JP2009018476A - Line printer, halftone processing method, program and dither mask - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a line printer having a plurality of printing heads arranged over a printing range, which suppresses deterioration of printing image quality even when positional deviation arises between the printing heads. <P>SOLUTION: A dither mask pattern table 52 formed by making a plurality of positional deviation assumption patterns where the direction and distance of the positional deviation of printing head chips HT1 to HT15 are assumed correspond to dither masks where dot dispersibility is optimized for the positional deviation is stored in a ROM 50 of a line head type printer 10 where the printing head chips HT1 to HT15 are connected. A control unit 20 of the printer 10 specifies the optimum dither mask on every predetermined pixel position from the dither mask pattern table 52 based on the detected positional deviation of the printing head chips HT1 to HT15, and performs halftone processing by using the optimum dither mask, so as to perform printing of image data D. <P>COPYRIGHT: (C)2009,JPO&INPIT

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. As a line printer having such a configuration, for example, a technique disclosed in Patent Document 1 below is known.

JP 2001-71495 A

  As described above, in a line printer having a configuration in which a plurality of print heads are arranged, one dot diameter is very small when high-quality printing is performed. 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, the print image quality is deteriorated. Since it is very difficult in practice to secure such an extremely high positioning accuracy, the deterioration of the print image quality due to the displacement of the print head has been an essential problem of such a line printer.

  Based on the above-described problems, the problem to be solved by the present invention is that even in the case of a line printer having a plurality of print heads arranged over a print range, even if a positional deviation occurs between the print heads, printing is performed. It is to suppress degradation of image quality.

  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.

Application Example 1 A line printer that prints predetermined image data by forming dots on the same raster by a plurality of print heads arranged over a print range,
Storage means for storing a plurality of dither masks associated with each of a plurality of misregistration assumption patterns presuming misregistration between the plurality of print heads;
In consideration of misalignment of the print head, a halftone process that performs halftone processing on the image data using at least a specific dither mask specified for each print head from among the plurality of stored dither masks A line printer comprising tone processing means.

  The line printer having such a configuration uses a specific dither mask specified in consideration of misregistration from a plurality of dither masks associated with each misregistration pattern between a plurality of print heads. Halftone processing is performed on Therefore, since the dispersibility of the printed dots can be ensured even if a positional deviation occurs, it is possible to suppress the deterioration of the print image quality due to the positional deviation.

[Application Example 2] In the line printer according to Application Example 1, the misregistration assumption pattern that is the same as or close to the misregistration is specified based on the misregistration, and the dither associated with the misregistration assumption pattern. A line printer comprising dither mask specifying means for specifying a mask as a specific dither mask.

  Since the line printer having such a configuration includes means for specifying the dither mask based on the positional deviation, it is possible to easily specify the dither mask and suppress deterioration in print image quality due to the positional deviation.

[Application Example 3] The line printer according to Application Example 2, further comprising detection means for detecting misregistration, and the dither mask specifying means specifies the dither mask based on the detected misregistration.

  Since the line printer having such a configuration includes the misregistration detection unit, it is possible to easily detect misregistration and suppress deterioration in print image quality due to misregistration.

Application Example 4 The line printer according to any one of Application Examples 1 to 3, wherein the plurality of misregistration assumption patterns are assumed to correspond to combinations of misalignment directions and distances of a plurality of print heads.

  In the line printer having such a configuration, a plurality of misregistration assumption patterns are assumed to correspond to combinations of misregistration directions and distances of a plurality of print heads. Degradation of image quality can be suppressed.

[Application Example 5] The line printer according to any one of Application Example 1 to Application Example 4, which is further detected by a temperature sensor that detects the temperature of a plurality of print heads and a dither mask when the dither mask is specified. A misregistration estimation unit that estimates a misregistration amount based on temperature, and the halftone processing unit performs a halftone process using a specific dither mask identified based on the estimated misregistration amount. Line printer.

  The line printer having such a configuration estimates the amount of misregistration based on the temperature of the print head, and performs halftone processing using a specific dither mask identified based on the misregistration amount. Therefore, even when the print head expands or contracts due to a temperature change and the nozzle position of the print head changes, it is possible to suppress the deterioration of the print image quality.

  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, a computer program for performing halftone processing, and a dither mask.

A. Example:
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.

  An optical sensor 76 is provided in the vicinity of the printer head 70 in parallel with the nozzle rows of the printer head 70. The optical sensor 76 is provided as a positional deviation detecting means for a print head chip, which will be described later, from dots printed on the printing paper P. In this embodiment, a line sensor using a CCD element is used, but another optical sensor such as a CMOS sensor may be used.

  The printer head 70 is provided with a temperature sensor 78. In this embodiment, the thermistor is used, but various temperature sensors such as a thermocouple and a platinum resistance temperature detector can be used.

  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 a misregistration detection unit 31, a dither mask specifying unit 32, a misregistration estimation unit 33, a halftone processing unit 34, and a print control unit 35 by developing the control program stored in the ROM 50 in the RAM 40 and executing it. To do. 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 further stores a dither mask pattern table 52 used in halftone processing described later.

  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.

  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. 3, 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 degraded 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 D is converted to 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 inks of four colors, 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 gradation data is obtained for each color of C, M, Y, and K in this way, the control unit 20 performs halftone processing as processing of the halftone processing unit 34 (step S170). 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. 4 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 broken arrow shown in FIG. 4 schematically represents that the gradation value of the image data is compared with the threshold value stored in the dither mask 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. 4 schematically shows 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 S170 is a dither mask created so as to suppress the deterioration of the print quality of the image data D even when the print head chips HT1 to HT15 are misaligned. Details thereof will be described later in “A-4. Outline of misregistration adjustment process”.

  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 35. According to the data, an image is printed by forming dots on the printing paper (step S180). 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. Overview of misalignment adjustment processing:
In the printer 10 of this embodiment, the print head chips HT1 to HT15 are arranged to constitute the printer head 70 as described above. In these print head chips HT1 to HT15, misalignment may occur. The misregistration means that the print head chips HT1 to HT15 are installed in a state deviated from the normal positions where they should originally be 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.

  Such positional deviation of the print head chips HT1 to HT15 will be described in detail with reference to FIG. The pattern 0 in the figure shows a case where the print head chip HT1 and the print head chip HT2 are installed at regular positions without causing a positional shift. The print head chips HT1 and HT2 are arranged in a staggered manner as described above, but in the middle column of the figure, for the sake of simplicity, the print head chips HT1 and HT2 are arranged linearly. The positional relationship is shown. As described above, in the situation where there is no positional deviation between the print head chips HT1 and HT2, as shown in the right column in the figure, the dots of the print image formed by the ink ejected from the print head chips HT1 and HT2 are A linear array is formed to form the same raster. That is, there is no deterioration in print image quality.

  On the other hand, the pattern 1 in the figure shows a case where the print head chip HT2 is shifted by one pixel from the normal position with respect to the print head chip HT1, as shown in the middle column of the figure. Yes. In such a case, as shown in the right column in the figure, the raster that should be formed in a straight line is formed in a form including a step at the joint between the print head chips HT1 and HT2. That is, the print image quality is deteriorated.

  Similarly, in the case of pattern 2, the print head chip HT2 is shifted by one pixel from the normal position with respect to the print head chip HT1, and in the case of pattern 3, the print head chip HT1 is set to the print head chip HT1. In the case 4 where the chip HT2 is set to be shifted to the right by one pixel from the normal position, in the case 4, the print head chip HT2 is set to be shifted to the left by one pixel from the normal position with respect to the print head chip HT1. Shows the case. In any of these cases, the print image quality deteriorates at the joint between the print head chips HT1 and HT2.

  Such positional deviations of the print head chips HT1 to HT15 are not limited to the single vertical and horizontal directions exemplified in FIG. 5 and the deviation of one pixel, but may occur as deviations of any distance in any direction.

  The printer 10 of the present embodiment performs the halftone process in step S170 using a dither mask that can suppress the deterioration of the print image quality even when the positional deviation of the print head chips HT1 to HT15 occurs. It can be carried out. A misregistration adjustment process for performing an image printing process using such a dither mask will be described below.

  FIG. 6 is a flowchart showing the flow of the misalignment adjustment process. The misalignment adjustment process is started when the user of the printer 10 instructs the misalignment adjustment process using the operation panel 96 and the liquid crystal display 98. This misalignment adjustment processing instruction is given as an initial setting when the user uses the printer 10 for the first time. When this process is started, the control unit 20 first detects the positional deviations of the print head chips HT1 to HT15 as the process of the positional deviation detection unit 31 (step S200). In this process, first, the control unit 20 drives the printer head 70 and the paper feed mechanism 80 to print a predetermined test pattern on the printing paper P. Then, the printing paper P on which the test pattern is printed is further conveyed toward the optical sensor 76, and the optical sensor 76 converts the test pattern into an electrical signal. By sampling this electric signal, a luminance signal and a chroma signal as a digital signal are obtained. By measuring the distance between the dots using these signals, the direction and distance of the positional deviation are detected.

  When the positional deviation is detected, the control unit 20 performs a dither mask specifying process as the process of the dither mask specifying unit 32 (step S210). This processing is performed for each connection position of the print head chips in consideration of the positional deviation of the print head chips HT1 to HT15 detected in step S200 from among the plurality of optimum dither masks stored in the dither mask pattern table 52. This is a process for specifying an optimal dither mask that can be applied to the above, and that can suppress deterioration in image quality due to misalignment.

  Here, the dither mask pattern table 52 will be described. The dither mask pattern table 52 stores a plurality of misregistration patterns presuming misalignment of the print head chips HT1 to HT15 and dither masks in association with each other. An example of the above-described dither mask pattern table 52 is shown in FIG. In this example, the five print head chip misregistration patterns shown in FIG. 5 and the five optimum dither masks OD0 to OD4 are stored in association with each other. Each optimum dither mask stored here is a dither mask that optimizes the dispersibility of dots in consideration of the shift of the associated print head chip. In the figure, for the sake of easy understanding, two identical dither masks are shown side by side as the optimum dither masks OD0 to OD4, but in reality, they are one dither mask.

  The above-mentioned optimum dither masks OD0 to OD4 are composed of a common dither mask Dm and selective dither masks Dl and Dr arranged on both ends thereof. The common dither mask Dm is a component that is commonly used in the positional deviation assumption patterns 0 to 4. The selected dither masks Dl and Dr are optimized for each misregistration pattern and are components that are selectively used according to the misalignment state. For example, as shown in the figure, the optimal mask associated with the misregistration assumed pattern 1 is formed by combining the left selected dither mask D11, the central common dither mask Dm, and the right selected dither mask Dr1. An optimal dither mask OD1 is configured.

  In FIG. 7, for the sake of simplification, a dither mask of 16 pixels in the horizontal direction and 16 pixels in the vertical direction is shown, but the size of the dither mask is 256 pixels × 256 pixels vertically, 256 pixels × 128 pixels vertically. For example, it may be set as appropriate in consideration of the dither mask creation efficiency, the efficiency of halftone processing using the dither mask, and the like.

  A method of creating such optimum dither masks OD0 to OD4 will be described. Various known optimization methods can be used as a method of creating the dither mask OD0 corresponding to the pattern 0 that is a pattern that does not cause the positional deviation of the print head chip. For example, the method shown in FIG. 16 of the publicly known document “Japanese Unexamined Patent Application Publication No. 2007-15359” can be performed using the graininess index shown in FIG. 11 of “Japanese Unexamined Patent Application Publication No. 2007-15359”. The method of FIG. 16 of this publicly known document is performed in the same flow as the method of FIG. The granularity index of FIG. 11 in the publicly known document obtains a power spectrum FS by Fourier transforming an image, and the obtained power spectrum FS is used as a sensitivity characteristic VTF (Visual Transfer Function) with respect to the visual spatial frequency of a human. This is an index obtained by integrating corresponding spatial weights with corresponding weights, and is expressed by the following equation (1). This granularity index indicates that the smaller the numerical value, the better the granularity (dot dispersibility). In place of the granularity index, other evaluation indexes related to dot dispersibility such as RMS granularity may be used.

  Regarding the method of creating the optimum dither masks OD1 to OD4 corresponding to the patterns 1 to 4 that are the patterns that cause the positional deviation of the print head chip, the positional deviation of the pattern 1 (the positional deviation of one pixel in the upward direction) occurs. The optimum dither mask OD1 will be described as an example with reference to FIG. In creating the dither mask, first, the original dither mask is read (step S300). As this dither mask, the optimum dither mask OD0 for a pattern that does not cause the above-described positional deviation can be used, for example.

  Next, a dither mask corresponding to each assumed misregistration pattern is set from the read dither mask OD0. This process will be described with reference to FIG. 9, taking as an example a case where the positional deviation of pattern 1 shown in FIG. 6 (the positional deviation of one pixel in the upward direction) occurs. Since the patterns 2 to 4 are the same as those in the case of the pattern 1, description thereof is omitted. FIG. 9 shows a state in which two dither masks OD0 having the same threshold configuration are consecutive in the left-right direction, shifted by one pixel in the upward direction. The pixel positions corresponding to each other in each dither mask OD0 have the same threshold value. In this way, two dither masks arranged in accordance with the assumed positional deviation are set as the dither mask A (step S302).

  Then, in the dither mask A, two pixel positions (pixel position p and pixel position q) are randomly selected from the portion of the selected dither mask Dr0 or D10 in the drawing (step S304), and the selected pixel position p is set. By replacing the set threshold value with the threshold value set at the selected pixel position q, the obtained dither mask is set as a dither mask B (step S306).

  Next, a granularity evaluation value Eva is calculated for the dither mask A (step S308). Here, the graininess evaluation value Eva is an evaluation value obtained as follows. Using the dither mask A, the dither method is applied to 256 images having gradation values of 0 to 255 to obtain 256 images represented by the presence or absence of dot formation. After calculating the above-mentioned graininess index for the 256 images obtained in this way, a value obtained by obtaining an average value thereof is set as a graininess evaluation value. In calculating the graininess evaluation value, the 256 graininess indices are not simply arithmetically averaged, but rather about a specific tone value (for example, a low tone area where dots are said to be relatively conspicuous). May be averaged over a large weighting factor.

  When the graininess evaluation value Eva for the dither mask A is obtained in this way, the graininess evaluation value Evb is similarly calculated for the dither mask B (step S310). Next, the graininess evaluation value Eva for the dither mask A and the graininess evaluation value Evb for the dither mask B are compared (step S312). If it is determined that the graininess evaluation value Evb is smaller (step S312: YES), the dither mask B in which the threshold values set at the two pixel positions are replaced than the original dither mask A. It is considered that the printing dot dispersion characteristics are more excellent. Therefore, in this case, dither mask B is replaced with dither mask A (step S314). On the other hand, when it is determined that the granularity evaluation value Evb of the dither mask B is larger than the granularity evaluation value Eva of the dither mask A (step S312: NO), the dither mask is not replaced.

  Thus, when the operation for replacing the dither mask B with the dither mask A is performed only when the granularity evaluation value Evb of the dither mask B is determined to be smaller than the granularity evaluation value Eva of the dither mask A, the granularity evaluation value is obtained. It is determined whether or not has converged (step S316). That is, since the original dither mask is optimized with no positional deviation, the granularity evaluation value takes a large value immediately after the above operation is started. However, when a smaller granularity evaluation value is obtained by replacing the threshold values set at the two pixel positions, a dither mask with the replaced threshold values is adopted, and the above-described operation for the dither mask is further performed. If the process is repeated, the obtained graininess evaluation value becomes smaller, and it is considered that the value will eventually stabilize. In step S316, it is determined whether or not the granularity 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, for example, when the granularity evaluation value Evb of the dither mask B is smaller than the granularity evaluation value Eva of the dither mask A. Can be determined that the granularity evaluation value has converged if the amount of decrease is stable over a plurality of operations and is below a certain value.

  If it is determined that the granularity evaluation value has not converged (step S316: NO), the process returns to step S304 to newly select two pixel positions and then repeat the following series of operations. 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 S316: YES), among the dither mask A at that time, the selected dither mask The threshold values of the pixel positions corresponding to Dr and Dl are adopted as the selection dither masks Dr1 and Dl1 of the optimum dither mask OD1 corresponding to the misregistration assumption pattern 1 (step S318). The optimum dither mask OD1 is obtained from the selected dither masks Dr1 and Dl1 thus obtained and the common dither mask Dm.

  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 S312, 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 dither mask A has better print dot dispersion characteristics than the dither mask B), the granularity When the result of adding the noise nz to the property evaluation 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 is repeated as many times as above (step S316: NO processing) and finally the noise nz is lowered 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.

  Note that the widths of the selected dither masks Dl and Dr may be appropriately set in consideration of the influence on human visual characteristics and the degree of freedom of selection of the threshold value constituting the dither mask.

  Here, the description returns to the process of step S210. In this process, the control unit 20 specifies a misregistration pattern stored in the dither mask pattern table 52 from the misregistration pattern that is the same as or similar to the misregistration detected in step S200. In this embodiment, the detected misregistration and each assumed misregistration pattern are regarded as vectors, and a vector representing the detected misregistration and a Euclidean distance between each vector representing the assumed misregistration pattern are calculated. The misregistration assumed pattern corresponding to the vector having the smallest Euclidean distance is specified as the same or approximate misregistration pattern. Then, the dither mask to be applied to each pixel position is specified using the dither mask associated with the specified misregistration expected pattern.

  A method of specifying a dither mask applied to each pixel position will be specifically described with reference to FIG. FIG. 10A shows a state in which the print head chips HT1 to HT5 are arranged. On the other hand, FIG. 10B shows the positional deviation between the print head chips HT1 to HT5. As shown in the drawing, the print head chip HT1 and the print head chip HT2 are displaced in the pattern 1 shown in FIG. Similarly, the print head chip HT2 and the print head chip HT3 are misaligned with the pattern 0 (no misalignment), the print head chip HT3 and the print head chip HT4 are misaligned with the pattern 2, the print head chip HT4 and the print head chip. The positional deviation of the pattern 3 is generated from HT5.

  FIG. 10C shows an optimum dither mask applied to each pixel position in consideration of the above-described misregistration pattern. In this example, the size of the dither mask has the same width as the pixel group formed by a single print head chip. As shown in the drawing, the control unit 20 applies the threshold values of the right end portion of the dither mask D1 and the left end portion of the dither mask D2 that are applied in the vicinity of the connection portion between the print head chips HT1 and HT2 where the displacement of the pattern 1 occurs. The selected dither masks Dr1 and Dl1 corresponding to the pattern 1 are specified from the dither mask pattern table 52 shown in FIG. Similarly, the selection dither masks Dr0 and D10 corresponding to the pattern 0 are provided at the pixel positions where the positional deviation of the pattern 0 occurs, and the selection dither mask Dr2 corresponding to the pattern 2 is provided at the pixel positions where the positional deviation of the pattern 2 occurs. For Dl2, the selected dither masks Dr3 and Dl3 corresponding to the pattern 3 are specified at the pixel position where the positional deviation of the pattern 3 occurs. Further, the common dither mask Dm is specified for the pixel position corresponding to the central portion of the print head chip where no positional deviation occurs. For the pixel positions corresponding to the left end of the print head chip HT1 and the right of the print head chip HT5 where no connection exists, the selection dither masks D10 and Dr0 are specified, respectively. Halftone processing is performed in step S170 using the dither masks D1 to D5 thus specified.

  In the present embodiment, the optimum dither mask is specified from the dither mask pattern table 52 for all print head chip connection locations, but it is not necessary to be limited to such an embodiment. Depending on the required printing quality and speed, a dither mask that is not optimal but is compatible to some extent may be specified. For example, when performing low image quality (high-speed) printing, the same dither mask is specified as the optimum dither mask for 13 locations among the connection locations of the print head chips HT1 to HT15, and the dither mask is specified in the remaining 1 location. If the dither mask is not optimally applied to the remaining one place, unless the print image quality is significantly degraded, the dither mask can be uniformly applied to all 14 connection places. Good.

  Further, in this embodiment, as the above-described misregistration assumption pattern, a pattern in which there is no misregistration of the print head chip and a pattern that is deviated by 1 pixel in the vertical and horizontal directions are assumed. Any amount such as 0.5 pixels, 1.5 pixels, etc. may be envisaged. Further, not only the vertical and horizontal shifts in a single direction, but also a combination of vertical and horizontal directions, for example, a misregistration pattern that is shifted by 0.5 pixels upward and 0.3 pixels left may be assumed. By doing so, it is possible to cope with various positional shifts.

  As described above, when the misregistration amount is assumed to be a numerical value that is not an integer, the optimum dither mask is created by virtually increasing the number of pixels of the image data D so that the misregistration amount becomes an integer. Just do it. For example, when assuming a positional shift of 0.5 pixels on the top and 0.5 pixels on the right, one pixel of the image data D is virtually 2 pixels in the vertical direction having the same gradation value, and left and right Treated as a total of 4 pixels, 2 pixels in the direction. Corresponding to this, one threshold value of the dither mask is treated as being virtually composed of four identical threshold values. Then, the optimum dither mask may be created on the assumption that the position is shifted by one virtual pixel on the upper side and one virtual pixel on the right side.

  In the present embodiment, the misregistration adjustment process is performed as the initial setting of the printer 10, but the present invention is not limited to this mode. For example, it may be performed periodically according to the usage time or elapsed time of the printer 10 or may be performed each time before the printer 10 is used. Alternatively, it may be performed at the factory shipment stage.

  In this embodiment, the optical sensor 76 is provided as the misalignment detection means so that the user of the printer 10 can easily adjust misalignment. However, the misalignment detection means is not limited to such an aspect. I can't. For example, a test pattern may be printed by applying the dither masks of all patterns stored in the dither mask pattern table 52, respectively. In this case, the user or manufacturer of the printer 10 can detect a misregistration corresponding to the result by specifying a print result in which the deterioration of the printed image is most suppressed by visual inspection or the like from these test patterns. . Note that the optimum dither mask can also be specified directly from the result.

  Of course, the printer 10 may be configured not to include a misregistration detection unit. For example, the positional deviation may be detected by printing a predetermined test pattern and actually measuring the dot deviation at the time of factory shipment. In this case, the manufacturer inputs the detected misalignment direction and distance to the printer 10 using the operation panel 96 or the like, and the printer 10 specifies the dither mask as processing of the dither mask specifying unit 32. Alternatively, the manufacturer may specify the optimum dither mask in consideration of the detected misregistration and specify and input the result to the printer 10.

  Further, in this embodiment, the print head chips HT1 to HT15 have a form in which the nozzles of each print head chip do not overlap with each other in the direction in which the print head chips are arranged. Is also applicable. In this case, among the adjacent print head chips, using the dither mask having a shape matching the arrangement of dots formed by one print head chip and dots formed by the other print head chip, the above steps are performed. The process of S210 may be performed.

  In the printer 10 having such a configuration, the dither mask associated with the misregistration assumed pattern that is the same as or approximate to the misalignment of the connection location is displayed from the dither mask pattern table 52 for each connection location of the print head chips HT1 to HT15. Identify. Then, halftone processing is performed on the image data D using the dither mask. Since the dither mask is created so that the dispersibility of the printed dots can be ensured even if the corresponding misregistration expected pattern occurs, it is possible to suppress the deterioration of the print image quality.

B. Variations:
A modification of the present invention will be described. FIG. 11 is a flowchart showing a modification of the flow of the image printing process shown in FIG. About the same process as FIG. 3, the same number as FIG. 3 is used for the last two digits of the process number, and description is abbreviate | omitted. In the present modification, when the color conversion process (step S420) is performed, the control unit 20 detects the temperatures of the print head chips HT1 to HT15 using the temperature sensor 78 (step S430).

  When the temperatures of the print head chips HT1 to HT15 are detected, the control unit 20 determines whether or not the detected temperature is within a predetermined range (step S440). This predetermined range is determined based on the temperature of the print head chips HT1 to HT15 when the print head chip position shift detection process (step S200) shown in FIG. 4 is performed.

  As a result, if the detected temperature exceeds the predetermined range (step S440: NO), the control unit 20 estimates the amount of displacement from the detected temperature as processing of the displacement estimation unit 33 (step S450). ). As described above, although the displacement detection is performed in the above step 200, the displacement amount is estimated because the print head chips HT1 to HT15 are expanded or contracted due to the temperature change exceeding a predetermined range. This is because the displacement amount of the print head chips HT1 to HT15 changes, and the influence is corrected. The estimated misregistration amount can be calculated by, for example, multiplying the misregistration amount detected in step S200 by the thermal expansion coefficient associated with the temperature change.

  Then, the control unit 20 re-specifies the dither mask specified in the dither mask specifying process (step S210) shown in FIG. 6 based on the estimated displacement amount as the processing of the dither mask specifying unit 32 (step S460). ). The method for specifying the dither mask here is the same as in step S210. The predetermined temperature range and the coefficient of thermal expansion described above may be appropriately set in consideration of the material of the print head chips HT1 to HT15.

  In this way, when the dither mask is specified again, or when the temperature detected in step S340 is within a predetermined range (step S440: YES), the control unit 20 performs halftone processing in the same manner as in FIG. 3 ( Step S470).

  The printer 10 having such a configuration estimates the amount of misregistration based on the temperatures of the print head chips HT1 to HT15, and specifies a dither mask used for halftone processing based on the estimated amount of misregistration. Therefore, even if the print head chips HT1 to HT15 expand or contract due to the temperature change and the position of the nozzle changes, it is possible to suppress the deterioration of the print image quality.

  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, it can also be realized in the form of a halftone processing method, a program, a dither mask, 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. 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 illustrates the assumption pattern of the position shift of a print head chip. 4 is a flowchart showing a flow of misalignment adjustment processing of the printer 10. It is explanatory drawing which shows an example of the dither mask pattern table. It is explanatory drawing which shows the example of the optimization method of a dither mask. It is explanatory drawing of the dither mask unit in optimization of a dither mask. It is explanatory drawing which shows the example of application of a dither mask. 4 is a flowchart illustrating a modification of the flow of the image printing process illustrated in FIG. 3.

Explanation of symbols

10 ... Printer 20 ... Control unit 30 ... CPU
31 ... Misregistration detection unit 32 ... Dither mask specifying unit 33 ... Misregistration estimation unit 34 ... Halftone processing unit 35 ... Print control unit 40 ... RAM
50 ... ROM
52 ... Dither mask pattern table 61-64 ... Ink cartridge 70 ... Printer head 71-74 ... Nozzle array 76 ... Optical sensor 78 ... Temperature sensor 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 ... printing paper MC ... Memory card Ni, Nj ... Nozzle OD0 to OD4 ... Optimal dither mask D1 to D5 ... Dither mask Dm ... Common dither mask Dr0 to Dr4, D10 to D14 ... Selected dither mask HT1 ~ HT15 ... Print head chip

Claims (8)

A line printer that prints predetermined image data by forming dots on the same raster by a plurality of print heads arranged over a print range,
Storage means for storing a plurality of dither masks associated with each of a plurality of misregistration assumption patterns presuming misregistration between the plurality of print heads;
In consideration of misalignment of the print head, a halftone process that performs halftone processing on the image data using at least a specific dither mask specified for each print head from among the plurality of stored dither masks A line printer comprising tone processing means. The line printer according to claim 1,
Further, based on the positional deviation, the positional deviation assumption pattern that is the same as or approximate to the positional deviation is identified, and the dither mask that is associated with the positional deviation assumption pattern is identified as the specific dither mask. Line printer with means. The line printer according to claim 2,
Furthermore, a detection means for detecting the positional deviation is provided,
The dither mask specifying means specifies a dither mask based on the detected misregistration.   The line printer according to any one of claims 1 to 3, wherein the plurality of misregistration assumption patterns are assumed to correspond to combinations of misalignment directions and distances of the plurality of print heads. The line printer according to any one of claims 1 to 4,
A temperature sensor for detecting temperatures of the plurality of print heads;
A displacement estimation means for estimating the amount of displacement based on the temperature detected by the temperature sensor when the dither mask is specified, and
The halftone processing unit performs the halftone processing using the specific dither mask specified based on the estimated amount of misalignment. A halftone processing method for forming dots on the same raster by a plurality of print heads arranged over a printing range and printing predetermined image data with a line printer,
Storing a plurality of dither masks associated with each of a plurality of misregistration assumption patterns presuming misregistration between the plurality of print heads;
A halftone processing method that performs halftone processing of the image data using a specific dither mask specified from the plurality of stored dither masks in consideration of the positional deviation. A halftone processing program for forming dots on the same raster by a plurality of print heads arranged over a printing range and printing predetermined image data with a line printer,
A storage function for storing a plurality of dither masks associated with each of a plurality of misregistration assumption patterns presuming misregistration between the plurality of print heads;
A program that causes a computer to realize a halftone processing function of performing halftone processing of the image data using a specific dither mask specified from the plurality of stored dither masks in consideration of the positional deviation. In a halftone process of a line printer that forms dots on the same raster by a plurality of print heads arranged over a print range, the dither mask is selected and used according to a positional deviation between the plurality of print heads. And
Obtained by applying the dither mask when there is the positional deviation in a predetermined direction, rather than the dispersibility of the dots obtained by applying the dither mask when there is no positional deviation between the plurality of print heads. Dither mask with excellent dispersibility of dots.

Images