JP2007124243A - Color conversion method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To create a color conversion table with a decreased creation man-hours wherein the physical appearance is the same in a shared region between the sRGB and the extended color space, the color conversion table being used to convert data in the sRGB and the extended color space into data in a color space of an output device. <P>SOLUTION: The data in the sRGB and the extended color space are once converted into data independently of the device, and colors are adjusted by using a plurality of shared parameters in a color space independently of the device. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a color conversion method for converting image data of a plurality of different color spaces into a color space of an arbitrary image output device.

  In recent years, image data input devices such as digital cameras have been improved in performance, and image data output from these input devices has been diversified. Conventionally, only data in the RGB color space called sRGB, which is the most common, has been used, but in recent years, there are many that support extended color spaces such as AdobeRGB.

  Conventionally, a printer driver of a color printer has been designed on the assumption that the RGB data of the input image data is expressed in the color space sRGB. Therefore, on the assumption that the input data is sRGB, each printer manufacturer is designed so that each output device has an optimum output, or the output impression is almost the same on any output device. Yes.

For example, Patent Document 1 proposes a method for realizing common color reproduction among a plurality of devices regardless of the difference in the color reproduction range of each device. Also, in Patent Document 2, as a method for realizing common color reproduction among a plurality of devices, when the color reproduction range of an input signal is mapped to the color reproduction range of a narrower device, the gradation of colors outside the color reproduction range is also disclosed. A method of mapping while preserving sex has been proposed.
JP 2004-153336 A JP2003-153020A

  However, in the above conventional example, the printer driver is designed on the assumption that the data of the input device is expressed in the sRGB space. That is, a color conversion table of input data (sRGB) → output data (device RGB) or input data (sRGB) → output (device CMYK) is prepared in advance, and printing is performed using the color conversion table as needed. It was like that.

  These color conversion tables are prepared for each output device, each recording medium, and each recording mode. Normally, these color conversion tables are tuned to a very fine part so as to obtain an optimum output by the output device. The color conversion tables are tuned so that the visual impression is almost the same.

  In such a configuration, when the input data is not sRGB data but data in an extended color space such as Adobe RGB, for example, it is necessary to prepare another color conversion table different from conventional RGB. In the conventional method, when the color conversion table for the extended color space is created, fine tuning is required in the same manner as the creation of the sRGB color conversion table. It will take time.

  The present invention has been made in view of the above-described conventional example, and when creating a color conversion table for sRGB and an extended color space, a color conversion table that makes the visual impression almost the same in the common area of both color spaces. The object is to provide a color conversion method that can be easily created.

  In order to achieve the above object, the color conversion method of the present invention comprises the following arrangement.

  That is, a color conversion method for converting data of a first input color space and data of a second input color space into a color space of an arbitrary output device, wherein the first input color space data is converted to a device non-device. A first color space conversion step for converting to data in a dependent color space; a second color space conversion step for converting data in the second input color space into data in a device-independent color space; A color adjustment step in which data in a device-independent color space is converted by a plurality of parameters and color adjustment is performed, and data adjusted in the color adjustment step is gamut matched to the color reproduction range of the output device. A gamut mapping step of performing mapping, and a step of calculating data specific to the output device corresponding to the gamut-mapped data, and the first and second color spaces The conversion in the conversion step is conversion that results in a common color reproduction in a common area of the first input color space and the second input color space, and the plurality of parameters used in the color adjustment step are: The parameters are common to the first input color space and the second input color space.

  The data of the first input color space and the second input color space is RGB data.

  Further, the RGBCMY hues of the first input color space and the second input color space are substantially the same hue.

  Furthermore, the device-independent color space is an L * a * b * space.

  The plurality of parameters used in the color adjustment step include (1) L * a * b * value after black and white color adjustment, and (2) 45 ° increments in the L * a * b * space. (3) The amount of change ΔL * C * h of the device-independent data corresponding to the position of the representative color of RGBCMY with the RGB values of the input color space, and (4) the target of the gray line Chromaticity a * and b *, (5) enlargement / reduction ratio in the a * direction and b * direction of the flesh color region, and (6) the total shift amount ΔL * a * b *.

  In the color adjustment step, (1) color adjustment in the brightness direction by the L * a * b * value parameter after black and white color adjustment, and (2) 45 ° increments in the L * a * b * space. (3) Lightness, saturation, hue adjustment of RGBCMY representative color position, (4) Gray line chromaticity adjustment, (5) Skin color area color adjustment, (6) Overall shift The color adjustment is performed in order.

  In the enlargement / reduction in the hue direction in increments of 45 ° in the L * a * b * space, the enlargement / reduction rate of all hues is calculated by interpolation calculation from the enlargement / reduction rate specified in 45 ° increments. It is characterized by being enlarged / reduced so as to have a continuous connection in each hue.

  Further, in the brightness, saturation, and hue adjustment of the RGBCMY representative color position, the brightness, saturation, and hue adjustment amounts are interpolated based on the relative positional relationship with the representative color for data other than the representative color. It is calculated by calculation, and the brightness, saturation, and hue are adjusted so as to have a continuous connection in all regions.

  Furthermore, in the color adjustment of the flesh color region, the region of a *> 0, b *> 0, and C * = (a * × a * + b * × b *) 1/2) <45 is flesh colored. It is regarded as an area, and the a * and b * values of data located in the skin color area are enlarged / reduced in the a * direction and the b * direction.

  According to the present invention, when converting the data of the first and second input color spaces into the color space data of an arbitrary output device, the data of each input color space is once converted into device-independent data, A color conversion table is created by adjusting colors using common parameters for data.

  Thereby, in the common area of the first and second input color spaces, there is an effect that it is possible to realize color reproduction with substantially the same visual impression.

  In addition, since color adjustment is performed using common parameters in the first and second input color spaces, it is only necessary to tune in one of the input color spaces, and there is no need to tune in both input color spaces. There are also advantages.

  Hereinafter, preferred embodiments of the present invention will be described more specifically and in detail with reference to the accompanying drawings.

  In this embodiment, when creating a color conversion table for a color printer that targets RGB data as input data, sRGB is assumed as the first input color space, and AdobeRGB is assumed as the second input color space. explain.

  The target color printer is, for example, an inkjet printer that performs recording by ejecting ink of four colors of cyan (C), magenta (M), yellow (Y), and black (K).

  FIG. 1 is a diagram showing image processing executed for recording with this ink jet printer.

  This image processing may be executed by a printer driver of a host computer connected to the ink jet printer, or may be executed by an MPU inside the ink jet printer capable of directly inputting RGB data.

  In FIG. 1, 10 represents input image data, which is RGB image data. The RGB data of the input image data is first converted to R′G′B ′ by the input gamma processing unit 11. The input gamma processing is processing by a one-dimensional lookup table used when color adjustment processing by the user in the printer driver, for example, processing such as brightness, color balance, and contrast is performed.

  The data R′G′B ′ after the input gamma processing is then converted to R ″ G ″ B ″ by the color conversion processing 12. This color conversion processing is performed by converting the RGB color data of the input image into the target color. This is a process using a three-dimensional lookup table for mapping to data in the device RGB space unique to the printer.Normally, the color of the printer is created by this color conversion process.

  The data R "G" B "after the color conversion processing is then converted into CMYK ink color data by the ink color separation processing 13. In this ink color separation processing, data in the device RGB space unique to the target color printer. The data CMYK after the ink color separation processing is then subjected to C′M ′ by the output gamma processing 14 after the ink color separation processing is performed so as to maximize the color gamut in the printer. The output gamma processing is performed to adjust the output characteristics of each ink, and C′M′Y′K ′ after the output gamma processing is a halftone. By processing 15, it is converted into N-valued data of C "M" Y "K" and recorded by an ink jet printer.

  The color conversion table created in this embodiment is a three-dimensional lookup table used in the color conversion process 12 shown in FIG.

  FIG. 2 is a block diagram showing an outline of the color conversion table creation method of this embodiment.

  In FIG. 2, reference numeral 20 denotes sRGB data, here, grid point data of an sRGB color conversion table (three-dimensional lookup table).

  FIG. 3 shows an sRGB color conversion table.

As shown in FIG. 3, the color conversion table is composed of N ³ grid point data obtained by dividing the R, G, and B axes into N sections. This table is a conversion table in which output device RGB values corresponding to each grid point RGB value are stored.

  Returning to FIG. 2 and continuing the description, the sRGB data 20 is input to the sRGB → L * a * b * conversion unit 22 and converted into L * a * b * data (L1a1b1). In the sRGB → L * a * b * conversion unit 22, the conversion is performed uniquely from the general relational expression of sRGB-L * a * b *. The device-independent data (L1a1b1) is input to the color adjustment unit 24. On the other hand, 21 represents AdobeRGB data, here, grid point data of the color conversion table for AdobeRGB (three-dimensional lookup table).

  The AdobeRGB data 21 is input to the AdobeRGB → L * a * b * conversion unit 23 and converted into L * a * b * data (L2a2b2). In the AdobeRGB → L * a * b * conversion unit 23, the conversion is performed uniquely from the relational expression of AdobeRGB-L * a * b *. The device-independent data (L2a2b2) is input to the color adjustment unit 24.

  The color adjustment unit 24 performs color adjustment according to the common parameters stored in the common parameter storage unit 25. The color adjustment method performed by the color adjustment unit 24 and the parameters stored in the common parameter storage unit 25 will be described in detail later.

  The device independent data (L1′a1′b1 ′) and (L2′a2′b2 ′) color-adjusted by the color adjustment unit 24 are input to the gamut mapping unit 26. The gamut mapping unit 26 maps the color-adjusted sRGB and AdobeRGB data to the color gamut of the output device.

  At this time, in the common region of sRGB and AdobeRGB, mapping is performed so that the visual impression is almost the same. The device-independent (L1 ″ a1 ″ b1 ″) and (L2 ″ a2 ″ b2 ″) that are gamut mapped are input to the device RGB calculation unit 27. Here, the device RGB corresponding to the input (L1 ″ a1 ″ b1 ″) and (L2 ″ a2 ″ b2 ″) is calculated. The device RGB corresponding to (L1 ″ a1 ″ b1 ″) is output as the color conversion table for sRGB data 28. The device RGB corresponding to (L2 ″ a2 ″ b2 ″) is the color conversion table for AdobeRGB data. 29 is output.

  Next, the color adjustment performed by the color adjustment unit 24 will be described in detail.

  First, parameters stored in the common parameter storage unit will be described.

The following six parameters are stored. That is,
(1) Black and white L * a * b * values of the output device (2) Hue direction enlargement / reduction ratio in 45 ° increments in L * a * b * space (3) Primary color (primary color) change ΔL , ΔC, Δh
(4) Gray line target chromaticity a *, b *
(5) Enlargement / reduction ratio of skin color area in a * direction and b * direction (6) Uniform shift amounts ΔL, Δa, Δb
It is.

  As shown in FIG. 4, the hue direction in increments of 45 ° in (2) is 0 ° on the line a *> 0 and b = 0 in the a * -b * plane of the L * a * b * space. The eight hue directions are 0 °, 45 °, 90 °, 135 °, 180 °, 225 °, 270 °, and 315 °.

Further, (3) PrimaryColor indicates a color corresponding to the following six RGB values when the number of bits of one pixel data is “8”. That is,
(A) Red (red): (R, G, B) = (255, 0, 0)
(I) Green: (R, G, B) = (0, 255, 0)
(C) Blue (blue): (R, G, B) = (0, 0, 255)
(D) Cyan: (R, G, B) = (0, 255, 255)
(E) Magenta: (R, G, B) = (255, 0, 255)
(F) Yellow: (R, G, B) = (255, 255, 0)
FIG. 5 is a flowchart showing color adjustment processing performed by the color adjustment unit 24.

  In step S501, the input device-independent data (L * a * b *) is compressed / decompressed according to the L * value of black and white (L * a * b *) of the output device. is there. For example, in this step, the L * value of the input data is linearly compressed / expanded between the black and white L * values of the output device.

  Step S502 is a step of performing enlargement / reduction in the hue direction in increments of 45 °. In this step, first, an enlargement / reduction ratio table in increments of 1 ° from 0 ° to 359 ° is prepared. Then, 45 ° increments stored in the common parameter storage unit 25 in the table values of the enlargement / reduction ratio table located at 0 °, 45 °, 90 °, 135 °, 180 °, 225 °, 270 °, 315 ° Set the parameter of the enlargement / reduction ratio. Then, for the table values of the enlargement / reduction ratio table other than 45 ° increments, a table value is obtained by interpolation calculation. In accordance with the enlargement / reduction ratio table obtained in this way, all input (L * a * b *) are enlarged / reduced. At this time, the L * value is left as it is, and only the a * and b * values are enlarged / reduced.

  Step S503 is a step of adjusting the hue (Δh), lightness (ΔL), and saturation (ΔC) of PrimaryColor.

  In this step, the hue is first adjusted.

  A hue change amount table in increments of 1 ° from 0 ° to 359 ° of hue is prepared, and the parameter (Δh) stored in the common parameter storage unit 25 at the position of the hue angle (X °) where the six primary colors are located. ) Is set. Table values other than PrimaryColor are calculated by interpolation calculation to create a hue change amount table. According to this hue change amount table, the hue is adjusted for all input L * a * b *. Also at this time, the L * value is left as it is, and conversion is performed only for the a * and b * values.

  Next, the brightness is adjusted.

  A brightness change amount table in increments of 1 ° from 0 ° to 359 ° of hue is prepared and stored in the common parameter storage unit 25 at the position of the hue angle (X °) where the six primary colors after hue adjustment are located. Parameter (ΔL) is set. Table values other than PrimaryColor are calculated by interpolation calculation to create a brightness change amount table. The brightness is adjusted for the L * a * b * data after the hue adjustment according to the brightness change amount table. At this time, the a * and b * values are left as they are, and only the L * value is adjusted. The lightness change amount changes the value of the lightness change amount table at the maximum saturation of the specified hue angle, and the lightness is adjusted so that the change amount decreases as the saturation decreases.

  Finally, adjust the saturation.

  A saturation change amount table in increments of 1 ° from 0 ° to 359 ° of hue is prepared, and stored in the common parameter storage unit 25 at the position of the hue angle (X °) where the six primary colors after brightness adjustment are located. Parameter (ΔC) is set. Table values other than PrimaryColor are calculated by interpolation calculation to create a saturation change amount table. The brightness is adjusted for the L * a * b * data after the brightness adjustment according to the saturation change amount table. At this time, the L * value is left as it is, and only the a * and b * values are adjusted. As the saturation change amount, the value of the saturation change table is changed at the maximum saturation of the specified hue angle, and the saturation is adjusted so that the change amount becomes smaller as the saturation becomes lower.

  Step S504 is a step of adjusting the gray line. For the (L * a * b *) data output from step S503, black and white are converted to the black and white (L * a * b *) values of the output device. Then, adjustment is performed so that the a * and b * values of the halftone gray line become the target gray chromaticity values a * and b * stored in the common parameter storage unit 25. Also at this time, the L * value is left as it is, and adjustment is performed only for the a * and b * values. In addition, adjustment is performed so that black, gray, and white are continuously connected.

  Step S505 is a step of adjusting the skin color area. For the L * a * b * data output from step S504, a *> 0, b *> 0, and C * = (a * × a * + b * × b *) 1/2 < The 45 area is regarded as a skin color area. Then, the a * and b * values of the data located in the skin color area are adjusted by enlarging and reducing in the a * direction and the b * direction. If you want to adjust the yellowness of the skin color area, adjust the enlargement / reduction ratio in the b * direction. If you want to adjust the redness of the skin color area, adjust the enlargement / reduction ratio in the a * direction.

  Step S506 is a step that shifts uniformly. In accordance with the parameters (ΔL, Δa, Δb) stored in the common parameter storage unit 25, all the L * a * b * data output from step S505 are converted.

  Color adjustment is performed by the above steps.

  Note that the parameters stored in the common parameter storage unit 25 are parameters that are carefully tuned using either sRGB or AdobeRGB data.

  According to the embodiment described above, it is possible to easily create color conversion tables for sRGB and AdobeRGB for a color printer that have substantially the same visual impression in the common area of both color spaces.

3 is a flowchart illustrating image processing in a color printer. It is a figure showing the outline | summary of the color conversion table preparation method. It represents a color conversion table (three-dimensional lookup table). It is a figure which shows the hue direction of a 45 degree unit. It is a flowchart which shows a color adjustment process.

Explanation of symbols

10 Input Image Data 11 Input Gamma Processing Unit 12 Color Conversion Processing Unit 13 Ink Color Separation Processing Unit 14 Output Gamma Processing Unit 15 Halftone Processing Unit 20 sRGB Data 21 AdobeRGB Data 22 sRGB → L * a * b * Conversion Unit 23 AdobeRGB → L * a * b * conversion unit 24 color adjustment unit 25 common parameter storage unit 26 gamut mapping unit 27 device RGB calculation unit 28 sRGB color conversion table 29 AdobeRGB color conversion table

Claims (9)

A color conversion method for converting data of a first input color space and data of a second input color space into a color space of an arbitrary output device,
A first color space conversion step of converting the data of the first input color space into data of a device-independent color space;
A second color space conversion step of converting the data of the second input color space into data of the device-independent color space, and converting the data of the device-independent color space by a plurality of parameters, A color adjustment process for color adjustment;
A gamut mapping step for performing gamut mapping in accordance with the color reproduction range of the output device with the data color-adjusted in the color adjustment step;
Calculating data specific to the output device corresponding to the gamut-mapped data,
The conversion in the first and second color space conversion steps is conversion that results in a common color reproduction in a common area of the first input color space and the second input color space.
The color conversion method, wherein the plurality of parameters used in the color adjustment step are parameters common to the first input color space and the second input color space.   The color conversion method according to claim 1, wherein the data of the first input color space and the second input color space is RGB data.   3. The color conversion method according to claim 1, wherein RGBCMY hues of the first input color space and the second input color space are substantially the same hue.   The color conversion method according to claim 1, wherein the device-independent color space is an L * a * b * space. The plurality of parameters used in the color adjustment step include
L * a * b * values after color adjustment of black and white,
The enlargement / reduction ratio in each hue direction in 45 ° increments in L * a * b * space,
The device-independent data change amount ΔL * C * h corresponding to the position of the RGBCMY representative color in the RGB value of the input color space;
Gray line target chromaticity a * and b *,
The enlargement / reduction ratio in the a * direction and b * direction of the skin color area,
Overall shift amount ΔL * a * b *,
The color conversion method according to claim 1, further comprising: In the color adjustment step,
Color adjustment in the lightness direction by the L * a * b * value parameter after the black and white color adjustment,
Expansion / reduction of hue direction in 45 ° increments in L * a * b * space,
RGBCMY representative color position brightness, saturation, hue adjustment,
Chromaticity adjustment of the gray line,
Skin color area color adjustment and overall shift,
The color conversion method according to claim 5, wherein color adjustment is performed in the order of.   In the enlargement / reduction in the hue direction in 45 ° increments in the L * a * b * space, the enlargement / reduction rate of all hues is calculated by interpolation calculation from the enlargement / reduction rate specified in 45 ° increments. 6. The color conversion method according to claim 5, wherein the color conversion method is performed so as to be expanded or reduced so as to have a continuous connection in hue.   In the RGBCMY representative color position brightness, saturation, and hue adjustment, the lightness, saturation, and hue adjustment amounts are calculated by interpolation calculation based on the relative positional relationship with the representative color for data other than the representative color. The color conversion method according to claim 6, wherein brightness, saturation, and hue are adjusted so as to have continuous connections in all regions. In the color adjustment of the skin color area,
a *> 0, b *> 0, and
C * = (a * × a * + b * × b *) 1/2) <45 is regarded as a skin color region,
7. The color conversion method according to claim 6, wherein the a * and b * values of the data located in the skin color area are enlarged / reduced in the a * direction and the b * direction.

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