JP2007190774A - Method and apparatus for image processing, and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus for image processing whereby a high-quality dot arrangement is achieved by taking characteristics of dot formation by a recording head into account, and whereby a good dot arrangement can be obtained also against a variation of strike positions caused by skewing, meandering and the like, and to provide an image forming apparatus. <P>SOLUTION: The image processing method includes a dot model information acquiring process of acquiring information on a dot model of dots formable at positions according to the positions on a coordinate by using the coordinate which regularly sections on a recording medium (20) by predetermined unit cells, and which enables the position of each cell to be specified as a dot formable position; and a dot arrangement determining process of quantizing multi-value input image data for every dot formable position on the coordinate, and determining the dot model of dots which should be arranged at respective positions on the basis of the information on the dot model acquired in the dot model information acquiring process. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image processing method and apparatus, and an image forming apparatus, and more particularly to a halftoning (intermediate gradation) technique used in a dot recording type image forming apparatus typified by an ink jet printer.

  An image forming apparatus that forms an image on a recording medium with dots, such as an ink jet printer, a thermal printer, or an LED printer, is known. In such an image forming apparatus, since an image is formed by a large number of dots formed by ink droplets or toner, dots are basically present or absent on a recording medium such as white paper. Depending on how the image is reproduced. In addition, since the number of inks and the like used is limited, a halftoning (intermediate gradation) technique is conventionally known as a method for expressing continuous gradation with the limited ink.

  For example, in Patent Document 1, in a recording apparatus that quantizes data of each pixel of an image to be recorded and controls ink ejection of each nozzle according to the quantization result, data on the ejection state of the nozzle corresponding to the pixel to be processed ( A technique has been proposed in which the quantization error is corrected based on the predicted ink spread after ink landing), thereby reducing image quality deterioration due to the ink ejection state.

Further, in Patent Document 2, even when a defective nozzle whose ink droplet landing position largely depends among a plurality of nozzles provided in the recording head, streaks are generated in the image. In order to obtain a high-quality image with reduced image quality, the effects of ink droplets ejected from each nozzle on the image to be formed are predicted, and the ink droplet ejection state is corrected based on the prediction results. There has been proposed a method of creating correction information to control the nozzle drive based on the correction information.
JP 2002-240327 A JP 2004-058282 A

  However, in the method proposed in Patent Document 1, the correspondence relationship between the position of a pixel on which image processing is to be performed and the nozzle where a dot is to be recorded is determined in advance, and the ejection characteristics of the nozzle corresponding to the target pixel are determined. From this, the actual landing result is predicted to correct the quantization error. For this reason, for example, when the landing position of the droplet discharged from the nozzle is greatly deviated from the ideal position, and a dot is formed at a position separated by one pixel or more (position to be ejected by another nozzle) However, there is a limit in absorbing an error based on the concept of “spreading of ink”, and there is a drawback that an appropriate dot arrangement cannot always be determined.

  In this regard, in Patent Document 2, there is a correction method in the case where there are defective nozzles in which the ink droplets are larger as the landing position deviates from a desired recording matrix among the plurality of nozzles provided in the recording head. Basically, this is an improved technique for conventional head shading correction, and density correction is performed in consideration of how much ink dots from one nozzle affect the peripheral position (adjacent matrix line). Is to do.

  Processing for determining the dot arrangement on the plane coordinates (nozzle coordinates) created by the relative movement between the nozzle array of the recording head and the recording medium is performed on the pixels on the image processing (quantization processing) when determining the dot arrangement. In this regard, the technique disclosed in Patent Document 2 is the same as that of Patent Document 1, and there is a significant shift between the position of the pixel when determining the dot arrangement and the position on the recording medium where the dots are actually formed (particularly, It cannot be said that the optimum dot arrangement is determined when the ejected droplets from different nozzles land on the same coordinates on the recording medium.

  Such a phenomenon will be described with reference to FIG. Here, a case where an image is formed by moving the line head relatively so as to be orthogonal to the recording medium is illustrated. In FIG. 6A, reference numeral 300 denotes a line head, and reference numeral 302 denotes a recording paper (recording medium). The flying directions of the liquid droplets discharged from the nozzles 1 to 6 are indicated by arrows. FIG. 5B is a diagram showing an example of a droplet ejection result on the recording paper 302. Dots k (k = 1 to 6) in the figure represent dots recorded by the nozzles k.

  Due to problems in the manufacture of the head, the ink droplets ejected from the ink jet head are landed out of the ideal landing position. For example, like the nozzle 5 in FIG. 38, the landing position may be shifted to the ideal landing position of the adjacent nozzle 4. In such a case, error diffusion processing considering dot spread is performed as in Patent Documents 1 and 2, and when the dot arrangement is determined sequentially from the position corresponding to the nozzle 1 as shown in FIG. At the nozzle 5, the dots 5 ejected from the nozzle 5 are ejected onto an area where the dot arrangement has already been determined (an area corresponding to the nozzle 4), and thus the dot 5 cannot be used effectively. is there.

  The fact that the dots 5 cannot be used is substantially the same as the fact that the nozzles corresponding to the nozzles 5 are missing (one nozzle is fewer). Since the dot 4 and the dot 5 can be used at the position where the dot 5 is landed, it is expected that the quality of dot arrangement is improved by effectively using these two types of dots.

  Further, in Patent Documents 1 and 2, the ejection characteristics of each nozzle (predicted ink spread after ink landing, etc.) are considered, but the characteristics of relative movement of the recording head and the recording medium, such as skewing and meandering of the recording medium, are considered. Variations in the landing position due to the above are not considered. That is, in Patent Document 1, the data that should be held as information representing the ejection characteristics of each nozzle is the number of nozzles, which is correction based on one-dimensional characteristic information along the nozzle arrangement direction.

  Also in the cited document 2, shading correction in the nozzle arrangement direction (one-dimensional) is performed, and the relative movement direction of the recording head and the recording medium (for example, the sub-scanning direction in the case of a full-line head having a page-wide recording area) ) Does not take into account that the landing characteristics change. Therefore, for example, due to the meandering in the sub-scanning direction during conveyance of the recording medium, dots are formed in each area of the recording medium at the front end part (image writing start area), the central part, and the rear end part (image end area). However, the techniques of Patent Documents 1 and 2 cannot sufficiently cope with the case where the characteristics of the above change.

  The present invention has been made in view of such circumstances, realizing high-quality dot arrangement in consideration of the characteristics of dot formation by the recording head, and against variations in the landing position caused by skewing, meandering, etc. Another object of the present invention is to provide an image processing method and apparatus and an image forming apparatus capable of obtaining a satisfactory dot arrangement.

  In order to achieve the above object, according to the first aspect of the present invention, a dot is formed when an image indicated by the input image data is formed on the recording medium by recording the dot on the recording medium according to the input image data. An image processing method for determining an arrangement, wherein the recording medium is regularly divided by a predetermined unit cell, and coordinates that can specify the position of each cell as a position where dots can be formed are used. In response, a dot model information acquisition step of acquiring dot model information of dots that can be formed at the position, and quantizing multi-value input image data for each dot formable position on the coordinates, the dot model information acquisition step And a dot arrangement determining step of determining a dot model of a dot to be arranged at each position on the basis of the dot model information acquired in (1).

  According to the present invention, since the dot arrangement can be determined at the coordinate position on the recording medium where the dots are actually formed, a high-quality dot arrangement can be realized.

  The “dot model” preferably includes information on at least one of the spread of dots formed on the recording medium (dot shape), dot position, dot density, and the presence / absence of satellites. More preferably, it includes at least information. The dot model information can include a dot model corresponding to “no dot” when no dot is formed or when a dot cannot be formed.

  A second aspect of the present invention is an aspect of the image processing method according to the first aspect, wherein the dot model information obtaining step is a recording in which a plurality of recording elements for recording dots on the recording medium are arranged. Information on dot models at different positions on the recording medium is acquired with respect to the relative movement direction of the head and the recording medium.

  According to the aspect of the second aspect, an appropriate dot model is selected according to the coordinate position in the relative movement direction corresponding to the difference in dot formation characteristics due to the difference in the coordinate position in the relative movement direction between the recording head and the recording medium. It becomes possible.

  The invention according to claim 3 is an aspect of the image processing method according to claim 2, wherein the dot model information obtaining step measures the dot model recorded on the recording medium by the recording head. A dot measurement step for obtaining the information, and an interpolation calculation step for obtaining a dot model at another position by interpolation calculation from dot model information for a plurality of positions obtained in the dot measurement step, To do.

  When obtaining dot model information (two-dimensional dot model information) for each position (dot formable position) on the two-dimensional coordinate set on the recording medium, a part of the coordinate position ( For example, the dot model information is acquired by actually measuring the dot shape, position, dot density, and the like of the recording dots at a plurality of positions set at appropriate intervals. And about the position which belongs to an unmeasured area | region, a dot model is estimated (calculated) by interpolation calculation from the dot model calculated | required by measurement.

  Thereby, it is possible to obtain substantially useful information of a two-dimensional dot model without requiring an excessive measurement time.

  The invention according to claim 4 is an aspect of the image processing method according to any one of claims 1 to 3, and includes a prohibition condition specifying step of specifying a combination of dots that cannot be simultaneously recorded, and the dot arrangement In the determining step, the dot model of the target position is determined from dot candidates that can be recorded in consideration of the combination of dots that cannot be simultaneously recorded.

  For example, when a plurality of dot sizes can be switched and recorded by one recording element, a situation where different dot sizes cannot be recorded at different positions from the same recording element at the same timing is conceivable. Alternatively, there may be a configuration in which adjacent recording elements cannot be driven simultaneously due to the structure of the recording head. According to the aspect of the fourth aspect, it is possible to determine an appropriate dot arrangement within a range that satisfies the conditions that can be actually recorded in consideration of such a condition that simultaneous recording is impossible.

  It is also possible to provide a program for causing a computer to execute each step of the image processing method according to the first to fourth aspects of the invention. Such a program can be applied as an operation program of a central processing unit (CPU) incorporated in a printer or the like, and can also be applied to a computer system such as a personal computer.

  Alternatively, the program may be configured as a single application software, or may be incorporated as a part of another application such as an image editing software. Such a program is recorded on a CD-ROM, a magnetic disk or other information storage medium (external storage device), and the program is provided to a third party through the information storage medium, or the program is recorded through a communication line such as the Internet. It is also possible to provide a download service.

  The invention according to claim 5 provides an image processing apparatus that achieves the object. That is, the image processing apparatus according to claim 5 determines a dot arrangement when an image indicated by the input image data is formed on the recording medium by recording dots on the recording medium according to the input image data. An image processing apparatus for processing, wherein the recording medium is regularly divided by predetermined unit cells, and each cell position is formed at a position corresponding to a position on a coordinate that can be specified as a position where dots can be formed. Dot model storage means for storing dot model information of possible dots, and quantized multi-value input image data for each dot formable position on the coordinates, and the dot model stored in the dot model storage means And dot arrangement determining means for determining a dot model of dots to be arranged at respective positions based on the information.

  According to the invention of claim 5, since the dot arrangement can be determined at the coordinate position on the recording medium where the dots are actually formed, a high quality dot arrangement can be realized.

  The invention according to claim 6 provides an image forming apparatus that achieves the object. That is, an image forming apparatus according to a sixth aspect includes a recording head in which a plurality of recording elements are arranged, a relative moving unit that relatively moves the recording head and the recording medium, and a predetermined unit cell on the recording medium. Dot model information that obtains dot model information of dots that can be formed at the position according to the position on the coordinates, using coordinates that can be regularly separated by using coordinates that can specify the position of each cell as a position where dots can be formed An acquisition unit; a dot model storage unit that stores information on the dot model acquired by the dot model information acquisition unit; and a quantized multi-value input image data for each dot formable position on the coordinates, and the dot model storage A dot arrangement determining means for determining a dot model of a dot to be arranged at each position based on the dot model information stored in the means; Based on the data of the dot arrangement on the coordinates determined by arrangement determining means, characterized in that and a recording control means for controlling driving of the recording elements.

  According to the image forming apparatus of the sixth aspect, since the dot arrangement can be determined at the coordinate position on the recording medium where the dots are actually formed, the high-quality dot arrangement can be determined, and the determined dot arrangement Accordingly, a high-quality image can be formed.

  A seventh aspect of the invention is an aspect of the image forming apparatus according to the sixth aspect of the invention, and is a dot measurement unit that acquires dot model information by measuring dots recorded on the recording medium by the recording head. And interpolation calculation means for obtaining dot models at other positions by interpolation calculation from dot model information for a plurality of positions obtained by the dot measurement means.

  According to the aspect of the seventh aspect, when acquiring dot model information (two-dimensional dot model information) for each position (dot-formable position) on the two-dimensional coordinates set on the recording medium. Then, actual recording dots (test patterns and the like) are measured at some positions on the coordinates (for example, a plurality of positions set at appropriate intervals in the relative movement direction), and dot model information is acquired. And about the position which belongs to an unmeasured area | region, a dot model is estimated (calculated) by the interpolation calculation from the dot model calculated | required by measurement.

  Thereby, it is possible to obtain substantially useful information of a two-dimensional dot model without requiring an excessive measurement time.

  An ink jet recording apparatus as an aspect of an image forming apparatus according to the present invention includes a liquid that includes a nozzle that discharges ink droplets for forming dots and a pressure generation unit (such as a piezoelectric element or a heating element) that generates discharge pressure. A liquid discharge head (corresponding to “recording head”) having a droplet discharge element array in which a plurality of droplet discharge elements (corresponding to “recording elements”) are arranged, and a recording head based on ink discharge data generated from image data And an ejection control means for controlling ejection of droplets from the nozzle, and an image is formed on the recording medium by the droplets ejected from the nozzle.

  As a configuration example of the recording head, a full-line type head having a recording element array in which a plurality of recording elements are arranged over a length corresponding to the entire width of the recording medium can be used. In this case, a combination of a plurality of relatively short recording head modules having recording element arrays that are less than the length corresponding to the entire width of the recording medium, and connecting them together, has a length corresponding to the entire width of the recording medium. There is an aspect in which a recording element array is configured.

  A full-line type head is usually arranged along a direction perpendicular to the relative feeding direction (relative conveyance direction) of the recording medium, but has a certain angle with respect to the direction perpendicular to the conveyance direction. There may be a mode in which the recording head is arranged along the oblique direction.

  A “recording medium” is a medium that can record an image by the action of a recording head (which can be called an image forming medium, a recording medium, or an image receiving medium), and is a continuous sheet, a cut sheet, a seal sheet, an OHP sheet. Such a resin sheet, a film, a cloth, an intermediate transfer medium, a printed circuit board on which a wiring pattern is printed, and other various media and shapes.

  “Conveyance means” means a mode in which the recording medium is transported to a stopped (fixed) recording head, a mode in which the recording head is moved relative to the stopped recording medium, or a movement of both the recording head and the recording medium Any of the embodiments are included.

  When a color image is formed by an inkjet head, a recording head may be arranged for each color of a plurality of colors (recording liquids), or a configuration in which a plurality of colors of ink can be discharged from one recording head may be adopted. .

  The present invention is not limited to the full-line head described above, but can also be applied to shuttle scan type recording heads (recording heads that perform droplet ejection while reciprocating in a direction substantially perpendicular to the recording medium conveyance direction). It is.

  According to the present invention, since the dot arrangement can be determined at the position on the recording medium where the dots are actually formed, the dot arrangement quality is improved.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[Image processing overview]
First, an overview of an image processing method according to an embodiment of the present invention will be described. 1A is a schematic diagram of an inkjet head, FIG. 1B is an example of nozzle coordinates created by relative movement of the head and the recording medium, and FIG. 1C is a dot actually recorded on the recording medium. An example of

  FIG. 1A shows a line head in which a plurality of nozzles 1 to 6 are arranged along the longitudinal direction (vertical direction in the drawing) of the head 10. In the conventional image processing method, the dot arrangement is determined based on a coordinate system (nozzle coordinates) as shown in FIG. 1B created by relative movement of the head 10 and the recording medium.

  The nozzle coordinates shown in FIG. 1B are a nozzle position i representing the position of the head 10 in the nozzle arrangement direction, and a position j in the recording medium feed direction (relative movement direction) perpendicular to the nozzle arrangement direction of the head 10. Is a coordinate system in which the position (i, j) of the pixel is specified. Here, one pixel, which is the minimum recording unit, is represented by one cell (cell), and the i coordinate (i = 1 to 6) of the pixel is paired with each position of nozzle numbers 1 to 6 of the head 10, respectively. It corresponds with one.

  On the other hand, in the embodiment of the present invention, as shown in FIG. 1C, the dot arrangement is determined based on the coordinates where the dots actually land on the recording medium 20 (referred to as “dot coordinates”). The dot coordinates are lattice coordinates that are regularly divided on the recording medium. Here, one pixel, which is the minimum recording unit, is represented by one cell (cell).

  In the illustrated example, the ideal dot size (design size) formed on the recording medium 20 by the ink droplets ejected from the nozzles 1 to 6 of the head 10 is larger than one square. For example, if the dots are formed so that the centers of the dots are located in the respective cells, the adjacent dots partially overlap each other, and a solid image without a gap can be obtained.

  However, in an actual head, a landing position deviation of ink droplets or a variation in droplet amount occurs due to various factors, and ideal dot formation is not necessarily guaranteed.

  FIG. 1C shows an example of a droplet ejection result when droplets are ejected by driving the nozzles 1 to 6 of the head 10 shown in FIG. 1A at an appropriate timing. In FIG. 1C, dot 1 is a dot formed by nozzle 1 of head 10, and dot 2 is a dot formed by nozzle 2. Hereinafter, similarly, the dot k (k = 1 to 6) in the figure represents the dot formed by the nozzle k of the head 10.

  When attention is paid to the dots 4 and 5, these dots 4 and 5 are formed on the same dot coordinates (x = 4). In the dot shape after landing, the coordinates of the cell (cell) to which the dot center (or dot density centroid) belongs are referred to as dot coordinates (dot formation position) to which the dot belongs.

  Considering based on the conventional nozzle coordinates described in FIG. 1B, since the dots 4 and 5 are ejected by different nozzles 4 and 5, respectively, these dots 4 and 5 are on different coordinates ( Dot 4 has nozzle coordinates i = 4 and dot 5 has nozzle coordinates i = 5). Therefore, when the dot arrangement is determined by the image processing algorithm based on the conventional nozzle coordinates, the arrangement of i = 4 dots and i = 5 dots is determined at different times according to the error diffusion processing order. Become.

  On the other hand, when considered based on the dot coordinates of this embodiment shown in FIG. 1C, since the dots 4 and 5 are on the same coordinate (x = 4), these two are at the same time (x = 4 pixel position) is determined, and it is possible to appropriately determine which one to use.

  FIG. 2 shows a table summarizing the correspondence between the dot coordinates x = 1 to 6, the number of dots that can be ejected at each coordinate position, and the number of nozzles that can form each dot. As shown in FIG. 2, dots 4 and 5 can be ejected with respect to the dot coordinate x = 4, and dots cannot be formed with respect to the dot coordinate x = 5 (see “ φ ”represents an empty set).

  As described above, in this embodiment, the set of dots that can be ejected at each position in the dot coordinate system and the information of the nozzles corresponding to each dot are grasped, and the dot arrangement is determined based on this information. The nozzle drive control corresponding to the dot arrangement is performed. Details of the dot arrangement determination method will be described later.

  In the example of FIG. 2, in order to simplify the description, the phenomenon that the dot landing position is shifted in the case of a single dot size has been described. However, the above-described concept based on the dot coordinates can also be applied to a change in dot size.

  For example, as shown in FIG. 3, a system capable of selectively ejecting dots of three types of dot sizes (small, medium, large) from each nozzle 1 to 6 and having different landing characteristics depending on the dot size And

  In FIG. 3, the small dot 6S formed by the nozzle 6 is formed at the position of the dot coordinate x = 4, the medium dot 6M is at the position of the dot coordinate x = 6, and the large dot 6L is at the position of the dot coordinate x = 5. It is formed.

  FIG. 4 is a chart summarizing the correspondence between the dot coordinates x = 1 to 6 corresponding to the change in dot size, the number of dots that can be ejected at each coordinate position, and the number of nozzles that can form each dot. is there. For convenience of illustration, a part of the table is omitted.

  Thus, in the case of a nozzle having different landing characteristics depending on the dot size, in the present embodiment, since the dots (6S, 6M, 6L) of each dot size can be handled so as to belong to different coordinates, more appropriate dot arrangement Can be determined.

  However, when handling multiple types of dot sizes that are ejected from the same nozzle as separate dots, conditions may occur where droplets cannot be ejected simultaneously (for example, small dots and large dots cannot be ejected at the same timing on scanning) Unless such a case where simultaneous droplet ejection is impossible is considered, it is expected that a dot arrangement that cannot actually be deposited is determined.

  In order to cope with such a problem, as shown in FIG. 5, it is desirable to obtain in advance a combination of dots in a relationship that prevents simultaneous droplet ejection, and to eliminate the case where there is a relationship incapable of droplet ejection. In addition, when there is a prohibited combination based on nozzles that prevents simultaneous driving of adjacent nozzles due to the structure of the head, not limited to the selection of the dot size, the prohibited combination of such nozzles is converted into a dot combination of dot coordinates. , It can be included in the condition (prohibition condition) when there is a relationship where the droplet cannot be ejected.

  Further, the method for determining the dot arrangement based on the dot coordinates as in the present embodiment is also very effective for a head in which the nozzle interval is not uniform.

  FIG. 6A shows an example of a head in which the nozzle interval is not uniform. According to the figure, the interval P1 between the nozzles 1 and 2 is larger than the interval P2 between adjacent nozzles 2 to 6 (P1> P2). An example of dot formation by the head 30 is shown in FIG. Regarding the relationship between the dot coordinates and the nozzle interval, it is preferable that the most frequent interval (in the case of FIG. 6A, the adjacent nozzle interval P2 of the nozzles 2 to 6) of the nozzle interval in the head is equal to the dot coordinate interval.

  In the illustrated example, dots 1 to 3 can be formed with respect to dot coordinates x = 1 to 3, respectively, and dot 4 or dot 5 can be formed with respect to dot coordinates x = 4. Further, it is possible to form the dot 6 for the dot coordinate x = 5 and the dot 7 for the dot coordinate x = 6.

  Thus, since it is only necessary to determine the correspondence between the dot coordinates on the recording medium, the dots that can land on each coordinate, and the nozzles for forming each dot, whether or not the nozzle spacing is uniform is determined in this embodiment. The image processing method in the form is not a problem.

  In addition to the non-uniformity of the nozzle pitch in manufacturing, the factors causing the non-uniform nozzle spacing include structural factors of the head. For example, when a line head is configured by connecting short heads, the nozzle interval at the joint between the short heads tends to be non-uniform. For this reason, conventionally, when a line head is configured by connecting short heads, a technique for increasing the nozzle interval is known, such as overlapping the nozzles at the connecting portions between the short heads. There is also known a technique for reducing the visibility of streaky density unevenness by switching nozzles that regularly (or irregularly) eject droplets in a portion where the density is not uniform.

  On the other hand, in the embodiment of the present invention, since an appropriate dot arrangement is determined on the dot coordinates based on the dot coordinates, it is not necessary to consider the difference in nozzle spacing. Accordingly, it is possible to determine an appropriate dot arrangement without considering any manufacturing device or discharge control distinction between the connecting portion between the short heads and the other portion.

  For the dot arrangement determination based on the dot coordinates, a dot arrangement determination method based on a dot model described later is suitable. The unevenness of the twist and the nozzle interval is generally smaller than the dot coordinate interval. Therefore, in order to appropriately determine the dot arrangement, a dot arrangement determination method based on a dot model that can appropriately use the above-described small deviation is desirable.

[Description of dot model]
Next, the dot model will be described. FIG. 7 shows an example of a dot model. In the figure, a circular hatched portion indicated by reference numeral 50 represents the position of the dot actually formed on the recording medium and its dot shape. As shown in FIG. 7, the dot model is a concept in which the shape of the dot 50 is divided by a lattice interval (cell corresponding to a pixel) of dot coordinates. In the example of FIG. 7, a 3 × 3 pixel matrix (mesh) composed of pixels (center cell in FIG. 7) representing the dot formation position and the surrounding 8 pixels is considered.

  Each pixel of this 3 × 3 pixel matrix is displayed as a set 11 to 33 of a column number x and a row number y with a column number first (as opposed to the normal matrix expression) as shown in the figure. And Therefore, the number set xy indicates the position of x columns and y rows. Hereinafter, the display of this number in the figure is omitted, but each pixel position is indicated by a set of this number.

  That is, the dot model illustrated in FIG. 7 includes the dot 50 included in each pixel 11 to 33 of a 3 × 3 pixel matrix including the pixel 22 to which the center of the dot 50 (or the density centroid) belongs and the surrounding 8 pixels. Are expressed based on the dot shape (dot spread image).

  FIG. 8 shows variations of the dot model. (A) represents a dot model of an ideal dot shape and a dot at a landing position. As shown in the figure, the ideal dot size is larger than the mesh (lattice) interval partitioned at the dot formation position, and the center of the dot coincides with the center of the pixel cell at the dot formation position. (B) shows a dot model example of a dot having a landing position shift in which the landing position is displaced downward from the ideal position.

  Further, (c) shows a dot model example of a dot having a landing position shift in which the landing position is displaced upward from the ideal position. (D) shows a dot model example of dots having different dot sizes. (E) is an example of a dot model in the case of having satellites. A small circle indicated by reference numeral 52 represents a satellite dot.

  As shown in FIGS. 8B to 8E, the dots that are actually formed may deviate from the ideal landing position, or the dot shape may differ from the ideal shape. 8B to 8E, the dot formation position is handled as the pixel 22 because the dot center (center of density) is in the cell of the pixel 22.

  When ink is actually ejected from a nozzle onto a recording medium, the dot position, dot shape, and the like are determined by the characteristics of the recording medium in addition to the ejection characteristics of the nozzle. For example, when the recording medium has a characteristic that ink is likely to bleed, the ink bleeds and spreads widely, and the dot shape may be lost. Thus, the actual dot model is determined by the ejection characteristics of the nozzles and the fixing characteristics of the recording medium.

  That is, dot size, dot density, and dot shape depend on ink characteristics and recording paper characteristics. The landing position depends on the nozzle characteristics. Satellites depend on ink characteristics and nozzle characteristics.

  The dot model takes into account the actual dot state, and the dot size, landing position, dot density (distribution), dot shape, presence / absence of satellites, and the like can be arranged at each dot placement position (in the example shown, pixel 11 , 21, 31, 12, 22, 32, 13, 23, 33) are expressed two-dimensionally (three-dimensional when the concentration direction is included).

  In the embodiment of the present invention, as illustrated in FIGS. 8A to 8E, in order to express various dot states, the dot placement possible positions 11 and 11 in FIG. A numerical representation of a dot model is used from the area and density of dots contained in 21, 31, 12, 22, 32, 13, 23, and 33, respectively.

  FIG. 9 shows an example in which the dot model is digitized. The value of each pixel (cell) in the dot model is determined in consideration of the area of overlap between the pixel (cell) and the dot and the dot density. In the example of FIG. 9, when the dot density is D, the value at the center dot formation position (the position of the pixel 22) is 1.0 × D (hereinafter, for convenience of description, the multiplication operation symbol (×) is omitted, and “ 1.0D "), and the values of the surrounding pixels are displayed in FIG. 9 depending on the overlapping area with the dots.

  In other words, as described with reference to FIG. 7, assuming that the pixel in i column and j row in the 3 × 3 pixel matrix is expressed as “pixel ij”, pixel 11 in FIG. 9 is 0.05D, pixel 12 is 0.9D, pixel 13 Is 0.5D, pixel 21 is 0.1D, pixel 23 is 0.7D, pixel 31 is 0.0D, pixel 32 is 0.3D, and pixel 33 is 0.2D. For each cell, the density distribution within the cell is not considered, and the average of the dot densities contained in each cell is set as the density achieved in each cell.

  As described above, the dot model is digitized (quantized), rasterized, and used to determine the dot arrangement.

  Although FIG. 7 to FIG. 9 show examples of 3 × 3 dot placement positions, the size of the dot model is not limited to the 3 × 3 size, and any size can be used. . Generally, it should be expressed by a size of N × M (N and M are integers of 2 or more).

  Further, with respect to the example of the 3 × 3 pixel matrix size described in FIGS. 7 to 9, as shown in FIG. 10, a finer mesh is formed for the pixels around the dot formation position (pixel 22). Is also possible.

  As shown in FIG. 10, it is possible to determine whether or not dots overlap each other at the end of the dot by dividing the area further finer than the dot coordinates for the pixels around the dot formation position (pixel 22). . At that time, it becomes possible to incorporate the non-linearity of density change due to dot overlap. That is, as shown in FIG. 10, the dot arrangement can be determined in consideration of the effect of overlapping the dots and overlapping the dots.

  In addition, the shape of the unit cell representing the pixel is not particularly limited, and there may be various modes such as a rectangle, a square, and other polygons.

  The method for determining the dot arrangement based on the dot coordinates can also be applied to the problem that the dot landing position differs depending on the sub-scanning position that occurs when the conveyance of the recording medium meanders (or skews). Misalignment in the sub-scanning direction due to fluctuations in the conveyance speed can be handled in the same way.

  FIG. 11 is a diagram illustrating an example in which the dot coordinates differ depending on the sub-scanning position. The figure shows how the landing positions of the dots respectively formed by the nozzle 1 and the nozzle 6 change depending on the sub-scanning position.

  In this way, when the dot landing position changes according to the sub-scanning position, the dot coordinates in the tables described in FIGS. 4 and 5 are created two-dimensionally, and the fluctuation of the dot landing position at each sub-scanning position is changed. This is reflected in both the dot model change and the dot position change, and the dot arrangement is determined using the two-dimensional dot coordinates.

  FIG. 12 is an explanatory diagram of a method for acquiring dot model information. In the figure, reference numeral 60 represents a line head, and reference numeral 62 represents a recording medium. It is assumed that the recording medium 62 is conveyed from the right to the left in the drawing with respect to the line head 60.

  As shown in FIG. 12, a plurality of dot measurement regions (here, regions 1 to 5) are set at different positions in the conveyance direction (sub-scanning direction) of the recording medium 62. In the illustrated example, strip-shaped dot measurement regions 1 to 5 having a length of the width of the recording medium 62 in the main scanning direction (vertical width in the figure) are set at regular intervals in the sub-scanning direction. The width in the sub-scanning direction of the measurement region and the distance between the regions do not necessarily have to be set at equal widths and equal intervals. The width of the dot measurement region in the sub-scanning direction and the interval between the regions are appropriately set according to the degree of variation in landing characteristics depending on the position in the sub-scanning direction.

  In FIG. 12, the position coordinates corresponding to the dot measurement areas 1 to 5 (for example, coordinates indicating the center positions of the areas 1 to 5) are X1, X2, X3, X4, and X5.

  In each of the regions 1 to 5 corresponding to X1 to X5, the dots ejected from the nozzles of the line head 60 are measured (actually measured), and the dot model corresponding to the nozzles in the regions 1 to 5 is determined. Thereby, the dot model information at each position for one line in the main scanning direction is obtained for each of the regions 1 to 5.

  On the other hand, for the areas other than these areas 1 to 5, a dot model is determined by interpolation calculation. For example, the dot model corresponding to the nozzle at the position X where X1 <X <X2 is determined by interpolation using the X1 dot model and the X2 dot model.

  Hereinafter, a procedure for determining a dot model by interpolation will be described.

  First, at positions X1 to X5 in FIG. 12, PDM (x, Ni, s) = (DI, y, xc, yc) is determined for the combination of each nozzle Ni (i is the nozzle number) and the dot size s with respect to Ni. To do. PDM (x, Ni, s) indicates a combination of dots in which droplets of dot size s ejected from the nozzle Ni are deposited on the recording medium at the position x in the X direction of the dot coordinates.

  Here, “DI” represents a dot image on a matrix of a predetermined size (Nx × Ny) to which the dot belongs, and “x, y” represents the dot coordinate to which the dot center (or density center of gravity) of the dot image DI belongs. "Xc, yc" represents the position on the dot image to which the dot center of the dot image DI belongs, respectively.

  A dot image represents a two-dimensional density distribution of target dots (dots specified by the above PDM (x, Ni, s)) that can express at least the density, shape, and presence / absence of satellites. It is treated as a set of cells that are included, continuous on the dot coordinates, and rectangular as a whole. It is desirable that the density distribution be expressed by a size that is 1 / integer of the cell size in dot coordinates.

  FIG. 13A is an explanatory diagram of a dot image. As shown in the figure, in a dot coordinate system partitioned by a unit cell 70 of a predetermined size into a two-dimensional matrix in the X direction (horizontal direction in the figure) and the Y direction (vertical direction in the figure) perpendicular thereto. Consider the two-dimensional density distribution of the target dot D.

The density of the target dot D shown in the drawing is distributed in 3 × 3 cells including the cell to which the dot center position C _D (XC, YC) belongs and the surrounding 8 cells on the dot coordinates. A coverage of the target dot D (inside the boundary line representing the outline of the dot) is included, and a set of 3 × 3 cells that are rectangular as a whole is handled as a dot image. FIG. 13B is an enlarged view of the dot image portion in FIG. In this example, a set of 3 × 3 cells is handled as a dot image, but the size of the dot image is not limited to this.

And the center position _{C D} of the target dots D and (XC, YC), the dot center position (XC, YC) of the X-coordinate of the cell on the dot coordinates including X, when the Y-coordinate and Y, PDM (x, Ni , S) = {DI, y, xc, yc} have the following relationships: y = Y, xc = XC-X, yc = YC-Y.

  Based on this PDM (x, Ni, s), PDM (x, Ni, s) at positions x other than X1 to X5 in FIG. 12 is determined by linear interpolation.

  14 and 15 are explanatory diagrams for explaining a linear interpolation calculation method. In these drawings, a dot image DI1 represents an example of a dot image of a dot D1 ejected at a position X1, and a dot image DI2 represents an example of a dot image of a dot D2 ejected at a position X2. In FIG. 14, the main scanning direction position (vertical cell position in the figure) on the dot coordinates to which the dot center (XC1, YC1) of the dot image DI1 belongs and the dot coordinates to which the dot center (XC2, YC2) of the dot image DI2 belongs. An example in which the upper main scanning direction position is the same is shown.

  FIG. 15 shows an example in which the main scanning direction position on the dot coordinates to which the dot center of the dot image DI1 belongs and the main scanning direction position on the dot coordinates to which the dot center of the dot image DI2 belongs are different.

  As shown in these drawings, the interpolation calculation is performed for the position X where X1 <X <X2, and the dot center position (XC1, YC1) of the dot image DI1 and the dot center position (XC2, YC2) of the dot image DI2. Is obtained by linear interpolation between the two points.

  That is, as shown in FIG. 16, the dot center position (XC1, YC1) when the dot image DI1 is placed on the dot coordinates and the position X1 in the X direction on the dot coordinates (where XC1 = X1 + xc1, YC1 = Y1 + yc1) and the dot center position (XC2, YC2) when the dot image DI2 is placed on the dot coordinates and the X direction position X2 on the dot coordinates (where XC2 = X2 + xc2, YC2 = Y2 + yc2) ), The dot center position (XC, YC) when the dot image DI at the X-direction position X on the desired dot coordinate is placed on the dot coordinate is the center position (XC1, YC1) and (XC2, YC2). Are obtained by linear interpolation calculation as points on a straight line passing between the two points.

The position Y in the Y direction on the dot coordinates passing through the position X in the X direction on the dot coordinates is y, and the following formula XC = α × XC1 + β × XC2
(Where α + β = 1, α: β = (X2−X) :( X−X1))
YC = α × YC1 + β × YC2
(Where α + β = 1, α: β = (Y2−Y) :( Y−Y1))
Meet.

  Next, the dot image DI is set in the X direction (XC-XC1) and the Y direction (YC-) so that the center position (XC1, YC1) of the dot image DI1 matches the center position (XC, YC) of the dot image DI. YC1) is calculated by shifting (moving) the dot image DI1 ′.

  Similarly, a dot image DI2 'obtained by shifting the dot image DI2 at the position X2 by (XC-XC2) in the X direction and (YC-YC2) in the Y direction is calculated.

  The dot image obtained by averaging the dot image DI1 ′ and the dot image DI2 ′ obtained as described above is determined as the dot image at the position x = X.

  By performing this calculation for each of the dot size s and the nozzle Ni, PDM (x, Ni, s) at the position x can be calculated. Based on the PDM (x, Ni, s) thus obtained, a dot model DM (x, y, s, t) is calculated. The variable t is a variable for distinguishing a plurality of dots when there are a plurality of dots that can be ejected at the position (x, y) (dot sizes are substantially the same).

  For the positions between the other dot measurement areas shown in FIG. 12 (X2 <X <X3, X3 <X <X4, X4 <X <X5), a dot model is obtained by interpolation calculation as described above.

  In this way, it is possible to obtain two-dimensional dot model information that can be applied even when the landing characteristics vary depending on the position in the sub-scanning direction.

[Specific example of error diffusion processing]
Next, a method for determining dot arrangement by error diffusion processing using the above dot model will be described.

  As described with reference to FIGS. 7 to 9, the dot model includes the dot shape, dot position error, dot size error, satellite presence / absence, and satellite at each dot formation position (pixel) of the dot coordinates set on the recording medium. It contains information about the dot formation state such as its shape when it has it.

  In the present embodiment, by using the above-described dot model, halftoning is performed by determining dot formation at each dot formation position to obtain a high-quality image. That is, in the dot arrangement determination method of the present embodiment, when forming an image by forming dots on a recording medium, an order for determining whether or not to form a dot at a position (x, y) on the recording medium. In accordance with (pixel processing order), dot formation at that position (x, y) is determined.

  As shown in FIG. 17, the image forming area on the recording medium is divided into cells (cells) representing dot-formable positions (pixels) and given coordinates (x, y) representing each cell (dots). Coordinate). In the example of FIG. 17, the position of the cell (pixel) at the position of x in the right direction and y in the downward direction is represented by (x, y) with the upper left corner of the image forming area as the origin.

  In the present embodiment, in the coordinate system of the two-dimensional pixel matrix as shown in FIG. 17, pixels to be processed (attention) in the order from the upper left to the right side and from the upper row to the lower row in order (raster order). It is assumed that dot formation is determined while moving (pixel).

  In the error diffusion process, the accumulated error in the quantized area is diffused to the unquantized area, and when the pixel of interest is quantized, the error including the accumulated error is quantized. It has the feature of diffusing into the quantization region.

  In FIG. 18, the shaded area is a quantized pixel area. The cells (pixels) belonging to the quantized region have been quantized, and the accumulated error in each pixel is zero. On the other hand, the white area in the figure is an unquantized area, and the cells (pixels) belonging to the unquantized area have not been quantized, and the accumulated error in each pixel does not occur. Is retained. Further, the positions marked with “*” in the figure represent the target pixel.

  When performing error diffusion processing by halftoning based on the dot model, considering the case where the dot is turned on at the target pixel (the position of the * mark in FIG. 18), for example, when forming the dot model shown in FIG. Since the model has a larger spread than the cell of one pixel, the density is also added to the surrounding quantized area (position where dots can be arranged) and the unquantized area due to the spread (see FIG. (See FIG. 19).

  As described above, since the error diffusion process is quantized so that the accumulated error of the quantized area becomes zero, the dot spread for the quantized area needs to be processed as an error. In particular, it is desirable that the error be eliminated near the position where it occurred.

  20A to 20D show an example in which different dot models are arranged at the target pixel position. As shown in these figures, the influence of the error on the quantized region differs depending on the spread of the dot model. The error on the quantized region corresponding to these dot models is shown in FIGS. As shown in b), it diffuses to a position adjacent to the error occurrence position. At this time, the error diffused to the target pixel position is included in the error diffusion from the subsequent target pixel position (FIG. 21C) and processed. Thereafter, the error is further diffused to the unquantized position with respect to the target pixel as shown in FIG.

  In the first diffusion process (“error diffusion 1”) shown in FIG. 21A, the pixel (x−1, y−1) in the quantized region adjacent to the target pixel (x, y) due to the spread of the dot model. ), (X, y-1), (x + 1, y-1), the error generated in the quantized region pixel (x-1, y), the target pixel (x, y), and unquantized This is a process of diffusing to the pixels (x + 1, y) and (x + 2, y) in the area.

  In the second diffusion process (“error diffusion 2”) shown in FIG. 21B, the pixel (x−1, y) on the right side of the target pixel (x, y) after the above “error diffusion 1”. Is accumulated in the pixel of interest (x, y) and other pixels (x-2, y + 1), (x-1, y + 1), (x, y + 1) in the unquantized region It is processing.

  In the third diffusion process (“error diffusion 3”) shown in FIG. 21C, the error generated as a result of the quantization process on the pixel of interest (x, y) is adjacent to the above “error diffusion 2”. This is a process of diffusing to pixels (x + 1, y), (x-1, y + 1), (x, y + 1), (x + 1, y + 1) in the unquantized area.

  Here, the error diffusion process has been described in three stages (error diffusion 1 to 3). However, in order to reduce the amount of calculation in mounting, it is necessary to omit a part of error diffusion from the essence of the present technology. It's not off.

  In the present embodiment, <1> the density of the dot model spreading at the already quantized position is diffused as an error to the unquantized position; <2> the error generated at each position of the dot model spreading at the already quantized position Due to the feature that the error distribution to the periphery changes depending on the magnitude of (dot model density), appropriate error diffusion according to the dot model is realized.

  A specific example of error diffusion processing will be described below. The meanings of the main symbols used in the explanation are as follows. The position (x, y) of the pixel of interest and the image signal of the multi-valued input image data are I (x, y). For example, in the case of input image data expressed by 8 bits (256 gradations), the image signal I (x, y) takes a value from 0 to 255.

  The threshold used for quantization determination is the threshold T (x, y) when there is one dot size that can be ejected. When there are a plurality of dot sizes that can be ejected, a threshold value T (x, y, s) is set using a variable s that distinguishes the dot size. The accumulated error E (x, y) represents the error accumulated at the position (x, y). The accumulated error is zero at the already quantized position. Cumulative spread S (x, y) represents a cumulative value when the spread portion of the dot model at the adjacent position or the peripheral position overlaps the position (x, y). This cumulative spread is only meaningful at unquantized positions.

  A dot model DM (x, y: ij) of a dot that can be ejected at the target position (x, y) is assumed. Here, “ij” represents a position on the dot model and indicates 11, 21, 31, 12, 22, 32, 13, 23, 33 in FIG. That is, “ij” represents relative coordinates with respect to (x, y). FIG. 22 shows a summary of the relationship between the dot position and the dot model.

  When there are a plurality of dots that can be ejected at the position (x, y) (dot sizes are substantially the same), the dot model DM (x, y, t: ij) is used. A plurality of dots are distinguished by a variable t.

If there are multiple dot sizes that can be ejected, the dot model DM (x, y, s, t: ij)
And s is a variable for distinguishing dot sizes, and t is a variable for distinguishing a plurality of dots.

  When two or more dots can be ejected simultaneously to the target position, the combinations of simultaneous droplet ejection are counted and each combination is treated as a separate “t”.

  For example, when two dots shown in FIGS. 23 (a) and (b) can be ejected independently, FIG. 24 (d) combining the two dots is added, and FIGS. 24 (b) and (c). , (D) are all combinations.

  In order to make a concise description as an algorithm, in addition to the case where droplet ejection is not performed as shown in FIG.

[Quantization determination 1 in the target pixel]
When the dot size that can be ejected is one, quantization is determined by the following [determination formula 1-1].

[Evaluation Formula 1-1] I (x, y) −E (x, y) −S (x, y)> T (x, y)
If the inequality of [Judgment Formula 1-1] is satisfied, it is determined that the dot is ON, and otherwise, the dot is OFF.

  In addition, when there are a plurality of dot sizes that can be ejected, the quantization is determined by the following [determination formula 1-2].

[Evaluation Formula 1-2] I (x, y) −E (x, y) −S (x, y)> T (x, y, s)
If the above inequality [Determination 1-2] is satisfied, it is determined that the dot size s dot is ON, otherwise the dot is OFF. Each s is sequentially determined, and if [Determination 1-2] is satisfied, the dot size s is determined to be dot ON, and the process ends. If [Determination 1-2] is not satisfied for all s, the dot is determined to be OFF. .

  A specific dot model determination method when the dot is turned ON by the determination process using the above [determination formula 1-1] or [determination formula 1-2] is any of the following.

  [1] One of t = 1, ..n is determined as a dot model by random numbers.

  [2] Among t = 1,.. N, t that minimizes the absolute value of the generated error for the already quantized region and the target pixel is determined as the dot model. That is, t that minimizes the following function is determined as a dot model.

[Formula 1-3]
F (t) = | [DM (x, y, s, t: 11) + DM (x, y, s, t: 21) + DM (x, y, s, t: 31)
+ DM (x, y, s, t: 12) − {I (x, y) −E (x, y) −S (x, y) −DM (x, y, s, t: 22)}] |
[Method of error diffusion processing]
Next, an error diffusion processing method will be described. By the quantization determination 1 described above, error diffusion processing is performed using the dot model determined for the pixel of interest as follows. When the pixel of interest is dot OFF, calculation is performed using the dot model DM (x, y, s, t) when t = 0.

(A. Calculation of initial value of generated error)
First, the initial value of the generated error at each position that occurs when the dot model at the position of interest (x, y) is determined is calculated as follows.

E (x-1, y-1) = DM (x, y, s, t: 11) ... [Formula A-1]
E (x, y-1) = DM (x, y, s, t: 21) ... [Formula A-2]
E (x + 1, y-1) = DM (x, y, s, t: 31) [Formula A-3]
E (x-1, y) = DM (x, y, s, t: 12) ... [Formula A-4]
E (x, y) = − {I (x, y) −E (x, y) −S (x, y) −DM (x, y, s, t: 22)} [Formula A-5]
Note that the above expression is not a normal mathematical equation, and when the same character variable is included on the left side and the right side, the calculation result on the right side is newly replaced with the value of the character variable on the left side. For example, in [Formula A-5], the left and right sides contain the same character variable E (x, y), and the calculation result on the right side is newly replaced with the value of E (x, y). Hereinafter, the same applies to the notation of arithmetic expressions in this specification.

  Subsequently, an error diffusion process at each position is performed. The error diffusion is performed in the order of the first diffusion → second diffusion → third diffusion as follows.

(B. First diffusion)
In the first diffusion (error diffusion 1) process, the error generated in the (y-1) row due to the spread of the dot model is diffused to the y row. That is, the error at each position is calculated by the following equation.

E (x + 2, y) = E (x + 2, y) + C31_1 * E (x + 1, y-1) [Formula B-1]
E (x + 1, y) = E (x + 1, y) + C31_2 * E (x + 1, y-1) + C21_1 * E (x, y-1) ... [Formula B-2]
E (x, y) = E (x, y) + C31_3 * E (x + 1, y-1) + C21_2 * E (x, y-1)
+ C11_1 × E (x-1, y-1) ... [Formula B-3]
E (x-1, y) = E (x-1, y) + C21_3 * E (x, y-1) + C11_2 * E (x-1, y-1) [Formula B-4]
However, C31_1, C31_2, and C31_3 are error diffusion coefficients for diffusing the error generated at the position 31 in the 3 × 3 pixel matrix (see FIG. 7) of the dot model to the adjacent position. Satisfy.

C31_1 + C31_2 + C31_3 = 1.0 ... [Formula B-5]
Similarly, C21_1, C21_2, and C21_3 are error diffusion coefficients for diffusing the error generated at the position 21, and satisfy the following equation [Formula B-6].

C21_1 + C21_2 + C21_3 = 1.0 [Formula B-6]
C11_1 and C11_2 are error diffusion coefficients for diffusing the error generated at the position 11, and satisfy the following equation [Equation B-7].

C11_1 + C11_2 = 1.0 [Formula B-7]
After the first diffusion process,
It is assumed that E (x + 1, y-1) = 0, E (x, y-1) = 0, E (x-1, y-1) = 0. This means that once the error in the upper row (y-1) of the pixel of interest (x, y) has been moved (diffused) to the y row, the accumulated error in the (y-1) row is set to zero. is doing.

(C. About the second diffusion)
Next, a second diffusion (error diffusion 2) process is performed. In this second diffusion, the accumulated error at the position (x-1, y) on the left side of the target pixel (x, y) is diffused to the unquantized position. The accumulated error at each position is calculated as follows. Calculated by

E (x, y) = E (x, y) + C12_1 × E (x-1, y) ... [Formula C-1]
E (x, y + 1) = E (x, y + 1) + C12_2 x E (x-1, y) ... [Formula C-2]
E (x-1, y + 1) = E (x-1, y + 1) + C12_3 x E (x-1, y) ... [Formula C-3]
E (x-2, y + 1) = E (x-2, y + 1) + C12_4 * E (x-1, y) ... [Formula C-4]
However, C12_1, C12_2, C12_3, and C12_4 are error diffusion coefficients for diffusing the accumulated error at position 12, and satisfy the following equation [Formula C-5].

C12_1 + C12_2 + C12_3 + C12_4 = 1.0 [Formula C-5]
After the second diffusion, E (x−1, y) = 0. This means that once the error has been diffused, the accumulated error of the quantized pixel (x−1, y) is set to zero.

(D. Third diffusion)
Next, third diffusion (error diffusion 3) is performed. The third diffusion is a process of diffusing the accumulated error at the target position (x, y) to other adjacent unquantized pixels, and the accumulated error at each position is calculated by the following calculation.

E (x + 1, y) = E (x + 1, y) + C22_1 × E (x, y) [Formula D-1]
E (x + 1, y + 1) = E (x + 1, y + 1) + C22_2 × E (x, y) (formula D-2)
E (x, y + 1) = E (x, y + 1) + C22_3 × E (x, y) [Formula D-3]
E (x-1, y + 1) = E (x-1, y + 1) + C22_4 x E (x, y) ... [Formula D-4]
However, C22_1, C22_2, C22_3, and C22_4 are error diffusion coefficients for diffusing the error at position 22, and satisfy the following equation [Formula D-5].

C22_1 + C22_2 + C22_3 + C22_4 = 1.0 [Formula D-5]
After the third diffusion operation, E (x, y) = 0. This means that once the error has been diffused, the accumulated error of the quantized pixel (x−1, y) is set to zero.

[About cumulative spread processing]
Next, a process for calculating the cumulative spread will be described.

  By determining the dot model of the pixel of interest (x, y), the cumulative spread to the surrounding unquantized positions is calculated by the following equation.

S (x + 1, y) = S (x + 1, y) + DM (x, y, s, t: 32) ... [Formula E-1]
S (x−1, y + 1) = S (x−1, y + 1) + DM (x, y, s, t: 13) [Equation E-2]
S (x, y + 1) = S (x, y + 1) + DM (x, y, s, t: 23) ... [Formula E-3]
S (x + 1, y + 1) = S (x + 1, y + 1) + DM (x, y, s, t: 33) [Formula E-4]
S (x, y) = 0 ... [Formula E-5]
[Conversion to nozzle drive information]
Conversion from the determined dot model to information enabling nozzle driving is performed by a conversion step corresponding to the reverse of the step at the time of dot model generation. That is, when the dot model is generated, the nozzle number Np and the scanning timing STq are associated with the dot arrangement possible position (x, y), and the dot model is determined from the nozzle drive signal corresponding to the dot that can be formed at each position. . At this time, the nozzle drive signal corresponding to the same position (x, y) (considering the case where size modulation is possible) Sr0, Sr1,.. , y, s, t).

  Using the opposite relationship, the determined dot model DM (x, y, s0, t0) is replaced with the nozzle number Np, scanning timing STq, nozzle drive signal (size modulation) Sr (simultaneous droplet ejection as one model When considered, there are a plurality of nozzle drive signals, which are converted into Sr0, Sr1, ..Srm). In this way, nozzle driving is performed according to the information of the obtained nozzle driving signal.

  The dot model is not limited to 3 × 3, and the error diffusion coefficient and diffusion range used for error diffusion at each position are not limited to the example of this embodiment.

  Moreover, it is also possible to put together the place which can process simultaneously among the 1st, 2nd, 3rd spreading | diffusion processes demonstrated in FIG.

  Furthermore, the conversion from the determined dot model to information that enables nozzle driving may be performed after the completion of halftoning (see FIGS. 25 and 26) or in the middle of halftoning (see FIGS. 28 and 29).

[Regarding other method of quantization determination at target pixel (quantization determination 2)]
In the above-described processing example of “quantization determination 1”, dot ON / OFF is determined by comparing with threshold T in [determination formula 1-1] or [determination formula 1-2]. Is also possible. “Quantization determination 2” described below determines the dot model of the pixel of interest (x, y) under the condition that the generated error is minimized without using a threshold value.

  That is, when the image signal at the pixel of interest is I (x, y), t of t = 0,. In the determination, t that minimizes the following function (a function representing the absolute value of the generated error) is obtained.

[Formula 2-1]
F (t) = | [DM (x, y, t: 11) + DM (x, y, t: 21) + DM (x, y, t: 31)
+ DM (x, y, t: 12) − {I (x, y) −E (x, y) −S (x, y) −DM (x, y, t: 22)}] |
When handling a combination of dots that are simultaneously ejected at the same position as one dot model, the following function is used, and the already quantized area of s = 0, .. m, t = 0, .. n S and t that minimize the generated error for the pixel of interest are determined as a dot model.

[Formula 2-2]
F (t) = | [DM (x, y, s, t: 11) + DM (x, y, s, t: 21) + DM (x, y, s, t: 31)
+ DM (x, y, s, t: 12)-{I (x, y) -E (x, y) -S (x, y) -DM (x, y, s, t: 22)}] |
[Example of processing flow 1]
Next, an example of the processing flow of the image processing method according to the present embodiment will be described. FIG. 25 shows an example of the entire processing flow.

  As shown in FIG. 25, when the processing is started, first, a dot model is determined from the nozzle number, the scanning timing, and the nozzle drive signal on the coordinates (dot coordinates) based on the dot landing position on the recording medium (step S110). ). In this step, as described with reference to FIG. 12, dot measurement is performed for a plurality of dot measurement areas on the recording medium to obtain a dot model for each area, and interpolation calculation is performed using the dot model based on this measurement. Find the dot model at. In this way, two-dimensional dot model information corresponding to the difference in the sub-scanning direction position is acquired. The obtained dot model information is stored in storage means such as a memory. The dot model information may be acquired in advance, or the dot model information may be updated by performing dot measurement at an appropriate timing as needed, or by reading the dot model data from the outside. A mode of acquisition is also possible.

  Next, the process proceeds to step S120 in FIG. 25, and initialization processing of each variable used in the calculation is performed. For example, initial values are given for the variables such as the cumulative error E (x, y) and the cumulative spread S (x, y) described with reference to FIGS.

  Next, the processing order (pixel processing order) for determining whether or not to form a dot at the position (x, y) on the dot coordinates is set (step S130 in FIG. 25). For example, as described with reference to FIG. 17, the raster order is set such that each line is processed in order from left to right in order from the top line in the image forming area to the bottom line.

  Next, the process proceeds to the dot formation determination process shown in step S140 of FIG. 25, and pixel dot formation is determined in accordance with the above processing order. The flow of this dot formation determination process is shown in FIG.

  FIG. 26 is a processing flow of a target pixel (dot formation determination processing subroutine). When the processing flow shown in the figure starts, first, quantization determination is performed on the target pixel (step S141). In this quantization determination process, the dot model is determined according to the determination method of “quantization determination 1” or “quantization determination 2” already described.

  When the dot model of the target pixel (including the case of dot OFF) is determined, the initial value of the generated error is calculated based on the dot model (step S142). Thereafter, the first diffusion process (step S143), the second diffusion process (step S144), and the third diffusion process (step S145) are performed in order, and after the error diffusion process consisting of steps S142 to S145, the accumulation is performed. A spread process is performed (step S146). Details of the processes of steps S142 to S146 are as already described.

  When the calculation of the cumulative spread of the surrounding unquantized positions is completed in step S146, the process exits the subroutine of FIG. 26 and proceeds to step S150 of FIG.

  In step S150, the target pixel is changed to the next position in accordance with a prescribed processing order (the order set in step S130).

  Next, it is determined whether or not the target pixel to be processed exists (step S160). If the target pixel exists (YES determination), the process returns to step S140, and the above-described steps S140 to S160 are performed. Repeated.

  When the dot formation determination is completed for all the pixels and the dot arrangement is determined, it is determined in step S160 that the target pixel does not exist. In this case (when NO is determined in step S160), the process proceeds to step S170, and the dot model determined at each position is converted into nozzle number, scanning timing, and nozzle drive signal information.

  At this time, the nozzles used and their sub-scanning positions are obtained from the dots used for dot arrangement using a table or the like, and converted into head drive signals (that is, actuator control signals) on the scanning sequence.

  Thus, a high-quality image can be formed by controlling the ejection operation from the nozzles based on the obtained nozzle number, scanning timing, and nozzle drive signal information. After the process of step S170 in FIG. 25, this process flow is terminated.

  According to the image processing method of this example, an error due to dot spread is diffused from the occurrence position to the vicinity thereof, so that an error of an appropriate position and amount is diffused, so that a high-quality dot arrangement can be obtained.

[Other examples of processing flow (Example 2)]
As described with reference to FIG. 3, when the landing characteristics differ depending on the dot size, each dot size is treated as a dot model belonging to a different position (x, y). However, when the dot sizes suitable for hitting from the same nozzle are handled as dot models at different positions, there are conditions in which droplets cannot be ejected simultaneously (for example, small dots and large dots cannot be ejected at the same timing in scanning).

  In order to solve this problem, a combination of dot models having a relationship incapable of simultaneous droplet ejection as shown in the table of FIG. 27 is obtained in advance, and the case of a relationship incapable of droplet ejection can be eliminated.

  FIG. 27 shows an example of dot model combinations that cannot be ejected simultaneously. A flow for determining the dot arrangement in consideration of the prohibition relationship based on such information is shown in FIGS.

  FIG. 28 is an overall process flow, and FIG. 29 is a subroutine flow of a dot formation determination process for the target pixel. In these drawings, the same or similar steps as those in the flowcharts shown in FIGS. 25 and 26 are denoted by the same step numbers, and the description thereof is omitted.

  In accordance with the processing order set in step S130 of FIG. 28, in step S140, it is determined whether or not to form a dot for the target pixel. That is, as shown in FIG. 29, the dot model of the pixel of interest is determined in steps S141 to S146, error diffusion processing is performed, and the cumulative spread of surrounding unquantized positions is obtained.

  Thereafter, the process proceeds to step S147, and the determined dot model is converted into a nozzle number, a scanning timing, and a nozzle drive signal.

  Subsequently, referring to the prohibited combination table from the dot model determined at the position, a dot model that cannot be used is extracted (step S148). A process of excluding the unusable dot model extracted in step S148 from the candidate dot model is performed (step S149), the process exits the subroutine of FIG. 29, returns to the flowchart of FIG. 28, and proceeds to step S150.

  In step S150, the target pixel is changed to the next position in accordance with a prescribed processing order (the order set in step S130).

  Next, it is determined whether or not the target pixel to be processed exists (step S160). If the target pixel exists (YES determination), the process returns to step S140, and the above-described steps S140 to S160 are performed. Repeated.

  When the dot formation determination is completed for all the pixels and the dot arrangement is determined, it is determined in step S160 that the target pixel does not exist. In this case (when NO is determined in step S160), this processing flow ends.

[Example of application to inkjet recording apparatus]
Next, an ink jet recording apparatus will be described as a specific application example of an image forming apparatus having an image processing function for converting image data into dot arrangement data using the image processing method described above.

  FIG. 30 is an overall configuration diagram of an ink jet recording apparatus showing an embodiment of an image forming apparatus according to the present invention. As shown in the figure, the ink jet recording apparatus 110 includes a plurality of ink jet recording heads (hereinafter referred to as “ink jet recording heads”) corresponding to black (K), cyan (C), magenta (M), and yellow (Y) inks. A printing unit 112 having 112K, 112C, 112M, and 112Y, an ink storage / loading unit 114 that stores ink to be supplied to each of the heads 112K, 112C, 112M, and 112Y, and recording paper as a recording medium The paper feeding unit 118 that supplies the paper 116, the decurling unit 120 that removes curl of the recording paper 116, and the nozzle surface (ink ejection surface) of the printing unit 112 are disposed so as to improve the flatness of the recording paper 116. A belt conveyance unit 122 that conveys the recording paper 116 while holding it, a print detection unit 124 that reads a printing result by the printing unit 112, and recorded And a discharge unit 126 for discharging recording paper (printed matter) to the outside.

  The ink storage / loading unit 114 includes ink tanks that store inks of colors corresponding to the heads 112K, 112C, 112M, and 112Y, and the tanks are connected to the heads 112K, 112C, 112M, and 112Y via a required pipe line. Communicated with. Further, the ink storage / loading unit 114 includes notifying means (display means, warning sound generating means) for notifying when the ink remaining amount is low, and has a mechanism for preventing erroneous loading between colors. ing.

  In FIG. 30, a magazine for rolled paper (continuous paper) is shown as an example of the paper supply unit 118, but a plurality of magazines having different paper widths, paper quality, and the like may be provided side by side. Further, instead of the roll paper magazine or in combination therewith, the paper may be supplied by a cassette in which cut papers are stacked and loaded.

  When a plurality of types of recording media (media) can be used, an information recording body such as a barcode or a wireless tag that records media type information is attached to a magazine, and information on the information recording body is read by a predetermined reader. It is preferable to automatically determine the type of recording medium to be used (media type) and to perform ink ejection control so as to realize appropriate ink ejection according to the media type.

  The recording paper 116 delivered from the paper supply unit 118 retains curl due to having been loaded in the magazine. In order to remove this curl, the decurling unit 120 applies heat to the recording paper 116 by the heating drum 130 in the direction opposite to the curl direction of the magazine. At this time, it is more preferable to control the heating temperature so that the printed surface is slightly curled outward.

  In the case of an apparatus configuration using roll paper, a cutter (first cutter) 128 is provided as shown in FIG. 30, and the roll paper is cut into a desired size by the cutter 128. Note that the cutter 128 is not necessary when cut paper is used.

  After the decurling process, the cut recording paper 116 is sent to the belt conveyance unit 122. The belt conveyance unit 122 has a structure in which an endless belt 133 is wound between rollers 131 and 132, and at least portions facing the nozzle surface of the printing unit 112 and the sensor surface of the printing detection unit 124 are horizontal (flat). Surface).

  The belt 133 has a width that is greater than the width of the recording paper 116, and a plurality of suction holes (not shown) are formed on the belt surface. As shown in FIG. 30, an adsorption chamber 134 is provided at a position facing the nozzle surface of the printing unit 112 and the sensor surface of the printing detection unit 124 inside the belt 133 spanned between the rollers 131 and 132. The recording paper 116 is sucked and held on the belt 133 by sucking the suction chamber 134 with a fan 135 to a negative pressure. In place of the suction adsorption method, an electrostatic adsorption method may be adopted.

  When the power of the motor (reference numeral 188 in FIG. 35) is transmitted to at least one of the rollers 131 and 132 around which the belt 133 is wound, the belt 133 is driven in the clockwise direction in FIG. The held recording paper 116 is conveyed from left to right in FIG.

  Since ink adheres to the belt 133 when a borderless print or the like is printed, the belt cleaning unit 136 is provided at a predetermined position outside the belt 133 (an appropriate position other than the print region). Although details of the configuration of the belt cleaning unit 136 are not illustrated, for example, there are a method of niping a brush roll, a water absorption roll, etc., an air blow method of blowing clean air, or a combination thereof. In the case where the cleaning roll is nipped, the cleaning effect is great if the belt linear velocity and the roller linear velocity are changed.

  Although a mode using a roller / nip conveyance mechanism in place of the belt conveyance unit 122 is also conceivable, if the roller / nip conveyance is performed in the printing area, the image is likely to blur because the roller contacts the printing surface of the sheet immediately after printing. There's a problem. Therefore, as in this example, suction belt conveyance that does not bring the image surface into contact with each other in the print region is preferable.

  A heating fan 140 is provided on the upstream side of the printing unit 112 on the paper conveyance path formed by the belt conveyance unit 122. The heating fan 140 heats the recording paper 116 by blowing heated air onto the recording paper 116 before printing. Heating the recording paper 116 immediately before printing makes it easier for the ink to dry after landing.

  Each of the heads 112K, 112C, 112M, and 112Y of the printing unit 112 has a length corresponding to the maximum paper width of the recording paper 116 targeted by the inkjet recording device 110, and the nozzle surface has a recording medium of the maximum size. This is a full-line type head in which a plurality of nozzles for ejecting ink are arranged over a length exceeding at least one side (full width of the drawable range) (see FIG. 31).

  The heads 112K, 112C, 112M, and 112Y are arranged in the order of black (K), cyan (C), magenta (M), and yellow (Y) from the upstream side along the feeding direction of the recording paper 116. 112K, 112C, 112M, and 112Y are fixedly installed so as to extend along a direction substantially orthogonal to the conveyance direction of the recording paper 116.

  A color image can be formed on the recording paper 116 by discharging different colors of ink from the heads 112K, 112C, 112M, and 112Y while the recording paper 116 is being conveyed by the belt conveyance unit 122.

  As described above, according to the configuration in which the full-line heads 112K, 112C, 112M, and 112Y having nozzle rows that cover the entire width of the paper are provided for each color, the recording paper 116 and the printing unit in the paper feeding direction (sub-scanning direction). An image can be recorded on the entire surface of the recording paper 116 by performing the operation of relatively moving the 112 once (that is, by one sub-scan). Thereby, it is possible to perform high-speed printing as compared with a shuttle type head in which the recording head reciprocates in a direction orthogonal to the paper transport direction, and productivity can be improved.

  In this example, the configuration of KCMY standard colors (four colors) is illustrated, but the combination of ink colors and the number of colors is not limited to this embodiment, and light ink, dark ink, and special color ink are used as necessary. May be added. For example, it is possible to add an ink jet head that discharges light ink such as light cyan and light magenta. Also, the arrangement order of the color heads is not particularly limited.

  The print detection unit 124 shown in FIG. 30 includes an image sensor (line sensor or area sensor) for imaging the droplet ejection result of the printing unit 112. From the droplet ejection image read by the image sensor, nozzle clogging or It functions as a means for checking ejection characteristics such as landing position errors.

  For the print detection unit 124 of this example, a CCD area sensor in which a plurality of light receiving elements (photoelectric conversion elements) are two-dimensionally arranged on the light receiving surface can be suitably used. It is assumed that the area sensor has an imaging range in which the entire area of the ink discharge width (image recording width) by at least the heads 112K, 112C, 112M, and 112Y can be imaged. A required imaging range may be realized by one area sensor, or a required imaging range may be secured by combining (connecting) a plurality of area sensors. Alternatively, a configuration in which the area sensor is supported by a moving mechanism (not shown) and the required imaging range is imaged by moving (scanning) the area sensor is also possible.

  Also, a line sensor can be used instead of the area sensor. In this case, it is preferable that the line sensor has a light receiving element array (photoelectric conversion element array) wider than at least the ink ejection width (image recording width) by each of the heads 112K, 112C, 112M, and 112Y.

  Test patterns or practical images printed by the heads 112K, 112C, 112M, and 112Y of the respective colors are read by the print detection unit 124, and ejection determination of each head is performed. The ejection determination includes the presence / absence of ejection, measurement of dot size, measurement of dot landing position, and the like.

  A post-drying unit 142 is provided following the print detection unit 124. The post-drying unit 142 is means for drying the printed image surface, and for example, a heating fan is used. Since it is preferable to avoid contact with the printing surface until the ink after printing is dried, a method of blowing hot air is preferred.

  When printing on porous paper with dye-based ink, the weather resistance of the image is improved by preventing contact with ozone or other things that cause dye molecules to break by pressurizing the paper holes with pressure. There is an effect to.

  A heating / pressurizing unit 144 is provided following the post-drying unit 142. The heating / pressurizing unit 144 is a means for controlling the glossiness of the image surface, and pressurizes with a pressure roller 145 having a predetermined uneven surface shape while heating the image surface, and transfers the uneven shape to the image surface. To do.

  The printed matter generated in this manner is outputted from the paper output unit 126. It is preferable that the original image to be printed (printed target image) and the test print are discharged separately. The ink jet recording apparatus 110 is provided with a sorting means (not shown) that switches the paper discharge path in order to select the prints of the main image and the prints of the test print and send them to the discharge units 126A and 126B. Yes. Note that when the main image and the test print are simultaneously formed in parallel on a large sheet, the test print portion is separated by the cutter (second cutter) 148. Although not shown in FIG. 30, the paper output unit 126A for the target prints is provided with a sorter for collecting prints according to print orders.

[Head structure]
Next, the structure of the head will be described. Since the structures of the respective heads 112K, 112C, 112M, and 112Y for each color are common, the heads are represented by reference numeral 150 in the following.

  FIG. 32A is a plan perspective view showing an example of the structure of the head 150, and FIG. 32B is an enlarged view of a part thereof. FIG. 32C is a plan perspective view showing another structure example of the head 150, and FIG. 33 is a cross-sectional view showing a three-dimensional configuration of one droplet discharge element (an ink chamber unit corresponding to one nozzle 151). It is (sectional drawing which follows the 33-33 line | wire in Fig.32 (a)).

  In order to increase the dot pitch printed on the recording paper 116, it is necessary to increase the nozzle pitch in the head 150. As shown in FIGS. 32A and 32B, the head 150 of this example includes a plurality of ink chamber units (liquid chambers) each including a nozzle 151 serving as an ink discharge port, a pressure chamber 152 corresponding to each nozzle 151, and the like. Droplet ejecting elements) 153 are arranged in a zigzag matrix (two-dimensionally), and thus are projected so as to be aligned along the longitudinal direction of the head (direction perpendicular to the paper feed direction). High density of nozzle spacing (projection nozzle pitch) is achieved.

  The configuration in which one or more nozzle rows are formed over a length corresponding to the entire width of the recording paper 116 in a direction substantially orthogonal to the feeding direction of the recording paper 116 is not limited to this example. For example, instead of the configuration of FIG. 32 (a), as shown in FIG. 32 (c), short head modules 150 ′ in which a plurality of nozzles 151 are two-dimensionally arranged are arranged in a staggered manner and connected. A line head having a nozzle row having a length corresponding to the entire width of the recording paper 116 may be configured.

  The pressure chamber 152 provided corresponding to each nozzle 151 has a substantially square planar shape (see FIGS. 32 (a) and (b)), and the nozzle 151 is provided at one of the diagonal corners. An outlet for supplying ink (supply port) 154 is provided on the other side. The shape of the pressure chamber 152 is not limited to this example, and the planar shape may have various forms such as a quadrangle (rhombus, rectangle, etc.), a pentagon, a hexagon, other polygons, a circle, and an ellipse.

  As shown in FIG. 33, each pressure chamber 152 communicates with the common channel 155 through the supply port 154. The common channel 155 communicates with an ink tank (not shown) as an ink supply source, and the ink supplied from the ink tank is distributed and supplied to each pressure chamber 152 via the common channel 155.

  An actuator 158 having an individual electrode 157 is joined to a pressure plate (vibrating plate also serving as a common electrode) 156 constituting a part of the pressure chamber 152 (the top surface in FIG. 33). By applying a driving voltage between the individual electrode 157 and the common electrode, the actuator 158 is deformed to change the volume of the pressure chamber 152, and ink is ejected from the nozzle 151 due to the pressure change accompanying this. For the actuator 158, a piezoelectric element using a piezoelectric body such as lead zirconate titanate or barium titanate is preferably used. When the displacement of the actuator 158 returns to its original state after ink ejection, new ink is refilled into the pressure chamber 152 from the common flow path 155 through the supply port 154.

  By controlling the driving of the actuator 158 corresponding to each nozzle 151 according to the dot arrangement data generated from the input image, ink droplets can be ejected from the nozzle 151. As described with reference to FIG. 30, while the recording paper 116 as a recording medium is conveyed in the sub-scanning direction at a constant speed, the ink ejection timing of each nozzle 151 is controlled according to the conveying speed, thereby It is possible to record a desired image.

  As shown in FIG. 34, the ink chamber units 153 having the above-described structure are arranged in a constant arrangement pattern along the row direction along the main scanning direction and the oblique column direction having a constant angle θ not orthogonal to the main scanning direction. The high-density nozzle head of this example is realized by arranging a large number in a lattice pattern.

  That is, with a structure in which a plurality of ink chamber units 153 are arranged at a constant pitch d along the direction of an angle θ with respect to the main scanning direction, the pitch P of the nozzles projected so as to be aligned in the main scanning direction is d × cos θ. Thus, in the main scanning direction, each nozzle 151 can be handled equivalently as a linear arrangement with a constant pitch P. With such a configuration, it is possible to realize a high-density nozzle configuration in which 2400 nozzle rows are projected per inch (2400 nozzles / inch) so as to be aligned in the main scanning direction.

  When the nozzles are driven by a full line head having a nozzle row having a length corresponding to the entire printable width, (1) all the nozzles are driven simultaneously, (2) the nozzles are sequentially moved from one side to the other. (3) The nozzles are divided into blocks, and the nozzles are sequentially driven from one side to the other for each block, etc., and one line (1 in the width direction of the paper (direction perpendicular to the paper conveyance direction)) Driving a nozzle that prints a line of dots in a row or a line consisting of dots in a plurality of rows is defined as main scanning.

  In particular, when driving the nozzles 151 arranged in a matrix as shown in FIG. 34, main scanning as described in (3) above is preferable. That is, nozzles 151-11, 151-12, 151-13, 151-14, 151-15, 151-16 are made into one block (other nozzles 151-21,..., 151-26 are made into one block, Nozzles 151-31,..., 151-36 as one block,..., And by sequentially driving the nozzles 151-11, 151-12,. One line is printed in the width direction of 116.

  On the other hand, by relatively moving the above-mentioned full line head and the paper, printing of one line (a line formed by one line of dots or a line composed of a plurality of lines) formed by the above-described main scanning is repeatedly performed. This is defined as sub-scanning.

  The direction indicated by one line (or the longitudinal direction of the belt-like region) recorded by the main scanning is referred to as a main scanning direction, and the direction in which the sub scanning is performed is referred to as a sub scanning direction. In other words, in the present embodiment, the conveyance direction of the recording paper 116 is the sub-scanning direction, and the direction orthogonal to it is the main scanning direction.

  In implementing the present invention, the nozzle arrangement structure is not limited to the illustrated example. In this embodiment, a method of ejecting ink droplets by deformation of an actuator 158 typified by a piezo element (piezoelectric element) is adopted. However, the method of ejecting ink is not particularly limited in implementing the present invention. Instead of the piezo jet method, various methods such as a thermal jet method in which ink is heated by a heating element such as a heater to generate bubbles and ink droplets are ejected by the pressure can be applied.

[Explanation of control system]
FIG. 35 is a block diagram showing a system configuration of the inkjet recording apparatus 110. As shown in the figure, the inkjet recording apparatus 110 includes a communication interface 170, a system controller 172, an image memory 174, a ROM 175, a motor driver 176, a heater driver 178, a print control unit 180, an image buffer memory 182, a head driver 184, and the like. It has.

  The communication interface 170 is an interface unit (image input unit) that functions as an image input unit that receives image data transmitted from the host computer 186. As the communication interface 170, a serial interface such as USB (Universal Serial Bus), IEEE 1394, Ethernet (registered trademark), a wireless network, or a parallel interface such as Centronics can be applied. In this part, a buffer memory (not shown) for speeding up communication may be mounted.

  Image data sent from the host computer 186 is taken into the inkjet recording apparatus 110 via the communication interface 170 and temporarily stored in the image memory 174. The image memory 174 is a storage unit that stores an image input via the communication interface 170, and data is read and written through the system controller 172. The image memory 174 is not limited to a memory composed of semiconductor elements, and a magnetic medium such as a hard disk may be used.

  The system controller 172 includes a central processing unit (CPU) and its peripheral circuits, and functions as a control device that controls the entire inkjet recording apparatus 110 according to a predetermined program, and also functions as an arithmetic device that performs various calculations. . That is, the system controller 172 controls the communication interface 170, the image memory 174, the motor driver 176, the heater driver 178, and the like, and performs communication control with the host computer 186, read / write control of the image memory 174 and ROM 175, and the like. At the same time, a control signal for controlling the motor 188 and the heater 189 of the transport system is generated.

  Further, the system controller 172 includes a dot measurement calculation unit 172A that performs calculation processing for generating landing position error data, dot shape data, and the like from the test pattern read data read from the print detection unit 124, and the measured dot state And a dot model creation unit 172B that creates dot model data from the above information. In addition, the dot model creation unit 172B functions as an interpolation calculation unit that performs the interpolation calculation described with reference to FIGS. The processing functions of the dot measurement calculation unit 172A and the dot model creation unit 172B can be realized by ASIC, software, or an appropriate combination.

  The ROM 175 stores programs executed by the CPU of the system controller 172, various data necessary for control (including test pattern data for creating dot models), and the like. The ROM 175 may be a non-rewritable storage unit. However, when various types of data are updated as necessary, it is preferable to use a rewritable storage unit such as an EEPROM. Further, by utilizing the storage area of the ROM 175, it is possible to use the ROM 175 as a dot model storage unit.

  The image memory 174 is used as a temporary storage area for image data, and is also used as a program development area and a calculation work area for the CPU.

  The motor driver 176 is a driver (driving circuit) that drives the conveyance motor 188 in accordance with an instruction from the system controller 172. The heater driver 178 is a driver that drives the heater 189 such as the post-drying unit 142 in accordance with an instruction from the system controller 172.

  The print control unit 180 cooperates with the image processing unit 190 in accordance with the control of the system controller 172 to generate various signals for generating ejection control signals from the image data (multi-value input image data) in the image memory 174. It functions as a signal processing unit that performs processing such as processing and correction, and also functions as a drive control unit that supplies the generated ink ejection data to the head driver 184 to control ejection driving of the head 150.

  The image processing unit 190 is a signal processing unit that generates dot arrangement data for each ink color from input image data, and performs high-quality dots by performing halftoning processing based on the above-described dot model for the input image data. It functions as an image processing apparatus (image processing means) that determines the arrangement.

  The image processing unit 190 of this example uses a binary (or multivalue) from density conversion processing (including UCR processing and color conversion) and, if necessary, pixel number conversion processing, density correction processing, and multivalued density data. ) Is a signal processing means for performing halftoning processing (intermediate gradation processing) or the like for conversion into dot arrangement data.

  35, the image processing unit 190 is illustrated as being separate from the system controller 172 and the print control unit 180. For example, the image processing unit 190 is included in the system controller 172 or the print control unit 180. A part thereof may be configured.

  The print control unit 180 also generates an ink ejection data (an actuator control signal corresponding to the nozzles of the head 150) based on the dot arrangement data generated by the image processing unit 190, and an ink ejection data generation unit 180A. And a drive waveform generator 180B. Each of these functional blocks (180A to 180B) can be realized by ASIC, software, or an appropriate combination.

  The ink discharge data generated by the ink discharge data generation unit 180A is given to the head driver 184, and the ink discharge operation of the head 150 is controlled.

  The drive waveform generator 180B is means for generating a drive signal waveform for driving the actuator 158 (see FIG. 33) corresponding to each nozzle 151 of the head 150, and the signal generated by the drive waveform generator 180B. (Drive waveform) is supplied to the head driver 184. Note that the signal output from the drive waveform generation unit 180B may be digital waveform data or an analog voltage signal.

  The print control unit 180 includes an image buffer memory 182, and image data, parameters, and other data are temporarily stored in the image buffer memory 182 when image data is processed in the print control unit 180. In FIG. 35, the image buffer memory 182 is shown in a mode associated with the print control unit 180, but can also be used as the image memory 174. Also possible is an aspect in which the print controller 180 and the system controller 172 are integrated and configured with one processor.

  An outline of the flow of processing from image input to print output is as follows. Image data to be printed is input from the outside via the communication interface 170 and stored in the image memory 174. At this stage, for example, RGB multivalued image data is stored in the image memory 174.

  The data format of the input image is not particularly limited, but is, for example, 8-bit RGB data. The input image is subjected to density conversion processing using a lookup table, and converted to density data (image signal I (i, j)) corresponding to the ink color of the printer. Note that (i, j) represents the position of the pixel, and density data is assigned to each pixel.

  Here, it is assumed that the resolution of the input image matches the resolution of the printer (nozzle resolution). If they do not match, pixel number conversion processing is performed on the input image in accordance with the printer resolution.

  Density conversion processing is general processing, and includes undercolor removal (UCR) processing or distribution processing to light ink in the case of a system using light ink (same color light ink).

  In the case of this example, it is converted into density data of four color inks of C (cyan), M (magenta), Y (yellow), and K (black). Alternatively, in the case of a system including other inks such as LC (light cyan) and LM (light magenta) in addition to the above four colors, it is converted into density data including the ink color.

  In the ink jet recording apparatus 110, a pseudo continuous tone image is formed by changing the droplet ejection density and dot size of fine dots with ink (coloring material) to the human eye. It is necessary to convert to a dot pattern that reproduces the gradation (shading of the image) as faithfully as possible. Therefore, the original image (RGB) data stored in the image memory 174 is sent to the print control unit 180 via the system controller 172 and converted into dot arrangement data for each ink color by the image processing unit 190.

  That is, in the case of this example, it is converted into dot arrangement data of four colors K, C, M, and Y. Thus, the generated dot arrangement data is stored in the image buffer memory 182. This dot arrangement data for each color is converted into KCMY droplet ejection data for ejecting ink from the nozzles of the head 150, and ink ejection data to be printed is determined.

  The head driver 184 outputs a drive signal for driving the actuator 158 corresponding to each nozzle 151 of the head 150 in accordance with the print contents based on the ink ejection data and the drive waveform signal given from the print control unit 180. The head driver 184 may include a feedback control system for keeping the head driving condition constant.

  In this way, when the drive signal output from the head driver 184 is applied to the head 150, ink is ejected from the corresponding nozzle 151. An image is formed on the recording paper 116 by controlling ink ejection from the head 150 in synchronization with the conveyance speed of the recording paper 116.

  As described above, based on the ink discharge data and the drive signal waveform generated through the required signal processing in the print control unit 180, control of the discharge amount and discharge timing of the ink droplets from each nozzle through the head driver 184. Is done. Thereby, a desired dot size and dot arrangement are realized on the recording paper 116 (recording medium).

  As described with reference to FIG. 30, the print detection unit 124 is a block including an image sensor. The print detection unit 124 reads an image printed on the recording paper 116, performs necessary signal processing, and the like to perform a print status (presence / absence of ejection, landing position). Error, dot shape, optical density, etc.) are detected, and the detection result is provided to the print controller 180 and the system controller 172.

  The print control unit 180 performs various corrections on the head 150 based on information obtained from the print detection unit 124 as necessary, and performs cleaning operations (nozzle recovery operation) such as preliminary ejection, suction, and wiping as necessary. Perform the controls to be implemented.

  In this example, the combination of the print detection unit 124, the dot measurement calculation unit 172A, and the dot model creation unit 172B corresponds to “dot model information acquisition means”, and the combination of the print detection unit 124 and the dot measurement calculation unit 172A is “dot measurement”. It corresponds to “means”. The print control unit 180 or a combination of the print control unit 180 and the system controller 172 corresponds to “recording control unit”.

  FIG. 36 is a block diagram illustrating a schematic configuration example of the image processing unit 190. As shown in FIG. 36, the image processing unit 190 mainly includes a quantization determination processing unit 202, a dot model determination processing unit 204, an error diffusion processing unit 206, and a cumulative spread processing unit 208. Each of these functional blocks (202 to 208) in the range surrounded by the one-dot chain line in the figure can be realized by ASIC, software, or an appropriate combination.

  The image processing unit 190 includes an image data input unit 210, an image data storage unit 212, a pixel processing order storage unit 214, a threshold enable unit 216, a dot model storage unit 218, a prohibition condition storage unit 220, and a cumulative error storage unit 222. A cumulative spread storage unit 224, a processing result storage unit 226, and an output unit 228. Note that the storage means (212 to 226) mentioned here can be realized by utilizing the storage areas of the storage means such as the image memory 174, the ROM 175, and the image buffer memory 182 described with reference to FIG.

  The image data input unit 210 is an interface unit that captures input image data. The image data input from the image data input unit 210 is stored in the image data storage unit 212. The image data storage unit 212 is a storage unit that stores the image signal I (x, y) necessary for the determination process of the quantization determination processing unit 202.

  The pixel processing order storage unit 214 indicates the pixel processing order (dot formation position order) when determining whether or not to form dots at each dot formable position on the dot coordinates during the halftoning process. Storage means for storing the array. If the order is clearly determined as in the raster order, the pixel processing order enabler 214 may be omitted.

  The threshold storage unit 216 enables a threshold used for determination of dot formation, for example, T (x, y) in [Determination Formula 1-1] and T (x, y, s) in [Determination Formula 1-2]. It is a storage means. As described in “quantization determination 2”, when the pixel dot model is determined without using the threshold value, the threshold value storage unit 216 can be omitted, and the quantization determination processing unit 202 and the dot model can be omitted. The decision processing unit 204 is configured as one integrated processing block.

  The dot model storage unit 218 is a storage unit that stores information on the dot model created by the dot model creation unit 172B described with reference to FIG. The prohibition condition storage unit 220 in FIG. 36 is a storage unit that stores data of combinations of dots that cannot be ejected simultaneously, and stores, for example, tables as described with reference to FIGS.

  The accumulated error storage unit 222 is a storage unit that stores the value of the accumulated error E (x, y) used for the calculation of the error diffusion processing unit 206. The cumulative spread storage unit 224 is a storage unit that stores the value of the cumulative spread S (x, y) used for the calculation of the cumulative spread processing unit 208.

  The quantization determination processing unit 202 is a processing unit that determines whether or not to form dots for each dot formable position in a predetermined processing order with respect to input image data. For example, the [determination formula described above] Quantization is determined using 1-1] or [Determination 1-2].

  Based on the determination result of the quantization determination processing unit 202, the dot model determination processing unit 204 performs processing for determining a dot model of a dot to be formed at the position. For example, as described above, there are a mode in which a random number is used for determination and a mode in which the function F (t) in [Equation 1-3] is used for determination.

  The error diffusion processing unit 206 is a processing unit that performs a process of diffusing a pixel generated by quantization of a target pixel to surrounding unquantized pixels. In the example of FIG. 36, the generated error initial value calculation unit 230, the first diffusion processing unit 231, the second diffusion processing unit 232, and the third corresponding to the error diffusion processing algorithm described in FIGS. A diffusion processing unit 233 is included. Specific processing examples of each processing block are as described with reference to FIGS.

  The cumulative spread processing unit 208 is a processing unit that calculates the value of the cumulative spread S (x, y) of the unquantized position used for quantization determination, and the calculation result is stored in the cumulative spread storage unit 224.

  The processing result storage unit 226 is a storage unit that stores a quantization result (dot arrangement) determined for each dot formable position through the processing of the dot model determination processing unit 204. The created dot arrangement data is output from the processing result storage unit 226 to the output unit 228.

  Next, a method for acquiring dot model information in the inkjet recording apparatus 110 configured as described above will be described.

  FIG. 37 is a flowchart showing a procedure for creating dot model information. As shown in the figure, first, dot coordinates are set on the recording medium (step S210), and dot model measurement areas are set at a plurality of positions in the sub-scanning direction on the recording medium as described in FIG. 12 (FIG. 37). Step S212). For example, the dot model measurement area is set as a strip-shaped area arranged at a constant interval in the sub-scanning direction.

  Next, a predetermined test pattern is printed in each dot model measurement area (step S214 in FIG. 37). The form of the test pattern is not particularly limited, but it is desirable that the test pattern be a pattern that can measure the shape of the dots by distinguishing the dots recorded by each nozzle.

  Next, the dot model information corresponding to each position in the main scanning direction is acquired by measuring the dots recorded in each dot model measurement area (step S216). In the case of this example, dot measurement is performed using the print detection unit 124 described with reference to FIG. Even if a predetermined number (multiple) of dots are formed from one nozzle, they are measured, statistical processing such as taking the average value of the measurement results of each dot is performed, and a dot model is created using the results Good.

  Next, dot model information is obtained by interpolation calculation for the position of an unmeasured area other than the dot model measurement area (step S218). As described with reference to FIGS. 12 to 16, the dot model of the unmeasured area is obtained by using the information of the dot model at the actually measured position.

  In this way, dot model information at each position on the dot coordinates is acquired. Each dot model is associated with information such as a nozzle number for ejecting droplets, scanning timing, and the like, and data indicating the corresponding relationship is stored (step S220 in FIG. 37).

  Further, as described with reference to FIGS. 5 and 27, a combination of dots that cannot be ejected simultaneously is specified, and the information (prohibition condition) is stored (step S222 in FIG. 37). In the case of this example, in the case of this example, the dot model and the data indicating the corresponding relationship are stored in the dot model storage unit 218 described with reference to FIG. The prohibition condition data is stored in the prohibition condition storage unit 220 of FIG.

  According to the ink jet recording apparatus 110 according to the present embodiment described above, since the dot arrangement can be determined at the coordinate position on the recording medium where the dots are actually formed, a high quality dot arrangement can be realized. In addition, by using dots of different dot sizes separately on the dot coordinates and using a prohibition table when simultaneous droplet ejection is not possible, dot placement quality can be improved even when the landing characteristics differ depending on the dot size.

  In the above embodiment, a page-wide line head has been described. However, the application of the present invention is not limited to a line head type printer, and can also be applied to multi-pass scanning by a shuttle scanning method and overlap scanning by a short head. is there. At this time, it is necessary to handle dot droplets for each scan in a unified manner, and to appropriately reflect them in a combination of tots that cannot be deposited simultaneously.

  Further, the memory capacity of the dot model, the dot coordinates, and the combination prohibition table can be reduced by utilizing the regularity when the same nozzle appears many times in the dot coordinates, the regularity of the fluctuation in the sub-scanning direction, and the like.

  Further, the memory capacity can be reduced by storing dot models at finer intervals and generating dot coordinate fineness data corresponding to fluctuations in the sub-scanning direction.

  Although the inkjet recording apparatus has been described as an example of the image forming apparatus in the above embodiment, the scope of application of the present invention is not limited to this. Other than the ink jet system, a thermal transfer recording apparatus including a recording head using a thermal element as a recording element, an LED electrophotographic printer including a recording head using an LED element as a recording element, and a silver salt photographic printer including an LED line exposure head The present invention can also be applied to various types of image forming apparatuses.

Explanatory drawing for demonstrating the conventional nozzle coordinate and the dot coordinate used by embodiment of this invention Chart illustrating dot coordinates, dots that can be ejected at each coordinate position, and the correspondence between nozzles corresponding to each dot Diagram showing an example of different landing characteristics depending on dot size Chart illustrating dot coordinates corresponding to changes in dot size, dots that can be ejected at each coordinate position, and nozzles corresponding to each dot Chart showing examples of dot combinations that cannot be simultaneously ejected The figure which showed the example when the nozzle interval is not uniform Explanatory diagram showing the concept of the dot model Explanatory drawing illustrating the variation of the dot model Explanatory drawing showing an example of numerical representation of the dot model Diagram showing an example of a dot model divided more finely than dot coordinates The figure which shows an example when dot coordinates differ with sub-scanning positions Figure showing an example of setting the dot measurement area Illustration of dot image Enlarged view of dot image Explanatory diagram showing how to calculate the dot model by interpolation Explanatory diagram showing how to calculate the dot model by interpolation Explanatory drawing used to explain the interpolation calculation method The figure which shows an example of the processing order of a pixel Explanatory diagram for explaining the quantized and unquantized regions Explanatory diagram illustrating the relationship between the dot model of the dot placed on the pixel of interest and the surrounding pixels Explanatory diagram illustrating the relationship between the dot model of the dot placed on the pixel of interest and the surrounding pixels Explanatory drawing for explaining the method of error diffusion processing A chart showing the correspondence between the positions on the dot coordinates and the positions on the dot model in FIG. Explanatory drawing showing an example when two dots A and B can be ejected independently Explanatory drawing which shows the example in the case of distinguishing four types of dot models using the variable t The flowchart which shows the 1st example of the processing flow of the image processing method by this embodiment. 25 is a flowchart showing a subroutine for dot formation determination processing in FIG. Chart showing examples of dot combinations that cannot be ejected simultaneously The flowchart which shows the 2nd example of the processing flow of the image processing method by this embodiment. 28 is a flowchart showing a subroutine for dot formation determination processing in FIG. 1 is an overall configuration diagram of an ink jet recording apparatus showing an embodiment of an image forming apparatus according to the present invention. Fig. 30 is a plan view of the main part around the printing unit of the inkjet recording apparatus shown in Fig. 30. Plane perspective view showing structural example of head Fig. 32 (a) main part enlarged view Plane perspective view showing another structure example of a full-line head Sectional drawing which follows the 33-33 line in Drawing 32 (a) FIG. 32A is an enlarged view showing the nozzle arrangement of the head shown in FIG. Main part block diagram which shows the system configuration | structure of the inkjet recording device which concerns on this embodiment. The block diagram which shows schematic structure of the image process part in FIG. Flow chart showing the acquisition procedure of dot model information in the ink jet recording apparatus of this example Explanatory drawing for demonstrating when the position where a dot lands greatly shifts (dot fluctuation) Explanatory diagram for explaining that there are dots that cannot be used to determine the dot placement due to dot distortion

Explanation of symbols

DESCRIPTION OF SYMBOLS 10,30 ... Head, 20 ... Recording medium, 50 ... Dot, 60 ... Line head, 62 ... Recording medium, 110 ... Inkjet recording device, 112 ... Printing section, 112K, 112C, 112M, 112Y ... Head, 114 ... Ink storage / Loading unit 116 ... Recording paper 122 ... Belt conveying unit 124 ... Print detection unit 150 ... Head 151 ... Nozzle 152 ... Pressure chamber 153 ... Ink chamber unit 158 ... Actuator 172 ... System controller 172A ... dot measurement calculation unit, 172B ... dot model creation unit, 180 ... print control unit, 190 ... image processing unit, 202 ... quantization determination processing unit, 204 ... dot model determination processing unit, 206 ... error diffusion processing unit, 208 ... Cumulative spread processing unit, 218 ... dot model storage unit, 220 ... prohibition condition storage unit

Claims (7)

An image processing method for determining a dot arrangement when forming an image indicated by the input image data on the recording medium by recording dots on the recording medium according to the input image data,
Dots of dots that can be formed at the positions according to the positions on the coordinates using coordinates that can be regularly separated on the recording medium by predetermined unit cells and the positions of the cells can be specified as positions where dots can be formed. Dot model information acquisition process to acquire model information;
Quantize multi-value input image data for each dot formable position on the coordinates, and determine the dot model of the dot to be placed at each position based on the dot model information acquired in the dot model information acquisition step A dot arrangement determining step to perform,
An image processing method comprising: The dot model information acquisition step includes
Dot model information is acquired at different positions on the recording medium with respect to the direction of relative movement between the recording head in which a plurality of recording elements for recording dots on the recording medium are arranged and the recording medium. The image processing method according to claim 1. The dot model information acquisition step includes a dot measurement step of acquiring dot model information by measuring dots recorded on the recording medium by the recording head;
An interpolation calculation step for obtaining a dot model at another position by interpolation calculation from information on dot models for a plurality of positions obtained in the dot measurement step;
The image processing method according to claim 2, further comprising: Including a prohibition condition specifying step of specifying a combination of dots that cannot be simultaneously recorded,
4. The dot model of a target position is determined from dot candidates that can be recorded in consideration of the combination of dots that cannot be simultaneously recorded in the dot arrangement determining step. An image processing method described in 1. An image processing apparatus that performs a process of determining a dot arrangement when an image indicated by the input image data is formed on the recording medium by recording dots on the recording medium according to the input image data,
The recording medium is regularly divided by predetermined unit cells, and the dot model information of dots that can be formed at the positions is stored according to the positions on the coordinates where the positions of the cells can be specified as the positions where dots can be formed. Dot model storage means for
Quantize multi-value input image data for each dot formable position on the coordinates, and based on the dot model information stored in the dot model storage means, dot models of dots to be placed at the respective positions An image processing apparatus comprising: a dot arrangement determining unit for determining. A recording head in which a plurality of recording elements are arranged;
Relative movement means for relatively moving the recording head and the recording medium;
Dots of dots that can be formed at the positions according to the positions on the coordinates using coordinates that can be regularly separated on the recording medium by predetermined unit cells and the positions of the cells can be specified as positions where dots can be formed. Dot model information acquisition means for acquiring model information;
Dot model storage means for storing dot model information acquired by the dot model information acquisition means;
Quantize multi-value input image data for each dot formable position on the coordinates, and based on the dot model information stored in the dot model storage means, dot models of dots to be placed at the respective positions A dot arrangement determining means for determining; a recording control means for controlling the drive of the recording element based on the dot arrangement data on the coordinates determined by the dot arrangement determining means;
An image forming apparatus comprising: Dot measuring means for obtaining dot model information by measuring dots recorded on the recording medium by the recording head;
Interpolation calculation means for obtaining dot models at other positions by interpolation calculation from dot model information about a plurality of positions obtained by the dot measurement means;
The image forming apparatus according to claim 6, further comprising:

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