Calibration of color devices

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 a first contribution using a first one-dimensional look-up table and the first portion, wherein the primary color value is used as an index to the first one-dimensional look-up table; determining a second contribution using a second one-dimensional look-up table and the second portion, wherein the primary color value is used as an index to the second one-dimensional look-up table; determining a third contribution using a third one-dimensional look-up table and the third portion, wherein the primary color value is used as an index to the third one-dimensional look-up table; and summing the first, second and third contributions.

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.

DETAILED DESCRIPTION

FIG. 1is a schematic block diagram of an embodiment of a printing apparatus that includes an interface31that receives print data, for example from a host computer, and stores the print data in a buffer memory33. A processor35is configured to process the print data to produce bit mapped raster data that is stored in a memory37. A print engine39prints an image pursuant to the bit map raster data generated by the processor35. The print engine39can be an electrophotographic print engine or an ink jet print engine, for example.

FIG. 2is a block diagram of an embodiment of a portion of an image processing pipeline for a color printer. At111input 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. At113gray component replacement is performed on printer primary color data, wherein some amounts of the non-black primary colors are replaced with black. At115each non-black primary color is adjusted using a plurality of one-dimensional tone reproduction curves (TRCs), as disclosed more particularly herein. At117black is adjusted (e.g., linearized) using a one-dimensional TRC. At119half-toning is performed, and at121printing takes place. In practice, the step113of gray component replacement can be merged into the step111of color data transformation by applying gray component replacement to each entry in color tables employed in the color data transformation. Additionally, the step115of adjusting each non-black primary color value using a plurality of one-dimensional TRCs and the step117of adjusting black using a one-dimensional TRC can also be merged into the step111of color data transformation by further adjusting each gray component replacement adjusted entry in the color tables. The one-dimensional TRCs employed in the disclosed embodiments can be implemented as one-dimensional look-up tables, for example.

The adjustments at115and117can 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. 3is a schematic flow diagram of an embodiment of a procedure that can be employed at115(FIG. 2) for adjusting non-black primary color values, such as CMY, using a plurality of one-dimensional TRCs for each non-black primary color.

At311a 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.

At313the GRAY component is subtracted from each of the non-black primary color inputs C, M, Y to arrive at versions C1, M1, Y1of the non-black primary colors from which the GRAY component has been removed.

At315the secondary colors red, green and blue are set to the minimums of the appropriate combinations (M1, Y1), (C1, Y1), (C1, M1) 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 C1, M1, Y1as calculated at313is 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.).

At317a 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 C1to obtain a primary component C2of the cyan color value C, red and blue are removed from M1to obtain a primary component M2of the magenta color value M, and red and green are removed from Y1to obtain a primary component Y2of the yellow color value Y. While the C, M, Y values form at most one secondary color in this example, the equations at317are generalized to include the three possible secondary colors.

At319or319A (FIG. 4) respective adjusted non-black primary color values are obtained by summing for each non-black primary color a respective contribution of a gray balancing related primary color TRC for balancing gray (which can be called a gray balancing contribution), a respective contribution of a secondary color related primary color TRC (which can be called a secondary color contribution), and a respective contribution of a primary component related primary color TRC (which can be called a primary contribution), wherein the index to each of the TRCs comprises the original primary color value.

In particular as to cyan, at319an adjusted cyan color value Cadj can be obtained by:

Cadj=(C⁢⁢2⁢/⁢C)*c⁢_⁢tbl⁡[C]+(green⁢/⁢C)*green_C⁡[C]+(blue⁢/⁢C)*blue_C⁡[C]+(GRAY⁢/⁢C)*graybal_C⁡[C]
wherein (C2/C) comprises the fractional portion or relative amount of the cyan color value C that comprises the primary component of the cyan color value C, c_tbl[ ] comprises a one-dimensional primary component related cyan TRC for adjusting (e.g., linearizing) cyan, (green/C) comprises the fractional portion or relative amount of the cyan color value C that comprises the green component of the cyan color value C, green_C[ ] comprises a one-dimensional green related cyan TRC, (blue/C) comprises the fractional portion or relative amount of the cyan color value C that comprises the blue component of the cyan color value C, blue_C[ ] comprises a one-dimensional blue related cyan TRC, (GRAY/C) comprises the fractional portion or relative amount of the cyan color value C that comprises the gray component of the cyan color value C, and graybal_C[ ] is a one-dimensional gray balancing related cyan TRC for balancing gray.

In this manner, (C2/C)*c_tbl[C] comprises a contribution of a primary component related cyan TRC for adjusting cyan, wherein the primary component related cyan TRC value c_tbl[C] as indexed by the cyan color value C is scaled or weighted by the relative amount of the cyan color value C that comprises the primary component of the cyan color value C. (green/C)*green_C[C] comprises a contribution of a green related cyan TRC, wherein the green related cyan TRC value green_C[C] as indexed by the cyan color value C is scaled or weighted by the relative amount of the cyan color value C that comprises the green component of the cyan color value C. (blue/C)*blue_C[C] comprises a contribution of a blue related cyan TRC, wherein the blue related cyan TRC value blue_C[C] as indexed by the cyan color value C is scaled or weighted by the relative amount of the cyan color value C that comprises the blue component of the cyan color value C. (GRAY/C)*graybal_C[C] comprises a contribution of a gray balancing related cyan TRC for balancing gray, wherein the gray balancing related cyan TRC value graybal_C[C] is scaled or weighted by the relative amount of the cyan color value C that comprises the gray component of the cyan color value C.

The first equation at319can be rewritten as follows:

This shows that in calculating an adjusted cyan value the division by C can be performed on each of the one-dimensional TRC values. Accordingly, at319A normalized TRCs c′_tbl[ ], green′_C[ ], blue′_C[ ] and graybal′_C[ ] (wherein c′_tbl[C]=c_tbl[C]/C, green′_C[C]=green_C[C]/C, blue′_C[C]=blue_C[C]/C and graybal′_C[C]=graybal_C[C]/C) can be employed to obtain an adjusted cyan color value Cadj:

In this manner, C2*c′_tbl[C] comprises a contribution of a normalized primary component related cyan TRC for adjusting (e.g., linearizing) cyan, wherein the normalized primary component related cyan TRC value c′_tbl[C] as indexed by the cyan color value C is scaled or weighted by the amount of the cyan color value C that comprises the primary component of the cyan color value C. (green)*green′_C[C] comprises a contribution of a normalized green related cyan TRC, wherein the normalized green related cyan TRC value green′_C[C] as indexed by the cyan color value C is scaled or weighted by the amount of the cyan color value C that comprises the green component of the cyan color value C. (blue)*blue′_C[C] comprises a contribution of a normalized blue related cyan TRC, wherein the normalized blue related cyan TRC value blue′_C[C] as indexed by the cyan color value C is scaled or weighted by the amount of the cyan color value that comprises the blue component of the cyan color value C. (GRAY)*graybal′_C[C] comprises a contribution of a normalized gray balancing related cyan TRC, wherein the normalized gray balancing related cyan TRC value graybal′_C[C] is scaled or weighted by the amount of the cyan color value that comprises the gray component of the cyan color value C.

In particular as to magenta, at319an adjusted magenta color value Madj can be obtained by:

Madj=(M⁢⁢2⁢/⁢M)*m⁢_⁢tbl⁡[M]+(red⁢/⁢M*red_M⁡[M]+(blue⁢/⁢M)*blue_M⁡[M]+(GRAY⁢/⁢M)*graybal_M⁡[M]
wherein (M2/M) comprises the fractional portion or relative amount of the magenta color value M that comprises the primary component of the magenta color value M, m_tbl[ ] comprises a one-dimensional primary component related magenta TRC for adjusting (e.g., linearizing) magenta, (red/M) comprises the fractional portion or relative amount of the magenta color value M that comprises the red component of the magenta color value M, red_M[ ] comprises a one-dimensional red related magenta TRC, (blue/M) comprises the fractional portion or relative amount of the magenta color value M that comprises the blue component of the magenta color value M, blue_M[ ] comprises a one-dimensional blue related magenta TRC, (GRAY/M) comprises the fractional portion or relative amount of the magenta color value M that comprises the gray component of the magenta color value M, and graybal_M[ ] is a one-dimensional gray balancing related magenta TRC for balancing gray.

In this manner, (M2/M)*m_tbl[M] comprises a contribution of a primary component related magenta TRC for adjusting magenta, wherein the primary component related magenta TRC value m_tbl[M] as indexed by the magenta color value M is scaled or weighted by the relative amount of the magenta color value M that comprises the primary component of the magenta color value M. (red/M)*red_M[M] comprises a contribution of a red related magenta TRC, wherein the red related magenta TRC value red_M[M] as indexed by the magenta color value M is scaled or weighted by the relative amount of the magenta color value M that comprises the red component of the magenta color value M. (blue/M)*blue_M[M] comprises a contribution of a blue related magenta TRC, wherein the blue related magenta TRC value blue_M[M] as indexed by the magenta color value M is scaled or weighted by the relative amount of the magenta color value M that comprises the blue component of the magenta color value M. (GRAY/M)*graybal_M[M] comprises a contribution of a gray balancing related magenta TRC for balancing gray, wherein the gray balancing related magenta TRC value graybal_M[M] is scaled or weighted by the relative amount of the magenta color value M that comprises the gray component of the magenta color value M.

The second equation at319can be rewritten as follows:

This shows that in calculating an adjusted magenta value the division by M can be performed on each of the one-dimensional TRC values. Accordingly, at319A normalized TRCs m′_tbl[ ], red′_M[ ], blue′_M[ ] and graybal′_M[] (wherein m′_tbl[M]=m_tbl[M]/M, red′_M[M]=red_M[M]/M, blue′M[M]=blue_M[M]/M and graybal′_M[M]=graybal_M[M]/M) can be employed to obtain an adjusted magenta color value Madj:

Madj=M⁢⁢2*m′⁢_⁢tbl⁡[M]+red*red′⁢_M⁡[M]+blue*blue′⁢_M⁡[M]+GRAY*graybal′⁢_M⁡[M]
wherein M2comprises the amount or portion of the magenta color value M that comprises the primary component of magenta, m′_tbl[ ] comprises a normalized one-dimensional primary component related magenta TRC for adjusting (e.g., linearizing) magenta, red comprises the red component of the magenta color value M, red′_M[ ] comprises a normalized one-dimensional red related magenta TRC, blue comprises the blue component of the magenta color value M, blue′_M[ ] comprises a normalized one-dimensional blue related magenta TRC, GRAY comprises the gray component of the magenta color value M, and graybal′_M[ ] is a normalized one-dimensional gray balancing related magenta TRC for balancing gray.

In this manner, M2*m′_tbl[M] comprises a contribution of a normalized primary component related magenta TRC for adjusting (e.g., linearizing) magenta, wherein the normalized primary component related magenta TRC value m′_tbl[M] as indexed by the magenta color value M is scaled or weighted by the amount of the magenta color value M that comprises the primary component of the magenta color value M. (red)*red′_M[M] comprises a contribution of a normalized red related magenta TRC, wherein the normalized red related magenta TRC value red′_M[M] as indexed by the magenta color value M is scaled or weighted by the amount of the magenta color value M that comprises the red component of the magenta color value M. (blue)*blue′_M[M] comprises a contribution of a normalized blue related magenta TRC, wherein the normalized blue related magenta TRC value blue′_M[M] as indexed by the magenta color value M is scaled or weighted by the amount of the magenta color value M that comprises the blue component of the magenta color value M. (GRAY)*graybal′_M[M] comprises a contribution of a normalized gray balancing related magenta TRC, wherein the normalized gray balancing related magenta TRC value graybal′_M[M] is scaled or weighted by the amount of the magenta color value M that comprises the gray component of the magenta color value M.

In particular as to yellow, at319an adjusted yellow color value Yadj can be obtained by:

Yadj=(Y⁢⁢2⁢/⁢Y)*y⁢_⁢tbl⁡[Y]+(red⁢/⁢Y)*red_Y⁡[Y]+(green⁢/⁢Y)*green_Y⁡[Y]+(GRAY⁢/⁢Y)*graybal_Y⁡[Y]
wherein (Y2/Y) comprises the fractional portion or relative amount of the yellow color value Y that comprises the primary component of the yellow color value Y, y_tbl[ ] comprises a one-dimensional primary component related yellow TRC for adjusting (e.g., linearizing) yellow, (red/Y) comprises the fractional portion or relative amount of the yellow color value Y that comprises the red component of the yellow color value Y, red_Y[ ] comprises a one-dimensional red related yellow TRC, (green/Y) comprises the fractional portion or relative amount of the yellow color value Y that comprises the green component of the yellow color value Y, green_Y[ ] comprises a one-dimensional green related yellow TRC, (GRAY/Y) comprises the fractional portion or relative amount of the yellow color value Y that comprises the gray component of the yellow color value Y, and graybal_Y[ ] is a one-dimensional gray balancing related yellow TRC for balancing gray.

In this manner, (Y2/Y)*y_tbl[Y] comprises a contribution of a primary component related yellow TRC for adjusting yellow, wherein the primary component related yellow TRC value y_tbl[Y] as indexed by the yellow color value Y is scaled or weighted by the relative amount of the yellow color value Y that comprises the primary component of the yellow color value Y. (red/Y)*red_Y[Y] comprises a contribution of a red related yellow TRC, wherein the red related yellow TRC value red_Y[Y] as indexed by the yellow color value Y is scaled or weighted by the relative amount of the yellow color value Y that comprises the red component of the yellow color value Y. (green/Y)*green_Y[Y] comprises a contribution of a green related yellow TRC, wherein the green related yellow TRC value green_Y[Y] as indexed by the yellow color value Y is scaled or weighted by the relative amount of the yellow color value Y that comprises the green component of the yellow color value Y. (GRAY/Y)*graybal_Y[Y] comprises a contribution of a gray balancing related yellow TRC for balancing gray, wherein the gray balancing related yellow TRC value graybal_Y[Y] is scaled or weighted by the relative amount of the yellow color value Y that comprises the gray component of the yellow color value Y.

The third equation at319can be rewritten as follows:

This shows that in calculating an adjusted yellow value the division by Y can be performed on each of the one-dimensional TRC values. Accordingly, at319A normalized TRCs y′_tbl[ ], red′_Y[ ], green′_Y[ ] and graybal′_Y[ ] (wherein y′_tbl[Y]=y_tbl[Y]/Y, red′_Y[Y]=red_Y[Y]/Y, green′_Y[Y]=green_Y[Y]/Y and graybal′_Y[Y]=graybal_Y[Y]/Y) can be employed to obtain an adjusted yellow color value Yadj:

Yadj=Y⁢⁢2*y′-⁢tbl⁡[Y]+red*red′⁢_Y⁡[Y]+green*green′⁢_Y⁡[Y]+GRAY*graybal′⁢_Y⁡[Y]
wherein Y2comprises the amount or portion of the yellow color value Y that comprises the primary component of yellow, y′_tbl[ ] comprises a normalized one-dimensional primary component related yellow TRC for adjusting (e.g., linearizing) yellow, red comprises the red component of the yellow color value Y, red′_Y[ ] comprises a normalized one-dimensional red related yellow TRC, green comprises the green component of the yellow color value Y, green′_Y[ ] comprises a normalized one-dimensional green related yellow TRC, GRAY comprises the gray component of the yellow color value Y, and graybal′_Y[ ] is a normalized one-dimensional gray balancing related yellow TRC for balancing gray.

In this manner, Y2*y′_tbl[Y] comprises a contribution of a normalized primary component related yellow TRC for adjusting (e.g., linearizing) yellow, wherein the normalized primary component related yellow TRC value y′_tbl[Y] as indexed by the yellow color value Y is scaled or weighted by the amount of the yellow color value Y that comprises the primary component of the yellow color value Y. (red)*red′_Y[Y] comprises a contribution of a normalized red related yellow TRC, wherein the normalized red related yellow TRC value red′_Y[Y] as indexed by the yellow color value Y is scaled or weighted by the amount of the yellow color value Y that comprises the red component of the yellow color value Y. (green)*green′_Y[Y] comprises a contribution of a normalized green related yellow TRC, wherein the normalized green related yellow TRC value green′_Y[Y] as indexed by the yellow color value Y is scaled or weighted by the amount of the yellow color value Y that comprises the green component of the yellow color value Y. (GRAY)*graybal′_Y[Y] comprises a contribution of a normalized gray balancing related yellow TRC, wherein the normalized gray balancing related yellow TRC value graybal′_Y[Y] is scaled or weighted by the amount of the yellow color value Y that comprises the gray component of the yellow color value Y.

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, a contribution of a gray balancing related primary color TRC is determined, a contribution of a secondary color related primary color TRC is determined and a contribution of a primary component related primary color TRC is determined, for example using the primary color value as an index into the TRCs and scaling the TRC values by the amount (e.g., relative or absolute, depending upon the values of the TRCs) of the non-black primary color that the particular component comprises. An adjusted non-black primary color value is obtained by combining the contribution of the primary component related primary color TRC, the contribution of the secondary color related primary color TRC and the contribution of the gray balancing related primary color TRC.

The one-dimensional primary component related primary color TRCs that are scaled to provide primary component related primary color TRC contributions can be obtained by printing cyan patches, magenta patches, and yellow patches at different coverages and linearizing them based on L*, ΔE, etc.

The one-dimensional green related cyan and yellow TRCs that are scaled to provide green related cyan and yellow TRC contributions can be obtained by printing green patches at different coverages and linearizing them based on L*, ΔE, etc. This can be done without applying the primary component related one-dimensional TRCs to the cyan and yellow components. 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, and 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, for example without applying the one-dimensional primary component related primary color TRCs.