JP4650946B2 - Image processing apparatus, image processing method, program, and recording medium - Google Patents

Description

  The present invention relates to a digital image processing apparatus, for example, a technique suitable for a printer or a copying machine.

  The image quality of recent printers has been remarkably improved, and the image quality demanded by users has been increasing year by year accordingly. Therefore, in recent printers, in addition to the conventional color materials such as CMYK, printers having the same color but a low density color material are increasing. There are various reasons for this.

  First, the first reason is the compatibility between the expansion of the color gamut and the graininess. For example, when cyan (hereinafter referred to as C), magenta (hereinafter referred to as M), and yellow (hereinafter referred to as Y) are printed in the same amount, the color will be close to black (hereinafter referred to as K), but generally K color material and CMY are overlapped. K (hereinafter, process black) has a different spectral reflectance and has a slightly different color. That is, the color gamuts that can be expressed by the process black and the K color material are different. For this reason, by outputting using CMYK four color materials, the color reproduction range that can be output by the printer is expanded as compared with the case where only CMY three color materials are used. However, on the other hand, since a general black color material has a higher contrast at the time of output than other colors, there is a problem that graininess deteriorates when a low density region is reproduced with a K color material. Here, when a K color material having a low density (hereinafter referred to as Lk) is used, the contrast at the time of output is lowered, and deterioration of graininess can be avoided while maintaining the expanded color gamut.

  Furthermore, the second reason is improved character quality. Regardless of the ink jet type or electrophotographic type printer, it is difficult to accurately output CMY in an overlapping manner. Therefore, black with CMY colors is displayed around the black output by overlapping CMY. This is inconvenient when outputting characters, and colored characters are less sharp than characters output in a single color. Here, in order to reproduce a character having a sufficient density, the problem can be solved by simply outputting the character portion with a K color material. However, if the characters do not have sufficient density, a malfunction occurs. This will be described with reference to FIG. For simplicity, consider performing binary error diffusion on one-dimensional image data. FIG. 1A shows the original data, each block is one pixel, and each number indicates the density value of the pixel. On the other hand, if error diffusion is performed with the threshold value 256, it is output as a dotted line as shown in FIG. If this is an edge of a character, the character edge is particularly rattled, resulting in poor readability of the character. This becomes more noticeable as the characters are smaller. Here, the threshold value is set, but the density when one pixel is output as a K color material and the density value when one pixel is output as an image must be set to be equal. Otherwise, the total density of input and output will be different.

  There is Patent Document 1 as an apparatus for solving this problem. In Patent Document 1, when a character is detected and the character has a low density, the character is all output as Lk. For example, assuming that the density of the Lk color material is half that of the K color material for simplicity, the image data shown in FIG. 1A is naturally a single line reproduced with a single Lk color (FIG. 1C). ) Is output. By using Lk in this way, it is possible to output low-density characters with high quality.

JP 2002-067355 A

  However, when the characters are relatively large, there is a problem that if the entire surface is reproduced with the Lk color material, the total amount of color material required is increased compared with the case where the entire surface is reproduced with the K color material, and the output cost increases.

The present invention has been made in view of the above problems,
An object of the present invention is to provide an image processing apparatus, an image processing method, a program, and a recording medium that reduce the total amount of color materials required for reproduction, that is, the cost required for reproduction, while suppressing the shakiness of character edges. There is to do.

The present invention provides a character edge determination unit that determines whether or not a target pixel of an input image is a character edge, a character internal determination unit that determines whether or not the target pixel is inside a character, and the target pixel If the pixel value of the pixel of interest is higher than a predetermined first threshold value, it is determined as a high density, and if the pixel value of the pixel of interest is lower than a predetermined second threshold value (<first threshold value), it is determined as a low density, If the pixel value of the target pixel is lower than the first threshold value and higher than the second threshold value, it is determined that the density is medium density, the pixel determined as the character edge, and the character inside is determined Intermediate region determining means for determining a target pixel determined to be inside the character as an intermediate region in a region adjacent to the selected pixel, and a character for determining whether the target pixel is a thick character pixel or a thin character pixel Depending on the determination result of the width determination means and each determination means Output means for reproducing an image using color materials having different densities, and the output means includes, for the target pixel determined to be high density by the density determination means, the inside, character edge, and intermediate area of the character. Reproduction using only high-density color materials, and for the target pixel that the density determination means determines to be low density and the character width determination means determines to be a fine character, the inside of the character, the character edge, All the intermediate areas are reproduced with a low-density color material, and the character edge is reduced to the target pixel that the density determination means determines to be medium density or low density and the character width determination means determines to be a bold character. Reproduction with a color material of density, reproduction of the inside of the character with a high density color material, reproduction of the intermediate area with a mixture of a low density color material and a high density color material, and the density determination means For the pixel of interest that the character width determining means determines to be a fine character, Serial character edges reproduced by low concentrations colorant, and most important feature that reproduces the intermediate region and the character inside mixed low concentration colorant and high density colorant.

According to the present invention, it is possible to perform appropriate processing according to the region and character type by changing the color material used according to the character edge, the character inside, and the density, and the image quality is improved.
In other words, all high-density characters are reproduced with high-density color material to improve the uniformity of the character density, and the occurrence of defects due to misalignment between the high-density color material plate and the low-density color material plate is suppressed. Can improve character quality.
By reproducing all low-density thin characters with high-density color material, the uniformity of the character density is improved, and the occurrence of defects due to misalignment between the high-density color material plate and the low-density color material plate is suppressed. Can improve character quality.
By reproducing the edges of medium and low density thick characters with a low density color material, it is possible to suppress the jagged edges of the characters, improving the quality of the characters, and reproducing the inside of the characters with a high density color material. The total amount of color material required can be suppressed, and the cost required for reproduction can be suppressed.
By reproducing the edges of the medium and low density fine characters with the low density color material, it is possible to suppress the jagged edges of the characters and improve the character quality, and both the high density color material and the low density color material inside the character. By playing back with, it is possible to suppress the occurrence of white spots inside the characters and improve the image quality.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
A preferred embodiment of the present invention will be described using a copying machine as an example.
FIG. 2 shows the configuration of an embodiment of the present invention. First, the original is read by the scanner 100. Reading is performed by irradiating a document with light emitted from a light source and reading the reflected light with a RGB three-color CCD. Although the read image data is an analog signal, it is converted into, for example, an 8-bit digital signal by A / D conversion.

  From the G signal of the read image data, the image area determination unit 101 determines what kind of area each area of the document is. Specifically, if the region of interest is a character border region, a character inner region, or a character border region or a character inner region, determine whether the character is a thick character or a thin character To do. Further, it is determined whether the region is a high concentration region, a medium concentration region, or a low concentration region. The image area determination means 101 will be described in detail later.

  Since the read data is a value obtained by measuring the intensity of the reflected light and does not necessarily show a linear response to the scanner characteristics, it is converted into density data corresponding to the scanner characteristics by the scanner γ correction means 102. Convert. The conversion to the signal linearly responding to the density is performed so that the conversion to CMYKLk data that responds linearly to the density in the subsequent color conversion means 104 can be performed with high accuracy. More specifically, a lookup table (hereinafter referred to as LUT) as shown in FIG. 3 is used for each of the three RGB colors and replaced with corresponding values. Note that the LUT is prepared in advance by measuring the characteristics of the scanner.

  Next, the filter correction unit 103 corrects the deterioration of Modulation Transfer Function (hereinafter referred to as MTF) generated during the reading of the scanner, that is, the blur of the image. The mechanism for MTF degradation is as follows. A general scanner reads an image while moving CCD light receiving elements arranged in a line in a direction perpendicular to a line direction (hereinafter referred to as a main scanning direction). At this time, since the movement in the sub-scanning direction is continuously performed while trying to obtain a signal of an area to be originally obtained, information on the peripheral area is convoluted, resulting in degradation of MTF. . In addition, the MTF deterioration is caused by the fact that the charge movement by the shift register of the CCD element is not performed as ideal, so the MTF deterioration also occurs in the main scanning direction, but generally compared with the MTF deterioration that occurs in the sub-scanning direction. Minor. The correction can be achieved by convolving and integrating a filter as shown in FIG.

  Next, the color conversion means 104 is performed because the input signal is RGB three colors and is an additive color mixing system, whereas the output requires a subtractive color mixing system. In general printers, conversion to CMYK 4 colors is often performed, but in this embodiment, conversion to 5 colors of CMYKLk with Lk added is further performed. The conversion means 104 will also be described in detail later.

  Next, the UCR unit 105 is assumed to reduce the amount of CMY based on the values of K and Lk obtained by the color conversion unit 104. Specifically, CMY to be output is determined by calculating Cout = Cin−K−Lk / 2, Mout = Min−K−Lk / 2, and Yout = Yin−K−Lk / 2. Cin, Min, and Yin are CMY values input from the color conversion unit 104, while Cout, Mout, and Yout are output CMY values of the UCR unit 105.

  Next, the printer γ correction means 106 corrects the signal in accordance with the output characteristics of the printer. Specifically, it is realized by replacing the LUT as shown in FIG. 5 with the corresponding values using CMYKLk 5 colors. It is assumed that the LUT is prepared in advance by measuring the characteristics of the printer.

  Further, the halftone processing means 107 converts the signal having the number of gradations of 8 bits 256 values into a 1 bit binary signal and outputs it by the printer 108. Although error diffusion is used for halftone processing, this is a general technique and will not be described in this embodiment.

  Next, the image area determination unit 101 will be described in detail. FIG. 6 shows the configuration of the image area determination means 101. As a premise, the image area determination unit 101 performs determination using the G signal among the RGB signals obtained from the scanner. First, the edge determination means 200 binarizes the pixel value of the input signal with a threshold value 128, and outputs true if it matches the eight patterns as shown in FIG. To do. Here, each 升 represents one pixel, and a black circle indicates a pixel that has become black due to binarization. Next, the halftone dot determining means 201 first determines a peak point if the following equation is established in a matrix centered on the target pixel as shown in FIG.

(2 * a11-a00-a22> th) and (2 * a11-a02-a20> th) and (2 * a11-a10-a12> th) and (2 * a11-a01-a21> th)
Note that th is a predetermined threshold value, and is set to 0 here. After that, if there is a pixel determined to be a peak point in 3 × 3 pixels centered on the target pixel, it is determined that all the 3 × 3 pixels are halftone dot areas, and true is output. In addition, it is determined that the pixel is not so. Further, the outputs of the edge determination means 200 and the halftone dot determination means 201 are determined to be true when the output of the edge determination means 200 is true by the subtracter 207 and the output of the halftone dot determination means 201 is negative, as the character edge area. Areas other than are output as no. That is, the edge determination means 200, the halftone dot determination means 201, and the subtracter 207 can be called character edge area determination means.

  Next, the density determination means 202 averages the pixel values of 3 × 3 pixels centered on the target pixel, and it is low density, medium density, or high depending on the range of the average value. Determine whether the concentration. The range used for the determination here should correspond to the boundary between the medium density and the high density with the pixel value that should correspond when the 1-dot K color material is output, and when the 1-dot Lk color material is output. The pixel value is a boundary value between low density and medium density. Here, the pixel value is 0 to 120, the density is low, 121 to 240 is the medium density, and 241 or more is the high density. By doing so, all the character areas determined to be low density are output with Lk color material, and all the character areas determined to be high density are output with K color material, so that no dot is placed inside the character. High-quality character output is possible without generating Here, the pixel value itself corresponding to the density value is used as the boundary. However, since the density of the pixel of interest is only an average, the boundary may be slightly shifted. In the present embodiment, the average density of the pixel values in the region of interest is used as the determination density. However, a median value or other statistics may be used.

  Next, the character inner area 203 scans in the main scanning direction, and once the character edge determination is true, it outputs true until a pixel for which the next character edge determination is true is input from there. Outputs no.

  Next, the intermediate area determination unit 204 determines a pixel for which the character internal determination is true in an area where a pixel for which the character edge determination is true and a pixel for which the character internal determination is true are adjacent to each other as an intermediate area.

  Next, the character width determination unit 205 sets the pixel of interest to a thick character pixel if there is no region determined to be true by the character edge determination in the region as shown in FIG. When there is a bold character pixel in the 5 × 5 pixel area, all the 5 × 5 pixel areas are determined to be thick character areas, and the others are determined to be thin character areas.

  Finally, the comprehensive judgment unit 206 performs output as shown in FIG. 9A according to the input. Further, when none of the output of the character edge determination means, the output of the intermediate area determination means 204, and the output of the character inside determination means 203 is 0, 0 is output.

  Next, the color conversion unit 104 will be described in detail. FIG. 8A is a diagram showing details of the color conversion means 104. The RGB signals that have been processed up to the previous stage are converted into five CMYKLk signals by the RGB → CMYKLk conversion means 300. Note that the color conversion means 300 outputs CMYKLk by matrix operation as shown in FIG. Also, the coefficient determination method from a00 to a44 outputs CMYKLk color mixture and calculates in advance from the spectral reflectance at that time.

  Next, the color replacement unit 301 performs a process of replacing K with Lk or Lk with K according to the output of the image area determination unit 101. In this embodiment, for simplicity, it is assumed that the density of the K color material is twice that of the Lk color material. When the K signal is replaced with the Lk signal, Lk = 2 × K, and when Lk is replaced with K, K = Lk ÷ 2 shall be replaced. Of course, the density of the color material of K and Lk does not need to be doubled, and the replacement ratio may be set according to the density ratio of the K color material and the Lk color material. The replacement according to the output of the image area determination means 101 is performed as shown in 9 (b). Since it is difficult to understand as it is, description will be made with reference to FIG. 9C in which 9 (a) and FIG. 9B are combined.

  If it is determined that the character has a high density, all characters are output using only K. By reproducing with a single color K, it is possible to suppress the occurrence of a defect in which characters appear double due to the shift of the launch positions of the K plate and the Lk plate (hereinafter referred to as plate displacement). Similarly, in the case of low density and narrow width, the occurrence of defects due to plate misregistration can be suppressed by reproducing all characters with Lk. In the case of low density, medium density, and wide width, the character edge is reproduced with Lk, the inside of the character is reproduced with K single color, and the intermediate area is not replaced, that is, K and Lk are mixed. As a result, the character edges are continuously reproduced without interruption, so that the character can be easily read. On the other hand, since the inside of the character is reproduced in K single color, the total color material amount can be suppressed as compared with the case of reproducing using Lk. The rationale for performing such processing is that accurate reproduction of the shape is not required in the character compared to the character edge. This is shown in FIG. FIG. 10A is an image in which intermediate density characters are represented by binary values when character edges are not continuously processed, and FIG. 10B is an image when character edges are processed continuously. As can be seen from this figure, if the edge of the character is not reproduced continuously, the play of the edge of the character is large, and if the edge of the character is reproduced continuously, the legibility is excellent. On the other hand, FIGS. 10 (c) and 10 (d) are reproduced by changing the number of lines inside the character after continuously processing the edges of the character. (C) is an image represented by a low number of lines, and (d) is an image represented by a high number of lines. The reason why such an image is presented is to show an image that is close to the case where characters of medium density are expressed in K single color and the case where K and Lk are mixed. As is clear from FIGS. 10C and 10D, the legibility of the characters hardly changes between the left and right images.

  Similarly, in the case of a medium density of narrow width, an Lk color material is used for continuous reproduction of the edge of a character, but this is not a sufficient line width. This is due to the possibility of remaining.

  In the above embodiment, K and Lk are described as color materials having different densities, but of course, the same processing is performed for other color materials such as C and light cyan, M and light magenta, and Y and light yellow. It is possible. In this embodiment, the color material having a low density and the color material having a high density have been described. However, the same processing can be performed using a color material having an intermediate density.

  In the above description, the use of the high-density color material and the low-density color material is switched in a binary manner. For example, the mixing ratio is controlled according to the density or the thickness of the line. You may be the technique of doing.

  The present invention also supplies a storage medium storing software program codes for realizing the processing procedures and functions of the above-described embodiments to a system or apparatus, and a computer (CPU or MPU) of the system or apparatus stores the storage medium. This can also be achieved by reading and executing the program code stored in. In this case, the program code itself read from the storage medium realizes the processing procedures and functions of the above-described embodiment. As a storage medium for supplying the program code, for example, a flexible disk, a hard disk, an optical disk, a magneto-optical disk, a magnetic tape, a nonvolatile memory card, a ROM, or the like can be used. Further, by executing the program code read by the computer, not only the procedure of the above-described embodiment is realized, but also an OS (operating system) operating on the computer based on the instruction of the program code. A case where part or all of the actual processing is performed and the functions of the above-described embodiments are realized by the processing is also included. Further, after the program code read from the storage medium is written into a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. This includes a case where the CPU or the like provided in the board or the function expansion unit performs part or all of the actual processing, and the functions of the above-described embodiments are realized by the processing.

It is a figure explaining the problem of a prior art. The structure of the Example of this invention is shown. The input / output characteristics of the scanner γ correction means are shown. An example of an MTF correction filter is shown. The input / output characteristics of the printer γ correction means are shown. The structure of an image area determination means is shown. It is a figure explaining edge determination, halftone dot determination, and character width determination. It is a figure explaining a color conversion means. It is a figure explaining the Lk and K process according to the image area determination result. It is a figure explaining that the image quality of a character improves by the process of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Scanner 101 Image area determination means 102 Scanner gamma correction means 103 Filter correction means 104 Color conversion means 105 UCR means 106 Printer gamma correction means 107 Halftone processing means 108 Printer

Claims (4)

Character edge determination means for determining whether or not the pixel of interest of the input image is a character edge, character internal determination means for determining whether or not the pixel of interest is inside a character, and the pixel value of the pixel of interest is If the pixel value of the pixel of interest is lower than a predetermined second threshold (<first threshold), the pixel is determined to have a low density if the pixel value is higher than a predetermined first threshold. If the pixel value is lower than the first threshold and higher than the second threshold, density determination means for determining medium density, pixels determined to be the character edge, and pixels determined to be inside the character In an adjacent region, an intermediate region determining unit that determines a target pixel determined to be inside the character as an intermediate region, and a character width determining unit that determines whether the target pixel is a thick character pixel or a thin character pixel Depending on the determination result of each determination means, the concentration difference An output means for reproducing an image by using that color material, and the output means,
For the target pixel determined by the density determination means to be high density, the inside, the character edge, and the intermediate area of the character are all reproduced using only the high density color material,
For the target pixel determined by the density determination means to be low density and the character width determination means determined to be a fine character, the inside, character edge, and intermediate area of the character are all reproduced with a low density color material,
For the pixel of interest that the density determination means determines as medium density or low density, and the character width determination means determines that it is a thick character, the character edge is reproduced with a low density color material, and the inside of the character is high. Reproduce with a color material with a density, and reproduce the intermediate area with a mixture of a low density color material and a high density color material,
For the pixel of interest that the density determination means determines as medium density and the character width determination means determines that it is a fine character, the character edge is reproduced with a low-density color material, and the intermediate area and the character interior are reduced. An image processing apparatus that reproduces a color material having a high density and a color material having a high density. A character edge determination step for determining whether or not a target pixel of the input image is a character edge, a character internal determination step for determining whether or not the target pixel is inside a character, and a pixel value of the target pixel If the pixel value of the pixel of interest is lower than a predetermined second threshold (<first threshold), the pixel is determined to have a low density if the pixel value is higher than a predetermined first threshold. If a pixel value is lower than the first threshold value and higher than the second threshold value, a density determination step for determining medium density, a pixel determined to be the character edge, and a pixel determined to be inside the character are An intermediate region determining step for determining a target pixel determined to be inside the character as an intermediate region in an adjacent region; and a character width determining step for determining whether the target pixel is a thick character pixel or a fine character pixel; Depending on the determination result of each determination step, the concentration difference An output step of reproducing an image by using that color material, said output step,
For the target pixel determined to be high density in the density determination step, the inside, the character edge, and the intermediate area of the character are all reproduced using only the high density color material,
For the pixel of interest determined by the density determination step as a low density and the character width determination step as a fine character, the inside, the character edge, and the intermediate area of the character are all reproduced with a low density color material,
For a pixel of interest that is determined to be medium density or low density in the density determination step and to be a bold character in the character width determination step, the character edge is reproduced with a low density color material, and the inside of the character is high. Reproduce with a color material with a density, and reproduce the intermediate area with a mixture of a low density color material and a high density color material,
For the pixel of interest that the density determination step determines as medium density and the character width determination step determines that it is a fine character, the character edge is reproduced with a low-density color material, and the intermediate area and the character interior are reduced. An image processing method, wherein reproduction is performed by mixing a color material having a high density and a color material having a high density.   A program for causing a computer to implement the image processing method according to claim 2.   A computer-readable recording medium recording a program for causing a computer to implement the image processing method according to claim 2.

Images