JP4779509B2 - Image forming method and image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus for forming an image by scanning a surface of a latent image carrier driven in the sub-scanning direction in a main scanning direction substantially orthogonal to the sub-scanning direction, and an image forming method in the apparatus. It is about.

  This type of image forming apparatus includes a latent image carrier, an exposure unit, and a developing unit, and forms a toner image on the latent image carrier as follows. In other words, the light source of the exposure unit is controlled based on the image data indicating the toner image, and the light beam from the light source is deflected in the main scanning direction by the deflector of the exposure unit while forming a light beam on the latent image carrier in a spot shape. By scanning, a latent image corresponding to the image data is formed on the surface of the latent image carrier. The latent image is developed with toner to form a toner image.

  Further, in order to reduce the size and speed of the deflector, it has been conventionally proposed to use the deflecting mirror surface as a deflector by vibrating the surface (see Patent Document 1). That is, in this apparatus, the deflection mirror supported by the torsion bar is vibrated, and the light beam emitted from the light source is reflected by the deflection mirror and reciprocally scanned on the surface of the latent image carrier.

JP 2002-182147 A (the third page and FIGS. 9 and 10)

  In such an image forming apparatus, the light beam from the light source can be scanned on the latent image carrier in both the forward and backward directions in the main scanning direction. However, in such an apparatus that uses a deflection mirror to scan spots in both the forward path and the backward path, there are the following problems in reproducing gradation using a dot concentration type threshold matrix. That is, in gradation reproduction using a dot concentration type threshold matrix, a single cell is formed using a plurality of pixels, and a pixel corresponding to a gradation level among a plurality of pixels of the cell is formed according to the threshold matrix. Only by performing exposure and development to form dots, gradation is realized by changing the size of a halftone dot constituted by such dots in accordance with the gradation level. That is, when the gradation level is low, a small number of pixels are exposed and developed to form small halftone dots, and when the gradation level is high, a large number of pixels are exposed and developed to form large halftone dots. Therefore, the gradation level to be realized is determined by the size of the halftone dots.

  However, in the image forming apparatus as described above, since the light beam is scanned in both directions in the main scanning direction while forming a light beam on the surface of the latent image carrier, the scanning pitch in the sub scanning direction is constant. In other words, the degree of spot overlap in the sub-scanning direction varies. In other words, the spot overlap in the sub-scanning direction increases at a narrow scanning pitch in the sub-scanning direction, whereas the spot overlap in the sub-scanning direction decreases at a wide scanning pitch in the sub-scanning direction. Therefore, even when forming halftone dots corresponding to the same gradation level, the gradation level is lowered because the halftone dots formed are small where the scanning pitch is narrow, whereas the halftone dots are formed where the scanning pitch is wide. Since the halftone dot is large, the gradation level becomes high. As a result, as will be described in detail later, when cells of even pixels of 4 or more are used in the sub-scanning direction, a halftone of the same gradation level is formed between both ends in the main scanning direction. There is a case where an image detrimental effect of density difference occurs.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems. A latent image is obtained by reciprocating scanning in the main scanning direction by a deflecting mirror surface while irradiating the surface of a latent image carrier driven in the sub-scanning direction in a spot shape. In the image forming apparatus for forming the image, even if the gradation reproduction is performed using the dot-concentrated threshold matrix for the cells of even pixels of 4 or more in the sub-scanning direction, the above-described image adverse effect is suppressed and the The purpose is to provide a technology that realizes accurate gradation reproduction.

  The image forming method according to the present invention includes a latent image carrier whose surface is driven in the sub-scanning direction, and a first beam in the main scanning direction that is substantially perpendicular to the sub-scanning direction with the light beam from the light source by the oscillating deflection mirror surface. A plurality of spot latent images are formed by irradiating the surface of the latent image carrier in a spot shape with a light beam scanned in the first direction. In addition, a latent image forming unit that forms a plurality of spot latent images by irradiating the surface of the latent image carrier with a light beam that scans in the second direction, and develops each of the plurality of spot latent images to form dots. In an image forming apparatus including a developing unit to be formed, an image is formed by expressing a gradation by forming a halftone dot by dots for a cell having a plurality of pixels using a dot concentration type threshold matrix. With image forming method In order to achieve the above object, a plurality of cells having an even number of four or more pixels in the sub-scanning direction are arranged adjacent to each other in the main scanning direction, and the cells are formed adjacent to each other in the main scanning direction. It is characterized in that cells adjacent to each other at at least one specific adjacent position among a plurality of adjacent positions are arranged so as to be shifted from each other by an odd number of pixels in the sub-scanning direction.

  The image forming apparatus according to the present invention has a latent image carrier whose surface is driven in the sub-scanning direction and a light beam from the light source in the main scanning direction substantially orthogonal to the sub-scanning direction by the oscillating deflection mirror surface. It is configured to be capable of scanning in both the first direction and the second direction opposite to the first direction, and a plurality of spot latent images are formed by irradiating the surface of the latent image carrier in a spot shape with a light beam scanned in the first direction. A latent image forming section that forms a plurality of spot latent images by irradiating the surface of the latent image carrier with a light beam that is formed and scanned in the second direction, and developing each of the plurality of spot latent images; An image forming apparatus including a developing unit that forms dots and using a dot-concentrated threshold matrix to express gradation by forming halftone dots with dots for cells having a plurality of pixels And the above purpose To achieve this, a plurality of cells having an even number of pixels of 4 or more in the sub-scanning direction are arranged adjacent to each other in the main scanning direction, and among the plurality of adjacent positions formed by the cells adjacent to each other in the main scanning direction. It is characterized in that at least one or more specific adjacent positions are arranged such that cells adjacent to each other are shifted from each other by an odd number of pixels in the sub-scanning direction.

  In the invention thus configured (image forming method and image forming apparatus), the first direction in the main scanning direction and the second direction opposite to the first direction while irradiating the light beam onto the latent image carrier in a spot shape. It is configured to be able to scan in both directions. For example, when forming a line latent image on the latent image carrier as shown in FIG. 6, the light beam is scanned in the first direction of the main scanning direction while being irradiated in a spot shape on the latent image carrier. A plurality of spot latent images are arranged along the main scanning direction to form a line latent image LI (+ X), while a light beam is scanned in a second direction opposite to the first direction to form a plurality of spot latent images. Line latent images LI (-X) are formed side by side along the main scanning direction. As a result, the line latent images LI (+ X) and LI (−X) are alternately formed in the sub-scanning direction.

  In the invention configured as described above, the light beam is reciprocated in the main scanning direction on the surface of the latent image carrier, and the surface of the latent image carrier is driven in the sub-scanning direction substantially orthogonal to the main scanning direction. is doing. Therefore, the scanning trajectory of the light beam on the latent image carrier (spot latent image formation trajectory) is, for example, shown by a one-dot chain line in FIG. 31, and the scanning pitch in the sub-scanning direction is not constant. Such non-uniformity of the scanning pitch in the sub-scanning direction is particularly noticeable near the end of the scanning locus in the main scanning direction. Therefore, as in the image forming method or the image forming apparatus according to the present invention, when gradation reproduction is performed using a dot-concentrated threshold matrix for a cell having an even number of pixels of 4 or more in the sub-scanning direction, main scanning is performed. In some cases, the halftone density is high at one end in the direction and the halftone density is low at the other end. The reason for this will be described in detail below.

  FIG. 8 shows a 1 × 4 cell (1 × 4 cell) of 1 pixel in the main scanning direction and 4 pixels in the sub scanning direction and a threshold matrix (1 × 4) having 1 to 4 threshold values corresponding to the 1 × 4 cell. FIG. 32 and FIG. 33 are diagrams for explaining the reason why the above-mentioned image adverse effect occurs. In this specification, a cell having N pixels in the main scanning direction and M pixels in the sub-scanning direction is referred to as an “N × M cell”, and a threshold matrix corresponding to the N × M cell is referred to as an “N × M threshold matrix. ". Here, the alternate long and short dash line in FIG. 32 indicates the scanning trajectory of the light beam on the latent image carrier (the formation trajectory of the spot latent image), the solid ellipse indicates the area exposed by the spot, and the thick solid rectangle indicates 1 ×. 4 cells are represented. Also, the reason for the occurrence of the image defect will be described using 1 × 4 cells for simplicity. First, the operation of the 1 × 4 threshold matrix will be described with reference to FIG. 8 before explaining the reason why the image adverse effect occurs using FIG.

  FIG. 8A is a diagram showing a 1 × 4 threshold matrix, and FIG. 8B is a diagram showing the growth of halftone dots in a 1 × 4 cell when the 1 × 4 threshold matrix is used. The 1 × 4 threshold value matrix shown in FIG. 8A is a dot concentration type threshold value matrix, and the size of the halftone dot is grown as the gradation level increases. Specifically, as the gradation level increases from 1 to 4, as shown in FIG. 8B, among the four pixels of the 1 × 4 cell, the hatched pixels are exposed according to each gradation level. As a result, the halftone dots grow. Then, the above-described image adverse effect when gradation reproduction is performed using such a 1 × 4 threshold matrix will be described with reference to FIGS. 32 and 33. FIG.

  32 and 33 show the case where a halftone of gradation level 2 is formed. In this case, a spot latent image is formed only on two pixels (two central pixels in FIG. 33) corresponding to the threshold values 1 and 2 among the four pixels constituting the 1 × 4 cell CL14. The scanning trajectory of the light beam corresponding to the spot latent image is relatively separated on the wide pitch side (left hand side in FIGS. 32 and 33), whereas on the narrow pitch side (right hand side in FIGS. 32 and 33). It is relatively close. For this reason, as shown in the “latent image formation” column of FIGS. 32 and 33, the pitch of the spot latent image formed in these pixels in the sub-scanning direction Y is wide on the wide pitch side, but narrow. Narrow on the pitch side. The two dots obtained by developing the latent image thus formed with toner are relatively separated on the wide pitch side as shown in the “halftone dot” column of FIG. 33, and the dot area ratio occupying the cell CL14 is large. It has become. In contrast, on the narrow pitch side, the two dots are relatively close, and the dot area ratio occupying the cell CL14 is smaller than that on the wide pitch side. Due to the difference in the pitch of the scanning locus in this way, “halftone dot” and “small dot” are formed even though halftone dots corresponding to the same gradation level are formed. Moreover, in the conventional apparatus in which the cells CL14 are simply arranged in the main scanning direction X, as shown in FIG. 32, the dot area ratio for the 1 × 4 cell CL14 at the left end is the largest, and the dot area ratio decreases as it goes to the right end. At the right end, the dot area ratio for the 1 × 4 cell CL14 is the smallest. Therefore, the density decreases in the direction from the left end to the right end (the arrow direction in FIG. 32) despite the formation of halftone dots corresponding to the same gradation level. As a result, a density difference occurs between the left end and the right end. In other words, an image detrimental effect occurs in which the halftone density is high at one end in the main scanning direction and the halftone density is low at the other end.

  In order to deal with such image defects, in the present invention, a plurality of cells having an even number of pixels of 4 or more in the sub-scanning direction are arranged adjacent to each other in the main scanning direction, and the cells are adjacent to each other in the main scanning direction. The cells adjacent to each other at at least one specific adjacent position among the plurality of adjacent positions to be formed are arranged so as to be shifted from each other by an odd number of pixels in the sub-scanning direction. Therefore, the phenomenon that the density decreases as it goes from the left end to the right end even though the halftone dots corresponding to the same gradation level are formed, and the density difference between the both ends in the main scanning direction is suppressed. It is possible to suppress the negative effects of image generation. The reason for this will be described using the example shown in FIG.

  In the example shown in FIG. 9, a specific adjacent position is provided for each adjacent position in the main scanning direction, and cells adjacent to each other at the specific adjacent position are arranged downstream of the upstream cells in the first direction in the main scanning direction. Are arranged so as to be shifted by one pixel in the sub-scanning direction. Therefore, for example, when a halftone of gradation level 2 is formed, the above-mentioned “halftone dot large” and “halftone dot small” appear alternately in the main scanning direction. Therefore, the phenomenon that the density decreases as it goes from the left end (left end in FIG. 9) to the right end (right end in FIG. 9) is suppressed. Therefore, it is possible to suppress the image adverse effect of density difference between both ends in the main scanning direction, and to realize good gradation reproduction.

A. Apparatus Configuration FIG. 1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. This image forming apparatus is a so-called tandem type color printer, and yellow (Y), magenta (M), cyan (C), and black (K) four-color photoconductors 2Y, 2M, and 2C as latent image carriers. 2K is provided in the apparatus main body 5. The apparatus forms a full-color image by superimposing the toner images on the photoreceptors 2Y, 2M, 2C, and 2K, or forms a monochrome image using only the black (K) toner image. That is, in this image forming apparatus, when a print command is given to the main controller 11 from an external device such as a host computer in response to an image formation request from the user, an engine controller is responded to the print command from the CPU 111 of the main controller 11. 10 controls each part of the engine unit EG to print an image corresponding to the print command on a sheet S such as copy paper, transfer paper, paper, and an OHP transparent sheet.

  The engine unit EG includes a charging unit, a developing unit, an exposure unit (latent image forming unit), and a cleaning unit corresponding to each of the four photoconductors 2Y, 2M, 2C, and 2K. As described above, for each toner color, there is provided a photoconductor (latent image carrier), a charging unit, a developing unit (developing unit), an exposure unit, and a cleaning unit, and an image forming unit that forms a toner image of the toner color. Is provided. The configuration of these image forming means (photosensitive member, charging unit, developing unit, exposure unit, and cleaning unit) is the same for all color components. Therefore, the configuration relating to yellow will be described here, and the other color components will be described. Are denoted by corresponding reference numerals, and description thereof is omitted.

  The photoreceptor 2Y is rotatably provided in the arrow direction (sub-scanning direction) in FIG. More specifically, the drive motor MT is mechanically connected to one end of the photoreceptor 2Y. The motor control unit 105 electrically connected to the drive motor MT controls the drive motor MT. As a result, the photoreceptor 2Y rotates.

  A charging unit 3Y, a developing unit 4Y, and a cleaning unit (not shown) are arranged around the photosensitive member 2Y driven in this way along the rotation direction. The charging unit 3Y is composed of, for example, a scorotron charger, and uniformly charges the outer peripheral surface of the photoreceptor 2Y to a predetermined surface potential by applying a charging bias from the charging control unit 103. The photosensitive member 2Y charged by the charging unit 3Y is irradiated with the scanning light beam Ly from the exposure unit 6Y (latent image forming unit) toward the outer peripheral surface. As a result, an electrostatic latent image corresponding to the yellow image data included in the print command is formed on the photoreceptor 2Y. Thus, the exposure unit 6Y operates in response to a control command from the exposure control unit 102Y. The configuration and operation of the exposure unit 6 (6Y, 6M, 6C, 6K) and the exposure control unit 102 (102Y, 102M, 102C, 102K) will be described in detail later.

  The electrostatic latent image formed in this way is developed with toner by the developing unit 4Y (developing unit). The developing unit 4Y contains yellow toner. When a developing bias is applied from the developing device controller 104 to the developing roller 41Y, the toner carried on the developing roller 41Y partially adheres to each surface portion of the photoreceptor 2Y according to the surface potential. As a result, the electrostatic latent image on the photoconductor 2Y becomes visible as a yellow toner image. As the developing bias applied to the developing roller 41Y, a DC voltage or a voltage obtained by superimposing an AC voltage on the DC voltage can be used. In particular, the photosensitive member 2Y and the developing roller 41Y are spaced apart from each other. In a non-contact development type image forming apparatus that develops toner by flying toner with a voltage waveform in which an alternating voltage such as a sine wave, a triangular wave, or a rectangular wave is superimposed on a direct current voltage in order to efficiently fly the toner Is desirable.

  The yellow toner image developed by the developing unit 4Y is primarily transferred onto the intermediate transfer belt 71 of the transfer unit 7 in the primary transfer region TRy1. The color components other than yellow are configured in the same manner as yellow, and magenta toner images, cyan toner images, and black toner images are formed on the photoreceptors 2M, 2C, and 2K, respectively, and primary transfer is performed. Primary transfer is performed on the intermediate transfer belt 71 in the regions TRm1, TRc1, and TRk1, respectively.

  The transfer unit 7 includes an intermediate transfer belt 71 stretched between two rollers 72 and 73, and a belt driving unit (not shown) that rotates the intermediate transfer belt 71 in a predetermined rotation direction R2 by rotating the roller 72. ). In addition, a secondary transfer roller 74 is provided at a position facing the roller 73 with the intermediate transfer belt 71 interposed therebetween. The secondary transfer roller 74 is configured to be brought into contact with and separated from the surface of the belt 71 by an electromagnetic clutch (not shown). Yes. When a color image is transferred to the sheet S, the primary transfer timing is controlled to superimpose the toner images to form a color image on the intermediate transfer belt 71, and the color image is taken out from the cassette 8 to be intermediate. The color image is secondarily transferred onto the sheet S conveyed to the secondary transfer region TR2 between the transfer belt 71 and the secondary transfer roller 74. On the other hand, when a monochrome image is transferred to the sheet S, only the black toner image is formed on the photoreceptor 2K, and the monochrome image is secondarily transferred onto the sheet S conveyed to the secondary transfer region TR2. In addition, the sheet S that has received the secondary transfer of the image in this way is conveyed toward the discharge tray portion provided on the upper surface portion of the apparatus main body via the fixing unit 9.

  The surface potential of each of the photoreceptors 2Y, 2M, 2C, and 2K after primary transfer of the toner image to the intermediate transfer belt 71 is reset by a neutralizing unit (not shown), and the toner remaining on the surface is further removed. Is removed by the cleaning unit, and then charged by the charging units 3Y, 3M, 3C, and 3K.

  In the vicinity of the roller 72, a transfer belt cleaner 75, a density sensor 76 (FIG. 2), and a vertical synchronization sensor 77 (FIG. 2) are arranged. Among these, the cleaner 75 can be moved toward and away from the roller 72 by an electromagnetic clutch (not shown). Then, the blade of the cleaner 75 is in contact with the surface of the intermediate transfer belt 71 that is stretched over the roller 72 while moving to the roller 72 side, and the toner that remains on the outer peripheral surface of the intermediate transfer belt 71 after the secondary transfer. Remove. The density sensor 76 is provided to face the surface of the intermediate transfer belt 71 and measures the optical density of the patch image formed on the outer peripheral surface of the intermediate transfer belt 71. Further, the vertical synchronization sensor 77 is a sensor for detecting the reference position of the intermediate transfer belt 71, and is a synchronization signal output in association with the rotational drive of the intermediate transfer belt 71 in the sub-scanning direction, that is, a vertical synchronization signal. It functions as a vertical sync sensor for obtaining Vsync. In this apparatus, the operation of each part of the apparatus is controlled based on the vertical synchronization signal Vsync in order to align the operation timing of each part and to superimpose toner images of each color accurately. Further, a color misregistration sensor 78 is disposed between the rollers 72 and 73, and detects the color misregistration amount of each color toner image.

  In FIG. 2, reference numeral 113 denotes an image memory provided in the main controller 11 for storing image data given from an external device such as a host computer via the interface 112, and reference numeral 106 is executed by the CPU 101. A ROM for storing calculation data, control data for controlling the engine unit EG, and the like, and a reference numeral 107 are RAMs for temporarily storing calculation results in the CPU 101 and other data. Reference numeral 108 denotes an FRAM (ferroelectric memory) for storing information on the usage status of each part of the engine.

  FIG. 3 is a main scanning sectional view showing a configuration of an exposure unit (latent image forming unit) provided in the image forming apparatus of FIG. The exposure unit 6Y (6M, 6C, 6K) has an exposure housing 61. A single laser light source 62Y is fixed to the exposure housing 61, and a light beam can be emitted from the laser light source 62Y.

  FIG. 4 is a diagram showing signal processing blocks in the image forming apparatus of FIG. The configurations and operations of the exposure unit 6 and the exposure control unit 102 will be described in detail below with reference to FIGS. The configuration of the exposure unit 6 and the exposure control unit 102 is the same for all color components, so the configuration relating to yellow will be described here, and the other color components will be denoted by corresponding reference numerals and description thereof will be omitted.

  The exposure unit 6Y (6M, 6C, 6K) has an exposure housing 61. A single laser light source 62Y is fixed to the exposure housing 61, and a light beam can be emitted from the laser light source 62Y. Then, the laser light source 62Y is ON / OFF controlled in accordance with the image signal as described below, and a light beam modulated in accordance with the image data is emitted from the laser light source 62Y. Hereinafter, a description will be given with reference to FIG.

  In this image forming apparatus, when an image signal is input from an external device such as the host computer 100, the main controller 11 performs predetermined signal processing on the image signal. The main controller 11 includes functional blocks such as a color conversion unit 114, an image processing unit 115, two types of line buffers 116A and 116B, a scanning mode switching unit 116C, a pulse modulation unit 117, a gradation correction table 118, and a correction table calculation unit 119. It has.

  In addition to the CPU 101, the ROM 106, the RAM 107, and the exposure control unit 102 shown in FIG. 2, the engine controller 10 detects a gradation characteristic that detects a gradation characteristic indicating the gamma characteristic of the engine unit EG based on the detection result of the density sensor 76. Part 123 is provided. In the main controller 11 and the engine controller 10, these functional blocks may be configured by hardware, or may be realized by software executed by the CPUs 111 and 101.

  In the main controller 11 to which the image signal is given from the host computer 100, the color conversion unit 114 converts the RGB gradation data indicating the gradation level of the RGB component of each pixel in the image corresponding to the image signal into the corresponding CMYK. Conversion into CMYK gradation data indicating the gradation level of the component. The CMYK gradation data output from the color conversion unit 114 is input to the image processing unit 115.

  The image processing unit 115 executes the following processing for each color component. FIG. 5 is a diagram showing the configuration of the image processing unit 115. The image processing unit 115 includes a gradation correction unit 1151 and a halftone processing unit 1152. The gradation correction unit 1151 performs gradation correction on the gradation data of each pixel input from the color conversion unit 114 to generate correction gradation data, and then the halftone processing unit 1152 performs the correction gradation data. Is subjected to halftoning processing to generate halftone gradation data. That is, the gradation correction of the gradation correction unit 1151 is performed by referring to the gradation correction table 118 registered in advance in the nonvolatile memory and inputting each pixel from the color conversion unit 114 according to the gradation correction table 118. The gradation data is converted into corrected gradation data indicating the corrected gradation level. The purpose of the gradation correction is to compensate for the change in the gamma characteristic of the engine unit EG configured as described above, and to keep the overall gamma characteristic of the image forming apparatus always ideal. In other words, in this type of image forming apparatus, the gamma characteristic of the apparatus varies from apparatus to apparatus, and even in the same apparatus, depending on the usage status. Therefore, in order to eliminate the influence of such gamma characteristic variations on the image quality, a gradation control process is executed to update the contents of the gradation correction table 118 based on the actual measurement result of the image density at a predetermined timing.

  In this gradation control process, for each toner color, a gradation patch gradation image prepared in advance for measuring the gamma characteristic is formed on the intermediate transfer belt 71 by the engine unit EG, and each gradation patch is obtained. The image density of the image is read by the density sensor 76, and based on a signal from the density sensor 76, the gradation characteristic detection unit 123 associates the gradation level of each gradation patch image with the detected image density ( The gamma characteristics of the engine unit EG are created and output to the correction table calculation unit 119 of the main controller 11. Then, the correction table calculation unit 119 compensates the actually measured gradation characteristic of the engine unit EG based on the gradation characteristic given from the gradation characteristic detection unit 123 to obtain an ideal gradation characteristic. The tone correction table data is calculated, and the content of the tone correction table 118 is updated to the calculation result. Thus, the gradation correction table 118 is changed and set. By doing so, this image forming apparatus can form an image with stable quality regardless of variations in gamma characteristics of the apparatus and changes over time.

  With respect to the corrected gradation data corrected as described above, the halftone processing unit 1152 generates halftone gradation data using a so-called dot concentration type threshold matrix MTX. That is, by forming one cell using a plurality of pixels and forming dots by performing exposure and development only on pixels corresponding to the gradation level among the plurality of pixels of the cell according to the threshold matrix MTX, The gradation is realized by changing the size of the halftone dots formed by such dots in accordance with the gradation level. Specific halftone processing will be described later in the section “B. Halftone processing”. The halftone gradation data generated by the halftone processing unit 1152 is input to two types of line buffers 116A and 116B.

  These line buffers 116A and 116B are common in that they store halftone gradation data constituting one line image data output from the image processing unit 15, but the readout order of gradation data is different. That is, the forward line buffer 116A outputs halftone gradation data constituting one line image data in the forward direction from the head, whereas the reverse line buffer 116B outputs in the reverse direction from the end. is there.

  The halftone gradation data thus output is input to the scanning mode switching unit 116C, and only the halftone gradation data output from one line buffer based on the scanning mode switching signal is scanned at an appropriate timing. 116C is output to pulse modulation section 117. In addition, the gradation data is input to the pulse modulation unit 117 at the timing and order corresponding to each color component by the scanning mode switching unit 116C.

  The gradation data after halftoning input to the pulse modulation unit 117 is a multi-value signal indicating the size and arrangement of toner dots (dots) of each color to be attached to each pixel. The modulation unit 117 uses the halftone gradation data to create a video signal for pulse width modulating the exposure laser pulse of each color image of the engine unit EG, and the engine controller 10 via a video interface (not shown). Output to. Based on this video signal, the laser light source 62Y of the exposure unit 6 is ON / OFF controlled. The same applies to the other color components.

  Next, returning to FIG. In the exposure housing 61, a collimator lens 631, a cylindrical lens 632, a deflector 65, and a scanning lens 66 are provided to scan and expose the light beam from the laser light source 62Y onto the surface (not shown) of the photoreceptor 2Y. It has been. That is, the light beam from the laser light source 62Y is shaped into collimated light of an appropriate size by the collimator lens 631, and then incident on the cylindrical lens 632 having power only in the sub-scanning direction Y. Then, by adjusting the cylindrical lens 632, the collimated light is imaged in the vicinity of the deflection mirror surface 651 of the deflector 65 in the sub-scanning direction Y. Thus, in this embodiment, the collimator lens 631 and the cylindrical lens 632 function as the beam shaping system 63 that shapes the light beam from the laser light source 62Y.

  The deflector 65 is formed by using a micromachining technique in which a micromachine is integrally formed on a semiconductor substrate by applying a semiconductor manufacturing technique, and includes a vibrating mirror that resonates and oscillates. That is, in the deflector 65, the light beam can be deflected in the main scanning direction X by the deflecting mirror surface 651 that resonates and vibrates. More specifically, the deflecting mirror surface 651 is pivotally supported around a swing shaft (torsion spring) that is substantially orthogonal to the main scanning direction X, and responds to an external force applied from an operating portion (not shown). Swings sine around the swing axis. This actuating unit applies an electrostatic, electromagnetic or mechanical external force to the deflection mirror surface 651 based on a mirror drive signal from a mirror drive unit (not shown) of the exposure control unit 102 to mirror the deflection mirror surface 651. Swing at the frequency of the drive signal. Note that any driving method such as electrostatic adsorption, electromagnetic force, or mechanical force may be adopted as the driving method by the operating unit, and since these driving methods are well known, description thereof is omitted here.

  Further, in the apparatus configured as described above, the light beam can be reciprocated in the main scanning direction, that is, the light beam can be scanned in both the (+ X) direction and the (−X) direction. . As described above, the gradation data constituting one line image data is temporarily stored in the storage unit (line buffers 116A and 116B), and the scanning mode switching unit 116C performs gradation data at an appropriate timing and order. Is supplied to the pulse modulation unit 117. For example, when the direction is changed to the (+ X) direction, as shown in FIG. 6A, the beam spot is read out from the line buffer 116A in the order of gradation data DT1, DT2,. Is irradiated onto the photoconductor 2 in the first direction (+ X) to form a line latent image LI (+ X). On the other hand, when the direction is switched to the (−X) direction, as shown in FIG. 6B, the grayscale data DTn, DT (n−1),. A beam spot is irradiated onto the photoconductor 2 in the second direction (−X) based on each gradation data, and a line latent image LI (−X) is formed.

B. Halftone Processing As described in the above section “A. Device Configuration”, in this embodiment, the halftone processing unit 1152 performs dot concentration type threshold matrix on the corrected gradation data output from the gradation correction unit 1151. Halftone gradation data is generated using MTX. Hereinafter, the halftone process according to the present embodiment will be described with reference to FIGS. FIG. 7 is a diagram showing halftone processing performed by the halftone processing unit 1152. In the present embodiment, a 1 × 4 cell CL14 having one pixel in the main scanning direction and four pixels in the sub-scanning direction is used, and only a pixel corresponding to a gradation level among a plurality of pixels included in the 1 × 4 cell CL14 is used. Exposure and development are performed to form dots. At this time, to which pixel a dot is to be formed is determined by a comparison result between the corrected gradation data and a threshold value matrix MTX described in detail below. That is, for each pixel, the correction gradation data is compared with the threshold value of the 1 × 4 threshold matrix MTX14, and if the correction gradation data is equal to or greater than the threshold value of the corresponding pixel, a spot latent image is formed in the pixel portion. If it is less, a spot latent image is not formed. For example, when the gradation level of the correction gradation data is 2, a spot is formed in the shaded portion of the 1 × 4 cell CL14 in FIG.

  FIG. 8 is a diagram illustrating the operation of the 1 × 4 threshold matrix MTX14. FIG. 8A shows the 1 × 4 threshold matrix MTX14, and FIG. 8B shows the growth of halftone dots in the 1 × 4 cell CL14 when the 1 × 4 threshold matrix MTX14 is used. FIG. A 1 × 4 threshold value matrix MTX14 shown in FIG. 8A is a dot concentration type threshold value matrix, and the size of a halftone dot is increased as the gradation level increases. Specifically, as the gradation level increases from 1 to 4, as shown in FIG. 8B, the hatched pixels in the four pixels of the 1 × 4 cell CL14 correspond to each gradation level. By being exposed, a halftone dot grows from the center of the cell.

C. Cell Arrangement Method FIG. 9 is a diagram showing an arrangement method of the 1 × 4 cell CL14 in the present embodiment. In FIG. 9, the alternate long and short dash line indicates a scanning line that is a trajectory of the light beam, the thick solid rectangle indicates the 1 × 4 cell CL14, and the solid ellipse indicates the light beam spot on the photoconductor. In the present embodiment, a plurality of 1 × 4 cells CL14 are arranged adjacent to each other in the main scanning direction X, and at a specific adjacent position TR among a plurality of adjacent positions formed by cells adjacent to each other in the main scanning direction X. Adjacent cells are arranged so as to be shifted from each other by an odd number of pixels in the sub-scanning direction Y. Specifically, a specific adjacent position TR is provided for each adjacent position in the main scanning direction X, and cells adjacent to each other at the specific adjacent position TR are cells upstream of the first direction (+ X) in the main scanning direction. The downstream cell is arranged so as to be shifted by one pixel in the sub-scanning direction Y.

D. Effects of the Invention in the Present Embodiment As described above, in the present embodiment, a plurality of 1 × 4 cells CL14 are arranged adjacent to the main scanning direction X, and a specific adjacent position TR is set for each adjacent position in the main scanning direction X. 1 × 4 cells CL14 cells adjacent to each other at the specific adjacent position TR are 1 × 4 cells on the downstream side with respect to the 1 × 4 cell CL14 on the upstream side in the first direction (+ X) in the main scanning direction. CL14 is arranged so as to be shifted by one pixel in the sub-scanning direction Y. Therefore, as shown in FIG. 9, for example, when a halftone of gradation level 2 is formed, the pitch relationship of the scanning trajectory involved in dot formation is reversed with the specific adjacent position TR as a boundary. That is, in the main scanning direction X, the “wide pitch” (or “narrow pitch”) is upstream on the upstream side of the specific adjacent position TR, while the “narrow pitch” (or “wide pitch”) is downstream. As a result, a halftone dot “large dot” and a relatively small halftone dot “small dot” appear relatively alternately in the main scanning direction X with respect to the cell CL14. Therefore, from the left end (left end in FIG. 9) to the right end (right end in FIG. 9) as described in “Problems to be Solved by the Invention” and “Means for Solving the Problems” The phenomenon that the concentration is lowered is suppressed. Therefore, it is possible to suppress the adverse effect of the image that a density difference occurs between both ends in the main scanning direction X, and to realize good gradation reproduction.

<Others>
The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. In the above-described embodiment, the specific adjacent position TR is provided for each adjacent position in the main scanning direction X of the cell when the cells are arranged. However, the arrangement of the specific adjacent position TR is not limited to this. Similar to the embodiment, it may be provided with periodicity or may be provided with non-periodicity. Further, as a mode in which the specific adjacent positions TR are periodically arranged, the specific adjacent positions TR may be provided for every two adjacent positions in the main scanning direction X as shown in FIG. . Here, in FIG. 10, the alternate long and short dash line represents a scanning line that is a trajectory for scanning the light beam, the thick solid rectangle represents the 1 × 4 cell CL14, and the solid ellipse represents the spot of the light beam on the photoconductor. Also in this case, as shown in FIG. 10, for example, when a halftone of gradation level 2 is formed, large halftone dots and small halftone dots appear alternately in the main scanning direction X. Therefore, it is possible to suppress the adverse effect of the image that a density difference occurs between both ends in the main scanning direction X, and to realize good gradation reproduction.

  Further, in the above-described embodiment, cells adjacent to each other at the specific adjacent position TR are set so that the downstream cell is only one pixel in the sub-scanning direction Y with respect to the upstream cell in the first direction (+ X) in the main scanning direction. However, the method of shifting the cells is not limited to this. For example, as shown in FIG. 11, the odd-numbered identification from the upstream side in the first direction (+ X) in the main scanning direction. In the adjacent position TR, the downstream cell is arranged so as to be shifted by one pixel in the sub scanning direction with respect to the upstream cell in the first direction (+ X) in the main scanning direction, while the first direction ( + X) at the even-numbered specific adjacent position TR from the upstream side, the downstream cell is arranged so as to be shifted by one pixel in the direction opposite to the sub-scanning direction with respect to the upstream cell in the first direction in the main scanning direction. May be. Here, in FIG. 11, the alternate long and short dash line represents a scanning line that is a trajectory of the scanning of the light beam, the thick solid line rectangle represents the 1 × 4 cell CL14, and the solid line ellipse represents the spot of the light beam on the photoconductor. Also in this case, as shown in FIG. 11, for example, when a halftone of gradation level 2 is formed, large halftone dots and small halftone dots appear alternately in the main scanning direction X. Therefore, it is possible to suppress the adverse effect of the image that a density difference occurs between both ends in the main scanning direction X, and to realize good gradation reproduction. That is, the effect of the present invention can be achieved if cells adjacent to each other in the sub-scanning direction at the specific adjacent position TR are shifted by an odd number of pixels.

  In the above embodiment, a 1 × 4 cell having four pixels in the sub-scanning direction and one pixel in the main scanning direction X is used. However, the cell type is not limited to this, and four or more cells in the sub-scanning direction are used. If the cell has an even number of pixels, the effects of the present invention can be achieved. For example, a 4 × 4 cell, an 8 × 6 cell, an 8 × 8 cell, or the like may be used as illustrated in a later embodiment.

  In the above-described embodiment, the present invention is applied to a so-called tandem color printer, but the scope of the present invention is not limited to this. For example, only a so-called four-cycle printer or single color printing is used. The present invention can also be applied to a monochrome printer.

  In the above embodiment, the present invention is applied to an image forming apparatus that temporarily forms a color image on an intermediate transfer medium such as an intermediate transfer belt and then transfers the color image to the sheet S. The present invention is also applicable to an apparatus that forms a color image by directly superimposing toner images on a sheet.

  In the above embodiment, the vibrating deflection mirror surface 651 is formed by using a micromachining technique. However, the method of manufacturing the deflection mirror surface is not limited to this, and the vibrating deflection mirror surface is used. The present invention can be applied to all so-called image forming apparatuses in which a light beam is deflected to scan the latent image carrier.

  Next, examples of the present invention will be shown. However, the present invention is not limited by the following examples as a matter of course, and it is of course possible to implement the present invention with appropriate modifications within a range that can meet the gist of the preceding and following descriptions. They are all included in the technical scope of the present invention.

  In the following, in order to show the effect of the present invention, a halftone image is formed in the case where a halftone image is formed according to Examples 1 to 14 which are examples of the present invention, and in Comparative Examples 1 to 4 using conventional gradation reproduction. The result when comparing with the case of forming an image will be described.

  In each of Examples 1 to 14 and Comparative Examples 1 to 4, 25% density cyan halftone images were created on both ends of the paper using Microsoft Word, and the density difference between these ends was measured. In addition, for the concentration measurement, a spectrolino manufactured by Gretag Macbeth was used, and an average of 5 points was adopted as a measured value in consideration of measurement error.

  Examples 1 to 6 and Comparative Examples 1 and 2 are 4 × 4 cells CL44, Examples 7 to 10 and Comparative Example 3 are 8 × 6 cells CL86, and Examples 11 to 14 and Comparative Example 4 are 8 × 8 cells. CL88 is used, and the cell arrangement method is changed. Also, a 4 × 4 threshold matrix MTX44, an 8 × 6 threshold matrix MTX86, and an 8 × 8 threshold matrix MTX88 are used corresponding to various cells, respectively. FIG. 12 shows threshold matrixes used in the examples and comparative examples. FIG. 12A shows a 4 × 4 threshold matrix MTX44, FIG. 12B shows an 8 × 6 threshold matrix MTX86, and FIG. 12 (c) shows an 8 × 8 threshold matrix MTX88. These threshold value matrices MTX44, 86, and 88 are all dot-concentrated type threshold value matrices, and the threshold values increase as they go outward from the center of the matrix as shown in FIGS. 12 (a) to 12 (c). It is comprised so that it may become. Note that the threshold value of each threshold value matrix corresponds to a density of 100% for 4 × 4, 48 to 100% for 8 × 6, and 64 to 100% for 8 × 8. Therefore, when forming a halftone with a density of 25%, a dot is formed in a pixel corresponding to a threshold value of 4 or less in the case of 4 × 4 cells CL44, and a threshold value of 12 or less in the case of 8 × 6 cells CL86. In the case of the 8 × 8 cell CL88, a dot is formed in a pixel corresponding to a threshold value of 16 or less. The resolution was 600 × 600 dpi (dot per inch) in the 4 × 4 cell CL44, and 1200 × 1200 dpi in the 8 × 6 cell CL86 and the 8 × 8 cell CL88.

  FIG. 13 is a diagram illustrating a cell arrangement method according to the first embodiment. In the first embodiment, a 4 × 4 cell CL44 is used, and a specific adjacent position TR is provided for each adjacent position in the main scanning direction X, and cells adjacent to each other at the specific adjacent position TR are arranged in the first direction (+ The downstream cell is arranged so as to be shifted by one pixel in the sub-scanning direction Y with respect to the upstream cell of X).

  FIG. 14 is a diagram illustrating a cell arrangement method according to the second embodiment. In the second embodiment, a 4 × 4 cell CL44 is used, and a specific adjacent position TR is provided for each adjacent position in the main scanning direction X, and cells adjacent to each other at the specific adjacent position TR are arranged in the first direction (+ The downstream cell is arranged so as to be shifted by one pixel in the sub-scanning direction Y with respect to the upstream cell of X). Further, the upstream cell is arranged so as to be shifted by one pixel in the first direction (+ X) in the main scanning direction with respect to the downstream cell in the sub-scanning direction Y.

  FIG. 15 is a diagram illustrating a cell arrangement method according to the third embodiment. In the third embodiment, a 4 × 4 cell CL44 is used, a specific adjacent position TR is provided for each adjacent position in the main scanning direction X, and cells adjacent to each other at the specific adjacent position TR are arranged in the first direction (+ The downstream cell is arranged so as to be shifted by one pixel in the sub-scanning direction Y with respect to the upstream cell of X). Further, the upstream cell is arranged so as to be shifted by 2 pixels in the first direction (+ X) in the main scanning direction with respect to the downstream cell in the sub-scanning direction Y.

  FIG. 16 is a diagram illustrating a cell arrangement method according to the fourth embodiment. In the fourth embodiment, a 4 × 4 cell CL44 is used, and a specific adjacent position TR is provided for each adjacent position in the main scanning direction X. In the odd-numbered specific adjacent position TR from the upstream side in the first direction in the main scanning direction X, While the downstream cells are arranged so as to be shifted by one pixel in the sub-scanning direction with respect to the upstream cells in the first direction in the scanning direction, the even-numbered specific adjacent positions from the upstream side in the first direction in the main scanning direction In TR, the downstream cell is arranged so as to be shifted by one pixel in the direction opposite to the sub-scanning direction with respect to the upstream cell in the first direction in the main scanning direction.

  FIG. 17 is a diagram illustrating a cell arrangement method according to the fifth embodiment. In the fifth embodiment, a 4 × 4 cell CL44 is used, and a specific adjacent position TR is provided for every two adjacent positions in the main scanning direction X, and cells adjacent to each other at the specific adjacent position TR are arranged in the first direction in the main scanning direction. The cells on the downstream side are arranged so as to be shifted by one pixel in the sub-scanning direction Y with respect to the cells on the upstream side of (+ X).

  FIG. 18 is a diagram illustrating a cell arrangement method according to the sixth embodiment. In the sixth embodiment, a 4 × 4 cell CL44 is used, and a specific adjacent position TR is provided for every three adjacent positions in the main scanning direction, and cells adjacent to each other at the specific adjacent position TR are arranged in the first direction ( + X) is arranged so that the downstream cell is shifted by one pixel in the sub-scanning direction Y with respect to the upstream cell.

  FIG. 19 is a diagram illustrating a cell arrangement method according to the seventh embodiment. In the seventh embodiment, 8 × 6 cells CL86 are used, and a specific adjacent position TR is provided for each adjacent position in the main scanning direction X, and cells adjacent to each other at the specific adjacent position TR are arranged in the first direction (+ The downstream cell is arranged so as to be shifted by one pixel in the sub-scanning direction Y with respect to the upstream cell of X).

  FIG. 20 is a diagram illustrating a cell arrangement method according to the eighth embodiment. In the eighth embodiment, 8 × 6 cells CL86 are used, and a specific adjacent position TR is provided for each adjacent position in the main scanning direction X. In the odd-numbered specific adjacent position TR from the upstream side in the first direction in the main scanning direction, main scanning is performed. While the downstream cells are arranged so as to be shifted by 3 pixels in the sub-scanning direction with respect to the upstream cells in the first direction, the even-numbered specific adjacent position TR from the upstream side in the first direction in the main scanning direction In this case, the downstream cells are arranged so as to be shifted by 3 pixels in the direction opposite to the sub scanning direction with respect to the upstream cells in the first direction in the main scanning direction.

  FIG. 21 is a diagram illustrating a cell arrangement method according to the ninth embodiment. In the ninth embodiment, 8 × 6 cells CL86 are used, and a specific adjacent position TR is provided for every two adjacent positions in the main scanning direction X, and cells adjacent to each other at the specific adjacent position TR are arranged in the first direction in the main scanning direction. The cells on the downstream side are arranged so as to be shifted by one pixel in the sub-scanning direction Y with respect to the cells on the upstream side of (+ X).

  FIG. 22 is a diagram illustrating a cell arrangement method according to the tenth embodiment. In the tenth embodiment, 8 × 6 cells CL86 are used, and a specific adjacent position TR is provided for each adjacent position in the main scanning direction X, and cells adjacent to each other at the specific adjacent position TR are arranged in the first direction (+ The downstream cell is arranged so as to be shifted by one pixel in the sub-scanning direction Y with respect to the upstream cell of X). Further, the upstream cell is arranged so as to be shifted by one pixel in the first direction (+ X) in the main scanning direction with respect to the downstream cell in the sub-scanning direction Y.

  FIG. 23 is a diagram illustrating a cell arrangement method according to the eleventh embodiment. In the eleventh embodiment, the 8 × 8 cell CL88 is used and a specific adjacent position TR is provided for each adjacent position in the main scanning direction X, and cells adjacent to each other at the specific adjacent position TR are arranged in the first direction in the main scanning direction ( + X) is arranged so that the downstream cell is shifted by one pixel in the sub-scanning direction Y with respect to the upstream cell.

  FIG. 24 is a diagram illustrating a cell arrangement method according to the twelfth embodiment. In the twelfth embodiment, the 8 × 8 cell CL88 is used and a specific adjacent position TR is provided for each adjacent position in the main scanning direction, and cells adjacent to each other at the specific adjacent position TR are arranged in the first direction (+ The downstream cells are arranged so as to be shifted by 3 pixels in the sub-scanning direction Y with respect to the upstream cells of X).

  FIG. 25 is a diagram illustrating a cell arrangement method according to the thirteenth embodiment. In the thirteenth embodiment, an 8 × 8 cell CL88 is used, and a specific adjacent position TR is provided for every two adjacent positions in the main scanning direction X, and cells adjacent to each other at the specific adjacent position TR are defined as the first in the main scanning direction. The cells on the downstream side are arranged so as to be shifted by one pixel in the sub-scanning direction Y with respect to the cells on the upstream side in the direction (+ X).

  FIG. 26 is a diagram illustrating a cell arrangement method according to the fourteenth embodiment. In the fourteenth embodiment, an 8 × 8 cell CL88 is used, and a specific adjacent position TR is provided for each adjacent position in the main scanning direction X, and cells adjacent to each other at the specific adjacent position TR are arranged in the first direction ( + X) is arranged so that the downstream cell is shifted by one pixel in the sub-scanning direction Y with respect to the upstream cell. Further, the upstream cell is arranged so as to be shifted by one pixel in the first direction (+ X) in the main scanning direction with respect to the downstream cell in the sub-scanning direction Y.

  FIG. 27 is a diagram showing a cell arrangement method in the first comparative example. In the first comparative example, the 4 × 4 cells CL44 are arranged adjacent to each other in the main scanning direction X, and no shift is provided between adjacent cells at any adjacent position in the main scanning direction X.

  FIG. 28 is a diagram illustrating a cell arrangement method in the second comparative example. In Comparative Example 2, 4 × 4 cells CL44 are arranged adjacent to each other in the main scanning direction X, and among the adjacent positions formed by the cells adjacent to each other in the main scanning direction X, upstream in the first direction in the main scanning direction. The odd-numbered adjacent positions from the side are arranged so that the downstream cell is shifted by 2 pixels in the sub-scanning direction with respect to the upstream cell in the first direction in the main scanning direction, while in the first direction in the main scanning direction At even-numbered adjacent positions from the upstream side, the downstream cells are arranged so as to be shifted by two pixels in the direction opposite to the sub-scanning direction with respect to the upstream cells in the first direction in the main scanning direction.

  FIG. 29 is a diagram showing a cell arrangement method in the third comparative example. In Comparative Example 3, 8 × 6 cells CL86 are arranged adjacent to each other in the main scanning direction X, and no shift is provided between adjacent cells at any adjacent position in the main scanning direction X.

  FIG. 30 is a diagram showing a cell arrangement method in Comparative Example 4. In Comparative Example 4, 8 × 8 cells CL88 are arranged adjacent to each other in the main scanning direction X, and no shift is provided between adjacent cells at any adjacent position in the main scanning direction X.

  Table 1 below is a table showing the measurement results of the density difference when halftones are formed on both ends of the paper according to the above examples and comparative examples. First, comparing the results of Examples 1 to 6 and Comparative Examples 1 and 2 using the 4 × 4 cell CL44, in Examples 1 to 6 of the present invention, the concentration difference between the left and right is 0.01. In contrast to the following, in Comparative Examples 1 and 2 using conventional gradation reproduction, the absolute value is 0.06 or more, and Examples 1 to 6 of the present invention are Comparative Examples 1 and 6. It can be seen that the density difference between the left and right is suppressed compared to 2. That is, according to Examples 1 to 6 of the present invention, it was found that good gradation reproduction can be realized by suppressing the image adverse effect of density difference between both ends in the main scanning direction.

  Next, comparing the results of Examples 7 to 10 and Comparative Example 3 using the 8 × 6 cell CL86, in Examples 7 to 10 of the present invention, the concentration difference between the left and right is 0.01 or less in absolute value. On the other hand, in the comparative example 3 using the conventional gradation reproduction, the absolute value is 0.05, and the examples 7 to 10 of the present invention are left and right as compared with the comparative example 3. It can be seen that the difference in the concentration of is suppressed. That is, according to Examples 7 to 10 of the present invention, it has been found that good gradation reproduction can be realized by suppressing the image adverse effect that a density difference occurs between both ends in the main scanning direction.

  Next, comparing the results of Examples 11 to 14 and Comparative Example 4 using the 8 × 8 cell CL88, in Examples 11 to 14 of the present invention, the concentration difference between the left and right is 0.01 or less in absolute value. On the other hand, in the comparative example 4 using the conventional gradation reproduction, the absolute value is 0.06, and the examples 11 to 14 of the present invention are more right and left than the comparative example 4. It can be seen that the difference in the concentration of is suppressed. That is, according to Examples 11 to 14 of the present invention, it has been found that good gradation reproduction can be realized while suppressing an image adverse effect that a density difference occurs between both ends in the main scanning direction.

1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus in FIG. 1. FIG. 2 is a main scanning sectional view showing a configuration of an exposure unit in the image forming apparatus of FIG. 1. FIG. 2 is a diagram showing a signal processing block in the image forming apparatus of FIG. 1. The figure which shows the structure of an image processing unit. FIG. 2 is a diagram showing a line latent image formed by the image forming apparatus of FIG. 1. FIG. 2 is a diagram showing halftone processing in the image forming apparatus of FIG. 1. The figure which shows operation | movement of a dot concentration type | mold threshold value matrix. The figure which shows arrangement | positioning of the cell in one Embodiment. The figure which shows arrangement | positioning of the cell in another embodiment. The figure which shows the arrangement | positioning of the cell in another embodiment. The figure which shows the dot concentration type | mold threshold value matrix in an Example and a comparative example. FIG. 3 is a diagram showing a cell arrangement of the first embodiment. FIG. 6 is a diagram showing a cell arrangement of Example 2. FIG. 6 is a diagram showing a cell arrangement of Example 3. The figure which shows the cell arrangement | positioning of Example 4. FIG. FIG. 10 is a diagram illustrating a cell arrangement according to a fifth embodiment. FIG. 10 is a diagram illustrating a cell arrangement according to a sixth embodiment. FIG. 10 is a diagram illustrating a cell arrangement according to a seventh embodiment. FIG. 10 is a diagram illustrating a cell arrangement according to an eighth embodiment. The figure which shows the cell arrangement | positioning of Example 9. FIG. FIG. 10 is a diagram showing a cell arrangement of Example 10. FIG. 15 is a diagram showing a cell arrangement of Example 11. The figure which shows the cell arrangement | positioning of Example 12. FIG. The figure which shows the cell arrangement | positioning of Example 13. FIG. The figure which shows the cell arrangement | positioning of Example 14. FIG. The figure which shows the cell arrangement | positioning of the comparative example 1. FIG. The figure which shows the cell arrangement | positioning of the comparative example 2. FIG. The figure which shows the cell arrangement | positioning of the comparative example 3. FIG. The figure which shows the cell arrangement | positioning of the comparative example 4. FIG. Explanatory drawing of the subject of this invention. Explanatory drawing of the subject of this invention. Explanatory drawing of the subject of this invention.

Explanation of symbols

  2, 2Y, 2M, 2C, 2K ... photosensitive member (latent image carrier), 6, 6Y, 6M, 6C, 6K ... exposure unit, 60 ... horizontal synchronization sensor, 62, 62Y, 62M, 62C, 62K ... laser light source 71: Intermediate transfer belt 651: Deflection mirror surface DT1, DT2, DT (n-1), DTn: Gradation data, Ly, Lm, Lc, Lk: Scanning light beam, LI (+ X), LI ( -X) ... Line latent image, MT ... Drive motor, X ... Main scanning direction, Y ... Sub-scanning direction, TR ... Specific adjacent position, MTX, MTX14, MTX44, MTX86, MTX88 ... Threshold matrix, CL, CL14, CL44, CL86, CL88 ... cell

Claims (6)

The latent image carrier whose surface is driven in the first direction and the oscillating deflection mirror surface are configured so that the light from the light source can be scanned in the second direction substantially orthogonal to the first direction, and the light is Using an image forming apparatus comprising a latent image forming unit that irradiates a latent image carrier to form a latent image and a developing unit that develops the latent image,
An image forming method for forming an image by forming a halftone dot using a threshold matrix at the time of developing the pixel for a cell composed of a plurality of pixels contributing to the formation of a halftone dot,
Reciprocatingly scanning the light beam in the second direction with the deflecting mirror surface to form the latent image;
Cells having an even number of pixels that are 4 or more with respect to the first direction are arranged adjacent to the second direction, and the cells having the even number of pixels are adjacent to each other in the second direction. Among the adjacent positions to be formed, a cell having the even number of pixels at the specific adjacent position has a length equal to the number of odd pixels in the first direction with respect to a cell adjacent through the specific adjacent position. An image forming method, wherein the image forming method is arranged so as to deviate in the direction of.   The image forming method according to claim 1, wherein the specific adjacent positions are periodically provided for each predetermined number of the adjacent positions.   A cell having the even number of pixels adjacent in the specific adjacent position is a cell having the even number of pixels downstream from the cell having the even number of pixels upstream in the second direction. The image forming method according to claim 1, wherein the image forming method is arranged so as to be shifted in the first direction by a length of an odd number of pixels in the first direction.   A cell having the even number of pixels adjacent at the specific adjacent position is an odd number in the first direction, and a cell having the even number of pixels downstream from the upstream cell in the second direction is an odd number in the first direction. The image forming method according to claim 1, wherein the image forming method is arranged so as to be shifted in a direction opposite to the first direction by a length of a pixel.   A cell having the even number of pixels on the downstream side of the cell having the even number of pixels on the upstream side in the main scanning direction at the odd numbered specific adjacent position from the upstream side in the second direction. The odd-numbered pixels in the direction of 1 are arranged so as to be shifted in the first direction, while the even-numbered specific adjacent position from the upstream side in the second direction is the upstream side in the second direction. The cell having the even number of pixels on the downstream side with respect to the cell having the even number of pixels is arranged so as to be shifted in the direction opposite to the first direction by the length of the odd number of pixels in the first direction. 3. The image forming method according to 1 or 2. The latent image carrier whose surface is driven in the first direction and the oscillating deflection mirror surface are configured so that the light from the light source can be scanned in the second direction substantially orthogonal to the first direction, and the light is A latent image forming unit that irradiates the latent image carrier to form a latent image; and a developing unit that develops the latent image;
Means for forming an image by forming the halftone dots using a threshold matrix when developing the pixels for a cell consisting of a plurality of pixels contributing to the formation of halftone dots;
Means for reciprocally scanning the light beam in the second direction with the deflecting mirror surface to form the latent image;
Cells having an even number of pixels that are 4 or more with respect to the first direction are arranged adjacent to the second direction, and the cells having the even number of pixels are adjacent to each other in the second direction. Among the adjacent positions to be formed, a cell having the even number of pixels at the specific adjacent position has a length equal to the number of odd pixels in the first direction with respect to a cell adjacent through the specific adjacent position. And an image forming apparatus provided with a unit arranged so as to be displaced in the direction of.

Images