JP2014188687A - Inkjet printer, inkjet printing and printing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an excellent image quality more than a conventional image quality, by making both compatible in restraint of density irregularity caused by displacement of a forming position of ink and excellent granularity.SOLUTION: An inkjet printer comprises: a halftone processing part for generating a first halftone by applying a first tither mask having a blue noise characteristic to a bit map image of regulating a recording amount of the ink belonging to a first group and generating a second halftone by applying a second tither mask having a characteristic in which a variation in the adjacent number of a halftone dot by displacement of a forming position of the halftone dot on printed matter is smaller than when applying the first tither mask, to the bit map image of regulating the recording amount of the ink belonging to a second group; and a discharge control part for realizing printing by discharging the ink to the printed matter from a nozzle possessed by a printing head based of the first halftone and the second halftone.

Description

  The present invention relates to an inkjet printer, inkjet printing, and a printing method.

  In an ink jet printer, halftone dots with ink (or ink, the same shall apply hereinafter) are difficult to be visually recognized in printing results, that is, halftone dots have a high granularity (graininess is not noticeable). The tone dots are dispersed and arranged, and for example, halftone processing is performed by a halftone method having blue noise characteristics as defined by Non-Patent Document 1. The blue noise characteristic means, for example, a characteristic that the spatial frequency of an image in which halftone dots are formed in order to reproduce an image having a constant gradation value has almost no component below a predetermined frequency. The human eye is sensitive to low frequency components below a certain level, but the visibility of high frequency components is not high. For this reason, an image having a blue noise characteristic is felt to be smooth (high graininess) with high image quality.

  Applicants have proposed two methods: halftone processing means using a blue noise dither mask having blue noise characteristics, and halftone means using a pair dot suppression dither mask in which the number of adjacent ink dots does not vary due to misalignment of ink dot formation on the print medium. Has been filed for a printing apparatus that includes at least one, and performs halftone processing using a paired dot suppression dither mask for printing on a printing medium in which cockling unevenness occurs (see Patent Document 1).

A printer is an output device that makes a hard copy record of data using a sequence of discrete graphic characters belonging to one or more predetermined character sets as a main format (JIS X0012-). 1990). In many cases, the printer can also be used as a plotter. A plotter is an output device (JIS X0012-1990) that directly creates a hard copy record of data in a two-dimensional graphic format on a removable medium.
An ink jet printer is a non-impact printing device in which characters are formed by jetting ink particles or droplets on paper (JIS X0012-1990).
A half-tone dot is an individual element constituting a gradation. Halftone dots can have a variety of shapes, such as squares, circles, and ellipses.

JP 2012-223594 A

"Digital halftoning" (by Robert Ulichney)

  In an inkjet printer, the printed material may bend due to the ink being absorbed. Such deflection is called “cockling”. When cockling occurs, the distance between the print head and the printing material is not constant, and the halftone dot forming position on the printing material is displaced. As the displacement of the halftone dot formation position on the printing material (deviation from the position to be originally formed), halftone dots are formed when the print head moves in the forward direction and when the print head moves in the backward direction. In bi-directional printing, a shift that occurs between dots formed during forward movement and dots formed during backward movement also applies. Also known is a shift caused by a so-called multi-pass printing method in which a single raster is formed by a plurality of main scans (movement of the print head). Further, such a shift in the formation position may also occur depending on the accuracy of attachment of ink ejection holes (hereinafter referred to as nozzles) included in the print head.

  Such a deviation in the formation position changes the coverage of the ink to cover the printing material as compared with the case where the deviation does not occur. Etc.) (see also the description regarding FIGS. 9 and 10 of Patent Document 1 as appropriate). In view of such problems, the above-mentioned Patent Document 1 generates cockling unevenness by performing a halftone process using a paired dot suppression dither mask when printing on a print medium (printed material) that generates cockling unevenness. Is suppressed.

  However, when halftone processing using a paired dot suppression dither mask is performed, there still remains a problem that the granularity of the printed result is lower than when halftone processing using a blue noise dither mask is performed. In other words, in the past, with regard to the printing results on a single substrate, it was an alternative to give priority to suppression of cockling unevenness at the expense of graininess or to give priority to improvement of graininess. There was room for further ingenuity to fully satisfy the requirements of

  The present invention has been made in order to solve at least the above-described problems, and realizes better image quality than before by coexisting suppression of density unevenness due to deviation in ink formation position and good graininess. An inkjet printer, inkjet printing, and a printing method that can be performed are provided.

  One aspect of the present invention is that an ink jet printer applies a first dither mask having a blue noise characteristic to a bitmap image that defines a recording amount of ink belonging to the first group, whereby a first half A second dither mask that generates a tone and has a characteristic that the variation in the number of adjacent halftone dots due to a shift in the formation position of the halftone dots on the printing material is less than that when the first dither mask is applied. By applying the recording amount of the ink belonging to the second group to the bitmap image that defines the halftone processing unit that generates the second halftone, and the first halftone and the second halftone, And a discharge control unit that realizes printing by discharging ink from the nozzles of the print head to the printing material.

  Halftone processing using the first dither mask having blue noise characteristics is effective in improving graininess. On the other hand, the halftone using the second dither mask has a characteristic that the variation in the number of adjacent halftone dots due to the shift of the halftone dot formation position on the printing material is less than that in the case where the first dither mask is applied. The treatment is effective in suppressing density unevenness (such as the above-mentioned cockling unevenness) caused by the positional deviation. In the present invention, these two processes are switched according to the type of ink. That is, when printing is performed using the ink belonging to the first group and the ink belonging to the second group, the halftone dots generated by the halftone process using the first dither mask and the second tone are formed on the substrate. Halftone dots generated by halftone processing using a dither mask are mixed. For this reason, as a whole of the printing result on the printing material, the suppression of uneven density due to the deviation of the formation position and the improvement of the graininess are compatible with each other in an appropriate balance, and the image quality becomes better than before.

In one aspect of the present invention, the ink belonging to the second group may be an ink having a lightness lower than that of the ink belonging to the first group.
It can be said that the ink having a low lightness is more conspicuous in density unevenness due to the deviation of the ink formation position than the ink having a high lightness. That is, the halftone process using the second dither mask is executed for the ink in which the cockling unevenness is easily noticeable, and the halftone process using the first dither mask is performed for the ink in which the cockling unevenness is not so conspicuous. Execute. Therefore, density unevenness (such as the above-mentioned cockling unevenness) due to the deviation of the formation position can be accurately suppressed, and an image quality with good graininess can be obtained.

  One aspect of the present invention is that the halftone processing unit is more susceptible to variations in the number of adjacent halftone dots due to a shift in the formation position of the halftone dots on the printed material than when the second dither mask is applied. A third ditone mask is generated by applying a third dither mask having less characteristics to a bitmap image that defines the recording amount of ink belonging to the third group, and the ejection control unit The above printing may be realized based on one halftone, second halftone, and third halftone. That is, the number of branches of the halftone process (the dither mask to be used) corresponding to the type of ink is not limited to two, and may be three, for example. According to this configuration, by switching the dither mask more finely according to the characteristics of each ink, in the printing result using the ink belonging to the first group, the ink belonging to the second group, and the ink belonging to the third group, A better image quality can be obtained.

  In one aspect of the present invention, the ink belonging to the third group may be an ink having a lightness lower than that of the ink belonging to the second group. According to this configuration, the halftone process using the third dither mask is performed on the ink belonging to the third group in which the cockling unevenness or the like is most noticeable, and then the cockling unevenness or the like is easily noticeable. Halftone processing using the second dither mask is executed for the ink belonging to the group, and halftone processing using the first dither mask is executed for the ink belonging to the first group where the cockling unevenness or the like is least noticeable. be able to.

  The technical idea according to the present invention is not realized only in the form of an ink jet printer, but may be embodied by other things. In addition, an invention of a method (printing method) including steps corresponding to the characteristics of the ink jet printer according to any one of the above-described aspects, an invention of a printing program for causing a predetermined hardware (computer) to execute the method, The invention of the recorded computer-readable recording medium can also be grasped. Further, the ink jet printer may be realized by a single device or may be realized by a combination of a plurality of devices. When the configuration of the ink jet printer is realized by a plurality of devices, they can be referred to as ink jet printing or an ink jet system.

FIG. 2 is a diagram schematically showing a hardware configuration and a software configuration. 6 is a flowchart illustrating print control processing. It is a flowchart which shows a halftone process. It is a figure for demonstrating halftone processing typically. It is a flowchart which shows the production | generation method of a pair dot control dither mask. It is a figure which shows the relationship between the gradation value S and a pair dot target value. It is a flowchart which shows the halftone process concerning a modification.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
1. FIG. 1 schematically shows a hardware configuration and a software configuration according to the present embodiment. FIG. 1 shows a personal computer (PC) 40 and a printer 10. The printer 10 corresponds to an ink jet printer. A system including the PC 40 and the printer 10 may be regarded as a printing apparatus or inkjet printing. The printer 10 includes a control unit 11 for controlling printing processing (liquid ejection processing). In the control unit 11, the CPU 12 expands the program data 14 a stored in the memory such as the ROM 14 to the RAM 13 and performs a calculation according to the program data 14 a under the OS, whereby firmware for controlling the own device is obtained. Executed. The firmware is a program for causing the CPU 12 to execute functions such as the print control unit 17. Further, the print control unit 17 has functions such as a halftone processing unit 17a and a discharge control unit 17b. Each of these functions will be described later.

Here, an image is a photograph, picture, illustration, figure, character, or the like that is visible to the human eye and that appropriately represents the original shape, color, and perspective. Image data means digital data representing an image. Examples of the image data include vector data and bitmap images. Vector data refers to image data stored as a set of commands and parameters that express geometric figures such as straight lines, circles, and arcs. A bitmap image (bit-mapped image) is image data described by an array of pixels. A pixel is a minimum element constituting an image to which a color or luminance can be independently assigned. Also, half-tone is an image composed of points with different screen line numbers, sizes, shapes, or densities. Halftones are generated by dithering, error diffusion, and the like.
In the following, image data that expresses an image arbitrarily designated by the user for printing on the printer 10 will be referred to as designated image data.

  The print control unit 17 inputs designated image data from, for example, a PC 40 or a storage medium inserted into the printer 10 from the outside, and generates a halftone from the designated image data. Printing based on the halftone can be realized. The storage medium inserted from the outside into the printer 10 is, for example, a memory card MC, and the memory card MC is inserted into a slot portion 19 formed in the housing of the printer 10. In addition, the print control unit 17 inputs designated image data from various external devices such as a scanner, a digital still camera, a portable terminal, and a server connected via a network. You can also.

  The printer 10 includes a carriage 20. The carriage 20 has a cartridge 21 for each of a plurality of types of ink. In the example of FIG. 1, a cartridge 21 corresponding to each ink of cyan (C), magenta (M), yellow (Y), black (K), and gray (Lk) is mounted. However, the specific types and number of liquids used by the printer 10 are not limited to those described above. For example, various inks such as light cyan, light magenta, orange, green, light gray, white, metallic, precoat liquid, etc. Liquid can be used. Further, the cartridge 21 may be installed at a predetermined position in the printer 10 without being mounted on the carriage 20.

The carriage 20 includes a print head 22 that ejects (discharges) ink supplied from each cartridge 21 from a number of nozzles. The print head 22 corresponds to a permanent head and is a mechanical part or an electric part of the printer main body that continuously or intermittently generates ink droplets (JIS Z8123-1: 2013).
The print control unit 17 generates a drive signal for driving and controlling the transport mechanism 16, a carriage motor (not shown), and the print head 22 based on the halftone.

  In the print head 22, piezoelectric elements for ejecting ink droplets (halftone dots or simply dots) from the nozzles are provided corresponding to the respective nozzles. The piezoelectric element is deformed when the drive signal is applied, and ejects dots from the corresponding nozzle. The transport mechanism 16 includes a paper feed motor and a paper feed roller (not shown), and is driven and controlled by the print control unit 17 to transport the printed material along the feed direction. The feed direction is the direction of the geometric vector related to the movement of the printed material when the printed material and the permanent head face each other.

  A printing substrate (print substrate) is a material that holds a printed image. The shape is generally rectangular, but there are round shapes (for example, optical disks such as CD-ROM and DVD), triangles, quadrangles, polygons, etc. Includes all paper and board varieties and processed products described in "Terminology". Specifically, sheet paper, roll paper, paperboard, paper, non-woven fabric, cloth, ivory, asphalt paper, art paper, colored paperboard, colored fine paper, inkjet paper, printing paper, printing paper, printing paper A, Printing paper B, printing paper C, printing paper D, India paper, thin leaf printing paper, thin leaf Japanese paper, back carbon paper, air mail paper, sanitary paper, embossed paper, OCR paper, offset paper, cardboard, chemical fiber paper, Processed paper, stencil paper, paper pattern, piece paper and craft paper, wallpaper base paper, paper thread base paper, paper string base paper, pressure-sensitive copying paper, photosensitive paper, thermal paper, crust paper, paperboard for cans, yellow paperboard, fake leather paper, ticket paper, Functional paper, cast coated paper, Kyoto paper, local paper, metallized paper, metal foil paper, glassine, gravure paper, kraft paper, craft stretch paper, craft ball, crepe paper, lightweight coated paper, insulation for cables , Decorative board base paper, building material base paper, kent paper, polishing base paper, synthetic paper, synthetic fiber paper, coated paper, capacitor paper, hybrid paper, reprint paper, bleached kraft paper, diazo photosensitive paper, paper tube base paper, magnetic recording paper, paper container Paperboard, dictionary paper, shading paper, heavy-duty kraft paper, pure white roll paper, securities paper, shoji paper, high-quality paper, information paper, food container base paper, book paper, calligraphy paper, white paperboard, white ball, newspaper roll , Blotting paper, water-soluble paper, drawing paper, streaked kraft paper, square eye paper, speaker cone paper, electrostatic recording paper, physiological paper, paper cotton paper, laminated board base paper, gypsum board base paper, adhesive paper base paper, semi-quality paper Paper, Cement bag paper, Ceramic paper, Solid fiber board, Tarfelt base paper, Tarpaulin paper, Alkali-resistant paper, Fireproof paper, Acid-resistant paper, Oil-proof paper, Towel paper, Floor paper, Corrugated cardboard, Cardboard base paper, Ground Paper, chipball, medium-quality paper, neutral paper, dust paper, matte art paper, tea-back paper, tissue paper, electrical insulation paper, classic paper, paste paper, transfer paper, toilet paper, statistics card paper, copy Board base paper, coated printing paper, coated paper base paper, bird cub, tracing paper, medium base paper, napkin base paper, flame retardant paper, NIP paper, tag paper, adhesive paper, carbonless paper, release paper, hatron paper, baryta Paper, paraffin paper, wax paper, vulcanized fiber, semi-paper, PPC paper, writing paper, fine coating printing paper, foam paper, continuous slip paper, copy paper, press board, moisture-proof paper, proof paper, waterproof paper, rust-proof paper , Unwrapping paper, bond paper, Manila ball, Mino paper, Shoin paper, milk carton base paper, imitation paper, oil paper, Yoshino paper, rice paper, cigarette paper, rye Liner, liner, sulfate paper, bifurcated kraft paper, roofing base paper, filter paper, Japanese paper, varnish paper, one-pump, lightweight paper, air-dried paper, wet strong paper, ashless paper, acid-free paper, finish paper or paperboard, double layer Paper or paperboard, three-layer paper or paperboard, multilayer paper or paperboard, non-size paper, size paper, woofer paper, wood grain paper or paperboard, machine finish paper or paperboard, machine glossy paper or board, plate glossy paper or board, Friction glossy finished paper or paperboard, calendered paper or paperboard, super calendered paper, lamin (paper or paperboard), single-sided colored paper or paperboard, double-sided colored paper or paperboard, twin wire paper or paperboard, rug paper, all rug paper, Mechanical pulp paper or paperboard, mixed straw pulp paper or paperboard, water-finished paper or paperboard, chipball, laminated chipball, millboard, high gloss millboard, etc. Paperboard, mechanical pulpboard, brown mechanical pulpboard, brown mixed pulpboard, artificial leather board, asbestos board, felt board, tar brown paper, water leaf paper, surface sized paper, press pan, press paper, wrinkled finish paper, beam Laminated ivory, blade coated paper, roll coated paper, gravure coated paper, size press coated paper, brush coated paper, air knife coated paper, extrusion coated paper, dip coated paper, curtain coated paper, hot Melt coated paper, solvent coated paper, emulsion coated paper, bubble coated paper, imitation art paper, Bible paper, poster paper, packaging tissue, base paper, carbon base paper, diazo photosensitive paper base paper, photographic printing paper base paper, Frozen food paper base: Direct contact paper, Frozen food paper base: Non-contact paper, Safety paper, Banknote paper, Insulating paper or paperboard, Laminated insulator , Electrical insulation paper for cables, paperboard for shoe soles, textile paper tube paper, stencil paper or paperboard, paperboard for pressing, paperboard for bookbinding, paperboard for clothing boxes, paper-type paper, recording paper, craft liner, tested liner, craft Tension liner, waste paper liner, envelope paper, paperboard for folding box, paperboard for coated folding wallbox, paperboard for bleached pulp backing folding box, typewriter paper, photocopied paper, spirit copy paper, calendar roll paper, corrugated paper, corrugated paper Processed paper, corrugated paper, double-layer tar paper, reinforced double-layer tar paper, upholstery paper or paperboard, cloth paper or paperboard, reinforced paper or paperboard, laminated paperboard, carton compact, upholstery, pulp molded product, wet Crepe, search card, carbon paper, multi-copy foam paper, back carbon foam paper, carbonless foam paper, envelope, postcard , Postcards with pictures, postal letters, postal letters with pictures, etc. In particular, functional paper is not limited to plant fibers, and a wide range of materials such as inorganic, organic, and metal fibers are used. Including, but not limited to, those mainly used as materials in advanced fields such as information, electronics, and medical use.

  By controlling the drive of the carriage motor by the print control unit 17, the carriage 20 (and the print head 22) moves (main scan) along the direction (scanning axis direction) intersecting the feeding direction. Accordingly, ink is ejected from each nozzle of the print head 22. Thereby, dots adhere to the substrate, and an image based on the halftone is reproduced on the substrate. The above “intersection” means orthogonal. However, the term orthogonal in the present specification does not mean only a strict angle (90 °), but includes an angle error that is acceptable in terms of product quality.

  Such a printer 10 can be said to be a serial printer. A serial printer is a printing device that prints one character at a time (JIS X0012-1990). The printer 10 can be expressed as a printer (referred to as a “serial ink jet printer”) that is a serial printer and an ink jet printer. The printer 10 further includes an operation panel 15. The operation panel 15 includes a display unit (for example, a liquid crystal panel), a touch panel formed in the display unit, various buttons and keys, accepts input from the user, and displays a necessary user interface (UI) screen on the display unit. Or display.

2. Print Control Process FIG. 2 is a flowchart showing a print control process for printing an image performed under the above-described configuration.
In step S100, the print control unit 17 receives an image print instruction from the user via the operation panel 15, and designates from any information source such as the PC 40, the storage medium, or the external device as described above according to the print instruction. Get image data. In other words, the user operates the operation panel 15 to arbitrarily designate an image to be printed by the printer 10 via the UI screen displayed on the display unit, and instructs the printer 10 to print the designated image. Of course, the user can also issue an image printing instruction by operating a portable terminal or the like that can remotely control the printer 10 from the outside. In addition, the user can instruct the printer 10 together with various print conditions such as the number of copies, paper size, print resolution, and the like.

  In step S110, the print control unit 17 executes resolution conversion processing for designated image data. However, the print control unit 17 first executes rasterization processing of designated image data as necessary. That is, when the format of the designated image data acquired in step S100 is not bitmap image but vector data, such vector data is rasterized and converted to raster format image data, that is, bitmap image. If the format of the designated image data acquired in step S100 is a bitmap image, there is no need for rasterization processing. In the resolution conversion process, the print control unit 17 converts the number of pixels of the designated image data as the bitmap image so as to match the required number of pixels in the X and Y directions calculated according to the paper size and the print resolution. . Here, as an example, the X direction corresponds to the scanning axis direction, and the Y direction corresponds to the feeding direction.

  In step S120, the print control unit 17 executes a color conversion process for the designated image data. That is, the color system of the designated image data is converted to the ink color system used by the printer 10. For example, when the designated image data is RGB data that expresses the color of each pixel as a gradation value (RGB value) for each of red (R), green (G), and blue (B), the RGB value for each pixel is CMYKLk. Each tone value is converted into a CMYKLk value. Each of RGB and each of CMYKLk are expressed by 256 gradations from 0 to 255, for example. Such a color conversion process can be executed by referring to an arbitrary color conversion lookup table that defines a plurality of correspondences between RGB values and CMYKLk values. As a result of step S120, a bitmap image defining the corresponding ink recording amount for each pixel is obtained for each CMYKLk.

  In step S130, the halftone processing unit 17a performs halftone processing (halftoning) on the designated image data (bitmap image for each CMYKLk) after color conversion. In the present embodiment, the halftone processing unit 17a selects a dither mask selected according to the ink type from a plurality of types of dither masks D1, D2,... (FIG. 1) stored in advance in a predetermined memory (for example, the ROM 14). Halftone processing is executed by the dithering used.

FIG. 3 is a flowchart showing the contents of the halftone process (step S130).
In step S200, the halftone processing unit 17a selects a bitmap image (for example, a bitmap image of C ink) related to one ink from the bitmap images for each CMYKLk.

  In step S210, the halftone processing unit 17a determines whether the ink corresponding to the currently selected bitmap image is ink belonging to the first group or ink belonging to the second group. If the ink belongs to the first group, the process proceeds to step S220. If the ink belongs to the second group, the process proceeds to step S230. Here, the ink belonging to the first group means an ink of a color in which density unevenness (such as the cockling unevenness) is difficult to be visually recognized by the user when the formation position is shifted, for example, each ink of CMYKLk Among these, Y ink corresponds. On the other hand, the ink belonging to the second group means a color ink in which unevenness in density (such as the above-mentioned cockling unevenness) is easily visible to the user when the formation position shift occurs, for example, for each ink of CMYKLk Among these, C ink, M ink, K ink, and Lk ink are applicable.

  Accordingly, if the currently selected bitmap image is related to Y ink, the process proceeds to step S220, whereas if it is related to any ink of CMKLk, the process proceeds to step S230. It can be said that the ease of visually recognizing the density unevenness depends on the brightness of the ink itself. Therefore, it can be said that the ink belonging to the second group is an ink having a lightness lower than that of the ink belonging to the first group.

  In step S220, the halftone processing unit 17a applies dithering by applying the blue noise dither mask D1 to the currently selected bitmap image (bitmap image defining the recording amount of ink belonging to the first group). By executing this, a halftone (first halftone) that defines halftone dot formation (dot on) or non-formation of halftone dots (dot off) is generated for each pixel. The blue noise dither mask D1 corresponds to a first dither mask having blue noise characteristics.

  On the other hand, in step S230, the halftone processing unit 17a applies the pair dot control dither mask D2 to the currently selected bitmap image (the bitmap image that defines the recording amount of ink belonging to the second group). By executing dithering, a halftone (second halftone) that defines halftone dot formation (dot on) or non-formation of halftone dots (dot off) is generated for each pixel. The paired dot control dither mask D2 is a second dither mask having characteristics in which the variation in the number of adjacent halftone dots due to the shift in the formation position of the halftone dots on the printed material is less than that when the blue noise dither mask is applied. It corresponds to.

  In step S240, the halftone processing unit 17a determines whether or not all the bitmap images for each CMYKLk have been selected in step S200. If all the bitmap images have been selected, the flowchart of FIG. Go to). On the other hand, if a bitmap image relating to unselected ink remains, the process proceeds to step S200, where a bitmap image relating to any one of the unselected inks is selected, and step S210 and subsequent steps are repeated.

  FIG. 4 schematically illustrates the processing content of FIG. According to FIG. 4, a bitmap image CD that defines the recording amount of C ink for each pixel, a bitmap image MD that defines the recording amount of M ink for each pixel, and a bit that defines the recording amount of K ink for each pixel. The bitmap images LkD that define the recording amounts of the map images KD and Lk ink for each pixel are halftones CHT, MHT, KHT, and LkHT, respectively, by the processing in step S230 (dithering using the pair dot control dither mask D2). Has been converted to In addition, the bitmap image YD in which the Y ink recording amount is defined for each pixel is converted into the halftone YHT by the process of step S220 (dithering using the blue noise dither mask D1). Note that the halftone generated in steps S220 and S230 is not limited to binary data defining dot on / off. When the print head 22 can eject dots of different sizes (for example, small dots, medium dots, large dots, etc.) with different ink amounts per dot, halftone is formed with large dots and medium dots. Alternatively, it may be 4-gradation data defining small dot formation or non-dot formation.

  In step S140, the ejection control unit 17b performs a process of rearranging the halftones CHT, MHT, YHT, KHT, and LkHT for each ink type obtained as a result of step S130 in the order to be transferred to the print head 22. According to the rearrangement process, each nozzle defined in the halftone is determined by which nozzle and at which timing according to the pixel position and the ink type. The ejection control unit 17b sequentially transmits the raster data (an example of a halftone) after the rearrangement process to the print head 22, thereby causing the nozzles to eject dots. Thereby, an image is reproduced on the substrate based on the halftone.

3. Description of Pair Dot Control Dither Mask The principle of pair dot control and the method of generating a pair dot control dither mask in this embodiment will be described.
3-1. Principle of pair dot control When a blue noise dither mask, which is a typical dot dispersion type dither mask that disperses dots as much as possible, is used, if there is a misalignment in the dot formation position, it is compared to a state where there is no misalignment. As a result, the dots are adjacent to each other or overlap each other, so that the coverage of the printed material by the ink decreases. Thereby, the brightness of an image becomes high and a change arises in a color. In contrast, the pair dot control dither mask is set to have a higher probability of forming dots in two adjacent pixels (pair pixels) than the blue noise dither mask. As a result, even when there is a deviation in the dot formation position, the variation in the coverage is smaller than in the case where there is no positional deviation (as compared with the case where a blue noise dither mask is used), and the brightness and color of the image. It has a characteristic that it is difficult to cause the change.

In order to give the above characteristics to the pair dot control dither mask, the pair dot control method described below is used in this embodiment. A pair dot is a name of both dots when a dot is formed in a pair pixel. As will be described later, the threshold arrangement of the pair dot control dither mask is set so that pair dots are generated with a significant probability even in a low density region. The significant probability that a pair dot occurs is a probability set as follows. In the paired dot control dither mask, when the gradation value of the bitmap image is in the range of 0 to 127/255, if the probability that the dot is arranged in one pixel is k (0 ≦ k ≦ 1), the dot is formed. The probability K of forming a dot in either the pixel adjacent to the right in the X direction (scanning axis direction) or the pixel adjacent in the Y direction (feeding direction) is about 0.8 × k ^2. It is said that. A state in which dots are formed in pixels adjacent to the right in the X direction of one pixel in which dots are formed, and dots formed in pixels adjacent in the Y direction under the right adjacent pair dots and one pixel in which dots are formed These states are referred to as lower adjacent pair dots, respectively.

Here, the gradation value is treated as corresponding to the probability that the dot is turned on (formed). If the bitmap image to be halftone processed is a uniform image having a gradation value of 26/255, the number of dots arranged is about 1 in 10 pixels (k = 0.1). In such a situation, the probability K of forming dots in the paired pixels is about K = 0.8 × k ² ≈0.008 in the paired dot control dither mask. Note, if you set randomly by the random number the arrangement of threshold 0-255 in the dither mask, the probability K for paired pixel dots are formed is substantially coincident to k ^2. In the blue noise dither mask with high dispersibility, priority is given to dot dispersibility in the low density region, and dots are formed in the paired pixels, and the probability is as close to 0 as possible.

The reason why the probability of forming a pair of dots in the pair dot control dither mask is made close to k ² is based on the following knowledge. That is, even if the dot interval is dispersed and arranged as far as possible with a blue noise dither mask or the like, if the deviation amount of the dot formation position is somewhat large, the probability that dots in a specific direction are adjacent to each other and become a pair dot is k. The knowledge of convergence to ² was obtained. This is because if the amount of deviation is large, two pixels that should originally be at a distance away from each other are adjacent to each other. If the distance between the two pixels is sufficiently large, the correlation between the presence or absence of dot formation in both pixels decreases, so the probability that dots are simultaneously formed in both pixels is simply the tone value of both (the probability of dot formation) k) is a value ^{k 2} obtained by multiplying. Accordingly, if the pair dot generation rate in a state where there is no shift is brought close to k ² in advance, the pair dot generation rate does not change much regardless of the shift, and the variation in the coverage can be suppressed. .

The coefficient 0.8 at the probability K = 0.8 × k ² in which dots are formed in the pair pixels is for adjusting the probability of pair dots. If the coefficient is 0.8, pair dots are generated. This means that the rate is suppressed to 80%. The coefficient can be set as appropriate within a predetermined range. For example, if the coefficient is in the range of 0.8 to 1.2, the fluctuation of the probability of occurrence of paired dots due to the deviation of the dot formation position can be suitably suppressed. This is desirable from the viewpoint of suppressing fluctuations in the dot generation probability.

3-2. Method for Generating Pair Dot Control Dither Mask FIG. 5 is a flowchart showing an example of a method for generating a pair dot control dither mask used in the present embodiment. The flowchart is realized by a function of a program for generating a pair dot control dither mask in predetermined hardware. In this example, providing a blue noise dither mask, from the blue noise dither mask, generating a dither mask close the probability that both dots are formed in the pair of pixels in k ² (pair dot control dither mask). Hereinafter, the dither mask being created is referred to as a working mask.

  First, a blue noise dither mask is prepared (step S300). In this example, a blue noise dither mask having a size of 64 × 64 pixels is used. In the blue noise dither mask of this example, 256 threshold values from 0 to 255 are stored in a matrix having a size of 64 × 64 pixels. Next, a process for counting the number of pair dots for each gradation value over the entire gradation range is performed for the current working mask (step S310). Specifically, this process is a process of counting the right adjacent pair dot number RPD [1, 2,... 127] and the lower adjacent pair dot number UPD [1, 2,. is there. In the following description, when parentheses are used as in (S), the value at the gradation value S is indicated, and when [] is used as in [a,. It is assumed that the sequence from a to x is represented. Further, the arrangement from the gradation range a to x is expressed as [a: x].

  Since all the threshold values are known for the working mask, the dot formation position at each gradation value can be examined in the gradation value range of 1 to 127/255. For this reason, it is easy to count the right adjacent pair dot number RPD (S) and the lower adjacent pair dot number UPD (S) for each gradation value S. Here, the count of the number of paired dots is limited to the gradation value of 1 to 127/255. The paired dot control dither mask used in this embodiment, that is, the number of paired dots in the gradation range of 1 to 127/255. This is to generate a mask having the occurrence probability with a predetermined characteristic. As the gradation value S increases, the number of pair dots also approaches the probability of occurrence here in the blue noise dither mask, so instead of counting the number of adjacent pair dots for the entire range, the gradation values 1 to 127 / The occurrence probability of pair dots may be adjusted within the range of 255. Of course, the method described below is also applicable to the case where the number of pair dots is counted and the probability of occurrence is adjusted for the entire gradation range.

In step S310, the number of right adjacent pair dots RPD [1: 127] and the number of lower adjacent pair dots UPD [1: 127] in a predetermined gradation range (here, 1 to 127/255) are counted, and then each gradation value is counted. It is determined whether or not the number of paired dots for each S is within the target range M (S) (step S320). The target range M (S) is a range set as follows. If the dither mask has a so-called white noise characteristic, dots are generated randomly, and if the probability of forming a dot in one pixel is k, the right or lower adjacent pixel In addition, the probability that dots are formed (this is referred to as the occurrence probability of pair dots) is k ² , respectively. When the gradation value of the image is 1,
k = 0.00392156 (= 1/255)
And the probability of occurrence of paired dots is
k ² = 0.0000154
It becomes. Therefore, when it is assumed that dots are randomly formed, a value H (hereinafter referred to as a predicted value) H predicted that a pair pixel exists in 64 × 64 pixels is:
H = k ² × 4096 = 0.063 ≒ 0
It is. This calculation is repeated in advance in the range of gradation values 1 to 127/255 to obtain a theoretical predicted value H [1: 127] of a paired dot, and this is multiplied by a coefficient of 0.8 for each gradation. The target value m [1: 127] of the paired dot in the value S is obtained in advance. In this example, the target value m (S) has a width of ± 20%, and this is called a target range M (S).

  FIG. 6 shows paired dot predicted values H [1:32] and target values m [1:32] when the gradation value S is 1 to 32. In FIG. As shown in the figure, the gradation value S = 10, the prediction value H (10) = 6, the target value m (10) = 5, the gradation value S = 20, the prediction value H (20) = 25, the target value. It can be seen that m (20) = 20.

  In step S320, the theoretical pair dot target range M [1: 127] thus obtained is compared with the right adjacent pair dot number RPD [1: 127] and the lower adjacent pair dot number UPD [1: 127]. To do. As a result of comparison, if it cannot be determined that both the number of paired dots RPD [1: 127] and URD [1: 127] are within the target range M [1: 127], the next step in the work mask is performed. Among the threshold values, a process of randomly replacing an appropriate number of threshold values (for example, two threshold values) is performed (step S330).

  When the threshold value in the working mask is changed, the number of pair dots in each gradation value changes, so the number of pair dots is corrected by changing the threshold value (step S340). Since the number of paired dots changes only within the range of gradation values corresponding to the replaced threshold value, it is not counted again in the gradation range of 1 to 127/255. And the threshold value q (p <q) are interchanged, only the right adjacent pair dot number RPD [p: q] and the lower adjacent pair dot number UPD [p: q] may be recounted. The threshold value to be replaced is selected at random. However, since the generation characteristics of the paired dots are to be adjusted in the range of gradation values 1 to 127/255, at least one of the threshold values to be replaced is within this range. It is desirable to set the threshold value in the range.

The number of pair dots thus counted is checked, and then it is determined whether or not the pair dot characteristics have been improved (step S350). Here, whether or not the paired dot characteristics are improved is determined as follows.
(A) If the right and lower adjacent pair dot numbers RPD [p: q] and UPD [p: q] are close to k ² by replacing the threshold values, it is determined that the number has improved.
(B) By swapped threshold, right and lower adjacent pairs dots UPD [p: q],: when the [p q] one of the other approaches k ² is not changed, improvement and to decide.
(C) When the gradation range [p: q] is improved or partially deteriorated, the number of pair dots generated in each gradation value of the gradation range and the predicted value at the gradation value If the sum of the differences is small, it is judged as an improvement.

If the above determination is made and it is determined that the paired dot characteristics are not improved, the process returns to step S330, and the process is executed again from the process of randomly changing the threshold value. If the threshold is replaced by two thresholds, and the combination is the entire range of gradations,
₄₀₉₆ C ₂ ways will exist. Even if it is limited to the range of gradation value 1 to 127/255,
There will be ₂₀₄₈ C ₂ ways. Therefore, there are a considerable number of combinations of threshold value replacement, and although it takes a considerable amount of time to complete all cases, replacement is found to improve the paired dot characteristics if performed sequentially (step S350, “YES”). ).

Therefore, if it is determined that the paired dot characteristics have been improved, it is next determined whether or not the graininess characteristics are satisfactory (step S360). Here, that the graininess characteristic is not a problem means that the graininess index GI shown below is within the target range, or is not within the target range, but has been improved from before the threshold replacement. ing. Since the granularity index GI is a known technique (for example, Japanese Patent Application Laid-Open No. 2007-15359), a detailed description is omitted, but the power spectrum FS is obtained by Fourier transforming the image to obtain the obtained power spectrum FS. This is an index obtained by adding a weight corresponding to the sensitivity characteristic VTF (Visual Transfer Function) with respect to the visual spatial frequency possessed by humans and integrating at each spatial frequency. Various formulas have been proposed as empirical formulas that give such VTF, and the following formula (1) shows typical empirical formulas. The variable L represents the observation distance, and the variable u represents the spatial frequency. The graininess index GI can be calculated by the calculation formula shown in the following formula (2) based on the VTF. The coefficient a _L is a coefficient for matching the obtained value with human senses. As is clear from the calculation method, the granularity index GI can be said to be an index indicating whether or not a human feels that dots are conspicuous. It can be said that the smaller the value of the graininess index GI, the more difficult it is to visually recognize dots in the print image quality, and the more excellent in that respect.

（１）

(1)

（２）

(2)

  The initially prepared blue noise dither mask is configured so that the graininess index GI becomes the smallest value. However, when the threshold value is randomly changed in step S330, the graininess of the work mask becomes blue noise. Lower than the mask. Therefore, a target range of the graininess index GI is provided within a range that is acceptable in view of human visual characteristics, and it is determined whether there is no problem in view of this range. Of course, since the graininess index GI is a value determined for each gradation value, an upper limit value is prepared for each gradation value, and if the graininess index GI at each gradation value is less than or equal to this upper limit value, the graininess characteristic is What is necessary is just to judge that it is in the target range.

  If there is a problem with the graininess characteristic, that is, it is not within the target range and has not been improved compared to before the replacement of the threshold value (step S360, “NO”), the process returns to step S330. The above process is repeated from the threshold value replacement. As a result of repeating the processing of steps S330 to S360, when it is determined that the paired dot characteristics are improved and the graininess characteristics are satisfactory (steps S350 and S360: “YES”), the loop of steps S330 to S360 is temporarily performed. The process returns to step S320, and it is determined whether or not the pair dot generation characteristic is within the target range.

  If it cannot be determined that the pair dot generation characteristic is within the target range (step S320, "NO"), the processing from step S330 described above is repeated. In the process shown in FIG. 5, steps S320 to S360 are repeatedly executed while changing the threshold value until the condition is satisfied. Therefore, while the number of times that the processing from step S330 to S360 is executed (hereinafter referred to as the number of loops) is small, the upper limit value of the graininess index GI in step S360 is increased, and as the number of loops increases. Alternatively, a process of bringing the upper limit value closer to the final target value may be performed. In this way, by changing the upper limit value according to the number of loops, it is possible to prevent the granularity index GI from falling into a local minimum value.

  In this way, the loop processing of steps S330 to S360 is executed several times, and there is no problem in the graininess characteristics, and the right adjacent pair dot number RPD [1: 127] and the lower adjacent pair dot number UPD [1: 127] are targeted. If it can be determined that it falls within the range M [1: 127] (step S320, “YES”), the paired dot control dither mask is completed, and the working mask at that time is stored as a paired dot control dither mask (step S370). Then, the pair dot control dither mask generation routine (FIG. 5) ends. In the above description, whether or not the generation characteristics of the paired dots are within the target range is determined in the range of 1 to 127/255 out of the entire range of gradation values where the generation of dots is possible. The dot control dither mask may be performed in the gradation range in which the pair dot generation probability is to be controlled. For example, it may be performed only in a lower density range (tone range corresponding to dot generation probability k = 0 to 0.25, 0.2 to 0.5, etc.).

By the method described above, a pair dot control dither mask can be obtained based on the blue noise dither mask. This pair-dot control dither mask is based on a blue noise dither mask, so if you analyze the distribution of dots formed in a range where the gradation value of the image is low as a spatial frequency, it is a low frequency region where human visual sensitivity is high Has almost no ingredients. Therefore, it is possible to provide a dither mask that can realize high image quality. In addition, in the above-described pair dot control dither mask, the probability of occurrence of a pair dot in which dots are formed in adjacent pixels is such that the dot formation probability k at that gradation value is about k ² × 0.8. Has been. As a result, even if the dot formation position is deviated, the change in the coverage is suppressed, and the change in brightness and color (density unevenness) due to the deviation of the dot formation position is accurately suppressed.

4). As described above, according to the present embodiment, the printer 10 conforms to the blue noise dither mask (first dither mask) and the blue noise dither mask that are particularly effective for realizing high image quality (granularity improvement). Paired dots that can achieve high image quality and suppress fluctuations in the number of adjacent dots (fluctuation in the coverage) and uneven density (such as the above-mentioned cockling unevenness) even if there is a deviation in the dot formation position. And a control dither mask (second dither mask). When the printer 10 generates a halftone from a bitmap image relating to ink (ink belonging to the second group) in which density unevenness due to deviation in the dot formation position is relatively conspicuous, a paired dot control dither mask is used. When the halftone is generated from the bitmap image relating to the ink (ink belonging to the first group) in which the density unevenness is relatively inconspicuous, the blue noise dither mask is used to perform dithering for each. did. Therefore, for all ink types used when reproducing an image based on the designated image data on the printing material, when using a blue noise dither mask or when using a pair dot control dither mask. In comparison, suppression of density unevenness due to the shift in the formation position and improvement in graininess are compatible, and better image quality can be provided to the user.

  The specific examples of the ink belonging to the first group and the ink belonging to the second group are not limited to those described above. For example, when light gray (LLk) ink is used, the LLk ink may be handled as one of the inks belonging to the first group.

5. Modifications The present invention is not limited to the above-described embodiment, and can be implemented in various modes without departing from the gist thereof. For example, the following modifications are possible. The contents appropriately combined with the above-described embodiments and modifications are also within the scope of the present disclosure.

Modification 1:
FIG. 7 is a flowchart showing halftone processing according to the first modification. The halftone process shown in FIG. 7 is different from the halftone process shown in FIG. 3 in that three types of dither masks to be switched according to the type of ink are prepared instead of two types.
Step S400 is the same as step S200. In step S410, the halftone processing unit 17a determines whether the ink corresponding to the currently selected bitmap image is the ink belonging to the first group, the ink belonging to the second group, or the third group. It is determined whether it is ink.

  In this modification, the ink belonging to the first group means a kind of ink in which density unevenness (cooking unevenness or the like) is most difficult to be visually recognized when the formation position is shifted, and each of the CMYKLk inks. Among them, Y ink corresponds. The ink belonging to the second group means an ink having a color whose density unevenness is more easily visible than the ink of the first group when the formation position shift occurs, and among the CMYKLk inks, C ink, M Ink and Lk ink are applicable. The ink belonging to the third group means an ink having a color whose density unevenness is more easily visible than the ink of the second group when the above-described formation position shift occurs, and K ink corresponds to each of the CMYKLk inks. . If the ink belongs to the first group, the process proceeds to step S420. If the ink belongs to the second group, the process proceeds to step S430. If the ink belongs to the third group, the process proceeds to step S440. It can be said that the ink belonging to the third group is an ink having a lightness lower than that of the ink belonging to the second group.

  Step S420 is the same as step S220. In step S430, the halftone processing unit 17a applies the pair dot control first dither mask to the currently selected bitmap image (bitmap image defining the recording amount of ink belonging to the second group) and dithers. A halftone (second halftone) is generated by executing the ring. In step S440, the halftone processing unit 17a applies the paired dot control second dither mask to the currently selected bitmap image (a bitmap image that defines the recording amount of ink belonging to the third group) and dithers. A halftone (third halftone) is generated by executing the ring. Step S450 is the same as step S240.

  When both the paired dot control first dither mask and the paired dot control second dither mask are applied with the blue noise dither mask due to the variation in the number of adjacent halftone dots due to the shift of the halftone dot formation position on the printed material Although the dither mask has less characteristics, the paired dot control second dither mask has such characteristics more strongly. Specifically, when the coefficient for defining the pair dot target value m when generating the pair dot control first dither mask is set to 0.8, for example, the pair dot control second dither mask is generated. A coefficient for defining the target value m of the pair dot at that time is set to a value closer to 1.0 than 0.8 (for example, 0.9). Thereby, the variation in the number of adjacent halftone dots due to the shift in the formation position of the halftone dots on the printed material has less characteristics than the case where the paired dot controlled first dither mask is applied. Is obtained. The paired dot control second dither mask corresponds to an example of a third dither mask.

  As described above, according to the first modification, among the inks in which the density unevenness due to the deviation in the formation position is easily noticeable, particularly for the ink that makes the density unevenness conspicuous, the paired dot control is particularly effective in suppressing the density unevenness. A halftone is generated using the second dither mask. Therefore, the density unevenness of the ink can be reliably suppressed, and a good image quality can be provided to the user. The number of dither masks to be switched according to the type of ink may be more than three.

Modification 2:
The printer 10 is not limited to a serial type ink jet printer. This is because deviations in dot formation positions and density unevenness due to such deviations may occur in other types of printers such as thermal transfer printing apparatuses, thermal sublimation types, and line printers. Therefore, the printer 10 may be a printer of these other methods. A line printer is a printing device that prints characters for one line as a unit (JIS X0012-1990).

Modification 3:
The case where the process of FIG. 2 is executed by the printer 10 has been described as an example, but at least a part of the process may be performed on the PC 40 side. For example, the printer driver 41 may execute the processes of steps S100 to S130 according to a program, output the halftone obtained as a result of these processes to the printer 10, and cause the printer 10 to execute printing according to the halftone.

DESCRIPTION OF SYMBOLS 10 ... Printer, 11 ... Control unit, 12 ... CPU, 13 ... RAM, 14 ... ROM, 14a ... Program data, 15 ... Operation panel, 16 ... Conveyance mechanism, 17 ... Print control part, 17a ... Halftone processing part, 17b DESCRIPTION OF SYMBOLS ... Discharge control part, 19 ... Slot part, 20 ... Carriage, 21 ... Cartridge, 22 ... Print head, 40 ... PC, 41 ... Printer driver, CD, MD, YD, KD, LkD ... Bitmap image, CHT, MHT, YHT, KHT, LkHT ... halftone, D1 ... dither mask (blue noise dither mask), D2 ... dither mask (paired dot control dither mask)

Claims (6)

A first halftone is generated by applying a first dither mask having blue noise characteristics to a bitmap image that defines the recording amount of ink belonging to the first group, and halftone dots on a substrate to be printed The second dither mask having a characteristic that the variation in the number of adjacent halftone dots due to the shift in the formation position of the first dither mask is less than that in the case where the first dither mask is applied, defines the recording amount of the ink belonging to the second group. A halftone processing unit that generates a second halftone by applying to the bitmap image;
An ejection control unit that realizes printing by ejecting ink from the nozzles of the print head to the substrate based on the first halftone and the second halftone;
An ink jet printer comprising:   2. The ink jet printer according to claim 1, wherein the ink belonging to the second group is an ink having a lightness lower than that of the ink belonging to the first group. The halftone processing unit has a third dither having a characteristic that the variation in the number of adjacent halftone dots due to a shift in the formation position of the halftone dots on the printed material is smaller than that in the case where the second dither mask is applied. A third halftone is generated by applying a mask to a bitmap image that defines the recording amount of ink belonging to the third group,
The inkjet printer according to claim 1, wherein the ejection control unit realizes the printing based on the first halftone, the second halftone, and the third halftone.   The ink jet printer according to claim 3, wherein the ink belonging to the third group is an ink having a lightness lower than that of the ink belonging to the second group. A first halftone is generated by applying a first dither mask having blue noise characteristics to a bitmap image that defines the recording amount of ink belonging to the first group, and halftone dots on a substrate to be printed The second dither mask having a characteristic that the variation in the number of adjacent halftone dots due to the shift in the formation position of the first dither mask is less than that in the case where the first dither mask is applied, defines the recording amount of the ink belonging to the second group. A halftone processing unit that generates a second halftone by applying to the bitmap image;
An ejection control unit that realizes printing by ejecting ink from the nozzles of the print head to the substrate based on the first halftone and the second halftone;
Ink-jet printing characterized by comprising. A first halftone is generated by applying a first dither mask having blue noise characteristics to a bitmap image that defines the recording amount of ink belonging to the first group, and halftone dots on a substrate to be printed The second dither mask having a characteristic that the variation in the number of adjacent halftone dots due to the shift in the formation position of the first dither mask is less than that in the case where the first dither mask is applied, defines the recording amount of the ink belonging to the second group. A halftone processing step of generating a second halftone by applying to the bitmap image;
An ejection control step of realizing printing by ejecting ink from a nozzle of the print head to a printing material based on the first halftone and the second halftone;
A printing method comprising:

Images