JP2009159284A - Color data conversion apparatus, color data conversion method and color data conversion program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the deterioration in color reproductivity because of turbidity of colors, partial blurring of hue of gradation or the like without disturbing continuity of gradation. <P>SOLUTION: A color correction means 1 determines, for each pixel, whether the color of each input pixel is a neighboring color of cyan, magenta, yellow or gray or not on the basis of RGB data of input pixels. When it determines that the color is a neighboring color of cyan, magenta, yellow or gray, it outputs RGB data of a correction color. When it determines that the color is not a neighboring color of any of them, it outputs the RGB data of the input pixels as it is. The color correction means 1 outputs data which reflects the value of the pixel in the RGB data as RGB data of a correction color. A color space conversion means 2 converts the RGB data output by the color correction means into CMYK data. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a color data conversion apparatus, a color data conversion method, and a color data conversion program for converting RGB data into CMYK data.

  In a computer such as a personal computer, a color image is generally expressed by RBG (Red Blue Green) data. Accordingly, an image input device such as a scanner outputs a color image expressed by RBG data to a computer, and an output device such as a liquid crystal display device inputs a color image expressed by RBG data.

  In the printer, CMYK (Cyan Magenta Yellow blacK) inks and toners are used. Therefore, when printing is performed based on the color image data of the computer, it is necessary to convert the RBG data into CMYK data. When converting RBG data into CMYK data (hereinafter referred to as color conversion), there is a method using a three-dimensional lookup table (color conversion lookup table) (see, for example, Patent Documents 1 and 2).

  In the method using a three-dimensional lookup table, assuming that RBG data as input data is composed of 8 bits, for example, a lattice point corresponding to the value indicated by the upper 4 bits of the input data is obtained. Then, CMYK data, which is output data, is determined by interpolation using the lower 4 bits of the input data for the data indicated by the obtained grid points.

  As interpolation methods, hexahedral interpolation, triangular prism interpolation, tetrahedral interpolation, and the like are known. In Patent Document 2, tetrahedral interpolation is said to be excellent in reproducibility of gray (R value of RBG data = G value = B value) having a high influence on the impression of image quality. When tetrahedral interpolation is performed, a three-dimensional RGB color space having R, G, and B values as coordinate axes is divided at equal intervals on each of the three axes. That is, it is divided into a plurality of cubes. The three-dimensional lookup table is a table in which converted pixel values (CMYK data in this example) are associated with the vertices of each cube, that is, lattice points.

  If the point in the RGB color space corresponding to the input RGB data is a grid point, the pixel value of the grid point is output as it is. If it is not a grid point, the cube including the point corresponding to the input RGB data is divided into six triangular pyramids (tetrahedrons). Then, for pixel values associated with each lattice point of the tetrahedron including points corresponding to the input RGB data, calculation is performed according to the position of the point corresponding to the input RGB data, Pixel values (CMYK data in this example) corresponding to the input RGB data are obtained.

Japanese Patent Laying-Open No. 2005-252993 (paragraphs 0038-0040) JP2007-158928 (paragraph 0029)

  Cyan (C), magenta (M), and yellow (Y), which are primary colors in a printer or the like, are preferably pure colors that do not include other colors when expressing gradation. Also, from the viewpoint of good color reproduction, when expressing gray in CMYK, it is preferable that grid points of colors other than gray be used for interpolation as much as possible.

  However, since the division into six tetrahedrons in tetrahedral interpolation is uniquely determined by the RGB data values, the tetrahedrons used for the interpolation are different when the shift of several gradations is different. There is. As a result, gradation continuity may be impaired. For example, when a cyan pure color RGB value (0, 255, 255) (= R value, G value, B value) is input, the CMYK value (255, 0, 0, 0) (= C value) that is a pure color. , M value, Y value, K value), the color reproducibility may be deteriorated due to color turbidity or blurring of a part of gradation in various reasons. As an example, if a rounding error has occurred in the pre-processing of the color conversion process, or if noise is superimposed on the image data when the image is read by the scanner, colors other than cyan will result from tetrahedral interpolation. Mixing may result in poor color reproducibility.

  Patent Document 1 discloses a technique for replacing a neighboring color (peripheral color), which is a color near a primary color (specific color), with a specific color. In other words, a neighboring color that is a color near the cyan specific color that is a pure cyan color and has the highest C value (for example, 255) is replaced with the cyan specific color. A neighboring color that is a color near the specific color of magenta that is a magenta pure color and has the highest M value (for example, 255) is replaced with the specific color of magenta. A nearby color that is a color near the specific color of yellow that is a pure yellow color and has the highest Y value (for example, 255) is replaced with the specific color of yellow. Then, the black neighboring color is replaced with a black specific color. By performing such replacement, a specific color portion in the image is made clear.

  However, in the method described in Patent Document 1, since the change between the specific color and a color other than the specific color and the neighboring color is suppressed, the continuity of gradation is impaired in the vicinity of the specific color.

  The present invention relates to a color data conversion apparatus, a color data conversion method, and a color data conversion program for converting RGB data into CMYK data, and without losing the continuity of gradation, the color turbidity and part of gradation An object of the present invention is to provide a color data conversion device, a color data conversion method, and a color data conversion program for converting RGB data into CMYK data that prevent deterioration of color reproducibility due to hue blurring.

  The color data conversion apparatus according to the present invention determines, for each input pixel, whether the color of the input pixel is a peripheral color of cyan, magenta, yellow, or gray based on the RGB data of the input pixel. Color correction means for outputting the RGB data of the correction color when it is determined that the peripheral color is yellow or gray, and outputting the RGB data of the input pixel as it is when it is determined that the peripheral color is not any of the peripheral colors; Color space conversion means for converting RGB data output by the color correction means into CMYK data, and the color correction means outputs data reflecting values in the RGB data of the pixels as RGB data of the correction color. And

  The color data conversion method according to the present invention determines, for each input pixel, whether the color of the input pixel is a peripheral color of cyan, magenta, yellow or gray based on the RGB data of the input pixel. When it is determined that the peripheral color is yellow or gray, RGB data reflecting the value in the RGB data of the pixel is output as RGB data of the correction color. The RGB data of the input pixel is output as it is, and the output RGB data is converted into CMYK data.

  A color data conversion program according to the present invention is a computer mounted on a color data conversion apparatus that converts RGB data including R, G, and B values into CMYK data including C, M, Y, and K values. In addition, for each input pixel based on the RGB data of the input pixel, a process for determining whether the color of the input pixel is a peripheral color of cyan, magenta, yellow, or gray, and cyan, magenta, yellow, or gray If it is determined that the color is a peripheral color, the process of outputting the RGB data reflecting the value in the RGB data of the pixel as the correction color RGB data, and if it is determined that the color is not any of the peripheral colors, It is characterized in that a process for outputting RGB data as it is and a process for converting RGB data into CMYK data are executed.

  According to the present invention, it is possible to convert RGB data that prevents deterioration of color reproducibility due to color turbidity or blurring of a portion of the gradation without loss of gradation continuity into CMYK data. .

  First, the outline of the present invention will be described with reference to the block diagram of FIG. The color data conversion apparatus shown in FIG. 1 includes a color correction unit 1 and a color space conversion unit 2. The color correction unit 1 determines whether the color of the input pixel is a peripheral color of cyan, magenta, yellow, or gray for each input pixel based on the RGB data of the input pixel. If it is determined that the peripheral color is cyan, magenta, yellow or gray, the RGB data of the correction color is output. If it is determined that the peripheral color is not any of the peripheral colors, the RGB data of the input pixel is output as it is. . The color correction unit 1 outputs data reflecting the value in the RGB data of the pixel as the RGB data of the correction color. The color space conversion unit 2 converts the RGB data output from the color correction unit into CMYK data.

  Further, as shown in FIG. 2, the color space conversion means 2 is configured to include, for example, a three-dimensional lookup table 2B and a conversion unit 2A. The three-dimensional lookup table 2B includes a grid point C value when a three-dimensional RGB color space having R, G, and B values as coordinate axes is divided into cubes at equal intervals in each of the three axes. M value, Y value, and K value are set. The conversion unit 2A uses the C value, M value, Y value, and K value of the tetrahedron lattice points in the cube to which the RGB data output from the color correction unit 1 belongs, and uses the C value, M value, Y value and K value are calculated. That is, the color space conversion unit 2 is configured to perform tetrahedron correction.

  Furthermore, it is preferable that the color correction unit 1 is configured to first determine whether or not the pixel color is a gray peripheral color in order to prioritize gray reproducibility. The color correction unit 1 outputs an average value of the R value, the G value, and the B value in the RGB data of the input pixel as the RGB data of the gray correction color.

  Further, the color correction unit 1 sets the G value and the B value to the maximum value (255 when RGB is represented by 8 bits) as RGB data of the cyan correction color, and R in the RGB data of the input pixel. The data with the R value as the value is output, and the RGB data of the magenta correction color is output with the R value and the B value as the maximum values and the G value in the RGB data of the input pixel as the G value. As RGB data of the correction color, data having the R value and the G value as the maximum values and the B value in the RGB data of the input pixel as the B value is output.

  The conversion unit 2A in the color correction unit 1 and the color space conversion unit 2 can be realized by a hardware circuit, but can also be realized by a CPU mounted on the color data conversion apparatus and a processing program executed by the CPU. .

  Embodiments of the present invention will be described below with reference to the drawings.

Embodiment 1. FIG.
FIG. 3 is a block diagram showing an output device including a color data conversion device according to the present invention. The output device including the color data conversion device shown in FIG. 3 receives RGB data and performs preprocessing such as saturation correction, brightness correction, and contrast correction, and RGB data processed by the preprocessing unit 11. A color conversion unit 12 that inputs and converts RGB color space data to CMYK color space data, and γ correction for correcting γ characteristics of a printer or the like with respect to CMYK data obtained by the color conversion unit 12 conversion; A post-processing unit 13 that executes screen processing, halftone processing including error diffusion processing, and the like, and a printing unit 14 that prints on recording paper based on CMYK data output from the post-processing unit 13 are included. The color data conversion device is realized by the color conversion unit 12.

  FIG. 4 is a block diagram illustrating a configuration example of the color conversion unit 12 illustrated in FIG. 3. 4 includes a CMY gray correction unit 21, a lookup table (LUT) conversion unit 22, and a memory 23.

  The CMY gray correction unit 21 receives the RGB data processed by the preprocessing unit 11, detects C, M, Y, and gray peripheral colors from the RGB data, and detects the detected peripheral colors as C, M, Y. , The data is converted so that each color becomes a pure color of gray. The LUT converter 22 converts RGB data into CMYK data by tetrahedral interpolation using a three-dimensional lookup table. That is, the CMYK value of each pixel is calculated.

  The memory 23 stores a three-dimensional lookup table. In the three-dimensional lookup table, CMYK values of grid points when a three-dimensional RGB color space having R, G, and B values as coordinate axes are divided at equal intervals on each of the three axes are set. ing. When the RGB color space is divided into 16, the capacity of the memory 23 is required to be 17 × 17 × 17 × 4 = 19652 (bytes).

  Next, processing executed by the CMY gray correction unit 21 will be described. The CMY gray correction unit 21 determines whether or not each of the C, M, Y, and gray peripheral colors is based on the RGB data, but executes determination processing in the order of gray detection → cyan detection → magenta detection → yellow detection. .

  The CMY gray correction unit 21 performs the following process in the gray detection process.

Let the RGB values of the input RGB data be (x ₁ , x ₂ , x ₃ ). Also, the gray detection condition
_{k = max (| x 1 -x} e |, | x 2 -x e |, | x 3 -x e |) <THR
And here,
x _e = (x ₁ + x ₂ + x ₃ ) / 3 (0 ≦ x ₁ , x ₂ , x ₃ ≦ 255).

When the RGB value satisfies the gray detection condition, the CMY gray correction unit 21 converts the RGB value into gray correction color data: (x _e , x _e , x _e ).

  The CMY gray correction unit 21 performs the following process in the cyan detection process.

Let the RGB values of the input RGB data be (x ₁ , x ₂ , x ₃ ). Also, the cyan detection condition is
x ₂ > THR1 and k = | x ₂ −x ₃ | <THR
And Here, (0 ≦ x ₁ , x ₂ , x ₃ ≦ 255).

When the RGB value satisfies the cyan detection condition, the CMY gray correction unit 21 converts the RGB value into cyan correction color data: (x ₁ , 255, 255).

  The CMY gray correction unit 21 performs the following process in the magenta detection process.

Let the RGB values of the input RGB data be (x ₁ , x ₂ , x ₃ ). In addition, the magenta detection condition is
x _1> THR2 and _{k = | x 1 -x 3 |} <THR
And Here, (0 ≦ x ₁ , x ₂ , x ₃ ≦ 255).

When the RGB value satisfies the magenta detection condition, the CMY gray correction unit 21 converts the RGB value into magenta correction color data: (255, x ₂ , 255).

  The CMY gray correction unit 21 performs the following process in the yellow detection process.

Let the RGB values of the input RGB data be (x ₁ , x ₂ , x ₃ ). Also, the yellow detection condition
x ₁ > THR3 and k = | x ₁ −x ₂ | <THR
And Here, (0 ≦ x ₁ , x ₂ , x ₃ ≦ 255).

When the RGB values satisfy the yellow detection condition, the CMY gray correction unit 21 converts the RGB values into yellow correction color data: (255, 255, x ₃ ).

  Note that THR, THR1, THR2, and THR3 are threshold values for detecting surrounding colors that are set in advance. Specifically, THR is a threshold value that is regarded as a common peripheral color for each color. THR1 is a threshold value for regarding the peripheral color of cyan. THR2 is a threshold value for regarding the surrounding color of magenta. THR3 is a threshold value for regarding the surrounding color of yellow. Each of THR, THR1, THR2, and THR3 is in the range of 0 to 255, but THR is a relatively small value (for example, 4). THR, THR1, THR2, and THR3 are determined in response to a request for how far a color from the primary color is regarded as a peripheral color.

Next, a specific example will be described. It is assumed that the input color INPUT (specifically, the value of INPUT) can take a value from 0 to 255 with 8 bits for each RGB value. In the RGB color space, the input color INPUT can be expressed as follows.
_{INPUT = (x 1, x 2} , x 3)

When the input color INPUT satisfies the following gray detection condition, the CMY gray correction unit 21 determines that the input color INPUT is the gray peripheral color GRAY _a .
_{k = max (| x 1 -x} e |, | x 2 -x e |, | x 3 -x e |) <THR
And Here, x _e = (x ₁ + x ₂ + x ₃ ) / 3, (0 ≦ x ₁ , x ₂ , x ₃ ≦ 255). When the gray detection condition is satisfied, that is, when the value of k is smaller than THR, the CMY gray correction unit 21 converts the output OUTPUT into gray correction color data: (x _e , x _e , x _e ). To do.

  When it is confirmed that the input data does not indicate a gray peripheral color, the CMY gray correction unit 21 performs detection processing in the order of cyan detection → magenta detection → yellow detection.

When the input color INPUT satisfies the following cyan detection condition in the cyan detection process for determining whether or not the input RGB data is data of cyan peripheral color, the CMY gray correction unit 21 sets the input color INPUT to The peripheral color of cyan is determined as CYAN _a .
x ₂ > THR1 and k = | x ₂ −x ₃ | <THR
When the cyan detection condition is satisfied, the CMY gray correction unit 21 sets the output OUTPUT to cyan correction color data: (x ₁ , 255, 255).

When it is confirmed that the input data does not indicate the cyan peripheral color, the CMY gray correction unit 21 performs a magenta detection process for determining whether the input RGB data is data of the magenta peripheral color. To do. When the input color INPUT satisfies the detection condition of the magenta peripheral color described above, the output OUTPUT is set to magenta correction color data: (255, x ₂ , 255). If it is confirmed that the input data does not indicate the magenta peripheral color, a yellow detection process is performed to determine whether the input RGB data is yellow peripheral color data.

When the input color INPUT satisfies the above yellow peripheral color detection condition, the output OUTPUT is set to yellow correction color data: (255, 255, x ₃ ). If the yellow peripheral color detection condition is not satisfied, the input color INPUT is output as it is as output OUTPUT.

Next, the operation of the LUT conversion unit 22 will be described. FIG. 5 is an explanatory diagram showing a cube as a result of dividing the RGB color space into 16 parts at equal intervals. Input pixel P _x of 256 gradations RGB data is represented by 8 bits, respectively, and belong to one of cubic RGB color space is 16 divided results at regular intervals. Let the lattice points of the cube be P _{0 to} P ₇ . Note that which of the 16 cubes the input pixel P _x belongs to is determined by the upper 4 bits of the 8 bits.

FIG. 6 is an explanatory diagram showing six tetrahedrons obtained by dividing a cube. Each of the six tetrahedrons includes a lattice point P ₀ closest to the origin in the RGB color space. In FIG. 6, six tetrahedrons are represented by Index0 to Index5.

Hereinafter, it is assumed that the input pixel P _x belongs to a tetrahedron represented by Index0. A tetrahedron composed of lattice points P _x , P ₄ , P ₅ , and P ₆ is defined as V ₀ . A tetrahedron composed of lattice points P _x , P ₀ , P ₅ , P ₆ is defined as V ₄ . A tetrahedron composed of lattice points P _x , P ₀ , P ₄ , and P ₆ is defined as V ₅ . A tetrahedron composed of lattice points P _x , P ₀ , P ₄ , and P ₅ is defined as V ₆ . V ₀ , V ₄ , V ₅ , and V ₆ are each a part of a tetrahedron represented by Index ₀ .

Furthermore, the CMYK values corresponding to grid points _{P 0} and _{A 0,} the CMYK values corresponding to grid points _{P 4} and _{A 4,} the CMYK values corresponding to the lattice point P5 ₀ and _{A 5,} corresponding to the grid points _{P 6} CMYK value that is referred to as _{a 6.} The CMYK value A _x corresponding to the input pixel P _x can be obtained as follows.

A _x = (A ₀ × (Volume of V ₀ ) + A ₄ × (Volume of V ₄ ) + A ₅ × (Volume of V ₅ ) + A ₆ × (Volume of V ₆ )) × 6/256 (1) )

Even when the input pixel P _x belongs to a tetrahedron other than the tetrahedron represented by Index ₀ , the CMYK value A _x corresponding to the input pixel P _x can be obtained by the same calculation.

  The post-processing unit 13 performs γ correction processing that refers to a table corresponding to the characteristics of the toner and ink used in the printing unit 14, and performs error diffusion processing and screen processing according to the interface of the printing unit 14. The printing unit 14 prints on recording paper using toner or ink.

Next, the operation of the output device including the color data conversion device shown in FIG. 3 will be described with reference to the flowchart of FIG.
First, the preprocessing unit 11 inputs input image data (RGB data) pixel by pixel, and performs preprocessing such as saturation correction, brightness correction, and contrast correction (step S100). Note that the processing order of saturation correction, brightness correction, and contrast correction is arbitrary.

  Next, the color conversion unit 12 determines whether the pixel processed by the preprocessing unit 11 is a peripheral color of cyan, magenta, yellow, or gray (step S200). The order of determination is the order of gray detection → cyan detection → magenta detection → yellow detection. When it is determined that the pixel processed by the preprocessing unit 11 is a peripheral color of cyan, magenta, yellow, or gray, the color conversion unit 12 replaces input data with correction color data as described above. (Step S300). If it is determined that cyan, magenta, yellow, and gray are not peripheral colors, the pixel data processed by the preprocessing unit 11 is output as it is.

  The LUT converter 22 uses the upper 4 bits of the pixel data (RGB data) output from the color converter 12 to determine which cube the pixel belongs to, and further uses the lower 4 bits of the RGB data. Then, it is determined which tetrahedron the cube belongs to.

  Next, the LUT conversion unit 22 reads out the CMYK values of the vertices (grid points) of the tetrahedron to which the pixel belongs from the three-dimensional lookup table stored in the memory 23. Then, using the CMYK values of the vertices of the tetrahedron, the CMYK values of the pixels are calculated by the calculation using the above equation (1) (step S400). Then, the calculated CMYK values of the pixels are output to the post-processing unit 13.

  The post-processing unit 13 performs γ correction processing by referring to a table corresponding to the output characteristics of the printing unit 14 such as a printer, that is, the characteristics of toner and ink used in the printer (step S500). Further, error diffusion processing and screen processing are performed in accordance with the interface of the printing unit 14. Then, the processed CMYK data is output to the printing unit 14. The printing unit 14 performs printing on the recording paper using toner and ink based on the CMYK data output from the post-processing unit 13 (step S600).

As described above, in this embodiment, the peripheral colors of cyan, magenta, yellow, and gray are replaced with correction colors that are pure colors of cyan, magenta, yellow, and gray. In this embodiment, assuming that the input RGB data is (x ₁ , x ₂ , x ₃ ), the cyan correction color data is (x ₁ , 255, 255). The magenta correction color data is (255, x ₂ , 255). The correction color data of yellow is (255, 255, x ₃ ). The data of the gray correction color is (x _e , x _e , x _e ). Note that x _e = [(x ₁ + x ₂ + x ₃ ) / 3].

  That is, the input RGB values are reflected in the cyan, magenta, yellow, and gray correction colors. Therefore, it is possible to prevent deterioration of color reproducibility due to color turbidity and blurring of a part of the gradation without impairing the continuity of gradation.

Embodiment 2. FIG.
In the first embodiment, the threshold value THR for determining whether or not it is a peripheral color of cyan, magenta, yellow, and gray is a common threshold value for each color, but in the second embodiment, for each color. The threshold value can be set according to the density.

  FIG. 8 is a block diagram illustrating a configuration example of the color conversion unit 32 according to the second embodiment. The color conversion unit 32 illustrated in FIG. 8 includes a memory 42 in addition to the CMY gray correction unit 41, the LUT conversion unit 22, and the memory 23. The configuration other than the color conversion unit 32 is the same as the configuration shown in FIG.

  In the memory 42, threshold values THR_c, THR_m, THR_y, and THR_bk at respective densities of 256 tones for cyan, magenta, yellow, and gray are stored in advance. Each of the threshold values THR_c, THR_m, THR_y, and THR_bk exists for each density of 256 gradations.

  Next, processing executed by the CMY gray correction unit 41 will be described. The CMY gray correction unit 41 determines whether or not it is a peripheral color of each of C, M, Y, and gray from the RGB data. As in the case of the first embodiment, gray detection → cyan detection → Determination processing is executed in the order of magenta detection → yellow detection.

  The CMY gray correction unit 41 performs the following process in the gray detection process.

Let the RGB values of the input RGB data be (x ₁ , x ₂ , x ₃ ). Also, the gray detection condition
_{k = max (| x 1 -x} e |, | x 2 -x e |, | x 3 -x e |) <THR_bk (e)
And Here, x _e = (x ₁ + x ₂ + x ₃ ) / 3 (0 ≦ x ₁ , x ₂ , x ₃ ≦ 255). Further, THR_bk (e) is a threshold value to be regarded as a gray peripheral color, and each of the values (0 to 255) that can be taken by x _e that is an average value of x ₁ , x ₂ , and x _3. Correspondingly, it is set in the memory 42. That is, 0 ≦ e ≦ 255. THR_bk (e) is a value in the range of 0 to 255, but is set to a relatively small value (for example, 4).

When the RGB value satisfies the gray detection condition, the CMY gray correction unit 41 converts the RGB value into gray correction color data: (x _e , x _e , x _e ).

  When the CMY gray correction unit 41 confirms that the input data does not indicate a gray peripheral color, the CMY gray correction unit 41 performs a cyan detection process. In the cyan detection process, the following process is executed.

Let the RGB values of the input RGB data be (x ₁ , x ₂ , x ₃ ). Also, the cyan detection condition is
x ₂ > THR1 and k = | x ₂ −x ₃ | <THR_c (xo)
And Here, (0 ≦ x ₁ , x ₂ , x ₃ ≦ 255).

THR_c (xo) is a threshold value to be regarded as a peripheral color of cyan, but it is stored in the memory 42 corresponding to each of values (0 to 255) that can be taken by xo that is an average value of x ₂ and x _3. Is set. That is, 0 ≦ xo ≦ 255. THR_c (xo) is a value in the range of 0 to 255, but is set to a relatively small value (for example, 4).

When the RGB values satisfy the cyan detection condition, the CMY gray correction unit 41 converts the RGB values into cyan correction color data: (x ₁ , 255, 255).

  When the CMY gray correction unit 41 confirms that the input data does not indicate a cyan peripheral color, the CMY gray correction unit 41 executes a magenta detection process. In the magenta detection process, the following process is executed.

Let the RGB values of the input RGB data be (x ₁ , x ₂ , x ₃ ). In addition, the magenta detection condition is
x _1> THR2 and _{k = | x 1 -x 3 |} <THR_m (xo)
And Here, (0 ≦ x ₁ , x ₂ , x ₃ ≦ 255).

THR_m (xo) is a threshold value to be regarded as a magenta peripheral color, and is stored in the memory 42 corresponding to each of values (0 to 255) that can be taken by xo that is an average value of x ₁ and x _3. Is set. That is, 0 ≦ xo ≦ 255. THR_m (xo) is a value within the range of 0 to 255, but is set to a relatively small value (for example, 4).

When the RGB value satisfies the magenta detection condition, the CMY gray correction unit 41 converts the RGB value into magenta correction color data: (255, x ₂ , 255).

  When the CMY gray correction unit 41 confirms that the input data does not indicate a magenta peripheral color, the CMY gray correction unit 41 performs a yellow detection process. In the yellow detection process, the following process is executed.

Let the RGB values of the input RGB data be (x ₁ , x ₂ , x ₃ ). Also, the yellow detection condition
x _1> THR3 and _{k = | x 1 -x 2 |} <THR_y (xo)
And Here, (0 ≦ x ₁ , x ₂ , x ₃ ≦ 255).

THR_y (xo) is a threshold value to be regarded as a yellow peripheral color, and THR_y (xo) is stored in the memory 42 corresponding to each of values (0 to 255) that can be taken by xo that is an average value of x ₁ and x _2. Is set. That is, 0 ≦ xo ≦ 255. THR_y (xo) is a value within the range of 0 to 255, but is set to a relatively small value (for example, 4).

When the RGB value satisfies the yellow detection condition, the CMY gray correction unit 41 converts the RGB value into yellow correction color data: (255, 255, x ₃ ). If it is determined that the surrounding color is not yellow, the pixel data processed by the preprocessing unit 11 is output as it is.

  The operation of each unit other than the CMY gray correction unit 41 is the same as that in the first embodiment.

  In the second embodiment, since a threshold value is set for each color according to the density, the setting of how far away from the primary color is regarded as the peripheral color is set for each color and the density. Can be done every time. Therefore, it is possible to more effectively prevent deterioration of color reproducibility due to color turbidity or blurring of a part of the gradation without impairing gradation continuity.

  In each of the above embodiments, tetrahedral interpolation is used as an interpolation method, but other interpolation methods such as hexahedral interpolation and triangular prism interpolation may be used.

It is a block diagram which shows the outline | summary of this invention. It is a block diagram which shows the outline | summary of this invention. It is a block diagram which shows the output device containing a color data converter. It is a block diagram which shows the structural example of the color conversion part in 1st Embodiment. It is explanatory drawing which shows the cube which is a result of having divided RGB color space into 16 at equal intervals. It is explanatory drawing which shows six tetrahedrons obtained by dividing | segmenting a cube. It is a flowchart which shows operation | movement of the output device containing a color data converter. It is a block diagram which shows the structural example of the color conversion part in 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Color correction means 2 Color space conversion means 2A Conversion part 2B Three-dimensional lookup table 11 Pre-processing part 12 Color conversion part 13 Post-processing part 14 Printing part 21 CMY Gray correction part 22 LUT conversion part 23 Memory 32 Color conversion part 41 CMY Gray correction unit 42 Memory

Claims (9)

A color data converter that converts RGB data including R value, G value, and B value into CMYK data including C value, M value, Y value, and K value,
Based on the RGB data of the input pixel, it is determined for each input pixel whether the color of the input pixel is a peripheral color of cyan, magenta, yellow or gray, and is a peripheral color of cyan, magenta, yellow or gray Color correction means for outputting the RGB data of the correction color when it is determined, and the RGB data of the input pixel as it is when it is determined that it is not any of the peripheral colors;
Color space conversion means for converting RGB data output by the color correction means into CMYK data;
The color correction unit outputs the data reflecting the value in the RGB data of the pixel as the RGB data of the correction color. The color data conversion apparatus according to claim 1, wherein the color correction means first determines whether or not the color of the pixel is a peripheral color of gray. The color data conversion apparatus according to claim 1, wherein the color correction unit outputs an average value of the R value, the G value, and the B value in the RGB data of the input pixel as the RGB data of the gray correction color. Color correction means
As the RGB data of the cyan correction color, the G value and the B value are set to the maximum value, and the R value in the RGB data of the input pixel is output as the R value.
As RGB data of the magenta correction color, the data having the R value and the B value as the maximum values and the G value in the RGB data of the input pixels as the G value is output,
4. The data with the R value and the G value as the maximum values and the B value in the RGB data of the input pixel as the B value is output as the RGB data of the correction color of yellow. 5. The color data converter described in 1. The color space conversion means includes a C value, an M value, and a grid point when a three-dimensional RGB color space having R, G, and B values as coordinate axes is divided into cubes at equal intervals in each of the three axes. Using the three-dimensional lookup table in which the Y value and K value are set and the C value, M value, Y value, and K value of the tetrahedral lattice points in the cube to which the RGB data output from the color correction means belongs, RGB The color data conversion apparatus according to claim 1, further comprising: a conversion unit that calculates C value, M value, Y value, and K value corresponding to the data. A color data conversion method for converting RGB data including R value, G value and B value into CMYK data including C value, M value, Y value and K value,
Based on the RGB data of the input pixel, it is determined for each input pixel whether the color of the input pixel is a peripheral color of cyan, magenta, yellow or gray, and is a peripheral color of cyan, magenta, yellow or gray If it is determined, the RGB data reflecting the value in the RGB data of the pixel is output as the correction color RGB data. ,
A color data conversion method comprising converting output RGB data into CMYK data. The color data conversion method according to claim 6, wherein first, it is determined whether or not the pixel color is a gray peripheral color. In a computer mounted on a color data conversion device that converts RGB data including R value, G value, and B value into CMYK data including C value, M value, Y value, and K value,
A process of determining whether the color of the input pixel is a peripheral color of cyan, magenta, yellow, or gray for each input pixel based on the RGB data of the input pixel;
If it is determined that the peripheral color is cyan, magenta, yellow or gray, a process of outputting RGB data reflecting a value in the RGB data of the pixel as RGB data of the correction color;
If it is determined that none of the surrounding colors, the process of outputting the RGB data of the input pixel as it is,
A color data conversion program for executing processing for converting RGB data into CMYK data. On the computer,
The color data conversion program according to claim 8, wherein first, a process of determining whether or not a pixel color is a gray peripheral color is executed.

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