Appearance-based technique for rendering colors on an output device
First Claim
1. A computer-implemented method of determining colorant values for rendering a color on a target printer whereinthe color is specified by a set of values that include a lightness value LLcol that correlates with luminance and a pair of chromaticity values c1col and c2col in a two-dimensional space that spans hue and chroma, andthe target printer responds to commands specifying amounts of a set of colorants including at least two primary colorants,the method comprising the steps of:
- determining, as a first function of c1col and c2col, (a) a first set of prescribed colorant values for printing a color characterized by the c1col and c2col pair at the maximum lightness that the target printer can print for c1col and c2col, and (b) a representation of the maximum lightness, referred to as LLtop, for c1col and c2col ;
determining a darkness value Dcol of the color on the basis of LLcol and the representation of LLtop for c1col and c2col ;
determining, as a second function of the darkness value, Dcol, a second set of colorant values;
for at least some values of Dcol, adjusting the first set of prescribed colorant values downwardly to define a color having the same hue as, but a lesser chroma than, the color characterized by c1col and c2col ; and
determining a third set of colorant values by combining the first and second sets of colorant values.
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Accused Products
Abstract
A target output device such as a printer is characterized in a manner that primary colorant amounts can be determined on the basis of a pair of chromaticity coordinates in a chromaticity space having a pair of chromaticity coordinates that span hue and chroma (saturation) and a luminance correlate (monotonic function of luminance). Lightness (or darkness) of the colors is handled largely independently. This makes it possible to store the colorant information in a two-dimensional table indexed by chromaticity coordinates. The information in the table is for colors that are characterized by the highest luminance that the target printer is capable of rendering for a given chromaticity (hue and chroma).
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Citations
18 Claims
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1. A computer-implemented method of determining colorant values for rendering a color on a target printer wherein
the color is specified by a set of values that include a lightness value LLcol that correlates with luminance and a pair of chromaticity values c1col and c2col in a two-dimensional space that spans hue and chroma, and the target printer responds to commands specifying amounts of a set of colorants including at least two primary colorants, the method comprising the steps of: -
determining, as a first function of c1col and c2col, (a) a first set of prescribed colorant values for printing a color characterized by the c1col and c2col pair at the maximum lightness that the target printer can print for c1col and c2col, and (b) a representation of the maximum lightness, referred to as LLtop, for c1col and c2col ; determining a darkness value Dcol of the color on the basis of LLcol and the representation of LLtop for c1col and c2col ; determining, as a second function of the darkness value, Dcol, a second set of colorant values; for at least some values of Dcol, adjusting the first set of prescribed colorant values downwardly to define a color having the same hue as, but a lesser chroma than, the color characterized by c1col and c2col ; and determining a third set of colorant values by combining the first and second sets of colorant values. - View Dependent Claims (2)
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3. A computer-implemented method of determining colorant values for rendering a color on a target printer wherein
the color is specified by a set of values that include a lightness value LLcol that correlates with luminance and a pair of chromaticity values c1col and c2col in a two-dimensional space that spans hue and chroma, and the target printer responds to commands specifying amounts of a set of colorants including at least two primary colorants, the method comprising the steps of: -
providing a two-dimensional table, referred to as the umbrella table, characterizing the target printer, which table includes, for each particular one of a plurality of c1-c2 pairs, an entry including (a) a set of colorant values for printing a color characterized by the particular c1-c2 pair at the maximum lightness that the target printer can print for the particular c1-c2 pair, and (b) a representation of the maximum lightness, referred to as LLtop, for the particular c1-c2 pair; determining, on the basis of the umbrella table, (a) a first set of prescribed colorant values for printing a color characterized by c1col and c2col at the maximum lightness, and (b) a representation of LLtop for c1col and c2col ; providing a table, referred to as the darkness table, which table includes for each particular one of a plurality of darkness values, an entry including a set of colorant values for printing a neutral component having a darkness that bears a predetermined relationship to the particular darkness value; determining a darkness value Dcol of the color on the basis of LLcol and the representation of LLtop for c1col and c2col ; for at least some values of Dcol, adjusting the first set of prescribed colorant values downwardly to define a color having the same hue as, but a lesser chroma than, the color characterized by c1col and c2col ; determining a second set of colorant values from the darkness value Dcol and the darkness tables; and determining a third set of colorant values by combining the first and second sets of colorant values. - View Dependent Claims (4, 5)
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6. A computer implemented method of determining colorant values for rendering a color specified by a set of tristimulus values to an output device that responds to commands specifying at least two primary colorants and a form of neutral colorant, the method comprising the steps of:
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mapping the tristimulus values to a set of values that include a lightness value LL that correlates with luminance and a pair of chromaticity values c1 and c2 in a two-dimensional space that spans hue and chroma; deriving a set of parameters as a first function of the chromaticity values only, the set of parameters including primary colorant values, wherein the first function has the property that for at least some particular values of c1 and c2, the set of parameters have the same values as the set of parameters for different particular values of c1 and c2, where the different particular values of c1 and c2 define a color having the same hue and lightness as, but a lesser chroma than, the color characterized by the particular values os c1 and c2; and deriving the desired values of the neutral colorant as a second function of the set of parameters and the lightness value. - View Dependent Claims (7, 8, 9, 10)
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11. A computer-implemented method for determining colorant values for rendering, on a target output device, colors specified by a pair of chromaticity values c1col and c2.sub. col in a two-dimensional space that spans hue and chroma, the method comprising:
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accessing a data structure stored in a computer memory, said data structure including, for each particular one of a plurality of c1-c2 pairs, an entry including (a) a representation of a set of colorant values for rendering a color characterized by the particular c1-c2 pair at the maximum lightness that the target output device can render for the particular c1-c2 pair, and (b) a representation of the maximum lightness, referred to as LLtop, for the particular c1-c2 pair; and using data from the entry for the particular c1-c2 pair to determine colorant values to render the color on the target output device. - View Dependent Claims (12, 13)
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14. A computer-implemented method for determining colorant values for darkening a rendered color on a target output device, the memory comprising:
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accessing a data structure stored in a computer memory, said data structure including, for each particular one of a plurality of darkness values, an entry including (a) a representation of a set of colorant values for rendering a neutral component having a darkness that bears a predetermined relationship to the particular darkness value, and (b) a representation of a chroma reduction factor for the color to be rendered and using data from the entry for the particular darkness value to determine colorant values to darken the rendered color on the target output device.
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15. A method of characterizing a target printer, the target printer being responsive to commands specifying values for at least one of a plurality of primary colorants, the method comprising the steps of:
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printing a set of color samples with the target printer using known colorant values substantially devoid of a neutral component so as to provide the lightest color that the target printer is capable of printing at the given hue and chroma; measuring at least some of the color samples to provide respective measurements of the form c1,c2,LL where LL is a lightness value that correlates with luminance and c1 and c2 are a pair of chromaticity values in a two-dimensional space that spans hue and chroma; and associating with the measured c1 and c2 values for at least some of the color samples, the colorant values that produced that color sample, and the measured lightness value for that color sample. - View Dependent Claims (16)
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17. A method of printing a color image on a reproducing printer having a first set of colorants with a first set of maximum chroma values so as to simulate the appearance of the same image on an original printer having a second set of colorants with a second set of maximum chroma values greater than the first set of maximum chroma values, the method comprising:
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defining a function of specified chroma value that falls off gradually to a value less than 100% for a specified chroma value of 100%, the value representing the ratio of one of the first maximum chroma values to a corresponding one of the second maximum chroma values; in response to specified hue and chroma values to be rendered by the reproducing printer, applying the function to the specified chroma value; determining colorant amounts for a color having the specified hue and the specified chroma, as possibly reduced by the function; and rendering a pixel in the image using the colorant amounts so determined.
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18. A method of printing a color image on a reproducing printer having a first set of colorants so as to simulate the appearance of the same image on an original printer having a second set of colorants, the second set of colorants allowing a greater darkness value than allowed by the first set of colorants, the method comprising:
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defining a function of specified darkness value that falls off gradually to a value less than 100% for a specified darkness value of 100%, the value representing the ratio of first maximum darkness value to a the second maximum darkness value; in response to specified values to be rendered by the reproducing printer, applying the function to the specified darkness value; determining colorant amounts for a neutral color having the specified darkness, as possibly reduced by the function; and rendering a pixel in the image using the colorant amounts so determined for a neutral component.
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Specification