Patent Publication Number: US-2013250362-A1

Title: Image processing apparatus and quantization method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to image processing of quantizing color material amount data. 
     2. Description of the Related Art 
     The following technique is well known as a technique for achieving excellent color development by enlarging the color gamut (color reproduction range) of an inkjet printer. 
     A technique disclosed in Japanese Patent Laid-Open No. 6-233126 (literature 1) uses particular color inks such as red (R), green (G), and blue (B) inks in addition to basic color inks including cyan (C), magenta (M), yellow (Y), and black (K) inks. This technique enlarges the color gamut of a red region by, for example, adding the R ink for reproducing a red color with chroma higher than that of a red color obtained by overlap of an M dot and a Y dot. 
     A technique disclosed in Japanese Patent Laid-Open No. 2004-155181 (literature 2) sets an appropriate print order according to an input color signal. Assume that a Y dot and C dot overlap each other. In this case, color development when a C dot overlaps a Y dot (YC order) is different from that when a Y dot overlaps a C dot (CY order). Therefore, a color which can be reproduced only in the YC order is printed in the YC order, and a color which can be reproduced only in the CY order is printed in the CY order, thereby enlarging the color gamut as compared with a fixed print order. 
     A technique disclosed in Japanese Patent Laid-Open No. 2005-088579 (literature 3) controls a dot arrangement so that a particular color dot and basic color dot do not overlap each other as much as possible, because the color development of a particular color ink suffers if a particular color dot and another color dot overlap each other. For example, to associate, with a predetermined 2×4 dot arrangement pattern, color material amount data which corresponds to the use amount of a print material and which has been quantized to nine values, and convert the data into binary data indicating whether to print a dot, a dot arrangement pattern different from that for another color is prepared for a particular color. This decreases the probability that a dot of another color overlaps that of a particular color to achieve a sufficiently good color development of the particular color ink, thereby enlarging the color gamut. 
     The above-described techniques have the following problems. The techniques described in literatures 1 and 3 require inks in addition to the basic color inks. The number of inks increases, and thus the printer structure is complicated and becomes large in size. 
     The technique described in literature 2 can enlarge, for example, a middle brightness region from yellow through green to cyan. The technique, however, cannot enlarge a low brightness region from yellow to black. Enlargement of the color gamut of the low brightness region is an issue for a printer using pigment inks which have been often used in recent years. 
     SUMMARY OF THE INVENTION 
     In one aspect, an image processing apparatus comprising: a quantization unit configured to quantize color material amount data of a black color, and quantize color material amount data of a chromatic color, wherein the quantization unit quantizes color material amount data of a first chromatic color so that a phase of a low spatial frequency component of black color quantization data obtained by quantizing the color material amount data of the black color is opposite to a phase of a low spatial frequency component of first quantization data obtained by quantizing the color material amount data of the first chromatic color, and a high spatial frequency component of the black color quantization data has no correlation with a high spatial frequency component of the first quantization data. 
     According to the aspect, it is possible to enlarge the color gamut of a low brightness region in a print image, thereby achieving excellent color development. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram for explaining the arrangement of an image processing apparatus and printing apparatus according to an embodiment. 
         FIG. 2  is a flowchart for explaining image processing executed by the image processing apparatus. 
         FIG. 3  is a block diagram showing the arrangement of an HT processing unit. 
         FIG. 4  is a flowchart for explaining HT processing for black. 
         FIGS. 5A and 5B  are views showing an example of an error diffusion matrix and cumulative error storage areas. 
         FIG. 6  is a block diagram showing the arrangement of a restrictive information calculation unit for calculation of black restrictive information. 
         FIG. 7  is a flowchart for explaining calculation of black restrictive information Kr. 
         FIG. 8  is a flowchart for explaining HT processing for chromatic colors. 
         FIG. 9  is a block diagram showing the arrangement of the restrictive information calculation unit for calculation of chromatic color restrictive information. 
         FIG. 10  is a flowchart for explaining calculation of chromatic color restrictive information KC1r. 
         FIGS. 11A and 11B  are views each showing a dot arrangement obtained by ideal printing. 
         FIGS. 12A ,  12 B,  13 A, and  13 B are views each showing a dot arrangement when misregistration has occurred. 
         FIGS. 14A and 14B  are views showing the arrangement of an HT processing unit according to the second embodiment. 
         FIG. 15  is a block diagram showing the arrangement of a restrictive information calculation unit for calculation of black restrictive information according to the third embodiment. 
         FIG. 16  is a flowchart for explaining calculation of black restrictive information Kr according to the third embodiment. 
         FIGS. 17A to 17D  are views each showing an example of a filter. 
         FIG. 18  is a block diagram showing the arrangement of the restrictive information calculation unit for calculation of chromatic color restrictive information according to the third embodiment. 
         FIG. 19  is a flowchart for explaining calculation of chromatic color restrictive information KC1r according to the third embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Image processing according to an embodiment of the present invention will be described in detail below with reference to the accompanying drawings. 
     First Embodiment 
     [Apparatus Arrangement] 
     The arrangement of an image processing apparatus and printing apparatus according to the embodiment will be described with reference to a block diagram shown in  FIG. 1 . 
     An image processing apparatus  11  is implemented by, for example, installing a printer driver on a general personal computer (PC). That is, the function of each unit (to be described later) of the image processing apparatus  11  is implemented when the microprocessor (CPU) of the PC uses a random access memory (RAM) as a work memory to execute the program of the printer driver. Note that a printer  12  can include the image processing apparatus  11  by providing, in the printer  12 , a one-chip microcontroller in which a program for executing the processing of each unit (to be described later) of the image processing apparatus  11  is embedded. 
     Image Processing Apparatus 
     An input image buffer  102  of the image processing apparatus  11  stores input image data to be printed. A color separation unit  103  refers to a color separation lookup table (LUT)  104  to perform color separation for input image data as RGB image data to obtain color material amount data (C, M, Y, and K data) corresponding to the ink colors of the printer  12 . The color material amount data are stored in a color separation image buffer  105 . 
     Based on values stored in a restrictive information buffer  107 , a halftone (HT) processing unit  106  performs halftone processing (HT processing) for the color material amount data (each color has multiple tones such as three or more tones) stored in the color separation image buffer  105  to obtain binary color material amount data for each color. The binary color material amount data are stored in an HT image buffer  108 . 
     The binary color material amount data stored in the HT image buffer  108  are input to the printer  12  via a serial bus  110  such as USB (Universal Serial Bus) for connecting the image processing apparatus  11  with the printer  12 . 
     A restrictive information calculation unit  109  creates restrictive information by predetermined calculation based on the binary color material amount data stored in the HT image buffer  108  and the multi-valued color material amount data stored in the color separation image buffer  105 , details of which will be described later. The unit  109  then updates the restrictive information buffer  107  with the created restrictive information. 
     The restrictive information buffer  107  stores a value (restrictive information) indicating whether a dot at a position on an image to be printed is easily made ON (a dot is easily formed). The restrictive information buffer  107  is prepared for each combination of black (black color) and a chromatic color. If there are one black color and N chromatic colors, NB restrictive information buffers  107  are prepared. Note that NB is given by: 
         NB=N ×( N− 1)/2+1  (1)
 
     Printer 
     The printer  12  is a printing apparatus adopting, for example, a thermal transfer or inkjet method, and moves a printhead  201  in the vertical and horizontal directions with respect to a print medium  202  to form, on the print medium  202 , an image indicated by the binary color material amount data input by the image processing apparatus  11  for each band. Note that the printhead  201  has one or more printing elements (nozzles in the inkjet method). Relative movement in the vertical and horizontal directions is implemented when a head control unit  204  controls a movement unit  203  to move the printhead  201 , and controls a conveyance unit  205  to convey the print medium  202 . 
     A pass separation unit  207  separates the binary color material amount data for each color input by the image processing apparatus  11  according to multi-pass printing. An ink color selection unit  206  selects an ink color corresponding to the color material amount data input by the pass separation unit  207 , from among the ink colors of the printhead  201 . Note that although the printhead  201  includes four color (process color) inks, that is, cyan (C), magenta (M), yellow (Y), and black (K) inks in the following example, the combination of colors is not limited to this. 
     [Image Processing] 
     Image processing executed by the image processing apparatus  11  will be described with reference to a flowchart shown in  FIG. 2 . 
     When image data is input, the image processing apparatus  11  stores the input image data in the input image buffer  102  (S 101 ). Note that the input image data is RGB image data including 8 bits for each of colors R, G, and B. 
     The color separation unit  103  of the image processing apparatus  11  performs color separation for the image data stored in the input image buffer  102  to obtain color material amount data (S 102 ). The color material amount data are stored in the color separation image buffer  105 . The color separation unit  103  uses a well-known technique to convert the input RGB image data into C, M, Y, and K color material amount data. 
         C= 3DLUT C ( R,G,B ); 
         M =3DLUT M ( R,G,B ); 
         Y= 3DLUT Y ( R,G,B ); 
         K =3DLUT K ( R,G,B );  (2)
 
     Note that 3DLUT X  indicates a three-dimensional LUT for generating color material amount data for the color X, which is included in the color separation LUT  104 . 
     The color material amount data is 8-bit image data for each of the colors C, M, Y, and K. The color material amount data, however, need only be multi-tone data, and the number of tones is not limited. As described above, since the printhead  201  includes the four inks, the input image data is converted into image data of four planes of C, M, Y, and K. If the printhead  201  includes inks, the number of which is larger than four, the input image data need only undergo color separation to obtain color material amount data corresponding to the number of inks. 
     The HT processing unit  106  of the image processing apparatus  11  executes HT processing for black to convert the sum of color material amount data K stored in the color separation image buffer  105  and the value stored in the restrictive information buffer  107  into binary data (S 103 ). The binary color material amount data K (to be referred to as color material amount data K′) having undergone the HT processing is stored in the HT image buffer  108 . Note that the HT processing unit  106  quantizes the multi-valued color material amount data to the binary color material amount data using, for example, an error diffusion method or minimized average error method. Note that the result of quantization of the color material amount data will hereinafter sometimes be referred to as “quantization color material amount data.” 
     As described above, the restrictive information buffer  107  stores restrictive information indicating whether a dot is easily formed at a position on an image to be printed. The restrictive information is updated as the HT image is updated. Note that at the start of the processing, zero is set as an initial value in the restrictive information buffer  107 . There are the following four kinds of restrictive information at a position (X, Y): 
     Kr(X, Y): restrictive information based on an HT image of black; 
     KC1r(X, Y), KC2r(X, Y): restrictive information based on overlap of HT images of black and one chromatic color; and 
     KC1C2r(X, Y): restrictive information based on overlap of black and two chromatic colors. 
     Note that the initial values are 
         Kr ( X,Y )=0, 
         KC 1 r ( X,Y )=0, 
         KC 2 r ( X,Y )=0, 
       and 
         KC 1 C 2 r ( X,Y )=0. 
     Note also that the pieces of restrictive information may be represented by Kr, KC1r, KC2r, and KC1C2r without explicitly indicating the position (X, Y). 
     The update operation of the restrictive information will be described later. An average of values stored in the restrictive information buffer  107  is zero, a value at a position where a dot is easily formed is positive, and a value at a position where a dot is hardly formed is negative. 
     The image processing apparatus  11  outputs, for each band, the color material amount data K′ stored in the HT image buffer  108  to the printer  12  (S 104 ). The restrictive information calculation unit  109  calculates restrictive information Kr based on the arrangement of K dots (S 105 ), and updates the value stored in the restrictive information buffer  107  with the restrictive information Kr (S 106 ). 
     The HT processing unit  106  of the image processing apparatus  11  performs HT processing for a chromatic color to convert the sum of the color material amount data C, M, or Y stored in the color separation image buffer  105  and the value stored in the restrictive information buffer  107  into binary data (S 107 ). The binary color material amount data C, M, or Y (to be referred to as color material amount data C′, M′, or Y′) having undergone the HT processing is stored in the HT image buffer  108 . Note that the HT processing unit  106  quantizes the multi-valued color material amount data to the binary color material amount data using, for example, an error diffusion method or minimized average error method. 
     The image processing apparatus  11  outputs, for each band, the color material amount data C′, M′, and Y′ stored in the HT image buffer  108  to the printer (S 108 ). The image processing apparatus  11  repeats the above processing for each band, and outputs color material amount data corresponding to the input image data to the printer  12 . 
     HT Processing Unit for Black 
       FIG. 3  is a block diagram showing the arrangement of the HT processing unit  106 . The HT processing for black will be described with reference to a flowchart shown in  FIG. 4 . 
     The HT processing unit  106  receives color material amount data K(x) of a pixel of interest (S 401 ), and causes an addition unit  301  to add the restrictive information to the color material amount data K(x) (S 402 ). Note that since the initial value of the restrictive information is zero, no restrictive information is substantially added in the HT processing for black. 
     The HT processing unit  106  causes a cumulative error addition unit  303  to add a cumulative error in error diffusion processing to the color material amount data K(x) (S 403 ), and causes a threshold setting unit  304  to set a quantization threshold Th (S 404 ). Note that the quantization threshold Th is set to 128 or the like. To avoid a dot generation delay, however, the threshold Th may be changed according to the color material amount data K(x) so that an average quantization error becomes small, as represented by: 
         Th ( x )= f ( K ( x ))  (3)
 
     As an example of the function f in equation (3), for example, Japanese Patent Laid-Open No. 2002-374412 (literature 4) proposes: 
         Th ( x )={ K ( x )×( N− 1)+128 }/N   (4)
 
     where N is a natural number of 2 or larger. 
     After that, a quantization unit  305  decides binary color material amount data K(x)′ of the pixel of interest according to expression (5) (S 405 ). The color material amount data K(x)′ is stored in the HT image buffer  108 . 
       if ( Kd&lt;Th ) 
         K ( x )′=0;
 
       if ( Kd≧Th ) 
         K ( x )′=255;  (5)
 
     where Kd represents the color material amount data K(x) having undergone the error addition operation. 
     The HT processing unit  106  causes an error calculation unit  306  to calculate an error Er(x) between the color material amount data Kd having undergone the error addition operation and the binary color material amount data K(x)′ (S 406 ) according to: 
         Er ( x )= Kd−K ( x )′  (6)
 
     The HT processing unit  106  causes an error diffusion unit  307  to diffuse the error Er(x) (S 407 ), and quantization of one pixel of black thus ends. A determination is made in step S 408  to repeat the processing in steps S 401  to S 407  until all the pixels of the color material amount data K stored in the color separation image buffer  105  are processed. 
       FIGS. 5A and 5B  show an example of an error diffusion matrix and cumulative error storage areas. The error diffusion matrix shown in  FIG. 5A  has a coefficient K 1  indicating an error diffused to a neighboring pixel in the main scanning direction with respect to a pixel D of interest, and coefficients K 2 , K 3 , and K 4  respectively indicating errors diffused to three neighboring pixels in the next row in the sub-scanning direction with respect to the pixel of interest. The values of the error diffusion coefficients are, for example, K 1 =7/16, K 2 =3/16, K 3 =5/16, and K 4 =1/16. The error diffusion coefficients need not be fixed and may be changed according to the tone of an image, as a matter of course. Furthermore, the number of pixels to which the error is diffused is not limited to four, and the error may be diffused to a larger number of pixels. 
     A cumulative error buffer  302  accumulates, for each pixel, errors diffused by the error diffusion unit  307 .  FIG. 5B  shows the storage areas of the cumulative error buffer  302 . The cumulative error buffer  302  has a storage area E 0 , and storage areas E(x) (1≦X≦W) the number of which is equal to the number W of pixels in the main scanning direction (horizontal direction) of the input image data. The error diffusion unit  307  stores the cumulative error in the cumulative error buffer  302  according to: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 if (x = 1) 
                   
               
               
                   
                   E(x) = E0 + Er(x) × 8/16; 
               
               
                   
                 if (1 &lt; x) 
               
               
                   
                   E(x − 1) = E(x − 1) + Er(x) × 3/16; 
               
               
                   
                 if (1 &lt; x &lt; W) 
               
               
                   
                   E(x) = E0 + Er(x) × 5/16; 
               
               
                   
                 if (x &lt; W) { 
               
               
                   
                   E(x + 1) = E(x + 1) + Er(x) × 7/16; 
               
               
                   
                   E0 = Er(x) × 1/16; 
               
               
                   
                 }; 
               
               
                   
                 if (x = W) { 
               
               
                   
                   E(x) = E0 + Er(x) × 13/16; 
               
               
                   
                   E0 = 0; 
               
               
                   
                 }; 
                 ...(7) 
               
               
                   
                   
               
            
           
         
       
     
     The cumulative error addition unit  303  adds, to the color material amount data K(x), a cumulative error stored in a storage area E(x) corresponding to a position x of the pixel of interest, as represented by: 
         Kd=K ( x )+ E ( x )  (8)
 
     The HT processing for black decides dot positions formed by a black color material, that is, the quantization color material amount data K′ representing the ON/OFF pattern (dot arrangement) of K dots. 
     Calculation of Black Restrictive Information 
       FIG. 6  is a block diagram showing the arrangement of the restrictive information calculation unit  109  for calculation of black restrictive information. Calculation of the black restrictive information Kr will be described with reference to a flowchart shown in  FIG. 7 . 
     A color separation image data filter  601  of the restrictive information calculation unit  109  executes, for the color material amount data K stored in the color separation image buffer  105 , filter processing represented by: 
         Kf=K*Fm   (9)
 
     where Fm represents a filter and represents convolution (S 701 ). 
     An example of the filter Fm shown in  FIG. 6  is an isotropic weighted average filter which has a size of 3×3 and in which coefficients are concentrically arranged. The filter Fm, however, is not limited to this. The size may be 5×5, 7×7, 9×9, 3×5, 5×7, or 5×9, and a nonisotropic filter in which filter coefficients are elliptically arranged may be used. Note that the filter Fm desirably has low-pass characteristics. 
     An HT data filter  602  performs, for the color material amount data K′ stored in the HT image buffer  108 , low-pass filter processing represented by: 
         K′   LPF   =K ′*LPF B   (10)
 
     where LPF B  indicates a low-pass filter (S 702 ). 
     The low-pass filter LPF B  shown in  FIG. 6  has the same arrangement as that of the filter Fm. As long as a filter has low-pass characteristics, the present invention is not limited to the arrangement (size and filter coefficients) shown in  FIG. 6 . 
     An addition unit  603  sets, as first restrictive information Kr, a difference value obtained by subtracting the color material amount data K′ LPF  having undergone the low-pass filter processing from the color material amount data Kf having undergone the filter processing (S 703 ) by: 
         Kr=Kf−K′   LPF   (11)
 
     The restrictive information calculation unit  109  updates the restrictive information buffer  107  with the restrictive information Kr (S 704 ). 
     By subtracting the average value of the color material amount data K′ from the average value of the color material amount data K, it is possible to obtain the restrictive information Kr including a small positive or negative value for a K dot ON region, and a large positive value for a K dot OFF region. If HT processing is executed after adding such restrictive information Kr to the color material amount data of a chromatic color, it is possible to control the HT processing so that chromatic color ON dots are hardly arranged in the K dot ON region and chromatic color ON dots are easily arranged in the K dot OFF region. Note that if addition of the color material amount data and the restrictive information causes the color material amount data to fall outside the range (for example, from 0 to 255), the data need only be set to fall within the range (for example, 0 or 255). 
     HT Processing for Chromatic Color 
     HT processing for chromatic colors will be described with reference to a flowchart shown in  FIG. 8 . 
     The HT processing unit  106  selects a chromatic color to undergo the HT processing (S 901 ). The selection order may be the order from a noticeable color to an unnoticeable color, or the descending order of the use amount of color. Assume, in this example, that the unit  106  selects cyan (C) as a first chromatic color. 
     The HT processing unit  106  receives color material amount data C(x) of a pixel of interest and corresponding restrictive information Kr(x) (S 902 ). The addition unit  301  calculates the sum of the color material amount data C(x) and the restrictive information Kr(x) (S 903 ) by: 
         C ( x )= C ( x )+ h   c1   Kr ( x )  (12)
 
     where h c1  is a real number. 
     The HT processing unit  106  causes the cumulative error addition unit  303  to add a cumulative error in error diffusion processing to the color material amount data C(x) (S 904 ), and causes the threshold setting unit  304  to set the quantization threshold Th (S 905 ). The quantization unit  305  then decides binary color material amount data C(x)′ of the pixel of interest according to expression (13) (S 906 ). The color material amount data C(x)′ is stored in the HT image buffer  108 . 
       if ( Cd&lt;Th ) 
         C ( x )′=0;
 
       if ( Cd≧Th ) 
         C ( x )′=255;  (13)
 
     where Cd represents the color material amount data C(x) having undergone the error addition operation. 
     The HT processing unit  106  causes the error calculation unit  306  to calculate an error Er(x) between the color material amount data Cd having undergone the error addition operation and the binary color material amount data C(x)′ (S 907 ) by: 
         Er ( x )= Cd−C ( x )′  (14)
 
     The HT processing unit  106  causes the error diffusion unit  307  to diffuse the error Er(x) (S 908 ), and quantization of one pixel of cyan thus ends. A determination is made in step S 909  to repeat the processing in steps S 902  to S 908  until all the pixels of the color material amount data C stored in the color separation image buffer  105  are processed. 
     The HT processing for cyan decides dot positions formed by a cyan color material, that is, quantization color material amount data C′ representing the ON/OFF pattern (dot arrangement) of C dots. 
     The HT processing unit  106  determines whether the HT processing is complete for all the chromatic colors (S 910 ). If the HT processing is complete for all the chromatic colors, the HT processing is terminated; otherwise, the restrictive information calculation unit  109  calculates restrictive information. 
     The restrictive information calculation unit  109  calculates restrictive information KC1r of the chromatic color based on the dot arrangement of the chromatic color (S 911 ). The restrictive information KC1r is used to make the low spatial frequency component of a paper white portion in the dot arrangement of K and the chromatic color (C in this example) be in phase with that of the dot arrangement of a chromatic color to undergo the HT processing next, details of which will be described later. Performing the HT processing for the next chromatic color with reference to the restrictive information KC1r can prevent paper white from occurring as much as possible. 
     The restrictive information calculation unit  109  updates the restrictive information buffer  107  with the restrictive information (S 912 ). The updated restrictive information (KC1r in this example) is referred to as information for deciding the dot arrangement of the next chromatic color. 
     The HT processing unit  106  returns the process to step S 901  to execute the HT processing for the next chromatic color. Before that, calculation of the restrictive information KC1r will be described. 
     Calculation of Chromatic Color Restrictive Information KC1r 
       FIG. 9  is a block diagram showing the arrangement of the restrictive information calculation unit  109  for calculation of the chromatic color restrictive information. Calculation (S 911 ) of the chromatic color restrictive information KC1r will be described with reference to a flowchart shown in  FIG. 10 . Note that the HT processing for the chromatic colors starts from cyan. 
     A multiplication unit  1001  performs multiplication processing for the color material amount data K′ and C′ stored in the HT image buffer  108  (S 1101 ). That is, the unit  1001  calculates the product KC (to be referred to as an HT product hereinafter) of the color material amount data C′ of cyan and the color material amount data K′ of black by: 
         KC= 255−(255 −C ′)×(255 −K ′)/255
 
       or  KC =Max ( C′,K ′)  (15)
 
     The above equation is used when 0 indicates dot OFF and 255 indicates dot ON. To the contrary, if 0 indicates dot ON and 255 indicates dot OFF, equation (15′) is used. 
         KC=C′×K′/ 255 
       or  KC =Min( C′,K ′)  (15′)
 
     An HT product average processing unit  1002  calculates the partial average of the HT product KC (S 1102 ). In calculation of the black restrictive information, filter processing is executed for the color material amount data K having undergone black color separation. It is not appropriate to perform filter processing for the product of the color material amount data K and C having undergone black and cyan color separation, respectively, or to perform multiplication for the color material amount data K and C after filter processing. This is because the decision of the dot arrangement of cyan depends on the dot arrangement of black, as follows. 
     Because of the maintainability of the density in the HT processing, the relationship of the density before and after the HT processing is represented by: 
         E[K′]=E[K ] and  E[C′]=E[C]   (16)
 
     Since the dot arrangement represented by K′ and that represented by C′ are exclusively decided, expression (17) presented below holds. 
         E[K′×C′]≠E[K′]×E[C′]=E[K]×E[C]   (17)
 
     As represented by expression (17), the density after the HT processing cannot be obtained based on data before the HT processing, and needs to be obtained based on the HT product. Note that this processing is executed to prevent a change in color by correction processing, and an average not for the whole image but for part of the image is obtained. For example, average filter processing represented by equation (18) is executed. 
         KCm=KC *LPF m   (18)
 
     An HT product filter  1003  performs filter processing for the HT product KC using the low-pass filter LPF B  (S 1103 ) by: 
         KC   LPF   =KC *LPF B   (19)
 
     For each pixel, an addition unit  1004  sets, as second restrictive information KC1r, a difference value obtained by subtracting the value calculated by the HT product filter  1003  from the partial average calculated by the HT product average processing unit  1002  (S 1104 ) by: 
         KC 1 r=KCm−KC   LPF   (20)
 
     The filters LPFm and LPF B  shown in  FIG. 9  are merely examples, and other filters may be used. 
     Note that the filter LPFm is used to obtain the average of the HT product for a partial region (a 5×5 pixel region in the example shown in  FIG. 9 ), and needs to be different from the filter LPF B  functioning as the low-pass filter of a pixel of interest. 
     Although  FIG. 9  and equation (19) show a case in which the same filter LPF B  as that used to calculate the black restrictive information is used, another filter may be used. Note that the filter desirably has low-pass characteristics. The processing executed by the HT product average processing unit  1002  is not limited to the filter processing as long as it is possible to obtain the average of the HT product for a partial region. By subtracting the filter of the HT product filter  1003  from that filter, the HT product average processing unit  1002  can execute filter processing including the processing in steps S 1102  to S 1104 . 
     By subtracting the value obtained by performing the filter processing for the HT product using the filter LPF B  from the partial average of the HT product obtained using the filter LPFm, it is possible to obtain restrictive information KC1r including a small positive or negative value for a region where the HT product is 255, and a large positive value for a region where the HT product is 0. If such restrictive information KC1r is added to the color material amount data of a chromatic color, and then HT processing is executed, it is possible to control the HT processing so that chromatic color ON dots are hardly arranged in the region where the HT product is 255 and chromatic color ON dots are easily arranged in the region where the HT product is 0. 
     HT Processing for Chromatic Color (Second Color) 
     Upon completion of the HT processing for cyan and calculation of the restrictive information KC1r, the HT processing unit  106  decides a chromatic color to undergo the HT processing next (S 901 ). Assume, in this example, that the unit  106  selects magenta (M) as a second chromatic color. 
     The HT processing unit  106  receives color material amount data M(x) of a pixel of interest and corresponding pieces Kr(x) and KC1r(x) of restrictive information (S 902 ). The addition unit  301  calculates the sum of the color material amount data M(x) and the pieces Kr(x) and KC1r(x) of restrictive information (S 903 ). For the second color, in addition to the black restrictive condition Kr, the restrictive information KC1r calculated for black and the first chromatic color (cyan in this example) is also added. 
         M ( x )= M ( x )+ h   m1   Kr ( x )+ h   m2   KC 1 r ( x )  (21)
 
     where h m1  and h m2  are real numbers. 
     The processing in steps S 904  to S 909  is the same as that for the first color, and a detailed description thereof will be omitted. In this example, in step S 910 , the HT processing is complete for cyan and magenta but is not complete for yellow. The restrictive information calculation unit  109 , therefore, calculates restrictive information. 
     The restrictive information calculation unit  109  calculates pieces KC2r and KC1C2r of chromatic color restrictive information based on the dot arrangement of the chromatic color (S 911 ). The restrictive information KC2r is used to make the low spatial frequency component of a paper white portion in the dot arrangement of K and the chromatic color (M in this example) be in phase with that of the dot arrangement of a chromatic color to undergo the HT processing next, details of which will be described later. Furthermore, the restrictive information KC1C2r is used to make the low spatial frequency component of a paper white portion in the dot arrangement of K and the chromatic colors (C and M in this example) be in phase with that of the chromatic color to undergo the HT processing next. Performing the HT processing for the next chromatic color with reference to the pieces KC2r and KC1C2r of restrictive information prevents paper white from occurring as much as possible. 
     The restrictive information calculation unit  109  updates the restrictive information buffer  107  with the restrictive information (S 912 ). The updated pieces (KC2r and KC1C2r in this example) of restrictive information are referred to as information for deciding the dot arrangement of the next chromatic color. 
     The HT processing unit  106  returns the process to step S 901  to execute the HT processing for the next chromatic color. Before that, calculation of the restrictive information KC1C2r will be described. Note that the restrictive information KC2r is calculated by the same processing as that for the restrictive information KC1r, and a description thereof will be omitted. 
     Calculation of Chromatic Color Restrictive Information KC1C2r 
     Assume that the HT processing has been executed for cyan and magenta in the order named. 
     The multiplication unit  1001  shown in  FIG. 9  performs multiplication processing for color material amount data K′, C′, and M′ stored in the HT image buffer  108  (S 1101 ). That is, the unit  1001  calculates the product KCM (HT product) of the color material amount data M′ of magenta, the color material amount data C′ of cyan, and the color material amount data K′ of black by: 
         KCM= 255−(255− M ′)×(255 −C ′)×(255 −K ′)/2552
 
       or 
         KC =Max( M′,C′,K ′)  (22)
 
     The above equation is used when 0 indicates dot OFF and 255 indicates dot ON. To the contrary, if 0 indicates dot ON and 255 indicates dot OFF, equation (22′) presented below is used. 
         KC=M′×C′×K′/ 255 2    
       or 
         KC =Min( M′,C′,K ′)  (22′)
 
     The HT product average processing unit  1002  calculates the partial average of the HT product KCM (S 1102 ) by: 
         KCMm=KCM* LPF m   (23)
 
     The HT product filter  1003  performs filter processing for the HT product KCM using the low-pass filter LPF B  (S 1103 ) by: 
         KCM   LPF   =KCM *LPF B   (24)
 
     For each pixel, the addition unit  1004  subtracts the value calculated by the HT product filter  1003  from the partial average calculated by the HT product average processing unit  1002 , and sets the subtraction result as the restrictive information KC1C2r (S 1104 ) by: 
         KC 1 C 2 r=KCMm−KCM   LPF   (25)
 
     HT Processing for Chromatic Color (Third Color) 
     Upon completion of the HT processing for cyan and calculation of the pieces KC2r and KC1C2r of restrictive information, the HT processing unit  106  decides a chromatic color to undergo the HT processing next (S 901 ). Assume, in this example, that the unit  106  selects yellow (Y). 
     The HT processing unit  106  receives color material amount data Y(x) of a pixel of interest and corresponding pieces Kr(x), KC1r(x), KC2r(x), and KC1C2r(x) of restrictive information (S 902 ). The addition unit  301  calculates the sum of the color material amount data Y(x) and the pieces Kr(x), KC1r(x), KC2r(x), and KC1C2r(x) of restrictive information (S 903 ) by: 
         Y ( x )= Y ( x )+ h   y1   Kr ( x )+ h   y2   KC 1 r+h   y3 KC2 r+h   y4   KC 1 C 2 r   (26)
 
     where h y1 , h y2 , h y3 , and h y4  are real numbers. 
     The processing in steps S 904  to S 909  is the same as that for the first and second colors, and a detailed description thereof will be omitted. The HT processing unit  106  determines in step S 910  that the HT processing is complete for all the chromatic colors, and terminates the HT processing. 
     [Effects of Enlargement of Color Gamut] 
     The above-described processing forms the dot pattern of black and that of a chromatic color so that the phases of the low spatial frequency components of the patterns are opposite to each other. Furthermore, the dot patterns of chromatic colors are formed so as to share the phase of the low spatial frequency component (in phase). With this operation, it is possible to exclusively arrange the black dots and the chromatic color dots from each other, and reproduce a color with higher chroma in a low brightness region, thereby enlarging the color gamut. 
     Furthermore, since the chromatic color dots are arranged so that the low spatial frequency component of a paper white portion in the dot arrangement of the chromatic color and black is in phase with that of a chromatic color to be printed next, it is possible to reduce exposure of paper white. As a result, it is possible to suppress a decrease in density and chroma due to exposure of paper white in a low brightness region. 
     [Effects of Resistance to Misregistration] 
     With the above-described processing, that is, the exclusive arrangement of black dots and chromatic color dots and arrangement of chromatic color dots in a paper white portion, it is possible to obtain the effects of resistance to misregistration. The effects of resistance to misregistration will be described.  FIGS. 11A ,  12 A, and  13 A each show a dot arrangement according to the embodiment.  FIGS. 11B ,  12 B, and  13 B each show a dot arrangement by another exclusive arrangement processing proposed by the applicant. 
     Assume that one side of each cell shown in each view is about 20 μm (corresponding to 1200 dpi). In the dot arrangements, W within a cell indicates a paper white portion, C/K indicates overlap of a cyan or magenta dot on a black dot, Y/K indicates overlap of a yellow dot on a black dot, and Y/C/K indicates overlap of three kinds of dots. Note that although each view shows paper white between dots diagonally adjacent to each other for the purpose of clarity, there is actually no paper white between dots diagonally adjacent to each other. 
       FIGS. 11A and 11B  each show a dot arrangement obtained by ideal printing. In either the dot arrangement according to the embodiment or the dot arrangement by the other exclusive arrangement processing, no paper white portion W appears. Note that in the dot arrangement ( FIG. 11B ) by the other exclusive arrangement processing, the overlapping area of black dots and chromatic color dots is smaller (nine cells in  FIG. 11A  and eight cells in  FIG. 11B ), and the color gamut of a low brightness region is wider. 
     In actual printing, the positions of dots shift due to variations, thereby causing misregistration.  FIGS. 12A and 12B  each show a dot arrangement when misregistration has occurred.  FIGS. 12A and 12B  each show a case in which the black dots have been shifted downward by 20 μm (one cell). 
     In either the dot arrangement ( FIG. 12A ) according to the embodiment or the dot arrangement ( FIG. 12B ) by the other exclusive arrangement processing, a paper white portion appears. The area of the paper white portion in  FIG. 12A  is decreased to half the area in  FIG. 12B  (two cells in  FIG. 12A  and four cells in  FIG. 12B ). This effect is obtained in this embodiment by arranging chromatic color dots so that the low spatial frequency component of the paper white portion is in phase with that of the chromatic color dot. 
       FIGS. 13A and 13B  each show a dot arrangement when misregistration has occurred.  FIGS. 13A and 13B  each show a case in which the black dots have been shifted downward by 20 μm (one cell) and the yellow dots have been shifted leftward by 20 μm (one cell). The area of the paper white portion in the dot arrangement ( FIG. 13A ) according to the embodiment is decreased to half the area in  FIG. 13B  (two cells in  FIG. 13A  and four cells in  FIG. 13B ). Moreover, the overlapping area of the black dots and chromatic color dots in  FIG. 13A  is slightly smaller than that in  FIG. 13B  ( 10  cells in  FIGS. 13A and 11  cells in  FIG. 13B ). This effect is obtained by exclusively arranging the black dots and chromatic color dots from each other, and making the chromatic color dots share the same low spatial frequency component. 
     As described above, in actual printing, a combination of the states shown in the above-described views appears in a print region. It is apparent based on the change amount of the rate of occurrence of a paper white portion and a change amount of the overlapping area of the black dots and chromatic color dots that a change in color before and after registration in the other exclusive arrangement processing is larger than that in the embodiment. That is, the processing of the embodiment reduces the color unevenness as compared with the other exclusive arrangement processing. 
     As described above, it is possible to enlarge the color gamut of a low brightness region, thereby achieving excellent color development and high tonality even if variations that may cause misregistration occur. 
     Modification of First Embodiment 
     Exclusive arrangement processing has been described above in which addition of restrictive information and color material amount data after color separation makes the phase of the low frequency component of the dot arrangement of black opposite to that of the low frequency component of the dot arrangement of a chromatic color, and the high frequency components of the dot arrangements of different colors have no correlation (the phase of the high frequency component of the dot arrangement of black has no correlation with that of the high frequency component of the dot arrangement of a chromatic color, as a matter of course). Note that restrictive information extracted from the low frequency component may be reflected on, for example, the threshold Th of the HT processing or the quantization error Er. 
     Second Embodiment 
     Image processing according to the second embodiment of the present invention will be described below. Note that in the second embodiment, the same components as those in the first embodiment have the same reference numerals, and a detailed description thereof will be omitted. 
     In the first embodiment, a case in which the HT processing unit  106  executes error diffusion processing has been described. In the second embodiment, a case in which the dither method capable of high-speed processing is used instead of the error diffusion method will be described. 
     The arrangement of an HT processing unit  106  according to the second embodiment will be explained with reference to  FIGS. 14A and 14B . 
     The HT processing unit  106  shown in FIG.  14 A receives color material amount data K(x) of a pixel of interest, and causes an addition unit  1601  to add restrictive information to the color material amount data K(x). Note that as in the first embodiment, the initial value of the restrictive information is zero, and thus no restrictive information is substantially added in HT processing for black. 
     The HT processing unit  106  causes a quantization unit  1603  to compare the color material amount data K(x) of the pixel of interest with an element (threshold) of a threshold matrix  1602  corresponding to the position of the pixel of interest, thereby outputting binary color material amount data K′(x).  FIG. 14B  shows an overview of binarization by dither processing in which the color material amount data K(x) is compared with a corresponding threshold Th(x) of the threshold matrix  1602 , and undergoes binarization to obtain the color material amount data K(x)′, as represented by: 
       if( K ( x )≦ Th ( x ))
 
         K ( x )′=0;
 
       if( K ( x )&gt; Th ( x )) 
         K ( x )′=255;  (27)
 
     The above quantization is performed for all the pixels of the color material amount data K, thereby deciding the ON/OFF pattern (dot arrangement) of K dots. Note that the threshold matrix  1602  is well known, and a threshold matrix such as the Bayer array, clustered dot type, and blue noise mask array is used. 
     HT processing for chromatic colors is obtained by replacing the error diffusion processing in steps S 904  to S 908  of the processing shown in  FIG. 8  with the above-described dither processing. Note that in general, the threshold matrix  1602  is desirably different from that for black. Furthermore, a different threshold matrix may be used for each chromatic color. Calculation of restrictive information is the same as that in the first embodiment, and a description thereof will be omitted. 
     Third Embodiment 
     Image processing according to the third embodiment of the present invention will be described below. Note that in the third embodiment, the same components as those in the first and second embodiments have the same reference numerals, and a detailed description thereof will be omitted. 
     In the first and second embodiments, a case in which restrictive information is generated using the same filter regardless of the density of an image has been described. In general, however, the spatial frequency characteristics of an HT processing image change depending on the density, and a problem may arise if restrictive information is generated using the same filter. For example, even if there is no problem in a low-density region of an image, granularity may be noticeable in a high-density region of the image. To the contrary, even if there is no problem in a high-density region of an image, the color unevenness may be noticeable in a low-density region of the image. To deal with this problem, in the third embodiment, a different filter is used to generate restrictive information according to the density value of an image, and the frequency characteristics of restrictive information are changed according to the density of the image, thereby preventing image quality depending on the density of the image from deteriorating. 
     HT processing according to the third embodiment is the same as that in the first or second embodiment except for calculation of restrictive information. Calculation of restrictive information according to the third embodiment will be described below. 
     Calculation of Black Restrictive Information 
       FIG. 15  is a block diagram showing the arrangement of a restrictive information calculation unit  109  for calculation of black restrictive information according to the third embodiment. Calculation of black restrictive information Kr according to the third embodiment will be described with reference to a flowchart shown in  FIG. 16 . 
     The arrangement of the restrictive information calculation unit  109  shown in  FIG. 15  is different from that in the first embodiment, in that a filter setting unit  2001  is included. The filter setting unit  2001  sets a filter to be used to calculate the restrictive information Kr, based on color material amount data K having undergone color separation. That is, based on the color material amount data K, the filter setting unit  2001  sets a color separation image data filter  601  (Fm) (S 2101 ), and sets an HT data filter  602  (LPF B ) (S 2102 ). Processing after that is the same as that in steps S 701  to S 704  shown in  FIG. 7 . 
     The filter setting unit  2001  sets the filter Fm based on the color material amount data K, for example, sets a two-dimensional Gaussian filter by: 
         Fm=F′m/ΣF′m   (28)
 
       where 
         F′m ( K,x,y )={1/(2πσ x ( K )σ y ( K ))} e   −p/2 ,
 
       and 
         p={x /(σ x ( K ))} 2 −2 {x/σ   x ( K ))}{ y /(σ y ( K ))}+{ y /(σ y ( K ))} 2 .
 
       FIGS. 17A to 17D  each show an example of a filter. Although a 3×3 square filter is shown for the sake of simplicity, a 5×5, 7×7, or 9×9 square or a 3×5, 5×7, or 5×9 rectangular may be possible. Note that the filter desirably has low-pass characteristics.  FIG. 17A  shows a filter Fm with σ x (30)=1.0 and σ y (30)=1.0.  FIG. 17B  shows a filter Fm with σ x (40)=0.9 and σ y (40)=0.9. 
     The filter setting unit  2001  sets the filter LPF B  based on the color material amount data K, for example, sets a two-dimensional Gaussian filter by: 
       LPF B =LPF′ B /ΣLPF′ B   (29)
 
       where 
       LPF′ B ( K,x,y )={1/(2πσ x ( K )σ y ( K ))} e   −p/2 ,
 
       and 
         p={x /(σ x ( K ))} 2 −2 {x/σ   x ( K ))}{ y /(σ y ( K ))}+{ y /(σ y ( K ))} 2 .
 
       FIG. 17C  shows a filter LPF B  with σ x (30)=1.0 and σ y (30)=1.0.  FIG. 17D  shows a filter LPF B  with σ x (40)=0.9 and σ y (40)=0.9. 
     As shown in  FIGS. 17A to 17D , the filter coefficients of the filters Fm and LFP B  change depending on the value of the color material amount data K. Note that although  FIGS. 17A to 17D  show a case in which the same filter coefficients are used in the filters Fm and LPF B , different filter coefficients may be used, as a matter of course. 
     Calculation of Chromatic Color Restrictive Information KC1r 
       FIG. 18  is a block diagram showing the arrangement of the restrictive information calculation unit  109  for calculation of chromatic color restrictive information according to the third embodiment. Calculation (S 911 ) of chromatic color restrictive information KC1r according to the third embodiment will be described with reference to a flowchart shown in  FIG. 19 . Note that the HT processing for chromatic colors has started from cyan. 
     The arrangement of the restrictive information calculation unit  109  shown in  FIG. 18  is different from that in the first embodiment, in that a filter setting unit  2301  is included. The filter setting unit  2301  sets a filter to be used to calculate the restrictive information KC1r, based on color material amount data K and C having undergone color separation. That is, based on the color material amount data K and C, the filter setting unit  2301  sets a filter LPFm for an HT product average processing unit  1002  (S 2401 ), and sets an HT product filter  1003  (LPF B ) (S 2402 ). Processing after that is the same as that in steps S 1101  to S 1104  shown in  FIG. 10 . 
     The filter setting unit  2301  sets the filter LPFm based on the color material amount data K and C, for example, sets a two-dimensional Gaussian filter by: 
       LPF m =LPF′ m /ΣLPF′ m   (30)
 
       where 
       LPF′ m ( K,C,x,y )={1/(2πσ x ( K,C )σ y ( K,C ))} e   −p/2 ,
 
       and 
         p={x /(σ x ( K,C ))} 2 −2 {x/σ   x ( K,C ))}{ y /(σ y ( K,C ))}+{ y /(σ y ( K,C ))} 2 .
 
     Note that the filter LPFm shown in  FIG. 9  is an example when σ x (K, C)=1 and σ y (K, C)=1. To obtain sufficient effects, the value of σ desirably becomes larger as K and C are smaller. The filter may be simply implemented by a function having one variable like σ(K+C). 
     The filter setting unit  2301  sets the filter LPF B  based on the color material amount data K and C, for example, sets a two-dimensional Gaussian filter by: 
       LPF B =LPP′ B /ΣLPF′ B   (31)
 
       where 
       LPF′ B ( K,C,x,y )={1/(2πσ x ( K )σ y ( K ))} e   −p/2 ,
 
       and 
         p={x /(σ x ( K,C ))} 2 −2 {x/σ   x ( K,C ))}{ y /(σ y ( K,C ))}+{ y /(σ y ( K,C ))} 2 .
 
     The filter LPF B  is as shown in  FIG. 17C  or  17 D but is not limited to this. Note that since σ is decided based on σ x (K, C) and σ y (K, C), it is only necessary to set σ x (10, 10)=0.9 and σ y (10, 10)=0.9 or σ x (20, 10)=1.0 and σ y (20, 10)=1.0. To obtain sufficient effects, the value of σ desirably becomes larger as K and C are smaller. The filter LPF B  preferably has low-pass characteristics. 
     For calculation of restrictive information KC2r or KC1C2r, σ for a filter need only be decided based on color material amount data having undergone color separation. For example, it is only necessary to set σ x (10, 10, 10)=0.8 and σ y (10, 10, 10)=0.8 or σ x (20, 10, 10)=0.9 and σ y (20, 10, 10)=0.9. To obtain sufficient effects, the value of σ desirably becomes larger as K, C, and M are smaller. The filter LPF B  preferably has low-pass characteristics. 
     Modification of Embodiments 
     In the above embodiments, there has been described the image processing apparatus adopting an inkjet printing method of causing the printhead  201  having a plurality of nozzles arranged in a predetermined direction to scan the print medium  202  in a direction intersecting the nozzle arrangement direction, and discharging inks on the print medium  202  to form an image. The present invention, however, is applicable to a printing apparatus (for example, the thermal transfer method) for performing printing using a method other than the inkjet method. In this case, a printhead in which printing elements for printing dots are arranged instead of the nozzles for discharging ink droplets is used. 
     The present invention is also applicable to a so-called full-line type printing apparatus for performing printing by moving the print medium  202  with respect to a printhead with a print width (length) corresponding to the width of the print medium  202 . 
     Other Embodiments 
     Aspects of the present invention can also be realized by a computer of a system or apparatus (or devices such as a CPU or MPU) that reads out and executes a program recorded on a memory device to perform the functions of the above-described embodiment(s), and by a method, the steps of which are performed by a computer of a system or apparatus by, for example, reading out and executing a program recorded on a memory device to perform the functions of the above-described embodiment(s). For this purpose, the program is provided to the computer for example via a network or from a recording medium of various types serving as the memory device (e.g., computer-readable medium). 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2012-070185, filed Mar. 26, 2012, which is hereby incorporated by reference herein in its entirety.