JP2009218780A - Image processing method, printer, color conversion device, and color conversion program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To associate color gamuts of image devices each other so that excellent color reproduction is performed while tone gradations are maintained when color conversion is performed based on the association. <P>SOLUTION: When there is an out-color-gamut area which is outside the color gamut of a second image device and in the color gamut of a first image device at a predetermined hue in a predetermined device independent color space, a peak value J<SB>0</SB>of a printer color gamut at the predetermined hue is detected. When a display color gamut is put close in shape to the printer color gamut, color conversion processing wherein saturation and lightness are mapped in the printer color gamut while both compressed is performed for a side of luminance higher than the peak value J<SB>0</SB>, and color conversion processing wherein saturation is compressed and mapped in the printer color gamut while luminance is held nearly constant is performed for a side of luminance lower than the peak value J<SB>0</SB>; and the correspondence between the display color gamut and printer color gamut is defined and image data of a display are converted into image data of a printer based upon the correspondence. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image processing method, a printer, a color conversion apparatus, and a color conversion program, and in particular, a second color image used in a second image device by using first image data used in the first image device. The present invention relates to an image processing method for converting data, a printer, a color conversion device, and a color conversion program.

Image devices such as displays and printers have different expressible color ranges (gamut) for each machine. For example, in order to represent image data expressed in the display color gamut in the printer color gamut, it is necessary to correlate colors that can be expressed only in the display color gamut with colors that can be expressed in the printer color gamut (gamut mapping). is there. This association includes a colorimetric type, a lightness-oriented type, and a saturation-oriented type. If the colorimetric type is used, the original image data is reproduced numerically and faithfully because it is associated with the nearest color gamut surface. However, if it is a lightness-oriented type, gradation can be maintained and a natural association without perceptual discomfort is possible, and if it is a saturation-oriented type, a color reproduction can be improved and an attractive association can be achieved. . Patent Documents 1 to 3 describe examples of various gamut mappings.
JP 2002-118762 A JP 2005-348053 A Japanese Patent Laid-Open No. 2002-314832

However, in the saturation-emphasized type, the gradation is impaired as compared with the lightness-oriented type. Even if it is a lightness-oriented type, it is difficult to perceive a sense of incongruity due to maintaining gradation, but it has a conversion source color gamut that has a wide color gamut in the saturation direction and a narrow color gamut in the saturation direction. If the image is compressed into a non-conversion destination color gamut, the color in the compressed area is lost and the color reproduction deteriorates.
The present invention has been made in view of the above problems, and in associating color gamuts between image devices, an image processing method, a printer, a color conversion apparatus, and a color processing method capable of performing good color reproduction while maintaining gradation The purpose is to provide a color conversion program.

  In order to solve the above-described problems, an image processing method according to the present invention converts first image data used in a first image device into second image data used in a second image device. It is a method, Comprising: It is set as the structure provided with a detection process, a color gamut conversion process, and a color conversion process.

  The color gamut conversion processing of the present invention is performed when there is an out-of-gamut region outside the color gamut of the second image device and within the color gamut of the first image device in a predetermined hue of a predetermined device-independent color space. First, in the detection step, the lightness in the maximum saturation of the color gamut of the second image device in the predetermined hue is detected. Next, in the color gamut conversion step, the detected lightness is used to detect the color gamut shape of the first image device closer to the color gamut shape of the second image device. A color gamut conversion process for compressing both the saturation and lightness on the higher lightness side than the lightness and mapping it within the color gamut of the second image device, and on a lightness side lower than the lightness detected in the detection step The color gamut of the first image device and the color of the second image device are obtained by performing a color gamut conversion process for mapping light intensity within the color gamut of the second image device while maintaining lightness substantially constant. It defines the correspondence with the color gamut. Then, based on the correspondence defined in the color gamut conversion step, the first image data is converted into second image data in the color conversion step.

  Here, various color spaces can be adopted as the device independent color space, and various absolute color spaces such as Lab, Jab, and XYZ can be adopted. The first and second image devices are not particularly limited, and may be any device that uses image data to handle a predetermined image. For example, various image devices such as a display, a printer, a scanner, and a digital camera can be employed. is there. Of course, the device does not have to be a separate body. For example, it can be said that a fax machine includes a scanner as a first image device and a printer as a second image device, and the present invention is applied to such a fax machine. It is also possible to apply. In addition, the above processing is performed within a predetermined hue and is not performed actively to change the hue. However, the processing may be changed depending on the nature of the device independent color space to be processed, the conversion of the space, the machine error of the imaging device, etc. It is not forbidden that the hues of the two image data and the first image data are different. Even if the hue is not changed actively, a hue change of about “0 to 10 degrees” may occur over the entire area where the gamut conversion process is performed. Note that the color gamut conversion processing according to the present invention can be performed not only for a single out-of-gamut region but also for a plurality of out-of-gamut regions.

  According to the above configuration of the present invention, when color gamut conversion is performed, both color saturation and lightness are compressed on the high lightness side with the lightness detected in the detection step as a boundary. Since the compression is performed while maintaining the lightness on the lightness side lower than the lightness detected in the detection step, the gradation is improved.

  In addition, as a selective aspect of the present invention, in consideration of the point that the saturation gradually decreases as the brightness departs from the brightness detected in the detection step in the shape of the color gamut shell cut at a predetermined hue angle. In the color conversion step, when the color gamut shape of the first image device is brought closer to the color gamut shape of the second image device on the lightness side higher than the lightness detected in the detection step, A color gamut conversion process may be performed in which the lightness compression rate is gradually decreased as the lightness approaches the lightness detected in the detection step so as to be mapped into the color gamut of the first image device. According to this configuration, since the lightness compression rate increases as the lightness rises from the lightness detected in the detection step, the color reproducibility is good in the high lightness region and the gradation is maintained in the low lightness region. The color gamut conversion process performed is realized.

  In addition, as a selective aspect of the present invention considering that different color gamut conversion processing is performed on the high brightness side and the low brightness side with the brightness detected in the detection step as a boundary, in the color gamut conversion step, The compression ratio and the compression direction on the higher brightness side than the brightness detected in the detection step when the color gamut shape of the first image device approaches the color gamut shape of the second image device are The color gamut conversion processing may be performed by compressing the compression rate and the compression direction on the lower lightness side than the lightness detected in the detection step so as to match the lightness detected in the detection step. According to this configuration, since the compression rate and the compression direction are continuous before and after the lightness detected in the detection step, color conversion around the lightness detected in the detection step is smooth. In addition, it is possible to prevent a lightness inversion phenomenon in the lightness detected in the detection step.

  Further, as a selective aspect of the present invention regarding the compression direction on the higher lightness side than the lightness detected in the detection step, in the color gamut conversion step, the lightness higher than the lightness detected in the detection step. To bring the color gamut shape of the first image device closer to the color gamut shape of the second image device on the degree side, toward the lightness detected in the detection step on the lightness axis of the device independent color space. A configuration may be adopted in which a color gamut conversion process is performed in which the image is compressed and mapped into the color gamut of the first image device at a compression rate corresponding to the distance from the color gamut outer shell of the second image device. According to this configuration, color gamut conversion processing is realized in which the lightness compression rate gradually increases and the chroma compression rate decreases as the lightness increases from the lightness detected in the detection step.

  In addition, as a selective aspect of the present invention regarding the compression destination of the out-of-gamut region, in the color gamut conversion step, the first image device may have a higher lightness side than the lightness detected in the detection step. In bringing the color gamut shape closer to the color gamut shape of the second image device, the compression destination of the out-of-gamut region is a predetermined range from the outer shell of the second image device, and the out-of-gamut region and the predetermined range are A color gamut conversion process that normalizes the predetermined range in the compression direction may be performed.

  The above-described image processing method includes various aspects such as being implemented as part of another method or being realized as an image processing apparatus having means corresponding to each process. The present invention can also be realized as an image processing system including the image processing apparatus, a program for causing a computer to implement functions corresponding to the above-described method configuration, a computer-readable recording medium storing the program, and the like. . The inventions of the image processing system, the image processing apparatus, the image processing program, and the medium on which the program is recorded also have the above-described operations and effects. Of course, the configurations described in claims 2 to 6 are also applicable to the system, the apparatus, the program, and the recording medium.

Hereinafter, embodiments of the present invention will be described in the following order.
(1) Configuration of the present invention:
(2) Printing process:
(3) LUT creation processing:
(4-1) Color gamut conversion processing:
(4-2) Modification of color gamut conversion process:
(5) Black spot correction processing:
(6) Smoothing processing of peak value fluctuation between hues:
(7) Summary:

(1) Configuration of the present invention:
FIG. 1 shows a schematic hardware configuration of a computer that executes a color conversion program according to the present invention, and FIG. 2 shows a schematic configuration when the color conversion program is realized as a color conversion module 21b on the OS of the computer. The figure is shown. The computer 10 includes a CPU 11 serving as the center of arithmetic processing. The CPU 11 can access a ROM 13 including a RAM 12 and a BIOS that can be used as a work area via a system bus 14. A hard disk drive (HDD) 15 as a non-volatile storage device is connected to the system bus 14, and the OS 20, application program (APL) 25, etc. stored in the HDD 15 are transferred to the RAM 12, and the CPU 11 is transferred to the ROM 13 and RAM 12. Run the software with appropriate access. That is, various programs are executed using the RAM 12 as a temporary work area.

  An operation input device 30 such as a keyboard and a mouse and a display 18 as a display device are also connected to the computer 10. Further, it can be connected to a printer 40 as a printing apparatus via a printer I / O 19b. Although the configuration of the computer 10 has been described in a simplified manner, a computer having a general configuration can be employed as a personal computer. Of course, the computer to which the present invention is applied is not limited to a personal computer. Further, the connection interface between the computer 10 and the printer 40 need not be limited to the above-mentioned ones, and various connection modes such as a serial interface, SCSI, and USB connection can be adopted. It is.

  In the present embodiment, each program type is stored in the HDD 15, but the recording medium is not limited to this. For example, it may be a flexible disk or a CD-ROM. Programs recorded on these recording media are read by the computer 10 via a flexible disk drive or a CD-ROM drive, and installed in the HDD 15 or directly executed. Then, it is read onto the RAM 12 to control the computer. The recording medium is not limited to this, and may be a magneto-optical disk or the like. It is also possible to use a non-volatile memory such as a flash card as a semiconductor device. When accessing and downloading an external file server via a modem or communication line, the communication line becomes a transmission medium. The present invention is utilized.

  On the other hand, as shown in FIG. 2, in the computer 10 according to the present embodiment, the color gamut conversion module 24, the printer driver (PRTDRV) 21, the operation input device driver (DRV) 22, and the display driver (DRV) 23 are included in the OS 20. It has been incorporated. The display DRV 23 is a driver that controls display of image data and the like on the display 18, and the operation input device DRV 22 is a driver that receives a code signal from the keyboard and mouse and receives a predetermined input operation. The color gamut conversion module 24 creates a correspondence relationship between the color gamuts (color reproduction gamuts) of different devices and, based on the correspondence relationship, changes the color gamut color of one device to the color gamut color of another device. Convert to Note that the color and gradation reproducibility can be adjusted when this conversion is performed (hereinafter, adjustment of color and gradation reproducibility is referred to as color gamut adjustment). In addition, the APL 25 can be executed on the OS.

  The APL 25 can read the image data 15a recorded on the displayed HDD 15 to the RAM 12 and execute processing such as retouching. The display DRV 23 can display an image on the display 18 based on the image data 15a read to the RAM 12. Display. When the user operates the operation input device 30, the operation content is acquired via the operation input device DRV 22 and the content is interpreted, and the APL 25 executes printing or retouching according to the operation content. Various processes are performed.

In addition, the user can reproduce and display an image formed in a predetermined device by performing setting processing by operating the operation input device 30 under execution of the APL 25. For example, when displaying the image data created in the sRGB color space, when the display color gamut is narrower than the sRGB color space, the portion that protrudes from the display color gamut is a predetermined color within the color gamut of the display 18. It can be adjusted to perform color conversion as expressed. Then, the gradation and color expressed in the image data are maintained in the display display.
In addition, the print result of the image data can be reproduced on the display. That is, the color conversion of the image data is performed so that only the colors that can be expressed in the color gamut of the printer 40 in the color gamut of the display 18 are expressed. Of course, when reproducing the print result, color conversion may be performed so that the gradation and color of the image data are maintained at the same time. Furthermore, the color image of the print result that has been adjusted and displayed in this manner may be printed by the printer 40. Hereinafter, as an example of color gamut adjustment executed by the color gamut conversion module, an example in which display is performed while adjusting the print result of the printer 40 will be described.

FIG. 3 shows an example of a screen for performing color gamut adjustment setting processing. In the setting process, for each device on the input side and output side, a profile (Device Model Profile) for converting the device color gamut in a predetermined standard color space (for example, sRGB color space) into a predetermined device independent color space. : DMP). Then, a profile (Gamut Map Model Profile: GMMP) that specifies a different color gamut adjustment or conversion method (Rendering Intent) is selected.
After performing the setting process, when the conversion of the color gamut is instructed, the color gamut conversion module 24 is activated, and the color gamut of the input device is changed to the color gamut of the output device while referring to the set DMP and GMMP. An LUT 15b for conversion is created and stored in the HDD 15. The creation of the LUT 15b will be described later. Next, when the user operates the mouse to instruct the display of the print result of the printer 40, an instruction to adjust the color gamut is made, and the image data 15a is transferred to the color gamut conversion module 24. When the image data 15a is transferred to the color gamut conversion module 24, color conversion processing is executed.

Here, the configuration of the color gamut conversion module 24 will be described with reference to FIG. The color gamut conversion module 24 of this embodiment includes the following modules M1 to M4. The XYZ value calculation module M1 converts the color gamut of the input device expressed by RGB values into CIE tristimulus values XYZ values while referring to the DMP of the input device. The XYZ value calculation module M2 converts the color gamut of the output device expressed by RGB values into CIE tristimulus values XYZ values while referring to the DMP of the output device. CA conversion module M3 went forward conversion of the color appearance (CA) conversion formula into XYZ values calculated in XYZ value calculating module M1, M2 calculates the ^J * ^a * ^{b * values,} J ^* a ^{* b *} Determine the color gamut of the input device in the color space. The color gamut mapping module M5 maps the color gamut of the input device determined by the CA conversion module M3 with reference to the GMMP to the color gamut of the output device calculated by the CA conversion module M3. The CA conversion module M4 calculates the XYZ value by performing the reverse transformation of the color appearance (CA) conversion formula on the J ^* a ^* b ^* value calculated by the color gamut mapping module M5, and outputs the color of the output device in the XYZ color space. Determine the area. In the color appearance transformation performed by the respective CA conversion module M3, M4, conversion in consideration of the input or observation conditions specified in the specified DMP for the output device (D _55, D _65, etc. source information) Executed.

  The profiles DMP1 to n and GMMP1 to GMMPn used in the modules M1 to M4 are stored in the HDD 15. Each DMP describes the correspondence for converting the color gamut of the device in the sRGB color space to the colorimetric values and the above observation conditions, and GMMP describes the conversion formulas and conditional expressions for associating the color gamuts. Has been. In this embodiment, the device color reproduction gamut described in the DMP is described as a color reproduction range in the sRGB color space, but may of course be expressed in another color space.

  The color gamut conversion performed using DMP includes forward conversion (corresponding to the AToB1Tag processing of the ICC profile) and backward conversion (corresponding to the BToA1Tagn processing of the ICC profile). The forward conversion is realized by a forward multidimensional LUT or a conversion formula for converting device colors into XYZ values. In reverse conversion, the result of forward conversion is used to reverse the forward multidimensional LUT, optimize the polynomial parameters by regression analysis, etc., and convert the generated XYZ values to device colors. This is realized by a multidimensional LUT or conversion formula to be performed.

In the color appearance conversion, the forward direction conversion of a color perception model such as CIECAM02 is applied to the XYZ values to obtain the color values in the color appearance space. The color value of this color appearance space (for example, J ^* a ^* b ^* value) can be calculated from the CIE tristimulus values XYZ. By using this J ^* a ^* b ^* , it is possible to perform color gamut mapping more suitable for human visual characteristics. The conversion for obtaining the color value J ^* a ^* b ^* of the color appearance space is performed by chromatic adaptation conversion, cone response conversion, and opposite color response conversion, and is converted so as to approach human visual characteristics. In the chromatic adaptation conversion, conversion corresponding to the light source under the observation environment is performed. The color perception model need not be limited to CIECAM02, CIECAM97s, etc., and human color perception parameters J (lightness), C (Chroma), Q (brightness), M (colorfulness), h (huequadrature or hueangle) , H (huequadrature or hueangle) as long as it is a color perception model, other perception models may be used.

  In the present embodiment, the color gamut conversion module 24 creates the LUT 15b and then executes the color gamut conversion process while referring to the LUT 15b. Of course, the color gamut conversion module 24 dynamically creates the LUT. It does not matter. For example, the color adjustment may be performed while creating the LUT in the order in which the color coordinates constituting the image data 15a are input. Further, the LUT 15b created in this way is stored in the HDD 15, and when the gamut conversion processing between the same gamuts is instructed, the color conversion processing is performed by referring to the created LUT. Is possible.

(2) LUT creation processing:
FIG. 5 is a flowchart of LUT creation work. Since this operation requires a lot of calculation processing, it is preferable to execute the calculation using a computer. The color gamut conversion processing in the present invention is performed by changing the lightness and saturation without positively changing the hue in a predetermined hue. That is, processing is performed on a plane that passes through the J axis and is parallel to the J axis in the J ^* a ^* b ^* color space. The area to be processed is a predetermined hue in which there is an area outside the conversion destination color gamut (printer color gamut) and within the conversion source color gamut (display color gamut). , Yellow phase to blue phase. Of course, this processing area is appropriately changed depending on the display, printer model, media used in the printer, ink type used in the printer, and the like.

The LUT created in the present embodiment is a correspondence relationship between RGB values and RGB values, but processing is performed in a Jab space, which is a device-independent color space, in order to consider color appearance. First, in step S200, the selected DMP1 for the display 18 is acquired to determine the color gamut of the display 18. For example, if the display 18 is expressed with 256 gradations for each color of RGB, the XYZ value calculation module M1 converts XYZ values into XYZ values using a known conversion formula while considering all combinations for each gradation of 256 colors. The color gamut of the display 18 in space is determined. The CA conversion module M3 further performs color appearance conversion in consideration of the observation conditions described in DMP1 with respect to the XYZ values converted in this way to calculate J ^* a ^* b ^* values. As described above, the LUT that defines the correspondence between the RGB values of the display 18 and the J ^* a ^* b ^* values is the first color conversion table. ^Then, J ^* a ^{* b *} gamut mapping module M5 which has received the value, the color gamut by performing the creation of a region encompassing the resulting ^J * ^a * ^{b *} values (for example, a three-dimensional convex hull and polygons, etc.) Confirm. The color gamut is a three-dimensional solid that encompasses the obtained J ^* a ^* b ^* values.

In step S210, the selected DMP2 is acquired for the printer 40, and the color gamut of the printer 40 is determined. If the CMYK gradation data of the printer 40 is expressed with 256 gradations for each color, DMP2 describes which range in the sRGB color space the color gamut that can be expressed with these gradations corresponds to. The XYZ value calculation module M2 determines the color gamut of the printer 40 in the XYZ color space if it is converted into XYZ values by a known conversion formula while considering the combination of 256 gradations of each RGB color in this range. Further, the CA conversion module M3 performs color appearance conversion in consideration of the observation conditions described in DMP2 with respect to the XYZ values to calculate J ^* a ^* b ^* values. The LUT that defines the correspondence between the RGB values of the printer 40 and the J ^* a ^* b ^* values is the second color conversion table. The ^{J * a} * ^b * gamut mapping module M5 which has received the value, the color gamut by performing the creation of the resulting ^{J * a} * ^b * including the value area (for example, a three-dimensional convex hull and polygons, etc.) Determine.

As described above, after determining the color gamuts of the display 18 and the printer 40, color gamut conversion processing is performed in step S220 to map the display color gamut in the printer color gamut in the J ^* a ^* b ^* color space. In the color gamut conversion process in the present application, mapping is performed in which both saturation and brightness are changed at high brightness, and saturation is changed while maintaining brightness at low brightness. Therefore, when reproducing the color of the display in the color gamut of the printer, color reproduction with high saturation is good at high lightness and looks good, and gradation expression in the dark part is good at low lightness. This color gamut conversion process will be described later.

  Since the correspondence between the display color gamut and the printer color gamut is defined by the above conversion, in step S230, representative points necessary for the interpolation calculation at the time of color conversion are extracted to create the LUT 15b. That is, the RGB values of the display 18 are converted into XYZ values based on the first color conversion table, the color gamut conversion processing is performed on the XYZ values, and the converted XYZ values are converted into the color reproduction gamut of the printer 40. By converting the RGB values of the printer 40 by reverse lookup based on the second color conversion table, an LUT 15b that associates the RGB values of the display 18 with the RGB values of the printer 40 in the sRGB color space is created. Since the XYZ values after the color conversion process are not necessarily the points described in the second color conversion table, the XYZ values after the color conversion process are searched from the second color conversion table for a close XYZ value. By specifying the XYZ values at several points surrounding the image and performing interpolation calculation from the RGB values corresponding to the specified XYZ values, the RGB values corresponding to the XYZ values after the color conversion process are determined.

  However, since conversion processing and calculation are performed before the RGB value is determined, the determined RGB value may be out of the printer color gamut in the sRGB color space. Therefore, in step S240, an area including the display 18 color gamut (for example, a three-dimensional convex hull or a polygon) is created to determine the color gamut in the sRGB color space, and a color gamut inside / outside determination process described later is performed. Become. The printer color gamut in the sRGB color space generated as a result has a one-to-one correspondence with the display color gamut in the sRGB color space. By converting the image data 15a with reference to the LUT 15b created as described above, an image color-converted into the color reproduction range of the printer 40 can be reproduced on the display 18.

  The above-mentioned DMP and GMMP can be added or changed as necessary, and it is easy to change the model of the display 18 or the printer 40, change the ink type or media type used in the printer 40, etc. Can respond.

(3) Printing process:
Here, the printing process using the LUT 15b will be described below with reference to the flowchart of FIG. When the PRTDRV 21 receives the print execution instruction, the image data acquisition module 21a acquires the image data 15a of the image A stored in the RAM 12 in step S100. In step S110, the color conversion module 21b is activated, and the RGB data of each dot in the image data 15a is converted into CMYK data. At this time, the color conversion module 21b generates CMYK data by interpolation with reference to the LUT 15b and the LUT 15c. The LUT 15b is created by performing a color gamut conversion process described later. Accordingly, the LUT 15b is a table that associates a predetermined color of the display 18 with a color having high color reproducibility while maintaining gradation in the printer 40. The LUT 15c is an LUT that converts sRGB values into CMYK values. In the present embodiment, the color gamut conversion module 24 creates the LUT 15b in advance, and the created LUT 15b is acquired and used by the color conversion module 21b. The color gamut conversion module 24 may create the LUT 15b and supply it to the color conversion module 21b in response to the request of the module 21b.

  When the color conversion module 21b performs color conversion to generate CMYK gradation data, the CMYK gradation data is transferred to the halftone processing module 21c in step S120. The halftone processing module 21c is a module that performs a halftone process for converting the CMYK gradation value of each dot and expressing it with the recording density of the ink droplets, and a head for attaching ink at the converted recording density. Generate drive data. In step S140, the generated print data is output via the printer I / O 19b. The print data generation module 21d receives the head drive data and rearranges them in the order used by the printer 40. That is, in the printer 40, an ejection nozzle array (not shown) is mounted as an ink ejection device, and in the nozzle array, a plurality of ejection nozzles are arranged in parallel in the sub-scanning direction, and therefore data separated by several dots in the sub-scanning direction. Are used simultaneously.

  Therefore, rasterization is performed in which data arranged in the main scanning direction is rearranged in order so that data to be used at the same time is buffered by the printer 40 at the same time. After the rasterization, print data is generated by adding predetermined information such as the resolution of the image, and is output to the printer 40 via the USB I / F 13. The printer 40 prints the image displayed on the display 18 based on the print data. In step S150, it is determined whether or not the above-described color conversion processing has been performed for all the rasters of the image data 15a, and the processes in and after step S100 are repeated until it is determined that the processing has been completed for all the rasters.

(4-1) Color gamut conversion processing:
Next, the color gamut conversion process in step S220 will be described in detail. FIG. 7 is a schematic diagram showing a state where the display color gamut and the printer color gamut are cut at a predetermined hue angle θ in the Jab space. In the figure, the horizontal axis represents saturation C (= (a ² + b ² ) ^1/2 ), and the vertical axis represents lightness J. In the present embodiment, in the color gamut conversion process, areas outside the printer color gamut and near the outer shell in the display color gamut are mapped near the outer shell of the printer color gamut. It should be noted that the internal area of the display color gamut excluding the vicinity of the outer shell of the printer color gamut is mapped to the same coordinate value.

In the color gamut conversion processing in the present embodiment, the boundary of lightness J ₀ of the coordinate indicating the maximum chroma C _max in the printer gamut cut at each hue angle (C _max, J _0), the high brightness side and the low brightness side Different mapping algorithms are adopted. Hereinafter, (C _max , J ₀ ) is referred to as a peak (cusp), and the brightness of the peak is referred to as a peak value. FIG. 8 is a diagram for explaining a compression algorithm in the present embodiment. The compression algorithm of the present embodiment is an algorithm that compresses both saturation and lightness on the higher lightness side than the peak value while placing importance on maintaining the lightness, such as SGCK (Sigmoidal Gaussian Cusp Knee). Compression algorithm), and on the lower brightness side than the peak value, an algorithm (second compression algorithm) that compresses the saturation while maintaining the brightness, such as BasicPhoto (registered trademark Microsoft), is adopted. It is. In the following description, SGCK and BasicPhoto will be described as examples.

In each compression algorithm, first, whether the display color gamut is within the printer color gamut is determined. In the SGCK inside / outside determination, the intersection (C _I , J _I ) between the straight line connecting (0, J ₀ ) and the color point (C, J) to be determined and the outer shell of the display color gamut is within the printer color gamut. It is determined whether or not there is. On the other hand, in the basic photo inside / outside determination, the intersection (C _I ', J') between the straight line J = J 'including the color point (C', J ') to be determined and the outer shell of the display color gamut is within the printer color gamut. It is determined whether or not. Here, in SGCK, the intersection (C _I , J _I ) is the determination target for the inside / outside determination, and in Basic Photo, the intersection (C _I ′, J ′) is the determination target for the inside / outside determination, because the SGCK compression direction is (0 , J ₀ ) direction and the compression direction of BasicPhoto is the J = J ′ direction. For other compression algorithms, the inside / outside determination of the intersection of the straight line extending in the compression direction from the determination target color point and the display color gamut outer shell is performed. Color points for which the intersection point is determined to be within the printer color gamut in this inside / outside determination are mapped without changing the coordinate values.

On the other hand, in order to determine whether or not the intersection (C _I , J _I ) or the intersection (C _I ′, J ′) in the inside / outside determination is out of the printer color gamut is a color point to be compressed. The compression target determination is performed. The compression target determination is performed by determining whether or not the color point to be processed is inside a predetermined uncompressed region P1 in the printer color gamut. The uncompressed area P1 in this embodiment is an area in which the printer color gamut is compressed to 90% in the saturation direction, and the saturation in the uncompressed area P1 is 90% or less of the saturation in the printer color gamut outer shell. The display color gamut located in the uncompressed area P1 is mapped to the same coordinate value of the printer color gamut without being compressed. On the other hand, the display color gamut that protrudes outside the uncompressed area P1 is linearly compressed and associated with the compressed area P2. The compression area P2 is an area set in the vicinity of the outer shell of the printer color gamut, and in this embodiment, the remaining printer color gamut excluding the non-compression area P1 corresponds to the compression area P2.

  In the present embodiment, the non-compressed area P1 is described as 90% of the printer color gamut. However, the present invention is not limited to this percentage, and any percentage greater than 0 and less than 100 can be adopted as appropriate. Further, it is not always necessary to specify the same percentage over the entire lightness range, and the non-compressed region P1 is a non-compressed region P1 as long as it is a region that includes the outer shell of the printer color gamut and extends inside the printer color gamut. Various ranges can be set by changing the size of the compression region P1.

When the compression target determination is completed, in the first compression algorithm, linear mapping as shown in FIG. 8 is performed on the color point determined as the compression target. FIG. 8 is a diagram for explaining the first compression algorithm. The mapping shown in the figure can be expressed by, for example, the following formula (1).

Here, (C, J) is the coordinate of the color point before compression, and (C _new , J _new ) is the coordinate of the color point after compression. In addition, on the straight line connecting (0, J ₀ ) and the color point (C, J), the distance from (0, J ₀ ) to the color point is set to D ₁ , (0, J ₀ ) to the uncompressed region P 1. _D 2 the distance to the outer shell of, as (0, _{J 0)} distance from to the outer shell of the display color gamut of _{D 3, (0, J 0} ) D 4 the distance to the outer shell of the printer gamut from is there.

That is, in the above formula (1), the distance from the outer shell of the non-compressed region P1 to the outer shell of the display color gamut (C ₁ -D ₂ ) D ₃ -D ₂ ) and the ratio of the distance (D ₄ -D ₂ ) from the outer shell of the non-compressed region P1 to the outer shell of the printer color gamut, so that the outer shell of the non-compressed region P1 and the display color gamut The positional relationship of the color point between the outer shell and the outer shell of the non-compressed region P1 is normalized and reproduced between the outer shell of the printer color gamut. As a result of the above compression, the color point having a lightness higher than the peak value among the color points outside the printer color gamut is mapped to the compression region P2 inside the printer color gamut by compressing the lightness and the saturation. In addition, the compression algorithm of Formula (1) is an example, and it does not matter even if it maps not only with linear mapping but with a nonlinear algorithm.

On the other hand, in the second compression algorithm, linear mapping as shown in FIG. 9 is performed on the color points determined to be compressed. FIG. 9 is a diagram for explaining the second compression algorithm. The mapping shown in the figure can be expressed by the following equation (2), for example.

Here, (C, J) is the coordinate of the color point before compression, and (C _new , J _new ) is the coordinate of the color point after compression. Further, the saturation of the outer shell of the uncompressed region P1 at lightness J is C ₁ , the saturation of the outer shell of the printer color gamut at lightness J is C ₂ , and the saturation of the outer shell of the display color gamut at lightness J is C _3. It is as. In the equation (2), the distance (C ₃ − from the outer shell of the non-compressed region P1 to the outer shell of the display color gamut with respect to the distance (C−C ₁ ) from the outer shell of the non-compressed region P1 to the color point. the C ₁₎ and multiplying the ratio of the distance from the outer shell of the uncompressed region P1 to the outer shell of the printer gamut (C 2 _{-C 1),} the outer shell and the outer shell of the display color gamut of the uncompressed region P1 The positional relationship between the color points is normalized and reproduced between the outer shell of the non-compressed region P1 and the outer shell of the printer color gamut. As a result of the above compression, among the color points outside the printer color gamut, color points having a lightness lower than the peak value are mapped to the compression region P2 within the printer color gamut by compressing the saturation while maintaining the lightness. . In addition, the compression algorithm of Formula (2) is an example, and it does not matter even if it maps not only with linear mapping but with a nonlinear algorithm.

Or more at the first compression algorithm described, as increased compression degree of brightness away from the peak value, the compression degree of brightness closer to the peak value is low, the brightness of the compression ratio in the peak J ₀ 0 It becomes. Of course, in the second compression algorithm, the lightness is maintained regardless of the lightness. That is, at the peak value, the compression rate and the compression direction of the first compression algorithm coincide with the compression rate and the compression direction of the second compression algorithm. Therefore, while adopting different compression algorithms across the peak value, continuity in the peak value is ensured between the mapped color points, and brightness reversal in the vicinity of the peak value is also prevented. Become. By preventing the reversal of lightness, it is possible to realize color gamut conversion that realizes a print result that gives a natural impression to human eyes that are more sensitive to lightness than saturation.

  FIG. 10 is a diagram for explaining the operational effect by using different compression algorithms with the above-described peak value as a boundary. This figure shows the gradation of green, and image data in which the brightness of green is continuously changed up and down is mapped and printed by each compression algorithm. FIG. 10A shows a printing result when mapping with BasicPhoto over the entire brightness range, FIG. 10B shows a printing result when mapping with SGCK over the entire brightness range, and FIG. Print results when mapping is performed using a compression algorithm are shown. In the printing result of FIG. 10A mapped with BasicPhoto, the gradation expression is poor at the upper high gradation, and in the printing result of FIG. 10B mapped with SGCK, there is a place where the brightness changes greatly in the dark part. For this reason, the gradation of the dark portion is poor and black crushing occurs. On the other hand, in FIG. 10C mapped by performing the color gamut conversion processing of the present embodiment, it can be seen that the gradation expression is rich in high saturation and the gradation is maintained in the dark portion.

FIG. 11 is a flowchart of the color gamut conversion process performed using the equations (1) and (2). In the figure, first, the hue angle θ to be processed is determined in step S300, and initial coordinates (C, J) at the hue angle θ are determined in step S305. And it obtains the peak value J ₀ of the printer gamut in the hue angle θ at step S310, the in step S315, performs outside judgment of the printer gamut to the color point of the initial coordinate. As a result of the inside / outside determination, if the inside of the printer color gamut is determined, the color is mapped to the color point of the printer color gamut having the same coordinate value as (C, J) in step S320. On the other hand, if it is determined that it is outside the printer color gamut, it is determined which compression algorithm to use in step S325 based on the peak value acquired in step S310. In step S330, the coordinates (C _new , J _new ) of the compression destination are determined according to the determined compression algorithm, and mapped to these coordinates.

In this way, when the mapping of the coordinates (C, J) is completed in steps S315 and S330, it is determined in step S335 whether or not the calculation of (C _new , J _new ) for all coordinates has been completed. Here, since this color gamut conversion process is a process as a process for creating an LUT, even if it is determined whether or not the calculation has been completed for all coordinates, it is necessary and sufficient to create the LUT. It is sufficient to determine whether or not the calculation for the coordinates has been completed. For example, the calculation is performed only on the integer coordinate value, or the calculation is performed only on the lattice point in consideration of the lattice point having a predetermined pitch in the Jab space. Various modes can be adopted.

  If it is not determined in step S335 that the calculation has been completed for all coordinates, the next calculation candidate coordinate (C, L) is set in step S345, and the processes in and after step S315 are repeated. If it is determined in step S335 that the calculation has been completed for all coordinates, it is determined in step S340 whether or not the gamut conversion processing has been completed for all hues, and it is determined that the gamut conversion processing has been completed for all hues. Steps S300 and after are repeated until

  Note that the color gamut conversion processing of the present invention is applicable not only when creating a static LUT as described above, but also when dynamically creating a mapping destination for an input sRGB value. . When continuously processing the color points of each hue angle as in the LUT creation, it is more efficient to determine the peak value in step S310 before the color gamut inside / outside determination in step S320 as in the flowchart described above. However, when the hue angle of the color point input in the dynamic mapping process changes randomly, it is more efficient to perform the peak value determination in step S310 after the color gamut inside / outside determination in step S320. Get better.

Incidentally, as a boundary the peak value J ₀ of the printer gamut in the embodiment described above, the high brightness side executes the color gamut conversion at a first compression algorithm, the low-brightness side in the second compression algorithm Although color gamut conversion is performed, in addition to switching the compression algorithm depending on whether the peak value is higher or lower than the peak value, for example, by switching the compression algorithm according to the similarity of the outer shell shape Also good. For example, if a color gamut having an upwardly convex outer shell shape is mapped to a color gamut having a downwardly convex outer shell shape, changes in lightness and saturation are emphasized, resulting in a sharp gradation. A big influence. In view of these circumstances, a brightness maintaining compression algorithm that maintains gradation is used when the bulge direction of the outer shell shape is different. Adopt a compression algorithm to compress the degree. Then, the gradation is appropriately maintained according to the color gamut shape.

(4-2) Modification of color gamut conversion process:
Next, a modification of the above-described color gamut conversion process will be described. In this modification, according to the degree of similarity of the color gamut shape between the printer color gamut and the display color gamut, the entire lightness is switched without switching the compression algorithm with the peak value or switching the compression algorithm as in the above-described embodiment. Select whether to apply a lightness-maintaining compression algorithm over a range. The determination of the similarity degree, utilizing the positional relationship between the peak value L _cusp 'of peak L _cusp and the display color gamut of the printer gamut. In other words, when the peak values are close to each other between the printer gamut and the display gamut, the gamut shapes are considered to be similar, and in mapping the color points outside the printer gamut in the display gamut into the printer gamut, The mapping is performed while maintaining the lightness substantially. Thus, by switching the compression algorithm based on the relationship between the peak value of the printer color gamut and the peak value of the display color gamut, the gradation after mapping is more appropriately maintained.

FIG. 12 shows a state in which the display color gamut Jab and the printer color gamut P are cut along the ab plane in the Jab space. (A) is a state cut at a green hue angle, (b) is a state cut at a magenta hue angle, and (c) is a state cut at a blue hue angle. In the figure, the horizontal axis represents saturation C, and the vertical axis represents lightness J. The peak value in each hue of the printer gamut has a different brightness value for each hue angle, with high brightness in green, medium brightness in magenta, and low in blue. Present in lightness. Therefore, if mapping between color gamuts where the peak value J ₀ and the peak value J ₀ ′ are close to each other, even if the color gamut conversion processing is performed with the lightness maintaining compression algorithm over the entire lightness range, If the mapping is between color gamuts where the peak value J ₀ and the peak value J ₀ ′ are separated, the brightness is shifted in addition to the saturation shift. Is preferable from the viewpoint of color reproduction.

FIG. 13 is a diagram for explaining how to switch the compression algorithm in the color gamut conversion processing of the present modification. As shown in the figure, in the color gamut conversion process of the present modification, first, the necessity of switching the compression algorithm is determined based on the peak value of each color gamut using the following equation (3).

Here J ₀ is the peak value of the printer gamut, J ₁ is the peak value of the display color gamut, J _w white point of the printer gamut (display color gamut), a. According to this equation (3), half of the white point interval of peak J ₀ as a threshold J _th, compression algorithms based on the magnitude relation between the difference J _sub and the threshold J _th between peak switching It is determined whether or not to perform. That is, when the difference between the peak values is larger than the threshold value (J _sub > J _th ), it is determined that the printer color gamut and the display color gamut are not similar, and the peak is the same as in the above-described embodiment. performing color gamut conversion processing is switched to a first compression algorithm to a value J ₀ at the boundary with the second compression algorithm. On the other hand, if the difference between the peak values is less than or equal to the threshold value (J _sub ≦ J _th ), it is determined that the printer color gamut and the display color gamut are similar, and for example, the lightness is maintained over the entire lightness range. In addition, the color gamut conversion process is executed by the second compression algorithm for compressing the saturation.

The compression direction of the first compression algorithm in the embodiment described above is the direction from the color point (C, J) to be processed toward the peak value (0, J _0p ) of the printer color gamut. In the example, a change corresponding to the difference J _th between the peak values may be added in this direction. Specifically, it is conceivable to set the compression direction in consideration of the angle according to the difference J _th as shown in the following formula (4).

Here, the vector n ₁ is a vector from the color point (C, L) to the peak value (0, L ₀ ) of the printer color gamut, and the vector n ₂ is from the peak value (0, L ₀ ) of the display. A vector toward the peak value (0, L ₀ ′) of the printer color gamut, and n _comp is a vector representing a compression direction in consideration of an angle corresponding to the difference L _th . By this equation (4), the n _comp direction obtained by multiplying the vector n ₁ by multiplying the vector n ₂ by a predetermined coefficient α is determined, and by compressing in the n _comp direction, the peak value of the lightness compression degree in the compression is determined. The difference between them will be reflected. That is, mapping can be performed in which the degree of lightness compression increases as the distance between peak values increases, and the degree of lightness compression decreases as the distance between peak values decreases. The embodiments lightness compression ratio in the peak value L ₀ of the printer gamut in the present modification becomes 0, the continuity with the compression direction and the compression ratio in peak L ₀ is maintained in the aforementioned It is the same.

FIG. 14 is a flowchart for performing a color gamut conversion process while selecting a mapping algorithm based on the equation (3). When the process is started, first, a hue angle θ at which a color point to be processed exists is determined in step S400, and initial coordinates (C, J) at the hue angle θ are determined in step S405. And it obtains the peak value J ₀ of the printer gamut in the hue angle θ at step S410, in step S415, performs outside judgment of the printer gamut to the color point of the initial coordinate. As a result of the inside / outside determination, if the inside of the printer color gamut is determined, the color is mapped to the color point of the printer color gamut having the same coordinate value as (C, J) in step S420. On the other hand, if it is determined that it is outside the printer color gamut, the peak value J ₀ of the printer color gamut and the peak value J ₀ ′ of the display at the hue angle θ are determined in step S425, and the difference between these peak values. J _sub is compared with the threshold value J _th . If the difference L _sub is smaller than L _{th as} a result of the comparison, it is determined in step S430 that the algorithm is not switched, and the process proceeds to step S435, where only the algorithm that maintains the lightness without switching the compression algorithm is used. Decide to do the mapping. On the other hand, when it is determined that the difference L _sub is larger than L _{th as} a result of the comparison, the process proceeds to step S440, and it is determined to perform mapping while switching the compression algorithm. Then, mapping is performed using the determined method. Then, in accordance with the determined compression algorithm, the coordinates (C _new , J _new ) of the compression destination are determined in step S345 and mapped to these coordinates.

When the mapping of the coordinates (C, J) is completed in steps S420 and S445 in this way, it is determined in step S450 whether or not the calculation of (C _new , J _new ) for all the coordinates has been completed. If it is not determined in step S450 that the calculation has been completed for all coordinates, the next calculation candidate coordinates (C, L) are set in step S455, and the processes in and after step S415 are repeated. If it is determined in step S450 that the calculation has been completed for all coordinates, it is determined in step S460 whether or not the gamut conversion processing has been completed for all hues, and it is determined that the gamut conversion processing has been completed for all hues. Step S400 and subsequent steps are repeated until

(5) Black spot correction processing:
In the color gamut conversion processing described above, an example has been described in which the brightness range is substantially the same in the display color gamut and the printer color gamut, that is, the black dots are substantially the same. However, in general, a printer compared with a display has a low black reproduction capability, and when the white points of the color gamuts of both devices are matched, the display color gamut becomes wider in the dark part and the black dots do not match. If the black dots do not match, a plurality of gradation values in the dark portion of the display color gamut are mapped to the black dots in the printer color gamut, reducing the gradation of the dark portion in the color gamut conversion process and causing color collapse. Therefore, in the mapping algorithm such as SGCK, in order to maintain the darkness of the dark portion, black point correction processing is performed in advance with an S-shaped curve or the like before mapping to match the black points between the color gamuts to be mapped. Has been done.

Here, the black point correction process using the S-curve of SGCK will be briefly described. SGCK S-curve correction is performed using the following equation (5).
In the above formula (5), J _O is the lightness of the color point that is input, the weight J _S lightness that S-curve correction using a sigmoid function like the J _O, the p _c which depends on the saturation of the input color point The coefficient, _JR, is the brightness after black point correction. In _JS , as shown in FIG. 15, the portion where the outer shell of the printer color gamut is inside the display color gamut is S-curve corrected by the sigmoid function, and the outer shell of the printer color gamut is outside the display color gamut. The part is linearly corrected. J _R is obtained by linear combination of the _{J O} and _{J S} in accordance with the _{p c,} saturation by _{p c} is considered. That is, in the _JR in the low saturation region, the effect of _JS appears strongly, and the gradation is sufficiently reproduced over the entire brightness region as shown in FIG. 16, but in the _JR as the saturation increases. effect of J _O becomes stronger, it leaves a site even after S-curve correction are mapped from the output device gamut in position outside the low brightness side as shown in FIG. 17.

  Therefore, in the present embodiment, the printer color gamut is roughly the region after the display color gamut is corrected by the SGCK S-curve, which is corrected to a lower lightness side than the black spot of the printer color gamut. The linear compression is applied to a predetermined area in the dark part. That is, after correcting with the SGCK S-curve, the dark part where the gradation is crushed is shifted to a higher lightness side than the black point of the printer color gamut, and the correction is performed to appropriately reproduce the gradation of the dark part of the display color gamut. I do. Of course, this correction is also made so as not to affect the medium brightness area and the high brightness area other than the dark part like SGCK. Although the present embodiment will be described as a black point correction process that complements the SGCK black point correction process, the black point correction process of the present invention is performed to complement a saturation-dependent black point correction process such as SGCK. Needless to say, the present invention is not limited to this case, and it is needless to say that it can be universally used as a process for matching the black dots between the color gamuts prior to the color gamut conversion process.

  FIG. 18 is a diagram for explaining the concept of the present embodiment. As shown in the figure, in the black point correction of the present invention, the black point of the display color gamut is matched with the black point of the printer color gamut with respect to the lightness after the S-curve correction of SGCK, and the entire lightness range of the display color gamut is obtained. Then, the lightness exceeding the black point is corrected so as to be output, and the portion where the output is lower than the black point of the printer color gamut is corrected to the high lightness side.

  The black spot correction itself can be achieved by simply associating the predetermined area that extends to the low brightness side of the printer color gamut with the predetermined area on the high brightness side of the black spot, but the gradation is maintained within the predetermined area. There are important and non-critical sites. Therefore, in the present embodiment, three regions having different compression rates and compression destinations are set in the dark portion of the display color gamut. The compression rate is constant in each region, and each region is linearly compressed with respect to the compression destination. The areas are set according to the importance of gradation, and for areas where gradation is important, the compression rate is lowered while securing a wider compression destination area, and for areas with low importance, the compression ratio is reduced. While compressing, the area of the compression destination is secured narrowly. Of course, you may set the compression rate and compression destination according to the importance level of gradation by nonlinear compression without setting the region, or perform non-linear compression with different compression rate in each region while setting the region. It doesn't matter.

The details of this black point correction processing will be described below with reference to FIG. First, the region setting procedure will be described. In the area setting, first, the black point J _DB of the printer color gamut on the lightness axis and the black point J _SB of the display color gamut after the S-curve correction are determined. This black point J _SB is a saturation-dependent value obtained by the above equation (5), and can be represented as a locus T ₁ representing the lightness that decreases as the saturation increases. Then, by multiplying the coefficient considering the peak value J ₁ of the display gamut in the trajectory T ₁ of the obtained black point J _DB alpha, calculates the trajectory T ₂ in consideration of the peak value. The coefficient α considering the locus T ₂ and the peak value J ₁ is determined by the following equation (6).

Next, as an axis of the straight line of symmetry of the black point of the printer gamut (J = J _DB), to determine the trajectory T ₃ with the trajectory T ₂ is moved line-symmetrically by the following equation (7).

Low brightness side is the first region A ₁ than the trajectory T ₂ determined above. Note that the color point that is the black point correction target of the present embodiment actually exists only in the region sandwiched between T ₁ and T ₂ of the first region A ₁ by the SGCK black point correction. Further, the trajectory _{T 2} and J _{= J DB} in a region surrounded by the second region _{A 2,} and the trajectory _{T 3} and J _{= J DB} and the region surrounded by the third region _{A 3.}

In forming the track T ₂ and track T ₃ in the above area forming, by considering the peak value, a region including the peak value J ₁ of the printer gamut, not a compressed black point correction in the present embodiment To be adjusted. That is, when the peak value J ₁ of the display gamut are present in high lightness, trajectory T ₃ is set to a higher brightness side, if the peak value J ₁ of the display gamut are present in low lightness, trajectory T ₃ is set to a lower brightness side. As a result, the peak value J as the brightness of _one lower second region A ₃ narrowing with brightness peak value J ₁ spreads the higher third region A _3, the compression of the peak value J ₁ is black point compensation It is possible to set an area that enables black point correction to maintain the gradation of the dark part to the maximum while preventing the fluctuation.

Next, a region serving as a compression destination of each region will be described with reference to FIGS. Figure 20 is a diagram for explaining a first area A ₁ of the compression target, Figure 21 is the figure, FIG. 22 for explaining the compression target second area A ₂ is a diagram for explaining a compression destination of the third region A ₃ . First, as shown in FIG. 20, the first area A ₁ is corrected to black point J _DB printer gamut uniformly. Next, as shown in FIG. 21, the second area A ₂ is corrected toward the area B ₂ on the low brightness side 75% of the third area A ₃ while being linearly compressed at a compression rate of 0.75. Correction formula of the second region A ₂ is represented by the following formula (8).

In the above equation (8), J _out is the lightness after the black point correction of this embodiment, and T ₁ (C) is the lightness of the locus T ₁ in the saturation C of the color point to be black point corrected. Then, the third area A ₃ is corrected toward the area B ₃ on the high brightness side 25% of the third area A ₃ while being linearly compressed at a compression rate of 0.25 as shown in FIG. Correction equation of the third region A ₃ is represented by the following formula (9).

In Expression (9), J _out is the lightness after the black point correction of the present embodiment, and T ₂ (C) is the lightness of the locus T ₂ in the saturation C of the color point that is the black point correction target.

Black point correction performed using the areas A _{1 to} A ₃ set as described above will be described with reference to FIG. FIG. 23 is a flowchart of the black point correction process. When the process is started, S-curve correction of SGCK is executed in step S500. Then, in step S505, it determines the black point _{J DB} printer gamut, the input color point (J, C) and black point _{J SB} in saturation corresponding to. Then compares the black point _{J DB} and black point _{J SB} in step S510. As a result of the comparison, if the black point J _DB is larger than the black point J _SB , the process proceeds to step S515, and the brightness J _out obtained by performing the S-curve correction of the equation (5) without performing the black point correction of this embodiment is output. To do. On the other hand, if the black point _{J DB} is less than the black point _{J SB} is configured to determine a trajectory _{T 1} proceeds to step S520, the above equation (6), determines the trajectory _{T 2} and track _{T 3} by the equation (7) . In step S525, the coordinate value (J _R , C) obtained by correcting the color point to be processed with the S-curve correction using equation (5) is compared with the locus T _{1 of the} black point L _SB . Result of the comparison, the coordinate value in the case of the trajectory _{T 1} below and outputs the black point _{J DB} of the printer gamut in step S530 as a _{J out.} On the other hand, if the coordinate values _(J R, C) is greater than the trajectory _{T 1} performs the comparison between the black point _{J DB} coordinate values _(J R, C) a printer gamut in step S535. If (J _R , C) is equal to or less than the black spot J _DB as a result of the comparison, the brightness J _out converted into the region B ₂ while being compressed at the compression rate of 0.75 according to the equation (8) in step S540 is output. . On the other hand, if the coordinate values _(J R, C) is larger than the black dots _{J DB} coordinate values at step S545 _(J R, C) and is compared with the trajectory T3. As a result of the comparison, if the coordinate value (J _R , C) is equal to or smaller than the trajectory T _3, the lightness J _out converted into the region B ₃ while being compressed at the compression rate 0.25 according to the equation (9) in step S550 Output. On the other hand, if the coordinate value (J _R , C) exceeds the trajectory T3, there is no need for black point correction, so the process proceeds to step S555, and the value is output as J _out . When the above black point correction is completed, the color point (J _out , C) after the black point correction is mapped to the printer color gamut by the above-described color gamut conversion processing.

  In the present embodiment, the example of correcting the brightness after the black point correction process using the SGCK S-curve has been described. However, the black point correction according to the present invention is performed in the color of the output device in the dark portion rather than the color gamut of the input device. If the gamut is narrow, the present invention can be applied to any color gamut conversion processing.

(6) Smoothing processing of peak value fluctuation between hues:
In the above color gamut conversion processing and black point correction processing, processing using the peak values of the printer color gamut and display color gamut is performed. FIG. 24 is a graph in which the peak value in each hue of the printer color gamut is plotted with respect to the peak value. In the figure, portions where the amplitude occurs in small increments are observed at hue angles of about 110 °, 130 °, and 260 °. In a portion where a small amplitude is generated, when the above-described color gamut conversion processing or black point correction processing is performed, inversion of brightness occurs before and after the amplitude, and a pseudo contour may occur. In order to prevent the occurrence of such a pseudo contour, it is preferable to smooth the gradation between hues. Hereinafter, processing for smoothing peak value fluctuations between hues for preventing the occurrence of pseudo contour will be described.

In the smoothing of the peak value variation between hues in the present embodiment, the peak value of the hue having the maximum saturation is used as a reference point for smoothing. In general, the primary color has a high color expression capability, so it is easy to obtain the maximum saturation. Therefore, first, the peak value of the primary color in the printer color gamut (in this embodiment, RGBCMY is assumed as the primary color of the additive color mixture and subtractive color mixture) is adopted as a smoothing reference point. ) To smooth the peak values between adjacent primary colors.

Here, L ₀ (θ) is a peak value in a predetermined hue θ (θ _B ? Θ? Θ _M ), L _0M is a peak value in a magenta hue θ _M , and L _0B is a peak value in a blue hue θ _B. The value Rate is a coefficient for linear interpolation between the peak values of adjacent primary colors. In addition, although the formula for smoothing the hue angle of blue to magenta is illustrated in the formula (10), the peak value between other primary colors can be smoothed by the same formula. FIG. 25 is a graph obtained by approximating the graph of FIG. 24 using the peak value of the primary color. It can be seen that the portion that was oscillating in small increments was smoothed. If interpolation is performed between the peak values of the default primary color in this way, peak value calculation processing and interpolation processing become very simple.

By the way, there is a case where the primary color does not take the maximum saturation, for example, as in the JCh color space. In such a case, first, a reference point having the maximum saturation is detected. That is, instead of the primary color, the peak value of a hue whose inclination of the peak value greatly varies is used as a reference point. That is, the hue angle θ that the degree of change in peak J ₀ to the change in hue varies greatly (hereinafter, referred to as the slope change point I.) To a plurality of points detected. Then, by interpolating between the peak values L ₀ (I) at each gradient fluctuation point I with a predetermined interpolation formula, the change in the peak value L ₀ with respect to the change in hue is smoothed.

In order to detect the gradient fluctuation point I, first, the peak value J ₀ of the color gamut obtained by cutting the printer color gamut at a predetermined hue angle is sequentially obtained while changing the hue angle θ by a predetermined amount, and the peak value J for the hue is obtained. ₀ fluctuation data is acquired. Then setting a predetermined hue range R, the degree of change of the slope of the peak value J ₀ at a predetermined hue range R within a predetermined hue range R of the acquired variation data, by a predetermined amount a predetermined hue range R Calculation is performed over the entire hue while shifting. The minimum value of the predetermined hue range R set at this time needs to be set wider than the small amplitude described above. Then, for example, twelve hue angles θ _{1 to} θ ₁₂ are selected in descending order of variation, and set as gradient variation points I _{1 to} I _{12 in} ascending order of hue angles. As a more specific method of detecting the gradient variation point, a point where the sign of the gradient changes before and after the gradient variation point, such as a, b, and c in FIG. It is possible that the inclination sign itself does not change as in 24d, but the inclination changes relatively large. In practice, the portion that is likely to be set at the above-described gradient variation point is often an extreme value or an inflection point.

Then, the peak value of the determined gradient change point is interpolated by the following interpolation formula (11).

In the equation (11), J ₀ (θ) of θ _n ? Θ? Θ _{n + 1} can be obtained. _Here, J 0 _{(I n)} is the peak value of the gradient change point _{I n, J 0} (θ) is the peak value of the hue angle theta, Rate 'is linear interpolation between peak values of adjacent reference points It is a coefficient. According to the above equation (11), the peak value between the peak values of each gradient fluctuation point is smoothed, and the amplitude can be eliminated in small increments. The interpolation between the peak values of the reference points is not limited to linear interpolation, and high-order interpolation such as cubic interpolation may be used.

FIG. 26 is a flowchart showing the flow of processing for smoothing peak value fluctuations between hues. In the present embodiment, in step S600, the peak value at each hue angle is obtained for every hue angle. In step S605, the inclination of the acquired peak value with respect to the hue change is detected, and the hue angles θ _{1 to} θ ₁₂ having a large inclination change are determined as the gradient change points I _{1 to} I ₁₂ in ascending order of the hue angle. In step S610, the peak values L (I ₁ ) to L (I ₁₂ ) at the gradient fluctuation points I _{1 to} I ₁₂ are linearly interpolated using the equation (11). If the above-described color gamut conversion processing or black point correction processing is performed using the peak value of each hue obtained as described above, the mapping vector direction is stabilized between the hues, resulting from the peak value change with respect to the hue change. Thus, the small amplitude is suppressed, and the pseudo contour is not generated.

  Further, in smoothing the peak value fluctuation between hues, in addition to the six primary colors described above, an intermediate value of each primary color is added to the reference point, or a reference point between the detected gradient change points I is added. If a plurality of values are added, the accuracy of approximation of the interpolated peak value to the actual peak value can be expected. Further, as a method of determining the gradient fluctuation point I described above, for example, only a portion where the gradient changes positively or negatively may be adopted as the gradient fluctuation point I. This is because when the sign of the slope does not change, approximation can be performed by interpolation or the like, and it is assumed that there is no significant deviation from the actual peak value. Of course, the interpolation is not limited to linear interpolation between peak values, but it is of course possible to perform interpolation in which each peak value is approximated nonlinearly by a cubic interpolation equation or the like. When cubic interpolation is used, approximation accuracy is improved when the fluctuation of the peak value at the reference point is moderate.

Furthermore, the above-described embodiments may be combined as a suitable modification when it is unclear whether the primary color is a gradient change point. That is, while setting the primary color RGBCMY as a temporary gradient change point, the peak value J ₀ of each hue included in the predetermined hue range R substantially centered on the primary color is sequentially obtained, and the obtained fluctuation data The degree of inclination variation is calculated. If the degree of change exceeds a predetermined reference, the primary color RGBCMY is adopted as the true gradient change point. On the other hand, if the degree of variation does not exceed a predetermined standard, there is a possibility that another true gradient variation point may exist, so the search for the hue in which the inclination of the peak value varies greatly is performed. . In this way, it is possible to reliably adopt the true gradient change point by performing the process of confirming whether the primary color is really the gradient change point, while assuming that the primary color is likely to be the gradient change point. Furthermore, when determining whether or not the primary color is a gradient change point, the amount of calculation can be minimized by making only the predetermined hue range R of each primary color a check target. In general, assuming that the primary color is often a gradient change point, the case where the primary color is not a gradient change point is an exception process.

  The above-described processing for smoothing the peak value between hues is preferably applied to a printer color gamut having uneven gamuts, particularly a color gamut such as photomat paper, but of course may be applied to the display color gamut. Absent. In particular, if the gradient change point and the primary color match, instead of sequentially calculating the peak value of each hue angle, the peak value of each hue angle is predicted by the hue gradation smoothing process. It is also possible to improve the processing speed.

(7) Summary:
As described above, in the present embodiment, there is an out-of-gamut region outside the color gamut of the second image device and within the color gamut of the first image device in a predetermined hue of a predetermined device-independent color space. when detects a peak value J ₀ of the printer gamut in a predetermined color, when brought close to the display gamut to the shape of the printer gamut, saturation and lightness in peak J higher brightness side than ₀ A color gamut in which both are compressed and mapped into the printer color gamut, and on the lower brightness side than the peak value J _{0, the} saturation is compressed while keeping the brightness substantially constant. A conversion process is performed to define a correspondence relationship between the display color gamut and the printer color gamut, and display image data is converted into printer image data based on the correspondence relationship. Therefore, in associating the color gamuts between the image devices, it is possible to perform good color reproduction while maintaining gradation.

  Note that the present invention is not limited to the above-described embodiments and modifications, and the configurations disclosed in the above-described embodiments and modifications are mutually replaced, the combinations are changed, the known technology, and the above-described implementations. Configurations in which the configurations disclosed in the embodiments and modifications are replaced with each other or combinations are also included.

It is a block diagram which shows the schematic hardware constitutions of the computer which executes the color conversion program concerning this invention. It is a block diagram which shows schematic structure in case a color conversion program is implement | achieved as a color conversion module on OS of the computer. It is an example of the screen which performs the setting process of color gamut adjustment. It is a conceptual diagram of a color conversion module. It is a flowchart of LUT creation work. It is a flowchart which shows a printing process. FIG. 4 is a schematic diagram illustrating a state where a display color gamut and a printer color gamut are cut at a predetermined hue angle θ in a Jab space. It is a figure explaining a 1st compression algorithm. It is a figure explaining a 2nd compression algorithm. It is a figure explaining the effect by using properly a compression algorithm on the boundary of a peak value. It is a flowchart of a color gamut conversion process. A state in which the display color gamut Jab and the printer color gamut P are cut along the ab plane in the Jab space is shown. It is a figure explaining the switching method of the compression algorithm in a color gamut conversion process. It is a flowchart which performs a color gamut conversion process, selecting a mapping algorithm. It is a figure explaining S-curve correction of SGCK. It is a figure explaining S-curve correction of SGCK in a low saturation area. It is a figure explaining S-curve correction of SGCK in a high saturation area. It is a figure explaining the concept of the black point correction process concerning this embodiment. It is a figure explaining the area | region setting in the black point correction process concerning this embodiment. It is a figure explaining the area | region of the compression origin and compression destination in the black point correction process concerning this embodiment. It is a figure explaining the area | region of the compression source and compression destination in the black point correction process concerning this embodiment. It is a figure explaining the area | region of the compression source and compression destination in the black point correction process concerning this embodiment. It is a flowchart of the black point correction process concerning this embodiment. It is the graph which plotted the peak value in each hue of a printer color gamut. It is the graph approximated using the peak value of the primary color. It is a flowchart of the process which smoothes the peak value fluctuation | variation between hues.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Computer, 11 ... CPU, 12 ... RAM, 13 ... ROM, 14 ... System bus, 15 ... Hard disk drive (HDD), 15a ... Image data, 15b ... Look-up table (LUT), 18 ... Display, 19b ... Printer I / O, 20 ... OS, 21 ... Printer driver (PRTDRV), 21a ... Image data acquisition module, 21b ... Color conversion module, 21c ... Halftone processing module, 21d ... Print data generation module, 22 ... Input device driver for operation (DRV), 23 ... Display driver (DRV), 24 ... Color gamut conversion module, 25 ... Application program (APL), 30 ... Input device for operation, 40 ... Printer

Claims (8)

An image processing method for converting first image data used in a first image device into second image data used in a second image device,
When there is an out-of-gamut region outside the color gamut of the second image device and within the color gamut of the first image device in a predetermined hue of a predetermined device-independent color space,
A detection step of detecting brightness at a maximum saturation of a color gamut of the second image device in the predetermined hue;
In compressing the color gamut shape of the first image device based on the color gamut shape of the second image device, both the saturation and the lightness are higher on the lightness side than the lightness detected in the detection step. A color gamut conversion process for compressing the image into the color gamut of the second image device, and compressing the saturation while maintaining the lightness substantially constant on the lightness side lower than the lightness detected in the detection step. A color gamut conversion step of defining a correspondence relationship between the color gamut of the first image device and the color gamut of the second image device by performing a color gamut conversion process for mapping within the color gamut of the second image device. When,
A color conversion step of converting the first image data into second image data based on the correspondence relationship;
An image processing method comprising:   In the color gamut conversion step, a compression target is used to bring the color gamut shape of the first image device closer to the color gamut shape of the second image device on the lightness side higher than the lightness detected in the detection step. 2. The image according to claim 1, wherein a color gamut conversion process is performed in which the lightness compression rate is gradually reduced as the lightness of the first image device approaches the lightness detected in the detection step so as to be mapped into the color gamut of the first image device. Processing method.   In the color gamut conversion step, the compression rate on the higher lightness side than the lightness detected in the detection step in bringing the color gamut shape of the first image device closer to the color gamut shape of the second image device. Gamut conversion processing is performed by compressing so that the compression direction matches the compression rate and the compression direction on the lightness side lower than the lightness detected in the detection step at the lightness detected in the detection step. The image processing method according to claim 1 or 2.   In the color gamut conversion step, in bringing the color gamut shape of the first image device closer to the color gamut shape of the second image device on the lightness side higher than the lightness detected in the detection step, the device The first image device is compressed toward the lightness detected in the detection step on the lightness axis of the independent color space, and compressed at a compression ratio according to the distance from the color gamut outer shell of the second image device. The image processing method according to any one of claims 1 to 3, wherein a color gamut conversion process for mapping in the color gamut is performed.   In the color gamut conversion step, the color gamut of the first image device is brought closer to the color gamut shape of the second image device on the lightness side higher than the lightness detected in the detection step. A color gamut conversion process is performed in which a compression destination of an out-of-gamut region is set to a predetermined range from an outer shell of the second image device, and the out-of-gamut region and the predetermined range are normalized in a compression direction with respect to the predetermined range. The image processing method as described in any one of Claims 1-4. The second image device is a printer to which the first image data is input;
A color conversion table created based on the correspondence defined in the color gamut conversion step according to any one of claims 1 to 5 is provided, and color conversion is performed based on the color conversion table. Printer characterized by. An image processing apparatus for converting first image data used in a first image device into second image data used in a second image device,
When there is an out-of-gamut region outside the color gamut of the second image device and within the color gamut of the first image device in a predetermined hue of a predetermined device-independent color space,
Detecting means for detecting lightness in the maximum saturation of the color gamut of the second image device in the predetermined hue;
When compressing the color gamut shape of the first image device based on the color gamut shape of the second image device, both the saturation and the lightness are higher on the lightness side than the lightness detected by the detection means. Gamut conversion processing for compressing the image and mapping it within the color gamut of the second image device, and compressing the saturation while maintaining the lightness substantially constant on the lightness side lower than the lightness detected by the detection means Color gamut conversion means for defining a correspondence relationship between the color gamut of the first image device and the color gamut of the second image device by performing a color gamut conversion process for mapping within the color gamut of the second image device. When,
Color conversion means for converting the first image data into second image data based on the correspondence relationship;
An image processing apparatus comprising: An image processing program for causing a computer to realize a function of converting first image data used in a first image device into second image data used in a second image device,
When there is an out-of-gamut region outside the color gamut of the second image device and within the color gamut of the first image device in a predetermined hue of a predetermined device-independent color space,
A detection function for detecting lightness in the maximum saturation of the color gamut of the second image device in the predetermined hue;
In compressing the color gamut shape of the first image device based on the color gamut shape of the second image device, both saturation and lightness are higher on the lightness side than the lightness detected by the detection function. Gamut conversion processing for compressing the image and mapping it within the color gamut of the second image device, and compressing the saturation while maintaining the lightness substantially constant on the lightness side lower than the lightness detected by the detection function A color gamut conversion function for defining a correspondence relationship between the color gamut of the first image device and the color gamut of the second image device by performing a color gamut conversion process for mapping within the color gamut of the second image device. When,
A color conversion function for converting the first image data into second image data based on the correspondence relationship;
An image processing program for causing a computer to realize the above.