JP2012201048A - Apparatus and method for forming image, and threshold matrix generating apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that can form an image in which striped density unevenness is suppressed in a single-pass system, and an image forming method and a threshold matrix generating apparatus.SOLUTION: A line head 44 is controlled to negate density variation in an array direction on a paper sheet 12 due to variation with time of dot forming positions corresponding to a plurality of timings.

Description

  The present invention relates to an image forming apparatus and an image forming method for generating an image by forming a plurality of dots on a recording medium, and a threshold matrix creating a threshold matrix used for halftone processing according to the image forming apparatus Relates to the device.

  In recent years, with the dramatic advancement of inkjet technology, color large format printing that achieves both high speed and high image quality using an inkjet image forming apparatus is becoming possible. This apparatus is used in a wide range of fields especially for sign / display applications, and can be applied to, for example, printing of storefront POP (Point Of Purchase), wall posters, outdoor advertisements / signboards, and the like. In the inkjet method, a printed matter can be obtained by ejecting droplets of a plurality of types of ink (for example, CMYK ink) onto a print medium to form a large number of dots.

  Of the inkjet methods, a single-pass method using a recording head (hereinafter, referred to as a line head) having a plurality of nozzles arranged along an arrangement direction (hereinafter may be referred to as a main direction) is particularly preferable. Attention has been paid. This is because the image on the recording medium can be completed by transporting the recording medium in the transport direction only once, and various specifications (high speed, low power, high image quality) required for sign display applications Because all can be compatible. On the other hand, in the single-pass method, due to some variation factor, the landing position of the ink droplets from each nozzle deviates from the target position in the main direction. There is a problem that unevenness is likely to occur).

  In order to solve this problem, various improved techniques such as a line head capable of suppressing the occurrence of the uneven stripe have been proposed even in the single pass method.

  Patent Document 1 discloses a line head having a structure in which a plurality of nozzles are arranged in a matrix at regular intervals along an oblique column direction. As a result, the dots are sequentially formed at a plurality of timings to complete the image, whereby the repetition period in the main direction can be shortened, and the occurrence of uneven stripes can be suppressed.

  In addition, various methods for suppressing the occurrence of uneven stripes have been proposed by correcting a control signal for controlling ejection of ink droplets from each nozzle.

  Patent Document 2 discloses an apparatus that corrects ink droplet ejection control signals by averaging characteristic data of a plurality of nozzles.

  Patent Document 3 discloses a device that calculates a gap between adjacent dots in the main direction from the amount of flight bending of each nozzle, and corrects a threshold matrix (dither matrix) used for halftone processing according to the size of the gap; A method is disclosed.

JP 2007-44967 A JP 2006-224403 A JP 2006-159810 A

  By the way, the fluctuation factors that shift the landing positions of the ink droplets from the respective nozzles include a permanent factor due to the molding processing accuracy of each nozzle and a temporary factor due to adhesion of dust near the nozzle.

  According to the research results of the present inventors, it has been found that the line head vibrates with respect to the main direction by driving the constituent parts of the image forming apparatus, and the landing position shifts with time. Then, due to the error in the landing position, the density of the dot arrangement becomes dense, and as a result, wavy line irregularities along the sub direction may occur.

  However, in the apparatus and method disclosed in Patent Document 3, the threshold value matrix is corrected under the premise that the dot gap does not fluctuate in one image forming operation, so that the occurrence of the wavy stripe unevenness cannot be suppressed. There was a problem.

  The present invention has been made to solve the above-described problems, and provides an image forming apparatus, an image forming method, and a threshold matrix creating apparatus capable of forming an image in which the occurrence of streak unevenness is suppressed in a single pass method. Objective.

  An image forming apparatus according to the present invention includes a plurality of dot forming elements arranged along an arrangement direction, and a dot forming unit that forms a plurality of dots on a recording medium using the plurality of dot forming elements; A transport unit that moves the dot formation unit and the recording medium relative to each other by transporting at least one of the dot formation unit and the recording medium in a predetermined transport direction; A dot formation control unit that controls the dot formation unit based on a control signal so that each dot is sequentially formed at a timing to generate an image row in the arrangement direction, and dot formation positions according to the plurality of timings The input image signal is supplied to the dot formation control unit so as to cancel out density fluctuations in the arrangement direction on the recording medium due to a time-dependent error in the recording medium. And having a signal converting unit for converting the signal.

  As described above, since the signal conversion unit is provided to convert the signal so as to cancel out the density fluctuation in the arrangement direction on the recording medium due to the time-dependent error of the dot formation position according to a plurality of timings, the dot formation position Thus, it is possible to form an image in which the generation of streaks due to the time-dependent error is suppressed.

  The signal conversion unit includes a group classification unit that classifies the image signal into a plurality of pixel groups according to a degree of the temporal error, and dots belonging to at least one pixel group classified by the group classification unit. It is preferable to include a dot number distribution unit that distributes the dots so that the density contribution to the image sequence is higher than the density contribution of the dots belonging to the remaining pixel groups to the image sequence.

  Furthermore, it is preferable that the group classification unit classifies the image signal into a plurality of pixel groups corresponding to the plurality of timings.

  The density contribution degree is a distribution ratio of dots of a single size, and the dot number distribution unit is configured such that the distribution ratio to the at least one pixel group is the distribution ratio to the remaining pixel groups. It is preferable to distribute the dots so as to be higher than the ratio.

  The density contribution degree is a distribution ratio of dots of a single size, and the dot number distribution unit is configured such that the distribution ratio to the at least one pixel group is the distribution ratio to the remaining pixel groups. It is preferable to distribute the dots so as to be lower than the ratio.

  Further, the plurality of dot forming elements can form dots of a plurality of sizes, the density contribution is a distribution ratio of dots for each size, and the signal conversion unit includes the at least one pixel group. It is preferable that a dot size assigning unit for assigning dots is further provided so that the distribution ratio to is different from the distribution ratio to each of the remaining pixel groups.

  In addition, it is preferable that the group classification unit classifies the image signal into the plurality of pixel groups by time-sharing time characteristics of the temporal error.

  Further, it is preferable that the group classification unit classifies the image signal into the plurality of pixel groups by spatially dividing the position characteristics of the density variation in the arrangement direction.

  Furthermore, it is preferable that the signal conversion unit further includes a halftone processing unit that converts the image signal into the control signal by a systematic dither method using a threshold matrix.

  Furthermore, it is preferable that the halftone processing unit changes the size of the threshold matrix according to the generation period of the temporal error.

  Furthermore, it is preferable that the size of the threshold value matrix in the transport direction is an integer multiple of the number of image columns corresponding to the generation period of the temporal error.

  In addition, the signal conversion unit classifies the image signal into a plurality of pixel groups according to the degree of temporal error, and corrects the image signal of at least one pixel group of the plurality of pixel groups. It is preferable to include a correction unit and a halftone processing unit that converts the image signal corrected by the signal correction unit into the control signal using halftone processing.

  Further, it is preferable that the time-dependent error is a short-cycle error that occurs when an entire image composed of a plurality of the image sequences is generated.

  Further, it is preferable that the short-cycle error is an error caused by vibration of the transport unit or the dot forming unit when generating the entire image.

  An image forming method according to the present invention includes a dot forming step of forming a plurality of dots on a recording medium using a plurality of dot forming elements arranged along an arrangement direction, and sequentially forming each dot at a plurality of timings. A generation step of generating an image row in the arrangement direction, the input step of inputting an image signal, and the recording medium associated with a temporal error in dot formation positions according to the plurality of timings A signal converting step for converting an input image signal into a control signal so as to cancel out the density fluctuation in the arrangement direction above, and a control step for controlling the plurality of dot forming elements based on the converted control signal It is characterized by providing.

  A threshold value matrix creating device according to the present invention is a device that creates a threshold value matrix for halftone processing according to an image forming apparatus, and the image forming apparatus forms a plurality of dots arranged along an arrangement direction. An element, and a dot forming unit that forms a plurality of dots on a recording medium using the plurality of dot forming elements, and at least one of the dot forming unit and the recording medium is conveyed in a predetermined conveying direction. A transfer unit that relatively moves the dot forming unit and the recording medium, and each dot is sequentially formed at a plurality of timings under the relative movement by the transfer unit to generate each image row in the array direction. And a dot formation control unit that controls the dot formation unit based on a control signal. A threshold value matrix creating unit that creates a threshold value matrix that converts an input image signal into the control signal provided to the dot formation control unit so as to cancel out density fluctuations in the arrangement direction on the recording medium. It is characterized by having.

  According to the image forming apparatus, the image forming method, and the threshold value matrix creating apparatus according to the present invention, the density variation in the arrangement direction on the recording medium due to the temporal error of the dot formation position corresponding to a plurality of timings is canceled. Therefore, it is possible to form an image in which the occurrence of streak unevenness due to a time-dependent error in the dot formation position is suppressed.

1 is a cross-sectional side view illustrating a configuration of an image forming apparatus according to an exemplary embodiment. FIG. 2 is a block diagram illustrating a system configuration of the image forming apparatus illustrated in FIG. 1. FIG. 2 is a plan perspective view illustrating a configuration example of the line head illustrated in FIG. 1. FIG. 4 is a schematic sectional view taken along line IV-IV in FIG. 3. It is a schematic explanatory drawing showing the correspondence of the example of arrangement | positioning of the some nozzle with which a line head is provided, and the discharge order on a paper. It is a schematic explanatory drawing showing the flow of the image processing in the image processing part shown in FIG. It is a schematic explanatory drawing of the halftone process by a systematic dither method. FIG. 3 is a functional block diagram of a threshold matrix creation unit shown in FIG. 2. It is a flowchart with which operation | movement description of the threshold value matrix preparation part shown in FIG. 8 is provided. FIG. 10A is a schematic explanatory diagram showing the amount of misalignment of dots to be formed. FIG. 10B is a graph showing fluctuation characteristics of the positional deviation amount of dots formed by ejection from the first nozzle 102a shown in FIG. FIG. 10C is a graph showing fluctuation characteristics of the positional deviation amount of dots formed by ejection from the second nozzle 102b shown in FIG. FIG. 11A is a schematic explanatory diagram illustrating the ink ejection order corresponding to each image position in the configuration example of the line head illustrated in FIG. 5. FIG. 11B is a schematic explanatory diagram showing a phase corresponding to the ink ejection order shown in FIG. 11A. It is a schematic explanatory drawing showing the result of having classified each cell of a threshold matrix. It is a graph showing the example of determination of the dot distribution ratio to each phase group. It is a detailed flowchart in step S6 of FIG. It is a detailed flowchart in step S67 of FIG. It is a schematic explanatory drawing showing the flow of the image processing in the image processing part which concerns on a 1st modification. It is a graph showing the example of allocation of the some dot size in the dot size allocation part shown in FIG. It is a schematic explanatory drawing showing the flow of the image process in the image process part which concerns on a 2nd modification. It is a graph showing the example of a correction | amendment for every phase group in the signal correction | amendment part shown in FIG. It is a block diagram showing the structure of the system of the image forming apparatus which concerns on a 3rd modification. 6 is a graph showing the position characteristics of density fluctuations in an image sequence ejected from a plurality of nozzles including the first nozzle and the third nozzle shown in FIG. 5. FIG. 22A is a schematic explanatory diagram illustrating the ejection order of ink corresponding to each image position in the configuration example of the line head illustrated in FIG. 5. 22B and 22C are schematic explanatory diagrams showing phases corresponding to the ink ejection order shown in FIG. 22A. 23A to 23C are schematic explanatory diagrams of dot patterns created according to the phase group classification according to the fourth modification.

  Hereinafter, the image forming method according to the present embodiment will be described in detail with reference to the accompanying drawings by giving preferred embodiments in relation to the image forming apparatus and the threshold value matrix creating apparatus for carrying out the image forming method.

  As shown in FIG. 1, the image forming apparatus 10 according to the present embodiment feeds and conveys a sheet 12 upstream of a sheet as a recording medium (hereinafter referred to as “sheet 12”) in the conveying direction. A paper feeding / conveying unit 14 is provided. Downstream of the paper feeding / conveying unit 14, a processing liquid application unit 16 that applies a processing liquid to a recording surface (hereinafter referred to as an image forming surface) of the paper 12 along the conveyance direction of the paper 12, and the image. An image forming unit 18 that forms an image by attaching ink to the forming surface, an ink drying unit 20 that dries the ink of the processing liquid layer formed on the paper 12, and the image of the processing liquid layer is fixed to the paper 12. An image fixing unit 22 to be discharged and a discharge unit 24 to discharge the paper 12 on which the image is fixed are provided.

  The paper feeding / conveying section 14 includes a stacking section 26 provided so that the sheets 12 can be stacked, a sheet feeding section 28 that feeds the sheets 12 stacked on the stacking section 26 one by one, and the sheet feeding section 28. A transport unit 30 that transports the fed paper 12 to the treatment liquid coating unit 16.

  The treatment liquid application unit 16 includes a treatment liquid application drum 32 that is rotatably provided, a treatment liquid application device 34 that applies the treatment liquid to the image forming surface of the paper 12, and a treatment liquid drying device 36 that dries the treatment liquid. With. As a result, a thin processing liquid layer is applied on the image forming surface of the paper 12.

  Between the processing liquid application unit 16 and the image forming unit 18, a first intermediate transport drum 38 that is rotatably provided is disposed. By rotating the first intermediate transport drum 38 with the paper 12 held on the surface of the first intermediate transport drum 38, the paper 12 supplied from the treatment liquid application unit 16 side is transported to the image forming unit 18 side. Is done.

  The image forming unit 18 includes a rotatable image forming drum 40 (conveying unit), and a head unit 42 that ejects ink droplets (droplets) onto the paper 12 conveyed by the image forming drum 40. Yes. The head unit 42 includes line heads 44 (dot forming portions) of at least basic colors Y (yellow), M (magenta), C (cyan), and K (black). The line heads 44 are arranged along the circumferential direction of the image forming drum 40. As a result, images of the respective colors are sequentially formed on the treatment liquid layer applied on the image forming surface of the paper 12. Since the treatment liquid has an effect of aggregating the color material (pigment) and latex particles dispersed in the ink solvent, it is possible to prevent the color material from flowing on the paper 12.

  Between the image forming unit 18 and the ink drying unit 20, a second intermediate conveyance drum 46 that is rotatably provided is disposed. By rotating the second intermediate transport drum 46 while the paper 12 is held on the surface of the second intermediate transport drum 46, the paper 12 supplied from the image forming unit 18 side is transported to the ink drying unit 20 side. The

  The ink drying unit 20 includes a rotatable ink drying drum 48, a plurality of hot air nozzles 50 for drying the treatment liquid layer of the paper 12, and a plurality of infrared heaters (heaters 52). Thereby, the solvent of the ink staying in the treatment liquid layer of the paper 12 is dried.

  Between the ink drying unit 20 and the image fixing unit 22, a third intermediate conveyance drum 54 that is rotatably provided is disposed. By rotating the third intermediate transport drum 54 while the paper 12 is held on the surface of the third intermediate transport drum 54, the paper 12 supplied from the ink drying unit 20 side is transported to the image fixing unit 22 side. The

  In the image fixing unit 22, an image fixing drum 56 that is rotatably provided, a heating roller 58 that is disposed in the vicinity of the surface of the image fixing drum 56, and a surface that is in pressure contact with the surface of the image fixing drum 56. The fixing roller 60 is provided. As a result, latex particles that aggregate in the treatment liquid layer are heated and pressurized to melt, and are fixed and fixed on the paper 12 as an image.

  The paper 12 on which the image on the image forming surface has been fixed through the above-described steps is conveyed to the discharge unit 24 provided on the downstream side of the image fixing unit 22 by the rotation of the image fixing drum 56.

  FIG. 2 is a block diagram showing the system configuration of the image forming apparatus 10 shown in FIG. In addition to the head unit 42 and the heater 52 (see FIG. 1), the image forming apparatus 10 includes a communication interface 62, a system controller 64, an image memory 66, a ROM 68, a motor driver 70, a motor 72, and a heater driver 74. A print controller 76, an image buffer memory 78, an image processor 80 (signal converter), a threshold matrix generator 82, a ROM 84, and a head driver 86 (dot formation controller).

  The communication interface 62 is an interface unit with the host device 90 that is used when a user gives an image forming instruction or the like to the image forming apparatus 10. A serial interface such as USB (Universal Serial Bus), IEEE 1394, Ethernet (registered trademark), a wireless network, or a parallel interface such as Centronics can be applied to the communication interface 62. In this part, a buffer memory (not shown) for speeding up communication may be mounted.

  An image signal sent from the host device 90 is taken into the image forming apparatus 10 via the communication interface 62 and temporarily stored in the image memory 66. The image memory 66 is a storage unit that stores an image signal input via the communication interface 62, and information is read and written through the system controller 64. The image memory 66 is not limited to a memory composed of semiconductor elements, and a magnetic medium such as a hard disk may be used.

  The system controller 64 includes a central processing unit (CPU) and its peripheral circuits, and functions as a control device that controls the entire image forming apparatus 10 according to a predetermined program and also functions as an arithmetic device that performs various calculations. . That is, the system controller 64 controls each unit such as the communication interface 62, the image memory 66, the motor driver 70, the heater driver 74, and the like. Further, the system controller 64 performs communication control with the host device 90, read / write control of the image memory 66 and ROM 68, and the like. Further, the system controller 64 generates a control signal for controlling the motor 72 and the heater 52 of the paper transport system. In addition to the control signal, the image signal stored in the image memory 66 is transmitted to the print control unit 76.

  The ROM 68 stores programs executed by the CPU of the system controller 64 and various data necessary for control. The image memory 66 is used as a temporary storage area for image signals, and is also used as a program development area and a calculation work area for the CPU.

  The motor driver 70 is a driver (drive circuit) that drives the paper transport motor 72 in accordance with an instruction from the system controller 64. The heater driver 74 is a driver that drives the heater 52 in accordance with an instruction from the system controller 64.

  On the other hand, the print control unit 76 includes a CPU and its peripheral circuits, and generates a discharge control signal from the image signal in the image memory 66 in cooperation with the image processing unit 80 under the control of the system controller 64. In addition to performing various processes and corrections for this purpose, the generated ink discharge data (control signal) is supplied to the head driver 86 to control the discharge drive of the head unit 42.

  Connected to the print controller 76 is a ROM 84 that stores programs executed by the CPU of the print controller 76 and various data necessary for control. The ROM 84 may be a non-rewritable storage means, but when various data are updated as necessary, it is preferable to use a rewritable storage means such as an EEPROM.

  The image processing unit 80 generates dot arrangement data for each ink color from an input image signal (hereinafter referred to as an input image signal). That is, the dot formation position (ink ejection timing) is determined by performing halftone processing on the input image signal. For this halftone process, a systematic dither method, an error diffusion method, a density pattern method, a random dot method, or the like can be applied. In the present embodiment, the description will focus on halftone processing using a systematic dither method.

  The threshold value matrix creating unit 82 creates a threshold value matrix Mt used for the halftone process by the image processing unit 80. As will be described later, the created threshold value matrix Mt has dot dispersion characteristics that cancel out density fluctuations in the arrangement direction on the paper 12 due to a temporal error in dot formation positions corresponding to a plurality of timings. The threshold value matrix creating unit 82 may create the threshold value matrix Mt each time an image formation instruction is given, or may store the created threshold value matrix Mt in a readable memory in advance.

  In FIG. 2, the image processing unit 80 and the threshold matrix creating unit 82 are illustrated as being separate from the system controller 64 and the print control unit 76. For example, the image processing unit 80 and / or the threshold value matrix creating unit 82 may be included in the system controller 64 or the print control unit 76 and constitute a part thereof.

  In addition, the print control unit 76 generates an ink discharge data (an actuator control signal corresponding to the nozzle of the line head 44) based on the dot arrangement data generated by the image processing unit 80, and an ink discharge data generation function. And a drive waveform generation function.

  The ink discharge data generated by the ink discharge data generation function is given to the head driver 86, and the ink discharge operation of the head unit 42 is controlled.

  The drive waveform generation function is a function that generates a drive signal waveform for driving an actuator corresponding to each nozzle of the line head 44. The signal (drive waveform) generated by the drive waveform generation function is supplied to the head driver 86.

  The print control unit 76 includes an image buffer memory 78, and data such as image signals and parameters are temporarily stored in the image buffer memory 78 when the print control unit 76 processes image signals.

  FIG. 3 is a perspective plan view showing a structural example of the line head 44 shown in FIG. 4 is a schematic cross-sectional view taken along line IV-IV in FIG.

  As shown in FIG. 3, the line head 44 includes a plurality of ink chamber units 100 (dot forming elements) arranged in a staggered matrix. Each ink chamber unit 100 includes a nozzle 102, a pressure chamber 104, and a supply port 106. In the pressure chamber 104 whose planar shape is approximately square, an outlet to the nozzle 102 side is provided at one of the diagonal corners, and an inlet (supply port 106) from the common channel 108 is provided at the other. Is provided.

  As shown in FIG. 4, each pressure chamber 104 communicates with the common flow path 108 via the supply port 106. The common channel 108 communicates with an ink tank (not shown) that is an ink supply source. As a result, the ink supplied from the ink tank is distributed and supplied to each pressure chamber 104 via the common channel 108.

  One surface of the pressure chamber 104 (corresponding to the top surface in the example of FIG. 4) is constituted by a pressure plate 110, which also serves as a common electrode. A piezoelectric element 112 as an actuator for applying pressure and deforming the pressure plate 110 is joined to the upper portion of the pressure plate 110. An individual electrode 114 is formed on the upper surface of the piezoelectric element 112.

  When a driving voltage is applied between the two electrodes, that is, the pressure plate 110 serving as a common electrode and the individual electrode 114, the piezoelectric element 112 sandwiched between the two electrodes is deformed. Due to this physical deformation, the volume of the pressure chamber 104 changes, so that ink is pushed out of the nozzle 102 and ejected as ink droplets. Then, after the ink droplet is ejected, when the displacement of the piezoelectric element 112 returns to the original state, the ink is again filled into the pressure chamber 104 from the common channel 108 through the supply port 106.

  Returning to FIG. 3, the feature of the arrangement of the nozzles 102 will be described. In this figure, the longitudinal direction and the short direction of the line head 44 are defined as an arrow X direction and an arrow Y direction, respectively. At this time, the conveyance direction (see FIG. 1) of the paper 12 is orthogonal to the arrow X direction and parallel to the arrow Y direction.

  The nozzles 102 in the L1th row are arranged at equal intervals along the arrow X direction at predetermined intervals (corresponding to 4 unit lengths). The nozzles in the L2 to L4 rows are also arranged in the same manner as in the L1 row. Hereinafter, the arrow X direction may be referred to as the “arrangement direction” of the nozzles 102 (ink chamber units 100).

  The nozzles 102 in the L2 row are arranged at positions shifted by one unit length in the left direction of the arrow X with reference to the positions of the nozzles 102 in the L1 row. The nozzles 102 in the L3 row are arranged at positions shifted by one unit length in the left direction of the arrow X with reference to the positions of the nozzles 102 in the L2 row. Each nozzle 102 in the L4th row is arranged at a position shifted by one unit length in the left direction of the arrow X with reference to the position of each nozzle 102 in the L3th row. As a result, the density of the substantial interval (projection nozzle pitch) between the nozzles 102 projected so as to be aligned along the longitudinal direction of the line head 44 is achieved.

  Note that various methods can be adopted as an ink droplet ejection mechanism by the line head 44. As shown in FIGS. 3 and 4, a method of ejecting ink droplets by deformation of an actuator composed of a piezo element (piezoelectric element) or the like may be applied. Further, a thermal jet method in which bubbles are generated by heating ink through a heating element such as a heater, and ink droplets are ejected by the pressure may be applied. Further, the present invention is not limited to the line head 44 and may be a multi-pass method in which an image is formed while reciprocating scanning in the width direction of the paper 12.

  FIG. 5 is a schematic explanatory diagram showing a correspondence relationship between an arrangement example of the plurality of nozzles 102 included in the line head 44 and the ejection order on the paper 12. For convenience of explanation, a case where 20 nozzles 102 (including the first nozzle 102a, the second nozzle 102b, and the third nozzle 102c) are used will be described as an example.

  Each cell in the rectangular grid shown in this figure represents an area for one pixel in the formed image. A blank cell represents an image position where an ink droplet has not yet been ejected (landed) at each ejection time (t). The arithmetic numbers written in the cells correspond to the time (ejection time t = 1 to 7) when the ink droplet is ejected (landed) at the image position.

  For example, a plurality of dots are sequentially formed by ink droplets ejected between ejection time points t = 1 to 4, and a first image row is generated. In addition, a plurality of dots are sequentially formed by ink droplets ejected between ejection time points t = 4 to 7, and a fourth image row is generated. In other words, each image sequence is generated (completed) by sequentially forming dots at a plurality of timings (four in this figure).

  The arrangement of the nozzles 102 is not limited to the example in FIG. 5, and the arrangement form (number and position) is not limited as long as an image sequence can be generated.

  FIG. 6 is a schematic explanatory diagram showing the flow of image processing in the image processing unit 80 shown in FIG. The image processing unit 80 basically includes a resolution conversion unit 120, a CMYK color conversion unit 122, and a halftone processing unit 124.

  An image signal (input image signal) input to the image processing unit 80 is multi-gradation data composed of a plurality of color channels. For example, 8-bit (256 gradations per pixel) RGB TIFF format data may be used.

  The resolution conversion unit 120 converts the resolution of the input image signal into a resolution corresponding to the image forming apparatus 10 using an image enlargement / reduction process that enlarges or reduces the image size. The first intermediate image signal obtained here has the same data definition as the input image signal, but the data size is different. Various known algorithms including interpolation calculation may be applied to the image enlargement / reduction processing.

  The CMYK color conversion unit 122 converts the first intermediate image signal acquired from the resolution conversion unit 120 into a device color signal (CMYK color signal) handled by the image forming apparatus 10 using a known color matching method. The second intermediate image signal obtained here corresponds to a multi-tone CMYK color signal.

  The halftone processing unit 124 converts the second intermediate image signal acquired from the CMYK color conversion unit 122 into a control signal (a signal used for controlling the head unit 42) for appropriately controlling ink ejection. The control signal obtained here is binary data (or multi-value data) for each CMYK that controls the presence / absence (on / off) of the ink ejection operation for each line head 44 in time series. In the present embodiment, an application example of the systematic dither method using the threshold matrix Mt will be described.

  FIG. 7 is a schematic explanatory diagram of halftone processing by a systematic dither method. As an example, the concept of binarization using a Bayer-type threshold matrix Mt is shown. First, each address of the multilevel CMYK color signal is associated with each matrix element of the threshold value matrix Mt. Then, the magnitude relationship between the pixel value of the pixel of interest and the threshold value of the matrix element of interest is compared, and if the pixel value is larger, “1 (on)” is assigned, otherwise Assigns “0 (off)”. In this way, the number of gradations of the image signal can be converted from multi-value to binary.

  Hereinafter, a method of creating the threshold value matrix Mt by the threshold value matrix creating unit 82 will be described in detail with reference to FIGS.

  FIG. 8 is a functional block diagram of the threshold matrix creating unit 82 shown in FIG.

  The threshold value matrix creating unit 82 sequentially specifies gradation levels that have not yet been set (specified levels lv described later), and gradation levels specified by the gradation level specifying unit 130. A constraint condition setting unit 132 that sets a constraint condition in the dot pattern DPT, a dot pattern determination unit 134 that determines a dot pattern DPTfix according to the specified gradation level, and a dot determined by the dot pattern determination unit 134 A dot pattern storage unit 136 that stores a pattern DPTfix, a threshold value conversion unit 138 (threshold determination unit) that creates a threshold matrix Mt based on the dot pattern DPTfix stored by the dot pattern storage unit 136, and the dot pattern determination unit Various intermediate data generated in the calculation process at 134 An intermediate data storage unit 140 to pay, and a density variation information storage unit 142 for storing the density variation information described later, and a matrix condition determining unit 144 for determining various conditions to create a threshold matrix Mt.

  The dot pattern determination unit 134 includes a group classification unit 146, a dot number distribution unit 148, an initial pattern creation unit 150, a dot position movement unit 152, an evaluation value map calculation unit 154, an overall evaluation value calculation unit 156, An update availability determination unit 158. Here, the initial pattern creation unit 150, the dot position movement unit 152, the evaluation value map calculation unit 154, the overall evaluation value calculation unit 156, and the update availability determination unit 158 optimize dot arrangement based on predetermined evaluation conditions. It functions as a dot pattern creation unit 160.

  The matrix condition determining unit 144 includes a matrix size determining unit 162 that determines the size of the threshold matrix Mt, and a dot recording rate determining unit 164 that determines a dot recording rate according to the gradation level.

  Hereinafter, an outline of the operation of the threshold value matrix creating unit 82 will be described with reference to the flowcharts of FIGS. 9 and 14 and the functional block diagram of FIG. The threshold value matrix Mt may be the same regardless of the color plate (for example, CMYK), or may be different for each color plate.

  Here, the threshold matrix Mt is created on the premise that an image is formed in the configuration example of the line head 44 shown in FIG. Note that the row direction of the matrix corresponds to the conveyance direction of the paper 12. The column direction of the matrix corresponds to the arrangement direction of the nozzles 102 (ink chamber units 100).

  In the present embodiment, in order to calculate the threshold value of each matrix element, a virtual rectangular grid is prepared that schematically illustrates the dot on / off state (see FIG. 7). Thereafter, the number of dots to be arranged on the rectangular lattice is determined according to the gradation level, and the dot arrangement order is optimized by sequentially repeating the dot arrangement and its evaluation. Then, based on the optimized dot arrangement order, the threshold value in each matrix element of the threshold value matrix Mt is determined.

  Hereinafter, for convenience of explanation, each region of the rectangular lattice is referred to as a “cell”. In addition, virtual dots (on states) arranged on the respective cells corresponding to the dots formed on the paper 12 may be simply referred to as “dots”.

  In FIG. 9, first, the matrix size determining unit 162 determines the size of the threshold matrix Mt (step S1). Prior to the execution of this step, information related to density fluctuation (hereinafter referred to as density fluctuation information) associated with a temporal error in dot formation position is stored in advance in the density fluctuation information storage unit 142. The term “density” in this specification means the optical density of an image formed on the paper 12.

  During the printing operation of the image forming apparatus 10, in order to generate an entire image composed of a plurality of image sequences, the entire apparatus may vibrate due to repeated driving of components. The vibration is transmitted to the head unit 42 (or the image forming drum 40), so that the landing positions of the ink droplets shift with time. That is, the density variation appears as wavy stripes along the conveyance direction (arrow Y direction) of the paper 12.

  Even if this vibration occurs when the head unit 42 or the image forming drum 40 itself is driven, the vibration is transmitted to the head unit 42 or the image forming drum 40 as the other components are driven. There may be. Further, the cause of the temporal variation is not limited to vibration, but may be a short-cycle error that occurs when generating the entire image.

  FIG. 10A is a schematic explanatory diagram showing the amount of misalignment of dots to be formed. The actual landing position is O ′ with respect to the target position O of the landing position (that is, the center position where the dot is formed). At this time, the difference in the arrow X direction between the landing position O ′ and the target position O is defined as a positional deviation amount δX. That is, when the positional deviation amount δX is a positive value, it means that it has landed to the right (in the positive direction of the arrow X) with respect to the target position O, and when the positional deviation amount δX is a negative value, Means that it has landed to the left (the negative direction of arrow X).

  FIG. 10B is a graph showing fluctuation characteristics of the positional deviation amount δX of dots formed by ejection from the first nozzle 102a shown in FIG. The horizontal axis of the graph is the position in the arrow Y direction (conveyance direction), and the vertical axis is the positional deviation amount δX.

  Each plot corresponds to each position on the leftmost image sequence shown in FIG. As can be seen from this figure, this plot has a periodicity with six pixels (cells) as one cycle with δX = 0 as a reference. The plots P1 to P3 have the same phase.

  FIG. 10C is a graph showing variation characteristics of the positional deviation amount δX of dots formed by ejection from the second nozzle 102b shown in FIG. The horizontal axis of the graph is the position in the arrow Y direction (conveyance direction), and the vertical axis is the positional deviation amount δX.

  Each plot corresponds to each position on the second image sequence from the left side shown in FIG. Similar to FIG. 10B, this plot has a periodicity with six pixels (cells) as one cycle with δX = 0 as a reference. Here, the plot P4 has the same phase as the plot P2 (see FIG. 10B), and the plot P5 has the same phase as the plot P3 (see FIG. 10B). That is, the plot of FIG. 10C is shifted (advanced) as a whole by the discharge timing (Δt = 1) for one time as compared with FIG. 10B.

  When the generation cycle (number of pixels) is acquired as the density variation information from the density variation information storage unit 142, the matrix size determination unit 162 sets a value that is an integer multiple of the number of image columns corresponding to the generation cycle to the row of the threshold matrix Mt. It is determined as the size in the direction (arrow Y direction). In the example of FIG. 10B, 6, 12, 18, etc. correspond.

  Note that, as the density fluctuation information, not only the generation period but also actual measurement data of the positional deviation amount δX and various kinds of information for calculating the generation period may be used. The conveyance speed of the paper 12 in the arrow Y direction is V [mm / s], and the sample interval (output resolution) is Sp [mm / pixel]. When the vibration frequency of the image forming apparatus 10 is F [Hz], the size of the threshold matrix Mt in the row direction may be determined as an integer closest to V / (Sp · F).

  As described above, it is assumed that the matrix size determining unit 162 determines the size of the threshold matrix Mt to be 6 × 8 (6 rows and 8 columns).

  Next, the group classification unit 146 classifies each cell corresponding to the threshold value matrix Mt into a plurality of phase groups (step S2).

  FIG. 11A is a schematic explanatory diagram showing the ink ejection order corresponding to each image position in the configuration example of the line head 44 shown in FIG. 5. In order to specify the row number of each cell, subscripts “A to F” are added to the left of the rectangular lattice. In addition, in order to specify the column number of each cell, subscripts “a to h” are appended above the rectangular lattice. Hereinafter, the same notation is used for each drawing (FIGS. 11B and 12A) for explaining this type of rectangular lattice. For convenience of explanation, for example, a cell in the upper left corner of a rectangular grid may be written as (A, a) to specify its position.

  In this figure, the arithmetic numbers written in each cell represent the ink ejection order in the entire image. For example, ejection is performed at the earliest timing to an image position corresponding to a cell in which “1” that is the minimum value is written. In addition, ejection is performed at the latest timing to the image position corresponding to the cell in which “9” which is the maximum value is written.

  FIG. 11B is a schematic explanatory diagram showing a phase corresponding to the ink ejection order shown in FIG. 11A. In this figure, the arithmetic numbers written in each cell represent the phase of the fluctuation characteristic of the positional deviation amount δX shown in FIGS. 10B and 10C. For example, the cells in the A row (eight cells) are classified into a set of four types of phases from “1” to “4”. Further, the cells in the a-th column (six) are classified into a set of all types of phases from “1” to “6”. Hereinafter, the time characteristic of the positional deviation amount δX is time-divided, and a set of cells grouped according to each phase is defined as “phase group”. Here, in the halftone process, each pixel of the image signal is associated with each cell of the threshold value matrix Mt. In other words, classifying each cell into a phase group is equivalent to classifying each pixel into a pixel group.

  As shown in FIG. 12, the eight cells labeled “1” in FIG. 11B are classified into “first phase group”. Eight cells labeled “2” are classified into “second phase group”. Eight cells labeled “3” are classified into “third phase group”. Hereinafter, similarly, the first to sixth phase groups (number of phases N = 6) are classified.

  Note that plots P1 to P3 (see FIG. 10B) and plots P4 and P5 (see FIG. 10C) respectively correspond to cells (positions) belonging to the first phase group. That is, the plots P1 to P3 correspond to cells in the A-th row and a-th column (A, a) of the threshold matrix Mt when halftone processing is performed on the input image signal using the systematic dither method. Position. Similarly, plots P4 and P5 are positions corresponding to the cells in the F-th row and the b-th column (F, b) of the threshold value matrix Mt.

  In this way, the group classification unit 146 classifies the input image signal into a plurality of phase groups according to the degree of the dot positional deviation amount δX (temporal error) (step S2).

  Next, returning to FIG. 9, the dot recording rate determination unit 164 determines the dot recording rate corresponding to each gradation level and each phase group (step S3). Here, the dot recording rate means a dot recording ratio (0 to 100%) with respect to the maximum number of dots capable of forming dots in each color of CMYK.

  The dot recording rate determination unit 164 determines a dot recording rate corresponding to the gradation level in advance, and determines the number of dots distributed to the first to sixth phase groups for each gradation level. At this time, the dot recording rate determination unit 164 distributes the dots so as to cancel the density fluctuation in the arrow X direction on the paper 12.

  In an image (printed material) formed on the paper 12, the print density is low in an image area where the dot distribution is sparse, and the print density is high in an image area where the dot distribution is dense. In the example of FIGS. 10B and 10C, the print density at the position becomes lower as the slope of the variation characteristic of the positional deviation amount δX is positive and the absolute value is larger. On the other hand, the negative the slope of the variation characteristic of δX and the larger the absolute value, the higher the print density at that position.

  FIG. 13 is a graph showing an example of determining the dot distribution ratio to each phase group. Here, the “dot distribution ratio” (distribution ratio) to each phase group refers to the ratio of the number of dots belonging to each phase group to the total number of dots corresponding to the dot recording rate. That is, the sum of the dot distribution ratios to the first to sixth phase groups is always 100%. When dots are evenly distributed to each phase group, the dot distribution ratio to each phase group is about 16.7%.

  As shown in the figure, when the dot size is single, the dot distribution ratio to the first phase group may be higher than the dot distribution ratio to the remaining phase groups. That is, the expected decrease in print density can be canceled out by relatively increasing the number of dots belonging to the first phase group and increasing the density contribution to each image row.

  Similarly, when the dot size is single, the dot distribution ratio to the fourth phase group may be lower than the dot distribution ratio to the remaining phase groups. That is, the expected increase in print density can be canceled out by relatively reducing the number of dots belonging to the fourth phase group and reducing the density contribution to each image row.

  The number of phase groups for which the dot distribution ratio is relatively high (and / or low) is not limited to one, and the dot distribution ratio is relatively high (and / or low) for a plurality of phase groups. Needless to say, it may be.

  Next, returning to FIG. 9, the gradation level designation unit 130 designates one gradation level that has not yet been set (step S4). Hereinafter, the designated gradation level may be referred to as “designated level lv”. In the present embodiment, since the total number of cells (matrix elements) in the threshold matrix Mt is 48, this corresponds to 48 gradations (lv = 0 to 47). This designation may be in accordance with the order of gradation levels (ascending order or descending order), or may be sequentially designated at random.

  Next, the constraint condition setting unit 132 sets a constraint condition in the dot pattern DPT (including DPTfix and DPTtmp described later) at the specified level lv (step S5). Here, the constraint condition is a condition that restricts the arrangement of dots at each gradation level in order to determine the threshold value in each cell of the threshold value matrix Mt without contradiction. Specifically, it is necessary to inherit the dot arrangement and order already determined at the gradation level lower than the designated level lv even at the designated level lv. In addition, it is necessary to select a dot position from among dot arrangements already determined at a gradation level higher than the designated level lv. The dot pattern DPTfix of each gradation level that has already been determined may be read and referred to so as to satisfy all such constraints.

  Next, the dot pattern determination unit 134 determines a dot pattern DPTfix [lv] corresponding to the designated level lv (step S6). A specific determination method will be described later. The dot pattern storage unit 136 acquires the dot pattern DPTfix [lv] determined by the dot pattern determination unit 134 and temporarily stores it.

  Next, the gradation level designating unit 130 determines whether or not the dot pattern DPTfix [lv] has been established at all gradation levels (step S7). If it is determined that it has not been confirmed, the process returns to step S4, and thereafter, steps S4 to S6 are repeated until it is confirmed.

  On the other hand, if it is determined that the threshold value has been determined, the threshold value conversion unit 138 creates a threshold value matrix Mt using the dot pattern DPTfix acquired from the dot pattern storage unit 136 (step S8). Specifically, at the specified level lv, the difference between the dot pattern DPTfix [lv + 1] and the dot pattern DPTfix [lv] is calculated, and a threshold value (= lv) is assigned to one cell in which a new dot has occurred. .

  In this way, the threshold matrix creation unit 82 creates the threshold matrix Mt.

  Next, a specific method for determining the dot pattern DPTfix [lv] corresponding to the designated level lv in step S6 of FIG. 9 will be described in detail with reference to the flowchart of FIG. 14 and the functional block diagram of FIG.

  In the present embodiment, a method of sequentially repeating the creation of a dot pattern DPT in which the dot arrangement is variously changed and the evaluation using evaluation values (an overall evaluation value EVA and an evaluation value map EV_MAP described later) is used. In this case, various search algorithms such as a structural algorithm and a sequential improvement algorithm can be used as an optimization problem for determining the dot pattern DPT.

  Here, an optimization method of the dot pattern DPT by the void-and-cluster method (hereinafter referred to as the VC method) will be described.

  First, the dot number distribution unit 148 determines the number of dots to be arranged in the cell for each phase group according to the designated level lv (step S61). Here, the dot number distribution unit 148 sets the number of dots arranged in the first to sixth phase groups at the designated level lv according to the dot recording rate and the dot distribution ratio (see FIG. 13) acquired from the dot recording rate determination unit 164. To decide.

  Next, the initial pattern creation unit 150 creates a dot pattern DPTini as initial data (step S62). The initial pattern creating unit 150 obtains the dot pattern DPTini by arranging the number of dots determined in step S61 on the cells in the rectangular lattice having the size determined in step S1.

  The arrangement of the dots may be determined randomly based on, for example, a random value generated using a pseudo-random number generation algorithm. Various algorithms such as Mersenne Twister, SFMT (SIMD-oriented Fast Mersenne Twister), and Xorshift method may be used as a pseudo-random number generation algorithm.

  Next, the evaluation value map calculation unit 154 calculates an evaluation value map EV_MAPtmp as temporary data from the dot pattern DPTini created in step S62 (step S63). The evaluation value map EV_MAPtmp includes each evaluation value calculated for each cell based on a predetermined evaluation function. The larger the evaluation value is, the better the performance is designed. As the evaluation function, a function obtained by quantifying various evaluation items including image quality items such as granularity and sharpness, ink usage, and the like may be applied. In particular, the dot pattern DPT in consideration of visibility can be determined by using an evaluation value obtained by quantifying the granularity according to the human visual response characteristic {eg, the Dooley-Shaw function).

  Next, the overall evaluation value calculation unit 156 calculates the overall evaluation value EVAtmp as temporary data from the evaluation value map EV_MAPtmp created in Step S63 (Step S64). The evaluation value map EV_MAP (EV_MAPtmp) is an individual evaluation value in each cell, whereas the overall evaluation value EVA (EVAtmp) is an evaluation value for the entire arrangement of dots. Various methods can be used for calculating the overall evaluation value EVA. For example, the sum for each cell of the evaluation value map EV_MAP may be used.

  Next, temporary data such as a dot pattern DPTtmp is stored (step S65). At this time, the intermediate data storage unit 140 overwrites and updates the dot pattern DPTini obtained in step S62 with respect to the initialized dot pattern DPT. Further, the intermediate data storage unit 140 overwrites and updates the overall evaluation value EVAtmp obtained in step S64 with respect to the initialized overall evaluation value EVA. Furthermore, the intermediate data storage unit 140 overwrites and updates the evaluation value map EV_MAPtmp obtained in step S63 with respect to the currently stored evaluation value map EV_MAP.

  Next, the update availability determination unit 158 substitutes 0 for a variable K that is an integer, and initializes K (step S66). Here, K is a counter representing the number of times of determination as to whether or not the dot pattern DPT needs to be updated.

  Next, the dot position moving unit 152 acquires the dot pattern DPTtmp by moving the dot position so as to satisfy the constraint conditions (step S67). This acquisition method will be described in more detail with reference to the flowchart of FIG.

  First, the dot position moving unit 152 selects one phase group to which the cell having the largest evaluation value map EV_MAP belongs (step S671). Hereinafter, the selected phase group is referred to as a “candidate group”. As another method, one phase group to which the cell having the smallest evaluation value map EV_MAP belongs may be selected.

  Next, the dot position moving unit 152 validates all replacement combinations in the candidate group (step S672). The exchange combination is a combination for exchanging the on / off state of dots between two cells. In the example of FIG. 12, since two combinations are selected from eight cells belonging to one phase group, 28 combinations are validated.

  Next, the dot position moving unit 152 determines a pair of cells (cell pair) having the maximum difference value of the evaluation value map EVA_MAP from the candidate groups (step S673). That is, the cell having the maximum value in the evaluation value map EVA_MAP and the cell having the minimum value are extracted one by one.

  Next, the dot position moving unit 152 determines whether or not the current dot pattern DPT changes by exchanging the cell pair selected in step S673 (step S674). Specifically, when the number of dots (ON state) in the selected cell pair is 0 or 2, the dot pattern DPT does not change. Further, when there is one dot (ON state) in the selected cell pair, the dot pattern DPT changes.

  If it is determined that there is no change, the exchange combination by the cell pair selected in step S673 is excluded (step S676), and another cell pair having the next largest difference value of the evaluation value map EVA_MAP is sequentially selected (step S676). S673).

  On the other hand, if it is determined to change, the dot position moving unit 152 further determines whether or not the constraint condition at the designated level lv is satisfied (step S675). When it is determined that the constraint condition is not satisfied, the exchange combination by the cell pair determined in step S673 is excluded (step S676), and another cell pair having the next largest difference value in the evaluation value map EVA_MAP is sequentially selected. (Step S673).

  When it is determined that the constraint condition is satisfied, the dot position moving unit 152 exchanges the on / off state of the cell pair with respect to the dot pattern DPT, and acquires a new dot pattern DPTtmp (step S677).

  In this way, the dot position moving unit 152 acquires the dot pattern DPTtmp by moving the dot position so as to satisfy the constraint conditions (step S67).

  Next, returning to FIG. 14, the evaluation value map calculation unit 154 calculates the evaluation value map EV_MAPtmp from the dot pattern DPTtmp (step S68). Then, the overall evaluation value calculation unit 156 calculates the overall evaluation value EVAtmp from the evaluation value map EV_MAPtmp (step S68). These calculation methods are the same as the calculation methods in steps S63 and S64.

  Next, the update possibility determination unit 158 compares the magnitude relationship between the overall evaluation value EVAtmp of the dot pattern DPTtmp and the overall evaluation value EVA that is the maximum value at the current time (step S69). If EVAtmp> EVA is not satisfied (that is, EVAtmp ≦ EVA), the process proceeds to the next step S71 without performing step S70.

  On the other hand, if EVAtmp> EVA is satisfied, it is determined that the dot pattern DPTtmp obtained in step S67 is the optimum dot pattern at the present time. At this time, the intermediate data storage unit 140 overwrites and updates the dot pattern DPTtmp obtained in step S67 with respect to the currently stored dot pattern DPT (step S70). Further, the intermediate data storage unit 140 overwrites and updates the overall evaluation value EVAtmp obtained in step S68 with respect to the currently stored overall evaluation value EVA (step S70). Further, the intermediate data storage unit 140 overwrites and updates the evaluation value map EV_MAPtmp obtained in step S68 with respect to the currently stored evaluation value map EV_MAP (step S70). Thereafter, the process proceeds to next Step S71.

  Next, the update availability determination unit 158 adds 1 to the current value of K (step S71).

  Next, the up / down determination unit 158 compares the current value of K with a predetermined value of Kmax (step S72). When the value of K is smaller, the update availability determination unit 158 further compares the magnitude relationship between the current overall evaluation value EVA and a predetermined allowable value EVA_OK (step S73). When EVA> EVA_OK is not satisfied (that is, EVA ≦ EVA_OK), the process returns to step S67 and steps S67 to S71 are sequentially repeated. In order to ensure sufficient convergence in this optimization calculation, for example, Kmax = 10000 can be set.

  When at least one of the first end condition (K> Kmax) or the second end condition (EVA> EVA_OK) is satisfied, the update availability determination unit 158 tentatively determines the dot pattern DPT at the present time. It determines as DPT (step S74).

  In this way, the dot pattern determination unit 134 determines the dot pattern DPT updated last as the dot pattern DPTfix [lv] (see step S6 in FIG. 9).

  As described above, since the signal conversion is performed so as to cancel out the density fluctuation in the arrangement direction (arrow X direction) on the paper 12 due to the time-dependent error of the dot formation position corresponding to a plurality of timings, the dot formation position Thus, it is possible to form an image in which the generation of streaks due to the time-dependent error is suppressed.

  You may comprise this Embodiment like the following 1st-3rd modification. The same components as those of the image forming apparatus 10 according to the present embodiment (see FIGS. 1 to 15) are denoted by the same reference numerals, and detailed description thereof is omitted.

<First Modification>
The image processing unit 80 </ b> A according to the first modification has a preferable configuration when the line head 44 can form dots of a plurality of sizes (so-called multi-size dots).

  FIG. 16 is a schematic explanatory diagram illustrating the flow of image processing in the image processing unit 80A according to the first modification.

  The image processing unit 80A includes the dot size allocation unit 200 in addition to the resolution conversion unit 120, the CMYK color conversion unit 122, and the halftone processing unit 124 described above. ) Is different. The dot size assigning unit 200 determines dot recording rates in a plurality of sizes in advance according to the gradation level. In the first modification, it is assumed that three types of dots of “large size”, “medium size”, and “small size” can be formed. The size of dots that can be formed is not limited to three types, and may be two types or four or more types.

  Hereinafter, the operation of the image processing unit 80A will be described. The image processing unit 80A sequentially performs resolution conversion, CMYK color conversion, and halftone processing on the input image signal to obtain a halftone signal that is binary data. Thereafter, the dot size assignment unit 200 generates a control signal by appropriately assigning “large size”, “medium size”, and “small size” to the halftone signal. FIG. 17 is a graph showing an example of assignment of a plurality of dot sizes in the dot size assignment unit 200.

  Then, the dot recording rate determination unit 164 determines that the dot distribution ratio for each size in at least one phase group corresponds to the dot distribution ratio for each size in the remaining phase groups according to the dot recording rate shown in FIG. Distribute differently.

  As shown in the figure, it is preferable to allocate a large number of large size dots to the cells belonging to the first phase group with respect to the determined dot pattern DPTfix. That is, by increasing the allocation ratio of the large size dots among the dots belonging to the first phase group and increasing the density contribution to each image row, it is possible to cancel the expected decrease in print density. .

  Similarly, it is preferable to allocate a large number of small-sized dots to the cells belonging to the fourth phase group with respect to the determined dot pattern DPTfix. That is, an expected increase in print density can be canceled by relatively increasing the small size allocation ratio among the dots belonging to the fourth phase group and lowering the density contribution to each image row.

  Even if comprised in this way, the effect similar to this Embodiment (refer FIG. 13) can be acquired. The number of phase groups for which the dot distribution ratio is relatively high (and / or low) is not limited to one, and the dot distribution ratio is relatively high (and / or low) for a plurality of phase groups. Needless to say, it may be.

  Note that, as in the first modification, the dot size allocation process and the halftone process may be sequentially performed independently, or the halftone process (a calculation process by a systematic dither method using the threshold matrix Mt) and You may do it at the same time.

<Second Modification>
The image processing unit 80B according to the second modified example is preferable in that the present invention can be applied to halftone processing (for example, error diffusion method, density pattern method, random dot method) other than the systematic dither method. It is.

  FIG. 18 is a schematic explanatory diagram illustrating the flow of image processing in the image processing unit 80B according to the second modification. The image processing unit 80B further includes a signal correction unit 202 and a halftone processing unit 204 in addition to the resolution conversion unit 120 and the CMYK color conversion unit 122 described above (image processing unit 80 in FIG. 6). And different. The signal correction unit 202 classifies the input image signal into a plurality of pixel groups, and corrects the image signal of at least one pixel group among the plurality of pixel groups. Since the classification of the pixel group is the same as the classification of the phase group described above, the following description will be given as “phase group”.

  Hereinafter, the operation of the image processing unit 80B will be described. The image processing unit 80B sequentially performs resolution conversion and CMYK color conversion on the input image signal, thereby acquiring a CMYK color signal that is multi-value data. Thereafter, the signal correction unit 202 corrects the CMYK color signal by performing gradation conversion processing on a predetermined pixel.

  FIG. 19 is a graph showing a correction example for each phase group in the signal correction unit 202 shown in FIG. In this manner, the signal correction unit 202 makes the gradation level (color signal value) of the pixels belonging to at least one phase group different from the gradation level (color signal value) of the pixels belonging to each remaining phase group. It may be corrected.

  As shown in the figure, the gradation level is preferably set higher for the pixels belonging to the first phase group than before the correction. That is, it is possible to cancel the expected decrease in print density by relatively increasing the gradation level of the pixels belonging to the first phase group and increasing the density contribution to each image sequence.

  Similarly, the gradation level of the pixels belonging to the fourth phase group may be set lower than that before correction. That is, the expected increase in print density can be canceled out by relatively lowering the gradation level of the pixels belonging to the fourth phase group and lowering the density contribution to each image sequence.

  Thereafter, the halftone processing unit 204 performs various halftone processes on the corrected CMYK color signal to obtain a control signal. Even if comprised in this way, the effect similar to this Embodiment (refer FIG. 13) can be acquired.

<Third Modification>
The image forming system 210 according to the third modified example is different from the present embodiment (the image forming apparatus 10 in FIG. 1) in that another apparatus executes the process of creating the threshold value matrix Mt.

  FIG. 20 is a block diagram illustrating a configuration of an image forming system 210 according to the third modification. The image forming system 210 includes an image forming apparatus 212 that forms an image, and a threshold matrix creating apparatus 214 that creates a threshold matrix Mt for halftone processing corresponding to the image forming apparatus 212.

  The threshold value matrix creating device 214, which is a device independent of the image forming device 212, includes a threshold value matrix creating unit 216. The threshold matrix creation unit 216 has a function equivalent to that of the threshold matrix creation unit 82 (see FIGS. 2, 6, and 8) according to the present embodiment.

  The image forming apparatus 212 basically has the same configuration as that of the image forming apparatus 10 according to the present embodiment (see FIG. 2), but does not include the threshold matrix creating unit 82 and is replaced with the ROM 84. An EEPROM 218 is provided.

  For example, the threshold matrix creation device 214 (threshold matrix creation unit 216) creates the threshold matrix Mt in advance based on the above-described density variation information in the image forming device 212. Thereafter, the threshold data 220 created by the threshold matrix creating device 214 is recorded in the EEPROM 218.

  By incorporating the EEPROM 218 into the image forming apparatus 212 so as to be able to read data, a separate calculation unit (threshold matrix creation unit 82 in FIG. 2) becomes unnecessary.

<Fourth Modification>
The group classification unit 146D (see FIG. 8) according to the fourth modification is different from the present embodiment in that the phase group definition method is different. In this embodiment, the time characteristics of the positional deviation amount δX (see FIGS. 10A to 10C) are grouped by time division. The threshold value matrix Mt using this classification method has a property of actively canceling density fluctuations following a time-dependent error in dot formation positions. That is, it is particularly effective when the reproducibility of the time-dependent error is high (the periodicity is strong).

  On the other hand, in the fourth modification, the position characteristics of density fluctuations in the arrangement direction (arrow X direction) are grouped by space division. The threshold value matrix Mt using this classification method has a property of relatively canceling density fluctuations within a predetermined block range. That is, even when the reproducibility of the error with time is not so high, it is particularly effective because it exhibits robustness when the reproducibility of the density fluctuation at the two-dimensional position is high.

  FIG. 21 is a graph showing the position characteristics of density fluctuations in an image sequence ejected from a plurality of nozzles including the first nozzle 102a and the third nozzle 102c shown in FIG. The horizontal axis of the graph is the position in the arrow X direction (arrangement direction), and the vertical axis is the optical density.

  As can be understood from this figure, each plot has a periodicity with four pixels (cells) as one cycle with the average optical density Do as a reference. Plots P11 and P12 correspond to the positions of dots formed at the same timing. That is, this period corresponds to the interval between the first nozzle 102a and the third nozzle 102c.

  FIG. 22A is a schematic explanatory diagram illustrating the ink ejection order corresponding to each image position in the configuration example of the line head 44 illustrated in FIG. 5. This figure is basically the same as FIG. 11A, with the row numbers expanded to (A to J) and the column numbers expanded to (a to p). In this case, an image is generated by ejecting ink droplets at the first to thirteenth timings.

  FIG. 22B is a schematic explanatory diagram illustrating a phase corresponding to the ink ejection order illustrated in FIG. 22A. In this figure, the arithmetic numbers written in each cell represent the phase of the optical density position characteristic shown in FIG.

  For example, the cells in the A column (16 cells) are classified into a set of four types of phases from “1” to “4”. In order to facilitate understanding of this feature, the cells corresponding to the phase of “1” are hatched instead of the numeral “1”. For example, the cells of 4 columns (total of 40) are classified into the phase of “1”. In this way, each cell {(A, a) to (J, p)} in the rectangular lattice may be classified into the first to fourth phase groups, and the direction of the arrow X (arrangement due to the temporal error) The effect of canceling density fluctuations in the direction) is achieved.

  Moreover, the phase group classification form is not limited to a line (parallelly arranged parallel lines), and may be divided into a plurality of line segments as shown in FIG. 22C. Thus, by shortening the cycle unit, the periodicity in the arrangement direction can be relaxed, and noise and graininess of the image can be suppressed. The length of the line segment and the number of divisions may be variously changed.

  23A to 23C are schematic explanatory diagrams of dot patterns DPT1 to DPT3 created according to the phase group classification according to the fourth modification. Here, the dot patterns DPT1 to DPT3 were created after classifying into the first to fourth phase groups shown in FIG. 22B.

  Regarding the dot pattern DPT1 (see FIG. 23A), the dot distribution ratio (concentration contribution) in the first to fourth phase groups was all set to 25%. Regarding the dot pattern DPT2 (see FIG. 23B), the dot distribution ratios in the first to fourth phase groups were 40%, 30%, 20%, and 10%, respectively. Regarding the dot pattern DPT3 (see FIG. 23C), the dot distribution ratios in the first to fourth phase groups were 50%, 30%, 10%, and 10%, respectively.

  As described above, even if the position characteristics of density fluctuations in the arrangement direction (arrow X direction) are grouped by being spatially divided, the same effects as those of the present embodiment can be obtained.

  In addition, this invention is not limited to embodiment mentioned above, Of course, it can change freely in the range which does not deviate from the main point of this invention.

  For example, although the present embodiment has been described mainly with respect to CMYK (four color plates), the design can be changed to any color plate type and plate number without being limited thereto. For example, a standard ink of CMYK may be combined with an optional ink such as light colors such as LC and LM and W (white).

  In this embodiment, only the paper 12 is conveyed by the rotation of the image forming drum 40. However, at least one of the head unit 42 and the paper 12 may be conveyed. This is because the present invention can be applied as long as both are relatively moved.

  Furthermore, it goes without saying that the size in the row direction or the column direction of the threshold value matrix Mt or the number of threshold levels is not limited to the present embodiment, and may be changed as appropriate.

DESCRIPTION OF SYMBOLS 10, 212 ... Image forming apparatus 40 ... Image forming drum 42 ... Head unit 44 ... Line head 80 (A, B) ... Image processing part 82, 216 ... Threshold matrix preparation part 86 ... Head driver 100 ... Ink chamber unit 102 ... Nozzle 104 ... Pressure chamber 106 ... Supply port 124, 204 ... Halftone processing unit 134 ... Dot pattern determination unit 138 ... Threshold conversion unit 142 ... Density fluctuation information storage unit 144 ... Matrix condition setting unit 146 (D) ... Group classification unit 148 ... Dot number distribution unit 150 ... initial pattern creation unit 152 ... dot position movement unit 154 ... evaluation value map calculation unit 156 ... overall evaluation value calculation unit 158 ... update availability determination unit 160 ... dot pattern creation unit 162 ... matrix size determination unit 164 ... Dot recording rate determination unit 200 ... dot size allocation unit 202 ... communication Correction unit 210 ... image forming system 214 ... threshold matrix generating apparatus

Claims (16)

A dot forming section comprising a plurality of dot forming elements arranged along the arrangement direction, and forming a plurality of dots on a recording medium using the plurality of dot forming elements;
A transport unit that relatively moves the dot formation unit and the recording medium by transporting at least one of the dot formation unit and the recording medium in a predetermined transport direction;
A dot formation control unit configured to control the dot formation unit based on a control signal so as to sequentially form dots at a plurality of timings and generate an image row in the arrangement direction under the relative movement by the conveyance unit; ,
The input image signal is supplied to the dot formation control unit so as to cancel out the density fluctuation in the arrangement direction on the recording medium due to the time-dependent error of the dot formation positions according to the plurality of timings. An image forming apparatus comprising: a signal conversion unit that converts the signal into the control signal. The image forming apparatus according to claim 1.
The signal converter is
A group classification unit for classifying the image signal into a plurality of pixel groups according to the degree of the temporal error;
Dots so that the density contribution to the image sequence of dots belonging to at least one pixel group classified by the group classification unit is higher than the density contribution to the image row of dots belonging to the remaining pixel groups. An image forming apparatus comprising: a dot number distribution unit that distributes. The image forming apparatus according to claim 2.
The group classification unit classifies the image signal into a plurality of pixel groups corresponding to the plurality of timings. The image forming apparatus according to claim 2 or 3,
The density contribution is a distribution ratio of dots of a single size,
The dot number distribution unit distributes dots so that the distribution ratio to the at least one pixel group is higher than the distribution ratio to the remaining pixel groups. The image forming apparatus according to claim 2 or 3,
The density contribution is a distribution ratio of dots of a single size,
The dot number distribution unit distributes dots so that the distribution ratio to the at least one pixel group is lower than the distribution ratio to the remaining pixel groups. The image forming apparatus according to claim 2 or 3,
The plurality of dot forming elements can form dots of a plurality of sizes,
The density contribution is a distribution ratio of dots for each size,
The signal conversion unit further includes a dot size allocation unit that allocates dots such that the distribution ratio to the at least one pixel group is different from the distribution ratio to the remaining pixel groups. Forming equipment. The image forming apparatus according to any one of claims 3 to 6,
The image forming apparatus according to claim 1, wherein the group classification unit classifies the image signals into the plurality of pixel groups by time-sharing time characteristics of the time-dependent error. The image forming apparatus according to any one of claims 3 to 6,
The image forming apparatus, wherein the group classification unit classifies the image signals into the plurality of pixel groups by spatially dividing the position characteristics of the density variation in the arrangement direction. The image forming apparatus according to any one of claims 1 to 8,
The image forming apparatus, further comprising: a halftone processing unit that converts the image signal into the control signal by a systematic dither method using a threshold matrix. The image forming apparatus according to claim 9.
The image forming apparatus, wherein the halftone processing unit changes a size of the threshold matrix according to a generation period of the temporal error. The image forming apparatus according to claim 10.
The size of the threshold matrix in the transport direction is an integer multiple of the number of image columns corresponding to the time-dependent error generation cycle. The image forming apparatus according to claim 1.
The signal converter is
A signal correction unit that classifies the image signals into a plurality of pixel groups according to a degree of the temporal error, and corrects the image signals of at least one pixel group of the plurality of pixel groups;
An image forming apparatus comprising: a halftone processing unit that converts the image signal corrected by the signal correction unit into the control signal using a halftone process. The image forming apparatus according to any one of claims 1 to 12,
The image forming apparatus according to claim 1, wherein the time-dependent error is a short-cycle error that occurs when an entire image composed of a plurality of the image sequences is generated. The image forming apparatus according to claim 13.
The image forming apparatus according to claim 1, wherein the short-cycle error is an error caused by vibration of the transport unit or the dot forming unit when the entire image is generated. A dot forming step for forming a plurality of dots on a recording medium using a plurality of dot forming elements arranged along the arrangement direction, and forming an image row in the arrangement direction by sequentially forming each dot at a plurality of timings An image forming method comprising:
An input step for inputting an image signal;
A signal conversion step of converting an input image signal into a control signal so as to cancel out a density variation in the arrangement direction on the recording medium due to a temporal error in dot formation positions according to the plurality of timings; ,
And a control step of controlling the plurality of dot forming elements based on the converted control signal. A threshold value matrix creating apparatus for creating a threshold value matrix for halftone processing according to an image forming apparatus,
The image forming apparatus includes a plurality of dot forming elements arranged along an arrangement direction, a dot forming unit that forms a plurality of dots on a recording medium using the plurality of dot forming elements, and the dot forming unit And at least one of the recording media in a predetermined transport direction, a transport unit that relatively moves the dot forming unit and the recording medium, and a plurality of timings under the relative movement by the transport unit. In the case of including a dot formation control unit that controls the dot formation unit based on a control signal so as to sequentially form dots and generate each image row in the arrangement direction,
The input image signal is supplied to the dot formation control unit so as to cancel out the density fluctuation in the arrangement direction on the recording medium due to the time-dependent error of the dot formation positions according to the plurality of timings. A threshold value matrix creating device comprising a threshold value matrix creating unit for creating a threshold value matrix to be converted into the control signal.

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