O , M _O , C _O ) with the density of black K, and when Y _O is the minimum density as shown in FIG. , As shown in FIG. The concentrations after substitution are Y _n , M _n , and
C _n, the K _n (where Y _n = 0).

It is known that the 100% UCR method has the following characteristics. -Working reliability is increased.・ Ink can be saved.・ Can cover some ink instability.

The energy savings of ink drying and the problems associated with drying are reduced. This is because the K ink amount can be roughly ⅓ to the sum of the Y, M, and C ink amounts. On the other hand, when obtaining the color reproduction characteristics of Y, M, C, and K, a small number of color patches by a discrete combination of Y, M, C, and K are actually created, and this color patch is actually measured, The colorimetric values are interpolated to estimate the color reproduction characteristics of Y, M, C, K other than the above-mentioned combinations, that is, the colorimetric values for a certain Y, M, C, K combination are estimated. Is being considered.

On the contrary, when a specific color coordinate is designated, the combination of Y, M, C, and K indicating the color coordinate is also interpolated by the value actually measured from the color patch. Can be estimated. Thus, Y, M,
If all combinations of C, K are estimated using color patches created based on discrete combinations of Y, M, C, K, the actual colorimetric values of Y, M, C, K Since the combination is estimated, the estimation accuracy is improved and the color reproduction characteristic is further improved.

FIG. 30 shows an example of a color patch created by a discrete combination of Y, M, C and K. In the figure, the maximum values of Y, M, C, and K are set to 255, and
This is a color patch when the density of the color is extracted in 5 steps in 64 steps. In addition, when a certain color coordinate is given, or when an electric signal corresponding to the color coordinate (for example, R, G, B color separation image signals) is given, Y is more than Y in any case. , M, C, K
A color output correction means for obtaining the combination of is required.

FIG. 31 shows an example thereof, in which a color masking device 10, which is one of color separation image correction devices, is provided in front of the printer 12, where R, G, B corresponding to color coordinates are provided.
After the color separation image signals of Y, M, C, and K are converted into Y, M, C, and K color signals, the images are supplied to the printer 12, and the images thereof are Y, M, C,
It is recorded on the recording medium 13 by K. By the way, the above-mentioned 100% UCR method has the above-mentioned advantages, but the saturation is generally lowered.

The density decreases as the proportion of black plates increases. However, the color reproduction characteristic is deteriorated. Therefore, the applicant of the present application can improve the color reproduction characteristic, which is a drawback of the 100% UCR method, and can estimate the color reproduction characteristic using less color patches when applying the color reproduction characteristic estimation method using color patches. The above-mentioned method was previously proposed (see Japanese Patent Laid-Open No. 2-136848).

In this system, among the combinations of Y, M, C, and K indicating the given color coordinates, the value of K is the minimum density value and the value of Y, M, C, and K is the minimum density value. Y,
A combination of M, C, K is obtained for each specific point of discretely given color coordinates, Y, M, C, K color patches are created by these discrete combinations, and the created color is created. The density values of Y, M, C and K corresponding to the color coordinates other than the specific point are estimated from the colorimetric values of the patch.

When arbitrary color coordinates are given, there are generally innumerable combinations of Y, M, C and K representing the color. In order to make this innumerable combination unique, K among the combinations showing the given color coordinates
The condition that the value of is the maximum density value is introduced. This condition is that at least one of Y, M and C is 0 or K
Is the maximum density (solid in printing). According to the improved type that includes the condition (improved type condition) that the value of K takes the maximum density value within the range that can express a certain color, the number of color patches is suppressed to the minimum. Reproducibility can be improved.

[0012]

However, in the improved 100% UCR method, in the case of gray color reproduction, K color is reproduced in accordance with the above conditions. In that case, the electrophotographic printer is used. In the case of poor gradation such as an ink jet printer and an ink jet printer, a defect that gradation is poor with a single color directly appears, and a problem that good gradation cannot be obtained occurs.

The present invention has been made in view of the above problems, and in order to produce good gradation of a printer having poor gradation, while making full use of the color gamut that can be created by Y, M, C, and K. It is an object of the present invention to provide a method for determining Y, M, C, K values of.

[0014]

Means for Solving the Problems The present invention for this purpose is, Y
(Yellow), M (Magenta), C (Cyan) and K
A color reproduction method that combines four colors (black) to reproduce colors.
Yes, a set of Y, M, C, K indicating the given color coordinates
Of the combinations, at least one of Y, M, and C is the maximum.
If K is included, the process of obtaining the target value including K, and K is set to 0
In case of, the process of obtaining the target value using only Y, M, and C is used.
, The concentration of K becomes the minimum, Y, M, C,
Estimate the density values of Y, M, C, K corresponding to the combination of K
The feature is that the color reproduction is performed in a fixed manner.

[0015]

There are innumerable combinations of Y, M, C and K indicating color coordinates, and the condition for determining the minimum concentration of K is that K = 0 or at least one of Y, M and C. It is equivalent to the maximum concentration. As described above, by minimizing the density of K, it is possible to reproduce a gray color by mixing at least three colors of Y, M, and C or four colors obtained by adding K to the gray color. It can cover bad sex.

Since one color can be determined and combinations for the remaining three colors can be determined, the number of color patches can be made sufficiently small as in the case of maximizing the density of K.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. As described above, there are generally innumerable combinations of Y, M, C, and K that represent a given color when given an arbitrary color coordinate. In order to reproduce this gray color by mixing a plurality of colors while introducing only one of these innumerable combinations, the condition that the density of K is minimized among the combinations showing the given color coordinates is introduced. To do.

That is, K can take various values even within the range showing the color coordinates, but if the minimum value of the possible values is determined as the value of K, other combinations of Y, M, and C can be obtained. The gray color can be determined, and the gray color can be reproduced by mixing at least three colors Y, M, and C or four colors obtained by adding K to the gray color. It is possible to cover the poor gradation.

This condition is that when any one of Y, M and C is the maximum density (as an absolute amount, not as a relative value) or when K is 0, that is, when Y is maximum (255)・ When M is the maximum (255) ・ When C is the maximum (255) ・ When K is 0 ... It is the condition that at least one of (1) is satisfied. The color solid determined by these becomes the color reproduction range.

It is to be noted that the condition that any one of Y, M, and C has the maximum density is that K is 0 even when these are maximum.
This is not always the case. When a color patch is created by this method, it becomes as shown in FIG. This figure shows
When the number of discrete points n of Y, M, C, and K is selected to 5, and the maximum quantization level is selected to 256 steps, at this time, the respective points (0, 64, 128, 192,
When the basic colors Y, M, C, K of (255) are actually combined and recorded on the recording medium, for example, printing paper with ink,
To obtain a color patch of.

The color patch of FIG. 1 is actually measured and its colorimetric value (given color coordinates) is converted into C using a conversion formula for another color system (for example, CIE LUV color system).
The values are converted into values in the IELUV color system and plotted for each color patch, as shown in FIG. The colorimetric value of each color patch corresponds to each grid point. However, this Figure 2
For convenience of explanation, is expressed on the two axes of saturation and lightness, and the value of cyan C is omitted. The same applies to the color system shown below.

The color patch described above may be arranged as shown in FIG. According to this FIG. 3, the number of color patches is further reduced as compared with FIG. When the color patches of FIG. 3 are actually measured and mapped on the CIE LUV color system, the result is as shown in FIG. The black circles are the values obtained by actual color measurement. Here, in the color system of FIG. 4, the space between the grids is reduced substantially linearly as the value of K increases, so when K is large, the white circle grid points are linearly interpolated based on the values of the immediately preceding grid points. Even if it is calculated by (for example, internal division), the error becomes small.

When all the white circle lattice points are interpolated, they become the same as the color system of FIG. 2, but this is the same as when the color patch of FIG. 1 is used as the color patch. In other words, not only the equation (1) is simply met, but if the color patch is reduced as the value of K is increased by utilizing the property of the colorimetric value, the interpolation calculation processing will be the same as in FIG. The number of patches can be reduced.

In order to further reduce the lattice spacing by a half by the electrical processing, interpolation calculation may be performed based on the mapped values. The interpolation process in this case is a non-linear interpolation process. An example of the interpolation process is shown in FIG. As shown in FIG. 5, if the black circles ● are grid points (sample points) and the triangles and the crosses are points to be interpolated, respectively, there are two grid points before and after as shown by the crosses. Different interpolation formulas are used in the case where there are one point and three points before and after as in the case of the cross mark.

CIE LUV is the color system of the points to be interpolated, and the values of the color system of each sample point are Li ', Ui', V
When i '(i = 1 to 4), the former case is interpolated by the following interpolation formula. L _m '=-(1/16) L1' + (9/16) L2 '+ (9/16) L3'- (1/16) L4' U _m '=-(1/16) U1' + ( 9/16) U2 '+ (9/16) U3'-(1/16) U4 'V _m ' =-(1/16) V1 '+ (9/16) V2' + (9/16) V3 ' -(1/16) V4 'In the latter case, the following interpolation formula is used.

L _m '= (5/16) L1' + (15/16) L2'- (5/16) L3'- (1/16) L4 'U _m ' = (5/16) U1 '+ (15/16) U2'- (5/16) U3'- (1/16) U4 'V _m ' = (5/16) V1 '+ (15/16) V2'- (5/16) V3' -(1/16) V4 'An example of the order of the interpolation processing is shown in FIG. Numbers I, II, III
Are interpolated in the order of.

By such an interpolation process, the number of grid points of the color system similar to the number of color patches actually used can be obtained. The interpolation processing may be interpolation processing by direct approximation. According to the above, the color coordinate values corresponding to the colorimetric values can be obtained without increasing the number of color patches. The values of the color coordinates in that case are data of grid points.

The values of color coordinates existing at points other than the grid points are estimated by the following method. First, the principle of the estimation method will be described. The simplest division space that divides an N-dimensional (N is an integer of 2 or more) space is a division space having (N + 1) vertices. For example, in two dimensions a triangle,
In three dimensions it is a triangular pyramid. Here, space 1 and space 2 are considered as shown in FIG. Here we consider two dimensions. In this case, the space 1 and the space 2 are a to i and a point a'to respectively.
i ′ is used to divide into triangles. For example, the divided space Δbde of the space 1 is divided into the divided space Δb′d′e ′ of the space 2.
It corresponds to.

Assuming that they correspond linearly in the corresponding divided spaces, the point P given to the space 1 is made to correspond to the point P'in the space 2. Here, for example, points surrounding the point P and the point P ′ are respectively Pi (xi, yi, zi,
...), Pi '(xi', yi ', zi', ...)
(I = 1 to N + 1), and the given point P is (x,
y, z ...), and the point P ′ to be obtained is (x ′, y ′,
z ′, ...), the matrix form can be shown as follows.

[0030]

[Equation 1]

Further, the point P to be obtained is 3 surrounding the point P.
The weighted average can be obtained from the points. As shown in FIG. 8, S (b): S (d): S (e) = S (b '): S
(D '): S (e'). It should be noted that which division space the given point P enters can be specified by checking which side of the boundary line (or boundary surface) of each division space it is.

In this example, by using the above-mentioned principle, a combination of basic colors showing a certain target color is obtained as follows. For simplification, the basic color will be described as two colors (for example, Y and M) also in this example. The Y, M coordinate system and the CIE LUV color system after the interpolation processing or the interpolation processing are as shown in FIGS. 9 and 10, respectively. In the figure, the grid points of white circles indicate the interpolated points.

Next, the space of the Y and M coordinate systems and CIE LUV
The color space is divided into triangles as shown in FIGS. 11 and 12, respectively. As a result, it is divided into 8 ² × 2 = 128 triangles. Next, as shown in FIG. 13, it is checked which side of the boundary line of each triangle the target value T'corresponding to the output color (target color) desired in the CIE LUV color system belongs to. This will identify the triangle.

Here, the triangle whose target value T'is formed by the grid points a'-c 'as shown in FIG. 14 is the triangle formed by the grid points a-c as shown in FIG. . Next ^{, the} coordinates of each vertex (three points) of the triangle of the CIE LUV color system containing the target value T'and the triangle of the Y ^, M coordinate system containing the target value T ', and the target value T' ( 2) Substituting into the equation, the target value T, and thus the basic color combination indicating the desired output color is calculated.

Below, the target value T is obtained by specifying the triangle and performing the operation of the equation (2) for each of the given target values T '. The given target value T'is CIEL as shown in FIG.
When it does not fall into any of the triangles of the UV color system, that is, when it is outside the color reproduction range, it is necessary to move this target value T ′ into the color reproduction range.

In this case, the target value T'is moved in the achromatic color direction as shown in FIG. 16, and the target value T'is set as a boundary between the straight line in the achromatic color direction and the color reproduction range as shown in FIG. The coordinate of the intersection point of is the target value T '. Then, the target value T ′ and the line segment L ′ including the target value T ′ are calculated, and the line segment L corresponding to the line segment L ′ in the Y ^, M coordinate system is calculated as shown in FIG. As a result, the target value T is calculated using the equation (1).

In the above example, the basic colors are two for the sake of simplicity of explanation, but the basic colors are three (Y, M,
Even in the case of C), the target value T (Y, M, C) can be similarly obtained. However, in this case, the space is divided into triangular pyramids, and the given target value T ′ is CIELUV.
After checking which triangular pyramid of the color system is included, the corresponding triangular pyramid of the Y, M, C coordinate system is determined.

Then, the target value T is calculated using the equation (2). Here, for example, as shown in FIG. 19, the target value T (Y,
(Y, M, C1) (Y2, M2, C2) (Y3, M3, C3) (Y4, M4, C4 ) And the target value T '(L, U, V) is entered in the CIEL
If the four vertices of the triangular pyramid of the UV color system are (L1, U1, V1) (L2, U2, V2) (L3, U3, V3) (L4, U4, V4), the target value T is You can ask.

[0039]

[Equation 2]

In the above, an example in which a 4 × 4 matrix is used as an inverse matrix has been shown, but the same is true even if a 3 × 3 matrix is used as follows, and the error can be reduced by the number of repeated operations. .

[0041]

[Equation 3]

Further, the target value T '(L, U, V) is the vertex
(L1, U1, V1), (L2, U2, V2), (L
3, U3, V3), (L4, U4, V4) can be discriminated as to whether or not it is inside the triangular pyramid.
That is,

[0043]

[Equation 4]

When α, β, and γ obtained in step (4) satisfy the following equation, they are inside (including on the surface) of the triangular pyramid, and when they do not satisfy, they are outside. α ≧ 0, β ≧ 0, γ ≧ 0, α + β + γ ≦ 1 Next, when the target value T ′ is outside the color reproduction range, it is necessary to move the target value T ′ into the color reproduction range as described above. is there. In the three-dimensional case, as shown in FIG. 20, points (L5, U5, V5) outside the color reproduction range and points (L0, U0, V0) within the color reproduction range toward the achromatic color direction are shown. The connecting straight line and the outermost vertex coordinates in the color reproduction range forming the triangular pyramid are (L1, U1, V1), (L2,
Letting (L _p , U _p , V _p ) be the intersection with the triangle _defined as (U2, V2), (L3, U3, V3), the intersection can be obtained as follows. First,

[0045]

[Equation 5]

Then, e, f, and g are obtained as

[0047]

[Equation 6]

Δ is obtained as

[0049]

[Equation 7]

Η is obtained as And

[0051]

[Equation 8]

In this case also, the intersection points (L _p , U _p , V _p )
Are vertex coordinates (L ₁ , U ₁ , V ₁ ), (L ₂ , U ₂ ,
The determination as to whether or not it is inside the triangle defined by V ₂ ), (L ₃ , U ₃ , V ₃ ) is performed as follows. First, the δ
Is not 0, When all of α ', β', and γ'obtained in (1) are 0 or more, they are inside the triangle (including on the side), and otherwise they are outside. The case of δ = 0 means that the straight line and the triangle are parallel and do not intersect.

Since the relationship between the color system and the signal value is known in this manner, the relationship between the three variables a, b, c and the signal value can be obtained as described above. The space (hexahedral) may be divided into triangular pyramids as shown in FIG. 21 or as shown in FIG. In these figures, points a to h and a'to h'in the space 1 and the space 2 are points corresponding to each other.

Using this estimation method, the target value T for the three-dimensional space satisfying the above-mentioned conditions that minimizes the density of K is obtained.
You should ask. Specifically, when Y is the maximum (255), the target value T is obtained by the above estimation for the other three colors of M, C, and K. Similarly, when M is the maximum (255), Y, C, and K
, The target value T for Y, M, K when C is maximum (255) and for Y, M, C when K is 0
Can be asked.

Next, an example of a color masking device (color separation image correction device) suitable for applying the method for estimating the color reproduction characteristic using the color patch will be described. The target value (combination of Y, M, C, K, that is, color correction data) calculated as described above is stored in advance in the main look-up table (MLUT). When the input system is a color CRT, Y, M, C, K associated with the color coordinate system of the basic color determined by R, G, B
The coordinate system becomes the given color coordinate.

The color coordinates are calculated.
R, G, B coordinate system, such as B, G coordinate system and Y, M,
Since the relationship between K and the coordinate system is as shown in FIG. 23, the processing of associating the input coordinate system, for example, the point t with t ′ and the point s with s ′ is performed. Therefore, in order to make these R, G, B coordinate systems correspond to the Y, M, C, K coordinate systems, a main lookup table (MLUT) is prepared.
When the coordinate system representing is input, the coordinate system of t'is referred to.

Color correction data other than grid points are calculated by interpolation. As described above, in the method according to the present invention, the data of the input coordinate system is predetermined. (a) M, C, K (Y is maximum concentration) (b) Y, C, K (M is maximum concentration) (c) Y, M, K (C is maximum concentration) (d) Y, M, C (K = 0) (3) In order to simplify the description, (d) will be described.

In this example, as shown in FIG. 24, a rectangular parallelepiped space W determined by three input image data R, G, B.
A rectangular parallelepiped formed from eight color correction data (known calculation color correction data P1 to P8 corresponding to Y, M, C, K (= 0)) including (interpolation point s is located at the diagonal vertex) The spatial area V of the shape is defined. Both the spatial regions W and V use P1 as a reference point. And 0, 32, 64, 96, 128, 160, 1 of each color
It is assumed that the color correction values as described above are applied to the combined colors at the points 92, 224, and 255. At this time, if the input image data R, G, and B each have a value of (100, 130, 150), the color correction data of the vertices (lattice points) of the space area surrounded by 8 points shown below are obtained. Used to interpolate.

Here, Pi (i = 1 to 8) on the left side indicates the coordinate value of each vertex of the spatial region V, and the right side indicates the color correction data Ki, Ci, Mi, Yi at that time. P1: (0, 96, 128, 128) = (K1, C1, M1, Y
1) P2: (0,128, 128, 128) = (K1, C1, M1, Y
1) P3: (0, 96, 160, 128) = (K1, C1, M1, Y)
1) P4: (0,128, 160, 128) = (K1, C1, M1, Y
1) P5: (0, 96, 128, 160) = (K1, C1, M1, Y)
1) P6: (0,128, 128, 160) = (K1, C1, M1, Y
1) P7: (0, 96, 160, 160) = (K1, C1, M1, Y
1) P8: (0, 96, 160, 160) = (K1, C1, M1, Y
1) The weighting coefficient for each vertex Pi of the spatial region V is calculated as follows.

In this example, the vertex of the correction value to be obtained and the volume of the spatial area W of the rectangular parallelepiped formed by the interpolation points s are used as the weighting factor Wi at the center of the correction value to be obtained. . Therefore, the weighting factor of the point P8 is a rectangular parallelepiped spatial region formed by s and P1 from (100,130,150)-(96,128,128) = (4,2,22) using the coordinates of P1 and the coordinates of s. Is 4 × 2 × 22 = 176, which is the weighting factor of the point P.

Similarly, the weighting factors of the remaining points P1 to P7 are calculated. P1 = 8400 P2 = 1200 P3 = 560 P4 = 80 P7 = 1232 P8 = 178 The sum of these weighting factors is the same as the volume V of the cubic space area, which is 32768 (a) in this example.
Therefore, the correction values K _s , C _s , M _s , Y at the s point
_s is a _{K s = 1 / a (P1K1} + P2K2 + P3K3 + P4K4 + P5K5 + P6K6 + P7K7 + P8K8) C s = 1 / a (P1C1 + P2C2 + P3C3 + P4C4 + P5C5 + P6C6 + P7C7 + P8C8) M s = 1 / a (P1M1 + P2M2 + P3M3 + P4M4 + P5M5 + P6M6 + P7M7 + P8M8) M s = 1 / a (P1Y1 + P2Y2 + P3Y3 + P4Y4 + P5Y5 + P6Y6 + P7Y7 + P8Y8). That is, the correction value of the point s to be obtained and the eight points surrounding it is K _i , C _i , M _i , Y _i (this corresponds to the interpolated values L _s ', U _s ' V _s ' of the color system). (Y, M, C, K coordinate system values) and each weighting coefficient is A _i ,

K _s = (1 / ΣA _i ) ΣA _i K _i C _s = (1 / ΣA _i ) ΣA _i C _i M _s = (1 / ΣA _i ) ΣA _i M _i Y _s = (1 / ΣA _i ) When the data of the input coordinate systems R, G, B which can be represented by ΣA _i Y _i are different, the data are interpolated by the weighting factors A _i calculated respectively corresponding to the input data.

Since this input data already contains a value that matches the conditional expression (3), the interpolator does not change it according to the conditional expression (3). The number of color correction data is actually set to the power of 2 in consideration of the capacity of the ROM.
Therefore, when using a 256 k-bit ROM, 32 points of color correction data per color (32 ³ =
You can have 32768 points).

FIG. 25 shows an example of the color masking device 10. The color correction data storage means 20 stores each color Y, M, K,
Color correction data for K is stored in each MLUT 21-24. Reference numeral 25 is a weighting coefficient storage means, which is also configured as an LUT. The input image data R, G, B are once supplied to the address signal forming means 40, and the upper 5 bits of the address signal corresponding to the input level are outputted, which are supplied to the color correction data storage means 20. Further, the weighting coefficient designating signal of the lower 3 bits outputted from this is supplied to the weighting coefficient storage means 25.

When the color coordinates determined by the input image data R, G, B deviate from the grid points in FIG. 21, four grid points in the Y, M, K, K coordinate system surrounding the color coordinates are located at the latter stage. As can be designated in the color correction data storage means 20, the 5-bit address signal is input image data R,
It is referred to and output by G and B. The address signal forming means 40 also comprises LUTs (PLUTs) 41 to 43, respectively. A bipolar ROM is suitable as the LUT. For these PLUTs 41 to 43, a controller 50
A 1-bit distribution signal is supplied from, but the details will be described later.

The color correction data referred to by the address signal corresponding to the input level of the input image data and the data indicating the weighting coefficient (hereinafter simply referred to as weighting coefficient) are sequentially supplied to the multiplication accumulating means 30 side eight times. It As described above, the multiplication and accumulating means 30 uses A _i B _i (B _i is Y, M,
K) (generic name of K) is sequentially executed and the sum of them is obtained. In this example, the multipliers 31 to 34 and the accumulators 35 to 38 are used.

Therefore, each of the multipliers 31 to 34 uses a ROM of 512 kbit, and the color correction data corresponding to these are used.
(8 bits) and the weighting coefficient A _i are supplied, and A _i , B
The multiplication process of _i is executed, and the multiplication output of the upper 8 bits is supplied to the accumulators (ALU) 35 to 38 in the subsequent stage, and the multiplication outputs are sequentially added. The accumulators 35 to 38 are operated with the accuracy of color bits, and the higher 8 bits are used as the cumulative calculation power (sum of products output). As a result, the same output as that obtained by dividing the cumulative calculation force by the weighting coefficient A _i is obtained.

The cumulative calculation powers of the upper 8 bits are respectively latch circuits.
Latched by 46-49. The latch pulse is generated by the controller 50. The configuration of each unit will be described in more detail. LUT 21 used as color correction data storage means 20
In the case of using a ROM having a capacity of 256 kbits, the numbers 24 to 24 extract only 32 points from the minimum level to the maximum level of the input image data. As a result, 32 points of color correction data can be stored for each point (thus, 3 ³ = 32 768 points for 3 colors).

Therefore, when the input level is 256 gradations, the distribution of 32 points is 0, 8, 16, ... 240, It is divided evenly so that a total of 32 pieces of 248 are obtained, and it becomes the 33rd point.
Do not use from 9 points to 255 points. Or 249-25
Points are treated as 248.

The color correction data at each distribution point is accurately calculated, and the plurality of calculated color correction data are stored in the respective LUTs 21-24. If the distribution points are set to 32 points in this way, 8-bit output general-purpose RO
Since M can be used, there is an advantage that the storage means can be constructed inexpensively. In the LUT 25 for the weighting coefficient storage means 4,
The weighting coefficient A _{i at} each distribution point is stored. Now, when 8 bits are allocated as described above, 8
The total of the individual weighting factors A _i is 8 × 8 × 8 = 512, but as described above, the commercially available general-purpose I
If K is to be used, the weighting factor is the theoretical value
If you have (maximum 512), the number of elements will increase, so in this example, the approximate value of the theoretical value compressed to about 1/2 is used as the actual value of the weighting coefficient.

In the example shown below, the sum of eight weighting factors is set to always be 256, and the maximum weighting factor of each is 255. In such a case, for example, in FIG. 22, when the interpolation point s is at the same position as P1, P1 to P1
Each weighting factor of 8 is, as shown by its theoretical value in (), Therefore, the sum of the weighting factors is 256.

When s is in the middle of P1 and P3 and is apart from P1 by 3 (thus, 5 from P3), the weighting factors of P1 to P8 are as follows. Therefore, each weighting factor is appropriately selected so that the sum of the weighting factors in this case is also 256.

Similarly, s is separated from the surfaces of P1 to P4 by 3 and separated from the surfaces of P1, P3, P5 and P7 by 1,
When the surface is away from the planes P1, P2, P5 and P6, the weighting factors P1 to P8 are as follows. Therefore, each weighting factor is selected so that the sum of the weighting factors in this case is also 256.

The above-mentioned 1-bit distribution signal is a control signal for designating the color correction data before and after including each point s. That is, for convenience of explanation, the relationship between 32 allocation points (lattice points) and the corresponding address signals is set as shown in FIG. Now the input image data level is 100
Then, it is necessary to form the address signal (12, 13) from which the color correction data storage means 20 outputs the color correction data (96 and 104) before and after including this input level.

Therefore, when the distribution signal is 0, the address signal (12) is output so that the smaller color correction data (96) is referred to. When the distribution signal is 1, the larger color correction data (96) is output. It is controlled so that the address signal (13) for referring to (104) is output.
However, when the distribution signal is 0 at the maximum value to be used (248 in this case), the color correction data of its own value is selected, and when the distribution signal is 1, the smaller one (in this case, 1) 240) is established.

Another interpolation method will be described. Now, as shown in FIG. 27, the input signal values R, G, B are the same as the LU.
It is assumed that the eight grid points of R, G, and B of T are surrounded by a cube having vertices A to H. In that case, as in the case of obtaining the target value, the cube can be divided into six triangular pyramids as shown by the one-dot chain line. Then, which triangular pyramid the input signal values R, G, B are included in can be similarly obtained. Now, the values of the input signal values R, G, B are (5, 1,
2), this point is included in the triangular pyramid formed by the vertices A, B, C, and G. FIG. 28 shows a triangular pyramid of the CIE LUV color system corresponding to this triangular pyramid. If A ′ to G ′ correspond to A to G, respectively,
The interpolation point P ′ also exists within the triangular pyramid T ′.

When this triangular pyramid T is determined, next, in FIG.
As shown in, the interpolation point P and the vertices A, B, C, and G are connected to form a total of four new triangular pyramids, and each volume VBC
GP, VACGP, VABGP, VABCP are required. From these volumes VBCGP, VACGP, VABGP, VABCP and the vertices A ', B', C ', G'of the color system of FIG. Note that VABGP is the volume of the triangular pyramid T.

P ′ = 1 / VABGP (VBCGP · A ′ + VACGP · B ′ + VABGP · C ′ + VABCP · G ′) In this way, for the arbitrary signal values R, G, B input from the LUT The value of the corresponding CIE LUV color system is obtained by the search and the interpolation calculation. In the present invention, the given color coordinates depend on the input device. As described above, if the input device is a display, its R,
Color coordinates are obtained from the values of G and B by calculation. If the input device is a printing system, it can be calculated from the Y, M, C and K four colors.

If the input device is a scanner system, color coordinates can be obtained from the R, G, B values.

[0080]

As described above, according to the present invention, the combination of Y, M, C, and K indicating the given color coordinates has the minimum K density. While the number of color patches corresponding to a specific point can be reduced, many gray colors are reproduced by mixing many colors, so deterioration of the gradation is noticeable in electrophotographic printers and inkjet printers with poor gradation. Becomes difficult,
The image quality can be improved.

[Brief description of drawings]

FIG. 1 is a diagram of a color patch according to an embodiment of the present invention.

FIG. 2 shows CIELU as the color patch colorimetric values shown in FIG.
Figure when mapped to V color system

FIG. 3 is a diagram showing another example of the color patch according to the embodiment of the invention.

FIG. 4 shows CIELU as the color patch colorimetric values shown in FIG.
Figure when mapped to V color system

FIG. 5 is an explanatory diagram of curve approximation.

6 is an explanatory diagram of target value expansion obtained by curve approximation in FIG.

FIG. 7 is a diagram showing the principle of target value estimation.

FIG. 8 is a diagram for explaining calculation by weighted average.

FIG. 9 is a diagram of a Y and M coordinate system after interpolation processing.

FIG. 10 is a diagram of the CIE LUV color system as well.

FIG. 11 is a diagram in which the YM coordinate system space is divided into triangles.

FIG. 12 is a diagram in which the CIE LUV coordinate system space is divided into triangles.

FIG. 13 is a diagram showing a combination T of Y and M corresponding to a target value T ′.

FIG. 14 is a diagram showing a target value T ′ in the CIE LUV color system.

FIG. 15 is a diagram showing a case where a target value T ′ exists outside the color reproduction range.

FIG. 16 is an explanatory diagram of moving a target value T ′ into a color reproduction range.

FIG. 17 is a combination T of Y and M corresponding to a new target value T ′.
Calculation of

FIG. 18 is a diagram showing the position of the CIE LUV color system of the new target value T ′.

[FIG. 19] Target values T and T ′ are set to Y, M, and C coordinate systems and CIEL
Figure shown with UV color system

FIG. 20 is a diagram showing a case where the target value T ′ exists outside the color reproduction range of the Y, M, and C coordinate systems.

FIG. 21 is a diagram showing an example of a method of dividing a space into triangles.

FIG. 22 is a diagram showing another example of a method of dividing a space into triangles.

FIG. 23 is a diagram showing a relationship between a B, G coordinate system and a Y, M, K coordinate system.

FIG. 24 is an explanatory diagram of interpolation processing.

FIG. 25 is a system diagram of a color masking device to which the present invention can be applied.

FIG. 26 is a diagram showing a relationship between an address used in the same device and color correction data.

FIG. 27 is a diagram for explaining a three-dimensional interpolation calculation.

[FIG. 28] Similarly, a diagram for explaining interpolation calculation.

FIG. 29: Illustration of 100% UCR method

FIG. 30 is an explanatory diagram of color patches using Y, M, C, and K.

FIG. 31 is a system diagram of a color image forming apparatus used for explaining the present invention.

[Explanation of symbols]

10 Color masking device 12 color printer 13 recording medium

Claims (2)

(57) [Claims] 1. Y (yellow), M (magenta), C
(Cyan) and K (black) are combined for color reproduction
A color reproduction method to be performed, which is a combination of Y, M, C, and K indicating a given color coordinate.
Of these, if at least one of Y, M, and C is maximum
Processing to obtain the target value including K, and when K is 0
In this case, use the process to obtain the target value using only Y, M, and C
The combination of Y, M, C, and K that minimizes the concentration of K
Estimate the density value of Y, M, C, K corresponding to the matching
A color reproduction method characterized by performing reproduction. 2. Y (yellow), M (magenta), C
(Cyan) and K (black) are combined for color reproduction
A color reproduction method to be performed, which is a combination of Y, M, C, and K indicating a given color coordinate.
Of these, if at least one of Y, M, and C is maximum
Processing to obtain the target value including K, and when K is 0
In this case, use the process to obtain the target value using only Y, M, and C
The combination of Y, M, C, and K that minimizes the concentration of K
For each specific point of discretely given color coordinates,
And the color of Y, M, C, K according to the combination obtained.
A color characterized by performing a color reproduction by creating a patch, estimating the density values of Y, M, C, and K corresponding to the color coordinates other than the specific point from the colorimetric value of the created color patch. How to reproduce.