BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a gray conversion method and a gray conversion apparatus for carrying out a gray conversion on image data representing a color image in order to output a color image having a halftone to a monochromatic printer. 
     2. Description of the Related Art 
     Recently almost all the digital images are color images having a halftone part owing to advance in image processing, reduction in price of data storage devices such as memories and hard discs, and the like. Though color printers are being popularized, color images having a halftone are often output through a monochromatic printer. Further color images are sometimes output through a monochromatic printer for a special effect. When image data representing a color image having a halftone is input into the monochromatic printer as it is, the print output through the monochromatic printer generally becomes too strong in contrast and becomes blackish or whitish. Accordingly, in order to improve finish of the print, it is necessary to convert the color image data to halftone image data having information on density of pixels and a proper γ-characteristic. 
     In the conversion process for converting color image data to density information data, gray conversion is a very important process which governs finish of the print. 
     The gray conversion is generally effected by γ-correction or color adjustment (e.g., RGB ratio adjustment). It is preferred that both the γ-correction and the color adjustment be carried out as the gray conversion. However, there has been developed no image application software which carries out both the γ-correction and the color adjustment though there has been developed image application software which carries out only the γ-correction or the color adjustment. 
     When an image is reproduced by a monochromatic printer on the basis of color image data, finish of the prints cannot be controlled by the printer. Accordingly, the color image data should be input into the printer after subjected to the gray conversion. Though the gray conversion may be carried out by a printer driver, there has not been developed a printer driver which carries out the RGB ratio adjustment which greatly affects the result of the gray conversion. Though there has been known a printer driver which carries out the γ-correction, the result of the γ-correction cannot be known unless prints are made on the basis of the corrected image data. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing observations and description, the primary object of the present invention is to provide a gray conversion method and a gray conversion apparatus which permits the operator to carry out RGB ratio adjustment on color image data control as the gray conversion while visually recognizing the result of the RGB ratio adjustment so that the color image data can be effectively and properly converted to halftone image data having information on density of pixels. 
     In accordance with a first aspect of the present invention, there is provided a gray conversion method for converting original color image data to halftone image data having information on density of pixels by a predetermined gray conversion, the halftone image data being to be input into a monochromatic printer when a color image represented by the original color image data is to be printed by the monochromatic printer, the method characterized in that 
     said predetermined gray conversion includes an RGB ratio adjustment for setting the ratio of R, G and B in the original color image data to an optimal value, 
     an RGB ratio adjustment is carried out on reference image data (e.g., image data derived from the original color image data or image data representing a color map) employing different ratios of R, G and B, 
     the results of the predetermined gray conversions including the RGB ratio adjustments employing the different ratios of R, G and B carried out on the reference image data are graphically shown on a screen, 
     the optimal value of the ratio of R, G and B is determined on the basis of comparison of the results of the predetermined gray conversions graphically shown on the screen, and 
     the predetermined gray conversion including an RGB ratio adjustment employing the optimal value of the ratio of R, G and B is carried out on the original color image data. 
     The reference image data may be, for instance, image data derived from the original color image data or image data representing a color map. 
     The “image data derived from the original color image data” may be any image data derived from the original color image data and may be the original color image data as it is. However it is preferred in view of the facility of gray conversion that the image data derived from the original color image data be image data representing a reduced original image. 
     The “color map” is obtained by arranging, in a pattern of a pallet, colors obtained by thinning, at a predetermined ratio, colors generated by all the combinations of densities of R, G and B, and does not depend upon the original color image data to be processed. 
     The RGB ratio adjustment is to adjust the ratio of coefficients α, β and χ in the following formula. 
     
       
         Gray=αRed+βGreen+χBlue  
       
     
     It is preferred that the gray conversion method in accordance with the first aspect of the present invention be carried out by a utility which is positioned between image application software and a printer driver and operates in combination with the printer driver, a printer driver, or a print control system of an OS (operating system). 
     It is further preferred that said predetermined gray conversion includes, in addition to the RGB ratio adjustment, a γ-correction in which the γ-curve and/or the γ-value is adjusted, a γ-correction is carried out on the reference image data employing different γ-curves and/or γ-values, the results of the γ-correction carried out on the reference image data employing the different γ-curves and/or γ-values are graphically shown on a screen, the optimal γ-curve and/or the γ-value is determined on the basis of comparison of the results of the γ-correction graphically shown on the screen, and the predetermined gray conversion including an RGB ratio adjustment employing the optimal value of the ratio of R, G and B and a γ-correction employing the optimal γ-curve and/or the γ-value is carried out on the original color image data. 
     In accordance with a second aspect of the invention, there is provided a gray conversion apparatus comprising a gray conversion means which carries out a predetermined gray conversion on original color image data to convert the original color image data into halftone image data having information on density of pixels, the halftone image data being to be input into a monochromatic printer when a color image represented by the original color image data is to be printed by the monochromatic printer, the apparatus characterized in that 
     the gray conversion means includes 
     an RGB ratio adjustment means which carries out an RGB ratio adjustment for setting the ratio of R, G and B in the original color image data to an optimal value and carries out an RGB ratio adjustment on reference image data (e.g., image data derived from the original color image data or image data representing a color map) employing different ratios of R, G and B, 
     a graphic display means which graphically shows the results of the predetermined gray conversions including the RGB ratio adjustments employing the different ratios of R, G and B carried out on the reference image data on a screen, and 
     an RGB ratio determining means which determines the optimal value of the ratio of R, G and B on the basis of comparison of the results of the predetermined gray conversions graphically shown on the screen, and 
     the gray conversion means carries out the predetermined gray conversion including an RGB ratio adjustment employing the optimal value of the ratio of R, G and B determined by the RGB ratio determining means on the original color image data. 
     It is preferred that the gray conversion means be in the form of a utility which is positioned between image application software and a printer driver and operates in combination with the printer driver, in the form of a printer driver, or in the form of a print control system of an OS. 
     It is further preferred that said gray conversion means further comprises 
     a γ-correction means which carries out a γ-correction on the reference image data employing different γ-curves and/or γ-values, 
     a graphic display means which graphically shows the results of the γ-corrections carried out on the reference image data employing the different γ-curves and/or γ-values on a screen, and 
     a γ-correction condition determining means which determines the optimal γ-curve and/or the γ-value on the basis of comparison of the results of the γ-correction graphically shown on the screen, and 
     the gray conversion means carries out the predetermined gray conversion including an RGB ratio adjustment employing the optimal value of the ratio of R, G and B and a γ-correction employing the optimal γ-curve and/or the γ-value on the original color image data. 
     In accordance with the present invention, since the influence of the RGB ratio adjustment on the gray conversion is graphically shown on a screen, the result of the gray conversion can be known without making prints on the basis of a selected ratio of R, G and B, whereby a gray conversion including an RGB ratio adjustment can be efficiently carried out on color image data. 
     Further when the gray conversion is carried out by a utility which is positioned between image application software and a printer driver and operates in combination with the printer driver, a printer driver, or a print control system of an OS, the gray conversion can be carried out independently of the image application software. 
     Further when the predetermined gray conversion includes a γ-correction in addition to the RGB ratio adjustment, the results of the γ-correction carried out on the reference image data employing the different γ-curves and/or γ-values are graphically shown on a screen, the optimal γ-curve and/or the γ-value is determined on the basis of comparison of the results of the γ-correction graphically shown on the screen, and the predetermined gray conversion including an RGB ratio adjustment employing the optimal value of the ratio of R, G and B and a γ-correction employing the optimal γ-curve and/or the γ-value is carried out on the original color image data, the result of the gray conversion can be known without making prints on the basis of a selected ratio of R, G and B and a selected γ-curve and/or γ-value, whereby a gray conversion including an RGB ratio adjustment and a γ-correction can be efficiently carried out on the color image data. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view of a frame showing a user interface of a gray conversion apparatus in accordance with an embodiment of the present invention, 
     FIG. 2 is a flow chart for illustrating processing when the default box of the RGB ratio adjustment means is checked, 
     FIG. 3 is a flow chart for illustrating processing when the percentage of one of R, G and B is fixed by the lock button, 
     FIG. 4 is a flow chart for illustrating the processing when the percentage of one of R, G and B is set by the slide bar, 
     FIG. 5 is a flow chart for illustrating the processing when the γ-value of one of R, G and B is set by the slide bar, 
     FIG. 6 is a flow chart for illustrating the processing when one of two default γ-values for each of R, G and B is selected by checking one of the radio buttons, 
     FIG. 7 is a flow chart for illustrating the processing of updating the γ-curves, 
     FIG. 8 is a flow chart for illustrating the processing of drawing the γ-curves, and 
     FIG. 9 is a flow chart for illustrating the processing of updating the bit map of the gray level representation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIG. 1, a gray conversion apparatus in accordance with an embodiment of the present invention comprises an RGB ratio adjustment means  1 , a γ-correction means  2  including γ-curve output means  2   a ,  2   b  and  2   c  and a gray level display means  3  which graphically shows the influence of the RGB ratio adjustment and the γ-correction on the gray conversion. 
     In the RGB ratio adjustment means  1 , the ratio of Red, Green and Blue is adjusted by controlling respective slide bars  4  with a mouse pointer (not shown). The RGB ratio is expressed in percentage, and the percentages for R, G and B are set so that the sum of their percentages is fixed to 100. That is, when the percentage of one of R, G and B is changed, the percentages of the other two colors are automatically changed so that the sum of the percentages of the three colors become 100. This process will be described in detail later with reference to FIG.  4 . 
     The RGB ratio adjustment means  1  is provided with three lock buttons  5 , each for one of R, G and B. When the lock button  5  for one of R, G and B is clicked, the percentage of the color is fixed with the percentages of the other two colors variable. This process will be described in detail later with reference to FIG.  3 . 
     The RGB ratio adjustment means  1  is further provided with a default box. When the default box is checked, the percentages of R, G and B are automatically set to preset values (the default values). This process will be described in detail later with reference to FIG.  2 . 
     In the γ-correction means  2 , three γ-curves  9   a ,  9   b  and  9   c  for R, G and B are displayed for given γ-values  6 . The γ-values  6  can be changed by controlling slide bars  7 . That is, when the γ-values  6  are changed by controlling slide bars  7 , γ-curves  9   a ,  9   b  and  9   c  corresponding to the changed γ-values are shown in the γ-correction means  2 . Two default γ-values  8  are set for each of R, G and B. One of two default γ-values  8  for each of R, G and B is selected by checking a radio button. One of the two default γ-values  8  for each of R, G and B is 1.0000 (linear) and the other is, for instance, 1.4000 which is recommended by the printer maker (ideal). When 1.0000 is selected, the γ-curve is a straight line inclined at 45° and the γ-correction becomes in effective. That is, input values are all output unchanged. 
     By controlling the slide bar  7 , the γ-value for each of R, G and B can be varied within the range of 0.2500 to 6.55535. 
     The γ-curves  9   a ,  9   b  and  9   c  are displayed for the γ-values selected. 
     Each process of the γ-correction will be described in detail later with reference to FIGS. 5 to  8 . 
     The gray level display means  3  comprises a color map  10  and a gray level representation  11 . As described above, the “color map” is obtained by arranging, in a pattern of a pallet, colors obtained by thinning, at a predetermined ratio, colors generated by all the combinations of densities of R, G and B. The gray level representation  11  shows the result of a gray conversion carried out on the respective pixels of the color map  10 . The gray level governs finish of the print when a color image is output through a monochromatic printer. The gray level representation  11  is updated each time the RGB ratio is changed or the γ-values are changed. The processing of updating the bit map of the gray level representation  11  will be described in detail later with reference to FIG.  9 . 
     FIG. 2 is a flow chart for illustrating the processing when the default box of the RGB ratio adjustment means  1  is checked. As shown in FIG. 2, when the default box is checked, the percentages of R, G and B are set to the respective default values (step S 1 ), and then the bit map of the gray level representation  11  is updated (step S 2 ). 
     FIG. 3 is a flow chart for illustrating the processing when the percentage of one of R, G and B is locked by the lock button  5 . As shown in FIG. 3, when the percentage of one of R, G and B is locked by the lock button  5 , the lock buttons  5  for the other two colors are automatically reset. (step S 3 ) 
     FIG. 4 is a flow chart for illustrating the processing when the percentage of one of R, G and B is set by the slide bar  4 . For the purpose of simplification, it is assumed here that the percentage of R is set by the slide bar  4 . 
     When the percentage of R is set by the slide bar  4 , it is determined whether the percentage of R has been locked. (step S 4 ) When it is determined that the percentage of R has been locked, the default values are reset (step S 6 ) after the lock button  5  for R is reset (step S 5 ). When it is determined that the percentage of R has not been locked, the default values are reset (step S 6 ) immediately after step S 4 . Then it is determined whether the percentage of G has been locked. (step S 7 ) When it is determined that the percentage of G has been locked, the percentage of B is changed so that the sum of the percentages of R, G and B becomes 100. (step S 8 ) Then the bit map of the gray level representation  11  is updated according to the percentages of R, G and B thus changed. (step S 12 ) When it is determined that the percentage of G has not been locked, it is determined whether the percentage of B has been locked. (step S 9 ) When it is determined that the percentage of B has been locked, the percentage of G is changed so that the sum of the percentages of R, G and B becomes 100. (step S 10 ) Then the bit map of the gray level representation  11  is updated according to the percentages of R, G and B thus changed. (step S 12 ) When it is determined that the percentage of B has not been locked, the percentages of G and B are changed so that the sum of the percentages of R, G and B becomes 100. (step S 11 ) Then the bit map of the gray level representation  11  is updated according to the percentages of R, G and B thus changed. (step S 12 ) 
     While visually checking the result of the gray conversion in this manner, the RGB ratio is adjusted. 
     FIG. 5 is a flow chart for illustrating the processing when the γ-values of R, G and B are set by the slide bar  7 . When the slide bar  7  for R, G or B is moved, the radio buttons (linear and ideal) for the color are reset (step S 13 ) and the γ-curve for the color is changed according to the γ-value set by the slide bar  7  (step S 14 ). At the same time, the bit map of the gray level representation  11  is updated according to the γ-value set by the slide bar  7 . (step S 15 ) 
     FIG. 6 is a flow chart for illustrating the processing when one of the two default γ-values  8  for each of R, G and B is selected by checking one of the radio buttons. When one of the two default γ-values  8  of a color is selected by checking one of the radio buttons, the other radio button is reset (step S 16 ) and the γ-curve for the color is changed according to the default γ-value selected (step S 17 ). At the same time, the bit map of the gray level representation  11  is updated according to the default γ-value selected. (step S 18 ) 
     While visually checking the result of the gray conversion in this manner, the γ-correction is adjusted. 
     FIG. 7 is a flow chart for illustrating the processing of updating the γ-curves. It is determined whether the γ-value is unchanged, or whether the newly set γ-value is equal to the preceding value. (step S 19 ) When it is determined that the γ-value is kept unchanged, the γ-curve is not updated. Whereas, when it is determined that the γ-value is changed, the value of 1/(newly set γ-value) is taken as Y (step S 20 ). Then the values of Buffer [X] are calculated according to formula, Buffer [X]=(X) Y , and stored. (steps S 21 , S 22  and S 23 ) X stands for integers from 0 to 255. Then γ-curves  9   a ,  9   b  and  9   c  are drawn according to the drawing processing shown in FIG. 8 (steps S 25  and S 26 ) with the values of X as the abscissa and the calculated values of Buffer [X] as the ordinate. (step S 24 ) 
     FIG. 9 is a flow chart for illustrating the processing of updating the bit map of the gray level representation. In this flow chart, the values of R-,G- and B-components of each pixel on the color map  10  are first extracted. (steps S 27  and S 28 ) Then the extracted values of R-, G- and B-components of each pixel on the color map  10  are subjected to γ-correction to calculate corrected values of R-, G- and B-components. (step S 29 ) The corrected values of R-, G- and B-components are subjected to the RGB ratio adjustment according to formula 
     
       
         Gray=αRed+βGreen+χBlue  
       
     
     and the gray level is calculated. (step S 30 ) Thereafter, the bit map is updated using the calculated gray level of each pixel. 
     Though, in the embodiment described above, the result of the RGB ratio adjustment and the γ-correction are graphically shown by the use of a color map  10 , for instance, the original color image data may be employed in place of the color map  10 . In this case, the influence of the RGB ratio adjustment and the γ-correction on the monochromatic image to be output from the printer can be directly shown. It is preferred in view of the facility of gray conversion that image data representing a reduced original image be used in place of the original color image data. 
     The present invention need not be limited to the embodiment described above, but the embodiment may be variously modified. 
     For example, though, in the embodiment described above, the gray conversion is carried out by a utility separate from the printer driver, the utility may be modularized and incorporated in a printer driver or a print control system such as a print manager of an OS. 
     Further, though in the embodiment described above, the RGB ratio and the γ-value are set by moving slide bars, the RGB ratio and the γ-value may be input as figures through a keyboard.
An original color image data is converted to halftone image data having information on density of pixels by a predetermined gray conversion. The predetermined gray conversion includes an RGB ratio adjustment, an RGB ratio adjustment is carried out on reference image data employing different ratios of R, G and B, the results of the predetermined gray conversions including the RGB ratio adjustments employing the different ratios of R, G and B carried out on the reference image data are graphically shown on a screen, an optimal value of the ratio of R, G and B is determined on the basis of comparison of the results of the predetermined gray conversions graphically shown on the screen, and the predetermined gray conversion including an RGB ratio adjustment employing the optimal value of the ratio of R, G and B is carried out on the original color image data.