Patent Publication Number: US-2011075171-A1

Title: Printing apparatus and calibration method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application No. 2009-227535 filed on Sep. 30, 2009. The entire disclosure of Japanese Patent Application No. 2009-227535 is hereby incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to printing apparatuses and calibration methods, and particularly relates to a printing apparatus that forms a printed image by forming ink dots based on ink amounts specified for each pixel of which image data is configured, and to a calibration method for such a printing apparatus. 
     2. Related Art 
     A calibration method in which, when executing calibration, a user sets desired environment information that he/she feels is optimal has been proposed (see JP-A-2001-213036). 
     SUMMARY 
     In calibration, increasing the number of color patches (color control patches) that are printed/measured generally makes it possible to attain a higher degree of accuracy; however, this also increases the amount of time, paper, and so on required for the calibration. Accordingly, some users have felt that the time required for calibration is too long, whereas other users have felt that the accuracy they desire cannot be achieved. There has also been a problem in that various color patches of colors unrelated to the colors expressed by an image that is to be printed are formed, and thus the accuracy of the colors expressed by the image that is to be printed cannot be ensured. 
     An advantage of some aspects of the invention is to provide a printing apparatus and a calibration method capable of carrying out calibration efficiently. 
     According to an aspect of the invention, a printing apparatus is adapted to form a printed image by forming a plurality of ink dots based on an ink amount specified for each of a plurality of pixels forming image data. The printing apparatus includes a determination unit, a patch printing unit, a color measurement unit, a correction data creation unit and a printing unit. The determination unit is configured to analyze the image data and to determine at least one color patch to be printed based on a result of the analysis. The patch printing unit is configured to print the at least one color patch. The color measurement unit is configured to perform color measurement on the at least one color patch to obtain a color value indicated in the at least one color patch. The correction data creation unit is configured to create correction data using the color value obtained by the color measurement unit. The printing unit is configured to form the ink dots based on the ink amount that has been corrected based on the correction data. In this manner, the color patch to be formed is determined based on the result of analyzing the image data to be printed, and thus it is possible to prevent the printing of excessive color patches and carry out calibration efficiently. 
     Moreover, it is preferable to form the color patch using a frequently-used ink that is used more often than other inks in the printing of the image data performed by the printing unit. This is because if the accuracy with respect to the inks that are used more frequently in the printing of the image data can be improved, the overall reproduction accuracy of the image data can be improved efficiently. Furthermore, it is preferable to form the color patch based on a frequently-appearing ink amount that is an ink amount appearing more frequently among ink amounts specified for each of the pixels in the image data. This, too, is because if the accuracy with respect to the ink amounts that are used more frequently in the printing of the image data can be improved, the overall reproduction accuracy of the image data can be improved efficiently. 
     Moreover, the location and size of the color patch may be determined based on the result of the analysis in addition to the color of the color patch. In other words, the location, size, and so on of the color patch to be printed may be determined based on a spatial distribution state of the pixels in the image data. Basically, it is desirable to form the color patch in a region in which the pixels of an ink amount corresponding to the color patch are heavily distributed, and it is desirable to form the color patch at a large size in the case where the pixels of an ink amount corresponding to the color patch are widely distributed. If the color patch is formed at a large size, the distribution range of color measurement points in the color patch can be increased. 
     In addition, the color patch may be determined based on a history of the color patches printed in the past in addition to the results of analyzing the image data. Accordingly, a recording medium that stores the color patches that have been printed by the printing unit in the past and history data that holds the color values obtained by performing color measurement on the color patches is also provided. By referring to the history data, it is determined that a different color patch than the color patch printed in a last printing cycle by the patch printing unit is to be printed. This makes it possible to prevent forming the same color patch in succession. The correction data creation unit creates the correction data by integrating the color values held in the history data with the color values obtained by performing color measurement on the different color patches than the color patches printed in the past. In other words, the correction data is created based on the color measurement values of a color patch printed in multiple periods of time. Doing so makes it possible to reduce the number of color patches printed in each period of time, and thus makes it possible to implement faster printing. Meanwhile, a case in which the image data is not characteristic can also be considered. In such a case, it is desirable to preferentially form color patches based on the history data. 
     Furthermore, the technical idea of the invention can be realized not only in a printing apparatus, but can also be realized in a printing method including steps carried out by each of the units of which the printing apparatus is configured. Of course, it goes without saying that in the case where the stated printing apparatus realizes the stated units by reading out a program, the technical idea of the invention can also be realized in a program that executes functions corresponding to the units, various types of recording media on which is the program recorded, and so on. Note that the technical idea of the invention can be realized not only in a printing apparatus and method, but also in a calibration apparatus and method incorporated into the printing apparatus and method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the attached drawings which form a part of this original disclosure: 
         FIG. 1  is a block diagram illustrating the hardware configuration of a computer. 
         FIG. 2  is a block diagram illustrating the software configuration of a computer. 
         FIG. 3  is a block diagram illustrating the hardware configuration of a printer. 
         FIG. 4  is a flowchart illustrating a printing process. 
         FIG. 5  is a diagram illustrating an example of a settings table PT. 
         FIG. 6  is a flowchart illustrating a calibration process. 
         FIG. 7  is a diagram illustrating an example of fixed patch data FPD. 
         FIG. 8  is a diagram illustrating an example of color measurement data MD. 
         FIG. 9  is a diagram and a table illustrating the creation of a correction table AT. 
         FIG. 10  is two graphs illustrating numbers of color patches. 
         FIG. 11  is a diagram illustrating an exemplary histogram. 
         FIG. 12  is a diagram illustrating the determination of the formation location and size of a color patch. 
         FIG. 13  is a diagram illustrating a UI window for accepting calibration settings. 
         FIG. 14  is a flowchart illustrating a calibration process according to a modified embodiment. 
         FIG. 15  is a diagram illustrating a UI window according to another modified embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, an embodiment of the invention will be described according to the following order: 1. Configuration of Calibration Apparatus and Printing Apparatus; 2. Printing Process; 3. Setting Process; and 4. Modified Embodiments. 
     1. Configuration of Calibration Apparatus and Printing Apparatus 
       FIG. 1  is a block diagram illustrating the configuration of the computer that executes a profile creation method according to an embodiment of the invention. In  FIG. 1 , a computer  10  is configured of a CPU  11 , a RAM  12 , a ROM  13 , a hard disk drive (HDD)  14 , a general interface (GIF)  15 , a video interface (VIF)  16 , an input interface (IIF)  17 , and a bus  18 . The bus  18  implements data communication among the various constituent elements  11  through  17  of which the computer  10  is configured, and controls the communication using a chipset or the like (not shown). Program data PD for executing various types of programs including an operating system (OS) is stored in the HDD  14 , and the CPU  11  executes computations based on the program data PD while expanding that program data PD in the RAM  12 . 
     The GIF  15  provides an interface compliant with, for example, the USB standard, and connects an external printer  20  to the computer  10 . The printer  20  according to this embodiment is an ink jet printer that forms a printed image by ejecting cyan (C), magenta (M), yellow (Y), and black (K) ink droplets based on ink amounts specified by the computer  10 . The VIF  16  connects the computer  10  to an external display  40 , and provides an interface for displaying images in the display  40 . The IIF  17  connects the computer  10  to an external keyboard  50   a  and a mouse  50   b , and provides an interface by which the computer  10  obtains input signals from the keyboard  50   a  and the mouse  50   b.    
       FIG. 2  is a block diagram illustrating the configuration of the printer  20 . The printer  20  includes an ASIC  21 , a print head  22 , a color measurement head  23 , an ejection control circuit  24 , a print head driving control circuit  25 , a color measurement head driving control circuit  26 , a paper feed motor driving control circuit  27 , a GIF  28 , and a bus  29 . The print head  22  is supplied with ink from CMYK ink cartridges (not shown) and ejects ink droplets of the CMYK ink onto printing paper based on control performed by the ejection control circuit  24 . The print head  22  is driven in the main scanning direction through the driving of a carriage motor that is controlled by the print head driving control circuit  25 . A two-dimensional image can be formed upon the printing paper by driving the print head  22  in the main scanning direction and transporting the printing paper in the sub scanning direction by the paper feed motor driving control circuit  27  driving a paper feed motor. By the paper feed motor driving control circuit  27  further driving the paper feed motor, the printing paper onto which an image has been formed is transported to a transport position at which color measurement can be carried out by the color measurement head  23 . 
     The color measurement head  23  is driven in the main scanning direction with respect to the printing paper by the color measurement head driving control circuit  26  controlling the driving of a color measurement head driving motor. The color measurement head  23  is provided with an optical sensor (not shown), and obtains (measures) the color values expressed by the printed image formed upon the printing paper (L*a*b* values in the CIELAB color space). When the color measurement head  23  carries out color measurement, any desired location upon the printing paper can be measured by moving the printing paper in the sub scanning direction using the paper feed motor driving control circuit  27  and the paper feed motor while moving the color measurement head  23  in the main scanning direction. The ASIC  21  is connected to the various constituent elements  24  through  28  via the bus  29 , and executes control of the various constituent elements  22  through  27  based on print control data inputted from the computer  10  via the GIF  28 . The ASIC  21  also obtains print status information, color measurement data MD, and so on from the various constituent elements  24  through  27  and outputs this information, data, and so on to the computer  10  via the GIF  28 . 
       FIG. 3  is a block diagram illustrating the software of a program executed by the computer  10  and data stored in the HDD  14 . A calibration program P 1  and a printer driver P 2  are executed by the computer  10 . The calibration program P 1  includes a settings management unit P 1   a , a color patch determination unit P 1   b , a color measurement unit P 1   c , and a correction data creation unit P 1   d . The printer driver P 2 , meanwhile, is configured of a size conversion unit P 2   a , a color conversion unit P 2   b , a halftone unit P 2   c , and a print data generation unit P 2   d . Image data ID of the image to be printed, a color conversion table LUT, a correction table AT, a settings table PT, the color measurement data MD, standard data SD, and fixed patch data FPD are stored in the HDD  14 . 
     2. Printing Process 
       FIG. 4  is a flowchart illustrating a printing process. In step S 100 , the image data ID to be printed is obtained. The image data ID according to this embodiment is image data in which each pixel holds a tone value for each of red (R), green (G), and blue (B) color elements. In step S 110 , the size conversion unit P 2   a  converts the size of the image data ID based on the size of the printing paper and the printing resolution. In step S 120 , the color conversion unit P 2   b  performs color conversion on the image data ID with reference to the color conversion table LUT. The color conversion table LUT defines the correspondence relationships between RGB tone values and CMYK ink amount gradation values indicating ink amounts using multiple grid points, and an ink amount gradation value corresponding to the RGB tone value of each pixel is calculated through an interpolation process. Once the image data ID has been converted into ink amount image data having an ink amount gradation value for each of the pixels, the ink amount image data IID is stored in the RAM  12  in step S 130 . In step S 140 , the settings management unit P 1   a  reads out the settings table PT from the HDD  14 , and obtains a cycle setting value FP held in the settings table PT. 
       FIG. 5  is a diagram illustrating an example of the settings table PT. The cycle setting value FP is a value for setting the time interval at which to execute calibration, and assumes, for example, a value from 1 day to 365 days. In step S 140 , the settings management unit P 1   a  reads out the correction table AT from the HDD  14 , and obtains an update date/time therefrom. In step S 150 , the settings management unit P 1   a  determines whether or not the amount of time indicated by the cycle setting value FP has elapsed since the update date/time in the correction table AT. In the case where the amount of time indicated by the cycle setting value FP has not elapsed since the update date/time in the correction table AT, the calibration process is not executed. On the other hand, in the case where the amount of time indicated by the cycle setting value FP has elapsed since the update date/time in the correction table AT, the calibration process is executed in step S 160 . 
       FIG. 6  is a flowchart illustrating the calibration process. In step S 161 , the color patch determination unit P 1   b  obtains a color patch setting mode setting value MP held in the correction table AT, and identifies the color patch setting based on that mode setting value MP. In this embodiment, the color patch setting can be set to either a fixed mode or an automatic mode, and, for example, mode setting values MP of “0” and “1” correspond to the fixed mode and automatic mode, respectively. In the case where the mode setting value MP is “0”, or in other words, the case where the color patch setting is the fixed mode, the fixed patch data FPD stored in the HDD  14  is obtained, and that fixed patch data FPD is outputted to the halftone unit P 2   c  (step S 162 ). Fixed patch data FPD is prepared on a printing resolution-by-printing resolution basis, and fixed patch data FPD corresponding to the printing resolution used when printing the image data ID is outputted. The halftone unit P 2   c  executes a halftone process, such as dithering or error diffusion, on the fixed patch data FPD (step S 163 ), after which the print data generation unit P 2   d  executes a process such as rasterizing on the post-halftone process data, and as a result, print control data capable of being controlled by the ASIC  21  of the printer  20  is created (step S 164 ). The print control data is outputted to the printer  20 , and the printer  20  prints a color patch or color patches (step S 165 ). To be more specific, the color patch is printed by the print head  22  forming ink dots in accordance with the CMYK ink amount specified by the fixed patch data FPD while scanning in the main and sub scanning directions. Accordingly, the various hardware elements that execute step S 165  constitute a patch printing unit in this embodiment. 
       FIG. 7  is a diagram illustrating an example of the fixed patch data FPD. The fixed patch data FPD is image data in which each pixel has a CMYK ink amount, and expresses a color patch group forming a primary color gradation for each of the CMYK inks. In this embodiment, CMYK ink can be ejected in an ink amount range from 0 to 255 gradations, and 18 color patches are formed for each ink by ink amount gradation values (0, 15, 31, 47 and so on up to 255) in which the ink amounts increase by 16 steps between 0 and 255 gradations. Accordingly, a total of 72 colors patches, or 18 gradations×4 colors, are printed. 
     When the color patches have been printed, color measurement is executed on the color patches (step S 166 ). To be more specific, the color measurement unit P 1   c  outputs data specifying the location of each color patch to the ASIC  21  of the printer  20 , and color measurement is carried out on each color patch by the color measurement head  23  moving in the main scanning direction and the printing paper moving in the sub scanning direction in a sequential manner. In the fixed mode, the color measurement is carried out in five locations (the upper-left corner, the upper-right corner, the center, the lower-left corner, and the lower-right corner) in each color patch, and the average of the values in those five locations is taken as a color measurement value of the color patch. The correction data creation unit P 1   d  stores the color measurement values obtained through the color measurement of the color patches in the color measurement data MD (step S 167 ). 
       FIG. 8  is a diagram illustrating an example of the color measurement data MD. In the color measurement data MD, color measurement values are held for each of the 18×4 color patches, and a measurement date/time indicating when each color measurement value was obtained as well as the formation location of each color patch are held in association therewith. In the case of the fixed mode, all of the 18×4 color patches are printed/measured together, and thus identical measurement dates/times are held for all of the color patches. Note that the color measurement data MD corresponds to “history data” according to the invention. The correction data creation unit P 1   d  obtains the standard data SD from the HDD  14 , calculates the deviation between the standard color values that should be expressed by each color patch as defined in the standard data SD and the color measurement values in the color measurement data MD, and creates the correction table AT based on that deviation (step S 168 ). 
       FIG. 9  is a schematic diagram illustrating the creation of the correction table AT. In  FIG. 9 , the standard value of the color patch (◯) and the color measurement value () are plotted with respect to the brightness of C ink (an L* value). For example, the standard value of C ink for a color patch whose ink amount gradation value is X 1  is α, and in the case where the color measurement value is β≠α, it is necessary to correct the ink amount gradation value of the C ink in the vicinity of the ink amount gradation value X 1 . In the case of an ink amount gradation value X 2  when a curve to which the color measurement values have been fitted indicates the standard value α of that color patch, when an ink amount gradation value X 1  for the C ink has been inputted, the standard value α can be reproduced by actually performing the printing using the ink amount gradation value X 2 . The ink amount gradation value X 1  is taken as an input value (a pre-correction gradation value) and the ink amount gradation value X 2  is taken as an output value (a post-correction gradation value), and the correspondence relationship between the input and output values is held in the correction table AT. When the aforementioned correspondence relationship has been stored for the CMYK inks, the current date/time is used as the update date/time for the correction table AT. At the same time, the existing correction table AT is deleted. The calibration process ends when the correction table AT has been created. 
     When the calibration process has ended, the ink amount image data IID is read out from the RAM  12  and the ink amount gradation values of the CMYK inks in each pixel of the ink amount image data IID are corrected based on the correction table AT (step S 180 ). Meanwhile, in the case where it has been determined in step S 150  that the amount of time indicated by the cycle setting value FP has not elapsed since the update date/time of the correction table AT and the calibration process is not to be carried out, steps S 180  and on are executed directly. The post-correction ink amount image data IID is outputted to the halftone unit P 2   c  (step S 190 ). The halftone unit P 2   c  executes a halftone process on the ink amount image data IID (step S 200 ), after which the print data generation unit P 2   d  executes a process such as rasterizing on the post-halftone process data, and as a result, print control data capable of being controlled by the ASIC  21  of the printer  20  is created (step S 210 ). The print control data is then outputted to the printer  20 , and as a result, the printer  20  prints a printed image corresponding to the image data ID (step S 220 ). In this manner, a printed image in which the standard values are reproduced across the entire range of darknesses in the CMYK inks can be formed. Note that because the calibration process is skipped before the amount of time indicated by the cycle setting value FP set by the user has elapsed, the calibration process can be prevented from being executed at an excessive frequency. However, 72 color patches are printed/measured in the fixed mode, and thus there are cases where the user will feel that this process takes too much time and wastes ink. Accordingly, this embodiment also provides an automatic mode. The automatic mode will be described hereinafter. 
     In the case where it has been determined in step S 161  of  FIG. 6  that the automatic mode is set, the color patch determination unit P 1   b  determines the number of color patches to be printed/measured in step S 169 . The number of color patches is determined based on the cycle setting value FP and an accuracy setting value AP set in the settings table PT. 
       FIG. 10  illustrates two graphs showing the relationship between the number of color patches, the cycle setting value FP, and the accuracy setting value AP. The number of color patches is obtained by multiplying the 72 color patches of the fixed mode by correction coefficients E 1  and E 2 . The correction coefficient E 1  is a linearly-increasing function of the cycle setting value FP, and is 1 when the cycle setting value FP is at a maximum value of 365 days. Accordingly, in this case, there is a trend for the number of color patches to increase in the case where the cycle setting value FP is high and the frequency with which the calibration process is executed is low. The correction coefficient E 2  is a linearly-increasing function of the accuracy setting value AP, and is 1 when the accuracy setting value AP is a maximum of 100%. Note that the accuracy setting value AP takes on a value from 0 to 100%, and a high value indicates that the user prioritizes accuracy, whereas a low value indicates that the user prioritizes the speed of printing. Accordingly, in this case, there is a trend for the number of color patches to increase as the user prioritizes accuracy and sacrifices speed. In the following step S 170 , the color patch determination unit P 1   b  reads out the ink amount image data HD from the RAM  12  and analyzes the ink amount gradation values of the CMYK inks in each pixel of the ink amount image data IID. Because the ink amount image data IID has been created in advance during the printing process, it is not necessary to create that data anew for the purpose of the analysis. Furthermore, it is not necessary to analyze the ink amount image data IID itself, and the analysis may be carried out based on reduced image data. 
       FIG. 11  is a diagram illustrating an example of a histogram created during the stated analysis. In this histogram, levels (with the width of each level being 16) whose central value is the ink amount gradation value (0, 15, 31, 47, and so on up to 255) of the color patch printed in the fixed mode are provided. When the histogram has been created, the standard deviation of the relative frequency of each of the levels in all of the inks is calculated (step S 171 ), and it is then determined whether or not the standard deviation is greater than a predetermined threshold (for example, 5%) (step S 172 ). In the case where the standard deviation is greater than the threshold, the image data ID to be printed can be considered to be characteristic and meet a prescribed condition (that is, there is a deviation in the color/darkness). In this case, in step S 173 , the level with the highest relative frequency is obtained for the number of color patches determined in step S 169 , and a color patch having an ink amount gradation value that indicates the central value of that level is determined to be formed. In this manner, color patches can be formed for inks and ink darknesses that are used more often in the ink amount image data IID. For example, in the case of a photograph of a landscape, a color patch corresponding to an ink amount by which the color of the sky can be reproduced is formed, thus particularly improving the accuracy with which the color of the sky can be reproduced. Note that the ink corresponding to the ink amount gradation value of the color patch determined in step S 173  corresponds to a frequently-used ink according to the invention, whereas the ink amount indicated by the ink amount gradation value corresponds to a frequently-appearing ink amount according to the invention. 
     Meanwhile, in the case where the image data ID to be printed is not characteristic (e.g., there is no considerable deviation in the color/darkness), it is determined that the color patches having the oldest measurement dates/times at which the color measurement values were stored in the color measurement data MD are to be formed (step S 174 ). In this case, too, it is determined that the color patches having the oldest measurement dates/times (higher level) color patches of a number equivalent to the number of color patches determined in step S 169  are to be formed. Next, the color patch determination unit P 1   b  determines the formation location and size of the color patch determined to be formed (step S 175 ). Because the color reproduction characteristics of the printer  20  change over time, it can be thought that the reliability of the color measurement values in the color patch having older measurement dates/times is low. Accordingly, it is possible to preferentially update color measurement values if the reliability thereof is low by printing/measuring color patches having older measurement dates/times. Naturally, a color patch that is different from the color patch printed in a last printing cycle (immediately previous thereto) is determined to be printed. 
       FIG. 12  is a diagram illustrating the determination of the formation location and size of the color patch. The color patch determination unit P 1   b  analyzes, for each color patch, a spatial distribution of pixels (pixels of interest) in the ink amount image data IID belonging to the level taken as the central value as the ink amount gradation value of the color patch determined to be formed. As shown in  FIG. 12 , the center of distribution coordinates of the location in which the pixels of interest are present and the standard deviation of the distances between the center of distribution coordinates and each pixel of interest are calculated. The calculated center of distribution coordinates are then taken as the center of distribution coordinates of the rectangular-shaped color patch and a length in proportion with the standard deviation is taken as the length of one side of the color patch. Note that in the case where the image data ID to be printed is not characteristic (e.g., there is no considerable deviation in the color/darkness), a color patch whose measurement date\time at which the color measurement values were stored in the color measurement data MD may be formed in a location that is different than the location stored in association with that color patch. Doing so makes it possible to spread the locations in which the same color patch is formed throughout multiple timings. When the formation location and size of all of the color patches determined to be formed have been determined, the color patch determination unit P 1   b  adjusts the locations of the color patches in step S 176 . 
     Here, in the case where the color patches overlap, the locations thereof are adjusted so that the color patches do not overlap. Doing so makes it possible to form the color patches in locations in which pixels having ink amount gradation values that resemble the ink amount gradation value of the color patches in the ink amount image data IID are present. The size of each color patch corresponds to the spread of the distribution of the pixels having ink amount gradation values that resemble the ink amount gradation values in each color patch. As described thus far, when the ink amount gradation values, location, and size of the color patch to be formed has been determined, the determined color patch is disposed relative to image data of the same number of pixels as the ink amount image data IID, thus generating automatic patch data APD (step S 177 ). The automatic patch data APD is then outputted to the halftone unit P 2   c  (step S 178 ). The processing performed thereafter prints the color patch, in the same manner as the fixed mode (up to step S 165 ). Note that the color patch is printed onto the same printing paper at the same printing resolution as used in the printing of the image data ID. 
     Although the color measurement of the color patch in the automatic mode is also carried out in the same manner as in the fixed mode, the color patch determination unit P 1   b  specifies the color measurement location of each color patch based on the location and size of the color patch determined/adjusted in steps S 173  to S 176 . As the size of the color patch increases, the color measurement unit P 1   c  sets the range of the five color measurement locations (the upper-left corner, the upper-right corner, the center, the lower-left corner, and the lower-right corner) to a wider range. In the case of the automatic mode, all 72 (18 gradations×4 colors) color patches are typically not printed/measured, and thus in step S 167 , the measurement dates/times and color measurement values in the color measurement data MD shown in  FIG. 8  are updated for only some of the color patches. Particularly in the case where the ink amount image data IID is not characteristic (e.g., there is no considerable deviation in the color/darkness), of the color measurement values in the color measurement data MD, the color measurement values of the color patches having old measurement dates/times are updated. The correction table AT is created in the automatic mode as well, in the same manner as in the fixed mode (step S 168 ). 
     However, because all 72 color patches are typically not printed/measured in the automatic mode, the correction table AT is created based on color measurement values having different measurement dates/times. As mentioned above, because the color reproduction characteristics of the printer  20  change over time, it is desirable to create the correction table AT based on color measurement values having new update dates/times. With respect to this point, in this embodiment, in the case where the ink amount image data IID is characteristic (e.g., there is a deviation in the color/darkness), color patches having ink amount gradation values (frequently-used inks/frequently-appearing ink amounts) corresponding to more pixels in the ink amount image data IID having similar ink amount gradation values are printed/measured. Accordingly, colors that are characteristic in the image data ID can be reproduced with high accuracy. On the other hand, in the case where the ink amount image data IID is not characteristic, color patches that have, of the color measurement values in the color measurement data MD, color measurement values that have old measurement dates/times are printed/measured. Accordingly, a calibration accuracy of a specified threshold can be prevented from dropping drastically, and an overall favorable color reproduction accuracy can be realized. 
     3. Setting Process 
       FIG. 13  is a diagram illustrating a UI window for accepting calibration settings. This UI window is displayed in the display  40  by the settings management unit P 1   a . Input signals from the keyboard  50   a  and the mouse  50   b  are accepted during the period in which the UI window is displayed. In this UI window, radio buttons R 1  for selecting either the fixed mode or the automatic mode are provided. The mode setting value MP is set by manipulating these radio buttons R 1 . Slider bars S 1  and S 2  are provided in the UI window for specifying the cycle setting value FP and the accuracy setting value AP. The cycle setting value FP can be set from 1 day (short) to 365 days (long), whereas the accuracy setting value AP can be set from 0% (fast) to 100% (accurate). An OK button B 1  is provided in the UI window, and the state that has been set is obtained by the settings management unit P 1   a  when the OK button B 1  has been pressed; the setting values MP, FP, and AP in the settings table PT are then updated in accordance with the state that has been set. In this manner, the user can specify the calibration settings, and a color patch is determined in accordance with those settings; accordingly, forming a number of color patches that the user feels is excessive, expending a large amount of processing time, and so on can be prevented. 
     4. Modified Embodiments 
       FIG. 14  is a flowchart illustrating a calibration process according to a modified embodiment. In this modified embodiment, the process moves to the fixed mode in the case where the ink amount image data IID has been determined as not being characteristic, even if the automatic mode is being executed. Doing so makes it possible to improve the overall reproduction accuracy in the case where the ink amount image data IID is not characteristic and it is not known what ink and what ink darkness should be focused on in the execution of the calibration process. 
       FIG. 15  is a diagram illustrating a UI window for accepting calibration settings according to another modified embodiment. In  FIG. 15 , radio buttons R 2 , through which the color patch size settings can be selected from among the automatic mode, a large fixed mode, and a small fixed mode, have been added. In the case where the automatic mode has been set, the size of the color patch is determined using the same method as that illustrated in the aforementioned embodiment in step S 175 . On the other hand, in the case where the large fixed mode or the small fixed mode has been set, a color patch is determined to be formed at a fixed size regardless of the standard deviation of the locations in which the pixels of interest are present. Of course, the size of the color patch formed in the case where the large fixed mode has been set is greater than the size of the color patch formed in the case where the small fixed mode has been set. 
     GENERAL INTERPRETATION OF TERMS 
     In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. Finally, terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies. 
     While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.