Printing apparatus, printing method, and image processing apparatus

Abstract

To suppress deterioration of image quality when dots are not formed due to factors such as clogging of a nozzle.SOLUTION: A printing apparatus acquires a position of a defective dot which is a pixel in which the dot cannot be formed by the head regardless of a position where the dot is formed, performs halftone processing of generating dot data indicating presence or absence of dot formation using image data including a plurality of pixels having a plurality of gradation values and a dither mask including a plurality of thresholds, searches for a threshold used in generation of a dot in a predetermined range in the vicinity of the defective dot, corrects a value of a changed threshold which is a threshold satisfying a search condition that the changed threshold is larger than a value of a target threshold which is a threshold used in the position of the defective dot to a value equivalent to the value of the target threshold, and performs the halftone processing in the pixel corresponding to the corrected threshold.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to printing techniques that perform halftoning using dithering. [Background technology]

[0002] When printing on a medium by ejecting droplets from multiple nozzles, nozzle clogging or other problems can cause droplets to not be ejected in certain locations, resulting in white streaks appearing in the printed image. When such missing dots occur, a known technique is to eject droplets onto pixels surrounding the row where the missing dots occur, making the missing dots less visible and preventing degradation of image quality. Known methods for correcting droplet ejection include a method of processing image data before halftone processing, as disclosed in Patent Document 1, and a method of correcting dot data indicating the on / off status of dots after halftone processing, as disclosed in Patent Document 2.

[0003] The method of Patent Document 1 processes the original image data by increasing the gradation values ​​of pixels that make up the image and are near the position where a missing dot occurs, thereby increasing the probability of dot formation, or by increasing the usage rate of large dots when a head that can form both large and small dots is used. Furthermore, Patent Document 2 modifies the dot data after halftone processing, even if it is assumed that dots will be formed using the dot data, for pixels where dots are missing because a faulty nozzle is responsible for dot formation, so that a substitute dot to replace the missing dot is formed in a pixel near the pixel that is responsible for dot formation by a normal nozzle and that is not supposed to form a dot. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-109480 [Patent Document 2] International Publication No. WO2008 / 102591 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the method in Patent Document 1 uses a probabilistic control method that increases the probability of dots being formed in rows near defective nozzle rows, so there is no guarantee that substitute dots will be generated in a 1:1 correspondence with the neighborhood of the missing dot, and there is a problem that substitute dots may occur quite far from the missing dot. In particular, when using a dithering method as a halftone method, which is simple and allows for high-speed processing, there is a problem that the intervals at which substitute dots are generated are unstable, and areas where substitute dots do not occur or areas where they occur excessively occur at a cycle that is easily noticeable to the human eye.

[0006] Furthermore, the method of modifying dot data after halftone processing poses the problem of requiring the addition of dedicated processing to search for pixels near the pixel where the missing dot occurred, where no dots will be formed, and then form substitute dots. Adding such processing increases the number of steps, resulting in slower speeds and higher costs. Furthermore, because substitute dots are generated in positions different from their original locations, the positions of the created substitute dots are not necessarily desirable for image quality. For this reason, it has been pointed out that it is necessary to consider separate fine-tuning methods to increase or decrease the number of substitute dots generated, or to optimize their positions. [Means for solving the problem]

[0007] The present disclosure can be embodied as a printing device equipped with 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 whether or not to form dots using image data consisting of multiple pixels with multiple gradation values ​​and a dither mask consisting of multiple thresholds; a defective position acquisition unit that acquires the position of a defective dot where the head cannot form the dot even though the dot is located at a position where the dot should be formed according to the dot data; a correction unit that searches for a threshold used in generating dots in a predetermined range near the defective dot, and corrects a changed threshold value that satisfies a search condition that the changed threshold value is greater than a target threshold value used at the position of the defective dot to a value equivalent to the target threshold value, and supplies the changed threshold value to the halftone processing at the pixel corresponding to the corrected threshold value; and a printing unit that drives the head according to the dot data.

[0008] The present disclosure can also be implemented as a method for printing by forming dots on a medium using a head. This method acquires the position of a defective dot where the head cannot form the dot despite being a position where dots should be formed, performs halftone processing to generate dot data indicating the presence or absence of dot formation using image data consisting of multiple pixels with multiple gradation values ​​and a teaser mask consisting of multiple thresholds, searches for a threshold value to be used in generating dots in a predetermined range near the defective dot, modifies the value of a modified threshold value, which is a threshold value that satisfies a search condition of being greater than the value of a target threshold value used at the position of the defective dot, to a value equivalent to the value of the target threshold value, performs the halftone processing on the pixel corresponding to the modified threshold value, and drives the head in accordance with the dot data.

[0009] The present disclosure can also be implemented as an image processing device that converts image data consisting of a plurality of pixels with a plurality of gradation values ​​into data. This image processing 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 consisting of a plurality of pixels with a plurality of gradation values ​​and a dither mask consisting of a plurality of thresholds, a defective position acquisition unit that acquires the position of a defective dot where the dot cannot be formed despite being a position where the dot should be formed in accordance with the dot data, and a correction unit that searches for a threshold used in generating dots in a predetermined range near the defective dot, and corrects the value of a changed threshold that satisfies a search condition that the changed threshold is greater than the value of a target threshold that is the threshold used at the position of the defective dot, to a value equivalent to the target threshold, and provides the changed threshold for the halftone processing at the pixel corresponding to the corrected threshold. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a schematic configuration diagram showing a printing device incorporating an image processing device according to an embodiment. [Figure 2] 10 is a flowchart showing an example of an image printing processing routine performed by the printing device. [Figure 3] 10 is a flowchart showing an example of a halftone processing routine. [Figure 4] FIG. 10 is an explanatory diagram showing an example of a dither mask used in halftone processing. [Figure 5] 10 is a flowchart showing an example of a threshold correction processing routine for correcting a threshold. [Figure 6] FIG. 2 is an explanatory diagram showing an overview of switching of dither threshold values ​​according to the first embodiment. [Figure 7] 10 is a flowchart showing an example of a dither threshold replacement process. [Figure 8] FIG. 10 is an explanatory diagram showing an overview of switching of dither threshold values ​​according to a second embodiment. [Figure 9] FIG. 11 is an explanatory diagram showing an overview of switching of dither threshold values ​​according to a third embodiment. [Figure 10] 10 is a flowchart illustrating a main part of processing in the fourth embodiment. [Figure 11]13 is a flowchart showing an example of a threshold correction processing routine according to the fifth embodiment. [Figure 12] FIG. 13 is an explanatory diagram showing an overview of switching of dither threshold values ​​in the fifth embodiment. [Figure 13] FIG. 10 is an explanatory diagram showing an outline of processing when forming multiple types of dots. DETAILED DESCRIPTION OF THE INVENTION

[0011] A. First embodiment: (A1) Device configuration: FIG. 1 is a schematic diagram of a printer 20, which is a printing device. The printer 20 is a so-called line printer, an inkjet printer that uses four colors of ink, as described below. As shown in the figure, the printer 20 includes a mechanism that uses a paper feed motor 74 to drive a paper feed roller 75 to transport a medium P, a mechanism that drives a head 90 located opposite the medium P to eject ink and form dots, and a detection device 70 that recognizes the image printed on the medium P. The printer 20 also includes a control unit 30 that handles signal exchanges between the detection device 70, the paper feed motor 74, the head 90, and an operation panel 99. In this embodiment, the paper feed roller 75 also serves as a platen, but the platen may be separate from the paper feed roller. In this case, the platen may be a flat platen with a flat surface. The paper feed rollers 75 may also be located upstream and downstream of the head 90.

[0012] The detection device 70 is a line sensor that can recognize the image on the medium P at a higher resolution than the printing resolution of the head 90, and from the image recognized by this detection device 70, the CPU 40 recognizes the positions of missing dots, which are pixels where dots are missing due to reasons such as clogging of the nozzle Nz, through processing described below. The missing dot acquisition unit is realized by including the processing performed by the CPU 40 using the detection device 70.

[0013] The head 90 is provided with a number of nozzles Nz extending across the width of the medium P, capable of ejecting color inks: cyan ink C, magenta ink M, yellow ink Y, and black ink K. Each of these 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 dot data and a drive signal COM. The actuator for ejecting ink from the nozzles Nz is not limited to piezoelectric elements; various configurations can be used, such as heater-type actuators that use ink bumping to eject ink, or laser-based actuators. Naturally, ink dots can be formed using a variety of methods, not limited to inkjet printing. These include thermal transfer and thermal dye-sublimation methods using ink ribbons, methods for forming a latent image on a photosensitive drum, and serial printers that eject ink from nozzles while reciprocating a printing head across the width of the medium.

[0014] This head 90 is supplied with ink of each color from color ink cartridges 82-85, each containing a different color ink, via ink supply pipes 92-95. In addition to the above-mentioned CMYK, light cyan ink Lc, light magenta ink Lm, and the like may also be used as ink colors. Naturally, special color inks such as red, blue, and green may also be used, as well as so-called metallic inks such as gold and pearl white. Furthermore, the head 90 may be equipped with an ink system for black and white printing.

[0015] The control unit 30 includes a CPU 40, ROM 51, RAM 52, and EEPROM 60, all of which are interconnected via a bus. The control unit 30 controls the overall operation of the printer 20 by loading programs stored in the ROM 51 and EEPROM 60 into the RAM 52 and executing them, and also functions as an input unit 41, halftone processing unit 42, and printing unit 46. The functions of the halftone processing unit 42 include those of a comparison unit 43 and a correction unit 44. Details of the operation of each of these units will be described later.

[0016] The printing unit 46 is a circuit for driving the head 90 and outputs to the head 90 a signal DD corresponding to dot data and a drive signal COM for simultaneously driving multiple piezo elements. The piezo elements are grouped by color (CMYK) and driven by a signal DD corresponding to the dot data held in a latch (not shown) and a drive signal COM output at a predetermined timing. When the signal DD is on (dot data value is 1) and the drive signal COM is applied, the piezo elements expand, pressurizing the ink in the ink chambers (not shown) and ejecting droplets from the nozzles Nz. Note that, because the printer 20 of this embodiment is a line printer, the nozzles Nz for each color are offset by a predetermined pitch in the feed direction of the medium P. Furthermore, to increase resolution in the medium width direction, the nozzles Nz for the same color ink are also arranged in a staggered arrangement, offset every other nozzle in the medium feed direction. Therefore, when forming dots at the same position in the feed direction of the medium P, the timing of droplet ejection from each nozzle Nz is different. For this reason, as will be described later, a rearrangement process is performed to match the dot data obtained by processing the gradation data of the image to be formed with the nozzle arrangement. Details of the processing of each functional unit, including the processing of the printing unit, will be described later with reference to flowcharts such as Figures 2 and 3.

[0017] A dither mask 61 is stored in the EEPROM 60. The dither mask 61 is used in halftone processing, which will be described later, and has dimensions of 256 horizontally (xd: medium width direction) by 64 vertically (yd: medium feed direction), as partially shown in FIG. 4. This dither mask 61 is an array of multiple threshold values ​​Thd. In this embodiment, these threshold values ​​Thd range from 1 to 255. Each threshold value Thd is arranged so that the spatial frequency of dots formed by comparison with this threshold value has so-called blue noise characteristics.

[0018] The blue noise characteristics of a dither mask are such that the largest frequency components are in the high-frequency range where one cycle is approximately two pixels long. This means that the threshold storage position is adjusted to generate the largest frequency components in the high-frequency range, taking into account the human visual characteristic of low sensitivity in the high-frequency range. When dots are generated using a dither mask with such blue noise characteristics, an image with excellent dot dispersion is obtained.

[0019] When a dither mask has blue noise characteristics, the distribution of dots formed is well dispersed, and the graininess of the image is sufficiently suppressed. If the pixel size is sufficiently small, a good image without a noticeable graininess can be obtained even if a dither mask with green noise characteristics, which has its largest frequency component slightly lower than the blue noise characteristics, is used. The dither mask 61 is designed to have predetermined spatial frequency characteristics such as these blue noise characteristics and green noise characteristics.

[0020] Of course, the size and characteristics of the dither mask 61 are arbitrary, and sizes and characteristics other than those of this embodiment can be used. For example, a dither mask having a size of 64 × 32 or more can be used to realize an ordered dither method, or a dot concentration type dither mask that realizes characteristics similar to halftone dots can be used.

[0021] 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 consisting of three color components: red (R), green (G), and blue (B). The image data ORG may also be acquired from a computer or the like connected via a wired or wireless connection, rather than from the memory card MC.

[0022] The printer 20 having the above hardware configuration drives the paper feed motor 74 to move the medium P in the feed direction while driving the head 90 to form ink dots of each color on the medium P. The control unit 30 drives the nozzles Nz at the appropriate timing based on the print data in accordance with the paper feed of the medium P, thereby forming ink dots of the appropriate color at the appropriate position on the medium P. In this way, the printer 20 is able to print a color image input from the memory card MC onto the medium P.

[0023] (A2) Printing process: The printing process in the printer 20 will now be described. Figure 2 is a flowchart showing the flow of the image printing process in the printer 20. This image printing process is initiated when the user issues a print instruction for a specific image stored on the memory card MC using the operation panel 99 or the like. When the printing process is initiated, the CPU 40 first reads and inputs the RGB format image data ORG to be printed from the memory card MC via the memory card slot 98 as processing of the input unit 41 (step S110).

[0024] When the image data ORG is input, the CPU 40 refers to a lookup table (not shown) stored in the EEPROM 60 and performs color conversion of the image data ORG from RGB format to CMYK format (step S120).

[0025] After color conversion processing, CPU 40 performs halftone processing in halftone processing unit 42, converting the image data into dot data in which the on / off status of each color dot is defined for each pixel (step S130). Details of the halftone processing will be described later. In this specification, "halftone processing" is not limited to binarization processing of on / off dots, but refers to general tone number conversion (reduction) processing including multi-value processing such as on / off of large and small dots or large, medium, and small dots. Furthermore, the image data provided to step S130 may have undergone image processing such as resolution conversion processing and smoothing processing.

[0026] After halftone processing, CPU 40 performs a rearrangement process to rearrange the dot pattern data to simultaneously drive each nozzle Nz of head 90 in accordance with the nozzle arrangement and paper feed amount of printer 20 (step S150). As described above, the rearrangement process rearranges the dot data obtained by halftone processing (step S130) in accordance with the arrangement of nozzles Nz in head 90. After performing the rearrangement process (step S150), CPU 40 drives head 90, paper feed motor 74, etc. to execute printing as processing of printing unit 46 (step S160).

[0027] As described above, the printer 20 forms an image captured from the memory card MC on the medium P. However, in this embodiment, there may be cases where the printed image contains missing dots at specific locations across the width of the medium P due to poor ink ejection from the nozzles Nz, resulting in a printout with visible white streaks. Anticipating such cases, this embodiment performs a process of reading the printed material (step S170) after printing (step S160). The printout is read using a detection device 70. The detection device 70 is precisely positioned and attached to the printer 20. Furthermore, because the resolution of the detection device 70 as a line sensor is higher (approximately twice the resolution) than the resolution of the ink dots formed by the head 90, it is possible to accurately detect the position in the X direction where the missing dots have occurred.

[0028] The data read by the detector 70 is used to determine whether missing dots due to missing dots are visible to the viewer of the printed material (step S180). If it determines that no missing dots have occurred or that the degradation in image quality due to missing dots has been corrected by the dither mask threshold correction process described below, and the missing dots are no longer visible, the process skips to "NEXT" and ends this processing routine without taking any action. On the other hand, if it determines that missing dots are visible, the process executes the dither mask threshold correction process (step S200). In this case, the printer 20 executes the process from step S130 described above again. That is, it performs halftoning using the ordered dither method (step S130), rearrangement (step S150), prints (step S160), and reads the printed material (step S170), and then executes the process of step S170 again.

[0029] Depending on the nature of the missing dots, even if the processes from step S130 onward are performed again, the missing dots may remain visible. In such cases, the dither mask threshold correction process may be performed with an expanded target range. If the missing dots are caused by multiple consecutive noises, correcting the dither mask threshold alone may not necessarily make the missing dots invisible. In such cases, an abnormality process (not shown) may be invoked to perform maintenance on the head 90, etc. If it is determined that the dot arrangement has been corrected to the extent that the missing dots are no longer visible through the dither mask threshold correction process (step S200) or head 90 maintenance, the process skips to "NEXT" and ends this processing routine. The printer 20 is intended for use in printing multiple copies of the same image, for example, hundreds of copies. If it is determined that printing without visible missing dots is possible, a high-speed printing process routine (not shown) is used to print a large number of copies at high speed. The medium P may be a single sheet of A4 or A3 paper, or may be a long sheet of paper such as roll paper, in which case the printer 20, which is a line printer, repeats the same image or forms a series of images along the length of the medium P. The image used to detect missing dots may be an image dedicated to detection.

[0030] (A3) Halftone processing details: Next, the halftone process will be described in detail with reference to Fig. 3. When this halftone process is started, first, a process is performed to obtain the dither mask 61 to be used for the current printing process from the dither masks 61 stored in the EEPROM 60 (step S131). A plurality of dither masks 61 with different sizes and noise characteristics are prepared, and a dither mask suitable for the image to be printed is selected.

[0031] Next, a process is performed to initialize the pixel position of the image to be subjected to halftone processing and the read position of the dither mask (step S132). The initial pixel position is the upper left of the image, which is the origin (0,0) when the pixel position is expressed as (X,Y). As shown in FIG. 4, the initial position of the dither mask is expressed as [xd,yd], with the upper left of the mask being the origin [0,0]. After initializing each position in this way, the processes from step STR1 to step STP1 are repeatedly executed for each pixel that forms the image.

[0032] First, a process is performed to read the pixel position (X, Y) and the pixel value DS corresponding to the gradation value of the pixel at that position (step S133). Next, a process is performed to calculate the corresponding threshold position [xd, yd] of the dither mask 61 based on this pixel position (X, Y). The threshold position [xd, yd] is found from the following equations (1) and (2) (step S134). Here, mod(A, B) is a function that returns the remainder when number A is divided by number B. In this embodiment, the size of the dither mask 61 is 256 × 64, so equations (1) and (2) are as follows. xd=mod(X,256) …(1) yd=mod(Y,64) …(2)

[0033] Using the threshold position [xd, yd] in the dither mask 61 thus obtained, the threshold Thd at that position is obtained (step S135), and this threshold Thd is compared with the pixel value DS (step S136). These processes implement 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 a value of 1 (step S137). If the pixel value DS is equal to or less than the threshold Thd, a dot is not formed, and the dot data DD is set to a value of 0 (step S138). The set dot data DD is then sequentially saved. The above process is repeated for pixel positions (X, Y) from the origin position (0, 0) to the end position of the image to be halftoned (steps STR1 to STP1). Through the above process, the original image data ORG is converted into dot data DD composed of on / off dots, and saved in preparation for printing.

[0034] Next, referring to FIG. 5, the threshold correction process (FIG. 2, step S200) when it is determined that a missing dot is visible will be described. The CPU 40 executes this process, thereby implementing the correction unit 44. This threshold correction routine first acquires the missing dot position Xf (step S201). The missing dot position is the position of a missing dot where a dot cannot be formed by the head 90, even though the missing dot position is the position where the dot should be formed according to the dot data. The missing dot position Xf can be easily acquired based on information acquired from the detection device 70. This process corresponds to the process performed by the missing dot position acquisition unit. Next, from this position Xf, a process is executed to acquire the position xd of the dither mask 61 in the halftone process (FIG. 3) (step S211). The position xd of the dither mask 61 is the position on the dither mask 61 that provided the threshold Thd referenced when determining whether the dot at the missing dot position Xf is to be turned on or off.

[0035] In the printer 20 of this embodiment, one nozzle Nz is responsible for one row in the Y direction (vertical direction). Therefore, missing dots due to clogging of the nozzle Nz occur continuously across the width of the medium P, i.e., at a specific position in the X direction. Therefore, the position xd on the dither mask 61 corresponding to the missing dot position Xf is identified using the above-described formula (1), while the y-direction position yd is set to a value of 0 as an initial value (step S221). Then, a threshold value Thd for the position [xd, yd] on the dither mask 61 is obtained (step S231). To illustrate an example of the dither mask 61 correction process, a portion of the dither mask 61 is illustrated in FIG. 6. In this example, if the position xd on the dither mask 61 corresponding to the position Xf on the image where the missing dot occurred is a value of 2, the threshold value Thd is obtained as 202 in the first step S231.

[0036] Next, a determination is made as to whether the threshold value Thd is smaller than a predetermined value ED (step S241). If the threshold value Thd is smaller than ED (step S241: "YES"), the dither threshold replacement process (step S250) is performed. If the threshold value Thd is equal to or greater than ED (step S241: "NO"), the process of step S250 is skipped and the process proceeds to step S261. This is for the following reason: In printing using ink, ink dots are circular for rectangular pixels. Therefore, when the proportion of dots formed per unit area is equal to or greater than a predetermined value, the density difference compared to when dots are formed in all pixels becomes sufficiently small. If this proportion that results in a sufficiently small density difference is, for example, 78% or greater, this corresponds to a case in which the input tone value DS is 200 or greater in the printer 20 of this embodiment. In other words, when the threshold value Thd of the dither mask 61 is equal to or greater than the value ED (200), a dot is formed in that pixel only if the pixel value DS, which is the input tone value of the pixel, exceeds the threshold value of 200 or greater, and it can be said that there is no major problem if a substitute dot is not generated for that pixel. Therefore, in step S241, the threshold value Thd is compared with the value ED (200 in this example), and if the threshold value Thd is equal to or greater than the value ED, the threshold replacement process for generating a substitute dot is omitted even if a missing dot would occur.

[0037] Taking FIG. 6 as an example, the threshold value Thd at the position [xd, yd]=[2,0] is 202, so the determination in step S241 is "NO," and the threshold value replacement process (step S250) is not executed. When the position [xd, yd]=[2,1], the threshold value Thd is 69, so the threshold value replacement process (step S250) is executed. The value ED that is the subject of this determination can be selected appropriately depending on the ink system. Of course, it is also possible to perform the dither threshold value replacement process (step S250) for all threshold values ​​Thd without performing the determination in step S241. Details of this dither threshold value replacement process will be described later.

[0038] Regardless of whether or not the dither threshold replacement process (step S250) is executed, the y-direction value yd of the dither mask 61 being judged is then incremented by 1 (step S261), and it is determined whether the value yd has exceeded 65 (step S271). If it has not exceeded the value, the process returns to step S231 again, and the above process (steps S231 to S271) is repeated, starting with obtaining the dither mask 61. If the value yd, which indicates the position of the threshold in the y direction, exceeds 65, it is determined that processing has been completed for all thresholds Thd arranged in the y direction of the dither mask 61, and the process exits to "NEXT" to end this processing routine.

[0039] The dither threshold replacement process (step S250) in this threshold correction process will be described in detail with reference to FIG. 7. In the following description of an example of threshold replacement, reference will be made to FIG. 6 as appropriate. In the threshold replacement process, the thresholds for the replacement range are first obtained (step S251). The replacement range can be of various sizes and shapes, but in the first embodiment, it is the range corresponding to both sides of pixel position Xf in the X direction on the image where a missing dot has occurred, that is, the range corresponding to the position (Xf-1) immediately before in the X direction and the position (Xf+1) immediately after in the X direction. This is shown as the replacement range SAO in FIG. 6. In the first embodiment, in step S251, thresholds are obtained for positions [xd-1] and [xd+1] on the dither mask 61 corresponding to the pixel at the position to be replaced. In the example shown in FIG. 6, if position yd in the y direction is value 1, then the threshold Thd at position [xd,1] is value 69, and the thresholds Thd at two positions within the replacement range SAO are values ​​142 and 163.

[0040] Next, a process is performed to search for a threshold value to be replaced (step S252). This process involves searching for a threshold value within the replacement range SAO that is greater than the threshold value at the target position, and then finding the largest threshold value among them. If only one threshold value greater than the threshold value at the target position is found within the replacement range, that threshold value becomes the replacement target. If there are two threshold values ​​greater than the threshold value at the target position, the larger of the two threshold values ​​becomes the replacement target. If multiple threshold values ​​have the same value, either one may be selected, or the threshold value that will not be adjusted in the bias adjustment process (step S257) described below may be selected. After the threshold value to be replaced has been found, a determination is made as to whether or not to perform replacement (step S253). If the threshold value to be replaced has been found, the determination is "YES," and a replacement process is performed in which the largest threshold value within the replacement range is replaced with the threshold value at the target position (step S254). In the replacement range SAO shown in FIG. 6, when yd = 1, both replacement targets are greater than the threshold value at the target position, and the threshold value 163 at the replacement position [xd+1,1] is the largest within the replacement range, so it is replaced with the threshold value 69 at the position [xd,1]. Similarly, when yd = 2, the threshold value 199 at the replacement position [xd-1,2] is replaced with the threshold value 44 at the position [xd,2]. Note that in this specification, "replacement" or "replacement process" refers to the process of correcting the threshold value 163 in the replacement range SAO (when yd = 1) 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 process of replacing the target threshold with the changed threshold value to be replaced is performed. Even if the target threshold value is replaced with the changed threshold value, no dots are formed at the target pixel, so whether or not replacement is performed has no effect on dot formation. Note that a value equivalent to the target threshold value is usually the same value as the target threshold value Thd, but any value within a range that does not cause a significant difference in the results in halftone processing using a dither mask may be treated as an equivalent value. For example, a value in the range of threshold value Thd ±α may be used. Here, α may be a value obtained by multiplying threshold value Thd by a few percent.The dither threshold replacement process (step S250) compensates for the degradation of image quality caused by missing dots, and even if the value of the corrected threshold differs slightly from the target threshold, the degradation of image quality caused by the missing dots is suppressed.

[0041] After the above replacement process, a bias adjustment process (step S257) is performed, and the replaced thresholds are saved as a new dither mask 61A to be applied to the missing dot locations (step S258). The bias adjustment process (step S257) is a process for adjusting the thresholds if it is determined that threshold replacement may cause a bias in the locations where dots are generated. In the first embodiment, at position yd=13, the maximum threshold found according to the replacement conditions, that is, the threshold value 234 at position [xd+1, yd], is the target of replacement. However, if the replacement is performed as is, threshold replacement will also occur at position [xd+1, yd-1] at this point. If the replacement at position yd=14 is considered first, it will be clear that threshold replacement will also occur at position [xd+1, yd+1]. In this case, the likelihood of dots being formed consecutively in the Y direction increases, so in the bias adjustment process (step S257), the threshold value at position yd=13 is replaced with the threshold value at position [xd+1,yd], thereby adjusting the dot formation state. Of course, this adjustment process may be performed after the threshold replacement process has been performed once for all target thresholds, or it may be performed by checking the magnitude of the thresholds for three consecutive pixels in the Y direction, as described above. Alternatively, the adjustment process may not be performed. Note that if it is determined in step S253 that replacement is not required (step S253: "NO"), steps S254 to S258 are not performed, and the dither threshold replacement process (step S250) ends.

[0042] After completing the dither threshold replacement process (step S250), as shown in FIG. 5, the threshold position yd is incremented by 1, and the above process is repeated until processing has been completed for the thresholds at all positions yd in the y direction. As a result, the threshold Thd used to determine dot on / off at the X-direction position Xf where a missing dot occurred and before and after it is replaced with a threshold that makes it easier for dots not to be formed at the missing dot position to be formed before and after the missing dot position. In the example shown in FIG. 6, for positions yd with values ​​of 1, 2, 5, and 9 to 15, the largest threshold value before and after the position where a missing dot occurs is replaced with the relatively small threshold value at the position of interest. This process can be easily seen from FIG. 6.

[0043] (A4) Advantages of the first embodiment: In the printer 20 of the first embodiment described above, if a missing dot occurs due to a clogged nozzle Nz or the like in the head 90, causing a white streak or the like to appear on the medium P, the location of the missing dot is identified and the threshold value for the corresponding position in the dither mask 61 is changed so that dots are more likely to be formed around the pixel location where the missing dot occurred. As a result, in subsequent printing, the effects of the missing dot can be suppressed and printing can continue. Moreover, once the threshold value is changed, the halftone processing using the dither method itself remains unchanged, so no extra time is required for halftone processing. As a result, printing can be performed while performing halftone processing at high speed.

[0044] Furthermore, in the threshold swapping process, the threshold used when determining whether or not to form dots for a pixel where dot dropout may occur is used to swap the threshold used when determining whether or not to form dots for the pixels before and after that pixel, which has the advantage that the dot formation determination results for a pixel where dot dropout may occur and the dot formation determination results for a pixel where the threshold has been swapped are likely to be the same. This not only reduces the occurrence of white streaks and other problems caused by dot dropout, but also reduces degradation of image quality. Moreover, because the search range for replacement targets is limited to two pixels before and after the pixel of interest, processing is simple and the time required for processing can be shortened.

[0045] B. Second embodiment: The printer 20 of the second embodiment has the same hardware configuration as the first embodiment, and the outline of the process it executes is also similar. However, in the second embodiment, the replacement range in the dither threshold replacement process shown in FIG. 7 is different. FIG. 8 illustrates the replacement range in the second embodiment and the actual replacement process. In the second embodiment, as shown in the figure, the replacement range SEO includes not only the two pixels before and after the pixel of interest in the X direction, but also two pixels before and after that, and four pixels diagonally shifted from the pixel of interest in both the X and Y directions. The replacement range SEO covers a total of eight pixels. In terms of the position of the dither mask 61, if the threshold corresponding to the pixel of interest is at position [xd, yd], the following eight positions constitute the replacement range SAO. These eight positions may be treated collectively or in several groups. In the first embodiment, the eight thresholds are treated collectively, and the largest threshold among them that is greater than the target threshold Thd is searched for.

[0046] The conditions for replacement include whether the threshold value has not already been replaced before this process, whether the threshold value is greater than the target threshold value Thd, and whether the threshold value is the largest value within the replacement range SEO. By excluding threshold values ​​corresponding to pixels that have already been replaced from the replacement decision targets, unnecessary processing can be avoided and processing can be sped up. In the illustrated example of the second embodiment, the replacement range is expanded, so that threshold replacement is also performed at positions yd=4, 7, and 8 in FIG. 8, further reducing the effects of missing dots. For example, when position yd=4, within the replacement range SEO, the threshold value 231 at position [xd-1, yd+1] satisfies the replacement conditions, and replacement is performed. The replacement results are illustrated as range SEF.

[0047] In this way, expanding the replacement range SEO increases the number of targets for which the threshold can be replaced, further reducing the impact of missing dots. In the second embodiment, the positions of dots generated by replacing the threshold are not limited to the columns adjacent to the pixel of interest to the left and right, but are also formed at positions [xd±2, yd] further outward. As in the second embodiment, by expanding the replacement range and searching for thresholds to be replaced starting from the inside, the occurrence rate decreases with increasing distance from the pixel of interest, but dots to compensate for missing dots are also formed in columns a certain distance away. This reduces adverse effects such as excessive dots being generated in columns adjacent to the pixel of interest to the left and right, which can easily result in vertical dot chains and appear as visible streaks. If the output resolution of the printer 20 is high, dots to compensate for missing dots may be formed in columns even further away.

[0048] In the example shown in FIG. 8 , as in the first embodiment, a bias adjustment process (step S257) is performed to prevent dots from being formed consecutively in the vertical direction. Specifically, if the threshold corresponding to the pixel of interest is at position [2, 7], the search range for the threshold to be replaced is the replacement range SEP, which is 8 pixels. Simply searching for the maximum threshold would result in selecting threshold 253 at position [4, 7]. However, in this case, the threshold at position [5, 7] has been replaced in the column xd=4, which could result in dots being formed closely together to compensate for the missing dots. Therefore, to prevent dot generation from being concentrated in the column xd=4, threshold 246 at position [0, 7] is selected as the modified threshold, and this is replaced with threshold 154 corresponding to the pixel of interest. The bias adjustment process is then performed to reduce the possibility of dots being formed closely together in the same column.

[0049] As explained above, the search range for dither threshold replacement in the second embodiment includes three locations on each column to the left and right of the target threshold (in the replacement range SEP, positions [1,6], [1,7], [1,8] and [3,6], [3,7], [3,8]), and one location on each side of the threshold corresponding to the pixel of interest, one pixel apart (in the replacement range SEP, positions [0,7], [0,7], [4,7]). Therefore, a simple estimate of the proportion of dots formed to compensate for missing dots is that the proportion formed in adjacent columns is three times the proportion formed in positions on either side, one pixel apart. This compensates for missing dots and suppresses the occurrence of white streaks, while also suppressing the problem of excessive dots being formed close to the location where the missing dot occurs, making it noticeable.

[0050] C. Third embodiment: The printer 20 of the third embodiment has the same hardware configuration as the first embodiment, and the outline of the processes it executes is also similar. However, in the third embodiment, the dither threshold replacement process (step S250 in FIG. 7) of the dither threshold correction process (step S200) is sometimes repeated multiple times. FIG. 9 illustrates the replacement range in the third embodiment and the actual replacement process. As shown in the figure, in the third embodiment, the replacement range is similar to the second embodiment, and thresholds corresponding to not only the two pixels before and after the pixel of interest in the X direction, but also two pixels before and after that, and four pixels shifted from the pixel of interest in both the X and Y directions, for a total of eight pixels, are treated as the replacement range SEQ. Furthermore, the search range is divided into groups as follows:

[0051] First priority group [xd-1,yd], [xd+1,yd] Second Priority Group [xd-2,yd], [xd+2,yd] Third priority group [xd-1,yd-1], [xd+1,yd-1] Fourth Priority Group [xd-1,yd+1], [xd+1,yd+1] Next, the target threshold Thd corresponding to the pixel where the missing dot occurred is first compared with the thresholds of the first priority group, and if a threshold greater than the target threshold is found, a replacement process is performed; if not found, the thresholds of the second priority group are then searched, and if no corresponding threshold is found in the second priority group, the third priority group is searched, and so on.

[0052] In this way, a search is performed for each group, and in the third embodiment, in the dither threshold replacement process, the following processes <1> and <2> are repeated until the end condition is met. (1) The thresholds in the replacement range SEQ are searched for sequentially from the first priority group, and if there is a threshold with a value greater than the target threshold, this is used as the target threshold and replaced with the target threshold. (2) The threshold before the swapping of the swapped position, i.e., the change threshold, is obtained, and the swapping range SEQ is searched for, and if a threshold larger than this is found, it is swapped.

[0053] As described above, in the third embodiment, when searching within the replacement range SEQ, the search range is divided into multiple groups, and pixels to be replaced are searched for sequentially. In this case, instead of searching for the largest threshold within the replacement range SEQ, if a threshold within the group being searched satisfies the condition of being greater than the target threshold (value 69 in the illustrated example), this threshold is selected as the change threshold to be replaced. This is because forming dots near the pixel of interest, even if the threshold value is only slightly different from the target threshold, can reduce the disruption of dot placement. Therefore, when searching the replacement range SEQ, unlike the examples shown in FIGS. 6 and 8, threshold value 163 at position [3,1] is determined to satisfy the condition, not threshold value 173 at position [0,1], and is replaced with target threshold value 69 (first stage).

[0054] 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 there is a threshold in the replacement range SEQ that is greater than the original threshold 163 at the replaced position [3,1]. As a result, threshold 163 replaces threshold 173 at position [1,0] (second stage). This replacement is repeated until a termination condition is met. In this embodiment, the termination condition is set to be whether comparison of all thresholds in the replacement range SEQ has been completed or whether a threshold that satisfies conditions <1> and <2> is greater than value ED. In this way, the process of sequentially replacing neighboring larger thresholds is repeated in a domino fashion, so to speak, until the termination condition is met.

[0055] In this way, the formation of substitute dots can be continued while alternating the formation positions of dots to replace missing dots. Furthermore, the difference between the original threshold value and the threshold value after replacement can be reduced, thereby minimizing disruption of dot placement. In the third embodiment, this process is repeated for the pixel xd=2 where a missing dot occurred, until the final replacement threshold value becomes equal to or greater than the value ED. This ensures that a replacement dot for the missing dot pixel is generated reliably when the input gradation value is equal to or less than the value ED. The number of additional replacement processes may be limited to one or two. Furthermore, the replacement range SEQ may be expanded even farther from the pixel where the missing dot occurred, for example, to a position three or more pixels away.

[0056] D. Fourth embodiment: Next, a fourth embodiment will be described. The printer 20 of the fourth embodiment performs the same processes, such as the dither threshold replacement process, as the various embodiments described above, except for some differences in the halftone process shown in FIG. 3. In the fourth embodiment, as shown in FIG. 10, instead of step S133 of the halftone process, the printer performs the following steps: (1) acquire a pixel position (X, Y) and an input tone value DI for that pixel position (step S133a); (2) determine whether that pixel position is near a pixel position where a dot has dropped (step S133b); and (3) execute two processes based on the determination result (steps S133c and S133d). Step S133c directly uses the input tone value DI as the pixel value DS, while step S133d corrects the input tone value DI to obtain the pixel value DS. The pixel position being the target of halftone processing being near the missing dot means that it is one of the pixel positions adjacent to the pixel where the dot has dropped in the X direction. Of course, the "neighborhood" may include not only the adjacent pixels on both sides but also pixel positions outside of the adjacent pixels.

[0057] In this embodiment, the pixel value DS is corrected by referencing a lookup table LUT, as described below, but correction may also be performed using a function or the like. For example, when the input gradation value DI is equal to or less than a predetermined value Tdi (e.g., 32), no correction is performed. Correction gradually begins from this predetermined value Tdi, and the pixel value DS corresponding to the input gradation value DI is corrected to a larger value. The correction amount ΔD increases as the input gradation value DI increases. This is because the larger the input gradation value DI, the more likely it is that a dot to complement a missing dot will not be formed. The correction amount ΔD is determined so that the image actually printed will be optimal. Therefore, the correction amount ΔD does not necessarily increase simply with the input gradation value; it is also possible that the pixel value DS will be smaller than the input gradation value DI. This type of correction can be combined with any of the methods of the first to third embodiments described above, but when combined with the third embodiment in particular, there is little chance that there will be a shortage of dots formed to compensate for missing dots, so there is a possibility that the absolute value of the correction amount ΔD will be reduced depending on the input gradation value DI, or the correction amount ΔD will be set to a negative value, which will ultimately reduce the pixel value DS.

[0058] The lookup table LUT used for such correction may correspond to the input gradation value DI and the pixel value DS, or may correspond to the input gradation value DI and the correction amount ΔD. In the latter case, the lookup table LUT is referenced, and the pixel value DS is obtained by adding or multiplying the input gradation value DI by the obtained correction amount ΔD. The relationship between the input gradation value DI and the pixel value DS does not need to be prepared in advance for the entire range of the input gradation value DI. For example, if the input gradation value DI is 8 bits, i.e., ranging from 0 to 255, this range may be divided into multiple sections, e.g., eight, and the relationship between the input gradation value DI and the pixel value DS at nine division points from 0, 32, 64, ..., 255 may be predetermined. The pixel value DS within each range may be internally interpolated using the values ​​at the division points. The interpolation may be linear interpolation or curved interpolation using values ​​at three or more adjacent division points.

[0059] This type of correction may be performed, for example, on the pixels on both sides of the column where the missing dot occurred (step S133b: "YES"), or may be performed on the pixels adjacent to the column on the outside. In this case, it is also preferable to change and optimize the contents of the lookup table LUT depending on the distance from the pixel where the missing dot occurred. A single lookup table LUT may be used, and the correction amount ΔD may be reduced for pixels at a large distance, or dedicated lookup tables LUTs may be prepared depending on the distance.

[0060] According to the fourth embodiment described above, it is possible to more appropriately form dots to compensate for missing dots in the vicinity of pixels where the missing dots occur. This reduces the occurrence of white streaks due to missing dots, and also makes it possible to more naturally arrange the surrounding dots, thereby minimizing degradation of image quality.

[0061] While the above describes an embodiment in which pixel values ​​DS of pixels surrounding a pixel with a missing dot are corrected, a similar effect can be achieved by relatively correcting the dither thresholds corresponding to the pixel. That is, the thresholds on both sides of the target threshold position [xd, yd], i.e., positions [xd±1, yd], may be corrected. In this case, the relationship between the input and output sides of the lookup table LUT can be interpreted in reverse to perform the correction. In other words, if the correction amount ΔD of the pixel value DS of the pixel adjacent to the target pixel increases the input tone value DI by a value α or β%, the threshold can be decreased by a value α or β%. This method of correcting corresponding thresholds has the advantage that it does not require a new step of changing the input tone value; all corrections can be performed simply by setting the thresholds. In this case, the thresholds can be rewritten before halftoning and the rewritten dither mask 61 can be saved in the EEPROM 60.

[0062] In this way, by correcting the dither mask 61, it is sufficient to modify the thresholds for, for example, 64 pixels arranged in the y direction (see Figure 4). Therefore, there is no need to correct the pixel values ​​DS of all pixels corresponding to the missing dot positions that make up the image data. Furthermore, when performing different corrections depending on the distance from the missing dot column, methods that correct pixel values ​​require that different correction tables that vary depending on the distance be stored during halftoning. However, methods that correct threshold values ​​simply correct the thresholds before halftoning is performed, eliminating the need to maintain multiple lookup tables LUTs during halftoning.

[0063] E. Fifth embodiment: Next, a fifth embodiment will be described. The printer of the fifth embodiment includes a head 90 having two rows of nozzles Nz, each of which forms a single ink color, aligned along the Y direction, instead of the head 90 having a row of nozzles Nz that form a single ink color aligned across the width of the medium P (X direction). In this head 90, two nozzles Nz that eject droplets of each color are aligned along the transport direction (Y direction) of the medium P. As viewed along the transport direction of the medium P, the two nozzles Nz alternately form dots. Therefore, missing dots resulting from a defective nozzle Nz do not occur consecutively. In other words, if a row of dots formed consecutively across the width of the medium P is called a raster, missing dots occur every other raster. Therefore, the dither threshold correction process is performed accordingly. Figure 11 is a flowchart outlining this process of replacing every other raster. In the fifth embodiment, the dither threshold correction processing routine described in the first embodiment with reference to FIG. 5 is the same as in the first embodiment, except for the addition of step S225. In step S225, it is determined whether the target threshold position yd in the dither mask 61 is even. This is the case when a missing dot occurs in a nozzle Nz responsible for an even-numbered row of a raster. In this example, even in the same column of the image, dots are formed by different nozzles Nz in the even-numbered raster and the odd-numbered raster. The missing dot occurs because the nozzle Nz responsible for the even-numbered raster is defective. Whether the defective nozzle Nz is responsible for the even-numbered raster or the odd-numbered raster can be easily determined by combining the drive amount of the paper feed motor 74, which is driven by processing by the CPU 40, with the detection results from the detection device 70.

[0064] FIG. 12 shows the processing that occurs when a dot is dropped by a nozzle Nz responsible for an even-numbered raster row. In the figure, the thresholds subject to replacement are indicated by hatching. The range in which the thresholds are replaced is indicated by the replacement range SER, enclosed by a dashed line. If the pixel of interest is located at [xd, yd], this replacement range SER includes six thresholds: [xd±1, yd], [xd±2, yd], and [xd±1, yd+1]. These six thresholds are subject to replacement. Therefore, the replacement ranges SER for each pixel of interest do not overlap. Note that yd is an even number. The replacement range SER may be narrower or wider. For example, the range may include eight thresholds, including the position [xd±2, yd+1]. Furthermore, as explained in the first to third embodiments, bias adjustment processing (FIG. 7, step S257) may also be performed to prevent dot formation from being biased to a specific side.

[0065] According to the fifth embodiment described above, even when multiple nozzles are responsible for forming dots in one row, it is possible to achieve the same effects as the previous embodiments. Because missing dots occur only every other raster or every several rasters, the effects of missing dots can be further suppressed by switching the threshold values.

[0066] F. Other Embodiments: (1) Another embodiment is a printing device equipped with a head for forming dots on a medium. The printing device includes a dither mask unit that prepares a dither mask consisting of multiple thresholds used in dither-based halftone processing; a halftone processing unit that converts input gradation values ​​for each pixel constituting image data into dot data representing the presence or absence of dot formation for each pixel by comparing the input gradation values ​​for each pixel with thresholds obtained from the dither mask according to the position of the pixel; a printing unit that drives the head according to the dot data; a defective dot acquisition unit that acquires the positions of defective dots, which are ON pixels for which dot formation is performed and for which dots are not formed by the head; and a correction unit that searches for a threshold used for the comparison in a predetermined range of pixels near the defective dot, and, if a change threshold is found that satisfies a search condition of being greater than a target threshold used for the comparison at the position of the defective dot, corrects the change threshold to the target threshold and uses it for the comparison in the halftone processing unit at the pixel corresponding to the corrected threshold. This makes it easier to form dots near defective dots where no dots are formed, thereby reducing degradation of image quality due to the defective dots. Moreover, because the threshold value of the dither mask is corrected, it is possible to suppress degradation of image quality due to defective dots where no dots are formed more easily than by correcting the input tone value. Note that suppression of degradation of image quality may be achieved by correcting the threshold value alone, or may involve correcting the input tone value.

[0067] In the first to fifth embodiments, a line printer is used as an example of the printer 20. However, the present invention can also be implemented in a so-called serial printer, in which the head forms an image by reciprocating in the width direction of the medium P (hereinafter referred to as the main scanning direction). In this case, if a defective dot occurs because some of the nozzles in the head are unable to form a dot, the defective dots will be arranged along the main scanning direction. Therefore, in this case, the position (X, Y) of the defective dot is determined using the detection device 70, and the y-direction position yd on the dither mask 61 corresponding to the y-direction position where the defective dot occurs is determined using the above-mentioned equation (2). The threshold value at the y-direction position yd is sequentially read in the x direction, and the threshold replacement process is performed as in the first to fifth embodiments. The replacement range for searching for the changed threshold value may also be set on both sides of the x direction, rather than in the y direction.

[0068] In the case of a serial printer, one raster is completed by the reciprocating movement of the head, and in this case, one raster is formed by dots made of droplets ejected from multiple nozzles. Therefore, in such a case, as in the case described as the fifth embodiment, the target threshold value can be extracted for every other column or every several columns, compared with the threshold values ​​within the replacement range, and replacement processing can be performed.

[0069] The head may be one that ejects droplets to form an image, or one that forms dots by other methods such as ink melting, dye sublimation, transfer, etc. The actuator that generates the pressure fluctuations for ejecting droplets may be a configuration that uses an electrostrictive element such as a piezoelectric element, or a configuration that uses a heater that heats the ink to generate bubbles.

[0070] The heads described above can also be of a type that forms multiple types of dots with different sizes. Using Figure 13, we will explain how to handle the formation of dots of different sizes. As an example, we will take a head that can form two types of dots. As shown in the top row of Figure 13, this head can form normal-sized dots (hereinafter referred to as M dots) and larger dots (hereinafter referred to as L dots). M dots are smaller than the size of the pixel frame. Therefore, if M dots are formed in all pixels when the input gradation value DI is 255, the density achieved on the medium P will remain at around 200, compared to the density of 255 when the medium P is filled. On the other hand, L dots are large enough to circumscribe the pixel frame.

[0071] The formation of these two types of dots is defined by the M dot input tone value DM and the L dot input tone value DL. The M dot input tone value DM indicates the proportion of M dots formed relative to the input tone value DI of the image data, and the L dot input tone value DL indicates the proportion of L dots formed, each ranging from 0 to 255. A formation proportion of 255 means that dots are formed in all pixels, resulting in a pixel coverage of 100%. The relationship between the proportion of dots formed relative to the input tone value DI and the achieved density is shown in the lower part of Figure 13. As shown, M dots are formed to a degree corresponding to the input tone value DI up to an input tone value DI of approximately 64. The rate of increase of the M dot input tone value DM gradually decreases once it exceeds 64. When the input tone value DI exceeds 160, it takes on a relatively flat value of approximately 128 to 135, eventually reaching 128. On the other hand, the L dot input tone value DL is 0 up to the input tone value DI of 64, and then increases to take on the value of the difference between the input tone value DI and the M dot input tone value DM. As a result, the value of the L dot input tone value DL increases sharply when the input tone value DI is 160 or greater.

[0072] In this way, since the M dots do not completely cover the pixel frame, when only M dots are formed according to the input gradation value DI, the density achieved remains at about 200 / 255, whereas when M dots and L dots are formed as shown in the figure according to the input gradation value DI, the density achieved is 255 / 255. As shown in the figure, reducing the proportion of M dots to form L dots can use the dither mask used when forming a single type of dot as it is if the so-called dither continuous method is used. Specifically, the formation of dots is determined as follows.

[0073] In each of the above-described embodiments, let The be the dither threshold after the replacement process of the target threshold around the pixel where dot omission occurs. At this time, if The < DL, form L dots, if The ≥ DL and The < (DL + DM), form M dots, if The ≥ DL and The ≥ (DL + DM), do not form dots, [[ID=1十二]]and make the judgment. By doing so, it is possible to easily judge whether to generate L dots and M dots by reflecting the characteristics of one dither mask. Note that the dither mask for M dots and the dither mask for L dots may be prepared individually.

[0074] This allows dot formation using the disclosed technique on a printer that forms two different sizes of dots. In this case, even if the value ED used to determine whether to swap the target thresholds at the pixel of interest is equal to or sufficiently close to the maximum input tone value, a dot that compensates for the effects of the missing dot is formed. This is because the proportion of dots formed near pixels where missing dots occurs cannot be increased above 100%. Therefore, rather than increasing the dot formation proportion, the density of pixels near the missing dot, i.e., the ink volume, can be increased by increasing the size of the dots formed. Note that the number of dot sizes is not limited to two; three or more sizes, such as large, medium, and small, may also be used. Furthermore, when using dark inks such as magenta and cyan inks and lighter inks such as light magenta and light cyan, the proportion of dark ink formed may be increased or decreased. Instead of increasing the dot size, the same effect can be achieved by placing two or more dots at the same pixel location.

[0075] (2) In the above configuration, the predetermined range may be a range that includes the pixels on both sides of the defective dot in a direction that intersects with the direction in which the defective dots are arranged. This makes it easier to form dots that complement the missing dots near the defective dots.

[0076] (3) In the configurations (1) and (2) above, the predetermined range may be a range including a total of eight pixels: the first and second pixels on either side of the defective dot in a direction intersecting the direction of the defective dots, the third and fourth pixels further outward from the two pixels on either side, and the fifth through eighth pixels on either side of the first and second pixels in the direction of the defective dots. This allows the thresholds of pixels other than the first and second pixels on either side of the defective dot to be corrected to the target threshold, increasing the likelihood of threshold correction, thereby increasing the likelihood of missing dots being corrected and further reducing degradation of image quality. Furthermore, the positioning of dots correcting missing dots can be prevented from becoming uneven, thereby also reducing degradation of image quality.

[0077] (4) In the configurations (1) to (3) above, the correction unit may search for threshold values ​​in the following order until the search condition is satisfied: threshold values ​​corresponding to the first and second pixels, threshold values ​​corresponding to the third and fourth pixels, and threshold values ​​corresponding to the fifth through eighth pixels. This allows the threshold values ​​to be corrected by assigning priorities to pixels within a predetermined range, making it easier to control the positions where dots that complement missing dots are likely to be formed.

[0078] (5) In the configurations (1) to (4) above, the correction unit may correct thresholds corresponding to pixels in a predetermined range that are greater than the change threshold to the change threshold that is the target of correction. This not only corrects the target threshold to the change threshold, but also corrects thresholds corresponding to pixels in a predetermined range that are greater than the change threshold to the change threshold that has been corrected to the target threshold, making it easier to form dots near missing dots. This also prevents dots that compensate for the effects of missing dots from concentrating near missing dots, which can lead to a decrease in image quality. This correction may be performed repeatedly until a specific termination condition is met, or may be performed a limited number of times, such as once or twice.

[0079] (6) In the configurations (1) to (5) above, the correction unit may not correct the changed threshold if the target threshold is larger than a predetermined value. This reduces the effort required for the correction process, as the threshold is corrected only for some of the pixels of the missing dots where no dots are formed. Also, if the target threshold is larger than a predetermined value, dots are unlikely to be formed in the first place, so the impact on image quality is small even if the threshold is not corrected. Of course, the threshold may be corrected in all cases.

[0080] (7) In the configurations (1) to (6) above, the dot formation state resulting from the threshold correction by the correction unit may be adjusted. In this way, if the threshold correction is left as it is, and dot formation is biased toward a specific location, for example, this can be adjusted.

[0081] (8) In the configurations (1) to (7) above, the adjustment of the dot formation state may involve replacing the threshold values ​​so that the corrected threshold values ​​do not line up in a predetermined number or more. By doing so, such an adjustment makes it less likely that a situation will occur in which the corrected threshold values ​​result in more than a predetermined number of dots lined up, thereby suppressing degradation of image quality. However, such an adjustment does not have to be performed. Furthermore, the adjustment of the dot formation state is not limited to replacing the threshold values ​​so that the corrected threshold values ​​do not line up in a predetermined number or more, and may involve preventing the creation of areas where the corrected threshold values ​​line up at a rate greater than a predetermined rate.

[0082] (9) In the above configurations (1) to (8), the adjustment of the dot formation state may be performed by increasing or decreasing a threshold value or input tone value corresponding to pixels within a predetermined range where dots may be formed. In this way, the possibility of dot formation can be adjusted by increasing or decreasing the threshold value or input tone value. Generally, when a defective dot occurs due to a defect such as noise and a replacement dot is formed in the surrounding area, dot formation may be insufficient or excessive. In such cases, dot formation can be corrected by increasing or decreasing the threshold value or input tone value corresponding to pixels where dots may be formed. The degree of increase or decrease may be adjusted by actually forming an image, or may be set in advance according to the tone values ​​of the image.

[0083] (10) The present disclosure can also be implemented as a method for printing by forming dots on a medium using a head. This method includes preparing a dither mask consisting of multiple thresholds used in halftone processing using a dither method, obtaining the positions of defective dots, which are pixels where dots are not formed by the head, comparing the input gradation values ​​of each pixel constituting image data with the thresholds obtained from the dither mask according to the position of the pixel, and performing halftone processing to convert the input gradation values ​​into dot data representing the presence or absence of dot formation for each pixel. During the halftone processing, a threshold value to be used for the comparison is searched for in a predetermined range of pixels near the defective dot, and if a modified threshold value is found that satisfies a search condition of being greater than a target threshold value used for comparison at the position of the defective dot, the modified threshold value is modified to the target threshold value and used for the comparison at the pixel corresponding to the modified threshold value in the halftone processing, and driving the head in accordance with the dot data.

[0084] This makes it easier to form dots near defective dots where no dots are formed, thereby suppressing degradation of image quality due to the defective dots. Moreover, because the threshold value of the dither mask is corrected, it is easier to suppress degradation of image quality due to defective dots where no dots are formed than by correcting the input tone value.

[0085] (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. This image processing device includes a dither mask unit that prepares a dither mask consisting of multiple thresholds used in halftone processing using a dither method; a halftone processing unit that converts input gradation values ​​for each pixel constituting the image data into dot data representing the presence or absence of dot formation for each pixel by comparing them with thresholds obtained from the dither mask according to the position of the pixel; a defective dot acquisition unit that acquires the positions of defective dots, which are pixels where dots are not formed, among the ON pixels for which dot formation is performed; and a correction unit that searches for a threshold used for the comparison at pixels within a predetermined range near the defective dot, and if a change threshold is found that satisfies a search condition of being greater than a target threshold used for the comparison at the position of the defective dot, corrects the change threshold to the target threshold and provides it for the comparison at the pixel corresponding to the corrected threshold in the halftone processing unit. This makes it easy to provide a dither mask that facilitates the formation of dots near defective dots where dots are not formed. As a result, by using dot data processed by this image processing device, it is possible to suppress degradation of image quality due to defective dots. Moreover, because the dither mask threshold value is corrected, it is possible to suppress degradation of image quality due to defective dots, where no dots are formed, more easily than by correcting the input gradation value.

[0086] (12) In each of the above embodiments, some of the configurations implemented by hardware may be replaced with software. At least a portion of the configurations implemented by software may also be implemented by a discrete circuit configuration. Furthermore, when some or all of the functions of the present disclosure are implemented by software, the software (computer program) may be provided in a form stored on a computer-readable recording medium. The term "computer-readable recording medium" is not limited to portable recording media such as floppy disks and CD-ROMs, but also includes internal storage devices within a computer, such as various RAMs and ROMs, and external storage devices fixed to a computer, such as a hard disk. In other words, the term "computer-readable recording medium" has a broad meaning, including any recording medium capable of fixing data packets, not just temporarily.

[0087] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0088] 20...printer, 30...control unit, 40...CPU, 41...input section, 42...halftone processing section, 43...comparison section, 44...correction section, 46...printing section, 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 pipe, 98...memory card slot, 99...operation panel, MC...memory card, Nz...nozzle

Claims

1. A printing device having a head for forming dots on a medium, a halftone processing unit that performs halftone processing to generate dot data that indicates whether or not dots are formed, using image data made up of a plurality of pixels having a plurality of gradation values ​​and a dither mask made up of a plurality of threshold values; a defective position acquisition unit that acquires the position of a defective dot where the dot cannot be formed by the head even though the defective dot is at a position where the dot should be formed in accordance with the dot data; a correction unit that searches for thresholds used in generating dots in a predetermined range near the defective dot, corrects the value of a changed threshold that satisfies a search condition of being greater than the value of a target threshold that is the threshold used at the position of the defective dot, to a value equivalent to the value of the target threshold, and provides the changed threshold to the halftone processing at the pixel corresponding to the corrected threshold; a printing unit that drives the head in accordance with the dot data; A printing device comprising:

2. 2. The printing device according to claim 1, wherein the predetermined range is a range that includes pixels on both sides of the defective dot in a direction that intersects with the arrangement direction of the defective dots.

3. 2. The printing device according to claim 1, wherein the predetermined range includes a total of eight pixels: first and second pixels that are pixels on either side of the defective dot in a direction intersecting the arrangement direction of the defective dot; third and fourth pixels that are pixels further outward from the pixels on either side; and fifth through eighth pixels that are pixels on either side of the pixels on either side in the arrangement direction.

4. 4. The printing device according to claim 3, wherein the correction unit searches for the threshold values ​​in the order of the threshold values ​​corresponding to the first and second pixels, the threshold values ​​corresponding to the third and fourth pixels, and the threshold values ​​corresponding to the fifth to eighth pixels, until the search condition is satisfied.

5. The printing device according to claim 1 , wherein the correction unit corrects thresholds that are larger than the change threshold among the thresholds corresponding to the pixels in the predetermined range to the change threshold that is the target of the correction.

6. The printing device according to claim 1 , wherein the correction unit does not correct the change threshold when the target threshold is greater than a predetermined value.

7. The printing device according to claim 1 , wherein the state of dot formation caused by the correction of the threshold value by the correction unit is adjusted.

8. 8. The printing apparatus according to claim 7, wherein the adjustment of the dot formation state involves rearranging the threshold values ​​so that the corrected threshold values ​​do not line up in a predetermined number or more.

9. 8. The printing device according to claim 7, wherein the adjustment of the dot formation state is performed by increasing or decreasing the threshold value or the tone value of pixels within the predetermined range that correspond to pixels where the dots can be formed.

10. A method for printing by forming dots on a medium using a head, comprising: The position of a defective dot where the dot cannot be formed by the head is acquired even though the position is a position where dot formation is performed, halftone processing is performed to generate dot data representing the presence or absence of dot formation using image data consisting of a plurality of pixels having a plurality of gradation values ​​and a teaser mask consisting of a plurality of threshold values; a search for thresholds used in generating dots in a predetermined range near the defective dot, and a modified threshold value that satisfies a search condition of being greater than a target threshold value that is a threshold value used at the position of the defective dot is modified to a value equivalent to the target threshold value, and the modified threshold value is used in the halftone processing at the pixel corresponding to the modified threshold value; driving the head in accordance with the dot data; Printing method.

11. An image processing device that processes image data and converts it into dot data for forming dots on a medium, a halftone processing unit that performs halftone processing to generate dot data that indicates whether or not dots are formed, using image data made up of a plurality of pixels having a plurality of gradation values ​​and a dither mask made up of a plurality of threshold values; a defective position acquisition unit that acquires the position of a defective dot where the dot cannot be formed even though the dot is to be formed in accordance with the dot data; a correction unit that searches for thresholds used in generating dots in a predetermined range near the defective dot, corrects the value of a changed threshold that satisfies a search condition of being greater than the value of a target threshold that is the threshold used at the position of the defective dot, to a value equivalent to the value of the target threshold, and provides the changed threshold to the halftone processing at the pixel corresponding to the corrected threshold; An image processing device comprising:

Images