JP5645789B2 - Color conversion apparatus, image forming apparatus, color conversion program, and image forming program - Google Patents

Description

  The present invention relates to a color conversion device that converts input RGB format image data into CMYK format image data, an image forming device that includes the color conversion device and outputs CMYK format image data converted by the color conversion device, and a color The present invention relates to a color conversion program for executing functions of a conversion device, an image formation program for executing functions of the image forming device, and a color conversion table used for color conversion.

In an image forming apparatus such as a color printer, when printing is performed, input RGB color image data is converted into CMYK format image data that can be output using a color conversion lookup table (color conversion table). Like to do.
Since the color conversion table is configured by associating RGB values with CMYK values, the color conversion table can be represented by an orthogonal lattice with each correspondence as a lattice point.
FIG. 27 shows correspondence between basic colors Rd (red), Gr (green), and Bl (blue) and Cy (cyan), Mg (magenta), and Ye (yellow), and achromatic color Wh. FIG. 5 is a schematic diagram of an RGB-CMYK color conversion table represented by an orthogonal lattice of a three-dimensional orthogonal coordinate system with (white) and Bk (black) as lattice points.
Since the CMY values corresponding to the RGB values are assigned in advance to the respective lattice points on the orthogonal lattice shown in FIG. 27, the CMYK values of the output device corresponding to the arbitrary RGB values related to the input device are smoothly derived. be able to.

However, the sense that humans can understand with the brain is that the direction along the achromatic color axis connecting Wh and Bk, the radiation (saturation) direction centered on the achromatic color axis and the concentricity (rather than the axial direction of each RGB channel) It is known to be sensitive to components in three directions (hue) direction. In other words, the sense that humans distinguish between colors is not only based on the size of the spatial distance, but rather depends on the coordinate position in the color space.
Specifically, the accuracy of the radial direction around the achromatic color axis and the variation of the distance in the concentric direction of the achromatic color axis are better than the variation in the direction along the R, G, B axes. It is an important factor for accurately grasping the color.
For this reason, the conventional orthogonal grid color conversion table in which RGB values and CMYK values are simply associated does not reflect the human sense of color and has a problem of poor color reproducibility. .

FIG. 28 is a diagram for explaining the problem of the conventional color conversion table, and is a grid point arrangement diagram when FIG. 27 is viewed from the achromatic color axis direction.
As shown in FIG. 28, it can be seen that in the radiation direction (saturation direction) centering on the achromatic color axis, the intervals between the lattice points existing on the same hue are unevenly arranged.
For example, five grid points are arranged on the achromatic color axis and each line segment of Rd, Gr, Bl, Cy, Mg, Ye, but on the line segment between the achromatic color axis and the Cy to Bl intermediate point. Only three grid points are arranged.
Further, when viewed from a concentric viewpoint, it can be seen that the intervals between the lattice points existing on the same saturation are uneven due to the distance from the achromatic axis (center axis). That is, it can be recognized that variations in hue that can be reproduced differ depending on the saturation.
Therefore, according to such a conventional color conversion table, it has been difficult to perform color adjustment that optimizes the saturation for each hue or optimizes the hue for each saturation.

Therefore, in order to solve such problems of the conventional color conversion table, a radial color conversion table is proposed that enables appropriate color conversion processing by adding necessary grid points to the conventional color conversion table. (See Patent Documents 2 and 3).
FIG. 29 is a grid point arrangement diagram when the radial color conversion table described in Patent Document 2 or Patent Document 3 is viewed from the achromatic color axis direction.
As shown in FIG. 29, according to this radial color conversion table, lattice points are arranged uniformly for each constant hue width, and lattice points are arranged equally for each constant saturation width. Therefore, it is possible to accurately adjust the color according to the color and hue felt by humans.

JP-A-9-224158 JP 2009-17097 A JP 2009-17098 A

However, the radial color conversion table described above has a configuration in which each grid point is arranged radially around the achromatic color axis, so that the coordinate position does not fundamentally match the grid point of the orthogonal grid, and so on. Consistency with a system that presupposes the use of other color conversion tables was not considered.
For this reason, calculation methods, programs, algorithms, etc. relating to color conversion and color adjustment used for conventional orthogonal grid color conversion tables cannot be used, and existing color conversion tables such as these have been adopted. It was difficult to use, mix and fuse with devices and systems.
In other words, many engineers acquired related technical knowledge, and refined algorithms through many years of improvement, and as a result, various varieties by using a conventional color conversion table that has facilitated the development of ASIC. Advantages (for example, cost reduction) could not be obtained.

  The present invention has been proposed in order to solve the problems of the conventional techniques as described above, and has been widely used while enabling accurate color conversion and color adjustment suitable for human senses. A color conversion table and an orthogonal grid type color conversion table with high consistency, compatibility, and affinity for existing devices and systems equipped with this color conversion table, and this orthogonal grid type color conversion table are generated and color converted. A color conversion apparatus for performing color conversion, an image forming apparatus provided with image data output means in the color conversion apparatus, a color conversion program for generating the orthogonal grid color conversion table and performing color conversion, and the color An object of the present invention is to provide an image forming program for outputting image data in addition to the function of a conversion program.

  In order to achieve the above object, the color conversion apparatus according to the present invention provides an RGB value of an input device and a CMYK of the output device between an input device expressed by RGB ternary values and an output device expressed by four CMYK values. A grid point arrangement unit for generating a radial color conversion table by arranging grid points as correspondences with values uniformly in a radial direction from a predetermined achromatic color axis, and the same hue plane of the radial color conversion table And a color conversion table reorganization processing unit for generating an orthogonal lattice type color conversion table in which the lattice points form an orthogonal lattice by stacking a hue layer composed of the complementary color hue plane in the order of hue, and image information for inputting RGB format image data An input unit and a color conversion processing unit that converts input RGB format image data into CMYK format image data using the orthogonal grid color conversion table are provided.

  An image forming apparatus according to the present invention includes the color conversion device and an image information output unit that outputs CMYK format image data color-converted by the color conversion device.

  In addition, the color conversion program of the present invention enables the computer to convert the RGB value of the input device and the CMYK value of the output device between the input device expressed by RGB three values and the output device expressed by four CMYK values. The grid point arrangement unit for generating the radial color conversion table by uniformly arranging the grid points as correspondences in the radial direction from a predetermined achromatic color axis, the same hue plane of the radial color conversion table, and its complementary color A color conversion table reorganization processing unit that generates an orthogonal lattice type color conversion table in which the lattice points form an orthogonal lattice by stacking hue layers composed of hue surfaces in order of hue, an image information input unit that inputs image data in RGB format, and The input RGB image data is made to function as a color conversion processing unit that converts the image data into CMYK format image data using the orthogonal grid color conversion table. A.

  The image forming program of the present invention has the function of the color conversion program, and causes the computer to output color-converted CMYK format image data by the function of the color conversion program.

  According to the color conversion device, the image forming apparatus, the color conversion program, the image forming program, and the color conversion table of the present invention, it is possible to perform color conversion and color adjustment suitable for human senses, and to an existing apparatus or system. High consistency, compatibility, and affinity.

1 is a block diagram illustrating a configuration of a color conversion apparatus and an image forming apparatus according to an embodiment of the present invention. It is the flowchart which showed the production | generation method of the orthogonal grid type | mold color conversion table which concerns on this embodiment. It is a figure of the orthogonal lattice which shows matching with the RGB value and CMY value in the color space of a three-dimensional orthogonal coordinate system. Color gamut obtained by dividing N (N = 4) between 6 adjacent points of Rd (red), Gr (green), Bl (blue), Cy (cyan), Mg (magenta), Ye (yellow) It is a figure which shows a saturation edge point group. It is a figure which shows the closed area | region (rectangular area | region BkPiWhPi ') formed by vector BkPi and vector BkPi'. It is a figure which shows the lattice point group arrange | positioned in the crossing position when the square area of BkPiWhPi 'is divided into M (M = 4). It is a figure which shows the orthogonal lattice (radial color conversion table) by which the lattice point of 325 is arrange | positioned. It is a figure which shows arrangement | positioning of the lattice point when seeing FIG. 7 from the direction of an achromatic color axis. FIG. 9 is a diagram in which an ID is assigned for each hue angle in the hue circle illustrated in FIG. 8. It is a figure when each hue layer is seen from the diagonal direction of the Wh side or Bk side of an achromatic color axis. It is the figure which represented each hue layer for every ID. It is the figure which looked at one hue layer from the achromatic color axis direction. It is the figure which looked at the hue layer shown in FIG. 12 from the diagonal direction. It is the figure which arranged each hue layer in ID order (hue order). It is a figure which shows an orthogonal lattice type | mold color conversion table. It is a figure which shows the first hue layer and the last hue layer. 5 is a flowchart illustrating a color conversion method in the color conversion apparatus or the image forming apparatus according to the present embodiment. It is the flowchart which showed the 1st interpolation method for calculating | requiring the output color value of the to-be-converted point. A diagram showing a hue plane Sa to which the conversion point A belongs, a virtual lattice point group Sa _{ij on} the hue plane Sa, and two hue layers Sn and Sn + 1 sandwiching the conversion point A (hue plane Sa) in the hue section. It is. 4 is a diagram for explaining a method of specifying an inclusion region of a conversion point A. FIG. It is the figure which showed the positional relationship in the area | region block BCDE of the to-be-converted point A. 5 is a diagram for explaining a method of specifying a triangular region including a conversion point A. FIG. It is a figure for demonstrating the method of calculating | requiring the to-be-converted point A based on the triangular area | region containing the to-be-converted point A. FIG. It is the flowchart which showed the 2nd interpolation method for calculating | requiring the output color value of the to-be-converted point. It is a figure for demonstrating the cube (8 points) interpolation method (a) and the triangular prism (6 points) interpolation method. It is a figure for demonstrating the method of calculating | requiring the to-be-converted point A based on mapping point A'n and A'n + 1. It is a schematic diagram of a color conversion table of RGB-CMYK represented by an orthogonal grid of a three-dimensional orthogonal coordinate system. It is a figure for demonstrating the problem of the conventional color conversion table. FIG. 5 is a grid point arrangement diagram when a radial color conversion table is viewed from the achromatic color axis direction.

(Color conversion device, image forming device)
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram illustrating a configuration of a color conversion apparatus and an image forming apparatus according to an embodiment of the present invention.
As illustrated in FIG. 1, the image forming apparatus 11 according to the present exemplary embodiment includes a color conversion apparatus including an image information input unit 101, a grid point arrangement unit 102, a color conversion table reorganization processing unit 103, a color conversion processing unit 104, and a storage unit 105. 10 and the image information output unit 106.
Hereinafter, each component will be described.

  The image information input unit 101 inputs image data such as a full color image. Specifically, RGB format image data is input from an external host computer (not shown), read and acquired by a scanner (not shown), or from a portable storage device such as a USB memory (not shown). Has the role of taking out.

The grid point arrangement unit 102 is a grid as an association between the RGB value of the input device and the CMYK value of the output device between the input device expressed by RGB ternary values and the output device expressed by four CMYK values. The points are evenly arranged in the radial direction from a predetermined achromatic color axis to generate a radial color conversion table Ta.
Specifically, in color conversion from RGB values to CMYK values, Rd (red), Gr (green), Bl (blue), Cy (cyan), Mg (magenta), Ye (yellow), Wh (white) , Bk (black) with a cubic grid line segment RdYe, line segment YeGr, line segment GrCy, line segment CyBl, line segment BlMg, and line segment MgRd divided into equal intervals to obtain a predetermined color A gamut saturation edge point is generated, and this gamut saturation edge point is divided into a combination of a saturation saturation point P and a complementary color saturation saturation point P ′ where a complementary color relationship is established, and for each combination, from BkPWhP ′ A radial color conversion table Ta is generated by arranging grid points at intersections when the line segment BkP and the line segment BkP ′ of the quadrangular region are divided at equal intervals.
The method for generating the radial color conversion table Ta will be described in detail in the corresponding section (step 1) of “Method for generating orthogonal grid color conversion table” described later.

The color conversion table reorganization processing unit 103 stacks the hue layers composed of the same hue surface and the complementary hue surface of the radial color conversion table Ta in the order of hue, and the orthogonal lattice type color conversion table Tb in which lattice points form an orthogonal lattice. Is generated.
That is, in the grid point group generated by the grid point arrangement unit 102, the grid point of the same hue plane and its complementary hue in the color conversion table that includes the achromatic axis and uses this as a common axis and converts RGB values into CMYK values. After grouping the group into one group, a hue layer is formed by combining each group with a set of lattice points on the hue of interest and its complementary color, and after disassembling and taking these out, The hue layers are layered in order of hue.
By doing so, the arrangement of the grid points that are radial with respect to the achromatic color axis is reconfigured to match the arrangement of the grid points of the color conversion table in the conventional orthogonal grid form.

When the hue layer composed of the combination of the target hue and its complementary hue is hierarchically stacked in order of hue, that is, when the hue angles are hierarchically stacked in the order of hue clockwise or counterclockwise, the hue angle is Since the entire circumference is 360 °, all the lattice points can be covered by stacking each hue layer by about 180 ° rotation.
In addition, a hue layer composed of a grid point group of a matrix formed by inverting the coordinates of the grid points in the row direction and the grid points in the column direction in the first hue layer (the hue layer composed of the first hue of interest and its complementary hue) It can be a final hue layer. For this reason, it is possible to reduce a calculation load related to generation of the final hue layer (see FIG. 16).

  The color conversion processing unit 104 converts the RGB format image data input by the image information input unit 101 into CMYK format image data using the orthogonal grid color conversion table Tb.

  The storage unit 105 includes a storage medium such as a memory and a hard disk. The storage unit 105 stores a radial color conversion table Ta and an orthogonal grid color conversion table Tb, or a CMYK format generated by the color conversion processing of the color conversion processing unit 104. Temporarily store image data.

The image information output unit 106 is configured by a so-called print engine or the like, and executes print processing of CMYK format image data stored in the storage unit 105 of the color conversion apparatus 10.
For example, when image data per page is stored in the storage unit 105, an exposure process is performed on a pre-charged photosensitive drum (not shown) to form a latent image of the image data, and toner is attached thereto. A printing process is executed through a process of forming a toner image, transferring and fixing the toner image on a printing paper or the like.
The image information output unit 106 also corresponds to a transmission device that outputs image data to an external device such as another image forming device.

(Generation method of orthogonal grid type color conversion table)
Next, a method for generating the orthogonal grid color conversion table Tb according to the present embodiment will be described.
FIG. 2 is a flowchart showing a method for generating an orthogonal grid color conversion table according to the present embodiment.
As shown in FIG. 2, first, a generation process of a radial grid point group (radial color conversion table Ta) is performed (step 1), and then a generated radial grid point group (radial color conversion table Ta) is decomposed. (Step 2), and a hierarchical structuring process is performed by stacking lattice point groups layer by layer (step 3), thereby generating an orthogonal lattice color conversion table Tb.

(Step 1: Generation of radial grid point group (radial color conversion table Ta))
In step 1, the grid point arrangement unit 102, between the grid points of the six reference colors, that is, the line segment RdYe, the line segment YeGr, the line segment GrCy, the line segment CyBl, the line segment BlMg, and the line segment MgRd. Is divided into equal intervals to increase the number of grid points (color gamut saturation edge points).
FIG. 3 is a diagram of an orthogonal grid showing correspondence between RGB values and CMY values in the color space of the three-dimensional orthogonal coordinate system.
The lattice point arrangement unit 102 has six adjacent points of Rd (red), Gr (green), Bl (blue), Cy (cyan), Mg (magenta), and Ye (yellow) with respect to the orthogonal lattice shown in FIG. A point obtained by dividing the interval into N equal parts (N is an arbitrary positive integer) to obtain a color gamut saturation edge point group, and extracting a hue angle of 180 ° from an arbitrary point (saturation saturation point P) Find a group. Alternatively, by extracting 4 points that are continuous in hue from among the 6 points, and dividing each point at a pitch of 1 / N, a point group corresponding to a hue angle of 180 ° can be obtained in the same manner. it can.
FIG. 4 shows that six adjacent points of Rd (red), Gr (green), Bl (blue), Cy (cyan), Mg (magenta), and Ye (yellow) are divided into N equal parts (N = 4). It is a figure which shows the color gamut saturation edge point group obtained by doing (that is, dividing | segmenting at equal intervals of pitch 1 / N).

Next, with black point Bk: {0,0,0} as the starting point, an arbitrary point (saturation saturation point P) in the gamut saturation edge point group and its complementary color point (complementary saturation saturation point P) Consider a closed region formed by two vectors whose end points are two points of ').
That is, consider a hue plane represented by a rectangular area composed of BkPWhP ′.
FIG. 5 is a diagram showing a closed region formed by the vector BkPi and the vector BkPi ′, that is, a quadrangular region made up of BkPiWhPi ′. Note that i is an integer corresponding to the division number N.

Then, the grid point arrangement unit 102 forms a grid point at the intersection position when the line segment BkP and the line segment BkP ′ of the quadrangular region are evenly divided, thereby generating a radial grid point group (radial color conversion table Ta). ) Is generated.
FIG. 6 shows a grid arranged at the intersection position when a quadrangular region (hue surface) composed of BkPiWhPi ′ is divided into M equal parts (M = 4) (that is, when divided at equal intervals of 1 / M pitch). It is a figure which shows a point group.
Specifically, a lattice point group on each hue plane is calculated based on the following equation (1).
Grid (x) = g · E [θ] + h · E [π + θ] (1)
(However, g and h show all the combinations of the decimal number sequence of the pitch (1 / M) from 0 to 1. M shall be a positive integer.)
The grid point arrangement unit 102 arranges all grid point groups by performing the above calculation processing for each combination of saturation saturation points Pi and complementary saturation saturation points Pi ′, that is, for possible values of i. As a result, a group of radial grid points (radial color conversion table Ta) arranged for each hue can be generated so that each grid point is evenly arranged in the radial direction around the achromatic color axis.

Here, a specific arrangement method of each grid point in the radial color conversion table Ta will be described with an example. In this example, the division number N between the six basic points and the division number M of the hue plane are N = 4 and M = 4.
Of the gamut saturation edge point group, the point group (Ex group) from which the hue angle of 180 ° is extracted can be obtained by the following equation (2).
Ex group = (1−ρ) · E [i] + ρ · E [i + 1] (2)
(However, ρ represents a numerical sequence given by a pitch 1 / N from 0 to 1.)

  For example, four points that are continuous in hue among the six basic colors are red {1,0,0}, magenta {1,0,1}, blue {0,0,1}, cyan {0,1,1 } And the adjacent points are divided by N = 4, the color gamut saturation edge point group in each line segment is calculated as shown in (i) to (iii) below.

(I) Red to magenta color gamut saturation edge point group: Ex [r-mg]
In this case, E [i] = {1,0,0}, E [i + 1] = {1,0,1}, ρ = {0,1 / 4,2 / 4,3 / 4,4 / 4} Is substituted into equation (1).
Thus, Ex [r-mg]: {1,0,0}, {1,0,1 / 4}, {1,0,1 / 2}, {1,0,3 / 4}, {1 , 0,1}.
That is, the number of gamut saturation edge points during this period is 5 (= N + 1).
(Ii) Magenta to blue color gamut saturation edge point group: Ex [mg-b]
In this case, E [i] = {1,0,1}, E [i + 1] = {0,0,1}, ρ = {0,1 / 4,2 / 4,3 / 4,4 / 4} Is substituted into equation (1). As a result, Ex [mg-b] becomes as follows.
Ex [mg-b]: {1,0,1}, {1 / 4,0,1}, {1 / 2,0,1}, {3 / 4,0,1}, {0,0, 1}
That is, the number of gamut saturation edge points during this period is 5 (= N + 1).
(Iii) Blue to cyan color gamut saturation edge point group: Ex [b-cy]
In this case, E [i] = {0,0,1}, E [i + 1] = {0,1,1}, ρ = {0,1 / 4,2 / 4,3 / 4,4 / 4} Is substituted into equation (1). Thereby, Ex [b-cy] becomes as follows.
Ex [b-cy]: {0,0,1}, {0,1 / 4,1}, {0,1 / 2,1}, {0,3 / 4,1}, {0,1, 1}
That is, the number of gamut saturation edge points during this period is 5 (= N + 1).

Accordingly, from (i) to (iii), when overlapping points occurring at the section connection location are excluded, they are combined, and a red to cyan color gamut saturation edge point group E [r corresponding to a hue angle of 180 ° is obtained. -cy] looks like this:
E [r-cy]: {1,0,0}, {1,0,1 / 4}, {1,0,1 / 2}, {1,0,3 / 4}, {1,0, 1}, {1 / 4,0,1}, {1 / 2,0,1}, {3 / 4,0,1}, {0,0,1}, {0,1 / 4,1} , {0,1 / 2,1}, {0,3 / 4,1}, {0,1,1}
That is, in this case (when N = 4), the number of gamut saturation edge points for a hue angle of 180 ° is 13 points (= 3N + 1).

  In addition to the above, in the numerical sequence setting of ρ, ex [r-mg is assumed that 0 to 1 is not a 1 / N pitch numerical sequence and 0 to (1-1 / N) is a 1 / N pitch numerical sequence. ] ', Ex [mg-b]', Ex [b-cy] 'are calculated and then combined, and the maximum saturation point of the complementary hue of red is added to obtain a hue angle of 180 ° It is possible to obtain a red to cyan color gamut saturation edge point group E [r-cy] corresponding to.

Next, the grid point arrangement unit 102 sets one point of the gamut saturation edge point group for each hue of 180 ° thus obtained and its complementary color point as an end point, and a black point as each start point. Lattice points are arranged at positions where the vectors obtained by dividing the two vectors by M into equal intervals are comprehensively added.
That is, when the division ratio is τ and the number 0 to 1 is divided at a pitch 1 / M, the number sequence is τ [4]: {0, 1/4, 2/4, 3/4 when M = 4. , 1}, and the number of grid points is 5 (= M + 1).
For example, when one point in the color gamut saturation edge point group is red {1,0,0}, the maximum saturation point of the complementary hue is {1,1,1} − {1,0,0 } = {0,1,1} (that is, cyan). Since the black point is Bk {0,0,0}, the grid point group Grid [{1,0,0}] on the hue plane composed of the red hue and the complementary hue is represented by the following equation (1). Can be expressed as:
Grid [{1,0,0}] = g ・ {1,0,0} + h ・ {0,1,1}
However, g and h are applied to all combinations for possible values of τ.

Therefore, when M = 4, the combination of g and h {g, h} is
{g, h}: {0,0}, {0,1 / 4}, {0,1 / 2}, {0,3 / 4}, {0,1}, {1 / 4,0}, {1 / 4,1 / 4}, {1 / 4,1 / 2}, {1 / 4,3 / 4}, {1 / 4,1}, {1 / 2,0}, {1/2 , 1/4}, {1 / 2,1 / 2}, {1 / 2,3 / 4}, {1 / 2,1}, {3 / 4,0}, {3 / 4,1 / 4 }, {3 / 4,1 / 2}, {3 / 4,3 / 4}, {3 / 4,1}, {1,0}, {1,1 / 4}, {1,1 / 2 }, {1,3 / 4}, {1,1}
Grid point group Grid [{1,0,0}]
Grid [{1,0,0}]: {0.25,0,0}, {0.5,0,0}, {0.75,0,0}, {1,0,0}, {0,0.25,0.25} , {0.25,0.25,0.25}, {0.5,0.25,0.25}, {0.75,0.25,0.25}, {1,0.25,0.25}, {0,0.5,0.5}, {0.25,0.5,0.5}, { 0.5,0.5,0.5}, {0.75,0.5,0.5}, {1,0.5,0.5}, {0,0.75,0.75}, {0.25,0.75,0.75}, {0.5,0.75,0.75}, {0.75, 0.75,0.75}, {1,0.75,0.75}, {0,1,1}, {0.25,1,1}, {0.5,1,1}, {0.75,1,1}, {1,1, 1}
Thus, 25 lattice points are arranged on one hue plane.

That is, (M + 1) ² lattice points are generated for one hue plane.
Similarly, the grid points are generated for each of the other color gamut saturation edge points in the range of 180 ° and arranged to form a radial grid point group (radial color conversion table Ta). It will be.
The total number of lattice points in the orthogonal lattice type color conversion table formed by stacking (3N + 1) layers with one hue plane as one hue layer can be calculated by (M + 1) · (3N + 1) ² . In the case of this example (when M = 4 and N = 4), the total number of lattice points is (4 + 1) ² · (3 × 4 + 1) = 325.

FIG. 7 is a diagram showing a state of an orthogonal lattice (radial color conversion table Ta) in which 325 lattice points are arranged, and FIG. 8 shows lattice points when FIG. 7 is viewed from the direction of the achromatic color axis. It is a figure which shows arrangement | positioning.
In other words, as shown in these figures, by devising the conversion points of each color value within the framework of the conventional orthogonal grid color conversion table, the grid points are evenly arranged radially around the achromatic color axis. A color conversion table Ta can be generated.
In this example, the division number N between the six basic points and the division number M of the hue plane are set to N = 4 and M = 4. However, these division numbers are not particularly limited to these values. It can be any value. For example, the number of divisions of N and M may be different.

However, considering that the radial conversion table Ta is formed based on the basic six colors (RGBCMY), M is preferably a multiple of six in terms of calculation, and an orthogonal lattice color conversion table Tb described later. From the viewpoint of high-speed processing performance in terms of memory configuration and circuit configuration, M is preferably a multiple of 8 in terms of operation. Further, considering these conditions together, M is a multiple of 24 in terms of operation ( More preferably, 24 is the least common multiple of 6 and 8.
When generating the cubic grid color conversion table, the number of layers (3N + 1) as the height (z) component and the number of grid points M + 1 per side of the hue plane as the vertical and horizontal (xy) components are the same. There is a need.
Therefore, in this case, when generating the radial color conversion table Ta, it is necessary to divide each side of the hue plane with M = 3N.
For example, when M = 24, the number of grids per side on one hue plane is 25 (= M + 1), and the number of grid points per hue plane is 625 (= 25 × 25).
In addition, to make this a cubic lattice, it is necessary to divide the basic 6 points by setting N to 8 (= 24 ÷ 3). Thereby, a cubic lattice composed of a lattice point group of 25 × 25 × 25 can be formed.

(Step 2: Decomposition processing of radial grid point group)
In step 2, the color conversion table reorganization processing unit 103 sets each lattice point in the radial color conversion table Ta as a hue layer composed of an arbitrary saturation saturation point P and its complementary color point (complementary saturation saturation point P ′). Disassemble the whole.
That is, one surface composed of the same hue surface and its complementary hue surface in the radial color conversion table Ta is decomposed for each hue layer as one hue layer.
FIG. 9 is a diagram in which an ID is assigned to each constant hue angle in the hue circle shown in FIG.
In FIG. 9, θ1 is assigned to the red hue, and thereafter, θ2, θ3,. . . , Θ12 and so on, IDs are added in order, θ1 + π is added to the hue of cyan, which is a complementary color of red, and thereafter, θ2 + π, θ3 + π,. . . , Θ12 + π, and so on.
Note that the hue of θ and the hue of θ + π shown in FIG. 9 are complementary to each other.

FIG. 10 is a diagram when each hue layer is viewed from an oblique direction on the Wh side or Bk side of the achromatic color axis. Note that FIG. 10 shows a state in which the hue layer is thinned out for each ID in consideration of easy visual recognition.
FIG. 11 is a diagram showing each hue layer for each ID.
The color conversion table reorganization processing unit 103 decomposes and extracts each of these hue planes from the lattice group forming the radial color conversion table Ta.
FIG. 12 is a view of one hue layer as viewed from the achromatic color axis direction, and FIG. 13 is a view of the hue layer shown in FIG. 12 as viewed from an oblique direction.
That is, as shown in these figures, in one hue layer, one hue plane related to an arbitrary hue (θi) and a hue plane related to a complementary hue (θi + π) are on the same plane. One hue layer (θi) is formed by axisymmetric alignment with the achromatic color axis.
FIG. 14 is a diagram in which the hue layers are arranged in the order of ID.
As in the present embodiment, when the division number N is 4, 13 hue layers from θ1 to θ13 can be extracted.

(Step 3: Hierarchical structure processing)
Then, the color conversion table reorganization processing unit 103 generates the orthogonal lattice color conversion table Tb in which the radial lattice point groups are hierarchically structured by stacking the extracted hue layers in the ID order (hue order).
FIG. 15 is a diagram showing an orthogonal grid color conversion table Tb.
That is, as shown in FIG. 15, the three-dimensional orthogonal lattice color conversion table Tb is generated by stacking the hue layers of S1 to S13 with IDs θ1 to θ13 in the ID order (descending order or ascending order). Can do.

FIG. 16 is a diagram illustrating an initial hue layer and a final hue layer.
As shown in FIG. 16, the first hue layer (θ1) and the final hue layer (θ13) are in a line-symmetric relationship with respect to the arrangement of the lattice points of each other with the achromatic color axis (Wh to Bk) as the symmetry axis. It holds.
For this reason, the final hue layer can be stacked by replacing only the coordinate information of the lattice point group in the first hue layer.
That is, instead of disassembling and extracting all the hue layers of θ1 to θ13, a hue layer of θ1 to θ12 is generated, and the lattice point group of the hue layer of θ1 is inverted with respect to the achromatic color axis. Thirteen hue layers θ1 to θ13 can also be generated by adding the hue layers (θ13).
In this way, it is possible to reduce processing for generating a lattice point group on the final hue layer and a processing load necessary for decomposing the generated final hue layer and extracting the lattice point group.

Here, a method of assigning the RGB value of each lattice point and the corresponding CMYK value when generating the orthogonal lattice color conversion table Tb will be described.
First, in an image forming apparatus such as a color printer that performs full color printing with four colors of CMYK, a color patch is created by combining various color values of CMYK values, and this is measured to measure CMYK values and device independence. Are associated with each other and further inversely converted to create a “correlation between chromaticity values of the device-independent color system and the corresponding CMYK values” in advance.
For example, color patches are set to CMYK values at linear equal intervals such as {0, 20, 40, 60, 80, 100}, and nonlinearly as {0, 10, 20, 40, 70, 100}. After setting CMYK values or stratifying based on the black amount, a radial grid point group based on the CMY values is generated, and various CMYKs are combined by combining the CMY values and the Bk values related to the stratified black amount. Various color patches are printed with values set, and a table is created in which CMYK values of the printed color patches are associated with device-independent chromaticity values obtained by measuring the colors.

Next, for each grid point of the radial color conversion table Ta, a chromaticity value (CIE-XYZ, CIE-Lab, etc.) of the device-independent color system corresponding to the RGB value is obtained. The chromaticity value of the device-independent color system corresponding to the RGB value is obtained by using a device profile (such as ICC Profile) that describes the characteristics and characteristics of the device from which the RGB value is derived, or the RGB value is sRGB or AdobeRGB. In the case of standard data whose specifications have already been disclosed, it can be obtained by substituting the RGB values of the respective grid points constituting the radial color conversion table according to the definition.
In addition, the color space defined by the RGB values is usually different in shape from the color space of the output device, and the mapping between the input and output devices is non-linearly coordinate-converted on the device-independent color space. Therefore, the value after being converted into the non-linear coordinates in this way can also be defined as the above-mentioned “chromaticity value of the device-independent color system corresponding to the RGB value”.

The “RGB values and corresponding chromaticity values of the device-independent color system” calculated for the grid points of the radial color conversion table Ta are the “chromaticity values of the device-independent color system” The CMYK value is obtained by substituting it into the “corresponding CMYK value association” and converting it, and performing interpolation when necessary.
Then, by associating the obtained CMYK values with the RGB values at the lattice points of the original radial color conversion table Ta, the assignment of the RGB values and the CMYK values at the corresponding lattice points of the orthogonal lattice type color conversion table Tb is completed. To do.

As described above, according to the color conversion device and the image forming apparatus of the present embodiment, while maintaining the characteristics of the radial color conversion table Ta based on the lightness, saturation, and hue components, this is converted into a conventional orthogonal lattice shape. The color conversion is performed using the orthogonal grid type color conversion table Tb adapted to the format of the color conversion table.
For this reason, accurate color reproduction and color adjustment that match the sense of human color can be achieved, and high consistency, compatibility, and affinity with existing devices and systems are achieved.

(Color conversion method)
Next, a color conversion method in the above-described color conversion apparatus or image forming apparatus will be described.
FIG. 17 is a flowchart illustrating a color conversion method in the color conversion apparatus or the image forming apparatus according to the present embodiment.
As shown in FIG. 17, first, the image information input unit 101 performs input processing of RGB format image data (step 11).

Next, when converting the RGB value of each pixel of the input image data into a CMYK value, the color conversion processing unit 104 specifies the region to which the pixel of interest belongs (step 12), and performs interpolation using the specified region to be assigned. The color conversion of the target pixel is performed by calculation (step 13).
The CMYK values obtained through the color conversion by interpolation are stored in the storage unit 105 (step 14).
When CMYK values for a certain number of pixels (for example, pixels for a unit page) are stored, the image information output unit 106 extracts the image data from the storage unit 105 and outputs it.

(Interpolation process)
Here, the interpolation processing in steps 12 to 13 will be described in detail.
Here, the first interpolation method and the second interpolation method will be described in detail as specific interpolation processing methods.
These interpolation methods enable color conversion processing of non-conversion points regardless of whether the conversion points coincide with the respective lattice points of the orthogonal lattice type color conversion table Tb. Thereby, it is possible to smoothly perform the color conversion process without going through the determination step of whether or not the non-conversion points coincide with the grid points.

(First interpolation method)
FIG. 18 is a flowchart showing a first interpolation method for obtaining the output color value of the target pixel which is the conversion point.
As shown in FIG. 18, here, first, two hue layers sandwiching the conversion point A in the hue section (adjacent hue layers, ie, radial hues in the vicinity of each of the clockwise and counterclockwise rotations of the conversion point A). A virtual lattice point group corresponding to the lattice point group on the hue plane of the conversion point A is obtained based on the coordinates of the lattice point group on the surface), the hue angles thereof, and the hue angle of the conversion point A (see FIG. Step 21).
FIG. 19 shows a hue surface Sa to which the conversion point A belongs, a virtual lattice point group Sa _{ij on} the hue surface Sa, and two hue layers Sn and Sn + sandwiching the conversion point A (hue surface Sa) in the hue section. FIG.
As shown in FIG. 19, the virtual lattice point (group) Sa _ij is regarded as a lattice point group existing on the hue plane Sa sandwiched between the hue layer Sn + 1 and the hue layer Sn + 1. Can do.

Therefore, the virtual lattice point group Sa _{ij on} the hue plane Sa of the conversion point A is divided into the lattice point group Sn _ij on the hue layer Sn, the lattice point group Sn + 1 _ij on the hue layer Sn + 1, and the hue layer Sn. Based on the hue angle Δφ _n with respect to the hue plane Sa and the hue angle Δφ _{n + 1} between the hue layer Sn + 1 and the hue plane Sa, it can be calculated by the following equation (3).
Sa _ij = Sn + 1 _ij · Δφ _n / (Δφ _n + Δφ _{n + 1} ) + Sn _ij · Δφ _{n + 1} / (Δφ _n + Δφ _{n + 1} )
(3)
In the above equation (3), Δφn and Δφn + 1 can be obtained by the following equation (4).
{Δφ _n , Δφ _{n + 1} } = {| θn−θa |, | θa−θn + 1 |} (4)
Incidentally, in terms of accuracy to _{{Δφ n, Δφ n + 1} } is sufficiently not small, sinΔφ _n ≒ Δφ _n, etc. If not regarded as _{sinΔφ n + 1 ≒ Δφ n +} 1, when obtaining greater accuracy Can calculate Sa _ij by the following equation (5).
Sa _ij = Sn + 1 _ij · HSn _ij · sin Δφ _n / (HS _{n ij} · sin Δφ _n + HSn + 1 _ij · sin Δφ _{n + 1} ) + Sn _ij HSn + 1 · sin Δφ _{n + 1} / (HSn · sin Δφ _n + HSn + 1 · sinΔφ _{n + 1} )
(5)
(However, H is a point corresponding to the foot of a perpendicular line drawn from the lattice point included in the lattice point group Sn _ij , Sn + 1 _ij to the achromatic color axis, and HSn _ij , HSn + 1 _ij is the achromatic color axis. (The distance from the upper point H to each grid point is shown.)

Next, on the hue plane Sa, BkA ↑ = p · BkP ↑ + q · BkP ′ ↑ (6)
A component ratio {p, q} is obtained (step 22). (However, ↑ indicates a vector. The same applies hereinafter.)
The component ratio {p, q} can be calculated by establishing the equation (6) for each channel component and solving it simultaneously. Here, it is assumed that the component ratio {p, q} = {0.7, 0.35} is calculated.
Subsequently, the component ratio {p, q} is divided by the number of divisions on one side of the hue layers Sn and Sn + 1 to extract the integer part and the fraction part (step 23), and the converted point is obtained using the integer part and the fraction part. The inclusion area of A is specified (steps 24 and 25).

FIG. 20 is a diagram for explaining a method of specifying the inclusion region of the conversion point A.
Here, a method for specifying the inclusion region of the conversion point A in the rectangular region BkPWhP ′ in an arbitrary hue layer will be described.
As shown in FIG. 20, since each side of the rectangular region of the hue layer is divided at a division pitch (1 / M = 1/4), the component ratio {0.7, 0.35} is set at this division pitch. By dividing, the integer part Ip and the fractional part Fp and the integer part Iq and the fractional part Fq relating to q are calculated.
As a result, Ip = 2, Fp = 0.8, Iq = 1, and Fq = 0.4 are calculated.
Here, the integer part indicates how many area blocks are counted from Bk, and the decimal part indicates the inter-lattice division ratio of the conversion point A in the included area block.

Therefore, when p = Ip + Fp and q = Iq + Fq, the conversion point A exists in the {Ip, Iq} = {2,1} -th region block BCDE from Bk, and the region block In BCDE, it can be recognized that the lattice points are arranged at positions to be divided at a division ratio of {Fp, Fq} = {0.8, 0.4}.
That is, this makes it possible to specify the inclusion region (affiliation region) of the non-conversion point A.

Then, an output color value (CMYK value) of the conversion point A is obtained based on the specified inclusion area (step 26).
Specifically, since the positional relationship of the conversion point A in the area block BCDE is as shown in FIG. 21, the output color value of the conversion point A is the grid points B, C, D, Based on the CMYK value for E, it can be calculated by the following equation (7).
A = (1-Fp). (1-Fq) .B + Fp. (1-Fq) .C + Fp.Fq.D + (1-Fp) .FqE (7)

The output color value of the conversion point A can be obtained by other methods.
First, after specifying the inclusion area block {Ip, Iq} of the conversion point A, the triangle area where the conversion point A exists is specified by comparing the magnitudes of Fp and Fq.
Specifically, when Fp ≦ Fq, it can be determined that the conversion point A is included in the ΔBDC region, and when Fp> Fq, the conversion point A is included in the ΔBDE region.
For example, as shown in FIG. 22, when Fp ≦ Fq, the triangular region including the conversion point A can be specified as ΔBDC.
Then, the CMYK value of the conversion point A can be calculated from the information of the three lattice points constituting the triangular area specified by such determination and the ratio of the conversion point A in the triangular area divided in area. it can.

FIG. 23 is a diagram for explaining a method for obtaining the conversion point A based on a triangular area including the conversion point A. FIG.
Specifically, if the triangle (region) that includes the conversion point A is ΔBCD, the triangle B is the three triangles that have the opposite side of each vertex of ΔBCD as the bottom and the conversion point A as the vertex. The coordinates of the conversion point A can be obtained by obtaining the area ratio, multiplying them by the vertex coordinates, and then combining them.
That is, the output color value of the conversion point A can be calculated by the following equation (8) based on the CMYK values relating to the respective grid points B, C, and D that are known coordinates.
A = α · B + β · C + γ · D (8)
(Where α = ΔACD area / ΔBCD area, β = ΔABD area / ΔBCD area, γ = ΔABC area / ΔBCD area)
The area of the triangle can be obtained by, for example, obtaining two vectors starting from one of the three points and ending with the remaining two points, and multiplying these outer products by 0.5. It is also possible to find the length of each side and apply the Heron formula.

(Second interpolation method)
FIG. 24 is a flowchart showing the second interpolation method.
Here, as shown in FIG. 25, the cube interpolation method (a) using eight points corresponding to the vertices of the rectangular parallelepiped or the cube including the conversion point A, or the diagonal direction of the rectangular parallelepiped or the cube is divided. The converted point A can be obtained based on a triangular prism interpolation method using six points corresponding to the vertices of the triangular prism that includes the converted point A among the possible triangular prisms.
That is, the mapping points A′n, A of the conversion point A in the adjacent hue layers Sn, Sn + 1 (that is, radial hue surfaces in the vicinity of the conversion point A in the clockwise and counterclockwise directions) sandwiching the hue surface Sa. An arrangement relationship between a rectangular area or triangle area including 'n + 1 and the conversion point A is obtained, and the conversion point A is obtained based on the internal ratio of Sa in Sn to Sn + 1.

As shown in FIG. 24, here, first, the maximum saturation of the hue of the conversion point A is obtained (step 31).
The maximum saturation point (saturation saturation point) P of the hue of the conversion point A (that is, the same hue as the non-conversion point A) can be calculated by the following equation (9).
P = (A−Min [A]) / Max [(A−Min [A])] (9)
(However, Min [A] represents the minimum value of the three components of the non-transformation point A: {Ax, Ay, Az} coordinates. A-Min [A] represents each coordinate of the transformation point A. The minimum value of the three components of the transformation point A coordinate is subtracted from the component, and Max [(A−Min [A])] is the maximum value of the three components of A−Min [A]. Show.)
Next, a complementary color having the maximum saturation obtained in the previous step is obtained (step 32).
The complementary color point (complementary color saturation saturation point) P ′ can be calculated by the following equation (10).
P ′ = {1,1,1} −P (10)
For example, the complementary color point P ′ when the maximum saturation point P = {1,0,0} (red) is P ′ = {1,1,1} − {1,0,0} = {0,1 , 1} (cyan).
Subsequently, a component ratio {p, q} such that BkA ↑ = p · BkP ↑ + q · BkP ′ ↑ is obtained (step 33), and the component ratio {p, q} is divided into one side of the hue layers Sn and Sn + 1. Divide by the number (M) to extract the integer part and the fractional part (step 34).
Steps 33 to 34 correspond to steps 22 to 23 in FIG. For this reason, detailed description of each of these steps is omitted.

Next, the region to which the mapping points A′n and A′n + 1 of the conversion point A in the two hue layers Sn and Sn + 1 sandwiching the conversion point A in the hue section belongs is specified (step 35).
This identification method uses the same method as the “method for identifying the inclusion region of the conversion point A” (steps 23 to 25) in the first interpolation method described above.
That is, the integer part obtained in step 34 {Ip, Iq} is used to identify the domain block B _n C _n D _n E _n includes mapping point A'n in the hue layer Sn.
Similarly, a region block B _{n + 1} C _{n + 1} D _{n + 1} E _{n + 1 including the} mapping point A′n + 1 in the hue layer Sn _{+ 1} is specified.
This makes it possible to obtain vertex 8 points required cubic interpolation method _{(B n C n D n E} n, B n + 1 C n + 1 D n + 1 E n + 1).
In the case of using the triangular prism interpolation method, a method of specifying the above-described triangular region with 6 points (B _n C _n D _n , B _{n + 1} C _{n + 1} D _{n + 1} ) corresponding to the apexes of the triangular prism (FIG. 22). And the corresponding explanation).

Subsequently, mapping points A′n and A′n + 1 are obtained (step 36).
The calculation of the mapping points A′n and A′n + 1 uses the same method as the method for obtaining the output color value of the conversion point A in the first interpolation method described above (see Expression 7).
That, B _n, and C _n, D _n, CMYK values are pre-assigned to E _n, the fractional part {Fp, Fq} the output color values of mapping point An 'using the following equation using the (11) Ask.
A'n = (1-Fp) · (1-Fq) · B n + Fp · (1-Fq) · C n + Fp · Fq · D n + (1-Fp) · Fq · E n ····· (11)
Similarly, the output color value of the mapping point A′n + 1 is obtained using the following equation (12).
A'n + 1 = (1-Fp) * (1-Fq) * _{Bn + 1} + Fp * (1-Fq) * Cn _{+ 1} + Fp * Fq * Dn _{+ 1} + (1-Fp) * Fq・ E _{n + 1} (12)

Based on the mapping points A′n and A′n + 1, the output color value of the conversion point A is obtained (step 37).
FIG. 26 is a diagram for explaining a method of obtaining the conversion point A based on the mapping points A′n and A′n + 1.
As shown in FIG. 26, the conversion point A is composed of a mapping point A′n on the hue layer Sn and a mapping point A′n + 1 on the hue layer Sn + 1 when viewed from the achromatic color axis direction. It can be regarded as an intersection where the straight line and the hue plane Sa intersect.

Therefore, the output color value of the conversion point A can be calculated based on the following equation (13) using the output color values of the mapping points A′n and A′n + 1.
A = (1−εl) · A′n + εl · A′n + 1 (13)
(However, the _{εl = HnA'n · sinΔφ n / (} HnA'n · sinΔφ n + Hn + 1A'n + 1 · sinΔφ n + 1), Hn is drawn from the mapping point A'n to the achromatic axis perpendicular line , Hn + 1 represents a perpendicular foot drawn from the mapping point A′n + 1 to the achromatic axis.)
Alternatively, the output color value of the conversion point A can be calculated by the following equation (14).
A = (1−εo) · A′n + εo · A′n + 1 (14)
(However, the _{εo = Δφ n / Δφ n +} Δφ n + 1.)

Note that sinΔφ _n ≈ when there is a margin in calculation time, such as when the hue (angle) difference between two hue layers sandwiching the conversion point A is relatively large, or when it is desired to pursue calculation accuracy. In the case where Δφ _n and sin Δ _{n + 1} = Δ _{n + 1} cannot be considered, the above equation (13) may be used.
On the other hand, if the difference in hue (angle) between the two hue layers sandwiching the conversion point A is sufficiently narrow, an error can be allowed for conversion accuracy, and sinΔφ _n is preferred when high-speed arithmetic is prioritized over conversion accuracy. ≒ [Delta] [phi _n, if it can be regarded as _{sinΔφ n + 1 ≒ Δφ n +} 1 is preferably used the above equation (14).

As described above, according to the second interpolation method, the conversion point A can be obtained based on the eight lattice points in the cubic interpolation method and the six lattice points in the triangular prism interpolation method.
Therefore, it is easy to specify the inclusion region of the non-conversion point A, and the memory consumption can be reduced due to the simplicity of the flow.
Therefore, it is possible to realize a higher-speed color conversion process than the first interpolation method for calculating the converted point A after calculating a large number of virtual lattice point groups.

(Color conversion program, image forming program)
Next, a color conversion program and an image forming program will be described.
The color conversion function of the computer (color conversion apparatus) in the above embodiment is realized by a color conversion program stored in a storage unit (for example, a ROM or a hard disk).
In addition, the color conversion function and the output function of the computer (image forming apparatus) in the above embodiment are realized by an image forming program stored in a storage unit (for example, a ROM or a hard disk).

The color conversion program is read by a computer control means (CPU (Central Processing Unit) or the like), and sends instructions to each component of the computer to perform predetermined processing such as image data input processing, grid point arrangement processing, A color conversion table reorganization process, a color conversion process, a storage process, and the like are performed. In addition to these processes, the image forming program causes image data output processing to be performed.
As a result, the color conversion function and the image forming function are realized by the cooperation of each component of the computer (color conversion apparatus, image forming apparatus) which is a hardware resource and the color conversion program or image forming program as software. Is done.

Note that the color conversion program for realizing the color conversion function and the image formation program for realizing the image forming function including the color conversion function are stored in a computer ROM, a hard disk or the like, or a computer-readable record. It can be stored in a medium such as an external storage device and a portable recording medium.
The external storage device is a memory expansion device that incorporates a recording medium such as a CD-ROM (Compact Disc-Read Only Memory) and is externally connected to a color conversion device or an image forming apparatus. On the other hand, the portable recording medium is a recording medium that can be mounted on a recording medium driving device (drive device) and is portable, and refers to, for example, a flexible disk, a memory card, a magneto-optical disk, and the like.

Then, the program recorded on the recording medium is loaded into a RAM (Random Access Memory) or the like of the computer and executed by the CPU (control means). By this execution, the functions of the color conversion device and the image forming apparatus of the above-described embodiment are realized.
Further, when a color conversion program or an image forming program is loaded by a computer, these programs held by another computer can be downloaded to its own RAM or an external storage device using a communication line. The downloaded program is also executed by the CPU, and realizes the color conversion function of the color conversion device or image forming apparatus of the above-described embodiment.

As described above, according to the color conversion apparatus, the image forming apparatus, the color conversion program, the image forming program, and the color conversion table (orthogonal grid type color conversion table) of the present embodiment, the characteristics (brightness and saturation) of the radial color conversion table are used. (Hue), in particular, while maintaining the continuity of the hue, an orthogonal grid color conversion table adapted to the conventional orthogonal grid color conversion table is configured, and color conversion processing is performed using this. .
In addition, the color value related to the non-conversion point can be converted through various interpolation processes regardless of whether or not it matches the grid point of the color conversion table.
For this reason, it is possible to accurately perform color conversion and color adjustment in accordance with the sense of human color such as hue and hue, and has high consistency, compatibility, and affinity with existing devices and systems.

That is, according to the present embodiment, grid points can be arranged along a direction in which human senses are sensitive without arranging many grid points in the color space. The conversion processing speed is increased, and the color conversion process can be performed even with an inexpensive control device.
Furthermore, it is easy to become familiar with devices, ASICs, and programs that use an orthogonal grid color conversion table that exists in many existing systems. In addition, it is easy to divert to an existing system, and a part of the existing system can be diverted, or the existing system can be renewed to a control system with high hue preservation. This helps prevent product deterioration and prolongs product life and maintains value.

As described above, the color conversion apparatus, the image forming apparatus, the color conversion program, the image forming program, and the color conversion table according to the present invention have been described with reference to the preferred embodiments. However, the present invention is limited only to the above-described embodiments. It goes without saying that various modifications can be made within the scope of the present invention.
For example, as an interpolation method for obtaining the conversion point A, tetrahedral interpolation performed based on a triangular pyramid formed by dividing a triangular prism that can be formed by dividing a rectangular parallelepiped or cube including the conversion point A along a diagonal direction, In a case where interpolation calculation cannot be performed in a space including the conversion point A, extrapolation using neighboring grid points can be employed.

  The present invention can be suitably used for an image forming apparatus such as a color printer.

DESCRIPTION OF SYMBOLS 1 Image processing apparatus 10 Color conversion apparatus 101 Image information input part 102 Grid point arrangement | positioning part 103 Color conversion table reorganization processing part 104 Color conversion processing part 105 Storage part 106 Image information output part Ta Radial color conversion table Tb Orthogonal lattice type color conversion table

Claims (7)

A grid point as a correspondence between the RGB value of the input device and the CMYK value of the output device between the input device expressed in RGB ternary and the output device expressed in CMYK quaternary A grid point arrangement unit that generates a radial color conversion table by arranging them uniformly in the radial direction from the axis;
A color conversion table reorganization processing unit for generating an orthogonal lattice type color conversion table in which the lattice points form an orthogonal lattice by stacking a hue layer composed of the same hue surface of the radial color conversion table and its complementary hue surface in order of hue; ,
An image information input unit for inputting image data in RGB format;
A color conversion apparatus comprising: a color conversion processing unit that converts input RGB format image data into CMYK format image data using the orthogonal grid color conversion table. The lattice point arrangement unit is
In color conversion from RGB values to CMYK values, Rd (red), Gr (green), Bl (blue), Cy (cyan), Mg (magenta), Ye (yellow), Wh (white), Bk (black) A predetermined color gamut saturation edge point is generated by dividing the line segment RdYe, the line segment YeGr, the line segment GrCy, the line segment CyBl, the line segment BlMg, and the line segment MgRd of the orthogonal lattice with each as a vertex at equal intervals. And
The color gamut saturation edge point is divided into a combination of a saturation saturation point P and a complementary saturation saturation point P ′ where a complementary color relationship is established,
The radial color conversion table is generated for each combination by arranging the grid points at intersections obtained by dividing a line segment BkP and a line segment BkP 'of a quadrangular region composed of BkPWhP' at equal intervals. The color conversion device described. The color conversion processing unit
For the pixels constituting the input RGB format image data, a virtual grid point group corresponding to the grid points on the adjacent hue layer sandwiching the hue of the pixel is arranged on the hue plane of the pixel, and the virtual grid points Identifying the belonging region that includes the pixel among the small regions formed by the group, and determining the CMYK value of the pixel based on the positional relationship of the pixel in the belonging region,
The color conversion apparatus according to claim 1, wherein the RGB value of the pixel is converted into the calculated CMYK value. The color conversion processing unit
For a pixel constituting the input RGB format image data, an assigned region including a mapping point of the pixel on an adjacent hue layer sandwiching the hue plane of the pixel is specified, and the positional relationship of the mapped point in the assigned region is determined. Based on the CMYK value of the mapping point, the CMYK value of the pixel is determined based on the CMYK value of the mapping point and the hue ratio between the hue plane of the pixel and the adjacent hue layer,
The color conversion apparatus according to claim 1, wherein the RGB value of the pixel is converted into the calculated CMYK value. An image forming apparatus including a predetermined color conversion device and an image information output unit for outputting CMYK format image data color-converted by the color conversion device.
The image forming apparatus according to claim 1, wherein the color conversion device is a color conversion device according to claim 1. Computer
A grid point as a correspondence between the RGB value of the input device and the CMYK value of the output device between the input device expressed in RGB ternary and the output device expressed in CMYK quaternary Lattice point arrangement unit that generates a radial color conversion table by arranging them uniformly in the radial direction from the axis,
A color conversion table reorganization processing unit for generating an orthogonal lattice type color conversion table in which the lattice points form an orthogonal lattice by stacking a hue layer composed of the same hue surface of the radial color conversion table and a complementary hue surface thereof in order of hue;
An image information input unit that inputs RGB format image data, and a color conversion processing unit that converts the input RGB format image data into CMYK format image data using the orthogonal grid color conversion table. A color conversion program characterized by An image forming program that has a function of a predetermined color conversion program and causes the computer to output color-converted CMYK format image data by the function of the color conversion program.
The image forming program according to claim 6, wherein the color conversion program is a color conversion program according to claim 6.