JP2002369023A - Color conversion table generator and color converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processor that can attain simple gamut mapping with excellent outward appearance. SOLUTION: A gamut in the L*a*b* space is deformed in a direction of the saturation and a point at the outside of gamut is mapped to the point in the closest point in the gamut. Using the deformed gamut in this way can avoid concentrating of mapping on a peak of the gamut.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color conversion table creation device and a color conversion device, and more particularly to a color conversion table creation device and a color conversion device capable of performing better gamut (color gamut) mapping.

[0002]

2. Description of the Related Art For example, when an image on a display in a computer system is printed by a printer, RGB (red, green, blue) signals output to the display are converted into CMYK (cyan, magenta) used for printer output. , Yellow, black)
It needs to be converted to a signal. Since both the RGB signal of the display and the CMYK signal of the printer depend on the characteristics of the device, it is necessary to express colors once in a color system independent of the characteristics of the device in order to perform accurate color conversion.

That is, once the RGB signal is converted into a color system such as L ^* a ^* b ^* which does not depend on the characteristics of the device, it is further converted into a CMYK signal.

In general, the range of colors that can be reproduced differs between a display and a printer. Therefore, an operation (gamut compression) for replacing a color that cannot be expressed by the printer with a color that can be expressed is required.

[0005] As a conventional technique for gamut compression,
For example, JP-A-10-84487 can be mentioned.
This is a method of performing conversion such that the color difference between the color data of the image before conversion and the color data of the image after conversion is reduced.

FIG. 16 shows a specific method of gamut compression.
It is a diagram for explaining, L^*a^*b ^*L space^*Axis direction
FIG. In the figure, the area indicated by hatching is
Indicates gamut.

When there is image data to be represented at the position of the starting point of the arrow in FIG. 16, the data is out of the gamut, and therefore, it is necessary to convert the data into data in the gamut. Here, by moving the position indicated by the image data from the start point of the arrow to the end point, a color that cannot be expressed is converted into a color that can be expressed.

Here, in order to minimize the color difference, the end point of the arrow is set so that the distance from the start point to the end point of the arrow is the shortest.

[0009]

However, such a gamut compression method has a problem called a blue shift and a lightning rod effect. Here, the blue shift is a phenomenon in which a vivid blue outside the gamut is preferably converted to a color that is biased toward cyan, but is not so converted, and is converted to purple with a small color difference. As shown in FIG. 17, the lightning rod effect is a phenomenon in which the converted data is concentrated on the sharp portion of the gamut.

Various methods have been studied to avoid such a problem (for example, see Japanese Patent Application Laid-Open No. 10-2585).
48, JP-A-10-290375).

However, in order to adopt such a method, it is necessary to accurately determine the boundary between the inside and outside of the gamut. In addition, processing outside the gamut and processing inside the gamut are separate, and the amount of calculation increases. Furthermore, it is necessary to appropriately set the constraint conditions so that the inside of the gamut and the outside of the gamut do not become discontinuous. However, since the boundary surface to which the constraint condition is applied is a three-dimensional free-form surface, it is difficult to actually set the boundary surface. There's a problem.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and has as its object to provide a color conversion table creating apparatus and a color conversion apparatus capable of performing simple and good-looking conversion.

[0013]

According to an aspect of the present invention, there is provided a color conversion table creating apparatus for printing a plurality of types of colors using color data in a first color space. Obtaining means for obtaining color data of the printed color in the second color space by measuring the color of the printed color, and conversion for deforming the second color space. Conversion means for performing color data conversion; and creation means for creating a table in which color data in the first color space where printing has been performed and color data converted by the conversion means are associated one-to-one. Is provided.

Preferably, the first color space is a space for expressing a device-dependent color, and the second color space is a space for expressing a device-independent color.

[0015] Preferably, the first color space is a color space represented by CMYK, and the second color space is L ^* a ^* b ^*.
Color space.

According to another aspect of the present invention, a color conversion table creating method includes a printing step of printing a plurality of types of colors using color data in a first color space, and measuring the printed colors. Obtaining the color data of the printed color in the second color space by:
A conversion step of performing a conversion for deforming the second color space on the acquired color data; a color data in the first color space on which printing is performed; and a color data converted in the conversion step. And a creating step of creating a table in which the tables are associated with each other on a one-to-one basis.

According to still another aspect of the present invention, a computer-readable recording medium records a color conversion table created by the above-described color conversion table creating method.

According to still another aspect of the present invention, a program for creating a color conversion table includes a printing step of printing a plurality of types of colors using color data in a first color space, and a color measurement of the printed colors. Thereby, an obtaining step of obtaining color data of the printed color in the second color space, a conversion step of performing conversion for deforming the second color space on the obtained color data, The computer is configured to execute a creation step of creating a table in which color data in the first color space in which printing has been performed and the color data converted in the conversion step are associated with each other on a one-to-one basis. .

According to still another aspect of the present invention, a computer-readable recording medium records the above-described color conversion table creating program.

According to still another aspect of the present invention, a color conversion apparatus comprises: a first color space data expressing a device-dependent color; and a second color space obtained by transforming a device-independent color space. Storage means for storing a table indicating a correspondence relationship with the data of the second color space, input means for inputting color data in a device-independent color space, and color data input by the input means as color data in a second color space. Conversion means for converting;
Reading means for reading data in the first color space corresponding to the converted color data from the storage means.

Preferably, the reading means reads data in the first color space corresponding to data closest to the converted color data.

[0022] Preferably, the color conversion device further includes an operation means for performing an interpolation operation using the data read by the reading means.

According to still another aspect of the present invention, a color conversion method includes a method of converting data of a first color space dependent on a device and data of a second color space obtained by transforming a color space independent of a device. A method for performing color conversion using storage means for storing a table indicating a correspondence relationship, comprising: an input step of inputting color data in a device-independent color space; A conversion step of converting the data into color data of a color space; and a reading step of reading data of a first color space corresponding to the converted color data from a storage unit.

According to still another aspect of the present invention, the color conversion program stores the data of the first color space dependent on the device and the data of the second color space obtained by transforming the color space independent of the device. A program for performing color conversion using storage means for storing a table indicating a correspondence relationship, comprising: an input step of inputting color data in a device-independent color space; And a reading step of reading the data of the first color space corresponding to the converted color data from the storage means.

According to still another aspect of the present invention, a computer-readable recording medium records the above-described color conversion program.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [First Embodiment] FIG. 1 is a block diagram showing a configuration of an image processing apparatus according to a first embodiment of the present invention.

Referring to the figure, an image processing apparatus includes a CPU 601 for controlling the entire apparatus, a printer 603 for printing color-converted color images, a display 605 for displaying color images, and an external device. LAN or modem card 6 for exchanging data with
07, an input device 609 including a keyboard and a mouse, and a flexible disk drive 611.
, A CD-ROM drive 613, a hard disk drive 615, a RAM 617, a ROM 619, and a reading device 621 that can read a color image.

A program for realizing the image data and an image processing method described later includes a flexible disk F and a CD.
-It can be installed in the image processing apparatus by communication via the ROM 613a, LAN, or modem.

This image processing apparatus performs the following operations (1) and (2). (1) CK (Chroma axis-Black axis) space and L ^* a ^*
Creation of table for associating with b ^* space (2) L ^* a ^* b ^* using table created in (1)
→ Creation of CMYK color conversion table (profile) One-to-one correspondence from L ^* a ^* b ^* color space to CMYK color space
In order to achieve better color reproduction, it has conventionally been necessary to rely on empirical rules and trial and error.

In this regard, the present inventor considered that the CMYK space is arranged in advance into a three-dimensional space having a one-to-one correspondence with L ^* a ^* b ^* . Once there is a one-to-one correspondence, L ^* a is the inverse transformation of CMYK → L ^* a ^* b ^{*
This is because *} b ^* → CMYK conversion can be easily obtained.

The coordinate system used to arrange the CMYK space in three dimensions is such that the chroma axis extends radially around the K axis representing approximately black (and gray). In this specification, this is called a CK space.

Since the CK space is a space in which CMYK is arranged so as to correspond to L ^* a ^* b ^* on a one-to-one basis, by changing the arrangement method (how to create the CK space), the K of the output image is changed.
Can be freely expressed from the conversion that maximizes the use amount to the conversion that uses a lot of CMY instead of K.

FIG. 2 is a diagram showing a concept of a CK space used in the image processing apparatus according to the present embodiment.

First, as shown in FIG.
Make a plane with. That is, with the origin at the center on the plane,
Take CMY axes radially. The portion sandwiched between the C-axis and the Y-axis covers the colors formed by combining C and Y. The same applies to a portion sandwiched between the M axis and the Y axis, and a portion sandwiched between the Y axis and the C axis. With this plane, all colors expressed using two colors of CMY (so-called high-key colors) can be expressed. Also, the same color does not appear twice on this plane.

Here, as shown in (B), a K axis perpendicular to the CMY plane is added to create a three-dimensional space. The color expressed in this space is obtained by adding black to the color of the CMY plane created in (A). The only exception is when black is maximum, the same color will not appear at different locations in this space. Moreover, this space covers the necessary colors.

In this way, a color space (CK space) that has a one-to-one correspondence with L ^* a ^* b ^* is created. L from CK space
The correspondence to ^* a ^* b ^* can be easily obtained by performing colorimetry. In addition, since the conversion from the CK space to the L ^* a ^* b ^* has a one-to-one correspondence, it can be easily inverted by various methods (the conversion from the L ^* a ^* b ^* to the CK space). Can be requested. That is, as soon as the coordinates in the CK space are determined, the CMYK values can be obtained.

When the conversion from L ^* a ^* b ^* to CMYK is determined based on the data of the color space arranged in this way, the conversion using only two colors of CMY and K (that is, K is Conversion that is the largest). That is, an image is represented by C and M, M and Y or Y and C and a K component (gray component).

As shown in FIG. 2B, if color data is managed in a space represented by a hexagonal prism, the calculation process becomes complicated. Therefore, as shown in FIG. To be stored.

FIG. 3 shows the CK recorded in the image processing apparatus.
It is a figure showing data of space. Referring to FIG. 3, the center is defined as the origin of the CMY plane, and the upward direction is defined as the direction in which the M component becomes dense. Similarly, the left direction is the direction in which the C component becomes darker. The rightward direction and the downward direction are both directions in which the Y component increases. Also, the direction in which the K component becomes dark is defined as the direction penetrating the paper surface.

By performing such data management,
The CK space can be represented by a three-dimensional array, and the calculation process can be simplified. In FIG. 3, the CMY axes are divided into eight in each axis direction. By increasing the number of divisions and recording more data, more accurate color conversion can be performed.

CMYK data and corresponding L ^* a ^* b ^* data are stored at arbitrary positions in the array as shown in FIG. 3, and for example, conversion of L ^* a ^* b ^* → CMYK is performed. Is performed, L ^* a closest to the L ^* a ^* b ^*
* b ^* value is read from the array and the CM stored at that position
YK data is obtained. Most of all L ^* a ^* b ^*
When the CMYK data corresponding to the data is stored, the memory capacity becomes enormous and it is not practical. Therefore, only the data at a predetermined interval is stored, and the data during the interval is referred to by an interpolation formula as shown in the formula (1). Thus, interpolation is performed.

[0042]

(Equation 1)

In equation (1), linear cubic interpolation is performed as an interpolation method, but another interpolation method may be used.

It is not necessary to brute the entire sequence for a given L ^* a ^* b ^* (if the brute force is (8 ×
16) ³ × 3 ≒ 7 million candidates will be searched). Therefore, a search may be performed at a coarse interval, a rough contact may be made, and then a search may be performed only in the vicinity.

FIG. 4 is a flowchart showing a color conversion table creation process performed by the image processing apparatus of FIG.

Referring to FIG. 4, in step S1,
A plurality of CMYK values for which colorimetry is required (ie, FIG.
Is determined. In step S3, using the printer 603, the determined CM
Printing of a color using YK as an ink component is performed.

Next, each of the colors printed in step S5 is measured by the reading device 621. Table A in which the color measurement results are associated with CMYK values
(See FIG. 3) is created in step S7.

In step S9, a table B for obtaining the value of CMYK from L ^* a ^* b ^* is created based on table A.

FIG. 5 is a flowchart of the CMYK determination process (S
It is a flowchart which shows the content of 1).

Referring to the figure, in step S101, a point in the CK space is selected. Next, in step S103, CK
From the definition of the space, the value of CMYK corresponding to the point is calculated. In step S105, the calculated CMYK value (that is, the color to be printed) is written to the memory. Step S10 until all necessary CMYK values are determined
The processing from 1 to S105 is repeatedly executed.

FIG. 6 shows the process of creating table A in FIG. 4 (S
It is a flowchart which shows the content of 7).

Referring to the figure, in step S201, the CMYK values of the measured colors are written in the corresponding positions in table A. The value of L ^* a ^* b ^* measured in step S203 is converted. This transformation is for transforming the L ^* a ^* b ^* space. Next, the value of L ^* a ^* b ^* converted in step S205 is written in the corresponding CMYK position.
The processing of steps S201 to S205 is repeatedly executed until the processing of all colors is performed.

FIG. 7 shows the process of creating table B in FIG. 4 (S
It is a flowchart which shows the content of 9).

Referring to the figure, in step S301, the value of L ^* a ^* b ^* to be registered in table B is determined. That is, if all the L ^* a ^* b ^* are registered in the table B, it is not preferable from the viewpoint of the memory capacity or the like. Therefore, the values of L ^* a ^* b ^* are determined at appropriate intervals. In step S303, the same conversion as in step S203 of FIG. 6 is performed on the determined L ^* a ^* b ^* . Step S30
5, the value of L ^* a ^* b ^* closest to the converted value is determined, and the corresponding CM is determined.
Determine YK.

In step S307, the converted L ^* a ^* b ^* values and CMYK values are written into table B.

By this processing, L ^* a ^* b ^{* is} converted to CMYK
A table B for performing the conversion is created.

FIG. 8 is a flowchart showing the CMYK value determination processing performed in step S305 of FIG.

Referring to the figure, the closest L ^* a ^* b ^* value is searched from table A in step S401. In step S403, an interpolation operation is performed on nearby L ^* a ^* b ^* values. In step S405, the closest one is selected, and in step S407, CMYK values are interpolated.

FIG. 9 is a flowchart showing a color conversion process performed by the image processing apparatus of FIG.

Referring to the figure, in step S501, the value of L ^* a ^* b ^* to be converted is input. Step S
At 502, the same conversion as in step S203 of FIG. 6 is performed on the input value.

By searching the table in step S503, the value of CMYK corresponding to the converted L ^* a ^* b ^* value is read, and output in step S505.

Next, the features of the color conversion processing performed by the image processing apparatus according to the present embodiment will be described. The color conversion in the present embodiment has the following features (1) and (2).

(1) Gamut mapping is performed simultaneously with color conversion. The conversion from L ^* a ^* b ^* to CMYK is to find an appropriate CMYK value for a given L ^* a ^* b ^* . At this time, if a CMYK having a value closest to the given color L ^* a ^* b ^* is searched, L ^* a ^* b
^CMYKs with matching ^* are searched. Outside the gamut, the closest combination of CMYK having L ^* a ^* b ^* is found.

Thus, given L ^* a ^* b ^*
It is not necessary to determine whether or not is within the gamut, and it is not necessary to convert it to L ^* a ^* b ^* within the appropriate gamut when it is outside the gamut. In addition, the continuity inside and outside the gamut is automatically maintained.

(2) Gamut mat in an intentionally distorted space
Do the ping. As described above, in the present embodiment,
Gamut compression and color conversion in one operation
It can now be used, but if you leave it as it is
Only mapping that minimizes the difference can be done. However
However, in the present embodiment, L ^*a^*b^*Space in advance
Search that minimizes color difference because it is distorted (deformed)
(= Gamut compression and color conversion
While at the same time)
Will be able to do it.

Since the colors that match in the undistorted space match in the distorted space, the colors in the gamut are correctly converted even if they are converted in the distorted space.
That is, it is converted into a CMYK combination having L ^* a ^* b ^* equal to the given L ^* a ^* b ^* . Further, the color reproduction range does not become small.

In the prior art, as shown in FIG. 17, a phenomenon called a lightning rod effect has occurred. That is, the range of the gamut of a printer or the like is often sharp at an acute angle, and if a point outside the gamut is mapped to the closest point in the gamut, many colors are mapped to the point where the gamut is sharp. there were.

To solve this problem, if the mapping is performed so as to make the brightness constant, the problem of the lightning rod effect can be avoided for the time being, but this is not preferable because the mapping is performed in too different colors.

As shown in FIG. 10, for example, FIG.
Performs a conversion to reduce the gamut shown in
When the mapping that minimizes the color difference is performed, it is possible to prevent the mapping from being concentrated on the sharp portion of the gamut as shown in FIG. Further, it is possible to perform mapping to a color that is closer to that in mapping with constant brightness.

Further, as shown in FIG. 11, when the transformation is performed so that the degree of distortion increases as the saturation increases, the concentration of mapping points can be further prevented.

Note that the same effect can be obtained by performing L ^* a ^* b ^* conversion that applies a distortion that extends the gamut in the lightness direction.

FIG. 12 is a diagram for explaining a method of setting the distortion in the saturation direction. If the distortion is constant at all the saturations as shown in FIG. 10, the graph becomes a straight line, and the distortion amount is changed as shown in FIG. 11, and the distortion amount increases as the saturation increases. If so, the graph becomes a curve.

In many printers, the saturation peaks in a dark color as compared with the display. In general, it is better to map bright and vivid colors to vivid colors, even at the expense of some lightness continuity. Is obtained.

In order to realize such mapping, as shown in FIG. 13, it is preferable to perform mapping in a space that is deformed such that distortion becomes greater at a point where the brightness is intermediate.
Note that a similar effect can be obtained by distortion in the saturation direction.

In any of the above-described space variations of the present embodiment, the colors in the gamut are converted in the same manner as in a space without distortion. Furthermore, conversion that satisfies the continuity inside and outside the gamut can be performed.

The bright blue outside the gamut is the color
Rather than converting to purple with close degrees, the higher the saturation, the more cyan
It is known that it is preferable to convert to a biased color
(Phenomenon called blue shift). On the other hand,
As for the color, the L ^*a^*b^*CMY that matches
It is better to convert to K.

Conversion that satisfies these two requirements can be easily realized by using a space in which isohues are intentionally distorted as shown in FIG. In other words, as shown in FIG. 14, only the space near blue needs to be distorted outside the gamut so that the more vivid the color, the closer to cyan the mapping is made.

[Second Embodiment] FIG. 15 is a block diagram showing a hardware configuration of a printer according to a second embodiment of the present invention.

Referring to the figure, the printer includes a CPU 201 for controlling the entire apparatus, a print engine 203 for performing printing, an input panel 205 for receiving input from a user, and a display unit 207 for displaying.
An interface 209 for externally inputting image data, a RAM 211 for temporarily storing data,
The ROM 213 stores programs and constants.

The printer according to the present embodiment also prints a color image by performing the color conversion processing according to the first embodiment.

The embodiments disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an image processing apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a specific example of a CK space.

FIG. 3 is a diagram illustrating an example of storing data in a CK space in a memory.

FIG. 4 is a flowchart illustrating a process of creating a color conversion table.

FIG. 5 is a flowchart showing the contents of a CMYK determination process (S1) of FIG. 4;

FIG. 6 is a flowchart showing the contents of a table A creation process (S7) in FIG. 4;

FIG. 7 is a flowchart showing the contents of a table B creation process (S9) in FIG. 4;

8 is a flowchart showing the contents of CMYK determination processing (S303) in FIG.

FIG. 9 is a flowchart illustrating a color conversion process according to the first embodiment.

FIG. 10 is a diagram showing a specific example of a distorted space.

FIG. 11 is a diagram showing a specific example of a distorted space.

FIG. 12 is a diagram for describing distortion in a saturation direction.

FIG. 13 is a diagram for explaining distortion in the brightness direction.

FIG. 14 is a diagram for explaining a distorted space for eliminating a blue shift.

FIG. 15 is a diagram illustrating a hardware configuration of a printer according to the second embodiment.

FIG. 16 is a diagram showing a specific example of gamut mapping.

FIG. 17 is a diagram for explaining a problem of conventional gamut mapping.

[Explanation of symbols]

601 CPU, 603 printer, 621 reading device.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2C262 AA24 AB11 AC04 BA01 BA09 BC01 BC19 EA04 EA13 FA13 5B057 AA11 CA01 CA08 CA12 CA16 CB01 CB08 CB12 CB16 CE18 CH07 DB02 DB06 DB09 DC25 5C077 LL12 LL19 MP01 MP08 PP33 PP36 PP36 PP38 SS02 TT02 5C079 HA18 HB02 HB08 HB11 KA04 LA02 LA21 LB02 MA04 NA03 PA03

Claims (13)

[Claims] 1. A printing means for printing a plurality of types of colors using color data in a first color space; and a second color space for the printed colors by measuring the colors printed. Acquisition means for acquiring the color data in the first color space; conversion means for performing transformation for deforming the second color space on the acquired color data; A color conversion table creation device, comprising: a creation unit that creates a table in which the color data of the color data and the color data converted by the conversion unit are associated with each other on a one-to-one basis. 2. The device according to claim 1, wherein the first color space is a space for expressing a device-dependent color, and the second color space is a device for expressing a device-independent color. Color conversion table creation device. 3. The color according to claim 1, wherein the first color space is a color space represented by CMYK, and the second color space is an L ^* a ^* b ^* color space. Conversion table creation device. 4. A printing step of printing a plurality of types of colors using color data in a first color space, and measuring the printed colors in a second color space of the printed colors. An acquisition step of acquiring color data of the following; a conversion step of performing transformation for deforming the second color space on the acquired color data; One-to-one correspondence between the color data and the color data converted in the conversion step.
And a creating step of creating a table associated with the color conversion table. 5. A computer-readable recording medium on which a color conversion table created by the color conversion table creating method according to claim 4 is recorded. 6. A printing step of printing a plurality of types of colors using color data in a first color space, and measuring the printed colors to obtain a second color space of the printed colors. An acquisition step of acquiring the color data in the step of: a conversion step of performing a conversion for deforming the second color space on the acquired color data; Color data and the color data converted in the conversion step are one-to-one.
A color conversion table creation program for causing a computer to execute a creation step of creating a table associated with a color conversion table. 7. A computer-readable recording medium on which the color conversion table creation program according to claim 6 is recorded. 8. A table showing a correspondence relationship between data in a first color space expressing a device-dependent color and data in a second color space obtained by transforming a color space expressing a device-independent color. Storage means for storing color data in a device-independent color space; conversion means for converting color data input by the input means into color data in a second color space; A readout unit for reading out data of the first color space corresponding to the obtained color data from the storage unit. 9. The color conversion apparatus according to claim 8, wherein said reading means reads out data in said first color space corresponding to data closest to said converted color data. 10. The color conversion device according to claim 8, further comprising an operation unit that performs an interpolation operation using the data read by the reading unit. 11. A storage unit for storing a table indicating a correspondence between data of a first color space dependent on a device and data of a second color space obtained by transforming a color space independent of a device. A color conversion method for performing color conversion, comprising: an input step of inputting color data in a device-independent color space;
And a reading step of reading the data of the first color space corresponding to the converted color data from the storage unit. 12. A storage means for storing a table indicating a correspondence relationship between data of a first color space dependent on a device and data of a second color space obtained by transforming a color space independent of a device. A program for causing a computer to execute a color conversion method, comprising: an input step of inputting color data in a device-independent color space; and inputting the color data input in the input step to a color in the second color space. A color conversion program for causing a computer to execute a conversion step of converting data into data, and a reading step of reading data of the first color space corresponding to the converted color data from the storage unit. 13. A computer-readable recording medium on which the color conversion program according to claim 12 is recorded.