Alice in Colorland: RGB vs CMYK

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Scrunching my face at the differences between the image on my screen and the printout on the paper in my hand, the reason for the two systems commonly used for generating colors -- RGB and CMYK -- finally hit me like a chessire cat landing with a thud on the top of my head.

RGB, the color system that I spend most of my time fooling with, is based on the three colors of light that can be used to create virtually any color. This "additive" color model creates yellow by mixing red and green unlike the "subtractive" color model that I discovered as a kid with my crayons in which I could create green by mixing blue and yellow or orange by mixing yellow and red. I made peace with the dichotomy of two sets of primary colors (red, green and blue or red, yellow and blue) -- one used for light, the other pigment -- decades ago. Then came CMYK.

CMYK is another subtractive color model. As when I was working with my crayons so long ago, I can combine its yellow and its blue-like (cyan) to create green, but the similarity ends more or less right there. For red, I have to mix yellow and magenta. Huh? Unlike the color systems that have come to make sense to me, the CMYK model has as its primary colors cyan (a light greenish blue), magenta (a purplish pink), yellow and ... black! Why black? Because no amount of mixing cyan, magenta and yellow will give you black. A muddy brown is the closest you'll come. And, while, the RGB model easily produces nearly any color you can imagine, the CMYK system is trickier to work with and far brighter people than I have tried to come up with ways to convert between the two. Even so, the translation is imperfect. The disparity between the image on my screen and that on the sheet of paper in my hand was testament to the difficulty of the conversion algorithm.

Black is easy for RGB. In a system in which colors are created by altering the intensity of three colors, no color at all is black. Maximum intensity of all three colors is white. Equal amounts of the three primaries (but more than nothing and less than maximum), gives you a large palette of shades of gray.

In RGB, each color is achieved by some combination of the three primaries. For 16-bit color, we might have five bits to represent 32 intensities of red, another five bits to reprsent 32 intensities of blue and six bits to represent 64 levels of green. That's 32 * 32 * 64. This system yields 65,536 unique colors.

If you use 32-bit color, you might have more than 4 billion colors (depending on whether each primary is given 8 bits or 10-11. Even higher depth computer screens are possible -- such as 40-bit and even 64-bit color, but the ability of the human eye to detect differences between more than nine quintillion colors is highly doubtful!

The CMYK system is used for printing while RGB is used, of course, for screens. The choice of these particular colors is no accident, as you might suspect.

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