JP6682238B2 - Image processing apparatus and image processing method - Google Patents

Description

  The present invention relates to an image processing apparatus and an image processing method for recording an image by ejecting ink on a recording medium.

  In the inkjet recording apparatus, a single-pass method is used in which an image is completed by performing relative scanning once between a long recording head and a recording medium. While such a single-pass method has the advantage of high-speed output, the ejection characteristics of the individual nozzles arranged in the recording head are likely to appear on the recording medium, and density unevenness often poses a problem.

  Japanese Patent Application Laid-Open No. 2004-242242 discloses a configuration in which unevenness in density is suppressed by relatively increasing the ejection amount of a nozzle at a position where white stripes occur in an image.

JP, 2010-228286, A

  By the way, in recent years, it has been required to cope with a recording medium having low ink absorbency such as used in offset printing. In a recording medium having low absorptivity, a new ink droplet may be applied to an adjacent position before the ink droplet applied to the recording medium is sufficiently absorbed. New density unevenness may occur. In the case of single-pass printing, the order in which the ink droplets are applied to the printing medium and the tendency of inquiries depend on the layout of the nozzles and the ejection characteristics of the printing head, so the state of density unevenness also depends on these. . Further, such density unevenness cannot be sufficiently mitigated even if the ejection amount is adjusted by employing Japanese Patent Application Laid-Open No. 2004-242242, because the inquiry between the ink droplets is not suppressed.

  The present invention has been made to solve the above problems. Therefore, it is an object of the present invention to provide an image processing apparatus and an image processing method capable of outputting a uniform image without density unevenness even when recording with a single pass on a recording medium having low absorptivity. is there.

The present invention For this purpose, the image data is multi-value data representing the input image density, refers to the threshold value matrix threshold corresponding to the position of the pixel is determined in advance, the multi-value for each pixel When the multi-valued data has a value equal to or larger than the threshold value by comparing the data and the threshold values set in the threshold value matrix, it is determined to be dot recording, and when the multi-valued data has a value smaller than the threshold value, A recording head including a generating unit that generates ejection data by performing a non-recording quantization process, and a nozzle that ejects ink arranged in a predetermined direction according to the ejection data generated by the quantization process; An image processing apparatus for recording an image by ejecting ink while relatively moving a recording medium in a direction intersecting with the predetermined direction, the method comprising: The stage, the recording head performs the quantization process using a recording threshold matrix determined based on a result of reading a test pattern recorded using data for recording a uniform dot pattern, In the recording threshold matrix, the average value of the threshold values corresponding to the nozzles corresponding to the low density position where the density of the test pattern is lower than the surroundings in the predetermined direction corresponds to the non-low density position that is not the low density position. Which is lower than the average value of the threshold values corresponding to the nozzles and which corresponds to the low density position in the ejection data corresponding to the gradation of the intermediate density generated using the recording threshold matrix, is used for the printing. The average of the number of consecutive non-printed pixels in the row of pixels to be recorded is the row of pixels in which the nozzle corresponding to the non-low density position is used for printing As non-recording pixel is greater than the average number of contiguous, characterized in that threshold value corresponding to the position of each pixel is set.

  According to the present invention, it is possible to output a uniform image with no density unevenness even when recording in a single pass on a recording medium having low absorptivity.

FIG. 3 is an internal configuration diagram of an inkjet recording apparatus that can be used in the present invention. FIG. 6 is a view of the recording head as viewed from the nozzle surface side. It is a block diagram which shows the control structure of the image processing system which can be used for this invention. FIG. 3 is a block diagram showing a process of image processing executed by a host device and a recording device. (A)-(c) is a figure for demonstrating the density nonuniformity generation mechanism. (A) And (b) is a figure which shows the correspondence of a nozzle layout and a threshold value matrix. FIG. 9 is an array state diagram of recording pixels when uniform image data is input. It is a figure for demonstrating the setting process of the threshold value matrix at the time of factory shipment. (A) And (b) is a figure which shows the threshold value matrix set at the time of shipment. It is a figure which shows another example of a threshold value matrix. It is a figure which shows another example of a nozzle layout. It is a figure which shows another example of a threshold value matrix. (A) And (b) is a figure which shows distribution of a threshold average value and a recording pixel number. (A)-(c) is a figure which shows another example of a threshold value matrix and a threshold value average value. FIG. 9 is an array state diagram of recording pixels when uniform image data is input. It is a figure which shows an example of the threshold value matrix prepared in 2nd Embodiment. FIG. 9 is an array state diagram of recording pixels when uniform image data is input. It is a distribution coefficient and a coefficient table used in the third embodiment. It is a block diagram of the image processing process in 4th Embodiment. It is a coefficient table used in the fourth embodiment. It is an internal block diagram of the inkjet recording device used in 5th Embodiment. It is a block diagram which shows the control structure of an image processing system. It is a flow chart which shows a processing process in calibration. It is a figure which shows the example of the some matrix prepared beforehand. (A) And (b) is a figure which shows another example of a system structure and a process.

(First embodiment)
<Schematic configuration of image processing system>
FIG. 1 is a perspective view for explaining the internal configuration of an inkjet recording device 104 that can be used in the present invention. The recording medium S sandwiched by the plurality of conveying rollers 81 is conveyed in the Y direction at a constant speed, and recording heads 11 to 14 for applying ink to the recording medium S according to the ejection data are arranged on the conveying path. ing. The recording head 11 ejects black ink, the recording head 12 ejects cyan ink, the recording head 13 ejects magenta ink, and the recording head 14 ejects yellow ink in the Z direction.

  FIG. 2 is a diagram showing a partial nozzle layout on the nozzle surfaces of the recording heads 11-14. Here, the recording head 11 for black ink will be described as an example, but the other recording heads 12 to 14 also have the same configuration. Nozzles 12 for ejecting ink as droplets are arranged on the nozzle surface 10 at a constant pitch (here, 600 dpi (dots / inch) in the X direction that intersects the Y direction, which is the transport direction. The nozzles 12 adjacent to each other in the X direction are arranged in a 3-nozzle cycle with a space D in the Y direction, that is, in this example, the nozzle surface 10 has a plurality of nozzles 12 in a 3-pixel cycle in the X direction. The nozzle row 1, the nozzle row 2, and the nozzle row 3 which are arranged in the above are arranged at intervals of D in the Y direction. Ink is ejected at the corresponding frequency.

  FIG. 3 is a block diagram showing the control configuration of the image processing system usable in the present invention. Here, an image processing system including the inkjet recording device 104 and the host device 100 will be described as an example.

  In the host device 100, the CPU 108 executes various processes according to various programs stored in the hard disk (HD) 107 or the ROM 110 while using the RAM 109 as a work area. Specifically, each software of the application 101, the printer driver 103, and the monitor driver 105 is operated via the operating system 102.

  The monitor driver 105 is software for executing processing such as creating image data to be displayed on the monitor 106. The printer driver 103 is software for converting the image data passed from the application software 101 to the OS 102 into image data that can be received by the recording device 104, and then transmitting the image data to the recording device 104.

  In the recording device 104, the controller 200 has a CPU 210 in the form of a microprocessor and a ROM 211 and a RAM 212. The controller 200 uses the CPU 210 to control the entire recording apparatus while using the RAM 212 as a work area according to programs and various parameters stored in the ROM 211. Specifically, the head drive circuit 202 is controlled to cause the received ejection data to correspond to the individual nozzles of the recording heads 11 to 14 to eject ink. Further, the conveyance roller 81 is rotated via the conveyance motor 206, and the recording medium S is conveyed at a conveyance speed corresponding to the ejection frequency of the recording heads 11-14.

  FIG. 4 is a block diagram for explaining image processing steps executed by the host apparatus 100 and the recording apparatus 104, respectively. In the host device 100, the user can use the application 101 to create image data to be recorded by the recording device 104. When the user inputs a print command, the image data created by the application 101 is transferred to the printer driver 103.

  The printer driver 103 performs the pre-stage process J0002, the post-stage process J0003, the γ correction process J0004, the quantization process J0005, and the print data creation process J0006 on the received image data in this order. Hereinafter, these processes will be described in detail.

  In the pre-stage process J0002, the application 101 converts the color gamut that can be displayed on the monitor 106 into a color gamut that can be expressed by the recording device 104. Specifically, the RGB image data represented by 8 bits is converted into the R′G′B ′ image data also represented by 8 bits by referring to the three-dimensional LUT stored in the ROM 110.

  In the post-stage processing J0003, the R′G′B ′ image data is converted into multivalued data indicating the image densities of the four color inks C, M, Y, and K ejected by the recording heads 11 to 14 mounted on the recording apparatus 104. Convert. Specifically, the 8-bit R′G′B ′ image data obtained in the pre-stage processing J0002 is referred to by the three-dimensional LUT stored in the ROM 110, so that the C, M, Y, and K Convert to 8-bit data.

  In the subsequent γ correction processing J0004, γ correction is performed on each of the C, M, Y, and K multivalued data obtained in the subsequent processing J0003. Specifically, each of the 8-bit C, M, Y, and K data is converted into 8-bit C'M'Y'K 'data by referring to the one-dimensional TUT prepared for each ink color. To do. By the γ correction processing J0004, the multivalued data of each of C, M, Y, and K and the density data represented by the recording medium have a linear relationship.

  In the quantization processing J0005, each of the γ-corrected 8-bit C′M′Y′K ′ data is converted into 1-bit data C, M, Y, K by adopting a predetermined quantization processing method. To do. In the quantized image data, dot recording (1) or non-recording (0) is defined as 1-bit information for each pixel corresponding to the recording resolution of the recording device 104.

  In the print data creation process J0006, the control information relating to the recording operation such as the recording medium information, the recording quality information and the paper feeding method is added to the 4-color 1-bit data generated by the quantization process J0005 to print the print data. To create. The print data generated as described above is supplied from the host device 100 to the recording device 104.

  After that, the binary image data supplied to the printing apparatus 104 is converted into ejection data associated with each nozzle of each of the print heads 11 to 14 and sent to the head drive circuit 202. Then, an image is recorded on the recording medium S by ejecting ink at appropriate timing from the individual nozzles arranged in the recording heads 11 to 14.

<Mechanism of density unevenness generation>
FIGS. 5A to 5C are diagrams for explaining the mechanism of density unevenness when single-pass printing is performed using the print head 11 on a print medium having a low absorptivity. As shown in FIG. 2, in the recording head 11, the nozzle row 1, the nozzle row 2, and the nozzle row 3 are arranged at intervals D in the Y direction.

  Here, let us consider a case where the recording head 11 is used to record a ruled line extending in the X direction, for example. At this time, the ink is first ejected from the nozzles of the nozzle row 1 which is the most upstream side in the conveyance direction, and the ink is ejected from the nozzles of the nozzle row 2 at the timing when the recording medium S is conveyed by a distance corresponding to the interval D. . Then, at the timing when the recording medium S is further conveyed by a distance corresponding to the distance D, ink is ejected from the nozzles of the nozzle row 3 located on the most downstream side in the conveying direction. As a result, on the recording medium S, a ruled line formed by a plurality of dots continuously arranged in the X direction is formed.

  FIG. 5A shows a state in which the ejection operation of only the nozzle row 1 is performed on the recording medium having low absorptivity to record an image of 33% duty. Ruled lines 1 extending in the Y direction are arranged in the X direction at intervals of every 3 pixels, and there is no deviation in the X direction between them.

  FIG. 5B shows a state in which a 66% duty image is recorded by performing the ejection operation with the nozzle rows 1 and 2 on the same recording medium as that in FIG. Next to the ink droplet ejected by the nozzle row 1, the ink droplet ejected by the nozzle row 2 before the ink droplet is absorbed by the recording medium S is recorded. Therefore, these ink droplets come into contact with each other and attract each other due to their surface tension. As a result, the ruled lines 1 and 2 extending in the Y direction move in the X direction so as to approach each other, and a line L is formed by combining these two ruled lines. A white paper area W is exposed between the lines L, but the white paper area W has a larger width than that in the case where no inquiry occurs, that is, when the data is recorded on a recording medium having high absorptivity.

  FIG. 5C shows a state in which a 100% duty image is recorded by performing the ejection operation in the nozzle rows 1 to 3 on the same recording medium as in FIGS. 5A and 5B. As described with reference to FIG. 5B, the ruled line 1 recorded by the nozzle row 1 and the ruled line 2 recorded by the nozzle row 2 are attracted to each other, and the ink droplets ejected from the nozzle row 3 are relatively large. It is landed almost at the center of the blank area W. Therefore, the ink droplets ejected from the nozzle row 3 are unlikely to come into contact with the ruled line 1 and the ruled line 2, and their positions do not shift in the X direction. As a result, the ruled lines 1 and 2 which are recorded close to each other and the ruled lines 3 which are recorded at a distance from them are alternately arranged in the X direction, and regular white stripes are generated in the image, resulting in uneven density. Is perceived as.

  Such density unevenness depends on the landing order on the recording medium S, and the ejection characteristics such as the ejection amount and ejection direction of each nozzle. For example, a white stripe appears at a position to be printed by a nozzle that ejects ink at a relatively late timing, such as the nozzle row 3, a nozzle that ejects a smaller amount of ink than other nozzles, or a nozzle whose ejection direction is displaced in the X direction. It is easy, and the density at that position is low, and it is easy to detect. The landing order and the ejection characteristics depend on the design layout of the nozzles in the print head, but also depend on design errors and the like, and the characteristics tend to vary from lot to lot. In the case of a long print head, a plurality of chips manufactured at the same time and having the same characteristics are connected and arranged in the X direction, so that in the print image, periodic density unevenness in the X direction may be detected. To do.

  In view of such a situation, the inventors of the present invention have made earnest studies, and as a result, grasp the tendency of density unevenness in each print head or chip in advance, and perform image processing corresponding to the nozzle layout and the ejection characteristics of each nozzle. Determined that it was effective. Therefore, in this embodiment, the dither processing is adopted as the quantization processing J0005, and the threshold matrix referred to in the processing is characterized so that the number of times of ejection of the nozzle at the position where white stripes are likely to appear is different from that of other nozzles. Relatively more than. The details will be described below.

  In the dither processing, the value of the multi-valued C′M′Y′K ′ data input to each pixel is compared with a preset threshold value associated with each pixel, and the dot recording at the pixel is performed. (1) or non-recording (0) is determined. When the input multi-valued data is 8-bit data having a value of 0 to 255, the threshold value can take a value of 1 to 255. Then, the pixel having a threshold value equal to or less than the signal value of the input multi-valued data is recorded (1), and the pixel having a threshold value larger than the signal value is not recorded (0). That is, a pixel for which a relatively small threshold value is set has a higher possibility of being judged as recording (1) than a pixel for which a relatively large threshold value is set.

  In view of such a situation, in the present embodiment, the threshold value of the pixel corresponding to the nozzle at the low density position where the white stripe is confirmed is set smaller than the threshold values of the pixels corresponding to the other nozzles. As a result, the number of ejections at the low density position can be made relatively larger than at other positions.

  FIGS. 6A to 6C are diagrams showing a nozzle layout in a print head, a density distribution obtained from an image actually printed by using the print head, and a threshold matrix corresponding to the density distribution. FIG. 6A is a diagram showing a correspondence relationship between individual nozzles and a threshold matrix. The six nozzle rows (nozzle rows 1 to 6) are laid out in a dispersed manner, and the plurality of nozzles are arranged in a cycle of 6 pixels in the X direction. Here, for the sake of explanation, a threshold matrix having a region of 12 pixels in the X direction and a region of 12 pixels in the Y direction is shown, but the actual threshold matrix has a larger region, and its size is particularly limited. Not a thing.

FIG. 6B is a diagram showing an example of a luminance distribution (density distribution) when an image actually recorded with the nozzle layout shown in FIG. 6A is read. The horizontal axis represents the pixel position (nozzle position) in the X direction, and the vertical axis represents the average value of the brightness values in the Y direction. In the figure, the larger the luminance value L ^* , the brighter, that is, the lower the density, and the more easily white stripes can be confirmed. In this embodiment, the pixel position (nozzle position) having the maximum luminance value L ^* is extracted as the low density position. Here, the second and eighth pixel positions from the left have the highest luminance (that is, the lowest density), and the pixel position is recognized as the low-density position. Then, in the threshold matrix, the threshold value corresponding to the low density position is set to be relatively smaller than the threshold values corresponding to other positions, that is, non-low density positions. That is, such a threshold matrix is prepared.

  FIG. 6C shows an example of a threshold matrix applicable when the brightness distribution as shown in FIG. 6B is obtained. The average value of the threshold is 125 for the pixel row corresponding to the low density position. On the other hand, in the other areas, the average value of the threshold is 175. In the region where the dither processing is performed using such a threshold matrix, the ejection frequency of the nozzles at the low density position can be made relatively higher than the ejection frequency of the nozzles at other positions.

  FIG. 7 shows a recording when image data having a signal value of 150 is uniformly input to a 12 × 12 pixel area in the configuration in which the quantization processing is performed using the threshold matrix shown in FIG. 6C. It is a figure which shows the arrangement state of a pixel (1). Pixels for which a threshold value of 150 or less is set are determined to be recording (1), and are shown in black in the figure. Pixels for which a threshold value greater than 150 is set are determined as non-printing (0), and are shown in white in the figure. As can be seen from the figure, the number of recording pixels corresponding to the low density position is larger than the number of recording pixels corresponding to other positions. Therefore, on the recording medium, the number of ink droplets ejected from the nozzle corresponding to the low density position can be made larger than that of the other nozzles, and the density unevenness can be suppressed. In the present embodiment, the threshold matrix information associated with such a recording head is stored in the ROM 211 of the recording apparatus in advance at the time of shipment of the recording apparatus.

FIG. 8 is a diagram for explaining the threshold matrix setting process at the time of factory shipment. In this setting step, first, a predetermined test pattern 80 is recorded using the target recording device. In the case of this embodiment, since the recording heads 11 to 14 respectively corresponding to the four color inks are mounted, the test patterns of four color types are output. The content of the test pattern 80 is not particularly limited, but a pattern in which individual nozzles are arranged with substantially the same number of dots in a similar cycle and white streaks and density unevenness are easily detected is preferable. The image data for printing such a test pattern may be prepared as binary data, but RGB data that produces a uniform dot pattern is input to the printer driver, and the series of processing shown in FIG. 4 is performed. You may make it record via. Of course, not only RGB data but also CMYK data L ^* a ^* b ^* data can be used. At this time, for the quantization processing, it is preferable to use a standard threshold matrix in which the threshold distribution is not biased for all nozzle rows.

  The size of the test pattern is also not particularly limited, but it is preferable that it has a width over the entire recording head in the X direction. Further, in the case of an ink such as white ink in which white stripes or density unevenness does not pose a problem in the image, the ink may be excluded from this step and a standard threshold matrix may be associated in advance. it can.

Next, the output test pattern 80 is read by a predetermined reading device 90 to obtain the luminance information L ^* corresponding to each pixel. The reading device may be a rear sensor or a line sensor, but it is desired that the reading resolution is equal to or higher than the recording resolution of the recording device. Here, the reading resolution is 600 dpi (ppi; pixels / inch), which is equal to the recording resolution. Through the reading process, multi-valued luminance data L ^* composed of 8 bits for each pixel is acquired in a two-dimensional array of 600 ppi. At this time, the reading area may be the entire area in which the test pattern is recorded, but the data of the end portion affected by the blank area may be excluded. However, in order to finally obtain the luminance distribution in the nozzle layout, it is preferable that image data of an integral multiple of the nozzle layout period is acquired in the X direction.

  Next, the plurality of acquired multi-valued luminance data are averaged among the plurality of pixels arranged in the Y direction, and the multi-valued data associated with each nozzle is calculated. Further, one cycle of the nozzle layout in the X direction is set as a unit, and averaging processing between the units is performed. As a result, an extreme density fluctuation due to a sudden ejection failure or the like can be suppressed, and a brightness distribution, that is, a density distribution peculiar to the nozzle layout within the cycle can be obtained. In the present embodiment, the brightness distribution as shown in FIG. 6B is obtained by such a process, and the threshold matrix as shown in FIG. 6C is selected based on the brightness distribution. Specifically, a plurality of threshold matrices are prepared in advance such that the low density positions and the variations in the thresholds are different from each other, and the information for associating with the selected threshold matrix is stored in the ROM 211 of the recording device.

  In the configuration of the present embodiment shown in FIG. 4, since the printer driver 103 performs the quantization processing, a plurality of threshold matrices are installed in advance with the printer driver, and information indicating which threshold matrix is used. Should be stored in the ROM 211. Further, when the recording apparatus performs the series of image processing steps shown in FIG. 4, the set threshold matrix may be directly stored in the ROM 211. Furthermore, instead of preparing a plurality of threshold matrixes in advance, an appropriate threshold matrix may be generated on the spot according to the acquired luminance distribution.

  By the way, in the above, the maximum value in the acquired luminance distribution is extracted as the low-density position, and the threshold matrix is prepared so that the threshold value of the position can be kept smaller than the threshold values of the other positions. Not limited to the configuration. For example, it is possible to extract the minimum value in the acquired luminance distribution as a high-density position and prepare a threshold matrix in which the threshold value at that position is larger than the threshold values at other positions.

  FIG. 9 is a diagram showing a threshold matrix in such a case. The average value of the thresholds is 175 for the pixel row corresponding to the high density position shown in gray. On the other hand, in the other areas, the average value of the threshold is 125. With respect to the area that has been subjected to the dither processing using such a threshold matrix, the ejection frequency of the nozzles at the high density position can be made relatively lower than the ejection frequency of the nozzles at other positions. Such a configuration is particularly effective when black stripes stand out in the image.

  In addition, the visibility of white stripes also changes depending on the gradation of the image. Therefore, in the threshold matrix, the degree to which the threshold at the low density position is kept lower than the thresholds at other positions can be adjusted for each gradation.

  FIG. 10 is a diagram showing another example of the threshold matrix usable + in this embodiment. Here, an example is shown in which the low-density position is extracted in a 3-pixel cycle. In FIG. 9B already described, the predetermined threshold values are evenly distributed to the pixels corresponding to the low density position and to the other pixels. On the other hand, in the example of FIG. 10, the threshold value is distributed to the pixel corresponding to the low density position so that the peak value is 128, and the threshold values are uniformly distributed to the other pixels.

  When the threshold matrix as shown in FIG. 10 is used, the ratio of the number of times the nozzles at the low density position eject ink and the number of times the nozzles at other positions eject ink change according to the gradation. Specifically, the number of discharges of all nozzles is equal in the low gradation region (signal value less than 128) where the graininess is easily noticeable, but at the low density position in the medium density region (signal value 128 or more) where white lines are easily noticeable. The number of discharges of the nozzle is larger than that of the nozzles at other positions. Therefore, it is possible to keep the dispersibility high in the highlight portion where the graininess is conspicuous and to bias the ejection operation to the nozzles in the low concentration position in the middle concentration portion where the white stripes are conspicuous. As a result, good uniformity can be obtained in all gradations.

  In the two types of threshold value matrixes shown in FIGS. 6C and 10, the low density position and the distribution of the threshold values are different, but the average value of the threshold values corresponding to the nozzles at the low density position is It is suppressed to be smaller than the average of the threshold values corresponding to the nozzles at the positions. As a result, it is possible to increase the number of dots printed at the position where white stripes are easily recognized as compared with other areas, and it is possible to reduce density unevenness.

  It should be noted that, in the above, for simplification of description, a threshold matrix in which regions of 12 pixels × 12 pixels are set and discrete thresholds are set has been described as an example. However, it is assumed that the actual threshold matrix has a larger image area and the thresholds of 1 to 255 are dispersed and arranged. At this time, in the X direction, a threshold matrix having a number of pixels that is an integral multiple of the nozzle layout period is desired. For example, in the print head 11 shown in FIG. 6A, the nozzles in each nozzle row are arranged in a cycle of 6 pixels in the X direction, and thus it is desirable to have a multiple of 6 pixels in the X direction. On the other hand, in the Y direction, the number of pixels is not particularly limited as long as a region in which the threshold value of 1 to 255 is appropriately dispersed to the extent that the texture does not appear is secured. Hereinafter, a case will be described in which a printhead having a more complicated nozzle layout is used and a threshold matrix of a larger size is adopted.

  FIG. 11 is a diagram showing a nozzle layout on the nozzle surface of the recording head used in this example. Here, the nozzle layout is shown as coordinates on the XY plane for 64 nozzles. The print head has 16 nozzle rows (nozzle rows 1 to 16) arranged in the Y direction, each nozzle row being composed of a plurality of nozzles having the same Y coordinate, and the nozzle layout cycle in the X direction is 16 pixels. There is.

  FIG. 12 is an example of a threshold matrix having an area of 64 pixels × 64 pixels which can correspond to the nozzle layout shown in FIG. In a region of 64 pixels × 64 pixels, threshold values of 1 to 255 are basically arranged in a highly dispersive state so as to have a blue noise characteristic. However, also in this threshold matrix, the average value of the corresponding threshold values is suppressed to be smaller than the average of the threshold values of the other regions in the low density position corresponding to the nozzle row 16, as in the above threshold matrix.

  FIGS. 13A and 13B are diagrams showing the distribution of the average value and the number of recording pixels in the Y direction of the threshold values corresponding to the individual nozzles arranged in the X direction in the threshold value matrix shown in FIG. Referring to FIG. 13A, among the 64 nozzles, the average value of the 1st, 17th, 33rd, and 49th nozzle positions corresponding to the low-density position is higher than the average value of the other nozzle positions. , But they are not equivalent to each other, but it can be seen that they are locally suppressed low.

  On the other hand, FIG. 13B shows a recording (1) when image data having a signal value 128 is uniformly input to a 64 × 64 pixel area in the configuration using the threshold matrix of FIG. 3] is a diagram showing the number of pixels set as “) for each nozzle. It can be seen that among the total of 64 nozzles, the print pixels of the 1st, 17th, 33rd, and 49th nozzles corresponding to the low-density position are larger than the print pixels of the other nozzles. As described above, even when a threshold matrix sufficiently larger than the cycle of the nozzle layout is used, if a relatively small threshold is set at each of the low density positions, density unevenness can be reduced as in the above embodiment. You can do it.

By the way, although the example in which only one peak of the brightness value L ^* exists in one cycle of the nozzle layout has been described above, a plurality of peaks appear in the cycle when the number of nozzles included in one cycle is large. In some cases.

FIGS. 14A to 14C show the threshold matrix set as the low density position and the average threshold value in the Y direction of the matrix when two peaks appear in the distribution of the luminance value L ^*. It is a figure. In order to support a nozzle layout having a 6-pixel cycle in the X direction, a threshold matrix having a region of 12 pixels × 12 pixels is illustrated here. In this example, the two low concentrations position, threshold the threshold luminance value of the peak L ^* corresponds to the largest low concentration position A, corresponding to a low concentration position B large luminance value L ^* is a second peak Is set to an even smaller value. Specifically, as shown in FIG. 14C, the average value of the threshold values corresponding to the low density position A having a larger peak luminance value L ^* is 107.5, and the peak low density position B lower than this is The average value of the threshold values corresponding to is 150.3, and the average value of the threshold values corresponding to other areas is 193.5.

  If such a threshold matrix is used, the number of ejections of the nozzle corresponding to the low density position B is made larger than that of the nozzles of other positions, and the number of ejections of the nozzle corresponding to the low density position A is made larger than this. Can be done. As a result, at each low density position, it is possible to increase the density according to the degree of white stripes, and it is possible to mitigate the density unevenness of the entire image.

  On the other hand, in order to make white stripes inconspicuous, it is also effective to make the non-recorded pixels on which dots are not recorded continuous in the Y direction as much as possible at the low density position. Hereinafter, the reason will be described. When the number of non-printed pixels continuous in the Y direction increases, the frequency of ink droplets ejected from the nozzles corresponding to that position adjacent to each other in the Y direction decreases on the print medium, and the ink drops easily become isolated in the Y direction. If ink droplets before absorption are present on both sides in the X direction, such ink droplets are attracted to the inks adjacent to these two sides and easily spread in the X direction. As a result, white stripes can be prevented from extending in the Y direction, and white stripes at that position can be suppressed. Then, in order to realize such a threshold matrix, it is sufficient that the thresholds are arranged so that the thresholds of small values are continuous in the Y direction as much as possible in the pixel row corresponding to the low density position.

  FIG. 15 is a diagram showing an arrangement state of recording pixels (1) when image data having a signal value 128 is uniformly input to a 64 × 64 pixel area in a configuration adopting such a threshold matrix. Is. At the nozzle position corresponding to the nozzle row 16, the number of continuous non-printing pixels in the Y direction is larger than that of other nozzle rows. With such a dot arrangement, it is possible to further suppress the occurrence of white lines.

  According to the present embodiment described above, a threshold value at which a position where white stripes are likely to appear in the print head to be used is extracted and the threshold value corresponding to the extracted position is set lower than the threshold values corresponding to other positions. Prepare a matrix. Then, this is set as a threshold matrix to be referred to in the dither processing, in association with the print head to be used. This makes it possible to increase the number of times the nozzles corresponding to the low-density position are ejected more than the nozzles at other positions, and to suppress the occurrence of white streaks and thus uneven density.

(Second embodiment)
In this embodiment, each of the recording heads 11 to 14 is configured to be capable of selectively recording either large dots or small dots. Also in the present embodiment, the same processing as in the first embodiment is performed up to the γ correction processing J0004, dither processing is adopted as the quantization processing J0005, and the threshold matrix has a feature.

  FIG. 16 is a diagram showing the threshold matrix for large dots and the threshold matrix for small dots prepared as the threshold matrix of this embodiment. The threshold matrix for large dots is set with a relatively higher threshold than the threshold matrix for small dots. Here, it is assumed that the pixel area shown in gray is extracted as the low density position in the test pattern reading process described with reference to FIG. In any of the threshold matrices, the average value of the threshold values set in the pixel areas corresponding to the low density positions is smaller than the average value of the threshold values set in the pixel areas in other positions.

  In the quantization processing J0005 of the present embodiment, the multi-valued data after the γ correction processing J0004 is performed is compared with the threshold values set in the threshold matrix for large dots, and large dot recording (1) or non-recording is performed. Determine (0). At the same time, the multi-valued data is also compared with the threshold values set in the threshold matrix B for small dots, and recording (1) or non-recording (0) of small dots is also determined. After that, the results of these two types of quantization are collated, and for pixels in which both large dots and small dots are recorded (1) and pixels in which only large dots are recorded (1), Outputs ejection data for dots. For pixels in which only small dots are recorded (1), ejection data for small dots is output. Further, non-ejection data is output for pixels in which both large dots and small dots are non-printed (0).

  FIG. 17 shows a case in which image data having a signal value 64 is input uniformly in the configuration in which the quantization processing is performed using the threshold matrix for large dots and the threshold matrix for small dots shown in FIG. FIG. 6 is a diagram showing an arrangement state of recording pixels of large dots and small dots. In any comparison process with any threshold matrix, pixels having a threshold value of 64 or less are determined as recording (1), and pixels having a threshold value greater than 64 are determined as non-recording (0). It Then, these two quantization results are collated to obtain the final quantization result. As can be seen from the figure, large dots are more likely to be recorded in the recording pixels corresponding to the low density position than in the recording pixels corresponding to other positions. For this reason, on the recording medium, at a position corresponding to a low density position where white stripes are likely to occur, it is possible to increase the ratio of recording large dots compared to other areas, and suppress density unevenness. .

  Note that, in FIG. 16, for simplification of description, a threshold matrix having an area of 9 pixels × 9 pixels and having three levels of threshold values (32, 64, 128) continuously set to the same value in the Y direction. Was described as an example. However, it is preferable that the actual threshold matrix has a larger image area and the thresholds of 1 to 255 are dispersed and arranged. At this time, the threshold matrix for large dots is set with a relatively higher threshold than the threshold matrix for small dots, and in either threshold matrix, the pixel area at the low density position is more than the other pixel areas. It suffices that the threshold value is set so that the average value becomes small. By doing so, it is possible to reduce white streaks from both sides of the number of dots and the dot size in the area corresponding to the low density position.

  Further, in the above, the configuration is such that the dot size can be switched in two steps such as large dot and small dot, but the effect of the present embodiment is not limited to such a configuration. The threshold matrix for larger dots is set with a relatively higher threshold than the threshold matrix for smaller dots, and in any threshold matrix, the pixel area at the low density position has a higher threshold than the other pixel areas. It suffices that the threshold value is set so that the average value becomes small.

(Third Embodiment)
Also in this embodiment, the number of ejections of the nozzle at the low density position is made relatively larger than that of the nozzles at other positions by providing the quantization processing J0005 shown in FIG. 4 with a characteristic. However, in this embodiment, the error diffusion process is adopted as the quantization process J0005 instead of the dither process.

  In the error diffusion processing, multi-valued errors that occur when the target pixel is quantized are distributed to surrounding pixels that have not been quantized yet. At this time, the distribution amount to each pixel is calculated according to a distribution coefficient as shown in FIG. On the other hand, FIG. 18B is a characteristic coefficient table of the present embodiment which is additionally used in FIG. The coefficient table is set based on the reading result in the test pattern reading process described in FIG. In the present embodiment, the coefficient shown in FIG. 18B is provided with a characteristic so that the number of times of ejection of a nozzle at a position where white stripes are likely to occur is relatively larger than that of other nozzles. Specifically, as shown in the figure, the coefficient (2) set in the pixel area shown in gray corresponding to the low density position is made larger than the coefficient (1) set in other pixel areas.

  When actually performing the error diffusion processing, the CPU 108 multiplies the error generated in the processing target pixel by the distribution coefficient shown in FIG. 18A and the coefficient shown in the coefficient table of FIG. The amount of error added to the pixel is calculated. Then, when such processing is performed using the coefficient table shown in FIG. 18B, an error about twice as large as that of the pixel areas at other positions is added to the pixel area corresponding to the low density position. It will be. That is, the signal value of the pixel region corresponding to the low-density position at the time of actual quantization can be made relatively larger than the signal values of the pixel regions at other positions. As a result, it is possible to reduce the density unevenness in the entire image by increasing the number of dots recorded in the pixel area where white stripes are likely to occur, compared to other areas.

(Fourth Embodiment)
In the present embodiment, the number of times of ejection of the nozzle row at the position where white streak is likely to occur is made relatively larger than that of the other nozzle rows by the newly added correction process instead of the quantization process.

  FIG. 19 is a block diagram for explaining the image processing steps executed by the host device 100 and the recording device 104 in the present embodiment. The difference from the block diagram shown in FIG. 4 is that a density correction process J0010 is added between the γ correction process J0004 and the quantization process J0005. The density correction processing J0010 multiplies each of the multi-valued (8-bit) C′M′Y′K ′ data input from the γ-correction processing J0004 by a predetermined correction coefficient and creates a new multi-valued (8-bit). ) C "M" Y "K" data is calculated and output.

  FIG. 20 is a coefficient table used in this embodiment. The coefficient table is prepared in association with each pixel based on the reading result in the test pattern reading process described in FIG. Here, for the sake of explanation, a region of 9 pixels in the X direction and 9 pixels in the Y direction is shown, and a pixel region corresponding to the low density position obtained in the test pattern reading process described in FIG. 8 is shown in gray.

  In this embodiment, the coefficient of the pixel area shown in gray is set to 1.2, and the coefficients of the other pixel areas are set to 1.0. The CPU 108 multiplies the multi-valued data (C'M'Y'K ') input to the pixels shown in gray by 1.2 and corrects the multi-valued data (C "M"). Y "K"). On the other hand, for the multi-valued data (C'M'Y'K ') input to the pixels shown in white, this is multiplied by 1.0 (that is, in the through state), and the corrected multi-valued data is obtained. Output as data (C "M" Y "K"). Then, in the subsequent quantization processing J0005, quantization is performed by general dither processing or error diffusion processing without adopting the special processing as in the above embodiment.

  According to the present embodiment as described above, the signal value of the multivalued data corresponding to the low density position can be set to about 1.2 times the signal value corresponding to the other position. As a result, it is possible to reduce the density unevenness in the entire image by increasing the number of dots recorded in the pixel area where white stripes are likely to occur, compared to other areas.

The individual coefficients stored in the coefficient table can be adjusted based on the reading result in the test pattern reading step described with reference to FIG. For example, when the distribution of the brightness values L ^* as shown in FIG. 14A is obtained in the reading process, the coefficient corresponding to the low density position A is 1.2 and the coefficient corresponding to the low density position B is 1. .1 and the coefficient corresponding to other positions can be set to 1.0. By doing so, the number of times of ejection of the nozzle corresponding to the low-density position B can be increased more than that of the nozzles of other positions, and the number of times of ejection of the nozzle corresponding to the low-density position A can be further increased. As a result, at each low density position, it is possible to increase the density according to the degree of white stripes, and it is possible to mitigate the density unevenness of the entire image.

(Fifth Embodiment)
FIG. 21 is a perspective view for explaining the internal configuration of the inkjet recording device 104 used in this embodiment. 22 is a block diagram showing the control configuration of the image processing system used in this embodiment. The difference from FIGS. 1 and 3 is that a scanner 4000 and its drive circuit 204 are newly provided. The scanner 4000 is arranged in parallel with the recording heads 11 to 14 at a position downstream of the recording heads 11 to 14 in the transport direction (Y direction) of the recording medium, and determines an image recorded by the recording heads as a recording resolution of the recording heads 11 to 14. It can be read with equal resolution (600 dpi). In the present embodiment, the pixel position extraction as described with reference to FIG. 8 is appropriately performed even after the recording device is loaded, and a threshold matrix can be newly set each time. Hereinafter, such processing for newly setting the threshold matrix is referred to as calibration.

  FIG. 23 is a flowchart for explaining processing steps in calibration. The series of steps in the calibration may be performed by the printer driver 103 of the host device 100 or the controller 200 of the recording device 104.

  When this process is started, first, in step S1, the printing apparatus 104 reads the image data for the test pattern stored in advance and prints it using the print heads 11-14. The image data may be binary data stored in the ROM of the recording device, or may be RGB data provided by the printer driver.

In step S2, the recording device 104 uses the scanner 4000 to read the test pattern output in step S1. As a result, multi-valued luminance data L ^* composed of 8 bits for each pixel is acquired in a two-dimensional array of 600 ppi. Note that the image data for the test pattern and the method of reading the test pattern can be the same as the factory shipment described with reference to FIG.

In step S3, predetermined image processing is performed on the read image data. Specifically, similar to the process described with reference to FIG. 8, the acquired multi-valued brightness data is averaged among a plurality of pixels arranged in the Y direction, and the multi-valued data associated with each nozzle is calculated. . Further, one cycle of the nozzle layout in the X direction is set as a unit, and averaging processing between the units is performed. As a result, the brightness distribution data unique to the nozzle layout as shown in, for example, FIG. 9A or FIG. 14A can be obtained. Then, the pixel position (nozzle position) corresponding to the peak of the brightness value L ^* is extracted.

  In step S4, based on the extraction result obtained in step S3, an appropriate one is selected from a plurality of threshold matrixes stored in advance and is registered.

  FIG. 24 is a diagram showing an example of a plurality of matrices prepared in advance. Here, six threshold value matrices in which the low-density positions are shifted by one pixel from each other are shown. In each of the threshold matrices, the threshold value at the low density position shown in gray is kept lower than the threshold values at other positions. In step S4, from the threshold matrices A to F prepared in advance, the low density position obtained in step S3 corresponding to the pixel position shown in gray is selected and set as a new threshold matrix. This is the end of the process.

  After that, when the normal recording operation is executed, the dither processing using the threshold matrix stored in step S4 is performed. As a result, it is possible to output an image in which white lines and uneven density are suppressed. Further, when the state of white stripes or density unevenness changes with the frequency of use of the printing apparatus, the calibration described in FIG. it can. With this configuration, even if the density unevenness changes with the use of the print head or the printing apparatus, it is possible to always maintain a uniform image output without white stripes or density unevenness.

  Note that, in the above, the case where a plurality of threshold value matrixes having mutually different low-density positions are prepared as shown in FIG. 24 has been described as an example, but a larger number of threshold value matrices having different characteristics may be prepared. . For example, even if the low-density position is the same, it is possible to prepare a plurality of threshold matrixes in which the threshold values to be set differ stepwise. Further, as described with reference to FIG. 10, it is possible to prepare a plurality of threshold matrixes in which the distributions of the thresholds are different in the plurality of pixels included in the low density position. Further, as described with reference to FIGS. 14A to 14C, it is possible to prepare a threshold matrix when a plurality of low density positions are detected. Furthermore, rather than storing such many kinds of threshold value matrices in advance, an appropriate threshold value matrix may be generated each time based on the multi-valued luminance distribution obtained in step S3. .

  Further, in the above, the configuration is such that an appropriate threshold value matrix is set according to the detected density unevenness state, but the configuration that can be changed according to the density unevenness state is not limited to the threshold value matrix. For example, the coefficient table used for the error diffusion processing may be changed according to the density unevenness state as in the third embodiment, or the coefficient table used in the nozzle row correction processing J0010 as in the fourth embodiment. May be changed according to the state of density unevenness. In any case, it is within the scope of the present invention as long as the parameter referred to in the image processing can be changed in association with the nozzle position in accordance with the detected density unevenness tendency.

  In the above description, the system including the host device 100 and the recording device 104 as shown in FIG. 3 is used, and the image processing shown in FIG. 4 is executed by each device. In this case, the host device 100 is the image processing device of the present invention. However, the present invention is not limited to such a form.

  25A and 25B are block diagrams showing another system configuration diagram and process steps applicable to the present invention. Here, the controller 300 performs the characteristic processing of the present invention on the image data created by the application on the server 400, and transfers the ejection data generated here to the recording device 104. In this case, the controller 300 is the image processing device of the present invention.

  With the configurations shown in FIGS. 25A and 25B, the controller 300 can independently perform a relatively heavy load including the characteristic processing of the present invention, and thus a large amount of image data can be processed at high speed. Can be processed.

  Further, as described above with reference to FIG. 1, a full-line type inkjet recording apparatus that records an image by transporting the recording medium S to the recording heads 11 to 14 fixed in the apparatus has been described as an example. However, the present invention is not limited to such a form. Even in the case of a serial type ink jet recording apparatus that records an image by intermittently repeating the relative movement of the recording head with respect to the recording medium and the conveying operation of the recording medium, the present invention is applicable if the so-called single-pass recording method is adopted. Works effectively.

  The present invention also supplies a program that implements one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read the program. It can also be realized by the processing to be executed. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

S recording medium
11 recording head
12 nozzles
100 host device
104 inkjet recording device

Claims (10)

The image data is multi-value data representing the input image density, refers to the threshold value matrix threshold corresponding to the position of the pixel is determined in advance, to the multi-value data and the threshold matrix for each pixel Quantization of comparing the predetermined threshold values to determine dot recording when the multi-valued data has a value equal to or greater than the threshold value, and non-recording when the multi-valued data has a value smaller than the threshold value A generation means for generating discharge data by performing processing ,
According to the ejection data generated by the quantization process, ink is ejected while relatively moving a recording head having a nozzle for ejecting ink arranged in a predetermined direction and a recording medium in a direction intersecting the predetermined direction. An image processing apparatus for recording an image by
The generating unit performs the quantization process using a recording threshold matrix determined based on a result of reading a test pattern recorded by the recording head using data for recording a uniform dot pattern. Done,
In the recording threshold matrix, the average value of the threshold values corresponding to the nozzles corresponding to the low density position where the density of the test pattern is lower than the surroundings in the predetermined direction corresponds to the non-low density position that is not the low density position. Which is lower than the average value of the threshold values corresponding to the nozzles and which corresponds to the low density position in the ejection data corresponding to the gradation of the intermediate density generated using the recording threshold matrix, is used for the printing. So that the average of the number of consecutive non-printed pixels in the row of pixels to be recorded is larger than the average of the number of consecutive non-printed pixels in the row of pixels in which the nozzle corresponding to the non-low density position is used for printing. In the image processing apparatus, a threshold value corresponding to the position of each pixel is set . As a result of reading an image recorded using the recording head, the recording threshold matrix has a second average value of the threshold values corresponding to the nozzle at the first low-density position where the density is the lowest in the predetermined direction. The average value of the threshold values corresponding to the nozzles at the second low density position lower than the average value of the threshold values corresponding to the nozzles at the second low density position having a low density is the first and second low density positions. The image processing apparatus according to claim 1 , wherein the threshold value associated with each of the nozzles is set so as to be lower than the average value of the threshold values corresponding to the non-nozzle. Wherein the recording threshold value matrix, as the arrangement of dots result of the recording operation on the recording medium in accordance with the ejection data having blue noise characteristics, or claim 1, characterized in that the threshold value is set 2. The image processing device according to item 2 . The recording head is capable of ejecting a first amount or a second amount of ink droplets larger than the first amount onto each pixel,
The generating unit, when equal image data is uniformly input, the read recorded images using a recording head results, corresponding to the low density position is lower concentration than the surrounding in the predetermined direction The number of times the nozzle ejects the second amount of ink is larger than the number of times the nozzle corresponding to the non-low density position other than the low density position ejects the second amount of ink. The image processing apparatus according to claim 1, wherein ejection data for ejecting one amount or the second amount of ink is generated. The generating unit associates the image data obtained by reading an image recorded using the recording head with each of the nozzles based on a density distribution obtained by averaging the image data in units of a cycle in which the nozzles are arranged. was by performing the quantization process, the image processing apparatus according to any one of claims 1 to 4, wherein the benzalkonium generate the ejection data. 6. The threshold value matrix for recording is selected and determined from storage means for storing a plurality of threshold value matrixes based on a result of reading the test pattern, according to any one of claims 1 to 5. The image processing device described. The recording head;
Storage means for storing the plurality of threshold matrices,
Wherein the recording head to record the test pattern, the test pattern based on the result of Tsu read using reading means, selecting and the recording threshold value matrix from the plurality of threshold matrices stored in the storage means Control means for determining
The image processing apparatus according to claim 6 , further comprising: The recording threshold matrix is based on the result of reading the test pattern recorded using the data for recording the uniform dot pattern generated by the quantization processing using the threshold matrix before correction, The image processing apparatus according to claim 1, wherein the image processing apparatus is generated by correcting the threshold matrix before correction. And before type recording head,
According to any one of claims 1 to 8, characterized in that further comprising recording head and said recording medium and moving means for relatively moving in a direction intersecting the predetermined direction of <br/> Image processing device. For image data that is multi-valued data indicating the input image density, a threshold value matrix in which threshold values corresponding to pixel positions are predetermined is referred to, and the multi-valued data and the threshold value matrix are set for each pixel. Quantization such that when the multivalued data has a value equal to or larger than the threshold value, dot recording is performed, and when the multivalued data has a value smaller than the threshold value, non-recording is performed. generates ejection data by performing the processing, the nozzles for ejecting the recording head and the recording medium arranged in the predetermined direction of the ink, and ejects ink while relative movement in the direction intersecting the predetermined direction that An image processing method for recording an image by
A recording step in which the recording head records a test pattern using data for recording a uniform dot pattern;
A reading step of reading the test pattern recorded in the recording step,
A generation step of generating a use threshold matrix used for recording based on the result read in the reading step;
Have
In the use threshold matrix, an average value of threshold values corresponding to nozzles corresponding to low density positions where the density of the test pattern is lower than the surroundings in the predetermined direction corresponds to non-low density positions that are not the low density positions. In the ejection data corresponding to the gradation of the intermediate density, which is lower than the average value of the threshold values corresponding to the nozzles, non-printed pixels are consecutive in the row of pixels in which the nozzles corresponding to the low density position are used for printing. The threshold value corresponding to the position of each pixel is set so that the average number of times is higher than the average number of times non-printed pixels continue in the pixel row in which the nozzle corresponding to the non-low density position is used for printing. An image processing method characterized by being provided .