Abstract:
A method of reconstructing the content of an ink color decomposition table is available as a method calibrating second- and higher-order colors. However, it is difficult to reconstruct the ink color decomposition table while maintaining a limitation upon the overall amount of ink allowed by the printing paper. Accordingly, a printer is made to form a color patch that corresponds to the signal value of a color to be calibrated, the signal value is calibrated based upon the measured color value of the color patch formed and a target value that corresponds to the signal value of the color to be calibrated, and multidimensional table data for calibration is created based upon the measured color value of the color patch formed.

Description:
FIELD OF THE INVENTION 
   This invention relates to a data processing apparatus and method and to an image processing apparatus. More particularly, the invention relates to calibration for the purpose of limiting fluctuation of color reproduction characteristics. 
   BACKGROUND OF THE INVENTION 
     FIG. 1  is a diagram useful in describing a technique for calibrating a color printer. 
   As shown in  FIG. 1 , an ink color decomposing processor  901  decomposes multivalued RGB image data, which is input thereto, into data of the color components C (cyan), M (magenta), Y (yellow) and K (black) corresponding to the colors of the colorants (referred to as “ink colors” below) of the color printer by interpolation processing such as tetrahedral interpolation based upon table information in an ink color decomposition table  907 . 
   A one-dimensional LUT  902  for calibration purposes corrects the grayscale characteristic of the image data in accordance with the color reproduction characteristic of the printer. Specifically, the LUT  902  corrects the multivalued CMYK data that is output from the ink color decomposing processor  901  to multivalued C′M′Y′K′ data. A calibration conforming to the characteristic of the color printer is achieved by this processing. 
   A halftoning processor  903  converts the multivalued C′M′Y′K′ data to a number of tones capable of being printed by the color printer. If the color printer is, e.g., a binary printer, then the C′M′Y′K′ data is binarized by the halftoning processing, whereby the processor  903  outputs binary C″M″Y″K″ data. 
   A color printer engine  904  performs printing based upon the C″M″Y″K″ data input thereto. 
   A sensor  906  investigates the color reproduction characteristic of the color printer engine  904 . On the basis of the color reproduction characteristic of each of the colors CMYK output from the sensor  906 , a one-dimensional LUT creating unit  905  creates a one-dimensional LUT of each of the colors CMYK in such a manner that a target color reproduction characteristic will be obtained. The result of this operation is written to the one-dimensional LUT  902 . 
   The technique using the one-dimensional LUT  902  shown in  FIG. 1  calibrates each of the colors CMYK independently and therefore implements a highly accurate calibration with regard to first-order colors (ink colors). However, a highly accurate calibration cannot be achieved with regard to colors other than first-order colors, such as the second-order colors R (red), G (green) and B (blue), and the third- and fourth-order colors, which form a gray line. 
   A method of calibrating second- and higher-order colors by reconstructing the content of the ink color decomposition table  907  is available as a method calibrating second-, third- and fourth-order colors, etc. However, it is difficult to reconstruct the ink color decomposition table  907  while maintaining a limitation upon the overall amount of ink allowed by the printing paper. 
   Furthermore, since the latest ink-jet printers employ a six-color ink system that uses light cyan and light magenta in addition to cyan, magenta, yellow and black (the conventional first-order colors) as the ink colors, it is even more difficult to maintain the above-mentioned limitation. The same holds true for other printers such as electrophotographic printers as well. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to solve the above-mentioned problems individually or collectively and create a multidimensional table for calibration taking second- and higher-order colors into consideration. 
   Another object of the present invention is to stabilize the tint of second- and higher-order colors easily and in a short period of time. 
   According to the present invention, the foregoing object is attained by providing a data processing apparatus for generating multidimensional table data, which is for subjecting image data to a color transformation, comprising a former arranged to cause a printer to form a color patch that corresponds to a signal value of a color to be calibrated; a sensor arranged to measure the color of the color patch formed; a corrector arranged to correct the signal value based upon result of color measurement by the sensor and a target value that corresponds to the signal value of the color to be calibrated; and a generator arranged to generate the multidimensional table data for calibration based upon the measured value of the color patch formed. 
   Further, according to the present invention, the foregoing object is attained by providing an image processing apparatus, wherein image data is subjected to a color transformation using multidimensional table data generated by the above data processing apparatus. 
   Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram useful in describing a color-printer calibration technique; 
       FIG. 2  is a block diagram illustrating the structure of an image processing apparatus according to an embodiment of the present invention; 
       FIG. 3  is a diagram useful in describing a method of creating a 3D calibration table; 
       FIG. 4  is a flowchart for describing calibration; 
       FIG. 5  is a graph illustrating the initial characteristics of the gray line of a 3D calibration table; 
       FIG. 6  is a graph illustrating a revised gray line of a 3D calibration table; 
       FIG. 7  is a diagram useful in describing an algorithm for correcting a gray line; 
       FIG. 8  is a diagram useful in describing a reference table; and 
       FIG. 9  is a block diagram illustrating another structure of an image processing apparatus according to a modification. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An image processing apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings. 
   [Structure] 
     FIG. 2  is a block diagram illustrating the structure of an image processing apparatus according to an embodiment of the invention. 
   As shown in  FIG. 2 , a color matching unit  101  applies color matching processing to RGB data to effect a color transformation to R′G′B′ data by three-dimensional interpolation processing such as tetrahedral interpolation or cubic interpolation, which is based upon the content of a color matching table  107 , in order to match the color of input image data and the color reproduction characteristic of a printer. 
   A 3D calibration unit  102  applies calibration processing to the R′G′B′ data to effect a color transformation to R″G″B″ data by three-dimensional interpolation processing such as tetrahedral interpolation or cubic interpolation based upon the content of a 3D calibration table  108 . 
   A color decomposing processor  103  executes color decomposition processing, which transforms the R″G″B″ data to CMYK data representing the colorants (referred to as “ink colors” below) of the printer, by three-dimensional interpolation processing such as tetrahedral interpolation or cubic interpolation based upon the content of a color decomposition table  109 . 
   An output gamma correction unit  104  corrects the gamma characteristic based upon a combination of the content of processing by a halftoning processor  105  and the characteristic of a color printer engine  106 . 
   The halftoning processor  105  executes halftoning processing in order to transform the C′M′Y′K′ multivalued data, which is output from the output gamma correction unit  104 , to a number of tones capable of being expressed by the color printer engine  106 . 
   The color printer engine prints an image on printing paper based upon C″M″Y″K″ data that is output from the halftoning processor  105 . 
   On the basis of a program and data stored in a ROM  111 , a CPU  110  controls the overall image processing apparatus using a RAM  112  as a working area, thereby controlling execution of processing for updating the 3D calibration table  108  and execution of image processing using the 3D calibration table  108  that has been updated. 
   The corresponding relationship between the R″G″B″ data, which forms a patch described later, and a calorimetric value of the formed patch is stored in a reference table  113 . 
   A color sensor  114 , which is mounted within the color printer engine  106 , measures the color of the color patch printed by the color printer engine  106 . A calibration table creating unit  115  retains the result of colorimetry performed by the color sensor  114 . The result of colorimetry may be stored in the RAM  112  if the RAM  112  has enough storage capacity. 
   This embodiment is such that by updating the content of the 3D calibration table  108 , the tint of a printout can be stabilized in simple fashion for second or higher-order colors without requiring limitation of the overall amount of ink in a case where the content of the color decomposition table  109  is changed, and without requiring complicated control of a six-color ink system or the like. 
   [Method of creating 3D calibration table] 
     FIG. 3 , which is a diagram useful in describing a method of creating the 3D calibration table  108 , illustrates the configuration of a system that includes a color printer. 
   As shown in  FIG. 3 , a computer  201  controls a color printer  203  for printing an image. A monitor  202  displays data and the like held in the computer  201 . It should be noted that the image processing apparatus of this embodiment shown in  FIG. 2  is mounted within a color printer  203 . 
     FIG. 4  is a flowchart useful in describing calibration executed in the system of  FIG. 3  under the control of the computer  201 . 
   First, on the basis of the reference table  113 , the CPU  110  generates R″G″B″ data that corresponds to a patch pattern of a gray line (S 302 ). Next, the R″G″B″ data that has been generated is processed by the color decomposing processor  103 , output gamma correction unit  104  and halftoning processor  105 , and a patch pattern  205  of the gray line is printed by the color printer  203  (S 303 ). Each patch of the patch pattern  205  printed undergoes colorimetry by the color sensor  114 , and the result of colorimetry is retained in the calibration table creating unit  115  (S 304 ). 
   The CPU  101  calculates a color difference ΔE between the colorimetry value of each patch retained in the calibration table creating unit  115  and a target value (of the calorimetric value) of the gray line stored in the reference table  113 , and determines whether the color difference ΔE falls within a tolerance ε (S 305 ). If ΔE&gt;ε holds, the CPU  101  revises the gray line (S 306 ). The details are set forth below. 
   The processing of steps S 302  to S 304  is repeated using the R″G″B″ data, which was revised at step S 306 , until the relation ΔE≦ε is established. When ΔE≦ε is established, a 3D calibration table is generated by three-dimensional interpolation using the R″G″B″ data of the gray line prevailing at this time (S 307 ). 
     FIG. 5  is a graph illustrating the initial characteristics of the gray line of the 3D calibration table  108 . Specifically, according to the initial characteristics shown in  FIG. 5 , the characteristic curves of R, G and B all overlap, and the output values of R″, G″ and B″ corresponding to input values α1, α2 and α3 of R″, G″ and B″ are α1, α2 and α3, respectively. 
     FIG. 6 , on the other hand, illustrates the characteristics of the revised gray line of the 3D calibration table  108 . In  FIG. 6 , the result is such that the R signal is emphasized and the G and B signals de-emphasized in comparison with the initial characteristics ( FIG. 5 ). The end result is that the color reproduction characteristic of the gray line of the color printer  203  is stabilized. 
     FIG. 7  is a diagram useful in describing an algorithm for correcting a gray line. 
   In  FIG. 7 , points P0 to P7 indicate the manner in which gray-line target values corresponding to the R″G″B″ data and stored in the reference table  113  and the results of previously printing and measuring the colors in the vicinity thereof are plotted in L*a*b* color space of CIE. 
   Further, a point P8 is obtained by plotting grayline calorimetric values (L8,a8,b8) acquired as the result of printing and measuring the color of a patch based upon signal values (R″,G″,B″) (=a2,a2,a2) of the gray line. 
   Indicated at each point are L*a*b* values representing the color at this point and R″G″B″ values that are the basis of this point. For example, the values corresponding to the point P0 are L*a*b*=(L0,a0,b0) and R″,G″,B″=(R″0,G″0,B″9). It should be noted that R″,G″,B″=(R″8,G″8,B″8) at point P8 is obtained by interpolation from the R″G″B″ values of points P0 to P7 in the vicinity of point P8. 
     FIG. 8  is a diagram useful in describing the reference table  113 . The reference table  113  is the original three-dimensional table. In order to simplify the description, however, the gray line connecting the white and black points is represented in  FIG. 8  by a cross section (two dimensions) passing through red and cyan. 
   It should be apparent from  FIG. 8  that according to this method of constructing the table, the grid spacing is made finer in the vicinity of the gray line while the grid spacing in the area remote from the gray line is made coarser. Thus the table is so designed as to raise the color-reproduction precision in the vicinity of the gray line in a table of the same size. 
   Accordingly, as a result of colorimetry of gray-line signal values (R″,G″,B″)=(a2,a2,a2), the color difference ΔE is expressed by the following equation, where P0 represents the target value of point P8:
 
Δ E=√[L 0− L 8) 2 +( a 0− a 8) 2 +( b 0− b 8) 2 ]
 
   If it is determined at step S 305  that the color difference ΔE exceeds the tolerance ε, (R″G″B″)=(β2r,β2b,β2g) shown in  FIG. 6  is calculated as ind by the following equations from the differences between the target R″G″B″ values (R″0,G″0,B″0) and R″G″B″ values (R″8,G″8,B″8), the latter of which are based upon the colorimetry values (L8,a8,b8):
 
β2 r=α 2+( R″ 0− R″ 8)
 
β2 g=α 2+( G″ 0− G″ 8)
 
β2 b=α 2+( B″ 0− B″ 8)
 
   Of course, the signal values of the gray line can be revised, in a manner similar to that described above, with regard to the input values a1 and a3 shown in  FIG. 6 . 
   The color reproduction characteristic of the gray line can be stabilized by the above calibration processing even in a case where the printing characteristic of the color printer  203  fluctuates. Furthermore, with regard to peripheral colors other than the gray line, an excellent color reproduction characteristic can be achieved if the 3D calibration table  108  is updated by performing a revision using three-dimensional interpolation that is based upon the calibrated gray line. 
   Thus, by updating the content of the 3D calibration table  108  of the 3D calibration unit (3D LUT)  102  disposed between the color matching unit  101  and the color decomposing processor  103 , the tint of a printout can be stabilized in simple fashion for second- or higher-order colors without requiring limitation of the overall amount of ink in a case where the content of the color decomposition table  109  is changed, and without requiring complicated control of a six-color ink system or the like. 
   Further, in this embodiment, the color sensor  114  for measuring tint is provided within the color printer  203 . As a result, it is unnecessary to separately provide a calorimeter or the like for measuring the tint of the color patches  205 , and the tint of second-order and higher colors can be stabilized automatically and in real time without labor being spent on colorimetry. It goes without saying that the color printer  203  may be a printer that uses any type of printing method, such as an ink-jet printer or electrophotographic printer. 
   [Modification] 
   The above embodiment has been described with regard to an example in which the 3D calibration unit  102  is provided between the color matching unit  101  and color decomposing processor  103  and a three-dimensional calibration is carried out based upon the content of the 3D calibration table  108 . However, a three-dimensional calibration using a 3D calibration table is not limited to that described above. 
     FIG. 9  is a block diagram illustrating the structure of an image processing apparatus in which the color matching unit  101  and color decomposing processor  103  are connected directly connected to each other. 
   In the arrangement of  FIG. 9 , the color matching table  107  and 3D calibration table  108  are combined and the color matching table  107  is updated by the table that results from the combination. Alternatively, the 3D calibration table  108  and color decomposition table  109  are combined and the color decomposition table  109  is updated by the table that results from the combination. As a result, processing equivalent to that of the above embodiment can be achieved. 
   Of course, the three tables consisting of the color matching table  107 , 3D calibration table  108  and color decomposition table  109  can be combined and processing equivalent to that of the above embodiment can be achieved by three-dimensional interpolation processing using the three-dimensional table that results from the combination. 
   The above embodiment has been described with regard to an example in which, after the gray line is revised, the 3D calibration table  108  is created based upon the revised gray line. However, the method of creating the 3D calibration table  108  is not limited to the above. Besides the gray line, a white-red-black line, for example, or a color such as skin color whose calibration is desired, can be revised, and the revised line or color may be incorporated to create the 3D calibration table  108 . 
   Further, it can make that the image processing apparatus has various calibration modes such as (i) a mode which calibrates the gray line, and (ii) a mode which calibrates both the gray line and a flesh color so that a user, who wants to shorten the calibration time, can choose the mode. 
   Note that in a case where the computer  201  and the color printer  203  are connected through the network, it can think that there is the color printer  203  in the remote place to the computer  201 . Therefore, it is convenient that a status of the color printer  203  is returned from the color printer  203  to the computer  201  after the calibration is instructed by the computer  201 . 
   For example, the computer  201  displays a status of the color printer  203  to the monitor  202 , when a response has been received from the color printer  203 . Note that the response indicates the status such as (i) forming patches, (ii) performing calorimetric by the sensor of the color printer  203 , (iii) generating a color table by using the calorimetric values obtained in the step (ii), and (iv) the end of calibration. In this case the color printer  203  can be used by other user, and therefore, it is desirable with the color printer  203  to send information, which indicates the above status of the color printer  203 , to other computers so as to inform the above status to a user of the other computer who tries to use the color printer  203 . According to the above structure, it is possible to inform a processing state of the color sensor in the color printer  203  to the computers as the status. 
   [Other Embodiment] 
   The present invention can be applied to a system constituted by a plurality of devices (e.g., host computer, interface, reader, printer) or to an apparatus comprising a single device (e.g., copy machine, facsimile). 
   Further, the object of the present invention can be also achieved by providing a storage medium storing program codes for performing the aforesaid processes to a system or an apparatus, reading the program codes with a computer (e.g., CPU, MPU) of the system or apparatus from the storage medium, then executing the program. 
   In this case, the program codes read from the storage medium realize the functions according to the embodiment, and the storage medium storing the program codes constitutes the invention. 
   Further, the storage medium, such as a floppy disk, a hard disk, an optical disk, a magneto-optical disk, CD-ROM, CD-R, a magnetic tape, a non-volatile type memory card, and ROM can be used for providing the program codes. 
   Furthermore, besides aforesaid functions according to the above embodiment are realized by executing the program codes which are read by a computer, the present invention includes a case where an OS (operating system) or the like working on the computer performs a part or entire processes in accordance with designations of the program codes and realizes functions according to the above embodiment. 
   Furthermore, the present invention also includes a case where, after the program codes read from the storage medium are written in a function expansion card which is inserted into the computer or in a memory provided in a function expansion unit which is connected to the computer, CPU or the like contained in the function expansion card or unit performs a part or entire process in accordance with designations of the program codes and realizes functions of the above embodiment. 
   The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention. Therefore, to apprise the public of the scope of the present invention, the following claims are made.