JP2004297667A - Image processing method and image processing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reproduce an excellent output image by carrying out pseudo halftone processing most suitable to a dot region in input image data by sufficiently taking into account the characteristic of an output device thereby more surely suppressing the pseudo contours. <P>SOLUTION: A flowchart indicating the procedures of the image processing method includes the steps of: outputting image data with a plurality of prescribed densities and different resolutions onto recording paper; measuring a reflection density of output image data; extracting image data with a reflection density closest to the prescribed density from the image data; calculating the number P of dot lines of the image data on the basis of the extracted resolution of the image data; and executing the pseudo medium tone processing on the basis of a prescribed predetermined number Q of dot lines which is less than the calculated number P of dot lines and closest to the number P of dot lines. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention is most suitable for multi-value gradation image data having a dot structure expressed by multi-value gradation, according to the halftone frequency of the dot region included in the multi-value gradation image data. The present invention relates to an image processing method and an image processing apparatus capable of executing pseudo halftone processing.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in an image processing apparatus such as a digital multifunction peripheral or a facsimile apparatus, it is determined whether image data composed of multi-valued gradations is a character / line drawing area or a photographic image area. 2. Description of the Related Art An image processing method for separating data into a character / line drawing area and a photographic image area and performing a pseudo halftone process according to each of the separated areas is widely used.
[0003]
However, since the above-described image processing method determines and separates a character / line drawing area and a photographic image area based on a threshold value of one halftone frequency, for example, the brightness gradually changes and the gradation is relatively continuous. There is a case where an image region having characteristics is separated into a character / line drawing region and a photographic image region. In this case, different pseudo halftone processing is performed on successive image areas, and as a result, a pseudo boundary line (pseudo contour) is formed at the boundary of the image areas subjected to different pseudo halftone processing. There is a risk that.
In addition, when processing is performed by the systematic dither method (which will be described later in detail) in which a halftone dot region having a threshold value equal to or higher than the halftone frequency is uniformly smoothed and processed into a screen, the number of halftone dots is much higher than the threshold value. For a halftone dot region, halftone components can be sufficiently removed, but on the other hand, there is a problem that sharpness of edges of characters and the like is lost and a clear image cannot be reproduced.
[0004]
In Patent Document 1, a plurality of dither threshold arrays of a dither matrix are provided in advance, and it is determined whether the image data is a character / line drawing area or a photographic image area by a well-known area determination method. In the case of a character / line drawing area, a threshold array capable of maintaining the resolution of the character / line drawing is selected from the plurality of threshold arrays, and pseudo halftone processing by a multi-value dither method is performed based on the selected threshold array. In the case of a photographic image area, a threshold array capable of retaining the gradation characteristics is selected from the plurality of threshold arrays, and pseudo halftone processing is performed based on the selected threshold array by a multi-value dither method. A method has been proposed. As a result, discontinuity at the image boundary is suppressed, and the occurrence of the pseudo contour can be effectively prevented.
[0005]
Here, the pseudo halftone processing will be described. Pseudo halftone processing is a method for expressing multi-valued gradations more naturally when expressing multi-valued gradation image data having a dot structure as, for example, a dot-like binary image using only white points and black points. This is an image processing method for expressing. Representative examples of the pseudo halftone processing include an organized dither method and an error diffusion method.
[0006]
The systematic dither method is a processing method for expressing a pseudo halftone by smoothing a halftone dot structure of multi-valued grayscale image data and then performing a halftone process (screen process) again. As an example, each pixel constituting a threshold matrix arranged according to a certain rule is generated as a black point if its pixel value is above a certain threshold, and a white point if it is below that threshold. There is a method of expressing the gradation in binary. Generally, this method is a processing method used when the resolution of the output device is inferior to the resolution of the input multi-level gradation image data. By changing (dithering) to a halftone dot area that can be output within the range of the resolving power, there is an advantage that the intermediate gradation is expressed while maintaining stable gradation characteristics.
[0007]
On the other hand, the error diffusion method allocates an error (difference between a pixel value of interest and a threshold value) that occurs when the pixel of interest is converted into, for example, black and white binary data based on a predetermined threshold value, to a pixel adjacent to the pixel of interest. This is a method of expressing an intermediate gradation by reducing the error comprehensively. This method is a processing method used when the resolution of the output device exceeds the resolution of the input multi-valued gradation image data, contrary to the dither processing method described above. The resolution characteristic and the gradation characteristic of the input multi-valued gradation image data can be better reproduced than by using the method. Further, since this processing method uses threshold data having no periodicity, moire does not occur.
[0008]
[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-196885
[Problems to be solved by the invention]
However, the image processing method described in Patent Document 1 cannot completely prevent the occurrence of the pseudo contour. In addition, there is a case where a good output image cannot be reproduced. The reason is described below.
[0010]
In an image processing apparatus such as a digital multifunction peripheral, there is a limit to a reproducible resolution. Therefore, if the input image data is within the range of the resolution, the image processing apparatus can reproduce a good output image. However, the amount of toner adhering to the recording paper increases or decreases due to the effects of the toner components of the output device, the characteristics of the photoconductor, and the charging characteristics. Therefore, even if the input image data is within the resolution range of the image processing device, it is theoretically possible. In some cases, the same resolution characteristics and gradation characteristics cannot be obtained. In addition, dot adhesion of the toner may occur depending on the paper quality, paper thickness, etc. of the recording paper, and in this case, the resolution characteristic and the gradation characteristic as the theory cannot be obtained. In particular, when an image is output at the maximum resolution of the image processing apparatus, the above-described phenomenon occurs remarkably.
As described above, if a stable output image cannot always be obtained, it is necessary to perform pseudo halftone processing in consideration of the characteristics of the output device. To this end, it is necessary to determine the threshold value of the dither matrix, the size of the matrix size, etc., after grasping the substantial resolution of the image processing device with due consideration of the characteristics of the output device. The image processing method described in Document 1 does not take such a thing into consideration. Therefore, a case may occur in which optimal pseudo halftone processing is not performed on the input image data. In this case, there is a possibility that the above-described pseudo contour may occur, and a good output image cannot be reproduced.
[0011]
Therefore, the present invention has been made in view of the above circumstances, and an object of the present invention is to perform pseudo halftone processing most suitable for a halftone dot region by sufficiently considering the characteristics of the output device. An object of the present invention is to provide an image processing method and an image processing apparatus that can more reliably suppress the occurrence of a pseudo contour and reproduce a good output image.
[0012]
[Means for Solving the Problems]
In order to solve the above-mentioned object, the present invention provides a method for executing pseudo halftone processing on multi-valued gradation image data having a halftone dot structure represented by multi-valued gradation by at least an error diffusion method or an organized dither method. A method of outputting image data of a plurality of predetermined densities having different resolutions and measuring a reflection density of each of the output image data; and a step of measuring a reflection density closest to the predetermined density from the image data. Image data having a reflection density is extracted, and a halftone frequency P of the image data is calculated based on the resolution of the extracted image data. The halftone frequency P is less than the calculated halftone frequency P And a pseudo-halftone processing step of performing a pseudo-halftone process based on a predetermined halftone frequency Q closest to the image processing method. There.
In this way, since the pseudo halftone processing is executed based on the halftone frequency corresponding to the substantial resolution of the output device by sufficiently considering the characteristics of the output device, the occurrence of the pseudo contour is more reliably suppressed. Thus, a good output image can be obtained. For this purpose, the reflection density measuring step includes outputting a plurality of image data of a predetermined density having different resolutions to recording paper or a predetermined image carrier, and measuring the reflection density of each of the output image data. It is desirable that
[0013]
Further, the pseudo halftone processing step performs a smoothing process on a halftone dot area having a halftone frequency greater than or equal to the halftone frequency Q in the multi-value halftone image data, and then performs the texture processing with the halftone frequency P. A pseudo-halftone process based on the dynamic dither method, and a pseudo-halftone process based on the error diffusion method for a halftone dot region having a halftone frequency less than the halftone frequency Q. Can be considered.
As a result, the pseudo halftone processing corresponding to the halftone dot area is executed, so that the occurrence of a pseudo contour that is visually recognizable to human eyes at the boundary of the area where different pseudo halftone processing has been performed is prevented. obtain.
[0014]
Further, even if the image processing apparatus is configured as an image processing apparatus using the above-described image processing method, the above problem can be solved.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings to facilitate understanding of the present invention. The following embodiments are examples embodying the present invention, and do not limit the technical scope of the present invention. FIG. 1 is a cross-sectional view of a digital multifunction peripheral according to an embodiment of the present invention, FIG. 2 is a diagram showing an example of binary image data having a 50% density, and FIG. FIG. 4 is a flowchart showing a procedure of the image processing method according to the embodiment of the present invention.
[0016]
First, an outline of an image processing system of a digital MFP 10 to which an image processing method according to an embodiment of the present invention is applied will be described with reference to FIG. The image processing method can be applied to a facsimile apparatus, a printer, and the like.
The digital multifunction peripheral 10 has a plurality of recording paper storage units 11 below, and a double-sided unit 12 is disposed on a side surface thereof. An image forming unit 13, a fixing device 3, and a laser scanner unit 5 are provided above the recording paper storage unit 11. Further, an image reading device 14 is provided on the upper part. A display operation unit such as a liquid crystal touch panel (not shown) is arranged on the upper surface of the main body of the digital multi-function peripheral 10, and the user operates the display operation unit to operate the display operation unit such as the resolution, image quality, and print density of the output image. Various operations such as setting and selection of image quality mode, selection of recording paper, start operation of image reading, transfer destination of read image data, and the like are performed.
[0017]
Original image light guided by the mirror 15 and the like from the image reading device 14 is photoelectrically converted by a CCD sensor or the like, and then input to an image processing unit (not shown) as digital image data (hereinafter, referred to as “image data”). You. Digital data transferred from a personal computer or the like is directly input to the image processing unit. In the image processing unit, pseudo halftone processing, smoothing processing, re-dotting processing (screen processing), and the like are performed on input image data according to a predetermined purpose. That is, the role of this image processing unit is to convert the input image data into image data that can be output by the image processing device.
[0018]
The image forming unit 13 is provided with a photosensitive drum 16 and a process unit 10 around which a charging device 17, a cleaning device 19, and the like are arranged. The transfer device 4 is disposed opposite to the transfer device 16. The photosensitive drum 16 has a photosensitive layer formed on the surface of, for example, an aluminum cylinder, and is driven to rotate clockwise in FIG. The charging device 17 charges the surface of the rotating photosensitive drum 16 to several hundred volts by corona discharge or the like. The laser beam emitted from the laser scanner unit 5 is applied to the surface of the photosensitive drum 16 on the downstream side of the charging device 17 in the rotation direction to form an electrostatic latent image. The charge on the photosensitive drum fluctuates depending on the charge output of the charging device 17 and the charging characteristics of the photosensitive drum 16, and even in the digital MFP 10, the charge is within a certain allowable range. , And is not always charged with an equal charge. This causes a variation in the amount of toner adhering to the photosensitive drum 16 and the amount of toner transferred to recording paper, which will be described later.
[0019]
In the developing device 18, the developing roller rotates, and the thin layer formed on the surface of the developing roller comes into contact with the surface of the photosensitive drum 16, and the charged toner adheres to the electrostatic latent image on the surface of the photosensitive drum to form a toner image. Form. The toner image is charged while being transported in the transfer device 4 in a state where the recording paper transported from the recording paper storage unit 11 is sandwiched between the photosensitive drum 16 and the transfer belt moving at the same speed as the peripheral speed. Then, the toner image is transferred onto the recording paper. A cleaning device 43 is disposed downstream of the transfer device 4, and removes toner and other deposits remaining on the surface of the photosensitive drum 16.
[0020]
The recording paper onto which the toner image has been transferred by the transfer device 4 is conveyed to a fixing device 3 provided on the downstream side in the transport direction, and the space between the fixing roller 1 of the fixing device 3 and the pressure roller 2 opposed thereto is transferred. As described above, the toner on the recording paper is melted and fixed by the heat of the fixing roller 1 having a heater therein, and the original image data is finally fixed on the recording paper.
[0021]
As described above, the toner adhered to the recording paper conveyed from the recording paper storage unit 11 is finally fixed in the fixing device 3. As described above, the charging characteristics of the photosensitive drum 16 and the charging device In the case where the amount of toner transferred to the recording paper varies due to the output specification 17 or the toner component, or when the dot collapse by the pressure roller 2 occurs, for example, the resolution and image quality of the output image are set by the user. However, even if the print density and the like are set, it is not possible to obtain the resolution characteristic and the gradation characteristic corresponding to the desired resolution. Desired resolution characteristics cannot be obtained.
[0022]
Such a decrease in the resolution characteristics and gradation characteristics of the output image due to various characteristics of the output device is not preferable. In the pseudo halftone processing by the dither processing method or the error diffusion method, the separation boundary of the halftone dot region is usually determined on the basis of the set resolution. If there is such a difference between the resolution and the resolution, the boundary of the halftone dot region where the pseudo halftone processing is performed cannot be accurately determined. Therefore, it is desirable to determine the boundary of the halftone dot region based on the substantial resolution of the output device.
[0023]
Here, using the flowchart of FIG. 4, a plurality of image data of a predetermined density having different resolutions are output, and a reflection density measuring step of measuring the reflection density of each of the output image data is performed. Image data having a reflection density closest to a predetermined density is extracted, and a halftone frequency P of the image data is calculated based on the resolution of the extracted image data. The halftone frequency P is smaller than the calculated halftone frequency P, and A pseudo halftone processing step of executing the pseudo halftone processing based on a predetermined predetermined halftone frequency Q closest to the halftone frequency P will be described.
[0024]
First, in step S10, as shown in FIG. 2, the staggered binary image data having a density of 50% (for example, 128 gradations in the case of image data expressed in 256 gradations) is 150 dpi. The image data 150 (FIG. 2A) having a resolution of 200 dpi, the image data 200 (FIG. 2B) having a resolution of 200 dpi, and the image data 600 (FIG. 2C) having a resolution of 600 dpi are recorded on recording paper or Output to a predetermined image carrier. The image carrier includes a photosensitive drum, an intermediate transfer member, and the like. Here, one cell of the image data shown in FIG. 2 is for making each resolution easy to understand, and one cell is one pixel (pixel) at a resolution of 600 dpi. In this embodiment, three types of image data having a resolution of 150 dpi, 200 dpi and 600 dpi are described as a reference for the sake of simplicity. It is desirable to prepare the image data. The resolution of the prepared image data is lower than the maximum resolution that can be output by the digital multi-function peripheral (hereinafter, referred to as “maximum resolution”). This is because, since image data having a resolution higher than the maximum resolution cannot obtain an output image maintaining its resolution characteristics in the first place, the substantial resolution of the digital multifunction peripheral cannot be specified. In addition, since image data having a normal 50% density tends to have a staggered pattern when pseudo halftone processing is performed by an error diffusion method, image data having a staggered pattern is prepared.
Outputting a plurality of image data with different resolutions under the condition of a constant density leads to the same result as outputting image data of a constant density by changing the resolution setting of the output device.
[0025]
Subsequently, in step S20, the reflection density of each image data output to the recording paper or a predetermined image carrier is measured. In this case, the reflection density of the image data output on the recording paper may be measured manually using a well-known reflection densitometer or spectral densitometer. A form in which the reflection density of the output image data is automatically measured may be used. Further, the reflection density of the image data output to the predetermined image carrier may be automatically measured by a reflection density sensor or a spectral density sensor provided in the digital MFP 10 in the same manner as described above.
[0026]
After measuring the reflection density in step S20, the reflection densities of the image data at each of the above resolutions are compared (S30), and the resolution is as high as possible and has a reflection density closest to 50% density within a predetermined allowable range. Image data is extracted, and the halftone frequency P of the image data is calculated based on the resolution of the extracted image data (S40).
[0027]
Here, FIG. 3 shows a reflection density characteristic graph of output image data when image data having a resolution of 150 dpi, 200 dpi, and 600 dpi is output. A represents a reflection density characteristic of 150 dpi, B represents a reflection density characteristic of 200 dpi, and C represents a reflection density characteristic of 600 dpi. The horizontal axis of the graph represents the density (%) of the input image data, and the vertical axis represents the reflection density (%) of the output image data.
Ideally, as shown by the broken line in FIG. 3, it is originally most preferable that the reflection density of the output image data coincides with the density of the input image data. An output image having a desired resolution characteristic cannot be obtained due to an increase or decrease in the amount of toner on the carrier or a crushed dot when printed on recording paper. As can be seen from FIG. 3, the higher the resolution of the input image data, the more noticeable the effect of the increase or decrease of the toner amount and the crushing of the dots on the output image. Reaches 80%. In the case of 50% density, the density characteristic A of 150 dpi has the highest possible resolution and the best density characteristic within a predetermined allowable range. Accordingly, the image data having the highest resolution among the image data having the reflection density regarded as 50% density is the image data having the resolution of 150 dpi. Normally, since monochrome image data has a halftone dot structure at an angle of 45 ° with respect to the main scanning direction, the halftone frequency P in this case is calculated as 106 (150 / √4). Note that the comparison in step S30 or the calculation in step S40 can be manually performed by a person, but it is also possible to cause a CPU, an ASIC, or the like included in the digital multi-function peripheral to perform the comparison operation processing.
[0028]
After the halftone frequency P is calculated, a predetermined halftone frequency Q smaller than the calculated halftone frequency P and closest to the halftone frequency P is specified (S50). ).
In general, the halftone frequency is predetermined to some extent depending on the paper quality and the application to be used (the halftone frequency determined in this way is the halftone frequency Q). For example, the number of lines is 65 for paper-turned paper used for newspapers, and 100, 120 or 133 for fine paper used for books and magazines, and 150 for coated paper used for calendars and catalogs. On art paper used for art books and photo books, images are expressed with a line number of 175 or 200. Therefore, as long as the image data is not special image data, image data having the above-described halftone frequency is usually handled. For example, when considered based on image data of 150 dpi (106 lines), the halftone frequency Q less than 106 lines and closest to 106 lines is 100.
[0029]
In steps S60 and S70, a predetermined pseudo halftone process is performed on the multi-value gradation image data input based on the halftone frequency. More specifically, after performing a smoothing process on a halftone dot area having a halftone frequency greater than or equal to the halftone frequency Q in the multi-valued gradation image data, the halftone is converted to the halftone dot with the halftone frequency P ( Pseudo halftone processing based on the systematic dithering method (screening) is performed (S60), and pseudo halftone processing based on the error diffusion method is performed on a halftone dot area having a halftone frequency less than the specified halftone frequency. (S70).
[0030]
【Example】
In the above-described embodiment, when image data having a resolution of 150 dpi, 200 dpi, and 600 dpi is output, it is assumed that the density characteristic A of 150 dpi exhibits the best density characteristic at 50% density. However, when an allowable range of, for example, ± 10% is provided for the reflection density, it is determined that the density characteristic B of 200 dpi also shows good density characteristics. In such a case, the halftone frequency P may be specified based on the image data of the density characteristic B with high resolution.
[0031]
【The invention's effect】
As described above, a reflection density measuring step of outputting image data of a plurality of predetermined densities having different resolutions and measuring a reflection density of each of the output image data; Image data having a reflection density is extracted, and a halftone frequency P of the image data is calculated based on the resolution of the extracted image data. The halftone frequency P is less than the calculated halftone frequency P And a pseudo-halftone processing step of performing a pseudo-halftone process based on a predetermined halftone frequency Q closest to the output device, so that the characteristics of the output device are sufficiently taken into consideration. Since the pseudo halftone processing is executed based on the halftone frequency corresponding to the typical resolution, it is possible to more reliably suppress the occurrence of the above-mentioned pseudo contour, thereby obtaining a good output image. To become.
[0032]
Further, the pseudo halftone processing step performs a smoothing process on a halftone dot area having a halftone frequency greater than or equal to the halftone frequency Q in the multi-value halftone image data, and then performs the texture processing with the halftone frequency P. The pseudo-halftone processing based on the dynamic dither method is performed, and the pseudo-halftone processing based on the error diffusion method is performed on a halftone area having a halftone frequency less than the halftone frequency Q. , It is possible to execute the pseudo halftone processing according to the halftone dot area, and it is possible to prevent the generation of a pseudo contour that can be visually recognized by human eyes at the boundary of the area where the different halftone processing has been performed.
[Brief description of the drawings]
FIG. 1 is a sectional view of a digital multifunction peripheral according to an embodiment of the present invention.
FIG. 2 is a diagram showing an example of binary image data having a 50% density.
FIG. 3 is a graph showing a reflection density characteristic of output image data.
FIG. 4 is a flowchart illustrating a procedure of an image processing method according to the embodiment of the present invention.
[Explanation of symbols]
A: Staggered binary image data of 50% density (150 dpi)
B: Staggered binary image data of 50% density (200 dpi)
C: Staggered binary image data of 50% density (600 dpi)
DESCRIPTION OF SYMBOLS 1 ... Fixing roller 2 ... Pressurizing roller 3 ... Fixing device 4 ... Display operation unit 5 ... Laser scanner unit 10 ... Digital multifunction device 11 ... Recording paper storage unit 12 ... Duplex unit 13 ... Image forming unit 14 ... Image reading device 16 ... Photosensitive drum 17 Charging device 18 Developing device 19 Cleaning device 20 Transfer device

Claims (5)

An image processing method for performing pseudo halftone processing on multi-valued gradation image data having a halftone dot structure expressed by multi-valued gradation by at least an error diffusion method or an organized dither method,
A reflection density measuring step of outputting image data of a plurality of predetermined densities having different resolutions and measuring a reflection density of each of the output image data;
Image data having a reflection density closest to the predetermined density is extracted from the image data, a halftone frequency P of the image data is calculated based on the resolution of the extracted image data, and less than the calculated halftone frequency P A pseudo halftone processing step of performing pseudo halftone processing based on a predetermined predetermined halftone frequency Q closest to the halftone frequency P;
An image processing method comprising: 2. The reflection density measuring step according to claim 1, wherein the image data of a plurality of predetermined densities having different resolutions are output to recording paper and the reflection densities of the respective image data output on the recording paper are measured. Image processing method. 5. The method according to claim 1, wherein the step of measuring the reflection density includes outputting a plurality of image data having different densities having different resolutions to a predetermined image carrier, and measuring the reflection densities of the respective image data output to the image carrier. 2. The image processing method according to 1. The pseudo halftone processing step performs a smoothing process on a halftone dot area having a halftone frequency greater than or equal to the halftone frequency in the multi-value halftone image data, and then performs an organized dithering with the halftone frequency P. And performing pseudo halftone processing based on an error diffusion method on a halftone dot region having a halftone frequency less than the halftone frequency Q. 4. The image processing method according to any one of claims 1 to 3. An image processing apparatus using the image processing method according to claim 1.

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