JP6083314B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to a technique for improving image quality at an image edge.

  In an electrophotographic image forming apparatus, an electrostatic latent image is formed on a photoreceptor, and the electrostatic latent image is developed with toner (coloring material) to be manifested as a toner image. The toner image is transferred from the photosensitive member onto a transfer medium, the toner image on the transfer medium is transferred to a recording paper, the toner image is thermally fixed on the recording paper, and printed out.

  In such a process, when the toner image on the photosensitive member is transferred to the transfer medium, a phenomenon (toner scattering during transfer) occurs in which the toner is scattered outside the image area.

  More specifically, after the surface of the photosensitive member is uniformly charged with a certain polarity (for example, “positive”), only the portion where the toner image is formed is irradiated with laser light, and the portion is uncharged. Since the toner is charged with the same polarity as the surface of the photoconductor, the toner does not adhere to the charged portion of the surface of the photoconductor in the development process, and the toner adheres only to the uncharged portion. A toner image is formed. Therefore, on the surface of the photoconductor after development, the periphery of the charged toner image (the surface of the photoconductor without the toner image) is also charged with the same polarity (FIG. 13).

  The toner particles 101 at the edge portion of the developed toner image receive a repulsive force F1 from toner particles of the same polarity inside the edge, but also receive a repulsive force F2 from the charge 102 on the photoconductor around the toner image. Therefore, these repulsive forces F1 and F2 antagonize, and the toner particles 101 at the edge portion adhere stably on the photoreceptor.

  However, near the transfer position, the gap between the photoconductor and the transfer medium (intermediate transfer belt) becomes very small, and the charge around the toner image leaks from the photoconductor before the toner image reaches the transfer position. As a result, the charging potential is lowered, the force F2 that has pushed the electrostatic repulsive force F1 between the toner particles is lowered, and the toner particles 101 are scattered at the edge portion of the toner image (FIG. 14).

  In particular, when toner near the edge scatters, the toner scatters on the white background where there is essentially no toner image, resulting in a loss of image quality due to thickening of characters and lines, white background crushing, edge blurring, etc. on the printed output image. Will be invited.

  For this reason, various techniques for reducing toner scattering have been proposed. For example, a technique for preventing toner scattering due to the characteristics of the toner itself (see, for example, Patent Documents 1 and 2), and a technique for reducing toner scattering at the time of transfer by a mechanical configuration such as a transfer unit (for example, see Patent Documents 3 and 4). )

  In addition, as a technique for improving the image quality of the image edge, a technique for increasing the toner adhesion amount at the contour portion of the toner image on the intermediate transfer belt more than the toner adhesion amount at the inner portion of the contour portion (see Patent Document 5), edge In addition, there is a technique (see Patent Document 6) in which dots of a predetermined size that are determined in advance according to the distance from the edge are assigned to pixels near the edge. However, these are techniques for emphasizing image edges, not techniques for suppressing toner scattering.

JP 2005-338776 A JP 2002-296831 A JP 2004-184984 A JP 2000-094734 A Japanese Patent Laid-Open No. 10-039653 JP 2010-130303 A

  If the toner characteristics are improved or the transfer mechanism is specially configured to suppress toner scattering, the cost increases. Therefore, a technique for suppressing toner scattering by image processing is desired. In particular, even in an image that has been halftone processed by a dither screen, it is desired to effectively suppress toner scattering without impairing the characteristics of the dot pattern of the dither screen.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of reducing toner scattering at an image edge in a halftone image by image processing.

  The gist of the present invention for achieving the object lies in the inventions of the following items.

[1] A pixel position detection unit that detects a position with respect to an image edge for each pixel of the print target image;
A halftone processing unit that generates a halftone image by performing a halftone process on the print target image;
A regulation value determining unit that determines a regulation value of the density of each pixel of the print target image based on the position detected by the pixel position detection unit for the pixel;
A density regulation unit that regulates the density of each pixel of the halftone image to be equal to or less than a regulation value determined by the regulation value decision unit for each pixel;
An electrophotographic image forming unit that forms an image based on an image signal of the halftone image that has been regulated by the density regulating unit;
An image forming apparatus comprising:

  In the said invention, the position with respect to the image edge of each pixel is calculated | required, and the regulation value of the density | concentration of each pixel is determined according to the position from an image edge. Then, the density of each pixel of the image after the halftone process is controlled to be equal to or less than the density regulation value determined for each pixel. For example, if the density of the pixels constituting the image edge is regulated lower than that of other pixels, the density of the pixels constituting the image edge is reduced, so that the repulsive force that the pixels on the image edge receive from the pixels on the inside is reduced, and the toner scattering Is reduced. In addition, since the density of each pixel of the image after the halftone process is controlled to be equal to or lower than the density control value determined for each pixel, the toner at the edge portion is not impaired without deteriorating the characteristics of the screen pattern. Scattering can be reduced.

[2] The density regulation unit replaces the density of each pixel of the halftone image with the regulation value when the density of the pixel exceeds the regulation value determined by the regulation value determination unit. Regulate to
The image forming apparatus according to [1], wherein
[ 3 ] The pixel position detection unit detects whether the position of each pixel is the position of an image edge,
The image forming apparatus according to [1] or [2] , wherein the restriction value for the pixel at the image edge position is smaller than the restriction value for the pixel not at the image edge position.

  In the above invention, the restriction value of the density of the pixels constituting the image edge is made smaller than the restriction value of the density for the other pixels. That is, the density of the pixels constituting the image edge is lowered.

[ 4 ] The pixel position detection unit detects whether the position of each pixel is the position of the image edge, the position adjacent to the inside of the pixel of the image edge, or another position,
The regulation value for the pixel at the position of the image edge is smaller than the regulation value for the pixel at the position adjacent to the inside,
The image forming apparatus according to [1] or [2] , wherein a restriction value for a pixel at a position adjacent to the inside is smaller than a restriction value for a pixel at the other position.

In the above invention, each pixel is classified into one of the pixel at the position of the image edge, the pixel adjacent to the inside of the pixel at the position of the image edge, and the other pixels. Then, the relationship between the density regulation value for the pixel at the image edge position <the density regulation value for the pixel adjacent inside the pixel at the image edge position <the density regulation value for the other pixels is set. The density of each pixel is regulated by these regulation values.

[ 5 ] The pixel position detection unit includes an inner filter centered on the target pixel, and an outer filter surrounding the inner filter,
When the calculation result of the inner filter satisfies a predetermined condition, the position of the target pixel with respect to the image edge is obtained from the calculation result of the inner filter,
The image forming apparatus according to [ 4 ], wherein when the calculation result of the inner filter does not satisfy the predetermined condition, the position of the target pixel with respect to the image edge is obtained from the calculation result of the outer filter.

  In the above invention, the size of the look-up table can be reduced by dividing the filter into an inner filter having a calculation range around the pixel of interest and an outer filter having a calculation range as the surrounding pixel. It can be determined whether the target pixel is a pixel at the position of the image edge, a pixel adjacent to the inside of the pixel at the position of the image edge, or another pixel.

[ 6 ] The image forming apparatus according to any one of [1] to [ 5 ], wherein the density regulating unit regulates a width of the image signal for each pixel.

In the above invention, the density of each pixel is regulated by regulating the width of the image signal given to the image forming unit.
[7] A program executed by an image forming apparatus having an electrophotographic image forming unit,
A pixel position detecting step for detecting a position with respect to the image edge for each pixel of the image to be printed;
A halftone processing step of generating a halftone image by performing a halftone process on the print target image;
A regulation value determining step of determining a regulation value of the density of each pixel of the print target image based on the position detected in the pixel position detection step for the pixel;
A density regulation step for regulating the density of each pixel of the halftone image to be equal to or less than the regulation value determined in the regulation value determination step for each pixel;
Causing the image forming unit to form an image based on an image signal of the halftone image after the restriction in the density restriction step;
Have
A program characterized by that.
[8] In the density regulation step, the regulation value is replaced by replacing the density of each pixel of the halftone image with the regulation value when the density of the pixel exceeds the regulation value determined in the regulation value determination step. Regulate to
[7] The program according to [7].
[9] In the pixel position detection step, it is detected whether the position of each pixel is a position of an image edge,
In the restriction value determining step, the restriction value for the pixel at the image edge position is set to be smaller than the restriction value for the pixel not at the image edge position.
The program according to [7] or [8], wherein
[10] In the pixel position detection step, it is detected whether the position of each pixel is the position of the image edge, the position adjacent to the inside of the pixel of the image edge, or another position,
In the regulation value determining step, a regulation value for a pixel at the position of the image edge is made smaller than a regulation value for a pixel at the position adjacent to the inside,
The restriction value for the pixel at the position adjacent to the inside is set smaller than the restriction value for the pixel at the other position
The program according to [7] or [8], wherein
[11] In the pixel position detection step, an inner filter centered on the target pixel and an outer filter surrounding the inner filter are used.
When the calculation result of the inner filter satisfies a predetermined condition, the position of the target pixel with respect to the image edge is obtained from the calculation result of the inner filter,
When the calculation result of the inner filter does not satisfy the predetermined condition, the position of the target pixel with respect to the image edge is obtained from the calculation result of the outer filter.
[10] The program according to [10].
[12] In the density regulation step, the width of the image signal for each pixel is regulated.
The program according to any one of [7] to [11], wherein

  According to the image forming apparatus of the present invention, toner scattering at the image edge can be reduced by image processing without impairing the characteristics of the dither screen dot pattern.

1 is a block diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 6 is an explanatory diagram schematically illustrating a method of preventing toner scattering adopted by the present invention. 2 is a block diagram illustrating a schematic configuration of a control unit of the image forming apparatus. FIG. It is a block diagram which shows the structure of a printing image process part. It is a figure which shows the structure of a pixel position detection part. It is a figure which shows notionally the operation | movement of a pixel position detection part. It is a figure which shows the other structure of a pixel position detection part. It is a figure which shows the result of having detected and classifying the pixel position from an image edge about each pixel which comprises the character of A. FIG. It is a figure which shows typically the flow and result of a density | concentration regulation with respect to a rectangular object of density 100% (solid). It is a figure which shows typically the flow and result of density regulation with respect to the rectangular object of density 75% (solid). It is the figure which illustrated the dither screen pattern in each density | concentration used by a halftone process. It is a figure which shows typically the flow and result of a density control process in the case of performing a halftone process after applying a density control (50/75/100%). FIG. 3 is a diagram schematically illustrating toner on the surface of a photoreceptor and electric charges around the toner. FIG. 6 is a diagram schematically illustrating a state where toner is scattered when approaching a transfer position.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows a schematic configuration of an image forming apparatus 3 according to an embodiment of the present invention. The image forming apparatus 3 forms an image on a sheet based on image data obtained by optically reading a document or image data obtained by rasterizing print data, and prints out the image. MFP). The image forming apparatus 3 employs an electrophotographic system as an image forming process.

  As described in the background art, when an image is formed by an electrophotographic method, toner scattering occurs at an image edge. Therefore, in the image forming apparatus 3, the toner scattering is reduced by reducing the toner amount of the pixels near the image edge.

  FIG. 2 shows how the toner amount is reduced at the image edge. FIG. 2A schematically shows the toner adhesion state when the processing for reducing the toner amount is not performed. FIG. 5B schematically shows the toner adhesion state when the toner amount is reduced for the pixels at the image edge. FIG. 6C schematically shows the toner adhesion state when the toner amount is reduced for the image edge pixel and the pixel adjacent to the image edge pixel (adjacent to the inside of the image). A pixel adjacent to the pixel at the image edge (adjacent to the inside of the image) is called an edge adjacent pixel. The image edge is a contour portion of a cluster of pixels having density. The image edge pixel is a pixel constituting the outline of a cluster of pixels having density.

  Compared to FIG. 6A, toner scattering is reduced in FIG. 5B, and toner scattering is further reduced in FIG. 4C than in FIG. For example, the toner amount of pixels at the image edge is limited to 50% or less, and the toner amount of pixels adjacent to the edge is limited to 75% or less.

  Returning to FIG. 1, the configuration of the image forming apparatus 3 will be described. The image forming apparatus 3 includes a document reading unit 4, an operation panel 5, an image forming unit 6, and a control unit 7.

  The document reading unit 4 performs a function of optically reading a document and generating image data. The document reading unit 4 includes, for example, a light source that emits light to the document, a line image sensor that receives the reflected light and reads the document for one line in the width direction, and sequentially reads the reading position in line units in the length direction of the document. An optical path composed of a moving unit for moving, a lens, a mirror, and the like for guiding reflected light from the document to the line image sensor to form an image, and a conversion unit for converting an analog image signal output from the line image sensor into digital image data And so on. The document reading unit 4 includes an automatic document feeder (ADF) 4A for automatically and continuously reading a plurality of documents set on the document table.

  The operation panel 5 receives operations such as job settings and execution instructions from the user, and functions to display various screens such as an operation screen, a setting screen, and a warning screen. The operation panel 5 includes a display unit such as a liquid crystal display (LCD), a touch panel provided on the display surface of the display unit, various operation switches such as a start button and a stop button, and the like. The touch panel detects a coordinate position where the display surface of the display unit is touched with a pen or a finger.

  The image forming unit 6 functions to form an image corresponding to the image data on a sheet and print it out. The image forming unit 6 includes a paper feeding unit 8 having a function of storing paper to be printed and a function of feeding out the stored paper one by one, and a toner image on the paper fed from the paper feeding unit 8. An electrophotographic image forming unit 9 for forming the image, a fixing device 10 for thermally fixing the toner image formed on the paper by the image forming unit 9 to the paper, and an image forming unit for the paper fed from the paper supply unit 8 9 and a fixing unit 10, and a transport unit 11 that transports the paper to a paper discharge area 12.

  The image forming unit 9 employs a so-called tandem method as a color image forming method. The image forming unit 9 includes an intermediate transfer belt 13, a plurality of image forming units 20C, 20M, 20Y, and 20K that form single-color toner images on the surface of the intermediate transfer belt 13, a cleaning device 14, and a secondary transfer. It has a roller 15.

  The intermediate transfer belt 13 is an annular belt having a width wider than that of the paper, and is carried by a plurality of rollers provided on the inner side thereof, and is driven so as to circulate in the direction of an arrow V shown in FIG. The image forming units 20C, 20M, 20Y, and 20K are disposed along the rotating intermediate transfer belt 13.

  The image forming unit 20C forms a C (cyan) toner image, the image forming unit 20M forms an M (magenta) toner image, and the image forming unit 20Y forms a Y (yellow) toner image. The unit 20K forms a toner image of K (black) color and transfers (primary transfer) to the intermediate transfer belt 13 respectively. A full color image is formed by superimposing the toner images formed by the image forming units 20C, 20M, 20Y, and 20K on the intermediate transfer belt 13.

  Each of the image forming units 20C, 20M, 20Y, and 20K includes a photosensitive drum 21, and a charging device 22, a laser scanning unit 23, a developing unit 24, and a cleaning unit 25 disposed around the photosensitive drum 21.

  During image formation, the photosensitive drum 21 is rotationally driven along the moving direction of the intermediate transfer belt 13. The photosensitive drum 21 is uniformly charged with a predetermined polarity (for example, plus) on the outer peripheral surface thereof by the charging device 22. The laser scanning unit 23 forms a latent image on the photosensitive drum 21 by irradiating the photosensitive drum 21 with laser light that is turned on / off according to an input image signal. The developing unit 24 performs a developing process in which a colorant (toner) is attached to the photosensitive drum 21 on which the latent image is formed to make it appear as a toner image.

  Transfer rollers 26 are provided at positions facing the photosensitive drums 21 of the image forming units 20C, 20M, 20Y, and 20K with the intermediate transfer belt 13 interposed therebetween. Each color toner image formed on the photosensitive drum 21 is transferred (primary transfer) to the intermediate transfer belt 13 by the electric field applied by the transfer roller 26.

  The cleaning units 25 of the image forming units 20C, 20M, 20Y, and 20K remove toner and residual charges remaining on the photosensitive drum 21 after transfer.

  The toner image formed on the intermediate transfer belt 13 is transferred (secondary transfer) to the sheet by the electric field applied by the transfer roller 15.

  The intermediate transfer belt 13 after the secondary transfer is cleaned by a cleaning device 14.

  The sheet on which the toner image is transferred is discharged to the discharge area 12 via the fixing device 10. The toner image is thermally fixed on the paper while passing through the fixing device 10.

  FIG. 3 is a block diagram illustrating a schematic configuration of the control unit 7. The control unit 7 includes a CPU (Central Processing Unit) 31 that comprehensively controls the operation of the image forming apparatus 3. The CPU 31 has a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, an image forming unit I / F 34, an operation panel I / F 35, an external data I / F 36, and a scan image processing unit through a bus. 37, a CMYK frame memory 38, a print image processing unit 40, and a print frame memory 39 are connected.

  The ROM 32 stores in advance an image forming apparatus (MFP) control program executed by the CPU 31 and various fixed data necessary for executing the control program. The RAM 33 is a work memory for temporarily storing various data when the CPU 31 executes a program.

  The operation panel 5 is connected to the operation panel I / F 35. The operation panel I / F 35 is an interface circuit for transmitting and receiving data between the operation panel 5 and the CPU 31. The operation panel 5 is controlled by the CPU 31.

  The external data I / F 36 is an interface circuit that receives print data described in a page description language (PDL) from an external device such as a personal computer. The received print data is processed by the CPU 31 executing a predetermined program. Here, various data processing including appropriate color conversion is performed on the received print data, and the processed data is converted into CMYK image data (bitmap format) having C, M, Y, and K as color components. It is converted and stored in the CMYK frame memory 38. The CMYK frame memory 38 is a memory capable of storing at least one page of CMYK image data.

  The scanned image processing unit 37 has a function of inputting image data (scanned image data) generated by reading a document with the document reading unit 4 from the document reading unit 4 and performing various image processes. Specifically, the scan image processing unit 37 performs appropriate color conversion on scan image data composed of R (red), G (green), and B (blue) color components input from the document reading unit 4. CMYK image data (bitmap format) is generated, and this data is stored in the CMYK frame memory 38.

  The print image processing unit 40 sequentially reads out CMYK image data stored in the CMYK frame memory 38, performs predetermined image processing, and outputs data (print image data) as a result of the image processing to the print frame memory. 39. The print image processing unit 40 detects halftone processing and the position of each pixel with respect to the image edge, determines a restriction value of density according to the pixel position, and determines the density of each pixel printed based on the restriction value. Concentration regulation processing is performed. Details of the print image processing unit 40 will be described later.

  The print frame memory 39 is a memory for storing at least one page of image data output to the image forming unit 6 and its accompanying information.

  The image forming unit I / F 34 activates the image forming unit 6 in response to a command from the CPU 31. Further, print image data for one page stored in the print frame memory 39 is sequentially read out, and this is synchronized with an image signal for driving the laser scanning unit 23 of the image forming unit 20 from the image forming unit 6. It performs the function of converting in accordance with the signal and outputting the image signal to the laser scanning unit 23.

  When print output is performed in the image forming apparatus 3, image data to be printed is input from the document reading unit 4 or the like, subjected to processing such as color conversion, and stored in the CMYK frame memory 38. The image data stored in the CMYK frame memory 38 is read by the print image processing unit 40, subjected to predetermined image processing, and stored in the print frame memory 39. The image data stored in the print frame memory 39 is read by the image forming unit I / F 34 and transferred to the image forming unit 6. The image forming unit 6 forms an image on a sheet according to the image data transferred from the image forming unit I / F 34 and prints it out.

  FIG. 4 is a block diagram illustrating a configuration of the print image processing unit 40. The print image processing unit 40 sequentially reads CMYK image data stored in the CMYK frame memory 38 and performs image processing using each read pixel as a target pixel.

  The print image processing unit 40 includes a binary conversion unit 41, a pixel position detection unit 42, a regulation value determination unit 43, a halftone processing unit 44, and an output value regulation unit 45. The halftone processing unit 44 and the output value regulating unit 45 are provided for each of C, M, Y, and K colors.

  The binary conversion unit 41 converts the pixel of interest into a binary value other than white and white and sends it to the pixel position detection unit 42. Here, binarization is performed by setting white as the case where the density of all the color components C, M, Y, and K is “0”, and setting the case where the density of at least one color component is not “0” as other than white.

  The pixel position detection unit 42 determines the position from the image edge for pixels other than white among the pixels binarized other than white and white. Specifically, pixel positions of pixels other than white are classified into three types: (a) a position constituting an image edge, (b) a position adjacent to the pixel of the image edge, and (c) another position. The pixel at the position (a) is a pixel at the image edge, and the pixel at the position (b) is an edge adjacent pixel. The pixel at the position (c) is referred to as an inner deep pixel.

  The restriction value determining unit 43 determines the restriction value (upper limit value) of the density of the target pixel according to the detected pixel position (classification). The image forming apparatus 3 employs a PWM modulation method in which the density of each pixel formed on the paper is controlled by the pulse width of an image signal applied to the image forming unit 6 (laser scanning unit 23 of the image forming unit 20). . The restriction value determination unit 43 determines the upper limit value of the pulse width (output PWM value) of the image signal when printing the target pixel as the restriction value of the density of the target pixel. The output of the regulation value determination unit 43 is input to the C, M, Y, and K output value regulation units 45.

  The halftone processing unit 44 performs a dither screen process using the in-dot gradation together, and outputs a density value (output PWM value) of the target pixel. Dither screen processing that uses in-dot gradation together is halftone processing that enables multi-tone expression that cannot be represented by a simple dither screen by using a function that adjusts the dot size in dot units. . In particular, a slight gradation difference can be expressed while securing the number of dots in a low density region.

  When the output PWM value of the target pixel input from the halftone processing unit 44 exceeds the control value of density determined by the control value determining unit 43 for the target pixel (the upper limit value of the output PWM value). In addition, the output PWM value of the target pixel is replaced with the density regulation value (the upper limit value of the output PWM value) of the target pixel. That is, the print image processing unit 40 applies the density regulation value to each pixel after the halftone process.

  FIG. 5 shows a detailed configuration of the pixel position detection unit 42. The pixel position detection unit 42 includes an inner filter 51 that operates on a 3 × 3 pixel at the center of the 5 × 5 matrix centered on the target pixel, and an outer peripheral portion of the 3 × 3 pixel at the center. , The first LUT (lookup table) 53 having the calculation result of the inner filter 51 as an input value, the second LUT 54 having the calculation result of the outer filter 52 as an input value, and the first LUT 53 And a selector 55 that selects one of the output values of the second LUT 54. Information indicating the presence or absence of white pixels in the inner filter 51 is input to the selector 55 as the selection signal 56.

  The first LUT 53 inputs the calculation result of the inner filter 51, and outputs a density regulation value corresponding thereto. The second LUT 54 receives the calculation result of the outer filter 52, and outputs a regulation value of the density corresponding thereto.

  The pixel position detection unit 42 separates the 5 × 5 filter arithmetic circuit centered on the pixel of interest into an inner filter 51 at the center and an outer filter 52 that targets pixels at the outer periphery, and the inner filter 51. And the outer filter 52 perform separate filter calculations. When the calculation result of the inner filter 51 satisfies a predetermined condition, the position of the target pixel with respect to the image edge is obtained from the calculation result of the inner filter 51, and when the calculation result of the inner filter 51 does not satisfy the predetermined condition, From the calculation result of the outer filter 52, the position of the target pixel with respect to the image edge is obtained.

  FIG. 6 conceptually shows the operation of the pixel position detector 42. If all the 8 pixels around the pixel of interest in the inner filter 51 are not pixels other than white, that is, if at least one of the surrounding 8 pixels is a white pixel (step S101; No), the pixel of interest Is determined to be at the position of the image edge (step S102).

  Specifically, the selector 55 selects the output value of the first LUT 53 and outputs it as a density regulation value. The first LUT 53 outputs the density regulation value (the upper limit value of the output PWM value) corresponding to the pixel at the image edge when at least one of the eight pixels around the center pixel (target pixel) is white. To do. For example, an output PWM value corresponding to a density of 50% is output as a density regulation value. Note that when all the eight pixels around the pixel of interest are other than white, the output value of the first LUT 53 is not used, so the output value of the first LUT 53 corresponding to the input value corresponding to this case may be arbitrary.

  If all the 8 pixels around the pixel of interest are pixels other than white (step S101; Yes), since the pixel of interest is not a pixel of the image edge, it is determined whether or not the pixel of interest is an edge adjacent pixel adjacent to the pixel of the image edge. Determination is made using the outer filter 52 (step S103). Here, if at least one of the 12 pixels excluding the pixels at the four corners in the outer periphery of the inner filter 51 is white (step S103; Yes), the target pixel is an edge adjacent pixel adjacent to the image edge pixel. (Step S104).

  Specifically, the selector 55 selects the output value of the second LUT 54 and outputs it as a density regulation value. The second LUT 54 outputs a density regulation value (upper limit value of the output PWM value) corresponding to the edge adjacent pixel with respect to an input value in which at least one of the 12 pixels is a white pixel. For example, an output PWM value corresponding to a density of 75% is output as a density regulation value.

  If all of the 12 pixels are pixels other than white (step S103; No), it is determined as an inner deep pixel (step S105). Specifically, the selector 55 selects and outputs the output value of the second LUT 54 as a density regulation value. The second LUT 54 outputs a density regulation value (upper limit value of the output PWM value) corresponding to the inner deep pixel with respect to an input value in which the above 12 pixels are all other than white pixels. For example, an output PWM value corresponding to 100% density (corresponding to no density regulation) is output as a density regulation value.

  By dividing the 5 × 5 filter into the inner filter 51 and the outer filter 52 in this way, the capacities of the first LUT 53 and the second LUT 54 can be reduced. As shown in FIG. 7A, when a calculation is performed with one 5 × 5 filter, a lookup table having a pattern of 2 to the 20th power is required. On the other hand, in the configuration of FIG. 5, the number of patterns of the first LUT 53 is 256 to the power of 2, and the number of patterns of the second LUT 54 is 4096 to the power of 2 to reduce the capacity of the lookup table. be able to. Further, although the 5 × 5 filter is divided into the inner filter 51 and the outer filter 52, since the calculation of the inner filter 51 and the outer filter 52 is executed simultaneously, it can be configured as a single-stage filter calculation.

  The pixel position detection unit 42 is not limited to the configuration illustrated in FIG. 5, and may be configured by connecting two stages of differential filters as illustrated in FIG. 7B. However, since it has a two-stage configuration, processing time is required compared to the case of the one-stage configuration.

  FIG. 8 shows the result of detecting the pixel position of each pixel constituting the image (object) of the character A and classifying the pixel into the image edge pixel, edge adjacent pixel, and inner depth pixel.

  Next, the operation of the print image processing unit 40 will be described using a processed image as an example.

  FIG. 9 schematically shows the flow and results of density regulation processing for a rectangular object 61 that is a 100% density (solid) image. In this case, halftone processing is not performed. FIG. 5A shows a rectangular object 61 having a density of 100% (solid). FIG. 5B shows the result of detecting the pixel position of each pixel of the rectangular object 61 by the pixel position detection unit 42. That is, each pixel constituting the rectangular object 61 is divided into a pixel at a position constituting an image edge (pixel at an image edge), a pixel at a position adjacent to a pixel at the image edge (edge adjacent pixel), and a pixel at another position ( The result is classified into (inner deep pixels).

  FIG. 6C is a diagram schematically showing the restriction value of the density of each pixel in terms of the pixel size, corresponding to the result of FIG. In this example, the upper limit value of the output PWM value is set to 50% for pixels at the image edge, 75% for pixels adjacent to the edge, and 100% (FullDot) for inner depth pixels.

  The output value restricting unit 45 restricts the density value of each pixel shown in FIG. 10A with the restricting value (upper limit value of the output PWM value) corresponding to each pixel shown in FIG. FIG. 4D shows the rectangular object 61 after density regulation. The dot size of each pixel in FIG. 4D corresponds to the density value after density regulation. In this way, the print image processing unit 40 reduces the dot diameter of the toner image corresponding to the image edge pixel and the pixel adjacent to the inside of the edge pixel to reduce the toner adhesion amount in the vicinity of the image edge and the edge (relative). To control). Thereby, toner scattering at the image edge can be reduced.

  Here, in order to control the toner adhesion amount of the pixel at the image edge and the pixel adjacent to the edge to be (relatively) reduced, strictly speaking, the density decreases in the vicinity of the image edge in the output image. In the case of 600 dpi, the width of the portion where the density is reduced (the portion where the edge pixel and the edge adjacent pixel are combined) is 0.1 mm or less, and is hardly recognized by human visual characteristics.

  FIG. 10 schematically shows the flow and results of density regulation processing for a rectangular object 63 that is a 75% density (solid) image. FIG. 5A shows a rectangular object 63 having a density of 75% (solid). FIG. 4B shows the result of detecting the pixel position of each pixel of the rectangular object 63 by the pixel position detection unit 42. That is, the result of classifying each pixel constituting the rectangular object 63 into a pixel of an image edge, an edge adjacent pixel, and an inner deep pixel is shown.

  FIG. 6C is a diagram schematically showing the restriction value of the density of each pixel in terms of the pixel size, corresponding to the result of FIG. In this example, the upper limit value of the output PWM value is set to 50% for pixels at the image edge, 75% for pixels adjacent to the edge, and 100% (FullDot) for inner depth pixels. FIGS. 10B and 10C are the same as FIGS. 9B and 9C.

  FIG. 4D shows a halftone image as a result of the halftone processing performed by the halftone processing unit 44 on the rectangular object 63 having a density of 75% (solid). FIG. 11 exemplifies a dither screen pattern at each density used in halftone processing.

  The output value regulating unit 45 shows the density value of each pixel of the image shown in FIG. 10D obtained by performing the halftone process on the rectangular object 63 having a density of 75% (solid), as shown in FIG. The density is regulated by the density regulation value (upper limit value of the output PWM value) corresponding to each pixel. The regulated image is as shown in FIG. The dot size of each pixel in FIG. 5E corresponds to the density value after regulation.

  Since the result of density regulation (FIG. 10 (e)) is the same as the dot arrangement in the halftone image (FIG. 10 (d)) where density regulation is not performed, the dot arrangement from the original shape of the object to be printed The divergence is small. That is, toner scattering at the edge portion of the image can be prevented without impairing the characteristics of the screen pattern used in the halftone process.

  By the way, as shown in FIG. 12, when halftone processing is performed on a rectangular object 63 having a density of 75% (solid) and density control (50/75/100%) is first applied, the processing result is as follows. The shape shown in FIG. 12E is obtained, and the deviation of the printed object from the original shape is increased. That is, when halftone processing is performed on the image of FIG. 12D to which the density regulation has been applied, a 50% screen pattern is applied to the edge pixels, and a 75% screen pattern is applied to the other pixels. Therefore, the arrangement of dots greatly deviates from the case where a 75% screen pattern is applied to the entire rectangular object 63.

  The print image processing unit 40 of the image forming apparatus 3 according to the present embodiment detects the pixel position with respect to the image edge in the original image, determines the density regulation value of each pixel, and obtains the original image by halftone processing. Since the upper limit of the density value of each pixel of the halftone image is regulated by the density regulation value determined for each pixel, the toner scattering at the edge of the image without affecting the characteristics of the screen pattern for the halftone image. It can be effectively reduced.

  The embodiment of the present invention has been described with reference to the drawings. However, the specific configuration is not limited to that shown in the embodiment, and there are changes and additions within the scope of the present invention. Are also included in the present invention.

  The concentration regulation values shown in the embodiment are merely examples, and the present invention is not limited to these. If the density regulation value for the pixel at the edge of the image is smaller than the density regulation value for the pixel adjacent to the edge and the density regulation for the pixel adjacent to the edge is smaller than the density regulation value for the inner depth pixel, the density regulation value is set for each image formation. What is necessary is just to set to the appropriate value according to the characteristic etc. of the part.

  In the embodiment, the density of the image edge pixel and the edge adjacent pixel is regulated to be low. However, only the image edge pixel may be regulated. On the other hand, the pixels adjacent to the inside of the edge adjacent pixels may be configured to restrict the density to a low level. The number of pixels from the image edge to the inside may be regulated as appropriate according to the characteristics of the image forming unit.

  Further, although the density of each pixel is regulated by the pulse width of the image signal, the method for regulating the density is not limited to this. For example, a method may be used in which the density is regulated by changing the intensity of laser light applied to the photosensitive drum 21.

  The image forming unit 6 exemplified in the embodiment has a method of forming an electrostatic latent image by irradiating a uniformly charged photosensitive drum with a laser beam, but the uniformly charged photosensitive drum. Alternatively, other methods such as a method of forming an electrostatic latent image by checking light through a liquid crystal shutter may be used.

  In the embodiment, the case where a color image is formed is illustrated, but the present invention is also applied to an image forming apparatus that forms a monochrome image.

  In the embodiment, the binary conversion unit 41 binarizes a pixel having a density of 0 in all color components of CMYK to white other than white and white, and density restriction values for each pixel of interest after binarization. This density regulation value was applied to all the color components of the target pixel. This prevents the following problems. For example, in the case where the color gradually changes due to gradation in one character, binarization is performed for each color component, and if the density is regulated by obtaining the position from the image edge for each color component, Since an edge is also generated at (location), density regulation acts on the edge, and the color changes from the original color.

DESCRIPTION OF SYMBOLS 3 ... Image forming apparatus 4 ... Document reading unit 4A ... Automatic paper feeder 5 ... Operation panel 6 ... Image forming part 7 ... Control part 8 ... Paper feed part 9 ... Image forming part 10 ... Fixing apparatus 11 ... Conveying part 12 ... Ejection Paper area 13 ... Intermediate transfer belt 14 ... Cleaning device 15 ... Secondary transfer roller 20 ... Image forming unit 21 ... Photoconductor drum 22 ... Charging device 23 ... Laser scanning unit 24 ... Developing unit 25 ... Cleaning unit 26 ... Transfer roller 31 ... CPU
32 ... ROM
33 ... RAM
34. Image forming unit I / F
35 ... Operation panel I / F
36 ... External data I / F
37 ... Scanned image processing unit 38 ... CMYK frame memory 39 ... Print frame memory 40 ... Print image processing unit 41 ... Binary conversion unit 42 ... Pixel position detection unit 43 ... Restriction value determination unit 44 ... Halftone processing unit 45 ... Output value Restriction part 51 ... inner filter 52 ... outer filter 53 ... first LUT
54 ... Second LUT
55: Selector 56 ... Selection signal 61 ... Rectangular object with 100% density (solid) 63 ... Rectangular object with 75% density (solid)

Claims (12)

A pixel position detection unit that detects a position with respect to an image edge for each pixel of the print target image;
A halftone processing unit that generates a halftone image by performing a halftone process on the print target image;
A regulation value determining unit that determines a regulation value of the density of each pixel of the print target image based on the position detected by the pixel position detection unit for the pixel;
A density regulation unit that regulates the density of each pixel of the halftone image to be equal to or less than a regulation value determined by the regulation value decision unit for each pixel;
An electrophotographic image forming unit that forms an image based on an image signal of the halftone image that has been regulated by the density regulating unit;
An image forming apparatus comprising:   The density regulation unit regulates the density of each pixel of the halftone image below the regulation value by replacing the density of the pixel with the regulation value when the density of the pixel exceeds the regulation value determined by the regulation value determination unit. Do
  The image forming apparatus according to claim 1. The pixel position detection unit detects whether the position of each pixel is the position of an image edge,
Regulation value for the pixel at the position of the image edge, the image forming apparatus according to claim 1 or 2, characterized in that less than regulatory limits for free pixel position of the image edge. The pixel position detection unit detects whether the position of each pixel is the position of the image edge, the position adjacent to the inside of the pixel of the image edge, or another position,
The regulation value for the pixel at the position of the image edge is smaller than the regulation value for the pixel at the position adjacent to the inside,
The image forming apparatus according to claim 1 or 2 regulation value for the pixel at the position adjacent to the inner is characterized in that less than regulation value for the pixels of the other positions. The pixel position detection unit includes an inner filter centered on the target pixel, and an outer filter surrounding the inner filter,
When the calculation result of the inner filter satisfies a predetermined condition, the position of the target pixel with respect to the image edge is obtained from the calculation result of the inner filter,
The image forming apparatus according to claim 4 , wherein when the calculation result of the inner filter does not satisfy the predetermined condition, the position of the target pixel with respect to the image edge is obtained from the calculation result of the outer filter. The concentration regulating unit, an image forming apparatus according to any one of claims 1 to 5, characterized in that to regulate the width of the image signal for each pixel.   A program executed by an image forming apparatus having an electrophotographic image forming unit,
  A pixel position detecting step for detecting a position with respect to the image edge for each pixel of the image to be printed;
  A halftone processing step of generating a halftone image by performing a halftone process on the print target image;
  A regulation value determining step of determining a regulation value of the density of each pixel of the print target image based on the position detected in the pixel position detection step for the pixel;
  A density regulation step for regulating the density of each pixel of the halftone image to be equal to or less than the regulation value determined in the regulation value determination step for each pixel;
  Causing the image forming unit to form an image based on an image signal of the halftone image after the restriction in the density restriction step;
  Have
  A program characterized by that.   In the density regulation step, the density of each pixel of the halftone image is regulated to the regulation value or less by replacing with the regulation value when the density of the pixel exceeds the regulation value determined in the regulation value determination step. Do
  The program according to claim 7.   In the pixel position detection step, it is detected whether the position of each pixel is the position of an image edge,
  In the restriction value determining step, the restriction value for the pixel at the image edge position is set to be smaller than the restriction value for the pixel not at the image edge position.
  9. The program according to claim 7 or 8, wherein:   In the pixel position detection step, it is detected whether the position of each pixel is the position of the image edge, the position adjacent to the inside of the pixel of the image edge, or another position,
  In the regulation value determining step, a regulation value for a pixel at the position of the image edge is made smaller than a regulation value for a pixel at the position adjacent to the inside,
  The restriction value for the pixel at the position adjacent to the inside is set smaller than the restriction value for the pixel at the other position.
  9. The program according to claim 7 or 8, wherein:   In the pixel position detection step, an inner filter centered on the target pixel and an outer filter surrounding the inner filter are used.
  When the calculation result of the inner filter satisfies a predetermined condition, the position of the target pixel with respect to the image edge is obtained from the calculation result of the inner filter,
  When the calculation result of the inner filter does not satisfy the predetermined condition, the position of the target pixel with respect to the image edge is obtained from the calculation result of the outer filter.
  The program according to claim 10.   In the density regulation step, the width of the image signal for each pixel is regulated.
  The program according to any one of claims 7 to 11, characterized in that: