JP2013257510A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus, even when printing density of a whole image is reduced, appropriately reproducing isolated pixel groups representing intermediate gradation and isolated points.SOLUTION: The image forming apparatus that exposes a photosensitive drum on the basis of image data to form an electrostatic latent image, develops the electrostatic latent image, and transfers the developed image to a recording medium, can adjust printing density by changing an exposure time T1 to be a reference. The image forming apparatus is configured to, when a reference exposure time T1 is set to a prescribed value or less, detect isolated points P1 and isolated pixel groups P2, expose regular printing pixels P3 other than the printing pixels detected as the isolated points P1 and the isolated pixel groups P2 for the reference exposure time T1, and expose the printing pixels detected as the isolated points P1 and the isolated pixel groups P2 for exposure times T2 and T3 that are longer than the reference exposure time T1.

Description

  The present invention relates to an image forming apparatus that forms an image on a recording medium using an electrostatic latent image formed by exposing a photoreceptor.

  As an image forming apparatus such as a printer or a multifunction device, an electrostatic latent image is formed on the surface of the photosensitive member by irradiating the photosensitive member with light, and the electrostatic latent image is developed by attaching toner to the surface. An apparatus for forming an image on a recording medium by transferring the image to a recording medium such as recording paper is known. An image to be printed is composed of two types of pixels (binary): a printing pixel (black pixel) to which toner is attached and a non-printing pixel (white pixel) to which toner is not attached. Dither processing and error diffusion processing are performed in order to represent an image having an intermediate gradation in binary. As a result, in the image to be printed, print pixels are scattered and print pixels are densely packed. And area.

  When printing such an image, for example, a print pixel having no print pixel (black pixel) in the periphery shown in FIG. 6A is generally called an isolated point, and the isolated point is compared with a portion where print pixels are densely packed. Therefore, it is difficult for toner to adhere. When the exposure amount (exposure time) of all the print pixels is the same, there is a problem that the isolated point is not reproduced (printed) because the toner is not sufficiently applied to the isolated point.

  In order to solve such a problem, for example, in Patent Documents 1 and 2, the isolated point is appropriately set by making the exposure amount for the isolated point relatively larger than the reference exposure amount of the normal print pixel that is not the isolated point. So-called isolated point emphasis control for reproducing (printing) is disclosed.

JP 2000-343744 A JP 2008-049694 A

  By the way, in the image forming apparatus that performs the isolated point emphasis control, it is considered that the density of the entire print image can be changed by changing the reference exposure amount in order to suppress the amount of toner used. It is done.

  However, when the print density is set thin and the reference exposure amount is set to a predetermined value or less, the isolated point is appropriately reproduced (printed) by the isolated point emphasis control, but FIGS. 7A, 7B and As shown in FIG. 7C, an isolated pixel group consisting of 2 to 4 points that are adjacent to each other and have no print pixels in the surrounding area is similar to the isolated point or is difficult to attach toner next to the isolated point, and expresses an intermediate gradation. There is a problem in that the isolated pixel group, which can be said to be an area, is not sufficiently filled with toner, and the intermediate gradation is not properly reproduced (printed).

  The present invention has been made paying attention to such problems, and its purpose is to provide isolated pixel groups and isolated points that represent intermediate tones even when the print density of the entire image is lowered. It is an object to provide an image forming apparatus that appropriately reproduces the above.

  In order to achieve the object, the present invention takes the following means.

That is, the image forming apparatus of the present invention provides a reference exposure in which a portion corresponding to each print pixel of the photoconductor is predetermined based on image data formed by the photoconductor and a plurality of print pixels and non-print pixels. An exposure control unit that forms an electrostatic latent image by exposing using an amount; a developing unit that develops the electrostatic latent image by attaching toner; a transfer unit that transfers the developed image to a recording medium; An image forming apparatus configured to change a print density by changing the reference exposure amount through the density setting unit.
The exposure control unit detects, as an isolated point, a print pixel having no print pixel within a predetermined comparison range around the plurality of print pixels when the reference exposure amount is set to a predetermined value or less, A print pixel group having a plurality of print pixels adjacent to each other and having no print pixel in the surrounding predetermined comparison range is detected as an isolated pixel group, and other than the isolated pixel and the print pixel detected as the isolated pixel group The normal print pixels are exposed with the reference exposure amount, and the print pixels detected as isolated points and isolated pixel groups are exposed with an exposure amount larger than the reference exposure amount.

  With this configuration, the isolated pixel group is exposed with an exposure amount larger than the reference exposure amount, which is the exposure amount of the normal print pixels other than the isolated pixel group and the isolated point, in the same manner as the isolated point. Even when the density is lowered, it is possible to appropriately reproduce (print) isolated pixel groups and isolated points expressing intermediate gradations.

  In order to reproduce (print) the isolated pixel group more appropriately by adjusting the degree of emphasis of the isolated pixel group according to the device characteristics and image data, the number of print pixels constituting the isolated pixel group can be changed. It is preferable that it is comprised.

  In order to reproduce (print) the isolated pixel group more appropriately by adjusting the degree of enhancement of the isolated pixel group according to the device characteristics and image data, the size of the surrounding predetermined comparison range can be changed. It is desirable that

  As a preferred specific configuration for mounting the exposure control unit, the exposure control unit is configured such that each pixel constituting the image data is any one of the isolated point, the print pixel belonging to the isolated pixel group, and the normal print pixel. Set by the pixel type discriminating unit for discriminating, the exposure value setting unit for setting the exposure value that associates the pixel type and the exposure amount, and the pixel type and the exposure value setting unit discriminated by the pixel type discriminating unit An exposure amount determination unit that determines the exposure amount of each pixel based on the exposure value, outputs print data indicating the exposure amount of each pixel, and an optical document that exposes the photoconductor based on the print data output from the exposure amount determination unit And a built-in unit.

  As described above, according to the present invention, not only an isolated point but also a print pixel group including a plurality of print pixels adjacent to each other and having no print pixel within a predetermined comparison range is set as an isolated print pixel group. Detects and emphasizes both isolated points and isolated print pixel groups, so that even if the overall print density is reduced, the isolated print pixel groups and isolated points that express intermediate tones can be properly reproduced (printed). Is possible. Therefore, it is possible to provide an image forming apparatus with improved print quality while suppressing the amount of toner used.

1 is a conceptual cross-sectional view schematically showing an internal configuration of an image forming apparatus according to an embodiment of the present invention. 1 is a conceptual block diagram illustrating an overall configuration of an image forming apparatus. The conceptual block diagram which shows the structure of FPGA in FIG. The conceptual diagram which shows typically the structure of the pixel discrimination | determination module in FIG. The conceptual block diagram which shows typically the structure of the LPH module in FIG. Explanatory drawing which shows an example of the pattern used when detecting an isolated point. Explanatory drawing which shows an example of the pattern used when detecting an isolated point. Explanatory drawing which shows an example of the pattern used when detecting an isolated point. Explanatory drawing which shows an example of the isolated pixel group comprised by two printing pixels. Explanatory drawing which shows an example of the isolated pixel group comprised by three printing pixels. Explanatory drawing which shows an example of the isolated pixel group comprised by four printing pixels. Explanatory drawing which shows an example of the pattern used when detecting the isolated pixel group comprised by two printing pixels. Explanatory drawing which shows an example of the pattern used when detecting the isolated pixel group comprised by two printing pixels. Explanatory drawing which shows an example of the pattern used when detecting the isolated pixel group comprised by two printing pixels. Explanatory drawing which shows the pattern used when detecting the isolated pixel group comprised by two printing pixels. Explanatory drawing which shows the pattern used when detecting the isolated pixel group comprised by three printing pixels. Explanatory drawing which shows the pattern used when detecting the isolated pixel group comprised by four printing pixels. The flowchart which shows typically the exposure control of this invention. Explanatory drawing which shows the operation result by exposure control of this invention.

  Hereinafter, an image forming apparatus (multifunction machine) according to an embodiment of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, the image forming apparatus 1 is a complex machine that forms an image on a recording sheet based on image data of a document read by a document reading unit 20, and its outer shape is defined by a main body housing 2. Yes. In the main body housing 2, a paper feeding unit 3, an image forming unit 4, a fixing unit 5 and the like are accommodated.

  The paper feed unit 3 includes a recording paper cassette 31 for storing recording paper, and stores recording media such as recording paper in a stacked state on a support plate 32 provided in the recording paper cassette 31. Be able to. The support plate 32 is rotatable around a support shaft 33 provided at one end thereof, and the end on the side opposite to the support shaft 33 is biased upward, whereby the uppermost recording is performed. The paper is pressed against the paper feed roller 34. When the paper feed roller 34 is rotated in this state, the recording paper stored in the recording paper cassette 31 is separated by the paper feed roller 34 and a separation pad (not shown) and formed inside the main body housing 2. One sheet is sent to the conveyance path 6 one by one.

  The recording paper sent from the recording paper cassette 31 to the conveyance path 6 is conveyed to the image forming unit 4 side through the conveyance path 6. The recording paper is temporarily stopped immediately before the image forming unit 4 by a registration roller 60 disposed at a position where the recording paper in the conveyance path 6 is sandwiched from both sides in the thickness direction, and an image is formed on the recording paper by the image forming unit 4. The image is sent to the image forming unit 4 in accordance with the timing.

  The image forming unit 4 includes a photosensitive drum 41, a charger 42, an optical writing unit 43, a developing device 44, a transfer device 45, and a cleaning device 46, and forms an image on recording paper by an electrophotographic method. A photosensitive member is formed on the surface of the cylindrical photosensitive drum 41, and the surface of the rotating photosensitive drum 41 is uniformly charged by the charger 42 during image formation. The optical writing unit 43 outputs light from an LED (Light-Emitting Diode) or a laser light source based on image data input from the outside, image data read by the document reading unit 20, and the like. In the present embodiment, an LED print head (LPH) is adopted for the optical writing unit 43. An electrostatic latent image is formed on the surface of the photosensitive drum 41 uniformly charged by the charger 42 by exposure with light output from the optical writing unit 43.

  The developing device 44 employs a non-magnetic one-component contact developing method, and includes a toner storage chamber 441, an agitator 442 provided in the toner storage chamber 441, a supply roller 443, and a developing roller 444 as a developer carrier. . The developing device 44 receives supply of toner from a toner cartridge 47 that contains toner. The agitator 442 rotates, whereby the toner is agitated in the toner storage chamber 441. Further, the supply roller 443 is also rotationally driven so that the toner is frictionally charged at the contact portion between the developing roller 444 and a regulating blade (not shown), and the toner is attracted to the surface of the developing roller 444.

  A toner image is formed on the surface of the photosensitive drum 41 on which the electrostatic latent image is formed by attaching toner supplied from the developing device 44. That is, the developing device 44, also called a developing unit, develops the electrostatic latent image by attaching toner to the photosensitive drum 41. A transfer unit 45, also called a transfer unit, transfers the developed image to a recording medium such as recording paper. The toner remaining on the surface of the photosensitive drum 41 after the transfer is removed by the cleaning device 46. The recording paper on which the toner image is transferred by the transfer unit 45 is conveyed to the fixing unit 5 through the conveyance path 6.

  The fixing unit 5 includes a heating roller 50 that is heated to a predetermined temperature, and a pressure roller 51 that contacts the heating roller 50 with a predetermined pressure. The recording paper on which the toner image has been transferred passes between the heating roller 50 and the pressure roller 51 to fix the toner, and is then discharged through the discharge roller 61 to a discharge tray formed outside the main body housing 2. 21 is discharged.

  FIG. 2 is a conceptual block diagram illustrating the overall configuration of the image forming apparatus according to the present embodiment. As shown in FIG. 2, the apparatus 1 mainly includes a CPU (Central Processing Unit) 70, an integrated chip set 71, a scanner unit 72 constituting an original reading unit 20, an FPGA (Field Programmable Gate Array) 73, and an optical writing unit. 43.

  The CPU 70 controls the operation of the apparatus 1 by executing a control program recorded in a flash ROM (Read Only Memory) 701 connected via the system bus 74. In addition to the CPU 70 and the flash ROM 701, an SRAM (Static Random Access Memory) 702, an SDRAM (Synchronous Dynamic Random Access Memory) 703, a MODEM 704, an operation panel unit 705, and an RTC (Real Time Clock) 706 are connected to the system bus 74. ing. The SRAM 702 stores data updated and referred to by the CPU 70. The MODEM 704 performs processing for modulating a transmission signal of data to be transmitted to an external facsimile apparatus through an NCU (Network Control Unit) substrate 707 and processing for demodulating a reception signal obtained by the NCU substrate 707. The SDRAM 703 stores image data obtained by the MODEM 704 or the like. The operation panel unit 705 is an operation unit operated by a person, and includes various keys that receive operations from the user and an LCD (liquid crystal display) that displays information to the user. The RTC 706 counts the date and time and outputs date and time data to the system bus 74 in response to a request from the CPU 70.

  The integrated chipset 71 communicates with a scanner unit 72 for reading a document, an FPGA 73 for driving an optical writing unit 43 for printing, and an external personal computer (hereinafter referred to as a personal computer) in accordance with a command from the CPU 70. It controls the driving of the network board 74 for transmitting and receiving image data to and from the device. Further, the integrated chip set 71 outputs image data to be printed to the FPGA 73 when a print instruction is issued from the CPU 70. The image data to be printed includes, for example, image data received from a communication device such as an external personal computer via the network board 74, image data indicating a document read by the scanner unit 72, image data received via the MODEM 704, and the like. Is mentioned. The integrated chip set 71 is connected to a second SDRAM 711 for storing image data.

  For example, as illustrated in FIG. 13, the image data is formed by arranging a plurality of print pixels (indicated by black circles) and a plurality of non-print pixels (indicated by white blanks) in a matrix. The print pixel is a pixel (black pixel) to which toner is attached. The non-printing pixel is a pixel (white pixel) to which toner is not attached. That is, the image data is binary image data. Dither processing and error diffusion processing are performed in order to represent an image having an intermediate gradation in binary. As a result, in the image to be printed, print pixels are scattered and print pixels are densely packed. And area. In order to attach the toner to the print pixels, a portion corresponding to the print pixels in the photosensitive drum 41 shown in FIG. 1 is exposed by the optical writing unit 43 using a predetermined reference exposure amount. In this embodiment, the exposure amount is defined by the length of the exposure time with the light amount being constant.

  The FPGA 73 shown in FIG. 2 determines the exposure amount (exposure time) of each pixel based on the image data, and outputs print data indicating the exposure amount of each pixel to the optical writing unit 43. That is, the FPGA 73 and the optical writing unit 43 form an electrostatic latent image by exposing a portion corresponding to each print pixel of the photosensitive drum 41 using a predetermined exposure reference amount based on the image data. An exposure control unit 80 is realized.

  When the user changes the print density, the CPU 70 detects this, and inputs a density change signal indicating the changed print density to the FPGA 73. Upon receiving the density change signal, the FPGA 73 changes the reference exposure amount to an exposure amount corresponding to the density change signal. In other words, the density setting unit 81 that changes the reference exposure amount is realized by the FPGA and each unit, and the print density can be changed by changing the reference exposure amount through the density setting unit 81.

  Since the ease of toner adhesion varies depending on the print pixel arrangement pattern, the exposure control unit 80 sets the exposure amount based on the reference exposure amount according to the print pixel arrangement pattern in order to properly attach the toner. Control to change. For example, as shown by a black circle in FIG. 6A, a print pixel having no print pixel within a predetermined surrounding range (for example, one dot) is called an isolated point. The isolated point has a characteristic that the toner is less likely to adhere compared to a portion where the print pixels are dense. Further, as indicated by black circles in FIGS. 7A, 7B and 7C, a print pixel group composed of 2 to 4 points having no print pixels within a predetermined range (for example, 1 dot) is called an isolated pixel group. For example, the isolated pixel group has the characteristic that the toner is less likely to adhere to the isolated point or similar to the isolated point. As shown in FIG. 13, if a print pixel having a print pixel within a predetermined range (for example, one dot) is called a normal print pixel, the normal print pixel includes an isolated point and an isolated pixel group. There is a characteristic that toner easily adheres. This characteristic causes a problem that isolated points and isolated pixel groups are not appropriately reproduced (printed) particularly when the print density is set low. This problem is particularly noticeable in the case of the non-magnetic one-component contact development method.

  Therefore, when the print density is set low, that is, when the reference exposure amount is set to a predetermined value or less, the exposure control unit 80 shown in FIG. 2 isolates isolated points and isolated points from a plurality of print pixels constituting the image data. A normal print pixel other than a print pixel detected as a pixel group is detected with a reference exposure amount (exposure time T1), and a print pixel detected as an isolated point and an isolated pixel group is exposed. The exposure is performed with an exposure amount (long exposure time) larger than the reference exposure amount.

  Specifically, as shown in FIG. 3, the exposure control unit 80 determines whether each pixel constituting the image data is an isolated point, a print pixel belonging to an isolated pixel group, or a normal print pixel. The pixel type discriminating unit 82, the exposure value setting unit 83 that sets an exposure value that associates the pixel type with the exposure amount, and the pixel type and exposure value setting unit 83 that are discriminated by the pixel type discriminating unit 82 are set. An exposure amount (exposure time) of each pixel is determined based on the exposure value, and an exposure amount determination unit 84 that outputs print data indicating the exposure amount of each pixel; and photosensitivity based on the print data output from the exposure amount determination unit 84 An optical writing unit 43 for exposing the body.

  Each of the above-described units 80 to 84 is realized by the following specific configuration.

  As illustrated in FIG. 3, the FPGA 73 includes a video module 730, an MPU (Micro Processor Unit) 731, a timing generator 732, an image module 733, a pixel determination module 734, and an LPH module 735. The MPU 731 performs serial communication with the CPU 70 and performs register access to the modules 730, 734, and 735. The MPU 731 sets an exposure value in which the pixel type and the exposure amount (exposure time) are associated with each other in the density reference value setting register 7351 of the LPH module 735 according to the density change signal from the CPU 70 (see FIG. 5). The timing generator 732 generates a one-line trigger signal and inputs it to the modules 733, 734, and 735. The video module 730 plays a role of relaying image data from the integrated chipset 71 to the image module 733. The image module 733 performs left / right print position adjustment processing, mask processing, and the like on the image data, and inputs the processed image data to the pixel determination module 734.

  The pixel determination module 734 determines the pixel type of each pixel based on the image data, and inputs a pixel type signal to the LPH module 735. As schematically illustrated in FIG. 4, the pixel determination module 734 includes a pixel data holding unit 7341 that holds pixel data to be compared that includes a plurality of D-flip-flops arranged in a matrix and a line memory. A pixel type determination circuit 7342 as a pixel type determination unit 82 that determines the pixel type of the target pixel by pattern matching that compares the pixel data to be compared with a preset pattern and outputs a pixel determination signal. .

  The pixel type determination circuit 7342 is configured to be able to change the size of a predetermined comparison range around the pixel of interest used when detecting an isolated point and an isolated pixel group in accordance with a detection matrix size switching signal input from the MPU 731. Has been. For example, in the discrimination of isolated points, the size of the main scanning and sub-scanning of the comparison range can be switched to any of 3 × 3 shown in FIG. 6A, 5 × 3 shown in FIG. 6B, and 7 × 3 shown in FIG. 6C. is there. The comparison range in the isolated pixel group can be switched to any one of the surrounding dots in the main scanning shown in FIG. 8A, the surrounding 2 dots in the main scanning direction shown in FIG. 8B, and the surrounding 3 dots in the main scanning direction shown in FIG. 8C. It is. The comparison range in the isolated pixel group is fixed at one peripheral dot in the sub-scanning direction.

  In addition, the pixel type determination circuit 7342 shown in FIG. 4 can change the number of print pixels constituting the isolated pixel group used when detecting the isolated pixel group in accordance with the pixel number switching signal input from the MPU 731. It is configured. The number of pixels can be any of 2 to 4 shown in FIGS. 7A, 7B, and 7C because the print pixels constituting the isolated pixel group are adjacent to each other. For example, when there are two isolated pixel groups, there are 8 patterns shown in FIG. 9, there are 12 patterns shown in FIG. 10 when there are 3 points, and there are 4 patterns shown in FIG. 11 when there are 4 points.

  The pixel type determination circuit 7342 shown in FIG. 4 stores a plurality of patterns in which the number of pixels constituting the isolated pixel group and the size of the comparison range are combined. For example, for the discrimination of isolated points, when the comparison range is 3 × 3, the pattern shown in FIG. 6A is used, and when the comparison range is 5 × 3, the pattern shown in FIG. 6B is used, and the comparison range is 7 In the case of × 3, the pattern shown in FIG. 6C is used. Further, when the comparison range is 1 dot in the main scanning, 1 dot in the sub scanning, and the number of pixels is 2 in the discrimination of the isolated pixel group, the pattern shown in FIG. 9 is used, and when the number of pixels is 3, When the pattern shown in FIG. 10 is used and the number of pixels is 4, the pattern shown in FIG. 11 is used. Even when the comparison range is 2 to 3 dots in the main scanning, a pattern in which the margin of the pattern is increased is used.

  As shown in FIG. 5, the LPH module 735 includes a density correction value output circuit 7351 that relays the pixel type signal, and a density reference value setting register in which an exposure value that associates the pixel type with the exposure amount (exposure time) is set. 7352 and an exposure amount determination unit 84 that determines the exposure amount (exposure time; STB (STroBe) time) of each pixel based on the pixel type and the exposure value, and outputs print data indicating the exposure amount of each pixel. A density calculation circuit 7353. The pixel type is indicated by a flag, and the exposure time is expressed by N-bit data. For example, the exposure time when the pixel type is “0; normal print pixel” is set to T1 (reference exposure time), the exposure time when the pixel type is “1; isolated point” is set to T2, and the pixel type is “ The exposure time for “2: isolated pixel group” is set to T3, and each exposure time is set so that the relationship of T1 <T2 and T1 <T3 is established.

  The operation of the image forming apparatus having the above configuration will be described. Here, an example in which the image data shown in FIG. 13 is printed will be described. It is assumed that the number of pixels in the isolated pixel group is two and the comparison range is set to one dot for both main scanning and sub scanning.

  As shown in FIG. 12, when the print density is set to be normal or higher than normal by the user (see ST0), and the standard exposure (standard exposure time T1) is set to a value larger than a predetermined value (ST1: NO reference), the exposure time of isolated points, isolated pixel groups, and normal printing pixels is set to the standard exposure time T1 (see ST2). A printing operation is performed in which toner is attached and developed, and the developed image is transferred to a recording sheet (see ST3).

  On the other hand, when the print density is set lower than usual by the user (see ST0 in FIG. 12) and the reference exposure amount (reference exposure time T1) is set to a predetermined value or less (see ST2 in FIG. 12: YES). As shown in FIG. 13, the isolated point P1 and the isolated pixel group P2 are detected from the image data by pattern matching (see ST4 in FIG. 12). The exposure time of the print pixel detected as the isolated point P1 is set to T2, and the exposure time of the print pixel detected as the isolated pixel group P2 is set to T3 (see ST5 in FIG. 12). The exposure time of the normal print pixels P3 other than the print pixels detected as the isolated point P1 and the isolated pixel group P2 is set to T1 (see ST6 in FIG. 12). Based on this setting, the photosensitive drum 41 is exposed by the optical writing unit 43, developed with toner attached thereto, and a printing operation for transferring the developed image onto a recording sheet is performed (see ST3 in FIG. 12).

As described above, the image forming apparatus according to the present embodiment prints each of the photosensitive drums 41 based on the photosensitive drum 41 that is a photosensitive member and the image data formed by a plurality of printing pixels and non-printing pixels. An exposure control unit 80 that exposes a portion corresponding to a pixel with a predetermined reference exposure amount (reference exposure time T1) to form an electrostatic latent image, and development that develops the electrostatic latent image by attaching toner. A developing device 44 as a part, a transfer unit 45 as a transfer part for transferring the developed image to a recording medium, and a density setting part 81 for setting a reference exposure amount (reference exposure time T1). An image forming apparatus 1 configured to change a print density by changing a reference exposure amount (reference exposure time T1),
When the reference exposure amount (reference exposure time T1) is set to a predetermined value or less, the exposure control unit 80 selects a print pixel having no print pixel within a predetermined comparison range around the plurality of print pixels as an isolated point P1. In addition, a print pixel group having no print pixels within a predetermined comparison range is detected as an isolated pixel group P2 from among a print pixel group including a plurality of print pixels adjacent to each other, and an isolated point P1 and an isolated pixel group P2 are detected. The normal print pixels P3 other than the print pixels detected as are exposed with the reference exposure amount (reference exposure time T1), and the print pixels detected as the isolated point P1 and the isolated pixel group P2 are used as the reference exposure amount (reference exposure time T1). The exposure is performed with a larger exposure amount (exposure times T2, T3).

  With this configuration, the isolated pixel group P2 is exposed with an exposure time T3 longer than the reference exposure time T1, which is the exposure time for normal print pixels other than the isolated pixel group P2 and the isolated point P1, as with the isolated point P1. Therefore, even when the overall printing density is lowered, it is possible to appropriately reproduce (print) the isolated pixel group P2 and the isolated point P1 expressing the intermediate gradation.

  Furthermore, in the present embodiment, the number of print pixels constituting the isolated pixel group P2 can be changed, so that the degree of enhancement of the isolated pixel group can be adjusted according to the characteristics of the device and the image data, and more appropriately It is possible to reproduce an isolated pixel group.

  Further, in the present embodiment, since the size of the surrounding predetermined comparison range can be changed, the degree of enhancement of the isolated pixel group can be adjusted according to the characteristics of the device and the image data, and the isolated pixel group can be more appropriately selected. Can be reproduced.

  In the present embodiment, the exposure control unit 80 determines whether each pixel constituting the image data is an isolated point P1, a print pixel belonging to the isolated pixel group P2, or a normal print pixel P3. The exposure value set by the determination unit 82, the exposure value setting unit 83 that sets the exposure value that associates the pixel type and the exposure amount, the pixel type determined by the pixel type determination unit 82, and the exposure value set by the exposure value setting unit 83 An exposure amount determining unit 84 that determines the exposure amount of each pixel based on the output amount, and outputs print data indicating the exposure amount of each pixel; and light that exposes the photosensitive drum 41 based on the print data output from the exposure amount determination unit 84 And a writing unit 43. According to this configuration, it is possible to incorporate control for emphasizing an isolated pixel group at a reduced cost into the conventional control for emphasizing only isolated points, and it is possible to provide a configuration that is easy to implement.

  As mentioned above, although embodiment of this invention was described based on drawing, it should be thought that a specific structure is not limited to these embodiment. The scope of the present invention is shown not only by the above description of the embodiments but also by the scope of claims for patent, and further includes all modifications within the meaning and scope equivalent to the scope of claims for patent.

  For example, in this embodiment, a photosensitive drum is used as the photosensitive member, but a photosensitive belt may be used.

  The specific configuration of each part is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

41 ... photosensitive drum (photosensitive member)
44. Developing device (developing part)
45. Transfer device (transfer section)
80 ... Exposure control unit 81 ... Density setting unit 82 ... Pixel type determination unit 83 ... Exposure value setting unit 84 ... Exposure amount determination unit T1 ... Reference exposure time (reference exposure amount)
T2, T3 ... exposure time P1 ... isolated point P2 ... isolated pixel group

Claims (4)

Based on image data formed by a photoconductor and a plurality of printing pixels and non-printing pixels, a portion corresponding to each printing pixel of the photoconductor is exposed using a predetermined reference exposure amount, and electrostatic latent image is obtained. An exposure control unit that forms an image; a developing unit that develops the electrostatic latent image by attaching toner; a transfer unit that transfers the developed image to a recording medium; and a density setting unit that sets the reference exposure amount An image forming apparatus configured to change a print density by changing the reference exposure amount through the density setting unit,
The exposure control unit detects, as an isolated point, a print pixel having no print pixel within a predetermined comparison range around the plurality of print pixels when the reference exposure amount is set to a predetermined value or less, A print pixel group having a plurality of print pixels adjacent to each other and having no print pixel in the surrounding predetermined comparison range is detected as an isolated pixel group, and other than the isolated pixel and the print pixel detected as the isolated pixel group The normal print pixels are exposed with the reference exposure amount, and the print pixels detected as isolated points and isolated pixel groups are exposed with an exposure amount larger than the reference exposure amount. Forming equipment.   The image forming apparatus according to claim 1, wherein the number of print pixels constituting the isolated pixel group is changeable.   The image forming apparatus according to claim 1, wherein the size of the surrounding predetermined comparison range is changeable.   The exposure control unit includes: a pixel type determining unit that determines whether each pixel constituting the image data is the isolated point, a print pixel belonging to the isolated pixel group, or the normal print pixel; An exposure value setting unit for setting an exposure value associated with an exposure amount; and a pixel type determined by the pixel type determination unit and an exposure value set by the exposure value setting unit to determine an exposure amount of each pixel; The exposure amount determination unit that outputs print data indicating the exposure amount of each pixel, and the optical writing unit that exposes the photosensitive member based on the print data output from the exposure amount determination unit. An image forming apparatus according to claim 1.

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