JP4581976B2 - Image processing method, image recording apparatus, and image recording medium - Google Patents

Description

  The present invention relates to an image recording apparatus such as an ink jet recording apparatus that discharges ink droplets to record an image on a recording medium, and more specifically, an image processing method based on image data, an image recording apparatus, and an image are recorded. The present invention relates to an image recording medium.

  An image recording apparatus for recording an image corresponding to digital image data (hereinafter referred to as image data) on an image recording medium such as recording paper (hereinafter referred to as recording paper), in addition to the one to which an electrophotographic process is applied And an ink jet recording apparatus (ink jet printer) that ejects ink droplets.

  In an inkjet recording apparatus, for example, full-color image recording is possible by using Y (yellow), M (magenta), (cyan), and K (black) ink liquids. In other words, a treatment liquid having a property of aggregating the coloring material and landing on the dots formed by the ink liquid is made to prevent ink bleeding.

  In the ink jet recording apparatus, a recording head in which a plurality of nozzles are arranged in the entire region along the direction (main scanning direction) orthogonal to the conveyance direction (sub scanning direction) of the recording paper is used. There has been a proposal for recording an image by ejecting ink droplets from a droplet ejection nozzle of a recording head without landing and landing on a recording sheet.

  By the way, in the ink jet recording apparatus, variations in the landing position of the ink liquid on the recording paper are caused by black streaks and white lines along the recording paper transport direction (sub-scanning direction) and the direction orthogonal to the transport direction (main scanning direction). It may appear as shading of the streaks. Such streaks are called banding or streak.

  On the other hand, some image recording apparatuses such as an ink jet recording apparatus can record only at a low gradation. In this case, the high gradation image data is quantized to a low gradation, and the low gradation data is recorded. A halftone process is performed to reproduce a halftone in a pseudo manner, and an image is recorded (dot formation) based on the quantized data.

  Here, the minimum unit of image data before quantization is referred to as a pixel, and the minimum unit of image data after quantization is referred to as a dot (corresponding to an ink droplet landed on a recording sheet).

  As the halftone processing for converting high gradation image data into low gradation image data, that is, quantization, there are a method using an error diffusion method and a threshold matrix (for example, dither screen, line screen). In these methods, pixels and dots correspond one-on-one. In addition, there is a halftone method in which one pixel is converted into a plurality of m × n dots and the gradation is represented by the number of dots, and this method is called a density pattern method.

  For proofreading, halftone processing using a line screen that is close to the halftone dot of the printing result using an actual printing plate is preferred, so halftone processing using a line screen has been proposed (for example, , Patent Document 1, Patent Document 2, and Patent Document 3.)

  When halftone processing using a line screen as described in Patent Documents 1 to 3 is applied to an ink jet recording apparatus, banding is performed on the line direction (the direction in which dots are likely to appear), which is the line tone of the line screen. By making the direction perpendicular to the direction, the banding can be made inconspicuous.

  However, due to halftone processing using a line screen, the bright line (low density area) of the image recorded on the recording paper has sparse dots and regularity due to using the same screen. In addition to this, dot arrangements having other keynotes (alignment) appear.

For example, as shown in FIG. 7 (A), and the gradation level 1, one dot D ₁ is formed on one region 90 which line screen process, as shown in FIG. 7 (B), one At low density (low gradation) such as gradation level 2 where two dots D ₁ and D ₂ are formed in one area 90, the dots formed in each area 90 have regularity, so There is a problem that not only M but also the main scanning direction (left and right direction on the paper surface), the sub scanning direction (up and down direction on the paper surface), and the direction orthogonal to the line direction M are generated, and the consistency is lost.

  On the other hand, in Patent Document 1 and the like, by changing the gradation level not in units of one dot but in units of three dots, for example, generation of a keynote in a direction different from the line direction is prevented.

However, changing the gradation level in units of a plurality of dots, such as 3 dots, reduces the gradation of the image formed on the recording paper. That is, there is a problem that when it is possible to express with 16 gradations, image recording can be performed only with 5 to 6 gradations.
Japanese Patent Laid-Open No. 2004-80065 JP 2004-15674 A JP 2004-166163 A

  The present invention has been made in view of the above facts, and when suppressing banding or the like by halftone processing using a so-called line screen, consistency is maintained even in a low gradation region without reducing gradation. An object of the present invention is to provide an image processing method, an image recording apparatus, and an image recording medium that prevent collapse and enable high-quality image recording.

An image processing method of the present invention that achieves the above object is an image processing method that enables image recording based on image data of a multi-tone image by dots formed on an image recording medium, wherein the basic tone direction is the main scanning direction or A first threshold value matrix that is inclined at a predetermined angle with respect to the sub-scanning direction, and a basic tone direction is the same as the first threshold value matrix, and in a direction orthogonal to the basic tone direction of the first threshold value matrix. A second threshold value matrix different from the first threshold value matrix, in which an array pattern of at least a low density region threshold value is axisymmetric with respect to a target line passing through the center of the first threshold value matrix, the second threshold value matrix and the first threshold value matrix, to apply alternately along the main scanning direction and the sub-scanning direction, against the image data of the multi-tone image Performing the halftone process Te by, for converting the quantized data for forming the dot on the recording medium, it is characterized.

According to the present invention, the basic threshold direction is the same as the first threshold value matrix whose tilt direction is inclined at a predetermined angle with respect to the main scanning direction or the sub-scanning direction, and is orthogonal to the basic tone direction of the first threshold value matrix. A second threshold matrix different from the first threshold matrix, in which the array pattern of at least the low-concentration region thresholds is axisymmetric with respect to a target line passing through the center of the first threshold matrix along the direction, alternately To perform halftone processing.

Thus, it is possible to suppress the regularity in a dot arrangement in a direction different from the trend direction of threshold value matrix is generated, can be prevented that the broken integrity.

  Further, as the second threshold value matrix, a threshold value matrix in which an array pattern of a plurality of threshold values including at least a low density region threshold value is axisymmetric with respect to the first threshold value matrix can be applied. When a plurality of threshold matrices having different threshold arrangement patterns are set as the second threshold matrix, the second threshold matrix may be selectively applied from the plurality of threshold matrices. .

  Furthermore, as the first threshold value matrix, an array pattern of at least a part of the threshold values excluding the threshold value in the low density region is a line with respect to a symmetric line passing through the center of the first threshold value matrix along the keynote direction. A threshold matrix that is symmetric can be applied, and when the plurality of threshold matrices having the same threshold pattern in the low density region along the key direction are set as the first threshold matrix, One threshold matrix can be selectively applied from the plurality of threshold matrices.

Such an image recording apparatus to which the present invention is applied is an image recording apparatus that records an image based on image data of a multi-tone image by dots formed on an image recording medium, the basic tone direction being the main scanning direction or A first threshold value matrix that is inclined at a predetermined angle with respect to the sub-scanning direction, and a basic tone direction is the same as the first threshold value matrix, and in a direction orthogonal to the basic tone direction of the first threshold value matrix. A second threshold value matrix different from the first threshold value matrix , in which at least a low density region threshold value array pattern is axisymmetric with respect to a target line passing through the center of the first threshold value matrix, applied alternately along the direction and the sub scanning direction, image processing means for converting the quantized data by performing halftone processing on the image data, the quantization de An image recording means for recording dots based on data on the image recording medium, it has good long as it contains.

Further, as an image recording apparatus, as a pre-Symbol second threshold matrix, to said first threshold matrix, including a threshold matrix is an array pattern axisymmetric multiple thresholds, including a threshold of at least the low density region Sometimes, the image processing means can selectively apply a second threshold matrix from the plurality of threshold matrices.

  Furthermore, as an image recording apparatus, at least a part of the first threshold value matrix excluding the threshold value in the low density region with respect to a symmetrical line passing through the center of the first threshold value matrix along the keynote direction. When the threshold arrangement pattern includes a plurality of threshold matrixes that are line symmetric, the image processing means can selectively apply the first threshold matrix from the plurality of threshold matrices.

  Further, as the image recording apparatus, an ink jet recording apparatus in which the image recording unit records the dots by discharging ink droplets containing a color material onto the recording medium can be used.

  According to the present invention, when banding is suppressed by halftone processing using a line screen, high consistency is obtained even in a low gradation region without reducing gradation, and high-quality image recording is achieved. The excellent effect that it becomes possible is obtained.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic configuration of an ink jet recording apparatus 10 applied as an image recording apparatus to the present embodiment.

  In the ink jet recording apparatus 10, a paper feed tray 14 is provided in the lower part of the housing 12. Recording paper 16 as an example of an image recording medium is stacked and stored in the paper supply tray 14. The recording paper 16 stored in the paper supply tray 14 is the uppermost layer by a pickup roll 18. Are taken out one by one.

  In the following, the recording paper 16 will be described as an example of the image recording medium. However, the present invention is not limited to this, and any image recording medium capable of image recording with an image recording apparatus can be applied.

  The recording paper 16 taken out from the paper feed tray 14 is transported along a paper feed transport path 22 formed in the housing 12 by a plurality of transport roller pairs 20.

  In the housing 12, an endless transport belt 28 stretched between a drive roll 24 and a driven roll 26 is disposed above the paper feed tray 14. The conveyor belt 28 is moved around by the rotational drive of the drive roll 24.

  A recording head array 30 serving as a droplet discharge head is disposed above the transport belt 28. The recording head array 30 is opposed to the flat portion of the conveyance belt 28 between the driving roll 24 and the driven roll 26, and the opposed region of the recording head array 30 is ejected from which ink droplets are ejected from the recording head array 30. It is an area.

  The recording paper 16 conveyed through the conveyance path 22 is held by the conveyance belt 28 and conveyed toward the discharge area. The recording head array 30 ejects ink droplets according to image information (image data) at the timing when the recording paper 16 passes through the ejection region, and attaches the ejected ink droplets to the recording paper 16. Image recording according to the image information is performed on the recording paper 16.

  Further, in the ink jet recording apparatus 10, the recording paper 16 is rotated while being held by the conveyor belt 28, so that the recording paper 16 passes through the ejection region a plurality of times, and so-called multipass image recording is possible.

  In addition, the recording paper 16 is opposed to the discharge area by providing the recording paper 16 by adsorbing and rotating it on the outer periphery of a cylindrical or column-shaped conveyance roll without providing the conveyance belt 28. In this case, the interval between the recording head array and the recording paper 16 is substantially uniform in the ejection region by, for example, curving the ejection region along the peripheral surface of the transport roll. What should I do?

  The recording head array 30 of the ink jet recording apparatus 10 applied to the present embodiment has an effective image recording area (ink droplet ejection area) having a length in the direction perpendicular to the conveyance direction of the recording paper 16 (the recording paper 16 4 ink jet recording heads corresponding to four colors of yellow (Y), magenta (M), cyan (C), and black (K) (hereinafter simply referred to as recording heads). 32 (32Y, 32M, 32C, 32K) are arranged along the transport direction.

  The recording head array 30 is longer than the width of the recording paper 16, and the recording head 32 of each color has nozzles for ejecting ink droplets closely arranged along the width direction of the recording paper 16 to perform recording. Ink droplets can be discharged toward the entire area along the width direction of the paper 16.

  Note that the recording head 32 has a plurality of rows such as four rows, for example, and is arranged two-dimensionally so that the nozzles in each row do not overlap along the conveyance direction of the recording paper 16. The nozzles are arranged closely with respect to the direction. Further, the configuration of the droplet discharge head to which the present invention is applied is not limited to this, and any configuration can be applied.

  In the recording head 32, an actuator of a droplet ejector (not shown) is driven according to image data, whereby ink droplets are ejected from the respective nozzles of the droplet ejector, and the ink droplets land on the recording paper 16. As a result, it adheres in a dot shape. At this time, for each recording head 32 (32Y, 32M, 32C, 32K), the drive timing of each actuator that ejects ink droplets is controlled according to the data based on the image information.

  As a result, the inkjet recording apparatus 10 can perform full-color image recording. As a configuration for ejecting ink droplets in the recording head 32, a known configuration such as a thermal method or a piezoelectric method can be applied.

  The recording head array 30 is immovable along a direction orthogonal to the conveyance direction, but may be configured to be movable as necessary, and thereby, by multi-pass image recording, higher resolution is achieved. It is possible to perform image recording and prevent a droplet discharge defect from appearing in the recording result. The recording head array 30 is not limited to this, and the recording head array 30 may be moved in the main scanning direction with the width direction of the paper 16 as the main scanning direction.

  An ink tank 34 (34Y, 34M, 34C, 34K) for storing Y, M, C, and K ink liquids is provided in the housing 12, and each of the recording heads 32 corresponds to the ink tank 34. A reservoir tank (not shown) is provided. The ink liquid in the ink tank 34 is replenished into the reservoir tank via an ink supply pipe (not shown) as the ink liquid in the reservoir tank is ejected from each of the recording heads 32 toward the recording paper 16. Is done.

  In the vicinity of the recording head array 30, four maintenance units 36 (36Y, 36M, 36C, 36K) corresponding to the four recording heads 32 (32Y, 32M, 32C, 32K) are arranged. When performing maintenance of the recording head 32 using the maintenance unit 36, the maintenance unit 36 is moved to a gap formed between the conveyance belt 28 and the recording head array 30, and each of the maintenance units 36 is moved to the recording head 32. It is made to oppose a nozzle surface (surface by the side of the conveyance belt 28).

  In this state, the maintenance unit 36 performs predetermined maintenance operations such as vacuum, dummy jet, wiping, and capping. The maintenance units 36 are divided into two groups of two, and are arranged on the upstream side and the downstream side in the conveyance direction of the recording paper 16 with the recording head array 30 interposed therebetween.

  In the ink jet recording apparatus 10, a charging roll 38 is provided on the conveyance path 22 side of the recording head array 30 so as to face the conveyance belt 28. The charging roll 38 is driven between the driven roll 26 while sandwiching the paper 16 together with the conveyance belt 28, and between a pressing position where the recording paper 16 is pressed against the conveyance belt 28 and a separation position separated from the conveyance belt 28. To be moved.

  The charging roll 38 is supplied with electric power of a predetermined voltage from a power source (not shown), thereby generating a potential difference with the grounded follower roll 26. By applying a charge to the recording paper 16 by this potential difference, the recording paper 16 is recorded. 16 is electrostatically attracted to the conveyor belt 28. The inkjet recording apparatus 10 is provided with a registration roll (not shown) on the upstream side of the charging roll 38 in the conveyance direction of the recording paper 16, and the recording paper 16 fed between the conveyance belt 28 and the charging roll 38 by this registration roll. Positioning is performed.

  A separation plate 40 is provided on the downstream side of the recording head array 30 in the conveyance direction of the recording paper 16. The recording paper 16 on which an image has been recorded is separated from the conveyance belt 28 by the separation plate 40. A cleaning roll (not shown) that sandwiches the conveyance belt 28 with the driving roll 24 is disposed below the peeling plate 40, and the surface of the conveyance belt 28 from which the recording paper 16 has been peeled is Cleaning is performed by a cleaning roll.

  A paper discharge tray 42 is provided in the upper part of the housing 12, and a paper discharge conveyance path 44 for conveying the recording paper 16 toward the paper discharge tray 42 is formed on the downstream side of the peeling plate 40 in the paper conveyance direction. Yes.

  The paper discharge conveyance path 44 includes a plurality of conveyance roller pairs 46, and the recording paper 16 peeled from the conveyance belt 28 by the peeling plate 40 is conveyed by the conveyance roller pairs 46 and discharged onto the paper discharge tray 42. And accumulated.

  Further, a reverse conveyance path 50 is formed between the paper feed tray 14 and the conveyance belt 28 by a plurality of conveyance roller pairs 48.

  The recording paper 16 on which the image is recorded on one side and sent to the paper discharge transport path 44 is reversed and sent to the paper feed transport path 22 by being sent to the reverse transport path 50. As a result, the inkjet recording apparatus 10 can record images on both sides of the recording paper 16.

  In the inkjet recording apparatus 10 configured as described above, when the image data is input, the recording paper 16 accommodated in the paper feed tray 14 is charged along with the driven roll 26 along the conveyance path 22. It is transported between the rolls 38. When the recording paper 16 is fed between the driven roll 26 and the charging roll 38, the recording paper 16 is held between the driven roll 26 and the charging roll 38 together with the conveying belt 28, and is held on the conveying belt 28 by being pressed against the conveying belt 28. .

  When the recording paper 16 held on the conveying belt 28 passes through an ejection area facing the recording head 32 of the recording head array 30 by the circular movement of the conveying belt 28, ink droplets from the recording head 32 according to image data. Is discharged, image recording based on the image data is performed.

  Here, when image recording is performed in only one pass, the recording paper 16 is peeled off from the transport belt 28 by the peeling plate 40, and the peeled recording paper 16 is transported along the paper discharge transport path 44 to be discharged. It is discharged to the tray 42.

  When performing multi-pass image recording, the recording belt 16 is passed through the ejection region a plurality of times by moving the conveyor belt 28 in a circular manner, and the recording paper 16 is conveyed when the image recording is completed. The paper is peeled off from the belt 28 and discharged to the paper discharge tray 42.

  On the other hand, FIG. 2 shows a schematic configuration of a control system of the inkjet recording apparatus 10. The control system of the inkjet recording apparatus 12 includes a control unit 60, a color conversion unit 62, an image processing unit 64, a recording data generation unit 66, and an image recording unit 68. The color conversion unit 62, the image processing unit 64, and the recording data generation unit 66 may be provided in an image processing apparatus such as a personal computer or a workstation that outputs image data to the inkjet recording apparatus 10, for example.

  The control unit 60 controls the color conversion unit 62, the image processing unit 64, the recording data generation unit 66, and the image recording unit 68 to form an image corresponding to the image data on the recording paper 16.

  Here, when the image data is input, the color conversion unit 62 performs color correction and density correction on the input image data in accordance with, for example, the characteristics of the recording paper 16 and the ink liquid, and also inputs the input data. If the processed image data is RGB data, it is converted to CMYK data. Such processing in the color conversion unit 62 can be applied with a general configuration that is performed using a preset conversion and correction table (LUT).

  The image processing unit 64 performs so-called halftone processing. That is, the image data having a relatively high gradation such as 256 gradations is converted into image data having the number of gradations that can be recorded by the image recording unit 68. This process is performed for each color of Y, M, C, and K.

  In a general inkjet image recording apparatus, the number of gradations that can be recorded is about 2 to 8. However, in the inkjet recording apparatus 10 applied to the present embodiment, as an example, when performing image recording, Basically, for each of Y, M, C, and K, there are two gradations, that is, the amount of ink droplets to be ejected (ejection droplet amount) is one type (normal amount).

  In the image processing unit 64, binarization processing is performed as quantization processing on each of the Y, M, C, and K image data, and the recording data generation unit 66 uses 2 in the image processing unit 64. The converted image data is converted into a data structure that can be decoded by the image recording unit 68, and the converted data is output to the image recording unit 68 in the recording order. The recording data creating unit 66 creates recording data to be output to the image recording unit 68 in consideration of the ejection timing and the data array mapped to the nozzle array of the recording head 32.

  The image recording unit 68 ejects ink droplets from each nozzle (droplet ejection nozzle) of the recording head 32 based on the data created by the recording data generation unit 66. At this time, the ejection of the ink droplets from each nozzle and the conveyance of the recording paper 16 are synchronized according to the data so that an image corresponding to the image data is recorded on the recording paper 16.

  As described above, the image processing unit 64 uses a binary data as quantized data obtained by performing the halftone process so that a predetermined gradation can be expressed. At this time, the image processing unit 64 performs screen processing using a line screen. That is, the image data of the number of gradations n (n is a natural number) is associated with each threshold value of a predetermined threshold matrix, and the target pixel and the threshold value corresponding to the target pixel are compared.

  Here, each pixel value of the image data of n gradations can take a value of 0 to (n−1). In the case of binarization, when the pixel value of the target pixel is equal to or greater than the threshold value, the pixel value of the pixel is quantized to 255, which is the maximum pixel value that each pixel can take, and the pixel value of the target pixel is When the threshold value is not reached, the pixel value is quantized to “0”, which is the minimum pixel value that each pixel can take.

  FIG. 3A shows a threshold matrix 70 as an example when halftone processing using a line screen is performed in this embodiment. The threshold value matrix 70 is a 4 × 4 size, a threshold value between 1 and 16 is set, and is a line screen applied when n = 16. Generally, image data on a computer has 256 gradations (n = 256) from 0 to 255. In this case, in order to use the threshold matrix 70, for example, the image data is first quantized to 16 gradations (for example, the image data is divided by 16 and the decimal part is rounded down), and then the threshold matrix 70 is used. It is possible to cope with the problem by binarizing or making corrections such as multiplying each value of the threshold matrix 70 by 16.

  This threshold value matrix 70 is such that the line direction M, which is the line tone, is inclined 45 degrees to the left with respect to the horizontal direction (left and right direction in FIG. 3). Here, as an example, the direction of the line tone is 45 degrees to the left, but the direction of the line tone is not limited to this and may be 45 degrees to the right, and may be in the main scanning direction or the sub-scanning direction. On the other hand, an angle within a predetermined range that approximates this angle may be used.

  By using such a threshold value matrix 70, the threshold value of the position along the line direction (arrow M direction) is set to be small. Further, by performing the quantization using the threshold value matrix 70, the pixel at the position along the line direction M is easily quantized to “n−1”, and along the line direction M in the low density region. Dots are easy to come out.

  As a result, in the inkjet recording apparatus 10, banding or the like along the main scanning direction or the sub-scanning direction is prevented from appearing in the image recorded on the recording paper 16, and the banding or the like appears to The quality is prevented from being deteriorated.

  On the other hand, in the image processing unit 64, in addition to the threshold matrix 70, a halftone process is performed using a threshold matrix set with reference to the threshold matrix 70. That is, halftone processing is performed using the first threshold matrix and the second threshold matrix as the threshold matrix 70.

  In the following description, when particularly distinguishing, the first threshold value matrix serving as a reference will be referred to as a threshold value matrix 70A, and the second threshold value matrix set based on the threshold value matrix 70A will be described as a threshold value matrix 70B.

  FIG. 3B shows the basics of the threshold matrix 70B. The threshold value matrix 70B is set so that the threshold value of at least the dot region F corresponding to the low density is a mirror image of the threshold value matrix 70A with respect to a straight line along the direction (arrow N direction) orthogonal to the line direction M. Has been.

That is, between the dot areas F ₁₁ , F ₂₂ , F ₃₃ , and F ₄₄ of the threshold value matrix 70 shown in FIG. 3A, the threshold values of the dot area F ₁₁ and the dot area F ₄₄ , and the dot area F ₂₂ and the dot threshold region _{F 33} is replaced.

In other words, the threshold matrix 70A of m × m, 70B, at least, the dot region _{F 11} and the dot area _{F (m-1) (m} -1) threshold, the dot area _{F 22} and the dot area _{F (m-2)} Each of the thresholds _(m-2) ,.

As the threshold matrix 70B against the threshold matrix 70A of the 4 × 4, further, as shown in FIG. 3 (C), the threshold value of the dot area _{F 13} and the dot regions _{F 24} and the dot regions _{F 31} and the dot area F ₄₂ threshold matrix 70C obtained by rearranging the threshold, as shown in FIG. 3 (D), the threshold value of the dot area _{F 12} and the dot regions _{F 34} and the threshold matrix obtained by rearranging the threshold dot areas _{F 21} and the dot regions _{F 43} 70D, as shown in FIG. 3 (E), the threshold value of the dot area _{F 13} with the threshold value of the dot area _{F 24,} the threshold value of the dot area _{F 31} and the dot regions _{F 42,} the dot region _{F 12} and the dot regions _{F 34} and, A threshold matrix 70E in which the threshold values of the dot area F ₂₁ and the dot area F ₄₃ are exchanged can be applied.

  The image processing unit 64 uses either the threshold matrix 70A or the threshold matrix 70B when forming two-gradation pixels by halftone processing.

  On the other hand, in the image processing unit 64, different threshold matrixes 70 are used instead of the same threshold matrix 70 between pixels adjacent in the main scanning direction or the sub scanning direction.

  That is, as shown in FIG. 4, the threshold matrixes 70A and 70B are alternately arranged in a tile shape and used. In FIG. 4, the threshold value matrix 70 </ b> A is blank, and the ground of the threshold value matrix 70 </ b> B is shaded.

  In the ink jet recording apparatus 10, an image is recorded on the recording paper 16 using the dots set in this way.

  In the inkjet recording apparatus 10 configured as described above, when image data is input, first, the color conversion unit 62 performs conversion processing (RGB-CMYK conversion), color correction processing, and the like on the input image data. The image data that has been subjected to color conversion and color correction is input to the image processing unit 64.

  When image data is input, the image processing unit 64 performs predetermined image processing. The inkjet recording apparatus 10 forms an image using ink liquids of C, M, Y, and K. From here, the image processing unit 64 converts the image data into Y, M, and C. , K is divided (separated) into data of each color component.

  Thereafter, the image processing unit 64 executes halftone processing using a line screen by alternately using preset threshold matrixes 70A and 70B. By performing this halftone process for each of Y, M, C, and K, data that enables gradation expression using binarized data is obtained.

  When the halftone process for all the Y, M, C, and K color data is completed, the recording data generating unit 66 prints the print data for all the Y, M, C, and K colors subjected to the halftone process. The recording data generation unit 66 generates the data and outputs the generated data to the image recording unit 68. As a result, an image corresponding to the image data is recorded on the recording paper 16.

  By the way, when halftone processing is performed on a low density image using a threshold matrix of one pattern, the pixels (dots) formed on the recording paper 16 are not only in the line direction M but also in the line direction. Other angles such as the direction orthogonal to the main direction, the main scanning direction, and the sub-scanning direction appear, and image consistency is lost.

  On the other hand, in the inkjet recording apparatus 10, as the threshold matrix 70 used for the halftone process, the threshold line matrix 70A in which the line direction M is inclined at 45 degrees with respect to the sub-scanning direction (or main scanning direction); For this threshold value matrix 70A, a threshold value matrix 70B is set in which a straight line along a direction orthogonal to the line direction M is used as a mirror surface and at least the threshold value of the low density region is changed to a mirror image.

  In the inkjet recording apparatus 10, the threshold matrixes 70A and 70B are alternately used along the main scanning direction and the sub-scanning direction.

  Thus, for example, in a 4 × 4 dot area, an image with one dot D having a low density (gradation level 1) corresponds to each of the threshold matrices 70A and 70B as shown in FIG. 5A. One dot D is recorded in the area to be printed.

  Further, as shown in FIG. 5B, in a low density (tone level 2) image with two dots D in a 4 × 4 dot area, the area corresponding to each of the threshold matrixes 70A and 70B. Two dots D are recorded.

  At this time, in the threshold value matrix 70A and the threshold value matrix 70B, the threshold values for the low density region are mirror images of each other. Therefore, in the regions corresponding to the adjacent threshold value matrices 70A and 70B, the positions of the dots D are different. The dot position is also shifted in the direction orthogonal to the line direction, the main scanning direction, and the sub-scanning direction.

  As a result, the keynote of the line direction M is clear, but the keynote at an angle different from the line direction M, such as the direction orthogonal to the line direction M, the main scanning direction, and the sub-scanning direction, becomes extremely thin. A pixel (dot D) having consistency along the direction is formed.

  The image recorded on the recording paper 16 is subjected to halftone processing using a line screen inclined at about 45 degrees with respect to the width direction (sub-scanning direction), so that the main scanning direction Of course, density unevenness due to banding along the sub-scanning direction is prevented.

  Therefore, even a low density image is recorded on the recording paper 16 with high quality.

  On the other hand, when such a halftone process is performed, any one of the threshold matrices 70B to 70D may be used as the threshold matrix 70B for the threshold matrix 70A. Further, any one of the threshold matrices 70B to 70E may be randomly arranged in a region corresponding to the threshold matrix 70B.

  Further, as the threshold matrix 70A, a threshold matrix in which a straight line along the line direction M is used as a mirror surface and the threshold is replaced with a mirror image can be used.

That is, as shown in FIG. 6A, a threshold value matrix 70F in which the threshold values of the dot area F ₂₁ and the dot area F _{12 and} the threshold values of the dot area F ₄₃ and the dot area F ₃₄ are exchanged is shown in FIG. As shown in FIG. 6C, a threshold value matrix 70G in which the threshold values of the dot area F ₃₁ and the dot area F _{13 and} the threshold values of the dot area F ₄₂ and the dot area F ₂₄ are interchanged. threshold of ₂₁ and the dot regions _{F 12,} the threshold value of the dot area _{F 43} and the dot regions _{F 34,} the threshold value of the dot area _{F 31} and the dot regions _{F 13} and the threshold matrix obtained by rearranging the threshold dot areas _{F 42} and the dot area _{F 24} Any of 70H can be used.

  At this time, the threshold matrixes 70A, 70F, 70G, and 70H can be randomly arranged and used in a region corresponding to the threshold matrix 70A.

  On the other hand, in the present embodiment, the threshold value matrix 70A and the threshold value matrix 70B are arranged and applied in tiles that are alternated along the main scanning direction and the sub-scanning direction. The halftone process may be performed by randomly applying 70A and the threshold matrix 70B in the main scanning direction and the sub-scanning direction.

  Thereby, it is possible to suppress the occurrence of the alignment of the arrangement of the dots D (pixels) even at low gradations such as gradation level 1 and gradation level 2.

  In the present embodiment described above, a threshold value of 1 to 16 is set with a size of 4 × 4, and the description has been given using the line screen applied when n = 16. A large matrix, for example, a threshold of 1 to 164 with a size of 8 × 8 is set, and a line screen applied when n = 64 may be used. In order to cope with 256 gradations generally used in image data, first, after quantizing into n gradations (for example, dividing the image data by (256 / n) and truncating the decimal point), It is possible to cope with quantization by a threshold matrix, or by correcting each value of the threshold matrix by (256 / n).

  Further, in the present embodiment, the halftone for the pixel and the dot is described as one-to-one. That is, using threshold matrixes 70A and 70B, pixels at density level 1 are dot arrays with threshold 1 turned on, pixels at density level 2 are dot arrays with thresholds 1 and 2 turned on,... A pixel at density level n is associated with a dot array in which thresholds 1 to n are turned on, and a multi-value pixel is replaced with a dot arrangement and quantized. By exchanging the threshold matrixes 70A and 70B, it is possible to suppress the occurrence of regularity in the dot arrangement in a direction different from the basic direction, and to prevent the consistency from being lost.

  Further, the present embodiment described above does not limit the configuration of the present invention. For example, in the present embodiment, as the image recording apparatus, the inkjet recording apparatus 10 that records an image with ink droplets ejected onto the recording paper 16 has been described as an example. However, the present invention is not limited to the inkjet recording apparatus 10. An ink jet recording apparatus having an arbitrary configuration can be applied.

  The present invention is not limited to the ink jet recording apparatus such as the ink jet recording apparatus 10, and uses an electrophotographic process type image recording apparatus and a ribbon (ink ribbon) for recording an image by forming dots on an image recording medium with toner. Thermal transfer type image recording apparatus for forming dots, thermal image recording apparatus for forming dots by heating using thermal paper as an image recording medium, and image recording medium with dots based on quantized data such as binarized data The present invention can be applied to an image recording apparatus having an arbitrary configuration for recording an image.

It is a schematic block diagram of the inkjet recording device which concerns on this Embodiment. It is a block diagram which shows schematic structure of the control system of an inkjet recording device. (A) is a schematic block diagram which shows an example of a 1st threshold value matrix, (B)-(E) is a schematic block diagram which shows an example of a 2nd threshold value matrix. It is the schematic which shows the arrangement | sequence when applying a threshold value matrix. (A) is a schematic diagram showing the arrangement of dots recorded at gradation level 1, and (B) is a schematic diagram showing the arrangement of dots recorded at gradation level 2. FIG. (A) thru | or (C) is a schematic block diagram which shows the other application example of a 1st threshold value matrix, respectively. (A) and (B) are schematic diagrams showing a conventional example, (A) shows an arrangement of dots recorded at gradation level 1, and (B) shows an arrangement of dots recorded at gradation level 2. ing.

Explanation of symbols

10 Inkjet recording device (image recording device)
16 Recording paper (image recording medium)
30 Recording Head Array 32 (32Y, 32M, 32C, 32K) Recording Head 60 Control Unit 62 Color Conversion Unit 64 Image Processing Unit 66 Recording Data Generation Unit 68 Image Recording Unit 70 Threshold Matrix 72A (70A, 70F, 70G, 70H) Threshold Value Matrix (first threshold matrix)
72B (70B, 70C, 70D, 70E) Threshold matrix (second threshold matrix)

Claims (10)

An image processing method that enables image recording based on image data of a multi-tone image by dots formed on an image recording medium,
A first threshold value matrix whose basic tone direction is inclined at a predetermined angle with respect to the main scanning direction or the sub-scanning direction;
At least with respect to a target line having the same direction as the first threshold matrix and passing through the center of the first threshold matrix along a direction orthogonal to the fundamental direction of the first threshold matrix. Using a second threshold matrix different from the first threshold matrix, in which the array pattern of the threshold values in the low density region is line symmetric ,
Applying the first threshold matrix and the second threshold matrix alternately along the main scanning direction and the sub-scanning direction;
By performing halftone processing on the image data of the multi-tone image, it is converted into quantized data that forms the dots on the recording medium,
An image processing method. The threshold matrix in which an array pattern of a plurality of threshold values including at least a threshold value in a low density region is axisymmetric with respect to the first threshold value matrix is applied as the second threshold value matrix. 2. The image processing method according to 1. When a plurality of threshold matrices having different threshold arrangement patterns are set as the second threshold matrix, the second threshold matrix is selectively applied from the plurality of threshold matrices. The image processing method according to claim 1 or 2. As the first threshold value matrix, an array pattern of at least a part of the threshold values excluding the threshold value in the low density region is line symmetric with respect to a symmetric line passing through the center of the first threshold value matrix along the keynote direction. The image processing method according to claim 1 , wherein a certain threshold value matrix is applied. Selected as the first threshold value matrix, the along the trend direction when the arrangement pattern of the threshold value of the low concentration range is set to the same plurality of threshold matrix, the first threshold value matrix from the plurality of threshold matrix The image processing method according to any one of claims 1 to 4, wherein the image processing method is applied in an automatic manner. An image recording apparatus that records an image based on image data of a multi-tone image by dots formed on an image recording medium,
A first threshold value matrix whose base tone direction is inclined at a predetermined angle with respect to the main scanning direction or the sub-scanning direction, and a base tone direction is the same as the first threshold value matrix, and the first threshold value matrix A second different from the first threshold matrix in which at least a low-density region threshold pattern is a target line with respect to a target line passing through the center of the first threshold matrix along a direction orthogonal to the basic tone direction. Image processing means for alternately applying the threshold value matrix along the main scanning direction and the sub-scanning direction, performing halftone processing on the image data, and converting the image data into quantized data;
Image recording means for recording dots based on the quantized data on the image recording medium;
An image recording apparatus comprising: When the second threshold value matrix includes a threshold value matrix in which an array pattern of a plurality of threshold values including at least a low density region threshold value is axisymmetric with respect to the first threshold value matrix, the image processing means includes: The image recording apparatus according to claim 6, wherein a second threshold matrix is selectively applied from the plurality of threshold matrices. As the first threshold value matrix, an array pattern of at least a part of the threshold values excluding the threshold value in the low density region is line symmetric with respect to a symmetric line passing through the center of the first threshold value matrix along the keynote direction. The image recording according to claim 6 or 7, wherein the image processing means selectively applies the first threshold matrix from the plurality of threshold matrices when including a plurality of threshold matrices. apparatus. 9. The image recording apparatus according to claim 6 , wherein the image recording unit records the dots by ejecting ink droplets including a color material onto the recording medium . 10. An image in which an image is recorded by the image processing method according to any one of claims 1 to 5 or the image recording apparatus according to any one of claims 6 to 9. recoding media.

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