Printing apparatus, printing method, and image processing apparatus

Abstract

The invention discloses a printing apparatus, a printing method, and an image processing apparatus. The purpose of the present invention is to suppress a decrease in image quality when a dot is not formed due to a main cause such as nozzle clogging. A printing apparatus acquires a position of a defect point, which is a pixel at which a point cannot be formed by a head despite the position at which the point is formed, and performs halftone processing for generating point data indicating the presence or absence of point formation using image data comprising a plurality of pixels having a plurality of gradation values and a dither mask comprising a plurality of threshold values. A threshold value used for generating points in a predetermined range near the defect point is searched, and a value of a change threshold value, which is a threshold value satisfying a search condition that is greater than a value of a target threshold value, which is a threshold value used at the position of the defect point, is corrected to a value equivalent to the value of the target threshold value. And providing the corrected threshold value to the halftone processing at the pixel corresponding to the corrected threshold value.

Description

Technical Field

[0001] The present disclosure relates to a printing technology for performing halftoning processing using a dithering method. Background Art

[0002] When printing on a medium by ejecting droplets from multiple nozzles, droplets may not be ejected to specific locations due to nozzle clogging or other malfunctions, sometimes resulting in white streaks in the printed image. When such dot dropouts occur, known techniques are used to eject droplets to pixels surrounding the row of dot dropouts, making the dropouts less visible and suppressing degradation in image quality. To correct the droplet ejection method, methods such as Patent Document 1, which processes image data prior to halftoning, and Patent Document 2, which corrects and addresses dot data indicating dot on / off status after halftoning, are known.

[0003] The method of Patent Document 1 increases the grayscale value of pixels near the location where a defect occurs in the pixels constituting the image to increase the probability of dot formation, or processes the original image data to increase the utilization rate of large dots while using a head capable of forming large and small dots. Furthermore, in Patent Document 2, even if dots are formed using dot data after halftoning, since defective nozzles are responsible for dot formation, the dot data is modified so that pixels with dot defects are replaced by pixels that are not formed in pixels that are handled by normal nozzles near the pixel.

[0004] Patent Document 1: Japanese Patent Application Laid-Open No. 2017-109480

[0005] Patent Document 2: International Publication No. WO2008 / 102591

[0006] However, the method of Patent Document 1 uses a probabilistic control method that increases the probability of forming dots near defective nozzle rows. This leads to the following problems: Replacement dots are not guaranteed to be generated in a 1:1 correspondence near defective points, and may sometimes be generated far from defective points. In particular, when using a simple and high-speed dithering method as a halftoning method, there is a problem: the intervals between replacement dots are unstable, and areas where no replacement dots are generated or where excessive replacement dots are generated may occur at a periodicity easily visible to the human eye.

[0007] Furthermore, methods for modifying dot data after halftone processing require the addition of dedicated processing to search for pixels near the pixel where the defect occurred and where no dots were formed, and to create replacement dots. This additional processing results in reduced speed and increased costs due to increased man-hours. Furthermore, since replacement dots are generated at positions different from the original ones, the resulting positions of the replacement dots may not be the desired locations for image quality. Therefore, the need to increase or decrease the number of replacement dots generated, or to explore fine-tuning methods to optimize their positions, has been identified. Summary of the Invention

[0008] The present disclosure can be implemented as a printing device having a head for forming dots on a medium. The printing device includes: a halftone processing unit that performs halftone processing to generate dot data indicating the presence or absence of dot formation using image data composed of multiple pixels having multiple grayscale values ​​and a dither mask composed of multiple threshold values; a defect position acquisition unit that acquires the position of a defective dot where the head cannot form the dot even though the dot formation is performed according to the dot data; a correction unit that searches for a threshold used for dot formation within a predetermined range near the defective dot, corrects the value of a modified threshold to a value equivalent to a target threshold, and applies the modified threshold to the halftone processing at the pixel corresponding to the modified threshold, wherein the modified threshold satisfies a search condition that the modified threshold is greater than the target threshold used at the defective dot position; and a printing unit that drives the head according to the dot data.

[0009] In addition, the present disclosure can also be implemented as a method for printing by forming dots on a medium using a head. This method obtains the location of a defective dot where the head cannot form the dot despite being a location where dot formation is performed, performs halftoning processing to generate dot data indicating whether or not dot formation is performed using image data consisting of multiple pixels having multiple grayscale values ​​and a dither mask consisting of multiple thresholds, searches for a threshold used for dot generation within a predetermined range near the defective dot, corrects the value of the change threshold to a value equivalent to the target threshold, and applies the modified threshold to the halftoning processing at the pixel corresponding to the corrected threshold. The change threshold is a threshold that satisfies a search condition: the search condition is that the change threshold is greater than the target threshold used at the location of the defective dot, and drives the head according to the dot data.

[0010] Furthermore, the present disclosure can also be implemented as an image processing device that processes image data and converts it into dot data for forming dots on a medium. The image processing device includes: a halftone processing unit that uses image data composed of multiple pixels having multiple grayscale values ​​and a dither mask composed of multiple threshold values ​​to perform halftone processing to generate dot data indicating whether or not a dot is formed; a defect position acquisition unit that acquires the position of a defect point where the dot cannot be formed even though the dot is formed according to the dot data; and a correction unit that searches for a threshold used for dot generation within a predetermined range near the defect point, corrects the value of a change threshold to a value equivalent to a target threshold, and applies the corrected threshold to the halftone processing at the pixel corresponding to the corrected threshold, wherein the change threshold satisfies a search condition that the change threshold is greater than the target threshold used at the defect point. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 1 is a schematic structural diagram showing a printing device incorporating the image processing device according to the embodiment.

[0012] Figure 2 This is a flowchart showing an example of an image printing process routine performed by the printing device.

[0013] Figure 3 This is a flowchart showing an example of a halftone processing routine.

[0014] Figure 4 This is an explanatory diagram showing an example of a dither mask used in halftoning processing.

[0015] Figure 5 This is a flowchart showing an example of a threshold value correction processing routine for correcting a threshold value.

[0016] Figure 6 This is an explanatory diagram showing an overview of replacement of the chattering threshold value according to the first embodiment.

[0017] Figure 7 This is a flowchart showing an example of a process for replacing a chattering threshold value.

[0018] Figure 8 This is an explanatory diagram showing an overview of replacement of the chattering threshold value according to the second embodiment.

[0019] Figure 9 This is an explanatory diagram showing an overview of replacement of the chattering threshold value according to the third embodiment.

[0020] Figure 10 This is a flowchart illustrating a main part of the processing in the fourth embodiment.

[0021] Figure 11This is a flowchart showing an example of a threshold value correction processing routine according to the fifth embodiment.

[0022] Figure 12 This is an explanatory diagram showing an overview of replacement of the chattering threshold value in the fifth embodiment.

[0023] Figure 13 It is an explanatory diagram showing an outline of processing when forming multiple types of dots.

[0024] Description of Reference Numerals

[0025] 20: Printer; 30: Control unit; 40: CPU; 41: Input unit; 42: Halftone processing unit; 43: Comparison unit; 44: Correction unit; 46: Printing unit; 51: ROM; 52: RAM; 60: EEPROM; 61, 61A: Dither mask; 70: Detection device; 74: Paper feed motor; 75: Paper feed roller; 82: Ink cartridge; 90: Head; 92: Supply tube; 98: Memory card slot; 99: Operation panel; MC: Memory card; Nz: Nozzle. DETAILED DESCRIPTION

[0026] A. First embodiment:

[0027] (A1) Device structure:

[0028] Figure 1 : This is a schematic diagram showing the structure of a printer 20 as a printing device. The printer 20 is a so-called line printer, an inkjet printer that uses four colors of ink, which will be described later. As shown in the figure, the printer 20 includes a mechanism for conveying the medium P by driving the paper feed roller 75 via the paper feed motor 74, a mechanism for ejecting ink and forming dots by driving the head 90 located opposite the medium P, and a detection device 70 for identifying the image printed on the medium P. Furthermore, the printer 20 includes a control unit 30 that manages the exchange of signals between the detection device 70, the paper feed motor 74, the head 90, and the operation panel 99. In this embodiment, the paper feed roller 75 also serves as a platen, but the platen can also be separate from the paper feed roller. In this case, a flat platen with a flat surface can also be used. In addition, the paper feed roller 75 can also be located upstream / downstream of the head 90.

[0029] The detection device 70 is a line sensor capable of recognizing an image on the medium P at a higher resolution than that achieved by printing with the head 90. The CPU 40 uses the processing described below to identify the positions of pixels missing due to factors such as nozzle clogging, or other factors, from the image recognized by the detection device 70. This processing, including the processing performed by the CPU 40 using the detection device 70, realizes a defective dot acquisition unit.

[0030] In the head 90, a plurality of nozzles Nz capable of ejecting cyan ink C, magenta ink M, yellow ink Y, and black ink K as color inks are arranged in the width direction of the medium P. Each of these plurality of nozzles Nz is provided with a piezoelectric element (not shown) as an actuator. The piezoelectric element is driven by a data signal DD corresponding to the dot data and a drive signal COM. Furthermore, the actuator for ejecting ink from the nozzles Nz is not limited to a piezoelectric element; various structures such as a heater type that utilizes sudden boiling of the ink for ejection, or a laser can also be used. Of course, the formation of ink dots is not limited to inkjet printing; various methods such as thermal transfer using an ink ribbon, thermal sublimation, forming a latent image on a photosensitive drum, or a serial printer in which a printing head reciprocates in the width direction of the medium and ejects ink from the nozzles can also be used.

[0031] Each color ink is supplied to the head 90 from ink cartridges 82-85 containing each color ink via ink supply tubes 92-95. In addition to the aforementioned CMYK inks, light cyan ink Lc, light magenta ink Lm, and the like can also be used. Of course, special color inks such as red, blue, and green, as well as so-called metallic inks such as gold and pearl white, can also be used. Furthermore, an ink system for black and white printing may be provided.

[0032] The control unit 30 includes a CPU 40, a ROM 51, a RAM 52, and an EEPROM 60, interconnected via a bus. The control unit 30 expands and executes programs stored in the ROM 51 and EEPROM 60 into the RAM 52. In addition to controlling the overall operation of the printer 20, the control unit 30 also functions as an input unit 41, a halftone processing unit 42, and a printing unit 46. The functions of the halftone processing unit 42 include functions as a comparison unit 43 and a correction unit 44. Details of the operation of each of these components will be described later.

[0033] The printing unit 46 is a circuit for driving the head 90. It outputs a signal DD corresponding to dot data and a drive signal COM for driving multiple piezoelectric elements at once to the head 90. The piezoelectric elements are grouped for each CMYK color and driven by a signal DD corresponding to the dot data stored in a latch (not shown) and a drive signal COM output at predetermined timing. When the drive signal COM is applied while the signal DD is on (dot data is a value of 1), the piezoelectric elements expand, pressurizing the ink within the ink chamber (not shown), causing droplets to be ejected from the nozzles Nz. Furthermore, in the printer 20 of this embodiment, since it is a line printer, the nozzles Nz for each color are staggered at predetermined intervals in the direction of transport of the medium P. Furthermore, to improve resolution across the width of the medium, the nozzles Nz for the same color ink are arranged in a staggered arrangement, with every other nozzle Nz staggered in the direction of transport of the medium P. Consequently, when forming dots at the same position in the direction of transport of the medium P, the timing of droplet ejection from the nozzles Nz varies. Therefore, as will be described later, a sorting process is performed to match the dot data obtained by processing the grayscale data of the image to be formed with the nozzle arrangement. Figure 2 、 Figure 3 The details of the processing of each functional unit including the processing of the printing unit will be described later.

[0034] A dither mask 61 is stored in the EEPROM 60. The dither mask 61 is used in the half-tone processing described later. Figure 4 As shown in the example, the dither mask 61 has a size of 256 in the horizontal direction (xd: the width of the medium) and 64 in the vertical direction (yd: the transport direction of the medium). Multiple threshold values ​​Thd are arranged in this dither mask 61. In this embodiment, the threshold values ​​Thd range from 1 to 255. Each threshold value Thd is configured so that the spatial frequency of a point formed by comparison with the threshold value exhibits a so-called blue noise characteristic.

[0035] The blue noise characteristics of the dither mask have a cycle length with a maximum frequency component in the high-frequency region around two pixels. This means that the threshold storage position is adjusted to produce the maximum frequency component in the high-frequency region, taking into account the human visual characteristic of low sensitivity in high-frequency regions. When dots are generated using a dither mask with such blue noise characteristics, images with excellent dot dispersion are obtained.

[0036] When the dither mask has a blue noise characteristic, the resulting dot distribution has good dispersion, effectively suppressing image graininess. If the image size is sufficiently small, even using a dither mask with a green noise characteristic, which has a maximum frequency component slightly lower than the blue noise characteristic, can produce a good image with minimal perceptible graininess. The dither mask 61 has predetermined spatial frequency characteristics such as these blue noise characteristics and green noise characteristics.

[0037] Of course, the size and characteristics of the dither mask 61 are arbitrary and can be other than those in this embodiment. For example, it can be a size of 64×32 or larger to implement an ordered dither method, or it can be a dot-concentrated dither mask that achieves characteristics close to those of halftone dots.

[0038] A memory card slot 98 is connected to the control unit 30, and image data ORG can be read and input from a memory card MC inserted into the memory card slot 98. In this embodiment, the image data ORG input from the memory card MC is data composed of three color components: red (R), green (G), and blue (B). Alternatively, the image data ORG can be obtained from a computer or the like connected via a wired or wireless connection rather than the memory card MC.

[0039] The printer 20, with the hardware configuration described above, drives the paper feed motor 74 to move the medium P in its conveyance direction and the drive head 90 to form ink dots of various colors on the medium P. The control unit 30 drives the nozzles Nz at appropriate timings based on print data in coordination with the feed of the medium P, thereby forming ink dots of appropriate colors at appropriate locations on the medium P. In this manner, the printer 20 can print a color image input from the memory card MC on the medium P.

[0040] (A2) Printing process:

[0041] The printing process in the printer 20 will be described. Figure 2 This is a flowchart illustrating the flow of image printing processing in printer 20. The user initiates the image printing process by using operation panel 99 or the like to instruct the user to print a predetermined image stored on memory card MC. When the printing process begins, CPU 40 first reads and inputs RGB image data ORG, the target for printing, from memory card MC via memory card slot 98 as part of processing by input unit 41 (step S110).

[0042] When the image data ORG is input, the CPU 40 refers to a lookup table (not shown) stored in the EEPROM 60 and converts the color of the image data ORG from the RGB format to the CMYK format (step S120 ).

[0043] When performing color conversion, the CPU 40 performs halftoning processing (step S130 ) as part of the halftoning process of the halftoning unit 42 , converting the image data into dot data that determines the presence or absence of dots of each color for each pixel. Details of this halftoning process will be described later. In this specification, "halftone processing" is not limited to binary processing for dot presence or absence; it generally refers to grayscale conversion (reduction) including multi-valued processing, such as the presence or absence of large and small dots, large, medium, and small dots. Furthermore, the image data provided to step S130 may have been subjected to image processing such as resolution conversion and smoothing.

[0044] When performing halftoning, the CPU 40 performs a sorting process (step S150) to sort the dot pattern data so that each nozzle Nz of the head 90 is driven simultaneously, in accordance with the nozzle arrangement and paper feed rate of the printer 20. As described above, the sorting process sorts the dot data obtained through the halftoning process (step S130) in accordance with the arrangement of the nozzles Nz in the head 90. After the sorting process (step S150) is completed, the CPU 40 drives the paper feed motor 74 and other components as part of the printing unit 46 to execute printing (step S160).

[0045] In the above, the printer 20 forms an image loaded from the memory card MC on the medium P. However, in this embodiment, the printed image may contain dot dropouts at specific locations along the width of the medium P due to poor ink ejection from the nozzles Nz, resulting in visible white streaks. To anticipate such a situation, in this embodiment, after printing (step S160), the printed material is read (step S170). This reading is performed using the detection device 70. The detection device 70 is precisely positioned and mounted on the printer 20. Furthermore, as a line sensor, its resolution is higher (approximately twice) than the resolution of the ink dots formed by the head 90, enabling accurate detection of the X-direction locations where dot dropouts have occurred.

[0046] Therefore, the data read from the detection device 70 is used to determine whether defective dots due to dot dropout are visible on the side of the printed material being viewed (step S180). Regarding whether dot dropout has occurred, if it is determined that the image quality degradation caused by dot dropout has been corrected through the dither mask threshold correction process (described later) and that dot dropout is no longer visible, nothing is done and the process exits to "NEXT," temporarily terminating this processing routine. On the other hand, if it is determined that dot dropout is visible, the dither mask threshold correction process is performed (step S200). In this case, the printer 20 repeats the process from step S130 described above. Specifically, halftoning using the ordered dithering method is performed (step S130), sorting is performed (step S150), printing is performed (step S160), the printed material is read (step S170), and the process of step S170 is repeated.

[0047] Depending on the way dots are missing, it is assumed that even if the processing below step S130 is performed again, there may be a situation where dot missing can be seen. In such a case, the target range can be expanded and the threshold correction processing of the dither mask can be performed. If dot missing occurs in multiple consecutive nozzles, the dot missing may not be made invisible only by the correction processing of the threshold of the dither mask. In such a case, an abnormal processing not shown in the figure can also be called to perform maintenance on the head 90, etc. If it is determined that the configuration of the dots has been corrected to the extent that dot missing is not visible through the correction processing of the threshold of the dither mask (step S200) and the maintenance of the head 90, etc., then exit to "NEXT" and end this processing routine. Assuming that the printer 20 prints multiple copies, for example, hundreds of copies of the same image, when it is determined that printing can be performed in a state where dot missing is not visible, multiple copies are printed at high speed according to the high-speed printing processing routine not shown. The medium P may be a single sheet of paper such as A4 or A3, or may be a long sheet of paper such as a roll. In this case, the printer 20, which is a line printer, repeats the same image along the length of the medium P or forms a series of images along the length. Furthermore, the image used for dot dropout detection may be a dedicated detection image.

[0048] (A3) Details of halftone processing:

[0049] Next, use Figure 3 The details of halftoning processing will be described below. When halftoning processing begins, the dither mask 61 used for the current printing process is first retrieved from the dither mask 61 stored in the EEPROM 60 (step S131). A variety of dither masks 61 are available, varying in size, noise characteristics, and other characteristics. A dither mask suitable for the image being printed is selected.

[0050] Next, the pixel position of the image to be processed by halftone and the read position of the dither mask are initialized (step S132). The initial position of the pixel position is the upper left corner of the image. If the pixel position is expressed as (X, Y), it is the origin (0, 0). Figure 4 As shown, the initial position of the dither mask is represented as [xd, yd] with the upper left corner of the mask as the origin [0, 0]. After each position is initialized, the processing of steps STR1 to STP1 is repeatedly performed for each pixel of all pixels forming the image.

[0051] First, the pixel position (X, Y) and the pixel value DS corresponding to the grayscale value of the pixel at that position are read (step S133). Next, based on the pixel position (X, Y), the threshold position [xd, yd] corresponding to the dither mask 61 is calculated. The threshold position [xd, yd] is calculated using the following equations (1) and (2) (step S134). Here, mod(A, B) is a function that returns the remainder when dividing the value A by the value B. Since the size of the dither mask 61 in this embodiment is 256×64, equations (1) and (2) are as follows.

[0052] xd=mod(X,256)……(1)

[0053] yd=mod(Y,64)……(2)

[0054] Based on the threshold position [xd, yd] thus determined in the dither mask 61, the threshold Thd at that position is obtained (step S135), and this threshold Thd is compared with the pixel value DS (step S136). This process implements the comparison unit 43. If the pixel value DS is greater than the threshold Thd, a dot is formed, and the dot data DD is set to 1 (step S137). If the pixel value DS is below the threshold Thd, a dot is not formed, and the dot data DD is set to 0 (step S138). The set dot data DD is then sequentially stored. The above process is repeated from the origin position (0, 0) to the end position of the image where the pixel position (X, Y) is halftoned (steps STR1 to STP1). Through this process, the original image data ORG is converted into dot data DD consisting of dot formation / non-formation, and stored for printing.

[0055] Next, use Figure 5 Threshold correction processing for cases where it is determined that point loss can be seen ( Figure 2, step S200) is described. The CPU 40 executes this processing to realize the correction unit 44. In this processing routine for performing threshold correction, the position Xf of the dot missing is first obtained (step S201). The position of the dot missing is the position of the defective point where the dot cannot be formed by the head 90 even though the dot is formed according to the dot data. Based on the information obtained from the detection device 70, the position Xf of the dot missing can be easily obtained. This processing corresponds to the processing based on the defect position acquisition unit. Next, the halftone processing ( Figure 3 ) is a process of acquiring the position xd of the dither mask 61 (step S211). The position xd of the dither mask 61 is a position on the dither mask 61 that provides a threshold value Thd referenced when determining whether a dot is formed or not at the dot-missing position Xf.

[0056] In the printer 20 of this embodiment, since one nozzle Nz is responsible for one column in the Y direction (longitudinal direction), dot omissions due to clogging of the nozzle Nz, etc., are continuously generated at specific positions in the width direction of the medium P, that is, in the X direction. Therefore, the position xd on the dither mask 61 corresponding to the position Xf of the dot omission is determined by the above-mentioned formula (1), and on the other hand, the value 0 is set as the initial value for the y-direction position yd (step S221). On this basis, the threshold value Thd of the position [xd, yd] on the dither mask 61 is obtained (step S231). In order to illustrate an example of the correction processing of the dither mask 61, Figure 6 exemplifies a portion of the dither mask 61. In this example, for example, when the position xd on the dither mask 61 corresponding to the position Xf on the image where dot deletion occurs is 2, the value 202 is acquired as the threshold Thd in the first step S231.

[0057] Next, a determination is made as to whether the threshold value Thd is less than a predetermined value ED (step S241). If the threshold value Thd is less than the value ED (step S241: "Yes"), the dither threshold value is changed (step S250). If the threshold value Thd is greater than the value ED (step S241: "No"), step S250 is skipped and the process moves to step S261. The reason for this is as follows. In ink printing, ink dots are circular compared to rectangular pixels. Therefore, when the proportion of dots formed per a certain area is greater than a predetermined value, the density difference compared to the state where dots are formed on all pixels becomes sufficiently small. When this proportion, for example, is greater than 78%, this corresponds to a situation where the input grayscale value DS is greater than 200 in the printer 20 of this embodiment. In other words, if the threshold Thd of the dither mask 61 is equal to or greater than the value ED (value 200), dot formation for that pixel is limited to cases where the pixel's input grayscale value, i.e., the pixel value DS, exceeds the threshold value 200 or greater. Therefore, even if a replacement dot is not generated for that pixel, there is no significant problem. Therefore, in step S241, the threshold Thd is compared with the value ED (value 200 in this example). If the threshold Thd is equal to or greater than the value ED, the threshold replacement process for generating a replacement dot is omitted, even if dot loss occurs.

[0058] When Figure 6 For example, since the threshold Thd at position [xd, yd] = [2, 0] is 202, the determination in step S241 is "No," and the threshold replacement process (step S250) is not executed. At position [xd, yd] = [2, 1], since the threshold Thd is 69, the threshold replacement process (step S250) is executed. The value ED used for this determination can be selected appropriately based on the ink system. Of course, the determination in step S241 can also be omitted, and the dither threshold replacement process (step S250) can be performed for all thresholds Thd. Details of this dither threshold replacement process will be described later.

[0059] Regardless of whether the dither threshold replacement process (step S250) has been executed, the y-direction value yd of the dither mask 61 being evaluated is then incremented by 1 (step S261), and a determination is made as to whether the y-direction value yd exceeds 65 (step S271). If not, the process returns to step S231 and repeats the above process from the acquisition of the dither mask 61 (steps S231 to S271). If the value yd indicating the y-direction position of the threshold exceeds 65, processing is deemed complete for all thresholds Thd arranged in the y-direction of the dither mask 61, and the process exits to "NEXT," terminating this processing routine.

[0060] according to Figure 7The following description of an example of using the threshold value replacement process will refer to the following description of the vibration threshold value replacement process (step S250) in detail. Figure 6 In the threshold replacement process, the threshold of the replacement range is first obtained (step S251). The replacement range can be of various sizes and forms, but in the first embodiment, it is the range corresponding to both sides of the pixel position Xf in the X direction on the image where the point loss occurs, that is, the previous position (Xf-1) in the X direction and the next position (Xf+1) in the X direction. Figure 6 In the first embodiment, in step S251, the threshold values ​​of the position [xd-1] and the position [xd+1] of the dither mask 61 corresponding to the pixel at the position to be replaced are acquired. Figure 6 In the example shown, if the position yd in the y direction is 1, the threshold Thd at the position [xd, 1] is 69, and the thresholds Thd at the two positions within the replacement range SAO are 142 and 163.

[0061] Next, a process of searching for a threshold value to be replaced is performed (step S252). This process searches for a threshold value that is larger than the threshold value of the focus position from the threshold values ​​within the replacement range SAO, and searches for the largest threshold value among them. If only one threshold value that is larger than the threshold value of the focus position is found in the replacement range, it becomes the object of replacement. If there are two, the one with the larger value becomes the object of replacement. If the values ​​of multiple thresholds are the same, either side can be selected, or the side that does not produce the adjustment in the later-described biased adjustment process (step S257) can be selected. After searching for the threshold value to be replaced, next, a judgment is made as to whether to replace it (step S253). If a threshold value to be replaced is found, the judgment is "yes", and a replacement process is performed to replace the threshold value of the maximum value in the replacement range with the threshold value of the focus position (step S254). In Figure 6In the replacement range SAO shown, when yd = 1, since both replacement targets are larger than the threshold at the target position, and since threshold 163 at the replacement position [xd+1,1] is the largest within the replacement range, it is replaced with threshold 69 at position [xd,1]. Similarly, when yd = 2, threshold 199 at the replacement position [xd-1,2] is replaced with threshold 44 at position [xd,2]. In this specification, "replacement" or "replacement processing" refers to the process of correcting the replacement target, or in the case of yd = 1, threshold 163 in the replacement range SAO, to a value equivalent to the threshold value 69 at the position corresponding to the target pixel (hereinafter referred to as the target threshold), regardless of whether or not the target threshold is replaced with the replacement target threshold, i.e., the threshold is changed. Since no dot is formed at the target pixel even when the target threshold is replaced with the change threshold, dot formation is not affected whether or not the replacement is performed. The value equivalent to the target threshold is typically the same as the target threshold Thd. However, in halftoning using a dither mask, values ​​within a range that does not significantly alter the result can be treated as equivalent. For example, a value within the range of ±α from the threshold Thd is acceptable. Here, α can be a percentage of the threshold Thd multiplied by several percent. The dither threshold replacement process (step S250) compensates for image quality degradation caused by defective dots resulting in dot dropout. Even if the corrected threshold value differs slightly from the target threshold, image quality degradation caused by the defective dots can be suppressed.

[0062] After the above replacement process is performed, a weight adjustment process is performed (step S257), and the replaced threshold is saved as a new dither mask 61A applicable to the area where dots are missing (step S258). The weight adjustment process (step S257) is a process that adjusts the threshold when it is determined that the replacement of the threshold may cause weighting in the area where dots are generated. In the first embodiment, it can be seen that at position yd = 13, according to the replacement conditions, the maximum value of the searched thresholds, that is, the threshold value 234 at position [xd+1, yd], becomes the target of replacement. However, if the replacement is performed directly, the threshold value at position [xd+1, yd-1] is also replaced at this time. When the replacement at position yd = 14 is discussed earlier, the threshold value at position [xd+1, yd+1] is also replaced. In this case, since the likelihood of dots forming continuously in the Y direction increases, the bias adjustment process (step S257) replaces the threshold at position [xd+1, yd] with the threshold at position [xd-1, yd] at position yd=13 to adjust the dot formation. Of course, this adjustment process can be performed after the threshold replacement process has been performed once for all target thresholds, or it can be performed after confirming the threshold values ​​at three consecutive pixels in the Y direction as described above. Alternatively, the adjustment process can be omitted. Furthermore, if step S253 determines that no replacement is necessary (step S253: "No"), steps S254-S258 are not executed, and the jitter threshold replacement process (step S250) ends.

[0063] After the jitter threshold value replacement process (step S250) is completed, Figure 5 As shown in FIG, the threshold position yd is incremented by 1, and the above process is repeated until the threshold processing for all positions yd in the y direction is completed. As a result, the threshold Thd used to determine the formation / non-formation of a dot at the position Xf in the X direction where the dot is missing and before and after it is replaced with a threshold that makes it easier for a dot that was not formed at the dot missing position to be formed before and after the dot missing position. Figure 6 In the example shown, for positions yd with values ​​1, 2, 5, 9 to 15, the maximum threshold is replaced with a smaller threshold at the focus position before and after the position where the point loss occurs. Figure 6 Its appearance can be easily read.

[0064] (A4) Effects of the first embodiment:

[0065] In the printer 20 of the first embodiment described above, when dot dropouts occur due to clogging of nozzles Nz of the head 90, resulting in white streaks or other issues on the medium P, the location of the dot dropout is determined, and the threshold value of the corresponding position on the dither mask 61 is changed to facilitate dot formation around the pixel location causing the dot dropout. Consequently, subsequent printing can be performed while suppressing the effects of the dot dropouts and continuing printing. Furthermore, since halftoning using the dithering method is maintained once the threshold value is changed, halftoning processing does not consume unnecessary time. Consequently, halftoning processing and printing can be performed at high speed.

[0066] Furthermore, in the process of replacing the threshold value, since the threshold value used when determining whether or not a dot is formed at a pixel that may cause dot dropout is used to replace the threshold value used when determining whether or not a dot is formed at the pixels before and after the pixel, the following advantage is achieved: the determination result of dot formation at the pixel that may cause dot dropout is easily the same as the determination result of dot formation at the pixel whose threshold value has been replaced. Therefore, not only can the generation of white streaks and the like caused by dot dropout be suppressed, but also the degradation of image quality can be suppressed. Moreover, since the search range for the replacement object is set to the two pixels before and after the pixel of interest, processing is facilitated and the time required for processing can be shortened.

[0067] B: Second embodiment:

[0068] The printer 20 of the second embodiment has the same hardware configuration as the first embodiment, and the outline of the processing executed is also the same, but in the second embodiment Figure 7 The change ranges in the jitter threshold value change processing shown are different. Figure 8 The figure illustrates the replacement range and actual replacement in the second embodiment. In the second embodiment, as shown, the replacement range includes not only the two pixels before and after the pixel of interest in the X direction, but also the threshold values ​​corresponding to the two pixels before and after it, as well as four pixels at positions offset in both the X and Y directions from the pixel of interest, totaling eight pixels, as the replacement range SEO. For example, using the position of the dither mask 61, when the threshold corresponding to the pixel of interest is at position [xd, yd], the following eight positions constitute the replacement range SEO. These eight positions can be processed collectively or divided into several groups. In the first embodiment, all eight thresholds are processed collectively, and the largest threshold value greater than the target threshold Thd is searched for.

[0069] As conditions for replacement, it is possible to consider a threshold value that has not been replaced before the processing, a threshold value that is greater than the target threshold value Thd, a maximum value within the replacement range SEO, etc. By removing the threshold value corresponding to the pixel that has been replaced from the determination target of replacement, it is possible to speed up the processing without performing unnecessary processing. In the example of the second embodiment shown in the figure, by expanding the replacement range, Figure 8 Positions yd = 4, 7, and 8 in the image can also be replaced with thresholds to further minimize the effects of missing points. For example, at position yd = 4, within the replacement range SEO, threshold 231 at position [xd-1, yd+1] satisfies the replacement criteria, and the replacement is performed. The replacement result is shown as range SEF.

[0070] In this way, when the replacement range SEO is expanded, the number of objects for which the threshold value can be replaced increases, and the influence of missing dots can be further suppressed. In the second embodiment, the position of the dot generated by replacing the threshold value is not limited to the left and right adjacent columns of the pixel of interest, but is also formed at the position [xd±2, yd] further outside. As in the second embodiment, when the replacement range is expanded and the threshold value set as the replacement object is searched sequentially from the inside, the farther away from the pixel of interest, the lower the generation rate, but to a certain extent, the columns far away will also form dots that make up for the missing dots. Therefore, the following disadvantages can be suppressed: too many dots are generated in the left and right columns adjacent to the pixel of interest, and the dots are easily connected in the vertical direction and become stripes and are seen. In the case where the output resolution of the printer 20 is high, dots that make up for the missing dots can also be formed in columns that are further away.

[0071] exist Figure 8 In the example shown, similar to the first embodiment, a weight adjustment process (step S257) is performed to reduce the likelihood of dots forming continuously in the vertical direction. Specifically, when the threshold corresponding to the pixel of interest is at position [2, 7], the search range for the threshold to be replaced is eight pixels within the replacement range SEP. Simply searching for the maximum threshold would select threshold 253 at position [4, 7]. However, in this case, the threshold at position [5, 7] might be replaced in the column with xd = 4, potentially leading to the formation of dots that are missing and close together. Therefore, to prevent dot generation from being biased toward the column with xd = 4, threshold 246 at position [0, 7] is selected as the change threshold and replaced with threshold 154 corresponding to the pixel of interest. This weight adjustment process is performed to reduce the likelihood of dots forming close together in the same column.

[0072] As described above, the second embodiment searches for replacement targets for the jitter threshold at three locations in each column to the left and right of the target threshold (positions [1, 6], [1, 7], [1, 8] and [3, 6], [3, 7], [3, 8] in the replacement range SEP) and one location on either side of the threshold corresponding to the pixel of interest (positions [0, 7] and [4, 7] in the replacement range SEP). Therefore, based on simple estimation, the proportion of dots formed in adjacent columns to compensate for dot loss is three times the proportion formed at the two sides of the threshold corresponding to the pixel of interest. This compensates for dot loss, suppresses the occurrence of white streaks, and also prevents problems such as excessive dots being formed close to the dot loss location and causing them to be conspicuous.

[0073] C. Third embodiment:

[0074] The printer 20 of the third embodiment has the same hardware configuration as the first embodiment, and the outline of the executed processing is also the same. However, in the third embodiment, the jitter threshold value replacement processing (step S200) in the jitter threshold value correction processing is repeated multiple times depending on the situation. Figure 7 : Step S250) is different. Figure 9 This figure illustrates the replacement range and actual replacement in the third embodiment. As shown in the figure, in the third embodiment, similar to the second embodiment, the replacement range includes not only the two pixels before and after the pixel of interest in the X direction, but also the two pixels before and after it, as well as four pixels offset from the pixel of interest in both the X and Y directions, for a total of eight pixels, which are processed as a replacement range SEQ. Furthermore, the search range is divided into groups as follows.

[0075] First priority group [xd-1, yd], [xd+1, yd]

[0076] Second priority group [xd-2, yd], [xd+2, yd]

[0077] The third priority group [xd-1, yd-1], [xd+1, yd-1]

[0078] Fourth priority group [xd-1, yd+1], [xd+1, yd+1]

[0079] On this basis, the object threshold Thd corresponding to the pixel where the point is missing is first compared with the threshold of the first priority group. If a threshold with a value larger than the object threshold is found, it is replaced. If not found, the threshold of the second priority group is replaced next. If the corresponding threshold is not found in the second priority group, the third priority group is further searched. The search is performed in this way.

[0080] In this way, the search is performed for each group. Furthermore, in the third embodiment, in the process of changing the jitter threshold, the following process is repeated: <1> 、 <2> Until the end condition is met.

[0081] <1> Search the thresholds of the replacement range SEQ in sequence from the first priority group. If there is a threshold with a value greater than the target threshold, it is used as the target threshold and replaced with the target threshold.

[0082] <2> The position to be replaced is obtained before replacement, that is, the change threshold value, and the replacement range SEQ is further searched. If there is a value greater than the threshold value, the position is replaced.

[0083] Thus, in the third embodiment, when searching within the replacement range SEQ, the search range is divided into a plurality of groups, and replacement pixels are searched in sequence. At this time, in the third embodiment, the maximum threshold value within the searched replacement range SEQ is not searched, but if there is a threshold value within the searched group that satisfies the condition of being larger than the object threshold value (value 69 in the illustrated example), it is selected as the change threshold value as the replacement object. This is because even if the difference between the threshold value and the object threshold value is small, the formation of a point near the pixel of interest can reduce the disorder of the point configuration. Therefore, when searching the replacement range SEQ, Figure 6 、 Figure 8 Unlike the example shown, it is determined that the threshold value 163 at the position [3, 1] rather than the threshold value 173 at the position [0, 1] satisfies the condition, and is replaced with the target threshold value 69 (first stage).

[0084] In this embodiment, since the search for all thresholds in the replacement range SEQ has not yet been completed, the search continues to see if a threshold greater than the original threshold 163 at the replaced position [3, 1] is within the replacement range SEQ. As a result, the threshold 173 at the position [1, 0] is replaced with the threshold 163 (second stage). This replacement is repeated until the termination condition is met. In this embodiment, the termination condition is set to the completion of the comparison of all thresholds in the replacement range SEQ or the satisfaction of the condition <1> 、 <2> The threshold value becomes larger than the value ED. In this way, the process of sequentially replacing the nearby larger threshold values ​​in a so-called domino manner is repeated until the end condition is met.

[0085] In this way, the formation position of the point used to replace the defect can be rotated while the formation of the replacement point continues. In addition, since the difference between the original threshold value and the threshold value after replacement can also be reduced, the disorder of the point configuration can be reduced. In the third embodiment, such a process is set to repeatedly overwrite the pixel with xd=2 where the point is missing until the threshold value of the last replacement target becomes above the value ED, so that when the input grayscale value is below the value ED, a replacement point for the pixel with the point missing can be reliably generated. In addition, the number of additional replacement processes can also be limited to once or twice. In addition, the replacement range SEQ can also be extended from the pixel where the point is missing to a farther distance, for example, to a position three pixels or more away.

[0086] D. Fourth embodiment:

[0087] Next, the fourth embodiment will be described. The printer 20 of the fourth embodiment has Figure 3 Except for a part of the halftone processing shown in FIG. 1 , other processing, that is, the dither threshold value replacement processing, etc., is performed in the same manner as the various methods shown in the first to third embodiments. Figure 10 As shown, instead of halftoning step S133, a process is performed to obtain the pixel position (X, Y) and the input grayscale value DI at that pixel position (step S133a), determine whether the pixel position is near the pixel position causing dot dropout (step S133b), and perform two processes based on the determination result (steps S133c and S133d). Step S133c uses the input grayscale value DI as the pixel value DS, while step S133d corrects the input grayscale value DI and uses it as the pixel value DS. The pixel position to be halftoned is near the defective point causing dot dropout, meaning it is located on either side of the pixel in the X direction. Of course, this is not limited to both sides; "nearby" can also include pixel positions outside the pixel.

[0088] In this embodiment, as described below, pixel values ​​DS are corrected by referring to a lookup table (LUT). However, correction can also be performed using a function or the like. For example, when the input grayscale value DI is below a predetermined value Tdi (e.g., 32), no correction is performed. Starting from this predetermined value Tdi, correction is gradually initiated, with the corresponding pixel value DS being corrected to a larger value relative to the input grayscale value DI. The correction amount ΔD increases as the input grayscale value DI increases. This is because the larger the input grayscale value DI, the more likely it is that no dots will be formed to compensate for missing dots. The correction amount ΔD is determined to optimize the image actually printed. Therefore, it does not necessarily increase simply with the input grayscale value; the pixel value DS may decrease relative to the input grayscale value DI. This correction method can also be combined with any of the methods described in the first through third embodiments. However, in particular, when combined with the third embodiment, since the likelihood of insufficient dots being formed to compensate for missing dots is low, the absolute value of the correction amount ΔD may be reduced based on the input grayscale value DI, or the correction amount ΔD may be negative, resulting in a decrease in the pixel value DS.

[0089] The lookup table LUT used for such correction can either establish a relationship between the input grayscale value DI and the pixel value DS, or establish a relationship between the input grayscale value DI and the correction amount ΔD. In the latter case, the pixel value DS can be obtained by simply adding or multiplying the correction amount ΔD to the input grayscale value DI after referring to the lookup table LUT. The relationship between the input grayscale value DI and the pixel value DS does not need to be prepared in advance for the entire range of the input grayscale value DI. For example, when the input grayscale value DI takes an eight-bit value, that is, a value in the range of 0 to 255, the range can be divided into multiple, for example eight, and the relationship between the input grayscale value DI and the pixel value DS is predetermined at nine division points of 0, 32, 64, ... 255. The pixel values ​​DS within each range are internally interpolated using the values ​​at the division points. The interpolation can be linear interpolation or curve interpolation using the values ​​at three or more adjacent division points.

[0090] The above correction can be performed, for example, on pixels adjacent to the column where the dot dropout has occurred (step S133b: "Yes"), or even on pixels adjacent to the column on the outer sides. In this case, the contents of the lookup table (LUT) are also preferentially modified and optimized based on the distance from the pixel where the dot dropout has occurred. A single lookup table (LUT) can be used to reduce the correction amount ΔD for pixels farther away, or dedicated lookup tables (LUTs) can be prepared based on distance.

[0091] According to the fourth embodiment described above, the degree of dot formation near pixels where dot omissions occur to complement them can be made more appropriate. This can suppress the occurrence of white streaks and other issues caused by dot omissions, and the arrangement of the surrounding dots can be made more rational, thereby suppressing image quality degradation.

[0092] While the above description focuses on an embodiment for correcting the pixel values ​​DS of pixels surrounding a pixel experiencing dot dropout, similar effects can be achieved by relatively correcting the dither thresholds corresponding to the corresponding pixels. Specifically, it is also possible to correct the thresholds on either side of the target threshold's position [xd, yd], that is, at positions [xd±1, yd]. In this case, the correction is performed by reversing the relationship between the input and output sides of the lookup table (LUT). Specifically, if the correction amount ΔD for the pixel values ​​DS of pixels adjacent to the target pixel is such that the input grayscale value DI increases by only a value α or a value β%, the threshold value can be reduced by only a value α or a value β%. This method of correcting the corresponding thresholds offers the advantage of requiring no additional steps such as changing the input grayscale value, allowing correction to be performed simply by setting all thresholds. In this case, the thresholds can be rewritten before halftoning and the rewritten dither mask 61 can be stored in the EEPROM 60.

[0093] Thus, the dither mask 61 can be corrected by, for example, correcting the thresholds of the sixty-four pixels arranged in the y direction (see Figure 4 ) is sufficient. Therefore, it is not necessary to correct the pixel values ​​DS of all pixels corresponding to the dot-missing positions constituting the image data. Furthermore, when performing different corrections depending on the distance from the dot-missing column, the pixel value correction method requires maintaining correction tables that differ according to distance during halftoning. However, the threshold correction method only requires correcting the threshold before executing halftoning, eliminating the need for multiple lookup tables (LUTs) during halftoning.

[0094] E. Fifth embodiment:

[0095] Next, the fifth embodiment will be described. Instead of a head 90 equipped with multiple nozzles Nz forming one ink color in rows across the width of the medium P, the printer of the fifth embodiment includes a head 90 equipped with two rows of nozzles Nz forming one ink color, arranged along the Y direction (the X direction) across the width of the medium P. In this head 90, two nozzles Nz eject droplets of each color along the conveyance direction (the Y direction) of the medium P. When viewed along the conveyance direction of the medium P, the two nozzles Nz alternately form dots. Therefore, missing dots, the result of dot omission caused by a defective nozzle Nz, are not continuously generated. In other words, when the row of dots formed continuously across the width of the medium P is referred to as a grid, dot omission occurs every other grid row. Therefore, the jitter threshold correction process is also performed based on this. Figure 11 This is a flowchart showing the outline of the replacement process for each grid. Figure 5 The jitter threshold correction processing routine described is identical except for the addition of step S225. In step S225, a determination is made as to whether the position yd of the target threshold in the jitter mask 61 is even. This occurs when a dot is missing from the nozzle Nz responsible for the even-numbered rows of the grid. In this example, even in the same column of the image, dots are formed by different nozzles Nz in the even-numbered and odd-numbered grids, and the nozzle Nz responsible for the even-numbered grids is defective and thus has a dot missing. Furthermore, by combining the drive amount of the paper feed motor 74 driven by the CPU 40 with the detection results of the detection device 70, it is possible to easily determine whether a defect has occurred in either the nozzle Nz responsible for the even-numbered grids or the nozzle Nz responsible for the odd-numbered grids.

[0096] exist Figure 12 , which is a diagram showing the processing when a point is missing due to the nozzle Nz of the grid responsible for the even-numbered rows. In the figure, the threshold value that is the object of the replacement process is shown by shading. In addition, the range of the threshold replacement is represented as a replacement range SER surrounded by a dotted line. If the pixel of interest is at position [xd, yd], the replacement range SER includes a total of six threshold values ​​of position [xd±1, yd], position [xd±2, yd], and position [xd±1, yd+1]. These six threshold values ​​are the objects of replacement. Therefore, the replacement range SER for each pixel of interest is not repeated. In addition, here, yd is an even number. In addition, the replacement range SER can be narrower or wider than it. For example, it can also be a range that takes the eight threshold values ​​including the position [xd±2, yd+1] as the object. In addition, as described in the first to third embodiments, the weight adjustment process can also be performed simultaneously ( Figure 7 , step S257), so that the formation of the point is not biased towards a specific side.

[0097] The fifth embodiment described above can achieve the same effects as the above embodiments even when multiple nozzles are responsible for forming dots in a row. Since dot dropout occurs only every one or more grids, the effect of dot dropout can be further suppressed by changing the threshold.

[0098] F. Other implementation methods:

[0099] (1) Another embodiment is a printing device for forming dots on a medium. The printing device includes: a dither mask unit that prepares a dither mask composed of a plurality of threshold values ​​used in halftone processing based on a dither method; a halftone processing unit that compares the input grayscale value of each pixel constituting image data with a threshold value obtained from the dither mask according to the position of the pixel, thereby converting the image data into dot data indicating whether a dot is formed for each pixel; a printing unit that drives the head according to the dot data; a defective dot acquisition unit that acquires the position of a defective dot, i.e., a pixel where the dot is not formed by the head, in an on-pixel for dot formation; and a correction unit that searches for the threshold value used for comparison in a predetermined range of pixels near the defective dot, and when a threshold value, i.e., a change threshold value, is found that satisfies a search condition that the value is greater than the target threshold value used for comparison at the position of the defective dot, the change threshold value is corrected to the target threshold value and provided to the comparison at the pixel corresponding to the corrected threshold value in the halftone processing unit. This makes it easier to form dots near defective dots, where no dots were formed. This reduces image quality degradation caused by defective dots. Furthermore, by correcting the dither mask threshold, image quality degradation caused by defective dots can be suppressed more easily than by correcting the input grayscale value. Furthermore, image quality degradation can be suppressed by simply correcting the threshold or by combining it with correcting the input grayscale value.

[0100] While the first through fifth embodiments illustrate a line printer as the printer 20, this method can also be implemented in a so-called serial printer, in which a head reciprocates in the width direction of the medium P (hereinafter referred to as the main scanning direction) while forming an image. In this case, if a defective dot occurs due to, for example, a nozzle in the head array being unable to form a dot, the defective dot is arranged along the main scanning direction. Therefore, in this case, the position (X, Y) of the defective dot is determined using the detection device 7. The y-direction position yd on the dither mask 61 corresponding to the y-direction position of the defective dot is determined using the aforementioned equation (2). The thresholds for the y-direction positions yd are sequentially read in the x-direction, and the threshold replacement process is performed in the same manner as in the first through fifth embodiments. The replacement range for the search threshold can also be set on both sides of the x-direction, rather than in the y-direction.

[0101] In the case of a serial printer, a grid is completed by the reciprocating motion of the head. In this case, a grid is formed by dots based on droplets ejected from multiple nozzles. Therefore, in this case, similar to the case described as the fifth embodiment, the target threshold is extracted for every one or more columns, compared with the threshold within the replacement range, and then the replacement process is performed.

[0102] The head can eject droplets to form an image, or it can form dots using other methods such as ink melting, sublimation, and transfer. The actuator that generates the pressure fluctuations needed to eject droplets can use an electrostrictive element such as a piezoelectric element, or a heater that heats the ink to generate bubbles (foam).

[0103] Among the above-mentioned heads, a head of a type that forms multiple dots of different sizes can also be used. Figure 13 The processing for forming such dots of different sizes will be described. As an example, a head capable of forming two types of dots will be listed. Figure 13 As shown in the top section of the figure, this head can form dots of normal size (hereinafter referred to as M dots) and larger dots (hereinafter referred to as L dots). M dots are smaller than the pixel frame. Therefore, if the input grayscale value DI is 255, when M dots are formed for all pixels, the density achieved on the medium P is such that the density when filling the medium P remains at approximately 200, compared to 255. Meanwhile, L dots are sized to circumscribe the pixel frame.

[0104] Based on the input gray value DM of point M and the input gray value DL of point L, it is specified how to form these two points. The formation ratio of point M with respect to the input gray value DM of point M and the input gray value DI of the image data, and similarly, the formation ratio of point L with the input gray value DL of point L, are respectively shown in the range of values 0 - 255. A formation ratio of value 255 means that points are formed in all pixels, and in this case, the pixel coverage rate is 100%. In Figure 13 The following shows the relationships such as the formation ratio of each point with respect to the input gray value DI and the achieved density. As shown in the figure, point M is formed to the extent corresponding to the input gray value DI until the input gray value DI is around 64. When the input gray value DM of point M exceeds 64, the increasing ratio gradually decreases. When the input gray value DI exceeds 160, a relatively flat value in the range of 128 to 135 is taken, and finally it is 128. On the other hand, the input gray value DL of point L is 0 until the input gray value DI is 64, and then it increases, taking the difference value between the input gray value DI and the input gray value DM. As a result, when the input gray value DI is above 160, the value of the input gray value DL of point L increases sharply.

[0105] In this way, since point M does not completely cover the pixel frame, the density achieved when only point M is formed according to the input gray value DI stays around 200 / 255, while the density achieved when point M and point L are formed as shown in the figure according to the input gray value DI is 255 / 255. As shown in the figure, by reducing the ratio of point M and forming point L, if the so-called dither continuity method is used, the dither mask used in the case of forming a single type of point can be directly used. Specifically, the formation of points is judged as follows.

[0106] In the above embodiments, the dither threshold after the replacement process of the object threshold around the pixel where point loss occurs is set to The. At this time, it is judged that,

[0107] If The < DL, then form point L,

[0108] If The ≥ DL and The < (DL + DM), then form point M,

[0109] If The ≥ DL and The ≥ (DL + DM), then no point is formed.

[0110] In this way, the characteristics of a dither mask can be reflected, and it is easy to judge whether to generate point L and point M. In addition, a dither mask for point M and a dither mask for point L can be separately prepared.

[0111] In this way, in a printer that forms two types of dots of different sizes, dot formation using the method disclosed herein can be performed. In this case, whether the value ED used to determine whether to replace the object threshold in the pixel of interest is equal to the maximum value of the input grayscale value, even if it is sufficiently close, a dot will be formed to compensate for the effect of the dot loss. This is because, in order to prevent the proportion of dots formed near the pixel where the dot loss occurs from increasing to more than 100%, the concentration of the pixels near the dot-missing pixel, that is, the amount of ink, can be achieved by increasing the size of the dots formed rather than increasing the proportion of dots formed. In addition, the dot size is not limited to two types, but can also be three or more types such as large, medium, and small. In addition, when using a dark ink such as magenta ink or cyan ink, and a lighter ink such as light magenta ink or light cyan ink that is lighter than it, the proportion of the dark ink formed can also be increased or decreased. Instead of increasing the dot size, the same effect can be achieved by arranging two or more dots at the same pixel position.

[0112] (2) In the above configuration, the predetermined range may include pixels on both sides of the defective point in a direction intersecting the arrangement direction of the defective point. This makes it easier to form the point to be filled in near the defective point.

[0113] (3) In the structure of (1) or (2) above, the predetermined range may be a range of eight pixels in total, including the first and second pixels, the third and fourth pixels, and the fifth to eighth pixels, wherein the first and second pixels are pixels on both sides of the defective point in a direction intersecting the arrangement direction of the defective point, the third and fourth pixels are pixels further outward of the pixels on both sides, and the fifth to eighth pixels are pixels on both sides of the first and second pixels in the arrangement direction. In this way, since the threshold values ​​of the pixels on both sides of the defective point, i.e., the pixels other than the first and second pixels, can be corrected to the target threshold value, the possibility of threshold correction can be increased, thereby increasing the possibility of completing the missing dots and further suppressing the degradation of image quality. In addition, the formation position of the dots for completing the missing dots can be suppressed from being biased, which also suppresses the degradation of image quality.

[0114] (4) In the configurations of (1) to (3) above, the correction unit may sequentially search for thresholds in the order of the thresholds corresponding to the first and second pixels, the thresholds corresponding to the third and fourth pixels, and the thresholds corresponding to the fifth to eighth pixels until the search condition is satisfied. In this manner, since the thresholds can be corrected for pixel labeling priorities within a predetermined range, it is easier to control the positions of points that are likely to be formed to complement an unformed point.

[0115] (5) In the configurations of (1) to (4) above, the correction unit may correct a threshold value greater than the change threshold value among the threshold values ​​corresponding to the pixels in the predetermined range to a change threshold value that is the target of correction. In this way, since not only the target threshold value is corrected to the change threshold value, but also the threshold value greater than the change threshold value among the threshold values ​​corresponding to the pixels in the predetermined range is corrected to the change threshold value that is corrected to the target threshold value, it is easier to form a point near the defective point. In addition, it is possible to suppress the concentration of points that compensate for the effect of the missing points near the defective point, thereby reducing the image quality. Such correction may be repeated until a specific end condition is satisfied, or may be performed only once or twice in a limited number of times.

[0116] (6) In the configurations of (1) to (5) above, the correction unit may not perform threshold value correction when the target threshold value is greater than a predetermined value. In this way, since threshold value correction is performed only on a portion of pixels in the defective point where no dot is formed, the effort required for the correction process can be reduced. Furthermore, when the target threshold value is greater than a predetermined value, since it is difficult to form a dot in the first place, even if threshold value correction is not performed, the impact on image quality is small. Of course, threshold value correction may be performed in all cases.

[0117] (7) In the configurations of (1) to (6) above, the state of dot formation resulting from the threshold correction by the correction unit can be adjusted. Thus, while the threshold correction is being maintained, it is possible to adjust the state of dot formation, for example, when dot formation is biased toward a specific location.

[0118] (8) In the configurations of (1) to (7) above, the adjustment of the state of dot formation can be performed by replacing the threshold value in such a manner that the corrected threshold value is not arranged in a predetermined number or more. If such an adjustment can be performed, the result of the threshold correction is less likely to result in a situation where the dots are arranged in a predetermined number or more, and degradation of the image quality can be suppressed. Alternatively, such an adjustment may not be performed. Furthermore, the adjustment of the state of dot formation is not limited to replacing the threshold value in such a manner that the corrected threshold value is not arranged in a predetermined number or more, and it is also possible not to form a portion where the corrected threshold value is arranged in a predetermined ratio or more.

[0119] (9) In the structures of (1) to (8) above, the state of dot formation can be adjusted by increasing or decreasing the threshold or input grayscale value corresponding to the pixels in the predetermined range that can form dots. In this way, the possibility of dot formation can be adjusted by increasing or decreasing the threshold or input grayscale value. Generally, when a defective dot is generated due to a malfunction of a nozzle or the like and a dot is formed to replace it in its periphery, there may be a case where insufficient dots are formed or an excessive number of dots are formed. In such a case, by increasing or decreasing the threshold or input grayscale value corresponding to the pixels that can form dots, the dot formation can be corrected. The degree of increase or decrease can be adjusted by actually forming an image, or it can be pre-set according to the grayscale value of the image.

[0120] (10) The present disclosure can also be implemented as a method for printing by forming dots on a medium using a head. The method prepares a dither mask composed of multiple threshold values ​​used in halftone processing based on a dither method, obtains the position of a defective dot, a pixel where the dot is not formed by the head, among the on-pixels for dot formation, compares the input grayscale value of each pixel constituting image data with the threshold value obtained from the dither mask according to the position of the pixel, and performs halftone processing to convert the pixel into dot data indicating whether a dot is formed for each pixel. During the halftone processing, the threshold value used for the comparison is searched for pixels in a predetermined range near the defective dot. When a threshold value, i.e., a change threshold value, is found that satisfies the search condition that the value is greater than the target threshold value used for comparison at the position of the defective dot, the change threshold value is corrected to the target threshold value and provided to the comparison at the pixel corresponding to the corrected threshold value in the halftone processing unit, and the head is driven according to the dot data.

[0121] This makes it easier to form dots near defective dots where no dots were formed, thus suppressing image quality degradation caused by defective dots. Furthermore, by correcting the dither mask threshold, image quality degradation caused by defective dots where no dots were formed can be suppressed more easily than by correcting the input grayscale value.

[0122] (11) The present disclosure can also be implemented as an image processing device that processes image data and converts it into dot data for forming dots on a medium. The image processing device includes: a dither mask unit that prepares a dither mask composed of multiple threshold values ​​used in halftone processing based on a dither method; a halftone processing unit that compares the input grayscale value of each pixel constituting the image data with a threshold value obtained from the dither mask according to the position of the pixel, thereby converting the image data into dot data indicating whether a dot is formed in each pixel; a defective point acquisition unit that obtains the position of a defective point, which is a pixel where the dot is not formed among the conductive pixels performing the dot formation; and a correction unit that searches for the threshold value used for the comparison in a predetermined range of pixels near the defective point, and when a threshold value, i.e., a change threshold value, that satisfies a search condition that the value is greater than the target threshold value used for the comparison at the position of the defective point is found, the change threshold value is corrected to the target threshold value and provided to the comparison at the pixel corresponding to the corrected threshold value in the halftone processing unit. In this way, a dither mask for making it easier to form dots near the defective point where the dot is not formed can be easily provided. As a result, using the dot data processed by the image processing device can suppress image quality degradation caused by missing dots. Furthermore, since the dither mask threshold is corrected, image quality degradation caused by missing dots (where no dots are formed) can be suppressed more easily than by correcting the input grayscale value.

[0123] (12) In each of the above embodiments, a portion of the structure implemented by hardware may be replaced by software. At least a portion of the structure implemented by software may also be implemented by a discrete circuit structure. In addition, when a portion or all of the functions of the present disclosure are implemented by software, the software (computer program) may be provided in the form of a computer-readable recording medium. "Computer-readable recording medium" is not limited to portable recording media such as floppy disks and CD-ROMs, but also includes various internal storage devices in computers such as RAM and ROM, and external storage devices such as hard disks that are fixed to the computer. In other words, "computer-readable recording medium" has a broad meaning that includes any recording medium that is not temporary but can be fixed to a data packet.

[0124] The present disclosure is not limited to the above-described embodiments and can be implemented in various structures without departing from its purpose. For example, the technical features in the embodiments corresponding to the technical features in the various modes described in the Summary of the Invention column can be appropriately replaced or combined to solve part or all of the above-mentioned problems or to achieve part or all of the above-mentioned effects. In addition, as long as the technical features are not described as necessary in this specification, they can be appropriately deleted.

Claims

1. A printing device, characterized in that The printing device includes a head for forming dots on a medium, and comprises: a halftone processing section that performs halftone processing to generate dot data indicating the presence or absence of dot formation using image data consisting of a plurality of pixels having a plurality of grayscale values ​​and a dither mask consisting of a plurality of threshold values; a defective position acquiring unit for acquiring a position of a defective point at which the head cannot form the dot even though the dot formation is performed according to the dot data; a correction unit that searches for a threshold value used in generating points within a predetermined range near the defect point, corrects a value of a change threshold value to a value equivalent to a target threshold value, and applies the change threshold value to the halftoning process at the pixel corresponding to the corrected threshold value, wherein the change threshold value satisfies a search condition that the change threshold value is greater than the target threshold value used at the position of the defect point; and The printing unit drives the head according to the dot data.

2. The printing device according to claim 1, wherein The predetermined range is a range including pixels on both sides of the defective point in a direction intersecting the arrangement direction of the defective point.

3. The printing device according to claim 1, wherein The predetermined range is a range of eight pixels in total, including the first pixel to the eighth pixel, the first pixel and the second pixel are pixels on both sides of the defect point in a direction intersecting the arrangement direction of the defect point, the third pixel and the fourth pixel are pixels further outward of the pixels on both sides, and the fifth pixel to the eighth pixel are pixels on both sides of the pixels on both sides in the arrangement direction.

4. The printing device according to claim 3, wherein The correction unit searches for the thresholds in the order of the thresholds corresponding to the first pixel, the second pixel, the third pixel, the fourth pixel, and the fifth pixel to the eighth pixel until the search condition is met.

5. The printing device according to claim 1, wherein The correction unit corrects a threshold value larger than the change threshold value among the threshold values ​​corresponding to pixels in the predetermined range to the change threshold value to be corrected.

6. The printing device according to any one of claims 1 to 5, characterized in that The correction unit does not perform the correction of the change threshold value when the target threshold value is larger than a predetermined value.

7. The printing device according to any one of claims 1 to 5, characterized in that A state of dot formation resulting from correction of the threshold value by the correction unit is adjusted.

8. The printing device according to claim 7, wherein: The adjustment of the state of dot formation is performed by replacing the threshold value so that the corrected threshold value does not become aligned by a predetermined number or more.

9. The printing device according to claim 7, wherein: The adjustment of the state of dot formation is performed by increasing or decreasing the threshold value corresponding to pixels in the predetermined range and capable of forming the dot, or the grayscale value of the pixel.

10. A printing method, characterized in that: Printing is done by forming dots on the medium through the head. Acquire the position of a defective point where the head cannot form the dot even though the dot is formed. A halftone process is performed to generate dot data indicating the presence or absence of dot formation using image data consisting of a plurality of pixels having a plurality of grayscale values ​​and a dither mask consisting of a plurality of threshold values. Searching for a threshold used in generating points within a predetermined range near the defect point, correcting the value of the change threshold to a value equivalent to the target threshold, and applying the change threshold to the halftone processing at the pixel corresponding to the corrected threshold, wherein the change threshold satisfies a search condition that the change threshold is greater than the target threshold used at the position of the defect point. The head is driven according to the dot data.

11. An image processing device, characterized in that: The image processing device processes image data and converts it into dot data for forming dots on a medium. a halftone processing section that performs halftone processing to generate dot data indicating the presence or absence of dot formation using image data consisting of a plurality of pixels having a plurality of grayscale values ​​and a dither mask consisting of a plurality of threshold values; a defective position acquiring unit for acquiring a position of a defective point where the dot cannot be formed even though the dot formation is performed according to the dot data; as well as A correction unit searches for a threshold used in generating points within a predetermined range near the defect point, corrects the value of the change threshold to a value equivalent to the value of the object threshold, and provides it to the halftone processing at the pixel corresponding to the corrected threshold, wherein the change threshold is a threshold that satisfies a search condition, and the search condition is that it is greater than the threshold used at the position of the defect point, that is, the value of the object threshold.

Images