JP4973353B2 - Control of ink discharge amount based on dot formation order - Google Patents

Description

  The present invention relates to a technique for printing an image by forming dots on a print medium.

  2. Description of the Related Art Printing apparatuses that print ink by forming ink dots by scanning on a printing medium are widely used as output apparatuses for images created by a computer or images taken by a digital camera. On the other hand, as such a printing apparatus, a printing apparatus including a plurality of print heads has been proposed.

JP-A-6-143895 JP-A-7-314656 JP 2002-166538 A JP-A-6-328678 JP 2002-307671 A JP 2002-166536 A

  However, in a printing apparatus having a plurality of print heads, when pigment ink is used, it has been confirmed through experiments and analyzes that ink aggregation (reduction of spots due to ink gathering) and bleeding (ink bleeding) occur remarkably. The present inventor found out.

  The present invention has been made to solve the above-described problems in the prior art, and an object thereof is to provide a technique for suppressing image quality deterioration caused by ink aggregation and bleeding.

In order to solve at least a part of the problems described above, the present invention provides a printing apparatus that performs printing on a print medium. This printing device
In each main scan, a preceding dot group formed by a preceding print head that ejects pigment ink droplets, which are ink droplets of pigment ink, to the print medium in advance, and subsequently ejects the pigment ink droplets to the print medium. A printing unit that performs printing by combining a plurality of dot groups including the subsequent dot group formed by the subsequent print head in a common print region,
The printing unit is configured such that the preceding dot density that is the dot density of the preceding dot group is higher than the subsequent dot density that is the dot density of the subsequent dot group.

  In the printing apparatus of the present invention, printing is performed such that the preceding dot density, which is the dot density of the preceding dot group formed in advance, is higher than the subsequent dot density, which is the dot density of the subsequent dot group. The flow of the pigment ink droplets ejected by the subsequent print head can be suppressed by the permeation prevention film formed by the pigment ink droplets ejected on the print medium by the print head. Thereby, since aggregation and bleeding of ink can be suppressed, the color reproduction range can be expanded by increasing the ink density while maintaining the printing image quality. “Dot density” means a value obtained by dividing the total area of dots formed on a print medium having a predetermined area by the predetermined area, and the dot density increases as the dot size and the number of dots increase.

In the above printing apparatus,
The printing unit may form dots so that the preceding dot density is higher than the subsequent dot density when the type of the printing medium is glossy paper.

  Since the formation of the permeation prevention film is remarkable when glossy paper is used, the present invention has a remarkable effect. In the present specification, “glossy paper” means that the surface (recording surface) has a 75 degree glossiness of 40% or more based on the provisions of JIS-P8142.

In the above printing apparatus,
In the printing unit, when a plurality of types of printing media are preset as the printing medium, and the type of the printing medium is glossy paper, the succeeding dot density of the preceding dot density among the plurality of types of printing media. You may make it form a dot so that ratio with respect to may become the highest.

In the above printing apparatus,
The printing apparatus may be configured to complete printing of each raster line in the common print area in one main scan,
Or
The printing apparatus may be a line printer that completes printing in the common printing area in one scan.

  The present invention has a remarkable effect in printing that ejects ink droplets without gaps on each raster line in order to complete each main scan line in one main scan, and ejects ink droplets without gaps between raster lines. In the line printer which performs, there is a further remarkable effect.

In the printing apparatus, the printing unit includes:
A halftone process is performed on the image data representing the gradation value of each pixel constituting the original image, thereby determining a dot formation state on each print pixel of the print image to be formed on the print medium. A dot data generation unit that generates dot data representing the determined dot formation state, and
According to the dot data, a print image is generated by combining dot groups formed in each of a plurality of pixel groups assumed to have different physical conditions regarding the dot formation in a common print area. A print image generator to
With
The halftone processing may be configured such that the distribution of the number of dots in each of the plurality of dot groups is determined according to the dot formation order of each of the plurality of dot groups in the common print region. Good.

  In this way, dot formation is performed by halftone processing configured to determine the distribution of the number of dots in each of the plurality of dot groups according to the dot formation order of each of the plurality of dot groups in the common print region. Since the state is determined, the distribution of the number of dots in each of the plurality of dot groups can be determined in accordance with various printing environments such as the characteristics of the printing medium and ink, and combinations thereof, thereby suppressing ink flow. Such a halftone process can also be realized by an error diffusion method. For example, the error diffusion method may be configured to approach the above-described distribution of the number of dots by adjusting at least one of the threshold value of the error diffusion method and the input gradation value.

In the above printing apparatus,
The halftone process determines the dot formation state using each threshold stored in a predetermined dither matrix,
The dot formation state is determined by adjusting at least one of the threshold value and the input gradation value according to the dot formation order of each of the plurality of dot groups in the common print region. It may be configured to approach the distribution.

In the above printing apparatus,
The halftone processing may be configured such that the earlier the dot formation order, the greater the distribution of the number of dots in each of the plurality of dot groups. In this way, a remarkable effect can be obtained in a printing environment in which the ink that has landed first prevents the ink that has landed later from being absorbed.

In the above printing apparatus,
In the halftone process, each of the plurality of dot groups is set in a region having a gradation value equal to or higher than a first gradation value set in advance and equal to or lower than a second gradation value set in advance. It may be configured so that the distribution of the number of dots approaches.

  With such a gradation value, ink flow tends to occur, but the dot granularity is conspicuous. Thus, it is possible to effectively suppress bleeding while maintaining suppression of dot granularity.

In the above printing apparatus,
In the predetermined dither matrix, each threshold value may be stored in each element so that each of the plurality of dot groups has a preset common characteristic.

  In this way, in each of the plurality of dot groups, it is possible to suppress bleeding and aggregation caused by the flow of ink that occurs in the low frequency region, and to produce a more remarkable effect. Furthermore, in the present invention, for example, the ink density is different between the preceding head dot pattern Dp1 and the succeeding head dot pattern Dp2, so that one dot pattern is conspicuous, resulting in a low frequency region resulting from the one dot pattern. Graininess degrades image quality.

In the above printing apparatus,
The common characteristic is a value represented by a graininess index calculated by a calculation process including a Fourier transform process,
The graininess index may be calculated based on a product of a VTF function determined based on a visual spatial frequency characteristic and a constant calculated in advance by the Fourier transform process.
Or
The common characteristic may be a value represented by RMS granularity calculated by a calculation process including a low-pass filter process.

  In the present invention, various forms such as a printing control device, a dither matrix, a dither matrix generation device, a printing device and a printing method using the dither matrix, and a printed material generation method, or the functions of these methods or devices are stored in a computer. The present invention can be realized in various forms such as a computer program to be realized, a recording medium on which the computer program is recorded, and a data signal that includes the computer program and is embodied in a carrier wave.

  In addition, the use of a dither matrix in a printing apparatus, a printing method, and a printed material generation method is such that dot formation is performed for each pixel by comparing the threshold value set in the dither matrix and the gradation value of the image data for each pixel. Although the presence / absence is determined, for example, the presence / absence of dot formation may be determined by comparing the sum of the threshold value and the gradation value with a fixed value. Furthermore, the presence / absence of dot formation may be determined according to the data generated in advance based on the threshold value and the gradation value without directly using the threshold value. In general, the dither method of the present invention only needs to determine the presence or absence of dot formation according to the gradation value of each pixel and the threshold value set at the corresponding pixel position of the dither matrix.

Below, in order to demonstrate the effect | action and effect of this invention more clearly, embodiment of this invention is described in the following orders.
A. Example of printing system configuration:
B. Halftone processing in the first embodiment of the present invention:
C. Halftone processing in the second embodiment of the present invention:
D. Halftone processing in the third embodiment of the present invention:
E. Variations:

A. Example of printing system configuration:
FIG. 1 is a block diagram illustrating an example of a configuration of a printing system. This printing system includes a computer 90 as a printing control device and a color printer 20 as a printing unit. The combination of the color printer 20 and the computer 90 can be called a “printing apparatus” in a broad sense.

  In the computer 90, an application program 95 operates under a predetermined operating system. A video driver 91 and a printer driver 96 are incorporated in the operating system, and print data PD to be transferred to the color printer 20 is output from the application program 95 via these drivers. The application program 95 performs desired processing on the image to be processed, and displays the image on the CRT 21 via the video driver 91.

  Inside the printer driver 96, there are a resolution conversion module 97 for converting the resolution of the input image into a print resolution, a color conversion module 98 for converting RGB into CMYK, and a dither matrix M and errors generated in the embodiments described later. A halftone module 99 that reduces the input gradation value to the number of output gradations that can be expressed by dot formation using the diffusion method, and print data to be transmitted to the color printer 20 using the halftone data are generated. The print data generation module 100, a color conversion table LUT used as a reference for color conversion by the color conversion module 98, and a recording rate table DT for determining the recording rate of dots of each size for halftone processing are provided. It has been. The printer driver 96 corresponds to a program for realizing a function for generating print data PD. A program for realizing the function of the printer driver 96 is supplied in a form recorded on a computer-readable recording medium. Examples of such a recording medium include a CD-ROM 126, a flexible disk, a magneto-optical disk, an IC card, a ROM cartridge, a punch card, a printed matter on which a code such as a bar code is printed, an internal storage device of a computer (RAM, ROM, etc. A variety of computer-readable media such as an external storage device and an external storage device.

  FIG. 2 is a schematic configuration diagram of the color printer 20. The color printer 20 includes a sub-scanning drive unit that transports the print medium P in the sub-scanning direction by the paper feed motor 22, a head drive mechanism that drives the print heads 10A and 10B to control ink ejection and dot formation, The paper feed motor 22, the carriage drive unit 24, the print head unit 60 including the print heads 10 </ b> A and 10 </ b> B, and the control circuit 40 that controls the exchange of signals with the operation panel 32. The control circuit 40 is connected to the computer 90 via the connector 56. In the color printer 20, the main scanning of the print heads 10A and 10B is not performed.

  FIG. 3 corresponds to the arrow AA in FIG. 2 and is an explanatory diagram showing the nozzle arrangement on the lower surfaces of the print heads 10A and 10B. On the lower surface of each of the print heads 10A and 10B, a black ink nozzle row K for discharging black ink, a cyan ink nozzle row C for discharging cyan ink, and a magenta ink nozzle for discharging magenta ink. Rows Mz and yellow ink nozzles Y for discharging yellow ink are formed.

  The plurality of nozzles Nz in each nozzle row are aligned at a constant nozzle pitch k · D along a direction perpendicular to the paper feed direction. Here, k is an integer, and D is a pitch (referred to as “dot pitch”) corresponding to the printing resolution in the sub-scanning direction. In this specification, it is also referred to as “nozzle pitch is k dots”. The unit [dot] at this time means the dot pitch of the printing resolution. Similarly, the unit of [dot] is used for the sub-scan feed amount.

  In each nozzle row C, Mz, Y, and K provided in each of the two print heads 10A and 10B, the nozzle pitch k is 2. On the other hand, since the two print heads 10A and 10B are arranged at positions shifted by half the nozzle pitch k in the direction perpendicular to the paper feed direction, the two print heads 10A and 10B pass through each pixel. Ink of each color can be ejected without causing it.

  FIG. 4 corresponds to the arrow BB in FIG. 2 and is an explanatory diagram showing the lateral surfaces of the print heads 10A and 10B. In FIG. 4, only the nozzle row that discharges yellow ink Y is shown for easy understanding. In FIG. 4, since the print medium P is fed in the paper feed direction indicated by the arrow, the ink droplets are first ejected by the print head 10A and then the ink is ejected by the print head 10B to the common print area. Will be. As can be seen from FIG. 3, the print head 10 </ b> A and the print head 10 </ b> B eject ink droplets to pixels adjacent to each other in a common print area, although they are different from each other. As described above, a print head that ejects ink droplets first with respect to a common print region is called a preceding head, and a print head that ejects ink droplets later is called a subsequent head. The set of dots (dot group) formed by the print head 10A and the set of dots (dot group) formed by the print head 10B are “each of the plurality of dot groups” in the claims. It corresponds to.

  FIG. 5 is an explanatory diagram showing the mechanism of a phenomenon called ink aggregation or bleeding. Ink agglomeration and bleeding occur through three stages. The first stage is a stage in which ink droplets are ejected to a plurality of adjacent pixels. At this stage, a plurality of ink droplets are present on the print medium P as balls. The second stage is a stage where a plurality of ink droplets are connected. At this stage, the ink becomes two lines in the paper feed direction and can move freely on this line. The third stage is a stage where an ink pool occurs. Due to the surface tension of the ink, the ink gathers at the center of the two lines, and the ink reservoir Ri is generated. By such ink flow, ink aggregation (reduction of spots due to ink gathering) and bleeding (ink bleeding) occur.

  FIG. 6 is an explanatory diagram showing the state of the ink flow mechanism when ink droplets are ejected by the preceding head and the succeeding head. When ink is ejected by the preceding head and an ink droplet film is formed on the print medium P, and ink droplets are ejected by the succeeding head, the ink droplet film formed by the preceding head (permeation prevention) The inventor of the present application has found a phenomenon in which the penetration of the ink droplets ejected by the subsequent head into the print medium P is inhibited by the film. The preceding head and the succeeding head eject ink droplets to pixels that are different from each other. However, ink droplets are ejected so as to fill each pixel, and there is a positional error. Will be. The print medium P (FIG. 6) of this embodiment includes a gloss layer PLs and a void-type ink absorption layer PLa. In the present embodiment, the gloss layer PLs uses extremely fine ultrafine silica in order to achieve high glossiness.

  According to the analysis of the present inventor, the permeation blocking membrane is formed as follows. When ink droplets containing a pigment that does not dissolve in the medium are ejected onto the glossy layer PLs by the preceding head, the pigment clogs the fine gaps in the glossy layer PLs. The permeation prevention membrane is generated by such clogging. When paying attention to such a mechanism, the permeation-preventing film becomes noticeable by ejecting the pigment ink onto a printing medium having a gloss layer that is miniaturized in order to achieve high glossiness. Furthermore, the inventor has predicted and confirmed experimentally that the ink fixing method is more prominent in the gap-type glossy paper that is permeation fixing than the swelling type glossy paper based on such a mechanism. Here, “glossy paper” means that the surface (recording surface) has a 75 degree glossiness of 40% or more (JIS-P8142).

  According to the analysis of the inventor of the present application, as the type of ink, a permeation-preventing film is remarkably generated with an aqueous pigment or an oily pigment, an aqueous thermosetting acrylic pigment, a soft solvent pigment, a solvent pigment, a UV curable pigment. The possibility of occurrence of pigments (although not as much as aqueous pigments and oil pigments) is predicted. On the other hand, since the dye ink dissolved in the solvent is not clogged, it was also found that the permeation preventing film hardly occurs.

  Based on the elucidation of such a mechanism, the present inventor has devised an optimal ink droplet ejection method considering such a phenomenon. The present invention controls the distribution of ink discharge amounts of the preceding head and the succeeding head. That is, the ejection distribution of the ink droplets by the preceding head is increased, and the ink ejection distribution of the succeeding head is decreased by that amount, thereby suppressing the flow of the ink droplets ejected to the succeeding head.

  The present invention can be implemented by various methods such as various halftone processing methods such as the dither method and error diffusion method, distribution of dot data to each recording scan after the halftone processing, and adjustment of the print head drive signal. is there. Hereinafter, the dither method will be described as an example.

  FIG. 7 is an explanatory diagram conceptually illustrating a part of the dither matrix M. In the illustrated matrix, 256 elements in the horizontal direction (main scanning direction), 64 elements in the vertical direction (sub-scanning direction), a total of 16384 elements, were selected uniformly from the range of gradation values 1 to 255. A threshold value is stored. Note that the size of the dither matrix M is not limited to the size illustrated in FIG. 7 and can be various sizes including a matrix having the same number of vertical and horizontal elements.

  FIG. 8 is an explanatory diagram showing the concept of dot formation using a dither matrix. For the sake of illustration, only some elements are shown. In the determination of the presence / absence of dot formation, as shown in FIG. 8, the gradation value of the image data is compared with the threshold value stored at the corresponding position in the dither matrix. If the gradation value of the image data is larger than the threshold value stored in the dither table, a dot is formed, and if the gradation value of the image data is smaller, no dot is formed. In FIG. 8, a hatched pixel means a pixel on which a dot is to be formed. In this way, if the dither matrix is used, it is possible to determine the presence or absence of dot formation for each pixel by a simple process of comparing the gradation value of the image data and the threshold value set in the dither matrix. The gradation number conversion process can be performed quickly. Furthermore, when the gradation value of the image data is determined, whether or not dots are formed in each pixel is determined solely by the threshold value set in the dither matrix. It is possible to positively control the dot generation state by the threshold storage position set in the matrix.

  As described above, since the systematic dither method can positively control the occurrence of dots by the threshold storage position set in the dither matrix M, the dot storage position can be adjusted by adjusting the threshold storage position setting. Dispersibility and other image quality can be controlled.

  FIG. 9 is an explanatory diagram conceptually illustrating a spatial frequency characteristic of a threshold value set for each pixel of a blue noise dither matrix having a blue noise characteristic as a simple example of adjustment of the dither matrix. The spatial frequency characteristic of the blue noise matrix is a characteristic in which the length of one cycle has the largest frequency component in a high frequency region near two pixels. Such spatial frequency characteristics are set in consideration of human visual characteristics. That is, the blue noise dither matrix is a dither matrix M in which the threshold storage position is adjusted so that the largest frequency component is generated in the high frequency region in consideration of the human visual characteristic that the sensitivity is low in the high frequency region.

  FIG. 9 further illustrates the spatial frequency characteristics of the green noise matrix as a dashed curve. As shown in the figure, the spatial frequency characteristic of the green noise matrix is a characteristic having the largest frequency component in the intermediate frequency region in which the length of one cycle is from 2 pixels to several tens of pixels. Since the threshold value of the green noise matrix is set so as to have such a spatial frequency characteristic, when it is determined whether or not dots are formed in each pixel with reference to the dither matrix M having the green noise characteristic, a unit of several dots is obtained. While dots are formed adjacent to each other, the dots are formed in a dispersed state as a whole. In printers where it is difficult to stably form fine dots of about one pixel, such as so-called laser printers, it is possible to identify isolated dots by determining the presence or absence of dot formation with reference to such a green noise matrix. Occurrence can be suppressed. As a result, it is possible to quickly output an image with stable image quality. In other words, a dither matrix that is referred to when determining the presence or absence of dot formation by a laser printer or the like is set with a threshold value adjusted to have green noise characteristics.

  FIG. 10 is an explanatory diagram conceptually showing a visual spatial frequency characteristic VTF (Visual Transfer Function) which is a sensitivity characteristic with respect to the visual spatial frequency of a human. By using the visual spatial frequency characteristic VTF, the human visual sensitivity is modeled as a transfer function called the visual spatial frequency characteristic VTF, thereby quantifying the graininess of the dots after the halftone process appealing to the human visual sense. It becomes possible. The value quantified in this way is called the graininess index. Formula F1 shows a typical experimental formula representing the visual spatial frequency characteristic VTF. The variable L in the formula F1 represents the observation distance, and the variable u represents the spatial frequency. Formula F2 is a formula that defines the graininess index. The coefficient K in the formula F2 is a coefficient for matching the obtained value with the human sense.

  Such quantification of the granularity that appeals to human vision enables fine optimization of the dither matrix for the human visual system. Specifically, Fourier transform is performed on the dot pattern assumed when each input gradation value is input to the dither matrix to obtain the power spectrum FS, and all the inputs are obtained after multiplication with the visual spatial frequency characteristic VTF. The graininess index that can be obtained by integrating with the gradation value (formula F2) can be used as an evaluation function of the dither matrix. In this example, the optimization can be achieved by adjusting the threshold storage position so that the evaluation function of the dither matrix becomes small.

  What is common to dither matrices such as blue noise dither matrix and green noise matrix set in consideration of such human visual characteristics is the region of the spatial frequency with the highest human visual sensitivity on the print medium. This is that the average value of the components within a predetermined low frequency range from 0.5 millimeters per millimeter to 2 millimeters per cycle with a center frequency of 1 millimeter per cycle is set to be small. For example, the average value of the components in a predetermined low frequency range is at least 5 cycles per millimeter to 20 cycles per millimeter with a frequency of 10 cycles per millimeter at which human visual sensitivity is almost zero. By having a frequency characteristic that is smaller than the average value, graininess can be suppressed in a region where human visual sensitivity is high, so that effective image quality improvement focusing on human visual sensitivity is performed. The inventor has confirmed that this is possible.

B. Halftone processing in the first embodiment of the present invention:
FIG. 11 is a flowchart showing a routine of print data generation processing in the first embodiment of the present invention. The print data generation process is a process performed by the computer 90 to generate print data PD to be supplied to the color printer 20.

  In step S100, the printer driver 96 (FIG. 1) inputs image data from the application program 95. This input process is performed in response to a print command from the application program 95. Here, the image data is assumed to be RGB data.

  In step S200, the resolution conversion module 97 converts the resolution of the input RGB image data (that is, the number of pixels per unit length) to a predetermined resolution.

  In step S300, the color conversion module 98 converts RGB image data into multi-tone data of ink colors that can be used by the color printer 20 for each pixel while referring to the color conversion table LUT (FIG. 1).

  In step S400, the halftone module 99 performs halftone processing. Halftone processing is processing (color reduction processing) that reduces 256 gradations, which is the number of gradations of multi-gradation data, to two gradations, which is the number of gradations that the color printer 20 can represent with each pixel. In the present embodiment, these two gradations are expressed as “no dot formation” and “dot formation”.

  FIG. 12 is a flowchart showing the flow of halftone processing in the first embodiment of the present invention. In step S410, the halftone module 99 (FIG. 1) reads the dot recording rate LVL from the recording rate table DT (FIG. 1) according to the input gradation value.

  FIG. 13 is an explanatory diagram showing a recording rate table DT (FIG. 1) used for determining level data used for halftone. The horizontal axis and the vertical axis indicate the input tone value and the dot recording rate, respectively. A straight line LVL indicates the relationship between the input gradation value and the dot recording rate. For example, when the input gradation value is the gradation value 64, the level data Lv1 is read.

  In step S410, the threshold th used in the dither halftone is also read from the dither matrix M (FIGS. 1 and 7).

  In step S420, a threshold adjustment process is performed. The threshold value adjustment process is a process for increasing or decreasing the threshold value in order to control the distribution of the ink discharge amount between the preceding head and the subsequent head. In this embodiment, the threshold value is increased or decreased so that the amount of ink discharged from the preceding head (print head 10A) is greater than the amount of ink discharged from the subsequent head (print head 10B). The increase / decrease of the threshold value is executed for each pixel depending on which of the preceding head and the succeeding head is responsible for ejecting ink droplets to each pixel.

  FIG. 14 is an explanatory diagram showing the dither matrix M in the first embodiment of the present invention. The numerical value stored in each element of the dither matrix M represents which of the preceding head and the succeeding head is responsible for dot formation. The element in which the numerical value “1” is stored corresponds to the pixel for which the print head 10A (preceding head) is responsible for dot formation, and the element in which the numerical value “2” is stored is the dot forming in the print head 10B (following head). Corresponds to the pixel in charge.

  FIG. 15 is an explanatory diagram showing the dot number distribution table Dn in the first embodiment of the present invention. The horizontal axis and the vertical axis represent the input tone value and the target number of dots on, respectively. A curve Td1 indicates the relationship between the input tone value of the print head 10A (leading head) and the target number of dots on. A curve Td2 shows the relationship between the input tone value of the print head 10B (subsequent head) and the target number of dots on. For example, when the input gradation value is 64, the number of dot-on targets of the preceding head is 5460, and the number of dots-on target of the succeeding head is 2730. Note that the straight line Ref indicates a reference value when it is assumed that the distribution to the preceding head and the subsequent head is the same. Further, the information indicating the distribution of the number of dots may be data for each input gradation value or an approximate calculation formula.

  The threshold adjustment process is a process of increasing or decreasing the threshold so as to probabilistically approach the number of dot-on targets. Specifically, for example, when the input gradation value is 64, the dot-on target number of dots to be formed by the preceding head is 5460, so the threshold value is reduced so that it approaches 5460 from the 4096 reference values. Adjusted to For example, when the threshold Th read from the dither matrix M is 40, the adjustment threshold Tha is 30 (= 40 × 0...) By multiplying the threshold by a coefficient 0.75 (= 4096/5460). 75) is calculated.

  In step S430, the level data LD read in step S410 is compared with the threshold value Tha adjusted in step S420. As a result of this comparison, when the level data Lv1 is larger than the threshold value Tha, “dot formation” is determined (step S440). On the other hand, when the level data LD (Lv1) is smaller than the threshold value Tha, “no dot formation” is determined (step S450).

  Thus, when the halftone process is completed for all pixels, the process proceeds to step S500 (FIG. 11). In step S500, print data PD is generated based on the formation state of large, medium, and small dots determined for each pixel.

  As described above, in the first embodiment, the reference for determining whether or not to form dots is based on the dot number distribution table Dn that stores the distribution of the number of dots formed by the preceding head and the number of dots formed by the subsequent head. Since the threshold value is adjusted, the number of dots to the preceding head and the succeeding head can be appropriately distributed. As a result, it is possible to directly control the distribution of the number of dots to the preceding head and the succeeding head to realize printing that suppresses the flow of ink on the printing medium P, thereby improving the image quality.

  In this embodiment, the threshold value is adjusted to appropriately distribute the number of dots to the preceding head and the succeeding head, but the input gradation value is adjusted to appropriately set the number of dots to the preceding head and the succeeding head. You may comprise so that it may distribute.

C. Halftone processing in the second embodiment of the present invention:
FIG. 16 is a flowchart showing a halftone processing routine in the second embodiment of the present invention. In the flowchart of the second embodiment, the level data selection process (step S405) is added, and the threshold adjustment process (step S420) is deleted. The halftone process of the second embodiment is substantially equivalent to the configuration in which the input tone value is adjusted to appropriately distribute the number of dots to the preceding head and the succeeding head.

  FIG. 17 is an explanatory diagram showing a recording rate table DT ′ used for determining level data used for halftone according to the second embodiment of the present invention. Two straight lines LVL1 and LVL2 represent level data used for determining whether or not to form dots of pixels recorded by the preceding head and the succeeding head, respectively. Specifically, the reading of the dot level data recorded by the preceding head is read according to the input gradation value based on the straight line LVL1, and the reading of the dot level data recorded by the subsequent head is performed by the straight line. Based on the LVL2, it is read according to the input gradation value.

  As described above, in the second embodiment, by selectively reading from the recording rate table DT ′, it is possible to substantially bias the input gradation value, so that the processing load of the halftone process is excessive. The invention of the present application can be applied.

D. Halftone processing in the third embodiment of the present invention:
The third embodiment of the present invention has been devised based on the following analysis of the present inventors. In each of the above-described embodiments, the ink discharge amount is differentiated depending on whether the preceding head dot pattern Dp1 or the subsequent head dot pattern Dp2 is used, which leads to different ink densities of the two. Such a difference in ink density results in conspicuousness of one dot pattern, and graininess in a low frequency region caused by the one dot pattern deteriorates image quality.

  Furthermore, the present inventor has found that the occurrence of low-frequency components in the preceding head dot pattern Dp1 and the subsequent head dot pattern Dp2 is noticeably manifested in bleeding and aggregation due to ink flow in a low-frequency region where human visual sensitivity is high. I also found out the role of From these viewpoints, the present inventors corresponded to the use of a dither matrix focusing on each dot pattern. Such a dither matrix was created based on the following analysis by the present inventors.

  FIG. 18 is an explanatory diagram showing a dot pattern formed using a conventional dither matrix. In FIG. 18, three dot patterns Dpa, Dp1, and Dp2 are formed by the dot pattern Dpa of the print image, the dot pattern Dp1 formed by the print head 10A (preceding head), and the print head 10B (following head), respectively. The dot pattern Dp2 is shown. The dot pattern Dpa of the print image includes a first pixel group dot pattern Dp1 (hereinafter referred to as a preceding head dot pattern Dp1) and a second pixel group dot pattern Dp2 (hereinafter referred to as a subsequent head dot pattern Dp2). Are combined in a common print area. In FIG. 18 to FIG. 20, the same distribution is used for easy understanding of the description. However, as described above, the dot densities of both Dp1 and Dp2 are different from each other according to the gradation value.

  As can be seen from FIG. 18, the dot pattern Dpa of the printed image shows relatively uniform dot dispersion, whereas the preceding head dot pattern Dp1 and the succeeding head dot pattern Dp2 cause dot density. ing. Such density of dots is generated by generating low-frequency components and recognized as a significant deterioration in image quality by the human eye. Such image quality degradation is caused by the fact that the conventional dither matrix is configured to improve the image quality of the dot pattern Dpa of the print image, but the preceding head dot pattern Dp1 and the subsequent head dot pattern As long as Dp2 can be combined without causing an error in the dot formation position as expected, and further without causing the flow of ink, it may not be obvious (FIG. 19). ).

  However, as described above, the occurrence of low-frequency components in the preceding head dot pattern Dp1 and the subsequent head dot pattern Dp2 causes noticeable bleeding and aggregation due to ink flow in a low-frequency region where human visual sensitivity is high. It will play a role to make it. Further, if the ink density of the preceding head dot pattern Dp1 and the subsequent head dot pattern Dp2 is different as described above, the graininess in the low frequency region caused by one dot pattern results in an image quality as a result of which one dot pattern stands out. It will deteriorate.

  From this point of view, the inventor of the present application has the granularity indexes Gg1 and Gg2 for each pixel group in order to suppress bleeding and aggregation caused by ink flow in a low-frequency region where human visual sensitivity is high. I came up with the idea of suppressing this.

  FIG. 20 is an explanatory diagram showing each dot pattern using the dither matrix generated by the dither matrix generation method of the second embodiment. According to the dither matrix generated by the dither matrix generation method of the second embodiment, the occurrence of low-frequency components in the preceding head dot pattern Dp1 and the subsequent head dot pattern Dp2 is suppressed, resulting in the flow of ink. Even if bleeding or agglomeration occurs, it can be avoided that it becomes noticeable in the low-circumference region. Furthermore, even if a relative misalignment between the dot formation positions of the print head 10A and the print head 10B occurs, there is an effect of suppressing the image quality from being excessively deteriorated due to sparse and sparse or dense and dense coincidence. .

  As described above, the third embodiment suppresses image quality deterioration due to the organic relationship between low-frequency density due to the dot pattern formed by each of the preceding head and the subsequent head and bleeding and aggregation caused by ink flow. Thus, there is an advantage that the image quality can be further improved.

  FIG. 21 is a flowchart showing the processing routine of the dither matrix generation method in the third embodiment of the present invention. In this example, a small dither matrix of 8 rows and 8 columns is generated for easy understanding. The granularity index (formula F2, FIG. 10) is used as an evaluation representing the optimality of the dither matrix.

  In step S100, a grouping process is performed. The grouping process is a group of pixels (first pixel group) in which dots are formed by the print head 10A (preceding head) and a group of pixels in which dots are formed by the print head 10B (following head) (second). And the dither matrix M is divided into elements corresponding to the respective pixel groups.

  FIG. 22 is an explanatory diagram showing divided matrices M1 and M2 in the third embodiment of the present invention. The division matrix M1 is a matrix in which elements for which the print head 10A (preceding head) is responsible for dot formation are extracted. The division matrix M2 is a matrix in which elements for which the print head 10B (following head) is responsible for dot formation are extracted.

  In step S200, a target threshold value determination process is performed. The target threshold value determination process is a process for determining a threshold value to be a storage element determination target. In this embodiment, the threshold value is determined by selecting in order from a threshold value having a relatively small value, that is, a threshold value having a value at which dots are easily formed. Thus, if the dots are selected in order from the threshold at which dots are likely to be formed, the elements stored in order from the threshold for controlling the dot arrangement in the highlight area where the graininess of the dots is conspicuous are fixed. This is because a large degree of design freedom can be given to a highlight region where graininess is conspicuous.

  In step S300, a storage element determination process is performed. The storage element determination process is a process for determining an element for storing a target threshold value. A dither matrix is generated by alternately repeating the threshold value determination process (step S200) and the storage element determination process (step S300). Note that the target threshold values may be all threshold values or some threshold values.

  FIG. 23 is a flowchart showing the processing routine of the storage element determination process in the third embodiment of the present invention. In step S310, the corresponding dot of the determined threshold is turned on. The determined threshold means a threshold at which the storage element is determined. In this embodiment, since the threshold value is selected in order from the value at which dots are likely to be formed as described above, when a dot is formed as the threshold value of interest, the pixel corresponding to the element storing the determined threshold value is not used. Dots are always formed. Conversely, at the smallest input tone value at which dots are formed at the threshold value of interest, no dots are formed at pixels corresponding to elements other than the element storing the determined threshold value.

  FIG. 24 is a black circle showing how dots are formed in each of the eight pixels corresponding to each element of the matrix in which threshold values (0 to 7) in which the first to eighth dots are likely to be formed are stored in the matrix. It is explanatory drawing shown. The dot pattern Dpa configured in this way is used to determine which pixel should form the ninth dot. The * mark will be described later.

  In step S320, a candidate storage element selection process is performed. The storage candidate element selection process is an element excluding elements for which a threshold value to be stored has been determined (in the example of FIG. 24, an element in which a threshold value (0 to 7) at which dots are easily formed in the first to eighth positions is stored). Are sequentially selected as storage threshold value candidates. In the example of FIG. 24, the element storing the * mark in the first row and the first column is selected as the first storage candidate of the target threshold value.

  In step S330, the corresponding dot of the storage candidate element is turned on. This process is performed in the form of being added to the dot group that was turned on as the corresponding dot of the determined threshold value in step S310.

  FIG. 25 is an explanatory diagram showing a matrix in which the dot formation state in which the corresponding dot of the storage candidate element and the corresponding dot of the determined threshold value are turned on, that is, the dot density matrix Dda that quantitatively represents the dot density. . The number 0 means that no dot is formed, and the number 1 means that a dot is formed (including the case where it is assumed that a dot is formed in a storage candidate element as described above). Means.

  FIG. 26 shows dot density matrices Dd1 and Dd2 in which the dot formation states in which the corresponding dots of the candidate elements and the corresponding dots of the determined threshold are turned on in the first pixel group and the second pixel group are quantified. It is explanatory drawing. As can be seen from FIG. 26, five dots are formed in the first pixel group, and four dots are formed in the second pixel group.

  In step S340, an evaluation value determination process is performed. In the present embodiment, the evaluation value determination process is a process of calculating an evaluation value based on the evaluation value calculation formula shown in FIG. The evaluation value calculation formula is configured as the sum of the first term for calculating the graininess index Ga with the evaluation for all the pixels and the second term for calculating the graininess index Gg1 and Gg2a with the evaluation for each pixel group. ing.

  The first term calculates the graininess index Ga with all pixels as ratings. In this embodiment, the graininess index Ga is calculated using the formulas F1 and F2 shown in FIG. Thereby, it is possible to realize an image quality with less graininess based on human visual sensitivity.

  The first term calculates the graininess indexes Gg1 and Gg2a with each pixel group as an evaluation. In this embodiment, the granularity indexes Gg1 and Gg2a are calculated using the formulas F1 and F2 shown in FIG. 10 as indexes indicating the granularity of the image quality of each pixel group. As a result, even with respect to the dot patterns formed by the preceding head and the succeeding head, it is possible to realize an image quality with less graininess based on human visual sensitivity.

  Note that the weighting coefficients Wa and Wg are values representing weighting for the first term and the second term, respectively. Specifically, when importance is attached to the granularity of all pixels, the value of the weighting coefficient Wa may be increased, and when importance is attached to the granularity of each pixel group, the value of the weighting coefficient Wg may be increased. good.

  In step S350, the evaluation value calculated this time is compared with the evaluation value calculated last time (stored in a buffer (not shown)). As a result of the comparison, when the evaluation value calculated this time is small (preferably), the evaluation value calculated in this buffer is stored (updated) in association with the storage candidate element, and the current storage candidate element is stored as the storage element. Temporarily determined (step S360).

  Such processing is performed for all candidate elements, and finally, the candidate storage elements stored in a buffer (not shown) are determined (step S370). Further, such processing is performed for all threshold values or all threshold values in a preset range, and the generation of the dither matrix is completed (step S400, FIG. 11).

  Thus, in the third embodiment, the dot formation by the preceding head and the succeeding head is appropriately controlled in consideration of the number of dots formed by the preceding head and the characteristics of the dot pattern formed by the succeeding head. In addition, it is possible to improve the image quality by realizing printing that suppresses the flow and graininess of the ink on the print medium P.

E. Variations:
As mentioned above, although several embodiment of this invention was described, this invention is not limited to such embodiment at all, and implementation in various aspects is possible within the range which does not deviate from the summary. It is. For example, the present invention can optimize a dither matrix for the following modifications.

E-1. In the above-described embodiment, an application example to a printing apparatus using two print heads is disclosed. For example, printing using four print heads as shown in the first modified example (FIG. 28). Can also be applied. In the first modification, four print heads with a nozzle pitch k of 4 are arranged in a staggered manner so that ink droplets can be ejected to each print pixel. In such a configuration, for example, as shown in FIG. 29, the pixel groups are divided into four pixel groups M21, M22, M23, and M24, and the number of dots is distributed based on the number-of-dots distribution table Dn ′ shown in FIG. It can be realized by setting. Further, in this modified example and the above-described embodiment (FIG. 15), the number of dots in each gradation value is allocated based on the information indicating the number of dots in each gradation value. You may make it distribute the number of dots based on the information showing (for example, each maximum value 100%).

  Further, in the distribution of the number of dots in each gradation value in the above-described embodiments and modifications, the number of both dots always increases as the gradation value increases. For example, the number of dots in one pixel group And at least one pixel group of another pixel group can be configured to compensate for the decrease. Such a degree of freedom is extremely difficult to obtain by assigning dot data to each recording scan after halftone processing as disclosed in JP-A Nos. 2002-307671 and 2002-166536.

E-2. In the above-described embodiments, the present invention is applied to a line printer that performs printing only by paper feeding. The present invention is applicable.

  FIG. 31 is a schematic configuration diagram of a color printer 20a that forms dots while performing main scanning of the print head and sub-scanning of the print medium. The color printer 20a reciprocates the carriage 30 in the axial direction (main scanning direction) of the paper feed roller 26 along the guide 34 by the carriage driving unit 24 in addition to the above-described sub-scanning driving units 22 and 26 (FIG. 2). And a head driving mechanism that drives the two print heads 10A and 10B mounted on the carriage 30 to control ink ejection and dot formation.

  FIG. 32 is an explanatory diagram showing how dots are formed by the color printer 20a. As shown in FIG. 32, the dot pitch Da of this modification is half of the dot pitch D of FIG. In other words, the print resolution in the sub-scanning direction is set to double. In this printing, the two print heads 10A and 10B can be considered as a virtual head 10AB configured by combining them. Unlike the printing in each of the embodiments described above, the virtual head 10AB cannot complete printing in one main scan, and completes printing in a plurality of main scans as shown in FIG. Such a printing method is called an interlaced printing method.

  Also in the interlaced printing method, paying attention to each main scan, for example, when the carriage 30 moves from the right to the left in FIG. 31 (main scan), the print head 10A becomes the preceding head and the print head 10B becomes the subsequent head. . On the other hand, for example, when the carriage 30 moves from left to right (main scanning), the print head 10B becomes the preceding head and the print head 10A becomes the subsequent head. Thus, it can be seen that the present invention is also applicable to the interlaced printing method. The preceding head and the succeeding head eject ink droplets to pixels that are separated from each other by the main scanning line. However, since printing is performed at a high resolution, the ink droplets are substantially ejected in a superimposed manner.

E-4. In the above-described embodiments and modifications, the optimality of the dither matrix is evaluated based on the granularity index and the RMS granularity. For example, as a simplified method, it corresponds to a pixel in which dot formation is sparse. The storage element may be determined so that the target threshold value is stored in the element to be processed. Further, for example, a Fourier transform may be performed on the dot pattern, and the dither matrix optimality may be evaluated using a VTF function. Specifically, the evaluation scale (Grainess scale: GS value) used by Dooley et al. Of Xerox may be applied to the dot pattern, and the optimality of the dither matrix may be evaluated based on the GS value. Here, the GS value is obtained by performing a predetermined process including a two-dimensional Fourier transform on the dot pattern to digitize the dot pattern, and performing integration after performing a filter process that multiplies the visual spatial frequency characteristic VTF. (Reference: Fine Imaging and Hardcopy, Corona, Japan Photographic Society, Japanese Imaging Society Joint Publishing Committee, P534). However, the former has an advantage that complicated calculation such as Fourier transform is unnecessary.

E-5. In the above-described embodiment, the evaluation process is performed for each storage element having one threshold value. However, the present invention can also be applied to a case in which a plurality of storage elements having a threshold value are simultaneously determined. . Specifically, for example, in the above-described embodiment, when the storage elements for the sixth threshold are determined and the storage elements for the seventh and eighth thresholds are determined, the seventh threshold storage element is also determined. The storage element may be determined based on the evaluation value when the dot is added to the storage element and the evaluation value when the dot is added to each of the storage elements of the seventh and eighth threshold values, or Only the storage element of the seventh threshold value may be determined.

E-6. In the above-described embodiment, the focus threshold value is determined by selecting in order from a relatively small threshold value, that is, a threshold value at which dots are likely to be formed, and the focus threshold value thus determined is stored in each element. Assuming that the dot formation state is assumed, the storage element of the target threshold value is selected from the plurality of storage candidate elements based on the matrix evaluation value representing the correlation with the calculated predetermined target state. Decide and create a dither matrix. However, the method is not limited to this method, and selection may be made in order from a relatively large value. However, the method of the embodiment has an advantage that a high degree of design freedom can be given to a highlight area where the graininess of dots is conspicuous as described above.

  Furthermore, the method is not limited to the method of sequentially determining threshold values, and a dither matrix as an initial state is prepared, and each threshold value is replaced while replacing a part of a plurality of threshold values stored in each element with threshold values stored in other elements. The dither matrix may be generated by determining the element in which is stored. In this case, the evaluation function can be set by including in the evaluation function (punishment function) a difference in dot density formed in each of the predetermined element group elements. It should be noted that the dot density matrix that is the reference for evaluation may be generated based on the smallest input gradation value at which dots are formed at the threshold value of interest, or may be generated based on an input gradation value higher than that. good.

1 is a block diagram illustrating an example of a configuration of a printing system. 1 is a schematic configuration diagram of a color printer 20. FIG. Explanatory drawing which shows the nozzle arrangement | sequence in the lower surface of print head 10A, 10B. Explanatory drawing which shows the horizontal surface of print head 10A, 10B. Explanatory drawing which shows the mechanism of the phenomenon called ink aggregation or bleeding. FIG. 4 is an explanatory diagram showing a state of an ink flow mechanism when ink droplets are ejected by a preceding head and a succeeding head. FIG. 3 is an explanatory diagram conceptually illustrating a part of the dither matrix M. Explanatory drawing which shows the idea of the presence or absence of the dot formation using a dither matrix. Explanatory drawing which illustrated notionally the spatial frequency characteristic of the threshold value set to each pixel of the blue noise dither matrix which has a blue noise characteristic. Explanatory drawing which showed notionally the visual spatial frequency characteristic VTF (Visual Transfer Function) which is a sensitivity characteristic with respect to the visual spatial frequency which a human has. 5 is a flowchart showing a routine of print data generation processing in the first embodiment of the present invention. 3 is a flowchart showing the flow of halftone processing in the first embodiment of the present invention. Explanatory drawing which shows the recording rate table DT utilized for the determination of the level data used for a halftone. Explanatory drawing which shows the dither matrix M in 1st Example of this invention. Explanatory drawing which shows the dot number distribution table Dn in 1st Example of this invention. The flowchart which shows the routine of the halftone process in 2nd Example of this invention. Explanatory drawing which shows the recording rate table DT 'utilized for the determination of the level data used for the halftone of 2nd Example. Explanatory drawing which shows the dot pattern formed using the conventional dither matrix. FIG. 6 is an explanatory diagram showing a dot pattern when no dot formation position error or ink flow occurs. Explanatory drawing which shows each dot pattern using the dither matrix produced | generated by the production | generation method of the dither matrix of 2nd Example of this invention. The flowchart which shows the processing routine of the production | generation method of the dither matrix in 3rd Example of this invention. Explanatory drawing which shows the division | segmentation matrices M1 and M2 in 3rd Example of this invention. The flowchart which shows the processing routine of the storage element determination process in 3rd Example of this invention. Explanatory drawing which shows a mode that the dot was formed in each of eight pixels corresponding to each element of the matrix in which the threshold value (0 to 7) in which the first to eighth dots are likely to be formed is stored in the matrix. Explanatory drawing which shows dot density matrix Dda. Explanatory drawing which shows the dot density matrix Dd1 and Dd2 which digitized the dot formation state by which the corresponding dot of the candidate element and the corresponding dot of the determined threshold value were turned on in the first pixel group and the second pixel group. Explanatory drawing which shows the dither matrix evaluation value calculation formula of 3rd Example of this invention. Explanatory drawing which shows the mode of application to the line printer which uses four print heads in the 1st modification. Explanatory drawing which shows four division | segmentation matrices M21, M22, M23, and M24 in a 1st modification. Explanatory drawing which shows the dot number distribution table Dn 'in a 1st modification. FIG. 10 is a schematic configuration diagram of a color printer 20a that forms dots while performing main scanning of a print head and sub-scanning of a print medium in a second modification. FIG. 3 is an explanatory diagram showing how dots are formed by the color printer 20a.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Print head 10A ... Print head 10B ... Print head 20, 20a ... Color printer 22 ... Motor 24 ... Carriage drive part 26 ... Roller 30 ... Carriage 32 ... Operation panel 34 ... Guide 40 ... Control circuit 56 ... Connector 60 ... Print head Unit 90 ... Computer 91 ... Video driver 95 ... Application program 96 ... Printer driver 97 ... Resolution conversion module 98 ... Color conversion module 99 ... Halftone module 100 ... Print data generation module

Claims (4)

A printing device for printing on a print medium,
In each main scan, a preceding dot group formed by a preceding print head that ejects pigment ink droplets, which are ink droplets of pigment ink, to the print medium in advance, and subsequently ejects the pigment ink droplets to the print medium. A printing unit that performs printing by combining a plurality of dot groups including the subsequent dot group formed by the subsequent print head in a common print region,
The printing unit
The preceding dot density that is the dot density of the preceding dot group is configured to be higher than the subsequent dot density that is the dot density of the subsequent dot group ,
Wherein when the type of the printing medium is glossy paper, the prior dot density forms dots to be higher than the subsequent dot density, the printing apparatus. The printing apparatus according to claim 1 ,
The printing apparatus is a printing apparatus that completes printing of each raster line in the common print area in one main scan. The printing apparatus according to claim 1 ,
The printing apparatus is a line printer that completes printing in the common printing area in one scan. A method for generating a printed material for forming a print image on a print medium,
In each main scan, a preceding dot group formed by a preceding print head that ejects pigment ink droplets of the pigment ink onto the print medium in advance, and subsequent printing that ejects the pigment ink droplets onto the print medium. A printing step of performing printing by combining a plurality of dot groups including a subsequent dot group formed by a head in a common print region;
The printing process, viewed contains a step preceding dot density is the dot density of the previous dot group is higher than the succeeding dot density is the dot density of the succeeding dot group,
The printing step further includes a step of forming dots so that the preceding dot density is higher than the subsequent dot density when the type of the printing medium is glossy paper .

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