298 CHAPTER 8 COLOR (Web host sites) MANIPULATION, CHANNELS, AND
298 CHAPTER 8 COLOR MANIPULATION, CHANNELS, AND LAYER MODES A Review of RGB and CMY Color You ve already worked with RGB images, and you know the letters stand for red, green, and blue. But do you know why those three colors are used? Why not red, yellow, and blue, like the paints you probably mixed as a child? The answer has two parts. The first part has to do with the structure of the human eye. Our color vision is made possible by light-sensitive cells in the retina called cones. There are three types of cones: some are sensitive to red, some to green, and some to blue. Every color we can see, we perceive as a combination of these three primary colors. The second part of the answer lies with how colors of transmitted light combine to make other colors. Transmitted colors are additive. The light of one color, combined with the light of another color, is perceived as a third color that is lighter than either component. Additive Colors Computer monitors use the additive technique to depict color. Each pixel in a computer monitor is a combination of red, green, and blue lights. These three colors combine efficiently into virtually any color that the eye can perceive. If you hold a magnifying glass up to your screen, you should be able to see the separate colors. It s easiest to see in white areas that s where all three colors are lit up. Figure 8-1 gives you an idea of how the primary colors combine to make other colors. Notice that the center of the circle, where all three colors combine, is white. (It may not look white to you, an optical illusion created by the other colors nearby. Try covering the other colors so that you see only the center portion. You ll see it really is white.) Figure 8-1. The primary (outer circles) and secondary (overlapping areas) additive colors
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