JP2016153223A - Image forming apparatus, image processing method, program and program recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve image recording which prevents the deterioration of an image over the whole gradations without causing an increase in cost even in the case where there is a defective ejection nozzle.SOLUTION: A main control unit for compensating a deterioration in an image compares pixel values corresponding to a defective ejection nozzle with a predetermined threshold when there is the defective ejection nozzle. Values that do not exceed the threshold among the pixel values corresponding to the defective ejection nozzle are defined as a value for stopping ejection by the defective ejection nozzle when converting multivalued data of an input image into an n value (n≥2), and a quantization error in converting the multivalued data of the input image into the n value due to the definition of being the value for stopping the ejection is developed to peripheral pixels around the defective ejection nozzle. A prescribed mask pattern corresponding to the sizes and arrangement of dots around the defective ejection nozzle is used on condition that the pixel values corresponding to the defective ejection nozzle are equal to or more than the threshold as a result of the comparison, and processing for replacing small dots among the dots around the defective ejection nozzle with large dots is executed by pattern matching.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus, an image processing method, a program, and a program recording medium.

  In an ink jet printer that forms an image by ejecting liquid such as ink from individual nozzles, “bad ejection” in which ink is not ejected from the nozzles due to factors such as clogging of foreign matter in one of the nozzles or drying of the ink. May occur. Therefore, in order not to deteriorate the image quality even when there are defective ejection nozzles, it is detected whether ink droplets are ejected normally from the nozzles, and the input image data is determined according to the result as normal printing. Compensation methods are known that perform different types of image processing and thereby compensate for image quality.

  In Patent Document 1 (Japanese Patent Laid-Open No. 2010-17918), when a defective ejection pixel is corrected, if there is a defective ejection nozzle when the input image data is quantized, recording by the nozzle is performed at the pixel position. An image processing method is disclosed in which ink droplets are not ejected, while quantization errors due to multi-level error diffusion processing are developed as they are in peripheral pixels to perform image compensation so as not to be excessively compensated. At the same time, when a droplet with an abnormal droplet size necessary for solid printing is formed at the development destination pixel, a process of thinning the droplet or changing the droplet size to a smaller size may be performed. It is disclosed.

  However, in the image processing method disclosed in Patent Document 1, when the carriage is scanned so as to be orthogonal to the conveyance direction of the recording medium, only the downstream side of the defective ejection nozzle can be corrected, so that it is printed as a dark color with high dot density. The problem of low compensation effect at the time is not solved.

  The present invention has been made in view of the above-described conventional problems, and its purpose is to include a light color having a low dot density to a dark color having a high dot density even when a defective ejection nozzle is present. In other words, it is possible to realize image recording that prevents image deterioration at all gradations without causing an increase in cost.

  The present invention relates to an image forming unit that forms an image on a recording medium by ejecting droplets from nozzles of a recording head, a pixel value corresponding to a defective ejection nozzle that cannot eject droplets from the nozzles, and a predetermined threshold value. Threshold comparison means for comparing the pixel value corresponding to the defective ejection nozzle, and at least a value that does not exceed the threshold value when the multi-value data of the input image is converted to n value (n ≧ 2). A pixel corresponding to the defective discharge nozzle is a quantization error when converting multi-valued data of the input image into n values (n ≧ 2) by setting the value to stop the discharge by the nozzle and setting the value to stop the discharge. And a first image compensation unit that executes a process for developing the pixels around the pixel, and as a result of the comparison, a pixel value corresponding to the defective nozzle is equal to or greater than the threshold value. so Second image compensation means for executing a process of replacing a small dot with a large dot among the dots around the defective ejection nozzle by pattern matching using a predetermined mask pattern according to the size and arrangement of the dots; An image forming apparatus including the image forming apparatus.

  According to the present invention, even when there is a defective ejection nozzle, image recording that prevents image deterioration in all gradations including a light color with a low dot density to a dark color with a high dot density, This can be realized without increasing the cost.

1 is a plan view schematically showing a serial type ink jet recording apparatus which is an embodiment of an image forming apparatus of the present invention. FIG. 2A is a block diagram schematically showing the configuration of the control unit of the image forming apparatus, and FIG. 2B is a functional block diagram of the main control unit. It is a flowchart explaining the compensation process procedure of the image data performed based on a program in a main control part, when a defective discharge nozzle is detected. FIG. 4 is a diagram showing the difference in dot diameter on paper, with the vertical axis representing the dot diameter on the paper and the horizontal axis representing the volume of one drop of ink. It is a figure explaining replacement of a dot by a mask pattern. It is a figure explaining replacement of a dot by a mask pattern. It is the same flowchart as FIG. 3 in a modification.

In the following, an embodiment of the present invention will be described, which has the following characteristics when printing image data with a head including a nozzle that is defectively ejected.
In short, in the present embodiment, the input image is converted into a dot pattern (n-value data) by multi-value error diffusion processing corresponding to n values (n ≧ 2), and the nozzles of the recording head are used. Based on the discharge status from the above, no droplets are discharged from the pixels corresponding to the recording positions at the defective discharge nozzles where the droplets cannot be discharged normally from some of the nozzles. At the same time, according to the comparison result between the pixel value of the pixel corresponding to the recording pixel position at the defective ejection nozzle and the “switching threshold” set for each sheet according to the density of the print image, the dot density, etc. In image processing that develops the quantization error in the multi-level error diffusion processing as it is to surrounding pixels, and in gradations that cause degradation in image quality due to compensation processing only downstream of the defective ejection nozzle in the multi-level error diffusion processing, Pattern matching processing is performed according to the dot size (for example, large droplets, medium droplets, small droplets) and arrangement around the defective ejection nozzle, and switching to image processing that replaces the dot size and arrangement allows all levels to be changed. Therefore, it is possible to apply image processing with a high compensation effect.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a plan view schematically showing a serial type ink jet recording apparatus which is an embodiment of an image forming apparatus of the present invention.
Here, in the serial type ink jet recording apparatus 1, a direction in which the carriage moves is referred to as a main scanning direction, and a direction in which the medium is conveyed is referred to as a sub scanning direction. Therefore, unless otherwise specified, the horizontal direction in the image data is referred to as the “main scanning direction” and the vertical direction is referred to as the “sub-scanning direction”.

A carriage 3 is movably held by a main guide member 2 and a secondary guide member that are horizontally mounted on the left and right side plates of the serial type ink jet recording apparatus 1.
The carriage 3 is reciprocated in the main scanning direction by the main scanning motor 5 via a timing belt 8 spanned between the driving pulley 6 and the driven pulley 7.

  A recording head 4 (4 a, 4 b) composed of a liquid discharge head is mounted on the carriage 3. For example, the recording head 4 ejects ink droplets of each color of Y (yellow), C (cyan), M (magenta), and K (black). The recording head 4 is mounted with a nozzle row 4n composed of a plurality of nozzles arranged in the sub-scanning direction orthogonal to the main scanning direction and with the droplet discharge direction facing downward.

Each of the recording heads 4a and 4b has two nozzle rows in which a plurality of nozzles are arranged.
For example, one nozzle row of the recording head 4a discharges K droplets, and the other nozzle row discharges C droplets. One nozzle row of the recording head 4b discharges M droplets, and the other nozzle row discharges Y droplets.
As the liquid discharge head constituting the recording heads 4a and 4b, for example, a piezoelectric actuator such as a piezoelectric element, or a thermal actuator that uses a phase change caused by film boiling of a liquid using an electrothermal transducer such as a heating resistor is used. it can.

The serial type ink jet recording apparatus 1 includes an endless transport belt 12 as transport means for electrostatically attracting the sheet 10 and transporting the sheet 10 at a position facing the recording head 4 (4a, 4b). The conveyor belt 12 is stretched between the conveyor roller 13 and the tension roller 14. The transport roller 13 is rotationally driven by a sub-scanning motor 16 via a timing belt 17 and a timing pulley 18. The conveyor belt 12 is driven by the conveyor roller 13 and moves around in the sub-scanning direction.
The conveyor belt 12 is charged (charged) by a charging roller when it moves around.

On one side of the carriage 3 in the main scanning direction, a maintenance / recovery mechanism 20 for maintaining and recovering the recording head 4 to the side of the conveyance belt 12 is provided. An empty discharge receptacle 21 for discharging is disposed.
As a means for transporting the paper 10, not only a transport belt 12 that uses electrostatic attraction, but also a transport means that uses a platen that supports the paper 10 and a transport roller 13 that transports the paper 10 may be used. In this case, the conveyance roller 13 is used on the paper discharge side instead of the tension roller 14, and the paper 10 is brought into contact with the conveyance roller 13 on both the paper supply side and the paper discharge side and conveyed.
In addition to the conveying belt 12 that electrostatically adsorbs, a means for conveying the paper 10 by using an air adsorbing means that sucks air from a hole opened in the platen can be used.

The maintenance / recovery mechanism 20 includes, for example, a cap member 20a for capping the nozzle surface of the recording head 4, a wiper member 20b for wiping the nozzle surface, and an empty discharge receiver for discharging droplets that do not contribute to image formation.
In addition, an ejection detection device 30 is disposed in a region outside the recording region between the conveyance belt 12 and the maintenance / recovery mechanism 20 and facing the recording head 4.

An encoder scale 23 having a predetermined pattern is disposed between both side plates along the main scanning direction of the carriage 3. On the other hand, the carriage 3 is provided with a main scanning encoder sensor 24 composed of a transmissive photosensor that reads the pattern of the encoder scale 23.
The encoder scale 23 and the main scanning encoder sensor 24 constitute a linear encoder (main scanning encoder) that detects the movement of the carriage 3.

  A code wheel 25 is attached to the shaft of the conveying roller 13, and on the other hand, a sub-scanning encoder sensor comprising a transmissive photosensor that detects a pattern formed on the code wheel 25 with the peripheral edge of the code wheel 25 sandwiched therebetween. 26 is provided. The code wheel 25 and the sub-scanning encoder sensor 26 constitute a rotary encoder (sub-scanning encoder) that detects the movement amount and movement position of the conveyor belt 12.

In the serial type inkjet recording apparatus 1 configured as described above, the paper 10 fed from the paper feed tray is attracted to the charged transport belt 12 and moved in the sub-scanning direction by the circular movement of the transport belt 12. Be transported.
When the sheet 10 reaches a predetermined position and stops, the recording head 4 is driven according to the image signal while moving the carriage 3 in the main scanning direction. Thus, ink droplets are ejected onto the paper 10 and recorded line by line.
The serial type ink jet recording apparatus 1 receives the recording end signal or the signal that the rear end of the paper 10 reaches the recording area, and finishes the recording operation and discharges the paper 10 to the paper discharge tray.

Next, an outline of the control unit of the serial type ink jet recording apparatus 1 will be described with reference to FIG.
2A is a block diagram schematically illustrating the configuration of the control unit 100 of the image forming apparatus, and FIG. 2B is a functional block diagram of the main control unit 100A.
That is, the control unit 100 includes, for example, a main control unit including a CPU 101 that controls the entire apparatus, a ROM 102 that stores programs executed by the CPU 101 and other fixed data, and a RAM 103 that temporarily stores image data and the like. (Computer) 100A is provided.

The control unit 100 includes a host I / F (interface) 106 that controls data transfer with a host (information processing apparatus) 200 such as a PC (personal computer), and an image output control unit 111 that drives and controls the recording head 4. And an encoder analysis unit 112. The encoder analysis unit 112 receives and analyzes detection signals from the main scanning encoder sensor 24 and the sub scanning encoder sensor 26.
The control unit 100 includes an I / O (input) between the main scanning motor driving unit 113 for driving and controlling the main scanning motor 5, the sub-scanning motor driving unit 114 for driving and controlling the sub-scanning motor 16, and various sensors and actuators 117. (Output unit) 116 and the like.
The control unit 100 also includes a droplet discharge detection unit 131 that detects discharge / non-discharge by the discharge detection device 30.

The image output control unit 111 outputs a drive waveform, a head control signal, print image data, and the like to a head driver 110 that is a head drive circuit for driving the recording head 4 mounted on the carriage 3 to output the recording head. Droplets corresponding to the print image data are ejected from the four nozzles.
The image output control unit 111 is a drive waveform generating unit that generates a drive waveform for driving and controlling the recording head 4, a head control signal for selecting a predetermined drive signal from the drive waveform, and data transfer that transfers print image data Including means.

The encoder analysis unit 112 includes a direction detection unit 120 that detects the movement direction of the carriage 3 from the detection signal, and a counter unit 121 that detects the movement amount of the carriage 3.
The control unit 100 controls the movement of the carriage 3 by controlling the driving of the main scanning motor 5 via the main scanning motor driving unit 113 based on the analysis result from the encoder analysis unit 112. Further, the feed control of the paper 10 is performed by driving and controlling the sub-scanning motor 16 via the sub-scanning motor driving unit 114.

The main control unit 100A of the control unit 100 is a computer and corresponds to the means for compensating for image degradation of the present invention. As shown in FIG. 2B, the main control unit 100A (CPU 101) sets the multivalued data of the input image to n values (n ≧ 2) for values that do not exceed at least the threshold value among pixel values corresponding to defective ejection nozzles. The quantization error when converting the multi-value data of the input image to n values (n ≧ 2) by setting the value to stop the discharge by the defective discharge nozzle when converting and the value to stop the discharge A first image compensator 101 (1) that executes processing for developing pixels around pixels corresponding to the defective ejection nozzle, and a threshold comparison unit 101 that compares a pixel value corresponding to the defective ejection nozzle with a predetermined threshold value. (2) and on the condition that the pixel value corresponding to the defective ejection nozzle is equal to or greater than the threshold value as a result of the comparison, a predetermined pattern corresponding to the shape and arrangement of dots around the defective ejection nozzle Used, and a second image compensation unit 101 performs processing to replace the small dots to large dots among the dots in the periphery of the defective ejection nozzle by pattern matching (3).
These function units are realized by function realizing means generated by the CPU 101 by the main control unit 100A reading the program.
When the main control unit 100 </ b> A performs droplet discharge detection of the recording head 4, the main control unit 100 </ b> A moves the recording head 4 to perform droplet discharge from a predetermined nozzle of the recording head 4, and detects a detection signal from the droplet discharge detection unit 131. Control for discriminating the droplet discharge state is also performed.

Next, image data processing (processing for compensating for image degradation) executed by the main control unit 100A of the serial type inkjet recording apparatus 1 described above will be described.
That is, in this processing procedure, first, the droplet discharge state is determined, that is, the presence or absence of a defective discharge nozzle is detected.
Specifically, this detection confirmation is, for example, a method of ejecting ink droplets on the paper surface, such as a nozzle check pattern, or a method of detecting ink droplets by the droplet ejection detection unit 131 provided in the serial type inkjet recording apparatus 1. Any known method may be used, and the detection method is not particularly limited.

There are also two types of detection timing: a method of detecting a defective ejection nozzle after print image data is input, and a method of detecting before print image data is input.
In the former, since defective ejection nozzles are detected immediately after printing image data is input and immediately before printing, there is an advantage in that defective ejection nozzles can be reliably detected and compensated. However, since it is necessary to perform detection determination processing after inputting the print image data, there is a demerit that it takes time from when the print start button is pressed to when printing is actually started.

On the other hand, the latter has an advantage that there is no waiting time until the printing is actually started after the print start button is pressed because the presence or absence of the defective ejection nozzle is detected before the print image data is input. However, there is a demerit that it is not possible to deal with a case where the defective ejection nozzle recovers or the number of defective ejection nozzles increases when there is time from detection to printing.
Therefore, it is important to obtain information on the presence or absence of defective ejection nozzles by performing ejection detection in consideration of the respective merits and demerits.

  Next, by comparing the information on which nozzles are performing defective ejection with the print quality when printing image data, to which pixel of the recording pixels the defective ejection nozzle corresponds (corresponds) For each pixel. As a result, in a pixel determined to be “recorded with a defective ejection nozzle”, a process is performed in which a dot is not generated (n-value data is set to 0) in the subsequent n-value conversion, and the quantization is performed. The error is developed to the pixels to be recorded by the peripheral nozzles that normally discharge, and the arrangement (density) and size of the ink droplets that are normally discharged are changed. As a result, the density around the nozzles with defective ejection is made constant, and the influence of the nozzles with defective ejection is alleviated.

  Note that, even when there is a pixel that does not generate a dot in the detection and confirmation described above, the dot density is low, and when a light color is expressed, the effect of improving the image quality by changing one dot is large. . Therefore, the improvement effect can be expected only by compensation processing on the downstream side of the defective ejection nozzle. However, for example, in an image in which the recording medium is filled with ink droplets, such as an image having a high dot density and close to solid coating, image quality can be sufficiently improved (compensated) if compensation processing cannot be performed upstream of the defective ejection nozzle. Cannot be expected.

  Therefore, in the serial type inkjet recording apparatus 1 (main control unit 100A), when the pixel value of the target pixel is less than a predetermined “threshold value” that is arbitrarily set, multi-value error diffusion processing is performed as image data compensation processing. Image processing is performed to expand the quantization error in to the surrounding pixels as they are. On the other hand, when the pixel value of the target pixel is higher than the threshold value, it is determined that the compensation process using the development of the quantization error by the multi-level error diffusion process is not sufficient. In that case, pattern processing is performed by comparing the shape and arrangement of the dots around the target pixel corresponding to the defective ejection nozzle with a predetermined mask pattern, and small dots are replaced with large dots that have an image quality improvement effect. Execute. That is, when the pixel value of the target pixel is equal to or greater than the threshold value, the coordinates of the pixel are stored, and the pattern matching is performed based on the stored coordinates after the multilevel error diffusion processing is completed.

A plurality of threshold values used in the above processing can be set, that is, can be changed according to the paper type to be used.
Actually, paper that is likely to bleed ink (so-called plain paper, etc.) has a high image quality improvement effect and can be set to a high threshold without ink compensation on the upstream side of the defective ejection nozzle due to ink bleed. On the other hand, in coated paper or the like whose surface is coated, the dot diameter is tightened small due to the influence of the surface coating of ink. Therefore, if there is no image compensation on the upstream side of the defective ejection nozzle, the improvement effect is low, and it is considered necessary to set the threshold value low.

FIG. 3 is a flowchart for explaining the image data compensation processing procedure executed by the main control unit 100A based on a program when a defective ejection nozzle is detected. Note that the program is recorded in a known or known computer-readable recording medium (herein, in particular, a program recording medium in order to be distinguished from a recording medium including paper).
In the serial type ink jet recording apparatus 1, the compensation processing is started by inputting input image data for one scanning performed by moving the carriage 3 to the main control unit 100 </ b> A of the serial type ink jet recording apparatus 1. The input image data is data that has already been converted from the RGB color space to the CMYK color space. That is, the input image data is image data having four pixel values of CMYK at a single pixel position. The conversion from the RGB color space to the CMYK color space may be performed by the printer driver 201 of the host 200 or may be performed inside the serial type ink jet recording apparatus 1.
First, the coordinates of the target pixel for image compensation processing are set (S101). Next, the color of the target pixel among the pixels having the set coordinates is set. In order to perform CMYK in order, here, for convenience, cyan is used when x = 0, magenta when x = 1, yellow when x = 2, and black when x = 3. First, x = 0, that is, a setting is made so as to make a determination on a cyan pixel (S102). Next, it is determined (or confirmed) whether the target pixel is a pixel recorded by the defective ejection nozzle (S103). Here, if the target pixel is a pixel recorded by the defective ejection nozzle (S103, Yes), no dot is generated for the pixel (n-value data is set to 0) (S104), and the input image data It is determined whether the pixel value (gradation value) of the pixel is equal to or greater than a predetermined threshold (S105).
Here, the pixel value (gradation value) is a gradation value for each color, and is a numerical value of 0 to 255 in this embodiment. For example, when the coordinates are 200 for cyan, 40 for magenta, 200 for yellow, and 100 for black, when x = 0, it is determined in step S105 whether the cyan pixel value 200 is equal to or greater than a predetermined threshold value.

If the pixel value of the pixel is greater than or equal to the threshold value (S105, Yes), the coordinates of the target pixel are stored in the storage means (S106), and the process proceeds to step S107.
On the other hand, if the pixel is not recorded by the defective ejection nozzle in step S103 (S103, No), a multi-value error diffusion process is performed to generate dots (n-value data is set in the pixel) (S107). Proceed to step S108.
In step S108, the quantization error (error generated when the pixel is quantized) is updated so as to be developed to the pixels to be recorded by the peripheral normal ejection nozzles (S108). Next, it is determined whether or not the determination is completed for all colors (whether x is 3 or more) (S109). If the determination has not been completed for all colors (S109, No), 1 is added to x (S110), that is, the process proceeds to the next color, and the processing from S103 is repeated. When the determination for all colors is completed (S109, Yes), it is determined whether or not the n-value conversion processing (processing for converting multi-value image data to n-value image data) for all pixels is completed (S111). ). Here, if there is a pixel for which n-value conversion processing has not ended (S111, No), the target pixel is moved (changed) (S112), and the processing from step S101 is repeated.

If the n-value conversion processing is completed for all the pixels (S111, Yes), it is determined whether or not there is a pixel (coordinate) stored for each color plate. First, the color of the target pixel is set to cyan (x = 0) (S113). Next, it is determined whether or not there is a pixel (coordinate) stored in the target color (S114). If there is a saved pixel (S114, Yes), pattern matching processing is performed on the neighboring pixels of the saved pixel of the target color image (S115), and the process proceeds to step S116. If there is no saved pixel in step S114 (S114, No), the process proceeds to step S116 as it is.
Here, if processing for all target pixels has been completed for the target color (S117, Yes), it is determined whether processing has been completed for all colors (whether x is 3 or more) ( S117). If the process has not been completed for all colors (S117, No), 1 is added to x (S118), that is, the process proceeds to the next color, and the processes from S114 are repeated. If the processing for all colors has been completed (S117, Yes), print image data for one scan is output, and this processing ends.

FIG. 4 shows the difference in dot diameter (dot size) on the paper 10 (the difference in paper, here the difference in dot diameter on the surface (paper surface) between plain paper and coated paper), although the explanation will be around. The vertical axis represents the dot diameter (μm) on the paper surface, and the horizontal axis represents the volume (pl) of one drop of ink.
That is, plain paper always has a larger dot diameter on the paper surface regardless of the volume per ink drop compared to coated paper, and the difference increases as the volume per ink drop increases. A tendency to open is observed.
Here, the threshold value used for determining the pixel value is not only the above-mentioned paper type, but also the dot shape (size) and arrangement after the multi-value error diffusion processing corresponding to the n value (n ≧ 2). (Density) may be set in advance, and may be set to a different value depending on the density at which the improvement effect of the image is weakened in the multi-value error diffusion processing or the gradation at which the dots are arranged.

The threshold used for determining the pixel value of the target pixel may be set according to the density of the ink used in the printing device. For example, Y (yellow) or the like has high brightness and low visibility, so that it is difficult to notice even if the target pixel hits a defective ejection nozzle. Therefore, the switching threshold value for Y can be set large. On the other hand, since B (black) has high visibility, the threshold value (switching threshold value) of the pixel value when determining “recording with defective ejection nozzles” is set smaller than Y.
As described above, the threshold values are the paper type (the type of recording medium), the ink color, the ink density when the ink is normally ejected, and the dot shape around the defective ejection nozzle after performing multi-value error diffusion. It can be changed by arrangement.

Next, the dot replacement by the mask pattern in step S115 in FIG. 3 will be described. At this stage, the dot size corresponds to the value of n-valued data, but for the sake of convenience, the dot size will be described below.
5 and 6 are diagrams for explaining the replacement of dots by the mask pattern in the present embodiment.
For the mask pattern, for example, a plurality of patterns are formed and used for each dot size in accordance with the arrangement of dots around the defective ejection nozzle.
FIG. 5A is a diagram illustrating an example of a mask pattern for replacing dots (here, a pattern in which the second small dot of the defective ejection nozzle is converted into one large dot). FIG. 5B is a diagram showing an image before replacement (thick line is a pattern matching execution range). FIG. 5C is a diagram illustrating an image after replacement (thick line indicates a pattern matching execution range).

Here, description will be made assuming that n in the n value (n ≧ 2) is a total of four values (n = 4) of three types of dot diameters and no droplets. Here, for the sake of convenience, the size of the dots is classified by “diameter”, but the dots may be, for example, large drops, medium drops, and small drops, and are not necessarily limited to circular ones.
The dot replacement (pattern matching) by the mask pattern is performed depending on the presence or absence of coordinates (pixel coordinates) stored as the target pixel, as described in the flowchart of FIG. That is, when pixel coordinates are not stored, pattern matching is not performed. However, in this case, compensation processing for defective ejection nozzles by multi-value error diffusion is performed, so that a print result with improved image quality can be obtained.

  On the other hand, when the pixel coordinates are stored, the second (or second) smaller dot diameter among the three kinds of dot diameters set in advance is converted into the maximum dot diameter. A mask pattern M1 (FIG. 5A) is prepared. Then, pattern matching is performed between the mask pattern M1 and the image data (FIG. 5B) that has become the n value after the multilevel error diffusion processing. Here, when the mask pattern M1 matches the image data, dot replacement is performed.

That is, the mask pattern M1 in FIG. 5A corresponds to 3 × 4 = 12 nozzles. Of these, the gray part corresponds to the defective ejection nozzle, no dot, the hatched part replaces dots (in the example shown, replacement is performed every other dot in the upper and lower stages), and the blank part is not replaced Means that.
More specifically, the pattern matching execution range (that is, the pixel corresponding to the defective discharge nozzle and the defective discharge nozzle corresponding portion of the mask pattern M1 are aligned with the n-value (four values in this case) image data shown in FIG. Pattern matching between the mask pattern M1 in FIG. 5A and the n-value image data.

As a result, as shown in FIG. 5C, among the image data of FIG. 5B, in the first implementation range A1, the second small dot in the third row from the top and the second from the left becomes the replacement target. It has been replaced with the largest dot as shown. In the second implementation range A2, since the mask pattern and the image data do not match, the replacement is not performed. In the third implementation range A3, the second smallest dot at the left end in the first row from the top, and the second and fourth dots from the left in the third row are replaced with the largest dots as shown in FIG. 5C. ing.
In this way, the image around the pixels corresponding to the defective ejection nozzle is converted into a dot pattern having a compensation effect (FIG. 5C). Here, as described above, the four dots are replaced with the maximum dot diameter.

In addition, since the above mask pattern is applied to the dot arrangement after performing the multi-value error diffusion processing corresponding to the n value (n ≧ 2), the mask is adapted to each dot size after the n value. A pattern can be set.
Also, the dots can be replaced by using the mask pattern M2 shown in FIG. 6 simultaneously with the mask pattern M1 shown in FIG. 5A. By doing in this way, the influence of a defective discharge nozzle can be relieve | moderated further.

  That is, FIG. 6A is a diagram showing a mask pattern M2 for converting the smallest dot into the second (second) smallest dot. FIG. 6B is an image before replacement (thick line is an implementation range of pattern matching by the pattern shown in FIG. 6A). FIG. 6C shows replacement by the mask pattern M1 in FIG. 5A and replacement by the mask pattern M2 in FIG. 6A. It is a figure which shows the image (Bold line is the implementation range of pattern matching) after performing each. Note that the mask patterns M2 and M1 are the same as those described for M1, except that the pattern and the size of the dot replacement are different.

Here, the mask patterns M1 and M2 are simultaneously applied to the same four-valued image data as in FIG. 5B. As a result, in addition to the dot replacement by the mask pattern M1 shown in FIG. 5C already described, the dot replacement by the master pattern M2 is further performed to obtain the dot of FIG. 6C.
Specifically, in the first implementation area A1, the third dot from the top and the third dot from the left that matches the master pattern M2, and the third dot from the left in the third stage in the second implementation area A2. However, in the third implementation area A3, the second and fourth dots from the left in the first row and the third dot from the left in the third row are replaced with the second smallest dot from the smallest dot, respectively. .

  In the present embodiment, as described above, the input image is converted into a dot pattern by multi-value error diffusion processing corresponding to n values (n ≧ 2), and the print head nozzles are used. Based on the discharge status (normal discharge, defective discharge) from the nozzle, the liquid droplets are not discharged from the pixels corresponding to the recording positions at the defective discharge nozzles where the droplets cannot be discharged normally from the nozzles. At the same time, in order to compensate for the deterioration of the image due to the defective ejection nozzle, the case where the image deterioration can be compensated by the multi-value error diffusion processing using a predetermined threshold is divided. The threshold value can be set by changing the pixel value of the pixel corresponding to the recording pixel position of the defective ejection nozzle for each sheet according to the density (tone value) of the print image, the dot density, and the like.

  In this way, pattern matching between image processing that unfolds the quantization error in the multi-level error diffusion processing as it is to surrounding pixels and a mask pattern that is formed in advance according to the shape and arrangement of dots around the defective ejection nozzle Process. As a result, it is possible to perform image processing with a high compensation effect on all gradations for pixels corresponding to defective ejection nozzles.

<Modification>
In the above-described embodiment, the multi-value error diffusion process is not performed when it is determined that the pixel is recorded by the defective ejection nozzle. In this modification, even when it is determined that the nozzle is a defective ejection nozzle, if the gradation value of the target pixel is greater than or equal to the threshold value, multi-value error diffusion processing is performed, and pattern matching is performed as a result of pattern matching. The dot of the target pixel is deleted.

FIG. 7 is a flowchart of a modified example. The same parts as in FIG. 3 which are the flow of the embodiment are denoted by the same reference numerals, and the description thereof is omitted.
If the processing from S101 to S103 is performed as in the above-described embodiment, and it is determined that the pixel is recorded by the defective ejection nozzle in S103, whether or not the pixel value (gradation value) of the target pixel is equal to or greater than a predetermined threshold value. Is determined (S121). The pixel values here are the same as in the above-described embodiment. When the pixel value is equal to or greater than a predetermined threshold (S121, Yes), multi-value error diffusion processing is performed to generate dots (n-value data). Thereafter, the coordinates of the target pixel are stored as in the above-described embodiment (S106). If the pixel value is less than or equal to the predetermined threshold (S121, No), no dot (n-value data) is generated (S123). Since the processing from S107 to S114 is the same as that in the above-described embodiment, the description thereof is omitted.

  When it is determined that there is a pixel (coordinate) stored in the target color in S114 (S114, Yes), pattern matching is performed between the stored pixel and its surrounding pixels (S124). Here, the difference from the above-described embodiment is that n-value data is also set in the saved pixel, and the value of the pixel saved as a result of pattern matching is set to 0 (a value that does not discharge). It is. Since the process of S116 is the same as that in the above-described embodiment, the description thereof is omitted.

  In this modification, as described above, the input image is converted into n-value data by multi-value error diffusion processing corresponding to the n-value (n ≧ 2). Based on the ejection status (normal ejection, defective ejection) from the nozzles of the recording head, for the pixels corresponding to the recording positions at the defective ejection nozzles where droplets cannot be ejected normally from the nozzles, image degradation due to the defective ejection nozzles In order to compensate, it is divided into a case where image deterioration can be compensated by multi-value error diffusion processing using a predetermined threshold value and a case where image deterioration cannot be compensated. When image deterioration cannot be compensated for by multi-value error diffusion processing, pattern matching processing is performed with a mask pattern formed in advance according to the shape and arrangement of dots around the defective ejection nozzle. As a result, image degradation is compensated by replacing surrounding pixels. Accordingly, it is possible to perform image processing with a high compensation effect on all the gradations for the pixels corresponding to the defective ejection nozzle.

In the above-described embodiment, the CPU 101 generates the image compensation unit 101 (1), the threshold comparison unit 101 (2), and the second image compensation unit 101 (3) as the main control unit 100A reads the program. Although implemented by function implementation means, these functions may be provided by hardware.
In the above-described embodiments and modifications, the input image data is described as CMYK four-color data. However, when the inkjet recording apparatus has more than four colors that can be ejected, the number corresponding to the color material is used. The determination may be made based on the pixel values of the data. In the case of one-color printing, the determination may be made based on the pixel value of one data.
In the above-described embodiment and modification, the n-value conversion processing is completed for all the pixels of the input image (S111, Yes), and the pattern matching processing is started. When the n-value conversion processing for the required number of lines is completed, the pattern matching processing may be started in parallel without waiting for the processing to be completed for all pixels. That is, the first image compensation unit 101 (1), the threshold comparison unit 101 (2), and the second image compensation unit 101 (3) may be performed in parallel.

DESCRIPTION OF SYMBOLS 1 ... Serial type inkjet recording device, 2 ... Main guide member, 3 ... Carriage, 4 (4a, 4b) ... Recording head, 10 ... Paper, 12 ... Conveyor belt, 2
0 ... Maintenance and recovery mechanism, 30 ... Discharge detection device, 100 ... Control unit, 100A ...
Main control unit 111... Image output control unit 112. Encoder analysis unit 113.
Main scanning motor drive unit, 114 ... sub-scanning motor drive unit, 131 ... droplet ejection detection unit, 2
00: Host (information processing apparatus), M1, M2: Mask pattern.

JP 2010-17918 A

Claims (11)

A threshold for comparing a predetermined threshold with an image forming unit that discharges droplets from a nozzle of the recording head to form an image on a recording medium, and a pixel value corresponding to a defective ejection nozzle that cannot normally discharge droplets from the nozzle. A comparison means;
Among the pixel values corresponding to the defective ejection nozzle, at least a value that does not exceed the threshold value is a value for stopping ejection by the defective ejection nozzle when the multi-value data of the input image is converted into n values (n ≧ 2). Processing for developing quantization error when converting multi-valued data of an input image into n values (n ≧ 2) due to the value for stopping the ejection to pixels around the pixel corresponding to the defective ejection nozzle First image compensation means for executing
As a result of the comparison, on the condition that the pixel value corresponding to the defective ejection nozzle is equal to or greater than the threshold value, a predetermined mask pattern corresponding to the size and arrangement of the dots around the defective ejection nozzle is used. Second image compensation means for executing processing for replacing small dots with large dots among the dots around the defective ejection nozzle by matching;
An image forming apparatus comprising: The image forming apparatus according to claim 1,
Having means to compensate for image degradation,
The means for compensating for the deterioration of the image performs processing for storing the coordinates of the pixel corresponding to the defective ejection nozzle in a storage unit on condition that the pixel value corresponding to the defective ejection nozzle is equal to or greater than the threshold value. An image forming apparatus that performs the processing by the second image compensation unit based on the stored coordinates of the pixel. The image forming apparatus according to claim 1 or 2,
The image forming apparatus, wherein the threshold value can be set according to a density of a printed image when ink is normally ejected onto a recording medium. The image forming apparatus according to claim 1 or 2,
2. The image forming apparatus according to claim 1, wherein the threshold value can be set according to the shape and arrangement of dots in the print image around the defective ejection nozzle after the multi-value error diffusion processing. The image forming apparatus according to claim 2,
The image forming apparatus, wherein the threshold value can be set according to a type of a recording medium. The image forming apparatus according to claim 2,
The image forming apparatus, wherein the threshold value can be set according to a color of ink ejected on a recording medium. The image forming apparatus according to claim 1,
The second image compensator includes a peripheral portion of the defective ejection nozzle corresponding to the n value after the multi-value error diffusion processing corresponding to the n value (an integer of n ≧ 2) is performed on the multi-value data of the input image. An image forming apparatus that replaces dots by using a mask pattern set individually for each dot size. An image forming step of forming an image on a recording medium by discharging droplets from a nozzle of the recording head;
A threshold value comparing step for comparing a predetermined threshold value with a pixel value corresponding to a defective ejection nozzle in which droplets cannot be ejected normally from the nozzle;
Among the pixel values corresponding to the defective ejection nozzle, at least a value that does not exceed the threshold value is a value for stopping ejection by the defective ejection nozzle when the multi-value data of the input image is converted into n values (n ≧ 2). Processing for developing quantization error when converting multi-valued data of an input image into n values (n ≧ 2) due to the value for stopping the ejection to pixels around the pixel corresponding to the defective ejection nozzle A first image compensation step of performing
As a result of the comparison, on the condition that the pixel value corresponding to the defective ejection nozzle is equal to or greater than the threshold value, a predetermined mask pattern corresponding to the size and arrangement of the dots around the defective ejection nozzle is used. A second image compensation step for performing a process of replacing a small dot with a large dot among the dots around the defective ejection nozzle by matching;
An image processing method comprising: The image processing method according to claim 8, wherein
A process of compensating for image degradation,
The step of compensating for the deterioration of the image is a step of performing a process of storing the coordinates of the pixel corresponding to the defective ejection nozzle in a storage unit on condition that the pixel value corresponding to the defective ejection nozzle is equal to or greater than the threshold value. And executing the second image compensation step based on the stored coordinates of the pixel.   A program for causing a computer of the image forming apparatus according to claim 1 to function as a means for compensating for the deterioration of the image.   A computer-readable program recording medium on which the program according to claim 10 is recorded.

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