If we add red and blue light together in roughly equal proportions, the result- ing mixture is a purplish color called magenta. Adding together red and green results in yellow, and adding together blue and green results in cyan, an aquamarine-like color. Another way to view this same information is to notice that magenta is characterized by the absence of green. It was made by adding together red and blue but no green. So magenta is the negative of green. Likewise yellow is negative blue and cyan is negative red. In the RGB space, we add selected amounts of light of the colors to black to get the desired color. In the cyan, magenta, yellow — or CMY — space, we start with
FIGURE 7.3 The RGB Color Space. This shows a simplifi ed diagram of how the RGB color space is laid out in three-dimensional Cartesian coordinates. Note that the blue axis is really on the bottom-left corner of the cube, and not visible in this drawing. The bottom-front, left corner is the origin, which is black since there is a zero amount of each light. The upper-rear, right corner is white, full amounts of each light.
white light and use different-strength fi lters colored cyan, magenta, or yellow to subtract different amounts of the opposite colors. Hence, the CMY space is known as subtractive and the colors are the subtractive primaries .
The CMY space is shown as a cube in Figure 7.4 . Note that the origin is white and the opposite corner is black — just the reverse of the RGB space. Note that again the fi neness of the scales is adjustable. In 8-bit color, there will be 256 steps, in 12-bit there will be 4096, and in 16-bit there will be 65,536. Respectively, the number of colors in each of these spaces will be 16,777,216 colors, 68,719,476,736 colors, and 281,474,976,710,656 colors.
The CMY space is used for situations where the light source is not a part of the display. For example, a paper print is viewed under ambient light. A transparency is viewed in a projector by means of the projector ’s lamp. The point is that we start with white light and then subtract selected amounts of light of within certain wavelength bands by means of varying the amounts of the three dyes (or fi lters): cyan, magenta, and yellow. The result is that the viewer is allowed to see the red, green, and blue light that remains.
A variant of CMY is CMYK, where the K stands for black. Note that equal amounts of cyan, magenta, and yellow dye result in gray. This is equivalent to the same amount of neutral or black dye. That is, 0.3 density units of cyan in combination with 0.3 density units of magenta dye, in combination with 0.3 density units of yellow dye is equivalent to 0.3 units of black dye (note the equivalence is not exact, but easily predicted). A patch of color comprising:
0.3 units of cyan dye, 0.5 units of magenta dye, and 0.6 units of yellow dye
FIGURE 7.4 The CMY Color Space. This shows a simplifi ed diagram of how the RGB color space is laid out in three-dimensional Cartesian coordinates. Note that the yellow axis is really on the bottom-left corner of the cube, and not visible in this drawing. The bottom-front, left corner is the origin, which is white since there is a zero amount of each dye and the full white light comes through. The upper-rear, right corner is black, full amounts of each dye.
C H A P T E R 7 : Color Space
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is equivalent to:
0.0 units of cyan dye, 0.2 units of magenta dye, 0.3 units of yellow dye and 0.3 units of black dye
The CMY system would use a total of 1.4 units of dye (0.3 ⫹ 0.5 ⫹ 0.6). The equivalent CMYK patch has a total of 0.8 units of dye (0.0 ⫹ 0.2 ⫹ 0.3 ⫹ 0.3). Remember from Beer’s Law the optical density is proportional to the amount of actual dye in the medium. The result is a savings of 0.6 units of dye. In large printing jobs, where the cost of ink is signifi cant, or in making multiple prints on a color printer, the difference can add up quickly. As a result, the CMYK system is used widely in printing applications.
It is important to note that the K dimension is not really a fourth dimen- sion. Instead it is derived from the C, M, and Y channels and so the CMYK system is really a three-dimensional system. And because of the simple rela- tionship among the primaries in the RGB and CMY color spaces, it is easy to convert from one to the other.
In summary, the additive and subtractive primary color spaces are intui- tive and work with primaries that are recognizable colors. They work well for identifying colors and for making basic calculations. The problem with them is that they do not react the way that people actually see. For example, if you had an RGB color patch but thought it was too light, you could reduce the amounts of red, green, and blue light in that patch in the hope of making it darker. The problem is that it requires three separate changes to do that and the resulting patch will probably appear to have shifted in hue (basic color) and possibly changed in its vibrancy at the same time. Through repeated adjustments you will eventually get the result you sought, but it will not be easy. And if you changed the whole image instead of only one small part of it, the rest of the image might have changed in curious ways.