Abstract:
A method of adjusting a non-black primary color value including determining first, second and third portions of the non-black primary color value; determining an adjusted first portion using a first one-dimensional look-up table, wherein the first portion is used as an index to the first one-dimensional look-up table; determining an adjusted second portion using a second one-dimensional look-up table, wherein the second portion is used as an index to the second one-dimensional look-up table; and summing the adjusted first portion, the adjusted second portion and the third portion.

Description:
BACKGROUND 
   The subject disclosure is generally directed to color printing. 
   Color printers typically employ a plurality of inks of different primary colors to generate a spectrum of colors. For example, many color printers use four ink colors: cyan (C), magenta (M), yellow (Y) and black (K). Color images are formed on a receiving substrate or medium by placing combinations of zero or more dots of C, M, Y or K ink at each pixel location. Cyan, magenta and yellow are typically employed since a wide range of colors can be produced by different combinations of these inks. 
   It can be difficult to calibrate color printers. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic block diagram of an embodiment of a printing system. 
       FIG. 2  is a schematic diagram of an embodiment of an image processing pipeline. 
       FIG. 3  is an embodiment of a procedure for adjusting secondary colors and gray using a plurality of one-dimensional look-up tables for each non-black primary color. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic block diagram of an embodiment of a printing apparatus that includes an interface  31  that receives print data, for example from a host computer, and stores the print data in a buffer memory  33 . A processor  35  is configured to process the print data to produce bit mapped raster data that is stored in a memory  37 . A print engine  39  prints an image pursuant to the bit map raster data generated by the processor  35 . The print engine  39  can be an electrophotographic print engine or an ink jet print engine, for example. 
     FIG. 2  is a block diagram of an embodiment of a portion of an image processing pipeline for a color printer. At  111  input color data is transformed to printer color space such as CMYK which employs the primary colors of cyan, magenta, yellow and black, for example using color tables. At  113  gray component replacement is performed on printer primary color data, wherein some amounts of the non-black primary colors are replaced with black. At  115  gray balancing and secondary color adjustment are performed for each non-black primary color using a plurality of one-dimensional tone reproduction curves (TRCs), as disclosed more particularly herein. At  117  linearization is performed. At  119  half-toning is performed, and at  121  printing takes place. In practice, the step  117  of linearization can be merged into the step  119  of half toning. The one-dimensional TRCs employed in the disclosed embodiments can be implemented as one-dimensional look-up tables, for example. 
   The adjustments at  115  and  117  can be employed to calibrate the printing apparatus, for example. For ease of reference, the embodiments disclosed herein employ C, M, Y, K primary colors. 
     FIG. 3  is a schematic flow diagram of an embodiment of a procedure that can be employed at  115  ( FIG. 2 ) for adjusting a gray component and a secondary color component of non-black primary color values, such as CMY, using a plurality of one-dimensional TRCs for each non-black primary color. 
   At  311  a GRAY component is set to the minimum of the original or input non-black primary colors C, M, Y values. As to each of such C, M, Y values, GRAY comprises the amount of a non-black primary color that contributes to the gray of the particular C, M, Y data and can be called a gray component of a non-black primary color. 
   At  313  the GRAY component is subtracted from each of the non-black primary color inputs C, M, Y to arrive at versions C 1 , M 1 , Y 1  of the non-black primary colors from which the GRAY component has been removed. 
   At  315  the secondary colors red, green and blue are set to the minimums of the appropriate combinations (M 1 , Y 1 ), (C 1 , Y 1 ), (C 1 , M 1 ) of the non-black primary color values from which the GRAY component has been removed. As to each of the C, M, Y values, green comprises the amounts of cyan and yellow that contribute to the green secondary color of the particular C, M, Y data, blue comprises the amounts of cyan and magenta that contribute to the blue secondary color of the particular C, M, Y data, and red comprises the amounts of magenta and yellow that contribute to the red secondary color of the particular C, M, Y data. It should be appreciated that in this example there is at most one non-zero secondary color for any set of C, M, Y values, since at least one of C 1 , M 1 , Y 1  as calculated at  313  is zero. The portion of a non-black primary color value that contributes to a secondary color can be conveniently called a secondary color component of the non-black primary color value (e.g., the green component of the cyan color value, the blue component of the cyan color value, etc.). For ease of reference, a secondary color component of a primary color value can sometimes be called a secondary component of a primary color (e.g., the blue component of magenta, the red component of magenta, etc.). 
   At  317  a primary component of each of the non-black primary color values is obtained by subtracting appropriate secondary color components from the non-black primary color values from which the GRAY component has been removed. In particular, green and blue are removed from C 1  to obtain a primary component C 2  of the cyan color value C, red and blue are removed from M 1  to obtain a primary component M 2  of the magenta color value M, and red and green are removed from Y 1  to obtain a primary component Y 2  of the yellow color value Y. While the C, M, Y values form at most one secondary color in this example, the equations at  317  are generalized to include the three possible secondary colors. 
   At  319  respective adjusted non-black primary color values are obtained by summing for each non-black primary color a respective adjusted gray component that has been adjusted for balancing gray, a respective adjusted secondary color component, and the primary component. 
   In particular as to cyan, at  319  a gray and secondary color adjusted cyan color value C_out can be obtained by:
 
 C _out= C 2+green —   C [green]+blue —   C [blue]+graybal —   C [GRAY]
 
wherein C 2  comprises the primary component of the cyan color value C, green_C[ ] comprises a one-dimensional green related cyan TRC that is indexed by the green component of the cyan color value C, blue_C[ ] comprises a one-dimensional blue related cyan TRC that is indexed by the blue component of the cyan color value C, and graybal_C[ ] is a one-dimensional gray balancing related cyan TRC for balancing gray that is indexed by the gray component.
 
   In particular as to magenta, at  319  a gray and secondary color adjusted magenta color value M_out can be obtained by:
 
 M _out= M 2+red —   M [red]+blue —   M [blue]+graybal —   M [GRAY]
 
wherein M 2  comprises the primary component of the magenta color value M, red_M[ ] comprises a one-dimensional red related magenta TRC that is indexed by the red component of the magenta color value M, blue_M[ ] comprises a one-dimensional blue related magenta TRC that is indexed by the blue component of the magenta color value M, and graybal_M[ ] is a one-dimensional gray balancing related magenta TRC for balancing gray that is indexed by the gray component.
 
   In particular as to yellow, at  319  a gray and secondary color adjusted yellow color value Y_out can be obtained by:
 
 Y _out= Y 2+red —   Y [red]+green —   Y [green]+graybal —   Y [GRAY]
 
wherein Y 2  comprises the primary component of the yellow color value Y, red_Y[ ] comprises a one-dimensional red related yellow TRC that is indexed by the red component of the yellow color value Y, green_Y[ ] comprises a one-dimensional green related yellow TRC that is indexed by the green component of the yellow color value Y, and graybal_Y[ ] is a one-dimensional gray balancing related yellow TRC for balancing gray that is indexed by the gray component.
 
   Generally, for each non-black primary color, a gray portion or component, a secondary color portion or component, and a primary portion or component are determined. A non-zero non-black primary color can include one or more of such components. For each non-black primary color, an adjusted gray component is obtained using the gray component as an index to a gray balancing related primary color TRC, and an adjusted secondary color component is obtained using the secondary color component as an index to a secondary color related primary color TRC. A gray and secondary color adjusted non-black primary color value is obtained by combining the primary component, the adjusted secondary color and the adjusted gray component. 
   The one-dimensional green related cyan and yellow TRCs that are indexed by the green component can be obtained by printing green patches at different coverages and linearizing them based on L*, ΔE, etc. Hue adjustment can be incorporated into the green related cyan and/or yellow TRCs to make green more cyan or more yellow, for example depending upon the desired calibration. If no hue adjustments to green are to be made, green_C[ ] and green_Y[ ] can be identical. 
   The one-dimensional blue related cyan and magenta TRCs and the red related magenta and yellow TRCs can be obtained similarly. 
   The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.