How to Programmatically Convert RGB Color to CMYK
RGB (Red-Green-Blue) and CMYK (Cyan-Magenta-Yellow-Black) are different ways of representing colors. RGB is commonly used for light-producing displays like computer monitors, and represents color as the sum of its red, green, and blue components. CMYK is commonly used for printed media, and represents color as having its components subtracted through absorption by ink on paper. Converting a color from RGB to CMYK while preserving its appearance is a non-trivial process because of the variability of the reflective properties of ink and paper, but you can make an approximation with a simple algorithm.
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Instructions
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Normalize the red, green, and blue components by dividing each by its maximum possible value. For example, in a system using 8-bit numbers, divide each component by 255. Normalization ensures that each of the values lies between zero and one.
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Subtract the red component from one and store this as the cyan component. Subtract the green component from one and store this as the magenta component. Subtract the blue component from one and store this as the yellow component. These values will change after the black component is calculated.
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Set the value of the black component to the smallest of the cyan, magenta, and yellow components. Check to see if the value of the black component is one; if it is, set the cyan, magenta, and yellow components to zero and exit the conversion process. Otherwise proceed to the next step.
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Subtract the black component from the cyan component and divide by the difference between one and the black component, assigning the result as the final value of the cyan component. Subtract the black component from the magenta component and divide by the difference between one and the black component, assigning the result as the final value of the magenta component. Subtract the black component from the yellow component and divide by the difference between one and the black component, assigning the result as the final value of the yellow component. This step is summarized symbolically as follows:
C = (C-B)/(1-B)
M = (M-B)/(1-B)
Y = (Y-B)/(1-B)
These normalized CMYK components may now be used to store the color. Scale them for the system you're using as appropriate.
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