JP2010278924A - Image processing apparatus, image forming apparatus, image processing method, program, and recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processing apparatus capable of preventing an occurrence of moires which are interference patterns of halftone patterns of an original with halftone patterns generated in a halftone processing in an image processing, and capable of obtaining an optimum copy output, to provide an image forming apparatus, to provide an image processing method, to provide a program, and to provide a recording medium. <P>SOLUTION: A difference detection section 103 calculates a difference value Dif<SB>_</SB>F(u, v) of a frequency component from a spatial frequency component of image data before halftone processing and a spatial frequency component of image data after halftone processing. A moire determining section 104 determines whether any moires occur based on the difference value Dif<SB>_</SB>F(u, v) of the frequency component. When it is determined that the moires occur, a filter coefficient calculation section 105 calculates a filter coefficient for suppressing the occurrence of the moires based on the difference value Dif<SB>_</SB>F(u, v) of the frequency component. A spatial filter processing section performs spatial filter processing by using the calculated filter coefficient. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image processing apparatus that performs image processing, an image forming apparatus, an image forming method, a program, and a recording medium.

  Image forming apparatuses such as copiers and multifunction peripherals perform halftone processing to generate halftone images when outputting image data obtained by scanning a document, and area gradation such as dithering or error diffusion Method to control the density of copy output. A halftone image is generated by halftone processing of image data and halftone processing when scanned image data is composed of halftones because dither patterns composed of a plurality of pixels are arranged in a certain direction. Halftones may interfere with each other, resulting in a regular interference pattern and appearing in the output result. This regular interference pattern is called moire and is one of the factors that degrade the output image quality.

  As a conventional technique for performing image processing for suppressing the occurrence of moiré, there is an image processing apparatus described in Patent Document 1. This image processing apparatus performs a Fourier transform on the image data after the scaling process to detect a specific spatial frequency that generates moiré in relation to the writing density, and a common multiple relationship with respect to the detected specific spatial frequency The generation of moire is suppressed by performing one spatial frequency filter process with a spatial frequency not present in the range.

  Further, the image processing apparatus described in Patent Document 2 detects only a halftone dot region having a frequency at which moire occurs, and performs strong smoothing processing in the case of a specific number of lines in which a problem such as moire occurs. Change to filter processing to suppress the occurrence of moire.

  The image processing apparatus described in Patent Document 3 obtains a difference signal between a gradation image and a halftone image obtained by converting the gradation image into halftone dots, and detects the difference signal to generate a moire detection signal. Then, by using the moire detection signal as a parameter, the gradation image and the low-frequency component signal of the gradation image are weighted and averaged to generate an image signal from which the moire factor has been removed, thereby suppressing the occurrence of moire.

JP-A-8-51536 JP 2001-119575 A JP-A-9-238259

  However, the image processing apparatuses described in Patent Document 1 and Patent Document 2 do not consider the image density and the screen angle of halftone processing. If the image density changes, the width between halftone dots in the image data changes, and the width between halftone dots generated by halftone processing also changes, so there is no interference and moire may not occur. . Also, if the screen angle of the halftone process changes, the width between the halftone dots generated by the halftone process changes depending on the direction, so that interference does not occur and moire may not occur. Since the image processing apparatuses described in Patent Document 1 and Patent Document 2 do not take these into consideration, even if no moire occurs, smoothing filter processing that is stronger than necessary is performed, so image quality such as blurred outlines is obtained. There is a problem that deterioration occurs.

  Further, the image processing apparatus described in Patent Document 3 performs weighted averaging of the low-frequency component signal from which the high-frequency component signal has been removed and the gradation image signal without considering the period of the detected moire detection signal. Therefore, even when high-frequency moire that does not need to be removed and is inconspicuous occurs, there is a problem that the high-frequency component of the image is reduced and the image quality is deteriorated such as the outline is blurred.

  An object of the present invention is to prevent occurrence of moire, which is an interference pattern between a halftone pattern of a document and a halftone pattern generated by halftone processing, and obtain an optimum copy output in image processing. An image processing apparatus, an image forming apparatus, an image forming method, a program, and a recording medium are provided.

The present invention includes a halftone processing unit that performs halftone processing on image data to generate a halftone image;
The position of the pixel indicated by the image data before halftone processing before the halftone processing is performed by the halftone processing unit, and the pixel indicated by the image data after halftone processing after the halftone processing is performed by the halftone processing unit When the position is expressed in the same coordinate system, an area selection unit that selects an area of a predetermined shape at the same position, and
A frequency conversion unit that converts pixel values indicated by pre-halftone image data and post-halftone image data of pixels included in the region selected by the region selection unit into frequency components;
A difference calculation unit that calculates a difference between frequency components based on the frequency component of the image data before halftone processing converted by the frequency conversion unit and the frequency component of the image data after halftone processing converted by the frequency conversion unit;
A determination unit that determines whether or not moire, which is an interference pattern, is generated based on the difference between the frequency components calculated by the difference calculation unit;
A filter coefficient calculation unit that calculates a filter coefficient for suppressing the occurrence of moire based on the difference between the frequency components calculated by the difference calculation unit when the determination unit determines that moire occurs;
An image processing apparatus comprising: a filter processing unit that performs filter processing on image data using the filter coefficient calculated by the filter coefficient calculation unit.

According to the present invention, the halftone processing unit performs halftone processing on the image data stored in the storage unit that stores the image data,
The filter coefficient calculation unit stores the calculated filter coefficient in the storage unit in association with the image data stored in the storage unit.

The present invention further includes a display control unit for displaying the image data on the display device,
During a period when the display control unit displays the image data stored in the storage unit on the display device, the halftone processing unit performs halftone processing on the image data stored in the storage unit. The filter coefficient calculation unit stores the calculated filter coefficient in the storage unit in association with the image data stored in the storage unit.

The present invention further includes a document type determination unit that determines the document type of the image data based on the image data,
The determination unit determines whether or not moire occurs when the document type determination unit determines that the document is a document including a printed photograph.

In addition, the present invention is an image forming apparatus including the image processing apparatus.
The present invention is also an image processing method executed by an image processing apparatus for processing an image,
A halftone processing step of performing halftone processing on the image data to generate a halftone image;
The pixel position indicated by the image data before halftone processing before the halftone processing is performed in the halftone processing step and the pixel indicated by the image data after halftone processing after the halftone processing is performed in the halftone processing step. When the position is expressed in the same coordinate system, an area selection step for selecting an area of a predetermined shape at the same position, and
A frequency conversion step of converting the pixel values indicated by the image data before halftone processing and the image data after halftone processing of the pixels included in the region selected in the region selection step into frequency components;
A difference calculating step for calculating a difference between frequency components based on the frequency component of the image data before halftone processing converted in the frequency conversion step and the frequency component of the image data after halftone processing converted in the frequency conversion step;
A determination step of determining whether or not moire as an interference pattern occurs based on the difference between the frequency components calculated in the difference calculation step;
A filter coefficient calculating step for calculating a filter coefficient for suppressing the occurrence of moire based on the difference between the frequency components calculated in the difference calculating step when it is determined that the moire occurs in the determining step;
The image processing method includes a filter processing step of performing a filter process on the image data using the filter coefficient calculated in the filter coefficient calculation step.

The present invention also provides a computer,
A halftone processing unit that performs halftone processing on image data to generate a halftone image;
The position of the pixel indicated by the image data before halftone processing before the halftone processing is performed by the halftone processing unit, and the pixel indicated by the image data after halftone processing after the halftone processing is performed by the halftone processing unit When the position is expressed in the same coordinate system, an area selection unit that selects an area of a predetermined shape at the same position, and
A frequency conversion unit that converts pixel values indicated by pre-halftone image data and post-halftone image data of pixels included in the region selected by the region selection unit into frequency components;
A difference calculation unit that calculates a difference between frequency components based on the frequency component of the image data before halftone processing converted by the frequency conversion unit and the frequency component of the image data after halftone processing converted by the frequency conversion unit;
A determination unit that determines whether or not moire, which is an interference pattern, is generated based on the difference between the frequency components calculated by the difference calculation unit;
A filter coefficient calculation unit that calculates a filter coefficient for suppressing the occurrence of moire based on the difference between the frequency components calculated by the difference calculation unit when the determination unit determines that moire occurs;
This is a program for functioning as a filter processing unit that performs filter processing on image data using the filter coefficient calculated by the filter coefficient calculation unit.

  The present invention is also a computer-readable recording medium on which the program is recorded.

  According to the present invention, the halftone processing unit performs halftone processing on the image data to generate a halftone image. The pixel position indicated by the image data before halftone processing before the halftone processing is performed by the halftone processing unit by the region selection unit, and the halftone processed image after the halftone processing is performed by the halftone processing unit When the position of the pixel indicated by the data is expressed in the same coordinate system, a region having a predetermined shape at the same position is selected. The frequency conversion unit converts the pixel values indicated by the pre-halftone image data and the post-halftone image data of the pixels included in the region selected by the region selection unit into frequency components. The difference calculation unit calculates the difference between the frequency components based on the frequency component of the image data before halftone processing converted by the frequency conversion unit and the frequency component of the image data after halftone processing converted by the frequency conversion unit. The The determination unit determines whether or not moire, which is an interference pattern, is generated based on the difference between the frequency components calculated by the difference calculation unit. When the filter coefficient calculation unit determines that the moire is generated by the determination unit, a filter coefficient for suppressing the generation of the moire is calculated based on the frequency component difference calculated by the difference calculation unit. Then, the filter processing unit performs filter processing on the image data using the filter coefficient calculated by the filter coefficient calculation unit.

  Therefore, in image processing, it is determined whether or not moire, which is an interference pattern between a halftone pattern of a document and a halftone pattern generated by halftone processing, is generated based on input image data. Therefore, it is possible to know the presence or absence of moire before outputting a copy or the like. When it is determined that moiré occurs, the filter coefficient for suppressing the occurrence of moiré is calculated and spatial filtering is performed, so that the occurrence of moiré can be prevented and an optimum copy output can be obtained.

  According to the invention, the halftone processing unit performs halftone processing on the image data stored in the storage unit that stores the image data, and the filter coefficient calculated by the filter coefficient calculation unit Are stored in the storage unit in association with the image data stored in the storage unit. Accordingly, an image captured by the scanner unit or the like is stored in a storage device such as a hard disk, filter coefficients are calculated for the image data stored in the storage device during a standby time, and the calculated filter coefficients are used as the image data. The data can be stored in the storage device in association with each other, and time-consuming processing can be performed efficiently.

  According to the present invention, when the user selects a preview function for image data, the display control unit displays the image data on the display device. The display control unit displays the image data stored in the storage unit on the display device, and the halftone processing unit stores the image data in the storage unit using a period during which the user performs the image data editing process. A halftone process is performed on the processed image data, and the filter coefficient calculation unit stores the calculated filter coefficient in the storage unit in association with the image data stored in the storage unit. Therefore, for example, before outputting a copy, a preview image of the image data is displayed on the display device, and whether or not moire occurs in the image data while the user is viewing the preview image displayed on the display device. Since it is possible to calculate the filter coefficient when it is determined that moiré is generated, time-consuming processing can be performed efficiently.

  According to the invention, the document type determination unit determines the document type of the image data based on the image data. When the determination unit determines that the document is a document including a printed photograph, the document type determination unit determines whether moire occurs. Moire may occur when a halftone image is included in the document image. Therefore, the determination result by the document type determination unit indicates that there is no possibility of moiré, for example, a document. In the case of a document that does not include a printed photograph, the processing by the region selection unit, frequency conversion unit, difference calculation unit, determination unit, and filter coefficient calculation unit can be omitted, and the processing time can be shortened.

  According to the invention, since the image processing apparatus is provided, when it is determined that moiré occurs in the input image data, a filter coefficient for suppressing the occurrence of moiré is calculated, and the calculated filter coefficient is used. Spatial filter processing can be performed, and a copy without moire can be output.

  According to the present invention, when the image processing is executed by the image processing apparatus that processes the image, the halftone processing step performs halftone processing on the image data to generate a halftone image. In the region selection step, the pixel position indicated by the image data before halftone processing before halftone processing is performed in the halftone processing step, and the halftone processed image after halftone processing is performed in the halftone processing step When the position of the pixel indicated by the data is expressed in the same coordinate system, a region having a predetermined shape at the same position is selected. In the frequency conversion step, the pixel values indicated by the pre-halftone image data and the post-halftone image data of the pixels included in the region selected in the region selection step are converted into frequency components, respectively. In the difference calculation step, the difference between the frequency components is calculated based on the frequency component of the image data before halftone processing converted in the frequency conversion step and the frequency component of the image data after halftone processing converted in the frequency conversion step. . In the determination step, it is determined whether or not moire that is an interference pattern occurs based on the difference between the frequency components calculated in the difference calculation step. In the filter coefficient calculation step, when it is determined in the determination step that moiré occurs, a filter coefficient for suppressing the occurrence of moiré is calculated based on the frequency component difference calculated in the difference calculation step. In the filter processing step, the image data is filtered using the filter coefficient calculated in the filter coefficient calculation step.

  Accordingly, it is possible to determine whether or not moire, which is an interference pattern between the halftone pattern of the document and the halftone pattern generated by the halftone process, is generated based on the input image data. It is possible to know the presence or absence of moire before outputting a copy or the like. When it is determined that moiré occurs, the filter coefficient for suppressing the occurrence of moiré is calculated and spatial filtering is performed, so that the occurrence of moiré can be prevented and an optimum copy output can be obtained.

  Further, according to the present invention, the program causes the computer to perform halftone processing on the image data to generate a halftone image, and the halftone processing before the halftone processing is performed by the halftone processing unit. When the pixel position indicated by the pre-tone image data and the pixel position indicated by the post-halftone image data after the halftone processing is performed by the halftone processing unit are represented in the same coordinate system, the same position A region selecting unit for selecting a region having a predetermined shape, and pixel values indicated by pre-halftone image data and post-halftone image data of pixels included in the region selected by the region selecting unit, respectively, as frequency components A frequency conversion unit that converts the image data, a frequency component of the image data before halftone processing converted by the frequency conversion unit, and image data after halftone processing converted by the frequency conversion unit A difference calculation unit that calculates a difference between frequency components based on the frequency component; a determination unit that determines whether or not moire that is an interference pattern occurs based on the difference between the frequency components calculated by the difference calculation unit; A filter coefficient calculation unit that calculates a filter coefficient for suppressing the occurrence of moire based on the difference between the frequency components calculated by the difference calculation unit when the determination unit determines that moire occurs; and a filter coefficient calculation The filter coefficient calculated by the unit is used as a filter processing unit that performs filter processing on the image data.

  Therefore, it is determined whether or not moire, which is an interference pattern between the halftone pattern of the original and the halftone pattern generated by the halftone process, is generated. A process capable of calculating a filter coefficient for suppressing occurrence and performing a spatial filter process can be realized by a program.

  Further, according to the present invention, since the computer-readable recording medium records the program, the image processing apparatus can be realized on the computer by the program read from the computer-readable recording medium.

It is a block diagram which shows the structure of the moire suppression filter coefficient calculation part 10 which concerns on one Embodiment of this invention. FIG. 10 is a diagram for explaining a pixel for which a total density complexity is calculated for a target pixel 301. It is an example of the graph which shows the difference value Dif_F (u, v) of the frequency component calculated by the difference detection part 103. It is a figure which shows the example of the moire determination with respect to the difference value Dif_F (u, v) of the frequency component shown in FIG. It is a figure which shows an example of the filter coefficient Filter31 calculated by the filter coefficient calculation part 105. FIG. It is a flowchart which shows the process sequence of the moire suppression filter coefficient calculation process which the moire suppression filter coefficient calculation part 10 performs, and the process which concerns on it. 1 is a block diagram illustrating a configuration of an image forming apparatus 1 according to an embodiment of the present invention. FIG. 6 is a diagram for explaining processing when the image forming apparatus 1 displays a preview. It is a figure which shows the example of the gamma curve which the output gradation correction | amendment part 21 uses. It is a block diagram which shows the structure of the image forming apparatus 1a which is other embodiment of this invention.

  FIG. 1 is a block diagram showing a configuration of a moire suppression filter coefficient calculation unit 10 according to an embodiment of the present invention. The moiré suppression filter coefficient calculation unit 10 is included in the image forming apparatus 1 that is one embodiment of the present invention and the image forming apparatus 1a that is another embodiment of the present invention. The image forming apparatus 1 will be described in detail with reference to FIG. 7, and the image forming apparatus 1a will be described in detail with reference to FIG.

  The moiré suppression filter coefficient calculation unit 10 causes the image data input to the image forming apparatuses 1 and 1a to be printed by a document type automatic determination unit 13 described later in FIG. 7 or a document type automatic determination unit 13a described later in FIG. The operation is performed when it is determined that the image data is a document image including. The manuscript including the printed photo is a printed photo manuscript on which a photo is printed, and a character printed photo manuscript on which characters and a photo are printed.

  Before describing the moiré suppression filter coefficient calculation unit 10, a document type determination method performed by the document type automatic determination unit 13 will be described. The document type automatic determination unit 13 uses a document type determination method including the following seven steps.

  In the first step, the minimum density value and the maximum density value in an n × m pixel block including the target pixel, for example, a 7 × 15 pixel block are calculated. In the second step, the maximum density difference is calculated using the calculated minimum density value and maximum density value. In the third step, a total density busyness that is the sum of absolute values of density differences between adjacent pixels, for example, a sum of values calculated in the main scanning direction and the sub-scanning direction is calculated as the total density busyness.

  FIG. 2 is a diagram for explaining a pixel for which the total density complexity is calculated for the target pixel 301. Among the pixels included in the block 30, the main scanning direction pixel 302 including the target pixel 301 and the sub scanning direction pixel including the target pixel 301 among the pixels included in the block 30. 303. Therefore, the total density busyness is a value obtained by summing the sum of absolute values of density differences between adjacent pixels in the main scanning direction and the sum of absolute values of density differences between adjacent pixels in the sub-scanning direction.

  In the fourth step, the calculated maximum density difference is compared with the maximum density difference threshold, and the calculated total density busyness is compared with the total density busyness threshold. When the maximum density difference <the maximum density difference threshold and the total density busyness <the total density busyness threshold, it is determined that the target pixel 301 belongs to the background photographic paper region. When the conditions of maximum density difference <maximum density difference threshold and total density busyness <total density busyness threshold are not satisfied, it is determined that the target pixel 301 belongs to the character halftone dot region.

  In the fifth step, regarding the pixel determined to belong to the background photographic paper region, when the target pixel 301 satisfies the maximum density difference <the background photographic paper determination threshold, it is determined that the pixel is the background pixel, and the maximum density difference <the background printing. When the paper determination threshold is not satisfied, it is determined that the pixel is a photographic paper pixel. In the sixth step, regarding the pixels determined to belong to the character halftone dot region, when the target pixel 301 satisfies the total density busyness <maximum density difference × character halftone dot determination threshold, the pixel is determined to be a character pixel, and the total sum When the density busyness <maximum density difference × character halftone dot determination threshold value is not satisfied, it is determined that the pixel is a halftone pixel.

  In the seventh step, the number of pixels classified into a background area consisting of background pixels, a photographic paper photograph area consisting of photographic paper pixels, a character area consisting of character pixels, and a halftone dot area consisting of halftone pixels is counted. The type of the entire document is determined by comparing the count value with respective threshold values for a predetermined background area, photographic paper photograph area, halftone dot area, and character area. For example, assuming that the detection accuracy is higher in the order of a character area, a halftone dot area, and a photographic paper photograph area, if the ratio of the character area is 30% or more of the total number of pixels, the ratio of the character original and the halftone area is the total number of pixels. When the ratio is 20% or more, it is determined that the original is a halftone dot original (hereinafter also referred to as “printed photo original”), and when the ratio of the photographic paper photo area is 10% or more of the total number of pixels, . Further, when the ratio of the character area and the ratio of the halftone dot area are each equal to or greater than the threshold value, it is determined that the character / halftone original (hereinafter also referred to as “character-printed photo original”). This determination method is a determination method described in, for example, Japanese Patent Application Laid-Open No. 2002-232708.

  Referring to FIG. 1, the moire suppression filter coefficient calculation unit 10 includes an area selection unit 101, a frequency conversion unit 102, a difference detection unit 103, a moire determination unit 104, and a filter coefficient calculation unit 105.

The area selection unit 101 uses the pre-halftone image data CMYK (before halftone processing).
Cyan-Magenta-Yellow-Black) 1 and rectangular regions having the same coordinates in the halftone processed image data CMYK2 after the halftone processing is performed are selected. The rectangular area of the same coordinate is a rectangle at the same position when the pixel position indicated by the image data CMYK1 before halftone processing and the pixel position indicated by the image data CMYK2 after halftone processing are represented in the same coordinate system. It is an area. The rectangular area, which is an area having a predetermined shape, has a size of, for example, 256 × 256 pixels. The number of rectangular areas to be selected is not limited to one, and a plurality of selections, for example, 25 rectangular areas may be selected by changing the position.

  The frequency conversion unit 102 performs Fourier transform on the pixel values of the pixels included in the rectangular regions selected by the region selection unit 101 to convert the pixel values into frequency components (hereinafter also referred to as “spatial frequency components”). The conversion to the frequency component is performed for each CMYK plane with respect to the pixel values indicated by the respective image data of the pixel data before halftone processing and the image data after halftone processing. The CMYK plane means that when one pixel is represented by four colors of cyan (Cyan: abbreviation C), magenta (Magenta: abbreviation M), yellow (Yellow: abbreviation Y), and black (Black: abbreviation Y). It is each image which consists of each color. The Fourier transform is performed using equation (1).

  Here, A (x, y) is the pixel value of the pixel at the position of the coordinate (x, y) in the XY coordinate system. M is the number of pixels in the X-axis direction in the rectangular area, and N is the number of pixels in the Y-axis direction in the rectangular area. C (u, v) is a Fourier series. u is a frequency in the X-axis direction, and v is a frequency in the Y-axis direction.

  For each rectangular area having the same coordinates, the difference detecting unit 103 calculates an intermediate value from a value AfterHT_F (u, v) obtained by dividing the absolute value of the frequency component of the image data after halftone processing by the number of pixels included in each rectangular area. The difference between the frequency components obtained by subtracting the value Before HT_F (u, v) obtained by dividing the absolute value of the frequency component of the pre-tone image data by the number of pixels in each rectangular area is calculated as the frequency component difference value Dif_F (u, v). To do. The frequency component difference value Dif_F (u, v) is calculated for each CMYK plane.

  FIG. 3 is an example of a graph illustrating the frequency component difference value Dif_F (u, v) calculated by the difference detection unit 103. 3A is an example of AfterHT_F (u, v), FIG. 3B is an example of BeforeHT_F (u, v), and FIG. 3C is a frequency component difference value Dif_F ( u, v) is an example. The difference values Dif_F (u, v) of the frequency components shown in FIG. 3C are “100” and “−30” in order from the lowest frequency of the variable u when the frequency of the variable v is the lowest value. , “−30”, “0”, “0”, “−40”..., And when the frequency of the variable v is the second lowest value, “−20”, “−4”, “7”, “0”, “20”, “20”..., And when the frequency of the variable v is the third lowest value, “0” in order from the lowest frequency of the variable u. , “47”, “110”, “178”, “50”, “30”..., And when the frequency of the variable v is the fourth lowest value, “−155” in order from the lowest frequency of the variable u. ”,“ −134 ”,“ 116 ”,“ −70 ”,“ 20 ”,“ −50 ”..., And when the frequency of the variable v is the fifth lowest value, In order from the lowest u frequency, “20”, “50”, “80”, “10”, “20”, “20”..., and when the frequency of the variable v is the sixth lowest value, In order from the lowest u frequency, “50”, “−70”, “3”, “−180”, “45”, “63”.

  The moire determination unit 104 serving as a determination unit sets a threshold for determining whether or not moire has occurred, and determines that moire occurs when the difference value Dif_F (u, v) of the frequency component is equal to or greater than the threshold. To output a moire determination signal. Moire is an interference pattern generated by interference between a half pattern generated by halftone processing and a halftone pattern of a document, that is, a half pattern of a document. Therefore, in the frequency component of the image data after halftone processing, the absolute value of the frequency component of the cycle corresponding to the cycle of moire tends to be locally larger than the absolute value of the frequency component in the vicinity of the frequency component. is there. Therefore, the presence / absence of moire can be determined by determining the presence / absence of a frequency at which the difference value Dif_F (u, v) of the frequency component is locally large. In addition, when the moire period is very long or very short, there is no concern about visual image quality degradation, so moire occurs in a specific frequency range excluding very long and very short frequencies. What is necessary is just to determine the presence or absence.

  FIG. 4 is a diagram illustrating an example of moire determination for the frequency component difference value Dif_F (u, v) illustrated in FIG. 3. The specific frequency range is a range in which a frequency indicated by a positive square root value of a sum of a value obtained by squaring the frequency variable u and a value obtained by squaring the frequency variable v is a threshold value TH_F1 or more and a threshold value TH_F2 or less. In the example shown in FIG. 4, the threshold value TH_F1 = 3 (Hz) and the threshold value TH_F2 = 5 (Hz), and the difference value Dif_F (u, v) of the frequency component in the region 41 sandwiched between two-dot chain lines is the target. become.

  In the example illustrated in FIG. 4, the threshold value TH_Dif_F of the difference value of the frequency components is “100”. The difference value Dif_F (u, v) of the frequency component is the three points marked with reference numeral “40” in FIG. 4, that is, the one where the frequency of the variable v is the third lowest value and the frequency of the variable u is lower The third value is “110”, the fourth value is “178”, and the frequency of the variable v is the fourth lowest value, and the variable u is the third lowest value, “116”. Yes, these are threshold values TH_Dif_F = “100” or more. Therefore, the moire determination unit 104 determines that moire occurs because the difference value Dif_F (u, v) of these three frequency components is equal to or greater than the threshold value TH_Dif_F.

  When the moire determination unit 104 determines that moire occurs, the filter coefficient calculation unit 105 calculates a filter coefficient used in the spatial filter process in order to suppress the occurrence of moire. First, of the frequency component difference value Dif_F (u, v), an excess value Dif_F_Over (u, v) is calculated for the frequency component difference value Dif_F (u, v) greater than or equal to the threshold value TH_Dif_F by Equation (2). To do.

  Next, the value BeforeHT_F (u, v) obtained by dividing the absolute value of the frequency component of the image data before halftone processing by the number of pixels in each rectangular area, and the excess value Dif_F_Over (u, v) calculated by Expression (2) ) Is substituted into equation (3) to calculate the filter coefficient intermediate value Filter_F (u, v).

  That is, a value obtained by subtracting Diff_F_Over (u, v) from Before HT_F (u, v) is divided by Before HT_F (u, v) to obtain a filter coefficient intermediate value Filter_F (u, v).

  Finally, an inverse Fourier transform is performed on the filter coefficient intermediate value Filter_F (u, v) to calculate a filter coefficient Filter (x, y). The inverse Fourier transform is performed using equation (4).

  The filter coefficient Filter (x, y) calculated by the equation (4) may be normalized to an integer by rounding off each coefficient after being multiplied by a constant. In addition, using only coefficients in an arbitrary range, other coefficients may be discarded. The number of coefficients is determined by, for example, the circuit scale. Or you may make it replace with the average value of the coefficient of each corresponding position so that it may become a coefficient arrangement | positioning symmetrical vertically and horizontally.

  FIG. 5 is a diagram illustrating an example of the filter coefficient 31 calculated by the filter coefficient calculation unit 105. 5A is an example of the excess value Dif_F_Over (u, v), FIG. 5B is an example of the filter coefficient intermediate value Filter_F (u, v), and FIG. This is an example of the filter coefficient 31.

  In the example shown in FIG. 5A, the excess value Dif_F_Over (u, v) is “10” when the frequency of the variable v is the third lowest value and the frequency of the variable u is the third lowest value. And the fourth lowest value is “78”, and the frequency of the variable v is the fourth lowest value and the frequency of the variable u is the third lowest value “16”, and all others are “0”. .

  In the example shown in FIG. 5B, the filter coefficient intermediate value Filter_F (u, v) is the third lowest value of the frequency of the variable v and the third lowest value of the frequency of the variable u. 92 ”and the fourth lowest value“ −0.16 ”, and the frequency of the variable v is the fourth lowest value and the frequency of the variable u is the third lowest value“ 0.80 ”. Are all “1”.

  In the example of the filter coefficient 31 shown in FIG. 5C, the filter coefficient Filter (x, y) is a 5 × 5 filter coefficient, and both the variable x and the variable y take values of 1 to 5. Filter (x, y) is (36) when (x, y) is (1, 1), “55” when (2, 1), “78” when (3, 1), and (4, 1). “55”, (5,1) is “36”, (1,2) is “55”, (2,2) is “80”, (3,2) is “90”, (4,2) “80”, (5,2) “55”, (1,3) “78”, (2,3) “90”, (3,3) “100”, (4,3) “90”, (5,3) “78”, (1,4) “55”, (2,4) “80”, (3,4) “90”, (4,4) “80”, (5,4) “55”, (1,5) “36”, (2,5) “55”, (3,5) “78”, (4,5) “55” and (5, 5) are “36”.

  FIG. 6 is a flowchart illustrating a moire suppression filter coefficient calculation process performed by the moire suppression filter coefficient calculation unit 10 and a processing procedure for the process. The moire suppression filter coefficient calculation unit 10 indicates that the document type determination signal from the document type automatic determination unit 13 indicates a document including a printed photograph, that is, a printed photograph document or a character printed photograph document. The process proceeds to Step A1. Steps A1 to A7 are moiré suppression filter coefficient calculation processing executed by the moiré suppression filter coefficient calculation unit 10, and steps A8 to A10 are processing related thereto.

  At the time of moving to step A1, image data that has been subjected to output gradation correction by an output gradation correction unit 21 described later, that is, image data before gradation reproduction processing is performed by a gradation reproduction processing unit 22 described later, The image data after the gradation reproduction processing is performed by the gradation reproduction processing unit 22 is obtained and stored in the memory 23. The image data that has undergone output gradation correction by the output gradation correction unit 21 is image data before halftone processing, and the image data that has undergone gradation reproduction processing by the gradation reproduction processing unit 22 is Image data after halftone processing. The process in which the gradation reproduction processing unit 22 performs the gradation reproduction process to obtain the image data after halftone processing is a halftone processing step.

  In step A1, which is an area selection step, the area selection unit 101 extracts a rectangular area. Specifically, a rectangular area having the same coordinates is selected from each image data of the pre-halftone process image data and the post-halftone process image data stored in the memory 23. In step A2, which is a frequency conversion step, the frequency conversion unit 102 performs a spatial frequency conversion process. Specifically, with respect to the image data before halftone processing and the image data after halftone processing, the pixel values of the pixels in the rectangular area selected by the area selection unit 101 are Fourier-transformed to obtain a spatial frequency component.

  In step A3, the difference detection unit 103 calculates an absolute value. Specifically, the absolute value of the spatial frequency component obtained by the frequency converter 102 is calculated for the pre-halftone image data and the post-halftone image data. In step A4, the difference detection unit 103 performs normalization processing. Specifically, the absolute value of the spatial frequency component calculated in step A3 is normalized by dividing it by the number of pixels included in each rectangular area.

  In step A5, the difference detection unit 103 calculates a difference. Specifically, the value obtained by normalizing the absolute value of the spatial frequency component obtained from the image data before halftone processing is subtracted from the value obtained by normalizing the absolute value of the spatial frequency component obtained from the image data after halftone processing. The difference is calculated as a difference value Dif_F (u, v) of the frequency component. Steps A3 to A5 are difference calculation steps.

  In step A6, which is a determination step, the moire determination unit 104 determines whether or not moire occurs. The moire determination unit 104 determines that moire occurs when the frequency component difference value Dif_F (u, v) is equal to or greater than a predetermined threshold within a predetermined frequency range, and proceeds to step A7. When the difference value Dif_F (u, v) of the frequency component is less than the predetermined threshold in the predetermined frequency range, the moire determination unit 104 determines that no moire has occurred, and proceeds to step A10.

  In step A7, which is a filter coefficient calculation step, the filter coefficient calculation unit 105 calculates a filter coefficient for suppressing the occurrence of moire. In step A8, which is a filter processing step, the spatial filter processing unit 20 described later with reference to FIG. 7 performs a spatial filter process using the filter coefficient calculated by the filter coefficient calculation unit 105. In step A9, the gradation reproduction processing unit 22 described later with reference to FIG. 7 performs gradation reproduction processing, and ends the moire suppression filter coefficient calculation process and the related process. Specifically, the gradation reproduction processing unit 22 performs halftone processing on the image data on which the spatial filter processing has been performed by the spatial filter processing unit 20, and stores the image data on which the halftone processing has been performed in the memory 23. Then, the moire suppression filter coefficient calculation process and the related process are finished.

  In step A10, the gradation reproduction processing unit 22 outputs the image data on which the gradation reproduction process has been performed, and ends the moire suppression filter coefficient calculation process and the process related thereto. Specifically, the halftone process is performed on the image data that has been subjected to the spatial filter processing by the spatial filter processing unit 20, and the image data that has been subjected to the halftone process is output to the color image output device 4. The coefficient calculation process and related processes are terminated.

  The moire suppression filter coefficient calculation process shown in FIG. 6 and the timing at which the process is executed are performed when the image forming apparatus 1 described later prints image data, displays a preview image, or the image forming apparatus described later. Reference numerals 1 and 1a denote filter coefficients in the background for image data stored in a storage device such as a hard disk. The preview image is an image displayed so that the user can grasp the rough contents of the image data, and is also simply referred to as “preview” hereinafter.

  When calculating the filter coefficient in the background, the moiré suppression filter coefficient calculation unit 10 determines whether or not moiré has occurred in the image data stored in the storage device during a time period when there is no job such as print output. If moire can occur, a filter coefficient is calculated for the image data, and the calculated filter coefficient is stored in the storage device in association with the image data. Then, when outputting image data, the image forming apparatuses 1 and 1a read out filter coefficients from the storage device, perform spatial filter processing using the read filter coefficients, perform halftone processing, and output the result.

  FIG. 7 is a block diagram showing a configuration of the image forming apparatus 1 according to the embodiment of the present invention. The image forming apparatus 1 is a digital color copying machine, for example, and includes a color image input device 2, a color image processing device 3, a color image output device 4, an operation panel 5, a control unit 6, a hard disk 7, and an image display device 8. Composed. The image processing method according to the present invention is executed by the color image processing apparatus 3.

  The color image input device 2 is configured by a reading device such as a scanner unit provided with, for example, a charge coupled device (hereinafter referred to as “CCD”). The color image input device 2 reads the reflected light image from the original as an RGB (Red-Green-Blue) analog signal by the CCD and sends it to the color image processing device 3.

  A color image processing apparatus 3 as an image processing apparatus includes an A / D (Analog-to-Digital) conversion unit 11, a shading correction unit 12, a document type automatic discrimination unit 13, an input tone correction unit 14, and a region separation processing unit 15. , Compression unit 16, decoding unit 17, color correction unit 18, black generation and under color removal unit 19, spatial filter processing unit 20, output gradation correction unit 21, gradation reproduction processing unit 22, memory 23, and moire suppression filter coefficient calculation The unit 10 is configured.

  The color image processing device 3 converts the image data represented by the signal received from the color image input device 2 into an A / D conversion unit 11, a shading correction unit 12, a document type automatic discrimination unit 13, an input tone correction unit 14, The region separation processing unit 15 and the compression unit 16 are sent in this order and temporarily stored in the hard disk 7. Thereafter, the image data read from the hard disk 7 includes a decoding unit 17, a color correction unit 18, a black generation and under color removal unit 19, a spatial filter processing unit 20, an output gradation correction unit 21, a gradation reproduction processing unit 22, and They are sent in the order of the moire suppression filter coefficient calculation unit 10 and sent to the color image output device 4 as CMYK digital color signals.

  The A / D conversion unit 11 converts RGB analog signals into digital signals and sends them to the shading correction unit 12. The shading correction unit 12 performs processing for removing distortion generated in the illumination system, the imaging system, and the imaging system of the color image input device 2 on the digital RGB signal received from the A / D conversion unit 11. The shading correction unit 12 also adjusts the color balance.

  A document type automatic determination unit 13 serving as a document type determination unit performs a γ correction process on the RGB signal from which distortion has been removed by the shading correction unit 12 (hereinafter also referred to as “RGB reflectance signal”) to perform a density signal. The document type of the read document is determined by the above-described document type determination method. For example, the automatic document type discrimination unit 13 determines whether the read original is a text original consisting of characters, a printed photo original consisting of a printed photo, or a text-printed photo original containing a mixture of characters and print photos. Determine if it exists.

  The document type determination result is obtained as a document type determination signal as an input tone correction unit 14, a region separation processing unit 15, a color correction unit 18, a black generation and under color removal unit 19, a spatial filter processing unit 20, and a tone reproduction processing unit. 22 and the moire suppression filter coefficient calculation unit 10, parameters corresponding to the document discrimination result are set, and the respective processes are performed.

  The input tone correction unit 14 performs image quality adjustment processing such as background density removal and contrast. The region separation processing unit 15 receives RGB digital signals from the input tone correction unit 14. Then, what kind of area each pixel indicated by the received RGB digital signal image data belongs to, for example, any one of a background area, a photographic paper photograph area, a character area, and a halftone dot area Separate whether it is a pixel. Based on the separation result, the region separation processing unit 15 sends a region identification signal indicating which region the pixel belongs to to the compression unit 16 and also receives the RGB digital signal received from the input tone correction unit 14 as it is. Send to.

  The compression unit 16 performs processing for encoding the RGB signal and the region identification signal received from the region separation processing unit 15. The compression unit 16 temporarily stores the encoded code obtained by encoding the RGB signal and the area identification signal in the hard disk 7 and manages it as filing data. The filing data is filed data.

  For example, JPEG (Joint Photographic Experts Group) is used as an RGB signal encoding method. As a method for encoding the region identification signal, for example, MMR (Modified Modified Reed) or MR (Modified Reed) which is a lossless compression method is used. The compressed area identification signal is stored in a management table provided in the storage device in association with the storage address or image data name of the compressed RGB signal, and image data reading and writing are controlled according to the association. . The image data name is identification information for identifying each image data.

  When a job such as copy output or print output is instructed, the decoding unit 17 reads the RGB signal encoding code and the region identification signal encoding code from the hard disk 7 and decodes the read encoding code. Do. The decoding unit 17 decodes the encoded code of the RGB signal into RGB image data, and decodes the encoded code of the region identification signal into the region identification signal. Then, the decoded region identification signal is sent to the color correction unit 18, the black generation and under color removal unit 19, the spatial filter processing unit 20, and the gradation reproduction processing unit 22. The color correction unit 18, the black generation and under color removal unit 19, the spatial filter processing unit 20, and the gradation reproduction processing unit 22 switch processing according to the region indicated by the received region identification signal. The decoded RGB image data is sent to the color correction unit 18.

The color correction unit 18 is a CMY (Cyan-
Magenta-Yellow) Removes color turbidity based on the spectral characteristics of the colorant. The black generation and under color removal unit 19 generates black (K) signals from the CMY three-color signals color-corrected by the color correction unit 18, and subtracts the K signals obtained by black generation from the original CMY signals. A new CMY signal is generated. That is, the black generation and under color removal unit 19 converts the CMY three-color signal received from the color correction unit 18 into a CMYK four-color signal and sends the converted signal to the spatial filter processing unit 20.

  The spatial filter processing unit 20 that is a filter processing unit performs spatial filter processing using a digital filter on the image data of the CMYK signal received from the black generation and under color removal unit 19 based on the region identification signal from the decoding unit 17, By correcting the spatial frequency characteristics, the output image is processed so as to prevent blurring and graininess deterioration. When the filter coefficient for suppressing the generation of moire is calculated by the moire suppression filter coefficient calculation unit 10 for the image data and the filter coefficient is stored in the memory 23, the spatial filter process using the filter coefficient is further performed. Do.

  Similar to the spatial filter processing unit 20, the gradation reproduction processing unit 22, which is a halftone processing unit, performs predetermined processing on the image data of the CMYK signal based on the region identification signal from the region separation processing unit 15. . For example, the region separated into characters by the region separation processing unit 15 has a high frequency enhancement amount in the sharp enhancement processing in the spatial filter processing by the spatial filter processing unit 20 in order to improve the reproducibility of black characters or color characters in particular. Increased. At the same time, the tone reproduction processing unit 22 performs binarization or multi-value processing on a high-resolution screen suitable for high frequency reproduction.

  In addition, for the region separated into halftone dot regions by the region separation processing unit 15, the spatial filter processing unit 20 performs low-pass filter processing for removing the input halftone component. Then, the output tone correction unit 21 performs output tone correction processing based on the characteristics of the color image output device 4, and then the tone reproduction processing unit 22 finally separates the image into pixels and outputs the respective levels. Gradation reproduction processing is performed so that the tone can be reproduced, that is, halftone processing is performed. With respect to the region separated into photographs by the region separation processing unit 15, binarization or multi-value processing is performed on the screen with an emphasis on gradation reproducibility.

  The moiré suppression filter coefficient calculation unit 10 includes image data that has undergone output gradation correction by the output gradation correction unit 21, that is, image data that has not been subjected to gradation reproduction processing by the gradation reproduction processing unit 22, and gradation The presence / absence of moiré is determined from the difference value Dif_F (u, v) of the frequency component from the image data after the gradation reproduction processing is performed by the reproduction processing unit 22. When it is determined that moiré occurs, a filter coefficient that suppresses the occurrence of moiré is calculated based on the difference value Dif_F (u, v) of the frequency component, and is fed back to the spatial filter processing unit 20 via the memory 23. .

  The moire suppression filter coefficient calculation unit 10 reads the image data and the area identification signal stored in the hard disk 7 and operates in the background during the standby time of the image forming apparatus 1. Waiting is a time period during which a job such as copy output or print output is not being executed. The calculated filter coefficient is stored in the memory 23 in association with the image data name, and is used by the spatial filter processing unit 20 when the image data stored in the hard disk 7 is output. The spatial filter processing unit 20 performs spatial filter processing with the calculated filter coefficient on the result of performing the spatial filter processing by the digital filter.

  The memory 23 is configured by a storage device such as a semiconductor memory, for example. The image data CMYK1 processed by the output gradation correction unit 21 and the image data CMYK2 processed by the gradation reproduction processing unit 22 are temporarily stored in the memory. 23. The image data CMYK1 processed by the output tone correction unit 21 is image data before halftone processing, and the image data CMYK2 processed by the tone reproduction processing unit 22 is image data after halftone processing. is there. The moire suppression filter coefficient calculation unit 10 reads out the image data before halftone processing and the image data after halftone processing from the memory 23, and determines whether or not moire has occurred.

  The halftone processed image data stored in the memory 23 is read from the memory 23 at a predetermined timing and sent to the color image output device 4. The moiré suppression filter coefficient calculation unit 10 stores the calculated filter coefficient in the memory 23, and the spatial filter processing unit 20 reads the filter coefficient stored in the memory 23 and performs spatial filter processing.

  The color image output device 4 is constituted by, for example, an output device that outputs a color image using an electrophotographic method or an ink jet method, and outputs image data to a recording medium such as paper. The operation panel 5 includes, for example, a display unit such as a liquid crystal display and an operation unit including setting buttons, and the color image input device 2, the color image processing device 3, and the color based on information input from the operation panel 5. The operation of the image output device 4 is controlled.

  The control unit 6 includes, for example, a central processing unit (hereinafter referred to as “CPU”) and a semiconductor memory, or a DSP (Digital Signal Processor), and controls each function of the color image processing device 3 and the hard disk 7. The hard disk 7 serving as a storage unit is configured by, for example, a magnetic disk device, stores data from the compression unit 16, and sends the stored data to the decoding unit 17. The image display device 8 which is a display device is configured by a liquid crystal display, for example, and displays a preview.

  FIG. 8 is a diagram for explaining processing when the image forming apparatus 1 displays a preview. In order to avoid duplication, the same processing as that described with reference to FIG. 7 will not be described in order to avoid duplication, and processing different from the processing described with reference to FIG. 7 will be described.

  The color correction unit 18 converts the RGB signal into an R′G′B ′ signal based on the display characteristics of the image display device 8, and sends the converted R′G′B ′ signal to the black generation and under color removal unit 19. The black generation and under color removal unit 19 passes the received image data of the R′G′B ′ signal to the spatial filter processing unit 20. “Through” means that the image data of the received R′G′B ′ signal is sent to the spatial filter processing unit 20 without being processed.

  The spatial filter processing unit 20 performs enhancement processing and smoothing processing on the image data of the R′G′B ′ signal based on the region identification signal, and sends it to the output gradation correction unit 21. The output tone correction unit 21 serving as a display control unit performs output γ correction processing for displaying the image data received from the spatial filter processing unit 20 on the image display device 8, and outputs the output γ correction processing to the image display device 8. .

  At this time, a different gamma curve may be selected depending on the image area based on the area identification signal, and the content of the output γ correction process may be different for each image area. The image area is a background area, a photographic paper photograph area, a halftone dot area, and a character area. For example, a gamma curve corresponding to the display characteristics of the image display device 8 is selected for a region other than the character region, and a gamma curve for clearly displaying characters is selected for the character region.

  FIG. 9 is a diagram illustrating an example of a gamma curve used by the output tone correction unit 21. The gamma curve is an input / output characteristic indicating the magnitude of the output signal with respect to the magnitude of the input signal. FIG. 9A is a diagram showing a gamma curve A corresponding to the display characteristics of the image display device 8. In the gamma curve A, the increase rate of the output signal is smaller than the increase rate of the input signal in the interval where the input signal is small, but the increase rate of the output signal is greater than the increase rate of the input signal in the interval where the input signal is large.

  FIG. 9B is a diagram in which a gamma curve B for displaying characters clearly is added. In the gamma curve B, the output signal hardly increases in the section where the input signal is smaller than the gamma curve A, and the increase rate of the output signal is larger than that in the gamma curve A and the input signal is larger in the section where the input signal is large. As it becomes, the rate of increase decreases. By using the gamma curve B, characters can be displayed clearly.

  When the preview mode for displaying the preview is selected, the decoding unit 17, the color correction unit 18, the black generation and under color removal unit 19, the spatial filter processing unit 20, and the output tone correction unit are used in the free time in which the preview is displayed. 21, the gradation reproduction processing unit 22 and the moire suppression filter coefficient calculation unit 10 operate to calculate filter coefficients for suppressing the occurrence of moire.

  The selection of the preview mode is performed by selecting the preview mode from a menu displayed on the operation panel, for example. In the idle time in which the preview is displayed, image data for preview is sent to the image display device 8 and the preview is displayed on the image display device 8, and the decoding unit 17, the color correction unit 18, and the black generation under color removal This is a time period when the unit 19, the spatial filter processing unit 20, and the output tone correction unit 21 are not operating.

  The processes of the decoding unit 17, the color correction unit 18, the black generation and under color removal unit 19, the spatial filter processing unit 20, the output gradation correction unit 21, and the gradation reproduction processing unit 22 in the idle time in which the preview is displayed are as follows: The processing is the same as that described with reference to FIG. 7, and description thereof is omitted to avoid duplication. The moire suppression filter coefficient calculation unit 10 determines the presence or absence of moiré from the difference value Dif_F (u, v) of the frequency components between the image data before halftone processing and the image data after halftone processing. When it is determined that moiré occurs, a filter coefficient that suppresses the occurrence of moiré is calculated based on the difference value Dif_F (u, v) of the frequency components and fed back to the spatial filter processing unit 20.

  As described above, the processes of the decoding unit 17, the color correction unit 18, the black generation and under color removal unit 19, the spatial filter processing unit 20, the output gradation correction unit 21, the gradation reproduction processing unit 22, and the moire suppression filter coefficient calculation unit 10. Is performed using a time period in which the user confirms the preview displayed on the image display device 8.

  FIG. 10 is a block diagram showing a configuration of an image forming apparatus 1a according to another embodiment of the present invention. The image forming apparatus 1a includes a color image input device 2, a color image processing device 3a, a color image output device 4, an operation panel 5, a control unit 6, a hard disk 7, and an image display device 8. The image processing method according to the present invention is executed by the color image processing apparatus 3a.

  A color image processing apparatus 3a, which is an image processing apparatus, includes an A / D (Analog-to-Digital) conversion unit 11, a shading correction unit 12, a document type automatic discrimination unit 13a, a compression unit 16, a decoding unit 17, and a color correction unit 18. A black generation and under color removal unit 19, a spatial filter processing unit 20, an output tone correction unit 21, a tone reproduction processing unit 22, a memory 23, a moire suppression filter coefficient calculation unit 10, an input processing unit 24, and a region separation unit 25. Consists of including. Among the components of the color image processing device 3a, the same components as those of the color image processing device 3 are denoted by the same reference numerals, and description thereof is omitted to avoid duplication.

  An automatic document type determination unit 13a corresponding to the automatic document type determination unit 13 shown in FIG. 7 is arranged at the subsequent stage of the decoding unit 17, and an area separation unit 25 corresponding to the area separation processing unit 15 shown in FIG. It is arranged in the subsequent stage. The input processing unit 24 is used instead of the input tone correction unit 14 shown in FIG.

  The input processing unit 24 performs the same processing as the processing performed by the input tone correction unit 14 on the image data received from the shading correction unit 12 and sends the image data to the compression unit 16. The document type automatic determination unit 13 a performs the same processing as the processing performed by the document type automatic determination unit 13 on the image data decoded by the decoding unit 17 and sends the image data to the region separation unit 25. The region separation unit 25 performs the same processing as the processing performed by the region separation processing unit 15 on the image data received from the document type automatic determination unit 13 a and sends the image data to the color correction unit 18.

  The image forming apparatus 1 illustrated in FIG. 7 associates the image data compressed and encoded by the compression unit 16 and the area identification signal and stores them in the hard disk 7. The image forming apparatus 1 a includes the color image input device 2. The image data read by the scanner unit provided in is temporarily stored in the hard disk 7 without performing the document type determination process and the area separation process. The image type read out from the hard disk 7 is subjected to document type determination processing by the document type automatic determination unit 13a, and region separation processing is performed by the region separation unit 25.

  A / D conversion unit 11, shading correction unit 12, document type automatic discrimination unit 13, input tone correction unit 14, region separation processing unit 15, compression unit 16, decoding unit 17, and color correction unit 18 of color image processing apparatus 3. , Black generation and under color removal unit 19, spatial filter processing unit 20, output tone correction unit 21, tone reproduction processing unit 22, and moire suppression filter coefficient calculation unit 10, and A / D of color image processing device 3a Conversion unit 11, shading correction unit 12, document type automatic discrimination unit 13a, compression unit 16, decoding unit 17, color correction unit 18, black generation and under color removal unit 19, spatial filter processing unit 20, output tone correction unit 21, Each function of the gradation reproduction processing unit 22, the moire suppression filter coefficient calculation unit 10, the input processing unit 24, and the region separation unit 25 is configured by hardware logic such as a DSP (not shown). Good to, these functions stores a program for realizing the storage device (not shown), and a program may be constituted by a CPU (not shown) to execute a stored in the storage device.

  The storage device includes a semiconductor memory such as a ROM (Read Only Memory) that stores the program and a RAM (Random Access Memory) that stores data used for control, and a semiconductor memory or a magnetic field that stores other necessary programs and data. It is composed of a disk device.

  In the above-described embodiment, the program is stored in the storage device of the color image processing devices 3 and 3a. However, the program is not limited to this storage device, and may be recorded on a computer-readable recording medium. Good. The recording medium may be a recording medium that can be read by providing a program reading device as an external storage device (not shown) in the color image processing devices 3 and 3a and inserting the recording medium therein, or other device, for example. It may be a storage device.

  Any recording medium may be used as long as the stored program is accessed from a computer and executed. Alternatively, any recording medium may be configured such that the program is read, the read program is stored in the program storage area of the storage device, and the program is executed.

The recording medium configured to be separable from the color image processing apparatuses 3 and 3a is, for example, a tape recording medium such as a magnetic tape / cassette tape, a magnetic disk such as a flexible disk / hard disk, or a CD-ROM (Compact Disk Read Only Memory). ) / MO (
Magnet-type optical disks (MDs) / MDs (Mini Discs) / DVDs (Digital Versatile Disks) / CD-Rs (Compact Disk Recordables) / Blu-rays and other optical discs, IC (Integrated Circuit) cards (including memory cards) ) / Card-type recording media such as optical cards, or mask ROM / EPROM (Erasable Programmable)
Read Only Memory) / EEPROM (Electrically Erasable Programmable Read Only
Memory) / a recording medium that carries a fixed program including a semiconductor memory such as a flash ROM.

The color image processing apparatuses 3 and 3a may be configured to be connectable to a communication network, and the program may be supplied via the communication network. The communication network is not particularly limited. For example, the Internet, intranet, extranet, LAN (Local Area Network), ISDN (Integrated Services Digital Network), VAN (Value Added Network), CATV (Community Antenna Television) communication network. A communication network such as a virtual private network, a telephone line network, a mobile communication network, or a satellite communication network can be used. Further, the transmission medium constituting the communication network is not particularly limited. For example, IEEE1394, USB (Universal Serial Bus), power line carrier, cable TV line, telephone line, ADSL (Asymmetric Digital Subscriber)
Line (wire) lines, etc., IrDA (Infrared Data Association) or infrared light used for remote control, Bluetooth (registered trademark), 802.11 wireless, HDR (High Data Rate), mobile phone network, satellite line, terrestrial digital It can also be used wirelessly such as on the network. The present invention can also be realized in the form of a computer data signal embedded in a carrier wave in which the program is embodied by electronic transmission.

  In the above-described embodiment, the digital color copying machine has been described as an example of the image forming apparatuses 1 and 1a. However, the image forming apparatuses 1 and 1a are not limited to the digital color copying machine, and the image forming apparatuses 1 and 1a have a copying function and printing. It may be a digital color multifunction peripheral or a digital monochrome multifunction peripheral having a function, a facsimile function, and a function such as a Scan to E-mail function that transmits image data acquired by scanning to the outside as an electronic mail.

  Further, the image forming apparatuses 1 and 1a may include a communication device configured by, for example, a modem or a network card, and may be capable of transmitting and receiving image data by facsimile. Alternatively, image data may be transmitted / received to / from a computer or other image forming apparatus 1, 1 a connected to the network via a network card and a LAN (Local Area Network) cable.

  When performing transmission by facsimile, the image forming apparatuses 1 and 1a read the image data compressed in a predetermined format, for example, a scanner, when the transmission procedure with the other party is ensured by the modem and the transmission is ensured. The compressed image data is read from the memory, necessary processing such as changing the compression format is performed, and sequentially transmitted to the other party via the communication line.

  In the case of receiving by facsimile, the image forming apparatuses 1 and 1a receive image data transmitted from the other party while performing a communication procedure using a modem, and the received image data is decoded by the decoding unit 17 and decoded. If necessary, the image data is subjected to rotation processing and resolution conversion processing, output tone correction and tone reproduction processing, and output to the color image output device 4.

  In this way, the gradation reproduction processing unit 22 performs halftone processing on the image data to generate a halftone image. By the area selection unit 101, the pixel position indicated by the image data before halftone processing before the halftone processing is performed by the gradation reproduction processing unit 22 and the halftone processing after the halftone processing is performed by the gradation reproduction processing unit 22. When the pixel position indicated by the image data after halftone processing is represented in the same coordinate system, rectangular regions at the same position are selected. The frequency conversion unit 102 converts the pixel values indicated by the image data before halftone processing and the image data after halftone processing included in the region selected by the region selection unit 101 into frequency components. The difference between the frequency components based on the frequency component of the pre-halftone image data converted by the frequency converter 102 and the frequency component of the post-halftone image data converted by the frequency converter 102 by the difference detector 103. A value Dif_F (u, v) is calculated. Based on the difference value Dif_F (u, v) of the frequency component calculated by the difference detection unit 103, the moire determination unit 104 determines whether or not moire as an interference pattern occurs. When the filter coefficient calculation unit 105 determines that moire is generated by the moire determination unit 104, the generation of moire is suppressed based on the difference value Dif_F (u, v) of the frequency component calculated by the difference detection unit 103. A filter coefficient Filter (x, y) is calculated. Then, the spatial filter processing unit 20 performs filter processing on the image data using the filter coefficient Filter (x, y) calculated by the filter coefficient calculation unit 105.

  Therefore, in image processing, it is determined whether or not moire, which is an interference pattern between a halftone pattern of a document and a halftone pattern generated by halftone processing, is generated based on input image data. Therefore, it is possible to know the presence or absence of moire before outputting a copy or the like. If it is determined that moiré occurs, the filter coefficient Filter (x, y) for suppressing the occurrence of moiré is calculated and spatial filtering is performed to prevent the occurrence of moiré and to produce an optimal copy output. Obtainable.

  Further, the gradation reproduction processing unit 22 performs halftone processing on the image data stored in the hard disk 7 that stores the image data, and the filter coefficient calculation unit 105 calculates the filter coefficients Filter (x, y) is stored in the hard disk 7 in association with the image data stored in the hard disk 7. Therefore, the image captured by the scanner unit or the like is stored in a storage device such as the hard disk 7, and the filter coefficient Filter (x, y) is calculated for the image data stored in the storage device during the standby time or the like. The filter coefficient Filter (x, y) can be stored in the storage device in association with the image data, and time-consuming processing can be performed efficiently.

  Further, when the user selects the image data preview function, the output gradation correction unit 21 displays the image data on the image display device 8. The output tone correction unit 21 displays the image data stored in the hard disk 7 on the image display device 8 and the tone reproduction processing unit 22 uses the period during which the user is editing the image data. Halftone processing is performed on the image data stored in the hard disk 7, and the filter coefficient Filter (x, y) calculated by the filter coefficient calculation unit 105 is associated with the image data stored in the hard disk 7. And stored in the hard disk 7. Therefore, for example, before outputting a copy, a preview image of the image data is displayed on the image display device 8, and while the user is viewing the preview image displayed on the image display device 8, there is moiré in the image data. Since it is determined whether or not moiré occurs and the filter coefficient Filter (x, y) can be calculated in advance, time-consuming processing can be performed efficiently.

  Further, the document type automatic determination unit 13 determines the document type of the image data based on the image data. When the moiré determination unit 104 determines that the original is an original including a printed photograph by the original type automatic determination unit 13, it is determined whether or not moire occurs. Moire may occur when a halftone image is included in the image of the document. Therefore, the determination result by the document type automatic determination unit 13 indicates that the document has no possibility of moiré, for example, When the document is a document that does not include a printed photograph, the processing by the region selection unit 101, the frequency conversion unit 102, the difference detection unit 103, the moire determination unit 104, and the filter coefficient calculation unit 105 can be eliminated, and the processing time can be reduced. It can be shortened.

  Further, since the color image processing devices 3 and 3a are provided, when it is determined that moire occurs in the input image data, a filter coefficient Filter (x, y) for suppressing the occurrence of moire is calculated, and the calculated filter is calculated. Spatial filter processing can be performed using the coefficient Filter (x, y), and a copy without moire can be output.

  Furthermore, in executing the image processing by the color image processing devices 3 and 3a for processing the image, in the flowchart shown in FIG. 6, in the process for obtaining the image data after halftone processing, the gradation reproduction processing unit 22 Is subjected to halftone processing to generate a halftone image. In step A1, the halftone process is performed by the process of obtaining the halftone processed image data and the position of the pixel indicated by the image data before the halftone process before the halftone process is performed in the process of obtaining the halftone processed image data. When the position of the pixel indicated by the halftone processed image data after being displayed is expressed in the same coordinate system, the rectangular regions at the same position are selected. In step A2, the pixel values indicated by the pre-halftone image data and the post-halftone image data of the pixels included in the region selected in step A1 are converted into frequency components, respectively. In steps A3 to A5, based on the frequency component of the pre-halftone image data converted in step A2 and the frequency component of the post-halftone image data converted in step A2, the difference value Dif_F (u , V). In step A6, based on the frequency component difference value Dif_F (u, v) calculated in steps A3 to A5, it is determined whether or not moire as an interference pattern occurs. In step A7, when it is determined in step A6 that moire occurs, the filter coefficient Filter for suppressing the occurrence of moire based on the difference value Dif_F (u, v) of the frequency components calculated in steps A3 to A5. Calculate (x, y). In step A8, the image data is filtered using the filter coefficient Filter (x, y) calculated in step A7.

  Therefore, in image processing, it is determined whether or not moire, which is an interference pattern between a halftone pattern of a document and a halftone pattern generated by halftone processing, is generated based on input image data. Therefore, it is possible to know the presence or absence of moire before outputting a copy or the like. If it is determined that moiré occurs, the filter coefficient Filter (x, y) for suppressing the occurrence of moiré is calculated and spatial filtering is performed to prevent the occurrence of moiré and to produce an optimal copy output. Obtainable.

  Further, the program causes the computer to perform halftone processing on the image data to generate a halftone image, and a halftone before the halftone processing is performed by the gradation reproduction processing unit 22. When the position of the pixel indicated by the pre-processing image data and the position of the pixel indicated by the image data after halftone processing after the halftone processing is performed by the gradation reproduction processing unit 22 are expressed in the same coordinate system A region selection unit 101 that selects a rectangular region at each position, and a pixel value indicated by image data before halftone processing and image data after halftone processing of pixels included in the region selected by the region selection unit 101, respectively, as frequency components The frequency conversion unit 102 that converts the frequency component into the frequency component of the pre-halftone image data converted by the frequency conversion unit 102 and the frequency component A difference detection unit 103 that calculates a difference value Dif_F (u, v) of the frequency component based on the frequency component of the image data after halftone processing, and a difference value Dif_F (u, f) of the frequency component calculated by the difference detection unit 103 v), the moire determination unit 104 that determines whether or not moire as an interference pattern occurs, and the frequency component calculated by the difference detection unit 103 when the moire determination unit 104 determines that moire occurs. Filter coefficient calculator 105 for calculating a filter coefficient Filter (x, y) for suppressing the occurrence of moire based on the difference value Dif_F (u, v) of the filter, and the filter coefficient calculated by the filter coefficient calculator 105 Using Filter (x, y), it functions as a spatial filter processing unit 20 that performs filter processing on image data. That.

  Therefore, it is determined whether or not moire, which is an interference pattern between the halftone pattern of the original and the halftone pattern generated by the halftone process, is generated. A process capable of calculating the filter coefficient Filter (x, y) for suppressing the generation and performing the spatial filter process can be realized by a program.

  Further, since the recording medium is a computer-readable recording medium in which the program is recorded, the color image processing apparatuses 3 and 3a can be realized on the computer by the program read from the computer-readable recording medium. it can.

DESCRIPTION OF SYMBOLS 1, 1a Image forming apparatus 2 Color image input apparatus 3, 3a Color image processing apparatus 4 Color image output apparatus 5 Operation panel 6 Control part 7 Hard disk 8 Image display apparatus 10 Moire suppression filter coefficient calculation part 11 A / D conversion part 12 Shading Correction unit 13, 13a Document type automatic discrimination unit 14 Input tone correction unit 15 Area separation processing unit 16 Compression unit 17 Decoding unit 18 Color correction unit 19 Black generation and under color removal unit 20 Spatial filter processing unit 21 Output tone correction unit 22 Gradation reproduction processing unit 23 Memory 24 Input processing unit 25 Region separation unit 101 Region selection unit 102 Frequency conversion unit 103 Difference detection unit 104 Moire determination unit 105 Filter coefficient calculation unit 301 Target pixel 302 Pixel in main scanning direction 303 Sub scanning direction Pixel

Claims (8)

A halftone processing unit that generates halftone images by performing halftone processing on image data;
The position of the pixel indicated by the image data before halftone processing before the halftone processing is performed by the halftone processing unit, and the pixel indicated by the image data after halftone processing after the halftone processing is performed by the halftone processing unit When the position is expressed in the same coordinate system, an area selection unit that selects an area of a predetermined shape at the same position, and
A frequency conversion unit that converts pixel values indicated by pre-halftone image data and post-halftone image data of pixels included in the region selected by the region selection unit into frequency components;
A difference calculation unit that calculates a difference between frequency components based on the frequency component of the image data before halftone processing converted by the frequency conversion unit and the frequency component of the image data after halftone processing converted by the frequency conversion unit;
A determination unit that determines whether or not moire, which is an interference pattern, is generated based on the difference between the frequency components calculated by the difference calculation unit;
A filter coefficient calculation unit that calculates a filter coefficient for suppressing the occurrence of moire based on the difference between the frequency components calculated by the difference calculation unit when the determination unit determines that moire occurs;
An image processing apparatus comprising: a filter processing unit that performs filter processing on image data using the filter coefficient calculated by the filter coefficient calculation unit. The halftone processing unit performs halftone processing on image data stored in a storage unit that stores image data,
The image processing apparatus according to claim 1, wherein the filter coefficient calculation unit stores the calculated filter coefficient in the storage unit in association with the image data stored in the storage unit. A display control unit for displaying the image data on the display device;
During a period when the display control unit displays the image data stored in the storage unit on the display device, the halftone processing unit performs halftone processing on the image data stored in the storage unit. The image processing apparatus according to claim 2, wherein the filter coefficient calculation unit stores the calculated filter coefficient in the storage unit in association with the image data stored in the storage unit. A document type discriminating unit for discriminating the type of the image data based on the image data;
4. The determination unit according to claim 1, wherein the determination unit determines whether or not moire occurs when the document type determination unit determines that the document is a document including a printed photograph. An image processing apparatus according to 1.   An image forming apparatus comprising the image processing apparatus according to claim 1. An image processing method executed by an image processing apparatus for processing an image,
A halftone processing step of performing halftone processing on the image data to generate a halftone image;
The pixel position indicated by the image data before halftone processing before the halftone processing is performed in the halftone processing step and the pixel indicated by the image data after halftone processing after the halftone processing is performed in the halftone processing step. When the position is expressed in the same coordinate system, an area selection step for selecting an area of a predetermined shape at the same position, and
A frequency conversion step of converting the pixel values indicated by the image data before halftone processing and the image data after halftone processing of the pixels included in the region selected in the region selection step into frequency components;
A difference calculating step for calculating a difference between frequency components based on the frequency component of the image data before halftone processing converted in the frequency conversion step and the frequency component of the image data after halftone processing converted in the frequency conversion step;
A determination step of determining whether or not moire as an interference pattern occurs based on the difference between the frequency components calculated in the difference calculation step;
A filter coefficient calculating step for calculating a filter coefficient for suppressing the occurrence of moire based on the difference between the frequency components calculated in the difference calculating step when it is determined that the moire occurs in the determining step;
An image processing method comprising: a filter processing step of performing filter processing on image data using the filter coefficient calculated in the filter coefficient calculation step. Computer
A halftone processing unit that performs halftone processing on image data to generate a halftone image;
The position of the pixel indicated by the image data before halftone processing before the halftone processing is performed by the halftone processing unit, and the pixel indicated by the image data after halftone processing after the halftone processing is performed by the halftone processing unit When the position is expressed in the same coordinate system, an area selection unit that selects an area of a predetermined shape at the same position, and
A frequency conversion unit that converts pixel values indicated by pre-halftone image data and post-halftone image data of pixels included in the region selected by the region selection unit into frequency components;
A difference calculation unit that calculates a difference between frequency components based on the frequency component of the image data before halftone processing converted by the frequency conversion unit and the frequency component of the image data after halftone processing converted by the frequency conversion unit;
A determination unit that determines whether or not moire, which is an interference pattern, is generated based on the difference between the frequency components calculated by the difference calculation unit;
A filter coefficient calculation unit that calculates a filter coefficient for suppressing the occurrence of moire based on the difference between the frequency components calculated by the difference calculation unit when the determination unit determines that moire occurs;
A program for functioning as a filter processing unit that performs filter processing on image data using the filter coefficient calculated by the filter coefficient calculation unit.   A computer-readable recording medium on which the program according to claim 7 is recorded.

Images