Abstract:
The color of magenta using newly developed magenta ink has characteristics L*≅41, a*≅82, and b*≅24, and has differences ΔL*≅4, Δa*≅0, and Δb*≅26 from the conventional magenta ink, i.e., the lightness value is low, and the color difference b* assumes a very small value. When the color of red is reproduced using such new magenta ink, its lightness and saturation values are low, and subdued red is reproduced, i.e., a visually favorable color cannot be obtained. Hence, when a color included in a first color gamut is input, and the input color is converted into the color of a second color gamut narrower than the first color gamut, color conversion is made using a three-dimensional lookup table having red defined by. L*≅45 to 50, a*≅67 to 70, and b*≅50 to 55.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a color processing apparatus and its method and, more particularly, to color processing for converting colors included within a broad color gamut into those in a narrow color gamut.  
       BACKGROUND OF THE INVENTION  
       [0002]     There is a gamut mapping technique that represents a color, which exists in a color space (e.g., an RGB color space of a digital camera) with a broad color gamut but does not exist in a color space (e.g., a CMYK color space of a printer) with a narrow color gamut, in the color space with the narrow color gamut. The general gamut mapping attains the above color representation by selecting a region where it seems like an identical color (e.g., by selecting an equal hue line, selecting a region where it seems like an identical color, and so forth), as disclosed in, e.g., Japanese Patent Laid-Open No. 2004-104604.  
         [0003]     The color of red is reproduced by a combination of color recording materials of magenta and yellow. The color of magenta using the color recording material of magenta has characteristics L*≅45, a*≅82, and b*≅2 on an L*a*b* color space. The color of yellow using the color recording material of yellow has characteristics L*≅88, a*≅4, and b*≅105 on the L*a*b* color space. The color of red is reproduced using these two color recording materials.  
         [0004]     However, new ink as a color recording material of magenta is developed (to be referred to as “new magenta ink” hereinafter), and the color of magenta using the new magenta ink has characteristics L*≅41, a*≅82, and b*≅24 on the L*a*b* color space. Therefore, the differences from the conventional magenta ink (to be referred to as “old magenta ink” hereinafter) are ΔL*≅4, Δa*≅0, and Δb*≅26, i.e., the lightness value is low, and b* assumes a very small value.  
         [0005]     Note that new ink as a color recording material of yellow is also developed (to be referred to as “new yellow ink” hereinafter), and its characteristics (color) are nearly the same as those of the conventional yellow ink (to be referred to as “old yellow ink” hereinafter).  
         [0006]     When the color of red is reproduced using the new magenta ink and new yellow ink with these characteristics, its lightness and saturation values are low, and subdued red is reproduced, i.e., a visually favorable color cannot be obtained. Hence, how to reproduce the visually favorable color of red is a question. As described in Japanese Patent Laid-Open No. 2004-104604 above, if an equal hue line is used, a close visual hue value can be selected on the color space with the narrow color gamut, but a saturation value often becomes low. In this case, a color with a subdued, light impression is obtained. In consideration of reproduction of a photo or the like, a color with an impression different from an original one is reproduced.  
         [0007]     Furthermore, no problem is experienced when an allowable range of an identical color exists. However, when the color gamut is different beyond the allowable range, no measure can be taken.  
       SUMMARY OF THE INVENTION  
       [0008]     The first aspect of the present invention discloses a technique that defines a color of red, which is to be reproduced in a second color gamut, by L*≅45 to 50, a*≅67 to 70, and b*≅50 to 55, when a color included on a first color gamut is converted into a color on a second color gamut narrower than the first color gamut.  
         [0009]     Further, the second aspect of the present invention discloses a technique that converts image data on a predetermined color space into image data corresponding to a color which has a high saturation value although a hue value thereof is changed, of those which are reproducible within a color space of a color recording material, upon converting image data on the predetermined color space into that on the color space of the color recording material, when memory color is represented.  
         [0010]     According to the present invention, a visually favorable color can be reproduced.  
         [0011]     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  
       [0012]      FIG. 1  is a view showing the structure of a printing apparatus;  
         [0013]      FIG. 2  is a block diagram showing the control arrangement of the printing apparatus;  
         [0014]      FIG. 3  is a flowchart showing the image signal processing (color processing) for printing;  
         [0015]      FIG. 4  shows the color gamut of a uniform color space by an a*b* plane; and  
         [0016]      FIG. 5  is a graph showing changes in color from white to reds A and B, and then to black while the ordinate plots lightness L* and the abscissa plots saturation.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     Preferred embodiments of the present invention will be described in detail hereinafter with reference to the accompanying drawings.  
         [0000]     [Structure of Printing Apparatus] 
         [0018]      FIG. 1  shows the structure of a printing apparatus (ink-jet color printer).  
         [0019]     The printing apparatus shown in  FIG. 1  is a device that prints an image by forming dots on a printing medium by ejecting ink droplets from nozzles of a print head  1  having a plurality of nozzle arrays. On the print head  1 , a plurality of different print head arrays  1 Y (for yellow ink),  1 M (for magenta ink), IC (for cyan ink),  1 K (for black ink),  1 LM (for light magenta ink), and  1 LC (for light cyan ink) are arranged. Each print head array ejects color ink different from each other. A color image is formed on a printing medium by mixing of different colors of these ink droplets. Print data is supplied from a controller (to be described later) of the printing apparatus to the print head  1  via a cable  9 .  
         [0020]     In this manner, the printing apparatus of this embodiment adopts an arrangement having inks of six colors. Furthermore, the printing apparatus may adopt an arrangement having inks of seven colors including yellow ink and light yellow ink (LY).  
         [0021]     For example, the cyan ink and light cyan ink are those of similar colors, and have different color recording material densities. The color recording material density of the cyan ink is higher than that of the light cyan ink. Note that a dye or pigment is often used as the color recording material. The differences between dark and light inks are not only the color recording material densities but also the color recording materials themselves often include different materials. Also, the dark and light inks may often have different solvent compositions in the inks, but they are still inks of similar colors.  
         [0022]     The print head  1  shown in  FIG. 1  is generally called a lateral alignment system, and print head arrays corresponding to a plurality of colors of inks are aligned in the main scan direction of a carriage  201  which mounts the print head  1 . In addition to the lateral alignment system, a case wherein print head arrays that eject a plurality of colors of inks are aligned in a direction perpendicular to the main scan direction of the carriage  201 , i.e., in the convey direction of the printing medium is called a lengthwise alignment system. Since the plurality of ink colors can be aligned within a space smaller than the lateral alignment system, the lengthwise alignment system is advantageous to attain a size reduction of the printing apparatus. Since the dot formation order of respective color inks remains the same even in a two-directional printing mode, color heterogeneity in the two-directional printing mode are smaller than the lateral alignment system.  
         [0023]     The scan speed and position (print position) of the carriage  201  which mounts the print head  1  are detected by a speed detector  5 , and the controller of the printing apparatus performs movement control of the carriage  201  in the main scan direction on the basis of this detection result. A power source that moves the carriage  201  is a carriage drive motor  8 , and the rotation of the carriage drive motor  8  is transmitted to the carriage  201  via belts  7  and  6 , thus moving the carriage  201  along a slide shaft  4 . Note that a print operation is performed during scanning of the carriage  201 .  
         [0024]     A recovery unit  400  has a function of always keeping the print head  1  in a good condition, and a cap array  420  of the recovery unit  400  closes an ink ejection face of the print head  1  in a non-print state, thus preventing drying and the like. Note that a position where the carriage  201  and recovery unit  400  oppose is called a home position (HP).  
         [0025]     The function of the recovery unit  400  during the print operation will be described below.  
         [0026]     In an actual print operation, not all nozzles in one print head array are always be used. Also, even when a plurality of colors of print head arrays are available, some print head arrays do not receive any print data, i.e., they are not used. When a given print head array does not eject any ink for a predetermined period of time while it is not capped, the surface of the print head array dries, and the ejection performance of ink drops due to adhesion of the color recording material, thus deteriorating an image. In order to prevent such phenomenon, nozzles in each print head array are controlled to eject ink at given time intervals in addition to the print operation, thus always keeping the surface of the print head array in an optimal condition. This operation is called preliminary ejection. The ink ejected by preliminary ejection is directed toward the cap array  420  in the recovery unit  400  so as to prevent contamination caused when it is scattered onto a printing medium or into the printing apparatus, and is sucked by a recovery pump (not shown) and is stored in a waste ink tank. Therefore, when preliminary ejection is made during the print operation, the carriage  201  must be returned to the HP to oppose the cap array  420  in both the one- and two-directional printing modes.  
         [0027]     A printing medium is fed in the sub-scan direction by a paper feed member (rubber rollers and the like) driven by a paper feed motor  305  (to be described later). A printing medium fed from a direction of an arrow A shown in  FIG. 1  reaches the printing position, and undergoes a print operation by the print head  1 . After that, the printing medium is exhausted in a direction of an arrow B by paper discharge mechanisms  2  and  3 .  
         [0028]     Also, inks are supplied from ink cartridges  10 K,  10 C,  10 LC,  10 M,  10 LM, and  10 Y to the print head  1  for respective colors. Note that in this embodiment, the dark and light cyan inks and dark and light magenta inks are respectively stored in identical cartridges.  
         [0000]     [Control Arrangement of Printing Apparatus] 
         [0029]      FIG. 2  is a block diagram showing the control arrangement of the printing apparatus.  
         [0030]     A controller  301  comprises a CPU  310  such as a microprocessor or the like, a ROM  311  which stores a control program to be executed by the CPU  310 , a color processing program (to be described later), and various data, a RAM  312  which is used as a work area when the CPU  310  executes various kinds of processing, and temporarily holds various data, and the like. The controller  301  controls the operation of the entire printing apparatus. The RAM  312  is assigned a reception buffer which temporarily stores print data input from a host computer  100  via an interface (I/F)  306 , and print buffers which temporarily store print data in correspondence with the print head arrays of the respective color inks.  
         [0031]     A head driver  302  drives the print head arrays of the respective color inks in accordance with print data of respective colors output from the controller  301 . Also, a motor driver  303  drives the carriage drive motor  8  and a motor driver  304  drives the paper feed motor  305  under the control of the controller  301 .  
         [0032]     An operation unit  307  comprises various keys operated by the user to set the operation of the printing apparatus, and a display for displaying the operation state of the printing apparatus and the like by the controller  301 .  
         [0000]     [Color Processing] 
         [0033]      FIG. 3  is a flowchart showing the image signal processing (color processing) for printing. Note that the printing apparatus may execute some or all steps of the color processing shown in  FIG. 3 , or the host computer  100  may execute these steps and may transmit the result to the printing apparatus. In this embodiment, either of these cases may be used.  
         [0034]     RGB original image signals obtained by an image input device such as a digital camera, scanner, or the like, or computer processing or the like are converted into R′G′B′ signals by color processing A (S 201 ). The color processing A converts original image signals RGB into image signals R′G′B′ corresponding to the color gamut of the printing apparatus.  
         [0035]     The R′G′B′ signals are converted into signals corresponding to the respective color inks by color processing B (S 202 ). Since the printing apparatus of this embodiment has a six-color arrangement, the converted signals are density signals C 1 , M 1 , Y 1 , K 1 , LC 1 , and LM 1  corresponding to cyan, magenta, yellow, black, light cyan, and light magenta. Practical color processing B uses a three-dimensional lookup table (3DLUT)  202  which receives R, G, and B inputs and generates C, M, Y, K, LC, and LM outputs, and as for an input value which deviates from a grid point, its output value is calculated based on those of surrounding grid points by interpolation.  
         [0036]     Next, the density signals C 1 , M 1 , Y 1 , K 1 , LC 1 , and LM 1  undergo gamma correction using a gamma correction table (S 203 ). Next, gamma-corrected density signals C 2 , M 2 , Y 2 , K 2 , LC 2 , and LM 2  are binarized by quantization (S 204 ), thus obtaining image signals C 3 , M 3 , Y 3 , K 3 , LC 3 , and LM 3  to be transferred to the print head arrays  1 C,  1 M,  1 Y,  1 K,  1 LC, and  1 LM. Note that the quantization (binarization) method adopts an error diffusion or dither method. The dither method binarizes using a predetermined dither pattern which has different thresholds for density signals of respective pixels.  
         [0000]     [Comparison of Color Gamuts] 
         [0037]      FIG. 4  shows the color gamut of a uniform color space (L*a*b* color space) by an a*b* plane.  
         [0038]     Referring to  FIG. 4 , bold solid lines are hue lines which indicate changes in color from white indicated by a symbol ◯ to primary colors (cyan, magenta, and yellow) and secondary colors (red A, blue, and green) similarly indicated by symbols ◯. Also, thin solid lines which connect the six colors of the primary and secondary colors in the hue order indicate boundaries of a color gamut when old magenta ink is used.  
         [0039]     On the other hand, a bold broken line is a hue line indicating a change in color from white to red B indicated by a symbol ● when new magenta ink is used. Also, thin broken lines that connect yellow, red B, and magenta indicate boundaries of a color gamut when the new magenta ink is used. As shown in  FIG. 4 , the color gamut when the new magenta ink is used (to be referred to as “color gamut B” hereinafter) is narrower than the color gamut when the old magenta ink is used (to be referred to as “color gamut A” hereinafter).  
         [0040]     In this connection, as a method of reproducing the color of red using the aforementioned hue lines, an intersection of the thin broken line that connects red B and magenta and the bold solid line that connects white and red A is defined as red (red C indicated by a symbol Δ). Since red C defined by this method can realize a maximum saturation value within the color gamut B, and is located on the hue line of red, it is suited as a hue value. However, the saturation value of red C is lower than that of red B, red C becomes visually subdued red, and it can hardly be favorable color reproduction. Depending on conditions, the lightness value of red C becomes higher than that of red B, and red C often becomes a ligther, subdued color in cooperation with the saturation drop.  
         [0041]      FIG. 5  shows changes in color from white to reds A and B and to black while the ordinate plots a lightness value L* and the abscissa plots a saturation (chroma) value. Since the saturation is a distance from an origin on the a*b* plane, a saturation value c of a color (a 1 *, b 1 *) on the a*b* plane is given by: 
   c =( a   1 *ˆ2 +b   1 *ˆ2)ˆ0.5   (1)  
         [0042]     In  FIG. 5 , a solid line indicates changes in color from white to red A, and then to black when the old magenta ink is used, and a broken line indicates changes in color from white to red B, and then to black when the new magenta ink is used. In the characteristics indicated by the solid line, a saturation value rises and reaches a maximum saturation value L*≅49, while a lightness value lowers from the neighborhood of L*=95, and after that, the lightness and saturation values lower and then reach black. On the other hand, in the characteristics indicated by the broken line, a saturation value rises and reaches a maximum saturation value L*≅45, while a lightness value lowers from the neighborhood of L*=95, and after that, the lightness and saturation values lower and then reach black. As shown in  FIG. 5 , when the new magenta ink is used, the lightness—saturation characteristics also have a narrow range in a red region.  
         [0000]     [Color Conversion Method of Red Region] 
         [0043]     As can be understood from the above description, when the new magenta ink is used, its color gamut B is narrower in the red region than the color gamut A when the old magenta ink is used, and the lightness—saturation characteristics also have a narrow range in the red region.  
         [0044]     When such new magenta ink is used, red B is reproduced by setting the hue value of red to be closer to yellow (i.e., using the hue line from white to red B in  FIG. 4 ) so as to utilize the maximum saturation of the color gamut B shown in  FIG. 4 . Note that red B whose hue value is closer to yellow appears yellowish compared to red A. Normally, since yellow has a higher lightness value than red, red B has a higher lightness value than red C obtained by maintaining the hue value from white to red A, and it appears lighter.  
         [0045]     That is, reds A and B are visually, apparently different colors. However, red B is visually favorable red rather than red C which maintains the hue value from white to red A and has lower saturation and lightness values. Therefore, when reproduction of so-called memory colors is important like in a photo image, not only the color accuracy but also colors with favorable impressions are important. Hence, it is preferable to adopt red B rather than red C in which only the hue and lightness values are simply adjusted.  
         [0046]     Therefore, when a table for representing a red region of the 3DLUT  201  used in the color processing A shown in  FIG. 3  is created on the basis of the hue line that connects white and red B, the color processing A which reproduces red B can be implemented.  
         [0047]     Practical numerical value ranges of red B. can be defined on the uniform color space L*a*b* as: 
 
L*≅45 to 50, a*≅67 to 70, b*≅50 to 55 
 
         [0048]     When L* is smaller than the above range, it is inappropriate since the reproduced color becomes visually dark and its impression impairs. Conversely, when L* is larger than the above range, such case does not correspond to this embodiment since the color gamut is narrow and such value does not exist.  
         [0049]     When a* is smaller than the above range, it is inappropriate since further saturation and lightness drops may occur. Conversely, when a* is smaller than the above range, it is inappropriate since the reproduced color becomes too yellowish.  
         [0050]     When b* is larger than the above range, it is inappropriate since the reproduced color becomes too yellowish. Conversely, when b* is smaller than the above range, the reproduced color becomes light since the saturation value lowers and the lightness value rises, and hence, it is inappropriate.  
         [0051]     Note that the central values of red B in this embodiment are: 
 
L*=46, a*=69, b*=52 
 
         [0052]     In this connection, in the following values of red A, since a* is as large as 74 but L* up to 49 is output, naturally appropriate red is reproduced: 
 
L*≅49, a*≅74, b*≅50 
 
         [0053]     A color difference ΔE between the aforementioned values of red A and the central values of red B is given by: 
 
 ΔE ={(49−46) 2 +(69−74) 2 +(50−52) 2 }ˆ0.5=6.2   (2) 
 
         [0054]     The value of ΔE at which different colors are visually recognized falls within the range from 2 to 3. Therefore, reds A and B which have the color difference ΔE=6.2 are sufficiently recognized as different colors. Note that the above L*a*b* values were measured using Spectrolinom available from GretagMacbeth AG. The colorimetric conditions at that time are as follows:  
         [0055]     Light source: D50  
         [0056]     Field angle: 2° 
         [0057]     White reference: absolute (correction by a white reference plate)  
         [0058]     Filter: none  
         [0059]     The aforementioned values of red are expressed on the L*a*b* space, but the same applies to those which are converted into other color spaces.  
         [0060]     According to this embodiment, determination of color in the red region, when a color which does not exist within a printable color gamut into a color gamut that can print using the new magenta ink, utilizes the hue line of red which deviates from that of red which can be reproduced by the old magenta ink to the side of yellow, and can be reproduced by the new magenta ink. As a result, the color in the red region which is preferred upon reproducing a photo image can be reproduced although they are not colorimetrically, visually identical colors.  
         [0000]     [Application to Printing Apparatus] 
         [0061]     In order to implement color conversion in consideration of color reproduction of the new magenta ink, the 3DLUT  201  for the color processing A need only be stored in the ROM  311  of the printing apparatus. Upon executing the color processing B by the host computer  100 , a printer driver having the 3DLUT  201  for the color processing A need only be installed in the host computer  100 .  
         [0062]     When a mode that attaches an importance on reproduction of memory colors is designated on a user interface provided by the printer driver, the above color conversion that attaches an importance on the saturation (and lightness) may be implemented. When the printer driver (printer controller) has a function of automatically determining which of photo, text, and graphics an object of an input image corresponds to, the above color conversion that attaches an importance on the saturation (and lightness) may be applied to image data corresponding to a photo image region that attaches an importance on reproduction of memory colors.  
         [0063]     In the printing apparatus, when the old and new magenta inks are likely to be used together, three-dimensional lookup tables for the color processing A which support both the inks are stored in the ROM  311 , and the three-dimensional lookup table used in the color processing A is switched upon operation of the operation unit  307 . Furthermore, when the new magenta ink is used in a printing apparatus released before the advent of the new magenta ink, for example, a three-dimensional lookup table for the new magenta ink may be downloaded from the host computer  100  to the RAM  312  and may be applied to the color processing A.  
       Other Embodiments  
       [0064]     Note that the present invention can be applied to an apparatus comprising a single device or to system constituted by a plurality of devices.  
         [0065]     Furthermore, the invention can be implemented by supplying a software program, which implements the functions of the foregoing embodiments, directly or indirectly to a system or apparatus, reading the supplied program code with a computer of the system or apparatus, and then executing the program code. In this case, so long as the system or apparatus has the functions of the program, the mode of implementation need not rely upon a program.  
         [0066]     Accordingly, since the functions of the present invention are implemented by computer, the program code installed in the computer also implements the present invention. In other words, the claims of the present invention also cover a computer program for the purpose of implementing the functions of the present invention.  
         [0067]     In this case, so long as the system or apparatus has the functions of the program, the program may be executed in any form, such as an object code, a program executed by an interpreter, or scrip data supplied to an operating system.  
         [0068]     Example of storage media that can be used for supplying the program are a floppy disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a CD-R, a CD-RW, a magnetic tape, a non-volatile type memory card, a ROM, and a DVD (DVD-ROM and a DVD-R).  
         [0069]     As for the method of supplying the program, a client computer can be connected to a website on the Internet using a browser of the client computer, and the computer program of the present invention or an automatically-installable compressed file of the program can be downloaded to a recording medium such as a hard disk. Further, the program of the present invention can be supplied by dividing the program code constituting the program into a plurality of files and downloading the files from different websites. In other words, a WWW (World Wide Web) server that downloads, to multiple users, the program files that implement the functions of the present invention by computer is also covered by the claims of the present invention.  
         [0070]     It is also possible to encrypt and store the program of the present invention on a storage medium such as a CD-ROM, distribute the storage medium to users, allow users who meet certain requirements to download decryption key information from a website via the Internet, and allow these users to decrypt the encrypted program by using the key information, whereby the program is installed in the user computer.  
         [0071]     Besides the cases where the aforementioned functions according to the embodiments are implemented by executing the read program by computer, an operating system or the like running on the computer may perform all or a part of the actual processing so that the functions of the foregoing embodiments can be implemented by this processing.  
         [0072]     Furthermore, after the program read from the storage medium is written to a function expansion board inserted into the computer or to a memory provided in a function expansion unit connected to the computer, a CPU or the like mounted on the function expansion board or function expansion unit performs all or a part of the actual processing so that the functions of the foregoing embodiments can be implemented by this processing.  
         [0073]     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.  
       CLAIM OF PRIORITY  
       [0074]     This application claims priority from. Japanese Patent Application No. 2004-271011 filed on Sep. 17, 2004, which is hereby incorporated by reference herein.