JP2017108314A - Image signal processing apparatus, image signal processing method, and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable execution of image correction processing of reducing toner consumption while preventing an increase in circuit scale and buffer memory.SOLUTION: An edge flag generation part (261) generates a flag indicating, for each of pixels, whether the pixel is a pixel corresponding to an edge of an image. An image analysis part (266) specifies, from the image, a correction target pixel the density of which is to be corrected on the basis of the flags generated corresponding respectively to the pixels of the image. An image correction part (267) corrects the density of the correction target pixel on the basis of the distance from the edge to the correction target pixel.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an image signal processing device, an image signal processing method, and a program.

In the field of image forming apparatuses adopting an electrophotographic system, reduction of toner consumption has been eagerly desired. For example, Patent Document 1 discloses a technique for saving toner consumption by reducing the exposure intensity for an image region having a certain area.
Further, it is known that an electrophotographic image forming apparatus has a phenomenon in which the amount of developed toner at the rear end portion of a latent image is larger than the amount of developed toner at the center portion of the latent image. This phenomenon is called a sweeping effect. For this sweeping effect, a technique for correcting the sweeping by performing a correction process on the image data and adjusting the exposure amount has been proposed (for example, Patent Document 2).
In addition to the problem of the sweeping effect, the electric field concentrates on the boundary between the exposed portion (electrostatic latent image) and the non-exposed portion (charged portion) formed on the photosensitive drum, so that the toner is formed on the edge of the image. Is known to be excessively attached. This phenomenon is called an edge effect and can occur in combination with the above sweeping effect. For this reason, when the exposure intensity is reduced in order to reduce excess toner for the image portion where the sweeping effect and the edge effect have occurred, it is necessary to perform a correction process suitable for each phenomenon. If correction processing suitable for each phenomenon cannot be performed, a decrease in image density, a decrease in dot reproducibility, a thin line, and the like will occur, leading to deterioration in image quality.

JP 2004-299239 A JP 2007-272153 A

  Here, it is desirable that correction processing for sweeping and edge effects be performed immediately before PWM (pulse width modulation) control for controlling the exposure amount. However, generally, image data that is the basis of PWM control is subjected to color conversion and halftone processing, and color information is converted to YMCK (yellow, magenta, cyan, black (key plate)), or image density The information is replaced with a dot pattern. Therefore, when detecting an image area where the phenomenon of sweeping effect and edge effect may occur, it is necessary to refer to a wide area and analyze a complicated pattern in order to reduce false detection. As described above, a large-scale circuit is required to refer to a wide area and to perform a complicated pattern analysis. Further, for example, in the sweeping effect, in order to detect the rear end portion of the latent image, the black continuous region and the white continuous region are discriminated in the vertical and horizontal directions in the reference window in the 20 × 20 pixel range around the target pixel. There is a need. In this case, in order to form a reference window, pixel values for 20 lines must be held, and a large-capacity buffer memory is required.

  The present invention has been made in view of such problems, and an image signal processing device that enables image correction processing that reduces toner consumption while preventing an increase in circuit scale, buffer memory, and the like, An object is to provide an image signal processing method and program.

  The image signal processing apparatus according to the present invention includes, for each pixel of the image signal, a generation unit that generates a flag indicating whether the pixel is a pixel corresponding to an edge of the image, and a generation unit that corresponds to each pixel of the image. Based on the flag generated in this way, the analysis means for specifying the correction target pixel whose density is to be corrected from the image, and the correction target pixel based on the distance from the correction target pixel to the edge Correction means for correcting the density of the light.

  According to the present invention, it is possible to perform image correction processing that reduces toner consumption while preventing an increase in circuit scale and buffer memory.

1 is a diagram illustrating a basic configuration of an electrophotographic image forming apparatus. It is a functional block diagram which shows the internal structure of a controller. It is explanatory drawing of a mode that an exposure apparatus is controlled by a drive signal and a light quantity adjustment signal. It is explanatory drawing of a mode that pixel density is adjusted by PWM control. It is explanatory drawing of a jumping development state and a contact development state. It is explanatory drawing of an edge effect. It is explanatory drawing of the image which the edge effect and the sweeping effect generate | occur | produced. FIG. 6 is an explanatory diagram of a toner distribution state when an edge effect and a sweeping effect occur. It is explanatory drawing of the generation | occurrence | production mechanism of the sweeping effect in a contact development state. It is a flowchart of the detection method of a correction object pixel from an edge flag. It is explanatory drawing of the detection method of a correction object pixel from an edge flag. It is another explanatory drawing of the detection method of a correction object pixel from an edge flag. It is explanatory drawing of the result of having detected the correction object pixel. It is a flowchart which shows the calculation method of correction processing amount. It is explanatory drawing of the edge flag production | generation method. It is explanatory drawing of the correction processing amount with respect to the distance from an edge.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, each structure shown in the following embodiment is an example, and is not limited to the illustrated structure.
FIG. 1 is a diagram showing a schematic configuration of an electrophotographic image forming apparatus as an application example of an image signal processing apparatus. The image forming apparatus 100 includes a photosensitive drum 110, a charging device 120, an exposure device 130, a controller 140, a developing device 150, a transfer device 160, a fixing device 170, an environment detection device 180, a document reading device 190, and the like. ing. A hatched portion in the drawing of the developing device 150 represents toner as a developer. Further, in the photosensitive drum 110, R in the figure represents a development area, and T in the figure represents a transfer position. In the image forming apparatus 100, the components that perform operations relating to image formation, excluding the controller 140, the environment detection device 180, and the document reading device 190, are printing units that actually perform printing, and are called so-called printer engines. Part.

  The environment detection device 180 acquires ambient environment information such as temperature and humidity inside and outside the image forming apparatus 100. The document reading device 190 includes at least a CCD (Charged Couple Device) and a CIS (Contact Image sensor). The document reading apparatus 190 may be configured to include a processing unit that performs predetermined image processing on the read image data.

  The photosensitive drum 110 is formed by forming an electrophotographic photosensitive member as an image carrier on the drum surface. The charging device 120 is a charging roller or the like that uniformly charges the electrophotographic photosensitive member on the surface of the photosensitive drum 110. The exposure apparatus 130 includes a laser beam scanner, a surface light emitting element, and the like. The exposure device 130 applies a light amount based on print data from the host computer 10, image data from the document reading device 190, and the like to the drum surface of the photosensitive drum 110 in which the electrophotographic photosensitive member is uniformly charged. Light is irradiated to expose the electrophotographic photoreceptor. In the present embodiment, the exposure apparatus 130 is constituted by a laser beam scanner provided with a semiconductor laser, and the exposure of the photosensitive drum 110 to the electrophotographic photosensitive member is performed by a laser beam. Although details will be described later, the exposure apparatus 130 causes a laser beam to be emitted by a drive signal and a light amount adjustment signal, which will be described later, generated by the controller 140 based on print data and image data. Then, the exposure device 130 irradiates the electrophotographic photosensitive member on the surface of the rotating photosensitive drum 110 with the light of the laser beam. As a result, an electrostatic latent image corresponding to the print data or image data is formed on the electrophotographic photosensitive member on the surface of the photosensitive drum 110.

  In addition to a toner container for storing and storing toner, the developing device 150 includes a developing roller 151 that is a developer carrier and a regulating blade 152 that functions as a toner layer thickness regulating member. Here, a non-magnetic one-component toner is used as the toner, but a two-component toner or a magnetic toner may be used. The layer thickness of the toner supplied to the developing roller 151 is regulated by the regulation blade 152 described above. The regulating blade 152 may be configured to apply a charge to the toner. Then, the toner regulated to a predetermined layer thickness and given a predetermined amount of electric charge is conveyed to the developing region R by the developing roller 151. The development region R is a region where the developing roller 151 and the photosensitive drum 110 are close to or in contact with each other and toner is attached thereto. The electrostatic latent image formed on the electrophotographic photosensitive member on the surface of the photosensitive drum 110 is developed with toner and converted into a toner image. The toner image formed on the surface of the photosensitive drum 110 is transferred onto the recording medium P, which is a recording sheet, by the transfer device 160 at the transfer position T. The toner image transferred onto the recording medium P is conveyed to the fixing device 170. The fixing device 170 applies heat and pressure to the toner image and the recording medium P to fix the toner image on the recording medium P.

  FIG. 2 is a functional block diagram showing the internal configuration of the controller 140. Details of the configuration of FIG. 2 will be described later. The controller 140 generates raster image data from the print data received from, for example, the host computer 10 by the rendering processing unit 200 described later, and stores the raster image data in the image memory 231. In the following description, the raster image data is abbreviated as “raster data”. Further, the controller 140 receives image data read by the document reading device 190 via a scanner I / F 280 described later, and stores the image data in the image memory 231 as raster data. Further, the raster data is processed into raster data suitable for printing by the image forming apparatus 100 by an image processing unit 260 described later of the controller 140. Although details will be described later, in this embodiment, the image processing unit 260 of the controller 140 uses a toner consumption amount with respect to raster data in order to suppress excessive toner adhesion due to an edge effect or a sweeping effect. Correction processing is executed to reduce the above.

  Here, when the edge effect is defined again, the edge effect is a boundary between an exposed area (exposed area) and an unexposed area (non-exposed area) in the electrophotographic photosensitive member on the surface of the photosensitive drum 110. This is a phenomenon in which toner adheres excessively at (edge). That is, since the surface potential of the exposed area is different from the surface potential of the non-exposed area, an electric field wraps around these boundaries, causing a phenomenon that toner is excessively adhered. Further, the sweeping effect is a phenomenon in which toner adheres excessively at the rear end portion of the image area in the conveyance direction of the electrostatic latent image. The excessive toner adhesion due to the edge effect and the sweeping effect not only reduces the reproducibility of the image density fixed on the recording medium P with respect to the original density but also causes excessive consumption of the toner. Let Therefore, if the excess toner associated with the edge effect and sweeping effect can be removed, not only the reproducibility of the image density with respect to the original density is prevented but also the toner is saved.

  The controller 140 generates a drive signal and a light amount adjustment signal to be output to the exposure device 130 based on the raster data that has been subjected to the correction processing for reducing the toner consumption as described above. The light amount adjustment signal is for adjusting the light emission amount of the semiconductor laser of the exposure apparatus 130 so as to be a constant target light amount, or for adjusting the light amount for each pixel with the target light amount as a reference and causing the light to be emitted. It is a control signal. The drive signal is a control signal for adjusting the light emission time of the semiconductor laser whose light emission amount is adjusted to the target light amount by the light amount adjustment signal by pulse width modulation. The amount of light emitted from the exposure device 130 is adjusted by the drive signal and the light amount adjustment signal, whereby the exposure amount of the electrophotographic photosensitive member of the photosensitive drum 110 is adjusted. Realized. A detailed configuration of the controller 140 will be described later.

Hereinafter, the operation of the controller 140 will be described together with related peripheral devices with reference to FIG.
The controller 140 includes a rendering processing unit 200, a CPU 210, a ROM 220, a RAM 230, an exposure amount adjustment unit 240, an exposure control unit 250, an image processing unit 260, a host I / F 270, and a scanner I / F 280. It is connected.

  The rendering processing unit 200 interprets the PDL data transmitted from the host computer 10 via the host I / F 270 to generate raster data of bitmap image data, and develops it in the image memory 231. The host I / F 270 is an interface for exchanging data with the host computer 10. The scanner I / F 280 determines the image area by detecting a character part from the image data sent from the document reading device 190, and generates an image area signal used for subsequent image processing. Further, the scanner I / F 280 performs table conversion in order to convert image data, which is luminance data read by the document reading device 190, into density data. The scanner I / F 280 performs a convolution operation with a digital spatial filter in accordance with the purpose such as edge enhancement. The processed image data is transferred to the image memory 231 via the system bus 290.

  The CPU 210 is a control unit that comprehensively controls the entire image forming apparatus 100. The CPU 210 controls the image processing unit 260 in accordance with a program stored in the ROM 220, and performs color conversion processing and halftone conversion processing on the image data read by the document reading device 190 at the time of copying. Further, the CPU 210 controls the image processing unit 260 to perform a process of specifying a pixel in which toner is excessive due to an edge effect or a sweeping effect among a plurality of pixels in the image. Further, the CPU 210 controls the image processing unit 260 to perform correction processing for reducing the edge effect and the sweeping effect on each pixel specified as a pixel in which the toner is excessive. Details of processing for specifying pixels in which toner is excessive due to the edge effect and sweeping effect performed by the image processing unit 260 under the control of the CPU 210, and correction processing for each of the specified pixels will be described later.

  The RAM 230 functions as a work memory for the CPU 210 and has an image memory 231. The image memory 231 is a storage area (page memory, line memory, etc.) in which image data (raster data) that is an object of image formation is developed. The RAM 230 may temporarily store output data from a color conversion unit 262 (described later) of the image processing unit 260 and output data from the halftone conversion unit 263. Further, the RAM 230 stores a correction parameter (information for determining a pixel width to be corrected) described later of the image processing unit 260, a correction coefficient (exposure amount reduction ratio) described later, and an LUT (look-up table). Is also retained.

  The exposure amount adjusting unit 240 executes APC (automatic light amount control) on the light source of the exposure apparatus 130 to set a target light amount, and generates the above-described light amount adjustment signal. The exposure control unit 250 generates the drive signal described above for performing PWM control on the exposure apparatus 130. Detailed configurations of the exposure adjustment unit 240 and the exposure apparatus 130 will be described later.

<Raster image data correction processing>
The image processing unit 260 includes an edge flag generation unit 261, a color conversion unit 262, a halftone conversion unit 263, a condition determination unit 264, a parameter setting unit 265, an image analysis unit 266, and an image correction unit 267. . Each of these components may have a hardware configuration or may be realized by a software configuration by the image signal processing program of the present embodiment.

  The edge flag generation unit 261 performs binarization processing, filtering processing, and pattern matching on image data read by the document reading device 190 and supplied via the scanner I / F 280, and includes an attribute flag including edge features. Is generated and the attribute flag is output. The attribute flag includes an edge flag which is a flag indicating an edge feature, a flag indicating that the pixel is a character pixel, a flag indicating that the pixel is a pixel of a photograph, and the like. Although a detailed description is not given here as a method for generating an attribute flag including an edge feature, for example, a Sobel kernel method for obtaining a first derivative, a Laplace kernel method for obtaining a second derivative, a Canny method using a smoothing filter, etc. It has been known. When PDL data is input from the host computer 10 via the host I / F 270, when the rendering processing unit 200 generates raster data from the PDL data, an attribute flag including an edge feature may be generated. In addition, the edge flag generation unit 261 may simultaneously output flags indicating features such as characters and halftone dots in addition to attribute flags including edge features.

  The color conversion unit 262 refers to, for example, RGB (red, green, blue) color space raster data in accordance with the toner color of the image forming apparatus 100 with reference to the above-described attribute flags, and the CMYK color space raster. Performs conversion to data. The halftone conversion unit 263 performs γ correction (gamma correction) and halftone processing while referring to the attribute flag. The γ correction is a process for performing density correction in order to match ideal gradation characteristics. As a specific configuration of the halftone conversion unit 263, a configuration by screen processing or a configuration by error diffusion processing can be given. In the screen processing, N-values are obtained using a predetermined plurality of dither matrices and input image data. In addition, the error diffusion processing performs N-value conversion by comparing the input image data with a predetermined threshold value, and the difference between the input image data and the threshold value at that time is subjected to N-value processing thereafter. Is a process of diffusing.

  Further, the halftone conversion unit 263 refers to the attribute flag and performs flag determination processing such as whether the pixel of interest is a pixel constituting a character, a pixel constituting a photograph, or a pixel constituting an edge, and determines the flag. It is also possible to switch the processing method according to the result. Examples of processing method switching include performing filter processing or performing smoothing processing in halftone processing according to the edge flag of the attribute flag. In addition, for example, it is possible to switch such as using a character table or a photo table as a reference table for γ correction by using an attribute flag. As for halftone processing, it is possible to switch to refer to the attribute flag and perform error diffusion processing if the pixel of interest is a character pixel and screen processing if the pixel of interest is a photo pixel. Further, for example, when the processing for smoothing the edge is performed by the above-described processing, it is necessary to delete the flag corresponding to the edge. For example, when processing for smoothing and fringing the edge portion by performing filtering processing or smoothing processing on the image data of the photograph, the edge is erased, so that the halftone conversion unit 263 has the attribute flag The edge flag in is deleted. When the edge is erased and the edge flag is deleted by performing various types of image processing such as filter processing and smoothing processing in this way, the image correction unit 267 reduces the edge effect and sweeping effect described later. The correction process is not performed.

  The condition determination unit 264 determines a correction condition in accordance with environment information acquired by the environment detection device 180 and device state information indicating the state of the image forming apparatus from the CPU 210. The parameter setting unit 265 sets a correction parameter (information for specifying a pixel width to be corrected) as preprocessing of correction processing for reducing the edge effect and the sweeping effect based on the correction condition. Details of the correction condition determination processing by the condition determination unit 264 and the correction parameter setting processing by the parameter setting unit 265 will be described later.

  Based on the attribute flag and the correction parameter, the image analysis unit 266 specifies a pixel that can generate an edge effect and a sweeping effect from the image data on the image memory 231, and determines the specified pixel as a correction target pixel. Determine as. Here, the edge effect and the sweeping effect are easily recognized when the image density exceeds a certain value. The edge effect occurs at the edge of the image area, and the sweeping effect occurs at the rear end of the image area in the electrostatic latent image transport direction. Therefore, the image analysis unit 266 specifies the pixels (correction target pixels) that can generate the edge effect and the sweeping effect in consideration of these points. Details of the correction target pixel specification and determination processing in the image analysis unit 266 will be described later with reference to the flowchart of FIG.

  The image correction unit 267 obtains a correction amount that can reduce the toner consumption for each pixel specified as a correction target pixel with respect to the edge effect and the sweeping effect as a result of the image analysis by the image analysis unit 266 described above. Specifically, the image correction unit 267 calculates a toner reduction amount for each pixel specified as the correction target pixel, and uses a correction coefficient (exposure reduction ratio) for each pixel corresponding to the toner reduction amount as a correction amount. Ask. Then, the image correction unit 267 performs a correction process for correcting the pixel density according to the correction coefficient for the correction target pixel specified as described above in each pixel of the raster data. The raster data after the correction processing is performed on the correction target pixel by the image correction unit 267 is sent to the exposure control unit 250.

  The exposure control unit 250 generates a drive signal for performing PWM control corresponding to each pixel based on the raster data after the correction processing is performed by the image correction unit 267 and sends the drive signal to the exposure apparatus 130. In addition, a light amount adjustment signal for setting a target light amount for each pixel is supplied from the exposure amount adjustment unit 240 to the exposure apparatus 130 at this time. Thereby, in the exposure apparatus 130, the emission time of the semiconductor laser is controlled by PWM control according to the drive signal. At this time, for the correction target pixel of the raster data, exposure with the exposure amount adjusted according to the exposure amount reduction ratio (correction coefficient) described above is performed with respect to the target light amount. Therefore, in the image forming apparatus 100, exposure amount adjustment (exposure amount reduction) that reduces the edge effect and sweeping effect is realized, and an increase in toner consumption due to the edge effect and sweeping can be efficiently reduced. become.

<Control of exposure apparatus>
Hereinafter, how the exposure amount in the exposure apparatus 130 is adjusted by PWM control using the light amount adjustment signal and the drive signal will be described with reference to FIG.
The exposure amount adjustment unit 240 has an integrated circuit (IC241) including a digital analog converter (DAC2021) and a regulator (REG2022) corresponding to 8 bits, and generates the light amount adjustment signal described above to expose the exposure apparatus 130. To send. The exposure apparatus 130 includes a VI conversion circuit 131 that converts voltage (V) into current (I), a laser driver IC 132, and a semiconductor laser 133.

  The IC 241 in the exposure amount adjustment unit 240 adjusts the voltage VrefH output from the REG 2022 based on the base signal indicating the drive current of the semiconductor laser 133 set by the CPU 210 in the controller 140, whereby the current base signal To the voltage signal. Specifically, the voltage VrefH is a reference voltage of the DAC 2021, and by adjusting the voltage VrefH by the base signal, the DAC 2021 outputs a light amount adjustment analog voltage 71 as a light amount adjustment signal corresponding to the base signal.

The VI conversion circuit 131 of the exposure apparatus 130 converts the light amount adjustment signal (light amount adjustment analog voltage 71) received from the exposure amount adjustment unit 240 into a current value Id and outputs it to the laser driver IC 132. Thereby, the semiconductor laser 133 emits light having a light amount corresponding to the light amount adjustment signal. Here, the IC 241 mounted on the exposure adjustment unit 240 outputs a light amount adjustment signal. However, the DAC 2021 is mounted on the exposure device 130 and a light amount adjustment signal is generated in the vicinity of the laser driver IC 132. It may be a configuration. The laser driver IC 132 switches the switch SW in accordance with the drive signal output from the exposure control unit 250. The switch SW performs ON / OFF control of light emission of the semiconductor laser 133 by switching between flowing the current IL to the semiconductor laser 133 or the dummy resistor R1 in accordance with the drive signal. Thereby, the light emission time of the semiconductor laser 133 is controlled by the PWM control according to the drive signal, and the exposure amount in the exposure apparatus 130 is adjusted.
<Image density control>
The pixel density control by the exposure device 130 will be described.
FIGS. 4A and 4B are diagrams for explaining how the pixel density is adjusted by controlling the exposure amount by PWM (pulse width modulation) control in the exposure apparatus 130. Each of SN01 to SN05 in FIG. 4A represents one pixel. FIG. 4A shows an image formed by dividing each of the pixels SN01 to SN05 into N (N is a natural number of 2 or more) subpixels and not exposing (thinning out) some of the subpixels. An example is shown. In FIG. 4A, the black portion in the drawing represents the exposed subpixel, and the white portion represents the unexposed (thinned out) subpixel. FIG. 4B shows the pixel density corresponding to each of the pixels SN01 to SN05. The pixel SN01 is 100%, the pixel SN02 is 75%, the pixel SN03 is 50%, the pixel SN04 is 75%, and the pixel SN05 is 87. The pixel density is 5%. In FIG. 4B, the pixel density is represented by a dot pattern for convenience. In the pixel density control for each of the pixels SN01 to SN05, the exposure control unit 250 thins out the light amount of 100% with respect to the target light amount by PWM control based on the drive signal (not to expose some subpixels). To be realized. For example, in the example in which one pixel is divided into 16 subpixels, when realizing the density of 50% indicated by the pixel SN03, for example, by driving the semiconductor laser 133 so as to expose only odd-numbered subpixels, Exposure is performed with a light amount of 50% with respect to 100% of the target light amount. Thereby, an image having a density of 50% like the pixel SN03 can be expressed.

  In this embodiment, the light amount adjustment is performed by pulse width modulation. For example, the exposure amount adjustment unit 240 in FIG. 3 converts the image data into a base signal and changes the light amount adjustment analog voltage, thereby changing the target. You may adjust exposure amount for every pixel on the basis of light quantity.

<Two types of development status>
Hereinafter, two types of development states observed in the developing device 150 will be described. 5A and 5B are explanatory diagrams of two types of development states. FIG. 5A shows a jumping development state, and FIG. 5B shows a contact development state.
In the jumping development state shown in FIG. 5A, the developing roller 151 and the photosensitive drum 110 are generated in the developing region R which is the closest part between the developing roller 151 and the photosensitive drum 110 which are maintained in a non-contact state. Development is performed by the development voltage applied between them. Here, the developing voltage indicates an AC bias voltage on which a DC bias is superimposed. The developing device 150 in the jumping development state has a gap GP between the developing roller 151 and the photosensitive drum 110 at the development position. If the gap GP is too small, leakage from the developing roller 151 to the photosensitive drum 110 is likely to occur, and it becomes difficult to develop the latent image. On the other hand, if the gap GP is too large, it is difficult for the toner to fly to the photosensitive drum 110. Therefore, the gap GP may be designed to be maintained in an appropriate size by an abutment roller (not shown) rotatably supported on the shaft of the developing roller 151.

  The contact development state shown in FIG. 5B is the development region R that is the closest part in a state where the photosensitive drum 110 and the developing roller 151 are in contact with each other, and between the developing roller 151 and the photosensitive drum 110. The development is performed by the development voltage (DC bias) applied to. 5A and 5B, the photosensitive drum 110 and the developing roller 151 rotate in the forward direction at different peripheral speeds. A DC voltage is applied as a developing voltage between the photosensitive drum 110 and the developing roller 151, and the polarity of the developing voltage is set to the same polarity as the charging potential of the surface of the photosensitive drum 110. Then, the toner thinned on the developing roller 151 is conveyed to the developing region R, and the electrostatic latent image formed on the surface of the photosensitive drum 110 is developed.

<Generation principle of edge effect and sweeping effect>
Hereinafter, the generation principle of the edge effect and the sweeping effect will be described.
First, the principle of edge effect generation will be described. As described above, the edge effect means that the electric field concentrates on the boundary between the exposed portion (electrostatic latent image) formed on the photosensitive drum 110 and the non-exposed portion (charged portion), thereby causing an edge of the image. This is a phenomenon in which toner adheres excessively. FIG. 6 is an explanatory diagram of the edge effect. As shown in FIG. 6, the electric field lines 601 from the non-exposed part on both sides of the exposed part wrap around the edge of the exposed part, so the electric field strength at the edge is stronger than the center of the exposed part. Become. As a result, more toner adheres to the edge portion than the center of the exposed portion.

  FIG. 7A is a diagram illustrating an example of an image in which an edge effect has occurred. In FIG. 7A, the downward arrow indicates the conveyance direction of the recording medium on which the image 700 is formed (the rotation direction of the photosensitive drum 110, the so-called sub-scanning direction). It is assumed that the image data that is the basis of the image 700 is image data having a uniform density as a whole. When the edge effect occurs, the toner concentrates and adheres to the edge portion 702 of the image 700. As a result, the density of the edge portion 702 becomes higher than that of the non-edge portion 701.

  FIG. 8A is a diagram illustrating a toner distribution state in the image 700 as a toner distribution curve. In FIG. 8A, a right-pointing arrow indicates the conveyance direction (sub-scanning direction) of the recording medium on which the image 700 is formed. The toner adhesion amount at the edge portion 802 downstream in the transport direction and the edge portion 803 upstream in the transport direction is larger than that of the non-edge portion 801, and the density is increased accordingly. Further, the toner in the edge portions 802 and 803 is excessive, which contributes to an increase in toner consumption. As described above, when the electric field concentrates on the edge portions 802 and 803, a phenomenon that toner adheres excessively to the edge portions 802 and 803 occurs. This edge effect is noticeable in the above-described jumping development state. In the contact development state, since the gap GP between the developing roller 151 and the photosensitive drum 110 is extremely short as described above, an electric field is generated from the photosensitive drum 110 toward the developing roller 151, and the edge is developed. This is because electric field concentration on the portions 802 and 803 is alleviated.

Next, the principle of generating the sweeping effect will be described. As described above, the sweeping effect is a phenomenon in which toner concentrates on the edge of the rear end portion of the image on the photosensitive drum 110. This sweeping effect is noticeable in the contact development state. This will be described in detail below.
FIG. 7B is a diagram illustrating an example of an image having a sweeping effect. In FIG. 7B, the downward arrow indicates the conveyance direction (sub-scanning direction) of the recording medium on which the image 710 is formed. Similar to the image 700 described above, it is assumed that the image data that is the basis of the image 710 is data of an image having a uniform density as a whole. When the sweeping effect occurs, the toner concentrates and adheres to the rear end portion 712 among the edges of the image 710. As a result, the density of the rear end portion 712 is higher than that of the non-edge portion 711.

  FIG. 8B is a diagram illustrating a toner distribution state in the image 710 as a toner distribution curve. In FIG. 8B, a right-pointing arrow indicates the conveyance direction (sub-scanning direction) of the recording medium on which the image 710 is formed. The toner adhesion amount at the edge portion 812 (the rear end portion 712 in FIG. 7B) on the downstream side in the transport direction is larger than that in the non-edge portion 811 (the non-edge portion 711 in FIG. 7B), and that much. The concentration will also increase. Further, the toner at the rear end portion of the edge portion 812 (the rear end portion 712 in FIG. 7B) on the downstream side in the transport direction is excessive, which contributes to an increase in toner consumption.

  FIG. 9A to FIG. 9C are diagrams for explaining the generation mechanism of the sweeping effect in the contact development state. In the contact development state, the peripheral speed of the developing roller 151 is faster than the peripheral speed of the photosensitive drum 110 so that the height of the toner on the photosensitive drum 110 becomes a predetermined height. As a result, the toner can be stably supplied to the photosensitive drum 110, and the image density is maintained at a target density. As shown in FIG. 9A, in the development region R, the electrostatic latent image 900 is developed by the toner 901 conveyed by the developing roller 151. In addition, since the developing roller 151 rotates faster with respect to the photosensitive drum 110, the positional relationship between the two surfaces is always shifted. When the rear end portion of the electrostatic latent image 900 enters the developing region R, the toner 901 on the developing roller 151 is in the drawing at the rear end portion of the electrostatic latent image 900 in the rotation direction from the starting position of the developing region R. It is located behind the toner 902 in which shading is drawn. Thereafter, as shown in FIG. 9B, the toner 901 on the developing roller 151 overtakes the toner 902 at the rear end until the toner 902 at the rear end exits the development region R. Then, as shown in FIG. 9C, the toner 901 is supplied to the toner 902 at the rear end portion of the electrostatic latent image 900 and adheres as the toner 903, so that the development amount at the rear end portion increases. This is the generation mechanism of the sweeping effect.

<Exposure amount adjustment processing to reduce edge effect and sweeping effect>
In the present embodiment, a process for adjusting the exposure amount by correcting raster data for forming an electrostatic latent image to reduce the edge effect and the sweeping effect will be described.
The image processing unit 260 performs preprocessing for adjusting the exposure amount. This pre-processing is executed by the CPU 210 controlling the image processing unit 260 according to a program. Hereinafter, this will be described in detail with reference to FIG. Here, correction of raster data generated by the rendering processing unit 200 and stored in the image memory 231 from image data sent from the host computer 10 will be described as an example.

  In the image processing unit 260, device state information indicating the state of the image forming apparatus 100 is input to the condition determining unit 264. Here, the apparatus status information includes information on the processing capability of the image forming apparatus 100, information on the process speed of the printer engine, and information on the surrounding environment such as the temperature and humidity inside and outside the image forming apparatus 100 acquired by the environment detection apparatus 180. included. In the apparatus status information, the photosensitive drum 110 predicted from the total number of output sheets (total number of printed sheets and total number of operating sheets) separately obtained in the controller 140 and the total operating time (cumulative time when printing is performed). Information indicating the durability of the member such as toner and toner is also included. The condition determination unit 264 determines a correction condition according to the received apparatus state information. Information about the determined correction condition (hereinafter referred to as correction condition information) is input to the parameter setting unit 265. Based on the received correction condition information, the parameter setting unit 265 sets a predetermined pixel width (the number of pixels from the edge, that is, the distance from the edge) to be subjected to correction processing for reducing the edge effect and the sweeping effect. And set as a correction parameter.

  Here, the correction parameter is changed according to the apparatus status information such as the processing capability of the image forming apparatus 100, the process speed of the printer engine, the ambient temperature and humidity of the image forming apparatus 100, the total number of output sheets, and the total operating time. There is a need. For this reason, the condition determination unit 264 uses, for example, the process speed of the printer engine that changes according to the type of paper to be printed, the processing capacity of the apparatus, the total number of output sheets and the total operating time, the ambient environment temperature and humidity as correction conditions. decide. For example, it is conceivable that the toner distribution curve in the vicinity of the edge portion of the printed image changes as the process speed of the printer engine changes according to the type of paper to be printed. Note that the toner distribution curve in the vicinity of the edge portion of the image is the curve as described with reference to FIGS. 8A and 8B. For example, if the process speed is high, the height of the overshoot portion of the toner distribution curve near the edge portion is reduced and the width is widened. Therefore, the distance from the edge (the pixel width to be corrected) is increased and increased. It is considered that the toner reduction amount with respect to the distance needs to be reduced. Similarly, the toner distribution curve in the vicinity of the edge portion becomes dull depending on the total number of operating sheets and the total operating time, so it is considered that the amount of toner reduction needs to be reduced. On the other hand, the electrostatic effect may be increased depending on the ambient temperature and humidity. In this case, the edge effect may be increased. Therefore, the distance from the edge is increased, and the toner reduction amount is increased with respect to the extended distance. It is considered necessary. As described above, the condition determination unit 264 determines the correction condition, and the parameter setting unit 265 sets the correction parameter (information for specifying the pixel width to be corrected, that is, the distance from the edge) based on the correction condition information. Yes.

  The image analysis unit 266 generates an edge effect and a sweeping effect from raster data on the image memory 231 based on the attribute flag including the edge feature from the edge flag generation unit 261 and the correction parameter from the parameter setting unit 265. Identify possible pixels. Here, the edge effect and the sweeping effect described above are easily visible when the optical density of the pixel becomes larger than a certain value due to an increase in the toner adhesion amount. The edge effect occurs at the edge of the image area, and the sweeping effect occurs at the rear end portion of the image area in the sub-scanning direction. Therefore, the image analysis unit 266 specifies these correction target pixels in the raster data on the image memory 231 in consideration of these. Then, the image correction unit 267 performs correction processing for reducing the edge effect and the sweeping effect on the correction target pixel in the raster data, and sends the raster data after the correction processing to the exposure control unit 250. send. Thereby, the exposure control unit 250 generates a drive signal for the exposure apparatus 130 based on the raster data after the correction processing, and the exposure apparatus 130 adjusts the exposure amount by PWM control based on the drive signal. As a result, the image forming apparatus 100 can efficiently reduce the toner consumption due to the edge effect and the sweeping effect.

  FIG. 10 shows a flowchart of processing when the image analysis unit 266 specifies the target pixel as the correction target pixel based on the attribute flag and the correction parameter. In the following description, steps S101 to S107 of each process in FIG. 10 are abbreviated as S101 to S107. The processing of the flowchart of FIG. 10 is performed by the image analysis unit 266, but may be realized by the CPU 210 executing the image forming processing program of the present embodiment.

  In the flowchart of FIG. 10, in S101, the image analysis unit 266 determines whether or not the pixel density (pixel value) of a certain target pixel in the raster data image is equal to or higher than a predetermined threshold. It is determined whether or not the pixel is a high density pixel. Specifically, the image analysis unit 266 determines whether or not the target pixel and a plurality of surrounding pixels centering on the target pixel are equal to or greater than a predetermined threshold value defined in advance. For example, when the density of the pixel value is represented by 8 bits, the predetermined threshold is, for example, a value of 250 out of 0 to 255 when the 8 bits are represented by a decimal number. In the case of this example, the image analysis unit 266 determines that the target pixel is a high-density pixel when the pixel values of the target pixel and the surrounding pixels have a value of 250 to 255. If the image analysis unit 266 determines in S101 that the pixel of interest is a high-density pixel, the image analysis unit 266 proceeds to S102 and the subsequent steps. If the image analysis unit 266 determines that the pixel of interest is not a high-density pixel, the image analysis unit 266 proceeds to S106. .

  Here, in the raster data image, for example, when the pixel value changes rapidly, the edge flag generation unit 261 detects the edge portion. When the pixel value of the target pixel is the value of the high density pixel or zero (0), pixels having the same density are continuously connected from the edge portion detected around the target pixel to the target pixel. It is thought that there is. On the other hand, when the pixel value of the target pixel is not the value of the high density pixel, it is considered that the high density pixel is not continuous from the target pixel to the edge portion, or the white area is continuous.

  Hereinafter, with reference to FIG. 11A to FIG. 11F, the relationship between the image and the edge, the attribute flag including the edge feature generated by the edge flag generation unit 261, and the edge portion when the edge is detected The pixel density from the target pixel to the target pixel will be described. FIG. 11A shows an image example in which an edge portion is included in an image of raster data input to the edge flag generation unit 261. FIG. 11A shows an example of an image 1100 having a size of 16 × 16 pixels, and each square in the lattice shape in the figure represents a pixel 1101, and diagonal lines and white lines in each square corresponding to each pixel 1101 are shown. Represents a pixel value (a shaded line has a high density and white has a low density). In this example, the edge flag generation unit 261 detects the edge portion of the image by applying, for example, a secondary differential filter to the raster data, and the feature (edge feature) of the detected edge portion is detected. Assume that an attribute flag is included. In the example of the image 1100 illustrated in FIG. 11A, the edge flag generation unit 261 includes an attribute flag including an edge feature for a portion where black squares are continuous as illustrated in FIG. Binary data ("1" edge flag 1104) is generated. FIG. 11B shows a reference window 1102 composed of 16 × 16 attribute flags 1105 respectively corresponding to 16 × 16 pixels of the image 1100 shown in FIG. Each square in the grid pattern of FIG. 11B represents the attribute flag 1105, and among the squares, the square represented by black indicates that the edge flag 1104 in the attribute flag 1105 is “1”. Represents. Further, an attribute flag 1103 in FIG. 11B indicates that the attribute flag corresponds to the target pixel. A reference window 1102 composed of 16 × 16 attribute flags 1105 as shown in FIG. 11B is a target of image analysis by the image analysis unit 266.

  As shown in FIGS. 11 (c) to 11 (f), the image analysis unit 266 includes a main scanning direction and a sub-scanning direction including the attribute flag 1103 of the target pixel from the reference window 1102 in FIG. 11 (b). The column of each attribute flag 1105 is cut out. Each of the arrows in FIGS. 11C to 11F represents a predetermined direction (hereinafter referred to as “reference direction”) that the image analysis unit 266 refers to the reference window 1102 when analyzing the image. .). In the following description of the present embodiment, the direction opposite to the reference direction is referred to as “reference reverse direction”. Note that the arrow direction in FIG. 11E corresponds to the main scanning direction, and the arrow direction in FIG. 11C corresponds to the sub-scanning direction. The image analysis unit 266 starts from the reference window 1102 in FIG. 11B for each of the four reference directions on the top, bottom, left, and right centered on the attribute flag 1103 of the target pixel as shown in FIGS. 11C to 11F. The column of the attribute flag 1105 is cut out. Then, the image analysis unit 266 performs the following image analysis using a flag string in which 16 attribute flags 1105 including the attribute flag 1103 of the target pixel in FIGS. 11C to 11F are connected. To identify pixels where edge effects and sweeping effects can occur.

  Returning to the flowchart of FIG. In step S102, the image analysis unit 266 determines whether there is an edge in the reference reverse direction from the attribute flag 1103 of the target pixel. If the image analysis unit 266 determines in S102 that there is an edge in the reference reverse direction from the attribute flag 1103 of the target pixel, the image analysis unit 266 proceeds to S103, whereas if it is determined that there is no edge in the reference reverse direction. Advances the process to S106.

  The determination process in S102 will be described using FIGS. 11C to 11F as examples. In the example of FIG. 11C in which the reference direction is the sub-scanning direction, in S102, the image analysis unit 266 determines whether there is an edge in the reference reverse direction as seen from the attribute flag 1103 of the target pixel. Here, in S102, whether or not there is an edge in the reference reverse direction as viewed from the attribute flag 1103 of the target pixel is determined by whether or not there is an SUM value (sum total) of the edge flag in each attribute flag 1105 in the reference reverse direction from the attribute flag 1103 of the target pixel. Judgment is made based on whether or not (value) is zero. When the SUM value is zero, no edge exists in the reference reverse direction from the target pixel. In the case of FIG. 11C, each attribute flag 1105 in the reference reverse direction as viewed from the attribute flag 1103 of the target pixel is not a black square indicating the presence of the edge flag of “1”, so the SUM value is zero. The image analysis unit 266 advances the process from S102 to S106. In the case of the example of FIG. 11D in which the reference direction is the reverse direction of the sub-scanning direction, in S102, the image analysis unit 266 determines whether there is an edge in the reference reverse direction from the attribute flag 1103 of the target pixel. In the example of FIG. 11D, the fourth attribute flag 1105 in the reverse direction from the attribute flag 1103 of the target pixel is a black square indicating the presence of the edge flag “1”. For this reason, the SUM value is not zero. Therefore, in this case, the image analysis unit 266 advances the process from S102 to S103. In the case of the example of FIG. 11E in which the reference direction is the main scanning direction, in S102, the image analysis unit 266 determines whether there is an edge in the reference reverse direction as seen from the attribute flag 1103 of the target pixel. In the case of the example in FIG. 11E, each attribute flag 1105 in the reference reverse direction as viewed from the attribute flag 1103 of the target pixel does not have a black square indicating the presence of the edge flag “1”, and the SUM value is zero. The image analysis unit 266 advances the process from S102 to S106. In the example of FIG. 11F in which the reference direction is the reverse direction of the main scanning direction, in S102, the image analysis unit 266 determines whether or not there is an edge in the reference reverse direction as seen from the attribute flag 1103 of the target pixel. . In the example of FIG. 11F, the sixth attribute flag 1105 in the reference reverse direction from the attribute flag 1103 of the target pixel is a black square indicating the presence of an edge flag of “1”, and the SUM value is Since it is not zero, the image analysis unit 266 advances the process from S102 to S103.

  In step S103, the image analysis unit 266 determines whether or not there are a predetermined number of edges or less in the reference direction from the attribute flag 1103 of the target pixel. If the image analysis unit 266 determines in S103 that there are not more than a predetermined number of edges in the reference direction from the attribute flag 1103 of the target pixel, the process proceeds to S104. On the other hand, if the image analysis unit 266 determines in S103 that there is no edge in the reference direction from the attribute flag 1103 of the target pixel or there are more than a predetermined number of edges, the process proceeds to S106. The predetermined number is a predetermined number of 1 or more. In the present embodiment, the predetermined number is, for example, “1”. Such a predetermined number is set because, for example, correction processing for reducing the edge effect and the sweeping effect is performed in an image having a pattern in which a portion in which a rapid change in pixel value occurs is repeated. This is to prevent this.

  The determination process in S103 will be described using FIGS. 11C to 11F as examples. In the example of FIG. 11C in which the reference direction is the sub-scanning direction, in S103, the image analysis unit 266 determines whether or not there are a predetermined number of edges or less in the reference direction as seen from the attribute flag 1103 of the target pixel. . In the case of the example in FIG. 11C, only the fourth attribute flag 1105 in the reference direction from the attribute flag 1103 of the target pixel is a black square indicating that the edge flag 1104 is “1”. The image analysis unit 266 advances the process from S103 to S104. In the example of FIG. 11D in which the reference direction is the reverse direction of the sub-scanning direction, in S103, the image analysis unit 266 determines whether there are a predetermined number or less edges in the reference direction as seen from the attribute flag 1103 of the target pixel. Judge. In the example of FIG. 11D, since there is no black square representing the presence of the edge flag 1104 of “1” in the reference direction from the attribute flag 1103 of the target pixel, the image analysis unit 266 proceeds from S103 to S106. Proceed with the process. In the example of FIG. 11E in which the reference direction is the main scanning direction, in S103, the image analysis unit 266 determines whether or not there are a predetermined number or less flags in the reference direction as seen from the attribute flag 1103 of the target pixel. . In the case of the example of FIG. 11E, the sixth flag in the reference direction from the attribute flag 1103 of the target pixel is a black square indicating that the edge flag 1104 is “1”. The unit 266 advances the process from S103 to S104. In the case of the example of FIG. 11F in which the reference direction is the reverse direction of the main scanning direction, in S103, the image analysis unit 266 determines whether there are a predetermined number or less edges in the reference direction as seen from the attribute flag 1103 of the target pixel. Judge. In the example of FIG. 11F, since there is no black square indicating the presence of the edge flag 1104 of “1” in the reference direction from the attribute flag 1103 of the target pixel, the image analysis unit 266 proceeds from S103 to S106. Proceed with the process.

  Note that there are cases where a plurality of edge flags “1” continue in the reference direction as viewed from the attribute flag 1103 of the target pixel. When a plurality of edge flags “1” are consecutive in this way, the image analysis unit 266 determines whether there are a predetermined number or less of edges in S103 described above, and the plurality of consecutive “1” edges. The edge flag is handled as one edge flag. For this reason, when determining whether or not there are a predetermined number of edges or less, the image analysis unit 266 fetches the attribute flag 1105 in order in the reference direction from the attribute flag 1103 of the target pixel, and the edge flag is “0”. Count the number of changes from "1" to "1". When the number of edge flags converted from “0” to “1” is equal to or less than the predetermined number, the image analysis unit 266 determines that there are equal to or less than the predetermined number of edges. For example, in the case of the example of FIG. 11C, when each attribute flag 1105 is fetched in order from the attribute flag 1103 of the target pixel in the reference direction, the edge flags of the attribute flags 1105 are arranged in the order of “00010000”. Accordingly, in the case of the example of FIG. 11C, the number of change points at which the edge flag has changed from “0” to “1” is one, so the image analysis unit 266 determines that there is one edge. To do. Here, it is assumed that a plurality of edge flags “1” are consecutively provided by filter coefficients at the time of edge extraction, and the edge flags of the attribute flags 1105 taken in order in the reference direction from the attribute flag 1103 of the pixel of interest. It is assumed that the order is “000111100”. Also in this case, since the number of change points at which the edge flag has changed from “0” to “1” is one, the image analysis unit 266 can determine that there is one edge. Note that it is desirable to set the number of each attribute flag 1105 to be captured when handling a plurality of consecutive “1” edge flags as one edge flag according to the characteristics and processing capability of the image forming apparatus 100. For example, if the number of captured attribute flags 1105 is too large, the pixel width to be corrected becomes too large and the processing load increases, so it is desirable to set the number in consideration of these. .

  Returning to the flowchart of FIG. In step S104, the image analysis unit 266 determines whether the distance from the attribute flag 1103 of the target pixel to the edge flag 1104 of “1” is within a predetermined distance range. Here, the value of the predetermined distance varies depending on the characteristics of the image forming apparatus 100 and the surrounding environment. In the present embodiment, information such as the characteristics of the image forming apparatus 100 and the surrounding environment is the apparatus state information described above, and corresponds to the correction conditions described above. Therefore, the value of the predetermined distance is prepared in advance for each correction condition described above and stored in the RAM 230 of the controller 140. The CPU 210 of the controller 140 sets a predetermined distance value corresponding to the correction condition to the image analysis unit 266 before executing the print job. As an example, when the predetermined distance is a distance corresponding to 4 pixels and FIG. 11C is taken as an example, the fourth edge flag 1104 from the attribute flag 1103 of the target pixel is “1”. The image analysis unit 266 determines that the distance is within the predetermined distance in S104. On the other hand, for example, in FIG. 11E, the edge flag 1104 of “1” is the sixth attribute flag 1103 of the target pixel, so the image analysis unit 266 is outside the predetermined distance range in S104. It is judged that. If the image analysis unit 266 determines in S104 that the distance between the attribute flag 1103 of the target pixel and the edge flag 1104 of “1” is within a predetermined distance range, the process proceeds to S105. On the other hand, if the image analysis unit 266 determines in S104 that the distance between the attribute flag 1103 of the target pixel and the edge flag 1104 of “1” is outside the predetermined distance range, the process proceeds to S106.

In S105, the image analysis unit 266 specifies that the target pixel corresponding to the attribute flag 1103 is a correction target pixel that is a target for toner reduction. Then, the image analysis unit 266 causes the RAM 230 to store information on the pixel position of the target pixel, the direction from the target pixel to the edge, and the distance from the target pixel to the edge. After S105, the image analysis unit 266 advances the process to S107.
When the process proceeds to S106, the image analysis unit 266 determines that the target pixel corresponding to the attribute flag 1103 is not a toner reduction target pixel. After S106, the image analysis unit 266 advances the process to S107.

  In S107, the image analysis unit 266 determines whether or not processing has been performed for all the directions described with reference to FIGS. 11C to 11F, and processing in any one direction is still performed. If it is determined that it is not, the process returns to S102. On the other hand, if the image analysis unit 266 determines in S107 that the process has been performed for all directions, the process of the flowchart of FIG. 10 ends.

  Here, four directions (up, down, left, and right) are given as an example as shown in FIGS. 11 (c) to 11 (f), but the direction is increased or decreased, for example, only two directions, eight directions, sixteen directions, and the like. May be. The two directions are, for example, the two upward and downward directions in FIG. 11C and FIG. In addition, the eight directions are, for example, eight directions obtained by adding four directions of 45 degrees obliquely to the four directions in the up, down, left, and right directions of FIGS. 16 directions including 8 directions for each. The image forming apparatus 100 according to the present embodiment sets each direction as described above based on one or both of the main scanning direction and the sub-scanning direction described above, and which of these directions is adopted. It can be determined according to the characteristics and processing capacity of the apparatus.

  Further, with respect to the size of the reference window 1102 in FIG. 11B and the cutout sizes in FIGS. 11C to 11F, the characteristics and processing capability of the image forming apparatus 100, the sweeping effect, and the edge effect are also obtained. Can be determined according to the range of As an example, the reference window 1102 can be reduced in the case of the image forming apparatus 100 with low processing capability, while the reference window 1102 can be increased in the case of the image forming apparatus 100 with high processing capability.

  FIGS. 12A to 11F are diagrams showing examples of images and edges that are different from the examples shown in FIGS. 11A to 11F. FIG. 12A shows an example of an image 1200 that includes an edge portion of the raster data image input to the edge flag generation unit 261, as in the example of FIG. FIG. 12B shows a reference window 1202 including attribute flags 1205 corresponding to the respective pixels 1201 of the image 1200 shown in FIG. 12A, as in the example of FIG. 11B. In the case of the image 1200 in FIG. 12A, a plurality of edge flags 1204 of “1” are generated as shown in FIG. 12 (c) to 12 (f) are similar to the examples of FIGS. 11 (c) to 11 (f), and the attribute flag of the target pixel is selected from the reference window 1202 in FIG. 12 (b). FIG. 10 is a diagram in which columns of attribute flags 1205 in the main scanning direction and sub-scanning direction including 1203 are cut out.

  FIG. 13 is a diagram showing the results of the analysis in S101 to S107 of FIG. 10 in the examples of FIGS. 11 (a) to 11 (f) and FIGS. 12 (a) to 12 (f) described above. is there. In FIG. 13, data A is an example of FIGS. 11 (a) to 11 (f), and data B is an example of FIGS. 12 (a) to 12 (f). The hatched cells in FIG. 13 indicate the conditions that cause the pixel of interest to be determined not to be subject to toner reduction in the flowchart of FIG. As shown in FIG. 13, for example, in the case of data A, a correction target pixel whose target pixel is targeted for toner reduction only when the reference direction is the direction (downward) shown in FIG. Identified as being. On the other hand, in the case of data B, it is determined that the pixel of interest is a pixel that is not subject to toner reduction in all examples of FIGS. 12 (c) to 12 (f). The image analysis unit 266 outputs the above-described determination result information to the image correction unit 267 for each pixel. That is, when the image analysis unit 266 determines that the target pixel is the correction target pixel, each piece of information on the position of the correction target pixel, the direction from the target pixel to the edge, and the distance from the target pixel to the edge Are transferred to the image correction unit 267 via the RAM 230. In the case of the example in FIG. 13, the image analysis unit 266 outputs an output value “4” indicating that the distance from the edge is four pixels as information on the distance in the correction target pixel in the example in FIG. The image is transferred to the image correction unit 267. On the other hand, when the image analysis unit 266 determines that the target pixel is a non-target pixel, for example, the image analysis unit 266 passes a value of “0” to the image correction unit 267 as the above-described distance information.

  A process in which the image correction unit 267 calculates the toner reduction amount from the determination result by the image analysis unit 266 described above will be described with reference to the flowchart of FIG. In the following description, steps S201 to S203 of each process in FIG. 14 are abbreviated as S201 to S203. The processing of the flowchart in FIG. 14 is performed by the image correction unit 267, but may be realized by the CPU 210 executing the image forming processing program of the present embodiment.

  In the flowchart of FIG. 14, the image correction unit 267 determines whether or not the target pixel is a correction target pixel that is a toner reduction target based on the determination result of the image analysis unit 266 as a process of S201. If the image correction unit 267 determines that the pixel of interest is a correction target pixel, the image correction unit 267 proceeds to S202. If the image correction unit 267 determines that the target pixel is not a correction target pixel, the image correction unit 267 ends the process of the flowchart in FIG.

  In step S202, the image correction unit 267 calculates an accurate distance from the target pixel determined to be the correction target pixel in step S201 to the edge. More specifically, the distance calculation here is a process of correcting a deviation between the actual edge position of the image and the position of the edge flag “1”. For example, when the edge flag generation unit 261 generates an edge flag, the actual edge position of the image differs from the position where the edge flag “1” is generated depending on the nature of the filter used for edge detection. Sometimes. For example, an edge flag generated by the edge flag generation unit 261 from an image 1509 in which an edge 1510 exists in a substantially central portion as shown in FIG. For example, when a first-order differential filter is used, the edge flag generation unit 261 has a first-order differential processing signal as shown in FIG. 15A having a peak corresponding to the position of the edge 1510 of the image 1509 in FIG. 1501 is generated. In this case, as shown in FIG. 15D, an edge flag 1511 of “1” corresponding to the position of the edge 1510 of the image 1509, for example, with a single pixel width 1520 is generated. On the other hand, for example, when a second-order differential filter is used, as shown in FIG. 15B, there are peaks at two positions inside and outside the edge 1510 of the image 1509 in FIG. Such a second derivative processing signal 1502 is generated. Then, based on the comparison between the second-order differential processing signal 1502 and the two threshold values 1503 and 1504, one pixel is provided on each of the inner side and the outer side across the edge 1510 as shown in FIGS. 15 (e) and 15 (f). Edge flags 1512 and 1513 of “1” having widths 1521 and 1522 are generated. Note that the edge flag 1512 is an outer edge flag generated at a position separated by one pixel on the outer side with respect to one pixel width 1520 including the edge 1510 of FIG. The edge flag 1513 is an inner edge flag generated at a position separated by one pixel on the inner side with respect to one pixel width 1520 including the edge 1510 of FIG. For this reason, the image correcting unit 267 calculates the distance from the target pixel to the edge according to the filter characteristics of the edge flag generating unit 261 when the secondary differential filter is used in the edge flag generating unit 261. The distance from the target pixel to the edge is corrected. For example, when the edge flag 1512 indicating the outer edge is set to be generated at a position one pixel outside the one pixel width 1520 including the edge 1510 in FIG. Correction is performed such that the distance of one pixel is subtracted from this distance. Further, when the edge flag 1513 indicating the inner edge is set to be generated at an inner position by one pixel with respect to the one pixel width 1520 including the edge 1510 in FIG. Correction is performed such that the distance of one pixel is added from the distance up to. As a result, the corrected distance matches the distance from the target pixel to the actual edge. After S202, the image correction unit 267 advances the process to S203.

  In S203, the image correction unit 267 calculates a toner reduction amount based on the distance calculated (corrected) in S202. Specifically, the image correction unit 267 corresponds to the excessive toner amount corresponding to the height of the overshoot portion of the toner distribution curve shown in FIGS. 8A and 8B described above as the toner reduction amount. Calculate the reduction amount. FIG. 16 is a diagram used for explaining the correspondence between the distance from the target pixel to the edge and the toner reduction amount. In FIG. 16, for example, when the distance from the lower edge to the target pixel 1601 of the image 1600 is expressed in pixel order as “1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12”, The toner reduction amount is “1, 2, 3, 4, 5, 4, 3, 3, 2, 2, 1, 1” in pixel order. In this example, the toner reduction amount peaks at a distance corresponding to 5 pixels from the lower edge of the image 1600. In this example, since the target pixel 1601 is at a distance corresponding to the fourth pixel from the lower edge, the toner reduction amount on the lower edge side with respect to the target pixel 1601 is, for example, “4”. These toner reduction amount values are changed according to the above-described correction conditions such as the characteristics of the image forming apparatus 100 and the surrounding environment, and are stored in the RAM 230 of the controller 140 for each of the correction conditions. And The toner reduction amount values are set by the CPU 210 for the image correction unit 267 according to each correction condition before executing a print job. Similarly, for example, when the distance from the right edge of the image 1600 to the target pixel 1601 is represented by “1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12” in pixel order, toner The reduction amount is “1, 2, 3, 4, 5, 5, 4, 3, 2, 2, 1” in pixel order. In this example, the target pixel 1601 is at a distance corresponding to the sixth pixel from the right end edge, and the toner reduction amount on the right end edge side with respect to the target pixel 1601 is, for example, “5”. Then, the image correction unit 267 calculates the toner reduction amount of the target pixel 1601 by adding the toner reduction amounts in the respective directions, and uses the correction coefficient (exposure amount reduction ratio) corresponding to the toner reduction amount to generate raster data. A density correction process is performed on the image and output to the exposure control unit 250.

  In the above-described embodiment, the example in which the controller 140 of the image forming apparatus 100 performs the correction process (and the pre-processing thereof) has been described, but the present invention is not limited to this. For example, the host computer 10 may perform the same processing and input corrected image data (raster data) to the image forming apparatus 100.

  As described above, according to the present embodiment, the image analysis unit 266 detects the toner reduction target pixel using the edge flag generated by the edge flag generation unit 261. In other words, the image analysis unit 266 performs analysis by holding a binary edge flag, not a multi-value pixel value, in a line buffer to form a reference window. The image correction unit 267 also calculates the toner reduction amount according to the distance from the target pixel to be reduced to the edge based on the binary edge flag instead of the multivalued pixel value. For this reason, the image forming apparatus 100 of the present embodiment can particularly reduce the circuit scale of the controller 140, and the controller 140 having the small circuit scale can reduce the consumption of excess toner due to the edge effect and the sweeping effect. Yes.

<Other embodiments>
The present invention supplies a program that realizes one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read and execute the program This process can be realized. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

  The above-described embodiments are merely examples of implementation in carrying out the present invention, and the technical scope of the present invention should not be construed as being limited thereto. That is, the present invention can be implemented in various forms without departing from the technical idea or the main features thereof.

  DESCRIPTION OF SYMBOLS 100 Image forming apparatus, 130 Exposure apparatus, 140 Controller, 210 CPU, 240 Exposure amount adjustment part, 250 Exposure control part, 260 Image processing part, 261 Edge flag generation part, 264 Condition judgment part, 265 Parameter setting part, 266 Image analysis Part, 267 Image correction part

Claims (16)

Generating means for generating, for each pixel of the image signal, a flag indicating whether the pixel is a pixel corresponding to an edge of the image;
Analysis means for specifying a correction target pixel whose density is to be corrected from the image based on the flag generated corresponding to each pixel of the image;
Correction means for correcting the density of the correction target pixel based on the distance from the correction target pixel to the edge;
An image signal processing apparatus comprising:   2. The correction unit according to claim 1, wherein the correction unit determines a toner reduction amount for the correction target pixel based on the distance, and corrects the density of the correction target pixel according to the toner reduction amount. Image signal processing device.   The image signal processing apparatus according to claim 2, further comprising execution means for causing the printing unit to print an image with a toner amount corresponding to the density of each pixel subjected to the correction. When the density of the target pixel in the image is equal to or higher than a predetermined threshold, the analysis unit refers to the flag corresponding to each pixel of the image and obtains a distance from the target pixel to the edge, When the distance from the target pixel to the edge is within a predetermined distance range, the target pixel is specified as the correction target pixel,
4. The correction unit according to claim 1, wherein the correction unit corrects the density of the correction target pixel based on a distance from the correction target pixel to the edge obtained by the analysis unit. 5. The image signal processing apparatus according to the item.   The said analysis means calculates | requires the said distance from the said attention pixel to the said edge with reference to the said flag respectively corresponding to each pixel which exists in the predetermined direction containing the said attention pixel. Image signal processing apparatus.   The analysis unit is configured to correspond to each pixel existing in the predetermined direction for at least one predetermined direction determined based on at least one of a main scanning direction and a sub-scanning direction when an image is printed. 6. The image signal processing apparatus according to claim 5, wherein the reference of the flag is performed.   7. The image signal processing apparatus according to claim 5, wherein the predetermined directions are four directions of up, down, left, and right with the target pixel having the density equal to or higher than the predetermined threshold as a center.   The analysis means has no change point at which the flag value changes in the direction opposite to the predetermined direction with respect to the target pixel, and the number of change points at which the flag value changes in the predetermined direction. 8. The image signal processing apparatus according to claim 5, wherein the target pixel is specified as a correction target pixel when the value is equal to or less than a predetermined number. 9.   9. The image signal processing apparatus according to claim 8, wherein the analysis unit detects a point where the flag is changed from 0 to 1 as the change point.   10. The image signal processing apparatus according to claim 4, wherein the correction unit performs the correction by the density according to a distance from the edge for each correction target pixel. 11. The generation unit generates the flag indicating whether or not the pixel is the edge based on a value after performing a predetermined filter process on the image signal,
The correction unit corrects the distance from the edge according to the filter processing used when generating the flag, and corrects the density of the correction target pixel based on the corrected distance. The image signal processing apparatus according to any one of claims 1 to 10.   The image signal processing apparatus according to claim 4, wherein the analysis unit sets the predetermined distance based on apparatus state information representing a state of the apparatus itself.   The device status information includes information on the processing capability of the device, information on the speed at which printing is performed, information on the total number of printed pages, information on the total operating time of the device, and information on the inside and outside of the device. 13. The image signal processing device according to claim 12, wherein the image signal processing device is at least one piece of information on the temperature of the device and information on the humidity inside and outside the device.   When the predetermined image processing for smoothing the edge is performed on the image area of the image signal after the flag is generated, the analysis unit is a pixel to be corrected based on the flag The image signal processing apparatus according to claim 1, wherein the image signal processing apparatus is not specified. A step of generating, for each pixel of the image signal, a flag indicating whether the pixel is a pixel corresponding to an edge of the image;
Analyzing means for identifying a correction target pixel whose density is to be corrected from the image based on the flag generated corresponding to each pixel of the image;
A correcting unit correcting the density of the correction target pixel based on a distance from the correction target pixel to the edge;
An image signal processing method comprising:   The program for functioning a computer as each means of the image signal processing apparatus of any one of Claims 1 thru | or 14.

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