KR100312726B1 - Method and Apparatus for Generation of Black and Color Halftoning Process in Consideration of Color Reconstructivity in the Color Printer - Google Patents

Abstract

The present invention relates to a three-dimensional look-up table (3D LUT), a maximal saturation and lightness table (MaxSL), a lightness to black table (LtoBk) and a tone. Brightness value (L ^* from the L ^* a ^* b ^* color value converted by converting the RGB color value input with the memory storing the tone correction table to CIE L ^* a ^* b ^* color value ), The saturation value (C ^* ) and the hue value (H ^* ) are calculated, and the maximum saturation value for the color value is read from the maximum saturation / brightness table (MaxSL) using the calculated color value as an index. In addition, after reading the first brightness value corresponding to the maximum saturation value, the first brightness value can be reproduced from the brightness vs. black table LtoBk using the first brightness value as an index, and then read. The first black value is defined as an initial value and is defined by the calculated brightness and saturation values. The black ink and CMY color appearing in the shadow area by generating the second black value, which is a black value suitable for the input RGB color value by using the sigmoid function, and then receiving the CMY color value and the first black value. In order to improve the combination characteristics of the inks, the black generation and the color reproduction characteristics of the color printer which vary the binarization threshold for the second black value and perform the binarization for the CMY color value according to the binarization result of the second black value, A color halftone processing apparatus and method.

According to the present invention, it is possible to calculate a black value that can adequately express the brightness and saturation of a color while maximizing the use of a relatively inexpensive black ink, and also when the black halftone treatment is performed, Since the combination characteristics of the CMY color inks can be improved, there is an advantage of providing an excellent printer output image.

Description

Method and Apparatus for Generation of Black and Color Halftoning Process in Consideration of Color Reconstructivity in the Color Printer

The present invention relates to a black generation and color halftone processing apparatus and method in consideration of the color reproduction characteristics of a color printer, and more particularly, a color calculated on each pixel of an input image and a printer gamut indicating color reproduction characteristics of a color printer. Black generation and color halftone processing considering color reproduction characteristics of one color printer to generate K using brightness and saturation of and to provide improved printer output image through color binarization algorithm suitable for generated CMY and K An apparatus and method are provided.

The color image is basically represented by an additive color mixture (additive mixed color) consisting of three colors of RGB (Red, Green, Blue) and reproduced in a soft copy device such as a color monitor. However, in hard copy devices such as digital color printers and color copiers, the CMY (Cyan, Magenta, Yellow) three colors are basically used for printing and printing of ink by dark color mixing (subtraction mixed color) and halftoning algorithms. Color reproduction is realized through the use of four colors, and in recent years, printing using four colors of black (K; blacK) added to these three colors improves the reproducibility of shadow areas in printed materials and at the same time expensive CMY. It is saving color ink.

In the case of four-color printing, in order to print a color image, a color separation algorithm for converting each pixel of an image input in three colors of RGB into CMY and K images and a color binarization algorithm suitable for the generated CMY and K images Through this process, a binary dot of 1 (white) and 0 (black) is required for the output of ink.

Equation 1 for converting an RGB color coordinate system into a CMY color coordinate system is well known.

1 is an exemplary diagram for describing each UCR algorithm for generating K.

In the algorithm for generating K, a method of converting RGB to CMY using Equation 1 without generating K, performing binarization on each CMY, and then printing by replacing the pixels printed by overlapping three colors with K There is a method of generating a separate K using an algorithm such as the Under Color Removal (UCR) method.

In the algorithm for generating K to replace a portion where three colors are printed without a separate K generation, a color theory that subtracts mixed colors by overlapping CMY inks generates K. It would seem reasonable to replace the overlapping portion of CMY with K, but in practice, the overlap of these inks cannot be K without theoretically producing a complete CMY ink, and K ink, by its nature, has If the contrast is large and mixed with other inks, there is a disadvantage in that the halftone pattern due to binarization is discontinuous and the visual error is increased.

In the UCR algorithm, since K has a certain subtractive characteristic over a radio wave length, a method of separating a certain amount of gray portions from the CMY to be printed and replacing the portion with K is used. There is a full black method and a skeleton black method. In the full black method, after converting to CMY through Equation 1, as shown in Equation 2, the minimum value is replaced with K in the converted CMY value, and the maximum K is added.

Here, mim (C, M, Y) means the minimum value of C, M, Y. This process can be expressed as a process of converting (a) of FIG. 1 to (b) of FIG. 1, which is the most economical method of minimizing the consumption of color ink, but in general, the K ink has a high density, so it is exactly If the image is not matched, chroma or saturation may be degraded.

In order to solve the problem of the full black method, UCR is performed only when the minimum value of Equation 2 exceeds a certain threshold in the skeleton black method, so as not to generate K in the bright highlight part. The reproducibility of dark shadow areas in the image is improved. This method can be expressed as Equation 3 and FIG.

However, this method has a disadvantage in that it is very difficult to set the proportional constant α for determining the shadow area.

A method having improved printing characteristics based on the above UCR method is proposed by US Patent Publication No. 5,402,245, which uses a nonlinear function to improve UCR characteristics. This algorithm is a mixture of full black algorithm and skeleton black algorithm. The algorithm generates K through a combination of two thresholds (t ₁ = 103, t ₂ = 230) and a nonlinear function such as Equation 4 and FIG. Doing. 2 is a waveform diagram illustrating a nonlinear function for removing color fading.

The color halftone algorithm using the four colors of CMYK described above simply generates K using RGB to CMY conversion and K-level simple conversion given by threshold and minimum value. Since the color reproducibility that appears cannot be taken into consideration, there is a disadvantage that a discontinuous surface due to a large contrast K ink can be generated.

Another method is to convert a color to a different coordinate system and then generate a CMYK. In this method, the RGB color is converted to another coordinate system, and then CMY and K are extracted to obtain accurate color reproduction. To split the color using CIE XYZ or CIE L ^* a ^* b ^* or HSL coordinates, use brightness and saturation.

The method using the CIE XYZ coordinate system is a method for calculating the amount of CMY and K inks that reproduce accurate colors based on the CMY and K inks used for color printing and the area of dots produced by the output of the ink. This is a method based on the Neuugebauer equation that divides the color by estimating the area of the four inks based on the assumption that the CIE XYZ values of the four inks and the input color's CIE XYZ coordinates are known.

In other words, the color of a point is based on the printing of four inks and can be estimated by using the area ratio shown by overprinting of the ink to give the color given by the output of the ink. 16 colors are obtained by outputting four inks, and the relative area thereof can be calculated as shown in Equation (5).

Here, A _Y , A _M , A _C and A _K represent the dot areas of the Y, M, C and K inks, respectively, and a means the relative area indicated by the output of the ink. The XYZ coordinates of the color estimated by the color mixing by this area ratio are calculated as in Equation (6).

Here, X _ymck , Y _ymck , and Z _ymck represent the CIE XYZ values of the colors obtained by the combined output of the four inks, and X _w , Y _w , Z _w represent the CIE XYZ values of the paper used for printing. By comparing the estimated CIE XYZ value with the XYZ value of the actual color, the area ratio of CMYK ink is obtained as a solution so that the original color can be accurately reproduced. However, it is difficult to solve the solution from this equation, and the color output due to the overlap of ink dots for one point affects other neighboring outputs due to the bleeding phenomenon, and much consideration should be given to this.

For this method, there is a method of outputting brightness from an RGB color and color-dividing it into CMY and K by using a black rate estimation function based on the brightness. In this method, after converting to CMY using RGB, the initial K value by the minimum value is obtained, the black ratio is calculated from the brightness of the color, and then the modified CMYK value is obtained.

For example, Nakamura uses the black power estimation function as shown in FIG. 3. 3 is a curve showing the black factor estimation function for lightness. Here, the black factor estimation function is the same as Equation 7 and can be obtained by the brightness L *.

The resulting output K is obtained by multiplying the black factor by K and modifying the CMY value again to obtain CMYK. According to this method, since only brightness is considered when reproducing color, black output is inevitable when reproducing a color having high saturation with low brightness.

A method of using saturation to overcome this problem is presented by US Patent Publication No. 5,502,579. In this method, the input RGB is converted to CIE L ^* a ^* b ^* , a C ^* representing saturation characteristics is determined from a ^* b ^*, and a black factor is calculated by a function. The assumption here is based on the assumption that we know the saturation and black factor of two points, one point (C ^* ₁ , Ak ₁ ) is the saturation and black rate indicated by printing only black ink, and the other point (C ^* ₂ , Ak ₂ ) is As a value for limiting the output of black ink, the black ratio is fixed at Ak ₂ for a color having a saturation above this saturation (C ^* ₂ ). In this case, the black factor for the saturation between two points can be obtained using a linear or nonlinear function.

U.S. Patent No. 5,502,579 calculates the black factor using a linear one-dimensional continuous function and a nonlinear sigmoid function as follows. In the case of the one-dimensional continuous function, the black factor can be calculated as shown in Equation 8.

In addition, in the case of a nonlinear function, the black factor may be calculated through Equation 9.

In the calculation result, if α 1, it is 1, and if α <0, it is 0. Backward propagation (back) to find the weight and threshold of the neural network that connects the black factor ( α ) and the color L ^* a ^* b ^* α input and the four outputs of CMYK obtained by the result of the saturation function. -propagation) algorithm is applied. In other words, a sample sample set is used to obtain weights and thresholds that minimize the back-propagation energy of the neural network for L ^* a ^* b ^* conversion pairs according to CMYK and its output. . This method has a high dependency on the two points applied to the calculation of the black factor and the sample applied when the neural network is optimized. In addition, in the case of color printer output, color saturation characteristics are different depending on the color, so that a lot of restrictions are placed on the K output for the two points to be set.

Accordingly, the present invention has been made to solve such a problem, and includes a printer gamut indicating color reproduction characteristics of a color printer and brightness (L ^* ) and saturation (C ^* ) for colors calculated at each pixel of an input image. To provide a black generation and color halftone processing apparatus and method in consideration of the color reproduction characteristics of a color printer capable of generating K by using and using the color binarization algorithm suitable for the generated CMY and K. Its purpose is.

1 is an exemplary diagram for explaining each UCR algorithm for generating K;

2 is a waveform diagram illustrating a nonlinear function for removing color fading;

3 is a curve showing the black factor estimation function for lightness,

4 is a block diagram showing a preferred embodiment of a black generation and color halftone processing apparatus in consideration of color reproduction characteristics of a color printer according to the present invention;

Figure 5 is a flow chart showing a preferred embodiment of the black generation and color halftone processing method in consideration of the color reproduction characteristics of the color printer according to the present invention,

6 is a flowchart showing a color halftone treatment process of the present invention;

7 is an exemplary view showing a triangular model based on brightness and saturation;

8 is a waveform diagram showing a sigmoid function by brightness and saturation.

<Explanation of symbols for the main parts of the drawings>

10: 3D lookup table storage 20: Maximum saturation / brightness table storage

30: brightness vs. black table storage 40: tone correction table storage

50: color conversion unit 60: saturation / color calculation unit

70 black generation unit 80 color halftone processing unit

81: threshold setting section 82: black / color printing section

In order to achieve the above object, a black generation and color halftone processing apparatus and method considering the color reproduction characteristics of a color printer according to the present invention includes a 3D look-up table (3D LUT) and a maximum saturation / RGB color values input with a brightness table (MaxSL; Maximal Saturation and Lightness table), a lightness to black table (LtoBk), and a memory storing a tone correction table. CIE L ^* a ^* b ^* color values are converted to L ^* a ^* b ^* color values, and then the brightness value (L ^* ), saturation value (C ^* ), and color value (H ^* ) are calculated. Read the maximum saturation value for the color value from the maximum saturation / brightness table MaxSL using the color value as an index, and read the first brightness value corresponding to the maximum saturation value, and then index the first brightness value. Brightness value given from the lightness versus black table (LtoBk) RGB color inputted using the sigmoid function defined by the calculated brightness and saturation values of the first black value as an initial value after reading the first reproducible black value. In order to improve the combination characteristics of the black ink and the CMY color ink appearing in the shadow area by generating the second black value suitable for the value and then receiving the CMY color value and the first black value, By varying the binarization threshold and performing binarization of the CMY color value according to the binarization result of the second black value, a black value that can express the brightness and saturation of the color properly while maximizing the use of relatively inexpensive black ink can be calculated. In addition, the color halftone treatment can improve the combination of black ink and CMY color ink in the shadow area. There is the advantage that can provide suhan printer output image.

Hereinafter, with reference to Figure 4 attached to a preferred embodiment of the black color generation and color halftone processing apparatus in consideration of the color reproduction characteristics of the color printer according to the present invention.

Figure 4 is a block diagram showing a preferred embodiment of the black generation and color halftone processing apparatus in consideration of the color reproduction characteristics of the color printer according to the present invention.

A preferred embodiment according to the apparatus of the present invention, as shown in Figure 4, 3D LUT (3D Look-Up Table) storage unit for representing the color reproduction characteristics of the color printer;

A maximum saturation / brightness table (MaxSL) storage unit having a maximum saturation value for each color obtained from the color gamut characteristics of the color printer and a brightness value corresponding to the maximum saturation value;

A lightness-to-black table (LtoBk; lightness to black table) storage unit 30 for storing the lightness-to-black table corresponding to each of the black (K) reproduction values for printing each brightness value on the human luminous characteristics;

A tone correction table storage section 40 of the color printer;

The RGB color values inputted with reference to the 3D lookup table and the tone correction table are converted into CMY color values through an RGB / CMY color coordinate conversion equation, and the converted CMY color values are converted to CIE L using forward interpolation. ^* a ^* b ^* color conversion unit 50 for converting a color value;

A saturation / color calculation unit 60 for calculating brightness values L ^* , saturation values C ^* , and color values H ^* from the converted L ^* a ^* b ^* color values;

After reading the maximum chroma value for the color value from the maximum chroma / brightness table MaxSL using the calculated color value as an index, and reading the first brightness value corresponding to the maximum chroma value, A color calculated by using the first brightness value as an index, reading a first black value that can reproduce the given first brightness value from the brightness vs. black table LtoBk, and then using the first black value as an initial value. A black generator 70 for generating and outputting a second black value suitable for the input RGB color value by using a sigmoid function defined by the brightness value and the saturation value of the black color value; And

In order to improve the combination characteristics of the black ink and the CMY color ink appearing in the shadow area by receiving the CMY color value and the second black value, the binarization threshold for the second black value is varied and the second black value is changed. It is configured to include a color halftone processing unit 80 for performing the binarization of the CMY color value according to the binarization result.

Here, the collar halftone processing unit 80,

After selecting the pixel of interest to be binarized in the order of raster scanning, the black (K) error value and the CMY color error value of the neighboring pixel adjacent to the pixel of interest are determined by the second black value and the CMY color value based on a predetermined color error diffusion method. And subtracting each from to calculate a corrected black value and a corrected CMY color value, and then determine whether the corrected black value is less than or equal to a first threshold value multiplied by a first proportional constant to an average value of the corrected CMY color values. If the value is less than or equal to the first threshold value, the value obtained by subtracting the maximum gray scale value by the second proportional constant from the intermediate gray scale value is set as the second threshold value; otherwise, the maximum gray scale value is set to the second threshold value. A threshold setting unit (81) for setting a value obtained by multiplying two proportional constants and the sum value of the intermediate gray level values as the second threshold value;

If the corrected black value is greater than or equal to the second threshold value, the pixel of interest is printed with only black ink with all CMY inks blocked, and then the error value for the black value of the pixel of interest and the CMY color value is determined. A black binarization printing unit 82 for calculating and storing;

If the corrected black value is smaller than the second threshold value, the corrected CMY color values are sorted in size order, and then the corrected value is determined by determining whether the largest value among the corrected CMY color values is greater than or equal to the intermediate gradation value. If the largest value among the CMY color values is greater than or equal to the halftone value, the pixel of interest is printed with all the ink blocked, and then the black value of the pixel of interest and the error value for the CMY color value are calculated and stored. And when the largest value among the modified CMY color values is smaller than the intermediate gray scale value, it is determined whether a second largest value among the modified CMY color values is greater than or equal to the intermediate gray scale value, or greater than the intermediate gray scale value. If the same, the pixel of interest is printed with only the corresponding color ink of the largest value of the corrected CMY color values and the corresponding ink of the second largest value, otherwise, the corrected CMY color Consists of a post, the black value of the target pixel and binarizing the color printing unit 83, which calculates and stores the error values for the color values CMY printing with only the color ink of the maximum value.

And, with reference to Figure 5 attached to a preferred embodiment of the black color generation and color halftone processing apparatus in consideration of the color reproduction characteristics of the color printer according to the present invention is as follows.

5 is a flow chart showing a preferred embodiment of the black generation and color halftone processing method in consideration of the color reproduction characteristics of the color printer according to the present invention.

According to a preferred embodiment of the present invention, as shown in Fig. 5, a three-dimensional retrieval table representing color reproduction characteristics of a color printer, a maximum saturation value for each color obtained from the color gamut characteristics of the color printer, and A brightness versus black table created by matching the maximum saturation / brightness table having a brightness value corresponding to the maximum saturation value with a black (K) reproduction value for printing each brightness value on human visual characteristics, and the tone of the color printer Converting an RGB color value input in a state in which a correction table is pre-stored to a CMY color value through an RGB / CMY color coordinate conversion equation (S10);

Referring to the 3D lookup table and the tone correction table and converting the converted CMY color values into CIE L ^* a ^* b ^* color values using forward interpolation (S20);

Calculating brightness values (L ^* ), saturation values (C ^* ), and color values (H ^* ) from the converted L ^* a ^* b ^* color values (S30);

Reading the maximum chroma value for the color value from the maximum chroma / brightness table MaxSL using the calculated color value as an index, and reading a first brightness value corresponding to the maximum chroma value ( S40);

Reading (S50) a first black value capable of reproducing the first brightness value given from the brightness vs. black table (LtoBk) using the first brightness value as an index;

Generating and outputting a second black value suitable for the input RGB color value by using a sigmoid function defined by the first black value as an initial value and a brightness value and a saturation value of the calculated color; 60;

In order to improve the combination characteristics of the black ink and the CMY color ink appearing in the shadow area by receiving the CMY color value and the second black value, the binarization threshold for the second black value is varied and the second black value is changed. According to the binarization result, it is configured to perform a color halftone processing step S70 of performing binarization on the CMY color value.

Here, the color halftone processing step (S70), as shown in Figure 6, the step of selecting the pixel of interest to be binarized in the order of raster scanning (S71);

Based on a preset color error diffusion method, the black (K) error value and the CMY color error value of the neighboring pixel adjacent to the pixel of interest are subtracted from the second black value and the CMY color value, respectively, to correct the corrected black value and the corrected CMY color value. Calculating (S72);

Determining whether the modified black value is less than or equal to a first threshold value obtained by multiplying an average value of the modified CMY color values by a first proportional constant (S73);

If the corrected black value is less than or equal to the first threshold value, a value obtained by subtracting the maximum gray scale value by the second proportional constant from the intermediate gray scale value is set as a second threshold value, otherwise, the maximum gray scale value Setting a value obtained by multiplying the second proportionality constant by the second gray scale value to the second threshold value and determining whether the corrected black value is greater than or equal to the second threshold value (S74). Wow;

If the corrected black value is greater than or equal to the second threshold value, the pixel of interest is printed with only black ink with all CMY inks blocked, and then the error value for the black value of the pixel of interest and the CMY color value is determined. Calculating and storing (S75);

If the corrected black value is less than the second threshold value, sorting the corrected CMY color values in size order (S76);

Determining whether the largest value among the modified CMY color values is greater than or equal to the intermediate gray level value (S77);

If the largest value among the corrected CMY color values is greater than or equal to the halftone value, the pixel of interest is printed with all the ink blocked, and then an error value for the black value of the pixel of interest and the CMY color value is calculated. Storing (S78); And

If the largest value among the modified CMY color values is smaller than the intermediate gray scale value, it is determined whether a second largest value among the modified CMY color values is greater than or equal to the intermediate gray scale value, and is greater than or equal to the intermediate gray scale value. Printing the pixel of interest with only the corresponding color ink of the highest value of the corrected CMY color values and the corresponding ink of the second largest value, otherwise printing only with the corresponding color ink of the largest value of the corrected CMY color values And calculating and storing an error value of the black value of the pixel of interest and the CMY color value (S79).

Here, in a preferred embodiment of the apparatus and method of the present invention, the intermediate gray value and the maximum gray value are preferably 128 and 225, respectively, and the first and second proportional constants are 0.5 and 0.15, respectively. desirable.

The operation and performance of the preferred embodiment of the black generation and color halftone processing apparatus and method in consideration of the color reproduction characteristics of the color printer according to the present invention configured as described above will be described in detail.

It can be seen that the gist of the present invention consists of two parts. That is, the present invention uses the printer color gamut representing the color reproduction characteristics of the color printer and the black value K by using the brightness values L ^* and saturation values C ^* for the colors computed in each pixel of the input image. And generating an improved printer output image through a color binarization algorithm suitable for the generated CMY color value and black value (K).

First, in order to implement a preferred embodiment according to the apparatus and method of the present invention, a 3D look-up table (3D LUT) representing the color reproduction characteristics of a color printer and the color gamut characteristics of the color printer are described. Maximal Saturation and Lightness table (MaxSL) having a maximum saturation value for each color to be obtained and a brightness value corresponding to the maximum saturation value, and black for printing each brightness value on human visual characteristics (K Lightness to Black table (LtoBk; Lightness to Black table) for storing the brightness vs. black table corresponding to the reproduction values, and the tone correction table of the color printer should be stored in the memory in advance. On the premise.

The color conversion unit 50 converts the RGB color values input by referring to the 3D search table and the tone correction table into CMY color values through an RGB / CMY color coordinate conversion equation, and forwards the converted CMY color values forward. interpolation) to CIE L ^* a ^* b ^* color values.

The saturation / color calculation unit 60 calculates the saturation value C ^* and the color value H ^* from Equation 10 from the converted L ^* a ^* b ^* color values.

In addition to the above description, as described above, the preferred embodiment of the present invention assumes that a printer gamut is given as a color reproduction characteristic of a color printer. Any form of gamut can be represented, but the CIE L ^* a ^* b ^* color value, brightness value, saturation value and color value of the input color should be estimated. The color gamut of the color printer here is obtained by making a 9 × 9 × 9 combination sample for the CMY color, printing it, and then measuring the value of CIE XYZ or CIE L ^* a ^* b ^* using a color measurement device.

When the color gamut of the color printer is obtained as described above, a maximum saturation / brightness table (MaxSL) having a maximum saturation value for the color hue represented by an angle of 0 to 359 ° and a corresponding brightness value can be created. .

To determine the human perceptual perception of K, we make a 16-level patch of K samples and measure the CIE L ^* values using a colorimeter. Correlate the K level for each sample with the measured value L ^* to generate an inverse function for estimating the K level for L ^{* from} 0 to 100 and store these values as a brightness versus black table (LtoBk).

These tables are stored in memory and their values are read using the brightness or color of the input color. The input RGB color value is converted into a CMY color value by Equation 1, and the CIE L ^* a ^* b ^* value for this color is estimated. Here, the CIE L ^* a ^* b ^* color values can be estimated through the forward interpolation for the 3D gamut represented by 9 × 9 × 9 and the printer's tone correction table, and in this interpolation method, three-dimensional triple linear interpolation is performed. trilinear interpolation and tetrahedral interpolation.

Thereafter, the black generation unit 70 reads the maximum saturation value α _s for the color value from the maximum saturation / brightness table MaxSL using the calculated color value as an index and the maximum value. After reading the first brightness value α _l corresponding to the saturation value, the first brightness value given from the lightness versus black table LtoBk can be reproduced using the first brightness value α _l as an index. After reading the first black value Bk, the black value suitable for the input RGB color value is determined by using the sigmoid function defined by the calculated brightness and saturation values of the first black value as an initial value. A second black value K, which is a value, is generated and output.

Equation 10 shows the color value and saturation value of the color from the interpolated CIE L ^* a ^* b ^* color values, and it expresses the brightness value, saturation value, and L ^* C ^* H ^* of the color value. Can be. Using the brightness value as an index, the level of K (that is, the first black value) at which the brightness value can be output is obtained from the lightness versus black table (LtoBk table). Using the color value as an index, α _s and α _l having the maximum saturation value for the color of the input color and the corresponding brightness value are read from the maximum saturation / brightness table.

When the color gamut of the printer is fixed, the color gamut may be represented by a triangular model by brightness and saturation as shown in FIG. 7. 7 is an exemplary view showing a triangular model by brightness and saturation. Using this model, the maximum saturation value (C ^* _max ) for the color and brightness of the input color can be calculated using Equation 11. For a 1 in Fig. 7 for the second and the L ^* ≥α _l of Equation (11) corresponds to _L ^* <α _l.

In addition, using the brightness of the input / output color as an index, the level of K that can express the brightness well is read from the brightness versus black table (LtoBk table) and expressed as the initial K _i . Using K _i and the values obtained above, K and CMY, which can best reproduce the input color, are obtained through Equation 2.

In this case, a sigmoid function defined by the calculated brightness and saturation values may be obtained through Equation 12.

Here, C ^* 'is used as a value adjusted by adjusting the C ^* of the estimated color for color enhancement.

By using this sigmoid function, the brightness and saturation of the input color can be considered simultaneously in generating K. That is, when the color of the input color is low, the value of the sigmoid function has a value close to 1 regardless of the brightness, and when the saturation is high, the function value is close to zero. Also, even with the same saturation, the sigmoid function has a value close to zero when the brightness is high, and the function value has a value greater than zero when the brightness is low. Fig. 8 is a waveform diagram showing the sigmoid function by brightness and saturation, and shows the difference of the sigmoid function values obtained according to the saturation for the case of brightness 50 and brightness 30. To generate K using the sigmoid function, we need the initial value of K for brightness. This initial value can be obtained from data having brightness measured by removing patches of gray scales of 0 to 255 only with K ink, excluding CMY ink. This data can be used to construct a relation that estimates the magnitude of K from the brightness inversely and minimize the error that occurs between the data and the estimated value to obtain the initial value of K for brightness. This value is stored as a K value for brightness and used as the initial value of K generation, K. The generated output K value may be obtained as shown in Equation 14.

Here, Ki = LtoBk [L *], which represents the initial value of K for brightness L * read from the brightness versus black table. If the saturation of the color is close to zero, the sigmoid function's value is close to one, and it is advantageous to print the color in K ink rather than CMY ink. In addition, since the K value generated as the output is estimated as the value of K for the brightness given by the actual measurement, the brightness can be preserved even if only K ink is output. However, if the color is high in saturation, the sigmoid function value approaches zero, and this color can be reproduced as CMY ink rather than K ink so that the corrected color can be reproduced as it is. In the case of generating K by using the algorithm of the present invention, if the saturation of the input color is small, it can be expressed only with pure brightness, and thus, K can be output to express the brightness best. The color loss is improved by minimizing the loss of saturation.

The procedure for obtaining the sigmoid function described above is as follows.

First, the calculated brightness value is determined as L ^* , the maximum saturation value for the calculated color value and the first brightness value corresponding to the maximum saturation value are respectively defined as α _s and α _l , and the calculated color value The sigmoid function defined by the calculated brightness and saturation values of the color when the maximum brightness value and the minimum brightness value for are set to L _max and L _min , respectively. Is,

Calculate as

here, And if If is The maximum saturation value (C ^* _max ) for the color value and brightness value of the input color is

If L ^* ≥α _l And if L ^* <α _l , Calculate as

In this case, the second black value may be obtained by multiplying the first black value by the sigmoid function as in Equation (14).

The color halftone processing unit 80 receives the CMY color value and the second black value and changes the binarization threshold for the second black value in order to improve the combination characteristics of the black ink and the CMY color ink appearing in the shadow area. And binarization of the CMY color value according to the binarization result of the second black value.

The CMYs and Ks produced by this method are converted to binary numbers, indicating that ink is taken and not taken by the color binarization algorithm. Here, a combination of K inks with high contrast and CMY inks used to reproduce colors should be combined to produce a harmonious output. In the color binarization algorithm presented for this purpose, the threshold for K is varied as shown in Equation 15 by comparing K and CMY values.

Here, β is 0.15. By varying the threshold value, β can improve the characteristics of the shadow area printed by K and the continuous surface of the color printed by CMY. If K is larger than the variable threshold, K is output and the error due to binarization is diffused by applying a binarization filter for the error of Stucky. If K is outputted, CMY is not outputted. When K is smaller than the threshold value, K is not output and the result is obtained by diffusing the K value as an error and binarizing the CMY image. In binarization of CMY images, the threshold is fixed at 128 and only two colors are printed to exclude all CMY inks printed on one pixel. Therefore, color is output by RGB by mixing two colors, CMY by one ink, and W (White) when not printing.

Terminologies used herein are terms defined in consideration of functions in the present invention, which may vary according to the intention or custom of a person skilled in the art, and the definition should be based on the contents throughout the present application. will be.

In addition, since the present invention has been described through the preferred embodiment of the present invention, in view of the technical difficulty aspects of the present invention, those having ordinary skill in the art can easily be different from another embodiment of the present invention. Since modifications may be made, it is obvious that both the embodiments and modifications cited in the above description belong to the claims of the present invention.

As described in detail above, a 3D Look-Up Table (3D LUT), a Maximal Saturation and Lightness Table (MaxSL), and a Lightness to Black Table (LtoBk) the RGB color values input in a state of having the memory that stores the table) and a tone correction table (tone correction table) from the CIE L ^* a ^* b ^* converted by converting the color values L ^* a ^* b ^* color value After calculating the brightness value (L ^* ), saturation value (C ^* ), and color value (H ^* ), the maximum color value from the maximum saturation / brightness table (MaxSL) is calculated using the calculated color value as an index. A first black value capable of reproducing the first brightness value given from the brightness versus black table (LtoBk) by reading the first brightness value corresponding to the maximum saturation value as well as reading the saturation value. After reading, set the first black value as initial value and calculate the color name. Using the sigmoid function defined by the value and the saturation value, the second black value, which is a black value suitable for the input RGB color value, is generated, and the black ink appearing in the shadow area by receiving the CMY color value and the first black value. According to the present invention, the binarization threshold for the second black value is varied and the binarization of the CMY color value is performed according to the binarization result of the second black value in order to improve the combination property of the ink and the CMY color ink. It is possible to calculate the black value that can express the brightness and saturation of the color properly while maximizing the black ink, and improve the combination characteristics of the black ink and the CMY color ink appearing in the shadow area when performing color halftone processing. Because of this, there is an advantage in providing a good printer output image.

Claims (17)

A 3D Look-Up Table (3D LUT) storage unit for expressing color reproduction characteristics of the color printer; A maximum saturation / brightness table (MaxSL) storage unit having a maximum chroma value for each color obtained from the color gamut characteristics of the color printer and a brightness value corresponding to the maximum chroma value; A lightness-to-black table (LtoBk; lightness to black table) storage unit for storing a lightness-to-black table corresponding to black (K) reproduction values for printing each brightness value on human visual characteristics; A tone correction table storage unit of the color printer; The RGB color values inputted with reference to the 3D lookup table and the tone correction table are converted into CMY color values through an RGB / CMY color coordinate conversion equation, and the converted CMY color values are converted to CIE L using forward interpolation. ^* a ^* b ^* color conversion for converting the color value unit; A saturation / color calculation unit configured to calculate a brightness value L ^* , a saturation value C ^* , and a color value H ^* from the converted L ^* a ^* b ^* color values; And Reading the maximum chroma value for the color value from the maximum saturation / brightness table MaxSL using the color value calculated by the saturation / color calculation unit as an index and corresponding to the maximum saturation value After reading the brightness value, the first brightness value can be read from the brightness vs. black table LtoBk using the first brightness value as an index, and then the first black value is read. A black generation unit that generates and outputs a second black value suitable for the input RGB color value by using a sigmoid function defined as an initial value and a brightness value and a saturation value of the calculated color. A black generation and color halftone processing apparatus in consideration of the color reproduction characteristics of a color printer, comprising. The method of claim 1, In order to improve the combination characteristics of the black ink and the CMY color ink appearing in the shadow area by receiving the CMY color value and the second black value, the binarization threshold for the second black value is varied and the second black value is changed. And a color halftone processing unit which takes into account color reproduction characteristics of a color printer, characterized by further comprising a color halftone processing unit for performing binarization on the CMY color value according to a binarization result. The method of claim 2, wherein the collar halftone processing unit, After selecting the pixel of interest to be binarized in the order of raster scanning, the black (K) error value and the CMY color error value of the neighboring pixel adjacent to the pixel of interest are determined based on a predetermined color error diffusion method, and the second black value and the CMY color value. And subtracting each from to calculate a corrected black value and a corrected CMY color value, and then determine whether the corrected black value is less than or equal to a first threshold value multiplied by a first proportional constant to an average value of the corrected CMY color values. If the value is less than or equal to the first threshold value, the value obtained by subtracting the maximum gray scale value by the second proportional constant from the intermediate gray scale value is set as the second threshold value; otherwise, the maximum gray scale value is set to the second threshold value. A threshold setting unit for setting a value obtained by multiplying a proportional constant by two and the intermediate gray level value as the second threshold value; If the corrected black value is greater than or equal to the second threshold value, the pixel of interest is printed with only black ink with all CMY inks blocked, and then the error value for the black value of the pixel of interest and the CMY color value is determined. A black binarization printing unit for calculating and storing; And If the corrected black value is smaller than the second threshold value, the corrected CMY color values are sorted in size order, and then the corrected value is determined by determining whether the largest value among the corrected CMY color values is greater than or equal to the intermediate gradation value. If the largest value among the CMY color values is greater than or equal to the halftone value, the pixel of interest is printed with all the ink blocked, and then the black value of the pixel of interest and the error value for the CMY color value are calculated and stored. And when the largest value among the modified CMY color values is smaller than the intermediate gray scale value, it is determined whether a second largest value among the modified CMY color values is greater than or equal to the intermediate gray scale value, or greater than the intermediate gray scale value. If it is the same, the pixel of interest is printed with only the corresponding color ink of the highest value among the corrected CMY color values and the corresponding ink of the second largest value, otherwise, the corrected CMY color value And a color binarization printing unit configured to calculate and store an error value for the black value of the pixel of interest and the CMY color value after printing with only the corresponding color ink having the largest value. Considered black production and color halftone processing equipment. The black generation and color halftone processing apparatus of claim 3, wherein the halftone value and the maximum grayscale value are 128 and 225, respectively. 4. The apparatus of claim 3, wherein the first proportional constant and the second proportional constant are 0.5 and 0.15, respectively, in consideration of color reproduction characteristics of the color printer. The color gamut of claim 1, wherein The color reproduction characteristics of color printers are characterized by measuring and measuring the values of CIE XYZ or CIE L ^* a ^* b ^* using a colormeasurement device after making and printing a 9 × 9 × 9 combination sample for CMY colors. Considered black production and color halftone processing equipment. The method of claim 1, The calculated brightness value is defined as L ^* , the maximum saturation value for the calculated color value and the first brightness value corresponding to the maximum saturation value are respectively defined as α _s and α _l , and for the calculated color value When the maximum and minimum brightness values are set to L _max and L _min , respectively, Sigmoid function defined by the brightness value and chroma value of the calculated color Is, Calculate as here, And if If is The maximum saturation value (C ^* _max ) for the color value and brightness value of the input color is If L ^* ≥α _l And calculate If L ^* <α _l Black generation and color halftone processing apparatus in consideration of the color reproduction characteristics of the color printer, characterized in that calculated as. The method of claim 7, wherein the second black value is, The black generation and color halftone processing apparatus in consideration of the color reproduction characteristics of the color printer, characterized in that the value multiplied by the first black value and the sigmoid function. A three-dimensional lookup table representing color reproduction characteristics of the color printer, a maximum saturation / brightness table having a maximum saturation value for each color obtained from the color gamut characteristics of the color printer and a brightness value corresponding to the maximum saturation value, and a human The three-dimensional retrieval table and the tone correction table in a state in which the brightness vs. black table created by matching the black (K) reproduction values for printing each brightness value on the luminous characteristics, and the tone correction table of the color printer are stored in advance. Converting an RGB color value input with reference to a CMY color value through an RGB / CMY color coordinate conversion equation; A brightness / saturation / color conversion step of calculating brightness values (L ^* ), saturation values (C ^* ), and color values (H ^* ) from the converted CMY color values with reference to the three-dimensional lookup table and the tone correction table. Wow; Reading a maximum chroma value for the color value from the maximum chroma / brightness table MaxSL using the calculated color value as an index, and reading a first brightness value corresponding to the maximum chroma value; And Reading a first black value capable of reproducing the first lightness value given from the lightness versus black table (LtoBk) using the first lightness value as an index; By using the sigmoid function defined by the brightness value and the saturation value of the calculated color as an initial value and the first black value, a second black value which is a black value suitable for the input RGB color value is generated. Black generation and color halftone processing method in consideration of the color reproduction characteristics of the color printer, characterized in that it comprises the step of outputting. The method of claim 9, wherein the brightness / saturation / color conversion step comprises: Referring to the three-dimensional lookup table and the tone correction table and converting the converted CMY color values into CIE L ^* a ^* b ^* color values using forward interpolation; And Calculating a color reproduction characteristic of the color printer, characterized by calculating brightness values (L ^* ), saturation values (C ^* ), and color values (H ^* ) from the converted L ^* a ^* b ^* color values. Considered black formation and color halftone treatment methods. The method of claim 9, In order to improve the combination characteristics of the black ink and the CMY color ink appearing in the shadow area by receiving the CMY color value and the second black value, the binarization threshold for the second black value is varied and the second black value is changed. The method of black generation and color halftone processing in consideration of the color reproduction characteristics of the color printer, characterized in that further comprising the step of performing the binarization of the CMY color value according to the binarization result. The method of claim 11, wherein the color halftone processing step, Selecting the pixels of interest to be binarized in raster scanning order; Based on a preset color error diffusion method, the black (K) error value and the CMY color error value of the neighboring pixel adjacent to the pixel of interest are subtracted from the second black value and the CMY color value, respectively, to correct the corrected black value and the corrected CMY color value. Calculating a; Determining whether the corrected black value is less than or equal to a first threshold value multiplied by a first proportional constant to an average value of the corrected CMY color values; If the corrected black value is less than or equal to the first threshold value, a value obtained by subtracting the maximum gray scale value by the second proportional constant from the intermediate gray scale value is set as a second threshold value, otherwise, the maximum gray scale value Determining whether the corrected black value is greater than or equal to the second threshold after setting a value obtained by multiplying the second proportionality constant by the second gray scale value to the second threshold value; If the corrected black value is greater than or equal to the second threshold value, the pixel of interest is printed with only black ink with all CMY inks blocked, and then the error value for the black value of the pixel of interest and the CMY color value is determined. Calculating and storing; If the corrected black value is less than the second threshold, sorting the corrected CMY color values by size; Determining whether the largest value among the modified CMY color values is greater than or equal to the halftone value; If the largest value among the corrected CMY color values is greater than or equal to the halftone value, the pixel of interest is printed with all the ink blocked, and then an error value for the black value of the pixel of interest and the CMY color value is calculated. Storing by; And If the largest value among the modified CMY color values is smaller than the intermediate gray scale value, it is determined whether a second largest value among the modified CMY color values is greater than or equal to the intermediate gray scale value, and is greater than or equal to the intermediate gray scale value. Printing the pixel of interest with only the corresponding color ink of the highest value of the corrected CMY color values and the corresponding ink of the second largest value, otherwise printing only with the corresponding color ink of the largest value of the corrected CMY color values And calculating and storing an error value of the black value of the pixel of interest and the CMY color value, in consideration of the color reproduction characteristics of the color printer. The method of claim 12, wherein the intermediate gray value and the maximum gray value, A black generation and color halftone processing method in consideration of the color reproduction characteristics of the color printer, characterized in that it is 128 and 225, respectively. The method of claim 12, wherein the first proportional constant and the second proportional constant, A black generation and color halftone processing method in consideration of color reproduction characteristics of a color printer, characterized in that it is 0.5 and 0.15, respectively. The method of claim 9, wherein the color gamut of the color printer, Color reproduction characteristics of color printers characterized by measuring and measuring the values of CIE XYZ or CIE L ^* a ^* b ^* using a color measurement device after making and printing a 9 × 9 × 9 combination sample for CMY colors. Black generation and color halftone treatment method taking into consideration. The method of claim 9, The calculated brightness value is defined as L ^* , the maximum saturation value for the calculated color value and the first brightness value corresponding to the maximum saturation value are respectively defined as α _s and α _l , and for the calculated color value When the maximum and minimum brightness values are set to L _max and L _min , respectively, Sigmoid function defined by the brightness value and chroma value of the calculated color Is, Calculate as here, And if If is The maximum saturation value (C ^* _max ) for the color value and brightness value of the input color is If L ^* ≥α _l And calculate If L ^* <α _l Black generation and color halftone processing method in consideration of the color reproduction characteristics of the color printer, characterized in that calculated as. The method of claim 9, wherein the second black value, The method of black generation and color halftone processing in consideration of the color reproduction characteristics of the color printer, characterized in that the multiplied by the first black value and the sigmoid function.