JP2010087693A - Apparatus, method and program for image processing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus, method and program for image processing, for executing gamut compression without extremely damaging luminance, saturation and gradation. <P>SOLUTION: Since a compression point (third compression point) corresponding to the color of an output device after conversion is obtained on the basis of points (first compression point and second compression point) obtained by compression by two kinds of different compression conditions, the faults of the compression by the respective compression conditions are compensated and adverse influence on the luminance, the saturation and the gradation accompanying the compression is suppressed. Also, since the third compression point is obtained on the basis of the compression by the two kinds of different compression conditions, the gradation corruption which is to easily occur when the compression by one compression condition is performed because of the shape of the enclosure of the color reproduction range of the output device (for instance, a sharp part or a hollowed part) is improved. Thus, the luminance, the saturation and the gradation are prevented from being extremely damaged by the gamut compression. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image processing apparatus, an image processing method, and an image processing program.

  Various devices such as a display (for example, a liquid crystal display device) and a printer exist as devices that express colors. Generally, different devices have different reproducible color gamuts (color reproduction ranges). Therefore, when the image displayed on the display is output from the printer, the output color (visible color) is different due to the difference in the color reproduction range between the display and the printer.

  Conventionally, color matching processing is performed to correct a difference in output color between different devices. General color matching processing includes gamut compression that compresses the color of an input device that is out of the color reproducible range of the output device into the color reproducible range of the output device.

In Patent Document 1, the maximum saturation point of the input color space having the same hue as the converted color and the maximum saturation point of the output device are derived, and the first anchor point (convergence point) of the converted color and the input device is derived. Is derived by deriving a first outline point where the line segment connecting the two and the outermost outline of the color gamut of the input space intersects, and mapping the first outline point to a point on the outermost outline of the color gamut of the output device. 2 outline points are derived, and a point on the line segment connecting the first anchor point and the first outline point is mapped to a point on the line segment connecting the second anchor point and the second outline point of the output device. There is described a color conversion processing method for converting a color to be converted into a color within the color gamut of the output device (that is, gamut compression) based on the rule to be applied.
JP 2003-323609 A

  In the color conversion processing method described in Patent Document 1 described above, since the maximum saturation point of the input device is mapped to the maximum saturation point of the output device, color turbidity and gradation near the maximum saturation point Can be prevented.

  However, such a color conversion processing method can be used for lightness and saturation when there is a drooping part in the outline of the color reproduction range of the output device, or when there is a part where the displacement of saturation is large relative to the change in hue. In addition, there is a problem that the gradation is impaired.

  The present invention has been made to solve the above-described problems, and is an image processing apparatus, an image processing method, and an image processing apparatus capable of performing gamut compression without significantly impairing lightness, saturation, and gradation. The object is to provide an image processing program.

  In order to achieve this object, the image processing apparatus according to claim 1 converts a color within a first color reproduction range indicated by input color data into a color within a second color reproduction range depending on an output device. A color conversion unit, wherein the color conversion unit deviates from the second color reproduction range when the color indicated by the input color data is out of the second color reproduction range. The first compression point when the compressed color is compressed to a color within the second color reproduction range according to the first compression condition is different from the first compression condition. Obtained by a first color compression means for calculating a second compression point when compressed to a color within the second color reproduction range according to a second compression condition, the original color and the first color compression means The color in the color space between the color of the first compression point and the color of the second compression point A weighting coefficient calculating means for calculating a weighting coefficient for each of the color of the output device corresponding to the first compression point and the color of the output device corresponding to the second compression point based on the relationship; Using the color of the output device corresponding to the first compression point and the color of the output device corresponding to the second compression point, each weighted by the calculated weighting coefficient, the color corresponding to the color of the output device after conversion is used. And a second color compression means for calculating the third compression point.

  The image processing device according to claim 2 is the image processing device according to claim 1, wherein the first compression condition minimizes a color difference between a color indicated by the input color data and a color within a second color reproduction range. The second compression condition is to use a point on the achromatic color axis in the color space as a convergence point for compression.

  The image processing device according to claim 3 is the image processing device according to claim 1 or 2, wherein each of a plurality of colors located on the surface of the second color reproduction range is on a color space indicating the color. Storage means for storing color reproduction range surface information in which coordinates and polar coordinates corresponding to the color are associated, and the first color compression means uses the color reproduction range surface information stored in the storage means. Then, the first compression point and the second compression point are calculated.

  The image processing method according to claim 4 is a method including a color conversion step of converting a color within a first color reproduction range indicated by input color data into a color within a second color reproduction range depending on an output device. In the color conversion step, when a color indicated by the input color data is out of the second color reproduction range, a color out of the second color reproduction range is used as a compression source, and the compression is performed. A first compression point when the original color is compressed into a color within the second color reproduction range by the first compression condition, and the second compression condition by a second compression condition different from the first compression condition. Corresponding to a first color compression step for calculating a second compression point when compressed to a color within the color reproduction range, the original color and the first compression point obtained by the first color compression step The positional relationship in the color space between the color of the output device and the color of the second compression point A weighting coefficient calculating step for calculating a weighting coefficient for each of the color of the first compression point and the color of the output device corresponding to the second compression point; and weighting by the weighting coefficient calculated by the weighting coefficient calculation step A second compression point corresponding to the color of the output device after conversion is calculated using the color of the output device corresponding to the first compression point and the color of the output device corresponding to the second compression point. And a color compression process.

  An image processing program according to claim 5 performs color conversion processing for converting a color in a first color reproduction range indicated by input color data into a color in a second color reproduction range depending on an output device in an image processing apparatus. A program for causing the color conversion processing to be performed when the color indicated by the input color data is out of the second color reproduction range when the color indicated by the input color data is out of the second color reproduction range. Is a compression source, and a first compression point when the color of the compression source is compressed into a color within the second color reproduction range by the first compression condition, and a second different from the first compression condition Obtained by a first color compression step for calculating a second compression point when compressed to a color within the second color reproduction range according to the compression condition, and the compression source color and the first color compression step. The color of the first compression point and the color of the second compression point A weighting coefficient calculating step for calculating a weighting coefficient for each of the color of the output device corresponding to the first compression point and the color of the output device corresponding to the second compression point based on a positional relationship in space; and the weighting coefficient Corresponding to the color of the output device after conversion using the color of the output device corresponding to the first compression point and the color of the output device corresponding to the second compression point weighted by the weighting coefficient calculated in the calculation step And a second color compression step for calculating a third compression point.

  According to the image processing apparatus of claim 1, based on the points (first compression point and second compression point) obtained by compression under two different compression conditions, the color of the output device after conversion is supported. Since the compression point (third compression point) is obtained, it is possible to compensate for the drawbacks of compression due to the respective compression conditions, and to suppress the adverse effects on the lightness, saturation, and gradation associated with the compression. Further, since the third compression point is obtained based on the compression under two different compression conditions, the compression of one is caused by the outline shape (for example, a sharp portion or a sharp portion) of the color reproduction range of the output device. It is possible to improve gradation loss that tends to occur when compression is performed under conditions. Therefore, there is an effect that it is possible to prevent the brightness, saturation, and gradation from being extremely impaired by gamut compression.

  Also, a weighting coefficient is calculated based on the positional relationship in the color space with the color of the compression source, the color of the first compression point, and the color of the second compression point, and the third compression point is calculated using the calculated weighting coefficient. Therefore, it is possible to obtain a third compression point in which the characteristics of the first compression point and the second compression point reflect the positional relationship of these compression points (first compression point and second compression point) with respect to the compression source. There is an effect.

  According to the image processing device of the second aspect, in addition to the effect produced by the image processing device according to the first aspect, the following effect can be obtained. Compression based on the compression condition (first compression condition) that minimizes the color difference between the color indicated by the input color data and the color within the second color reproduction range, and compression of points on the achromatic color axis in the color space. The compression based on the compression condition (second compression condition) used as a point is the compression for obtaining the first compression point and the second compression point, respectively. Therefore, the compression that minimizes the color difference from the compression source and the compression that uses the point on the achromatic color axis in the color space as the convergence point make up for each other's disadvantages that can be caused by each compression. Further, there is an effect that it is possible to prevent the saturation and gradation from being extremely impaired.

  According to the image processing device of the third aspect, in addition to the effect produced by the image device according to the first or second aspect, the following effect is obtained. For each of a plurality of colors located on the surface of the color reproduction range (second color reproduction range) of the output device, color reproduction that associates coordinates in the color space indicating the color with polar coordinates corresponding to the color Since the range surface information is stored in the storage unit, there is an effect that the processing when calculating the first compression point and the second compression point can be made efficient.

  According to the image processing method of the fourth aspect, the same effect as that of the image processing apparatus according to the first aspect can be obtained.

  According to the image processing program of the fifth aspect, the same effect as that of the image processing apparatus according to the first aspect can be obtained.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a block diagram showing an electrical configuration of a printer 1 as an image processing apparatus according to an embodiment of the present invention.

  As shown in FIG. 1, the printer 1 is configured to be able to print image data input from a personal computer (hereinafter referred to as “PC”) 2 on a predetermined print medium. Although details will be described later, the printer 1 of this embodiment has a function of converting color image data input from the PC 2 into color image data within the color gamut (color reproduction range) of the printer 1.

  The printer 1 includes a CPU 11, a ROM 12, a RAM 13, a flash memory 14, a print head, and the like. The printer 1 performs printing on a print medium (for example, a paper medium), and inputs various data. And an operation panel 16 having a user operation unit that allows a user to input an operation such as selecting a print medium or selecting a print medium.

  The printer 1 also includes an interface (I / F) 17 that can be connected to the PC 2 via the cable 5. Therefore, the printer 1 can obtain (input into the printer 1) image data stored in the PC 20 via the cable 5 and the I / F 17.

  The CPU 11 is an arithmetic processing unit that controls the entire printer 1, and the ROM 12 stores various control programs executed by the CPU 11 and fixed value data referred to during the execution. The ROM 12 stores an image processing program 12a as a control program. The image processing program 12a is a program that executes the processing of the flowcharts shown in FIGS.

  The ROM 12 stores an output RGB → Lab conversion table 12b and an output Lab → RGB conversion table 12c.

  The output RGB → Lab conversion table 12b is a table for converting device values (RGB values) in the printer 1 into values in a uniform color space (L * a * b * values (hereinafter referred to as “Lab values”)). For each axis of input values (R, G, B), the range from 0 to 255 is equally divided (for example, 9 divisions), and the output value is stored at each intersection of the grid passing through the division point. .

  The output RGB → Lab conversion table 12b is created by a known method. Specifically, an RGB value corresponding to each intersection of the grid is input, and the color of the patch formed on the recording paper by the printer 1 at that time is measured by a colorimeter to obtain a Lab value.

  The output Lab → RGB conversion table 12 c is a table for converting Lab values into RGB values of the printer 1. A method of creating the output Lab → RGB conversion table 12c is also known, and the corresponding RGB values when the L *, a *, and b * values are changed after the RGB → Lab color conversion is performed. Created by asking.

  The RAM 13 has a working area in which various register groups necessary for executing a control program in the CPU 11 are set, a temporary area that temporarily stores data being processed, and the like, and is a rewritable memory that can be accessed randomly. It is. The RAM 13 has an input image data memory 13a, an input RGB → Lab memory 13b, and an output image data memory 13c. The input image data memory 13a is an area for storing color image data (RGB data) input from the PC 2.

  The input RGB → Lab memory 13b is an area for storing an RGB → Lab conversion table (not shown) on the input device side embedded in the input image data. In this embodiment, since the display (not shown) of the PC 2 serves as an input device, the RGB → Lab conversion table stored in the input RGB → Lab memory 13b is a table for converting RGB values in the display of the PC 2 into Lab values. It is. The RGB → Lab conversion table on the input device side is created in the same manner as the output RGB → Lab conversion table 12b described above.

  The output image data memory 13c stores output image data (CMYK data) obtained as a result of performing color conversion processing (see FIG. 4) described later on the input image data stored in the input image data memory 13a. It is an area.

  The flash memory 14 is a rewritable nonvolatile memory and includes an output color gamut surface value table memory 14a. The output gamut surface value table memory 14a is an area for storing an output gamut surface value table 140 (see FIG. 2) created by an output gamut surface value table creating process (see FIG. 3) described later.

  The output color gamut ground surface value table 140 is a uniform color space value (Lab value in the present embodiment) for each color located on the surface (outside) of the color gamut (color reproduction range) of the printer 1 as an output device. And the hue-saturation plane in the uniform color space (in this embodiment, the L * a * b * color system color space is used, and the a * -b * plane) is converted into polar coordinates. Is a table in which the values of the elevation angle θ and the declination φ are associated with RGB values that are device values. FIG. 2 is a schematic diagram showing the contents of the output color gamut surface value table 140.

  Here, a method of creating the output color gamut surface value table 140 will be described with reference to FIG. FIG. 3 is a flowchart showing an output color gamut surface value table creation process. This output color gamut surface value table creation process is a process executed by the CPU 11 of the printer 1 and is performed before the shipment of the printer 1.

  When the output color gamut surface value table creation process is started, first, the surface grid of the output RGB → Lab conversion table 12b is subdivided by interpolation (S21). By subdividing the surface grid, the amount of information in the output color gamut surface value table 140 can be increased.

  When the surface grid is subdivided, it is close to (R, G, B) = (255, 255, 255) which is a white spot and (R, G, B) = (0, 0, 0) which is a black spot. It is preferable to divide as finely as possible. When the L * a * b * color system color space is expanded in polar coordinates, the corresponding points (that is, (L, θ, φ)) become sparser in the region closer to the white point and black point, so the vicinity of the white point and black point Since the number of corresponding points can be increased by more finely dividing, the surface of the uniform color space can be expressed more accurately.

  Further, when the surface grid of the output RGB → Lab conversion table 12b is subdivided by interpolation, it is preferable to increase the division interval as the distance from the white point and black point increases. Since the corresponding points become denser as they move away from the white point and the black point, the corresponding points can be arranged on an average by making the interval wider as they move away from the white point and the black point.

After the processing of S21, Lab values (L _o , a _o , corresponding to RGB values (R _o , G _o , B _o ) located on the surface grid subdivided by interpolation based on the output RGB → Lab conversion table 12b. After obtaining b _o ), the values of elevation angle θ _o and declination angle φ _o in the polar coordinate system are obtained using equation [1] (S22). In this example, since the convergence point is a point on the achromatic color axis and the center (L = 50) of the L range (0 to 100), the numerator of θ _{o in} the formula [1] is L _o −. 50.

After the processing of S22, the obtained values of elevation angle θ _o and declination angle φ _o , corresponding Lab values (L _o , a _o , b _o ), and RGB values (R _o , G _o , B _o ), It is registered in the output color gamut ground surface value table 140 (S23), and it is confirmed whether processing has been completed for all data on the surface grid (surface grid subdivided by interpolation) (S24).

  As a result of checking in the process of S24, if the process is not yet completed (S24: No), the process proceeds to S22, and the processes of S22 and S23 are repeatedly executed. On the other hand, as a result of the confirmation in the processing of S24, when the processing is completed for all the data on the surface grid subdivided by interpolation (S24: Yes), the output color gamut surface value table creation processing is performed. finish.

  Next, a color conversion process in which the printer 1 of the present embodiment converts color image data input from the PC 2 into color image data in the color gamut of the printer 1 will be described with reference to FIG.

  FIG. 4 is a flowchart showing color conversion processing executed by the CPU 11 of the printer 1. This color conversion process is activated when color image data (input image data) to be printed is input from the PC 2.

As shown in FIG. 4, in this color conversion process, first, input image data stored in the input image data memory 13a in the RAM 13 is read at the time of input from the PC 2 (S1). Next, based on the RGB → Lab conversion table (not shown) stored in the input RGB → Lab memory 13b when the input image data is input, the RGB values (R _i , G _i , B) of one pixel included in the input image data are input. _i ) is converted into Lab values (L _i , a _i , b _i ) (S2).

After the process of S2, based on the output color gamut surface value table 140, it is confirmed whether the converted Lab values (L _i , a _i , b _i ) are values within the color gamut (color reproduction range) of the printer 1 (S3). Here, Lab values _{(L i, a i, b} i) is, if the value of the color gamut of the printer 1 (S3: Yes), on the basis of the output Lab → RGB conversion table 12c, Lab values _(L i, a _i , b _i ) are converted into RGB values (R ₀ , G ₀ , B ₀ ) (S4), and the process proceeds to S5.

On the other hand, if the Lab value (L _i , a _i , b _i ) is a value out of the color gamut of the printer 1 (S3: No) as a result of confirmation in the process of S3, the Lab value (L _i , a _i , A gamut compression process is performed to perform compression so that b _i ) is a value within the color gamut of the printer 1 (S7), and the process proceeds to S5. Details of the gamut compression process (S7) will be described later with reference to FIG.

In S5, the RGB values (R ₀ , G ₀ , B ₀ ) obtained by the processing in S4 or S7 are converted into CMYK values using a conversion table (not shown) (S5). The CMYK value obtained by the process of S5 is stored in the output image data memory 13c.

  After the process of S5, it is confirmed whether or not the process has been completed for all data (all pixels) in the input image data (S6). If not completed yet, the process proceeds to the process of S2, and color conversion for the next pixel is performed. I do. On the other hand, as a result of checking in the process of S6, if the process has been completed for all data in the input image data (S6: Yes), this color conversion process is terminated. By executing this color conversion process, input image data having a color within the color gamut of the input device is converted into output image data having a color within the color gamut of the printer 1.

  Next, the gamut compression process (S7) described above will be described with reference to FIG. FIG. 5 is a flowchart showing the gamut compression process (S7) executed in the color conversion process (see FIG. 4).

As shown in FIG. 5, in the gamut compression process (S7), first, using the formula [2], from the Lab values (L _i , a _i , b _i ) converted by the process of S2, L * a * The values of elevation angle θ _i and declination angle φ _i in the polar coordinate system of the b * color system color space are obtained (S11). In this example, since the convergence point is the point on the achromatic color axis and the center (L = 50) of the range (0 to 100) of L, the numerator of θ _{i in} the formula [2] is L _i −. 50.

After the process of S11, the closest elevation angle θ _o and the values of the elevation angle θ _i and the deflection angle φ _i obtained by the process of S11 from the output color gamut surface value table 140 based on the equation [3], and The value of the deflection angle φ _o is obtained (S12), and based on the output color gamut surface value table 140, the first Lab value (L _{o1) is calculated} from the elevation angle θ _o and the deflection angle φ _o obtained by the processing of S12. , A _o1 , b _o1 ) and the first RGB value (R _o1 , G _o1 , B _o1 ) are obtained (S13).

The first Lab values (L _o1 , a _o1 , b _o1 ) obtained by the process of S13 are equal to the Lab values (L _i , a _i , b _i ) converted by the process of S2 in the embodiment. This is a compression point when compression is performed with a point on the achromatic color axis in the color space and a Lab value (50, 0, 0) as the convergence point.

After the processing of S13, the second color difference from the Lab values (L _i , a _i , b _i ) converted by the processing of S2 is the smallest from the output color gamut surface value table 140 based on Expression [4]. Lab values (L _o2 , a _o2 , b _o2 ) and second RGB values (R _o2 , G _o2 , B _o2 ) are obtained (S14).

  In equation [4], the constant k is a value from 0 to 0.5. As the value of the constant k is closer to 0, the saturation is weighted, and as the value of the constant k is closer to 0.5, the lightness is weighted. The value of the constant k is a value stored in the printer 1 in advance. Note that the user may be able to change the value of the constant k.

After the processing of S14, the first Lab value (L _o1 , a _o1 , b _o1 ), the first RGB value (R _o1 , G _o1 , B _o1 ), the second Lab value (L _o2 , a _o2 , b _o2 ) and the second RGB value (R _o2 , G _o2 , B _o2 ), using the formula [5], the formula [6], and the formula [7], the RGB value indicating the color output by the printer 1 (R ₀ , G ₀ , B ₀ ) are converted (S15), and the gamut compression process (S7) is terminated.

ΔE ₁ obtained by the equation [5] is converted into the Lab value (ie, the compression source) Lab value (L _i , a _i , b _i ) and the first Lab value (in the uniform color space). L _o1 , a _o1 , b _o1 ), and ΔE ₂ obtained by equation [6] is the original Lab value (L _i , a _i , b _i ) and second value in the uniform color space. The color difference from the Lab value (L _o2 , a _o2 , b _o2 ).

According to the equation [7], the weighting coefficient multiplied by the first RGB value and the second RGB value changes according to the color difference from the compression source in the uniform color space, and the obtained RGB value (R ₀ , G ₀ , B ₀ ) changes.

According to Equation [7], the color difference from the compression source becomes smaller in the uniform color space, that is, the nth RGB value having n with a smaller value of ΔE _n (n = 1 or 2). The weighting is greater than the other RGB value. Therefore, the RGB value (R ₀ , G ₀ , B ₀ ) indicating the color output by the printer 1 is the compression source (that is, the input image data of the input image data) among the two compression points by two different types of gamut compression. ) The value reflects the compression point characteristics close to the RGB values (R _i , G _i , B _i ).

In the present embodiment, since the second RGB value is a value corresponding to a compression point obtained by compression that minimizes the color difference in the uniform color space, the points on the achromatic color axis in the uniform color space are converged. A larger weight is given compared to the first RGB value corresponding to the compression point obtained by compression as a point. Therefore, the color difference between the color signal (R _i , G _i , B _i ) of the input image data and the color signal (R ₀ , G ₀ , B ₀ ) of the printer 1 after compression increases, and the saturation and lightness are impaired. Can be suppressed.

  Here, FIG. 6 is a schematic diagram showing a compression result by the above-described gamut compression processing. FIG. 6A is a schematic diagram showing a compression result in a lightness (L) -saturation (C) plane at a predetermined hue in the L * a * b * color system color space. ) Is a schematic diagram showing a compression result on the ab plane at a predetermined lightness (L) in the L * a * b * color system color space. In FIGS. 6A and 6B, the solid line Gi is the boundary line of the input color gamut, and the dotted line Go is the boundary line of the output color gamut.

  According to the printer 1 of the present embodiment, as shown in FIGS. 6A and 6B, the color A of the input image data out of the output color gamut has a convergence point on the achromatic color axis as L. The first compression point P1a when compression is performed with the center point (L = 50) of the range (0 to 100) and the second compression point P1b when compression that minimizes the color difference are performed, respectively. After being obtained by the processing of S13 and S14 in the gamut compression processing (see FIG. 6), the color A ′ is obtained as the final compression point by using the equations [5] to [7] by the processing of S15.

  Therefore, the first compression point P1a obtained by compression with the central point (L = 50) in the L range (0 to 100) as the convergence point is a color having a large color difference from the compression source color A. Considering the second compression point P1b when the compression that minimizes the color difference is performed, the color difference from the compression source can be made smaller and closer to the compression source color.

  Furthermore, according to the present embodiment, when the color A ′ is obtained, the second compression point P1b having a smaller color difference from the compression source is multiplied by a larger weighting coefficient, so that the compression is preferably performed. The color difference from the original can be made smaller.

  In addition, the second compression point P1b obtained by the compression that minimizes the color difference from the compression source is located in the vicinity of the sharp portion SH1 of the output color gamut where the compression points due to other compression sources tend to concentrate. By considering the first compression point P1a, the compression points concentrated on the pointed portion SH1 can be dispersed, and the gradation can be prevented from being lost due to the compression.

  Further, as shown in FIG. 6A, the color B of the input image data located in the vicinity of the blurred portion SH2 of the output color gamut is the second obtained by the compression that minimizes the color difference from the compression source. Assuming that the compression point P2b is the final compression point, there is no compression point located near the bottom of the squeezed portion SH2, and gradation collapse occurs.

  However, according to the printer 1 of the present embodiment, the color B is considered in consideration of the first compression point P2a obtained by compression with the center point (L = 50) of the L range (0 to 100) as the convergence point. Since 'is the final compression point, the compression point is allocated in the bottom direction of the blurred portion SH2, so that the gradation collapse caused by the compression that minimizes the color difference from the reduction source can be suppressed. it can.

  When there are two or more compression points at which the color difference is minimum, according to the value of the constant k in the above equation [4], that is, whether the lightness is weighted or the chroma is weighted. Accordingly, it is assumed that one compression point can be selected. Thus, even when there are two or more compression points at which the color difference is minimum, the compression point can be obtained by the above-described gamut compression processing (see FIG. 5).

  As described above, according to the printer 1 of the present embodiment, the compression and the point on the achromatic color axis that minimizes the color difference from the compression source in the uniform color space (in this embodiment, the range of the point L (0 To 100) (L = 50)), the compression corresponding to the color of the final output device (printer 1) based on the compression points obtained by two different compression conditions, ie, compression with the convergence point as the convergence point. Since points are obtained, the disadvantages of compression due to the respective compression conditions are compensated, and it is possible to prevent lightness, saturation and gradation from being extremely impaired by gamut compression.

  In addition, even if there is a pointed part or a squeezed part on the surface (outside) of the output color gamut, compression is performed under one compression condition by using compression points obtained by two different compression conditions. This can improve the gradation loss that was likely to occur when

  Also, the Lab value of the compression source, the first Lab value obtained by the compression with the point on the achromatic color axis as the convergence point, and the second Lab obtained by the compression that minimizes the color difference from the compression source. The final compression point is obtained using the weighting coefficient obtained based on the positional relationship between the value and the color space, and is obtained by compression with the original color and the point on the achromatic color axis as the convergence point. It is possible to obtain the final compression point reflecting the positional relationship in the color space with the color of the compression point obtained and the color of the compression point obtained by the compression that minimizes the color difference from the compression source.

  In particular, the present embodiment is configured to increase the weighting for the compression point where the color difference from the compression source Lab value is smaller, so the color difference between the compression source color and the final compression point is greatly different. It is suppressed.

  Further, according to the printer 1 of the present embodiment, since the surface value of the color space of the output color gamut of the printer 1 is provided as the output color gamut surface value table 140, the compression that minimizes the color difference from the compression source. And a compression point by compression with a point on the achromatic color axis as a convergence point can be efficiently obtained.

  Although the present invention has been described based on the above embodiments, the present invention is not limited to the above embodiments, and various modifications can be easily made without departing from the spirit of the present invention. It can be done.

  For example, in the above embodiment, the color conversion process of FIG. 4 and the gamut compression process of FIG. 5 have been described as processes executed by the printer 1, but an image output device other than the printer 1 (for example, a liquid crystal display device) or the like The processing may be executed by a printer server or a personal computer.

  In the above-described embodiment, the first compression point is obtained by compression using the Lab value (50, 0, 0) as the convergence point. However, the first compression point is not limited to the Lab value (50, 0, 0). A similar effect can be obtained if compression is performed with a point on the axis as the convergence point.

  In the above embodiment, the L * a * b * color system color space is used as the uniform color space, but other uniform color spaces such as a Luv color space may be applied.

  Moreover, in the said embodiment, although the process using RGB value was performed in the gamut compression process (refer FIG. 5), you may use a CMYK value. At this time, the output color gamut surface value table 140 is created using CMYK values instead of RGB values.

  In the above embodiment, the output color gamut surface value table 140 is created by the output color gamut surface value table creation process (see FIG. 3) executed before shipment of the printer 1. The output color gamut surface value table 140 may be created by executing the output color gamut surface value table creation process every time an instruction is given.

  In the above embodiment, the input device profile (in the above embodiment, the RGB → Lab conversion table) is described as being embedded in the input image data. However, the profile is stored in advance in the flash memory 14 for each input device. It is good also as a structure to carry out.

1 is a block diagram illustrating an electrical configuration of a printer as an image processing apparatus according to an embodiment of the present invention. It is a schematic diagram which shows the content of the output color gamut surface value table. It is a flowchart which shows an output color gamut surface value table creation process. It is a flowchart which shows a color conversion process. 5 is a flowchart showing a gamut compression process executed in the color conversion process of FIG. 4. It is a schematic diagram which shows the compression result by a gamut compression process.

Explanation of symbols

1 Printer (an example of an output device, an example of an image processing apparatus)
14a Output color gamut surface value table memory (an example of storage means)
140 Output color gamut surface value table (example of color reproduction range surface information)
S7 An example of color conversion means, an example of a color conversion process, an example of a color conversion step S13 An example of a part of the first color compression means, an example of a part of the first color compression process, a part of the first color compression step Example S14 Example of first color compression unit, example of part of first color compression process, example of part of first color compression step S15 example of weighting coefficient calculation unit, example of second color compression unit, calculation of weighting coefficient Example of process, example of second color compression process, example of weighting coefficient calculation step, example of second color compression step

Claims (5)

An image processing apparatus comprising color conversion means for converting a color within a first color reproduction range indicated by input color data into a color within a second color reproduction range depending on an output device,
The color conversion means includes
When the color indicated by the input color data is out of the second color reproduction range, the color out of the second color reproduction range is set as the compression source, and the color of the compression source is set as the first color. A first compression point when compressed to a color within the second color reproduction range according to the compression condition, and a color within the second color reproduction range according to a second compression condition different from the first compression condition. First color compression means for calculating a second compression point when compressed;
Corresponding to the first compression point based on the positional relationship in the color space between the color of the compression source and the color of the first compression point and the color of the second compression point obtained by the first color compression means. Weighting coefficient calculating means for calculating a weighting coefficient for each of the color of the output device and the color of the output device corresponding to the second compression point;
The output device after conversion using the color of the output device corresponding to the first compression point and the color of the output device corresponding to the second compression point each weighted by the weighting coefficient calculated by the weighting coefficient calculation means And a second color compression means for calculating a third compression point corresponding to each color. The first compression condition is to minimize a color difference between a color indicated by the input color data and a color within a second color reproduction range;
The image processing apparatus according to claim 1, wherein the second compression condition is to use a point on an achromatic color axis in a color space as a convergence point of compression. For each of a plurality of colors located on the surface of the second color reproduction range, a memory for storing color reproduction range surface information in which coordinates on the color space indicating the color and polar coordinates corresponding to the color are associated with each other With means,
3. The first color compression unit calculates the first compression point and the second compression point using the color reproduction range surface information stored in the storage unit. The image processing apparatus described. An image processing method including a color conversion step of converting a color in a first color reproduction range indicated by input color data into a color in a second color reproduction range depending on an output device,
The color conversion step includes
When the color indicated by the input color data is out of the second color reproduction range, the color out of the second color reproduction range is set as the compression source, and the color of the compression source is set as the first color. A first compression point when compressed to a color within the second color reproduction range according to the compression condition, and a color within the second color reproduction range according to a second compression condition different from the first compression condition. A first color compression step of calculating a second compression point when compressed;
Based on the positional relationship in the color space between the color of the compression source, the color of the output device corresponding to the first compression point obtained by the first color compression step, and the color of the second compression point, the first A weighting coefficient calculating step of calculating a weighting coefficient for each of the color of the compression point and the color of the output device corresponding to the second compression point;
The color of the output device after conversion using the color of the output device corresponding to the first compression point and the color of the output device corresponding to the second compression point weighted by the weighting coefficient calculated in the weighting coefficient calculation step And a second color compression step of calculating a third compression point corresponding to the image processing method. An image processing program for causing an image processing apparatus to execute a color conversion process for converting a color in a first color reproduction range indicated by input color data into a color in a second color reproduction range depending on an output device. ,
The color conversion process
When the color indicated by the input color data is out of the second color reproduction range, the color out of the second color reproduction range is set as the compression source, and the color of the compression source is set as the first color. A first compression point when compressed to a color within the second color reproduction range according to the compression condition, and a color within the second color reproduction range according to a second compression condition different from the first compression condition. A first color compression step for calculating a second compression point when compressed;
Corresponding to the first compression point based on the positional relationship in the color space between the color of the compression source and the color of the first compression point and the color of the second compression point obtained by the first color compression step. A weighting coefficient calculating step for calculating a weighting coefficient for each of the color of the output device and the color of the output device corresponding to the second compression point;
Using the color of the output device corresponding to the first compression point and the color of the output device corresponding to the second compression point weighted by the weighting factor calculated in the weighting factor calculation step, An image processing program comprising: a second color compression step for calculating a third compression point corresponding to a color.

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