KR20010026635A - Color Error Diffusion Method Using Multi-threshold in Color Printing System - Google Patents

Abstract

PURPOSE: A method for diffusing multi-stage color error in color printing system is provided to achieve an enhanced color printing quality by preventing colors of identical line from being overlapped with each other. CONSTITUTION: A method comprises a first step(S10) of designating pixels in the order of raster scanning, and converting chromatic component into cyan, magenta and yellow components; a second step(S20) of calculating modified cyan component value by performing error diffusion with respect to cyan; a third step(S30) of judging whether the modified cyan component value is higher than a first threshold value; a fourth step(S40) of determining the cyan component of the subject pixel as cyan, subtracting the maximum gradation value from the modified cyan component value, and storing the result as a cyan gradation error value, if the modified cyan component value is higher than or equal to the first threshold value; a fifth step(S50) of judging whether the modified cyan component value is lower than the first threshold and higher than or equal to the second threshold, if the modified cyan component value is lower than the first threshold; a sixth step(S60) of determining the cyan component of the subject pixel as light cyan, calculating an average threshold, subtracting the modified cyan component value from the average threshold and storing the subtraction result as a cyan gradation error value if the modified cyan component value is lower than the average threshold, and subtracting the average threshold value from the modified cyan component value and storing the result as a cyan gradation error value if else, if it is judged in the fifth step that the modified cyan component value is lower than the first threshold and higher than or equal to the second threshold; a seventh step(S70) of determining the cyan component of the subject pixel as white and storing the modified cyan component value as a cyan gradation error value, if the modified cyan component value is lower than the second threshold.

Description

Color Error Diffusion Method Using Multi-threshold in Color Printing System

The present invention relates to a color halftone processing method in an image system, and more particularly, in a six-color color error diffusion method, by setting a threshold value in multiple steps to prevent overlapping of toner or ink to subdivide the gradation expression step. The present invention relates to a multi-stage color error diffusion method of a color printing system capable of improving print quality.

The error diffusion method, which is one of the halftone processing techniques, assigns a gray scale value larger than the threshold value as a dot for printing based on a fixed threshold value, and assigns a non-printing dot to a gray scale value other than the threshold value. A continuous gradation image is converted and processed into a halftone image through a threshold process. In this case, a lack of accuracy may occur as a result of using a fixed threshold value. Accordingly, in the error diffusion method, a target pixel value and a thresholded binary pixel value of a continuous grayscale image may occur. This problem is overcome by compensating the error of the liver by diffusing the adjacent pixels. In other words, the above-mentioned problem is solved by propagating an error between the binarization target pixel value and the thresholded binary pixel value of the continuous grayscale image being processed at present to adjacent pixels participating in the weight determination. This approach is even better when it comes to expressing continuous tones over a small area surrounded by some few dots.

In general, the error diffusion method has a relatively higher complexity than the alignment dither method, and thus requires more hardware components in implementation, but has an excellent gradation expression capability that can offset this in terms of image quality.

However, the error diffusion method has a disadvantage in that worm artifacts, start-up artifacts, and the like are generated, and based on an intermediate brightness value, that is, 127 gray scale images of 8 bits, 127 An error pattern is accumulated incorrectly in a document area in which adjacent gray scale values are widely distributed, and thus a constant pattern is continuously generated, resulting in severe visual rejection. In order to perform color printing by applying the error diffusion method as described above, a color printing system using a CMY color system is used.

Basically, the color printing system performs color printing by mixing three colors of inks, cyan (C), magenta (M), and yellow (Yellow; Y), based on the CMY color coordinate system. In this case, Cyan (C), Magenta (M), Yellow (Y), Blue (Blue = Cyan + Magenta; C + M), and Red (Red = Magenta + Yellow; M) for one pixel. + Y), green (Yellow + Cyan; Y + C), and black (Black = Cyan + Magenta + Yellow; C + M + Y) colors can be printed, and any color ink can be printed. If you do not shoot (white), you can print eight colors.

Accordingly, in order to perform the color error diffusion binarization process, the pixel of interest is designated in the order of raster scanning from the input color image, and the color components of the pixel of interest are separated and cyan (C), magenta (M), and yellow (Yellow). ; Y) Convert to series. As the input color image is generally composed of RGB color coordinates, it is necessary to convert it to CMY color coordinates. The equation for converting an RGB color coordinate system to a CMY color coordinate system uses Equation 1, which is well known.

In this case, the conventional method of spreading the color error for three-color printing is a method of applying the error diffusion independently to cyan (Cyan; C), magenta (M), and yellow (Yellow; Y). . However, such a three-color printing system has a disadvantage in that black colors are not reproduced properly, and thus, a four-color color printing system with black ink has been introduced, and in recent years, a light cyan; Six color printing systems with C ') and light magenta (M') are being actively introduced.

As with four or six color printing systems, toner or ink increases in number, and the toner or ink applied to the paper is peeled off or stained. In addition, in the case of a liquid like ink, when overlapping due to the properties of the material, the paper is crushed or the ink is consumed more and the range of gradation expression is narrowed. In particular, when printing or copying a document with evenly distributed CMYKC'M 'colors, using a single threshold value, toners or inks of the same color, ie, C and C' or M and M 'are the same pixel There is a problem that occurs when overlapping.

Accordingly, the present invention has been made to solve such a problem, in the color error diffusion method of the six-color color printing system, by setting the threshold value of the same series color in multiple stages to have different values C or C 'or It is an object of the present invention to provide a multi-stage color error diffusion method of a color printing system which improves color printing quality by blocking overlapping colors of the same series such as M and M '.

1a and 1b is a flow chart showing a preferred embodiment of a multi-stage color error diffusion method of a color printing system according to the present invention,

2A and 2B are exemplary diagrams showing an error diffusion mask proposed by Floyd-Ssteinberg and Stucky, respectively.

3 is an exemplary diagram showing division of a printing color gamut by a color error diffusion method of a conventional six-color printing system using a fixed threshold value.

4 is an exemplary diagram illustrating division of a printing color gamut according to the present invention using a plurality of thresholds.

In order to achieve the above object, the multi-stage color error diffusion method of the color printing system according to the present invention includes cyan, cyan, magenta, m, yellow, y, and light cyan. '), In the error diffusion method of a color printing system that includes a light magenta (M') as a printing primary color, when one pixel is expressed using a plurality of colors, in particular a cyan color consisting of magenta and light magenta When expressing series and magenta series composed of magenta and bright magenta, set multiple thresholds for one pixel and express only the designated color within the set threshold section to prevent different colors from being duplicated. It is characterized by improving color printing quality by preventing overlapping of series colors.

Hereinafter, a preferred embodiment of the multi-stage color error diffusion method of the color printing system according to the present invention will be described with reference to FIGS. 1A and 1B.

1A and 1B are flowcharts showing a preferred embodiment of the multi-stage color error diffusion method of the color printing system according to the present invention.

As shown in Figs. 1A and 1B, a preferred embodiment of the multi-stage color error diffusion method of the color printing system according to the present invention, the gray level error value between the pixel value of the binarized target image having the continuous gray level and the thresholded binary pixel value In an error diffusion method of a color printing system in which a gray level error value is diffused to adjacent pixels according to a predetermined weight to perform binarization, a color component of a pixel of interest is specified by specifying the pixel of interest in the order of raster scanning in an input color image. Separating and converting into a cyan (Cyan; C) component, a magenta (M) component, a yellow (Yellow; Y) component (S10);

Calculating a corrected cyan component value by performing an error diffusion process on the cyan component (C) (S20);

Determining whether the corrected cyan component value is greater than or equal to a first threshold (th1, herein 170) (S30);

If the corrected cyan component value is greater than or equal to the first threshold value th1, after determining that the cyan component of the pixel of interest is printed in cyan, the maximum gray value is subtracted from the correct cyan component value, and the cyan gray error value is determined. Storing at step S40;

If the modified cyan component value is less than the first threshold th1, whether the modified cyan component value is less than the first threshold th1 and greater than or equal to a second threshold th2 (here, 85). Determining (S50);

If the corrected cyan component value is less than the first threshold th1 and greater than or equal to the second threshold th2, it is determined that the cyan component of the pixel of interest is printed with light cyan (C ′). An average threshold value obtained by averaging a first threshold value th1 and the second threshold value th2 is calculated, and if the corrected cyan component value is smaller than the average threshold value, the corrected cyan component value is subtracted from the average threshold value. Storing as a cyan gradation error value; otherwise, subtracting the average threshold value from the corrected cyan constituent value and storing it as a cyan gradation error value (S60);

If the corrected cyan component value is smaller than the second threshold value th2, after determining that the cyan component of the pixel of interest is processed to white, storing the corrected cyan component value as a cyan gray scale error value (S70). Wow;

Calculating a corrected magenta component value by performing an error diffusion process on the magenta component (M) (S80);

Determining whether the corrected magenta component value is greater than or equal to a first threshold value th1 (S90);

If the corrected magenta component value is greater than or equal to the first threshold th1, after determining that the magenta component of the pixel of interest is to be printed in magenta, the maximum gray scale value is subtracted from the correct magenta component value to increase the magenta gray scale error value. Storing at step S100;

If the corrected magenta component value is less than the first threshold th1, determining whether the corrected magenta component value is less than the first threshold th1 and greater than or equal to a second threshold th2 ( S110);

If the corrected magenta component value is less than the first threshold value th1 and greater than or equal to the second threshold value th2, after determining that the magenta component of the pixel of interest is printed in bright magenta M ′, the An average threshold value obtained by averaging a first threshold value th1 and the second threshold value th2 is calculated, and if the corrected magenta component value is smaller than the average threshold value, the corrected magenta component value is subtracted from the average threshold value. Storing as a magenta gradation error value; otherwise, subtracting the average threshold value from the corrected magenta constituent value and storing it as a magenta gradation error value (S120);

If the corrected magenta component value is smaller than the second threshold value th2, determining that the magenta component of the pixel of interest is processed in white, and then storing the corrected magenta component value as a magenta gradation error value (S130). Wow;

Calculating a corrected yellow component value by performing an error diffusion process on the yellow component (Y) (S140);

Determining whether the corrected yellow component value is greater than or equal to a halftone value (S150);

If the corrected yellow component value is greater than or equal to the intermediate gray scale value, determining to print the yellow component of the pixel of interest in yellow, and then subtracting the maximum gray scale value from the corrected yellow component value as a yellow gray error value (S160);

If the corrected yellow component value is smaller than the intermediate gray scale value, it is determined to process the yellow component of the pixel of interest as white, and then storing the corrected yellow component value as a yellow gray error value (S170). do.

Here, the first threshold value th1 and the second threshold value th2 are preferably 170 and 85, respectively, based on the case where the number of bits per pixel is 8 bits, respectively. The gradation value is preferably 128 and 255, respectively.

The operation of the preferred embodiment of the multi-stage color error diffusion method of the color printing system according to the present invention configured as described above will be described in detail with reference to the accompanying drawings.

As is well known, a representative method of the known error diffusion methods is a method proposed by Floyd and Steinberg, and another representative method is a method proposed by Stucki.

FIG. 2A is an error diffusion mask proposed by Floyd and Steinberg, and FIG. 2B is an error diffusion mask proposed by Stucky.

Error diffusion method by Floyd Steinberg is 2 The use of 3 error diffusion masks not only requires relatively little memory, but also has some advantages in terms of image quality.However, due to the size of the error diffusion masks, the surrounding pixels are properly considered. Failure to do so often results in incorrect error accumulation. As an alternative to solving this problem, Stucky has shown 3 in FIG. Five error diffusion masks are known.

For example, the error diffusion mask proposed by Floyd-Steinberg is shown in FIG. 2A, and the numbers described in each cell of the error diffusion mask are represented by In other words, the error diffusion method is the most basic premise that the error diffusion method should store the gray level error value of the previous line through the line memory.

As shown in Equation 2, the pixel value I (v, h) of interest at the current coordinate position (v, h) and the gray level error values E (v-1, h-1) and E (v-1, h of adjacent pixels ), E (v-1, h + 1), and E (v, h-1) by applying weights of 1/16, 5/16, 3/16, and 7/16, respectively, depending on the degree of correlation Calculate the value I (v, h). Where v is vertical coordinate and h is horizontal coordinate.

After calculating the corrected pixel value I '(v, h) through Equation 2, the binarized result value O (v, h) with respect to the current coordinate position (v, h) is calculated using the conditional expression as shown in Equation 3 below. Then, the tone error value E (v, h) for the position is calculated.

In other words, if the corrected pixel value of interest I '(v, h) is greater than or equal to the preset fixed threshold value (TH`_F, which becomes 127 when referring to a 256-gradation image having 8 bits per pixel), After outputting white (WHITE) with the result value O (v, h), the maximum grayscale value at the pixel value I (v, h) of interest becomes 255 when referring to a 256 grayscale image with 8 bits of pixels. ) Is subtracted and specified as the gradation error value E (v, h) for the corresponding position, otherwise, black (BLOCK) is output as the binarization result value O (v, h), and the pixel value I (v, h) is designated as the gradation error value E (v, h) for the corresponding position.

The present invention basically includes Cyan (C), Magenta (M), Yellow (Y), Black (K), Light Cyan (C '), Light Magenta (Light Magenta). M ') is based on a six-color printing system consisting of printing primary colors.

FIG. 3 is an exemplary diagram showing division of a printing color gamut by a color error diffusion method of a conventional six-color printing system using a fixed threshold, and FIG. 4 is a division of a printing color gamut according to the present invention using a plurality of thresholds. It is an exemplary diagram showing.

In the conventional color error diffusion method of the conventional six-color printing system, as shown in FIG. 3, since only a fixed threshold value is used for each color component, the same series colors such as C and C 'and M and M' overlap with each other. In order to prevent such overlapping phenomenon, the present invention uses a plurality of threshold values as shown in FIG. 4.

First, in step S10, the pixel of interest is separated by specifying the pixel of interest in the order of raster scanning from the input color image, and then the cyan (C) component, the magenta (M) component, and the yellow (Yellow; Y) Convert to component.

Subsequently, in step S20, an error diffusion process is performed on the cyan component C to calculate a correct cyan component value.

Next, in step S30, it is determined whether the corrected cyan component value is greater than or equal to the first threshold value th1.

As a result of the determination in step S30, if the corrected cyan component value is greater than or equal to the first threshold value th1, in step S40, after determining that the cyan component of the pixel of interest is printed in cyan, the correct cyan color is determined. The maximum gray value is subtracted from the component value and stored as a cyan error value.

On the other hand, if the corrected cyan component value is smaller than the first threshold value th1 as a result of the determination in step S30, the corrected cyan component value is smaller than the first threshold value th1 in step S50. It is determined whether it is greater than or equal to two thresholds th2.

As a result of the determination in step S50, when the corrected cyan component value is less than the first threshold th1 and greater than or equal to the second threshold th2, in step S60, the cyan component of the pixel of interest is bright. After determining to print in cyan (C '), an average threshold is obtained by averaging the first threshold th1 and the second threshold th2 so that the corrected cyan component value is less than the average threshold. If so, the corrected cyan component value is subtracted from the average threshold and stored as a cyan gradation error value. Otherwise, the average threshold value is subtracted from the corrected cyan component value and stored as a cyan gradation error value.

As a result of the determination in step S50, if the corrected cyan component value is smaller than the second threshold value th2, in step S70 it is determined that the cyan component of the pixel of interest is treated as white, and then the corrected cyan component The value is stored as a cyan error value.

By setting the threshold values of the same series of colors of cyan (C) and light cyan (C ') to have different values, the color printing quality can be improved by blocking the phenomenon of C and C' overlapping.

In the following process, as in the turquoise series process, the following error diffusion process is performed in order to prevent the phenomenon of M and M 'overlapping in the magenta series.

In step S80, an error diffusion process is performed on the deep red component M to calculate a corrected deep red component value.

In step S90, it is determined whether the corrected magenta component value is greater than or equal to the first threshold value th1.

As a result of the determination in step S90, if the corrected magenta component value is greater than or equal to the first threshold value th1, in step S100, after determining that the magenta component of the pixel of interest is printed in magenta, the correction magenta The maximum gray value is subtracted from the component value and stored as a magenta gray error value.

On the other hand, if it is determined in step S90 that the corrected magenta component value is less than the first threshold value th1, then in step S110 the corrected magenta component value is smaller than the first threshold value th1 and is equal to the first threshold value th1. It is determined whether it is greater than or equal to two thresholds th2.

As a result of the determination in step S110, when the corrected magenta component value is less than the first threshold th1 and greater than or equal to the second threshold th2, in step S120, the magenta component of the pixel of interest is bright. After determining to print in magenta (M '), an average threshold value obtained by averaging the first threshold value th1 and the second threshold value th2 is calculated so that the corrected magenta component value is less than the average threshold value. If so, the corrected magenta component value is subtracted from the average threshold value and stored as a magenta gradation error value. Otherwise, the average threshold value is subtracted from the corrected magenta component value and stored as a magenta gradation error value.

On the other hand, if it is determined in step S110 that the corrected magenta component value is less than the second threshold value th2, in step S130 it is determined that the magenta component of the pixel of interest is treated as white, and then the correction is made. The magenta component value is stored as a magenta tone error value.

By setting the threshold values of the magenta (M) and the bright magenta (M '), which are the same color, in different stages, color printing quality can be improved by blocking the overlapping phenomenon of M and M'.

On the other hand, when referring to the six-color color printing system, yellow performs the error diffusion process according to the general color error diffusion method.

In step S140, an error diffusion process is performed on the yellow component Y to calculate a corrected yellow component value. In step S150, it is determined whether the corrected yellow component value is greater than or equal to the intermediate gray value.

If the corrected yellow component value is greater than or equal to the intermediate gradation value as a result of the determination in step S150, in step S160, after determining that the yellow component of the pixel of interest is printed in yellow, the maximum value of the corrected yellow component value is determined. The gray scale value is subtracted and stored as a yellow gray scale error value.

As a result of the determination in step S150, if the corrected yellow component value is smaller than the intermediate gray scale value, in step S170, it is determined that the yellow component of the pixel of interest is processed in white, and then the corrected yellow component value is yellow in gray scale. Store as an error value.

Terminologies used herein are terms defined in consideration of functions in the present invention, which may vary according to the intention or customs of those skilled in the art, and the definitions 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 above, cyan (C), magenta (M), yellow (Y), light cyan (C '), and light magenta (M') are used as printing primary colors. In the error diffusion method of a color printing system including a single pixel when using a plurality of colors, in particular, when representing a cyan series composed of magenta and light magenta, and a magenta series composed of magenta and light magenta, According to the present invention, a plurality of threshold values are set for a pixel, and only specified colors are expressed within the set threshold section, thereby preventing overlapping of different colors. There is an advantage that can improve the image quality.

In addition, since the applied toner or ink can be suppressed from peeling off or staining, it is advantageous to obtain a high quality color halftone image by having a relatively natural gray scale expressing ability.

Claims (4)

Color printing system including Cyan (C), Magenta (M), Yellow (Y), Light Cyan (C '), Light Magenta (M') as printing primary colors In the error diffusion method, Specifying the pixel of interest in the input color image in order of raster scanning, separating the color component of the pixel of interest, and converting the color component of the pixel of interest into a cyan (Cyan) component, a magenta (M) component, and a yellow (Yellow; Y) component. Wow; Calculating a corrected cyan component value by performing an error diffusion process on the cyan component (C); Determining whether the corrected cyan component value is greater than or equal to a first threshold value; If the correct cyan component value is greater than or equal to the first threshold value, it is determined that the cyan component of the pixel of interest is printed in cyan, and then the maximum gray value is subtracted from the correct cyan component value and stored as a cyan gradation error value. Steps; If the modified cyan component value is less than the first threshold value, determining whether the modified cyan component value is less than the first threshold value and greater than or equal to a second threshold value; If the corrected cyan component value is less than the first threshold and greater than or equal to the second threshold, the first threshold is determined after printing the cyan component of the pixel of interest in light cyan (C '). Calculates an average threshold value obtained by averaging a value and the second threshold value, and if the corrected cyan component value is less than the average threshold value, the corrected cyan component value is subtracted from the average threshold value and stored as a cyan gradation error value. Otherwise, subtracting the average threshold value from the corrected cyan component value and storing it as a cyan gradation error value; And And if the correct cyan component value is less than the second threshold, determining that the cyan component of the pixel of interest is to be processed in white, and then storing the correct cyan component value as a cyan gradation error value. Multistage color error diffusion method of color printing system. The method of claim 1, Calculating a corrected magenta component value by performing an error diffusion process on the magenta component (M); Determining whether the corrected magenta component value is greater than or equal to a first threshold value; If the corrected magenta component value is greater than or equal to the first threshold, determining that the magenta component of the pixel of interest is to be printed in magenta, and then subtracting the maximum gray value from the correct magenta component value and storing it as a magenta gray error value. Steps; If the corrected magenta component value is less than the first threshold, determining whether the corrected magenta component value is less than the first threshold and greater than or equal to a second threshold; If the corrected magenta component value is less than the first threshold and greater than or equal to the second threshold, the first threshold is determined after printing the magenta component of the pixel of interest as light magenta (M ′). Calculates an average threshold value obtained by averaging a value and the second threshold value, and if the corrected magenta component value is less than the average threshold value, the corrected magenta component value is subtracted from the average threshold value and stored as a magenta gradation error value. Otherwise, subtracting the average threshold value from the corrected magenta component value and storing it as a magenta gradation error value; And If the corrected magenta component value is less than the second threshold, determining that the magenta component of the pixel of interest is to be processed in white, and then storing the corrected magenta component value as a magenta gradation error value. Multistage color error diffusion method of color printing system. The method of claim 2, Calculating a corrected yellow component value by performing an error diffusion process on the yellow component (Y); Determining whether the corrected yellow component value is greater than or equal to a halftone value; If the corrected yellow component value is greater than or equal to the intermediate gray scale value, determining to print the yellow component of the pixel of interest in yellow, and then subtracting the maximum gray scale value from the corrected yellow component value as a yellow gray error value ; And If the correction yellow component value is smaller than the intermediate gray value, determining that the yellow component of the pixel of interest is processed in white, and then storing the corrected yellow component value as a yellow gray error value. Multistage color error diffusion method of color printing system. The method of claim 3, wherein the first threshold value and the second threshold value are 170 and 85, respectively, based on the case where the number of bits per pixel is 8 bits, respectively, and the intermediate gray value and the maximum gray value are 128, respectively. And 255, the multi-stage color error diffusion method of the color printing system.