JP2017201759A - Color conversion device, color conversion method, and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the accuracy reduction due to interpolation processing even when a plurality of peak points of an output value of color separation LUT are present or a plurality of start points to receive the output value exist.SOLUTION: Disclosed is an image processing device for color-separating a signal component of an input image to the signal component handled by an output device. This image processing device includes: interpolation means for determining the output value of the signal component handled by the output device corresponding to each pixel value of the input image by interpolation processing based on color separation LUT of multiple dimensions in accordance with the signal component of the input image; and conversion means for performing non-linear conversion of the pixel value of the input image using one-dimensional conversion LUT before interpolation processing in the interpolation means. In input and output characteristics of at least one dimension out of multiple dimensions, the color separation LUT includes a plurality of peak points where the output value takes the maximum or minimum value or a plurality of rising points where the output value rises with respect to an input value. The conversion LUT has the input/output characteristics for enhancing non-linearity corresponding to each of the plurality of peak points or each of the plurality of rising points in the color separation LUT.SELECTED DRAWING: None

Description

  The present invention relates to image processing for converting an input image signal into a signal corresponding to a plurality of color components handled by an output device.

  Printers represented by inkjet and electrophotographic systems receive image signals (usually RGB color signals) as input and print by converting the color signals into signals representing the amount of color material installed in the printer. Data is being generated. This conversion is generally called color conversion processing or color separation processing, and is interpolated using a multi-dimensional LUT (the number of dimensions is determined according to the signal component of the input image) as the mainstream method. There is a method for processing.

  The multidimensional LUT used in the above method usually holds only values on grid points (output values corresponding to input values) obtained by thinning out the input color space in order to save the storage capacity of the apparatus. For example, a three-dimensional LUT used when an input image is an RGB 8-bit color signal usually has about 17 × 17 × 17 grid points thinned out. Then, an output value between lattice points that cannot be obtained directly from the three-dimensional LUT is derived by interpolation processing. At this time, an error occurs in the interpolation processing, and there is a problem that the color reproduction accuracy between grid points is lowered.

  In this regard, as a technique for improving the color reproduction accuracy of the interpolation process, it may be possible to cope with a large non-linearity by performing a non-linear conversion using a one-dimensional LUT before the interpolation process. In Patent Document 1, a different one-dimensional LUT is provided for each RGB channel input to the three-dimensional LUT, and the RGB color signals are made non-uniform in the interval of the grid points with respect to each one-dimensional axial direction. A method for improving the linearity of an output signal between lattice points has been proposed.

JP 2014-187604 A

  However, the characteristics of nonlinear conversion for enhancing the linearity of the interpolation process not only differ depending on the input color component (for example, RGB), but also differ depending on the output color component (for example, CMYK). In addition, when the output color component of the multidimensional LUT includes a special color such as RGB, the input / output characteristics may have a plurality of peaks. For example, among special color inks that are used in a printer of a type in which many inks having different shades and hues are mounted, a larger amount of ink may be used in two regions, a gray halftone region and a high saturation region. In this case, the non-linearity is greatly different in each of these regions (the output value has two peaks). In addition, depending on the special color ink, there may be a plurality of locations where ink values begin to enter the input RGB values (locations where ink values rise). In such a case, even if a conventional one-dimensional LUT for nonlinear conversion is provided for each channel of color components input to the multidimensional LUT, a decrease in color reproduction accuracy during interpolation processing is inevitable.

  An image processing apparatus according to the present invention is an image processing apparatus that color-separates signal components of an input image into signal components handled by an output device, and is based on a multi-dimensional color separation LUT corresponding to the signal components of the input image Interpolation means for determining an output value of a signal component handled by the output device corresponding to each pixel value of the input image by interpolation processing, and before the interpolation processing in the interpolation means, the pixel value of the input image, Conversion means for performing non-linear conversion using a one-dimensional conversion LUT, and the color separation LUT has a peak portion where an output value has a maximum or minimum value in the input / output characteristics of at least one of the plurality of dimensions. Alternatively, the conversion LUT includes a plurality of rising points where the output value rises with respect to the input value, and the conversion LUT includes the plurality of peak points or rising points in the color separation LUT. Having input and output characteristics of increasing the nonlinearity in response to each occurrence Ri, characterized in that.

  According to the present invention, in the input / output characteristics of a multidimensional LUT used for color separation processing, even when there are a plurality of output value peak points or a plurality of output value start points, a decrease in accuracy due to interpolation processing is suppressed. can do.

1 is a block diagram illustrating an example of a configuration of a printing system according to Embodiment 1. FIG. It is an example of the graph showing the characteristic of the one-dimensional conversion LUT corresponding to green ink. 3 is a flowchart illustrating a rough flow of print processing according to the first embodiment. 4 is a flowchart illustrating details of color separation processing + compositing processing according to the first exemplary embodiment. It is a figure explaining a mode that color conversion is carried out from RGB to CMYKGyRGB. 6 is a flowchart illustrating a flow of creating a conversion LUT and a color separation LUT according to the first embodiment. It is a figure which shows an example of a LUT division | segmentation mask. It is a figure showing an example of a plurality of sets of LUT division masks. It is a figure which shows a mode that a base LUT is divided | segmented. It is a flowchart which shows the flow of the optimization process of LUT. It is a comparison figure with the method concerning a present Example, and the conventional method. It is a figure which shows an example of the other variation of a LUT division | segmentation mask. FIG. 10 is a block diagram illustrating an example of a configuration of a printing system according to a second embodiment. 10 is a flowchart illustrating a flow of color separation processing + composition processing according to the second embodiment. FIG. 10 is a block diagram illustrating an example of a configuration of a printing system according to a third embodiment. 10 is a flowchart illustrating a flow of color separation processing + composition processing according to the third embodiment. FIG. 10 is a block diagram illustrating an example of a configuration of a printing system according to a fourth embodiment. 14 is a flowchart illustrating a flow of color separation processing + composition processing according to the fourth embodiment. FIG. 10 is a block diagram illustrating an example of a configuration of a printing system according to a fifth embodiment. 10 is an example of a graph showing characteristics of a conversion LUT used in Example 5. 10 is a flowchart illustrating a flow of color separation processing according to a fifth embodiment.

  Embodiments of the present invention will be described below with reference to the drawings. It should be noted that the following embodiments do not limit the present invention, and not all combinations of features described in the present embodiment are essential to the solution means of the present invention. In addition, about the same structure, the same code | symbol is attached | subjected and demonstrated.

  According to the present invention, the input / output characteristics of a multi-dimensional LUT (look-up table) used in color separation processing can include a plurality of peak points of output values, and a plurality of points where output values start to enter or decrease. It is a premise. Here, the peak part means the part where the output value takes the maximum value or the minimum value, and the part where the output value starts to enter (rise) or starts to decrease means the output value when the input value is increased. Means a place where 0 changes to a significant value other than 0. Furthermore, the minimum value may be treated as the maximum value. For example, if the maximum value is found for the data obtained by inverting the ink value with respect to the axis of the ink value 0 and adding the maximum value of the ink value (255 when the ink value is 8 bits), the minimum value is found. Can do.

  Based on the above premise, in the present embodiment, a mode in which nonlinear transformation is performed using a plurality of one-dimensional LUTs corresponding to a plurality of peak locations and rising locations in a multi-dimensional LUT used in color separation processing will be described. In each embodiment of the present specification, the case where printing is performed by an ink jet printer will be described as an example. However, the printer method is not limited to this. For example, an electrophotographic printer, a sublimation printer, a UV printer using a color material that is cured by irradiation with ultraviolet rays, or a 3D printer that forms a three-dimensional object may be used. The present invention can also be applied to an image display device such as a display or a projector. It can also be applied to image processing software such as photo editing software and CG production software.

(Configuration of output device)
FIG. 1 is a block diagram illustrating an example of a configuration of a printing system according to the present embodiment. The printing system includes an image processing apparatus 10 and a printing apparatus (printer) 20, and is connected by a printer interface or a circuit. The image processing apparatus 10 is a general personal computer, for example, and an image processing function described below is realized by an installed printer driver. That is, each unit of the image processing apparatus 10 described below is realized by a CPU in a computer executing a predetermined program stored in a memory (ROM or the like) or a hard disk (HDD). Note that the printer 20 may include the image processing apparatus 10.

  The image processing apparatus 10 receives input of image data representing an image to be printed via the input terminal 101. The input image data is sent to the color matching processing unit 102. Here, it is assumed that the input image data is RGB image data in which each pixel is represented by a color signal of 8 bits (256 gradations) for each RGB.

  The color matching processing unit 102 performs color matching processing on the input image data and corrects the color of the RGB image. By color matching processing, uniform color reproduction can be obtained even when a printer or a recording medium having different color reproduction characteristics is used. In color matching processing, a three-dimensional color matching LUT 103 stored in an HDD (not shown) or the like is used. The color matching LUT 103 is an LUT in which output values (RGB values) are described only on grid points obtained by thinning input values expressed in 256 tones for RGB into 17 × 17 × 17 points. The output value between grid points is derived by linear interpolation.

  The color separation processing unit according to the present exemplary embodiment includes two modules, and the printer handles each pixel value of the RGB image by interpolation processing using a three-dimensional LUT (hereinafter, color separation LUT) corresponding to the input RGB image. The output value corresponding to the signal component (here, the ink color component) is converted. In this embodiment, before performing the interpolation process using the color separation LUT, nonlinear conversion using the one-dimensional conversion LUT is performed in order to increase the accuracy of the interpolation calculation. The first color separation processing unit 104a and the second color separation processing unit 104b correspond to 8 colors corresponding to the inks of the color materials (eight colors here) included in the printer 20 from the image data corrected by the color matching processing unit 102. Plain 8-bit ink value image data is generated. Here, the eight color inks are cyan (C), magenta (M), yellow (Y), black (K), gray (Gy), red (R), green (G), and blue (B). .

The first color separation processing unit 104a includes a first nonlinear conversion unit 104a _-1 and a first interpolation unit 104a _-2 . The first nonlinear conversion unit 104 _a-1 performs nonlinear conversion on the image data after the color matching process using the first conversion LUT 105 _a-1 . Thereafter, the first interpolation unit 104 _a-2 performs an interpolation operation on the non-linearly converted image data using the first color separation LUT 105 _a-2 . The image data color-separated by the first color separation processing unit 104 a is sent to the synthesis processing unit 106. The second color separation processing unit 104b includes a second nonlinear conversion unit 104 _b-1 and a second interpolation unit 104 _b-2 . The second nonlinear conversion unit 104 _b-1 performs nonlinear conversion on the image data after the color matching process using the second conversion LUT 105 _b-1 . Thereafter, the second interpolation unit 104 _b-2 performs an interpolation operation on the non-linearly converted image data using the second color separation LUT 105 _b-2 . The image data color-separated by the second color separation processing unit 104b is sent to the synthesis processing unit 106.

In the non _- linear conversion processing in the first non-linear conversion unit 104 _a-1 and the second non _- linear conversion unit 104 _b-1 , an output value corresponding to an input value (for each ink color) is prepared for each RGB color component of the input image. A one-dimensional conversion LUT that defines 8 bits) is used. This one-dimensional conversion LUT has values only at a number of lattice points (here, 12 lattice points) smaller than 256 gradations that can be expressed by 8 bits, and values between lattice points are obtained by linear interpolation. In this embodiment, two sets of one-dimensional conversion LUTs (first conversion LUT 105 _a-1 and second conversion LUT 105 _b-1 ) corresponding to each of all ink types (eight types) for each RGB channel, a total of 48. Conversion LUTs are prepared. FIG. 2 is an example of a graph representing the characteristics of a conversion LUT for nonlinear conversion corresponding to green ink. In each graph of (a) to (f), the horizontal axis represents the input RGB value before nonlinear conversion, and the vertical axis represents the output RGB value after nonlinear conversion. (A) to (c) correspond to the first conversion LUT 105 _a-1 , and (d) to (f) correspond to the second conversion LUT 105 _b-1 . In particular, it can be seen that the input / output characteristics change relatively greatly at the two portions indicated by reference numerals 201 and 202 in the two graphs (b) and (e) corresponding to the G channel. As described above, by using a plurality of conversion LUTs having different input / output characteristics for one input channel of one kind of ink color, nonlinearity corresponding to each of a plurality of peak locations in the G-axis direction and locations where ink values start to enter. Can be increased.

The interpolation processing in the first interpolation processing unit 104 _a-2 and the second interpolation calculation unit 104 _b-2 is, for example, describing ink values of eight colors only on the lattice points thinned out to 12 × 12 × 12, respectively. Using the three-dimensional color separation LUT, the value between the lattice points is obtained by linear interpolation. In this case, the first color separation LUT 105 _a-2 and the second color separation LUT 105 _b-2 are prepared for each ink color. In this embodiment, a total of 16 three-dimensional LUTs are required, 8 for each of the first color separation LUT 105 _a-2 and the second color separation LUT 105 _b-2 . The one-dimensional conversion LUT and the three-dimensional color separation LUT described above may be stored separately in the HDD or the memory, or may be stored together.

  The combination processing unit 106 adds and combines the ink value image generated by the first color separation processing unit 104a and the ink value image generated by the second color separation processing unit 104b for each ink type. As a result, an 8-plane 8-bit ink value composite image corresponding to the 8-color ink provided in the printer 20 is generated.

  An output gamma (OPG) processing unit 107 performs a gamma correction process on the ink value synthesized image generated by the synthesis processing unit 106. In the gamma correction process, a one-dimensional OPGLU 108 stored in an HDD (not shown) is referred to. The OPGLUT 108 is set in advance for each ink type so that the brightness of the printed matter changes linearly with respect to the pixel value of the ink value image when recording is performed using only each of the eight ink colors. Note that L * defined by CIELAB is used as the brightness evaluation value.

  The halftone processing unit 109 converts the pixel value of the ink value image of each color obtained by the OPG processing unit 107 into a binary value (or a binary number that is two or more and less than the number of input gradations) (quantum processing (quantum). Process). For the halftone process, for example, a known dither matrix method is used. Halftone image data obtained by the halftone process is sent to the printer 20 via the output terminal 110.

  The inkjet printer 20 forms an image on the recording medium according to the HT image data generated by the image processing apparatus 10 by moving the recording head 111 vertically and horizontally relative to the recording medium 112. The recording head 111 has a plurality of recording elements (nozzles). In this embodiment, inks of eight colors of cyan (C), magenta (M), yellow (Y), black (K), gray (Gy), red (R), green (G), and blue (B) are recorded. It is mounted on the head 111. The moving unit 113 moves the recording head 111 under the control of the head control unit 114. The transport unit 115 transports the recording medium 112 under the control of the head control unit 114. In this embodiment, it is assumed that a multi-pass printing method is used in which an image is completed by performing scanning a plurality of times on the printing medium 112 by the printing head 111. The pass separation processing unit 116 generates scanning data for each ink color based on the halftone image data for each color generated by the image processing apparatus 10 and the pass mask acquired from the pass mask DB 117. The ink color selection unit 118 selects an ink color from the ink colors mounted on the recording head 111 based on the generated scan data for each color.

(Overview of print processing)
Next, an outline of printing processing in the printing system according to the present embodiment will be described. FIG. 3 is a flowchart illustrating a rough flow of the printing process in the printing system according to the present embodiment.

  First, in step 301, RGB image data is acquired via the input terminal 101. The acquired RGB image data is sent to the color matching processing unit 102. In subsequent step 302, the color matching processing unit 102 performs color matching processing with reference to the three-dimensional color matching LUT 103.

  Next, in step 303, the type of ink (ink type of interest) to be subjected to color separation processing among all eight colors is determined. In step 304, first, the first color separation processing unit 104a and the second color separation processing unit 104b each generate ink value image data for the ink type of interest. If the ink type of interest is cyan, for example, ink value image data corresponding to cyan is generated. Then, the two ink value images obtained by the color conversion are synthesized by the synthesis processing unit 106. Details of the color separation process and the composition process will be described later.

  Next, in step 305, the OPG processing unit 106 performs gamma correction processing on the ink value image data obtained by the combining processing in step 304. In this gamma correction process, a one-dimensional OGLUT 108 corresponding to the ink type of interest is applied. For example, if the ink type of interest is cyan, an OPGLUT 108 in which a value is set in advance so that the brightness of the printed matter changes linearly with respect to the pixel value of the ink value image when recording using only cyan ink is used. Used.

  In step 306, it is determined whether or not the OPG process has been completed for all eight ink types. If processing has been completed for all ink types, the process proceeds to step 307. On the other hand, if there is an unprocessed ink type, the process returns to step 303, the next ink type is determined as the ink type of interest, and the processes in steps 304 to 306 are repeated. In this embodiment, the order of the ink type of interest is the order of cyan, magenta, yellow, black, gray, red, green, and blue, but is not limited to this.

  In step 307, the halftone processing unit 109 performs halftone processing on the image data of each ink color that has been subjected to OPG processing, and converts the image data into halftone image data. The halftone image data is output from the output terminal 110 in an arbitrary size such as the entire image or the bandwidth for each unit recording area, and is sent to the printer 20.

  In step 308, the halftone image data input via the input terminal 119 is sent to the pass separation processing unit 116, and converted into scan data for each ink color. In step 309, image formation based on the scan data is started for each ink color. Specifically, an ink color suitable for each scan data is selected by the ink color selection unit 118, and the recording head 111 moves relative to the recording medium 112 based on the selected ink color at a constant driving interval. Each nozzle is driven to record an image on a recording medium. The recording medium 112 is transported by a predetermined transport amount for each scan, and the entire image is formed.

  The above is the outline of the printing process in the printing system of this embodiment.

(Color separation process + composition process)
Next, the process of obtaining an ink value image for the ink type of interest by separately performing color separation in two modules and synthesizing the results, which is a feature of this embodiment, will be described in detail with reference to another flow. To do. FIG. 4 is a flowchart showing details of the color separation process + synthesis process according to this embodiment.

In step 401, the first color separation processing unit 104a acquires two types of LUTs for the ink type of interest. Specifically, the first nonlinear conversion unit 104 _a-1 acquires the first conversion LUT 105 _a-1 of the ink type of interest, and the first interpolation unit 104 _a-2 acquires the first color separation LUT 105 _a- of the ink type of interest. _{Get 2} In subsequent step 402, the second color separation processing unit 104b acquires two types of LUTs for the ink type of interest. Specifically, the second nonlinear conversion unit 104 _b-1 acquires the second conversion LUT 105 _b-1 for the ink type of interest, and the second interpolation unit 104 _b-2 acquires the second color separation LUT 105 _{b- for} the ink type of interest _{. Get 2}

  In step 403, an RGB value of interest as a processing target (hereinafter referred to as an RGB value of interest) is acquired from image data in an RGB color space that has undergone color matching processing.

Next, in step 404, the nonlinear conversion processing and interpolation processing (first color separation processing) by the first color separation processing unit 104a is performed, and in step 405, the nonlinear conversion processing and interpolation processing (second color processing by the second color separation processing unit 104b). The decomposition process is executed. Specifically, in the first color separation processing unit 104a, first, a nonlinear conversion process using the first conversion LUT 105a _-1 is performed on the target RGB value. Further, an interpolation process using the first color separation LUT 105 _a-2 is executed on the nonlinear RGB value of interest. Similarly, in the second color separation processing unit 104b, first, a nonlinear conversion process using the second conversion LUT 105b _-1 is performed on the target RGB value. Further, an interpolation process using the second color separation LUT 105 _b-2 is executed on the nonlinear RGB value of interest. FIG. 5 is a diagram illustrating a state in which RGB signal components are color-converted into CMYKGyRGB signal components in each color separation processing unit. FIG. 5 shows an example in which both a one-dimensional conversion LUT and a three-dimensional color separation LUT are prepared for each ink type. The first color separation processing unit 104a and the second color separation processing unit 104b have the same structure except for the contents (grid point values) used in the LUT, and FIG. 5 shows the first color separation processing unit 104a. And the second color separation processing unit 104b. That is, the RGB value of interest is subjected to nonlinear conversion for each ink type, and then the RGB value subjected to nonlinear conversion is similarly interpolated for each ink type to obtain the ink value for the ink type of interest in CMYKGyRGB. Thus, the ink value for each ink type obtained in each color separation processing unit is sent to the synthesis processing unit 106.

Thereafter, in step 406, the synthesis processing unit 106 synthesizes (adds) the output value of the first interpolation processing unit 104a _{-2 and} the output value of the second interpolation unit 104b _-2 to correspond to the target RGB value. An interpolation value (ink value) is generated.

  In step 407, it is determined whether or not processing has been completed for all pixel values (RGB values) in the input image data. If there is an unprocessed pixel value, the process returns to step 403 to determine the next target RGB value and continue the process. On the other hand, when the processing has been completed for all the pixel values, the present processing is terminated and an interpolation value (ink value) corresponding to each generated RGB value is output to the OPG processing unit 107.

  The above is the content of the color separation process + synthesis process according to the present embodiment.

(LUT creation)
Next, a method of creating a one-dimensional conversion LUT (the number of grid points is 12) and a three-dimensional color separation LUT (the number of grid points is 12 × 12 × 12) used in each of the color separation processing units described above will be described. The color separation LUT used in each color separation process of the present embodiment includes a plurality of peak positions and input value start positions of input / output characteristics of a color separation LUT (hereinafter referred to as a base LUT) indicating an interpolation target as a basis. It is an LUT obtained by dividing so as to belong to different LUTs. That is, it is an LUT (partial LUT) that represents a part of the input / output characteristics of the base LUT. The base LUT in the present embodiment is a color separation LUT having 33 × 33 × 33 grid points designed in advance by the developer of the printer 20 or the like. This will be described in detail below.

  FIG. 6 is a flowchart showing a flow of creating a one-dimensional conversion LUT and a three-dimensional color separation LUT (partial LUT) according to the present embodiment.

  First, in step 601, the base LUT is acquired. In step 602, the mth set is acquired from the plurality of sets of masks used for dividing the base LUT. At this time, the variable m is an integer from 1 to M (M is 1 or more). Here, a mask (LUT division mask) used for LUT division will be described with reference to FIGS.

  FIG. 7 is a diagram illustrating an example of the LUT division mask. In FIG. 7, a pair of (a1) and (a2) is a mask in the case of dividing the input value (any of RGB in this embodiment) in a gradation. A pair of (b1) and (b2) is a mask in the case where all input values are distributed to one side and no input value is distributed to one side. In either case, the vertical axis represents the division ratio (or distribution ratio), and takes a value in the range of 0.0 to 1.0. In any of the masks indicated by the pair (a1) and (a2) and the pair (b1) and (b2), the sum of the division ratios is 1.0 over the entire range of input values. FIG. 8 is a diagram illustrating an example of a plurality of sets of LUT division masks. Here, an example in the case of a total of eight sets from the first to the eighth is shown. In each set, either the mask by the pair (a1) and (a2) or the mask by the pair (b1) and (b2) is assigned to the input R value, input G value, and input B value, respectively. ing. In the present embodiment, one of the LUT division mask sets among these eight sets is acquired in order (for example, in order from the first set).

  In step 603, the base LUT is divided using the mth set of LUT division masks acquired in step 602. FIG. 9 is a diagram showing how the base LUT is divided into two LUTs by the LUT division mask. In FIG. 9, a graph (in) is a one-dimensional graph showing the relationship between the output value (ink value) with respect to the input value of the G axis in the base LUT when the ink color is green, for the sake of simplicity. It is. When a pair of masks (a1) and (a2) (any one of the first, second, fifth, and sixth sets in the example of FIG. 8) is applied to the base LUT, Each is divided into two LUTs as shown in graph (out1) and graph (out2). In this example, there are two peak locations, a reference numeral 901 and a reference numeral 902 in the graph (in), such as a reference numeral 903 in the graph (out1) and a reference numeral 904 in the graph (out2). That is, the base LUT is divided into two LUTs so that the first peak location belongs to the LUT of the graph (out1) and the second peak location belongs to the LUT of the graph (out2). Similarly, the LUT division mask set is applied to the other input axes and ink colors, and the base LUT is divided. Although an example of a one-dimensional input value has been described here, it is originally necessary to obtain an ink value for each ink color with respect to the input RGB value of the base LUT. Therefore, in practice, each of the results obtained for the input R value, the input G value, and the input B value is multiplied by 1/3, and the sum thereof is used as the ink value after the LUT division mask is applied. As a result, the sum of the base LUT before the division and the LUTs after the LUT division becomes equal, and can be handled as ink values in the base LUT.

  In step 604, the value of the variable X corresponding to the color separation processing unit targeted for LUT creation is initialized. In this embodiment, the value of the variable X is an integer of 1 or 2. “1” corresponds to the first color separation processing unit 104a, and “2” corresponds to the second color separation processing unit 104b. Here, it is assumed that “1” is set as the initial value. In the subsequent step 605, a variable i representing the loop count and a constant N representing the upper limit of the loop count are initialized. Specifically, the variable i is set to 1, and the constant N is set to 1000.

Next, in step 606, the LUT used in the color separation processing unit corresponding to the variable X is initialized. For example, when X = 1, the first conversion LUT 104 _a-1 and the first color separation LUT 104 _a-2 are initialized. Specifically, it is as follows. First, for the conversion LUT, the positions and initial values of 12 grid points are set. Here, the lattice point positions are equally spaced, that is, a total of 12 points of 0, 23, 46,. The initial value of each grid point is given as a random number between 0 and 255. However, the grid point values are set after rearrangement so as to increase monotonously with respect to the grid point position. For the color separation LUT (partial LUT), initial values of 12 × 12 × 12 lattice points are set. In this embodiment, 12 × 12 × 12 initial values are set by thinning out the lattice point values of the base LUT (the number of lattice points is 33 × 33 × 33).

  In step 607, the one-dimensional conversion LUT optimization process corresponding to the variable X is executed. In step 608, optimization processing of a three-dimensional color separation LUT (partial LUT) corresponding to the variable X is executed. Details of these optimization processes will be described later.

  In step 609, it is determined whether or not the evaluation value E in the LUT optimization process obtained in the i-th loop is the best evaluation value so far. If it is determined that the evaluation value is the best, the process proceeds to step 610. On the other hand, if it is not the best evaluation value, the process proceeds to step 611.

  In step 610, the one-dimensional LUT and the three-dimensional LUT obtained in the i-th loop are updated and held as the best LUT. In addition, information on the best evaluation value held together with these best LUTs is also updated.

  In step 611, it is determined whether the value of the loop count i has reached the upper limit. If the upper limit has not been reached (if the value of variable i is less than N), variable i is incremented (i = i + 1), the process returns to step 606, and the processing is continued. On the other hand, if the value of the number of loops i has reached the upper limit (value of variable i = N), the process proceeds to step 612.

  In step 612, it is determined whether or not the value of the variable X corresponding to the color separation processing unit has reached the upper limit (2 in this embodiment). If the upper limit has not been reached (if the value of variable X is less than 2), variable X is incremented (X = X + 1) and the process returns to step 604 to continue the process. On the other hand, if the value of variable X has reached the upper limit (value of variable X = 2), the process proceeds to step 613.

  In step 613, it is determined whether the value of the variable m representing the mth set of LUT division masks has reached the upper limit M (8 in this embodiment). If the upper limit has not been reached (if the value of the variable m is less than M), the variable m is incremented (m = m + 1), the process returns to step 602, and the processing is continued. On the other hand, if the value of the variable m has reached the upper limit M (value of the variable m = M), the process proceeds to step 614.

  In step 614, the evaluation values of the eight best LUTs created under each of the first to eighth sets of LUT partition masks are compared, and the LUT having the best evaluation value is determined as the LUT used in each color separation processing unit. Is done.

  As described above, a one-dimensional conversion LUT and a three-dimensional color separation LUT (partial LUT) having optimal grid point values are created.

(LUT optimization processing)
FIG. 10 is a flowchart showing the flow of optimization processing of the conversion LUT and color separation LUT (partial LUT) in the above-described steps 607 and 608.

  In step 1001, one target lattice point (target lattice point p) is determined from all lattice points of the target LUT. At the stage immediately after the start of processing, the point at the position “0” in the case of a one-dimensional conversion LUT, and the lattice point at the position (0, 0, 0) in the case of a three-dimensional color separation LUT (partial LUT) Is selected as a target lattice point p.

  In step 1002, the value of the variable s is initialized. Here, the variable s is a variable representing a fluctuation range for finding an optimal lattice point value. Although what kind of value the initial value of the variable s is set to is arbitrary, for example, a value such as “8” is set.

  In step 1003, three types of LUTs (L0, L1, L2) are created using the lattice point value at the target lattice point p and the fluctuation range given by the variable s. Here, L0 is an LUT that does not change the value of the target lattice point p as it is, L1 is an LUT that is + s to the value of the target lattice point p, and L2 is an LUT that is −s to the value of the target lattice point p. is there.

  In step 1004, the respective evaluation values E0, E1, and E2 when the interpolation values are calculated using the three types of LUTs (L0, L1, and L2) obtained in step 1103 are derived. Here, the evaluation values are 33 × 33 × 33 calculated using the 33 × 33 × 33 lattice point values of the base LUT and the color separation LUT (partial LUT) having 12 × 12 × 12 lattice points. A difference from the interpolation value corresponding to each grid point value is obtained, and a sum of values obtained by squaring the difference (sum of squared differences) is used. In this case, the input value of the color separation LUT having 12 × 12 × 12 grid points is a value obtained by performing nonlinear conversion using a one-dimensional conversion LUT. Since the evaluation value becomes smaller as the error is smaller, the smallest evaluation value among E0, E1, and E2 is the best evaluation value. Note that the evaluation method is arbitrary, and instead of using the above-mentioned sum of squared differences, evaluation using the maximum value of error may be performed. Further, the error may be evaluated after converting the ink value to a value in a uniform color space such as L * a * b *. Further, the smoothness of changes in ink values and L * a * b * values may be used as evaluation values. Furthermore, you may use the comprehensive evaluation value which combined these evaluation values by the product-sum operation.

  In step 1005, the evaluation values E0, E1, and E2 derived in step 1004 are compared. Then, the LUT having the best evaluation value (here, the smallest) is held (updated) as the best LUT at the present time.

  In step 1006, it is determined whether the best evaluation value has converged. As a criterion for determination, if the evaluation value of the best LUT currently held is equal to the evaluation value E0 without fluctuation, it is assumed that the evaluation value has converged. As a result of the determination, if the best evaluation value has not converged, the process returns to step 1002 to continue the processing. On the other hand, if the best evaluation value has converged, the process proceeds to step 1007.

  In step 1007, the value of the variable s is updated. In this embodiment, a value obtained by dividing the previous value by “2” is set as the value of the new variable s. However, it is only necessary to reduce the value stepwise, and the method of updating the variable s is not limited to this.

  In Step 1008, it is determined whether or not the value of the variable s is less than “1.0”. If the value of the variable s is “1.0” or more, the process returns to step 1002 to continue the processing. On the other hand, if the value of the variable s is less than “1.0”, the process proceeds to step 1009.

  In step 1009, it is determined whether or not processing has been completed for all grid points of the target LUT. If there is an unprocessed grid point, the process returns to step 1002 to determine the next grid point as the target grid point p and continue the process. On the other hand, if the processing has been completed for all grid points, this processing is terminated.

  As described above, the conversion LUT and the color separation LUT (partial LUT) are optimized.

(Effect of this embodiment)
Here, a case where the effect of the present embodiment can be particularly expected will be described again. In the color separation LUT, generally, a lattice point value (for example, an ink value) increases relatively monotonously with a change in an input value (for example, an RGB value), reaches a peak in a certain region, and then becomes saturated or gradually. Often decreases. However, for example, recent ink jet printers are increasingly equipped with special color inks other than the four basic colors of CMYK and dark and light inks having the same hue and different densities. The purpose of this is to expand various performances such as expansion of the color reproduction range, reduction of graininess, and reduction of ink consumption due to reduction of ink ejection amount. It has been found that such a color separation LUT corresponding to ink colors other than the four basic colors has input / output characteristics different from those of the prior art. For example, the color separation LUT of inks having different hues from the four basic colors and high saturation (for example, red, green, and blue special colors) may have a curve input / output characteristic including a plurality of peaks. FIG. 11 is a diagram for explaining the interpolation processing in the case where the color separation LUT has input / output characteristics having two peaks by comparing the method according to the present embodiment with the conventional method. FIG. 11A shows the conventional method, and FIG. 11B shows the method of this embodiment. In FIG. 11A, the graph on the left side shows one-dimensional ink values on the G axis of the green color separation LUT, which is the special color, and is the same as the graph (in) in FIG. And the interpolation processing result of the conventional method using this LUT is a graph on the right side. On the other hand, the graphs arranged at the top and bottom of the left end in FIG. 11B show the ink values on the G axis corresponding to the two partial LUTs obtained by dividing the same green color separation LUT as in FIG. This is a one-dimensional graph, which is the same as the graph (out1) and graph (out2) in FIG. 9 described above. Then, the graph at the right end of FIG. 11B is a combination of the interpolation processing results of this embodiment using these two partial LUTs (the central graph in FIG. 11B). When the graph on the right side of FIG. 11A (conventional method) is compared with the graph on the right end of FIG. 11B (this example), the change in the first peak is more significant in this example. It is clear that the input / output characteristics of the original color separation LUT can be reproduced more accurately. As described above, the present embodiment is particularly effective in a printer that forms an image using a color material having a plurality of peak portions in the input / output characteristics of the color separation LUT.

<Modification>
The mask used for dividing the base LUT is not limited to that shown in FIG. FIG. 12 shows an example of another variation of the LUT division mask. First, the pair of (c1) and (c2) has a region in which all input values are distributed to only one of the input values at both ends, and in a range where the input value takes an intermediate value. It is a mask when distributing in gradation. A pair of (d1) and (d2) is a mask in the case of distributing to two with a certain value as a boundary. A pair of (e1) and (e2) is a mask when the input values are evenly distributed. In any of these mask pairs, the sum of the division ratios is 1.0 over the entire range of input values, as in the mask of FIG.

  Further, in this embodiment, the case where the input / output characteristics of the base LUT include two peak locations has been described as an example. However, for example, the location where the ink value starts to enter (the rising location where the ink value transitions to a value other than 0) The same applies when there are two). Specifically, the base LUT of input / output characteristics having two rising points is divided into a partial LUT from the first rising point to a point before the second rising point and a partial LUT after the second rising point. And split into Then, interpolation processing is performed on each of the divided partial LUTs, and the results are synthesized. Further, there may be three or more peak points and rising points. When three or more peak locations or rising locations are included, the LUT may be divided corresponding to each peak location and the like, and the number of color separation processing units corresponding to that may be provided. The LUT division mask used in this case is a combination of a plurality of masks according to the number of divisions such that the sum of the division ratios is 1.0 in the entire range of input values.

  Further, the configuration of the recording head 111 includes eight colors of cyan (C), magenta (M), yellow (Y), black (K), gray (Gy), red (R), green (G), and blue (B). Although an example including color inks has been shown, the number and types of inks are not limited thereto. For example, in addition to light cyan, light magenta, light yellow, light gray, light red, light green, light blue, etc., which are light color inks with a low density, or the already mentioned special color inks of red, green, and blue, Orange, pink, white or the like may be used as the special color ink. A colorless and transparent clear ink or a metallic metallic ink may be used. Various types of inks such as dyes, pigments, and UV may be used.

  Further, among the inks mounted on the recording head 111, for example, the present invention may be applied only to the special color ink, and the conventional color conversion processing may be applied to the four basic colors of CMYK.

  The input image data is an RGB color image, but may be a CMYK color image, for example. Furthermore, components other than color, for example, components representing gloss may be included. In these cases, the color separation LUT is a four-dimensional or more LUT.

  Further, in the interpolation processing, mathematical expressions indicating similar input / output characteristics may be used instead of using a plurality of color separation LUTs generated by dividing the base LUT. In other words, from the color separation LUT (partial LUT) corresponding to each of a plurality of peak locations and locations where ink values start to be entered, formulas of curves representing the input / output characteristics (approximate value curve formulas) are obtained respectively. Interpolation calculation between grid points may be performed using the obtained mathematical formula.

  According to the present embodiment, even if the input / output characteristics of the color separation LUT include a plurality of output value peak points and output value start points, it is possible to reduce a decrease in accuracy due to the interpolation calculation.

  Next, instead of preparing in advance a plurality of color separation LUTs obtained by dividing the base LUT, an embodiment in which these are generated in the course of image forming processing in an output device such as a printer will be described as a second embodiment. To do. In addition, description is simplified or abbreviate | omitted about the part which is common in Example 1, and suppose that it demonstrates centering around difference below.

  FIG. 13 is a block diagram illustrating an example of the configuration of the printing system according to the present embodiment. The difference from the block diagram of FIG. 1 according to the first embodiment is that an LUT generation unit 1300 that generates an LUT used in each color separation processing unit based on a base LUT 1301 separately held in the image processing apparatus 10 is provided. It is. The base LUT 1301 is the same as the three-dimensional base LUT in the first embodiment. The LUT generation unit 1300 divides the base LUT 1301 into two LUTs according to the peak location and the like, and uses the one-dimensional conversion LUT and 3D used by the first color separation processing unit 104a and the second color separation processing unit 104b therefrom. A dimensional color separation LUT is generated.

  Other configurations are the same as those in the first embodiment, and the contents of the printing process in the printing system are as described in the flow of FIG. Hereinafter, a color separation process that differs from the case of the first embodiment will be described. FIG. 14 is a flowchart illustrating the flow of the color separation process + synthesis process according to the present embodiment.

First, in step 1401, the LUT generation unit 1300 acquires a base LUT 1301 serving as a reference from an HDD (not shown) or the like. In subsequent step 1402, the LUT generation unit 1300 performs LUT generation processing using the acquired base LUT. Here, the content of the LUT generation processing is the same as the conversion LUT and color separation LUT creation flow described in the flow of FIG. In step 1403, the LUTs obtained by the LUT generation process are used as the first conversion LUT 105a _-1 and the second conversion LUT _b-1 , and the first color separation LUT 105a _-2 and the second color separation LUT. Set as _b-2 (stored in HDD or the like).

  Each processing of Steps 1404 to 1410 corresponds to Steps 401 to 407 in the flow of FIG. 4 of the first embodiment, and there is no particular difference, so the description is omitted.

The above is the content of the color separation process + synthesis process according to the present embodiment. In FIG. 13, arrows directed to the _first conversion LUT 105 _a-1 and the second conversion LUT _b-1 are indicated by broken lines. This is because the first conversion LUT _a-1 and the second conversion LUT _b-1 are prepared in advance and optimized for a plurality of color separation LUTs that can be derived from the base LUT 1301. It means that it doesn't matter.

  As described above, the LUT used in each color separation processing unit may be generated in the image forming process in the output device such as a printer. In this case, the same effect as that of the first embodiment can be obtained.

  Embodiments 1 and 2 are modes in which nonlinear conversion and interpolation processing are separately performed in a plurality of color separation processing units associated with a plurality of color separation LUTs generated according to peak locations in the input / output characteristics of the base LUT. there were. Next, a mode in which the LUT to be used is switched in each one nonlinear conversion unit and interpolation unit included in one color separation processing unit will be described as a third embodiment. In addition, description is simplified or abbreviate | omitted about the part which is common in Example 1, and suppose that it demonstrates centering around difference below.

  FIG. 15 is a block diagram illustrating an example of the configuration of the printing system according to the present embodiment. Unlike the block diagram of FIG. 1 according to the first embodiment, there is one color separation processing unit 1500 including a nonlinear conversion unit 1510 and an interpolation unit 1520. An LUT designating unit 1530 for designating a one-dimensional conversion LUT used in the nonlinear conversion process and a three-dimensional color separation LUT used in the interpolation process is provided. The composition processing unit 1540 generates an interpolation value (ink value) for the input RGB value by cumulatively adding the output values from the interpolation unit 1520.

  Other configurations are the same as those in the first embodiment, and the contents of the printing process in the printing system are as described in the flow of FIG. Hereinafter, the color separation process + synthesis process in this embodiment will be described. FIG. 16 is a flowchart illustrating the flow of the color separation process + synthesis process according to the present embodiment. In this flow, in one color separation processing unit 1500, the first and second LUTs are read in order, nonlinear conversion processing and interpolation processing are executed, and the interpolation processing result is added and synthesized by the synthesis processing unit 1540. An interpolation value of the target RGB value is generated. This will be described in detail below.

  In step 1601, an RGB value to be noted as a processing target (hereinafter referred to as an attention RGB value) is acquired from the image data in the RGB color space that has been subjected to the color matching process, input from the color matching processing unit 102.

In step 1602, the nonlinear conversion unit 1510 and the interpolation unit 1520 obtain a one-dimensional conversion LUT and a three-dimensional color separation LUT specified by the LUT specification unit 1530. Here, the LUT designation unit 1530 designates the first conversion LUT 105 _a-1 and the first color separation LUT 105 _a-2 .

In step 1603, the nonlinear conversion unit 1510 performs a nonlinear conversion process (first nonlinear conversion process) on the target RGB value using the acquired first conversion LUT _a−1 . The non-linearized attention RGB value is sent to the interpolation unit 1520.

In step 1604, the interpolation unit 1520 executes an interpolation process (first interpolation process) on the nonlinear RGB value of interest using the acquired first color separation LUT _a-2 . The interpolation value obtained as a result of the first interpolation processing is sent to the synthesis processing unit 1540.

  In step 1605, the synthesis processing unit 1540 holds the output value from the color separation processing unit 1500 (the result of the first interpolation processing) as a provisional synthesis value in preparation for the addition processing performed in step 1609. (Stored in a memory not shown). At this time, it is assumed that the composite value is initialized ("0" is set) in advance.

In step 1606, the nonlinear conversion unit 1510 and the interpolation unit 1520 acquire the one-dimensional conversion LUT and the three-dimensional color separation LUT specified by the LUT specification unit 1530. Here, the LUT designation unit 1530 designates the second conversion LUT 105 _b-1 and the first color separation LUT 105 _b-2 .

In step 1607, the non-linear conversion unit 1510 performs a non _- linear conversion process (second non-linear conversion process) on the target RGB value using the acquired second conversion LUT _b-1 . The non-linearized attention RGB value is sent to the interpolation unit 1520.

In step 1608, the interpolation unit 1520 performs an interpolation process (second interpolation process) on the nonlinear RGB value of interest using the acquired second color separation LUT _b-2 . The interpolation value obtained by the second interpolation processing is sent to the synthesis processing unit 1540.

  In step 1609, the composition processing unit 1540 adds the output value from the color separation processing unit 1500 (result of the second interpolation process) to the composition value (result of the first interpolation process) held in step 1605. The addition result thus obtained becomes an interpolation value of the target RGB value.

  In step 1610, it is determined whether or not processing has been completed for all pixel values (RGB values) in the input image data. If the process has been completed for all pixel values, this process ends. On the other hand, if there is an unprocessed pixel value, the process returns to step 403 to determine the next target RGB value and continue the process.

  The above is the content of the color separation process + synthesis process according to the present embodiment.

  As described above, the same effects as those of the first embodiment can be obtained by a configuration in which the LUT to be used is switched in one color separation processing unit.

  Next, based on the configuration according to the third embodiment (the configuration in which one color separation processing unit switches and uses the LUT), the configuration according to the second embodiment (the LUT to be used is changed from the base LUT in the course of the image forming process). An embodiment in which the configuration to be generated is combined will be described as a fourth embodiment. In addition, description is simplified or abbreviate | omitted about the part which is common in a previous Example, and suppose that it demonstrates centering on difference below.

  FIG. 17 is a block diagram illustrating an example of the configuration of the printing system according to the present embodiment. A difference from the block diagram of FIG. 15 according to the third embodiment is that an LUT generation unit 1700 that generates an LUT used by the color separation processing unit 1500 based on a base LUT 1701 separately held in the image processing apparatus 10 is provided. Is a point. Note that the base LUT 1701 is the same as the three-dimensional base LUT in the first embodiment (base LUT 1301 in the second embodiment). The LUT generation unit 1700 divides the base LUT 1701 into two LUTs as in the LUT generation unit 1300 of the second embodiment, and first and second conversion LUTs that are sequentially read and referenced by the color separation processing unit 500, and First and second color separation LUTs are generated. In FIG. 17, the arrows directed to the first conversion LUT 105a-1 and the second conversion LUTb-1 are indicated by broken lines, but the meaning is the same as in the case of the second embodiment, and may be set in advance. It means that it doesn't matter.

  The contents of the printing process in the printing system are the same as those in the third embodiment, and are as described in the flow of FIG. FIG. 18 is a flowchart illustrating the flow of the color separation process + synthesis process according to the present embodiment. The present embodiment is based on the configuration of the third embodiment and adds the configuration of the second embodiment in which LUT generation is performed during the image forming process. Therefore, the content of the color separation process + synthesis process of the present embodiment is a combination of the flow of FIG. 16 in the third embodiment and the flow of FIG. 14 in the second embodiment. Hereinafter, it demonstrates along the flow of FIG.

  First, in step 1801, the LUT generation unit 1700 acquires a base LUT 1701 serving as a reference from an HDD (not shown) or the like. In subsequent step 1802, the LUT generation unit 1700 executes the LUT generation process using the acquired base LUT. The content of the LUT generation process is the same as the conversion LUT and color separation LUT creation flow described in the flow of FIG. Each LUT obtained by the LUT generation processing is set as the first conversion LUT 105a-1 and the second conversion LUTb-1, and as the first color separation LUT 105a-2 and the second color separation LUTb-2 (HDD or the like). Stored).

  Each processing of steps 1804 to 1813 corresponds to steps 1601 to 1610 in the flow of FIG. 16 of the third embodiment, and there is no particular difference, and thus description thereof is omitted.

  The above is the content of the color separation processing according to the present embodiment.

  As described above, the same effect as that of the first embodiment can be obtained by the configuration in which the LUT to be used is generated from the base LUT in the course of the image forming process based on the configuration in which the LUT to be used in one color separation processing unit is switched. be able to.

  In the first to fourth embodiments, it is assumed that the input / output characteristics of the base LUT include a plurality of peak values of output values and locations where the output values start to enter, and a one-dimensional conversion LUT for dealing with large nonlinearities. This is a mode in which nonlinear conversion is performed by using a plurality of inks per one input channel of one ink color. Next, similarly to the base LUT, a mode in which nonlinear conversion is performed using only one one-dimensional LUT having a plurality of peaks in its input / output characteristics will be described as a fifth embodiment. In addition, description is simplified or abbreviate | omitted about the part which is common in a previous Example, and suppose that it demonstrates centering on difference below.

  FIG. 19 is a block diagram illustrating an example of the configuration of the printing system according to the present embodiment. As in the third embodiment, there is only one color separation processing unit, and there is also one each of the one-dimensional conversion LUT and the three-dimensional color separation LUT referred to by the nonlinear conversion unit 1910 and the interpolation unit 1920, respectively. . That is, since the interpolation process performed on the target pixel value (for example, RGB value) in the input image data is performed only once, there is no synthesis processing unit that exists in the first to fourth embodiments. In this case, the color separation LUT used in the interpolation processing in the interpolation unit 1920 is the base LUT itself that appeared in the first to fourth embodiments. Other configurations are the same as those in the first embodiment, and the contents of the printing process in the printing system are as described in the flow of FIG.

  FIG. 20 is an example of a graph showing the characteristics of a one-dimensional conversion LUT (for green ink) used in this embodiment. In each of the graphs (a) to (c), the horizontal axis indicates the input RGB value, and the vertical axis indicates the output RGB value after nonlinear conversion. In the graph of FIG. 20 (b), the characteristics change relatively greatly between the reference numerals 2001 and 2002, and it is possible to increase the nonlinearity at a plurality of locations in the G-axis direction with one conversion LUT. It has become.

  FIG. 21 is a flowchart showing the flow of color separation processing according to the present embodiment. Unlike the previous embodiment, the content does not include the synthesis process.

  In step 2101, the color separation processing unit 1900 acquires two types of LUTs. Specifically, the nonlinear conversion unit 1910 acquires a one-dimensional conversion LUT 1911, and the interpolation unit 1920 acquires a three-dimensional color separation LUT 1921.

  In the subsequent step 2102, the target RGB value is acquired from the input image data in the RGB color space that has been subjected to the color matching process.

  In step 2103, the non-linear conversion unit 1910 executes non-linear conversion processing using the conversion LUT 1911 on the target RGB value.

  Further, in step 2104, the interpolation unit 1920 executes an interpolation process using the color separation LUT 1921 for the nonlinear RGB value of interest. Thereby, an interpolation value (ink value) corresponding to the target RGB value is generated.

  In step 2105, it is determined whether or not processing has been completed for all pixel values (RGB values) in the input image data. If the process has been completed for all pixel values, this process ends. On the other hand, if there is an unprocessed pixel value, the process returns to step 2102 to determine the next target RGB value and continue the process.

  The above is the content of the color separation processing according to the present embodiment.

  As described above, in the non-linear conversion process, the same effect as that of the first embodiment can be obtained by using a single one-dimensional conversion LUT including a plurality of locations where input / output characteristics greatly change per input channel of one ink color. Can be obtained.

[Other Examples]
Further, the present invention supplies a program for realizing one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus execute the program. It can also be realized by a process of reading and executing. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

Image processing apparatus 10
First color separation processing unit 104a
Second color separation processing unit 104b
Compositing processing unit 106

Claims (15)

An image processing apparatus for color-separating signal components of an input image into signal components handled by an output device,
Interpolation means for determining an output value of a signal component handled by the output device corresponding to each pixel value of the input image by an interpolation process based on a multi-dimensional color separation LUT corresponding to the signal component of the input image;
Conversion means for nonlinearly converting the pixel value of the input image using a one-dimensional conversion LUT before the interpolation processing in the interpolation means;
With
The color separation LUT includes a plurality of peak points where the output value has a maximum or minimum value in the input / output characteristics of at least one of the plurality of dimensions, or a plurality of rising points where the output value rises with respect to the input value,
The conversion LUT has input / output characteristics that increase nonlinearity corresponding to each of the plurality of peak points or rising points in the color separation LUT.
An image processing apparatus.   The conversion means performs non-linear conversion on each pixel value of the input image using the plurality of conversion LUTs corresponding to each of the plurality of peak portions or rising portions in the color separation LUT. The image processing apparatus according to 1. The interpolation means uses the plurality of partial LUTs obtained by dividing the color separation LUT corresponding to each of the plurality of peak points or rising points in the color separation LUT, and the input image that has been nonlinearly transformed Interpolation means for performing an interpolation process using the pixel value of
Combining the interpolation processing results respectively obtained by the interpolation means, and generating the output value corresponding to the pixel value of the input image;
Further comprising
The sum of the grid point values at the same position in each of the plurality of partial LUTs is equal to the grid point value at the corresponding position in the color separation LUT.
The image processing apparatus according to claim 1, wherein the image processing apparatus is an image processing apparatus. The interpolation means is constituted by means for performing a plurality of interpolation processes corresponding to each of the plurality of partial LUTs,
The said synthesis | combination means synthesize | combines the result of the interpolation process each received from the means to perform the said some interpolation process, The said output value corresponding to the pixel value of the said input image is produced | generated. The image processing apparatus described. Storage means for storing the color separation LUT;
The image processing apparatus according to claim 3, further comprising: an LUT generation unit configured to generate the plurality of partial LUTs by dividing the color separation LUT stored in the storage unit. The LUT generation unit divides the color separation LUT using a plurality of masks according to the number of the plurality of peak portions or rising portions in the color separation LUT,
The image processing apparatus according to claim 5, wherein the plurality of masks are masks such that a sum of division ratios of the masks is 1.0 in the entire range of input values. The signal component of the input image is a color component represented in an RGB color space,
The LUT generation means performs the division according to the RGB value of the input image.
The image processing apparatus according to claim 5, wherein the image processing apparatus is an image processing apparatus. A designating unit for designating the conversion LUT used by the conversion unit and the partial LUT used by the interpolation unit;
The conversion means performs nonlinear conversion using the conversion LUT specified by the specification means,
The image processing apparatus according to claim 2, wherein the interpolation unit performs an interpolation process using the partial LUT specified by the specifying unit.   9. The method according to claim 3, wherein the interpolation unit performs the interpolation process using mathematical expressions representing input / output characteristics of the plurality of partial LUTs instead of the plurality of partial LUTs. The image processing apparatus according to item. The output device is a printing apparatus;
The output value corresponding to the signal component handled by the output device is a value representing the amount of color material used in image formation in the printing apparatus. Image processing device. The printing apparatus is an inkjet printing apparatus,
The value representing the amount of the color material is an ink value.
The image processing apparatus according to claim 10. The ink used by the printing device includes a special color ink,
The image processing apparatus according to claim 11, wherein the nonlinear conversion in the conversion unit and the interpolation processing in the interpolation unit are performed only for the spot color ink.   The image processing apparatus according to claim 12, wherein the special color ink is further a light color ink. An image processing method for color-separating signal components of an input image into signal components handled by an output device,
An interpolation step of determining an output value of a signal component handled by the output device corresponding to each pixel value of the input image by an interpolation process based on a multi-dimensional color separation LUT corresponding to the signal component of the input image;
A conversion step of nonlinearly converting the pixel value of the input image using a one-dimensional conversion LUT before the interpolation processing in the interpolation step;
Including
The color separation LUT includes a plurality of peak points where the output value has a maximum or minimum value in the input / output characteristics of at least one of the plurality of dimensions, or a plurality of rising points where the output value rises with respect to the input value,
The conversion LUT has input / output characteristics that increase nonlinearity corresponding to each of the plurality of peak points or rising points in the color separation LUT.
An image processing method.   A program for causing a computer to function as the image processing apparatus according to any one of claims 1 to 13.

Images