Graphic designers and photographers know the pain of translating RGB image information on screen into CMYK print on paper. Even with the best understanding of the alchemy of color management, RGB displays and CMYK printing processes simply cannot recreate the same hues and intensities of every color. Apple has filed for a patent on an idea to help those whose bread and butter still involves printing: make the display CMYK.

Most displays use the additive red-green-blue color model, which attempts to mimic how our eyes interpret color. By mixing various intensities of red, green, and blue light in each pixel location, displays can reproduce some 16.7 million combinations of hue and intensity. Even the best displays, however, can only reproduce a limited subset of the colors most humans can perceive.

Printing processes, on the other hand, use a subtractive cyan-magenta-yellow color model. By increasing the amount of cyan, magenta, or yellow pigments or dyes, wavelengths are gradually subtracted from white light to produce a range of colors. Photographic prints use CMY dyes, whereas many inkjets and most commercial printing processes add black ink ('B' already stands for blue, so 'K' stands for black, hence CMYK) to reduce the amount of ink needed to make black. This also accommodates for the fact that most CMY inks alone can't make a very deep, rich black.

This diagram compares the gamuts of sRGB, AdobeRGB, CMYK, and Hexachrome to the CIE color space.

Due to the differences between transmissive light and reflected light, as well as the fact that inks don't necessarily match the theoretical cyan, magenta, and yellow colors needed to reproduce a wide range of colors, CMYK processes generally reproduce a much smaller subset of the colors of a typical RGB display. Color management attempts to mitigate this discrepancy, but it is far from perfect. Inkjet manufacturers and printing ink manufacturers have attempted to expand the gamut of possible colors by adding additional colors to the mix.

However, Apple's senior scientist Gabriel Marcu and director of display engineering Wei Chen considered a different solution. They propose building a display that uses CMY subtractive filters to generate colors instead of RGB subpixels. Apple filed a patent application for their invention, "Subtractive Display," in September 2009, which was published by the USPTO late last week.

If the filters are formulated correctly, the display could more closely match the color gamut of traditional printing processes, making it easier for designers and photographers to better visualize the final product onscreen. Such a display would have limited applications, since print production is generally on the decline as more media is consumed on computers, HDTVs, and mobile devices like the iPad or smartphones. But for those that depend on reliable, accurate print output, such a display could be indispensable.