JP2006088373A - Printing that suppresses noise contained in printed images - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for suppressing deterioration of image quality due to image noise in printing in which ink is ejected onto a printing medium.
The present invention is a print control apparatus that controls a printing unit to perform printing by forming dots on a print medium. The print control apparatus includes dot data indicating the presence / absence of formation of each type of dot in each pixel of the print image, and forms dots with any of a plurality of control contents set by operating a plurality of setting parameters. A print data generation unit configured to generate print data configured to perform; and a print image evaluation unit configured to digitize image noise included in an output image output from the print unit according to the print data. The print data generation unit has a test pattern output mode in which a plurality of test patterns are output for each of a plurality of control contents using the same input image. The print image evaluation unit quantifies image noise included in a plurality of evaluation image data generated from a plurality of test patterns.
[Selection] Figure 1

Description

  The present invention relates to a technique for printing an image on a print medium by ejecting ink droplets.

  In recent years, printers that form dots by ejecting ink droplets from a print head onto a print medium while performing main scan of the print head and sub-scan of the print medium are widely used as output devices of computers. In such a printer, in order to reduce print image noise such as graininess and banding caused by factors such as dot size and position errors, and sub-scan feed amount errors, it is an effort to reduce these multiple factors individually. Has been made.

JP 2003-219158 A

  However, it has not been considered to reduce the noise of the printed image by paying attention to the synergistic effect of such a plurality of factors. Furthermore, the synergistic effect of multiple factors is an effect that can occur not only in printers that perform main scanning of the print head and sub-scanning of the printing medium, but also in printers that perform printing by forming dots on the printing medium. is there.

  The present invention has been made to solve the above-described problems in the prior art, and an object of the present invention is to provide a technique for suppressing deterioration in image quality due to image noise in printing in which ink is ejected onto a printing medium. .

The present invention is a print control device that controls a printing unit to perform printing by forming dots on a print medium,
It includes dot data that indicates the presence or absence of the formation of each type of dot in each pixel of the printed image, and is configured to form the dot with any of a plurality of control contents set by operating a plurality of setting parameters. A print data generation unit for generating the print data,
A print image evaluation unit that digitizes image noise included in an output image output by the printing unit in accordance with the print data;
With
The print data generation unit has an output mode for outputting a plurality of test patterns for each of the plurality of control contents using the same input image,
The print image evaluation unit quantifies image noise included in a plurality of evaluation image data generated from a plurality of test patterns output from the printing unit.

  According to the printing control apparatus of the present invention, image noise is digitized for each of a plurality of test patterns output for each of a plurality of control contents set by operating a plurality of setting parameters. It is possible to select an optimum one from a plurality of control contents. Each type of dot means a dot of each size formed with each color ink.

  The present invention realizes an improvement in print quality by a novel idea related to optimization of print control processing. Conventionally, tolerances and other specifications are determined for each setting parameter of print control processing such as ink dot diameter, paper feed amount, color reduction processing content, and interlacing processing content without considering the interaction. By setting each setting parameter so as to satisfy the specifications, the individual differences of the printers are absorbed and the minimum print quality is guaranteed.

  On the other hand, the present invention was created by the inventors paying attention to the point that each setting parameter has an organic relationship. The present invention is characterized in that each setting parameter is not set independently of other setting parameters but is set as a combination of a plurality of setting parameters. Such setting is made possible by quantifying the image noise for each test pattern output with the control content set by operating a plurality of setting parameters.

  The present invention further has a feature that it can cope with not only individual differences at the time of manufacturing the printer but also individual differences caused by aging and a new printing environment (for example, a new printing medium). The final selection of the control content in the present invention may be performed automatically, for example, with the control content having the smallest numerical value as an initial setting, or may be performed semi-automatically via a user as will be described later. Also good.

In the print control apparatus, the print image evaluation unit further selects a control content having a relatively small numerical value from among the plurality of control content according to the digitized image noise, and the selection is made. Display a selection screen allowing the user to select one of the control contents on the display unit, and receive the selection by the user;
The print data generation unit may generate print data configured to form the dots with the control content selected by the user, at least as an initial setting.

  In this configuration, a relatively small amount of image noise is automatically selected from among a plurality of available control details, so that the user can realize a control content with less image noise from the selected range. A combination of settings can be selected. As a result, it is possible to realize optimal print control in consideration of individual differences among printers while suppressing an increase in the burden on the user. This configuration has an advantage of enabling setting that also takes into account user preferences.

In the print control apparatus, the printing unit includes a print head including a plurality of nozzles for ejecting ink and a plurality of drive elements for driving the plurality of nozzles, and the main scanning of the print head And forming a dot by discharging ink from the print head onto the print medium while performing sub-scanning of the print medium,
The plurality of setting parameters include a voltage waveform for driving the plurality of driving elements, a paper feed amount for performing sub-scanning of the print medium, and a content of processing for generating the dot data. It may include at least two of the three setting parameters.

  It has been confirmed by the inventor that these three setting parameters have a significant interaction with respect to printing in which the main scanning of the print head and the sub-scanning of the print medium are performed. For this reason, if at least two of the three setting parameters are included in the setting parameters, the present invention can achieve a remarkable effect.

  The present invention relates to a printing apparatus, a computer program for causing a computer to implement the functions of the method or apparatus, a recording medium storing the computer program, a data signal including the computer program and embodied in a carrier wave, It can be realized in various forms such as a computer program product.

Next, embodiments of the present invention will be described in the following order based on examples.
A. Configuration of a printing system in an embodiment of the present invention:
B. Image noise to be suppressed in the embodiments of the present invention:
C. High image quality processing in the first embodiment of the present invention:
D. High image quality processing in the second embodiment of the present invention:
E. Variations:

A. Configuration of a printing system in an embodiment of the present invention:
FIG. 1 is an explanatory diagram showing the configuration of a printing system 10 according to an embodiment of the present invention. The printing system 10 includes a user interface unit 18 for image processing and printing processing, a personal computer PC that performs image processing and printing processing in response to input from the user interface unit 18, and an output device that outputs a print image. A color printer 20 and a scanner 30 that inputs the output print image and generates image data are provided.

  The personal computer PC includes an image processing application program 300 that executes image processing, a print data generation unit 100 that generates print data PD and setting data to be transmitted to the color printer 20, and image data input from the scanner 30. A print image evaluation unit 200 that evaluates a print image in response to the print image, and an interface unit 15 that manages an interface between the scanner 30, the user interface unit 18, and the color printer 20. The user interface unit 18 includes a display 18a that displays an operation display screen, which will be described later, and a keyboard 18b and a mouse 18c that receive input from the user.

  FIG. 2 is a block diagram illustrating a configuration of the print data generation unit 100 according to the embodiment of the present invention. The print data generation unit 100 includes a resolution conversion module 107, a color conversion module 108, a color reduction module 109, an interlace data generation unit 110, a color conversion table LUT, and a recording rate table DT.

  Programs for realizing the functions of the print data generation unit 100 and the print image evaluation unit 200 are supplied in a form recorded on a computer-readable recording medium. Such recording media include flexible disks, CD-ROMs, magneto-optical disks, IC cards, ROM cartridges, punch cards, printed matter printed with codes such as barcodes, computer internal storage devices (such as RAM and ROM). A variety of computer-readable media such as a memory) and an external storage device can be used.

  FIG. 3 is a schematic configuration diagram of the color printer 20. The color printer 20 includes a sub-scan feed mechanism that transports the printing paper P in the sub-scan direction by the paper feed motor 22 and a main scan feed that reciprocates the carriage 31 in the axial direction (main scan direction) of the platen 25 by the carriage motor 24. A mechanism, a head drive mechanism that drives a print head unit 60 (also referred to as “print head assembly”) mounted on the carriage 31 to control ink ejection and dot formation, and a paper feed motor 22 and a carriage motor. 24, a control circuit 40 that controls the exchange of signals with the print head unit 60 and the operation panel 32. The control circuit 40 is connected to the computer PC via the connector 56.

  The sub-scan feed mechanism that transports the printing paper P includes a gear train (not shown) that transmits the rotation of the paper feed motor 22 to the platen 25 and a paper transport roller (not shown). The main scanning feed mechanism for reciprocating the carriage 31 is an endless drive belt 36 between the carriage motor 24 and a slide shaft 34 that is laid in parallel with the axis of the platen 25 and slidably holds the carriage 31. And a position sensor 39 for detecting the origin position of the carriage 31.

  FIG. 4 is a block diagram showing the configuration of the color printer 20 with the control circuit 40 as the center. The control circuit 40 is configured as an arithmetic logic circuit including a CPU 41, a programmable ROM (PROM) 43, a RAM 44, and a character generator (CG) 45 that stores a dot matrix of characters. The control circuit 40 further includes an I / F dedicated circuit 50 dedicated to interface with an external motor and the like, and a head connected to the I / F dedicated circuit 50 to drive the print head unit 60 and eject ink. A drive circuit 52 and a motor drive circuit 54 for driving the paper feed motor 22 and the carriage motor 24 are provided. The I / F dedicated circuit 50 incorporates a parallel interface circuit and can receive print data PD supplied from the computer PC via the connector 56. The color printer 20 executes printing according to the print data PD.

  The paper feed amount of the printing paper P is adjusted when the color printer 20 is manufactured or when the printing mode is set. This is because the paper feed amount of the printing paper P varies depending on the type of the platen 25 and the printing paper P. For example, the paper feed amount that varies according to the surface condition of the platen 25 caused by individual differences is adjusted during the manufacture of the color printer 20 so as to compensate for the variation amount. On the other hand, the paper feed amount that varies depending on the type of the print paper P is adjusted when the paper type is selected in the setting of the print mode so as to compensate for the variation amount.

  Such adjustment can be made by, for example, applying a bias signal to the motor drive circuit 54 that drives the paper feed motor 22. However, with such a method, it is not possible to compensate for the paper feed amount that fluctuates in accordance with the secular change of the sub-scan feed mechanism. Furthermore, variations in the paper feed amount for each paper feed cannot be compensated.

  The print head unit 60 has a print head 28 and can be mounted with an ink cartridge. The print head unit 60 is attached to and detached from the color printer 20 as one component. That is, when the print head 28 is to be replaced, the print head unit 60 is replaced.

  FIG. 5 is an explanatory diagram showing the nozzle arrangement on the lower surface of the print head 28. On the lower surface of the print head 28, a cyan ink nozzle row C for discharging cyan ink, a light cyan ink nozzle row LC for discharging light cyan ink, and a magenta ink nozzle row M for discharging magenta ink. A light magenta ink nozzle array LM for discharging light magenta ink, a yellow nozzle array Y for discharging yellow ink, a dark yellow nozzle array DY for discharging dark yellow ink, and a black ink A black ink nozzle row K for this purpose is formed. Here, the light cyan ink is an ink having a lower density than the cyan ink. The light magenta ink is an ink having a lower density than the magenta ink. It is an ink having a low brightness as compared with the dark yellow ink.

  Each nozzle is provided with a piezo element (described later) as a drive element for driving each nozzle to eject ink droplets. During printing, ink droplets are ejected from each nozzle while the print head 28 moves in the main scanning direction.

  FIG. 6 is an explanatory diagram showing the structure of the nozzle Nz and the piezo element PE. The piezo element PE is installed at a position in contact with the ink passage 68 that guides ink to the nozzle Nz. In the present embodiment, by applying a voltage between the electrodes provided at both ends of the piezo element PE, one of the side walls of the ink passage 68 is deformed to eject the ink droplet Ip from the tip of the nozzle Nz at a high speed. .

  FIG. 7 is an explanatory diagram showing the relationship between the two types of drive waveforms of the nozzle Nz when ink is ejected and the two ink droplets IPs and IPm ejected. FIG. 7A shows a driving waveform for ejecting small ink droplets IPs capable of forming small dots by itself, and FIG. 7B shows a medium ink droplet IPm capable of forming medium dots by itself. The drive waveform for discharging is shown.

The small ink droplet IPs can be ejected from the nozzle Nz through two processes, an ink absorption process and an ink ejection process, as described below.
(1) Ink supply process (d1s): In this process, the ink passage 68 (FIG. 6) is expanded and ink is supplied to the ink passage 68 from an ink tank (not shown). Expansion of the ink passage 68 is performed by lowering the potential applied to the piezo element PE and contracting it.
(1) Ink ejection process (d2): In this process, the ink passage 68 is compressed and ink is ejected from the nozzles Nz. The ink passage 68 is compressed by increasing the potential applied to the piezo element PE and expanding it.

  The middle ink droplet IPm can be ejected in the same manner as the small ink droplet IPs by lowering the potential relatively slowly as shown in FIG. 7B in the ink absorption process. This is because more ink can be supplied from an ink tank (not shown) if the ink passage 68 is slowly expanded by slowly lowering the potential.

  As described above, when the rate of decreasing the potential is increased, the ink interface Me shifts to the inner side of the nozzle Nz as shown in FIG. Ink droplets become smaller. On the other hand, when the potential decrease rate is slowed down, the ink interface Me moves to the inside of the nozzle Nz as shown in FIG. Becomes larger. In this embodiment, the size of the ink droplet is changed by changing the potential change rate in the ink supply process in this way.

  FIG. 8 is an explanatory diagram showing a state in which dots of three sizes of large, medium and small are formed at the same position using the small ink droplet IPs and the medium ink droplet IPm. The drive waveform W1 is a waveform for ejecting small ink droplets IPs, and the drive waveform W2 is a waveform for ejecting medium ink droplets IPm. As can be seen from FIG. 8, in this embodiment, after the drive waveform W1 for ejecting the small ink droplet IPs is output, the drive waveform W2 for ejecting the middle ink droplet IPm is output after a predetermined time has elapsed. Yes.

  The reason why the two drive waveforms W1 and W2 are output to the piezo element PE at such timing is to make the landing positions of the small ink droplet IPs and the medium ink droplet IPm coincide. That is, as can be seen from FIG. 8, when small ink droplets IPs having a relatively slow average flying speed are ejected first, and medium ink droplets IPm having a relatively fast average flying speed are ejected after a certain period of time, the landing position Can be matched. The average flying speed means an average value of the flying speed from ejection to landing, and decreases when the deceleration rate is large.

  In this embodiment, the size and landing position of the ink droplet Ip are adjusted by mounting the print head unit 60 on the color printer 20. This is because the size and landing position of the ink droplet Ip vary depending on individual differences in the nozzle rows of the print head 28 such as the characteristics of the piezo element PE and the size and shape of the ink passage 68. Specifically, the head drive circuit 52 (FIG. 4) is adjusted by adjusting the drive signal for each nozzle row based on the characteristic information of the print head 28. The characteristic information of the print head 28 is stored in a memory (not shown) provided in the print head unit 60.

  The reason why the drive signal is adjusted for each nozzle row is because it is customarily manufactured as a unit for each nozzle row, and it is known that the characteristics are approximated for each nozzle row. Therefore, variation in the characteristics of each nozzle in each nozzle row is not compensated. The drive signal is adjusted by, for example, adjusting the time d1s and d1m for the dot diameter, and by adjusting the height of the peak voltage for the dot landing position.

  As described above, the color printer 20 guarantees the print image quality by adjusting separately without considering the interaction between the paper feed amount, the size of the ink droplet Ip, and the landing position, and within the preset tolerance range. Is configured to do.

B. Image noise to be suppressed in the embodiments of the present invention:
FIG. 9 is an explanatory diagram showing how banding noise occurs in a printed image formed by ink dots. The numbers in the circles indicate the numbers of the nozzles in charge of forming dots at the pixel positions. In this example, banding noise is generated as white stripes or black stripes because the landing positions of dots formed by the No. 5 nozzle are shifted upward due to manufacturing errors or the like. The banding noise greatly deteriorates the print image quality because it is noise in a frequency band with high human visual sensitivity.

  FIG. 10 is a graph showing human visual sensitivity VTF. The visual sensitivity VTF is high in the low spatial frequency band. On the other hand, since white stripes and black stripes have a long and low spatial frequency in the main scanning direction, it can be seen that they are easily noticed as noise by human eyes. Furthermore, it can be seen that, among white stripes and black stripes, white stripes are particularly easily noticed by human eyes, so that suppression of white stripes is effective in suppressing noise that is easily noticeable by human eyes.

  It has been found that white stripes are likely to occur when the ink dot diameter is small and the paper feed amount of the printing paper P is large. In such a case, a gap is likely to be generated between the ink dots. On the other hand, when the paper feed amount is small, black streaks are likely to occur. Furthermore, if the dispersion of the ink dot landing positions in the sub-scanning direction is large, white stripes are likely to be divided and banding noise tends to be suppressed. On the other hand, ink dot mottle is generated and graininess is deteriorated. The potential for problems is high. A mottle is a settlement caused by variations in the landing positions of ink dots.

  Thus, it can be seen that the paper feed amount, the size of the ink dots, and the landing position have an organic relationship and affect image noise that causes deterioration in print image quality such as banding noise and graininess.

C. High image quality processing in the first embodiment of the present invention:
FIG. 11 is a flowchart showing the contents of the image quality improvement processing in the first embodiment of the present invention. This high image quality processing is based on the evaluation of image noise included in the print image quality of the printed material printed in this manner while printing the test pattern with the print control parameter (also called the setting parameter) varied. Determine the optimal print control parameters. In this embodiment, the ink dot diameter and the paper feed amount are selected as the print control parameters.

  In step S100, the print image evaluation unit 200 (FIG. 1) causes the color printer 20 to print a predetermined test pattern group (FIG. 12) in response to a command from the user. In the present embodiment, the predetermined test pattern group is a test pattern group including a total of 45 patches in which test patterns including patches of five different densities are printed with nine different settings.

  FIG. 13 is a flowchart showing a test pattern group printing method according to the first embodiment of the present invention. In step S110, the print image evaluation unit 200 determines the setting contents. The setting content is determined based on a table prepared in advance in the print image evaluation unit 200.

  FIG. 14 is a table showing a table for storing the setting contents for printing the test pattern group in the first embodiment of the present invention. This table includes “dot diameter” and “sub-scan feed amount” as print control parameters representing setting contents. The setting contents are simply shown as a combination of a nominal value and bias values ε and δ with respect to the nominal value for easy understanding.

  The print image evaluation unit 200 first selects setting 1 and sets “dot diameter” and “sub-scan feed amount” as “nominal value + ε” and “nominal value + δ” as print control parameters, respectively.

  In step S120, the print image evaluation unit 200 transmits the setting data to the color printer 20 via the I / F unit 15 (FIG. 1). In response to the reception of the setting data, the color printer 20 sets “dot diameter” and “sub-scan feed amount” to “nominal value + ε” and “nominal value + δ”, respectively.

  In step S <b> 130, the print image evaluation unit 200 transmits print data PD for printing five types of patches having different densities to the color printer 20. The color printer 20 prints five types of patches having different densities in the above setting state in response to reception of the print data PD. Such print processing is performed for all nine types of settings for the same color patch (step S140). As a result of such printing processing, a predetermined test pattern group as shown in FIG. 12 is printed.

  In this embodiment, “setting data” and “print data PD” correspond to “print data” in the claims. The setting data may be transmitted to the color printer 20 as a part of header information of the print data PD, for example. In this way, it is possible to reduce the burden on the user when it is desired to change the setting for each print job.

  In step S200 (FIG. 11), the print image evaluation unit 200 inputs a print image on which a predetermined test pattern group is printed as image data using the scanner 30 (FIG. 1). In this input operation, the print image evaluation unit 200 requests the user to input a print image on which a predetermined test pattern group is printed from the scanner 30 through the display 18a, and the user responds to this request. Done.

  In step S300 (FIG. 11), the print image evaluation unit 200 evaluates the input image data. This evaluation is a process for quantifying the image noise. The quantification of the image noise is performed by separating the image noise included in the print image into graininess noise and banding noise, and multiplying each noise by a predetermined sensitivity coefficient and summing them up.

  FIG. 15 is a flowchart showing the contents of the print image evaluation process in the first embodiment of the present invention. In step S310, the print image evaluation unit 200 generates a winner spectrum. The Wiener spectrum can be easily obtained by, for example, squaring the two-dimensional Fourier transform result of the brightness of the image data, or as the two-dimensional Fourier transform of the autocorrelation function of the brightness of the image data. The Wiener spectrum can quantify (numerize) graininess noise and banding noise.

  In step S320, the print image evaluation unit 200 separates the granularity noise and the banding noise included in the winner spectrum. This separation can be realized by utilizing the presence or absence of isomorphism of image noise and the presence or absence of concentration on a specific frequency. Isotropy is a property in which the nature of image noise does not depend on a specific direction.

  A separation method that focuses on the isotropy of image noise is a method that uses the fact that banding noise has directionality while graininess noise has isotropy. For example, based on a uniform patch Wiener spectrum, focusing on power fluctuations according to direction and isolating banding noise with directionality from isotopic background noise (here, granular noise) Can do.

  The separation method focusing on the concentration on a specific frequency is a method using the fact that banding noise is concentrated on a specific frequency. For example, it is possible to set a threshold value that changes according to the average level of power based on a uniform patch Wiener spectrum, and to determine that noise having a power larger than the threshold value is banding noise and extract it separately.

  FIG. 16 is an explanatory diagram showing how the total noise amount is calculated from the granularity noise and the banding noise separated and extracted in this way. FIG. 16A shows the granularity noise value of each patch and the integrated noise value of each setting. FIG. 16B shows the banding noise value of each patch and the integrated noise value of each setting. FIG. 16C shows the total noise amount calculated from the two integrated noises of each setting.

  The density 1, density 2, density 3, density 4, and density 5 in FIGS. 16 (a) and 16 (b) are the noise values of the patches from the left end to the right end of each setting shown in FIG. For example, for the patch at the left end of setting 1 shown in FIG. 12, the graininess noise value is calculated to be 4.1. The integrated noise in FIGS. 16A and 16B is a value representing the magnitude of noise in each setting, and is the largest value among the noise values of each patch. The value representing the magnitude of noise in each setting is the largest value in each patch because it is preferable to make the print processing system a highly robust system based on the worst-case print image quality. is there.

  In step S330, the print image evaluation unit 200 performs weighting processing. The weighting process is a process of multiplying the separated granularity noise value and the banding noise value by a weighting coefficient. The reason why the weighting process is performed is that the granularity noise and the banding noise are different in nature and have different effects on the visual sensitivity, and the evaluation differs depending on the user's preference.

  For example, in the setting 1, the weighting process is performed as follows. In setting 1, the integrated noise values of the graininess noise and the banding noise are “8.8” and “2.8”, respectively. The coefficient 1 means that the integrated noise of the graininess noise is multiplied by the coefficient 0.6, and the integrated noise of the banding noise is multiplied by the coefficient 0.4. As a result, a value of “6.4” is calculated. For coefficients 2 and 3, the values 5.8 and 5.2 are calculated in the same manner.

  In step S340, the print image evaluation unit 200 determines the total noise amount. The total noise amount is determined by selecting, for each setting, the largest value among the values calculated using the coefficient 1, the coefficient 2, and the coefficient 3. The reason why the largest value is selected is to make the print processing system a highly robust system while taking into consideration the influence on visual sensitivity and the variation in user's preference.

  In step S400 (FIG. 11), the print image evaluation unit 200 narrows the settings. In this embodiment, the setting is narrowed down by selecting three settings with the smallest total noise amount. Specifically, in the example of FIG. 16, setting 2, setting 4, and setting 7 are selected. The print image evaluation unit 200 displays the three selected settings together with the image of each patch on the display 18a.

  FIG. 17 is an explanatory diagram showing a user interface screen W1 representing an image of each patch displayed on the display 18a. The user interface screen W1 shows an image of each patch printed by setting 2, setting 4, and setting 7. The user can select the most preferred setting by clicking either setting 2, setting 4, or setting 7 using the mouse 18c (step S500).

  In step S <b> 600 (FIG. 11), the print image evaluation unit 200 operates the color printer 20 via the I / F unit 15 so that the selected setting becomes the initial setting of the color printer 20. Specifically, the color printer 20 is operated so that the head driving circuit 52 and the motor driving circuit 54 are set to the selected setting.

  As described above, according to the first embodiment of the present invention, the image noise included in the print image in which a plurality of organically related control parameters such as the dot diameter and the paper feed amount are changed is quantified and quantified. Since the user can select a preferable setting from a plurality of settings narrowed down according to the image noise, it is possible to easily realize the optimization of the setting in consideration of the organic relationship between the plurality of print control parameters.

D. High image quality processing in the second embodiment of the present invention:
The second embodiment of the present invention differs from the first embodiment in that not only the dot diameter and the paper feed amount but also a color reduction process is included as an organically related print control parameter. The reason why the color reduction process is included is that the content of the color reduction process also has a great influence on the graininess noise and banding noise due to the organic relationship with the dot diameter and the paper feed amount. In the present embodiment, 256 ink gradation values are reduced to four gradations of large dots, medium dots, small dots, and no dots.

  FIG. 18 is an explanatory diagram showing a relationship between the dot recording rate table and the ink discharge amount. FIG. 18A is a diagram showing the relationship between the gradation value of multi-gradation data and the dot recording rate of dots of each size, and is the same as FIG. FIG. 18B is an explanatory diagram showing the relationship between the gradation value and the weight of ink ejected to a predetermined area. The predetermined area is assumed to be an area composed of 255 pixels. The ink weight is assumed to be 10 ng for small dots, 20 ng for medium dots, and 30 ng for large dots.

FIG. 18B is a plot of the following product for each gradation value.
(1) The recording rate of dots of each size (for example, in the gradation value G2, small dots are 25% and medium dots are 50%)
(2) Ink weight (10 ng for small dots, 20 ng for medium dots, 30 ng for large dots)
(3) Number of pixels in a predetermined area (255 pixels)
For example, when the gradation value is 255 (maximum gradation value), the above product is 7650 ng (= 100% × 30 ng × 255 pixels).

  As can be seen from FIG. 18B, when the gradation value increases from 0 to 255, the ink discharge amount increases from 0 ng to 7650 ng along the straight line Wi. As described above, in this embodiment, in order to make the explanation easy to understand, it is assumed that the ink weight ejected to a predetermined region and the gradation value have a linear relationship.

As can be seen from FIGS. 18A and 18B, the weight of ink ejected to the predetermined area increases as follows according to the increase in the gradation value.
(1) In the region from the gradation value 0 to the gradation value G1, the ink weight increases linearly as the dot recording rate of small dots increases.
(2) In the area from the gradation value G1 to the gradation value G2, the dot recording rate of small dots is constant, and the ink weight increases linearly as the dot recording rate of medium dots increases.
(3) In the area from the gradation value G2 to the gradation value G3, the dot recording rate of small dots and medium dots is constant, and the ink weight increases linearly as the dot recording rate of large dots increases.
(4) In the area from the gradation value G3 to the maximum gradation value, the dot recording rate of small dots and medium dots starts to decrease, and the ink weight is linearly replaced by replacing the small dots and medium dots with large dots. Will increase.

In the present embodiment, such a dot recording rate profile is generated as a result of the following trade-off.
(1) In order to suppress graininess (roughness of the image), it is preferable to increase the dot recording rate of relatively small dots by decreasing the recording rate of relatively large dots that are easily visible. Such characteristics are particularly remarkable in a low gradation region.
(2) In order to reduce banding (striped image quality degradation), it is preferable to reduce the dot recording rate of relatively small dots by replacing relatively small dots with relatively large dots. Such characteristics are particularly remarkable in a high gradation region.
As a result of such a trade-off, in this embodiment, the profile is set with the upper limit value of the dot recording rate of small dots being 25% and the upper limit value of the dot recording rate of medium dots being 50%.

  FIG. 19 is an explanatory diagram showing the relationship between the dot recording rate of small dots and medium dots and the occurrence of banding. FIG. 19A is the same diagram as FIG. 9A, and the numbers in the circles indicate the numbers of the nozzles in charge of forming dots at the pixel positions, as in FIG. 9A. .

  FIG. 19C shows a state where the same ink amount is ejected to the same area as FIG. 19B. For this reason, the dot pattern of FIG. 19C and the dot pattern of FIG. 19B express the same gradation. However, in FIG. 19C, the six small dots in FIG. 19B are replaced with three medium dots.

  In the dot pattern of FIG. 19C, it can be seen that banding is suppressed as compared to that of FIG. This is because the white stripes generated in FIG. 19B are divided by two medium dots formed by the 4th nozzle and the 5th nozzle. It has been found that white stripes are less noticeable as image quality degradation when divided as shown below.

  Thus, it can be seen that banding is likely to occur if the small dot recording rate is increased alone, or if the small dot recording rate is increased while the medium dot recording rate is extremely low. However, when medium dots are frequently used, the graininess (roughness of the image) tends to occur. This is because medium dots are easier to visually recognize than small dots.

  For this reason, the dot recording rate for each size is determined to an optimum value as a result of the trade-off between banding and graininess as described above. However, when the dot diameter varies depending on the setting, there arises a problem that the upper limit value of the small dot that should have been optimally set for the color reduction process is not optimal. In addition, the color reduction process indirectly interacts with the paper feed amount having an organic interaction with the dot diameter.

  FIG. 20 is an explanatory diagram showing a plurality of dot recording rate tables DT (n) used in the second embodiment of the present invention. FIG. 20 shows that a plurality of dot recording rate tables DT (n) includes three dot recording rate tables DTn, DT1, and DT2. The dot recording rate tables DTn, DT1, and DT2 can be selected as print control parameters for color reduction processing.

  FIG. 21 is an explanatory diagram showing a dot recording rate table setting method in the second embodiment of the present invention. FIG. 21A shows a dot recording rate profile of each size used for setting the dot recording rate table. For example, the dot recording rate profiles for small dots include profiles SDn, SD1, and SD2. The difference between these profiles SDn, SD1, and SD2 is the upper limit value of the dot recording rate. The upper limit of the dot recording rate is set to 25% in the profile SDn, but is set to 30% (L1) which is 5% higher than the profile SDn in the profile SD1, and only 5% higher than the profile SDn in the profile SD2. Low 20% (L2) is set. With the adjustment of the small dot recording rate profile, the medium dot recording rate profiles MDn, MD1, and MD2 are also adjusted.

  FIG. 22 is a table showing a table for storing the setting contents for printing the test pattern group in the second embodiment of the present invention. This table includes a “dot recording rate table” in addition to “dot diameter” and “sub-scan feed amount” as print control parameters representing setting contents.

  As described above, in the second embodiment, the print control parameters include not only the dot diameter and the paper feed amount but also the color reduction process, so that the number of setting options to be selected increases. Since the choices can be narrowed down, it is possible to easily realize the optimization of the setting in consideration of the organic relationship of the added print control parameters while suppressing an increase in the burden on the user.

  In this embodiment, “print data configured to form dots with any of a plurality of control contents set by operating a plurality of setting parameters” means a plurality of dot recordings as setting parameters. It means print data including print data PD generated by selecting a rate table and setting data (dot diameter and sub-scan feed amount).

E. Variations:
The present invention is not limited to the above-described examples and embodiments, and can be implemented in various modes without departing from the gist thereof. For example, the following modifications are possible.

E-1. In each of the above-described embodiments, a control content having a relatively small numerical value among a plurality of control details is selected according to image noise, and the user selects the final control content from among the selected control details. However, the final selection may be automatically performed. However, if the user makes a final selection, there is an advantage that the user's preference can be taken into consideration, and if the user makes a final selection automatically, the user's burden is reduced. Has the advantage of becoming even lighter.

E-2. In each of the above-described embodiments, three parameters such as the voltage waveform for driving the nozzle drive element, the paper feed amount for performing sub-scanning of the print medium, and the content of processing for generating dot data are set parameters. Although used, other parameters such as an interlacing method, color conversion, and color separation may be included in the setting parameters. However, since the three parameters used in the embodiment have been confirmed to have a significant interaction, if at least two of the three setting parameters are included in the setting parameters, the present invention Can have a remarkable effect.

E-3. The control content may be selected for each printing situation such as the printing environment (for example, the type of printing paper and ink, the compatibility of the printing paper and ink) and the printing target image (for example, a natural image or DTP). In such a case, it is preferable that the control content selected according to the printing environment and the print target image is automatically selected in the printing process.

E-4. In the above-described embodiment, the test pattern is output only once. However, the test pattern may be set stepwise by outputting a plurality of times. For example, a rough setting may be performed at the initial stage and gradually detailed settings may be performed. In such a case, the next test pattern group may be determined by analyzing the sensitivity of the image noise with respect to the variation of each setting parameter.

  For example, when image noise variation due to adjustment of the dot diameter and paper feed amount is large among the three parameters, the setting parameter may be determined by varying the adjustment of the dot diameter and paper feed amount in detail. Furthermore, the setting fluctuation range may be determined in consideration of human visual sensitivity VTF. In this way, the burden on the user can be further reduced.

E-5. In the above-described embodiment, an ink jet printer including a piezo element has been described as an example. However, various types of printers such as a printer that discharges ink with bubbles generated in ink by energizing a heater provided in a so-called nozzle are used. Applicable to printing devices.

E-6. The present invention can be applied not only to color printing but also to monochrome printing. Further, the present invention can be applied to printing that expresses multiple gradations by expressing one pixel with a plurality of dots.

E-7. In the above embodiment, a part of the configuration realized by hardware may be replaced with software, and conversely, a part of the configuration realized by software may be replaced by hardware. For example, the control circuit 40 in the printer 20 may execute part or all of the print image evaluation unit shown in FIG. In this case, part or all of the functions of the computer PC as a print control apparatus for creating print data are realized by the control circuit 40 of the printer 20.

  When some or all of the functions of the present invention are realized by software, the software (computer program) can be provided in a form stored in a computer-readable recording medium. In the present invention, the “computer-readable recording medium” is not limited to a portable recording medium such as a flexible disk or a CD-ROM, but an internal storage device in a computer such as various RAMs and ROMs, a hard disk, and the like. An external storage device fixed to the computer is also included.

1 is an explanatory diagram illustrating a configuration of a printing system 10 according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating a configuration of a print data generation unit 100 according to an embodiment of the present invention. 1 is a schematic configuration diagram of a color printer 20. FIG. FIG. 2 is a block diagram illustrating a configuration of the color printer 20 with a control circuit 40 as a center. FIG. 4 is an explanatory diagram showing nozzle arrangement on the lower surface of the print head. Explanatory drawing which shows the structure of the nozzle Nz and the piezo element PE. Explanatory drawing which showed the relationship between the two types of drive waveforms of the nozzle Nz at the time of ink discharge, and the ink droplets IPs and IPm of two sizes discharged. Explanatory drawing which shows a mode that the dot of three sizes of large, medium, and small is formed in the same position using small ink drop IPs and medium ink drop IPm. Explanatory drawing which shows a mode that banding noise generate | occur | produces in the printing image formed by the ink dot. The graph which shows human visual sensitivity VTF. The flowchart which shows the content of the image quality improvement process in 1st Example of this invention. Explanatory drawing which shows the predetermined test pattern group in 1st Example of this invention. 3 is a flowchart showing a method for printing a predetermined test pattern group in the first embodiment of the present invention. The table | surface which shows the table which stores the setting content in the printing of the test pattern group in 1st Example of this invention. 3 is a flowchart showing the contents of print image evaluation processing in the first embodiment of the present invention. Explanatory drawing which shows a mode that a total noise amount is calculated from the granularity noise and banding noise which were separated and extracted. Explanatory drawing which shows the user interface screen W1 showing the image of each patch displayed on the display 18a. FIG. 4 is an explanatory diagram showing a relationship between a dot recording rate table and an ink discharge amount. Explanatory drawing which shows the relationship between the dot recording rate of a small dot and medium dots, and generation | occurrence | production of banding. Explanatory drawing which shows the some dot recording rate table DT (n) utilized in 2nd Example of this invention. Explanatory drawing which shows the setting method of the dot recording rate table in 2nd Example of this invention. The table | surface which shows the table which stores the setting content in the printing of the test pattern group in 2nd Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Printing system 15 ... Interface part 18 ... User interface part 20 ... Color printer 22 ... Paper feed motor 24 ... Carriage motor 25 ... Platen 28 ... Print head 30 ... Scanner 31 ... Carriage 32 ... Operation panel 34 ... Sliding shaft 36 ... Drive belt 38 ... pulley 39 ... position sensor 40 ... control circuit 41 ... CPU
44 ... RAM
DESCRIPTION OF SYMBOLS 50 ... Dedicated circuit 52 ... Head drive circuit 54 ... Motor drive circuit 56 ... Connector 60 ... Print head unit 68 ... Ink path 100 ... Print data generation part 107 ... Resolution conversion module 108 ... Color conversion module 109 ... Subtractive color module 110 ... Interlace data generation unit 125 ... Platen 200 ... Print image evaluation unit 300 ... Image processing application program

Claims (6)

A printing control apparatus that controls a printing unit to perform printing by forming dots on a printing medium,
It includes dot data that indicates the presence or absence of the formation of each type of dot in each pixel of the printed image, and is configured to form the dot with any of a plurality of control contents set by operating a plurality of setting parameters. A print data generation unit for generating the print data,
A print image evaluation unit that digitizes image noise included in an output image output by the printing unit in accordance with the print data;
With
The print data generation unit has an output mode for outputting a plurality of test patterns as the print data for each of the plurality of control contents using the same input image,
The print control apparatus, wherein the print image evaluation unit quantifies image noise included in a plurality of evaluation image data generated from a plurality of test patterns output as the output image. The print control apparatus according to claim 1,
The print image evaluation unit further selects a control content having a relatively small value among the plurality of control contents in accordance with the digitized image noise, and selects the control contents from the selected control contents. Display on the display a selection screen that allows the user to select one, and receive the selection by the user;
The print data generation unit generates print data configured to perform the dot formation with the control content selected by the user, at least as an initial setting. The print control apparatus according to claim 1 or 2,
The printing unit includes a print head that includes a plurality of nozzles for ejecting ink and a plurality of drive elements for driving the plurality of nozzles. While performing scanning, dots are formed by ejecting ink from the print head onto the print medium,
The plurality of setting parameters include a voltage waveform for driving the plurality of driving elements, a paper feed amount for performing sub-scanning of the print medium, and a content of processing for generating the dot data. A print control apparatus including at least two of the three setting parameters. A printing apparatus that performs printing by forming dots on a print medium,
A printing unit that forms dots by ejecting ink from a print head onto a print medium; and
A printing control apparatus according to any one of claims 1 to 3,
A printing apparatus comprising: A printing control method for controlling a printing unit to perform printing by forming dots on a printing medium,
It includes dot data that indicates the presence or absence of the formation of each type of dot in each pixel of the printed image, and is configured to form the dot with any of a plurality of control contents set by operating a plurality of setting parameters. A print data generation process for generating the print data;
A print image evaluation step for quantifying image noise included in an output image output by the printing unit according to the print data;
With
The print data generation step has an output mode in which a plurality of test patterns are output as the print data for each of the plurality of control contents using the same input image,
The printing image evaluation step includes a step of quantifying image noise included in a plurality of evaluation image data generated from a plurality of test patterns output as the output image. . A computer program for causing a computer to execute processing for controlling a printing unit in order to form dots on a print medium and perform printing,
It includes dot data that indicates the presence or absence of the formation of each type of dot in each pixel of the printed image, and is configured to form the dot with any of a plurality of control contents set by operating a plurality of setting parameters. Print data generation function for generating print data,
A print image evaluation function for quantifying image noise included in an output image output by the printing unit according to the print data;
Having a program for causing the computer to realize
The print data generation function has a function of outputting a plurality of test patterns as the print data for each of the plurality of control contents using the same input image,
The computer program according to claim 1, wherein the print image evaluation function includes a function of digitizing image noise included in a plurality of evaluation image data generated from a plurality of test patterns output as the output image.

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