JP2013180564A - Image forming method and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming method and an image forming apparatus which improve an image quality by reducing visibility of streak-shaped density irregularities due to non-ejection and ejection abnormalities, and which improve an image quality by reducing visibility of random streaks due to enlargement of an ejection direction error of specific nozzles.SOLUTION: In a constitution equipped with a plurality of nozzles capable of striking droplets on the same striking position regarding a sub scanning direction, halftone processing is performed by a resolution of 1/N (N is an integer not smaller than 2) of an output resolution to an input image (10) (step S12). Nozzles are assigned so that dots composing the same line in the sub scanning direction include dots struck from different nozzles. Moreover, a correspondence between the assigned nozzles and striking positions in an output solution image is set (step S14). An output image (16) of the output resolution is generated on the basis of the nozzle allocation.

Description

  The present invention relates to an image forming method and an image forming apparatus, and more particularly to a halftone processing technique for converting multi-gradation image data into image data having a gradation value less than the original gradation value.

  2. Description of the Related Art An ink jet recording apparatus that forms a color image on a recording medium has a configuration including a full line type ink jet head in which nozzles are provided over a length corresponding to the entire width of the recording medium.

  According to the single pass method in which an image is formed over the entire surface of the recording medium by relatively moving the full-line type inkjet head and the recording medium only once, the recording medium is intermittently moved at a predetermined feed pitch in the sub-scanning direction. As compared with the serial method in which the head is scanned in the main scanning direction and the image is formed in the same direction, the image can be formed at a higher speed.

  On the other hand, when a discharge abnormality occurs in a certain nozzle among a number of nozzles provided in a full-line type ink jet head, streaky density unevenness along the sub-scanning direction occurs in the image formed on the recording medium. End up. Such streak-like density unevenness significantly reduces the image quality, so a countermeasure is required.

  In Patent Document 1, in multi-pass image formation using a serial type head, the dots of the first pixel group formed during forward movement and the dots of the second pixel group formed during double movement have blue noise characteristics or A technique is disclosed that is configured to have any one of green noise characteristics and suppresses image degradation caused by positional deviation between dots formed during forward movement and dots formed during double movement.

  In Patent Document 2, a plurality of line heads capable of ejecting ink of the same color and density are arranged on a print head, one pixel is configured by a dot pattern of n rows × n columns, and one pixel is n times the nozzle pitch. A technique for converting a dot pattern after halftone so that the number of dots allocated to each head is substantially equal when expressed at a resolution of 1 is disclosed.

  With such a configuration, even when a new ejection failure nozzle is generated during operation, density unevenness and streaks are prevented from newly occurring.

JP 2008-23893 A JP 2006-82528 A

  In general halftone processing, processing that makes poor visual graininess (e.g., evaluated by frequency characteristics in a two-dimensional space) inconspicuous is performed by an error diffusion method, a dither matrix method, or the like.

  However, there are many methods that do not take into consideration the occurrence of non-ejection and the ejection direction error of an actual nozzle, and it is necessary to consider this. As a method of consideration thereof, it is necessary to optimally design the droplet size, or the optimum design of the number of single rings, the number of colors, and the drawing (nozzle) density, which are factors related to more fundamental system specifications.

  Suppressing streaks by increasing the number of shingling, the number of colors, and the drawing density is not preferable because it leads to a decrease in productivity and an increase in production cost. In particular, assuming use in the printing field, a single-pass method is often employed, and a method of drawing the same line in the paper feed direction with about one or two nozzles in the paper feed direction (sub-scanning direction) In view of the situation where many are employed, suppression of streaks by a method other than the above is desired.

  Although the technique disclosed in Patent Document 1 is an effective method when a uniform landing error occurs in a two-dimensional space, the same effect is exhibited even when applied to image formation by a full-line inkjet head. I can't.

  In the technique disclosed in Patent Document 2, since the nozzle arrangement density is n times the image resolution, it is difficult to increase the image resolution depending on the structure of the head, even if an attempt is made to increase the image resolution. .

  The present invention has been made in view of such circumstances, and improves the image quality by reducing the visibility of streaky density unevenness (single streaks) due to non-ejection and ejection abnormality (first object). An object of the present invention is to provide an image forming method and an image forming apparatus that improve the image quality by reducing the visibility of random stripes due to an increase in the ejection direction error of a specific nozzle (second object).

  In order to achieve the above object, an image forming method according to the present invention comprises two or more nozzle rows composed of a plurality of nozzles arranged along the first direction over the entire length of the recording medium in the first direction. An image forming method using a droplet ejection unit having nozzles that can eject droplets at the same droplet ejection position, wherein the output resolution of the output image is 1 / Nth of the input image data. A halftone processing step of performing halftone processing at a resolution of (N is an integer of 2 or more) and a unit pixel in the halftone image after the halftone processing is performed in a second direction orthogonal to the first direction Nozzles belonging to each nozzle row are allocated so that droplets are ejected from at least two or more nozzles for one line along the line, and the correspondence between the allocated nozzles and the droplet ejection positions at the output resolution is A nozzle allocation step is because, in the second direction, includes: a droplet ejection step of performing droplet ejection at the output resolution by relatively moving the droplet ejection means and the recording medium.

  According to the present invention, for one line in the second direction, the nozzles are assigned in units of low resolution pixels during halftone processing so that droplets are ejected from nozzles belonging to at least two or more nozzle rows. Since the correspondence between the assigned nozzle and the pixel of the output image is determined and droplets from different nozzles are mixed on one line along the second direction, even if a discharge abnormality occurs in a specific nozzle, Since droplets from other nozzles exist on one line in the second direction, the visibility of streaky density unevenness along the second direction can be reduced.

6 is a flowchart showing a flow of an image forming method according to an embodiment of the present invention. Schematic plan view showing an example of multi-head nozzle arrangement Schematic plan view showing another nozzle arrangement of the multihead Explanatory diagram showing the relationship between output resolution and resolution in halftone processing, (a) multi-head nozzle arrangement, (b) explanatory diagram showing output resolution, (c) explanatory diagram of resolution in halftone processing Flow chart showing an example of halftone processing Illustration of error diffusion method Explanatory diagram schematically showing the relationship (parallel arrangement) between halftone image and nozzle allocation, (a) schematic diagram showing halftone image, (b) schematic diagram showing output image Explanatory drawing which shows the example of the real image to which the halftone process and nozzle allocation process shown in FIG. 7 were applied Explanatory drawing of the real image shown in FIG. An explanatory view schematically showing another example (series arrangement) of the relationship between the halftone image and the nozzle assignment, (a) a schematic view showing the halftone image, and (b) a schematic view showing the output image. Explanatory drawing which shows the example of the real image to which the halftone process and nozzle allocation process shown in FIG. 10 were applied Explanatory drawing of the real image shown in FIG. Explanatory drawing which shows typically other examples (random arrangement) of a halftone image Schematic plan view showing a configuration example of a multi-head having three nozzle rows, (a) an example of multiple heads, (b) an example of one head An explanatory view schematically showing the relationship between a halftone image and nozzle assignment when a multi-head having three nozzle rows is applied, (a) a schematic view showing a halftone image, and (b) an output image. Pattern diagram An explanatory view schematically showing another example of a relationship between a halftone image and nozzle assignment when a multi-head having three nozzle rows is applied, (a) a schematic view showing a halftone image, (b) Schematic diagram showing the output image An explanatory view schematically showing another example of a relationship between a halftone image and nozzle assignment when a multi-head having three nozzle rows is applied, (a) a schematic view showing a halftone image, (b) Schematic diagram showing the output image 1 is an overall configuration diagram showing a schematic configuration of an ink jet recording apparatus according to an embodiment of the present invention. FIG. 18 is a plan view showing a configuration of a printing unit in the ink jet recording apparatus shown in FIG. FIG. 18 is a block diagram showing the configuration of a control system in the ink jet recording apparatus shown in FIG.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings.

[Outline of image forming method]
FIG. 1 is a flowchart showing the flow of an image forming method according to the present invention. In the image forming method described below, the same droplet ejection position (halftone dot) is applied in the sub-scanning direction (the direction indicated by symbol S in FIG. 2 and the second direction), which is the recording medium conveyance direction. The present invention is applied to a configuration (see FIG. 2) that includes a plurality of nozzles that can be dropped.

  As shown in FIG. 1, when a multi-tone input image 10 is input, a color separation process for generating a color separation image separated for each color is performed (step S10: a color separation processing step).

  In the color separation process, for example, when image data represented by 8-bit data of R (red), G (green), and B (blue) (gradation value (density value) is 0 to 255) is input. , RGB are converted into C (cyan), M (magenta), Y (yellow), and K (black), and a predetermined process such as a gamma conversion process for correcting a linear deviation of the output value with respect to the input value is performed. A C / M / Y / K image (separated image) 12 for each color is formed.

  Next, the separation image 12 is converted to a resolution of 1 / N of the output resolution in the sub-scanning direction, and further, halftone processing is executed (halftone processing step: step S12). Although details of the halftone processing step will be described later, a multi-tone input image is converted to a tone value lower than the tone value of the input image. For example, a binary halftone image (halftone dot image) 14 representing on / off of each pixel (dot) is generated from an input image represented by gradation values from 0 to 255.

  In addition to binary, the halftone image 14 is ternary (when one pixel is composed of two or more dots, or four dots), four-value (when one pixel is composed of two or more dots, or three dots) It is also possible to express with

  When a halftone image (halftone image before nozzle allocation) 14 having a resolution of 1 / N (N is an integer of 2 or more) is generated by the halftone processing step (step S12), the nozzle allocation processing is executed. (Nozzle allocation step in step S14).

  In the nozzle assignment step, one of a plurality of nozzles is assigned to each pixel of the halftone image 14 before nozzle assignment (halftone image after nozzle assignment (not shown in FIG. 1, reference numeral 15 is assigned to FIG. 7). Further, an output image 16 for each CMKY color in which a resolution of 1 / N of the output resolution is converted into the output resolution is generated.

  In the nozzle allocation process, one nozzle from a plurality of nozzles that can eject droplets at the same droplet ejection position in the sub-scanning direction for each pixel of the halftone image or for each of a plurality of consecutive pixels (each unit pixel). Is assigned. In this nozzle allocation, pixels (dots) constituting one line in the sub-scanning direction are performed by droplet ejection from two or more nozzles (one line in the sub-scanning direction only by droplet ejection from one nozzle). In order not to form pixels (dots), the relationship between the assigned nozzle and the droplet ejection position at the output resolution is determined.

  The output image 16 for each CMKY color generated by the nozzle assignment step includes information on the ejection timing, color, and dot size. The output image 16 is sent to the printer, and predetermined processing is executed on the printer side (step S16: printer-side processing step).

  In the printer-side processing step, a driving voltage supplied to the inkjet head is generated based on the output image 16. The drive voltage generated in the printer-side process is supplied to the inkjet head at a predetermined drive cycle, and droplets are ejected from each nozzle provided in the inkjet head.

  FIG. 2 shows an example of a configuration in which a plurality of nozzles can eject droplets at the same droplet ejection position in the sub-scanning direction (shown with reference numeral S) (a configuration in which one pixel can form a plurality of nozzles). FIG. 2 is a schematic plan view showing an example of nozzle arrangement of a head (ink jet head, hereinafter simply referred to as a head).

  The head 20 shown in the figure includes a first head 20-1 and a second head 20-2. In the first head 20-1, a plurality of nozzles 22 are arranged in a line at regular intervals along the main scanning direction (indicated by reference numeral M and shown in the first direction).

  Similarly to the first head 20-1, the second head 20-2 has a plurality of nozzles 24 arranged at equal intervals in the main scanning direction, and the main head of each nozzle 24 of the second head 20-2. The position in the scanning direction coincides with the position of each nozzle 22 of the first head 20-1.

  That is, in the output image, the dots constituting the pixels 26 on one line in the sub-scanning direction can be formed by the nozzles 22 of the first head 20-1, and the nozzles 24 ( It can also be formed by a nozzle having the same position in the main scanning direction as the first head 20-1.

  FIG. 3 is a schematic plan view showing another nozzle arrangement of the multi-head. In the following description, the same or similar parts as those described above are denoted by the same reference numerals, and the description thereof is omitted.

  The head 20 ′ shown in the figure has a first nozzle row 28 and a second nozzle row 30. The first nozzle row 28 and the second nozzle row 30 shown in FIG. 3 correspond to the nozzle row of the first head 20-1 and the nozzle row of the second head 20-2 shown in FIG. 2, respectively.

  That is, the configuration in which a plurality of nozzles can eject droplets at the same droplet ejection position in the sub-scanning direction may be configured using a plurality of heads, or a plurality of nozzle rows along the main scanning direction are provided in one head. You may comprise.

  The nozzle arrangement of the heads 20 and 20 ′ illustrated in FIGS. 2 and 3 may be called a tandem nozzle. Such a multi-head (tandem nozzle) drives a plurality of nozzle rows in parallel to form an image at twice the speed without increasing the droplet ejection frequency compared to the case of one head (one nozzle row). be able to.

[Detailed description of halftone processing]
Next, the halftone process shown in step S12 of FIG. 1 will be described in detail. 4A and 4B are explanatory diagrams showing the relationship between the resolution of the output image and the resolution in the halftone process, where FIG. 4A shows the nozzle arrangement of the multi-head and FIG. 4B shows the resolution of the output image. FIG. 4 is an explanatory diagram of resolution at the time of (c) halftone processing. In the following description, it is assumed that the head 20 having the form shown in FIG. 2 is applied.

  In the halftone processing shown in this example, halftone processing is performed at a low resolution of 1 / N of the output resolution in the sub-scanning direction. For example, the resolution of the halftone process shown in FIG. 4C is half that of the output resolution shown in FIG. In FIG. 4C, reference numeral 26 'is assigned to one pixel in the halftone process.

  In the embodiment shown in FIG. 4, the resolution in the halftone process is the resolution obtained by dividing the output resolution by the number of heads. However, the halftone process may be performed at a resolution obtained by dividing the output resolution by a number exceeding the number of heads. Is possible.

  FIG. 5 is a flowchart showing the flow of halftone processing. First, a resolution conversion process for converting the resolution to 1 / N of the output resolution is performed on the color separation image (C / M / Y / K image) (step S20: resolution conversion process).

  For the resolution conversion process, a method of thinning out one of the two pixels to be processed, a method of obtaining an average pixel value of the two pixels to be processed, a method based on a predetermined mask pattern, or the like can be applied.

  Next, the density value (pixel value) of the processing target pixel is compared with a predetermined threshold value for each pixel (step S22 comparison processing), and the on / off state of the processing target pixel is determined. A halftone image ((C / M / Y / K HT image) 14 (see FIG. 1) is generated.

  On the other hand, a quantization error is calculated by subtracting the threshold value from the pixel value of the pixel to be processed (step S24: error calculation step), and the unprocessed area around the pixel to be processed is based on the error diffusion coefficient matrix stored in advance. An error is diffused to the pixels (step S26: error diffusion process) This procedure is executed for all the pixels to be processed, and each pixel is turned on or off.

  FIG. 6 is an explanatory diagram of error diffusion processing applied to the halftone processing. In FIG. 6, pixels with diagonal hatching represent processed pixels 42, and pixels with dot hatching represent processing target pixels 44.

  Note that a pixel illustrated by a bold line is one pixel in the halftone process. One pixel in the output image is a pixel illustrated by a thin line, and is one-half of the one pixel in halftone processing (resolution is double).

  In the error diffusion process, the quantization error generated in the processing target pixel 44 is diffused to the unprocessed pixels 46, 48, and 50 at a predetermined ratio. When processing the pixel 46 in which the error of the pixel 44 is diffused, a value obtained by adding the error to the original pixel value is compared with a predetermined threshold value.

  The halftone processing is not limited to the error diffusion method, and other methods such as a dither matrix method and a threshold matrix method can be applied.

[Detailed description of nozzle assignment processing]
Next, the nozzle assignment processing step shown in step S14 of FIG. 1 will be described in detail. FIG. 7 is an explanatory diagram schematically showing the relationship (parallel arrangement) between the halftone image after nozzle assignment and nozzle assignment. FIG. 7A is a schematic diagram showing the halftone image 15 after nozzle allocation, and FIG. 7B is a schematic diagram showing the output image 16.

  “1” shown in FIGS. 7A and 7B means a pixel formed by the nozzle belonging to the first head 20-1 in FIG. 2, and “2” means a nozzle belonging to the second head 20-2. It means the pixel to be formed. In the following description, one pixel is assumed to be composed of one dot.

  In the halftone image 15 after nozzle assignment shown in FIG. 7A, the first head 20-1 and the second head 20-2 are alternately used in the sub-scanning direction, and one line in the same direction is the first line. Pixels (dots) formed by droplet ejection of the head 20-1 and pixels formed by droplet ejection of the second head 20-2 are alternately arranged.

  In FIG. 7A, the nozzles belonging to the first head 20-1 or the nozzles belonging to the second head 20-2 are assigned in units of one pixel of the halftone image 15, and are shown in the upper left corner of the figure. As described above, it is also possible to assign nozzles using two or more pixels continuous in the sub-scanning direction of the halftone image 15 as unit pixels.

  In the output image 16 shown in FIG. 7B, the halftone image 15 after nozzle allocation is converted to a resolution of N times (2 times in the illustrated example). The output image 16 is composed of pixels generated by the same head (nozzle row) in one line in the main scanning direction.

  FIG. 8 is an explanatory diagram illustrating a relationship between an original image, a halftone image, and an output image when the parallel processing described above is applied as the nozzle assignment processing.

  An original image 10 shown in FIG. 8 is composed of three black (K) alphabetic characters (ABC). The original image 10 is subjected to color separation processing to generate a color separation image 12. The separation image 12 is subjected to resolution conversion processing and halftone processing to generate a halftone image 14 (before nozzle assignment) (halftone image 15 after nozzle assignment), and the resolution at the time of halftone processing is The output image 16 (parallel processing) is generated by converting into the output resolution.

  FIG. 9 is a diagram for explaining the effect when parallel processing is applied to nozzle allocation processing. In the nozzle allocation process for obtaining the output image 16 shown in the figure, particularly when non-ejection (ejection abnormality) independent of the head (nozzle array) occurs, a streak shape along the sub-scanning direction due to the occurrence of the ejection abnormality. The effect of reducing the visibility of the density unevenness is obtained.

  “Non-ejection” here means a state in which no dots are formed by ejecting liquid from the nozzle, “non-ejection” includes “non-ejection”, and furthermore, dots are formed by ejecting liquid from the nozzle. However, this includes a state in which an abnormality occurs in the dot size and the dot formation position (ejection direction).

  FIG. 10 is an explanatory diagram schematically illustrating another example (serial processing) between the halftone image after nozzle allocation and the nozzle allocation. FIG. 10A is a schematic diagram showing a halftone image 15 ′ after nozzle allocation, and FIG. 10B is a schematic diagram showing an output image 16 ′.

  The halftone image 15 ′ after nozzle assignment shown in FIGS. 10A and 10B includes pixels formed by nozzles in which one line in the main scanning direction belongs to the first head 20-1, and the second head 20-. 2 is composed of pixels formed by nozzles belonging to 2, and pixels formed by nozzles belonging to the first head 20-1 and pixels formed by nozzles belonging to the second head 20-2 are alternately arranged. Nozzle assignment is made.

  Further, in the output image 16 ′ shown in FIG. 10B, the pixels formed by the first head 20 and the pixels formed by the second head 20-2 are arranged in a staggered manner.

  FIG. 11 is an explanatory diagram illustrating a relationship between an original image, a halftone image, and an output image when the above-described serial processing is applied as the nozzle assignment processing. Moreover, FIG. 12 is a figure explaining the effect in case a serial process is applied to a nozzle allocation process.

  The nozzle allocation process for obtaining the output image 16 ′ shown in FIG. 12 is a streak-like line along the sub-scanning direction due to the occurrence of the ejection abnormality, particularly when the frequency of ejection abnormality of a specific head (nozzle row) is high. An effect of reducing the visibility of density unevenness can be obtained.

  FIG. 13 is an explanatory diagram schematically showing another example (random arrangement) of the halftone image 15 ″ after nozzle allocation. The following method is an example of realizing the random arrangement shown in FIG. First, an integer uniform random number (α) generated on a computer is generated for each pixel.

  Generally, a computer can generate an integer uniform random number. Examples of integer uniform random numbers that can be generated by a computer include a random function (rand ()) used in C language and the like.

  Next, a remainder obtained by dividing α by 2 (the number of nozzles that can eject droplets at the same droplet ejection position) is β, and a nozzle indicated by a value λ (= β + 1) obtained by adding 1 to β is assigned. When the number of nozzles that can eject droplets at the same droplet ejection position is 2, the value of β is “0” or “1”, so the value of λ is either “1” or “2”.

  When a pattern having periodicity exists in the ejection abnormality, if a simple pattern such as “series arrangement” or “parallel arrangement” described above is employed, the ejection abnormality pattern may be emphasized.

  According to such a random arrangement, an effect is exhibited without emphasizing a pattern of ejection abnormality with respect to ejection abnormality of a pattern having such periodicity.

  In the serial arrangement and the random arrangement, it is preferable that each line in the main scanning direction processed as the same line at the time of halftone processing is configured by the same number of droplets. That is, it is preferable that one line in the main scanning direction is configured by the same number of droplets ejected from each head (each nozzle row).

  It is preferable that the nozzle rows are assigned such that the number of dots ejected from the nozzles belonging to each head (nozzle row) on the same line in the sub-scanning direction is the same.

  By adopting such nozzle allocation, ejection abnormality occurs in one of the first head 20-1 (first nozzle row 28) and the second head 20-2 (second nozzle row 30). However, there is no bias, and robustness can be secured against streaky density unevenness along the second direction.

[Description of other examples of multi-head]
Next, a case where there are three nozzles capable of droplet ejection at the same droplet ejection position in the sub-scanning direction will be described. FIG. 14 is a schematic plan view showing a configuration example of a multi-head having three nozzle rows, FIG. 14 (a) shows an example having three heads, and FIG. 14 (b) shows one head. It is an example which comprises three nozzle rows.

  The head 120 shown in FIG. 14A includes a first head 120-1, a second head 120-2, and a third head 120-3, and is the same as the nozzle 122 of the first head 120-1 in the main scanning direction. The nozzle 124 of the second head 120-2 and the nozzle 125 of the third head 120-3 are located at the positions.

  The head 120 ′ shown in FIG. 14B includes a first nozzle row 128, a second nozzle row 130, and a third nozzle row 131, and is located at the same position in the main scanning direction as the nozzles 124 of the first nozzle row 128. The nozzle 126 of the second nozzle row 130 and the nozzle 125 of the third nozzle row 131 are located.

  FIG. 15 shows a halftone image and nozzle assignment when a multi-head having three nozzle rows that can be ejected at the same droplet ejection position in the sub-scanning direction shown in FIGS. 14A and 14B is applied. It is explanatory drawing which shows typically the relationship. FIG. 15A is a schematic diagram showing a halftone image after nozzle allocation, and FIG. 15B is a schematic diagram showing an output image.

  Note that “1” in FIGS. 15A and 15B is formed by nozzles belonging to the first head 120-1 in FIG. 14A or nozzles belonging to the first nozzle row 128 in FIG. 14B. “2” represents a pixel formed by a nozzle belonging to the second head 120-2 in FIG. 14A or a nozzle belonging to the second nozzle row 130 in FIG. “3” represents a pixel formed by a nozzle belonging to the third head 120-3 in FIG. 14A or a nozzle belonging to the third nozzle row 131 in FIG.

  The halftone image 115 after nozzle assignment shown in FIG. 15A is halftone processed at a resolution of one third of the resolution of the output image, and one line in the main scanning direction is the first head. Pixels formed by nozzles belonging to 120-1 (first nozzle row 128) (illustrated with “1”), pixels formed by nozzles belonging to the second head 120-2 (first nozzle row 128) (Shown with “2”), the pixels formed by the nozzles belonging to the third head 120-3 (third nozzle row 131) (shown with “3”) are arranged in this order. The nozzles are assigned such that

  As in the case of the two-head (two-nozzle row) configuration, one pixel in the halftone image may be used as a unit pixel for nozzle assignment, or two or more pixels continuous in the sub-scanning direction may be used as unit pixels for nozzle assignment. It is also possible (see FIG. 7A).

  In the output image 116 shown in FIG. 15B, the resolution of the halftone image 115 after nozzle assignment shown in FIG. 15A is converted into the resolution of the output image, and the nozzles of the halftone image 115 after nozzle assignment are displayed. Allocation is maintained.

  According to this aspect, in particular, when the frequency of ejection abnormality that does not depend on a specific head (nozzle row) is high, the effect of reducing the visibility of streaky density unevenness along the sub-scanning direction due to the occurrence of the ejection abnormality. Is obtained.

  FIG. 16 shows a halftone image and nozzle assignment when a multi-head having three nozzle rows that can be ejected at the same droplet ejection position in the sub-scanning direction shown in FIGS. 14A and 14B is applied. FIG. 16A is a schematic diagram illustrating a halftone image, and FIG. 16B is a schematic diagram illustrating an output image.

  The halftone image 115 ′ after nozzle assignment shown in FIG. 16A is further compared with the halftone image 115 after nozzle assignment shown in FIG. 15A, and pixels constituting one line in the main scanning direction. Are formed from nozzles belonging to different heads (nozzle rows).

  For example, the first row from the top in FIG. 16A (one row along the main scanning direction on the downstream side in the sub-scanning direction) is formed by the nozzles belonging to the first head 120-1 (first nozzle row 128). And pixels formed by nozzles belonging to the second head 120-2 (second nozzle row 130).

  The second row from the top in the figure shows pixels formed by nozzles belonging to the first head 120-1 (first nozzle row 128) and nozzles belonging to the second head 120-2 (second nozzle row 130). And pixels formed by nozzles belonging to the third head 120-3 (third nozzle row 131).

  Furthermore, the third row from the top in the figure belongs to the pixels formed by the nozzles belonging to the second head 120-2 (second nozzle row 130) and the third head 120-3 (third nozzle row 131). It is composed of pixels formed by nozzles.

  In the output image 116 ′ shown in FIG. 16B, the resolution of the halftone image 115 ′ after nozzle assignment shown in FIG. 16A is converted into the resolution of the output image, and the halftone image 115 after nozzle assignment is obtained. 'Nozzle assignment is maintained.

  This mode is an intermediate mode between the mode described above and the mode described below, and the same effect as both modes can be obtained.

  FIG. 17 shows a halftone image and nozzle assignment when a multi-head having three nozzle rows that can be ejected at the same droplet ejection position in the sub-scanning direction shown in FIGS. 14A and 14B is applied. FIG. 17A is a schematic diagram illustrating a halftone image, and FIG. 17B is a schematic diagram illustrating an output image.

  In the halftone image 115 ″ after nozzle assignment shown in FIG. 17A, a part of one line oblique to the main scanning direction and the sub-scanning direction is only by nozzles belonging to the same head (nozzle row). What is formed is included.

  For example, pixels formed by nozzles belonging to the third head 120-3 (third nozzle row 131) are arranged in an oblique direction.

  In the output image 116 ″ shown in FIG. 17B, the resolution of the halftone image 115 ″ after the nozzle assignment shown in FIG. 17A is converted into the resolution of the output image, and the halftone image 115 after the nozzle assignment. "Nozzle allocation" is maintained.

  According to this aspect, particularly when the frequency of ejection abnormality of a specific head (nozzle row) is high, an effect of reducing the visibility of streaky density unevenness along the sub-scanning direction due to occurrence of the ejection abnormality is obtained. It is done.

  In the three-head configuration illustrated in FIG. 14A and the three-nozzle row configuration illustrated in FIG. 14B, the random arrangement illustrated in FIG. 13 can be applied.

[Effect of the image forming method according to the present embodiment]
According to the image forming method configured as described above, in a configuration including a plurality of nozzles capable of forming pixels (dots) at the same droplet ejection position (halftone dot) in the sub-scanning direction, 1 / N of the output resolution. Halftone processing is executed with a resolution of (N is an integer of 2 or more), and one line in the sub-scanning direction is configured by droplet ejection from nozzles belonging to two or more heads (nozzle rows). Even if a discharge abnormality occurs in the nozzles of the nozzle row), the visibility of streaky density unevenness due to the discharge abnormality can be reduced.

  The parallel arrangement in which one line in the main scanning direction is formed by droplet ejection from nozzles belonging to the same head (nozzle row) is particularly effective for the occurrence of random ejection abnormalities independent of the head (nozzle row). .

  On the other hand, the serial arrangement in which one line in the main scanning direction is formed by droplet ejection from nozzles belonging to two or more heads (nozzle rows) is particularly effective when the occurrence frequency of ejection abnormality of a specific head is high. Demonstrate.

  Further, when droplet ejection is performed using nozzles belonging to two or more heads (nozzle rows) in the main scanning direction and the sub-scanning direction, an effect is exhibited particularly for ejection abnormalities that occur periodically.

[Device configuration example]
Next, an apparatus configuration example to which the above-described image processing method is applied will be described.

<Overall configuration>
FIG. 18 is an overall configuration diagram of the ink jet recording apparatus according to the embodiment of the present invention. An ink jet recording apparatus 200 (liquid ejection apparatus) shown in FIG. 18 is an on-demand type ink jet recording apparatus, and a recording medium transport unit 214 (relative movement means) that holds and transports the recording medium 212 and a recording medium transport unit 214. Inkjet heads 216K-1, 216K-2, 216C- that discharge color inks corresponding to K (black), C (cyan), M (magenta), and Y (yellow) to the recording medium 212 held in And a printing unit 217 (droplet ejecting means) including 1,216C-2, 216M-1, 216M-2, 216Y-1, and 216Y-2.

  The recording medium transport unit 214 includes an endless transport belt 218 provided with a number of suction holes (not shown) in a recording medium holding region where the recording medium 212 is held, and a transport roller (drive) around which the transport belt 218 is wound. A roller 220 and a driven roller 222) and a back side of the conveyance belt 218 in the recording medium holding area (a surface opposite to the recording medium holding surface on which the recording medium 212 is held), and a non-printing area provided in the recording medium holding area. A chamber 224 communicating with the illustrated suction hole and a vacuum pump 226 for generating a negative pressure in the chamber 224 are included.

  The carry-in unit 228 into which the recording medium 212 is carried is provided with a pressure roller 230 for preventing the recording medium 212 from floating, and the discharge roller 232 from which the recording medium 212 is discharged is also provided with a pressure roller 234. It has been.

  The recording medium 212 carried in from the carry-in unit 228 is given a negative pressure from the suction hole provided in the recording medium holding area, and is sucked and held in the recording medium holding area of the transport belt 218.

  On the conveyance path of the recording medium 212, a temperature adjustment unit 236 for adjusting the surface temperature of the recording medium 212 to a predetermined range is provided on the upstream side of the printing unit 217 (upstream side in the recording medium conveyance direction). A reading device (reading sensor) 238 for reading an image recorded on the recording medium 212 is provided on the rear side of the recording medium 217 (on the downstream side in the recording medium conveyance direction).

  The recording medium 212 carried in from the carry-in section 228 is sucked and held in the recording medium holding area of the transport belt 218, and after temperature adjustment processing by the temperature adjustment section 236 is performed, image recording is performed in the printing section 217.

  As shown in FIG. 18, the inkjet heads 216K-1, 216K-2, 216C-1, 216C-2, 216M-1, 216M-2, 216Y-1, 216Y-2 are arranged from the upstream side in the recording medium conveyance direction. They are arranged in this order. When the recording medium 212 passes directly under the inkjet heads 216K-1, 216K-2, 216C-1, 216C-2, 216M-1, 216M-2, 216Y-1, 216Y-2, Thus, each color ink of KCMY is ejected to form a desired color image.

  The printing unit 217 is not limited to the above-described form. For example, inkjet heads 216LC and 216LM corresponding to LC (light cyan) and LM (light magenta) may be provided. Further, the arrangement order of the inkjet heads 216K-1, 216K-2, 216C-1, 216C-2, 216M-1, 216M-2, 216Y-1, and 216Y-2 can be changed as appropriate.

  The recording medium 212 on which the image has been recorded is discharged from the discharge unit 232 after the recorded image (test pattern) is read by the reading device 238.

<Configuration of the printing unit>
FIG. 19 is a plan view illustrating a configuration example of the printing unit 217. The inkjet head 216 (216K-1, 216K-2, 216C-1, 216C-2, 216M-1, 216M) is viewed from the drawing surface side of the recording medium 212. -2, 216Y-1, 216Y-2) is a view of a surface on which an image is formed.

  In the following description, when it is not necessary to distinguish the inkjet heads 216K-1, 216K-2, 216C-1, 216C-2, 216M-1, 216M-2, 216Y-1, and 216Y-2 for each color, It may be described as an inkjet head 216.

  An inkjet head 216 shown in the figure is a full-line head having a plurality of nozzles (not shown) over a length corresponding to the entire width of the recording medium 212, and the recording medium 212 and the inkjet head 216 are relatively moved once. By scanning only, an image can be formed over the entire area of the recording medium 212 (single-pass method).

  Here, the “full width” of the recording medium 212 is the total length of the recording medium 212 in the direction (main scanning direction) orthogonal to the conveyance direction (sub-scanning direction) of the recording medium 212. It is good also as the full length in the same direction of the image formation area | region in which is formed.

  As shown in FIG. 19, as a structural example of the ink jet head 216, a structure in which a plurality of ink jet head modules are connected along the longitudinal direction (main scanning direction) can be mentioned. Examples of connecting a plurality of head modules include a form in which the head modules are connected in one line along the main scanning direction, and a form in which the head modules are connected in a zigzag pattern in two lines.

  Although the detailed structure of the ink jet head 216 is not shown, the ink jet head 216 includes a nozzle that discharges a liquid, a liquid chamber that communicates with the nozzle, and a discharge force generating element that generates a discharge force. ing. As the ejection force generating element, a piezoelectric system in which a piezoelectric element is provided on a wall constituting the liquid chamber, and the liquid chamber is deformed by the deformation of the piezoelectric element to eject the liquid can be applied.

  Further, as the discharge force generating element, a thermal method in which a heater is provided in the liquid chamber, the liquid in the liquid chamber is heated by the heater, and the liquid is discharged using a film boiling phenomenon can be applied.

  For the nozzle arrangement of the inkjet head 216, a matrix arrangement in which a plurality of nozzles are arranged along an oblique direction that is not orthogonal to the main scanning direction and the nozzle rows in the oblique direction are arranged along the main scanning direction may be applied. it can.

  By applying such a matrix arrangement, the substantial nozzle density in the main scanning direction can be increased. The nozzle arrangement is not limited to the matrix arrangement, and other arrangements such as a mode in which the nozzles are arranged in one line along the main scanning direction, and a two-row staggered arrangement may be applied.

<Description of control system>
FIG. 20 is a block diagram showing the configuration of the control system of the ink jet recording apparatus shown in FIG. As shown in the figure, the inkjet recording apparatus 200 includes a communication interface 300, a system control unit 302, a conveyance control unit 304, an image processing unit 306, a head drive unit 308 (droplet ejection control unit), an image memory 310, A ROM 312 is provided.

  The communication interface 300 is an interface unit that receives raster image data sent from the host computer 314. The communication interface 300 may be a serial interface such as USB (Universal Serial Bus) or a parallel interface such as Centronics. The communication interface 300 may include a buffer memory (not shown) for speeding up communication.

  The system control unit 302 includes a central processing unit (CPU) and its peripheral circuits, and functions as a control device that controls the entire inkjet recording apparatus 200 according to a predetermined program, and also functions as an arithmetic device that performs various calculations. Further, it functions as a memory controller for the image memory 310 and the ROM 312.

  That is, the system control unit 302 controls each unit such as the communication interface 300 and the conveyance control unit 304, performs communication control with the host computer 314, read / write control of the image memory 310 and the ROM 312 and the like. A control signal to be controlled is generated.

  The image data sent from the host computer 314 is taken into the ink jet recording apparatus 200 via the communication interface 300 and subjected to predetermined image processing by the image processing unit 306.

  The image processing unit 306 has a signal (image) processing function for performing various processing and correction processing for generating a print control signal from the image data, and the generated print data (dot data) is transferred to the head drive unit. It is a control part supplied to 308.

  An image processing unit 306 illustrated in FIG. 20 includes a color separation processing unit (step S10) illustrated in FIG. 1 and a halftone processing unit (halftone processing) that performs halftone processing (step S12). Means), a nozzle assignment processing unit (nozzle assignment means) for executing the nozzle assignment process (step S14), and the like.

  The halftone processing unit includes a resolution conversion unit that executes the resolution conversion step (step S20) of FIG. 5, a comparison unit that executes comparison processing (step S22), and an error calculation unit that executes error calculation processing (step S24). There may be a mode including an error diffusion processing unit that executes error diffusion processing (step S26) and an error diffusion matrix storage unit that stores an error diffusion matrix.

  When required signal processing is performed in the image processing unit 306, the ejection droplet amount (droplet ejection amount) and ejection of the inkjet head 216 via the head driving unit 308 based on the print data (halftone image data). Timing control is performed.

  Thereby, a desired dot size and dot arrangement are realized. The head driving unit 308 shown in FIG. 8 may include a feedback control system for keeping the driving conditions of the inkjet head 216 constant.

  That is, the head driving unit 308 includes a printer side processing unit that executes the printer side processing illustrated in FIG.

  FIG. 20 illustrates an aspect in which the image processing unit 306 is included in the ink jet recording apparatus 200, but there may be an aspect in which a pixel processing apparatus capable of realizing the function of the image processing unit 306 is provided separately from the ink jet recording apparatus 200. .

  The conveyance control unit 304 controls the conveyance timing and conveyance speed of the recording medium 212 (see FIG. 18) based on the print data generated by the image processing unit 306. 8 includes a motor that drives the driving roller 220 (22) of the recording medium transport unit 214 that transports the recording medium 212, and the transport control unit 304 functions as a driver of the motor. Yes.

  The image memory (temporary storage memory) 110 functions as temporary storage means for temporarily storing image data input via the communication interface 300, a development area for various programs stored in the ROM 312 and a calculation work area for the CPU. (For example, a work area of the image processing unit 306). As the image memory 310, a volatile memory (RAM) capable of sequential reading and writing is used.

  The ROM 312 stores programs executed by the CPU of the system control unit 302, various data necessary for control of each unit of the apparatus, control parameters, and the like, and data is read and written through the system control unit 302. The ROM 312 is not limited to a memory made of a semiconductor element, and a magnetic medium such as a hard disk may be used. Alternatively, a removable storage medium that includes an external interface may be used.

  The parameter storage unit 318 stores various control parameters necessary for the operation of the inkjet recording apparatus 200. The system control unit 302 appropriately reads parameters necessary for control, and updates (rewrites) various parameters as necessary.

  The program storage unit 320 is a storage unit that stores a control program for operating the inkjet recording apparatus 200. The system control unit 302 (or each unit of the device) reads a necessary control program from the program storage unit 320 when executing control of each unit of the device, and the control program is executed as appropriate.

  The display unit 322 is means for displaying various information sent from the system control unit 302, and a general-purpose display device such as an LCD monitor is applied. Note that lighting of the lamp (blinking and extinguishing) may be applied to the display form of the display unit 322. Further, sound (sound) output means such as a speaker may be provided.

  As the input interface (I / F) 324, information input means such as a keyboard, a mouse, and a joystick is applied. Information input via the input interface 324 is sent to the system control unit 302.

  The configuration of the ink jet recording apparatus 200 described with reference to FIGS. 18 to 20 is an example, and the addition, deletion, and change of the configuration can be made as appropriate.

  The above-described image forming method and apparatus can be changed, added, and deleted as appropriate without departing from the spirit of the present invention.

[Invention disclosed in this specification]
As will be understood from the description of the embodiments of the invention described in detail above, the present specification includes disclosure of various technical ideas including at least the following aspects.

  (First Aspect): Two or more nozzle rows composed of a plurality of nozzles arranged along the first direction are provided over the entire length in the first direction of the recording medium, and each nozzle row has the same droplet ejection position. An image forming method using a droplet ejection unit having a nozzle capable of droplet ejection at a resolution of 1 / N (N is an integer of 2 or more) of the output resolution of an output image with respect to input image data. At least two or more nozzles for one line along the second direction orthogonal to the first direction for each unit pixel in the halftone processing step for performing the halftone processing and the halftone image after the halftone processing is performed Nozzle assignment processing step in which nozzles belonging to each nozzle row are assigned so that droplet ejection is performed, and correspondence between the assigned nozzles and droplet ejection positions at the output resolution is determined, and the second direction Along, an image forming method comprising: a droplet ejection step of performing droplet ejection at the output resolution by relatively moving the droplet ejection means and the recording medium.

  According to the first aspect, with respect to one line in the second direction, nozzles are provided for each low-resolution unit pixel during halftone processing so that droplets are ejected from nozzles belonging to at least two or more nozzle rows. Is assigned, and the correspondence between the assigned nozzle and the pixel of the output image is determined, and droplet ejection from different nozzles is mixed on one line along the second direction. However, since droplets from other nozzles exist on one line in the second direction, the visibility of streaky density unevenness along the second direction can be reduced.

  In such an aspect, there may be an aspect in which the first direction is the main scanning direction and the second direction is the sub-scanning direction.

  The “unit pixel” of the halftone image may be one pixel or two or more pixels continuous in the second direction.

  (Second Aspect): The nozzle assignment step assigns nozzles so that the number of dots on each line in the first direction of the output image processed as one line in the main scanning direction at the time of halftone processing is the same. An image forming method.

  According to this aspect, even if ejection abnormality occurs in any one of the plurality of nozzle rows, there is no bias, and robustness can be secured against streaky density unevenness along the second direction. it can.

  (Third aspect): The image forming method in which the nozzle assignment step assigns the nozzle rows so that the number of dots ejected from the nozzles belonging to each nozzle row on the same line in the first direction is the same.

  According to such an aspect, as in the second aspect, even if ejection abnormality occurs in any one of the plurality of nozzle arrays, there is no bias and the stripe-shaped density unevenness along the second direction is prevented. Robustness can be secured.

  (Fourth aspect): The image forming method in which the nozzle assignment step assigns the nozzles so that the droplets from the nozzles belonging to the same nozzle row in the second direction do not continue.

  According to this aspect, even when a nozzle belonging to a specific nozzle row has an ejection failure, droplets are ejected from nozzles belonging to another nozzle row on the same line in the sub-scanning direction. In this case, even when ejection abnormalities increase, streaky density unevenness along the second direction is difficult to be visually recognized.

  (Fifth aspect): An image forming method in which the nozzle assigning step assigns nozzles belonging to the same nozzle row in the first direction.

  According to this aspect, even when a random ejection abnormality that does not depend on the nozzle row occurs, streaky density unevenness along the second direction is difficult to be visually recognized.

  (Sixth aspect): The image forming method in which the nozzle assigning step assigns nozzles belonging to different nozzle rows in the first direction.

  According to this aspect, even when the frequency of occurrence of abnormal discharge in a specific nozzle row is high, streaky density unevenness along the second direction is difficult to be visually recognized.

  (Seventh aspect): The image forming method in which the nozzle assigning step assigns nozzles randomly in the second direction.

  According to this aspect, even when a periodic ejection abnormality occurs, streaky density unevenness along the second direction is difficult to be visually recognized.

  (Eighth aspect): The image forming method in which the halftone processing step performs halftone processing at a resolution obtained by dividing the output resolution by the number of nozzle rows.

  According to this aspect, it is possible to suppress a decrease in image quality due to the resolution of the halftone process becoming too low. Further, the nozzle allocation process does not become complicated.

  (9th aspect): It has two or more nozzle rows composed of a plurality of nozzles arranged along the first direction over the entire length in the first direction of the recording medium, and each nozzle row has the same droplet ejection position. Droplet ejection means having a nozzle capable of droplet ejection, relative movement means for relatively moving the droplet ejection means and the recording medium along a second direction orthogonal to the first direction, and input image data And halftone processing means for performing halftone processing at a resolution of 1 / N (N is an integer of 2 or more) of the output resolution of the output image, and for each unit pixel in the halftone image after the halftone processing is performed In addition, for one line along the second direction orthogonal to the first direction, nozzles belonging to each nozzle row are assigned so that droplets are ejected from at least two or more nozzles, and the assigned nozzles and output resolution are assigned. An image forming apparatus comprising: a nozzle assigning unit that is determined to correspond to a droplet ejection position and a droplet ejection control unit that controls the droplet ejection unit to perform droplet ejection at an output resolution using the allocated nozzle .

  In this mode, it is preferable that the nozzle allocation unit allocates the nozzles so that the number of dots on the same line in the first direction is the same.

  The mode in which the nozzle assigning means assigns the nozzles so that the number of dots ejected from the nozzles belonging to each nozzle row on the same line in the first direction is the same as the nozzle assigning step is A mode in which nozzle allocation is performed so that droplets from nozzles belonging to the same nozzle row in two directions do not continue is preferable.

  Further, the mode in which the nozzle assigning unit assigns nozzles belonging to the same nozzle row in the first direction, the mode in which the nozzle assigning unit assigns nozzles belonging to different nozzle rows in the first direction, and the nozzle assigning unit in the second direction It is preferable to assign nozzles at random.

  In such an aspect, it is preferable that the halftone processing unit performs the halftone process at a resolution obtained by dividing the output resolution by the number of nozzle rows.

  (Tenth aspect): An image forming apparatus in which the droplet ejection means includes a plurality of inkjet heads each including one or more nozzle rows.

  (11th aspect): The image forming apparatus provided with the inkjet head in which the droplet ejection means was equipped with the some nozzle row.

  20,20-1,20-2,120,120 ', 120-1,120-2,120-3,216,216K-1,216K-2,216C-1,216C-2,216M-1,216M -2, 216Y-1, 216Y-2 ... head, 28, 30, 128, 130, 131 ... nozzle array, 22, 24, 122, 124, 125 ... nozzle, 214 ... recording medium transport unit, 302 ... system control unit 306: Image processing unit 308: Head driving unit

Claims (11)

Two or more nozzle rows composed of a plurality of nozzles arranged along the first direction are provided over the entire length of the recording medium in the first direction, and each nozzle row can eject droplets at the same droplet ejection position. An image forming method using a droplet ejection means having a nozzle,
A halftone processing step of performing halftone processing on the input image data at a resolution of 1 / N (N is an integer of 2 or more) of the output resolution in the output image;
For each unit pixel in the halftone image that has been subjected to the halftone process, each nozzle is ejected from at least two or more nozzles for one line along the second direction orthogonal to the first direction. A nozzle assignment step in which nozzles belonging to a row are assigned, and the correspondence between the assigned nozzles and the droplet ejection position at the output resolution is determined;
A droplet ejection step of performing droplet ejection at an output resolution by relatively moving the droplet ejection means and the recording medium along the second direction;
An image forming method comprising:   The nozzle allocation step performs nozzle allocation so that the number of dots on each line in the first direction of the output image processed as one line in the main scanning direction during halftone processing is the same. 2. The image forming method according to 1.   The image forming method according to claim 1, wherein the nozzle assigning step assigns the nozzles so that the number of dots ejected from the nozzles belonging to each nozzle row on the same line in the first direction is the same. .   4. The image forming method according to claim 1, wherein in the nozzle allocation step, nozzle allocation is performed so that droplet ejection from nozzles belonging to the same nozzle row in the second direction is not continuous. 5.   The image forming method according to claim 4, wherein the nozzle assigning step assigns nozzles belonging to the same nozzle row in the first direction.   The image forming method according to claim 4, wherein the nozzle assigning step assigns nozzles belonging to different nozzle rows in the first direction.   4. The image forming method according to claim 1, wherein the nozzle assigning step assigns nozzles at random in the second direction. 5.   The image forming method according to claim 1, wherein the halftone processing step performs halftone processing at a resolution obtained by dividing an output resolution by the number of nozzle rows. Two or more nozzle rows composed of a plurality of nozzles arranged along the first direction are provided over the entire length of the recording medium in the first direction, and each nozzle row can eject droplets at the same droplet ejection position. Droplet ejection means comprising a nozzle;
Relative movement means for relatively moving the droplet ejection means and the recording medium along a second direction orthogonal to the first direction;
Halftone processing means for performing halftone processing on input image data at a resolution of 1 / N of the output resolution in the output image (N is an integer of 2 or more);
For each unit pixel in the halftone image that has been subjected to the halftone process, each nozzle is ejected from at least two or more nozzles for one line along the second direction orthogonal to the first direction. Nozzle assigning means to which nozzles belonging to a row are assigned, and the correspondence between the assigned nozzles and the droplet ejection positions at the output resolution is determined;
Using the assigned nozzle, droplet ejection control means for controlling the droplet ejection means to perform droplet ejection at an output resolution;
An image forming apparatus.   The image forming apparatus according to claim 9, wherein the droplet ejection unit includes a plurality of inkjet heads each including at least one nozzle row.   The image forming apparatus according to claim 9, wherein the droplet ejection unit includes an inkjet head including the plurality of nozzle rows.