CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority under 35 USC 119 from Japanese Patent Application No. 2003-368758, the disclosure of which is incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an image processing apparatus on which an image forming engine for forming a full-color image on a recording medium by using color materials corresponding to color data on the basis of C, M, Y, K data is mounted.  
         [0004]     2. Description of the Related Art  
         [0005]     It is known to provide an image forming apparatus that receives RGB data output from a host computer or the like, converts the RGB data to CMYK data, records an electrostatic latent image onto a photosensitive member in correspondence with the color data, develops the image by using toners of the colors, and transfers the image onto a recording medium (recording paper), thereby obtains a full-color image.  
         [0006]     In the case of generating CMYK data from RGB data, first, CMYK data is generated from RGB data. Next, when amounts of C, M, and Y are equal to each other, the equal amounts are replaced with K data. The K data amount is set as a base and subtracted from the original C, M, and Y data, thereby generating each of the C, M, and Y data.  
         [0007]     As a conventional method of generating K data, the difference between the maximum and minimum values of CMY data is regarded as saturation information. When amounts of C, M, and Y data are equal to each other, it is determined that saturation is low, and K data is generated so as to be maximum.  
         [0008]     In such a K data generating method, however, there is a case such that K data becomes the maximum in a color having rather low saturation. As a result, excellent color reproduction is not achieved, and a problem occurs such that tone reproducibility deteriorates and spreading occurs in a low saturation part in which the K data is generated maximally.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention has been made in view of the above circumstances and provides an image processing apparatus capable of generating K data on the basis of CMY data of gray tones which becomes achromatic color according to the degree of color tone (degree of lightness, darkness, or the like) and realizing improved color reproducibility.  
         [0010]     The invention provides an image processing apparatus on which an image forming engine for forming a full color image onto a recording medium by using color materials corresponding to data of color tones on the basis of C (cyan), M (magenta), Y (yellow), and K (black) data is mounted, the image processing apparatus including: a color converting component for converting R (red), G (green), and B (blue) data input from the outside to C (1) , M (1) , and Y (1)  data which depends on a characteristic of the image forming engine; a lightness acquiring component for acquiring lightness from the C (1) , M (1) , and Y (1)  data obtained by the conversion by the color converting component; a memory for storing a lightness-gray value characteristic table which depends on the characteristic of the image forming engine, is preset, and indicates gray value balance of the C (1) , M (1) , and Y (1)  data with respect to lightness; a gray value reading component for reading a gray value at the lightness acquired by the lightness acquiring component, from the memory; a first arithmetic component for obtaining C (2) , M (2) , and Y (2)  data by subtracting the gray values read by the gray value reading component from the corresponding C (1) , M (1) , and Y (1)  data derived by conversion by the color converting component; a K data generating component for generating K data on the basis of the C (2) , M (2) , and Y (2)  data; and a second arithmetic component for calculating C, M, and Y data by subtracting a base amount determined on the basis of the K data generated by the K data generating component from the C (1) , M (1) , and Y (1)  data obtained by color conversion by the color converting component.  
         [0011]     According to the invention, first, the color converting component converts R (red), G (green), and B (blue) data input from the outside to C (1) , M (1) , and Y (1)  data. At this time, the color reproducibility depends on the characteristic of the image forming engine applied, so that correction has to be made with factors which are set for the image forming engine.  
         [0012]     The lightness acquiring component acquires lightness of the C (1) , M (1) , and Y (1)  data generated by conversion of the color converting component. The lightness is determined mostly by the balance of the C (1) , M (1) , and Y (1)  data. For example, when the amount of color data as a whole is small (light), the lightness becomes the maximum. When the amount of color data as a whole is large (dark), the lightness becomes the minimum. The degree of a change in the lightness is an almost linear characteristic, so that lightness can be obtained from the balance of the C (1) , M (1) , and Y (1)  data.  
         [0013]     As already described in Description of the Related Art, in theory, when amounts of the C (1) , M (1) , and Y (1)  data are balanced and equal to each other, gray is expressed. However, there are cases in which gray is expressed in an unbalanced state depending on the characteristic of the image forming engine. A data value does not move from the origin inherently (theoretically) but the data largely changes according to density of the colors.  
         [0014]     The present inventor herein has found out that the gray balance has a close relationship with lightness.  
         [0015]     Consequently, in the memory, a lightness-gray value characteristic table which depends on the characteristic of the image forming engine, is preset, and indicates gray value balance of the C (1) , M (1) , and Y (1)  data with respect to lightness is prestored. The gray value reading component reads a gray value at the lightness acquired by the lightness acquiring component, from the memory.  
         [0016]     The second arithmetic component calculates C (2) , M (2) , and Y (2)  data by subtracting the gray value read by the gray value reading component from the C (1) , M (1) , and Y (1)  data obtained by color conversion by the color converting component.  
         [0017]     In this case, by subtracting the gray value, the C (2) , M (2) , and Y (2)  data become small values (values close to zero). Consequently, to raise the values, for example, it is preferable to add the smallest value among the C, M, and Y data indicative of the gray values.  
         [0018]     By using the difference between the maximum and minimum values of the C (2) , M (2) , and Y (2)  data as a reference, the K data generating component generates K data by using, as factors, a K data insertion start point according to lightness and the ratio of K data according to lightness.  
         [0019]     The K data insertion start point according to lightness denotes the upper limit value of lightness at which K data is generated. The ratio of K data according to lightness denotes a characteristic curve (by which the most appropriate value is determined according to the characteristic of the image forming engine) in which an amount of conversion to K data gradually increases as lightness decreases from the upper limit value (as it becomes darker).  
         [0020]     Finally, the second arithmetic component calculates C, M, and Y data by subtracting a base amount determined on the basis of the K data generated by the K data generating component from the C (1) , M (1) , and Y (1)  data obtained by color conversion by the color converting component.  
         [0021]     According to the foregoing, the most appropriate K data is generated in consideration of the balance of the gray value which depends on the characteristic of the image forming engine and changes with lightness. Further, by subtracting the K data generated in such a manner as a base amount, the optimum C, M, and Y data is generated. Thus, C, M, Y, and K data with excellent color reproducibility can be obtained.  
         [0022]     According to another aspect, the invention provides an image processing method in an image processing apparatus on which an image forming engine for forming a full color image onto a recording medium by using color materials corresponding to data of color tones on the basis of C (cyan), M (magenta), Y (yellow), and K (black) data is mounted, the image processing method comprising: (a) converting R (red), G (green), and B (blue) data input from the outside to C (1) , M (1) , and Y (1)  data which depends on a characteristic of the image forming engine; (b) acquiring lightness from the C (1) , M (1) , and Y (1)  data obtained by the conversion in (a); (c) storing a lightness-gray value characteristic table which depends on the characteristic of the image forming engine, is preset, and indicates gray value balance of the C (1) , M (1) , and Y (1)  data with respect to lightness into a memory; (d) reading a gray value at the lightness acquired in (b), from the memory; (e) obtaining C (2) , M (2) , and Y (2)  data by subtracting the gray values read in (d) from the corresponding C (1) , M (1) , and Y (1)  data derived by conversion in (a); (f) generating K data on the basis of the C (2) , M (2) , and Y (2)  data; and (g) calculating C, M, and Y data by subtracting a base amount determined on the basis of the K data generated in (f) from the C (1) , M (1) , and Y (1)  data obtained by color conversion in (a).  
         [0023]     As described above, the invention produces excellent effects such that K data is generated on the basis of CMY data of gray tones which becomes achromatic color according to the degree of color tone (degree of lightness, darkness, or the like) so that color reproducibility can be improved. 
     
    
     BRIEF DESCRITPION OF THE DRAWINGS  
       [0024]     Preferred embodiment of the present invention will be described in detail based on the following figures, wherein:  
         [0025]      FIG. 1  is a schematic configuration diagram of an image forming apparatus according to an embodiment of the invention;  
         [0026]      FIG. 2  is a functional block diagram for control of a color converting process in a controller of the image forming apparatus;  
         [0027]      FIG. 3  is a color balance characteristic diagram when C=M=Y in the L*a*b* colorimetric system;  
         [0028]      FIG. 4  is a c* distribution characteristic diagram in the characteristics of  FIG. 3 ;  
         [0029]      FIG. 5  is a c*-L* characteristic diagram in the characteristics of  FIG. 3 ;  
         [0030]      FIG. 6  is a CMY gray value characteristic diagram in the characteristics of  FIG. 3 ;  
         [0031]      FIG. 7  is a three-dimensional characteristic diagram of the L*a*b* calorimetric system showing a start point of insertion (subtraction) of K data and a ratio of insertion (subtraction) of K data;  
         [0032]      FIG. 8  is a flowchart showing a color converting process control routine in the embodiment of the invention;  
         [0033]      FIG. 9  is a flowchart showing a lightness acquisition control subroutine in step  206  in  FIG. 8 ;  
         [0034]      FIG. 10  is a flowchart showing a CMY gray value acquisition control subroutine in step  208  in  FIG. 8 ;  
         [0035]      FIG. 11  is a characteristic diagram showing a color change according to the density of each of the colors in the L*a*b* calorimetric system; and  
         [0036]      FIG. 12  is a c*-L* characteristic diagram in the characteristics of  FIG. 11 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]      FIG. 1  is a schematic diagram of a tandem-type color image forming apparatus according to an embodiment of the present invention.  
         [0038]     A color image forming apparatus  10  has an optical scanner  12  disposed upright in an approximate center of the apparatus. From the optical scanner  12 , on the basis of color data of C (cyan), M (magenta), Y (yellow), and K (black) generated by a controller  54 , light beams (hereinbelow, also called scan beams) AY, AM, AC, and AK corresponding to the colors are output to the left in  FIG. 1 .  
         [0039]     On the left side of the optical scanner  12  in  FIG. 1 , in order from the top in the vertical direction, photosensitive drums  14 Y,  14 M,  14 C, and  14 K (hereinbelow, also generically called photosensitive drums  14 ) are disposed in series. Near the photosensitive drums  14 Y,  14 M,  14 C, and  14 K, corresponding chargers  16 Y,  16 M,  16 C, and  16 K (hereinbelow, also generically called chargers  16 ) and developers  18 Y,  18 M,  18 C, and  18 K (hereinbelow, also generically called developers  18 ) are disposed, respectively.  
         [0040]     By the optical scanner  12 , photosensitive drums  14 , chargers  16 , developers  18 , and the like, an engine part  10 A of the color image forming apparatus  10  is constructed.  
         [0041]     The photosensitive drum  14  is charged by the charger  16  and its drum surface is scanned with and exposed to an optical beam from the optical scanner  12 , thereby forming an electrostatic latent image.  
         [0042]     The electrostatic latent image is developed by the developer  18 , thereby obtaining toner images of the colors. In combinations of Y and M colors, and C and K colors, the toner images are transferred to primary intermediate transfer members  20 YM and  20 CK respectively. The images are transferred from the primary intermediate transfer members  20 YM and  20 CK to a secondary intermediate transfer member  22  where the images are overlapped. Near the secondary intermediate transfer member  22 , a transfer charger  24  for charging the secondary intermediate transfer member  22  and a cleaner  26  for collecting discarded toner from the secondary intermediate transfer member  22  are disposed. The primary intermediate transfer members  20 YM and  20 CK, the secondary intermediate transfer member  22 , and the above-described image forming components are rotated by the driving force of a drive motor (MOT)  27 . The driving of the motor  27  is controlled by a signal from the controller  54 .  
         [0043]     Toner images colorized on the secondary intermediate transfer member  22  are transferred together from the secondary intermediate transfer member  22  onto a recording sheet  30  fed from a paper feeding cassette  28  in the lower part of the apparatus. When the recording sheet  30  is sent to a fixer  32  disposed at the top of the secondary intermediate transfer member  22  in  FIG. 1 , the transfer image becomes a fixed image.  
         [0044]     By this process, a color image is formed on the recording sheet  30  by the color image forming apparatus  10  and the recording sheet  30  is ejected onto an ejection tray  34  at the top of the apparatus.  
         [0045]     In this configuration, the controller  54  generates CMYK data to be transmitted to the optical scanner  12 , from image data of R (red), G (green), and B (blue) input from the outside.  
         [0046]      FIG. 2  is a control block diagram for generating the CMYK data in the controller  54 . As described above, the controller  54  executes various control such as the driving control of the motor  27 , charging control of the charger  16 , development control of the developer  18 , fixing control of the fixer  32 , and carriage control of the recording sheet  30 . With reference to  FIG. 2 , only the control for generating CMYK data from RGB data will be described.  
         [0047]     The block diagram of  FIG. 2  shows functional blocks but does not limit the hardware configuration of the invention.  
         [0048]     RGB data from the outside is input to an RGB-CMY color converting component  100 . In the RGB-CMY color converting component  100 , an RGB-CMY conversion map shown in Table 1 is stored, and CMY data corresponding to the RGB data is read from the map and sent to the RGB-CMY color converting component  100 . In the embodiment, each of the R, G, and B data is 8-bit (256-shade) data, each of the C, M, and Y data is percentage (%) data, and, in reality, there are 256 3  conversion patterns.  
                                                                         TABLE 1                                   R (8 bits)   G (8 bits)   B (8 bits)   C(%)   M(%)   Y(%)                                         0   0   0   100   100   90            0   0   16   100   100   86            0   0   17   98   98   85           .   .   .   .   .   .           .   .   .   .   .   .           .   .   .   .   .   .           255   255   240   0   0   8           .   .   .   .   .   .           .   .   .   .   .   .           .   .   .   .   .   .           255   255   255   0   0   0                      
 
         [0049]     The data read by the RGB-CMY color converting component  100  is transmitted as C (1) , M (1) , and Y (1)  data to a lightness acquiring component  102 .  
         [0050]     The lightness acquiring component  102  acquires lightness based on the C (1) , M (1) , and Y (1)  data. To acquire the lightness, a lightness reading component  104  is connected to the lightness acquiring component  102 . A CMY-lightness characteristic map memory  106  is connected to the lightness reading component  104  and stores a CMY-lightness characteristic map shown in Table 2. As the lightness, a numerical value indicated by L* in the L*a*b* calorimetric system is applied.  
                                   TABLE 2                                   C(%)   M(%)   Y(%)   L* (lightness)                            0    0    0   95            0    0    20   93           .   .   .   .           .   .   .   .           .   .   .   .            80    80    60   32            80    80    80   30           .   .   .   .           .   .   .   .           .   .   .   .           100   100   80   26           100   100   100   20                      
 
         [0051]     The CMY-lightness characteristic map is determined on the basis of the balance between C and M colors which are conspicuous to human eyes and the Y color which is not conspicuous to human eyes, and the characteristics of the engine part  10 A of the image forming apparatus. The maximum lightness in the embodiment is 95, and the minimum lightness is 20.  
         [0052]     In Table 2, six levels (0%, 20%, 40%, 60%, 80%, and 100%) are set for each of the C (1) , M (1) , and Y (1)  data, and a map of 6 3  patterns is used by the combinations of the colors.  
         [0053]     For example, when the C (1) , M (1) , and Y (1)  data is 80%, 80%, and 60%, respectively, 32 can be read as lightness from Table 2.  
         [0054]     On the other hand, when the C (1) , M (1) , and Y (1)  data is 88%, 87%, and 76%, respectively, corresponding lightness does not exist in Table 2.  
         [0055]     In such a case, approximate values are selected. Approximate values are selected in such a manner that ranges of 0&lt;CMY&lt;10, 10≦CMY&lt;30, 30≦CMY&lt; 50 ,  50 ≦CMY&lt;70, 70&lt;CMY&lt;90, 90≦CMY≦100 are determined by using the levels as references, a range in which the data is actually included is extracted, and lightness is set. Since “88%, 87%, 76%” is close to “80%, 80%, 80%” in Table 2, 30 is selected as lightness.  
         [0056]     The lightness L* acquired by the lightness acquiring component  102  is transmitted to a CMY gray value acquiring component  108 . The CMY gray value acquiring component  108  acquires Cgray, Mgray, and Ygray data at which a combination of the CMY data becomes a gray value on the basis of the lightness L*.  
         [0057]     For this purpose, the CMY gray value acquiring component  108  is connected to a CMY gray value reading component  110 . The CMY gray value reading component  110  has the role of reading the Cgray, Mgray, and Ygray data from the L-CMY gray value characteristic map (refer to Table 3) stored in an L-CMY gray value characteristic map memory  112  on the basis of the lightness L* input from the CMY gray value acquiring component  108 , and transmitting the data to the CMY gray value acquiring component  108 .  
                                   TABLE 3                                   L* (lightness)   Cgray   Mgray   Ygray                           95    0    0    0           90    5    5    4           .   .   .   .           .   .   .   .           .   .   .   .           35    82    81   70           30    88    87   76           25    94    93   83           20   100   100   90                      
 
         [0058]      FIG. 3  is a characteristic diagram of the L*a*b* colorimetric system in the characteristic of the engine part  10 A of the image forming apparatus  10  in the embodiment and shows a color balance change characteristic when a data value in a state where C=M=Y is displaced from 0% to 100%. As understood from  FIG. 1 , although the data value does not move from the origin inherently (theoretically), the data largely changes according to density of the colors.  
         [0059]      FIG. 5  shows the relation between the distance c* from the origin (a*=b*=0) in the  FIG. 3  and the Z axis (L* axis) in  FIG. 3 . It is understood that when the distance from the origin of lightness changes, an influence is exerted on lightness. In other words, it can be said that, in many cases, when a gray value of CMY is obtained according to the lightness L*, C≠M≠Y.  
         [0060]     Based on the above, the grounds of Table 3 will be described. When a gray value of CMY is obtained according to the lightness L*, the occupation ratios of data of C, M, and Y are different from each other. When the lightness L* is known, an accurate CMY gray value can be obtained.  
         [0061]     As shown in  FIG. 5 , even when amounts of C, M, and Y are equal to each other, the lightness L* changes according to the density.  
         [0062]     In the case of showing the above (the characteristic of  FIG. 1 ) by the relation between the change amount c* and L*, as shown in  FIG. 5 , even when the amounts of C, M, and Y are equal to each other, the lightness L* changes according to the density.  
         [0063]     When the CMY gray value is obtained in such a characteristic (characteristic of dependency on the engine part  10 A), it is expressed as shown by a CMY gray characteristic diagram of  FIG. 6 . Table 3 shows the CMY gray value map obtained by expressing the characteristic diagram of  FIG. 6  by numerical values and stored.  
         [0064]     Accurate CMY gray values (Cgray, Mgray, and Ygray data) obtained by the CMY gray value acquiring component  108  are transmitted to a CMY correction value calculating component  114  for generating K.  
         [0065]     To the CMY correction value calculating component  114  for generating K, the C (1) , M (1) , and Y (1)  data obtained by conversion of the RGB-CMY color converting component  100  is input. Basically, the CMY correction value calculating component  114  generates C (2) , M (2) , and Y (2)  data as references to generate K data by subtracting the CMY gray values (Cgray, Mgray, and Ygray data) from the C (1) , M (1) , and Y (1)  data.  
         [0066]     However, if the Cgray, Mgray, and Ygray data is simply subtracted from the C (1) , M (1) , and Y (1)  data, the C (2) . M (2) , and Y (2)  data itself becomes small values, and precision of calculation deteriorates.  
         [0067]     Consequently, in the embodiment, all of the C (2) , M (2) , and Y (2)  data are raised by the minimum values of the Cgray, Mgray, and Ygray data. The operation is expressed as shown by the following equation (2). 
 
 C   (2)   ·M   (2)   ·Y   (2)   =C   (1)   ·M   (1)   ·Y   (1) +Min( Cgray·Mgray·Ygray )− Cgray·Mgray·Ygray   (2) 
 
         [0068]     When the C (1) , M (1) , and Y (1)  data is 88%, 87%, and 76%, respectively, and gray is expressed, in the case where Cgray, Mgray, and Ygray data corresponding to the lightness L* at the values is 88%, 87%, and 76% (in this case, the data coincides with the C (1) , M (1) , and Y (1)  data), the C (2) , M (2) , and Y (2)  data is 76%, 76%, and 76%, respectively, in the equation (2).  
         [0069]     On the other hand, when the C (1) , M (1) , and Y (1)  data is 75%, 75%, and 75% and gray is not expressed in practice, in the case where Cgray, Mgray, and Ygray data corresponding to the lightness L* at the values is 78%, 77%, and 66%, respectively, the C (2) , M (2) , and Y (2)  data is 63%, 64%, and 66%, respectively, in the equation (2).  
         [0070]     The C (2) , M (2) , and Y (2)  data obtained by calculation in the CMY correction value calculating component  114  for generating K is transmitted to a K generating component  116 .  
         [0071]     The K generating component  116  generates K data by using, as a reference, the difference between the maximum value and the minimum value of the C (2) , M (2) , and Y (2)  data.  
         [0072]     That is, the K data is calculated on the basis of the equation (3). 
 
 K =Min( C   (2)   ·M   (2)   ·Y   (2) )−(α Z +β)  (3) 
        where Z is the difference between the maximum and minimum values of the C (2) ·M (2) ·Y (2)  data, and     α and β are coefficients determined by the start point of lightness and the ratio when K data is inserted.        
 
         [0075]     It is understood from the equation (3) that, when the difference between the maximum and minimum values of the C (2) ·M (2) ·Y (2)  data is zero, the data K becomes the maximum. However, the difference between the maximum and minimum values of the C (2) , M (2) , and Y (2)  data (63%, 64%, 66%) is not zero because of correction, so that the K data does not become the maximum in a slightly colored part.  
         [0076]     The coefficients α and β will now be described.  
         [0077]      FIG. 7  shows the relation between an insertion amount of the K data and lightness by three-dimensional axes (x axis denotes a*, y axis denotes b*, and z axis indicates L*). As shown in  FIG. 7 , a region of inserting K data can be expressed by an almost conical shape, and a characteristic curve (determined by the characteristic of the engine part  10 A) in which as the lightness decreases from the apex (upper limit value according to lightness) of the conical-shape characteristic decreases, the amount of conversion to K data gradually increases is obtained.  
         [0078]     The K data obtained by the equation (3) is applied as K data at the time of forming an image and is also transmitted to a base eliminating component  118 .  
         [0079]     To the base eliminating component  118 , the C (1) , M (1) , and Y (1)  data obtained by the conversion in the RGB-CMY color converting component  100  is input. Basically, by subtracting the K data, CMY data is generated.  
         [0080]     Coefficients γ and σ as shown in the equation (4) are added so as to correspond to α and β used for generating the K data. 
 
 C·M·Y=C   (1)   ·M   (1)   ·Y   (1) −(γ K +σ)  (4) 
        where γ and σ are coefficients determined by the start point of lightness and the ratio at the time of erasing K data        
 
         [0082]     By the equation (4), CMY data to be applied as CMY data for forming an image is generated.  
         [0083]     The operation of the embodiment will be described hereinbelow with reference to the flowchart of  FIG. 8 .  
         [0084]     First, in step  200 , whether RGB data is input from the outside (of the host computer or the like) or not is determined. If the determination is affirmative, the procedure proceeds to step  202  where an RGB-CMY converting process (generation of C (1) , M (1) , and Y (1)  data) is executed (refer to Table 1). If the determination in step  200  is negative, the routine is finished.  
         [0085]     When the converting process in step  202  is finished, the procedure advances to step  204  where the C (1) , M (1) , and Y (1)  data is read by the lightness acquiring component  102 . After that, the procedure advances to step  206 , and the lightness L* is acquired on the basis of the C (1) , M (1) , and Y (1)  data. Control for acquiring the lightness L* in step  206  (refer to  FIG. 9 ) will be described later.  
         [0086]     In step  208 , on the basis of the lightness L* acquired in step  206 , C, M, and Y gray values (Cgray, Mgray, and Ygray data) are acquired. Control for acquiring the C, M, and Y gray values (Cgray, Mgray, and Ygray data) (refer to  FIG. 10 ) in step  208  will be described later.  
         [0087]     In step  210 , the C (1) , M (1) , and Y (1)  data and the C, M, and Y gray values (Cgray, Mgray, and Ygray data) are read by the CMY correction value calculating component  114  for generating K data. After that, the procedure shifts to step  212  where C (2) , M (2) , and Y (2)  data necessary for generating K data is calculated using the equation (2) on the basis of the C (1) , M (1) , and Y (1)  data and the C, M, and Y gray values (Cgray, Mgray, and Ygray data).  
         [0088]     In step  214 , the C (2) , M (2) , and Y (2)  data is read by the K data generating component  116 . In step  216 , on the basis of the C (2) , M (2) , and Y (2)  data, K data is calculated by the equation (3). The procedure shifts to step  218  where the calculated K data is stored.  
         [0089]     In step  220 , the base eliminating component  118  reads the calculated K data and the C (1) , M (1) , and Y (1)  data. In step  222 , on the basis of the K data and the C (1) , M (1) , and Y (1)  data, C, M, and Y data is calculated by the equation (4). The procedure shifts to step  224  where the C, M, and Y data is stored.  
         [0090]     As a result, generation of the C, M, Y, and K data from the RGB data is completed. In step  226 , whether it is a time for output or not is determined. If the determination is affirmative, the procedure proceeds to step  228  where the stored C, M, Y, and K data is read and output to the optical scanner  12  (refer to  FIG. 1 ).  
         [0091]     A lightness acquisition control subroutine in step  206  (refer to  FIG. 8 ) will now be described with reference to the flowchart of  FIG. 9 .  
         [0092]     First, in step  250 , a variable “n” (the number of data) is set to 1. In step  252 , the n-th C (1) , M (1) , and Y (1)  data is transmitted to the lightness reading component  104 .  
         [0093]     In step  254 , on the basis of the n-th C (1) , M (1) , and Y (1)  data, the lightness reading component  104  extracts the lightness L* from the CMY-lightness characteristic map (refer to Table 2) stored in the CMY-lightness characteristic map memory  106 , and the procedure shifts to step  256 . In step  256 , the extracted lightness L* is sent back from the lightness reading component  104  to the lightness acquiring component  102 , and the procedure shifts to step  258 .  
         [0094]     In step  258 , whether the process has been finished for all data or not is determined. If the determination is negative, the procedure shifts to step  260  where the variable “n” is incremented. The procedure returns to step  252  and is repeated. If the determination in step  258  is affirmative, it is determined that acquisition of the lightness L* of all data has been completed and the procedure returns.  
         [0095]     Based on the flowchart of  FIG. 10 , the CMY gray value acquisition control subroutine in the step  208  (refer to  FIG. 8 ) will now be described.  
         [0096]     First, in step  270 , the variable “n” (the number of data) is set to 1. In step  272 , the n-th lightness L* is transmitted to the CMY gray value reading component  110 .  
         [0097]     In step  274 , on the basis of the n-th lightness L*, the CMY gray value reading component  110  extracts C, M, and Y gray values (Cgray, Mgray, and Ygray data) from the L-CMY characteristic map (refer to Table 3) stored in the L-CMY characteristic map memory  112 , and the procedure shifts to step  276 . In step  276 , the extracted C, M, and Y gray values (Cgray, Mgray, and Ygray data) are sent back from the CMY gray value reading component  110  to the CMY gray value acquiring component  108 , and the procedure shifts to step  278 .  
         [0098]     In step  278 , whether the process has been finished for all data or not is determined. If the determination is negative, the procedure shifts to step  280  where the variable “n” is incremented. The procedure returns to step  272  and is repeated. If the determination in step  278  is affirmative, it is determined that acquisition of the lightness L* of all data has been completed and the procedure returns.  
         [0099]     Although  FIGS. 9 and 10  show that data is sequentially processed by software, data may be continuously processed as in a so-called pipeline process.  
         [0100]     As described above, in the embodiment, considering that gray is not obtained even when amounts of C, M, and Y are equal to each other and the CMY data indicative of a gray value (CMY gray values) changes according to lightness, by calculating the C, M, and Y data (C (2) , M (2) , and Y (2)  data) for generating K data in consideration of unbalance of the C, M, and Y gray values depending on the characteristic of the engine part  10 A of the image forming apparatus  10  at the time of generating K data, optimum K data can be obtained.  
         [0101]     In the embodiment, in the case of acquiring the lightness L* from the C (1) , M (1) , and Y (1)  data subjected to the converting process on the basis of input RGB data, the table shown in Table 2 is generated and stored in the CMY-lightness characteristic map memory  106  in advance. Alternatively, the lightness L* may be obtained by an arithmetic expression (refer to Equation (1)) using the C (1) , M (1) , and Y (1)  data as main parameters. 
 
 L=Lmax−z (0.45 a×C   (1) +0.45 b×M   (1) +0.1 c×Y   (1) )  (1) 
 
         [0102]     The basis for Equation (1) is that, as shown in  FIG. 11 , when a data value gradually increases (to a maximum of 100%) from the origin (data value=0%) in the L*a*b* colorimetric system, the distance from the origin increases.  
         [0103]      FIG. 12  shows the relation between the distance from the origin and the L*axis as the z axis of  FIG. 11 . As shown in  FIG. 12 , as the distance from the origin increases, the more the lightness decreases. An amount of change of the Y color which is not conspicuous to human eyes is smaller than those of the other colors (C and M colors).  
         [0104]     It is understood from  FIG. 12  that the maximum value of lightness is generally 95% and a change in the lightness of the Y color is about 10% (=95−85) and, on the other hand, a change in the lightness of the C and M colors reaches about 45% (95−50). Since dependency on the lightness varies among the colors (C:M:Y=45%:45%: 10%=0.45:0.45:0.1), the values are used as amounts of dependency on lightness and are integrated together with factors a, b, and c depending on the characteristic of the image forming engine into the C (1) , M (1) , and Y (1)  data, respectively, thereby enabling the degree of influence exerted on the lightness of the C (1) , M (1) , and Y (1)  data to be obtained (the factor z denotes a correction factor depending on the characteristic of the image forming engine). By subtracting the degree of influence from the maximum value Lmax of the lightness, lightness L can be obtained.  
         [0105]     The invention described above is characterized in that the lightness acquiring component acquires lightness from a pre-stored table of lightness based on the C (1) , M (1) , and Y (1)  data depending on the characteristic of the image forming engine.  
         [0106]     The table is characterized in that the C (1) , M (1) , and Y (1)  data is divided into ranges, and lightness is set for each of divided ranges.  
         [0107]     Specifically, when the balance of all of the C (1) ·M (1) ·Y (1)  data is considered, the data amount becomes huge. Consequently, when each color data is expressed by numerical values of 0 to 100 and the numerical values are divided into about 6 levels (0, 20, 40, 60, 80, and 100), the number of combinations of levels becomes 6 3  (=216), and the data amount can be reduced.  
         [0108]     More specifically, it is sufficient to preliminarily determine lightness in each of ranges of 0≦CMY&lt;10, 10≦CMY&lt;30, 30≦CMY&lt;50, 50≦CMY&lt;70, 70≦CMY&lt;90, and 90≦CMY≦100 by using the dividing points as references, extract a range in which the data is actually included, and set the lightness in the range.  
         [0109]     The invention is also characterized in that the lightness acquiring component acquires lightness L by arithmetic operation of the following equation (1) when lightness as a reference of white color is set as Lmax and correction factors depending on the characteristic of the image forming engine are set as a, b, c, and z. 
 
 L=Lmax−z (0.45 a×C   (1) +0.45 b×M   (1) +0.1 c×Y   (1) )  (1) 
 
         [0110]     Although a table is pre-stored in the above example, alternatively, the lightness may be obtained by the arithmetic expression (Equation (1)) in which the factors depending on the characteristic of the image forming engine are preliminarily determined.  
         [0111]     The basis for the arithmetic expression is that, in the case where the origin is C=Y=M=0% in the L*a*b* colorimetric system, as a data value gradually increases (to the maximum of 100%), the distance from the origin of a point expressing L*a*b* at that time increases.  
         [0112]     The relation between the distance from the origin and the lightness L* is that, the more the distance from the origin increases, the more the lightness decreases. An amount of change of the Y color which is not conspicuous to human eyes is smaller than those of the other colors (C and M colors).  
         [0113]     The maximum value of lightness L* is generally 95%, and an amount of change of the lightness of Y color is about 10 points (=95−85), while a change in the lightness of the C and M colors is about 45 points (95−50). Since dependency on the lightness varies among the colors (C:M:Y=45%:45%:10%=0.45:0.45:0.1), the values are used as amounts of dependency on the lightness and are integrated together with factors a, b, and c depending on the characteristic of the image forming engine into the C (1) ·M (1) ·Y (1)  data, respectively, thereby enabling the degree of influence exerted on the lightness of the C (1) ·M (1) ·Y (1)  data to be obtained (the factor z denotes a correction factor depending on the characteristic of the image forming engine). By subtracting the degree of influence from the maximum value Lmax of the lightness, lightness L can be obtained.
The present invention provides an image processing apparatus on which an image forming engine for forming a full-color image by using color materials corresponding to data of color tones on the basis of C (cyan), M (magenta), Y (yellow), and K (black) data is mounted. The image processing apparatus includes: a color converting component for converting R (red), G (green), and B (blue) data input from the outside to C (1), M (1), and Y (1) data which depends on a characteristic of the image forming engine; a lightness acquiring component for acquiring lightness from the C (1), M (1), and Y (1) data obtained by the conversion of the color converting component; a memory for storing a lightness-gray value characteristic table; a gray value reading component; a first arithmetic component for obtaining C (2), M (2), and Y (2) data; a K data generating component; and a second arithmetic component for calculating C, M, and Y.