JP5056223B2 - Line printer, halftone processing method, printing method - Google Patents

Description

  The present invention relates to a line printer that prints predetermined image data by forming dots on the same raster by a plurality of print heads.

  In an inkjet line printer, while feeding printing paper, ink droplets are ejected from the nozzles of a printer head arranged in a line in a direction perpendicular to the paper feeding direction and adhered to the paper, thereby allowing characters and images to be printed. Print.

  Among such line printers, for example, in a thermal printer, a printer head is generally formed by arranging a plurality of print heads. The reason for using a plurality of print heads in this way is to improve the yield and yield of printer heads that are manufactured by cutting out from a disk-shaped silicon substrate. In such a line printer having a configuration in which a plurality of print heads are arranged, when performing high-quality printing, one dot diameter is very small. For example, when printing at 600 dpi, one dot diameter is about 40 μm. Therefore, even when the installation position of the print head is slightly deviated from the normal position due to a manufacturing error or the like, the print image quality is deteriorated.

  For example, a technique disclosed in Patent Document 1 below is known as a technique for suppressing degradation of print image quality due to such a print head misalignment.

JP-A-2005-219508

  Japanese Patent Application Laid-Open No. 2004-133867 discloses that the adjacent print heads overlap so that the dot creation locations of one print head and the dot creation locations of the other print head are mixed. A technique is disclosed in which printing is performed such that deterioration in print image quality due to misalignment between heads is not noticeable.

  However, the technique described above only shares the dot formation in the overlap area between the adjacent print heads, thereby avoiding sudden deterioration of the print image at the connection portion of the print head and making it inconspicuous. In the overlap region, the print image quality still deteriorated.

  In light of the above-described problems, the problem to be solved by the present invention is that a positional deviation between print heads occurs in a line printer having a plurality of print heads arranged so as to partially overlap the print range. Even in such a case, the deterioration of the print image quality at the overlap portion of the print head is suppressed.

SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.
[Mode 1] A line printer for printing predetermined image data,
A plurality of print heads arranged so as to have overlapping portions partially overlapping in the print head arrangement direction over the print range of the image data;
Printing means for printing the image by performing halftone processing on the image data and forming dots on a print medium;
The printing means includes a first pixel group in which the dots are formed by one print head of the plurality of print heads, and a dot in which the dots are formed by the other print head in a print region corresponding to the overlapping portion. Two pixel groups are combined with each other to form one print image, the first pixel group, the second pixel group, and a pixel group in which the dots are formed by the plurality of print heads; Perform halftone processing using a dither mask created to have blue noise characteristics or green noise characteristics,
The width of the dither mask in the direction of the print head arrangement is 1 / M (M is an integer of 1 or more) times the number of pixels corresponding to the print head arrangement pitch.
Line printer.

Application Example 1 A line printer that prints predetermined image data,
A plurality of print heads arranged so as to have overlapping portions partially overlapping in the print head arrangement direction over the print range of the image data;
Printing means for printing the image by performing halftone processing on the image data and forming dots on a print medium;
The printing means includes a first pixel group in which the dots are formed by one print head of the plurality of print heads, and a dot in which the dots are formed by the other print head in a print region corresponding to the overlapping portion. A line printer that forms a single print image by combining two pixel groups and performs halftone processing in consideration of the dispersibility of the dots for each of the first pixel group and the second pixel group.

  In the line printer having such a configuration, in a print region corresponding to an overlapping portion of a plurality of print heads, dots are formed by the first pixel group in which dots are formed by one print head of the plurality of print heads and the other print head. A halftone process in consideration of dot dispersibility is performed for each of the second pixel groups. Therefore, even when a positional deviation occurs between a plurality of print heads, the dot dispersibility of the first pixel group and the second pixel group is maintained. However, it is possible to secure a good dot dispersibility to some extent, and to suppress the deterioration of the print image quality due to the positional deviation.

[Application Example 2] The line printer according to Application Example 1, wherein the printing unit forms dots by the dot dispersion of the first pixel group, the dot dispersion of the second pixel group, and a plurality of print heads. A line printer that performs halftone processing using a dither mask created in consideration of the dot dispersibility of a pixel group.

  The line printer having such a configuration is a dither created in consideration of the dot dispersibility of the first pixel group, the dot dispersibility of the second pixel group, and the dot dispersibility of the entire pixel group combining these. Halftone processing is performed using a mask. Therefore, even when a positional deviation occurs between a plurality of print heads, the dot dispersibility of the first pixel group and the second pixel group is maintained. Good dot dispersibility can be ensured to some extent, and deterioration of print image quality due to misregistration can be suppressed.

Application Example 3 In the line printer according to Application Example 1, the printing unit includes a pixel group in which dots are formed in consideration of dot dispersibility by a plurality of print heads in a print region corresponding to the overlapping portion. A line printer that performs printing by distributing to the first pixel group and the second pixel group in consideration of the dot dispersion of each of the first pixel group and the second pixel group.

  In the line printer having such a configuration, in the print area corresponding to the overlapping portion of the print head, the pixel group of the entire image formed in consideration of dot dispersibility is divided into each of the first pixel group and the second pixel group. In consideration of dot dispersibility, printing is performed by sorting the first pixel group and the second pixel group. Therefore, even when a positional deviation occurs between a plurality of print heads, the dot dispersibility of the first pixel group and the second pixel group is maintained. Good dot dispersibility can be ensured to some extent, and deterioration of print image quality due to misregistration can be suppressed.

Application Example 4 In the line printer according to Application Example 3, the printing unit performs distribution between the first pixel group and the second pixel group based on a threshold value of a dither mask used for halftone processing. Printer.

  The line printer having such a configuration sorts the first pixel group and the second pixel group based on the dither mask threshold. Since the threshold value of the dither mask is generated in consideration of dot dispersibility, the dot dispersibility of each of the first pixel group and the second pixel group can be kept good. Therefore, it is possible to suppress the deterioration of the print image quality due to the positional deviation.

Application Example 5 In the line printer according to Application Example 4, the printing unit distributes the first pixel group and the second pixel group at positions corresponding to the threshold values in ascending order of the threshold value of the dither mask. A line printer configured to assign pixels to a first pixel group or a second pixel group at a predetermined ratio.

  The line printer having such a configuration assigns pixels at positions corresponding to the threshold values to one of the first pixel group and the second pixel group in a predetermined ratio in ascending order of the dither mask threshold value. The dither mask threshold value is generated so that dot dispersibility is improved by forming dots in the order of the pixel position having the smallest threshold value to the pixel position having the largest threshold value, so that the first pixel group and the second pixel group Each dot dispersibility can be kept good. Therefore, it is possible to suppress the deterioration of the print image quality due to the positional deviation in the overlapping portion of the print head.

Application Example 6 In the dot printer according to any one of Application Example 3 to Application Example 5, the printing unit distributes the first pixel group and the second pixel group, and the first pixel group and the first pixel group. A line printer that performs the mixing ratio of the two pixel groups continuously or stepwise along the arrangement direction of the print heads.

  The line printer having such a configuration performs the first pixel group and the second pixel group by changing their mixture ratio continuously or stepwise along the arrangement direction of the print heads. Therefore, even when misalignment occurs between a plurality of print heads, the change in dot dispersibility of the entire image gradually or continuously stepwise in a print region corresponding to the overlapping portion of the print heads. Since it changes, the deterioration of dot dispersibility can be made inconspicuous.

  In addition to the configuration as the line printer described above, the present invention can also be configured as a method in which a computer performs halftone processing and a printing method.

A. First embodiment:
A first embodiment of the present invention will be described.
A-1. General configuration of the printer 10:
FIG. 1 is an explanatory diagram showing a schematic configuration of a printer 10 as an embodiment of the present application. The printer 10 is an ink jet line printer, and includes a control unit 20, ink cartridges 61 to 64, a printer head 70, a paper feed mechanism 80, and the like as shown in the figure. The ink cartridges 61 to 64 correspond to inks that exhibit cyan (C), magenta (M), yellow (Y), and black (K) colors. Of course, the type and number of inks are not limited to this.

  The printer head 70 is a line head type printer head, and includes a plurality of thermal nozzles arranged in a line on the lower surface thereof. Each ink in the ink cartridges 61 to 64 is supplied to nozzles provided on the lower surface of the printer head 70 through an introduction pipe (not shown), and ink is ejected from these nozzles to print on the printing paper P. Details of the printer head 70 will be described later with reference to FIG.

  The paper feed mechanism 80 includes a paper feed roller 82, a paper feed motor 84, and a platen 86. The paper feed motor 84 rotates the paper feed roller 82 to convey the printing paper P disposed between the printer head 70 and the flat platen 86 in a direction perpendicular to the axial direction of the paper feed roller 82.

  The control unit 20 includes a CPU 30, a RAM 40, and a ROM 50, and controls operations of the printer head 70 and the paper feed motor 84 described above. The CPU 30 operates as the halftone processing unit 31 and the print control unit 32 by expanding and executing the control program stored in the ROM 50 in the RAM 40. The functions of these functional units will be described in detail later. The ROM 50 stores a control program for controlling the operation of the printer 10, and a dither mask storage unit 52 that is an area for storing a dither mask used in halftone processing described later is secured. .

  The control unit 20 includes a memory card slot 92 for inserting a memory card MC in which image data D is recorded, a USB interface 94 for connecting a device such as a digital camera, and operations for performing various operations related to printing. A panel 96 and a liquid crystal display 98 for displaying a UI (user interface) are connected.

A-2. Detailed configuration of the printer head 70:
FIG. 2 is an explanatory diagram showing a detailed configuration of the printer head 70. As shown in the figure, the printer head 70 of this embodiment has 15 sets of print head chips HT1 to HT15 in which nozzle rows 71 to 74 for ejecting inks of colors C, M, Y, and K are formed. Are formed side by side. The length of one print head chip is about 20 mm. The same ink ejected from these head chips forms dots on the same raster by adjusting paper feed and ink ejection timing. The print head chips HT1 to HT15 of the present embodiment are formed in a staggered pattern in consideration of the problem of the strength of the end of the print head chip and the problem of the installation space of the accessory device, but are formed in a straight line. May be.

  The nozzle rows 71 to 74 are each formed by arranging the nozzles in a staggered manner. The odd-numbered nozzles Ni (i: odd number) and the even-numbered nozzles Nj (j: even number) are arranged at a density of 800 dpi. The ink ejected from the odd-numbered nozzles Ni and the even-numbered nozzles Nj is ejected by adjusting the timing with the paper feed mechanism 80, thereby forming dots on the same raster. Therefore, the nozzle rows 71 to 74 each have a total nozzle density of 1,600 dpi in total.

  As shown in FIG. 2, the printer head 70 has overlap portions OL1 to OL14 in which adjacent print head chips overlap in the arrangement direction. The print head chips HT1 to HT15 have dots in the same portion of the print paper P from either nozzle row of print head chips (for example, print head chips HT1 and HT2) that overlap each other in these overlap portions OL1 to OL14. The positions are adjusted so that can be formed. Note that the widths of the overlap portions OL1 to OL14 are all set equal.

  In such overlapping portions OL1 to OL14, when dots are formed at predetermined positions of the printing paper P, ink droplets are ejected from the nozzles of the print head chips that overlap each other to the predetermined positions. Is done. That is, in the print region corresponding to the overlap portions OL1 to OL14, a pixel group in which dots are formed by one of the overlapping print head chips and a pixel group in which dots are formed by the other print head chip are mutually connected. Are combined to form a single printed image.

  In this embodiment, the positions of the pixel group in which dots are formed by the above-described one print head chip and the pixel group in which dots are formed by the other print head chip are determined in advance. FIG. 3 illustrates allocation of a pixel group in which dots are formed by the print head chip HT1 and a pixel group in which dots are formed by the print head chip HT2. As shown in FIG. 3A, the above-described pixel group allocation in the present embodiment is such that the number of pixels (shaded portions in the drawing) formed by the print head chip HT1 in the overlap portion OL1 is the print head. It is set to decrease irregularly step by step toward the chip HT2. Although description is omitted, the same setting is made for pixels in which dots are formed by the print head chips HT2 to HT15.

  However, this allocation is not limited to such an embodiment. For example, as shown in FIG. 3B, the number of pixels (shaded portions in the figure) formed by the print head chip HT1 is equal to the print head. You may set so that it may decrease regularly in steps as it goes to chip | tip HT2. Such a change rate is not limited to a step change, but may be a continuous change. Of course, as shown in FIG. 3 (c), it may be allocated in a regular regular pattern (checkered pattern in the figure). In this way, various settings can be made for the allocation of pixel groups in the overlap portion.

  In such a printer head 70, a positional deviation may occur between the print head chips HT1 to HT15. The misregistration means that the print head chips HT1 to HT15 are installed in a state shifted in an arbitrary direction in the vertical and horizontal directions from a normal position where the print head chips HT1 to HT15 should be originally installed as a problem of manufacturing accuracy. For example, since the printer head 70 of the printer 10 has a nozzle pitch of 1,600 dpi, even if the installation position of any of the print head chips HT1 to HT15 is shifted by only 16 μm with respect to the adjacent print head chip, the print image is printed. In this case, one dot shift occurs. That is, the print image quality is deteriorated. The printer 10 according to the present embodiment can suppress the deterioration of the print image quality in the print areas corresponding to the overlap portions OL1 to OL14. This will be described in detail in “Outline of Print Processing” and “A-4. Method for Creating Dither Mask”.

  The printer 10 of the present embodiment is a thermal ink jet printer, but is not limited thereto, and dots are formed on the same raster by a plurality of print heads to print predetermined image data. Any line printer may be used. For example, it may be an ink jet printer of another ink discharge method such as a piezo type, or may be a printer of another printing method such as a dot impact type printer.

A-3. Overview of image printing process:
In FIG. 4, the printer 10 performs predetermined image processing on the image data D stored in the memory card MC, thereby converting the image data D into dot data expressed by the presence or absence of dot formation. It is a flowchart which shows the flow of the process to print.

  When the image printing process is started, the control unit 20 reads the image data D to be printed from the memory card MC (step S100). Here, the image data is assumed to be RGB color image data, but the present invention is not limited to color image data, and can be similarly applied to monochrome image data.

  When the image data D is read, the control unit 20 performs resolution conversion processing (step S110). The resolution conversion process is a process of converting the resolution of the read image data into a resolution (printing resolution) at which the printer 10 is to print an image. When the print resolution is higher than the resolution of the image data, the resolution is increased by performing an interpolation operation to generate new image data between pixels. Conversely, when the resolution of the image data is higher than the print resolution, the resolution is lowered by thinning out the read image data at a certain ratio. In the resolution conversion process, by performing such an operation on the read image data D, the resolution of the image data is converted into the print resolution.

  When the resolution of the image data D is converted into the print resolution in this way, the control unit 20 performs color conversion processing (step S120). With color conversion processing, RGB color image data expressed by a combination of R, G, and B gradation values is converted into image data expressed by a combination of gradation values of each color used for printing. It is processing to do. As described above, the printer 10 prints an image using four color inks of C, M, Y, and K. Therefore, in the color conversion processing of the present embodiment, processing is performed for converting image data expressed by RGB colors into data expressed by gradation values of C, M, Y, and K colors.

  When the gradation data is obtained for each color of C, M, Y, and K in this way, the control unit 20 refers to the dither mask stored in the dither mask storage unit 52 as the processing of the halftone processing unit 31 and performs half processing. Tone processing is performed (step S130). This process is a process of determining the presence or absence of dot formation for each pixel in order to generate dots at an appropriate density according to the gradation value of the gradation data. In this embodiment, a dither method is used. The dither method is a method of determining the presence or absence of dot formation for each pixel by comparing the threshold value set in the dither mask and the gradation value of the image data for each pixel.

  The above-described dither method will be described in detail with reference to FIG. FIG. 5 is an explanatory diagram conceptually showing a state in which the presence or absence of dot formation for each pixel is determined with reference to the dither mask. When determining the presence or absence of dot formation, first, the pixel to be determined is selected, and the gradation value of the image data for this pixel is compared with the threshold value stored at the corresponding position in the dither mask. The thin dashed arrows shown in FIG. 5 schematically indicate that the gradation value of the image data and the threshold value stored in the dither mask are compared for each pixel. For example, for the pixel in the upper left corner of the image data, the gradation value of the image data is 97 and the threshold value of the dither mask is 1, so it is determined that a dot is formed on this pixel. An arrow indicated by a solid line in FIG. 5 schematically represents a state in which it is determined that a dot is to be formed in this pixel and the determination result is written in the memory.

  On the other hand, for the pixel on the right side of this pixel, the gradation value of the image data is 97, and the threshold value of the dither mask is 177. Since the threshold value is larger, it is determined that no dot is formed for this pixel. In the dither method, the image data is converted into data representing the presence / absence of dot formation for each pixel by determining whether or not to form a dot for each pixel while referring to the dither mask.

  Note that the dither mask used in step S130 prints the image data D in the overlap portions OL1 to OL14 of the print head chips HT1 to HT15 even when the print head chips HT1 to HT15 are misaligned. This is a dither mask created so as to suppress degradation of image quality, and details thereof will be described later in “A-4. Dither Mask Creation Method”.

  When data representing the presence / absence of dot formation for each pixel is obtained from the gradation data of each color of C, M, Y, and K by the halftone process, the control unit 20 performs the control as a process of the print control unit 32. According to the data, an image is printed by forming dots on the printing paper (step S140). That is, the paper feed motor 84 shown in FIG. 1 is driven, and ink droplets are ejected from the printer head 70 based on the dot data in accordance with this movement. As a result, an ink dot of an appropriate color is formed at an appropriate position, and the image data D is printed.

A-4. To create a dither mask:
An example of a method for generating a dither mask referred to in the halftone process in step S130 will be described. The dither mask used by the printer 10 of this embodiment has the following two characteristics.

  [First characteristic]: The pixel position of the dither mask applied in the overlap portions OL1 to OL14 of the print head chips HT1 to HT15 is classified into the pixel position of the first pixel group and the pixel position of the second pixel group. Is possible. Here, the first pixel group refers to a pixel group in which dots are formed by one print head chip (for example, the print head chip HT1) among the plurality of print head chips, and the second pixel group is A pixel group in which dots are formed by the other print head chip (for example, print head chip HT2).

  [Second characteristic]: a dither mask, a dither mask obtained by extracting a threshold set at the pixel position of the first pixel group from the dither mask (dither mask at the pixel position of the first pixel group), Each of the dither masks extracted from the threshold values set at the pixel positions of the second pixel group (dither masks at the pixel positions of the second pixel group) has blue noise characteristics or green noise characteristics. Here, the “dither mask having blue noise characteristics” refers to the following dither mask. That is, while the dots are irregularly generated, the set spatial frequency component of the threshold value is a dither mask having the largest component in a high frequency region in which one period is two pixels or less. “Dither mask with green noise characteristics” means that dots are generated irregularly, and the spatial frequency component of the set threshold is the largest in the intermediate frequency region where one period is 2 pixels to several tens of pixels. A dither mask having components. Note that these dither masks may form dots in a regular pattern as long as they are close to a specific brightness.

  FIG. 6 is a flowchart showing a flow of processing for generating a dither mask having such two characteristics. Here, a description will be given of a method of making corrections so that the above-mentioned [first characteristic] and [second characteristic] can be obtained based on an existing dither mask having blue noise characteristics. However, instead of modifying the original mask, it is also possible to generate a dither mask having [first characteristic] and [second characteristic] from the beginning. Here, a case where a mask having a blue noise characteristic is used as a base will be described. However, when a dither mask having a green noise characteristic is used as a base, a dither mask having the above characteristics can be obtained in substantially the same manner. be able to.

  In creating the dither mask, first, the original dither mask is read (step S200). Such a dither mask has a blue noise characteristic as a whole, but the first pixel group (a dither mask obtained by extracting a threshold value set in the pixel position of the first pixel group from the dither mask) and the second pixel group. These pixel groups (the dither mask obtained by extracting the threshold value set at the pixel position of the second pixel group described above from the dither mask) are all dither masks having no blue noise characteristics.

  Next, a mask A is set from the read dither mask (step S202). This mask A is, for example, a range including the overlap portion of the print head chip, taking into account the repeating unit of the overlap portion and the size of the read dither mask. It is set to a size that is an integer of 1 or more times. A mask A in this embodiment is shown in FIG. In this example, the width of a single dither mask is set to be equal to the pitch of the print head chip. Therefore, as shown in the figure, a range (two pitches of the print head chip) in which two read single dither masks are arranged is set as the mask A. Pixel positions corresponding to each other in the two single dither masks have the same threshold value. By setting the size of the mask A in such a manner, the corresponding relationship between the positions of the illustrated print head chips HT2 and HT3 and the mask A is repeatedly applied to the print head chips HT4 and HT5 (not illustrated). Can do.

  Then, two pixel positions (pixel position p and pixel position q) are randomly selected from the dither mask A (step S204), and the threshold value set for the selected pixel position p and the selected pixel position q are set. The obtained threshold value is replaced with a mask B (step S206).

  Next, the graininess evaluation value Eva for the mask A is calculated (step S208). Here, the graininess evaluation value is an evaluation value obtained as follows. First, the dither method is applied to 256 images having gradation values of 0 to 255 to obtain 256 images expressed by the presence or absence of dot formation. Next, each image is decomposed into an image of the first pixel group and an image of the second pixel group. Here, the image of the first pixel group is an image made up of pixels in the shaded area shown in FIG. 7, and the image of the second pixel group is an image made up of pixels in the white portion shown in FIG. is there. As a result, for each gradation value from “0” to “255”, an image of the first pixel group, an image of the second pixel group, and an image (total image) obtained by superimposing these images are obtained. It will be. For the 768 (= 256 × 3) images obtained in this way, after calculating the graininess index, the value obtained by obtaining the average value thereof is taken as the graininess evaluation value.

  The above-mentioned graininess index is a known index for evaluating the dot dispersibility of an image, and is shown, for example, in FIG. 11 of the known document “Japanese Patent Laid-Open No. 2007-15359”. Here, the granularity index is obtained by obtaining a power spectrum FS by Fourier transforming an image, and assigning a weight corresponding to a sensitivity characteristic VTF (Visual Transfer Function) to a human visual spatial frequency to the obtained power spectrum FS. The index obtained by integrating at each spatial frequency is expressed by the following equation (1). This granularity index indicates that the smaller the numerical value, the better the granularity (dot dispersibility).

  In calculating the graininess evaluation value, the 768 graininess indexes are not simply arithmetically averaged, but are calculated for each of the first pixel group image, the second pixel group image, and the total image. You may weight and average. Alternatively, a specific gradation value (for example, a low gradation region in which dots are relatively conspicuous) may be averaged by applying a large weighting factor. In step S208 of FIG. 6, such a graininess evaluation value is obtained for the mask A, and the obtained value is set as the graininess evaluation value Eva.

  When the graininess evaluation value Eva for the mask A is obtained, the graininess evaluation value Evb is similarly calculated for the mask B (step S210). Next, the granularity evaluation value Eva for the mask A and the granularity evaluation value Evb for the mask B are compared (step S212). If it is determined that the graininess evaluation value Evb is smaller (step S212: YES), the mask B in which the threshold values set at the two pixel positions are replaced than the original mask A. It is considered to have more preferable characteristics. Therefore, in this case, mask B is read as mask A (step S214). On the other hand, if it is determined that the granularity evaluation value Evb of the mask B is larger than the granularity evaluation value Eva of the mask A (step S212: NO), the mask is not replaced.

  In this way, if the operation of replacing the mask B with the mask A is performed only when it is determined that the granularity evaluation value Evb of the mask B is smaller than the granularity evaluation value Eva of the mask A, has the granularity evaluation value converged? It is determined whether or not (step S216). That is, in the original dither mask, since the dots of the first pixel group and the dots of the second pixel group are generated in a biased manner, immediately after starting the above operation, the granularity evaluation value is Take a big value. However, if a smaller granularity evaluation value is obtained by replacing the threshold values set at two pixel positions, a mask with the replaced threshold value is adopted, and the above-described operation is repeated for this mask. Therefore, it is considered that the obtained graininess evaluation value becomes smaller and eventually stabilizes at a certain value. In step S216, it is determined whether or not the graininess evaluation value is stable, in other words, whether or not it is considered that the graininess has stopped decreasing. Whether or not the granularity evaluation value has converged is determined, for example, by calculating the amount of decrease in the granularity evaluation value when the granularity evaluation value Evb of the mask B is smaller than the granularity evaluation value Eva of the mask A. In addition, if the amount of decrease is stably below a certain value over a plurality of operations, it can be determined that the granularity evaluation value has converged.

  If it is determined that the granularity evaluation value has not converged (step S216: NO), the process returns to step S204, and after selecting two new pixel positions, the series of subsequent operations is repeated. While the operation is repeated in this manner, the granularity evaluation value eventually converges, and if it is determined that the granularity evaluation value has converged (step S216: YES), the mask A at that time is the [first This is a dither mask having [characteristic] and [second characteristic]. Therefore, a portion corresponding to the above-described single dither mask constituting the mask A is adopted as a dither mask used in the printer 10 (step S218).

  However, in this method, the granularity evaluation value may converge to the local optimum value before it becomes a sufficiently small value. To avoid that, a technique known as simulated annealing may be introduced. Specifically, for example, in step S212, the graininess evaluation value Eva is added with a noise nz having an appropriate amplitude that changes every time and compared. That is, it is determined whether Eva + nz> Evb. By comparing the noise nz in this way, even when the granularity evaluation value Evb is larger than the granularity evaluation value Eva (the mask A is more excellent in print dot dispersion characteristics than the mask B), the granularity evaluation is performed. If the result of adding the noise nz to the value Eva is larger than the graininess evaluation value Evb, the dither mask is replaced.

  When the dither mask is replaced in this way, the graininess evaluation value temporarily increases (the print dot dispersion characteristic deteriorates). However, the amplitude of the noise nz to be applied is sufficiently reduced while being gradually reduced. If the loop (step S216: NO processing) is repeated as many times as possible, and the noise nz is finally reduced to zero, the number of loops until convergence increases, but does not fall into a local optimum value. Therefore, the granularity evaluation value can be converged to a lower value.

  In this embodiment, the graininess evaluation value Eva used for dither mask evaluation is calculated based on the graininess index which is a subjective evaluation value using the visual sensitivity characteristic VTF. It is also possible to calculate based on evaluation values concerning various dot dispersibility, for example, RMS granularity which is a standard deviation of density distribution.

  In this embodiment, in consideration of ease of understanding, the halftone process of step S130 is performed using a dither mask having the same width as the pitch of the print head chips HT1 to HT15. The width of the dither mask may be set as appropriate. However, it is efficient to apply the dither mask so that a predetermined location of the dither mask always corresponds to the joint portion of the print head chips HT1 to HT15 in the region of the joint portion of the print head chips HT1 to HT15. In order to obtain such a configuration, the width of the dither mask is preferably set to 1 / M (M is an integer of 1 or more) times the pitch of the print head chips HT1 to HT15. For example, when a dither mask having a width that is 1/2 the nozzle pitch is used, the following configuration may be used. First, the control unit 20 determines whether or not the pixel area where the halftone process is performed is a pixel position having the overlap portions OL1 to OL14. As a result, when the pixel position has the overlap portions OL1 to OL14, the dither mask (the entire image and the first pixel group) obtained by the method shown in FIG. The dither mask optimized for the image formed by the above and the image formed by the second pixel group) is used to perform halftone processing on the pixel area. On the other hand, in the case where the pixel position does not have the overlap portions OL1 to OL14, the dither mask (the entire image) which is stored in the dither mask storage unit 52 and which is read in step S200 in FIG. A halftone process is performed on the pixel region using an optimized dither mask.

  The printer 10 having such a configuration uses the first pixel group, the second pixel group, and a dither mask created so that good dot dispersion can be obtained for the entire pixel group obtained by combining these. Perform halftone processing. Therefore, even when a positional deviation occurs between a plurality of print heads, the dot dispersibility of the first pixel group and the second pixel group is maintained. Can be ensured, and deterioration in print image quality due to misalignment can be suppressed.

B. Second embodiment:
A second embodiment of the present invention will be described. In the first example, as described above, the positions of the first pixel group and the second pixel group in the overlap portions OL1 to OL14 of the print head chips HT1 to HT15 are set in advance, and based on the setting, Good dot dispersibility is obtained for each of an image composed of a first pixel group, an image composed of a second pixel group, and an image composed of a pixel group obtained by adding the first pixel group and the second pixel group. An image is printed by performing a halftone process using a dither mask. On the other hand, the second embodiment uses a dither mask that provides good dot dispersibility for an image composed of the entire pixel group obtained by adding the first pixel group and the second pixel group, This embodiment is different from the first embodiment in that the entire pixel group is divided into the first pixel group and the second pixel group in consideration of the dot dispersibility of each of the one pixel group and the second pixel group. Hereinafter, these different points will be described in detail.

  FIG. 8 is a flowchart showing the flow of the halftone process in step S130. When this process is started, the control unit 20 first moves the attention area (step S300). This attention area is a pixel area corresponding to the size of a dither mask used for halftone processing, and the first attention area is, for example, data of an area located at the end of the print image.

  When the attention area is moved, the control unit 20 determines whether or not dots can be formed for each pixel position using the dither mask (step S310). The dither mask used here is a dither mask in which the dot dispersibility of the entire pixel corresponding to the dither mask is optimized.

  When determining whether or not dots can be formed, the control unit 20 determines whether or not the region of interest is a region including an overlap portion of the print head chip (step S320). As a result, if the region includes an overlap portion (step S320: YES), the control unit 20 refers to the dot distribution rate of the adjacent print head chips HTn and HTn + 1 for each pixel position (step S330). The dot distribution rate is stored in the ROM 50 in advance, but may be stored in a rewritable EEPROM.

  The above-mentioned distribution ratio may be set arbitrarily, but it is desirable to change continuously or stepwise according to the pixel position from the viewpoint of improving the image quality. As a specific example of the distribution ratio, FIG. 9 shows a pixel group (hereinafter referred to as a first pixel group) in which dots are formed by the print head chip HT1 in the overlap portion OL1 of the print head chips HT1 and HT2, and the print head chip. An example of a method of allocating to a pixel group in which dots are formed by HT2 (referred to as a second pixel group) is illustrated. As shown in the drawing, the print head chip HT1 and the print head chip HT2 are arranged in an overlapping manner from the pixel position X90 to the pixel position X98 to form an overlap portion OL1.

  In the present embodiment, as shown in the figure, the distribution ratio of the first pixel is changed so as to continuously decrease from 90% to 10% from the pixel position X90 to the pixel position X98. %, 90%, 70%, 70%, and 50% may be changed in stages. As described above, by changing continuously or in steps, the change in dot dispersibility of the entire image including the first pixel group and the second pixel group changes gradually or continuously in steps. Therefore, the deterioration of dot dispersibility due to the positional deviation of the print head chip can be made inconspicuous.

  When the distribution rate is set in this way, the control unit 20 calculates the number of dot distributions of the adjacent print head chips HTn and HTn + 1 for each pixel position (step S340). The number of dot distributions is calculated by multiplying the total number of dots for each pixel position by the distribution rate. In the specific example shown in FIG. 9, the total number of dots for each pixel position is 40 as shown in the figure. For example, the number of distributed dots of the first pixel group at the pixel position X90 is 40 × 90% = 36. .

  When the distribution dot numbers of the print head chips HTn and HTn + 1 are calculated in this way, the dot formation positions of the print head chips HTn and HTn + 1 are allocated with reference to the threshold value of the dither mask applied to the overlap portion OLn (step S350). This process is performed, for example, by assigning pixels at positions corresponding to the threshold values to either one of the print head chips HTn or HTn + 1 in the order from the smallest threshold value of the dither mask applied to the overlap portion OLn. . Specifically, as illustrated in FIG. 9, for example, at the pixel position X90, 36 threshold values 40 of the 40 dither mask threshold values corresponding to the pixel position are extracted in ascending order, The pixel positions corresponding to these threshold values are set as the first pixel group, and the remaining pixel positions are set as the second pixel group.

  When the dot formation positions of the print head chips HTn and HTn + 1 are assigned in this way, or when it is determined in step S320 that the area does not include the overlapped portion (step S320: NO), the control unit 20 causes all the print areas to be printed. It is determined whether or not the above processing is completed (step S360). As a result, if all the print areas have been completed (step S360: YES), the control unit 20 completes the halftone process, returns the process to the image print process of FIG. 4 described above, and continues the dot formation process ( Step S140) is performed. On the other hand, if all the printing areas are not completed (step S360: NO), the process returns to step S300, and the above-described series of processes is repeated.

  In FIG. 8, the control unit 20 performs the halftone process for all the print areas, and then performs the dot formation process of FIG. 4. However, the halftone process and the dot formation process are performed for each print area. You may repeat and.

  When printing is performed by allocating dot formation positions to the print head chips HTn and HTn + 1 by such a method, the dither mask threshold values optimized for the entire pixels are formed in the order of pixel positions having a smaller threshold value and larger pixel positions. In this way, the dot dispersibility of each of the first pixel group and the second pixel group can be kept good in the overlap portions OL1 to OL14. . Therefore, even if a positional deviation occurs in the print head chips HT1 to HT15, it is possible to suppress deterioration in print image quality in the print area of the overlap portions OL1 to OL14.

  As mentioned above, although the Example of this invention was described, this invention is not limited to such an Example, Of course, in the range which does not deviate from the summary of this invention, it can implement in a various aspect. For example, the present invention is not limited to the line printer shown in the embodiments, and can be configured as a printer driver installed in a computer. Of course, the present invention can also be realized in the form of a halftone processing method, a program, a computer-readable recording medium storing the program, and the like.

It is explanatory drawing which shows schematic structure of the printer 10 as an Example of this application. 2 is an explanatory diagram showing a detailed configuration of a printer head 70. FIG. It is explanatory drawing which illustrates about allocation of the pixel group in which a dot is formed by print head chip HT1, and the pixel group in which a dot is formed by print head chip HT2. 3 is a flowchart showing a flow of image printing processing of the printer 10. It is explanatory drawing which showed notionally the mode of determining the presence or absence of dot formation about each pixel, referring a dither mask. It is explanatory drawing which shows the example of the optimization method of the dither mask as 1st Example. It is explanatory drawing of the setting method of the dither mask used as the origin used for the optimization method of the dither mask as 1st Example. It is a flowchart which shows the flow of the halftone process as 2nd Example. It is explanatory drawing which illustrates the distribution method of the 1st pixel group and 2nd pixel group in the overlap part of the print head chip as 2nd Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Printer 20 ... Control unit 30 ... CPU
31 ... Halftone processing unit 32 ... Print control unit 40 ... RAM
50 ... ROM
52 ... Dither mask storage unit 61-64 ... Ink cartridge 70 ... Printer head 71-74 ... Nozzle array 80 ... Paper feed mechanism 82 ... Paper feed roller 84 ... Paper feed motor 86 ... Platen 92 ... Memory card slot 94 ... USB interface 96 ... Operation panel 98 ... Liquid crystal display P ... Print paper MC ... Memory card Ni, Nj ... Nozzle HT1 to HT15 ... Print head chips OL1 to OL14 ... Overlap portion

Claims (2)

A line printer for printing predetermined image data,
A plurality of print heads arranged so as to have overlapping portions partially overlapping in the print head arrangement direction over the print range of the image data;
Printing means for printing the image by performing halftone processing on the image data and forming dots on a print medium;
The printing means includes a first pixel group in which the dots are formed by one print head of the plurality of print heads, and a dot in which the dots are formed by the other print head in a print region corresponding to the overlapping portion. Two pixel groups are combined with each other to form one print image, the first pixel group , the second pixel group, and a pixel group in which the dots are formed by the plurality of print heads; There blue noise characteristics, or have the line halftone processing using the dither mask that is created to have green noise characteristics,
The line printer in which the width of the dither mask in the direction of the print head arrangement is 1 / M (M is an integer of 1 or more) times the number of pixels corresponding to the print head arrangement pitch . A plurality of print heads are arranged so as to have overlapping portions partially overlapping in the print head arrangement direction over the print range of the image data, and the plurality of print heads in a print region corresponding to the overlapping portions The first pixel group in which dots are formed by one of the print heads and the second pixel group in which dots are formed by the other print head are combined with each other and printed by a line printer that forms one print image. A halftone processing method for
A dither mask created so that the first pixel group , the second pixel group, and the pixel group in which the dots are formed by the plurality of print heads have a blue noise characteristic or a green noise characteristic. The halftone process is performed using a dither mask whose width in the direction of the print head arrangement is 1 / M (M is an integer of 1 or more) times the number of pixels corresponding to the print head arrangement pitch. Method.

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