JP2000013627A - Color converting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow even a person who is not an expert to perform color correction according to simple visual decision and to maintain the balance and continuity of color density of not only a part that undergoes the color correction but also as an entire image. SOLUTION: An outputted image is visually compared with an image supplied from a maker side and a spectral reflectance ratio Rλ, is adjusted according to the visual decision to correct an error at the time of converting XYZ into RGB. In the adjustment at the time, an L that is a part of a parameter of the spectral reflectance ratio and controlled variables ΔL, Δa and Δb of (a) and (b) data are set. In profile calibration, an adjustment spectral reflectance ratio R'λ is operated based on the set value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a color conversion method for converting color data between different color systems.

[0002]

2. Description of the Related Art
As a means for outputting a full-color image, a printing machine is used to output a large amount (tens of thousands to hundreds of thousands of copies). In some cases, a printing machine prints a small amount (several thousand copies or less).

[0003] In any case, in printing by a printing machine,
Using a photosensitive lithographic printing plate (PS plate) as a master,
In this state, an image is formed on an S plate and is printed using ink of each color of C (cyan), M (magenta), Y (yellow), and K (black) in a state of being mounted on a rotary press.

In general, in a printing press, when the color is not stabilized at the time of startup, the consumables such as ink paper are used for an additional 7 hours, so that the printing contents (color balance of images, character calibration, etc.) are used. When it is necessary to check the color proof, the same image data is output by a full color printer connected to a personal computer or the like (color proof output).

[0005] By inspecting the color proof, it is possible to proofread the color balance of an image and the like and characters.

Here, in the color signal conversion processing in the color proof, a color system (control values by CMYK) used for the original image data and a color system corresponding to human visual perception (for example, CIEXYZ, CIELab). ) Is described using an LUT (lookup table), a matrix, or the like, and color conversion of the actual image data is performed using the LUT. For example, in an LUT, it is generally necessary to output a large number of color patches from a device (printing and color proof) using a colorimeter, measure the colors, and create a conversion table based on the data, which requires a large amount of time. There is a disadvantage that it requires.

If this LUT can be dealt with by making one for one model or recording method, there is no problem even if it takes a little time, but in actuality, the individual differences of the device,
The color reproduction characteristics of a device such as a printer change due to environmental factors such as a lot difference of materials used and ambient temperature and humidity. In particular, when high color accuracy is required for a color proof or the like, it is necessary to recreate the LUT so that the color reproduction characteristics of the device are accurately reflected for each individual printer and installation environment.

Each time, the work of measuring a large number of patch colors and creating a conversion table is extremely burdensome.

[0009] For this reason, it is desired as a practical method to visually determine the color of a color proof image and to correct (color correct) a color table (LUT) based on a small amount of measurement data.

For this purpose, a method of partially changing a numerical value described in a color system (for example, CIEXYZ, CIELab) corresponding to the human visual sense in the LUT is used.

However, in the above simple method, unless color correction is performed while maintaining continuity between the colors of the device, an undesired image such as a tone jump may occur. In addition, since it is necessary to adjust each color data, if the colors are totally shifted, the color correction work becomes enormous.

There is also a method of adjusting X, Y, Z and L, a, b by one-dimensional curve conversion. This can be corrected over the entire color range, but it is difficult to balance the correction amounts of the three coordinate axes. For example, if only X is adjusted and the colors are matched in a certain color gamut, they do not always match in another color gamut. Therefore, a skilled person can select an appropriate measurement point in a relatively short time and perform color correction with continuity, but some of them may not be able to be reproduced by conversion. In this case, the connection of colors is completely lost only by partial color correction.

In the present invention, in consideration of the above fact, even a person who is not an expert can correct the color by simple visual judgment.
To obtain a color adjustment method for an output image that can accurately reflect the color reproduction characteristics of the device as well as the color-corrected portion and maintain the color density balance and color density continuity of the entire image. Is the purpose.

[0014]

According to the present invention, the correspondence between the color data of the first color system and the color data of the second color system includes a spectral reflectance. And a color conversion method for converting the color data of the first color system into the color data of the second color system using the table. The second
The spectral reflectance is changed based on the color data of the color system, and the table is corrected based on the changed spectral reflectance.

According to the first aspect of the present invention, when the color data is converted from the first color system to the second color system and output to the printer, the difference between the printer and the reference machine at the time of creating the table is obtained. Differences in color density occur due to differences in gravity, evaluation environment, and the like.

In this case, even if only a portion having a color density difference is corrected, appropriate correction cannot be easily performed because there is no connection between surrounding colors. Further, a color that cannot be reproduced between color data may occur.

Therefore, the spectral reflectance is changed based on the color data of the second color system, and the conversion from the first color system to the second color system is performed based on the changed spectral reflectance. Correct the time table.

Specifically, the spectral reflectance is corrected by multiplying the spectral reflectance by a correction coefficient depending on the wavelength, and a second color system table value is corrected based on the corrected spectral reflectance data. To correct.

That is, all the related data in the table is corrected so that the color connection is maintained as a whole, instead of the partial correction of the color data of the second color system. Correction work equivalent to the work to be performed can be easily performed.

According to a second aspect of the present invention, there is provided the color data of the first color system and the color data of the standard color system, the color data of the standard color system and the color data of the second color system, Are constructed by a predetermined table including the spectral reflectance as a parameter, and the color data of the first color system is converted through the color data of the standard color system. A color conversion method for converting color data into color data of the second color system, comprising outputting an image based on the color chart data of the first color system, and comparing the output image with a color chart image. Then, the table at the time of conversion from the first color system to the standard color system is corrected, an image is output based on the color chart data of the second color system, the output image,
When comparing with a color chart image and correcting the table at the time of conversion from the standard color system to the second color system, an important color is selected as a comparison target, and the color data of the selected target is selected. And changing the table at the time of conversion from the standard color system to the second color system based on the changed spectral reflectance.

The first color system (for example, CMYK system) is changed to the second color system.
When converting into a color system (for example, RGB system), it is preferable to interpose a color system corresponding to human visual sense such as CIEXYZ, CIELAB as a standard color system.

An image (for example, a printed matter) output by CMYK color data is inspected for image quality such as gradation and color balance based on an image (color proof) output by RGB color data. In this case, the values of the color system corresponding to the human visual sense, such as CIEXYZ and CIELAB, must match for each corresponding pixel.

Therefore, color chart data of the first color system and color chart data of the second color system are prepared in advance. In this case, when both color chart data are output by the printing conditions and the color proof reference machine, respectively, the values of the standard color system match for each corresponding pixel.

First, an image is output based on the color chart data of the first color system, and this output image is compared with the original color chart image.

As a result of this comparison, the table at the time of conversion from the first color system to the standard color system is corrected. As a result, the characteristics of the output image based on the color data of the first color system are reflected at the time of conversion to the standard color system.

Next, an image is output based on the color chart data of the second color system, the output image is compared with the color chart image, and the standard color system is changed to the second color system. Correct the table at the time of conversion. In this case, correction based on the characteristics and environment of the recording paper on which the output image is recorded is, of course, performed, but separately from this, an important color is selected as a comparison target, and based on the color data of the selected target, Change the spectral reflectance. At this time, a color-dependent connection can be maintained by setting a correction coefficient depending on the wavelength based on the color data, applying the correction coefficient to each data in the table, and changing the spectral reflectance. Therefore, by correcting the table at the time of conversion from the standard color system to the second color system based on the changed spectral reflectance, the color data from the standard color system to the second color system is corrected. In this conversion, appropriate color conversion is performed without generating color discontinuity and colors without reproducibility.

Thus, the images output with the respective color data of the first color system and the second color system match,
For example, as described above, in relation to a printed matter and a color proof, the printed matter can be inspected by the color proof.

In the above-described process, when a correction coefficient for each wavelength of the spectral reflectance is obtained based on the target color data, a coefficient reflecting a human visual characteristic, for example, a color function such as CIExyz is used. Accurate correction with good operability is possible.

Further, by changing the correction strength for each color corresponding to the LUT grid point, it is possible to selectively correct a specific color gamut or to selectively increase (weaken) the correction. .

In the correction of the color conversion table between the second color system and the standard color system, an example in which the spectral reflectance is corrected has been described. The same can be applied to the case of correcting the color conversion table.

[0031]

FIG. 1 shows an overall configuration of a user-side print processing system 100 and a maker-side calibration processing system 102 according to the present embodiment. (User side print processing system 100) In the user side print processing system 100, a LAN 104 is constructed,
In FIG. 1, three computers 106, 108, 110
Are connected to the LAN 104. In these computers 106, 108 and 110, the scanner 11
2, a process such as a layout for a printed material is performed based on the image data read. The image is not limited to the scanner 112 but may be directly taken from a storage medium such as an FD, an MO, or a DVD, or an image may be obtained through a communication line.

A plurality of (two in this embodiment) printing machines 114 are connected to the LAN 104, and a printout is performed based on print image data created by the computers 106, 108, and 110. Is created.

Here, the printed matter is a sheet of paper on which an image whose gradation is expressed by only one bit (that is, white or black) is recorded, and this image is formed by the white or black dots (dots). By adjusting the density (or size) of each color, the gradation of each color is expressed, and the image is formed by overlapping the inks of each color.

As described above, since printing on a printed material is generally made up of halftone dots, a halftone dot recorder 116 is interposed between the LAN 104 and the printing machine 114.
Halftone processing is performed based on the image data (CMYK-based data that is the first color system) input from 104. The halftone-processed one-bit image is output as a printing plate for each of CMYK colors, inked by a printing machine, and output in color.

Each of the computers 106, 108, 1
A printer 118 is connected to 10, and print images created by the computers 106, 108, and 110 are printed (color proof). The operator can judge whether or not the actual printed matter is appropriate by looking at the color proof image.

Here, the color proof image output from the printer 118 uses, for example, a photosensitive material and has a gradation corresponding to full color. That is, it has a characteristic that the density changes continuously according to the exposure amount for each photosensitive material to be applied. For this reason, color reproduction generally differs from that of the printed matter.

That is, when a color proof image is printed by the printer 118, the original data (CMYK
In the profile data 122 recorded on the external hard disk 120.
The CMYK system (first color system) is converted into the CIEXYZ system (standard color system) based on the UT 124 (see FIG. 3A), and further based on the standard LUT 126 (see FIG. 3B). The image is converted to an RGB system (second color system). At this time, if there is an error relating to color reproducibility between the device at the time of creating the respective standard LUTs 122 and 124 and the actual device, the color density, color balance, etc. will change.

Therefore, in this embodiment, CMYK → X
When converting YZ (first color system → standard color system), XYZ → R
At the time of GB (standard color system → second color system) conversion, calibration is performed in advance so that there is no change in color balance or the like, and each of the computers 106, 108, and 110 has (stores). ) Profile data 122
The standard LUTs 123 and 126 are corrected so as to be suitable for each actual device.

At the time of this correction, a color chart (paper) 127 and color chart data (a floppy disk 128 is used as a recording medium in FIG. 1) supplied in advance from the manufacturer are used.

FIG. 2 shows a correction process based on the printing and printer color chart 127 supplied from the manufacturer. Note that the digital image data 130 in FIG. 2 includes image data read by the scanner 112 shown in FIG. 1 or image data read from another recording medium, image data obtained through a communication line, and the like, all of which are applied during normal operation. Contains.

The correction of the error at the time of CMYK → XYZ (CIE) conversion is as follows. Note that this correction is performed when the printing press 114 is installed, and does not need to be performed thereafter unless there is no replacement of peripheral devices and the like.

First, in order to perform printing based on the printing color chart 127 supplied from the maker, halftone processing is performed by the halftone recorder 116. The dot data thus obtained is input to the printing press 114, and printing is started. Here, it is preferable that the image of the printed matter printed by the actual printing press 114 coincides with the image printed by the standard printing press 132 (see FIG. 1) on the maker side. (Color density, color balance, etc.) occur.

For this reason, the image of the color chart 127 supplied from the manufacturer is printed, and the printing result is compared with the colorimeter 1.
The standard LUT 124 (see FIG. 3A) recorded in the print profile data 122 is corrected based on the difference from the color chart 127.

Next, correction of an error at the time of XYZ → RGB conversion is as follows.

Also in this case, a printer color chart 127 (paper) and color chart data (a floppy disk 128 is used as a recording medium as shown in FIG. 1) supplied from the manufacturer in advance are used. ing.

The procedure is to output an image from the printer 118 based on the printer color chart data based on the RGB data.

Thus, the output image is visually compared with the image supplied from the maker, and the amount of correction of the color data for the target color is determined by this visual judgment. The adjustment at this time is performed by using the computer 106, L, a, and b data, which are color systems corresponding to human visual perception.
Keyboards 106A, 108A, 11 of 108, 110
By operating 0A, the respective correction amount target values ΔL, Δa, Δb of L, a, b are set. In the profile calibration, the expressions (14) to (16) correspond to the above ΔL, Δ
The adjusted spectral reflectance R′λ is calculated so as to obtain the values of a and Δb. The method for correcting the spectral reflectance is characterized by a correction intensity coefficient γx using the X component Xλ of the color matching function as a weighting factor and the color coordinates (L, a, b) as parameters, as shown in equation (1), for example. Function that is used. Similarly, the Y and Z components are characterized by correction intensity coefficients γy and γz.

The initial value of γ is obtained as follows.

From the XYZ values of the color of interest used in the visual judgment described in the original profile data,
L, a, and b values are obtained by equation (7).

Further, the corrected L ', a', and b 'values are obtained by the equations (14) to (16), and the Y', X ', and Z' values are obtained by inversely calculating the equations (11) to (13). Ask. Furthermore (1
The correction intensity coefficient is obtained by the equations (7) to (19).

Using this corrected intensity coefficient, the spectral reflectance characteristic for each point in the table is corrected by equation (1).

The correction intensity coefficient can be described in the form of a function using the L, a, and b values as variables. This method will be described later.

[0053]

(Equation 1) Here, R′λ: reflectance (after correction) Rλ: reflectance (before correction) xλ, yλ, zλ: color matching functions γx, γy, γz: correction intensity coefficients L, a, b: colorimetric system It is.

Using the calculated adjusted spectral reflectance R'λ, the standard LUT is corrected based on the equations (8) to (10), and the actual profile data 122 is created.

As described above, the correction amount target values ΔL, Δa,
Δb is set visually for the color of interest by the operator, but since the correction amount of the spectral reflectance is calculated based on these and the XYZ values are changed over the entire color data, correction by visual observation is necessary. Not only a predetermined portion of the image, but also the entire image centered on the predetermined portion can be corrected by a continuous change, so that the color connection is compensated. Note that the correction amount target values ΔL, ΔL
If there is a colorimeter, a and Δb can be corrected by using the colorimeter without visual observation. (Manufacturer-side calibration processing system 102) As shown in FIG.
40 and a standard printer 1
32 are prepared. The printing machine 132 is a machine that prints on a predetermined sheet based on CMYK (first color system) color data, and is a printing machine 1 installed on the user side.
It has the functions of 14 standard machines.

On the other hand, the printer 142 is an RGB system (second
This is a machine for printing on predetermined paper (photosensitive material) based on the color data of the (color system), and has a function as a standard machine of the printer 118 installed on the user side.

The CMYK color data is input to the computer 140 (or stored in advance), and its image format is a color chart format.

On the basis of the color data, the printing machine 132 prints the color data without converting it.
At 42, printing is performed after conversion into the RGB color system. At the time of conversion to the RGB system, the standard color system (in the present embodiment,
CIEXYZ system).

Finished color chart (paper) 127
Are classified into those for a printing press and those for a printer, respectively, and are supplied to the user side. In addition, this color chart 127
Is measured by a colorimeter 144 to generate digital image data. This digital image data is recorded on a floppy disk 128 and supplied to a user together with the color chart (paper) 127. .

The operation of this embodiment will be described below with reference to FIGS.
This will be described according to the flowchart of FIG.

First, according to FIG. 4, CMYK → XYZ (C
The LUT correction procedure at the time of IE) conversion will be described. The LUT correction at the time of the CMYK → XYZ (CIE) conversion is performed by the standard printing machine 132 (the manufacturer side) when the printing machine 114 is installed.
This is performed in order to deal with the above, and is rarely performed thereafter except when the peripheral device is replaced.

In step 200, the color chart data for printing is read from the floppy disk 128. Then, in step 202, the read CMYK data is sent to the dot recorder 116 and converted into dot data (dot processing). .

In the next step 204, the dot recorder 1
16 based on the halftone data generated in
14 to print. In this case, it is preferable to print a predetermined number of copies because the image quality, color tone, and the like are not stable at the beginning of printing.

In step 206, an image (stable image) close to the final copy on which the predetermined number of copies have been printed is picked up and colorimetrically measured. In the next step 208, the standard LUT recorded as a part of the profile data supplied from the manufacturer is corrected based on the colorimetric result, and then in step 210, the profile data is updated.

The above describes the characteristics of the actual printing press 132 as C
This is to reflect when converting the MYK data to the CIEXYZ data, whereby the correspondence between the actual printing press 114 and the standard printing press 132 is established. In addition, since the color of the actually printed image is measured, correction based on the type of printing paper and the environment at the time of printing is also performed.

Next, in this embodiment, the spectral reflectance is changed based on the printer color chart data (matching with the printing color chart data) supplied from the manufacturer, and the LUT 126 ( FIG. 3 (B)
), That is, the profile data 122 is updated.

That is, as shown in FIG. 5, at step 250, first, the printer color chart data (RGB) is read from the floppy disk 128, and then at step 252, the printer 118 is read based on the read printer color chart data. Print with.

In step 260, the printed output image and the color chart image are visually compared,
The parts (targets) where important colors and color densities are clearly different, parts where colors are not connected, and the like (targets) are picked up, correction amount target values ΔL, Δa, and Δb are determined.
08A and 110A are input (step 262).

That is, even a non-expert operator judges a difference in color density and the like that can be easily recognized.
What is necessary is just to input the control amount.

In the next step 264, the temporary LUT is corrected by calculating the spectral reflectance using the above equation (1) based on the input correction amount target value. Update and create the correction profile 122.

Here, this correction is not for correcting only the picked-up portion, but for correcting the spectral reflectance itself to a continuous curve. Therefore, not only the picked-up portion but also the entire image based on this is balanced. It can be corrected well. Therefore, it is possible to correct the pickup portion while maintaining the connection of colors, and it is possible to accurately correct even a non-expert.

For example, as a result of the comparison in step 260, if there is a color, the control amount in step 262 is 0, γx = γy = γz = 0, and the above equation (1) 'λ = Rλ.

A specific color gamut can be controlled by using the correction amount intensity coefficients γx, γy, γz as functions of L, a, b values.

[0074] Also, for example, a color brightness L is, the L _H to select a color gamut range of L _L, if such as strong correction amount of this region, γx, γy, γz, the following It can be represented by the following equation.

[0075] γx (L, a, b) = 4γx 0 (L H -L) (L-L L) / (L H -L L) 2 ··· (20) γy (L, a, b) = _{4γy 0 (L H -L) (} L-L L) / (L H -L L) 2 ··· (21) γz (L, a, b) = 4γy 0 (L H -L) (L-L _{L) / (L H -L L} ) 2 ··· (22) in addition, the L value, min value L _h and max value L _L are predetermined, L _L ≦ L ≦ L _h is satisfied Is a condition.

The actual profile 136 has items as shown in Table 1 below.

[0077]

[Table 1]

[0078]

As described above, the color conversion method according to the present invention enables color correction by simple visual judgment, even for non-experts.
This has an excellent effect that the balance of color density and the continuity of color density can be maintained for the entire image.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a user-side printing system and a maker-side calibration processing system according to the present embodiment.

FIG. 2 is a process diagram showing a flow of calibration.

3A is a printing LUT, and FIG. 3B is a printing LU.
T.

FIG. 4 is a control flowchart showing a conversion procedure of CMYK → CIEXYZ.

FIG. 5 is a control flowchart showing a conversion procedure from CIEXYZ to RGB.

[Explanation of symbols]

100 User side print processing system 102 Manufacturer side calibration processing system 1
02 106, 108, 110 Computer 112 Scanner 114 Printing machine (user side) 118 Printer (user side) 122 Profile data 126 Printer LUT 127 Color chart 128 Floppy disk 140 Computer 132 Printing machine (manufacturer side) 142 Printer (manufacturer side)

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2G020 AA08 DA05 DA14 DA24 DA34 DA43 5B057 AA11 CE18 5C077 LL11 MM27 MP08 PP05 PP15 PP31 PP33 PP42 PP43 PQ20 PQ23 SS02 TT08 5C079 HA19 HB03 HB05 HB11 LA00 LB01 MA07

Claims (2)

[Claims] 1. A correspondence between color data of a first color system and color data of a second color system is constructed by a predetermined table including spectral reflectance. A color conversion method for converting color data of a first color system into color data of a second color system using the table, based on the converted color data of the second color system. Changing the spectral reflectance, and correcting the table based on the changed spectral reflectance. 2. The correspondence between the color data of the first color system and the color data of the standard color system, and the correspondence between the color data of the standard color system and the color data of the second color system. , Which is constructed by a predetermined table including the spectral reflectance as a parameter, and converts the color data of the first color system into the second color system through the color data of the standard color system. A color conversion method of converting an image based on the color chart data of the first color system, comparing the output image with a color chart image, Correcting the table at the time of conversion from the color system to the standard color system, outputting an image based on the color chart data of the second color system, comparing the output image with the color chart image, Correct the table when converting from the standard color system to the second color system In this case, the important color is selected as a comparison target, the spectral reflectance is changed based on the color data of the selected target, and the second standard color system is changed based on the changed spectral reflectance. A color conversion method comprising: correcting a table at the time of conversion to a color system.